10. Questions On Phase Conversion & Tap Changing Transformers
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Phase Conversion - Transformers Questions an...
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Phase Conversion - Transformers Questions and Answers by Manish 5-7 minutes
This set of Transformers Multiple Choice Questions & Answers (MCQs) focuses on “Phase Conversion”. 1. Only Scott connection is used for _________ a) Converting three-phase to two-phase conversion b) Converting three-phase to single-phase conversion c) Converting single-phase to two-phase conversion d) None of the mentioned View Answer Answer: a Explanation: Scott connection is used for obtaining twophase supply as it is needed for various special purposes like supplying two-phase electric arc furnaces and can easily obtained from three-phase supply by this method. 2. In Scott connection, according to the vector diagram two windings are placed at ______________ a) At 1200 to each other b) Perpendicular with respect to each other c) At 600 to each other
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Phase Conversion - Transformers Questions an...
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d) Can’t say View Answer Answer: b Explanation: For a transformer vector diagram, if one winding is kept along the perpendicular axis of another winding then one can obtain two-phase supply from threephase supply by using Scott connection. Physically this can be obtained by appropriate turns ratio. 3. What is the ratio of voltage/turn of two windings of a transformer in operated in Scott connection? a) ½ b) 1/√2 c) 1 d) 1/√3 View Answer Answer: c Explanation: The transformer primaries must have √3 N1/2 (teaser) and N1 turns; this would mean equal voltage/turn in each transformer. A balanced 2-phase supply could then be easily obtained by having both secondaries with equal number of turns, N2. 4. The primaries of two transformers in Scott connection are in turns ratio of ___________ a) √3/2: 1 b) 2/√3: √2 c) 1:1 d) Can’t say View Answer 2 of 6
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Phase Conversion - Transformers Questions an...
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Answer: a Explanation: A 2-phase supply could thus be obtained by means of transformers; one connected perpendicular according to the vector diagram is called the teaser transformer and the other is connected across, with turns ratio √3/2: 1 respectively. 5. The secondaries of two transformers are in ratio ____ a) 1:1 b) √3/2: 1 c) √3: 1 d) √2: 1 View Answer Answer: a Explanation: Though transformer primaries are in the ratio of √3/2: 1 with respect to each other secondary windings of these transformers are in ratio of 1:1. This simply means that both of these transformers have equal voltage per turns. 6. In Scott connection neutral point is located at _______________ a) Three phase side at teaser b) Two-phase side c) Three phase side secondary d) Anywhere View Answer Answer: a Explanation: Neutral point if required can be located on three phase side of a transformer at teaser in Scott 3 of 6
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Phase Conversion - Transformers Questions an...
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connection which is used to obtain two-phase supply from three-phase supply. Neutral point is located at teaser winding dividing the winding in the ratio 1:2. 7. For single-phase to three-phase change in transformer which of the following connection is most suitable? a) Scott connection b) Scott connection with resistor network c) Scott connection with some energy storing device d) Any of the mentioned View Answer Answer: c Explanation: A single-phase power pulsates at twice the frequency, while the total power drawn by a balanced 3-phase load is constant. Thus a 1-phase load can never be transferred to a 3-phase system as a balanced load without employing some energy-storing device (capacitor, inductor or rotating machine). 8. For 3/6-phase converter transformers __________ a) Scott connection is used b) Each secondary winding is divided c) Each primary winding is divided d) Can’t be done View Answer Answer: b Explanation: For converting 3-phase to 6-phase, threephase delta connection is used on primary side. Each secondary phase is divided into two equal halves with appropriate polarity and corresponding 6-phase voltage is 4 of 6
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Phase Conversion - Transformers Questions an...
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obtained. 9. In 3/6-phase connection of transformers _______________ a) Secondaries are two stars in phase sequence b) Secondaries are two deltas in phase sequence c) Secondaries are two stars in phase opposition d) Secondaries are two deltas in phase opposition View Answer Answer: c Explanation: Six-phase voltages (characteristic angle 360°/6 = 60°) are obtained by means of two stars in phase opposition, each star being formed from three respective half-windings. This is done without Scott connection. 10. 3/6-phase circuit is employed for providing DC path in some circuits. a) True b) False View Answer Answer: a Explanation: In certain applications like thyristors and rectifiers six-phase supply is required for providing path for the DC current. Therefore, it becomes necessary to convert three-phase AC supply into six-phase. 11. Two single-phase furnaces let’s say, A and B are supplied at 100 V by means of a Scott-connected transformer combination from a 3-phase 6600 V system. The voltage of furnace A is leading. Calculate on of the line current on the 3-phase side, when the furnace A takes 400 5 of 6
3/19/18, 7:58 AM
Phase Conversion - Transformers Questions an...
