EE6002 POWER SYSTEM TRANSIENTS LT PC 3 0 0 3 UNIT I ... · wave excitation - double frequency transients - basic transforms of the RLC circuit transients. Different types of power

REGULATION: 2013 ACADEMIC YEAR: 2018-2019

JIT-JEPPIAAR/EEE/Mr..A.ANTONY CHARLES. IIId Yr/SEM 06 /EE6002/POWER SYSTEM TRANSIENTS /UNIT 1-

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UNIT I INTRODUCTION AND SURVEY 9

Review and importance of the study of transients - causes for transients. RL circuit transient with

sine wave excitation - double frequency transients - basic transforms of the RLC circuit transients.

Different types of power system transients - effect of transients on power systems – role of the

study of transients in system planning.

UNIT II SWITCHING TRANSIENTS 9

Over voltages due to switching transients - resistance switching and the equivalent circuit for

interrupting the resistor current - load switching and equivalent circuit - waveforms for transient

voltage across the load and the switch - normal and abnormal switching transients. Current

suppression - current chopping - effective equivalent circuit. Capacitance switching - effect of

source regulation - capacitance switching with a restrike, with multiple restrikes. Illustration for

multiple restriking transients - ferro resonance.

UNIT III LIGHTNING TRANSIENTS 9

Review of the theories in the formation of clouds and charge formation - rate of charging of thunder

clouds – mechanism of lightning discharges and characteristics of lightning strokes – model for

lightning stroke - factors contributing to good line design - protection using ground wires - tower

footing resistance - Interaction between lightning and power system.

UNIT IV TRAVELLING WAVES ON TRANSMISSION LINE COMPUTATION OF

TRANSIENTS 9

Computation of transients - transient response of systems with series and shunt lumped parameters

and distributed lines. Traveling wave concept - step response - Bewely’s lattice diagram - standing

waves and natural frequencies - reflection and refraction of travelling waves.

UNIT V TRANSIENTS IN INTEGRATED POWER SYSTEM 9

The short line and kilometric fault - distribution of voltages in a power system - Line dropping and

load rejection - voltage transients on closing and reclosing lines - over voltage induced by faults -

switching surges on integrated system Qualitative application of EMTP for transient computation.

TOTAL: 45 PERIODS

TEXT BOOKS:

1. Allan Greenwood, ‘Electrical Transients in Power Systems’, Wiley Inter Science, New

York, 2nd Edition, 1991.

2. Pritindra Chowdary, “Electromagnetic transients in Power System”, John Wiley and Sons

Inc., Second Edition, 2009.

3. C.S. Indulkar, D.P.Kothari, K. Ramalingam, ‘Power System Transients – A statistical

approach’, PHI Learning Private Limited, Second Edition, 2010.

REFERENCES:

1. M.S.Naidu and V. Kamaraju, ‘High Voltage Engineering’, Tata McGraw Hill, Fifth Edition,

2013.

2. R.D. Begamudre, ‘Extra High Voltage AC Transmission Engineering’, Wiley Eastern

Limited, 1986.

3. Y.Hase, Handbook of Power System Engineering,” Wiley India, 2012.

4. J.L.Kirtley, “Electric Power Principles, Sources, Conversion, Distribution and use,” Wiley,

2012

EE6002 POWER SYSTEM TRANSIENTS LT PC

3 0 0 3



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Subject Code: EE6002 Year/Semester: III /06

Subject Name: Power System Transients Subject Handler: Mr. A. Antony Charles

UNIT I INTRODUCTION AND SURVEY

Review and importance of the study of transients - causes for transients. RL circuit transient with sine

wave excitation - double frequency transients - basic transforms of the RLC circuit transients. Different

types of power system transients - effect of transients on power systems – role of the study of transients

in system planning.

PART*A

Q.No Questions

1. Define the term transients. (May/June2014) BTL3

The term transient originates from electric circuit theory where it denotes the voltage and current

component that occurs during the transition from one (typically sinusoidal) steady state to another steady

state.

2 Define power system transients. (Nov/Dec 2018) BTL1

• In power system the term transient is used in a slightly different way: it denotes those phenomena

in voltage and current with a short duration.

• There is no clear limit, but phenomena with duration of less than one cycle (of the power

frequency,50Hz or 60 Hz system) are generally referred to as transients.

3 What are the three classes of power system phenomena? BTL2

• Events that can be classified by their fundamental frequency magnitude.

• Events that present significant changes in the fundamental frequency magnitude but of short

duration.

• Events of very short duration for which the fundamental frequency magnitude does not offer important information.

4 Name any two effects of transients in power system. (Nov/Dec 2018) BTL3

• Internal causes (devices switching and arcing)

• External causes (lightning and poor electrical connections)

5 Define lightning induced transients. BTL1

• Lightning is commonly known for externally generated transients. It produces extremely

powerful, short duration transients on power distribution system either as direct strike or a near

hit.

• In most cases, a lightning strike induced surge on local power distribution line causes damage to

susceptible equipment.



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• The large electric fields generated during discharge of lightning can couple into the power system

network, creating induced transients.

6

What are the causes of transients? (April/May 2017) BTL2

• The vast majority of transients are produced due to switching devices, static discharge and arcing.

• A sudden change in an electrical circuit due to this action will generate a transient over voltages

due to the stored energy contained in inductances (L)and capacitance’s (C). The size and duration

of this transients purely depend on the value of L an C in the circuit and the applied waveform.

7 How line energizing of transmission lines causes transients? BTL4

• Energizing of transmission lines is another possible cause of oscillatory transients.

• The transmission lines can be modeled as a lumped capacitor which would result in the same

oscillations as for capacitor energizing.

• The voltage at the end of the line approaches 2.0 p.u and this is main concern from an insulation

point of view. The over voltage at the source side is approximately 1.4p. u

8 Classify transient based on their origin. BTL3

• Broadly the transients are categorized in two ways based upon their origin.

• Atmospheric Origin, that is lightning. Switching origin, that is all switching operations load

rejection and faults.

9 Define transient due to Electromagnetic phenomena. BTL1

The electromagnetic action is assumed to be unaffected by the mechanical state of the system in which its

occurs. For this type of condition, the generators are assumed to run at constant speed.

10 Define impulsive transients. BTL1

• An impulsive transient is a sudden change in the steady state condition of voltage, current or both

that is unidirectional in polarity.

• Impulsive currents are normally characterized by their rise and decay times.

• They are damped quickly by the resistive circuit elements and do not propagate far from their

source.

11 Define random transient. BTL1

• The source powering the circuit, or the system is frequently the cause of transient induced problem

or failures.

• These transients are difficult to deal with due to their nature they are totally random, and it is difficult

to define their amplitude, duration and energy constant.

• These transients are generally caused by switching parallel loads on the same branch of a power

distribution system and can also be caused by lightning.

12 What are the causes of fast transient over voltages? BTL2

• Disconnect or operations and faults with in gas insulated substations (GIS).

• Switching of motors and transformers with short connections to the switch gear.

• Certain lightning conditions.



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13

Give the basic importance aspects of transient over voltages. BTL3

• Frequency of the recovery voltage transient.

• Amplitude of the power frequency recovery over voltages circuit damping constant.

14 What are the effects of lightning transients? (April/May 2018) BTL2

• A direct or indirect lightning stroke on a transmission line produces a steep raise in voltage wave

on the line. The voltage of this wave may rise from zero to peak value about 1 µs.

• The travelling waves produced due to lightning transients will shatter the insulators and may even wreck poles.

15 Define recurring transients. BTL1

• Transients may be formed form energy stored in the circuit and inductance and capacitance when

electrical conditions in the circuit are abruptly changed.

• Switching induced transients are good examples of change current di/dt in an inductor will

generate voltage equal to L(di/dt).

• The energy transient is equal to usually exists a high power impulse for relatively short time.

