Transformer Zero % impedance 1

[KnicksJets]

I know that the zero percent impedance of a transformer is important primarily for fault calculation and could effect reactive output. However, if a transformer spec doesn't specify this value, how do we obtain this? We have an old transformer (750 MVA, 24 KV to 230 KV GSU-Westinghouse) with minimal documentations and thus no zero sequence impedance value. Is it typically same as the normal percent impedance (Z%) of the transformer?

Also, say we are replacing a transformer with zero % impedance = 13% with another having only 12.5% zero % impedance, what are the changes required (short circuit calc might be affected, any effect to protection scheme etc?)

Would appreciate any insights.

 

[IndianTechEngg1001]

Please see the attached file from BS form which you can get the values of Z%.

If you want to replace transformer and your %Z also changing you will encounter change in short circuit level at secondary. likewise Protection will also be affected. Check if it can take the new SC burden.

 

[KnicksJets]

Appreciate the prompt response and confirming our concern. I have two follow up questions:

1. The change in SC fault shouldn't be too high, do you agree? The X/R between the existing and new are pretty different and we already expect the fault to be higher in the new. THe difference in zero % impedance is about 0.5% only.

2. When you say protection will be affected, I am assuming the breaker to clear the fault has to be reevaluated for the increased SC. Correct? Should we be concerned about our relay protection too? A thought is that reactive output being affected might lead to other design confiuration changes.

Thanks for the response and the attachment.

 

[IndianTechEngg1001]

I forgot your mail point

It is easy to calculate the maximum current that a transformer can deliver under symmetrical fault conditions. By way of example, consider a 2 MVA transformer with an impedance of 5%. The maximum fault level available on the secondary side is:

2 MVA x 100/5 = 40 MVA

and from this figure the equivalent primary and secondary fault currents can be calculated.

The calculation above deals with a balanced 3-phase fault. Non-symmetrical faults (phase-earth, phase-phase etc) lead to more complex calculations requiring the application symmetrical component theory. This in turn involves the use of positive, negative and zero sequence impedances (Z1, Z2 and Z0 respectively).

The zero sequence impedance is dependent upon the path available for the flow of zero sequence current and the balancing ampere turns available within the transformer. Generally, zero sequence current requires a delta winding, or a star connection with the star point earthed. Any impedance in the connection between the star point and earth increases the overall zero sequence impedance. This has the effect of reducing the zero sequence current and is a feature that is frequently put to practical use in a distribution network to control the magnitude of current that will flow under earth fault conditions.

The positive and negative sequence impedances (Z1 and Z2) of a transformer are identical. I actually do not know where and how they are used in calculations. If someone can help on these I will be glad

 

[Marmite]

The magnetic field produced by zero sequence currents is radically different than those produced by positive or negative sequence currents, and therefore the zero sequence impedance is likely to be very different to the normal percentage impedance. When zero sequence currents flow in one winding without corresponding ampere turns balance in another winding the impedance is very high.
You could possibly test the transformer to find out the zero sequence impedance.
Regards
Marmite

 

[prc]

pdshah's question was what is the zero sequence impedance of a 750 MVA GSU.
Positive sequence impedance can be found out from the rating plate on the transformer.Zero sequence impedance of this unit will be same as positive sequence impedance as this unit must be a shell type transformer with LV in delta.