Impedance of typical transformers

[LMAO]

I was just wondering what impedance of a typical 12.4k to 600V, 7.5MVA transformer is? Is 5% a good estimate?

 

[jim dungar]

It depends on what you are doing with the results. Do you want to estimate high or low?

For transformers above 300kVA the normal range is 5% to 5.75%Z, depending on the manufacturer. Then there is the +/- 7.5% manufacturing tolerance on top of these normal values.

 

[rcwilson]

5%, 7.5 MVA at 600V is about 144 kA short circuit with an infinite bus at 12.47 kV. It would take a relatively small utility system with fault MVA = about 335 MVA to even get down to 100 kA. At 5.75% short circuit currents are still 125 kA.

That's why common impedances I have seen in this size range are 6-6.5%. We sometimes pay extra to get as high as 7.5% on 10-15 MVA units to control short circuit levels.

7.5 MVA is about 7200 amps at 600V. You have an 8,000 amp breaker?

 

[LMAO]

It depends on what you are doing with the results. Do you want to estimate high or low?

For transformers above 300kVA the normal range is 5% to 5.75%Z, depending on the manufacturer. Then there is the +/- 7.5% manufacturing tolerance on top of these normal values.

Did you mean 0.75% tolerance?

 

[kingpb]

Acceptable IEEE standard for manufacturing is +/- 7.5% on the impedance, unless specified otherwise.

Meaning the impedance you select, or if none is specified, then the value after testing will be within that range, and you can't say they didn't meet spec.

For a 7.5MVA transformer with a HV side of 13.8kV and below and a LV side of 2.4kV and below, is going to be around 6.75% unless you specify something else; with an X/R ratio of around 14.

The manufacturing tolerance is used in studies where the negative value would be for SC, and the positive value would be for LF, MS, stability, harmonics, and power flow.

 

[LMAO]

Acceptable IEEE standard for manufacturing is +/- 7.5% on the impedance, unless specified otherwise.

Meaning the impedance you select, or if none is specified, then the value after testing will be within that range, and you can't say they didn't meet spec.

For a 7.5MVA transformer with a HV side of 13.8kV and below and a LV side of 2.4kV and below, is going to be around 6.75% unless you specify something else; with an X/R ratio of around 14.

The manufacturing tolerance is used in studies where the negative value would be for SC, and the positive value would be for LF, MS, stability, harmonics, and power flow.

OK, I thought the impedance was 6.75% ? 7.5%! That's why I was getting confused.
So the impedance is 6.75% ? (6.75 ? 0.075)% = 6.75% ? 0.50625%. Correct?

 

[jim dungar]

So the impedance is 6.75% ? (6.75 ? 0.075)% = 6.75% ? 0.50625%. Correct?

Yep,