IMPORTANT TWO MARKS AND SIXTEEN MARKS QUESTION AND ANSWER

Transcription

1 Year/Sem:IV/VII IMPORTANT TWO MARKS AND SIXTEEN MARKS QUESTION AND ANSWER PREPARED BY J.SHANMUGAPRIYAN ASST.PROF/EEE CCET KARUR 1

2 SYLLABUS UNIT I PROTECTIVE RELAYS Principles and need for protective schemes nature and causes of faults types of faults Power system earthing - Zones of protection and essential qualities of protection Protection scheme construction and characteristics of relays over current relays directional, distance and differential relays under frequency relays negative sequence relays static relays microprocessor based relays. UNIT - II APPARATUS PROTECTION Apparatus protection generator and transformer protection protection of bus bars, transmission lines, CT's & PT's and their application in protective schemes UNIT - III THEORY OF CIRCUIT INTERRUPTION Physics of arc phenomena and arc interruption.restriking voltage & Recovery voltage, rate of rise of recovery voltage, current chopping, interruption of capacitive current, resistance switching DC circuit breaking. UNIT - IV CIRCUIT BREAKERS Switch gear fault clearing process interruption of current Types of Circuit Breakers Air blast, oil, SF6 and Vacuum circuit breakers comparative merits of different circuit breakers Testing of circuit breakers Circuit breaker ratings. UNIT - V PROTECTION AGAINST OVER VOLTAGES Causes of over voltages methods of protection against over voltages ground wires, Peterson coil, surge absorbers, surge diverters relay co-ordination selection of protective system Insulation co-ordination. REFERENCE BOOKS 1 Sunil S. Rao, Switchgear and Protection, Khanna publishers, New Delhi, C.L. Wadhwa, Electrical Power Systems, New Age International (P) Ltd., M.L. Soni, P.V. Gupta, V.S. Bhatnagar, A. Chakrabarti, A Text Book on Power System Engineering, DhanpatRai& Co., Badri Ram, Vishwakarma, Power System Protection and Switchgear, Tata McGraw Hill, B. Ravindranath, and N. Chander, Power System Protection & Switchgear, New Age Publishers,

3 UNIT I PROTECTIVE RELAYS TWO MARKS 1. What is Relay? The protective relaying is used to give an alarm or to cause prompt removal of any element of power system when that element behaves abnormally. The relays are compact and self contained devices. 2. What are the main reasons for main protection failure? Failure in circuit breakers. Failure in protective relay. Failure in tripping circuit. Failure in dc tripping voltage. Loss of voltage or current supply to the relay. 3. Name the methods of backup protection. Relay Backup Protection BreakerBackup Protection Remote Backup Protection Centrally Co-Coordinated Backup Protection 4. Give the essential quantities of protective relaying. Reliability Selectivity Speed And Time. Sensitivity Stability Adequateness Simplicity and Economy. 5. Classify the protective relays. Electro Magnetic Attraction Type Relays. Induction Type Relays Directional Type Relays. Relays Based On The Timing Differential Relays Distance Type Relays. 6. Define plug setting multiplier. The ratio of actual current in the relay coil to the pickup current is called plug-setting multiplier P.S.M= fault current in the relay coil /pickup value 3

4 7. What are the advantages of electromagnetic relays? It can be used for both A.C And D.C Fast operation The Operating time varies with current High operating speed. Compact, Reliable, Simple, Robust 8. What are the disadvantages of electromagnetic relays? The Directional Feature Is Absent Due To fast Operation The Transients can Affect The Working. 9. What are the applications of electromagnetic relays? A.C and D.C Equipments. Over/Under Current And Over/ Under Voltage Protection Differential Protection Used As A Auxiliary Relays. Earth Fault Protection 10. Name the types of Induction type Relays. Shaded Pole Type Watt Hour Meter Type Induction Cup Relay The Above Relays Are Classified Based On The Construction. 11. Define Differential Relay. A differential relay is defined as the relay that operates when the phasor difference of two or more similar value electrical quantities exceeds a predetermined value. 12. What are the types of differential relay? Current Differential Relay Biased Beam Relay Or Percentage Differential Relay Voltage Balance Differential Relay 13. Define Maximum Torque Angle. The angle by which the current supplied to the relay leads the voltage supplied to the relay so as to obtain the maximum torque is called maximum torque angle. It is denoted by τ. 14. Name The Types Of Relay Based On The Voltage And Current. Impedance relay (measurement of Z) Reactance relay.(measurement of X) Admittance relay (measurement of Y) 4

5 15. Draw The Operating Characteristics Of Impedance Relay. 16. Draw the operating characteristics of reactance relay. 17. Draw the operating characteristics of mho relay. 18. Define static relays. The relays do not use moving parts and use the solid state electronic components such as diodes, transistors are called static relays. 5

