Generator Chracteristics

Generator Chracteristics
The three most important characteristics or curves of a d.c generator are
1. No-load saturation characteristic (E0/If)
It is also know as Magnetic characteristic or Open circuit Characteristic (O.C.C). It shows the reation between the no-load generated e.m.f in armature, E0 and the field or exciting current If at a given fixed speed. It is just te magnetisation curve for the material of the electromagnets.Its shape is practically the same for all generators whether separately-excited or self-excited.

A typical no load saturation curve is shown in Figure.It has generator output voltage plotted against field current.The lower straight line portion of the curve represents the air gap because the magnetic parts are not saturated. When the magnetic parts start to saturate, the curve bends over until complete saturation is reached. Then the curve becomes a straight line again.

Separately-excited Generator
The No-load saturation curve of a separately excited generator will be as shown in the above figure.It is obvous that when If is increased from its initial small value, the flux and hence generated e.m.f Eg increase irectly as curent so long as the poles are unsaturated.This is represented by straight portion in figure.But as the flux denity increases,the poles become saturated, so a greater increase If is required to produce a given increase in voltage than on the lower part of the curve.That is why the upper portion of the curve bends.

Self-excited Generator (Series or Shunt )

The O.C.C curve for self-excited generators whether shunt or series wound is shown in above figure.Due to the residal magnetism in the poles, some e.m.f (=OA) is gnerated even when If =0.Hence, the curve starts a little way up.The slight curvature at the lower end is due to magnetic inertia.It is seen that the first part of the curve is practically straight.This is due to fact that at low flux densities reluctance of iron path being negligible,total reluctance is given by the air gap reluctance which is constant.Hence,the flux and consequently,the generated e.m.f is directly proportional to the exciting current.However, at high flux densities, where μ is small,iron path reluctance becomes appreciable and straight relation between E and If no longer holds good.In other words,after point B, saturation of pole starts.However, the initial slope of the curve is determined by air-gap width.O.C.C for higher speed would lie above this curve and for lower speed,would lie below it.

Compound-wound Generator

If the series field amp-turns are such as to produce the same voltage at rate load as at no load.then th generator is flat-compounded. It should be noted, however, that even in the case of a flat-cmpounded generator, the voltage is not constant form no load to rated load. At half load, the voltage is actually greater than the rated voltage as seen from figure.

If the series field amp-turns are such that the rated voltage is greater than the no-load voltage, then generator is over-compounded.If rated voltage is less than the no -load voltage, then the generator is under-compound.

2. Internal or Total characteristic (E/Ia)
It gives the relation between the e.m.f E atually induces in the armature (after allowing for the demagnetising effect of armature reaction) and the armature current Ia.

Separately-excited Generator
Let us consider a separately-excited generator giving its rated no-load voltage of E0 for a certain constant field current.If there were no armature reaction and armature voltage drop,then this voltage would have remained constant as shown in figure by the horizontal line 1. But when the generator is loaded, the voltage falls due to these two causes, thereby gving slightly dropping characteristics.If we subtract from E0 the values of voltage drops due to armature reaction for different loads, then we get the value of E-the e.m.f actually induced in the armature under load conditions.Curve 2 is plotted in this way and is known as the internal characteristic. Series Generator

In this genarator, because field windings are in series with the armature, they carry full armature current Ia. As Ia is increased, flux and hence generated e.m.f. is also increased as shown by the curve. Curve Oa is the O.C.C. The extra exciting current necessary to neutralize the weakening effect of armature reaction at full load is given by the horizontal distance ab. Hence, point b is on the internal characteristic.

3. External characteristic (V/I)
It is also referred to as performance characteristic or sometimes voltage-regulating curve.
It gives relation between the terminal voltage V and the load current I.This curve lies below the internal characteristic because it takes in to account the voltage drop over the armature circuit resistance.The values of V are obtained by subtracting IaRa from corresponding values of E.This characteristic is of great importnce in judging the suitability of a generator for a particular purpose.It may be obtained in two ways (i) by making simultaneous measurements with a suitable voltmeter and an ammeter on a loaded generator or (ii) graphically from the O.C.C provided the armature and field resistances are known and also if the demagnetising effect or the armature reaction is known.

Figure above shows the external characteristic curves for generators with various types of excitation. If a generator, which is separately excited, is driven at constant speed and has a fixed field current, the output voltage will decrease with increased load current as shown. This decrease is due to the armature resistance and armature reaction effects. If the field flux remained constant, the generated voltage would tend to remain constant and the output voltage would be equal to the generated voltage minus the IR drop of the armature circuit. However, the demagnetizing component of armature reactions tends to decrease the flux, thus adding an additional factor, which decreases the output voltage.

In a shunt excited generator, it can be seen that the output voltage decreases faster than with separate excitation. This is due to the fact that, since the output voltage is reduced because of the armature reaction effect and armature IR drop, the field voltage is also reduced which further reduces the flux. It can also be seen that beyond a certain critical value, the shunt generator shows a reversal in trend of current values with decreasing voltages. This point of maximum current output is known as the breakdown point. At the short circuit condition, the only flux available to produce current is the residual magnetism of the armature.

To build up the voltage on a series generator, the external circuit must be connected and its resistance reduced to a comparatively low value. Since the armature is in series with the field, load current must be flowing to obtain flux in the field. As the voltage and current rise the load resistance may be increased to its normal value. As the external characteristic curve shows, the voltage output starts at zero, reaches a peak, and then falls back to zero.

The combination of a shunt field and a series field gives the best external characteristic as illustrated in Figure. The voltage drop, which occurs in the shunt machine, is compensated for by the voltage rise, which occurs in the series machine. The addition of a sufficient number of series turns offsets the armature IR drop and armature reaction effect, resulting in a flat-compound generator, which has a nearly constant voltage. If more series turns are added, the voltage may rise with load and the machine is known as an over-compound generator.

D.C Generator E.M.F Equation,Terminal Voltage and Ratings

Generator E.M.F Equation
Let
Φ = flux/pole in weber
Z = total number of armture conductors
= No.of slots x No.of conductors/slot
P = No.of generator poles
A = No.of parallel paths in armature
N = armature rotation in revolutions per minute (r.p.m)
E = e.m.f induced in any parallel path in armature
Generated e.m.f Eg = e.m.f generated in any one of the parallel paths i.e E.
Average e.m.f geneated /conductor = dΦ/dt volt (n=1)
Now, flux cut/conductor in one revolution dΦ = ΦP Wb
No.of revolutions/second = N/60
Time for one revolution, dt = 60/N second
Hence, according to Faraday's Laws of Electroagnetic Induction,
E.M.F generated/conductor is

For a simplex wave-wound generator
No.of parallel paths = 2
No.of conductors (in series) in one path = Z/2
E.M.F. generated/path is

For a simplex lap-wound generator
No.of parallel paths = P
No.of conductors (in series) in one path = Z/P
E.M.F.generated/path

In general generated e.m.f


where A = 2 - for simplex wave-winding
= P - for simplex lap-winding

Terminal Voltage
DC generator output voltage is dependent on three factors : (1) the number of conductor loops in series in the armature, (2) armature speed, and (3) magnetic field strength. In order to change the generator output, one of these three factors must be varied. The number of conductors in the armature can not be changed in a normally operating generator, and it is usually impractical to change the speed at which the armature rotates. The strength of the magnetic field, however, can be changed quite easily by varying the current through the field winding. This is the most widely used method for regulating the output voltage of a DC generator.

DC Generator Ratings
A DC generator contains four ratings.
Voltage: Voltage rating of a machine is based on the insulation type and design of
the machine.
Current: The current rating is based on the size of the conductor and the amount of
heat that can be dissipated in the generator.
Power: The power rating is based on the mechanical limitations of the device that is used to turn the generator and on the thermal limits of conductors,bearings, and other components of the generator.
Speed: Speed rating, at the upper limit, is determined by the speed at which mechanical damage is done to the machine. The lower speed rating is based on the limit for field current (as speed increases, a higher field current is necessary to produce the same voltage).

Generator Efficiency

Generator Efficiency
Various power stages in the case of a d.c generator are shown below



Following are the three gnerator efficiencies

1. Mechanical Efficiency


2. Electrical Efficiency

3.Overall or Commercial Efficiency

It is obvious that overall efficiency is the product of mechanical and electrical efficiencies. For good generators,its value may be as high as 95%.

Condition for Maximum Efficiency
In general generator efficiency = Output / (Output + losses)
The condtion for maximum efficiency of generator is given by

i.e Variable loss = Constant loss.

Generator Losses - Copper,Hysteresis, Eddy Current and Mechanical losses

Generator Losses
In dc generators, as in most electrical devices, certain forces act to decrease the efficiency. These forces, as they affect the armature, are considered as losses and may be defined as follows:
1. Copper loss in the winding 2. Magnetic Losses 3. Mechanical Losses

Copper loss
The power lost in the form of heat in the armature winding of a generator is known as Copper loss. Heat is generated any time current flows in a conductor.
loss is the Copper loss, which increases as current increases. The amount of heat generated is also proportional to the resistance of the conductor. The resistance of the conductor varies directly with its length and inversely with its cross- sectional area. Copper loss is minimized in armature windings by using large diameter wire.Copper loss is again divided as

(i) Armature copper loss
= Armature copper loss. Where Ra =resistance of armature and interpoles and series field winding etc. This loss is about 30 to 40% of full -load losses.

(ii) Field copper loss : It is the loss in series or shunt field of generator.
is the field copper loss in case of series generators, where Rse is the resistance of the series field widing.

is the field copper loss in case of shunt generators.


This loss is about 20 to 30% of F.L losses.
(iii) The loss due to brsh contact resistance.It is usually inluded in the armture copper loss.

Magnetic Losses (also known as iron or core losses)
(i) Hysteresis loss (Wh)
Hysteresis loss is a heat loss caused by the magnetic properties of the armature. When an armature core is in a magnetic field, the magnetic particles of the core tend to line up with the magnetic field. When the armature core is rotating, its magnetic field keeps changing direction. The continuous movement of the magnetic particles, as they try to align themselves with the magnetic field, produces molecular friction. This, in turn, produces heat. This heat is transmitted to the armature windings. The heat causes armature resistances to increase. To compensate for hysteresis losses, heat-treated silicon steel laminations are used in most dc generator armatures. After the steel has been formed to the proper shape, the laminations are heated and allowed to cool. This annealing process reduces the hysteresis loss to a low value.


(ii) Eddy Current Loss (We)
The core of a generator armature is made from soft iron, which is a conducting material with desirable magnetic characteristics. Any conductor will have currents induced in it when it is rotated in a magnetic field. These currents that are induced in the generator armature core are called EDDY CURRENTS. The power dissipated in the form of heat, as a result of the eddy currents, is considered a loss.

Eddy currents, just like any other electrical currents, are affected by the resistance of the material in which the currents flow. The resistance of any material is inversely proportional to its cross-sectional area. Figure, view A, shows the eddy currents induced in an armature core that is a solid piece of soft iron. Figure, view B, shows a soft iron core of the same size, but made up of several small pieces insulated from each other. This process is called lamination. The currents in each piece of the laminated core are considerably less than in the solid core because the resistance of the pieces is much higher. (Resistance is inversely proportional to cross-sectional area.) The currents in the individual pieces of the laminated core are so small that the sum of the individual currents is much less than the total of eddy currents in the solid iron core.

As you can see, eddy current losses are kept low when the core material is made up of many thin sheets of metal. Laminations in a small generator armature may be as thin as 1/64 inch. The laminations are insulated from each other by a thin coat of lacquer or, in some instances, simply by the oxidation of the surfaces. Oxidation is caused by contact with the air while the laminations are being annealed. The insulation value need not be high because the voltages induced are very small.

Most generators use armatures with laminated cores to reduce eddy current losses.

These magnetic losses are practically constant for shunt and compound-wound generators, because in their case, field current is constant.

Mechanical or Rotational Losses
These consist of
(i) friction loss at bearings and comutator.
(ii) air-friction or windage loss of rotating armature
These are about 10 to 20% of F.L losses.

Careful maintenance can be instrumental in keeping bearing friction to a minimum. Clean bearings and proper lubrication are essential to the reduction of bearing friction.Brush friction is reduced by assuring proper brush seating, using proper brushes, and maintaining proper brush tension. A smooth and clean commutator also aids in the reduction of brush friction.

Usually, magnetic and mechanical losses are collectively known as Stray Losses. These are also known as rotational losses for obvious reasons.

As said above, field Cu loss is constant for shunt and compound generators.Hence, stray losses and shunt Cu loss are constant in their case.These losses are together known as standing or constant losses Wc.

Hence, for shunt and compound generators,
Total loss = armature copper loss + Wc
Armature Cu loss is known as variable loss because it varies with the load current.
Total loss = Variable loss + constant losses Wc

Armature Reaction in Generator, Compensating Windings and InterPoles

Armature Reaction :
All current-carrying conductors produce magnetic fields. The magnetic field produced by current in the armature of a dc generator affects the flux pattern and distorts the main field. This distortion causes a shift in the neutral plane, which affects commutation. This change in the neutral plane and the reaction of the magnetic field is called armature reaction.