16 Define Ferro-resonance (April/May 2018) BTL2

• Resonance in an electrical system occurs when inductive reactance of the circuit becomes equal

to the capacitive reactance.

• Under resonance, the impedance of the circuit and the power factor is unity. Resonance causes

high transient voltage in the power system.

• If generator emf wave is distorted, the trouble of resonance may occur due to fifth or higher

harmonics. This phenomenon is referred as ferro resonance.

17 What is meant by lightning? BTL2

• Lighting is an unavoidable event that affects power systems through several mechanisms. Lighting

is the most well-known of the externally generated transients.

• Most lightning transients are not actually a result of direct lightning strikes, they most often

“induced” onto conductors as lightning strikes near the power line.

• The large electric fields generated during a discharge can couple into the power system, creating

induced transients.

18 List out the power system transients with respect to time duration. BTL5

Sl. No. Nature of the transient Time Duration

1 Lightning 0.1µs – 1.0 ms

2 Switching 10 µs to less than a second

3 Sub synchronous resonance 0.1 ms – 5 seconds

4 Transient stability 1 ms – 10 seconds

5 Long term dynamics 0.5 – 1000 seconds



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6 Tie-line regulation 10 – 1000 seconds

7 Daily load managements, operator action Upto 24 hrs

19 What is impulsive high-frequency transients? BTL2

The high frequency impulsive transient has duration below 50 ns and the frequency of the high frequency

oscillatory type ranges between 0.5 MHz and 5MHz.

20 What is meant by attenuation and distortion of travelling waves? BTL2

The decrease in the magnitude of the wave as it propagates along the line is called attenuation. The

elongation or change of wave shapes that occur is called distortion.

21 Define switching transients. BTL1

The switching transient is initiated whenever there is sudden change of circuit conditions. This transient is most frequently developed due to switching operations such as the closing of a switch (or) circuit breaker to energies a load. The opening of a circuit breaker to clear a fault.

22 What is meant by insulation failure? BTL2

The insulation failure between line to earth which cause high voltage in the system. Suppose a line at potential V is earthed at point C, the earthing of line causes to equal voltages-V travel along the main wire and return wire. Due to insulation failure, the current to earth is twice the ratio of voltage to impedance.

PART * B

1. Explain the classification of transients. (13M) BTL2

Answer: Page: 1.3-V. THIYAGARAJAN

Definition: Broadly the transients are studied in two categories, based upon their origin--------(2M)

Electromagnetic and Electromechanical transients---------------------------(4M)

Transient classification with respect to frequency groups-------------------(2M)

Classification depending on frequency and its nature-----------------(2M)

Explanation: A direct or indirect lightning stroke on a transmission line produces a steep wave in the voltage

on the line. The voltage wave may rise from zero to peak value in a about 1 µs-------(3M)

2. Explain the switching transients of RL circuit with sine wave excitation .(April/May 2017) (13M)

BTL2


Definition: The ideal switch charges from close to open position instantaneously and the sinusoidal current

is always interrupted at current zero.--------------------------------(2M)

Switching an RL circuit-----------------------------(4M)



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------------------------------------(4M)

Explanation: Transient current components is determined by inductance and the resistance only not

influenced by the network. The steady state current component is the particular solution of the non-

homogenous differential equation.-------------------------------(3M)

3.

Explain about the causes and sources for transient in a power system. (Nov/Dec 2018) (13M) BTL2


Definition: Transients produced due to internal causes may increase the system voltage to twice the normal

value. By providing the proper insulation of the equipment, the switching transients----------(2M)

Current chopping-------------------(3M)

Switching operation of Un loaded line--------(3M)

Ferro resonance and energisation of a loaded line ----------(2M)

Explanation: Transient in a power system are mainly due to the oscillations set up by sudden changes in the

circuit due to the breaker operation. This circuit changes may be normal switching operation it may be fault

condition. --------------(3M)

4. (i)Discuss the various types of power system transients.(APRIL/MAY 2017) (8M)

(ii)Briefly discuss the various sources transients on power system. (APRIL/MAY 2017) (5M)BTL3

Types of power system transients:


Definition: When switch operates, it completely isolates the load from the supply. Two halves of the circuit

behave independently-------------------(2M)

First order and second order transients------------(2M)

Basic transforms on RLC circuit--------------------(2M)

Explanation:

When switching devices operate they can change the topology circuit may be connected or disconnected it

is called switching surge transients.--------------------(2M)

Sources transients on power system:



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Definition: Transient activity is believed to account for 80% all electrically related features or down time

or running outages----------------------------(2M)

Characteristics of the transient voltage waveform

Explanation:

A system approach to transient voltage surge suppression can result in dramatic performance in terms of

return on investment. The most effective means of improving your power quality.-------------------(3M)

5. Classification of transients on frequency ranges. (13M) BTL3


Definition: An electrical transient is a temporary excess of voltage and current in an electrical circuit when

a fault occurs on a system or switch opens or closes---------------------------------(2M)

Impulsive low frequency transient------------(2M)

Medium frequency impulsive transient-------------------(4M)

Subsidence transients-------------------------(2M)

Explanation: Transients may not be propagated easily as the low frequency types but have a higher

amplitude and low arcing faults on the power distribution----------------------------------------(3M)

6. With suitable examples explain ferro resonance.(13M) BTL4


Definition: Ferro resonance or nonlinear resonance is a type of resonance in electric circuits which occurs

when a circuit containing a nonlinear inductance is fed from a source that has series capacitance, and the

circuit is subjected to a disturbance such as opening of a switch---------------------------------(2M)

Three phase transformers with no load or light load------------(4 M)

Underground cable ------------------(2M)

Single phase switching phases switched one at time-------------------------(2M)

Explanation: Ferro resonance is a rare condition most likely to occur with three-phase delta-connected

transformers. Not nearly as often, ferro resonance has also been documented in Y-Y transformers and in

aerial three-pot banks served by long-dedicated aerial circuits.----------------------------------------(3M)

7

Explain in detail, the need for study of transients in system planning.(Nov/Dec 2018)(13M)BTL4




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Definition: In an R-L-C circuit, both electromagnetic and electrostatic energies are involved, hence any

sudden change in the conditions of the circuit involves the redistribution of these two forms of energy. The

transient currents produced due to this redistribution are known as double-energy transient-----------(2M)

Transient over voltages ------------(4M)

Over damped and natural oscillations ------------------(2M)

Oscillatory transients------------------------(2M)

Explanation: Impulsive transients can excite the natural frequency of power system circuits and produce

oscillatory transients. An oscillatory transient is a sudden, non–power frequency change in the steady-state

condition of voltage, current, or both, that includes both positive and negative polarity values.----------(3M)

8. Explain the resistance switching.(13M)BTL2


Definition: Resistive switching refers to the physical phenomena where a dielectric suddenly changes its

resistance under the action of a strong electric field or current. The change of resistance is non-volatile and

reversible. -----------------------------------------------(2M)

Damped restriking voltage-------------(2M)

Oscillatory restriking voltage ------------------(4M)

Recovery voltage-------------------------(2M)



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Explanation: When the fault occurs, the contacts of the circuit breaker are open, and an arc is struck

between the contacts. With the arc shunted by the resistance R, a part of arc current is diverted through the

resistance. This results in the decreases of arc current and an increase in the rate of deionization of the arc

path. --------------(3M)

9

What are the source of transients? Also explain how transient affect the power system.(13M)

BTL1


Definition: Power system transients are outward manifestation of sudden change in system conditions when

a switch opens or closes, or fault occurs on a system -----------------------------------------------(2M)

Sources / Causes of transient -------------(2M)

Current chopping------------------(2M)

Switching surges -------------------------(4M)

Explanation: It is the phenomenon of current interruption before the natural current zero is reached.

