The utility is providing a 1500 KVA 1

[VIKRAM DESAI]

The utility is providing a 1500 KVA 11KV delta to 415V Y pad mounted transformer. There are 3 generators each rated at 415V 300KW running parallel. can any one give me size of zigzag transformer with NGR to provide artificial neutral on 11KV side?
Thanks in advance

 

[7anoter4]

Sorry, VIKRAM DESAI, I don't understand why if it is an utility transformer you have to supply the grounding transformer. However, without capacitive current of 11 kV supply network you cannot calculate the NGR.
Let's say the capacitive current between a phase and ground is 30 A.
Then Ig=>3*Ico=90 A[100 A may be o.k.]
RNG=VLL/SQRT(3)/Ig NGR resistance
VLL=11000 V RNG=11000/SQRT(3)/100≈64 ohm
According to Post Glover the current and time rating of the Zigzag, when used with an NGR, should be the same as the NGR.
The transformer required power: 11*100=1100 kVA
NGR power required will be: RNG*Ig^2=640 kW.
You have to state the time rating according to your system-short-time, long time or steady-state.
It depends on insulation type, the public access possibility and other[fire protection,arc flash protection and other].
In order to calculate the rated power see IEEE 32.
For instance for 10 sec time rating the rating power is 3%
NGR rated power =3*640/100=19.2 kW and the transformer power 3*1100/100=33 kVA

 

[7anoter4]

See also:

https://www.postglover.com/download/high-resistance-grounding-application-guide/

 

[prc]

I think transformer rating will be 11/ root3x 100 kVA

 

[7anoter4]

Thank you, prc.You are right.S=3*Ico*VLL/sqrt(3) .My mistake.

 

[prc]

The post glover &I-gard guides are also showing the same mistake in the formulae given.