22524: SAP Past Paper PDF - Summer 2023 - Switchgear and Protection

MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous) m (ISO/IEC - 27001 - 2013 Certified) __________________________________________...

MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous) m (ISO/IEC - 27001 - 2013 Certified) ________________________________________________________________________________________________________________ SUMMER – 2023 EXAMINATION sb Model Answer – Only for the Use of RAC Assessors 22524: SAP Subject Name: Switchgear And Protection te Important Instructions to examiners: 1) The answers should be examined by key words and not as word-to-word as given in the model answer al scheme. 2) The model answer and the answer written by candidate may vary but the examiner may try to assess the lc understanding level of the candidate. le 3) The language errors such as grammatical, spelling errors should not be given more Importance (Not applicable for subject English and Communication Skills). a 4) While assessing figures, examiner may give credit for principal components indicated in the figure. The figures drawn by candidate and model answer may vary. The examiner may give credit for any equivalent r.c figure drawn. 5) Credits may be given step wise for numerical problems. In some cases, the assumed constant values may vary and there may be some difference in the candidate’s answers and model answer. om 6) In case of some questions credit may be given by judgement on part of examiner of relevant answer based on candidate’s understanding. 7) For programming language papers, credit may be given to any other program based on equivalent concept. 8) As per the policy decision of Maharashtra State Government, teaching in English/Marathi and Bilingual (English + Marathi) medium is introduced at first year of AICTE diploma Programme from academic year 2021-2022. Hence if the students write answers in Marathi or bilingual language (English +Marathi), the Examiner shall consider the same and assess the answer based on matching of concepts with model answer. Sub Q. Marking Q. Answer No. Scheme N. 1. Attempt any FIVE of the following: 10 Marks a) State any two functions of protective system. Ans: Functions of Protective System: 1. To switch ON or OFF the electric power during normal conditions for operation and maintenance. 2. To protect the power system by isolating the faulty parts due to abnormal 1 Mark for conditions within shortest possible time. each of any 3. To provide the reliable and continuous supply to consumers. two functions 4. To isolate only faulty sections of system without affecting the healthy = 2 Marks sections of system. 5. Constantly monitor the electrical quantities of system so that occurrence of fault should be minimized. 6. To prevent the occurrence of major faults. 7. To ensure maximum safety conditions from generation stage to consumer’s premises. b) List any four applications of HRC fuse. Ans: Applications of HRC Fuse: 1. HRC fuses are used to protect the circuit from short circuits in HV switchgear. ½ Mark for 2. Used for backup protection to the circuit breaker. each of any 3. Used to protect electrical devices like motors, transformers etc. four 4. Motor stators also require this kind of fuse. applications 5. Used to protect high selectivity radial and ring networks. = 2 Marks 6. Used for protection of semiconductor devices. Page 1 of 17  MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous) m (ISO/IEC - 27001 - 2013 Certified) ________________________________________________________________________________________________________________ SUMMER – 2023 EXAMINATION sb Model Answer – Only for the Use of RAC Assessors 22524: SAP Subject Name: Switchgear And Protection te 7. Used for protection of cables. al 8. Used for protection of LV distribution system against overload and short circuit. lc 9. Used for protection of automobiles circuits. le 10. Used for protection of capacitor banks. 11. Used for protection of small loads in circuits having high fault levels. a 12. Used for protection of AC drives. r.c 13. Used for protection of DC drives. 14. Used for protection of UPS system. om 15. Used for protection of electronic equipment. OR Equivalent Answer c) State function of RCBO and MPCB. Ans: Functions of RCBO: 1. It gives protection against short circuit fault. 1 Mark for 2. It gives protection against over-current or overload fault. any one 3. It gives protection against leakage current fault, electrical shock. function 4. It can give protection against current imbalance. 