Current chopping mainly occurs in air blast CB’s, because they retain same extinguishing power irrespective

of the magnitude of current to be interrupted. --------------(3M)

PART *C

1. Explain about the effects of transient activity .(15M) BTL2


Definition: Transients can interrupt the normal timing of the motor and result in micro-jogging, motors will

accelerate the failures and electrical distribution systems produce hysteresis loss in motors----------(3M)

Electrical distribution equipment----------------------(3M)

Electronics equipment and lightning------------------(3M)

Switching surge and dielectric loss------------------(2M)

Explanation: Transients may not be propagated easily as the low frequency types but have a higher

amplitude and low arcing faults on the power distribution----------------------------------------(4M)

2. Describe about the classification depend on control the transients. (15M) BTL2


Definition: Depending upon the control to the transients produced in power system, there are three types

of transients---------------------------- (3M)

Single transient ----------------------(3M)



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Recurrent transients ------------------(3M)

Random transients -----------------(2M)

Explanation: Catastrophic equipment failure, immediate operation cause power system components

failure and system undergo abnormal condition -------------------------------------(4M)

3.

Explain about the technical definition of transients and double frequency transients. (15M) BTL2


Definition: A sub cycle disturbances in the AC waveform that is evidenced by a sharp brief the continuity

waveform. Transients may be of either polarity and may be additive or subtractive energy of the nominal

waveform---------(3M)

Recovery voltage device equipment----------------------(3M)

Oscillatory frequency ranges------------------(3M)

Under damped and critically damped curves------------------(2M)

Explanation: Transient waveforms may contain one or more oscillatory components can be characterized

by the natural frequencies of the equipment’s depend on the power system equipment----------(4M)



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UNIT II SWITCHING TRANSIENTS

Over voltages due to switching transients - resistance switching and the equivalent circuit for interrupting the

resistor current - load switching and equivalent circuit - waveforms for transient voltage across the load and

the switch - normal and abnormal switching transients. Current suppression - current chopping - effective

equivalent circuit. Capacitance switching - effect of source regulation - capacitance switching with a restrike,

with multiple restrikes. Illustration for multiple restriking transients - ferro resonance.

Q.No. PART*A

1.

Define switching transients. BTL1

• The transients due to either closing or opening operation of a switching device is called switching

transient.

• Fuse can perform opening operations only.

• After closing operation, transient currents will flow through the system and after an opening operation,

when a power frequency current is interrupted a transient recovery voltage (TRV) will appear across

the terminals of the interrupting devices.

2

Give the two categories of switching operations. BTL3

• Energization and

• De-energization

3

Define transient recovery voltage. (TRC)BTL1

• A transient voltage is developed across contacts of a switch when they start to open.

• This voltage, known as transient recovery voltage (TRV), is present immediately after the current

zero, and in actual system its duration is in the order of milli seconds.

4

What are the two components of recovery voltage? BTL2

• A transient component which occurs immediately after a current zero.

• A steady state component which is voltage that remains after the transient dies out.

5 What is magnification of capacitance switching? BTL2

• The magnitude and frequency of the inrush current resulting from energizing capacitor bank is a function of the following:



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• The point of the wave of the applied voltage where the contacts were closed.

• The capacitance and inductance of the circuit

• The charge on the capacitor at closing time

• Any damping resistance contained in the circuit.

For capacitance switching, failures are often reported which are the direct result of inrush currents and over-

voltages that have propagated themselves into lower voltage network causing damage especially to

electronic equipment connected to the circuit.

6

Define normal switching transients. BTL1

• Normal switching transients are circumstances in which voltage or current with in normal peak values.

• Closing switch or circuit in a dominantly capacitive or inductive network results in rush currents which

can cause problems for protection system.

• A transient is created whenever a sudden change occurs in a power circuit, especially during power

switching, either closing or opening a circuit.

7

What is the effect of load switching? BTL2

• The most important function performed by switching devices is switch ON and OFF loads in frequent

number of times.

• The impedance of the load circuit being switched can be series combination of resistances and reactance

a parallel combination of resistance and reactance or a series parallel combination of resistance and

reactance.

• It is to be known that the recovery voltage is one of the principle factors for determining the ability of a

circuit breaker to clear a circuit successfully.

8

Give the principle area of interest associated with the switching of series capacitors relates to their rapid

re-insertion. BTL3

• Surge-arrester sizing.

• Line breaker TRV.

• Line energization.

• Bank insertion and by-pass.

• Single phase reclosing.

• Line protection.

9 Mention the two-important cause of current interruption. BTL3

• Single line fed bus fault and

• Transformer secondary fault

10

What is the cause of back-to-back capacitor switching? BTL2

The inrush currents associated with back to back capacitor switching must be evulated with respect to the

capacitor switch capabilities. Standard specify inrush currents magnitude and frequency limits for general and

define purpose breakers.

11 What are the effects of shunt capacitor switching? BTL2

• Bring about severe harmonic distortion and resonance with load-generation harmonics.



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• Increase the transient inrush current of power transformers in the system create over voltages and

prolong its decay rate.

• Stress the capacitor’s themselves due to switching transients.

• Discharge into external fault,and produce damaging over voltages across current transformers secondary

terminals.

12

Define load switching. BTL1

• Loads are frequently switched in a power system.

• For industrial system contactors are designed to switch normal loads, such as motors, pumps and

furnaces and so on frequently.

• Load break switches and circuit switches are the devices that can interrupt the load current but not fault

current.

• The frequency of normal load switching in utility system is usually very low.

13 Give the examples of normal switching transients. BTL3

Simple switching transient include circuit closing transients, initiated by clearing a short circuit.

Inductance and capacitance with little damping the simple switching.

14

What are the phenomena of current chopping? BTL2

• The current carried by a power switch does not normally stop when it contact separate rather the current

continues to flow through an arc until it reaches periodic zero.

• The analyses have been based on the assumption that current would be extinguished at a normal current

zero.

15

What is the effect of un-loaded high voltage transmission lines?BTL2

• Switching un loaded high voltage transmission lines in and out services is same functioning as switching

a capacitor bank because an unloaded transmission line has a dominantly capacitive behavior.

• A small voltage jump occurs because of Ferranti-rise effect when a transmission line is energized after

closing a switch device, the resulting voltage wave reflects and causes the doubling the voltage at the

open end of the line.

16

Define multiple restrikes phenomena in capacitor switching. BTL1

• When a couple of re-ignitions occurs is called multiple restrikes, very high voltages build up across the

interrupting chamber and it is most likely flash over takes place on the outside chamber of the interrupter.

• The circuit breaker is short-circuited out of the system in the way and cannot contain function anymore

this is a very dangerous and unwanted situation.

17

How restrike free operation is performed to avoid over voltages? BTL3

High voltage circuit breakers, which have to perform capacitive current switching, should be restrike-free to

avoid over voltages.

18

How ferro resonance cause overvoltage’s? BTL3

• In a resonant circuit, inductive and capacitive reactance of the circuit is equal to each other.

• The only opposition to current is circuit resistances, resulting in the undesired voltages and over

currents at resonance frequency.



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• This resonance effect presents one stable state operation are mitigated by system frequencies control

by pure resistances.

19

Define ‘reignition ‘and ‘restriking’ phenomena in capacitor switching. BTL1

The shunt capacitor switching, it is important that the terms ‘reignition ‘and ‘restriking’ are defined.In general,

dielectric breakdown can occur in an interrupter at any time after initial arc extinction .When switching

capacitive circuits ,this breakdown is termed as ‘reignition’ if it occurs within 0.25 cycle of initial arc extinction,

but any time after this period the phenomena is called ‘restriking’

20

Classify the modes of ferro resonant. BTL3

The steady state condition can be classified into four types.