You know that for proper commutation, the coil short-circuited by the brushes must be in the neutral plane. Consider the operation of a simple two-pole dc generator, shown in figure. View A of the figure shows the field poles and the main magnetic field. In this the flux is distributed symmetrically with respect to polar axis,which is the line joining the centres of NS poles.The Magnetic neutral axis or plane (M.N.A)coincides with the geometrical neutral axis or plane (G.N.A).M.N.A may be defined as the axis along which no e.m.f is produced in the armature conductors or the axis which is perpendicular to the flux passing through armature.

The armature is shown in a simplified view in views B and C with the cross section of its coil represented as little circles. The symbols within the circles represent arrows. The dot (0)represents the point of the arrow coming toward you, and the cross (+) represents the tail, or feathered end, going away from you. When the armature rotates clockwise, the sides of the coil to the left will have current flowing toward you, as indicated by the dot. The side of the coil to the right will have current flowing away from you, as indicated by the cross. The field generated around each side of the coil is shown in view B of figure. This field increases in strength for each wire in the armature coil, and sets up a magnetic field almost perpendicular to the main field.

Now you have two fields — the main field, view A, and the field around the armature coil, view B. View C of figure shows how the armature field distorts the main field and how the neutral plane is shifted in the direction of rotation. If the brushes remain in the old neutral plane, they will be short- circuiting coils that have voltage induced in them. Consequently, there will be arcing between the brushes and commutator. To prevent arcing, the brushes must be shifted to the new neutral plane.Due to this brush shift,the armature conductors and hence armature current is redistribued. Some armature conductors which were earlier under the influence of N-pole come under the influence of S-pole and vice-versa.

Compensating Windings and InterPoles
Shifting the brushes to the advanced position (the new neutral plane) does not completely solve the problems of armature reaction. The effect of armature reaction varies with the load current. Therefore, each time the load current varies, the neutral plane shifts. This means the brush position must be changed each time the load current varies.

In small generators, the effects of armature reaction are reduced by actually mechanically shifting the position of the brushes. The practice of shifting the brush position for each current variation is not practiced except in small generators. In larger generators, other means are taken to eliminate armature reaction. COMPENSATING WINDINGS or INTERPOLES are used for this purpose.Their function is to neutralize the cross magnetizing effect of armature reaction. The compensating windings consist of a series of coils embedded in slots in the pole faces. These coils are connected in series with the armature in such a way that the current in them flows in opposite direction to that flowing in armature conductors directly below the pole shoes.

The series-connected compensating windings produce a magnetic field, which varies directly with armature current. Because the compensating windings are wound to produce a field that opposes the magnetic field of the armature, they tend to cancel the effects of the armature magnetic field. The neutral plane will remain stationary and in its original position for all values of armature current. Because of this, once the brushes have been set correctly, they do not have to be moved again.

It should be carefully noted that compensating winding must provide sufficient m.m.f so as to counter balance the armature m.m.f. Let

Zc = No.of compensating conuctors/pole face
Za = No.of active armature conductors/pole
Ia = Total armature current
Ia/A = Current/armature conductor
ZcIa = Za (Ia/A) or Zc = Za/A

Another way to reduce the effects of armature reaction is to place small auxiliary poles called "interpoles" between the main field poles. The interpoles have a few turns of large wire and are connected in series with the armature. Interpoles are wound and placed so that each interpole has the same magnetic polarity as the main pole ahead of it, in the direction of rotation. The field generated by the interpoles produces the same effect as the compensating winding. This field, in effect, cancels the armature reaction for all values of load current by causing a shift in the neutral plane opposite to the shift caused by armature reaction. The amount of shift caused by the interpoles will equal the shift caused by armature reaction since both shifts are a result of armature current.

Ferro Resonance - Introduction,Classification and Characterstics

Introduction
The term "Ferro-resonance ", which appeared in the literature for the first time in 1920, refers to all oscillating phenomena occurring in an electric circuit which must contain at least:

• a non-linear inductance (ferromagnetic and
  saturable),
• a capacitor,
• a voltage source (generally sinusoidal),
• low losses.

Power networks are made up of a large number of saturable inductances (power transformers, voltage measurement inductive transformers (VT), shunt reactors), as well as capacitors cables, long lines, capacitor voltage transformers, series or shunt capacitor banks,voltage grading capacitors in circuit-breakers,metalclad substations). They thus present scenarios under which ferroresonance can occur.

The main feature of this phenomenon is that more than one stable steady state response is possible for the same set of the network parameters. Transients, lightning overvoltages,energizing or deenergizing transformers or loads, occurrence or removal of faults, live works, etc...may initiate ferroresonance. The response can suddenly jump from one normal steady state response (sinusoidal at the same frequency as the source) to an another ferroresonant steady state response characterised by high overvoltages and harmonic levels which can lead to serious damage to the equipment.

A practical example of such behaviour (surprising for the uninitiated) is the deenergization of a voltage transformer by the opening of a circuit-breaker. As the transformer is still fed through grading capacitors accross the circuit-breaker, this may lead either to zero voltage at the transformer terminals or to permanent highly distorted voltage of an amplitude well over normal voltage.

To prevent the consequences of ferroresonance (untimely tripping of protection devices,destruction of equipment such as power transformers or voltage transformers, production losses,...), it is necessary to:

• understand the phenomenon,
• predict it,
• identify it and
• avoid or eliminate it.

Little is known about this complex phenomenon as it is rare and cannot be analysed or predicted by the computation methods (based on linear approximation) normally used by electrical
engineers. This lack of knowledge means that it is readily considered responsible for a number of unexplained destructions or malfunctionings of equipment.

A distinction drawn between resonance and ferroresonance will highlight the specific and some times disconcerting characteristics of ferroresonance.

Practical examples of electrical power system configurations at risk from ferroresonance are used to identify and emphasise the variety of potentially dangerous configurations.Well-informed system designers avoid putting themselves in such risky situations.

Ferro Resonance
The main differences between a ferroresonant circuit and a linear resonant circuit are for a given ω :

• its resonance possibility in a wide range of
  values of C,
• the frequency of the voltage and current waves
  which may be different from that of the sinusoidal
  voltage source,
• the existence of several stable steady state
  responses for a given configuration and values

Classification of ferroresonant modes
Experience of waveforms appearing on power systems, experiments conducted on reduced system models, together with numerical simulations, enable classification of ferroresonance states into four different types.

This classification corresponds to the steady state condition, i.e. once the transient state is over, as it is difficult for a ferroresonant circuit to distinguish the normal transient state from ferroresonant transient states. However, this in no way implies that transient ferroresonance phenomena do not present a risk for electrical equipment. Dangerous transient overvoltages can occur several system periods after an event (for example following energizing of an unloaded transformer) and persist for several power system cycles.

The four different ferroresonance types are:

• fundamental mode,
• subharmonic mode,
• quasi-periodic mode,
• chaotic mode.

The type of ferroresonance can be identified:

• either by the spectrum of the current and voltage signals,
• or by a stroboscopic image obtained by measuring current i and voltage v at a given point of the system and by plotting in plane v, i the instantaneous values at instants separated
  by a system period.

The characteristics of each type of ferroresonance are defined below.
Fundamental mode
Voltages and currents are periodic with a period T equal to the system period, and can contain a varying rate of harmonics. The signal spectrum is a discontinuous spectrum made up of the fundamental f0 of the power system and of its harmonics (2f0, 3f0 ...). The stroboscopic image is reduced to a point far removed from the point representing the normal state.

Subharmonic mode
The signals are periodic with a period nT which is a multiple of the source period. This state is
known as subharmonic n or harmonic 1/n.Subharmonic ferroresonant states are normally of odd order. The spectrum presents a fundamental equal to f0/n (where f0 is the source frequency and n is an integer) and its harmonics (frequency f0 is thus part of the spectrum).A stroboscopic plotted line reveals n points.

Quasi-periodic mode
This mode (also called pseudo-periodic) is not periodic. The spectrum is a discontinuous spectrum whose frequencies are expressed in the form: nf1+mf2 (where n and m are integers
and f1/f2 an irrational real number). The stroboscopic image shows a closed curve.

Chaotic mode
The corresponding spectrum is continuous, i.e. it is not cancelled for any frequency. The stroboscopic image is made up of completely separate points occupying an area in plane v, i known as the strange attractor.

Since it is not possible to discuss the different case studies here iam giving the links related to different cases of ferroresonance

Ferroresonace - Link1 Link2

Examples of ferroresonance in a high voltage power system - click here

Modeling Ferroresonance Phenomena in an Underground Distribution System - click here

Examples of Ferroresonance in Distribution sysems - click here

Summary of a recent ferroresonance study - click here

Classification of Generators - Series,Shunt,Compound

Classification of Generators :
Generators are usually classified according to the way in which their fields are excited.The field windings provide the excitation necessary to set up the magnetic fields in the machine. There are various types of field windings that can be used in the generator or motor circuit. In addition to the following field winding types, permanent magnet fields are used on some smaller DC products.Generators may be divided in to (a) Separately-excited generators and (b) Self-excited generators.
(a) Separately-excited generators are those whoe field magnets are energised from an independent external source of DC current.
(b) Self-excited generators are those whose field magnets are energused by the current produced by the generators themselves.Due to residual magnetism, there is always present someflux in the poles.When the armature is rotated, some e.m.f and hence some induced current is produced which is partly or fully passed through the field coils thereby strengthening the residual pole flux.

Self-excited generators are classed according to the type of field connection they use. There are three general types of field connections — SERIES-WOUND, SHUNT-WOUND (parallel), and COMPOUND-WOUND. Compound-wound generators are further classified as cumulative-compound and differential-compound.

Series-wound generator
In the series-wound generator, shown in figure, the field windings are connected in series with the armature. Current that flows in the armature flows through the external circuit and through the field windings. The external circuit connected to the generator is called the load circuit

A series-wound generator uses very low resistance field coils, which consist of a few turns of large diameter wire.

The voltage output increases as the load circuit starts drawing more current. Under low-load current conditions, the current that flows in the load and through the generator is small. Since small current means that a small magnetic field is set up by the field poles, only a small voltage is induced in the armature. If the resistance of the load decreases, the load current increases. Under this condition, more current flows through the field. This increases the magnetic field and increases the output voltage. A series-wound dc generator has the characteristic that the output voltage varies with load current. This is undesirable in most applications. For this reason, this type of generator is rarely used in everyday practice.

Shunt wound
In this field winding is connected in parallel with the armature conductors and have the full voltage of the generator applied across them.The field coils consist of many turns of small wire. They are connected in parallel with the load. In other words, they are connected across the output voltage of the armature.

Current in the field windings of a shunt-wound generator is independent of the load current (currents in parallel branches are independent of each other). Since field current, and therefore field strength, is not affected by load current, the output voltage remains more nearly constant than does the output voltage of the series-wound generator.

In actual use, the output voltage in a dc shunt-wound generator varies inversely as load current varies. The output voltage decreases as load current increases because the voltage drop across the armature resistance increases (E = IR).

In a series-wound generator, output voltage varies directly with load current. In the shunt-wound generator, output voltage varies inversely with load current. A combination of the two types can overcome the disadvantages of both. This combination of windings is called the compound-wound dc generator.

Compound-wound generator :
Compound-wound generators have a series-field winding in addition to a shunt-field winding, as shown in figure. The shunt and series windings are wound on the same pole pieces. They can be either short-shunt or long-shunt as shown in figures. In a comound generator, the shunt field is stronger than the series field.When series field aids the shunt field, generator is said to be commutatively-compounded.On the other hand if series field opposes the shunt field,the generator is said to be differentially compounded.

In the compound-wound generator when load current increases, the armature voltage decreases just as in the shunt-wound generator. This causes the voltage applied to the shunt-field winding to decrease, which results in a decrease in the magnetic field. This same increase in load current, since it flows through the series winding, causes an increase in the magnetic field produced by that winding.

By proportioning the two fields so that the decrease in the shunt field is just compensated by the increase in the series field, the output voltage remains constant. This is shown in figure, which shows the voltage characteristics of the series-, shunt-, and compound-wound generators. As you can see, by proportioning the effects of the two fields (series and shunt), a compound-wound generator provides a constant output voltage under varying load conditions. Actual curves are seldom, if ever, as perfect as shown.

Armature and its Windings

Armature :
Gramme -Ring armature
The old Gramme-Ring armature,now obselete is shown in figure view A. Each coil is connected to two commutator segments as shown. One end of coil 1 goes to segment A, and the other end of coil 1 goes to segment B. One end of coil 2 goes to segment C, and the other end of coil 2 goes to segment B. The rest of the coils are connected in a like manner, in series, around the armature. To complete the series arrangement, coil 8 connects to segment A. Therefore, each coil is in series with every other coil.

View B shows a composite view of a Gramme-ring armature. It illustrates more graphically the physical relationship of the coils and commutator locations.

The windings of a Gramme-ring armature are placed on an iron ring. A disadvantage of this arrangement is that the windings located on the inner side of the iron ring cut few lines of flux. Therefore, they have little, if any, voltage induced in them. For this reason, the Gramme-ring armature is not widely used.

Drum-type armature :
A drum-type armature is shown in figure.The armature windings are placed in slots cut in a drum-shaped iron core. Each winding completely surrounds the core so that the entire length of the conductor cuts the main magnetic field. Therefore, the total voltage induced in the armature is greater than in the Gramme-ring. You can see that the drum-type armature is much more efficient than the Gramme-ring. This accounts for the almost universal use of the drum-type armature in modem dc generators.