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kW at 0.707 pf lagging and B takes 800 kW at unity pf. a) 110 A b) 99 A c) 250 A d) 149 A View Answer Answer: b Explanation: N1/N2= 6600/100= 60 √3/2 (N1/N2) = 57.16 Furnace currents are= Ia = 400*1000/ (100*0.707) = 5658 A Similarly, Ib= 8000 A On 3-phase side, IA= 5658/57.16 = 99 A. Sanfoundry Global Education & Learning Series – Transformers. To practice all areas of Transformers, here is complete set of 1000+ Multiple Choice Questions and Answers.
6 of 6
3/19/18, 7:58 AM
Tap Changing Transformers Questions and Answers
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Tap Changing Transformers Questions and Answers by Manish 5-6 minutes
This set of Transformers Multiple Choice Questions & Answers (MCQs) focuses on “Tap Changing Transformers-1”. 1. Why tapping is necessary? a) To provide a neutral point b) To vary secondary voltage c) To control real and reactive power flow in the network d) For neutral point, variation in secondary voltage, controlling power flow View Answer Answer: d Explanation: Voltage variation in power systems is a normal phenomenon owing to the rapid growth of industries and distribution network. System voltage control is therefore essential for many purposes as listed above. 2. Common range pf tap changing is __________ a) 5% b) 7%
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Tap Changing Transformers Questions and Answers
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c) 15% d) 10% View Answer Answer: a Explanation: Adjustment is normally carried out by off-circuit tap changing, the common range being 5% in 2.5% steps. Daily and short-time control or adjustment is carried out by means of on-load tap changing gear, which makes this application easier. 3. How tap changing is achieved? a) Voltage variation with constant flux and constant voltage turn b) With varying flux c) Either by voltage variation or by flux variation d) Can’t be done by variation in voltage or flux View Answer Answer: c Explanation: Tap changing may be achieved in one of the three conditions, viz. (i) voltage variation with constant flux and constant voltage turn, (ii) with varying flux, (iii) a mix of (i) and (ii). 4. Negative tapping means ________________ a) less turns than positive tap b) less turns than principal tap c) less turns than zero tap d) more turns than principal tap 2 of 6
3/19/18, 7:58 AM
Tap Changing Transformers Questions and Answers
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View Answer Answer: b Explanation: The principal tapping is one to which the rating of the winding is related. A positive tapping means more turns and a negative tapping implies less turns than those of the principal tap. Tap changing may be achieved by three conditions. 5. Tapping is done at _________________ a) primary only b) secondary only c) primary or secondary side d) on both sides View Answer Answer: c Explanation: Tapping can be done to either primary or secondary sides. The taps may be placed on the primary or secondary side which depends on construction. Tapping is thus so useful for various applications. 6. Bushing insulators are required when ____________ a) tapping is done near the neutral end b) tapping is done at the line ends c) tapping is done d) they are not required View Answer Answer: b Explanation: The taps may be placed on the primary or secondary side which partly depends on construction. If tapping is near the line ends, fewer bushings insulators are 3 of 6
3/19/18, 7:58 AM
Tap Changing Transformers Questions and Answers
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required. If the tappings are placed near the neutral ends, the phase-to-phase insulation conditions are eased. 7. For achieving large voltage regulation tapping should be done at _____________ a) at the upper ends b) at the lower ends c) at the centre d) anywhere View Answer Answer: c Explanation: For achieving large voltage variation, tappings should be placed near the centres of the phase windings to reduce magnetic asymmetry. It is generally done on that winding which is placed outside. 8. In case of large voltage variations transformers are tapped at _____________ a) LV b) HV c) Both LV and HV d) Can’t be tapped View Answer Answer: b Explanation: Centre tapped arrangement cannot be put on LV windings placed next to the core (as in core type transformer) because of accessibility and insulation considerations. The HV winding placed outside the LV winding is easily accessible and can, thus, be tapped easily. 9. Tapping of HV winding is advantageous for 4 of 6
3/19/18, 7:58 AM
Tap Changing Transformers Questions and Answers