• Fundamental mode

• Sub harmonic mode

• Quasi periodic mode

• Chaotic mode

21 Define fundamental mode of ferro resonance. BTL1

• The signals have a distorted wave form, but are periodic with a period equal to the source period.

• The signal spectrum is made up of the fundamental frequency of the system and its harmonics.

22 Define Quasi-periodic mode of ferro resonance.BTL1

In this mode ,the signals are not periodic. The signal spectrum is still a discontinuous spectrum.

23 Define Chaotic mode of ferro resonance.BTL1

• The signals show an irregular and unpredictable behavior.

• The signal spectrum is continuous.

PART * B

1. Explain with appropriate waveform,

(i) Current suppression.

(ii) Current chopping

(iii) Capacitance switching with one and multiple restrikes.

(iv) Ferro resonance. BTL2


Definition: Current suppression or chopping occurs when the circuit is forced to zero. The result of the per

armature extinction of the power frequency current before the current is reached. To increase the instability as

the current approaches zero. ---------(2M)

Interruption of low magnesting current---------------------------(4M)



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Current chopping transient------------------------(2M)

Scenes of ferro resonance ----------------------(2M)

Explanation: When a small current is interrupted by a circuit breaker the current may be brought to zero

abruptly and ahead of the normal current zero. This is known as a current chopping.---------------(3M)

2 Explain in detail about, (i)Resistance switching (ii)Load switching with their equivalent circuits. BTL2


Definition: To reduce a restriking voltage, RRRV (Rate of Rise of Restriking Voltage) and severity of the

transient oscillations a resistance is connected across the contacts of the circuit breaker. This is known as

resistance switching -------------------------------(2M)

Analysis of resistance switching---------------------------(4M)

Equivalent circuit for the resistance switching------------------------(2M)

Equivalent circuit for the resistance switching ----------------------(2M)

Explanation: Switching on and off the loads is frequently performed by switching devices, which is many

instances can be represented by a parallel RL circuit.-------------------(3M)

3 Explain the load switching in both normal and abnormal condition with a neat sketch.(Nov/Dec

2018)(13M) BTL2


Definition: Switching ON and Switching OFF the loads is frequently performed by switching devices, which

is many instances can be represented by a parallel circuit-------------------------------(2M)

Resistive load switching-----------------------------------(4M)

Effect of power factor -------------------------------------(2M)

Example of load switching--------------------------------(2M)



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Explanation: Low power factor loads will be inductive, high power factors loads are resistive. When such load

is switched off the effective capacitance of the load becomes important in determining the form of

transient-----------------------------------------(3M)

4 What is capacitance switching? Explain in briefly the effect of source regulations with a capacitance

restrike. (Nov/Dec 2018) (13M)BTL1


Definition: The shunt capacitors are employed to correct a lagging power factor to provide voltage supports for

a system switching in and out of the capacitor banks is called capacitance switching-----(2M)

Effect of source regulation on capacitance switching-----------------------------------(4M)

Capacitance switching with a restrike -------------------------------------(2M)

Capacitance switching with a multiple restrike --------------------------------(2M)

Explanation: The capacitor banks or shunt capacitors plays a vital role in power systems. The shunt capacitors

are connected between transmission line and ground commonly available in the ground and substation in the

power systems. ----------------------(3M)

PART * C

1. Describe briefly about the ferro resonance. (13M) BTL3


Definition: In the presence of system capacitance, certain transformer and reactor combinations can give rise

to ferro resonance phenomena, due to non linearity and saturation of the reactance. This is characterized by

peaky current surges of short duration which generate over voltage ---------- -----(2M)



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Scenes of ferro resonance -----------------------------------(4M)

Ferro resonance situations in practice------------------(4M)

Explanation: In the presence of system capacitance certain transformer and reactor combinations can give rise

to ferro resonance phenomena, due to nonlinearity and saturation of the reactance. This is characterized by

peaky current surges of short duration which generate over voltages. ----------------------(5M)

2 With a neat sketch explain the capacitance switching with multiple restrikes. (13M) BTL3


Definition: When the capacitor is disconnected, the potential of source side circuit breaker will return to this

lower value but will do so by way of an oscillation involving the source inductance and stray capacitance

adjacent to the circuit breaker on the source side. ---------- -----(4M)

Capaictence switching with restrikes-----------------------------------(4M)

Capaictence switching with multiple restrikes

------------------(4M)

Explanation: At the time the transient voltage reaches its peak, identified +3Vm in above fig, the transient

current in the lower trace passes through zero. Some breakers are capable of interrupting at such a current

zero. If this happens, the high voltage is left in trapped on the capacitor ----------------------(3M)



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UNIT III LIGHTNING TRANSIENTS

Review of the theories in the formation of clouds and charge formation - rate of charging of thunder clouds –

mechanism of lightning discharges and characteristics of lightning strokes – model for lightning stroke - factors

contributing to good line design - protection using ground wires - tower footing resistance - Interaction between

lightning and power system.

Q.No. Questions

1.

How lightning over voltages are caused in power system? BTL3

• Lightning over voltages on the power system are caused by the large current that flows to ground during

a lightning discharge from a cloud to earth.

• During such a discharge the object could be a part of the discharge path to ground or it could be indirectly

affected by induced over voltages.

2

Define lightning mechanism. BTL1

• The concentration of electric charge existing within the cloud must be limited to the particular limit and

in most case it will be smaller.

• When compared to such clouds, the earth can be considered as infinite.

• I t is known as fact that the electrical gradient of the earth is 100 volts per centimeter and before the

discharge electrical gradient with in the cloud must be higher than 100 volts per centimeter.

• Therefore, the lightning stroke starts from the cloud rather than from the ground.

3 What are the specification of tower footing resistance? BTL2



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• Direct lightning strokes on the line conductors injecting a current wave on the line, lightning strokes on

the transmission towers or on the ground wires.

• Lightning strokes on the transmission towers or on the ground wires.

4

Give the Wilson theory of lightning. BTL3

• Presence of a large numbers of ions in the atmosphere i.e both the positive and negative ions.

• Ions attach themselves with the minute particles of dust and extremely small drops of water to form large

ions as contrasted with unattached ions.

• Number of small ions and large ions per cubic centimeter ranges from 30 to 1000 respectively.

5

Give the Simpson’s theory of lightning. BTL3

• When a large water drops of radius greater than 0.25 centimeter falls through ascending air, the drops

become flattened or unstable and large number of small drops are formed.

• Terminal or steady state velocity of large drops is eight meters per second which constitute the limiting

relative velocity of rain drops.

• No water drops will reach earth in the ascending air with the velocity greater than eight meters per

second.

• Drop breaks or combines with ions in atmosphere it results in positively charged droplets and negatively

charged air.

6 Define direct lightning stroke. BTL1

The direct lightning stroke is the one which strikes either the phase conductors or the tower or the shield wire

generates very high voltage in the power line.

7 Define In direct lightning stroke. BTL1

A very high voltage can be generated in the power line due to the stroke which hits the nearby the ground. Such

strokes are called as indirect lightning strokes or induced lightning strokes.

8

What is the effect of direct strokes? BTL2

• If flash hits an overhead line, the current injected into the conductor is divided the strike point, giving rise

two voltage waves that propagate in opposite directions.

• The prospective the magnitude of the voltages can be estimated by multiplying the current flows in each

direction by the characteristic impedance of the line which is normally the range is 400Ω to 500Ω.

9

How the degree of hazard is minimized in lightning? BTL3

• The degree hazard depends on the circumstances to minimize the chances of being struck by lightning

during thunderstorm.

• One should be sufficiently far away from tall objects likely to be struck remain inside building or be well

insulated.

10

Give the effects of indirect lightning strokes. BTL3

• When the subject is close to a parallel hit or other tall object.

• Due to an intense electric field from a stroke which can induce sufficient current to cause death.

• When lightning terminating on the earth sets up high potential gradients over the ground surface in an

outwards direction from the point or object struck.