Armature Windings :
Drum-type armatures are wound with either of two types of windings - the Lap Winding or the Wave Winding. The difference beween the two is merely due to the different arrangement of the end connections at the front or commutator end of armature.Each winding can be arranged progressively or retrogressively and connected in simplex,duplex and triplex.The following rules,however,apply to both types of the windings:

(i)The front and back pitch are each approximately equal to the pole-pitch i.e windings should be full-pitched.This results in increased e.m.f round the coils.For special purposes,fractional-pitched windings are deliberately used.
(ii)Both pitches should be odd, otherwise it would be difficult to place the coils properly on the armature.For example if YB and YF were both even,then all the coil sides and conductors would lie either in the upper half of slots or in the lower half.Hence, it would become impossible for one side of the coil to lie in the upper half of one slot and the other side of the same coil to lie in the lower half of some other slot.
(iii) The number of commutator segments is equa to the number of slots or coils because the front ends of conductors are joined to the segments in pairs.
(iv) The winding must close upon itself i.e if we start from agiven point and move from one coil to another,then all conuctors should be traversed and we should reach the same point again without a break or discontinuty in betwen.

Lap Winding :
View A This type of winding is used in dc generators designed for high-current applications. The windings are connected to provide several parallel paths for current in the armature. For this reason, lap-wound armatures used in dc generators require several pairs of poles and brushes.

In lap winding, the finishing end of one coil is connected to a commutator segment and to the starting end of the adjacent coil situated under the same pole an so on,till all the coils have been connected.This type of winding derives its name from the fact it doubles or laps back with its succeding coils.Following points regarding simplex lap winding should be noted:

1. The back and front pitches are odd and of opposite sign.But they can't be equal. They differ by 2 or some multiple thereof.
2. Both YB and YF shpuld be nearly equal to a pole pitch.
3. The average pitch YA = (YB + YF)/2.It equals pole pitch = Z/P.
4. Commutator pitch YC = ±1.
5. Resultant pitch YR is even, being the arithmetical difference of two odd numbers i.e YR = YB - YF.
6. The number of slots for a 2-layer winding is equal to the number of coils.The number of commutator segments is also the same.
7. The number of parallel paths in the armature = mP where 'm' is the multiplicity of the winding and 'P' the number of poles.Taking the first condition, we have YB = YF ± 2m where m=1 fo simplex lap and m =2 for duplex winding etc.

• If YB > YF i.e YB = YF + 2, then we get a progressive or right-handed winding i.e a winding which progresses in the clockwise direction as seen from the comutator end.In this case YC = +1.
• If YB < size="1">F i.e YB = YF - 2,then we get a retrogressive or left-handed winding i.e one which advances in the anti-clockwise direction when seen from the commutator side.In this case YC = -1.
• Hence, it is obvious that for
  

The figures below shows the simplex lap winding in circular form and in development form.

Wave Winding
View B, shows a wave winding on a drum-type armature. This type of winding is used in dc generators employed in high-voltage applications. Notice that the two ends of each coil are connected to commutator segments separated by the distance between poles. This configuration allows the series addition of the voltages in all the windings between brushes. This type of winding only requires one pair of brushes. In practice, a practical generator may have several pairs to improve commutation.

When the end connections of the coils are spread apart as shown in Figure a wave or series winding is formed. In a wave winding there are only two paths regardless of the number of poles. Therefore, this type winding requires only two brushes but can use as many brushes as poles. Because the winding progresses in one direction round the armature in a series of 'waves' it is know as wave winding.If, after passing once round the armature,the winding falls in a slot to the left of its starting point then winding is said to be retrogressive.If, however, it falls one slot to the right, then it is progressive.

The figures below shows the simplex wave winding in circular form and in development form.

Points to note in case of Wave winding :

1. Both pitches YB and YF are odd and of the same sign.
2. Back and front pitches are nearly equal to the pole pitch and may be equal or differ by 2, in which case, they are respectively one more or one less than the average pitch.
3. Resultant pitch YR = YF + YB.
4. Commutator pitch, YC = YA (in lap winding YC = ±1 ). Also YC = (No.of commutator bars ± 1 ) / No.of pair of poles.
5. The average pitch which must be an integer is given by YA = (Z ± 2)/P = (No.of commutator bars ± 1)/No.of pair of poles.
6. The number of coils i.e NC can be found from the relation NC = (PYA ± 2)/2.
7. It is obvious from 5 that for a wave winding, the number of armature conductors with 2 either added or subtracted must be a multiple of the number of poles of the generator.This restriction eliminates many even numbers which are unsuitable for this winding.
8. The number of armature parallel paths = 2m where 'm' is the multiplicity of the winding.

Pole pitch,Coil pitch,Back pitch,Front pitch,Resultant pitch,Commutator pitch,Conductor and Coil

Pole-pitch :

It may be variously defined as

(i) The periphery of the amature divided by the number of poles of the generator i.e the distance between two adjacent poles .

(ii)It is equal to the nmber of armature conductors per pole

Conductor

The length of a wire lying in the magnetic field and in which an e.m.f is induced,is called a conductor as,for example length AB or CD in fig.

Coil and Winding Element

With reference to right side fig, the two conductors AB and CD along wth their end connections constitute one coil of the armature winding.The coil may be single turn or multi-turn coil.A single turn coil wil have two conductors.But a multi-turn coil may have many conductors per coil side.

Coil-span or Coil-pitch (Ys)

It is the distance, measured in tems o armature slots between two sides of a coil.It is,in fact, the periphery of the armature spanned by the two sides of the coil.

If the coil-pitch is equal to the pole-pitch,then winding is called full-pitched.It means that coil span is 180 electrical degrees.In this case,the coil sides lie under opposite poles,hence the induced e.m.fs in them are additive.Therefore,maximum e.m.f is induced in he coil as a whole.it being the sum of the e.m.fs induced in the two coil sides.

If the coil-pitch is less than the pole-pitch,then the winding is fractional-pitched.In tis case,there is a phase difference between the e.m.fs in the two sides of the coil.

Pitch of a Winding (Y)

In general, it may be defined as the distance round the armature between two successive conductors which are directly connected together.Or it is the istance between the beginings of two consecutive turns.

Y = YB - YF -------- for lap winding

Y = YB + YF -------- for wave winding

Back Pitch (YB)

The distance, measured in terms of the armature conductors,whic a coil advances on the bak of the armature is called back pitch.

Front Pitch (YF)

The number of armature conductors or elements spanned by a coil on the front is called the front pitch.

Alternatively, the front pitch ma be defined as the distance between the second conductor of one coil and the first conductor of the next coil which are connected together at the front i.e commutator end of the armature. Both front and back pitches for lap winding are shown in fig.

Resultant Pitch (YR)

It is the distance between the begining of one coil and the begining of the next coilto which it is connected.It is also shown in fig.

Commutator Pitch (YC)

It is the distance between the segments to which the two ends of a coil are connected.From fig it is clear that for lap winding , Yc is the difference of YB and YF where as for wave winding it is the sum of the two.

Rankine Cycle,Processes and its Variations

Rankine Cycle :
The Rankine cycle is a thermodynamic cycle. Like other thermodynamic cycles, the maximum efficiency of the Rankine cycle is given by calculating the maximum efficiency of the Carnot cycle. It is named after William John Macquorn Rankine, a Scottish polymath.

Processes of the Rankine cycle

There are four processes in the Rankine cycle, each changing the state of the working fluid. These states are identified by number in the diagram above.

• Process 4-1: First, the working fluid is pumped (ideally isentropically) from low to high pressure by a pump. Pumping requires a power input (for example mechanical or electrical).
• Process 1-2: The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a saturated vapor. Common heat sources for power plant systems are coal (or other chemical energy), natural gas, or nuclear power.
• Process 2-3: The superheated vapor expands through a turbine to generate power output. Ideally, this expansion is isentropic. This decreases the temperature and pressure of the vapor.
• Process 3-4: The vapor then enters a condenser where it is cooled to become a saturated liquid. This liquid then re-enters the pump and the cycle repeats.
  The exposed Rankine cycle can also present vapor overheating , which reduces the amount of liquid condensed after the expansion in the turbine.

Rankine cycles describe the operation of steam heat engines commonly found in power generation plants. In such vapour power plants, power is generated by alternately vaporizing and condensing a working fluid (in many cases water, although refrigerants such as ammonia may also be used).
The working fluid in a Rankine cycle follows a closed loop and is re-used constantly. Water vapour seen billowing from power plants is evaporating cooling water, not working fluid. (NB: steam is invisible until it comes in contact with cool, saturated air, at which point it condenses and forms the white billowy clouds seen leaving cooling towers).

Variables and Equations

heat input rate (energy per unit time)

mass flow rate (mass per unit time)

mechanical power used by or provided to the system (energy per unit time)

η thermodynamic efficiency of the process (power used for turbine per heat input, unitless)

h1,h2,h3,h4 these are the "specific enthalpies" at indicated points on the T-S diagram

Each of the first four equations are easily derived from the energy and mass balance for a control volume. The fifth equation defines the thermodynamic efficiency of the cycle as the ratio of net power output to heat input.

Real Rankine cycle (non-ideal)
In a real Rankine cycle, the compression by the pump and the expansion in the turbine are not isentropic. In other words, these processes are non-reversible and entropy is increased during the two processes (indicated in the figure as ΔS). This somewhat increases the power required by the pump and decreases the power generated by the turbine. It also makes calculations more involved and difficult.

Variations of the basic Rankine cycle
Two main variations of the basic Rankine cycle are used in modern practice.
Rankine cycle with reheat
In this variation, two turbines work in series. The first accepts vapor from the boiler at high pressure. After the vapor has passed through the first turbine, it re-enters the boiler and is reheated before passing through a second, lower pressure turbine. Among other advantages, this prevents the vapor from condensing during its expansion which can seriously damage the turbine blades.
Regenerative Rankine cycle
The regenerative Rankine cycle is so named because after emerging from the condenser (possibly as a subcooled liquid) the working fluid is heated by steam tapped from the hot portion of the cycle. This increases the average temperature of heat addition which in turn increases the thermodynamic efficiency of the cycle.
Organic Rankine cycle
The Organic Rankine Cycle (ORC) uses organic fluids (such as toluene) in place of water and steam. For example, this allows use of lower temperature heat sources such as solar ponds, which typically operated at around 70-90 °C. The efficiency of the cycle is much lower as a result of the lower temperature range, but this can be worthwhile, because of the lower cost involved in gathering heat at this lower temperature.
Reverse Rankine cycle
A Rankine cycle that is driven in reverse, via net work input, is the vapor-compression refrigeration cycle. Its purpose is to move heat, rather than produce work.

For more details click on the below links
ttp://en.wikipedia.org/wiki/Rankine_cycle
http://www.taftan.com/thermodynamics/RANKINE.HTM

Generator Equivalent circuit and Main parts

Generator Equivalent circuit :

The equivalent circuit of a generator and load is shown in the diagram to the right. To determine the generator's VG and RG parameters, follow this procedure:

• Before starting the generator, measure the resistance across its terminals using an ohmmeter. This is its DC internal resistance RGDC.
• Start the generator. Before connecting the load RL, measure the voltage across the generator's terminals. This is the open-circuit voltage VG.
• Connect the load as shown in the diagram, and measure the voltage across it with the generator running. This is the on-load voltage VL.
• Measure the load resistance RL, if you don't already know it.
• Calculate the generator's AC internal resistance RGAC from the formula

Note 1:The AC internal resistance of the generator when running is generally slightly higher than its DC resistance when idle. The above procedure allows you to measure both values. For rough calculations, you can omit the measurement of RGAC and assume that RGAC and RGDC are equal.
Note 2: If the generator is an AC type (distinctly not a dynamo), use an AC voltmeter for the voltage measurements.

Maximum power :
The maximum power theorem applies to generators as it does to any source of electrical energy. This theorem states that the maximum power can be obtained from the generator by making the resistance of the load equal to that of the generator. However, under this condition the power transfer efficiency is only 50%, which means that half the power generated is wasted as heat and Lorentz force or back emf inside the generator. For this reason, practical generators are not usually designed to operate at maximum power output, but at a lower power output where efficiency is greater.

Terminology:
The parts of a dynamo or related equipment can be expressed in either mechanical terms or electrical terms. Although distinctly separate, these two sets of terminology are frequently used interchangeably or in combinations that include one mechanical term and one electrical term. This causes great confusion when working with compound machines such as a brushless alternator or when conversing with people who are used to working on a machine that is configured differently than the machines that the speaker is used to.

Mechanical

• Rotor: The rotating part of an alternator, generator, dynamo or motor.
• Stator: The stationary part of an alternator, generator, dynamo or motor.

Electrical

• Armature: The power-producing component of an alternator, generator, dynamo or motor. The armature can be on either the rotor or the stator.
• Field: The magnetic field component of an alternator, generator, dynamo or motor. The field can be on either the rotor or the stator and can be either an electromagnet or a permanent magnet.

Main Parts of Generator :
Actual generator consists of the following essential parts
1. Magnetic frame or Yoke 2. Pole-cores and Pole-Shoes 3.Field Poles 4. Field Coils 5. Armature Core 6. Armature Windings or Conductors 7.Commutator 8. Brushes and Bearings

In the above figure,views A through E, shows the component parts of dc generators.