5. It automatically breaks the circuit when flow of current through it exceeds a certain level. Functions of MPCB: 1. Protection against faults such as short circuits, line-to-ground faults and line-to-line faults. The MPCB can interrupt any electrical fault that is below 1 Mark for its breaking capacity. any one 2. Motor overload protection, when motor draws electric current above its function nameplate value for an extended period of time. Overload protection is normally adjustable in MPCBs. 3. Protection against phase unbalances and phase loss. Both conditions can severely damage a three-phase motor, so the MPCB will disconnect the motor in either case as soon as the fault is detected. 4. Thermal delay to prevent the motor from being turned back on immediately after an overload, giving the motor time to cool down. An overheated motor can be permanently damaged if it is turned back on. 5. Fault Signalling – Most models of motor protection circuit breakers have a LED display that is turned on whenever the MPCB has tripped. This is a visual indication for nearby personnel that a fault has occurred and the electric motor must not be connected again until the fault is addressed. 6. Automatic Reconnection – Some MPCB models allow a cool down time to be input in case there is an overload, after which the motor will restart automatically. d) State the need of directional relay. Ans: Need of Directional Relay: In power system (power equipment / distribution lines / transmission lines etc.) fault can be in two different directions from relay and it is highly desirable for a relay to respond differently for faults in the forward or reverse 2 Marks direction. In fact, the relay should respond only when the fault is on one side, Page 2 of 17  MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous) m (ISO/IEC - 27001 - 2013 Certified) ________________________________________________________________________________________________________________ SUMMER – 2023 EXAMINATION sb Model Answer – Only for the Use of RAC Assessors 22524: SAP Subject Name: Switchgear And Protection te while for failures on the other side it remains inactive. In order to avoid al disconnection of unnecessary circuits as normal overcurrent relay cannot provide this function directional relay is needed. Directional relay enables lc better discrimination of the faulty part of the network than with overcurrent le protection. e) List any four protection scheme used for alternator. a Ans: r.c Protection Scheme Used for Alternator: 1. Mertz price protection. om 2. Differential protection. ½ Mark for 3. Percentage differential protection. each of any 4. Over current protection. four schemes 5. Earth fault protection. = 2 Marks 6. Inter turn fault protection. 7. Negative phase sequence protection against unbalanced loading. 8. Overheating protection. 9. Balanced earth fault protection. 10. Restricted earth fault protection. 11. Protection against loss of excitation. 12. Reverse power protection. 13. Protection against over speed. 14. Short circuit protection. f) Draw time-current characteristics of IDMT relay. Ans: Time-Current Characteristics of IDMT Relay: 2 Marks for labeled diagram g) List any four faults related to busbar. Ans: Faults Related to Busbar: 1. Failure of support insulators resulting in earth faults. 2. Failure of circuit breakers to interrupt the fault current. ½ Mark for 3. Foreign object accidentally falling across the busbar. each of any 4. Flash over across support insulator causing flash over due to over-voltage. four faults 5. Heavily polluted insulator, causing flash-over. = 2 Marks 6. Earthquake, mechanical damages. 7. Failure of other related equipment. 8. Errors in operation and maintenance. Page 3 of 17  MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous) m (ISO/IEC - 27001 - 2013 Certified) ________________________________________________________________________________________________________________ SUMMER – 2023 EXAMINATION sb Model Answer – Only for the Use of RAC Assessors 22524: SAP Subject Name: Switchgear And Protection te 2. Attempt any THREE of the following: 12 Marks al a) State importance of feeder reactor. Write any two disadvantages of it. Ans: lc Importance of Feeder Reactor: le 1. If a fault occurs on any feeder, the voltage drop in its reactor will not affect the bus-bars voltage so there is a little tendency for the generator to lose 1 Mark for a synchronism. each of any r.c 2. The fault on a feeder will not affect other feeders and consequently, the two points effects of the fault are localized. = 2 Marks om 3. When the fault occurs on any one feeder, then the voltage drops occur only in its reactors and the bus bar is not affected much. 4. Since most of the short circuits occurs on feeders, feeder reactor is most required reactor in power system protection. Disadvantages of Feeder Reactor: 1. There is constant power loss and voltage drop in reactors even during 1 Mark for normal operation. each of any 2. If the number of generators is increased, the size of feeder reactors will two have to be increased to keep short-circuit currents within ratings of the disadvantages feeder circuit breakers. = 2 Marks 3. Every feeder needs a reactor hence the number or reactors increases. 