Short Circuit MVA values.

[raisinbran]

When the taps of a transformer are raised (or lowered), does the available short circuit MVA at the transformer secondary raise (or lower) or stay the same as calculated at nominal tap settings?
Raisinbran

 

[jghrist]

The short circuit MVA changes because the percent impedance changes, but not in a predictable manner. I have tried various equations to relate impedance with tap settings, but nothing matches test results very well for transformers where I have test results at different taps.

A couple test examples:

12 MVA 100-12.47 kV transformer:

105 kV tap - 8.84%
102.5 kV tap - 8.84%
100 kV tap - 8.87%
97.5 kV tap - 8.97%
95 kV tap - 9.12%

15 MVA 100-12.47 kV transformer:

105 kV tap - 9.18%
102.5 kV tap - 9.10%
100 kV tap - 9.08%
97.5 kV tap - 9.13%
95 kV tap - 9.22%

 

[ScottyUK]

It depends to some extent on the tapping winding design: the ones described by jghrist sound like five true taps on the winding. Other designs, usually types with large numbers of tap positions, use a separate tapping winding with multiple taps which can be connected to buck or boost the main winding. The minimum impedance is usually at the nominal tap when the tapping winding is effectively not in service. The relationship between tap position and impedance is non-linear.


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