Purpose of standby E/F protection in SUT/UT?

Purpose of standby E/F protection in SUT/UT? 
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1 Answer

Answer :

Back up for LV winding, LV neutral CT- CDG 12 – resistance earthing – relay set high time delay to discriminate with LV feeder and trip transformer if sustained E/F, also protects neutral earthing resistor.
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Related questions

What is E/F current limit for SUT and UT?

Description : What is E/F current limit for SUT and UT?

Last Answer : 400a limited by 10 ohms resistor.  

Why UT, SUT secondary is rated for 6.9kV where as bus voltage is 6.6kV?

Description : Why UT, SUT secondary is rated for 6.9kV where as bus voltage is 6.6kV? 

Last Answer : The no load secondary 6.9kV voltage level adequately takes into account voltage drop during loaded condition to cater station buses at 6.6kV level. 

Why REF is provided in the LV side of SUT and UT?

Description : Why REF is provided in the LV side of SUT and UT? 

Last Answer : The LV sides of the two transformers are earthed through the resistance. This will limit the flow of current in case of LV earth fault. Hence the differential protection may not act for a LV earth fault. Hence ref protection is provided. 

Why the neutral is earthed through earthing resistance in case of UT and SUT?

Description : Why the neutral is earthed through earthing resistance in case of UT and SUT? 

Last Answer : This is done to limit the earth fault current.  

What is the vector group of GT, UT, SUT?

Description : What is the vector group of GT, UT, SUT?

Last Answer : Yd11 Dy1 Yy0 

During unit operation, can we parallel UT & SUT continuously?

Description : During unit operation, can we parallel UT & SUT continuously?

Last Answer : No, due to switchgear limitation.  

What is the coverage of differential protection for SUT?

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

Last Answer : Covers from 230 kV bushing to 6.6 kV breaker end.  

Why main generator/UT is not provided with separate overfluxing protection?

Description : Why main generator/UT is not provided with separate overfluxing protection?

Last Answer : Since GT is provided with overfluxing protection, it is adequate to protect main generator / UT also. Main generator can withstand higher degree of overfluxing. If a generator CB is used, separate overfluxing protection is essential for main generator. 

What are the protections for SUT?

Description : What are the protections for SUT? 

Last Answer : a) Over current protection for phase and earth fault b) Differential protection c) HV and LV restricted earthfault protection  d) HV side directional back up over current protection for phase ... protection f) Over fluxing protection g) Buchholz and high oil, winding temperature protection. 

Why vector group of SUT is chosen as Yn-Yo-Yo?

Description : Why vector group of SUT is chosen as Yn-Yo-Yo?

Last Answer : To facilitate momentary paralleling of SUT with UT on 6.6kV buses.  

Why twin secondary SUT?

Description : Why twin secondary SUT? 

Last Answer : As per is, the rating single secondary power transformer is limited to 25MVA (6.6 kV) or 40 MVA (11 kV), in order to limit the 3 phase symmetrical fault level with in 26-40 kA (contribution from grid and local machines) 

During unit operation, can we parallel 2 SUT?

Description : During unit operation, can we parallel 2 SUT? 

Last Answer : No, due to switchgear limitation. 

Why REF is now is used for HV side also in GT/SUT?

Description : Why REF is now is used for HV side also in GT/SUT?

Last Answer : The E/F setting of differential is poor.  

Which of the following equations is correct for Soderberg Criteria? a. (σ m / S ut ) + (σ a / S e ) = (1 / N f ) b. (σ m / S ut ) - (σ a / S e ) = (1 / N f ) c. (σ m / S yt ) + (σ a / S e ) = (1 / N f ) d. (σ m / S ut ) - (σ a / S e ) = (1 / N f )

Description : Which of the following equations is correct for Soderberg Criteria? a. (σ m / S ut ) + (σ a / S e ) = (1 / N f ) b. (σ m / S ut ) - (σ a / S e ) = (1 / N f ) c. (σ m / S yt ) + (σ a / S e ) = (1 / N f ) d. (σ m / S ut ) - (σ a / S e ) = (1 / N f )

Last Answer : c. (σ m / S yt ) + (σ a / S e ) = (1 / N f )

The capacity of highest storage tank with efficient standby pump should be higher of the following figures a)1/4 the maximum water consumption in 24 hours b)1/3 the normal water consumption in 24 hours c)1/4 the normal water consumption in 24 hours. d) a or b* e) b or c f) a of c

Description : The capacity of highest storage tank with efficient standby pump should be higher of the following figures a)1/4 the maximum water consumption in 24 hours b)1/3 the normal water consumption in 24 hours c)1/4 the normal water consumption in 24 hours. d) a or b* e) b or c f) a of c

Last Answer : d) a or b*

What are the protections for UT?

