Sizing unit transformer 1

[magneticted]

Looking at a 6 Mw 11 Kv synchronous machine efffy 97% PF to be 0.9 leading to 0.9 lagging for continous operation,and require a 33kV / 11 kV power transformer. KVA rating of transformer, would this be based on KVA rating of motor

 

[edison123]

I would say 7.2 to 7.5 MVA trafo. You need to add the auxiliary load requirements, if any.

* Women are like the police. They can have all the evidence in the world and yet they still want a confession - Chris Rock *

 

[burnt2x]

magneticted,

Agree with edison123.

Trafo efficiency should be included in the computations (95% - 91% used by edison123) to arrive at 7.2 - 7.5 MVA!

Thanks edison123.

 

[ScottyUK]

Transformer efficiency? Your estimate (91% - 95%) seems very low. Are you forgetting that the transformer has to provide the reactive load for this motor too?


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If we learn from our mistakes I'm getting a great education!

 

[lansford]

Two other considerations are:
Bracing of transformer windings due to starting inrush
Voltage at motor at starting so motor accelerates
May require a larger transformer than what is dictacted due to thermal loading
JIM

 

[motorspert]

I think I agree with Scotty, the transformer losses wouldnt be as high as 5%.

I would specify 7.5MVA though, seems a good size. But tell the Tx manufacturer what the starting regime is, starting current, duration of start and frequency of starts etc.

If the power factor is really going to wander about from 0.9lag to 0.9lead then regulation needs to be considered as the volts will creep up when the motor is running at leading power factor, are you fitting an on-line tap changer?