4. Feeder reactors do not offer any protection for bus bar faults. 5. If short circuit occurs at busbars no protection is provided to generators by feeder reactor. b) Define: - arc voltage, restriking voltage, recovery voltage and RRRV. Ans: i) Arc Voltage: The voltage that appears across the contacts of circuit breaker during the arcing period is called as the arc voltage. ii) Restriking voltage: The transient voltage that appears across the contacts of the circuit breaker at or near current zero during the arcing period is 1 Mark for called as the restriking voltage. each iii) Recovery voltage: The normal power frequency r.m.s voltage that appears definition across the contacts after the final arc extinction is called recovery voltage. = 4 Marks iv) RRRV: The RRRV (Rate of Rise of the Restriking Voltage) is defined as the slope of the steepest tangent to the restriking voltage curve. It is expressed in volts per micro-second. c) A 3Φ 66 kV / 33 kV star / delta transformer is protected by Mertz-Price protection scheme. CT’s on LV side have a ratio of 600/5. Find CT ratio on HV side. Ans: The CTs on LV side are connected in star as transformer windings are in delta, whereas those on HV side are in delta as the transformer windings are in star on 1 Mark that side. Assume CT line current on LV side to be 5 A and then that on HV side will also 1 Mark be 5 A. But HV side CTs are in delta. Hence the HV side CT current will be (CT line current)/√3 = 5/√3 A. Assume line current (for convenience) of 600 A on LV side (delta side) of Page 4 of 17  MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous) m (ISO/IEC - 27001 - 2013 Certified) ________________________________________________________________________________________________________________ SUMMER – 2023 EXAMINATION sb Model Answer – Only for the Use of RAC Assessors 22524: SAP Subject Name: Switchgear And Protection te transformer. When transformed to HV side the line current will be I Lht given by al √3 x 66 x ILht = √3 x 33 x 600 1 Mark lc ILht = (33/66) x (600) = 300 A. On HV side CT primary current is 300 A and CT secondary current is 5/√3 A, le Hence the CT ratio is 300 / (5/√3) = (300√3/5) = 519.61 / 5 = 103.92 / 1. 1 Mark a d) Explain short circuit protection of 3Φ IM. r.c Ans: The short circuit protection of three phase induction motor may be in one or om combination form of the following according to application of motor: 1. External protection against short circuit: External protection device is 2 Marks for normally done using different types of fuses, MCBS, MCCBS, MPCBS and each of any short circuit relays. This kind of protection device is compulsory and legal two points and placed under safety regulations. = 4 Marks 2. Built-in motor protection: This is usually done with thermal overload protection to avoid damage and breakdown of motor. The built-in protector always requires an external circuit breaker while some built-in motor protection types even require an overload relay. There are two types commonly used namely: (i) Thermostats (ii) Thermistors or positive temperature coefficient Sensors (PTC). 3. Advanced External Motor Protection Systems: Due to the increasing use of electric motors in various aspects of industry, there has been significant advancement in external motor protection against damage of short circuits. Advance type sensor-relay combination like PT100 temperature monitoring of stator and bearings, insulation resistance and temperature monitoring sensors, digital / electronic / numerical relays etc. are applied to motor protection against short circuit. 