Auto Transformer impedance 1

[mLp85]

Hi,

I am doing some fault studies on a grid with 66kV/33kV Auto transformers YNa0, solidly earthed. I have their positive sequence impedances, but not any info on zero sequence impedance or X/R ratio. Has anyone had experience with these vector groups and what typical zero seq impedances and X/R ration could be expected?

 

[prc]

1) Please confirm that there is no stabilizing tertiary winding. Zero sequence impedance is approximately 0.9 of positive sequence impedance.
2) Please inform transformer MVA and positive impedance (%) for furnishing X/R ratio.

 

[mLp85]

Thanks for the prompt response.

1) I can confirm there is no tertiary (delta) winding; and
2) MVA is 12.5 for ONAN operation.

 

[bacon4life]

@PRC is that an assumption specifically for autotransformers? I am asking because my distribution simulation software has a hard coded assumption that distribution substation transformers always have the zero sequence impedance as 0.9 of the positive sequence impedance. However, that assumption is a somewhat poor match for my fleet of delta-wye transformers where I have test reports that include the zero sequence impedance. I have delta-wye 115/12 kV transformers with ratios ranging from 0.75 to 1.25.

 

[davidbeach]

Where we don't have zero sequence test data for the 115/12 type transformers we assume R0 = R1 and X0 = 0.85*X1. The results are close enough to what relay events show that we haven't found a need to revisit that assumption.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[prc]

1) bacon4life- The ratio of zero sequence impedance to positive sequence impedance varies between 0.8-1.0 for three limbed core type transformers. Z0 varies for the 3-limbed and 5-limbed cores. It also varies depending on the side from where the circuit is excited. It will not vary between auto connection or two winding types.
2) Z0 for transformers with various connections and core construction are covered in Table 1 of IEC 60076-8 Transformer Application Guide. Zo for various special auto-connections, refer to B.A.Cogbill,
" A zero sequence equivalent circuit of auto connection which yields neutral shift"- AIEE December 1956