Why differential protection for PHT motors?

Why differential protection for PHT motors?
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1 Answer

Answer :

For more than 2500 kW motors it is required to provide differential protection. It is biased Relay against internal phase fault or earth fault very fast. Insensitive to starting current and stalling current. 
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Related questions

What is the 2 stage stalling protection for PHT motor?

Description : What is the 2 stage stalling protection for PHT motor?

Last Answer : Because locked rotors withstand time of motor is less than starting time of motor under reduced voltage conditions. Stage 1 = 350% 6 sec for starting at rated voltage (Because starting time = 6 ... time under reduced voltage condition (Since starting current is less, stage 1 will not operate) 

Which of the following operating characteristics for DC motors is considered to give high starting torque? A. series B. shunt C. cumulative-compound D. differential-compound

Description : Which of the following operating characteristics for DC motors is considered to give high starting torque? A. series B. shunt C. cumulative-compound D. differential-compound

Last Answer : Answer: A

What FCN was implemented to avoid reactor trip on 220V-DC failure of PHT and PPP breakers?

Description : What FCN was implemented to avoid reactor trip on 220V-DC failure of PHT and PPP breakers?

Last Answer : The breaker close position supervision relay VAA 21 is changed by VAJC type, contacts position do not change if 220V-DC is lost now.

Why bi-directional flow is chosen for PHT system?

Description : Why bi-directional flow is chosen for PHT system?

Last Answer : Bi-directional flow is chosen for PHT system because, a. Uniform temperature gradient is facilitated so there will be no differential thermal expansion. b. It facilitates fuelling even when the ... , which facilitates uniform neutron flux and this intern gives rise to maximum fuel burns up. 

What are the main futures of PHT system?

Description : What are the main futures of PHT system?  

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What are the materials in contact with PHT system?

Description : What are the materials in contact with PHT system?

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How coolant flow is maintained in the event of loss of power to the PHT system?

Description : How coolant flow is maintained in the event of loss of power to the PHT system?

Last Answer : When the PHT power fails, it takes two minutes for DG to come to full power. The circulation is maintained by flywheel, which increases the de-acceleration and maintains the flow for two minutes. If ... which more denser cold water comes down and less denser hot water goes up by convection method. 

Explain differential protection?

Description : Explain differential protection?

Last Answer : Differential protection is based on the fact that any fault within an electrical equipment would cause the current entering it, to be different, from that leaving it. Thus, we can compare the two ... phase or both and issue a trip output if the difference exceeds a pre- determined set value.

What are the problems in bus zone differential protection?

Description : What are the problems in bus zone differential protection?

Last Answer : 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 ... the bus makes circuit complicated. Setting of relays need a change with large load changes.

What are the problems arising in differential protection in power transformer and how are they overcome?

Description : What are the problems arising in differential protection in power transformer and how are they overcome?

Last Answer : 1. 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. 2. ... harmonic restraining unit is added to the relay which will block it when the transformer is energized.

What is the coverage of differential protection for SUT?

Description : What is the coverage of differential protection for SUT?

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What is differential protection?

Description : What is differential protection? 

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Why delta – delta CT’s are used for star – star transformer differential protection?

Description : Why delta – delta CT’s are used for star – star transformer differential protection?

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Why inter-turn protection is provided along with differential protection in generator?

Description : Why inter-turn protection is provided along with differential protection in generator?

Last Answer : Inter-turn protects two separate windings from the fault of the generator.  

For which of the following ratings of the transformer differential protection is recommended ? (a) above 30 kVA. (b) equal to and above 5 MVA (c) equal to and above 25 MVA (d) none of the above

Description : For which of the following ratings of the transformer differential protection is recommended ? (a) above 30 kVA. (b) equal to and above 5 MVA (c) equal to and above 25 MVA (d) none of the above

Last Answer : (b) equal to and above 5 MVA

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Last Answer : (a) merz price percentage differential protection

Explain the “Differential Protection Scheme” used for alternators with neat labeled diagram.

Description : Explain the “Differential Protection Scheme” used for alternators with neat labeled diagram.

Last Answer : Differential Protection Scheme for Alternator: Figure shows the differential protection provided to 3 phase alternator. Here identical CTs are placed on both sides of 3 phase alternator and ... causing its operation. Relay sends trip signal to the CB, thereby alternator circuit is tripped.

Explain differential protection scheme for busbars with neat sketch.

Description : Explain differential protection scheme for busbars with neat sketch.

Last Answer : Differential Protection Scheme for Bus bar:  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 ... of these currents will flow through the relay coil causing opening of circuit breaker of bus-bar.

List two limitations of Differential-Protection scheme for transformer.

Description : List two limitations of Differential-Protection scheme for transformer.

Last Answer : Limitations of Differential Protection Scheme of Transformer:  1) Due to the magnetization characteristics of the CTs used, the ratio errors change with respect to the circulating currents.  2) ... & improper operation.  5) Inrush of magnetizing current may lead to inadvertent operation.