Yoke
Yoke is a outer frame. It serves two purposes.
(i) It provides mechanical support for the poles and acts as a protecting cover for the whole machine and
(ii) It carries the magnetic flux produced by the poles.

In small generators where cheapness rather than weight is the main consideration, yokes are made of cast iron. But for large machines usually cast steel or rolled steel is employed.The modern process of forming the yoke consists of rolling a steel slab round a cylndrical mandrel and then welding it at the bottom. The feet and the terminal box etc, are welded to the frame afterwards.Such yokes possess sufficient mechanical strength and have high permeability.

Pole Cores and Pole Shoes
The field magnet consist of pole ores and pole shoes. The pole shoes have two purposes
(i) they spread out the flux in the air gap and also, being larger cross section,reduce the reluctance of the magnetic path
(ii) they support the exciting coils (field coils)

Field Poles
The pole cores can be made from solid steel castings or from laminations. At the air gap, the pole usually fans out into what is known as a pole head or pole shoe. This is done to reduce the reluctance of the air gap. Normally the field coils are formed and placed on the pole cores and then the whole assembly is mounted to the yoke.

Field Coils
The field coils are those windings, which are located on the poles and set up the magnetic fields in the machine. They also usually consist of copper wire are insulated from the poles. The field coils may be either shunt windings (in parallel with the armature winding) or series windings (in series with the armature winding) or a combination of both.

Armature Core or Stack
The armature stack is made up thin magnetic steel laminations stamped from sheet steel with a blanking die. Slots are punched in the lamination with a slot die. Sometimes these two operations are done as one. The laminations are welded, riveted, bolted or bonded together.

It houses the armature conductors or coils and causes them to rotate and hence cut the magnetic flux of the field magnets.In addition to this its most important function is to provide a path of low reluctance to the flux through the armature from a N-pole to a S-pole.It is cylindrical or drum shaped and is built up of usually circular sheet stee discs or laminations approximtely 5mm thick.It is keyed to the shaft.

Armature Windings
The armature windings are usually former-wound. These are first wound in the form of a flat rectangular coils and are then pulled into their proper shape in a coil puller.Various conductors of the coil are insulated rom each other.The conductors are placed in the armature slots which are lined with tough insulating material.This slot insulation is folded over above the armature conductors placed in the slot and is secured in place by special hard wood or fibre wedges.

Commutator
A commutator is an electrical switch that periodically reverses the current in an electric motor or electrical generator.It converts the alternating current induced in the armature conductors into unidirectional current in the external load circuit.

It typically consists of a set of copper contacts, fixed around the circumference of the rotating part of the machine (the rotor), and a set of spring-loaded carbon brushes fixed to the stationary part of the machine (the stator) that complete the electrical circuit from the rotor's windings to the outside of the machine. Friction between the copper contacts and the brushes eventually causes wear to both surfaces. The carbon brushes, being made of a softer material, wear faster and are designed to be replaced easily without dismantling the machine. The copper contacts are usually inaccessible and, on small motors, are not designed to be repaired. On large motors the commutator may be re-surfaced with abrasives. Each segment of the commutator is insulated from the adjacent segments; a large motor may contain hundreds of segments.

Brushes and Bearings
The brushes whose function is to collect currnet from commutator,are usually made of carbon or graphite and are in the shape of a rectangular block. These brushes are housed in brush-holders usually of the box type variety.

Because of their reliability, ball-bearings are frequently employed,though for heavy duties,roller bearings are preferable.The ball and rollers are generally packed in hard oil for quieter operation and for reduced bearing wear,sleeve bearings are used which are lubricated by ring oilers fed from oil reservoir in the bearing bracket.

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Electrical Generator,Generator construction and working

Electrical Generator :
An electrical generator is a device that converts mechanical energy to electrical energy, generally using electromagnetic induction. The source of mechanical energy may be a reciprocating or turbine steam engine, water falling through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, or any other source of mechanical energy.

In 1831-1832 Michael Faraday discovered that a potential difference is generated between the ends of an electrical conductor that moves perpendicular to a magnetic field. He also built the first electromagnetic generator called the 'Faraday disc', a type of homopolar generator, using a copper disc rotating between the poles of a horseshoe magnet. It produced a small DC voltage, and large amounts of current.

The Dynamo was the first electrical generator capable of delivering power for industry. The dynamo uses electromagnetic principles to convert mechanical rotation into an alternating electric current. A dynamo machine consists of a stationary structure which generates a strong magnetic field, and a set of rotating windings which turn within that field. On small machines the magnetic field may be provided by a permanent magnet; larger machines have the magnetic field created by electromagnets.

The energy conversion in generator is based on the principle of the production of dynamically induced e.m.f. Whenever a conductor cuts magneticic flux , dynamically induced e.m.f is produced in it according to Faraday's Laws of Electromagnetic induction.This e.m.f causes a current to flow if the conductor circuit is closed. Hence, two basic essential parts of an electrical generator are (i) a magnetic field and (ii) a conductor or conductors which can so move as to cut the flux.

Simple loop Generator construction :
To see the construction and working of a simple loop generator Click here
Simple loop generator is having a single-turn rectangular copper coil rotating about its own axis in a magnetic field provided by either permanent magnet or electro magnets.In case of without commutator the two ends of the coil are joined to slip rings which are insulated from each other and from the central shaft.Two collecting brushes ( of carbon or copper) press against the slip rings.Their function is to collect the current induced in the coil. In this case the current waveform we obtain is alternating current ( you can see in fig). In case of with commutator the slip rings are replaced by split rings.In this case the current is unidirectional (observe in fig).

Generator working :
In figure see the case when the coil is rotating in anticlock-wise direction with out commutator. As the coil assumes successive positions in the field, the flux linked with it changes.Hence, an e.m.f is induced in it which is proportional to the rate of change of flux linkages (e=-N dΦ/dt). When the plane of the coil is at right angles to lines of flux then flux linked with the coil is maximum but rate of change of flux linkages is minimum.

It is so because in this position, the coil sides do not cut or shear the flux, rather they slide along them i.e they move parallel to them.Hence,there is no induced e.m.f in the coil.Generaly this no e.m.f is taken as the starting position i.e zero degrees position.The angle of rotation or time wil be measured from this position.

As the coil continues rotating further, the rate of change of flux linkages (and hence induced e.m.f in it ) increases till the coil rotates 90° from its startinig position. Here the coil plane is vertical (see in fig) i.e parallel to the lines of flux.As seen, the flux linked with the coil is minimum but rate of change of flux linkages is maximum. Hence , maximum e.m.f is induced in the coil when in this position.

In the next quarter revolution i.e from 90° to 180°,the flux linked with the coil gradually increases but the rate of change of flux linkages decreases.Hence,induced e.m.f decreases gradually till it becomes zero.

So,we find that in the first half revolution of the coil, no e.m.f is induced in it at 0°, maximum when the coil is at 90° position anno e.m.f when coil is at 180°.The direction of this induced e.m.f can be found by applying Fleming's Right hand rule.

In the next half revolution i.e from 180° to 360°, the variations in the magnitude of e.m.f are similar to those in the first half revolution.Its value is maximum when coil is at 270° and minimum when the coil is at 360° position.But it wil be found that th direction of induced current is reverse of the previous direction of flow.

Therefore,we find that the current which we obtain from such a simple generator reverses its direction after every half revolution.Such a current undergoing periodic reversals is known as alternating current.It should be noted that alterating current not only reverses its direction, it does not even keep its magnitude constant while flowing in any one direction.The two half- cycles may be called positive and negative half-cycles respectively.

Now see when the coil is rotating with commutator.In this case the slip rings are replaced by split rings.The split rings are made out of a conducting cylinder which is cut into two halves or segments insulated from each other by a thin sheet of mica or some other insulating material (you can see in fig). As before, the coil ends are joined to these segments on which rest the carbon or copper brushes.

In case of split rings, the positions of the segments of split rings have also reversed when the current induced in the coil reverses i.e when the curent direction reverses the brushes also comes in contact with reverse segments as that of positive half-cycle.Hence, this current is unidirectional.It should be noted that the position of the brushes is so arranged that the change over of segments from one brush to other takes place when the plane of the rotating coil is at right angles to the plane of the lines of flux.It is so because in that position, the induced e.m.f in the coil is zero.You can observe this in two cases by pausing the waveform.

Another important point is that now the current induced in the coil is alternating as before.It is only due to the rectifying action of the split-rings (also called commutator) that it becomes unidirectional in the external circuit.

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Fleming's Right hand Rule

Fleming's right hand rule (for generators) shows the direction of induced current flow when a conductor moves in a magnetic field.

The right hand is held with the thumb, first finger and second finger mutually at right angles, as shown in the diagram .

• The Thumb represents the direction of Motion of the conductor.
• The First finger represents the direction of the Field.
• The Second finger represents the direction of the induced or generated Current (in the classical direction, from positive to negative).

Thermal Power Plant Layout and Operation

Thermal Power Plant Lay out :

The above diagram is the lay out of a simplified thermal power plant and the below is also diagram of a thermal power plant.

The above diagram shows the simplest arrangement of Coal fired (Thermal) power plant.


Main parts of the plant are
1. Coal conveyor 2. Stoker 3. Pulverizer 4. Boiler 5. Coal ash 6. Air preheater 7. Electrostatic precipitator 8. Smoke stack 9. Turbine 10. Condenser 11. Transformers 12. Cooling towers
13. Generator 14. High - votge power lines

Basic Operation :A thermal power plant basically works on Rankine cycle.
Coal conveyor : This is a belt type of arrangement.With this coal is transported from coal storage place in power plant to the place near by boiler.

Stoker : The coal which is brought near by boiler has to put in boiler furnance for combustion.This stoker is a mechanical device for feeding coal to a furnace.

Pulverizer : The coal is put in the boiler after pulverization.For this pulverizer is used.A pulverizer is a device for grinding coal for combustion in a furnace in a power plant.

Types of Pulverizers
Ball and Tube Mill
Ball mill is a pulverizer that consists of a horizontal rotating cylinder, up to three diameters in length, containing a charge of tumbling or cascading steel balls, pebbles, or rods.
Tube mill is a revolving cylinder of up to five diameters in length used for fine pulverization of ore, rock, and other such materials; the material, mixed with water, is fed into the chamber from one end, and passes out the other end as slime.
Ring and Ball
This type consists of two rings separated by a series of large balls. The lower ring rotates, while the upper ring presses down on the balls via a set of spring and adjuster assemblies. Coal is introduced into the center or side of the pulverizer (depending on the design) and is ground as the lower ring rotates causing the balls to orbit between the upper and lower rings. The coal is carried out of the mill by the flow of air moving through it. The size of the coal particals released from the grinding section of the mill is determined by a classifer separator. These mills are typically produced by B&W (Babcock and Wilcox).

Boiler : Now that pulverized coal is put in boiler furnance.Boiler is an enclosed vessel in which water is heated and circulated until the water is turned in to steam at the required pressure.

Coal is burned inside the combustion chamber of boiler.The products of combustion are nothing but gases.These gases which are at high temperature vaporize the water inside the boiler to steam.Some times this steam is further heated in a superheater as higher the steam pressure and temperature the greater efficiency the engine will have in converting the heat in steam in to mechanical work. This steam at high pressure and tempeture is used directly as a heating medium, or as the working fluid in a prime mover to convert thermal energy to mechanical work, which in turn may be converted to electrical energy. Although other fluids are sometimes used for these purposes, water is by far the most common because of its economy and suitable thermodynamic characteristics.

Classification of Boilers
Bolilers are classified as
Fire tube boilers : In fire tube boilers hot gases are passed through the tubes and water surrounds these tubes. These are simple,compact and rugged in construction.Depending on whether the tubes are vertical or horizontal these are further classified as vertical and horizontal tube boilers.In this since the water volume is more,circulation will be poor.So they can't meet quickly the changes in steam demand.High pressures of steam are not possible,maximum pressure that can be attained is about 17.5kg/sq cm.Due to large quantity of water in the drain it requires more time for steam raising.The steam attained is generally wet,economical for low pressures.The outut of the boiler is also limited.

Water tube boilers : In these boilers water is inside the tubes and hot gases are outside the tubes.They consists of drums and tubes.They may contain any number of drums (you can see 2 drums in fig).Feed water enters the boiler to one drum (here it is drum below the boiler).This water circulates through the tubes connected external to drums.Hot gases which surrounds these tubes wil convert the water in tubes in to steam.This steam is passed through tubes and collected at the top of the drum since it is of light weight.So the drums store steam and water (upper drum).The entire steam is collected in one drum and it is taken out from there (see in laout fig).As the movement of water in the water tubes is high, so rate of heat transfer also becomes high resulting in greater efficiency.They produce high pressure , easily accessible and can respond quickly to changes in steam demand.These are also classified as vertical,horizontal and inclined tube depending on the arrangement of the tubes.These are of less weight and less liable to explosion.Large heating surfaces can be obtained by use of large number of tubes.We can attain pressure as high as 125 kg/sq cm and temperatures from 315 to 575 centigrade.

Superheater : Most of the modern boliers are having superheater and reheater arrangement. Superheater is a component of a steam-generating unit in which steam, after it has left the boiler drum, is heated above its saturation temperature. The amount of superheat added to the steam is influenced by the location, arrangement, and amount of superheater surface installed, as well as the rating of the boiler. The superheater may consist of one or more stages of tube banks arranged to effectively transfer heat from the products of combustion.Superheaters are classified as convection , radiant or combination of these.

