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

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

Answer :

The E/F setting of differential is poor.  
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Related questions

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. 

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

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

Last Answer : Yd11 Dy1 Yy0 

Why in transformer the LV CB also be tripped along with HV CB for a primary side fault?

Description : Why in transformer the LV CB also be tripped along with HV CB for a primary side fault?

Last Answer : Auxiliary transformer 415v delta star transformer, if HV CB alone tripped then back feeding from LV side (say DG runs parallel with transformer)—arcing voltage at the fault on primary—fault fed for more time – more damage.  

Why the tapings are placed on HV side?

Description : Why the tapings are placed on HV side?

Last Answer : 1.A fine voltage regulation is possible with hV side as it has large number of turns. 2.LV side carries large current 3. HV side is placed outside so its feasible 4. In case of step ... large no. of turns the flux and flux density decreases. Hence core losses decreases results in higher efficiency.

Why SC test is generally performed on HV side of a transformer?

Description : Why SC test is generally performed on HV side of a transformer?

Last Answer : The rated current is less on HV side. This will also permit to use ammeter and wattmeter of lower current range.

Why the tap changers are always connected to HV side of the transformer?

Description : Why the tap changers are always connected to HV side of the transformer? 

Last Answer : During tap changing action the load current has to be shifted from one tap to another tap. In case HV wining the load current will be less. Hence lesser arcing will take place. 

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 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 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) 

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.  

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.  

During unit operation, can we parallel 2 SUT?

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

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.  

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.  

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

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.

What is REF relay?

Description : What is REF relay?

Last Answer : It is restricted earth fault relay. When the fault occurs very near to the neutral point of the transformer, the voltage available to drive the earth circuit is very small, which may not be ... of the transformer is restricted to cover only around 85%. Hence the relay is called REF relay.

Why stabilising resistor in REF or residual E/F scheme?

Description : Why stabilising resistor in REF or residual E/F scheme? 

Last Answer : Required against CT saturation under heavy through fault currents.  

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Description : Why our GT having off load tap changer?

Last Answer : Because our station is base load station.  

What is the advantage of Pressure relief device in TELK type GT over explosion vent of BHEL, even though in both cases oil will be expelled out during sudden pressure rise?

Description : What is the advantage of Pressure relief device in TELK type GT over explosion vent of BHEL, even though in both cases oil will be expelled out during sudden pressure rise?

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What is the type of lightning arrestor for GT?

Description : What is the type of lightning arrestor for GT? 

Last Answer : Zn O (zinc oxide) types.  

Why off load tap changer was chosen for GT?

Description : Why off load tap changer was chosen for GT?

Last Answer : Because our plant works on base load always.  

How selection of cooling fluid in GT done?

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Last Answer : a) There are 5 factors are there. b) Density c) Coefficient of thermal expansion d) Viscosity e) Specific heat f) Thermal conductivity. 

What are the effects of GT over fluxing?

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