Through-fault capability requirements for unit auxiliary transformers

The magnitude and duration of short circuit currents for faults on the low sides of unit auxiliary transformers (UATS) are calculated taking into account the generators´ initial loadings, time constants, and saturation. The stresses imposed on UATS by these currents are compared with the short circuit withstand capabilities provided by present standards. In many cases it is found that the standard capabilities are not adequate for the short circuit duties encountered in UAT applications. A method is presented to specify the short circuit withstand capabilities when the standards do not apply.