6 19. What are the advantages of static relays? The moving parts are absent. The Power Consumption Is Low. The Response Is Very Quick Maintenance Is Less The Sensitivity Is High. The Testing And Servicing Required Is Less. 20. What are the disadvantages of static relays? Characteristics are temperature dependent. Reliability is unpredictable. Additional d.c supply is required. Less robust compared to electro magnetic relays. susceptible to the voltage fluctuations 21. Give the types of static relays. Frequency Relay. Negative Sequence Relay. Microprocessor Based Relay. Static Distance Relay. Static Differential Relay. Static Time Over current Relay and Directional O/C Relay. 22. What are the types of frequency relays? Under frequency relays Over frequency relay 23. What are the components present in the static relays? Rectifier Filter Level detector Amplifier 24. What are the types of comparators used in static relays? Rectifier bridge type Hall effect type 25. What are the types of induction type relays? Induction cup type Induction disc type 6

7 26. What are the conditions to be satisfied for the operation of directional over current relay? The direction of current in the circuit must reverse to operate directional element. The current value in the reverse direction must be greater than the current setting. The high value of current must persist for a time period which is greater than the time setting of the relay. 27. Define pickup current. The minimum current at which the armature gets attracted to close the trip circuit is called pickup current. 28. Define impedance relay. Impedance relays also called as distance relay. The operation is dependent on the ratio of voltage to current, which is expressed in terms of impedance.as the ratio V/I affect the performance of these relays, the relays are also called ratio relays. 29. What is Selectivity? It is the ability of the protective system to select correctly that part of the system in trouble and disconnect the fault part without disturbing the rest of the system. 30. What is Sensitivity? It is the ability of the relay system to operate with low value of actuating quantity. 31. What is reliability? It is the ability of the relay system to operate under the pre-determined conditions. Without reliability, the protection would be rendered largely ineffective and could even become a liability. 32. What is simplicity? The relaying system should be simple so that it can be easily maintained. Reliability is closely related to simplicity. The simpler the protection the greater will be its reliability. 33. What are the types of electromagnetic attraction type relays? Attracted armature type relay Solenoid type relay Balanced beam type relay 34. What is inverse time relay? An inverse time relay is one in which the operating time is approximately inversely proportional to the magnitude of the actuating quantity. At higher values of current less than pickup, the relay never operates. At higher values, the time of operation of the relay decreases steadily with the inverse of current. 7

8 35. What is 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. 36. What is definite time lag relay? In this type of relay, there is a definite elapse between the instant of pickup and the closing of relay contacts. 37. What are the conditions must be satisfied for the operation of induction type directional over current relay? Current flow in a direction such as to operate the directional element. Current in the reverse direction exceeds the preset value. Excessive current persists for a period corresponding to the time setting of over current element. 38. What are the functions of protective relays To detect the fault and initiate the operation of the circuit breaker to isolate the defective element from the rest of the system, thereby protecting the system from damages consequent to the fault. 39. Define protected zone. Are those which are directly protected by a protective system such as relays, fuses or switch gears. If a fault occurring in a zone can be immediately detected and or isolated by a protection scheme dedicated to that particular zone. 40. What is primary protection? Is the protection in which the fault occurring in a line will be cleared by its own relay and circuit breaker. It serves as the first line of defense. 41. What is back up protection? Is the second line of defense, which operates if the primary protection fails to activate within a definite time delay. 42. Name the different kinds of over current relays. Induction type non-directional over current relay, induction type directional over current relay & current differential relay. 43. Define energizing quantity. It refers to the current or voltage which is used to activate the relay into operation. 8

9 44. Define operating time of a relay. It is defined as the time period extended from the occurrence of the fault through the relay detecting the fault to the operation of the relay. 45. Define resetting time of a relay. It is defined as the time taken by the relay from the instant of isolating the fault to the moment when the fault is removed and the relay can be reset. 46. What is biased differential bus zone reduction? The biased beam relay is designed to respond to the differential current in terms of its fractional relation to the current flowing through the protected zone. It is essentially an overcurrent balanced beam relay type with an additional restraining coil. There straining coil produces a bias force in the opposite direction to the operating force. 47. What is the need of relay coordination? The operation of a relay should be fast and selective, ie, it should isolate the fault in the shortest possible time causing minimum disturbance to the system. Also, if a relay fails to operate, there should be sufficiently quick backup protection so that the rest of the system is protected. By coordinating relays, faults can always be isolated quickly without serious disturbance to the rest of the system. 48. Mention the short comings of Merz Price scheme of protection applied to a power transformer. In a power transformer, currents in the primary and secondary are to be compared. As these two currents are usually different, the use of identical transformers will give differential current, and operate the relay under no-load condition. Also, there is usually a phase difference between the primary and secondary currents of three phase transformers. Even CT s of proper turn-ratio are used; the differential current may flow through the relay under normal condition. 49. What is a programmable relay? A static relay may have one or more programmable units such as microprocessors or microcomputers in its circuit. 16 MARK QUESTIONS. 1.What are the functions of protective relaying and explain it. 1.To sound an Alarm (or) to close the trip circuit of circuit breaker. 2.To disconnect the abnormally operating part. 3.To disconnect the faulty part quickly 4.Tolocalise the effect of fault by disconnecting the faulty part 5.To disconnect the faulty part quickly so as to improve the system stability Refer Page No:485Text Book :Switchgear Protection and Power systems Sunil S Rao Functions 9