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_____________ a) step-up transformer b) step-down transformer c) both type of transformers d) never advantageous View Answer Answer: b Explanation: It is not possible to tap other than an integral number of turns and this may not be feasible with LV side tappings. An example can illustrate this, 250 V phase winding with 15 V/turn cannot be tapped closer than 5%. It is therefore essential to tap the HV windings which is advantageous in a step-down transformer. 10. Tap changing do not causes change in losses of a transformer. a) True b) False View Answer Answer: b Explanation: Axial mmf unbalance is minimized by thinning out the LV winding or by arranging parts of the winding more symmetrically. For very large tapping range, a special tapping coil can be employed. Tap changing causes changes in leakage reactance, core loss, I2R loss and perhaps some problems in parallel operation of dissimilar transformers. 11. Off-circuit tap changer is used when transformer is ________________ 5 of 6
3/19/18, 7:58 AM
Tap Changing Transformers Questions and Answers
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a) deenergized b) energized c) can be used in either energy conditions d) never used View Answer Answer: a Explanation: The cheapest method of changing the turn ratio of a transformer is the use of off-circuit tap changer. As the name off-circuit tap changer indicates, it is required to deenergize the transformer before changing the tap. Sanfoundry Global Education & Learning Series – Transformers. To practice all areas of Transformers, here is complete set of 1000+ Multiple Choice Questions and Answers.
6 of 6
3/19/18, 7:58 AM
Transformers Test - Sanfoundry
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sanfoundry.com
Transformers Test - Sanfoundry by Manish 5-6 minutes
This set of Transformers test focuses on “Tap Changing Transformers – 2”. 1. Face plate carrying suitable studs is mounted on _____________ a) lower yoke b) upper yoke c) away from tapped positions d) anywhere View Answer Answer: b Explanation: The face plate carrying the suitable studs can be mounted at a convenient place on the transformer such as upper yoke or located near the tapped positions on the windings. The movable contact arm A may be rotated by handwheel mounted externally on the tank. 2. Why microswitch is provided to open the circuit breaker? a) To prevent inadvertent operation b) To make circuit energized again c) To avoid unauthorised operation
1 of 6
3/19/18, 7:58 AM
Transformers Test - Sanfoundry
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d) For various other purposes View Answer Answer: a Explanation: To prevent inadvertent operation, an electromagnetic latching device or microswitch is provided to open the circuit breaker so as to deenergize the transformer as soon as the tap changer handle is moved; well before the contact of the arm with the stud (with which it was in contact) opens. 3. With the introduction of on-load tap changers, operating efficiency ___________ a) improves b) decreases c) depends on the application d) stays constant View Answer Answer: a Explanation: On-load tap changers are used to change the turn ratio of transformer to regulate system voltage while the transformer is delivering load. With the introduction of on-load tap changer, the operating efficiency of electrical system gets considerably improved. 4. During the operation of on-load tap changer main circuit ______________ a) can be opened b) always closed c) depends on application type d) depends on the construction 2 of 6
3/19/18, 7:58 AM
Transformers Test - Sanfoundry
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View Answer Answer: b Explanation: During the operation of an on-load tap changer the main circuit should not be opened to prevent (dangerous) sparking and no part of the tapped winding should get short-circuited. All forms of on-load tap changing circuits are provided with an impedance, which is introduced to limit short circuit current during the tap changing operation. 5. On-load tap changers are generally _______________ a) resistor type b) reactor type c) can be resistor type or reactor type d) can’t say View Answer Answer: c Explanation: All forms of on-load tap changing circuits are provided with an impedance, which is introduced to limit short circuit current during the tap changing operation. The impedance can either be a resistor type or centre-tapped reactor can also be used. The on-load tap changers can in general be classified as resistor or reactor type. 6. In modern type, all tap-changers are made of _____________ a) resistor type b) reactor type c) can be either resistor or reactor type d) can’t say 3 of 6
3/19/18, 7:58 AM
Transformers Test - Sanfoundry
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View Answer Answer: a Explanation: The impedance can either be a resistor or centre-tapped reactor. The on-load tap changers can in general be classified as two types- resistor or reactor type. In modern designs the current limiting is almost invariably carried out by a pair of resistors. 7. Why an energy storage device is provided on on-load tap changing transformers? a) To ensure transition is started b) To store additional energy c) To ensure that transition once started is completed d) To check the extra energy produced due to transition View Answer Answer: c Explanation: To ensure that the transition once started gets completed, an energy storage (usually a spring device) is provided which acts even if the auxiliary power supply happens to fail. It thus, serves an important role. 8. Highest tap changers available as on today are ___________ a) 1475 kV impulse b) 2000 kV impulse c) 575 kV impulse d) 955 kV impulse View Answer Answer: a Explanation: At present tap changers are available for the 4 of 6
