Buried Tertiary Rating & Impedance 1

[dpc]

We are trying to calculate some fault currents for a substation with a three-winding transformer that includes a buried tertiary. There is no data on the nameplate or in the test report regarding the impedances related to the tertiary winding or its kVA rating. I presume this is not required since the winding has no leads brought out.

Is there some reasonable approximation that can be made as to the impedance between the tertiary (4160 V) and primary (115 kV) winding?

 

[davidbeach]

Does Conrad St. Pierre's book provide any guidance?

 

[prc]

dpc, if you give kVA ratings and impedance between primary and secondary, I can make reasonable guesstimate.

 

[dpc]

David - I didn't think about that! - I'll check.

prc - MVA rating is 15/20/25, High side is 105 kV. Low side is 34.5 kV Impedance H-X is 8.5%

Thanks,

dpc

 

[prc]

dpc, Believe 8.5 % is on 15 MVA base.So HV-TV can be 12.5 %, LV-TV =3.5 % all on 15 MVA base.This is assuming tertiary is the innermost winding,the normal practice.

 

[dpc]

prc,

Yes, it's an ANSI rated unit - 8.5% @ 15 MVA. Thank you very much for your assistance. I appreciate it.

Best Regards,

Dave

 

[RAgrawal]

Hi PRC,

Can you please share, how did you calculate the various impedance?

 

[prc]

As I mentioned in my posting, I did not estimate but guesstimate. I know that in this range of ratings, normally HV-TV=150% of HV-LV and LV-TV =40% of HV-LV

 

[dpc]

prc - your guesstimate seems to be right on! Just to close the loop on this - we finally did receive an estimate from the transformer manufacturer:

"The estimated positive sequence impedances at 15MVA base are

LV-TV = 3.63%

HV-TV = 13.60%

The estimated zero sequence impedances at 15MVA base are

LV-TV = 3.42%

HV-TV = 12.27%"

 

[prc]

Thank you dpc for the information