10 2. With a neat diagram, explain the zones of protection. Various protections zones are: 1. Generator protection Zones 2. Generator transformer unit protection Zones 3. Bus Bar protection Zones 4. Transmission line protection Zones Refer Page No:487Text Book :Switchgear Protection and Power systems Sunil S Rao :Fig,No 25.1 (a),(b) in page NO 487, What are the essential quantities of protective relaying and explain it. The qualities of protective relaying are named as 1. Selectivity,Discrimination 2. Sensitivity,Power consumption 3. Reliability 4. Adequateness 5.Speed,Time 6.Stability 7.System Security Refer Page No:490Text Book: Switchgear Protection and Power systems Sunil S Rao 4.What is fault? Explain the nature, types and causes of faults. Definition: A fault in its electrical equipment is defined as a defect in its electrical circuit due to which flow of current is diverted from the intended path. Explanation Types of Faults Causes Of Faults Refer Page No:486 Text Book :Switchgear Protection and Power systems Sunil S Rao Refer table No 25.1 for nature and types of fault. 5. What is meant by directional feature of a directional over current relay? Describe the construction and principle.(apr 04,AU). Description about over current protection Features of a directional over current relay Principle of a directional over current relay Construction of a directional over current relay Refer Page No: 515,528 Text Book: Switchgear Protection and Power systems Sunil S Rao 10

11 6. Explain the negative sequence currents for the operation of relays.(apr 04,AU). Principle Explanation Refer Page No:5-41, Book : Protection and Switchgear Uday A Bakshi 7.Explain the operation of directional and non-directional induction type over current relay.(apr 05 AU) Description about over current protection Features of a directional,non Directional over current relay Principle of a directional,non Directional over current relay Construction of a directional,non directional over current relay Refer Page No:515,528 Text Book :Switchgear Protection and Power systems Sunil S Rao 8. Draw the characteristics of the following distance relays in the R-X diagram and explain. i) Mho relayii) Reactance relay iii)impedance relay Definition of aove three relay Characteristics R-X Diagram Refer Page No: Text Book: Switchgear Protection and Power systems Sunil S Rao 9. How electromagnetic relays differ from static relays and describe the block diagram for static relays. Comparison between Electromagnetic Relay and Static relay Definition of static relay Block Diagram of Static relay Description Advantages Refer Page No:7-12,7-14, Book : Protection and Switchgear Uday A Bakshi 10. Describe the principle of operation of microprocessor based static relays. Principle Operation of Microprocessorrelay Advantages Refer Page No:7-25, Book : Protection and Switchgear Uday A Bakshi 11

12 1. Name the fault occurs in generator. Stator faults Rotor faults Due to abnormal conditions UNIT II -APPARATUS PROTECTION 2. What are the faults occurs in stator? Phase to earth fault Phase to phase fault Inter turn faults involving turns of same phase winding. 3. Name the possibilities of abnormal condition. Overloading Over speeding Unbalanced loading Over voltage Failure of prime mover Loss of excitation Cooling system failure 4. Give the disadvantages of basic differential scheme. This circuit inaccurately with heavy external faults.the CT S may saturate and cause unequal secondary currents and the difference of secondary currents may approach the pickup value to operate the relay unnecessarily. 5. What are the functions of auxiliary relay? Tripping of the main circuit breaker of generator Tripping of the field circuit breaker Tripping of the natural circuit breaker if it is present Shunt down of the prime mover Turn on of co2 gas if provided Operation of alarm to indicate the occurrence of the fault 6. Name the possible transformer faults. Overheating Winding faults Open circuits Through faults Over fluxing 12

13 7. What are the problems encountered in differential protection. Unmatched characteristics of C.T s Ratio change due to tap change Difference in lengths of pilot wires Magnetizing current inrush 8. Name the factors affect the magnitude and direction of the magnetizing Current inrush Size of the transformer Size of the power system Type of magnetic material used for the core The amount of residual flux existing The method by which transformer is energized 9. Tabulate the typical values of harmonic contents in a magnetizing current. Order of harmonic content Amplitude as a %of fundamental Name the types of faults can be protected by The Bucholz Relay Local Overheating Entrance Of Air Bubbles In Oil Core Bolt Insulation Failure Short Circuit Laminations Loss Of Oil Short Circuit Between Phases Winding Short Circuit 11. What are bus-bar faults in transformer? Failure of circuit breaker Earthquake and mechanical damage Accidents due to foreign bodies falling across the bus bars Flashover due to heavily polluted insulator 13

14 Errors in the operation and maintenance of switchgear 12. Give the difficulties in the bus bar protection. current levels for different circuits are different large number of circuits to be protected with large load changes, relay setting need to be changed due to various bus sections, the scheme become complicated 13. Define current transformer Current transformer is one, which is treated as a series transformer under virtual short circuit conditions. The windings carry full current. Secondary must be always shorted 14. Define potential transformer Potential transformer is one, which is treated as a parallel transformer under open circuit conditions. Full line voltage appears across its terminals. Secondary is nearly under open circuit conditions. 15. Define ratio error Large error is introduced in the measurements done by the instrument transformers % Ratio error = nominal ratio- actual ratio Actual ratio X Define phase angle error. In the power it is must that the phase of secondary current is to be displaced by exactly 180 from that of primary current for CT. While phase of secondary voltage is to be displaced by exactly 180 from that of primary voltage of P.T. but actually this is not so it is called phase angle error. 17. What are the applications of C.T and P.T? Circulating current differential protection Over current phase fault protection Distance protection. 18. List out the advantages of instrument transformer. This is used to measure high voltage and current Several instrument can be fed through single transformer It is used in pilot lights. 19. List out the disadvantages of instrument transformer. It can be used only for AC circuits and it is not used in DC circuits. 14