11 How the measurement of stroke current is characterized? BTL3

Measurement of stroke currents at ground have shown that the high currents is characterized by a fast rise to crest

(1to 10µsec) followed by a longer decay time of 50-1000µsec to half-time.



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12

Define lighting phenomenon. BTL1

• It is a peak electric discharge of charge accumulated in the clouds between the cloud and ground or

between two neighboring clouds or with in the same cloud.

• This phenomenon of discharge charge from charged cloud is called lighting phenomenon.

• The electrode separation between clouds to ground is very large.

13

What is ground to cloud lightning? BTL2

The lightning mostly originates in the thunder cloud and terminate the ground called ground to cloud lightning.

It is a major cause of danger and damage to extra high voltage transmission up to a few hours to complete a

destruction of a line.

14

What is the effect of indirect lightning strokes? BTL2

• Lightning and switching surges can occur occasionally in electric power transmission lines.

• These phenomena can produce high voltage level in a very short time that can damage insulation or can

cause severe flash over.

• Lightning surge may be caused by either direct strokes or by the field radiated from direct lightning

,called indirect strokes.

15

What is the effect of induced lightning surges? BTL2

• The induce lightning surges can cause significant damage to electric power components,

telecommunication equipment, computer networks etc.

• This result in severe damages of equipment, interruption of services, increased operation and maintenance

cost.

• Therefore, adequate lightning or surge protection of electrical and electronics system from

electromagnetic disturbances has been increase important.

16 What is the need of protection lightning? BTL2

Protecting over head transmission lines against lightning strokes is one of the most important tasks to safe guard

electric power systems, since lightning is a usual cause of faults in over head line.

17

Define flash over. BTL1 The tower has to carry vast impulse currents during occurrence of direct strokes. If the tower footing resistance is substainal the potential of the tower rise to a huge value as a result of flash over may take place along the insulator strings. This is known as flash over.

18

What is the effect of tower footing resistance on lightning performance? (April/May 2018) BTL2

The tower footing resistance for fast front surges is not well understood .The impulse ground resistance is less

than measured or calculated resistance because significant ground currents cause voltage gradients sufficient to

break down the soil around the ground rod.

19

What is the need of surge arrester? BTL2

Surge arrester are installed on transmission lines between phase and earth in order to improve then lightning

performance and reduce the failure rate. Surge arrester are semiconductor with a linear resistance from few ohms

to several mega ohms.

20 Give the condition of ideal lightning arrester. BTL3

• Conduct electric current at certain voltage above the rated voltage.

• Hold the voltage with a little change for the duration of over voltages and



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• Substantially cease condition at very nearly the same condition voltage at which conduction started.

21

Give the main characters tics of a surge arrester. BTL3

• Rated voltage which must be 1.25*MCOV;

• Protection level;

• Capacity to withstand the energy of transient voltages.

PART-B

1.

Discuss the mechanism of lightning strokes and over voltages of transmission lines.(Nov/Dec 2018)(13M)

BTL2


Definition: An electric discharge between the cloud and earth between clouds or within the same cloud is knmown

as lightning. The discharge of the charged cloud to the ground is called lightning phenomenon---------- -----(2M)

Lightning phenomenon -----------------------------------(4M)

Lightning mechanism-------------------------------------(4M)

Explanation: The velocity of this streamer is about3% that of light. Thought the discharge current in the return

streamer is relatively large since its continues only for a few micro seconds it contains less energy and hence its

streamer is called a cold lightning strokes----------------------(3M)

2

Discuss the different theories of charge formation of thunder clouds. (13M) BTL2


Definition: During thunder storms positive and negative charges separated by the heavy air currents with ice

crystals in the upper part and rain in the lower cloud ---------- -----(2M)

Simpsons theory of cloud model -----------------------------------(4M)

Reynolds and masons modified theory------------------------------------(4M)



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Explanation: Thunder clouds consists of a uniform mixture of positive and negative charges .Due to air currents

and hail stones the charge separate vertically----------------------(3M)

3

Explain in detail how the charges are formed in the clouds. (13M) BTL2


Definition: The factors that contribute to the formation or accumulation of charge in the clouds are too many and

uncertain. But during thunderstorms, positive and negative charges become separated by the heavy air currents

with ice crystals in the upper part and rain in the lower parts of the cloud---------- -----(2M)

Rate of changing thunder clouds -----------------------------------(4M)

Mechanism of lightning strokes ------------------------------------(4M)

Explanation: Lightning phenomenon is a peak discharge in which charge accumulated in the clouds discharges

into a neighboring cloud or to the ground. The electrode separation, i.e., cloud-to-c1oud or c1oud-to-ground is

very large, perhaps 10 km or more. The mechanism of charge formation in the clouds and their discharge is

quite a complicated and uncertain process----------------------(3M)

4

Discuss the interaction of lightning in power system.(13M)BTL2


Definition: The parameters and characteristics of lightning include the amplitude of the currents, the rate of

rise, the probability distribution of the above, and the wave shapes of the lightning voltages and currents and

capacitor charging currents---------------------------------(2M)

Direct lightning strokes -----------------------------------(4M)

In direct lightning strokes ------------------------------------(4M)

Explanation: The lightning current oscillograms indicate and initial high current portion which is characterized

by short front times up to 10 μs. The high current peak may last for some tens of microseconds followed by a

long duration low current portion lasting for several milliseconds----------- (3M)

5

Discuss in detail, that the factors contribute to good line design. How the ground wires are protected from

lightning transients. (Nov/Dec 2018)(13M) BTL1


Definition: In lightning studies, the strokes to ground are of importance, as they induce over voltages on

transmission lines and in nearby object. ---------------------------------(2M)

Factors affecting mechanical design of overhead transmission lines -----------------------------------(4M)

Protection against the transmission line design ------------------------------------(4M)



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Explanation: The return stroke current I (z, t) is assumed as function of vertical co-ordinate z and time t and

the initial current I( 0, t) is of importance in engineering applications. The electromagnetic fields associated with

the lightning current and the lightning induced voltages equations ----------- (3M)

6

Explain the counter poise method of protection. (13M) BTL2


Definition: Effective protection against direct strokes requires a shield to prevent lightning from striking the

electrical conductor together with adequate drainage facilities over an insulated structure so that the

discharge can drain to the ground without affecting the conductors ---------------------------------(2M)

Ground wires -----------------------------------(4M)

Requirement of ground wires for effective protection ------------------------------------(4M)

Explanation: Ground wire is a conductor run parallel to the main conductor of the transmission line supported

on the same tower and earthed at every equally and regularly spaced towers. It is run above the main conductor

of the line. The ground wire shields the transmission line conductor from induced charges, from clouds as well

as from a lightning discharge. ------------------------------(3M)

PART*C

1.

Give the mathematical models for lightning discharges and explain them. (15M) BTL3


Definition: During the charge formation process, the cloud may be considered to be a nonconductor. Hence,

various potentials may be assumed at different parts of the cloud. If the charging process is continued, it is

probable that the gradient at certain parts of the charged region exceeds the breakdown strength of the air or

moist air in the cloud. Hence, local breakdown takes place within the cloud. ----------------------(4M)

Isokeraunic level-----------------------------------(4M)

Transmission line parameters ------------------------------------(4M)

Explanation: In lightning studies, the strokes to ground are of importance, as they induce over voltages on

transmission lines and in nearby objects. Hence, a model based on lightning return stroke is presented here

which assumes that the lightning channel is straight, vertical and normal to the ground plan---------------(3M)

2

Explain the different characteristics of the lightning strokes.(15M) BTL2


Definition: When lightning strikes a power line a current is injected into the power system. This is a very

useful concept. What voltages this current gives rise, will depend upon its wave shape and the impedances

through which it flows. ----------------------(4M)



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Stroke A lightning mechanism -----------------------------------(4M)

Stroke B lightning mechanism ------------------------------------(4M)

Explanation: Direct impact of the lightning strokes on the building. A lightning stroke can fall on an overhead

line can spread over several kilometers across the electric power line. ---------------(3M)

UNIT-IV TRAVELLING WAVES ON TRANSMISSION LINE COMPUTATION OF TRANSIENTS

Computation of transients - transient response of systems with series and shunt lumped parameters and distributed

lines. Traveling wave concept - step response - Bewely’s lattice diagram - standing waves and natural frequencies -

reflection and refraction of travelling waves.

