Principles of Power Systems By V.K Mehta PDF Free Download

Principles of Power Systems By V.K Mehta PDF Free Download

Now you can read & learn electrical power systems in offline.You can download Principles of Power Systems By V.K Mehta. This is a very nice book with attractive colors & amazing images.All concepts of power systems are explained very clearly.You can get idea behind every power concept in single read.You can use principle of power system by vk mehta free download in doc format as text in image can be copied.

We are sharing only the link of this book which is already on internet.We respect copy right policy of principle of power system by vk mehta book publishers.

What You Get In Principles of Power Systems By V.K Mehta 


CONTENTS
Introduction 
Generating stations 
Variable load on power stations 
Economics power generation 
Tariff 
Power factor improvement 
Supply systems 
Mechanical design of overhead lines 
Electrical desing of overhead lines 
Performance of transmission lines 
Underground cables 
Distribution system general 
D.C Distribution 
A.C Distribution 
Voltage control 
Introduction to switchgear 
Symmetrical fault calculations 
Unsymmetrical fault calculations 
Circuit breakers 
Fuses 
Protective relays 
Protection of Alternators and transformers 
Protection of busbars and lines 
Protection againts overvoltages 
Substations 
Neutral Grounding 


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Power Factor | It's Calculation, Power Factor Improvement Methods

Power Factor | Calculation and Power Factor Improvement Methods


What Is Electrical Power Factor ?

The cosine of angle between voltage and current in an a.c. circuit is known as power factor. In an a.c. circuit, there is generally a phase difference φ between voltage and current. The term cos φ is called the power factor of the circuit. If the circuit is inductive, the current lags behind the voltage and the power factor is referred to as lagging. However, in a capacitive circuit, current leads the voltage and power factor is said to be leading.



Consider an inductive circuit taking a lagging current I from supply voltage V; the angle of lag being φ. The phasor diagram of the circuit is shown in Fig. 6.1. The circuit current I can be resolved into two perpendicular components, namely ;

(a) I cos φ in phase with V
(b) I sin φ 90°  out of phase with V

The component I cos φ is known as active or wattful component, whereas component I sin φ is called the reactive or wattless component. The reactive component is a measure of the power factor. If the reactive component is small, the phase angle φ is small and hence power factor cos φ will be high. Therefore, a circuit having small reactive current (i.e., I sin φ) will have high power factor and vice-versa. It may be noted that value of power factor can never be more than unity.

(i) It is a usual practice to attach the word ‘lagging’ or ‘leading’ with the numerical value of power factor to signify whether the current lags or leads the voltage. Thus if the circuit has a p.f. of 0·5 and the current lags the voltage, we generally write p.f. as 0·5 lagging.

(ii) Sometimes power factor is expressed as a percentage. Thus 0·8 lagging power factor may be expressed as 80% lagging.

Power Triangle 

The analysis of power factor can also be made in terms of power drawn by the a.c. circuit. If each side of the current triangle oab of below figure is multiplied by voltage V, then we get the power triangle OAB shown in figure where
OA = VI cos φ and represents the active power in watts or kW
AB = VI sin φ and represents the reactive power in VAR or kVAR
OB = VI and represents the apparent power in VA or kVA
The following points may be noted form the power triangle :



(i) The apparent power in an a.c. circuit has two components viz., active and reactive power at right angles to each other.
OB² = OA² + AB²
or (apparent power)² = (active power)² + (reactive power)²
or (kVA)²= (kW)² + (kVAR)²

(ii) Power factor, cos φ = OA/OB = active power/apparent power = kW/kVA
Thus the power factor of a circuit may also be defined as the ratio of active power to the apparent power. This is a perfectly general definition and can be applied to all cases, whatever be the waveform.

(iii) The lagging reactive power is responsible for the low power factor. It is clear from the power triangle that smaller the reactive power component, the higher is the power factor of the circuit.
kVAR = kVA sin φ = (kW * sin φ)/cos φ
∴ kVAR = kW tan φ

(iv) For leading currents, the power triangle becomes reversed. This fact provides a key to the power factor improvement. If a device taking leading reactive power (e.g. capacitor) is connected in parallel with the load, then the lagging reactive power of the load will be partly neutralized, thus improving the power factor of the load.