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Transformers Test - Sanfoundry
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highest insulation level of 1475 kV (peak) impulse and 630 kV power frequency voltage. Efforts are underway to develop tap changers suitable for still higher insulation levels. 9. Which of the following is/are part/s of tap changing circuit? a) Automatic voltage regulator b) A time delay relay c) Compounding elements d) AVR, time delay relay, compounding elements View Answer Answer: d Explanation: The main components are an automatic voltage regulator, a time delay relay, and compounding elements. The time delay here, prevents unwanted initiation of a tap change by a small transient voltage fluctuation. It may be set for a delay upto 1 min. 10. Compact tap changers are made of _____________ a) thyristorized tap changers b) vacuum switches in diverter switch c) metallic tap changers d) rubbered tap changers View Answer Answer: b Explanation: More compact tap changers with high reliability and performance are being made by employing vacuum switches in the diverter switch. Also, now thyristorized tap changers are available widely in market for special 5 of 6
3/19/18, 7:58 AM
Transformers Test - Sanfoundry
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applications where a large number of operations are desired. Sanfoundry Global Education & Learning Series – Transformers. To practice all areas of Transformers for tests, here is complete set of 1000+ Multiple Choice Questions and Answers.
6 of 6
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Phase Conversion - Transformers Questions and Answers by Manish 5-7 minutes
This set of Transformers Multiple Choice Questions & Answers (MCQs) focuses on “Phase Conversion”. 1. Only Scott connection is used for _________ a) Converting three-phase to two-phase conversion b) Converting three-phase to single-phase conversion c) Converting single-phase to two-phase conversion d) None of the mentioned View Answer Answer: a Explanation: Scott connection is used for obtaining twophase supply as it is needed for various special purposes like supplying two-phase electric arc furnaces and can easily obtained from three-phase supply by this method. 2. In Scott connection, according to the vector diagram two windings are placed at ______________ a) At 1200 to each other b) Perpendicular with respect to each other c) At 600 to each other
1 of 6
3/19/18, 7:58 AM
Phase Conversion - Transformers Questions an...
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d) Can’t say View Answer Answer: b Explanation: For a transformer vector diagram, if one winding is kept along the perpendicular axis of another winding then one can obtain two-phase supply from threephase supply by using Scott connection. Physically this can be obtained by appropriate turns ratio. 3. What is the ratio of voltage/turn of two windings of a transformer in operated in Scott connection? a) ½ b) 1/√2 c) 1 d) 1/√3 View Answer Answer: c Explanation: The transformer primaries must have √3 N1/2 (teaser) and N1 turns; this would mean equal voltage/turn in each transformer. A balanced 2-phase supply could then be easily obtained by having both secondaries with equal number of turns, N2. 4. The primaries of two transformers in Scott connection are in turns ratio of ___________ a) √3/2: 1 b) 2/√3: √2 c) 1:1 d) Can’t say View Answer 2 of 6
3/19/18, 7:58 AM
Phase Conversion - Transformers Questions an...
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Answer: a Explanation: A 2-phase supply could thus be obtained by means of transformers; one connected perpendicular according to the vector diagram is called the teaser transformer and the other is connected across, with turns ratio √3/2: 1 respectively. 5. The secondaries of two transformers are in ratio ____ a) 1:1 b) √3/2: 1 c) √3: 1 d) √2: 1 View Answer Answer: a Explanation: Though transformer primaries are in the ratio of √3/2: 1 with respect to each other secondary windings of these transformers are in ratio of 1:1. This simply means that both of these transformers have equal voltage per turns. 6. In Scott connection neutral point is located at _______________ a) Three phase side at teaser b) Two-phase side c) Three phase side secondary d) Anywhere View Answer Answer: a Explanation: Neutral point if required can be located on three phase side of a transformer at teaser in Scott 3 of 6
3/19/18, 7:58 AM
Phase Conversion - Transformers Questions an...