Q.No. PART*A

1.

What is the basic characters tics of transmission lines? BTL1

• Basically a transmission lines is a distributed system that supports propagating electromagnetic waves for

signal transmission.

• One of the main characters tics obtained by the transmission lines is the delay response due to the finite

wave velocity.

2

What is the representation of transmission lines parameters? BTL1

A transmission line may be explained in terms of distributed circuit parameters, also called line parameters the

inductance parameters which represent the series loop inductance per unit length of line,capaictence parameter

which is shunt capacitance per unit length between conductors.

3

Why overhead lines are not represented in lumped elements? BTL2

A representation of overhead lines by means of lumped elements is not helpful in making us understand the wave

phenomena because electromagnetic waves have a travel time Only when the physical dimensions of a certain part



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=

of the power system are small compared with the wave length of the transients, the travel time of the

electromagnetic waves can be neglected.

4

What are the specifications of a travelling wave?(Nov/Dec 2018)BTL2

• Any disturbance on a transmission line or system such as sudden opening or closing of line, or a fault

results in the development of overvoltage or overcurrent at that point.

• This disturbance propagates as travelling waves to the ends of the line or to a termination such as a

substation usually these travelling waves are high frequency disturbances and travelling as waves.

• They may be reflected, transmitted, attenuated during propagation until the energy is absorbed.

5

What are the damages caused by the travelling waves? BTL2

• The high peak (or) crest voltage of the surge may cause flashover in the internal winding their by spoil

the windings insulation.

• The steep wave front of the surge may cause internal flashover between inter turns of the transformer.

6

What is meant by surge impedance of a line and why is it also called the natural impedance? BTL2

The ratio of voltage to current which has the dimension of impedance is called as surge impedance of

the line.

E /I=√(L/C) = Zc = Zn ( natural impedance)

It is also called the natural impedance because this impedance does not depend on load impedance but depends

only on the line constants.The value of this impedance is 400Ω to 600Ω for an overhead line and 40 to 60Ω for a

cable.

7

What is the application Bewley’s lattice diagram?BTL2

With the use of Bewley’s diagram one can know at a glance the position the direction of motion of every

incidence of reflected and transmitted wave on the system at every instant of time.

8 What is Velocity of Wave Propagation?BTL2

The velocity of propagation of any electromagnetic disturbance in air is the same as that of light, namely

2.998*1010 cm per sec. The only difference for transmission line is that the conductor provides a guide.

9

What is reflection and refraction of travelling waves?BTL2

• Reflection involves a change in direction of waves when they bounce off a barrier; refraction

of wave involves a change in direction of wave as they pass from one medium to another, and

diffraction involves a change in direction of wave as they pass through an opening or around

a barrier in their path.

10

What is step response of traveling waves?BTL2

• The unique characteristic of the short circuit is that is impossible to develop any voltage across

it.

• When a travelling wave of voltage reaches a short circuit the reflected voltage wave must

precisely cancel out the incident wave so that the refracted wave is zero



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‘Y’ p

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11

What is attenuation and distortion of travelling waves?BTL2

• In general in addition to the effects of reflection at transition points, travelling waves are also

subjected to both attenuation (decrease in magnitude) and distortion (change is shape) as they

propagate along the transmission line.

• They are caused by losses in the energy of the wave due to resistance, leakage, dielectric, and

corona.

• Corona is the main cause of attenuation at high voltages. It reduces the magnetic and the steepness of the

wave fronts within a few miles to a safe voltage

12

What is lattice Diagram? (Nov/Dec 2011)BTL2

The lattice diagram determines the voltage at a given point and time in a transmission system. It is a

useful aid to keep track of travelling voltage wave or current wave as it reflects back and forth from

the ends of the line.

13 What is attenuation factor ‘k’?BTL2

It is also common for an attenuation factor ‘k’ to be defined corresponding to the length of a particular line.

14

Define propagation constant ‘r’?BTL1

The propagation constants of a line γ of a line of series impedance ‘Z’ and shunt

admittance

γ = √

15

What are the properties of bewley’s lattice diagram? (May/June 2012)BTL2

• All waves travel downward, because time always increases.

• The position of any wave at any time can be deduced directly from the diagram.

• The total potential at any point at any instant of time is the superposition of all the waves

which have arrived at that point up until the instant of time, displaced in position from each

other by intervals equal to the difference in their time of arrival.

• The history and origin of the wave is easily traced. It is possible to find where it comes

from and just what other waves went into its composition.

• Attenuation is included, so that the wave arriving at the far end of a line corresponds

to the value entering to the value entering multiplied by the attenuation factor of the

line.

16

What is step response of traveling waves?BTL2

• The unique characteristic of the short circuit is that is impossible to develop any voltage across

it.

• Thus when a travelling wave of voltage reaches a short circuit the reflected voltage wave must

precisely cancel out the incident wave so that the refracted wave is zero.

• If the incident voltage wave is V1 and the incident wave I1, The reflected voltage and current

wave will be –V1 and +I2.

17 What is Velocity of Wave Propagation?BTL2



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The velocity of propagation of any electromagnetic disturbance in air is the same as that of light, namely

2.998*1010 cm per sec. The only difference for transmission line is that the conductor provides a guide

18 Give the distributed inductance and capacitance parameters of the transmission life.BTL3

Associated disturbance inductance and capacitance parameters the free space permeability is depend on the coaxial

cable.

19

What are the different cases for arriving discontinuity of a travelling wave?BTL2

• Line terminated by resistance.

• Line terminated by inductance.

• Line terminated by capacitance.

20

What are the characteristics obtained in lossless transmission lines?BTL2

• Series resistance of the conductors

• Skin effect for high frequencies.

• Leakage currents across string insulators

• Influence of the ground resistance

• Corona losses

21

Define SWR.BTL1

Standing wave ratio is the ratio of the amplitude of partial standing waves at an antinode to the amplitude at an adjacent node is an electrical transmission line.

22

What are standing waves?BTL2

• A standing wave, also known as stationery waves, is a wave that remains constant position.

• This phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a stationery medium as a result of interference between two waves travelling in opposite direction.