(v) The power factor of a circuit can be defined in one of the following three ways :

(a) Power factor = cos φ = cosine of angle between V and I

(b) Power factor = R/Z= Resistance/Impedance

(c) Power factor = VI/(VI * cosφ) = Active power/Apparent Power

(vi) The reactive power is neither consumed in the circuit nor it does any useful work. It merely flows back and forth in both directions in the circuit. A wattmeter does not measure reactive power.

Disadvantages of Low Power Factor

The power factor plays an importance role in a.c. circuits since power consumed depends upon this
factor.

P = VL IL cos φ

It is clear from above that for fixed power and voltage, the load current is inversely proportional to the power factor. Lower the power factor, higher is the load current and vice-versa. A power factor less than unity results in the following disadvantages :

(i) Large kVA rating of equipment. The electrical machinery (e.g., alternators, transformers, switchgear) is always rated in kVA.
Now, kVA = kW/cos φ
It is clear that kVA rating of the equipment is inversely proportional to power factor. The smaller the power factor, the larger is the kVA rating. Therefore, at low power factor, the kVA rating of the equipment has to be made more, making the equipment larger and expensive.

(ii) Greater conductor size. To transmit or distribute a fixed amount of power at constant voltage, the conductor will have to carry more current at low power factor. This necessitates large conductor size. 

(iii) Large copper losses. The large current at low power factor causes more I²R losses in all the elements of the supply system. This results in poor efficiency.

(iv) Poor voltage regulation. The large current at low lagging power factor causes greater voltage drops in alternators, transformers, transmission lines and distributors. This results in the decreased voltage available at the supply end, thus impairing the performance of utilization devices. In order to keep the receiving end voltage within permissible limits, extra equipment (i.e., voltage regulators) is required.

(v) Reduced handling capacity of system. The lagging power factor reduces the handling capacity of all the elements of the system. It is because the reactive component of current prevents the full utilisation of installed capacity. The above discussion leads to the conclusion that low power factor is an objectionable feature in the supply system.

Causes of Low Power Factor

Low power factor is undesirable from economic point of view. Normally, the power factor of the whole load on the supply system in lower than 0·8. The following are the causes of low power factor:

(i) Most of the a.c. motors are of induction type (1φ and 3φ induction motors) which have low lagging power factor. These motors work at a power factor which is extremely small on light load (0·2 to 0·3) and rises to 0·8 or 0·9 at full load.

(ii) Arc lamps, electric discharge lamps and industrial heating furnaces operate at low lagging power factor.

(iii) The load on the power system is varying ; being high during morning and evening and low at other times. During low load period, supply voltage is increased which increases the magnetization current. This results in the decreased power factor.

Power Factor Improvement Methods 


The low power factor is mainly due to the fact that most of the power loads are inductive and, therefore, take lagging currents. In order to improve the power factor, some device taking leading power should be connected in parallel with the load. One of such devices can be a capacitor. The capacitor draws a leading current and partly or completely neutralists the lagging reactive component of load current. This raises the power factor of the load.
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Instantaneous Relay Operation

Instantaneous Relay Working Operation 

What Is Instantaneous Relay ?

An instantaneous relay is one in which there is no time delay provided intentionally. More specifically ideally there is no time required to operate the relay. Although there is some time delay which can not be avoided.

Instantaneous relay. An instantaneous relay is one in which no intentional time delay is provided. In this case, the relay contacts are closed immediately after current in the relay coil exceeds the minimum calibrated value. Figure shows an instantaneous solenoid type of relay. Although there Will be a short time interval between the instant of pickup and the closing of relay contacts, no intentional time delay has been added.

Instantaneous Relay Working Operation


The instantaneous relays have operating time less than 0-1 second. The instantaneous relay is effective only Where the impedance between the relay and source is small compared to the protected section impedance. The operating time of instantaneous relay is sometimes expressed in cycles based on the power-system frequency
e.g. one-cycle would be If 50 second in a 50-cycle system.

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Hysteresis Motor Construction & Working

Hysteresis Motor Construction & Working 

Hysteresis Motor: Single-phase cylindrical (non-salient-pole) synchronous-induction or shaded.pole motors are classed as hysteresis motors. A hysteresis motor has neither a salient-pole rotor nor direct excitation. but nevertheless it rotates at synchronous speed. This type of motor runs into synchronism and runs on hysteresis torque.