Reheater : Some of the heat of superheated steam is used to rotate the turbine where it loses some of its energy.Reheater is also steam boiler component in which heat is added to this intermediate-pressure steam, which has given up some of its energy in expansion through the high-pressure turbine. The steam after reheating is used to rotate the second steam turbine (see Layout fig) where the heat is converted to mechanical energy.This mechanical energy is used to run the alternator, which is coupled to turbine , there by generating elecrical energy.

Condenser : Steam after rotating staem turbine comes to condenser.Condenser refers here to the shell and tube heat exchanger (or surface condenser) installed at the outlet of every steam turbine in Thermal power stations of utility companies generally. These condensers are heat exchangers which convert steam from its gaseous to its liquid state, also known as phase transition. In so doing, the latent heat of steam is given out inside the condenser. Where water is in short supply an air cooled condenser is often used. An air cooled condenser is however significantly more expensive and cannot achieve as low a steam turbine backpressure (and therefore less efficient) as a surface condenser.

The purpose is to condense the outlet (or exhaust) steam from steam turbine to obtain maximum efficiency and also to get the condensed steam in the form of pure water, otherwise known as condensate, back to steam generator or (boiler) as boiler feed water.

Why it is required ?
The steam turbine itself is a device to convert the heat in steam to mechanical power. The difference between the heat of steam per unit weight at the inlet to turbine and the heat of steam per unit weight at the outlet to turbine represents the heat given out (or heat drop) in the steam turbine which is converted to mechanical power. The heat drop per unit weight of steam is also measured by the word enthalpy drop. Therefore the more the conversion of heat per pound (or kilogram) of steam to mechanical power in the turbine, the better is its performance or otherwise known as efficiency. By condensing the exhaust steam of turbine, the exhaust pressure is brought down below atmospheric pressure from above atmospheric pressure, increasing the steam pressure drop between inlet and exhaust of steam turbine. This further reduction in exhaust pressure gives out more heat per unit weight of steam input to the steam turbine, for conversion to mechanical power. Most of the heat liberated due to condensing, i.e., latent heat of steam, is carried away by the cooling medium. (water inside tubes in a surface condenser, or droplets in a spray condenser (Heller system) or air around tubes in an air-cooled condenser).

Condensers are classified as (i) Jet condensers or contact condensers (ii) Surface condensers.
In jet condensers the steam to be condensed mixes with the cooling water and the temperature of the condensate and the cooling water is same when leaving the condenser; and the condensate can't be recovered for use as feed water to the boiler; heat transfer is by direct conduction.

In surface condensers there is no direct contact between the steam to be condensed and the circulating cooling water. There is a wall interposed between them through heat must be convectively transferred.The temperature of the condensate may be higher than the temperature of the cooling water at outlet and the condnsate is recovered as feed water to the boiler.Both the cooling water and the condensate are separetely with drawn.Because of this advantage surface condensers are used in thermal power plants.Final output of condenser is water at low temperature is passed to high pressure feed water heater,it is heated and again passed as feed water to the boiler.Since we are passing water at high temperature as feed water the temperature inside the boiler does not dcrease and boiler efficincy also maintained.

Cooling Towers :The condensate (water) formed in the condeser after condensation is initially at high temperature.This hot water is passed to cooling towers.It is a tower- or building-like device in which atmospheric air (the heat receiver) circulates in direct or indirect contact with warmer water (the heat source) and the water is thereby cooled (see illustration). A cooling tower may serve as the heat sink in a conventional thermodynamic process, such as refrigeration or steam power generation, and when it is convenient or desirable to make final heat rejection to atmospheric air. Water, acting as the heat-transfer fluid, gives up heat to atmospheric air, and thus cooled, is recirculated through the system, affording economical operation of the process.

Two basic types of cooling towers are commonly used. One transfers the heat from warmer water to cooler air mainly by an evaporation heat-transfer process and is known as the evaporative or wet cooling tower.

Evaporative cooling towers are classified according to the means employed for producing air circulation through them: atmospheric, natural draft, and mechanical draft. The other transfers the heat from warmer water to cooler air by a sensible heat-transfer process and is known as the nonevaporative or dry cooling tower.

Nonevaporative cooling towers are classified as air-cooled condensers and as air-cooled heat exchangers, and are further classified by the means used for producing air circulation through them. These two basic types are sometimes combined, with the two cooling processes generally used in parallel or separately, and are then known as wet-dry cooling towers.

Evaluation of cooling tower performance is based on cooling of a specified quantity of water through a given range and to a specified temperature approach to the wet-bulb or dry-bulb temperature for which the tower is designed. Because exact design conditions are rarely experienced in operation, estimated performance curves are frequently prepared for a specific installation, and provide a means for comparing the measured performance with design conditions.

Economiser : Flue gases coming out of the boiler carry lot of heat.Function of economiser is to recover some of the heat from the heat carried away in the flue gases up the chimney and utilize for heating the feed water to the boiler.It is placed in the passage of flue gases in between the exit from the boiler and the entry to the chimney.The use of economiser results in saving in coal consumption , increase in steaming rate and high boiler efficiency but needs extra investment and increase in maintenance costs and floor area required for the plant.This is used in all modern plants.In this a large number of small diameter thin walled tubes are placed between two headers.Feed water enters the tube through one header and leaves through the other.The flue gases flow out side the tubes usually in counter flow.

Air preheater : The remaining heat of flue gases is utilised by air preheater.It is a device used in steam boilers to transfer heat from the flue gases to the combustion air before the air enters the furnace. Also known as air heater; air-heating system. It is not shown in the lay out.But it is kept at a place near by where the air enters in to the boiler.

The purpose of the air preheater is to recover the heat from the flue gas from the boiler to improve boiler efficiency by burning warm air which increases combustion efficiency, and reducing useful heat lost from the flue. As a consequence, the gases are also sent to the chimney or stack at a lower temperature, allowing simplified design of the ducting and stack. It also allows control over the temperature of gases leaving the stack (to meet emissions regulations, for example).After extracting heat flue gases are passed to elctrostatic precipitator.

Electrostatic precipitator : It is a device which removes dust or other finely divided particles from flue gases by charging the particles inductively with an electric field, then attracting them to highly charged collector plates. Also known as precipitator. The process depends on two steps. In the first step the suspension passes through an electric discharge (corona discharge) area where ionization of the gas occurs. The ions produced collide with the suspended particles and confer on them an electric charge. The charged particles drift toward an electrode of opposite sign and are deposited on the electrode where their electric charge is neutralized. The phenomenon would be more correctly designated as electrodeposition from the gas phase.

The use of electrostatic precipitators has become common in numerous industrial applications. Among the advantages of the electrostatic precipitator are its ability to handle large volumes of gas, at elevated temperatures if necessary, with a reasonably small pressure drop, and the removal of particles in the micrometer range. Some of the usual applications are: (1) removal of dirt from flue gases in steam plants; (2) cleaning of air to remove fungi and bacteria in establishments producing antibiotics and other drugs, and in operating rooms; (3) cleaning of air in ventilation and air conditioning systems; (4) removal of oil mists in machine shops and acid mists in chemical process plants; (5) cleaning of blast furnace gases; (6) recovery of valuable materials such as oxides of copper, lead, and tin; and (7) separation of rutile from zirconium sand.

Smoke stack :A chimney is a system for venting hot flue gases or smoke from a boiler, stove, furnace or fireplace to the outside atmosphere. They are typically almost vertical to ensure that the hot gases flow smoothly, drawing air into the combustion through the chimney effect (also known as the stack effect). The space inside a chimney is called a flue. Chimneys may be found in buildings, steam locomotives and ships. In the US, the term smokestack (colloquially, stack) is also used when referring to locomotive chimneys. The term funnel is generally used for ship chimneys and sometimes used to refer to locomotive chimneys.Chimneys are tall to increase their draw of air for combustion and to disperse pollutants in the flue gases over a greater area so as to reduce the pollutant concentrations in compliance with regulatory or other limits.

Generator : An alternator is an electromechanical device that converts mechanical energy to alternating current electrical energy. Most alternators use a rotating magnetic field. Different geometries - such as a linear alternator for use with stirling engines - are also occasionally used. In principle, any AC generator can be called an alternator, but usually the word refers to small rotating machines driven by automotive and other internal combustion engines.

Transformers :It is a device that transfers electric energy from one alternating-current circuit to one or more other circuits, either increasing (stepping up) or reducing (stepping down) the voltage. Uses for transformers include reducing the line voltage to operate low-voltage devices (doorbells or toy electric trains) and raising the voltage from electric generators so that electric power can be transmitted over long distances. Transformers act through electromagnetic induction; current in the primary coil induces current in the secondary coil. The secondary voltage is calculated by multiplying the primary voltage by the ratio of the number of turns in the secondary coil to that in the primary.

To see the complete operation of the plant in flash player Click here

Electricity Power Blackout and Outage tips

Electricity Power Blackout and Outage tips :

• Assemble an emergency kit with:
  (i) plenty of water (in general a minimum of 4 litres per person per day is needed);Water can be partially supplemented with canned or tetra pak juices.
  (ii) ready-to-eat foods that do not need refridgeration.. Don't forget the manually operated can opener;
  (iii) flashlights;
  (iv) portable radio;
  (v) alkaline batteries, stored separately from electronic equipment (such as radios) in case of battery leakage."Heavy duty batteries" are not recommended for emergency use, as they have much less power capability, a shorter shelf life and are much more prone to leaking.
  (vi) money. Remember bank machines will not operate during a blackout. You may want to keep a small amount of cash ready for this situation.
• Place the emergency kit in a pre-designated location so that you can find it in the dark.
• Do not use candles for lighting. Candles are in the top three causes of household fires.
• Turn off all but one light or a radio so that you'll know when the power returns.
• Check that the stove, ovens, electric kettles, irons, air conditioners and (non-wall or ceiling mounted) lights are off. This can be serious safety issues if you forget you have left some of these devices on. Also by keeping them turned off will prevent heavy start-up loads which could cause a second blackout when the utilities restart the power.
• Turn off or unplug home electronics and computers to protect them from damage when the electricity returns, in case of power surges.
• Listen to local radio and television for updated information. (The reason for having a battery powered (ie. portable) radio.)
• Keep refrigerator and freezer doors closed. A full modern freezer will stay frozen for up to 48 hours; partially full freezers for 24 hours. Most food in the fridge will last 24 hours except dairy products, which should be discarded after six hours. These estimates decrease each time the refrigerator door is opened.
• Do not ration water (or juice). If you are thirsty you need the fluids. If it is hot you need to drink plenty of fluids even if you do not feel thirsty.
• Remember to provide plenty of fresh, cool water for your pets.
• Keep off the telephone unless it is an emergency, or for short periods if it is for an important purpose such as checking up on your loved ones, particularly people who have disabilities or infirmaties.
• In summer: open windows at opposing ends of a room to create a cross breeze in the absence of air conditioning and electric fans.
• In summer: close blinds, curtains, drapes, windows and doors on the sunny side of your home to block out the heat from the sun.
• In winter: open blinds, curtains and drapes during the day on the sunny side of your home to let sunlight and its heat during the sunny days, and close during the night. Otherwise keep them closed to keep the heat in. You may also want to use window insulation kits or plastic sheeting to add extra insulation to keep the heat in.
• In winter: make sure you have extra blankets. Also make sure you have a bucket and a wet mop to soak up any water from frozen and burst water pipes.
• While generally unnecessary and expensive, if you are using a gas-powered generator, run it in a well-ventilated area and not in a closed areas such as a room or garage. They can give off deadly carbon monoxide fumes. And do not hook up the generator to your local wiring, instead plug in the items you want or need into the generator. For short-term use a much safer and cheaper alternative is an Inverter with built-in battery.
• Do not use propane or other combustion-type heaters indoors due to the probability of toxic carbon monoxide buildup.

Other notes:

• Water pressure may drop and even stop above a certain height in high-rise buildings due to their water pumps losing power.
• Remember that electrical devices such as elevator will not work. You can not predict when a blackout will strike to make a choice about using elevators, but if a blackout does strike, check the elevators of any of the building you are in to hear if there are people stuck; in which case call up the fire department to get the people out.
• Electrically operated garage doors will not work. While landlords may be able to hoist the heavy door up manually, some may not want to do so for security purposes or because it volates the conditions of their insurance policies.

Latest Power Outages,Causes and factors contributing to it

Latest Power Outages :
Electricity Blackout in Germany on November 4th 2006 -even France, Italy, Spain and other countries were affected.

One of the worst and most dramatic power failures in three decades plunged millions of Europeans into darkness over the weekend, halting trains, trapping dozens in lifts and prompting calls for a central European power authority. The blackout, which originated in north-western Germany, also struck Paris and 15 French regions, and its effects were felt in Austria, Belgium, Italy and Spain. In Germany, around 100 trains were delayed.

Additional Power Outages

09/24/2006 On September 24th afternoon 1.30pm Pakistan was hit by a nationwide blackout. Millions of homes across Pakistan were left without power for several hours. Power has been restored in capital Islamabad after over a two-hour breakdown. The outage was caused due to a fault that occurred during maintenance of a high-tension transmission line.