4. Instantaneous High Set Overcurrent Relay: It is used which trips instantaneously the related circuit breaker of short circuited motor. OR Equivalent Answer 3. Attempt any THREE of the following: 12 Marks a) List different types of faults occurred on a power system. Draw necessary sketches (any four). Ans: Types of Faults Occurred on a Power System: Power system faults: 1. Open circuit fault a) One conductor (single phase) open type fault b) Two conductors (two phase) open type fault 1 Mark for c) Three conductors (three phase) open type fault each of any 2. Short circuit fault four faults a) Symmetrical faults with related i) Three phase (L-L-L) short circuit fault diagram ii) Three phase to ground fault = 4 Marks b) Unsymmetrical faults i) Single line to ground fault Page 5 of 17  MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous) m (ISO/IEC - 27001 - 2013 Certified) ________________________________________________________________________________________________________________ SUMMER – 2023 EXAMINATION sb Model Answer – Only for the Use of RAC Assessors 22524: SAP Subject Name: Switchgear And Protection te ii) Double line to ground fault al iii) Line to line fault lc Types of Sr.No Diagram le Faults a One conductor r.c (single phase) 1 open type fault om Two Conductors 2 (Two Phase) Open Type Fault Three Conductors 3 (Three Phase) Open Type Fault Three Phase (L- L-L) Short 4 Circuit Fault Three Phase to Ground (L-L-L- 5 G) Fault Single Line to Ground (L-G) 6 Fault: Page 6 of 17  MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous) m (ISO/IEC - 27001 - 2013 Certified) ________________________________________________________________________________________________________________ SUMMER – 2023 EXAMINATION sb Model Answer – Only for the Use of RAC Assessors 22524: SAP Subject Name: Switchgear And Protection te Double Line to al Ground (L-L- G) Fault: lc 7 le a r.c Line to Line (L- om L) Fault: 8 OR Equivalent Answer / Diagrams b) Explain working of vertical break isolator with neat sketch. Ans: Working of Vertical Break Isolator: The figure shows vertical break isolator. It has two sets of insulators mounted on I section channel, which is usually earthed. Each insulator has fixed terminals for 2 Marks for connecting it to concerned lines. The fixed contact is mounted on top of one set of explanation insulators and moving contact of blade type is mounted on the other. The moving contact is operated by an operating mechanism installed on base. Whenever isolator is required to be opened, operating mechanism is operated and because of it moving contact pushes upwards thus disconnecting the circuit. 2 Marks for sketch c) Define – Relay time, reset current, pick-up current, current setting. Ans: Relay Time: It is the time interval between occurrence of fault and closure of relay contacts. Reset Current: The value of current below which the relay resets and comes back to its original position is called as reset current or dropout. Page 7 of 17  MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous) m (ISO/IEC - 27001 - 2013 Certified) ________________________________________________________________________________________________________________ SUMMER – 2023 EXAMINATION sb Model Answer – Only for the Use of RAC Assessors 22524: SAP Subject Name: Switchgear And Protection te Pick-up Current: The threshold value of operating current above which the al relay operates. 1 Mark for OR each lc It is the minimum current in the relay coil at which the relay definition le starts to operate. = 4 Marks Current Setting: The method of adjusting Pickup current value is known as a current setting. r.c d) Draw neat sketch of Buchholz relay. State any four points related to its om construction. Ans: Construction of Buchholz Relay: 2 Marks for diagram Buchholz relay consists of an oil filled chamber. 1. Buchholz relay is a gas actuated relay installed in oil immersed transformers for protection against all kinds of faults. 2 Marks for 2. Buchholz relay is in the form of a domed vessel placed in the connecting any four pipe between the main tank and the conservator. points 3. In Buchholz relay there are two hinged floats, one at the top and other at the bottom in the chamber. Each float is accompanied by a mercury switch. 4. The mercury switch on the upper float is connected to an alarm circuit during incipient faults. 5. The lower mercury switch located in the direct path of the flow of oil from the transformer to the conservator is arranged to trip the circuit breaker in case of severe internal faults. e) With neat sketch explain pilot wire protection scheme for transmission line. Ans: Pilot Wire Protection Scheme for Transmission Line: Page 8 of 17  MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous) m (ISO/IEC - 27001 - 2013 Certified) ________________________________________________________________________________________________________________ SUMMER – 2023 EXAMINATION sb Model Answer – Only for the Use of RAC Assessors 22524: SAP Subject Name: Switchgear And Protection te al 2 Marks for sketch lc le a r.c om 2 Marks for Figure shows the single line diagram of Merz price voltage balance system for explanation pilot wire protection of three phase