PART-B

1. Discuss the transient response of system with series and shunt lumped parameters and distributed lines

.(13M)BTL3


Definition: There is an impedance ZS in between the source and line which normally is composed of the transient

reactance x’ of the synchronous machine and any resistance that can be included in the circuit breaker during

switching operation. ----------------------(3M)

Lumped and distributed parameters -----------------------------------(3M)

Computation of transients in transmission line -----------------------------------(4M)

Explanation: Transient response of a system voltage reaches the maximum value of the series and shunt lumped

parameters .For short line ignore the shunt capacitance and conductance all together for medium and long

transmission lines appropriate with the lumped and for long lines ----(3M)



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2

With a neat sketch, explain bewley’s lattice diagram.(13M)BTL3


Definition: Lattice diagram have been introduced by bewley’s and are of great help while making calculations

and travelling waves, when a travelling waves a terminal point is may be reflected and transmitted of both voltage

and current----------------------(3M)

Travelling waves -----------------------------------(3M)

Standing waves -----------------------------------(4M)

Explanation: The mechanism by which a mechanical wave propagates itself through a medium involves particle

interaction; one particle applies a push or pull on its adjacent neighbour, causing a displacement of that neighbour

from the equilibrium or rest position. ------------(3M)

3 Derive the reflection and refraction coefficients of a travelling waves .(13M)BTL2


Definition: When a traveling wave on a transmission line reaches a junction with another line, or a termination,

then part of the incident wave is reflected, and a part of it is transmitted beyond the junction or termination. The

incident wave, reflected wave and the transmitted wave ----------------------(3M)

Reflection coefficient for voltage------------------------------------(3M)

------------------------(4M)

Explanation: When a voltage surge E arrives at the junction J, which is on open circuit, it is reflected without a

change in sign (i.e. E). Also, a current surge (- I) of opposite sign to the incident (I) is reflected so that the

transmitted current is zero. -------------------------------(3M)

4

Write short notes on standing waves and natural frequency.(13M)BTL3




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Definition: A standing wave also known as a stationary wave, is a wave that remains in a constant. Position. This

phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a

stationary medium as a result of interference between two waves travelling in opposite directions----------(3M)

Nodes and antinodes------------------------------------(3M)

Standing waves Expression-----------------------------------(4M)

Explanation: The SWR is usually defined as a voltage ratio called the VSWR, for voltage standing wave ratio.

For example, the VSWR value 1.2 : 1 denotes a maximum standing wave amplitude that is 1.2 times greater than

the minimum standing wave value. -------------------------------(3M)

5 Explain the various type of traveling wave concept in step response.(13M)BTL2


Definition: Simple wave or a travelling-wave, also sometimes called a progressive wave is a disturbance that

varies both with time and distance. Travelling waves are observed when a wave is not confined to a given space

along the medium.--------------------------(3M)

Travelling wave expression ------------------------------------(3M)

Sine wave expression-----------------------------------(4M)

Explanation: This crest is followed by a trough which is in turn followed by the next crest. Infect, one would

observe a distinct wave pattern (in the form of a sine wave) travelling through the medium. This type of wave

pattern which is seen travelling through a medium is sometimes referred to as a travelling wave.--------------(3M)

6 Derive the reflection and refraction of the travelling waves .(13M)BTL4 (April/May 2018)(Nov/Dec 2018)


Definition: When a traveling wave on a transmission line reaches a junction with another line, or a termination,

then part of the incident wave is reflected back, and a part of it is transmitted beyond the junction or termination.

The incident wave, reflected wave and the transmitted wave --------------(3M)

Reflection coefficient for voltage ------------------------------------(3M)

Refraction coefficient or transmission coefficient for current -----------------------------------(4M)

Explanation: When a voltage surge E arrives at the junction J, which is on open circuit, it is reflected without

a change in sign (i.e. E). Also, a current surge (- I) of opposite sign to the incident (I) is reflected so that the

transmitted current is zero. If the line is fed from a constant voltage source E, then as the reflected voltage surge



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(E) arrives at the generator end, since the generator maintains the voltage at its end at voltage E, it send a voltage

surge of -E back to the line so as to keep its voltage at E.-------------------------------(3M)

PART*C

1 With a neat sketch explain transient and load analysis of transients diagram. (15M) BTL4 (April/May 2018)


Definition: All waves travel downhill, because time always increases. The position of any wave at any time can be

deduced directly from the diagram. The total potential at any point, at any instant of time is the superposition of all

the waves which have arrived at that point up until that instant of time, displaced in position from each other by

intervals equal to the difference in their time of arrival --------------------------(3M)

Reflections at three substations-----------(3M)

Travelling at standing waves -----------------------------------(5M)

Explanation: The mechanism by which a mechanical wave propagates itself through a medium involves particle

interaction; one particle applies a push or pull on its adjacent neighbor, causing a displacement of that neighbor from

the equilibrium or rest position. As a wave is observed travelling through a medium, a crest is seen moving along

from particle to particle-------------------------------------(4M)

3 Explain the behavior of travelling waves at line terminations for

(i)Open circuited line

(ii)Short circuited line. BTL3 (April/May 2017)


Definition: An open circuit at the end of a transmission line demands that the current at that point be zero at all

times thus when a current wave of + I arrives at the open circuit , a current wave of – I is at once initiated to

satisfy the boundary condition .This will travel towards the source with a voltage of +V.--------------(3M)

Incident voltage wave ------------------------------------(4M)

Line terminated with the capacitor-----------------------------------(4M)

Explanation: The voltage across the inductor initially rises to double the value of the incident wave and

decays exponentially. This is of importance when long lines are terminated with inductors or transformers on open

circuit. ------------------------------(4M)



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UNIT V TRANSIENTS IN INTEGRATED POWER SYSTEM

The short line and kilometric fault - distribution of voltages in a power system - Line dropping and load rejection - voltage transients on closing and reclosing lines - over voltage induced by faults -switching

surges on integrated system Qualitative application of EMTP for transient computation.

Q.No. PART*A

1.

Define kilometric fault.(Nov/Dec2018) BTL2



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Kilometric fault is the fault located beyond the terminals and thus the current can be easily interrupted

due to the added impedance of the line. This added impedance not only limits the current but also

supports some of the system voltage.

2

What are the causes of over voltage? BTL2

i) Over voltage are induced when a ground fault occurs on one of the conductors.

ii)A line to ground fault can produce an overvoltage on an un faulted phase as high as 2.1 times the normal line to neutral voltage on a three phase line.

3

What is meant by switching surges? BTL2

The disturbance produced by the switching operation in a system which sets up travelling waves which

travel along the connected lines to and for transmission line. These disturbances are called as switching

surges.

4

Define reflection coefficient. BTL1

The reflection coefficient (a) is given by the ratio of the voltage of reflected wave to the voltage of incident wave of a transmission line due to the travelling waves caused by switching surges.

Reflection coefficient a = Vr/Vi

Where Vr = is the reflected wave. Vi= is the incident wave.

5

Define transmission coefficient. BTL1

It is defined as the ratio of voltage of transmitted wave to the voltage of incident wave. Transmission coefficient = Vt /Vi.

6

What is meant by EMTP? BTL2

The EMTP is a comprehensive computer program designed to solve electrical transient problems in

lumpy circuits, distributed circuits. This program is capable of solving steady-state circuit problems.

Transients analysis can be carried out in circuits without any arbitrary configuration of lumped

parameters (R, L, & C). Transmission lines with distributed parameters, transposed (or) un transposed,

can be included in the network.

7

What are the effects of load rejection in power systems? BTL2

• Sudden load rejection on large power systems causes the speeding up of generator prime movers.

• The speed governors and automatic voltage regulates will intervene to restore the normal

conditions.

• Initially both the frequency and voltage increases.

8

Write the network equation to model a transmission network for EMTP calculation.BTL3

[G][V(T)] = [ I(T)-[I]]

Where [G] is the nodal conductance matrix.



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[V(T)] is the node voltages.

[I(T)] is the vector of current sources

is the vector of past history terms .

9

What are the effects of transients when a switch a switch is closed? BTL2

• When a switch is suddenly closed immediately prior to the circuit being completed, a certain voltage across the switch contacts.

• At the moment the contacts made by pre striking discharge, this voltage appears.

10

Define fault surge voltage. BTL1

• Right after initiation of the line to ground fault a transient called fault surge occurs but is died out

by few milli seconds.

• Then a sustained dynamic over voltage is observed and this is called “temporary voltage”

11

Define travelling waves caused surges. BTL1

• A transient on a distributed parameter line is called surge because the transient is caused by

travelling waves.

• An overvoltage’s due to surges is in general much greater than the temporary over voltages, thus

insulation of a transmission system is determined by the surge voltages.

12

List the various types of surges on overhead line. BTL3

• Switching surges

• Fault initiation and fault clearing surges

• Lightning surges

13

Give the technology of circuit breakers must be able to make. BTL3

• Withstand the high thermal energy of the arc before current zero.