15 20.A 11000: 110 potential transformer is used along with a voltmeter reading 87.5V. Estimate the value of line voltage. V1/V2=11000/110 V2=87.5 V3=8750V 21. What are the Merz price protection schemes for transformer? Star delta transformer Star Star transformer delta -Star transformer 22. What are the drawbacks of unbalanced loading of alternators? Unbalanced loading of the stator winding develops negative sequence current in itself. This gives rise to additional heating of the stator winding. 23. What are the basic bus bar protection schemes? Frames leakage protection Circulating current protection. 24. What are different types of potential transformer? Instrument potential transformer Capacitance potential transformer. 25. What are the various faults that would affect an alternator? (a) Stator faults Phase to phase faults Phase to earth faults Inter turn faults (b) Earth faults Fault between turns 3, Loss of excitation due to fuel failure (c) Over speed Loss of drive Vacuum failure resulting in condenser pressure rise, resulting in Shattering of the turbine low pressure casing (d) Fault on lines Fault on bus bars 15

16 26. What is power swing? During switching of lines or wrong synchronization surges of real and reactive power flowing in transmission line causes severe oscillations in the voltage and current vectors. It is represented by curves originating in load regions and traveling towards relay characteristics. 27. Why neutral resistor is added between neutral and earth of an alternator? In order to limit the flow of current through neutral and earth a resistor is introduced between them. 28. What is the backup protection available for an alternator? Overcurrent and earth fault protection is the backup protections. 29. What are faults associated with an alternator? (a) External fault or through fault (b) Internal fault Short circuit in transformer winding and connection Incipient or slow developing faults 30. What are the main safety devices available with transformer? Oil level gauge, sudden pressure delay, oil temperature indicator, winding temperature indicator. 31. What are the limitations of Buchholz relay? Only fault below the oil level are detected. Mercury switch setting should be very accurate, otherwise even for vibration, there can be a false operation. The relay is of slow operating type, which is unsatisfactory. 32. What are the problems arising in differential protection in power transformer and how are they overcome? Difference in lengths of pilot wires on either sides of the relay. This is overcome by connecting adjustable resistors to pilot wires to get equipotential points on the pilot wires. Difference in CT ratio error difference at high values of short circuit currents that makes the relay to operate even for external or through faults. This is overcome by introducing bias coil. Tap changing alters the ratio of voltage and currents between HV and LV sides and the relay will sense this and act. Bias coil will solve this. Magnetizing inrush current appears wherever a transformer is energized on its primary side producing harmonics. No current will be seen by the secondary. CT s as there is no load in the circuit. This difference in current will actuate the differential relay. A harmonic restraining unit is added to the relay which will block it when the transformer is energized. 16

17 33. What is REF relay? It is restricted earth fault relay. When the fault occurs very near to the neutral point of the transformer, the voltage available to drive the earth circuit is very small, which may not be sufficient to activate the relay, unless the relay is set for a very low current. Hence the zone of protection in the winding of the transformer is restricted to cover only around 85%. Hence the relay is called REF relay. 34. What is over fluxing protection in transformer? If the turn s ratio of the transformer is more than 1:1, there will be higher core loss and the capability of the transformer to withstand this is limited to a few minutes only. This phenomenon is called over fluxing. 35. Why bus bar protection is needed? Fault level at bus bar is high The stability of the system is affected by the faults in the bus zone. A fault in the bus bar causes interruption of supply to a large portion of the system network. 36. What are the merits of carrier current protection? Fast operation, auto re-closing possible, easy discrimination of simultaneous faults. 37. What are the errors in CT? Ratio error Percentage ratio error = [(Nominal ratio Actual ratio)/actual ratio] x 100 The value of transformation ratio is not equal to the turns ratio. Phase angle error: Phase angle =180/_[(I m Cosθ-I 1 Sin θ )/ni s ] 38. What is field suppression? When a fault occurs in an alternator winding even though the generator circuit breaker is tripped, the fault continues to fed because EMF is induced in the generator itself. Hence the field circuit breaker is opened and stored energy in the field winding is discharged through another resistor. This method is known as field suppression. 39. What are the causes of bus zone faults? Failure of support insulator resulting in earth fault Flashover across support insulator during over voltage Heavily polluted insulator causing flashover Earthquake, mechanical damage etc. 17

