POWER SYSTEM TRANSIENTS UNIT I INTRODUCTION AND …of the same cloud is known as lightning. Lightning is a huge spark and takes place when clouds are charged to such a high potential

POWER SYSTEM TRANSIENTS

UNIT – I

INTRODUCTION AND SURVEY

1. Define power system transients(Nov/Dec 2011)The power system transient is the outward manifestation of a sudden change in circuitconditions as when a switch opens or closes or a fault occurs on a system. The transient periodis very short.

2. State the principle of superpositionAccording to the superposition principle, in any linear system if a stimulus S1 produces aresponse R1 and a stimulus S2 produces a response R2 then S1 and S2 applied simultaneouslywill evoke a response R1+ R2.The principle is not restricted to two stimuli but is true for any finite number.

3. Mention the application of superposition principle in the determination of transient.a) Due to the opening of switch in a circuit, the current is interrupted. The net current flowing

in the circuit is zero due to this opening. (i.e) an injected current I2 flows in oppositedirection to the existing current I1.

b) According to superposition if the emf and the injected current I2 are appliedsimultaneously, the total response will be the sum of individual responses. The combinedresponse will give the circuit’s response to the interruption and will include all transienteffects.

4. Mention the limitations of superposition principle.The superposition principle can be applied only in linear circuits. For the nonlinear devicessuch as saturable devices like iron cored reactor and unloaded transformer, this is notapplicable. Since here the current is not directly proportional to voltage. For nonlinear resistorssuch as protective devices, the principle of superposition should not be applied.

5. Mention the sources of power system transients(Nov/Dec 201, 2013)(May/June 2012)The sources of transients are classified as,a) Internal sourcesb) External sources

The internal sources are classified as

a) Switching surges i.e., due to the opening and closing of a switch in power systemb) Insulation failuresc) Arcing groundd) Ferro resonance

6. What is current chopping?Current chopping is the production of high voltage transient across the contacts of air blastcircuit breaker. When breaking low currents i.e., unloaded transformer or reactor magnetizingcurrent, the powerful deionizing effect of air blast causes the current zero well before thenatural current zero is reached. This phenomenon is called chopping and produces high voltagetransients across the breaker circuits.

7. What is meant by ferroresonance?Resonance causes high transient voltage in the power system. In usual transmission lines thecapacitance is very small so that resonance rarely occurred in power system at normalfrequency. However if generator emf wave is distorted, the trouble of resonance may occur dueto 5th or higher harmonics. This phenomenon is called as ferroresonance, since the inductanceinvolved is usually iron cored.

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EE6002- POWER SYSTEM TRANSIENTS Page 1 of 23

SRI VIDYA COLLEGE OF ENGG & TECH QUESTION BANK UNIT-I

8. What are the causes of switching surges?The making and breaking of electric circuits with switch gear may result in abnormal transientover voltages in a power system having large inductance and capacitances. The differentsituation under which this happens are summarized asa) Interruption of low inductive currents by high speed circuit breakerb) Interruption of small capacitance currents (switching operation of unloaded lines).c) Ferroresonanced) Energisation of loaded line.

9. What is meant by arcing ground?The phenomenon of intermittent arc taking place in line to ground fault of a three phase systemwith consequent production of transients is known as arcing ground. Arcing ground can beprevented by earthing the neutral.If the neutral of three phase wires was not earthed in long high voltage transmission lines aserious problem called arcing ground is produced. The arcing ground produces severeoscillations of three to four times the normal voltage.

10. What is meant by lightning?An electric discharge between cloud and earth, between clouds or between the charge centersof the same cloud is known as lightning. Lightning is a huge spark and takes place when cloudsare charged to such a high potential with respect to earth or neighboring cloud.

11. What are types of lightning?a) Direct strikeb) Indirect stroke

12. What are the types of power system transients?(Nov/Dec 2011)a) Ultra-fast transientsb) Medium fats transientsc) Slow transients

13. What are the effects of lightning? (Nov/Dec 2011)Lightning produces a steep fronted voltage wave on the line. The voltage of this wave may risefrom zero to peak value in about 1µs and decay to half the peak value in about 5µs

14. What is meant insulation failure?The insulation failure between line and earth which high voltage in the system. Suppose a lineat potential V is earthed at point C, the earthing of line causes two equal voltages – v travelalong the main wire and return wire. Due to insulation failure, the current to earth is twice theratio of voltage to impedance.