07/12/2006 Electricity Blackout in Auckland (New Zealand) - 700,000 people without electricity for up to 10 hours. An earth wire, which snapped in high winds, fell into Transpower's Otahuhu substation, damaging 110 kilovolt supply lines. The cause - a simple metal shackle.

11/25/2005 Electricity Blackout in Münsterland - 250,000 people without electricity for up to six days. Ice and storm had caused serious damage to the network , leading to the blackout.

10/24/2005 -11/11/2005 Hurricane Wilma caused loss of power for most of South Florida and Southwest Florida, with hundreds of thousands of customers still powerless a week later, and full restoration not complete.

09/12/2005 A blackout in Los Angeles affected millions in California.

08/29/2005 Millions of Louisiana, Mississippi and Alabama residents lost power after a stronger Hurricane Katrina badly damaged the power grid.

08/26/2005 On 1.3 Million People in South Florida lost power due to downed trees and power lines caused by the then minimal Hurricane Katrina. Most customers affected were without power for four days, and some customers had no power for up to one week.

08/22/2005 All of southern and central Iraq, including parts of the capital Baghdad, all of the second largest city Basra and the only port Umm Qasr went out of power for more than 7 hours after a feeder line was sabotaged by insurgents, causing a cascading effect shutting down multiple power plants.
08/18/2005 Almost 100 million people on Java Island, the main island of Indonesia which the capital Jakarta is on, and the isle of Bali, lost power for 7 hours. In terms of population affected, the 2005 Java-Bali Blackout was the biggest in history.

05/25/2005 On most part of Moscow was without power from 11:00 MSK (+0300 UTC). Approximately ten million people were affected. Power was restored within 24 hours.

09/04/2004 On five million people in Florida were without power at one point due to Hurricane Frances, one of the most widespread outages ever due to a hurricane.

12/20/2003 Apower failure hit San Francisco, affecting 120,000 people.

09/27/2003- 09/28/2003 Italy blackout - a power failure affected all of Italy except Sardinia, cutting service to more than 56 million people.

09/23/2003 A power failure affected 5 million people in Denmark and southern Sweden.

09/02/2003 A power failure affected 5 states (out of 13) in Malaysia (including the capital Kuala Lumpur) for 5 hours starting at 10 am local time.

08/28/2003 There was a 2003 London blackout on which won worldwide headlines such as "Power cut cripples London" but in fact only affected 500,000 people.

Direct Causes and Contributing Factors to power outage:

• Failure to maintain adequate reactive power support
• Failure to ensure operation within secure limits
• Inadequate vegetation management
• Inadequate operator training
• Failure to identify emergency conditions and communicate that status to neighboring systems
• Inadequate regional-scale visibility over the bulk power system.

Conclusions and Recommendations:

• Conductors contacting trees
• Ineffective visualization of power system conditions and lack of situational awareness
• Ineffective communications
• Lack of training in recognizing and responding to emergencies

System Enhancement & Elimination of Bottlenecks

• Insufficient static and dynamic reactive power supply: FACTS
• Need to improve relay protection schemes and coordination
• On-Line Monitoring and Real-Time Security Assessment
• Increase of Reserve Capacity : HVDC / Generation

Power Failures,Protection from outages and Restoration

Power Failures :
A power outage (Also power cut, power failure or power loss) is the loss of the electricity supply to an area.

The reasons for a power failure can for instance be a defect in a power station, damage to a power line or other part of the distribution system, a short circuit, or the overloading of electricity mains. While the developed countries enjoy a highly uninterrupted supply of electric power all the time, many developing countries have acute power shortage as compared to the demand. Countries such as Pakistan have several hours of daily power-cuts in almost all cities and villages except the metropolitan cities and the state capitals. Wealthier people in these countries may use a power-inverter or a diesel-run electric generator at their homes during the power-cut.

A power outage may be referred to as a blackout if power is lost completely, or as a brownout if the voltage level is below the normal minimum level specified for the system, or sometimes referred to as a short circuit when the loss of power occurs over a short time (usually seconds). Systems supplied with three-phase electric power also suffer brownouts if one or more phases are absent, at reduced voltage, or incorrectly phased. Such malfunctions are particularly damaging to electric motors. Some brownouts, called voltage reductions, are made intentionally to prevent a full power outage. 'Load shedding' is a common term for a controlled way of rotating available generation capacity between various districts or customers, thus avoiding total wide area blackouts.

Power failures are particularly critical for hospitals, since many life-critical medical devices and tasks require power. For this reason hospitals, just like many enterprises (notably colocation facilities and other datacenters), have emergency power generators which are typically powered by diesel fuel and configured to start automatically, as soon as a power failure occurs. In most third world countries, power cuts go unnoticed by most citizens of upscale means, as maintaining an uninterruptible power supply is often considered an essential facility of a home.
Power outage may also be the cause of sanitary sewer overflow, a condition of discharging raw sewage into the environment. Other life-critical systems such as telecommunications are also required to have emergency power. Telephone exchange rooms usually have arrays of lead-acid batteries for backup and also a socket for connecting a diesel generator during extended periods of outage.

Power outages may also be caused by terrorism (attacking power plants or electricity pylons) in developing countries. The Shining Path movement was the first to copy this tactic from Mao Zedong.

Live Examples of breakdown in interconnected grid system
In most parts of the world, local or national electric utilities have joined in grid systems. The linking grids allow electricity generated in one area to be shared with others. Each utility that agrees to share gains an increased reserve capacity, use of larger, more efficient generators, and the ability to respond to local power failures by obtaining energy from a linking grid.

These interconnected grids are large, complex systems that contain elements operated by different groups. These systems offer the opportunity for economic savings and improve overall reliability but can create a risk of widespread failure. For example, a major grid-system breakdown occurred on November 9, 1965, in eastern North America, when an automatic control device that regulates and directs current flow failed in Queenston, Ontario, causing a circuit breaker to remain open. A surge of excess current was transmitted through the northeastern United States. Generator safety switches from Rochester, New York, to Boston, Massachusetts, were automatically tripped, cutting generators out of the system to protect them from damage. Power generated by more southerly plants rushed to fill the vacuum and overloaded these plants, which automatically shut themselves off. The power failure enveloped an area of more than 200,000 sq km (80,000 sq mi), including the cities of Boston; Buffalo, New York; Rochester, New York; and New York City.

Similar grid failures, usually on a smaller scale, have troubled systems in North America and elsewhere. On July 13, 1977, about 9 million people in the New York City area were once again without power when major transmission lines failed. In some areas the outage lasted 25 hours as restored high voltage burned out equipment. These major failures are termed blackouts.

The worst blackout in the history of the United States and Canada occurred August 14, 2003, when 61,800 megawatts of electrical power was lost in an area covering 50 million people. (One megawatt of electricity is roughly the amount needed to power 750 residential homes.) The blackout affected such major cities as Cleveland, Detroit, New York, Ottawa, and Toronto. Parts of eight states—Connecticut, Massachusetts, Michigan, New Jersey, New York, Ohio, Pennsylvania, and Vermont—and the Canadian provinces of Ontario and Québec were affected. The blackout prompted calls to replace aging equipment and raised questions about the reliability of the national power grid.

The term brownout is often used for partial shutdowns of power, usually deliberate, either to save electricity or as a wartime security measure. From November 2000 through May 2001 California experienced a series of planned brownouts to groups of customers, for a limited duration, in order to reduce total system load and avoid a blackout due to alleged electrical shortages. However, an investigation by the California Public Utilities Commission into the alleged shortages later revealed that five energy companies withheld electricity they could have produced. In 2002 the commission concluded that the withholding of electricity contributed to an “unconscionable, unjust, and unreasonable electricity price spike.” California state utilities paid $20 billion more for energy in 2000 than in 1999 as a result, the head of the commission found.

The commission also cited the role of the Enron Corporation in the California brownouts. In June 2003 the Federal Energy Regulatory Commission (FERC) barred Enron from selling electricity and natural gas in the United States after conducting a probe into charges that Enron manipulated electricity prices during California’s energy crisis. In the same month the Federal Bureau of Investigation arrested an Enron executive on charges of manipulating the price of electricity in California. Two other Enron employees, known as traders because they sold electricity, had pleaded guilty to similar charges. See also Enron Scandal.

Despite the potential for rare widespread problems, the interconnected grid system provides necessary backup and alternate paths for power flow, resulting in much higher overall reliability than is possible with isolated systems. National or regional grids can also cope with unexpected outages such as those caused by storms, earthquakes, landslides, and forest fires, or due to human error or deliberate acts of sabotage.

Protecting the power system from outages
In power supply networks, the power generation and the electrical load (demand) must be very close to equal every second to avoid overloading of network components, which can severely damage them. In order to prevent this, parts of the system will automatically disconnect themselves from the rest of the system, or shut themselves down to avoid damage. This is analogous to the role of relays and fuses in households.

Under certain conditions, a network component shutting down can cause current fluctuations in neighboring segments of the network, though this is unlikely, leading to a cascading failure of a larger section of the network. This may range from a building, to a block, to an entire city, to the entire electrical grid.

Modern power systems are designed to be resistant to this sort of cascading failure, but it may be unavoidable (see below). Moreover, since there is no short-term economic benefit to preventing rare large-scale failures, some observers have expressed concern that there is a tendency to erode the resilience of the network over time, which is only corrected after a major failure occurs. It has been claimed that reducing the likelihood of small outages only increases the likelihood of larger ones. In that case, the short-term economic benefit of keeping the individual customer happy increases the likelihood of large-scale blackouts.

Power Analytics
Power Analytics is the term used to describe the management of electrical power distribution, consumption, and preventative maintenance throughout a large organization’s facilities, particularly organizations with high electrical power requirements. For such facilities, electrical power problems – including the worst-case scenario, a full power outage – could have a devastating serious impact. Additionally, it could jeopardize the health and safety of individuals within the facility or in the surrounding community.

Power Analytics use complex mathematical algorithms to detect variations within an organization’s power infrastructure (measurements such as voltage, current, power factor, etc.). Such variations could be early indications of longer-term power problems; when a Power Analytics system detects such variations, it will begin to diagnose the source of the variation, surrounding components, and then the complete electrical power infrastructure. Such systems will – after fully assessing the location and potential magnitude of the problem – predict when and where the potential problem will occur, as well as recommend the preventative maintenance required preempting the problem from occurring.

Restoring power after a wide-area outage
Restoring power after a wide-area outage can be difficult, as power stations need to be brought back on-line. Normally, this is done with the help of power from the rest of the grid. In the absence of grid power, a so-called black start needs to be performed to bootstrap the power grid into operation.

For more information on this visit the below links :
http://www.windows.ucar.edu/spaceweather/cold_start.html
http://blackout.gmu.edu/home.html

Electric Power Systems and its components

Electric Power Systems :
Electric Power Systems, components that transform other types of energy into electrical energy and transmit this energy to a consumer. The production and transmission of electricity is relatively efficient and inexpensive, although unlike other forms of energy, electricity is not easily stored and thus must generally be used as it is being produced.

Components of an Electric Power System
A modern electric power system consists of six main components:

1. The power station
2. A set of transformers to raise the generated power to the high voltages used on the transmission lines
3. The transmission lines
4. The substations at which the power is stepped down to the voltage on the distribution lines
5. The distribution lines
6. the transformers that lower the distribution voltage to the level used by the consumer's equipment.

Power Station
The power station of a power system consists of a prime mover, such as a turbine driven by water, steam, or combustion gases that operate a system of electric motors and generators. Most of the world's electric power is generated in steam plants driven by coal, oil, nuclear energy, or gas. A smaller percentage of the world’s electric power is generated by hydroelectric (waterpower), diesel, and internal-combustion plants.

Transformers
Modern electric power systems use transformers to convert electricity into different voltages. With transformers, each stage of the system can be operated at an appropriate voltage. In a typical system, the generators at the power station deliver a voltage of from 1,000 to 26,000 volts (V). Transformers step this voltage up to values ranging from 138,000 to 765,000 V for the long-distance primary transmission line because higher voltages can be transmitted more efficiently over long distances. At the substation the voltage may be transformed down to levels of 69,000 to 138,000 V for further transfer on the distribution system. Another set of transformers step the voltage down again to a distribution level such as 2,400 or 4,160 V or 15, 27, or 33 kilovolts (kV). Finally the voltage is transformed once again at the distribution transformer near the point of use to 240 or 120 V.

Transmission Lines
The lines of high-voltage transmission systems are usually composed of wires of copper, aluminum, or copper-clad or aluminum-clad steel, which are suspended from tall latticework towers of steel by strings of porcelain insulators. By the use of clad steel wires and high towers, the distance between towers can be increased, and the cost of the transmission line thus reduced. In modern installations with essentially straight paths, high-voltage lines may be built with as few as six towers to the kilometer. In some areas high-voltage lines are suspended from tall wooden poles spaced more closely together. For lower voltage distribution lines, wooden poles are generally used rather than steel towers. In cities and other areas where open lines create a safety hazard or are considered unattractive, insulated underground cables are used for distribution. Some of these cables have a hollow core through which oil circulates under low pressure. The oil provides temporary protection from water damage to the enclosed wires should the cable develop a leak. Pipe-type cables in which three cables are enclosed in a pipe filled with oil under high pressure (14 kg per sq cm/200 psi) are frequently used. These cables are used for transmission of current at voltages as high as 345,000 V (or 345 kV).