18 40. What are the problems in bus zone differential protection? Large number of circuits, different current levels for different circuits for external faults. Saturation of CT cores due to dc component and ac component in short circuit currents. The saturation introduces ratio error. Sectionalizing of the bus makes circuit complicated. Setting of relays need a change with large load changes. 41. What is static relay? It is a relay in which measurement or comparison of electrical quantities is madein a static network which is designed to give an output signal when a threshold condition is passed which operates a tripping device. 16 mark question: 1. Write notes on the following: I) Buchholz relay II) Use of Impedance relay on Transmission line protection. Principle Explanation Limitations Refer Page No:596,559 Text Book :Switchgear Protection and Power systems Sunil S Rao 2)With a neat sketch explain the differential system of protection applied to Star / Delta connected power Transformer Principle Description Refer Page No:8-5, Book : Protection and Switchgear Uday A Bakshi 3.What are the problems encountered in Differential protection system? 1. Un matched characteristics of CTs 2. Ratio change due to tap change 3. Difference in lengths of pilot wires 4. Magnetizing current inrush 5. Size of power transformer 6. Size of power system 7. Type of magnetic material used for core 18

19 8. The amount of residual flux existing before energizing the transformer 9. The method by which transformer is energized Refer Page No:8-11, Book : Protection and Switchgear Uday A Bakshi 4.Draw and Explain the protection scheme of an 3 phase AC Induction motor. Explanation Refer Page No:9-27, Book : Protection and Switchgear Uday A Bakshi 5. Explain how radialfeeders are protected by graded time lag differential relays. Explonation Refer Page No:10-11, Book : Protection and Switchgear Uday A Bakshi 6.What is biased differential protection?explain how it can be used to protect an alternator against internal faults. Principle Connection of CTs for differential protection of generator Refer Page No:621 Text Book :Switchgear Protection and Power systems Sunil S Rao 19

20 Unit III THEORY OF CIRCUIT INTERRUPTION TWO MARKS 1. What is resistance switching? It is the method of connecting a resistance in parallel with the contact space(arc). The resistance reduces the restriking voltage frequency and it diverts part of the arc current. It assists the circuit breaker in interrupting the magnetizing current and capacity current. 2. What do you mean by current chopping? When interrupting low inductive currents such as magnetizing currents of the transformer, shunt reactor, the rapid deionization of the contact space and blast effect may cause the current to be interrupted before the natural current zero. This phenomenon of interruption of the current before its natural zero is called current chopping. 3. What are the methods of capacitive switching? Opening of single capacitor bank Closing of one capacitor bank against another 4. What is an arc? Arc is a phenomenon occurring when the two contacts of a circuit breaker separate under heavy load or fault or short circuit condition. 5. Give the two methods of arc interruption? High resistance interruption:-the arc resistance is increased by elongating, and splitting the arc so that the arc is fully extinguished Current zero method:-the arc is interrupted at current zero position that occurs 100 times a second in case of 50Hz power system frequency in ac. 6. What is restriking voltage? It is the transient voltage appearing across the breaker contacts at the instant of arc being extinguished. 7. What is meant by recovery voltage? The power frequency rms voltage appearing across the breaker contacts after the arc is extinguished and transient oscillations die out is called recovery voltage. 8. What is RRRV? It is the rate of rise of restriking voltage, expressed in volts per microsecond. It is closely associated with natural frequency of oscillation. 20

21 9. How the arc is initiated? 1. by high voltage gradient at the cathodes, resulting in field emission 2. By increase of temperature, resulting in harmonic emission. 10. Give the two theories which are used to explain the phenomenon of Current, zero interruption. Energy balance theory Voltage race theory 11. Give the methods of producing initiation electrons. Field emission Thermal emission 12. Define arc voltage. The voltage across an arc voltage and is given by arryton s equation Ea=A+B/ I Where A, B are constants & I is arc current 13. What are the three parts of arc extinction? Arcing phase Current zero phases Post arc phase 14. Give the factors on which the active recovery voltage depends upon Power factor The armature reaction The circuit conditions 15. Give the factors on which the RRRV depends upon. The active recovery voltage The natural frequency of oscillation. 16. How the resistance of high resistance interruption method can be increased Lengthening the arc Cooling the arc Splitting the arc Constraining the arc. 17. How the current zero interruption can be obtained. Lengthening the gap Cooling Blast effect 18. How to protect the circuit breakers from the recovery voltages? 21

22 Resistance switching can protect the circuit breakers.connecting a resistor across the breaker contact does this. This resistor in turn reduces the high voltage that is appearing across the contacts of the circuit breaker. 19. What is meant by deionization? An arc consists of ionized particles of gases. This arc can be interrupted if the contact gap could be deionized. The process of making the ionized particles to lose their ions is known as deionization. 20. Why is it difficult to interrupt large D.C currents at high voltage? Since D.C is a steady unidirectional current it does not have a natural zero and therefore it is difficult to interrupt large D.C currents at high voltages. 21. Draw the diagram of a D.C switch. 22. Give the condition for critical restriking voltage damping. Critical restriking voltage damping isobtained, if R= Give the method employed in energy balance theory. The method employed in energy balance theory is that if the rate at which heat generated between the contacts is lower than the rate at which heat between contacts is dissipated, the arc will be extinguished otherwise it will restrike. L / C 26. Define ionization. The electrons from the cathode make many collisions with the atoms and molecules of the gases and vapors existing between the two contacts during towards the anode. 27. What is the relation between arc resistance and length of arc? The arc resistance is a function of length of arc which is nothing but separation between the contacts.more the length, more is the arc resistance. 28. What is the relation between arc resistance and cross section of arc? If the area of cross section of the arc is less then arc resistance is large. 29. How the arc is maintained? 22

