Determine positive sequence Zpt, Zst from transformer test report (with buried delta tertiary)

[raar]

This is my first experience with a Wye-Gnd to Wye-Gnd transformer with a internal grounded delta tertiary winding (no external terminals available for connection). I am trying to model this transformer for short circuit, load flow, downstream arc flash and it seems most modeling software require Zps, Zpt, Zst for positive and zero sequence along with the related X/R's. The test report clearly provides the T-model impedance's for the zero sequence circuit and so I am able to calculate the zero sequence Zps, Zpt, and Zst and X/R for each. The report also provides the positive sequence Zps and X/R, but I am not sure how to determine the positive sequence Zpt and Zst and X/R from the test data.

The report references IEEE C57.12.90-2015, which I have a copy of, but in section 9.3.3.2 for Three-winding transformers the test report does not clearly define Z12, Z23, Z31, nor Z1, Z2, Z3, therefore I am not exactly sure how to get started with determining the other impedance's. Could someone please provide some general direction or a clue on how to proceed with the determining the positive sequence Zpt, Zst, and related X/R's?

 

[prc]

Approximate positive sequence impedance between circuits for such a connection will be 1.18 times the zero sequence impedance.

 

[raar]

Hi prc, thank you for the response, that is interesting. I have access to study where a similar transformer is modeled, but the positive to zero sequences are:
|Zps1| ~= 1.007 * |Zps0|
|Zpt1| ~= 1.312 * |Zpt0|
|Zst1| ~= 0.141 * |Zst0|

The above relationships seem generally consistent with your suggestion except the Zst which is quite different. I don't know the tertiary MVA rating, the report just says "Buried".

 

[Kiribanda]

Your transformer is Ynynd0. Normally with such buried delta winding transformers,
High side winding MVA = Low side winding MVA + Buried delta MVA rating
for all cooling classes.
So knowing the MVA rating of high side & low sides, you can easily calculate the MVA rating of the buried delta winding.
Also is it possible to upload a copy of the test report please?

 

[bacon4life]

Kiribanda- The Ynynd0 transformers I am familiar with have equal MVA ratings on the high and low side, with the tertiary winding being 10% to 30% of that rating. Maybe this varies by region.

 

[prc]

bacon4life- you are right. Buried tertiary will have 1/3 MVA of HV or LV winding. In the case of auto-transformers, this may go down as mentioned by you.

 

[raar]

Per the report the phase relationship is YNyn0+d1. As bacon4life mentioned the report lists identical ratings for the high voltage and low voltage connections. Unfortunately I do not own the report, it is not mine to share so I shouldn't upload a copy, and understand that makes things more difficult.

One thought that occurred when speaking with colleague is perhaps those positive sequence impedances (Zpt1, Zst1) just don't matter? I believe I have the zero sequence reasonably correct because the T-model was provided and were used to derive the Zps0, Zpt0, and Zst0.