Explain differential protection of busbar.

Description : Explain differential protection of busbar.

Last Answer : Differential protection Scheme for bus bar: 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 ... . The difference of these currents will flow through the relay coil causing opening of circuit breaker.

Write down limitation of differential protection of transformer.

Description : Write down limitation of differential protection of transformer.

Last Answer : Limitations of differential protection of transformer: 1) Due to the magnetization characteristics of the CTs used, the ratio errors change with respect to the circulating currents. 2) The pilot wires ... current (higher imbalance) due to which accuracy of sensing & operation is decreased. 

A series resistance is added to the over-current relay in a bus bar differential protection scheme which:

Description : A series resistance is added to the over-current relay in a bus bar differential protection scheme which: a. Helps to develop some voltage across it which otherwise should appear across the relay and is ... the effects of CT saturation and lowers the pickup value of the relay d. All of the above 

Last Answer : A series resistance is added to the over-current relay in a bus bar differential protection scheme which: a. Helps to develop some voltage across it which otherwise should appear across the relay and is ... the effects of CT saturation and lowers the pickup value of the relay d. All of the above 

The single phasing relays are used for the protection of (a) single phase motors only (b) two phase motors only (c) two single phase motors running in parallel (d) three phase motors

Description : The single phasing relays are used for the protection of (a) single phase motors only (b) two phase motors only (c) two single phase motors running in parallel (d) three phase motors

Last Answer : (d) three phase motors

For which of the following protection from negative sequence currents is provided ? (a) Generators (b) Motors (c) Transmission line (d) Transformers

Description : For which of the following protection from negative sequence currents is provided ? (a) Generators (b) Motors (c) Transmission line (d) Transformers

Last Answer : (a) Generators

The delay fuses are used for the protection of ________ . (a) motors (b) power outlet circuits (c) fluorescent lamps (d) light circuits

Description : The delay fuses are used for the protection of ________ . (a) motors (b) power outlet circuits (c) fluorescent lamps (d) light circuits

Last Answer : (a) motors

A relay used for protection of motors against overload is :

Description : A relay used for protection of motors against overload is :  (A) Impedance relay (B) Electromagnetic attraction type (C) Thermal relay (D) Buchholz's relay

Last Answer : D

An AC generator operating in parallel loses its excitation without tripping the circuit breaker. This will __________. A. not affect the faulty generator due to the compensation of the other generators B. cause the slip rings to melt C. cause a low voltage differential to develop between the armature and the bus D. cause high currents to be induced in the field windings

Description : An AC generator operating in parallel loses its excitation without tripping the circuit breaker. This will __________. A. not affect the faulty generator due to the compensation of the other generators B. ... between the armature and the bus D. cause high currents to be induced in the field windings

Last Answer : Answer: D

Figure 'A' of the diagram shown in the illustration represents a ______________. EL-0092 A. differential transformer B. saturable reactor C. synchro system D. magnetic amplifier

Description : Figure 'A' of the diagram shown in the illustration represents a ______________. EL-0092 A. differential transformer B. saturable reactor C. synchro system D. magnetic amplifier

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Basic operating characteristics of the operational amplifier such as gain and stability are the function of its _____________. A. differential input stage B. power output stage C. feedback circuit D. supply voltages

Description : Basic operating characteristics of the operational amplifier such as gain and stability are the function of its _____________. A. differential input stage B. power output stage C. feedback circuit D. supply voltages

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Description : To provide its unique characteristics to analog circuits, the operational amplifier is made up of a/an ______________. A. voltage amp, current amp and output amp B. input amp, power amp and output amp C. scaling amp, power amp and voltage amp D. differential amp, voltage amp and output amp

Last Answer : Answer: D

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Description : Mention any 2 applications of differential relays.

Last Answer : Protection of generator and generator-transformer unit: protection of large motors and bus bars

What is biased differential bus zone reduction?

Description : What is biased differential bus zone reduction?

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Mention any two applications of differential relay.

Description : Mention any two applications of differential relay.

Last Answer : Protection of generator & generator transformer unit; protection of large motors and busbars .

Why high impedance circulating current differential?

Description : Why high impedance circulating current differential? 

Last Answer : Under through faults, CT’s of different phases saturates differently. Net spill current will operate low impedance CAG relay, so high impedance scheme with CAG relay and stabilising resistor used. 

Why generator differential provided?

Description : Why generator differential provided? 

Last Answer : For generator phase to phase faults and 3 phase faults. 

Will main generator differential relay pick up for generator earth faults?

Description : Will main generator differential relay pick up for generator earth faults? 

Last Answer : No earth fault current limited to 5A, while differential setting is 10%. 

What is a differential amplifier? Also, explain CMRR.

Description : What is a differential amplifier? Also, explain CMRR.

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Inverse definite minimum time lag relay is also called ___________ (A) pilot relay. (B) differential relay. (C) over current relay. (D) directional overcurrent relay.