Hysteresis Motor Construction & Working

Hysteresis-type lamination, shown in figure are usually made of hardened. high retentive steel rather than commercial. law retentivity dynamo steel.

Working of Hysteresis Motor: As a result of a rotating magnetic field produced by phase splitting or a shaded-pole stator. eddy currents are induced in the steel of the rotor which travel across the two bar paths of the rotor as shown in figure A high-retentivity steel produces a high hysteresis lose. and an appreciable amount of energy is consumed from the rotating field in reversing the current direction of the rotor. At the same time the rotor magnetic field set up by the eddy cur- rents causes the rotor to rotate. A high starting torque is produced a: a result of the high resistance (proportional to hysteresis). As the rotor approaches synchronous speed. the frequency of current reversal in the cross-bars decreases, and the rotor becomes permanently magnetized in one direction as a result of the high retentivity of the steel rotor. Consequently the motor continues to rotate at synchronous speed.


An extremely important use of this type of motor is for the rotation of gyroscopc rotors in inertial navigation and control system. Here the requirement is for as near absolute accuracy as can be achieved. One major component of the instrument accuracy that contains the gyroscope is that the gyroscopic moment be absolutely constant. This constancy requires a synchronous motor that is driven by a regulated constant-frequency source.

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EMF Method or Synchronous Impedance Method | Voltage Regulation of Synchronous Generator [Alternator]

Voltage Regulation of Synchronous Generator [Alternator] By EMF Method or Synchronous Impedance Method

EMF method: This method is also known as synchronous impedance method.Here the magnetic circuit is assumed to be unsaturated. In this method the MMFs (fluxes) produced by rotor and stator are replaced by their equivalent emf, and hence called emf method.To predetermine the regulation by this method the following information is to be determined.Armature resistance/phase of the alternator, open circuit and short circuit characteristics of the alternator.
Here we discuss Voltage Regulation of Synchronous Generator [Alternator] by EMF Method or Synchronous Impedance Method.this is better method than direct loading but not best methods to find out voltage regulation.

Synchronous Impedance Method:

To perform  voltage regulation by emf method we need to calculate the following data.
1.Armature Resistance per phase [Ra]
2.Open Circuit characteristics which is a graph between open circuit voltage [Vo.c.] and field current.
3.Short circuit characteristics which is a graph between short circuit current [Is.c.] and field current.
Voltage Regulation Synchronous Generator by Synchronous Impedance Method
In Synchronous Impedance Method we need to calculate OC and SC characteristics to find Synchronous Impedance.so..follow these steps to find out OC & SC test values.
Open Circuit Characteristic (O.C.C.):-

The open-circuit characteristic or magnetization curve is really the B-H curve of the complete magnetic circuit of the alternator. Indeed, in large turboalternators, where the air gap is relatively long, the curve shows a gradual bend. It is determined by inserting resistance in the field circuit and measuring corresponding value of terminal voltage and field current. Two voltmeters are connected across the armature terminals. The machine is run at rated speed and field current is increased gradually to If1 till armature voltage reaches rated value or even 25% more than the rated voltage. Figure illustrates a typical circuit for OC test.The major portion of the exciting ampere-turns is required to force the flux across the air gap, the reluctance of which is assumed to be constant. A straight line called the air gap line can therefore be drawn as shown, dividing the excitation for any voltage into two portions,

(a) that required to force the flux across the air gap, and
(b) that required to force it through the remainder of the magnetic circuit.
The shorter the air gap, the steeper is the air gap line.




Procedure to conduct OC test:
(i) Start the prime mover and adjust the speed to the synchronous speed of the alternator.
(ii) Keep the field circuit rheostat in cut in position and switch on DC supply.
(iii) Keep the TPST switch of the stator circuit in open position.
(iv) Vary the field current from minimum in steps and take the readings of field current and
stator terminal voltage, till the voltage read by the voltmeter reaches up to 110% of rated voltage. Reduce the field current and stop the machine.
(v) Plot of terminal voltage/ phase vs field current gives the OC curve.


Short Circuit Characteristic (S.C.C.):-
The short-circuit characteristic, as its name implies, refers to the behaviour of the alternator when its armature is short-circuited. In a single-phase machine the armature terminals are short-circuited through an ammeter, but in a three phase machine all three phases must be short-circuited. An ammeter is connected in series with each armature terminal, the three remaining ammeter terminals being short-circuited. 

