impedance of three winding transformer

[salem]

three winding transformer with following parameters,
- 132/11/11 kV (both the 11 kV windings are loaded).
- 120/60/60 MVA
- YN yn0 yn0
1- what is the recommended/practical Z% between HV-LV1 , HV-LV2 , and LV1-LV2 ? to limit the short circuit to 15 kA.
2- is there is a requirement for NER for neutral 11 kV to limit the earth fault current?
3- is there is a requirement for additional delta winding?

if any reference(s) for the above?.

 

[beyond86]

1- what is the recommended/practical Z% between HV-LV1 , HV-LV2 , and LV1-LV2 ? to limit the short circuit to 15 kA.

To limit short current to 15 kA for 120 MVA transformer, positive Z% should be not less than 41%. Actually, I have never seen transformers with such impedance. It would be a huge core.

2- is there is a requirement for NER for neutral 11 kV to limit the earth fault current?

If you are going to ground neutral point of the secondary side, you should use NER. In my country, networks 11 kV work with isolated neutral point.

3- is there is a requirement for additional delta winding?

I think, for large transformers, delta winding is must be. Delta winding helps to catch odd harmonics, which come from grid. Also, it helps to make voltage on secondary side more linear.

 

[prc]

1)Assumed that 15 kA refers to the LV side and LVs are not paralleled.

2)LV current for 60 MVA 11kV works out to approx 3 KA. So to limit SC current to 15KA, HV-LV1 impedance =HV-LV2 impedance = approx 20% on 60 MVA base. Then LV1-LV2 impedance will be about 38% on 60 MVA base. Such a transformer is technically feasible though impedance is a bit on the higher side. For high impedance transformers, the core will be less and copper will be more. Normal impedance for such rating is 12.5-15 %

3)NGR is required if only you want to limit LG fault current to lower values. (from arc flash requirements, I think)

4) If you are not using NGR and your systems are solidly grounded ones, you can avoid the stabilizing delta winding.

 

[prdpks2000]

request transformer Data Sheet and GA

 

[salem]

prdpks2000, actually information in data sheet need to be specified.