Description : Inverse definite minimum time lag relay is also called ___________ (A) pilot relay. (B) differential relay. (C) over current relay. (D) directional overcurrent relay.

Last Answer : Ans: B Inverse definite minimum time lag relay characteristic is inverse but minimum time is fixed. The operating time is inversely proportional to the magnitude of actuating quantity.

Unless it can be shown that a 45°C ambient temperature will not be exceeded, motors within a machinery space must be designed for an ambient temperature of _____________. A. 45°C B. 50°C C. 55°C D. 60°C

Description : Unless it can be shown that a 45°C ambient temperature will not be exceeded, motors within a machinery space must be designed for an ambient temperature of _____________. A. 45°C B. 50°C C. 55°C D. 60°C

Last Answer : Answer: B

As shown in the illustration, the electrical symbol is used to indicate which of the listed types of motors? EL-0054 A. Shunt motor. B. Series motor. C. Compound motor. D. Tri-field motor.

Description : As shown in the illustration, the electrical symbol is used to indicate which of the listed types of motors? EL-0054 A. Shunt motor. B. Series motor. C. Compound motor. D. Tri-field motor.

Last Answer : Answer: C

An advantage of DC motors over AC motors is that they _____________. A. are less expensive B. require less maintenance C. offer infinite speed variation D. all of the above

Description : An advantage of DC motors over AC motors is that they _____________. A. are less expensive B. require less maintenance C. offer infinite speed variation D. all of the above

Last Answer : Answer: C

Which of the listed motors will operate at the highest RPM, assuming that each operates at the same frequency? A. A four-pole synchronous motor under normal load. B. A four-pole induction motor under no load. C. A six-pole synchronous motor under normal load. D. A six-pole induction motor under full load.

Description : Which of the listed motors will operate at the highest RPM, assuming that each operates at the same frequency? A. A four-pole synchronous motor under normal load. B. A four-pole induction motor under ... A six-pole synchronous motor under normal load. D. A six-pole induction motor under full load.

Last Answer : Answer: A

Which of the following is a characteristic of fractional horsepower repulsion start motors? A. They start with a rotating stator field. B. The short circuiting ring is removed from the commutator while starting. C. The brushes are removed from the commutator while starting. D. They have a low starting torque.

Description : Which of the following is a characteristic of fractional horsepower repulsion start motors? A. They start with a rotating stator field. B. The short circuiting ring is removed from the commutator while ... brushes are removed from the commutator while starting. D. They have a low starting torque.

Last Answer : Answer: B

Universal motors will operate on AC or DC current, and are generally found in _____________. A. portable tools B. large pump motors C. turbo electric main motors D. forced draft fans

Description : Universal motors will operate on AC or DC current, and are generally found in _____________. A. portable tools B. large pump motors C. turbo electric main motors D. forced draft fans

Last Answer : Answer: A

Electric motors intended for use outside the engine room and boiler room are frequently rated to run at a designed ambient temperature of 40°C. What is the equivalent temperature in degrees Fahrenheit? A. 54.2°F B. 72.0°F C. 104.0°F D. 129.6°F

Description : Electric motors intended for use outside the engine room and boiler room are frequently rated to run at a designed ambient temperature of 40°C. What is the equivalent temperature in degrees Fahrenheit? A. 54.2°F B. 72.0°F C. 104.0°F D. 129.6°F

Last Answer : Answer: C

Electrical failures in motors are caused by the breakdown of insulation, which may be caused by _____________. A. penetration of moisture B. accumulation of dirt C. overheating D. all of the above

Description : Electrical failures in motors are caused by the breakdown of insulation, which may be caused by _____________. A. penetration of moisture B. accumulation of dirt C. overheating D. all of the above

Last Answer : Answer: D

The armature cores of the D.C. motors are constructed with laminations to ___________. A. eliminate hysteresis B. minimize brush sparking C. reduce eddy current losses D. compensate for armature reaction

Description : The armature cores of the D.C. motors are constructed with laminations to ___________. A. eliminate hysteresis B. minimize brush sparking C. reduce eddy current losses D. compensate for armature reaction

Last Answer : Answer: C

The failure of ball bearings used in electric motors can result from _____________. A. failure to clean the bearing before repacking B. failure to maintain proper alignment C. excess lubrication forcing the bearing full of grease and eliminating all areas of expansion D. all of the above

Description : The failure of ball bearings used in electric motors can result from _____________. A. failure to clean the bearing before repacking B. failure to maintain proper alignment C. excess lubrication forcing the bearing full of grease and eliminating all areas of expansion D. all of the above

Last Answer : Answer: D

Some large A.C. motors are equipped with with heaters. These could be ___________. I. resistance heaters bolted directly to the frame II. low voltage windings embedded in the motor windings A. I only B. II only C. either I or II D. neither I or II

Description : Some large A.C. motors are equipped with with heaters. These could be ___________. I. resistance heaters bolted directly to the frame II. low voltage windings embedded in the motor windings A. I only B. II only C. either I or II D. neither I or II

Last Answer : Answer: C