transmission line. The pair of CTs in each line is connected in series with a relay, in such a way that under normal conditions, their secondary voltages are equal and opposite. Because current entering is equal to current leaving, they cancel out and no current flows through relay coil. Suppose a fault occurs at point F, the current entering and leaving are different, hence causing current to flow through the relay, which gives trip signal to the circuit breaker for protection of transmission line. 4. Attempt any THREE of the following: 12 Marks a) Explain with neat sketch working principle of distance relay. Ans: Distance Protection Scheme for Transmission Line: 2 Marks for sketch Impedance or distance protection scheme uses impedance relay. The relay operation is based on the impedance (or distance) between the relay and point of fault. Figure shows arrangement for distance protection for typical transmission line. The voltage element of impedance relay receives supply from PT secondary and current element receives supply from CT secondary. It measures Impedance at 2 Marks for relay location ( Z = V / I ) The protection zone of line is between A and B. working Under normal working conditions, the impedance of line is ZL. The impedance relay is so designed that, it operates only when line impedance becomes less than ZL. When fault occurs between points A & B, the impedance of line becomes less than ZL and impedance relay operates which trips the CB and line is protected. b) Explain reverse power protection of 3Φ alternator. Ans: Reverse Power Protection of 3Φ Alternator: The turbine drives alternator. Alternator is connected to supply system through transformers bus bars etc. and similarly so many alternators are connected to Page 9 of 17  MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous) m (ISO/IEC - 27001 - 2013 Certified) ________________________________________________________________________________________________________________ SUMMER – 2023 EXAMINATION sb Model Answer – Only for the Use of RAC Assessors 22524: SAP Subject Name: Switchgear And Protection te supply system. But when input to any one turbine is stopped, then that 4 Marks for al alternator starts rotating as synchronous motor and this is the reverse power explanation condition which is undesirable hence can be detected by using directional relay lc in any one phase. le The induction type directional relay is used for the reverse power protection of alternator. Here the shunt magnet coil and series magnet coil are exited from a alternator to whom protection is to be provided. When power flow direction is r.c correct, the disc rotates in the normal direction and does not close trip contacts. But when the power flow reverses, the disc rotates in opposite direction causing om closure of trip contacts. c) With neat sketch explain working of restricted earth fault protection scheme of transformer. Ans: Restricted Earth Fault Protection: 2 Marks for sketch Referring to Figure, the star connected neutral earthed side is protected by restricted earth fault protection. An earth fault F1 beyond the transformer causes the currents I2 and I1 to flow in CT secondary. Therefore, the resultant current in earth fault relay is negligible and relay does not operate for the faults beyond its 2 Marks for region. For earth fault within the transformer star connected winding F2 only I2 explanation flows and I1 is negligible. So earth fault relay operates. When fault occurs very near to neutral point, the voltage available for driving earth fault relay is very small. Hence the practice is to set the relay such that it operates for earth fault current of the order of 15% of rated current. Such setting protects restricted portion of winding; hence the name is restricted earth fault protection. 4. d) List any four major faults related to 3Φ IM. Draw sketch of single phasing preventer. Ans: Faults Related to 3Φ IM: A) Electrical / magnetic sections: 1. Electrical supply failure due to single phasing, under voltage, unbalanced voltages and reversal of phases. 2. Short circuit faults between turns of a stator coil due to failure of insulation. 3. Short circuit faults between stator coils due to failure of insulation. ½ mark for 4. Short circuit faults between stator coil/s and body of motor due to failure of each any 4 insulation. faults Page 10 of 17  MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous) m (ISO/IEC - 27001 - 2013 Certified) ________________________________________________________________________________________________________________ SUMMER – 2023 EXAMINATION sb Model Answer – Only for the Use of RAC Assessors 22524: SAP Subject Name: Switchgear And Protection te 5. Open circuit in stator winding/coils or their terminal connections. = 2 Marks al 6. Loose or broken rotor bars. 7. Damaged core stampings/teeth. lc B) Mechanical section: le 1. Unbalanced rotor. 