The machine is run at rated speed and field current is increased gradually to If2 till armature current reaches rated value. The armature short-circuit current and the field current are found to be proportional to each other over a wide range, as shown in Figure, so that the short circuit characteristic is a straight line. Under short-circuit conditions the armature current is almost 90° out of phase with the voltage, and the armature mmf has a direct demagnetizing action on the field.The resultant ampere − turns inducing the armature emf are, therefore, very small and is equal to the difference between the field and the armature ampere − turns. 

This results in low mmf in the magnetic circuit, which remains in unsaturated condition and hence the small value of induced emf increases linearly with field current. This small induced armature emf is equal to the voltage drop in the winding itself, since the terminal voltage is zero by assumption. It is the voltage required to circulate the short circuit current through the armature windings. The armature resistance is usually small compared with the reactance.



Short-Circuit Ratio:

The short-circuit ratio is defined as the ratio of the field current required to produce rated volts on open circuit to field current required to circulate full-load current with the armature short-circuited.

Short-circuit ratio = If1/If2

Determination of synchronous impedance Zs:

As the terminals of the stator are short circuited in SC test, the short circuit current is circulated against the impedance of the stator called the synchronous impedance. This impedance can be estimated form the oc and sc characteristics.The ratio of open circuit voltage to the short circuit current at a particular field current, or at a field current responsible for circulating the rated current is called the synchronous impedance.

synchronous impedance Zs = (open circuit voltage per phase)/(short circuit current per phase)
for same If
Hence Zs = (Voc) / (Isc)
for same If
From figure synchronous impedance Zs = V/Isc

Armature resistance Ra of the stator can be measured using Voltmeter Ammeter method. Using synchronous impedance and armature resistance synchronous reactance and hence regulation can be calculated as follows using emf method.

Zs =√(Ra)² + (XS)² and Synchronous reactance Xs =  ( Zs)² - (Ra)²

Hence induced emf per phase can be found as 
Eph = √ [ (V cos  Ø+ IRa)²+ (V sin  Ø ± IXS)²]
where
V = phase voltage per phase = Vph ,
I = load current per phase
in the above expression in second term + sign is for lagging power factor and
– sign is for leading power factor.

% Regulation = [(Eph – Vph / Vph )] x 100

where Eph = induced emf /phase, Vph = rated terminal voltage/phase.


Synchronous impedance method is easy but it will not give accurate results. This method gives the value of regulation which is greater (poor) than the actual value and hence this method is called pessimistic method. The complete phasor diagram for the emf method is shown in above figure.


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Top 10 Difference Between Core And Shell Type Transformers - Types of Transformers

Top 10 Difference Between Core  And Shell Type Transformers - Types of Transformers

In our previous articles we have discusses about differences between lap,wave winding this tutorial we are sharing major differences between core type transformer and shell type transformer. In this tutorial comparisons between shell type and core type transformers are discussed.There are two major types of transformers based on construction.
They are,
1.Core Type Transformers
2.Shell Type Transformers

Types of Transformers

Difference Between Shell And Core Type Transformers

Core Type TransformersShell Type Transformers
1. In core type transformer winding is placed on
two core limbs.
1. In shell type transformer winding is placed on mid arm
of the core.It is installed on mid-limb of the core.
Other limbs will be used as mechanical supporting
2. Core type transformers have only one magnetic flux path.2. Shell type transformers have two magnetic flux path.
3. It has better cooling since more surface is exposed to
atmosphere.
3. Cooling is not effective in shell type when compared
to core type transformer.
4. It is very useful when we need large size low voltage.4. It is very useful when we need small size high voltage.
5. In core type transformer output is less. Because of losses.
So efficiency will be less than shell type transformer.
5. In core type transformer output is high. Because of
less losses.So efficiency will be more.
6. The winding is surrounded considerable part of core.6. Core is surrounded considerable part of winding of
transformer.
7. It has less mechanical protection to coil.7. It has better mechanical protection to coil.
8. Core has two limbs.8. Core has three limbs.
9. This transformer is easy to repair,Easy to maintain.9. This transformer is not easy to repair.We need a
skilled technician to maintain this type of transformer.
10. In this type transformer concentric cylindrical
winding are used
10. In this type transformer sandwiched winding are used.

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Back EMF | Back EMF Significance in DC Motor

Back EMF | Back EMF Significance in DC Motor

What is Back EMF in DC Motor?