Description : What are the protections for UT?  

Last Answer : a) Differential protection b) LV restricted earthfault protection c) LV back up earthfault and over current protection d) Buchholz and high oil, winding temperature protection. 

During unit operation can we have one UT feeding both unit 6.6 kV loads?

Description : During unit operation can we have one UT feeding both unit 6.6 kV loads? 

Last Answer : No, logically prevented.  

Where fault level is more, whether in generator bus duct or UT bus duct? Why?

Description : Where fault level is more, whether in generator bus duct or UT bus duct? Why?

Last Answer : Fault level is more in UT bus duct. Because the fault currents fed by the both generator and GT adds up within UT bus duct in case there is a fault in UT bus duct. 

Component F in the illustrated device is for _________. EL-0033 A. short circut protection B. latching the trip unit closed after resetting the breaker C. overload protection D. providing a flexible connection between the input terminal G and the tripping unit E

Description : Component F in the illustrated device is for _________. EL-0033 A. short circut protection B. latching the trip unit closed after resetting the breaker C. overload protection D. providing a flexible connection between the input terminal G and the tripping unit E

Last Answer : Answer: C

Why 100% winding protection is felt essential for main generator stator E/F protection? (Used in NAPS onwards?)

Description : Why 100% winding protection is felt essential for main generator stator E/F protection? (Used in NAPS onwards?)  

Last Answer : At MAPS 4% of winding is not protected. Earlier felt that the Electro magnetic stress due high external fault currents near 4% of neutral may not be high to cause E/F here. But now felt that the mechanical stress can leads to E/F. 

Why in DG E/F protection, we do not open class IV CB’s or supply CB’s?

Description : Why in DG E/F protection, we do not open class IV CB’s or supply CB’s?

Last Answer : Delta of aux. Transformer prevents E/F currents from grid into DG neutral.  

Why do we use O/C & E/F protection on both sides of transformer?

Description : Why do we use O/C & E/F protection on both sides of transformer?

Last Answer : Power in feed exists on both ends.  

The purpose of the automatic bus transfer shown in the illustration is to ___________. EL-0014 A. stepdown voltage to the lighting distribution panel B. provide emergency power to the lighting distribution panel C. energize the emergency switchboard from the main switchboard D. provide overcurrent protection to the lighting distribution panel

Description : The purpose of the automatic bus transfer shown in the illustration is to ___________. EL-0014 A. stepdown voltage to the lighting distribution panel B. provide emergency power to ... emergency switchboard from the main switchboard D. provide overcurrent protection to the lighting distribution panel

Last Answer : Answer: B

The purpose of a magnetic relay is to _____________. A. open a circuit only in the event of overload B. remotely open and close contacts C. provide overcurrent protection during starting D. relay voltages at increased power

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The purpose of a motor undervoltage protection device is to _____________. A. prevent high armature current when power is restored B. start the motor at a very low voltage C. trip the load off the motor in case of fire D. protect personnel from low voltage shocks

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Purpose of start up protection? Is it always in service?

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Last Answer : Trips the generator. If generator is excited with internal fault the over current 50s trip the generator to prevent major damage. The earth fault relay 64c also. The relays are polarised dc armature ... initially during start up. Start up protection is cutout as soon as generator CB is closed.  

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Last Answer : Class-B avoids load rejection. For modern machines, the inertia is less and easily gets damaged on overload. Therefore trip only for internal faults.  Low forward interlock prevents the risk of run away if a CIES valve fails to close. 

What is the purpose of back up impedance protection in main generator?

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Last Answer : Covers inter-phase faults both externals to the GT and in GT. This also covers partially faults inside generator, time delayed to coordinate with 230 kV-distance protection. 

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