23 After the initiation of the arc by field emission and thermionic emission the electrons will traveling towards the anode collide with one another electrons to dislodge them and thus arc is maintained. 30. Write short notes about high resistance method? In high resistance method the arc resistance is increased with time. This will reduce the current to such a value which will be insufficient to maintain the arc. Thus the current is interrupted and the arc is extinguished. This method is used in DC circuit breakers. 31. Write short notes about low resistance method? The low resistance or current zero method is employed for arc extinction in a.c circuits. In this method arc resistance is kept low until current zero where extinction of arc takes place naturally and is prevented from restriking. 32. Define prospective current. Prospective current is defined as the current that would flow in the circuit if circuit breakers were replaced by solid conductor. 33. What is meant by transient recovery voltage? It is the voltage that appears between the contacts immediately after final arc interruption. This cause high dielectric stress between the contacts. 34. What is called restriking? If the breakdown occurs after one fourth of a cycle, the phenomenon is called restriking. In restriking high voltage appear across the circuit breaker contacts during capacitive current breaking. 35. What is the cause for power frequency and oscillatory transient component? The oscillatory component is due to inductance and capacitance in the circuit.the power frequency component is due to the system voltage. 36. Define a.c and d.c circuit breaking. There is different in case of the d.c and a.c circuits.in a.c circuits,the current passes through zero twice in one complete cycle.when the currents are reduced to zero, the breakers are operated to cut off the current. This is not the case with dc circuits, as natural current zero point is not available. 37. What is meant by recombination? If a gas contains positive ions and electrons then there is tendency between them to combine and form a neutral atom. This is called recombination. This will assist arc extinction as combination of positive ions and negative charges takes place inside. 38. Define diffusion. 23

24 The electrons from highly ionized space diffuse to the surrounding weakly ionized space which is an important process in building up dielectric strength. 39. Give the functions of a shunt resistor. It reduces rate of rise of restriking voltage and thus reduces duties of the breaker. It reduces the transient voltages during switching out inductive or capacitive loads. 40. What are the three factors which depend by arc resistance? Degree of ionization Length of arc Cross- section of arc. 41. What is the relation between arc resistance and degree of ionization? If there is less number of ionized particles between the contacts then the arc resistance increases. 16 marks Questions 1.Write short note on circuit interruption due to capacitor switching. Principal Explanation Waveforms Refer Page No:10-11, Book : Protection and Switchgear Uday A Bakshi 2. Define the term restriking voltage.derive an expression restriking voltage and explain the effect of this on the operation of C.B under fault condition Definition Derivation Waveforms Refer Page No:10-11, Book : Protection and Switchgear Uday A Bakshi 3.Explain the arc phenomena for initiation of arc,maintenance of arc and the methods used for interrupting the arc. Arc Phenomena Initiation of arc Maintenance of arc Methods of arc interruption Refer Page No:10-11, Book : Protection and Switchgear Uday A Bakshi 24

25 4.Explain the need for the pre insertion resistors across the CB contacts and derive the same. Need for preinsertion of resistors Derivation Refer Page No:10-11, Book : Protection and Switchgear Uday A Bakshi 5.What are the theories involved to the arc interruption method and explain any one. Two theories are Slepians theory Cassie s theory Explanations Refer Page No:11-41, Book : Protection and Switchgear Uday A Bakshi 6.Explain about the interruption of Inductive currents. Wave forms Explanations Refer Page No:11-30, Book : Protection and Switchgear Uday A Bakshi 25

26 UNIT IV CIRCUIT BREAKER 1. What are the uses of switchgear? Switchgear is used for switching and for isolation under normal or abnormal conditions for heavier electrical equipments and majority of distribution and transmission networks. Switchgear used is a combination of a circuit breaker and its control circuit. 2. Write the two functions of switchgear in any power system. To permit plant and distributors to be conveniently put into and taken out of service. To enable the same plant and lines when these become faulty to be rapidly and safely isolated by automatic means. 3. What is a circuit breaker? Make or break a circuit either manually or by remote control under normal conditions. Break a circuit automatically under fault conditions. 4. What is the function of isolating fluid in circuit breaker? It extinguishes the arc drawn between the contacts when the circuit breaker opens. It provides insulation between the contacts and from each contact to earth. 5. What are the types of circuit breaker? Oil circuit breaker. air circuit breaker air blast circuit breaker SF 6 circuit breaker vacuum circuit breaker 6. Give the disadvantages of oil circuit breaker. It is inflammable and may cause fire hazards. There is a possibility of its forming and explosive mixture with air. 7. Give the advantages of minimum oil circuit breaker. Reduction in tank size. Reduction in total weight. Reduction in cost. 8. What are the two different designs of the arcing chambers in terms of vent? Axial vent Radial vent. 26

