Assessment of Transformer Modeling Impact on Transient Recovery Voltage in Transformer Limited Faults

SUMMARY High-voltage circuit breakers directly connected to transformers without an appreciable capacitance must function to clear faults limited by these equipments. Although the current magnitude of such transformer-limited faults ( TLF) is usually only a fraction of the maximum interrupting capability of the circuit breaker, the transient recovery voltage (TRV) conditions are expected to be more severe than those associated with higher levels of fault currents. The assessment of this type of interruption has not been very well documented and the associated TRV parameters not fully covered by the current circuit breaker standards. As the frequency transformer response is the dominant feature of the TRV parameters for these cases, in this type of analysis, its modeling should be as accurate as possible in order to achieve reliable results. Unfortunately, detailed information about a frequency transformer model to be used in transient studies is not generally available. Some studies found in the literature assumes that the transformer TRV response is predominantly single frequency and so represents the transformer by a single frequency equivalent circuit that may lead to inaccurate results. Based on these considerations, this paper presents the results of a series of transformer-limited fault TRV studies. The goal is to compare the influence of different approaches in transformer modeling and their impact in the TRV parameters. The transformer models included conventional single frequency models and more complex ones based on system identification procedures from field measurements of transformer frequency response. All the studies considered an actual power substation belonging to a power utility in Brazil (CEMIG GT – Companhia Energetica de Minas Gerais). As the main conclusion it can be pointed out the knowledge of the actual transformer response may provide further details relating to the actual frequencies presented in the switching transient. Further studies considering distinct topologies are needed to develop general guidelines to support standard reviews regarding TLF.