Inherent transient recovery voltage of power transformers-a model-based determination procedure

Knowledge of the inherent transient recovery voltage (TRV) associated with power-transformer terminal faults is essential for correct application of circuit breakers. Instead of establishing standards for TRV parameters as a function of power transformer ratings, standardized model-based determination procedures are proposed. The dynamic properties of the transformer short-circuit impedance Z_x allow a straightforward calculation of the TRV waveshape without high computational effort. Test setups for lightning impulse (LI), frequency response analysis (FRA) and low-voltage current injection (LVCI) are presented and discussed. A sample calculation introduces the TRV calculation from LI test data and comparison with LVCI and FRA results. It is concluded that the presented procedure should be incorporated into the transformer type-test routines to provide highly valuable information about the TRV of each specific transformer.