27 9. How are circuit breakers rated? The number of poles Rated voltage and current Rated frequency Rated making capacity Short time rating Operating duty 10. Define rated current. It is the maximum value of current in rms amperes which shall carry exceeding the temperature limits of the various parts of the breaker. continuously without 11. Define rated frequency The rated frequency of a breaker is the frequency for which it is designed to operate. 12. Define making capacity of the circuit breaker. Making capacity is the product of the making current it can make and carry instantaneously at the rated service voltage. 13. Define breaking capacity of the circuit breaker The breaking capacity of a breaker is the product of the breaking current and recovery voltage. 14. Give the short circuit operating duty of a circuit breaker. B-3-MB-3-MB B-breaking operation 3-duration in minutes M-making operation 15. Define short time rating of a circuit breaker Short time rating is defined as the period for which the circuit breaker is able to carry fault current while remaining closed. 16. How is the capacitance current interrupted? Due to the opening of a long transmission line on no load. Due to disconnection of a capacitor bank. 17. On which basis the circuit breakers are classified? Interrupting medium According to service Way of operation 27

28 Action Method of control Way of mounting Tank construction Contacts 18. What are the main features that affect the performance of a plain break oil circuit breaker? Length of break Speed of contact movement Head of oil above contacts Clearance to earthed metal 19. What are the problems associated with oil circuit breakers? Current chopping Capacitive current interruption Rate of rise of restriking voltage 20. Mention the disadvantages of SF 6 circuit breaker In case of leakage in the circuit breaker tank, SF 6 gas being heavier than air settles in the surroundings and may lead to suffocation of the operating personnel.however,it is nonpoisonous. 21. What are the different types of test conducted in circuit breaker? short circuit test dielectric test 22. Classify dielectric test. Frequency test one minute dry wet power 23. What are the two types of air blast circuit breaker? The cross blast where the blast cuts the arc The axial blast where the blast is along the arc 24. What is meant by dielectric test? Dielectric test mainly conducted to check power frequency and impulse voltage withstand test. 25. Classify routine tests conducted on circuit breakers. power frequency voltage test millivolt drop test operation test 26. What are isolating switches? 28

29 Isolating switches are switches which, when opened, provide an isolating distance and switch more less at no load. 27. Mention three major duties to be performed by a circuit breaker under short circuit. The fault current must be safely interrupted by the circuit breker. The fault section of the system must be isolated by circuit breaker as quickly as possible keeping minimum daily. The fault circuit only must be isolated without affecting the healthy one. 28. Write the classification of circuit breakers based on the medium used for arc extinction? Air break circuit breaker Oil circuit breaker Minimum oil circuit breaker Air blast circuit breaker SF6 circuit breaker Vacuum circuit breaker 29. What is the main problem of the circuit breaker? When the contacts of the breaker are separated, an arc is struck between them. This arc delays the current interruption process and also generates enormous heat which may cause damage to the system or to the breaker itself. This is the main problem. 30. What are demerits of MOCB? Short contact life Frequent maintenance Possibility of explosion Larger arcing time for small currents Prone to restricts 31. What are the advantages of oil as arc quenching medium? It absorbs the arc energy to decompose the oil into gases, which have excellent cooling properties It acts as an insulator and permits smaller clearance between line conductors and earthed components 32. What are the hazards imposed by oil when it is used as an arc quenching medium? There is a risk of fire since it is inflammable. It may form an explosive mixture with arc. So oil is preferred as an arc quenching medium. 33. What are the advantages of MOCB over a bulk oil circuit breaker? It requires lesser quantity of oil It requires smaller space 29

30 There is a reduced risk of fire Maintenance problem are reduced 34. What are the disadvantages of MOCB over a bulk oil circuit breaker? The degree of carbonization is increased due to smaller quantity of oil There is difficulty of removing the gases from the contact space in time The dielectric strength of the oil deteriorates rapidly due to high degree of Carbonization. 35. What are the types of air blast circuit breaker? Arial-blast type cross blast Radial-blast 36. What are the advantages of air blast circuit breaker over oil circuit breaker? The risk of fire is diminished The arcing time is very small due to rapid buildup of dielectric strength between contacts The arcing products are completely removed by the blast whereas oil deteriorates with successive operations 37. What are the demerits of using oil as an arc quenching medium? The air has relatively inferior arc quenching properties The air blast circuit breakers are very sensitive to variations in the rate of rise of restriking voltage Maintenance is required for the compression plant which supplies the air blast 38. What is meant by electro negativity of SF6 gas? SF6 has high affinity for electrons. When a free electron comes and collides with a neutral gas molecule, the electron is absorbed by the neutral gas molecule and negative ion is formed. This is called as electro negativity of SF6 gas. 39. What are the characteristic of SF6 gas? It has good dielectric strength and excellent arc quenching property. It is inert, non-toxic, noninflammable and heavy. At atmospheric pressure, its dielectric strength is 2.5 times that of air. At three times atmospheric pressure, its dielectric strength is equal to that of the transformer oil. 40. Write the classifications of test conducted on circuit breakers. Type test Routine test Reliability test 30

31 Commissioning test 41. What are the indirect methods of circuit breaker testing? Unit test Synthetic test Substitution testing Compensation testing Capacitance testing 42. What are the advantages of synthetic testing methods? The breaker can be tested for desired transient recovery voltage and RRRV. Both test current and test voltage can be independently varied. This gives flexibility to the test The method is simple With this method a breaker capacity (MVA) of five time of that of the capacity of the test plant can be tested. 16 MARK QUESTIONS: 1. What are the physical, chemical and dielectric properties of SF6 gas? Physical Properties Chemical Properties Dielectric Properties Refer Page No:98-100Text Book :Switchgear Protection and Power systems Sunil S Rao 2. Explain the working principle of Air break circuit Breaker. Principle Construction Advantages Applications Refer Page No:83Text Book :Switchgear Protection and Power systems Sunil S Rao 3. Show view of working portion of a typical Bulk oil circuit breaker Principle Construction Advantages Applications 31