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connection which is used to obtain two-phase supply from three-phase supply. Neutral point is located at teaser winding dividing the winding in the ratio 1:2. 7. For single-phase to three-phase change in transformer which of the following connection is most suitable? a) Scott connection b) Scott connection with resistor network c) Scott connection with some energy storing device d) Any of the mentioned View Answer Answer: c Explanation: A single-phase power pulsates at twice the frequency, while the total power drawn by a balanced 3-phase load is constant. Thus a 1-phase load can never be transferred to a 3-phase system as a balanced load without employing some energy-storing device (capacitor, inductor or rotating machine). 8. For 3/6-phase converter transformers __________ a) Scott connection is used b) Each secondary winding is divided c) Each primary winding is divided d) Can’t be done View Answer Answer: b Explanation: For converting 3-phase to 6-phase, threephase delta connection is used on primary side. Each secondary phase is divided into two equal halves with appropriate polarity and corresponding 6-phase voltage is 4 of 6
3/19/18, 7:58 AM
Phase Conversion - Transformers Questions an...
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obtained. 9. In 3/6-phase connection of transformers _______________ a) Secondaries are two stars in phase sequence b) Secondaries are two deltas in phase sequence c) Secondaries are two stars in phase opposition d) Secondaries are two deltas in phase opposition View Answer Answer: c Explanation: Six-phase voltages (characteristic angle 360°/6 = 60°) are obtained by means of two stars in phase opposition, each star being formed from three respective half-windings. This is done without Scott connection. 10. 3/6-phase circuit is employed for providing DC path in some circuits. a) True b) False View Answer Answer: a Explanation: In certain applications like thyristors and rectifiers six-phase supply is required for providing path for the DC current. Therefore, it becomes necessary to convert three-phase AC supply into six-phase. 11. Two single-phase furnaces let’s say, A and B are supplied at 100 V by means of a Scott-connected transformer combination from a 3-phase 6600 V system. The voltage of furnace A is leading. Calculate on of the line current on the 3-phase side, when the furnace A takes 400 5 of 6
3/19/18, 7:58 AM
Phase Conversion - Transformers Questions an...
about:reader?url=https://www.sanfoundry.com/...
kW at 0.707 pf lagging and B takes 800 kW at unity pf. a) 110 A b) 99 A c) 250 A d) 149 A View Answer Answer: b Explanation: N1/N2= 6600/100= 60 √3/2 (N1/N2) = 57.16 Furnace currents are= Ia = 400*1000/ (100*0.707) = 5658 A Similarly, Ib= 8000 A On 3-phase side, IA= 5658/57.16 = 99 A. Sanfoundry Global Education & Learning Series – Transformers. To practice all areas of Transformers, here is complete set of 1000+ Multiple Choice Questions and Answers.