2. Damaged bearings. a 3. End play in shaft, bent shaft. r.c 4. Cooling/ventilation system failures, damaged fan. 5. Failure/disturbances of alignment. om 6. Foundation arrangement disturbed. 7. Worn out brushes leading to abnormal operation with sparking etc. OR 1. Prolonged overloading. 2. Single phasing. 3. Stalling. 4. Phase to phase faults / phase fault. 5. Phase to ground fault. 6. Inter- turn faults. 7. Earth faults. 8. Reversal of phases. 9. Failure of bearings / Rotor Jam. 10. Supply under-voltage. 11. Supply over voltage. 12. Unbalanced Supply Voltage. 13. Faults in stator and associated circuit. 14. Faults in rotor or associated circuit. 15. Open circuit fault in stator / rotor circuit. Single Phasing Preventer: 2 Marks for sketch 4. e) Explain with neat sketch differential protection scheme of bus bar. Ans: Differential Protection Scheme for Bus Bar: 2 Marks for sketch Page 11 of 17  MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous) m (ISO/IEC - 27001 - 2013 Certified) ________________________________________________________________________________________________________________ SUMMER – 2023 EXAMINATION sb Model Answer – Only for the Use of RAC Assessors 22524: SAP Subject Name: Switchgear And Protection te al lc 2 Marks for le explanation a r.c Under normal conditions, the sum of the currents entering the bus bar zone is equal to those leaving it and no current flows through the relay coil. If a fault om occurs within the protected zone, the currents entering the bus will no longer be equal those leaving it. The difference of these currents will flow through the relay coil causing opening of circuit breaker of bus-bar. 5 Attempt any TWO of the following: 12 Marks 5. a) Compare ACB and MCB on any six points. Ans: Comparison Between ACB and MCB: Sr. ACB MCB No. 1. An air circuit breaker (ACB) is an A miniature circuit breaker (MCB) electrical switching device that is an electrical safety device that uses air as the arc quenching protects the electrical circuit from medium. excessive current, which may be caused by overload or short circuit. 2. The current rating of ACB is The current rating of MCB is less. 1 Mark for more than MCB. each of any 3. ACB has comparatively much The breaking capacity of MCB is six points high breaking capacity. lower than ACB. = 6 Marks 4. Operational accuracy of ACB is Operational accuracy of MCB is comparatively less. better. 5. ACBs are suitable for high rating MCBs are suitable for low rating LT panels / loads. LT panels / loads. 6. ACB is not compatible to use at MCB is compatible to use at the the load end because of its less load end due to its frequent frequent operation. operation with high accuracy. 7. The short circuit current for ACB The short circuit current rating for is high. MCB is less. 8. For ACB, the time required for For MCB, the time required for sensing the short circuit is about sensing the short circuit is about 9 30 milliseconds. milliseconds. 9. The adjustment of tripping The adjustment of tripping threshold of ACB is little threshold of MCB is easy. complex than MCB. 10. ACB have a tripping relay MCB has a tripping release mechanism for its operation. mechanism for its operation. 11. ACB have a high rupturing MCB has a low rupturing capacity. Page 12 of 17  MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous) m (ISO/IEC - 27001 - 2013 Certified) ________________________________________________________________________________________________________________ SUMMER – 2023 EXAMINATION sb Model Answer – Only for the Use of RAC Assessors 22524: SAP Subject Name: Switchgear And Protection te capacity. al 12. ACB is more expensive as MCB is less expensive as compared to MCB. compared to ACB. lc 5 b) Draw a block diagram of microprocessor based over current protection. le State function of each block. Ans: a Microprocessor Based Over Current Protection: r.c om 3 Marks for block diagram Function of CT: CTs are the devices that measure the current in a circuit. CTs are needed because the circuit's current is much higher than the relay can handle. CTs step down the current to a low level safe to connect to the relay. Function of Current to Voltage (I to V) Converter: ½ Mark for As the microprocessor accepts signals in voltage form, the current signal each of any derived from the CT is converted into a proportional voltage signal using six block current to voltage (I to V) converter. functions