Why twin secondary SUT? 

1 Answer

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) 

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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. 

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. 

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

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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. 

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.  

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

Last Answer : Yd11 Dy1 Yy0 

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

Last Answer : No, due to switchgear limitation.  

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Last Answer : No, due to switchgear limitation. 

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

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Last Answer : 400a limited by 10 ohms resistor.  

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

Last 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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