Answer / pillay

a)Loose terminations at the auto transformer itself .
b)intermittent failure of the change over timer or starter.
c)excessive load at time of starting .
d)insulation failure of the auto transformer.

Answer / k.prakashchandra

Autotransformer is for starting of large motor.If you use a
starter additionally ,it requires a certain minimum voltage
for the hold coil.As this voltage was high enough for heavy
starting current which burnt the autotransformer.

Answer / amit kumar singh

The one reason may be that at the time of starting,
induction motor sucts very high inrush current for the
establishment of magnetic field. Thus the total MVA
requirement at the time of motor starting is about 6 to 7
times the rated MVA of motor (if voltage is considered to
remain constant at the time of starting).

As, a transformer can only withstand 1.5 times its rated
MVA (i.e. also only for short duration), the heavy
requirement of MVA by motor could have damaged the
transformer (in case of failure of tap changer operation to
reduce voltage applied before starting of motor).

Answer / d.lakshmi narayana

To check the transformer tap voltage
persantage.(or)Unbalance the tap voltage (or)To take proper
tap voltage for motor starting load

Just a silly question but just to make my curious mind bac to normal - Why was not 67 and 67N relays built into a single relay as i have noticed that the R,Y,B CT wiring end up into the 67 relay while the common of the R,Y,B CT wiring end up into the 67N relay. Then it was better if a single relay was manufactured just like the distance relay (21). Another question i would like to ask is - how do we know where the 67 and 67 N relay has to be placed.why is it not placed at the transformer bay? In the substaion that i am working -i have noticed that it is placed between a breaker and a common double bus bar and this line bay is connected to another substation at a remote place which is furthur connected to a load. B U S______CT for 67 & 67N ______Breaker _____ remot s/s. B A R

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1 Answers   HAL,

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What is the formulae of Megger value for a A.C.Motor

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Send the control circuit digram of a star delta stater?

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Sir we have 12.5 MVA(110/11KV) transformer (Y-Y) connected,which tripped showing earth fault,when we charged transformer it stood OK ,but on connecting 11 KV feeders it tripped showing earth fault.our service engineers found that the B phase and R phase phase connecting to neutral broken from HV OLTC(on load tap changer),and R phase and Neutral is ok.Dissolved gas analysis shows that accetline content is high,tan delta value is 12.My dout is it is an internal fault of transformer,why REF or diffrential relay is not acted?

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why we are maintain phase to phase clearence

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wht is meant by sh6

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why any inductive load need free wheeling diode?

4 Answers   Essar,

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