Function of Rectifier: = 3 Marks The ac voltage proportional to the load current is converted into dc through a precision rectifier as the microprocessor accepts dc voltage proportional to the load current. Function of Multiplexer: Using a multiplexer, the microprocessor can sense the fault currents of a number of circuits. If the fault current in any circuit exceeds the pick-up value, the microprocessor sends a tripping signal to the circuit breaker of that faulty circuit. Function of A/D Convertor: A/D convertor converts analog signal into digital form for further processing of signal. Function of Microprocessor Kit: The microprocessor sends signal to the ADC for starting the conversion. The microprocessor reads the end of conversion signal to examine whether the conversion is over or not. As soon as conversion is over, the microprocessor Page 13 of 17  MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous) m (ISO/IEC - 27001 - 2013 Certified) ________________________________________________________________________________________________________________ SUMMER – 2023 EXAMINATION sb Model Answer – Only for the Use of RAC Assessors 22524: SAP Subject Name: Switchgear And Protection te reads the current signal in digital form and then compares it with the pickup al value. The microprocessor first determines the magnitude of the fault current and then selects the corresponding time of operation. Then it goes in delay lc subroutine and sends a trip signal to the circuit breaker after the predetermined le time delay. a 5 c) List any three difficulties experienced in differential protection of r.c alternator. State the remedies to overcome each. Ans: om Difficulties Experienced in Differential Protection of Alternator with Remedies: Sr. Difficulties Experienced in Differential Relay How are they No. in Alternator Protection: overcome? 1 The differential protection provides very fast Additional sensitive 2 Marks for protection against phase-to-phase faults and earth fault relay each of any phase to ground faults. If neutral is not grounded should be provided. three or grounded through resistance, error may cause. difficulties with related 2 When differential relay is used for protection, the Use Biased remedy CT’s should be identical in design, otherwise the Differential = 6 Marks ratio error may occur. protection. 3 There may be unequal length of leads of CT wire Use Biased connections, causing error. Differential protection 4 Unequal secondary burden on CT. Use Biased Differential protection 6 Attempt any TWO of the following: 12 Marks a) Related to vacuum circuit breaker – i) Draw neat sketch ii) Write any four importance points related to its construction. iii) Give any two advantages and two disadvantages. Ans: Vacuum Circuit Breaker: 2 Marks for diagram Page 14 of 17  MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous) m (ISO/IEC - 27001 - 2013 Certified) ________________________________________________________________________________________________________________ SUMMER – 2023 EXAMINATION sb Model Answer – Only for the Use of RAC Assessors 22524: SAP Subject Name: Switchgear And Protection te al lc le a r.c om ½ Mark for each of any Points Related Construction of Vacuum Circuit Breaker: four points 1. It consists of fixed contact, moving contact and arc shield mounted inside = 2 Marks a vacuum chamber (vacuum interrupter). 2. The vacuum circuit breaker comprises a steel arc chamber in the center, symmetrically arranged ceramic insulators. 3. The movable member is connected to the control mechanism by stainless steel bellows. This enables the permanent sealing of the vacuum chamber so as to eliminate the possibility of the leak. 4. A glass vessel or ceramic vessel is used as the outer insulating body. 5. The arc shield prevents the deterioration of the internal dielectric strength by preventing metallic vapors from falling on the inside surface of the outer insulating cover. 6. The material used for current-carrying contacts plays an important role in the performance of the vacuum circuit breaker. Alloys like, Copper-bismuth or copper-chrome is used as material to make VCB contacts. 7. The arc shields are supported to the insulating housing such that they cover these shields and is prevented from condensing on the insulating enclosure. OR Equivalent Answer / Diagram ½ Mark for Advantages of Vacuum Circuit Breaker: each of any 1. The vacuum circuit breaker does not require any additional filling of the oil two or gas. They do not need period refilling. advantages 2. The inertia of the movable guide rod is small and can be used frequently. = 1 Mark 3. The vacuum circuit breaker unit is compact and self-contained. It can be installed in any required orientations. 