6 of 6
3/19/18, 7:58 AM
Tap Changing Transformers Questions and Answers
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sanfoundry.com
Tap Changing Transformers Questions and Answers by Manish 5-6 minutes
This set of Transformers Multiple Choice Questions & Answers (MCQs) focuses on “Tap Changing Transformers-1”. 1. Why tapping is necessary? a) To provide a neutral point b) To vary secondary voltage c) To control real and reactive power flow in the network d) For neutral point, variation in secondary voltage, controlling power flow View Answer Answer: d Explanation: Voltage variation in power systems is a normal phenomenon owing to the rapid growth of industries and distribution network. System voltage control is therefore essential for many purposes as listed above. 2. Common range pf tap changing is __________ a) 5% b) 7%
1 of 6
3/19/18, 7:58 AM
Tap Changing Transformers Questions and Answers
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c) 15% d) 10% View Answer Answer: a Explanation: Adjustment is normally carried out by off-circuit tap changing, the common range being 5% in 2.5% steps. Daily and short-time control or adjustment is carried out by means of on-load tap changing gear, which makes this application easier. 3. How tap changing is achieved? a) Voltage variation with constant flux and constant voltage turn b) With varying flux c) Either by voltage variation or by flux variation d) Can’t be done by variation in voltage or flux View Answer Answer: c Explanation: Tap changing may be achieved in one of the three conditions, viz. (i) voltage variation with constant flux and constant voltage turn, (ii) with varying flux, (iii) a mix of (i) and (ii). 4. Negative tapping means ________________ a) less turns than positive tap b) less turns than principal tap c) less turns than zero tap d) more turns than principal tap 2 of 6
3/19/18, 7:58 AM
Tap Changing Transformers Questions and Answers
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View Answer Answer: b Explanation: The principal tapping is one to which the rating of the winding is related. A positive tapping means more turns and a negative tapping implies less turns than those of the principal tap. Tap changing may be achieved by three conditions. 5. Tapping is done at _________________ a) primary only b) secondary only c) primary or secondary side d) on both sides View Answer Answer: c Explanation: Tapping can be done to either primary or secondary sides. The taps may be placed on the primary or secondary side which depends on construction. Tapping is thus so useful for various applications. 6. Bushing insulators are required when ____________ a) tapping is done near the neutral end b) tapping is done at the line ends c) tapping is done d) they are not required View Answer Answer: b Explanation: The taps may be placed on the primary or secondary side which partly depends on construction. If tapping is near the line ends, fewer bushings insulators are 3 of 6
3/19/18, 7:58 AM
Tap Changing Transformers Questions and Answers
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required. If the tappings are placed near the neutral ends, the phase-to-phase insulation conditions are eased. 7. For achieving large voltage regulation tapping should be done at _____________ a) at the upper ends b) at the lower ends c) at the centre d) anywhere View Answer Answer: c Explanation: For achieving large voltage variation, tappings should be placed near the centres of the phase windings to reduce magnetic asymmetry. It is generally done on that winding which is placed outside. 8. In case of large voltage variations transformers are tapped at _____________ a) LV b) HV c) Both LV and HV d) Can’t be tapped View Answer Answer: b Explanation: Centre tapped arrangement cannot be put on LV windings placed next to the core (as in core type transformer) because of accessibility and insulation considerations. The HV winding placed outside the LV winding is easily accessible and can, thus, be tapped easily. 9. Tapping of HV winding is advantageous for 4 of 6
3/19/18, 7:58 AM
Tap Changing Transformers Questions and Answers
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_____________ a) step-up transformer b) step-down transformer c) both type of transformers d) never advantageous View Answer Answer: b Explanation: It is not possible to tap other than an integral number of turns and this may not be feasible with LV side tappings. An example can illustrate this, 250 V phase winding with 15 V/turn cannot be tapped closer than 5%. It is therefore essential to tap the HV windings which is advantageous in a step-down transformer. 10. Tap changing do not causes change in losses of a transformer. a) True b) False View Answer Answer: b Explanation: Axial mmf unbalance is minimized by thinning out the LV winding or by arranging parts of the winding more symmetrically. For very large tapping range, a special tapping coil can be employed. Tap changing causes changes in leakage reactance, core loss, I2R loss and perhaps some problems in parallel operation of dissimilar transformers. 11. Off-circuit tap changer is used when transformer is ________________ 5 of 6
3/19/18, 7:58 AM
Tap Changing Transformers Questions and Answers
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a) deenergized b) energized c) can be used in either energy conditions d) never used View Answer Answer: a Explanation: The cheapest method of changing the turn ratio of a transformer is the use of off-circuit tap changer. As the name off-circuit tap changer indicates, it is required to deenergize the transformer before changing the tap. Sanfoundry Global Education & Learning Series – Transformers. To practice all areas of Transformers, here is complete set of 1000+ Multiple Choice Questions and Answers.