Supplementary Equipment
Any electric-distribution system involves a large amount of supplementary equipment to protect the generators, transformers, and the transmission lines themselves. The system often includes devices designed to regulate the voltage or other characteristics of power delivered to consumers.

To protect all elements of a power system from short circuits and overloads, and for normal switching operations, circuit breakers are employed. These breakers are large switches that are activated automatically in the event of a short circuit or other condition that produces a sudden rise of current. Because a current forms across the terminals of the circuit breaker at the moment when the current is interrupted, some large breakers (such as those used to protect a generator or a section of primary transmission line) are immersed in a liquid that is a poor conductor of electricity, such as oil, to quench the current. In large air-type circuit breakers, as well as in oil breakers, magnetic fields are used to break up the current. Small air-circuit breakers are used for protection in shops, factories, and in modern home installations. In residential electric wiring, fuses were once commonly employed for the same purpose. A fuse consists of a piece of alloy with a low melting point, inserted in the circuit, which melts, breaking the circuit if the current rises above a certain value. Most residences now use air-circuit breakers.

Thermal power plant,Advantages and Disadvantages

Thermal power plant or Steam power plant :
A generating station which converts heat energy of coal combustion in to electrical energy is known as Thermal power plant or Steam power plant. Some of its advantages and disadvantages are given below.

Advantages

1. The fuel used is quite cheap.
2. Less initial cost as compared to other generating plants.
3. It can beinstalled at any place iirespective of the existence of coal. The coal can be transported to the site of the plant by rail or road.
4. It require less space as compared to Hydro power plants.
5. Cost of generation is less than that of diesel power plants.

Disadvantages

1. It pollutes the atmosphere due to production of large amount of smoke and fumes.
2. It is costlier in running cost as compared to Hydro electric plants.

Power Station or Power Plant and classification

Power Station or Power Plant :
A power station or power plant is a facility for the generation of electric power. 'Power plant' is also used to refer to the engine in ships, aircraft and other large vehicles. Some prefer to use the term energy center because it more accurately describes what the plants do, which is the conversion of other forms of energy, like chemical energy, gravitational potential energy or heat energy into electrical energy. However, power plant is the most common term in the U.S., while elsewhere power station and power plant are both widely used, power station prevailing in many Commonwealth countries and especially in the United Kingdom.

At the center of nearly all power stations is a generator, a rotating machine that converts mechanical energy into electrical energy by creating relative motion between a magnetic field and a conductor. The energy source harnessed to turn the generator varies widely. It depends chiefly on what fuels are easily available and the types of technology that the power company has access to.

Classification of Power plants :
Power plants are classified by the type of fuel and the type of prime mover installed.
By fuel

• In Thermal power stations, mechanical power is produced by a heat engine, which transforms thermal energy, often from combustion of a fuel, into rotational energy
• Nuclear power plants use a nuclear reactor's heat to operate a steam turbine generator.
• Fossil fuel powered plants may also use a steam turbine generator or in the case of Natural gas fired plants may use a combustion turbine.
• Geothermal power plants use steam extracted from hot underground rocks.
• Renewable energy plants may be fuelled by waste from sugar cane, municipal solid waste, landfill methane, or other forms of biomass.
• In integrated steel mills, blast furnace exhaust gas is a low-cost, although low-energy-density, fuel.
• Waste heat from industrial processes is occasionally concentrated enough to use for power generation, usually in a steam boiler and turbine.

By prime mover

• Steam turbine plants use the pressure generated by expanding steam to turn the blades of a turbine.
• Gas turbine plants use the heat from gases to directly operate the turbine. Natural-gas fuelled turbine plants can start rapidly and so are used to supply "peak" energy during periods of high demand, though at higher cost than base-loaded plants.
• Combined cycle plants have both a gas turbine fired by natural gas, and a steam boiler and steam turbine which use the exhaust gas from the gas turbine to produce electricity. This greatly increases the overall efficiency of the plant, and most new baseload power plants are combined cycle plants fired by natural gas.
• Internal combustion Reciprocating engines are used to provide power for isolated communities and are frequently used for small cogeneration plants. Hospitals, office buildings, industrial plants, and other critical facilities also use them to provide backup power in case of a power outage. These are usually fuelled by diesel oil, heavy oil, natural gas and landfill gas.
• Microturbines, Stirling engine and internal combustion reciprocating engines are low cost solutions for using opportunity fuels, such as landfill gas, digester gas from water treatment plants and waste gas from oil production.

Other sources of energy :

Other power stations use the energy from wave or tidal motion, wind, sunlight or the energy of falling water, hydroelectricity. These types of energy sources are called renewable energy.

For more details on this click on the below link :
http://en.wikipedia.org/wiki/Power_plant

Combination of Two Port Networks

Combination of Two Port Networks :

• Series connection of two 2-port networks: Z = Z1 + Z2
• Parallel connection of two 2-port networks: Y = Y1 + Y2
• Cascade connection of two 2-port networks: A = A1 . A2

Reciprocal and Non reciprocal Networks :
A two port network is called reciprocal if the open circuit transfer impedances are equal Z12 =Z21 . Consequently, in a reciprocal two port network with current I feeding oe port , the open circuit voltage measured at other port is the same, irrespective of the ports. The voltage is equal to V = Z12*I = Z21*I. Networks containing resistors,inductors and capacitors are generally reciprocal. Networks that additionally have dependent sources are generally nonreciprocal.

Z parameters in terms of remaining parameters given by :

where D is the determinant of the corresponding matrix.

For examples and more details visit the site given below :
http://fourier.eng.hmc.edu/e84/lectures/ch3/node11.html

Two Port Networks - Z , Y , h , g , ABCD Parameters

Two Port Networks :

A pair of terminals at which a signal (voltage or current) may enter or leave is called a port.

A network having only one such pair of terminals is called a one port network.

A two-port network (or four-terminal network, or quadripole) is an electrical circuit or device with two pairs of terminals.Examples include transistors, filters and matching networks. The analysis of two-port networks was pioneered in the 1920s by Franz Breisig, a German mathematician.

A two-port network basically consists in isolating either a complete circuit or part of it and finding its characteristic parameters. Once this is done, the isolated part of the circuit becomes a "black box" with a set of distinctive properties, enabling us to abstract away its specific physical buildup, thus simplifying analysis. Any circuit can be transformed into a two-port network provided that it does not contain an independent source.

A two-port network is represented by four external variables: voltage and current at the input port, and voltage and current at the output port, so that the two-port network can be treated as a black box modeled by the the relationships between the four variables , , and . There exist six different ways to describe the relationships between these variables, depending on which two of the four variables are given, while the other two can always be derived.

Note: All voltages and currents below are complex variables and represented by phasors containing both magnitude and phase angle. However, for convenience the phasor notation and are replaced by V and I respectively.

The parameters used in order to describe a two-port network are the following: Z, Y, A , h, g. They are usually expressed in matrix notation and they establish relations between the following parameters:
Input voltage V1
Output voltage V2
Input current I1
Output current I2

Z-model : In the Z-model or impedance model, the two currents I1 and I2 are assumed to be known, and the voltages V1and V2can be found by:

where

Here all four parameters Z11,Z12 ,Z21 , and Z22 represent impedance. In particular, Z21 and Z12 are transfer impedances, defined as the ratio of a voltage V1(or V2) in one part of a network to a current I2(or I1 ) in another part . Z12 = V1 / I2 . Z is a 2 by 2 matrix containing all four parameters.

Y-model : In the Y-model or admittance model, the two voltages V1 and V2 are assumed to be known, and the currents I1 and I2 can be found by:

where

Here all four parameters Y11,Y12 ,Y21 , and Y22 represent admittance. In particular, Y21 and Y12 are transfer admittances. Y is the corresponding parameter matrix.

ABCD -model : In the A-model or transmission model, we assume V1 and I1 are known, and find V2 and I2 by:

where

Here A and D are dimensionless coefficients, B is impedance and C is admittance. A negative sign is added to the output current I2 in the model, so that the direction of the current is out-ward, for easy analysis of a cascade of multiple network models.

H-model : In the H-model or hybrid model, we assume V2 and I1 are known, and find V1 and I2 by:

where

Here h12 and h21 are dimensionless coefficients, h11 is impedance and h22 is admittance.

g model :In g model or inverse hybrid model, we assume V1 and I2 are known, and find V2 and I1 by :


where



Here g12 and g21 are dimensionless coefficients, g22 is impedance and g11 is admittance.

For more details on this topic visit the following links :
http://fourier.eng.hmc.edu/e84/lectures/ch3/node11.html
http://en.wikipedia.org/wiki/Two-port_network
http://web.cecs.pdx.edu/~ece2xx/ECE222/Slides/TwoPortsx4.pdf

Vector Diagrams - RLC Series and Parallel circuit

Vector Diagrams :

RLC Series circuits
When we add a resistance to a series LC circuit, as shown in the schematic diagram to the right, the behavior of the circuit is similar to the behavior of the LC circuit with no resistance, but there are some variations.We wil see the affects of added resistance with the parameters given below

• f = 1 MHz
• e = 10 vrms
• L = 150 µh
• C = 220 pf
• R = 100 Ω

With these measured voltages across R L and C we get are

• vL = 39.1v
• vC = 30.0v
• vR = 4.15v

We must take into account the different phase angles between voltage and current for each of the three components in the circuit. The vector diagram to the right, while not to scale, illustrates this concept.

Since this is a series circuit, the current is the same through all components and is therefore our reference at a phase angle of 0°. This is shown in red in the diagram. The resistor's voltage, vR, is in phase with the current and is shown in green. The blue vector shows vL at +90°, while the gold vector represents vC, at -90°. Since they oppose each other diametrically, the total reactive voltage is vL - vC. It is this difference vector that is combined with vR to find vT (shown in cyan in the diagram).

We already know that vT = 10 vrms. Now we can see that vT is also the vector sum of (vL - vC) and vR. In addition, because of the presence of R, the phase angle between vT and i will be arctan((vL-vC)/vR), and can vary from -90° to +90°.

For calculations and more information visit the following site :
http://www.play-hookey.com/ac_theory/ac_rlc_series.html

RLC Parallel circuits
The schematic diagram to the right shows three components connected in parallel, and to an ac voltage source: an ideal inductance, and ideal capacitance, and an ideal resistance. We will use the following values for our components

• VAC = 10 vrms.
• f = 1 MHz. ( = 6283185.3 rad/sec)
• L = 150 µh. (XL = 942.4778 )
• C = 220 pf. (XC = 723.43156 )
• R = 1000

According to Ohm's Law:
iL =vL/XL = 10/942.4778 = 0.01061033 = 10.61033 mA.
iC =vC/XC = 10/723.43156 = 0.013823008 = 13.823008 mA.
iR = vR/R = 10/1000 = 0.01 = 10 mA.

If we measure the current from the voltage source, we find that it supplies a total of 10.503395 mA to the combined load — only about half a milliamp more than iR alone.
So we now have 10 mA of resistive current and just over 3.2 mA of reactive current, and yet the measured total current is just over 10.5 mA.

As usual, the vectors, shown to the right, tell the story. Since this is a parallel circuit, the voltage, v, is the same across all components. It is the current that has different phases and amplitudes within the different components.

Since voltage is the same throughout the circuit, we use it as the reference, at 0°. Current through the resistor is in phase with the voltage dropped across that resistor, so iR also appears at 0°.

Current through an inductor lags the applied voltage, so iL appears at -90°. Current through a capacitor leads the applied voltage, so iC appears at +90°. Since iC is greater than iL, the net reactive current is capacitive, so its phase angle is +90°.

Now the total current, iT, is the vector sum of reactive current and resistive current. Since iR is significantly greater than the difference, iC - iL, the total impedance of this circuit is mostly resistive, and the combined vector for iT is at only a small phase angle, as shown in the diagram.

For calculations and more information visit the following site :
http://www.play-hookey.com/ac_theory/ac_rlc_parallel.html

The above are the cases of ideal inductor and capacitor.If those are not ideal the vector diagram wil be as shown below

(a)Series RLC circuit

(b)In (a) XL > XC, and the driving voltage (V) leads the current by a phase angle of f.

(c)In (b) XC > XL, and the driving voltage (V) lags the current by a phase angle of f.

For more details see the below site :
http://www.physics.sjsu.edu/becker/physics51/ac_circuits.htm

Parallel and series Resonance

Parallel Resonance :
A condition of resonance will be experienced in a tank circuit when the reactances of the capacitor and inductor are equal to each other. Because inductive reactance increases with increasing frequency and capacitive reactance decreases with increasing frequency, there will only be one frequency where these two reactances will be equal.



In the above circuit, we have a 10 µF capacitor and a 100 mH inductor. Since we know the equations for determining the reactance of each at a given frequency, and we're looking for that point where the two reactances are equal to each other, we can set the two reactance formulae equal to each other and solve for frequency algebraically:
XL = 2*pi*f*L Xc = 1/2*pi*f*c

Equating these two and solving for frequency f gives

a formula to tell us the resonant frequency of a tank circuit, given the values of inductance (L) in henrys and capacitance (C) in Farads. Plugging in the values of L and C in our example circuit, we arrive at a resonant frequency of 159.155 Hz.