15. What is meant by subsidence transients?When a disturbance such as a fault occurs on the primary of transformer, then subsidencetransient is produced. Due to this sudden reduction of voltage produces on the primary.

16. Show the power system transients with respect to time duration.S.No Nature of the transient phenomena Time duration1. Lightning 0.1µs – 1.0ms2. Switching 10µs to less than a second3. Sub synchronous resonance 0.1ms – 5 seconds4. Transient stability 1ms – 10 seconds5. Long term dynamics 05. – 1000 seconds6. Tie-line dynamics 0.5 – 1000 seconds7. Daily load managements, operating action Upto 24hrs

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17. List out the frequency ranges of transients.S.No Origin of Transient Frequency Range1. Restrikes on disconnectors, and faults in GIS 100 kHz – 50 MHz2. Lightning surges 10 kHz – 3MHz3. Restrikes in circuit breakers 10 kHz – 1 MHz4. Transient Recovery Voltage

(a) Terminal faults(b) Short-line faults

50/60 Hz – 20 kHz50/60 Hz – 100 kHz

5. Fault clearing 50/60 Hz – 3 kHz6. Fault initiation 50/60 Hz – 20 kHz7. Fault Energisation 50/60 Hz – 3 kHz8. Load rejection 0.1 Hz – 3 kHz9. Transformer Energisation (DC) 0.1 Hz – 3 kHz10. Ferro resonance (DC) 0.1 Hz – 1 kHz

18. What is an impulsive low-frequency transient?The impulsive low-frequency transient rises in 0.1 ms and lasts more than 1 ms.

19. What is an impulsive high-frequency transient?The high frequency impulsive transient has duration below 50 ns and the frequency of highfrequency oscillatory type ranges between 0.5 and 4 MHz.

20. What is meant by first order transients?The energy storage elements in electrical circuits are inductors and capacitors. The first ordertransients occur when the circuit contains only one energy storage element that is inductance orcapacitance.

21. List out the common internal sources of transient activity.(a) Photocopiers(b) PC Power Supplies(c) Laser Printers(d) Electronic Ballasts(e) Welders(f) Power factor correction Equipment(g) Power Supplies(h) Temperature controllers(i) Motor Controllers(j) Pumps(k) Inverters(l) Compressors(m)Generators(n) Variable Speed Motors(o) Lightning Power Supplies.

22. List the types of power system transients.(Nov/Dec 2012, 2013) (May/June 2012)(i) Depending upon the duration of transients

(a) Ultra-fast transients(b) Medium fats transients(c) Slow transients

(ii) According to the sources of transients(a) Switching transients(b) Lightning transients

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(iii) Depending on its nature(a) Impulsive transients(b) Oscillatory transients

(iv) Depending upon the control to the transients(a) Single transients under our control(b) Recurrent transients(c) Random transients

23. Define surge impedance loading. (Nov/Dec 2011)The surge impedance loading or SIL of a transmission line is the MW loading of a transmissionline at which a natural reactive power balance occurs.

24. Draw the double frequency transient circuit with an example(April/may 2011)

The double frequency transient is initiated by opening the circuit breaker shown in figure.In this diagram L1 and C1 are the inductance and stray capacitance on the source of the breakerL1 and C2 represent the inductive load and its stray capacitance.Example : an unloaded transformer.

25. List the effects of transients on power system. (April/May 2011)(i) A direct or indirect lightning stroke on a transmission line produce a steep fronted

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

(ii) The travelling wave produced due to lightning transients will shatter the insulator andmay even wreck poles.

(iii) If the travelling waves produced due to lightning hit the windings of transformer orgenerator it may cause considerable damage. The inductance of the windings opposesand sudden passage of electric charge through it. Therefore the electric charges ‘pilesup’ against transformer.

26. What is known as voltage surge? (Nov/Dec 2010)An uninterrupted voltage increase that lasts more than a few seconds is usually called a"voltage surge" rather than a spike. These are usually caused by malfunctions of the electricpower distribution system.

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POWER SYSTEM TRANSIENTS UNIT I INTRODUCTION AND …of the same cloud is known as lightning. Lightning is a huge spark and takes place when clouds are charged to such a high potential