Improved Autotransformer Model for Transient Recovery Voltage (TRV) Studies

A three-phase ungrounded fault supplied by a single autotransformer produces a transient recovery voltage (TRV) that is often the worst case for a circuit breaker (CB). This is the so-called “transformer limited fault.” The transformer model is extremely important for computing this TRV because it is the key element in the circuit. Unfortunately, data that are needed for more sophisticated autotransformers models (e.g. frequency-response analysis (FRA) data) are not readily available to the study engineer. Consequently, time-domain simulations are generally based on simplified second order (RLC) transformer models whose parameters are estimated from curves provided in IEEE Standard C37.011. This paper provides a more robust autotransformer model that is based on measurement data that are more readily available. The model is validated using FRA data from factory tests. TRV simulation results are compared with those produced by simplified second-order models. In addition, capacitance measurement data for twenty-two 230:115-kV autotransformers are provided to expand the body of capacitance data available to the industry.