6 of 6
3/19/18, 7:58 AM
Transformers Test - Sanfoundry
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sanfoundry.com
Transformers Test - Sanfoundry by Manish 5-6 minutes
This set of Transformers test focuses on “Tap Changing Transformers – 2”. 1. Face plate carrying suitable studs is mounted on _____________ a) lower yoke b) upper yoke c) away from tapped positions d) anywhere View Answer Answer: b Explanation: The face plate carrying the suitable studs can be mounted at a convenient place on the transformer such as upper yoke or located near the tapped positions on the windings. The movable contact arm A may be rotated by handwheel mounted externally on the tank. 2. Why microswitch is provided to open the circuit breaker? a) To prevent inadvertent operation b) To make circuit energized again c) To avoid unauthorised operation
1 of 6
3/19/18, 7:58 AM
Transformers Test - Sanfoundry
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d) For various other purposes View Answer Answer: a Explanation: To prevent inadvertent operation, an electromagnetic latching device or microswitch is provided to open the circuit breaker so as to deenergize the transformer as soon as the tap changer handle is moved; well before the contact of the arm with the stud (with which it was in contact) opens. 3. With the introduction of on-load tap changers, operating efficiency ___________ a) improves b) decreases c) depends on the application d) stays constant View Answer Answer: a Explanation: On-load tap changers are used to change the turn ratio of transformer to regulate system voltage while the transformer is delivering load. With the introduction of on-load tap changer, the operating efficiency of electrical system gets considerably improved. 4. During the operation of on-load tap changer main circuit ______________ a) can be opened b) always closed c) depends on application type d) depends on the construction 2 of 6
3/19/18, 7:58 AM
Transformers Test - Sanfoundry
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View Answer Answer: b Explanation: During the operation of an on-load tap changer the main circuit should not be opened to prevent (dangerous) sparking and no part of the tapped winding should get short-circuited. All forms of on-load tap changing circuits are provided with an impedance, which is introduced to limit short circuit current during the tap changing operation. 5. On-load tap changers are generally _______________ a) resistor type b) reactor type c) can be resistor type or reactor type d) can’t say View Answer Answer: c Explanation: All forms of on-load tap changing circuits are provided with an impedance, which is introduced to limit short circuit current during the tap changing operation. The impedance can either be a resistor type or centre-tapped reactor can also be used. The on-load tap changers can in general be classified as resistor or reactor type. 6. In modern type, all tap-changers are made of _____________ a) resistor type b) reactor type c) can be either resistor or reactor type d) can’t say 3 of 6
3/19/18, 7:58 AM
Transformers Test - Sanfoundry
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View Answer Answer: a Explanation: The impedance can either be a resistor or centre-tapped reactor. The on-load tap changers can in general be classified as two types- resistor or reactor type. In modern designs the current limiting is almost invariably carried out by a pair of resistors. 7. Why an energy storage device is provided on on-load tap changing transformers? a) To ensure transition is started b) To store additional energy c) To ensure that transition once started is completed d) To check the extra energy produced due to transition View Answer Answer: c Explanation: To ensure that the transition once started gets completed, an energy storage (usually a spring device) is provided which acts even if the auxiliary power supply happens to fail. It thus, serves an important role. 8. Highest tap changers available as on today are ___________ a) 1475 kV impulse b) 2000 kV impulse c) 575 kV impulse d) 955 kV impulse View Answer Answer: a Explanation: At present tap changers are available for the 4 of 6
3/19/18, 7:58 AM
Transformers Test - Sanfoundry
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highest insulation level of 1475 kV (peak) impulse and 630 kV power frequency voltage. Efforts are underway to develop tap changers suitable for still higher insulation levels. 9. Which of the following is/are part/s of tap changing circuit? a) Automatic voltage regulator b) A time delay relay c) Compounding elements d) AVR, time delay relay, compounding elements View Answer Answer: d Explanation: The main components are an automatic voltage regulator, a time delay relay, and compounding elements. The time delay here, prevents unwanted initiation of a tap change by a small transient voltage fluctuation. It may be set for a delay upto 1 min. 10. Compact tap changers are made of _____________ a) thyristorized tap changers b) vacuum switches in diverter switch c) metallic tap changers d) rubbered tap changers View Answer Answer: b Explanation: More compact tap changers with high reliability and performance are being made by employing vacuum switches in the diverter switch. Also, now thyristorized tap changers are available widely in market for special 5 of 6
3/19/18, 7:58 AM
Transformers Test - Sanfoundry
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applications where a large number of operations are desired. Sanfoundry Global Education & Learning Series – Transformers. To practice all areas of Transformers for tests, here is complete set of 1000+ Multiple Choice Questions and Answers.
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