32 Refer Page No:132Text Book :Switchgear Protection and Power systems Sunil S Rao 4. Explain the construction and working of double arc extinction chamber air blast circuit breaker. (ABCB). Construction Advantages Applications Refer Page No: 89 Text Book :Switchgear Protection and Power systems Sunil S Rao 5. Show the constructional layout of SF6 breaker and is advantages and disadvantages. Principle Construction Advantages Applications Refer Page No: Text Book :Switchgear Protection and Power systems Sunil S Rao 6. Explain the principle of operation of vacuum circuit breaker. Construction Advantages Applications Refer Page No: 143 Text Book :Switchgear Protection and Power systems Sunil S Rao 32

33 Unit-5 Protection against over voltages TWO MARKS 1. Why protection is needed for power system? An electrical power system consists of generators, transformers, transmission and distribution lines, etc. Short circuits and other abnormal conditions often occur on power system. The heavy current associated with short circuits is likely to cause damage to equipment if suitable protective relays and circuit breakers are not provided for the protection of each section of the power system. That is why protection is needed. 2. What are the possible causes of fault in power system? Faults on transmission lines are caused by Over voltages due to lightning or switching surges External conducting objects falling on overhead lines Birds Certain foreign particles such as fine cement dust In case of cables, transformers, generators and other equipment, faults are caused by Failure of solid insulation due to aging, heat, moisture or over voltage, mechanical damage, accidental contact with earth, flash over due to over voltages etc. 3. What are the types of faults? Two broad classifications of faults are Symmetrical faults Three phase fault Unsymmetrical faults Line to Ground fault Line to Line fault Double Line to Ground fault 4. Define symmetrical fault. When this type of fault occurs, it gives equal current in all the three phases. Ex. Three phase to ground fault. 5. Define unsymmetrical fault. When this type of fault occurs, it gives unequal current in all the three phase. Ex. Line to ground, Line to Line and Double Line to ground 6. Mention the frequency of different types of faults occurring in over head lines. 33

34 i. Line to Ground 85% ii. Line to Line 8% iii. Double Line to Ground 5% iv. Three Phase 2% 7. What are the essential qualities of protection? The basic requirements of protective system are as follows: i. Selectivity or Discrimination ii. Reliability iii. Sensitivity iv. Stability v. Fast operation 8. What are the types of protective scheme? The most common protective schemes which are usually used for protection of a modern power system i. Over current protection ii. Distance protection iii. Carrier-Current protection iv. Differential protection 9. What is primary protection? If a fault occurs, it is the duty of the primary protective scheme to clear the fault. It acts as a first line of defence. 10. What is back up protection? The backup protection is designed to clear the fault if the primary protection fails. It acts as a second line of defence. 11. What is protective zone? A power system is divided into a number of zones from the protection point of view. Each element of the power system has a separate protective scheme for its protection. The elements which come under a protective scheme are said to be in the zone of protection of that particular scheme. Similarly, a relay has its own zone of protection. 12. What is earthing? The process of connecting the metallic frame (i.e. non-current carrying part) of electrical equipment or some electrical part of the system(e.g. neutral point in a star connected system, one conductor of the secondary of a transformer) to earth is called as grounding or earthing. 13. Mention the types of earthing. Grounding or earthing may be classified as i. Equipment grounding ii. System grounding 34

35 14. What is neutral grounding? The process of connecting neutral point of 3-phase system to earth either directly or through some circuit element (e.g. resistance, reactance) is called neutral grounding. 15. What is the main purpose of neutral grounding? It provides protection to personnel and equipment from abnormal condition. 16. What are the advantages of neutral grounding? i. Voltages of the healthy phases do not exceed line to ground voltages i.e. they remain nearly constant ii. The high voltages due to arcing grounds are eliminated. iii. It provides greater safety to personnel and equipment. iv. It provides improved service reliability. 17. What are the methods of neutral grounding? The methods commonly used for neutral grounding are i. Solid or effective grounding ii. Resistance grounding iii. Reactance grounding iv. Peterson-coil grounding 18. What are limitations of reactance grounding? i. In this system, the fault current required to operate the protective device is higher than that of resistance grounding for the same fault conditions. ii. High transient voltages appear under fault conditions. 19. What is the use of Peterson coil? It is otherwise called as arc suppression coil. It is used to prevent an arcing ground. 20. Calculate the reactance of Peterson coil suitable for a 33 kv, 3-phase transmission line having a capacitance to earth of each conductor as 4.5 µf. Assume supply frequency to be 50 Hz. Supply frequency f = 50 Hz. Line to earth capacitance, C= 4.5 µf For Peterson coil grounding, reactance X L of the Peterson coil should be equal to X C /3 where X C is line to earth capacitive reactance. Reactance of Peterson coil, X L = X 3 C = 1 3ωC 1 = 3 2πf C 1 = = Ω 6 3 2π