We know whenever conductor cuts the magnetic field,e.m.f will induce in conductor.This also applies for conductors in armature too.When the armature of a d.c. motor rotates under the influence of the driving torque, the armature conductors move through the magnetic field and hence e.m.f. is induced in them as in a generator. The induced e.m.f. acts in opposite direction to the applied voltage  V (Lenz’s law) and in known as back e.m.f or counter e.m.f. denoted with  Eb. 

The back emf  Eb(= PΦZN/60 A) is always less than the applied voltage V, although this difference is small when the motor is running under normal conditions.

Back EMF in DC Motor Circuit Diagram



Significance of Back EMF In DC Motor:


It is seen in the generating action, that when a conductor cuts the lines of flux, emf. gets induced in the conductor. The question is obvious that in a dc. motor, after a motoring action, armature starts rotating and armature conductors cut the min flux.So is there a generating action exiting in a motor ? The answer to this question is 'Yes'

After a motoring action, there exists a generating action.There is an induced e.m.f in the rotating armature conductors according to Faraday's law of electromagnetic induction. This induced e.m.f. in the armature always acts in the opposite direction of the supply voltage. This is according to the Lenz’s law which states that the direction of the induced e.m.f. is always so as to oppose the cause producing it. In a dc. motor, electrical input i.e. the supply voltage is the cause and hence this induced e.m.f. opposes the supply voltage. This e.m.f tries to set up a current through the armature which is in the opposite direction to that, which supply voltage is forcing through the conductor.

So as this e.m.f. always opposes the supply voltage, it is called back e.m.f. and denoted as Eb Though it is denoted as Eb, Basically it gets generated by the generating action which we have seen earlier in case of generators.So its magnitude can be determined by the emf. equation which is derived earlier. So,

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Long transmission lines,Analysis With Rigorous Method

Long transmission lines,Analysis By Rigorous method

What are called as Long transmission lines?

Answer: When the length of an overhead transmission line is more than 150 km and line voltage is very high (> 100 kV), it is considered as a long transmission line. For the treatment of such a line, the line constants are considered uniformly distributed over the whole length of the line and rigorous methods are employed for solution.

Long transmission lines:

It is well known that line constants of the transmission line are uniformly distributed over the entire length of the line. However, reasonable accuracy can be obtained in line calculations for short and medium lines by considering these constants as lumped. If such an assumption of lumped constants is applied to long transmission lines (having length excess of about 150 km), it is found that serious errors are introduced in the performance calculations. Therefore, in order to obtain fair degree of accuracy in the performance calculations of long lines, the line constants are considered as uniformly distributed throughout the length of the line. Rigorous mathematical treatment is required for the solution of such lines.



Above shows the equivalent circuit of a 3-phase long transmission line on a phase-neutral basis. The whole line length is divided into n sections, each section having line constants 1/n th of those for the whole line. The following points may by noted :

(i) The line constants are uniformly distributed over the entire length of line as is actually the case.
(ii) The resistance and inductive reactance are the series elements.
(iii) The leakage susceptance (B) and leakage conductance (G) are shunt elements. The leakage susceptance is due to the fact that capacitance exists between line and neutral. The leakage conductance takes into account the energy losses occurring through leakage over the insulators or due to corona effect between conductors.

Admittance =√G ²+B²

(iv) The leakage current through shunt admittance is maximum at the sending end of the line and decreases continuously as the receiving end of the circuit is approached at which point its value is zero.

Analysis of  Long Transmission Line (Rigorous method)

Below shows one phase and neutral connection of a 3-phase line with impedance and shunt admittance of the line uniformly distributed.


Consider a small element in the line of length dx situated at a distance x from the receiving end.
Let z = series impedance of the line per unit length
y = shunt admittance of the line per unit length
V = voltage at the end of element towards receiving end
V + dV = voltage at the end of element towards sending end
I + dI = current entering the element dx
I = current leaving the element dx
Then for the small element dx,
z dx = series impedance
y dx = shunt admittance
Obviously, dV = I z dx
or dV/dx = I z  ...(i)
Now, the current entering the element is I + dI whereas the current leaving the element is I. The difference in the currents flows through shunt admittance of the element i.e., 
dI = Current through shunt admittance of element = V y dx
dI/dx= V y  ...(ii)
Differentiating eq. (i) w.r.t. x, we get,










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