What happens at resonance is quite interesting. With capacitive and inductive reactances equal to each other, the total impedance increases to infinity, meaning that the tank circuit draws no current from the AC power source! We can calculate the individual impedances of the 10 µF capacitor and the 100 mH inductor and work through the parallel impedance formula to demonstrate this mathematically:

The resonant frequency we obtain calculating with above formula is 159.155 Hz.
Calculating XL and Xc with this frequency values are XL = Xc = 100 Ω
Now, we use the parallel impedance formula to see what happens to total Z:
ZParallel = 1 /(1/ZL + 1/Zc)

Calculating impedance with above formula we get
ZParallel = 1/0 Undefined!
We can't divide any number by zero and arrive at a meaningful result, but we can say that the result approaches a value of infinity as the two parallel impedances get closer to each other. What this means in practical terms is that, the total impedance of a tank circuit is infinite (behaving as an open circuit) at resonance.

Series Resonance :

A similar effect happens in series inductive/capacitive circuits. When a state of resonance is reached (capacitive and inductive reactances equal), the two impedances cancel each other out and the total impedance drops to zero!

At f = 159.155Hz ZSeries = ZL + Zc = 0Ω.

With the total series impedance equal to 0 Ω at the resonant frequency of 159.155 Hz, the result is a short circuit across the AC power source at resonance

For detail calculations visit the following site:
http://www.allaboutcircuits.com/vol_2/chpt_6/2.html

http://www.allaboutcircuits.com/vol_2/chpt_6/3.html

Electrical Resonance and Resonant frequency

Electrical Resonance :
Electrical resonance occurs in an electric circuit at a particular resonant frequency when the impedance between the input and output of the circuit is at a minimum (or when the transfer function is at a maximum). Often this happens when the impedance between the input and output of the circuit is zero and when the transfer function equals one.

Resonance with capacitors and inductors :
Resonance of a circuit involving capacitors and inductors occurs because the collapsing magnetic field of the inductor generates an electric current in its windings that charges the capacitor, and then the discharging capacitor provides an electric current that builds the magnetic field in the inductor, and the process is repeated continually. An analogy is a mechanical pendulum. In some cases, resonance occurs when the inductive reactance and the capacitive reactance of the circuit are of equal magnitude, causing electrical energy to oscillate between the magnetic field of the inductor and the electric field of the capacitor.

At resonance, the series impedance of the two elements is at a minimum and the parallel impedance is a maximum. Resonance is used for tuning and filtering, because it occurs at a particular frequency for given values of inductance and capacitance. It can be detrimental to the operation of communications circuits by causing unwanted sustained and transient oscillations that may cause noise, signal distortion, and damage to circuit elements.

Parallel resonant or near-to-resonance circuits can be used to prevent the wastage of electrical energy, which would otherwise occur while the inductor built its field or the capacitor charged and discharged. As an example, asynchronous motors waste inductive current while synchronous ones waste capacitive current. The use of the two types in parallel makes the inductor feed the capacitor, and vice versa, maintaining the same resonant current in the circuit, and converting all the current into useful work.

Since the inductive reactance and the capacitive reactance are of equal magnitude, ωL = 1/ωC, so:

where ω = 2πf, in which f is the resonant frequency in hertz, L is the inductance in henries, and C is the capacitance in farads when standard SI units are used.

Resonant freqency :
A resonant frequency is a natural frequency of vibration determined by the physical parameters of the vibrating object. This same basic idea of physically determined natural frequencies applies throughout physics in mechanics, electricity and magnetism, and even throughout the realm of modern physics.The lowest resonant frequency of a vibrating object is called its fundamental frequency. Some of the implications of resonant frequencies are:

1. It is easy to get an object to vibrate at its resonant frequencies, hard to get it to vibrate at other frequencies.
2. A vibrating object will pick out its resonant frequencies from a complex excitation and vibrate at those frequencies, essentially "filtering out" other frequencies present in the excitation.
3. Most vibrating objects have multiple resonant frequencies.

Resonant frequency is given by the expression :


For more details see the following links :
http://en.wikipedia.org/wiki/Electrical_resonance
http://hyperphysics.phy-astr.gsu.edu/hbase/sound/reson.html#resdef
http://www.allaboutcircuits.com/vol_2/chpt_6/1.html

Capacior Transient Response

Capacior Transient Response :
Because capacitors store energy in the form of an electric field, they tend to act like small secondary-cell batteries, being able to store and release electrical energy. A fully discharged capacitor maintains zero volts across its terminals, and a charged capacitor maintains a steady quantity of voltage across its terminals, just like a battery. When capacitors are placed in a circuit with other sources of voltage, they will absorb energy from those sources, just as a secondary-cell battery will become charged as a result of being connected to a generator. A fully discharged capacitor, having a terminal voltage of zero, will initially act as a short-circuit when attached to a source of voltage, drawing maximum current as it begins to build a charge. Over time, the capacitor's terminal voltage rises to meet the applied voltage from the source, and the current through the capacitor decreases correspondingly. Once the capacitor has reached the full voltage of the source, it will stop drawing current from it, and behave essentially as an open-circuit.

When a battery is connected to a series resistor and capacitor, the initial current is high as the battery transports charge from one plate of the capacitor to the other. The charging current asymptotically approaches zero as the capacitor becomes charged up to the battery voltage. Charging the capacitor stores energy in the electric field between the capacitor plates. The rate of charging is typically described in terms of a time constant RC.



Example :




When the switch is first closed, the voltage across the capacitor (which we were told was fully discharged) is zero volts; thus, it first behaves as though it were a short-circuit. Over time, the capacitor voltage will rise to equal battery voltage, ending in a condition where the capacitor behaves as an open-circuit. Current through the circuit is determined by the difference in voltage between the battery and the capacitor, divided by the resistance of 10 kΩ. As the capacitor voltage approaches the battery voltage, the current approaches zero. Once the capacitor voltage has reached 15 volts, the current will be exactly zero. Let's see how this works using real values:




The capacitor voltage's approach to 15 volts and the current's approach to zero over time is what a mathematician would call asymptotic: that is, they both approach their final values, getting closer and closer over time, but never exactly reaches their destinations. For all practical purposes, though, we can say that the capacitor voltage will eventually reach 15 volts and that the current will eventually equal zero.

Equations for RC Circuits :
Time constant = Tc = RC


For more details of this topic visit the below sites :
http://www.allaboutcircuits.com/vol_1/chpt_16/2.html
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capchg.html#c1
http://www.aplac.hut.fi/courses/bee/exercises.pdf

Inductor Transient Response

Inductor Transient Response :

Inductors have the exact opposite characteristics of capacitors. Whereas capacitors store energy in an electric field (produced by the voltage between two plates), inductors store energy in a magnetic field (produced by the current through wire). Thus, while the stored energy in a capacitor tries to maintain a constant voltage across its terminals, the stored energy in an inductor tries to maintain a constant current through its windings. Because of this, inductors oppose changes in current, and act precisely the opposite of capacitors, which oppose changes in voltage. A fully discharged inductor (no magnetic field), having zero current through it, will initially act as an open-circuit when attached to a source of voltage (as it tries to maintain zero current), dropping maximum voltage across its leads. Over time, the inductor's current rises to the maximum value allowed by the circuit, and the terminal voltage decreases correspondingly. Once the inductor's terminal voltage has decreased to a minimum (zero for a "perfect" inductor), the current will stay at a maximum level, and it will behave essentially as a short-circuit.

When a battery is connected to a series resistor and inductor, the inductor resists the change in current and the current therefore builds up slowly. Acting in accordance with Faraday's law and Lenz's law, the amount of impedance to the buildup of current is proportional to the rate of change of the current. That is, the faster you try to make it change, the more it resists. The current builds up toward the value it would have with the resistor alone because once the current is no longer changing, the inductor offers no impedance. The rate of this buildup is often characterized by the time constant L/R . Establishing a current in an inductor stores energy in the magnetic field formed by the coils of the inductor.

Example :

When the switch is first closed, the voltage across the inductor will immediately jump to battery voltage (acting as though it were an open-circuit) and decay down to zero over time (eventually acting as though it were a short-circuit). Voltage across the inductor is determined by calculating how much voltage is being dropped across R, given the current through the inductor, and subtracting that voltage value from the battery to see what's left. When the switch is first closed, the current is zero, then it increases over time until it is equal to the battery voltage divided by the series resistance of 1 Ω. This behavior is precisely opposite that of the series resistor-capacitor circuit, where current started at a maximum and capacitor voltage at zero. Let's see how this works using real values:

Just as with the RC circuit, the inductor voltage's approach to 0 volts and the current's approach to 15 amps over time is asymptotic. For all practical purposes, though, we can say that the inductor voltage will eventually reach 0 volts and that the current will eventually equal the maximum of 15 amps.

Equations for RL Circuits :

Time constant = TC = L/R


For more details of this topic visit the below sites :

http://www.allaboutcircuits.com/vol_1/chpt_16/3.html

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/indtra.html#c2

http://www.aplac.hut.fi/courses/bee/exercises.pdf --- Exercise problems

http://www.allaboutcircuits.com/vol_1/chpt_16/4.html - Voltage & current calculations

Faraday's Law and Lenz's Law

Faraday's Law :

Any change in the magnetic environment of a coil of wire will cause a voltage (emf) to be "induced" in the coil. No matter how the change is produced, the voltage will be generated. The change could be produced by changing the magnetic field strength, moving a magnet toward or away from the coil, moving the coil into or out of the magnetic field, rotating the coil relative to the magnet, etc.

Lenz's Law :
When an emf is generated by a change in magnetic flux according to Faraday's Law, the polarity of the induced emf is such that it produces a current whose magnetic field opposes the change which produces it. The induced magnetic field inside any loop of wire always acts to keep the magnetic flux in the loop constant. In the examples below, if the B field is increasing, the induced field acts in opposition to it. If it is decreasing, the induced field acts in the direction of the applied field to try to keep it constant.

For more explanation on these topics click on the below links :
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html#c2
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html#c1

Transient Response and Time constant

Transient Response :
It if defined as the response of a system caused by a sudden change of DC voltage, current, or load.These sudden changes are mostly found as the result of the operation of switching devices. Engineers use voltage regulators and surge suppressors to prevent transients in electricity from affecting delicate equipment.


Time constant (τ) :
In physics and engineering, the time constant usually denoted by the Greek letter τ, (tau), characterizes the frequency response of a first-order, linear time-invariant (LTI) system. Examples include electrical RC circuits and RL circuits. It is also used to characterize the frequency response of various signal processing systems – magnetic tapes, radio transmitters and receivers, record cutting and replay equipment, and digital filters – which can be modeled or approximated by first-order LTI systems.


Other examples include time constant used in control systems for integral and derivative action controllers, which are often pneumatic, rather than electrical.


Time constant is the time required for a physical quantity to rise from zero to 1-1/e (that is, 63.2%) of its final steady value when it varies with time t as 1 - e-kt. The time required for a physical quantity to fall to 1/e (that is, 36.8%) of its initial value when it varies with time t as e-kt. Generally, the time required for an instrument to indicate a given percentage of the final reading resulting from an input signal. Also known as lag coefficient.


Time constants in electrical circuits :
In an RL circuit, the time constant τ (in seconds) is

where R is the resistance (in ohms) and L is the inductance (in henries).


Similarly, in an RC circuit, the time constant τ is:

where R is the resistance (in ohms) and C is the capacitance (in farads).

For more details on this topic visit the following sites :
www.en.wikipedia.org/wiki/Time_constant
www.answers.com/topic/time-constant

Admittance and Susceptance

Admittance :
In electrical engineering, the admittance (Y) is the inverse of the impedance (Z). The SI unit of admittance is the siemens. Oliver Heaviside coined the term in December 1887.Admittance is a measure of how much current is admitted in a circuit. The admittance has its most obvious utility in dealing with parallel AC circuits.

where
Y is the admittance, measured in siemens
Z is the impedance, measured in ohms.

Therefore the expression for admittance in terms of voltage and current can also be wriiten as
Y = I/V = G + j B
G, the real part of the admittance, is the conductance of the circuit, and B, the imaginary part of the admittance, is the susceptance of the circuit. The units of admittance are called siemens or mhos (reciprocal ohms).

Substituting the expression of Z = R + j X in Y = 1/Z after simplifying and equating to G + j B the expressions are



The magnitude of admittance is given by:


Susceptance :
In electrical engineering, the susceptance (B) is the imaginary part of the admittance. In SI units, the susceptance is measured in siemens. Oliver Heaviside first defined this property, which he called permittance, in June 1887.Susceptance is the measure of how much a circuit is susceptible to conducting a changing current.

It can also be defined as the opposite of reactance.Just as there's capacitive reactance and inductive reactance, so too there is capacitive susceptance (BC) and inductive susceptance (BL). Just like with conductance, both of these are the reciprocal of their corresponding reactances. That is to say, capacitive susceptance is 1 divided by the capacitive reactance, and inductive susceptance is 1 divided by the inductive reactance.

Thus, capacitive susceptance can be simplified into the following equation: BC = 2*pi*f*C.

Inductive susceptance, meanwhile, becomes exactly like the formula for capacitive reactance, except that it of course uses inductance rather than capacitance: BL = 1/2*pi*f*L

For more details on this topic visit the following sites :
http://en.wikipedia.org/wiki/Admittance
http://www.allaboutcircuits.com/vol_2/chpt_5/5.html http://www.geocities.com/siliconvalley/2072/eleccsa.htm