• Rapidly removes the arc after current zero in order to withstand the TRV.

14

Mention the events leading to the generation of temporary over voltages. BTL2

• Load rejection and line dropping

• Ferranti effect

• Ground fault

• Harmonic over voltages due to magnetic saturation

15

What are the three main components of load rejection? BTL2

The phenomena that occurs after load rejection in the three main components are:

• Generators

• Transformers

• Transmission lines/cables.

16 Define harmonic over voltages. BTL1



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• Harmonic oscillations in power system are initiated by system non linearities whose primary

sources is that of saturated magnetizing characteristics of transformers and shunt reactors.

• Magnetizing current of these components increases rapidly and contains a high percentage of

harmonics for voltages above the rated voltages.

• Saturated transformers inject large harmonics current in a system.

17

How line fault generates fault clearing surge?BTL2

• It is also called line to ground fault originally due to lightning and is sustained by an electrical arc

of with the energy is supplied from a generator in the system.

• Therefore, when the energy from the generator is shut down by opening the CB, the arc is

distinguish and fault itself is cleared.

• At this stage the fault clearing surge is generated.

18

What is need of arrester?BTL2

• An arrester is mainly to protect power components from a lightning over voltages.

• It is expected to apply arrester to suppress a switching over voltage on an UHV systems.it is

greatly enhanced by the development of arrester.

19

What are the possibilities of occurrence high power frequency over voltages?BTL2

• When the length of the line increases

• When the source reactance increases

• When the surge impedances of the line is lowered as direct result of an increased number of the

connected lines.

20

Give the various events that occurring in reclosing operations. BTL3

• A fault occurs, usually in one phase

• The transmission line is deenergized

• On unfaulted phases, the current is capacitive when remote end is already switched off.

PART* B

1. Explain short line kilometric fault. (13M)BTL2


Definition: Faults occurring between the transmission line from the breakers are the short line fault

the line impedance and admittance system voltage ----------(3M)

Surge impedance across transmission line ------------------------------------(3M)

Circuit breaker at a recovery voltage at receiver end -------------------------(4M)

Explanation: The voltage at the circuit breakers the line side terminal voltage across the line short

circuit condition the fault will occur across successful operation -------------------------------(3M)

2 What is line dropping? Explain its construction and operation of transients.(13M) BTL2




5/QB+Keys/Ver1.0

6.35

Definition: In three phase circuit capacitance switching, the determination of the voltage trapped after switching off is based on the ratio of the positive sequence capacitance C1 (total capacitance per phase) to the zero sequence capacitance.----------(3M) Load Rejection ------------------------------------(3M)

Voltage Transients on Closing and Reclosing Lines------------------------(4M)

Explanation: During the restrike, the voltage across the switch will be divided into source side

and to the line based on their respective surge impedances. The wave travels up and down the line

corresponding to the oscillatory swing of the bank in the case of lumped capacitance-------------------------

------(3M)

3 Explain in detail about the switching surges on integrated system. (Nov/Dec 2018)(13M)BTL2


Definition: The 138kV bus is fed through the generator transformer. There are lines connected to both the

buses shows a single phase representation. A fault at short distance down the line of 354kV bus is

considered for the analysis.----------------------------------------. ------------------(3M)

First generation encounter ------------------------------------(3M)

Generation and switching transients -------------------------(4M)

Explanation: The lines are represented by the resistances R1 and R2 under transient conditions until it is

modified by reflection from points down the line..-------------------------------(3M)



5/QB+Keys/Ver1.0

6.36

4 Explain EMTP for transient computation .(13M) BTL2


Definition: The EMTP is a computer program designed to solve electrical transient problems in lumped

circuits, distributed circuits-------------------------(3M)

Equivalent circuit of the breakers ------------------------------------(3M)

EMTP for transient and applications -------------------------(4M)

Explanation: Nonlinear resistors (for surge arresters) and nonlinear inductors (for saturable devices) can

be represented. It is also possible to open or close switches to simulate breaker operations, flashovers etc.

Both voltage and current sources are available with sine, ramp, or step functions. Trapped charges can be

recognized.-------------------------------(3M)

5 Explain the over voltage induced by faults. (13M) BTL2


Definition: Where the line is connected to a major load on a generating station, sudden load rejection will

result in over speeding of the machines and a rise in voltage until such times as this can be checked by the

governors and exciters.------------------------(3M)

Voltage Transients on Closing and Reclosing Lines ------------------------------------(3M)

Fault network analysed by the superimposed network------------------------(4M)

Explanation: Over voltage may induced by the symmetrical faults such a ground fault of three phase

conductors line to ground fault will be given-------------------------------(3M)

6 (i)Explain the voltage transients on closing and reclosing lines. (9M) BTL2

(ii)Describe the cause of over voltages induced by various power factor. (4M) BTL2


Definition: There is an impedance ZS in between the source and line which normally is composed of the

transient reactance x’ of the synchronous machine and any resistance that can be included in the circuit

breaker during switching operation. A shunt impedance ZSh is also in the circuit which can represent the

shunt compensating reactor. --------------------(3M)

Travelling wave concept------------------------------------(3M)

Transmission line loss------------------------(4M)



5/QB+Keys/Ver1.0

6.37

Explanation: The total voltage at the open end is now (a+a-a3-a3) = 2(a-a3) which is the sum of all

voltages marked on the inclined lines up to 3T. Proceeding in this manner, we observe that in order to

keep the voltage at +1, continued reflection--------------------------(3M)

PART*C

1.

With a neat sketch explain switching surges on integrated system. (April/May 2018) (15M)BTL4


Definition: Also, the incident surge voltage E is related to the incident surge current I by the surge

impedance of the line Z1. Similarly the transmitted surge voltage ET is related to the transmitted surge

current IT by the surge impedance of the line Z2 and the reflected surge voltage ER is related to the

reflected surge current IR by the surge impedance of the line Z1.

Surge voltage and impedance on transmission lines --------------------(4M)

Reflected wave voltage ------------------------------------(4M)

-------------------------------------(4M)

Explanation: The voltage surge - E is accompanied by a current surge - I. The surge voltage - E as it

reaches the open junction J, is reflected again without a change in sign, and accompanied by a current + I

so as to make the transmitted current again zero. Once these voltage and current waves reach the

generator, the instantaneous voltage and current will be zero, and the line would once again be

uncharged. -------------------------------(3M)

2

Explain about load rejection in integrated power system.BTL2(April/May 2017)(15M)


Definition: In three phase circuit capacitance switching, the determination of the voltage trapped after switching off is based on the ratio of the positive sequence capacitance C1 (total capacitance per phase) to the zero sequence capacitance C0 (capacitance from line-to-ground). Voltage transients on closing and reclosing lines --------------------------(4M)



5/QB+Keys/Ver1.0

6.38

Over voltages by faults ------------------------------------(3M)

Switching Surges on a integrated systems ------------------------(4M)

Explanation: The secondary side switching of transformers connected to high voltage lines can itself

create over voltages, even without restriking. This comes about again as a result of the nonlinear properties

of the transformer core.------------------------------(4M)

3

Describe the causes of over voltages induced by various faults in a power system.BTL3(Nov/Dec

2017)(15M)


Definition: Voltage transients on closing and reclosing lines --------------------------(3M)

Over voltages by faults ------------------------------------(4M)

----------------------------------(4M)

Explanation: The secondary side switching of transformers connected to high voltage lines can itself

create over voltages, even without restriking. This comes about again as a result of the nonlinear properties

of the transformer core.------------------------------(4M)

Documents

EE6002 POWER SYSTEM TRANSIENTS LT PC 3 0 0 3 UNIT I ... · wave excitation - double frequency transients - basic transforms of the RLC circuit transients. Different types of power