4. The small operating mechanism, small size and light in weight. 5. They are compact, reliable and have long service life. 6. Rapid recovery of high dielectric strength of current due to interruptions. That only a half cycle or less arcing occurs or after proper contact separation. 7. The control power is small, and the action noise is small during switch operations. 8. The arc extinguish time is short, arc voltage is low, the arc energy is small Page 15 of 17  MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous) m (ISO/IEC - 27001 - 2013 Certified) ________________________________________________________________________________________________________________ SUMMER – 2023 EXAMINATION sb Model Answer – Only for the Use of RAC Assessors 22524: SAP Subject Name: Switchgear And Protection te and the contact loss is small. al 9. With multiple reclosing functions, suitable for the requirement of the application of a distribution network. lc 10. The arc extinguishing medium or insulating medium does not use oil, there le is no danger of fire hazard and explosion and it is safe. a 11. Extinguish the arc is sealed container and the arc and hot gas are not exposed. As a separate component, the arc extinguishing chamber is simple r.c and convenient. 12. The contact head is divided into a completely sealed structure, and its om performance will note reduced by the influence of moisture, dust, gases, etc. Reliable work, stale on-off performance. 13. After the arc is broken, the medium between the fractures recovers quickly and the medium does not need to be replaced. ½ Mark for 14. They require very less maintenance. each of any two Disadvantages of Vacuum Circuit Breaker: disadvantages 1. It needs additional surge suppressor for the interruption of low magnetizing = 1 Mark current in a certain range. 2. The requirement of high technology for the production of the vacuum interrupters. 3. Loss of vacuum due to transit damage or it is failure makes the entire interrupter useless and it cannot be repaired on-site. 4. When an inductive or capacitive load is disconnected, overvoltage is easily caused due to interception process, and reignition etc. and the corresponding overvoltage protection should be adopted. 5. They are more expensive than other interrupting devices. 6. Generally, they are useful up to 33kV only. b) Describe any six fundamental quality requirements of protective relaying. Ans: Quality Requirements of Relay: 1. Selectivity: It is the ability of protective system to select correctly that part of system in trouble and disconnect the faulty part without disturbing the rest of the system. 2. Speed: The relay system should disconnect the faulty section as fast as possible to prevent the electrical apparatus from damage and for system 1 Mark for stability. each of any 3. Sensitivity: It is the ability of the relay system to operate with low value of six points actuating quantity. = 6 Marks 4. Reliability: It is the ability of the relay system to operate under predetermined conditions. 5. Simplicity: The relay system should be simple so that it can be easily maintained. 6. Economy: The most important factor in the choice of particular protection scheme is the economic aspect. The protective gear should not cost more Page 16 of 17  MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous) m (ISO/IEC - 27001 - 2013 Certified) ________________________________________________________________________________________________________________ SUMMER – 2023 EXAMINATION sb Model Answer – Only for the Use of RAC Assessors 22524: SAP Subject Name: Switchgear And Protection te than 5% of the total cost of equipment to be protected. al c) A plant consists of two 10 MVA generators of reactance 18% each and two lc 5 MVA generators of 12% each. All are connected to bus bar to supply a le load through three step up transformers of 8 MVA each having reactance of 8%. Determine fault MVA on HV side of transformer. a Ans: r.c 1 Mark for om Diagram Assume base kVA = 10000 kVA 1 Mark % Reactance related to base KVA % X = (Base kVA / Rated kVA) x % Reactance on Rated kVA 2 Marks for XG1 = (10000/10000) x 18% = 18 % calculations XG2 = (10000/10000) x 18% = 18 % of XG1, XG2, XG3 = (10000/5000) x 12% = 24 % XG3, XG4 and XG4 = (10000/5000) x 12% = 24 % and XT XT = (10000/8000) x 8% = 10% For Fault F (HT side) Total reactance is %X = (XG1 || XG2 || XG3 || XG4 ) + XT = (18 || 18 || 24 || 24) +10 = 15.14 % X = 15.14 % 1 Mark S.C. kVA = Base kVA x (100/ % X) = 10000 x (100 /15.14) = 66050 kVA S.C. MVA = 66.05 MVA 1 Mark Page 17 of 17