Case study on transformer models for calculation of high frequency transmitted overvoltages

Events such as lightning, switching of vacuum circuit breaker or switching operations in gas insulated substation (GIS) generate high frequency overvoltages. An equipment in a transmission or a distribution system has to be protected against such phenomena. In the design stage of insulation coordination, which is usually based on electromagnetic transient simulations, the overvoltages, which are transmitted through transformers, should be accurately calculated in order to design an adequate protection of the system components. Since those overvoltages include high frequency components, the traditional, low frequency transformer models cannot be used for accurate calculation of transmitted overvoltages. Therefore, it is particularly important to have a proper transformer model, accurate also for representing the high frequency transformer behavior. Two different transformer models for high frequency, are developed in an EMTP-type software program. The first model named “Black box” derives solely from the values measured on the transformer terminals and does not require any knowledge of the transformer inner geometry. The second model named “Grey box”, is based on a lumped RLC parameters network, whose values are derived from the simple geometry of the transformer window and from the nameplate data. In this paper, we propose to analyze the capabilities of a “black box” model and a “grey box” model to characterize a transformer at high frequencies. The case study is done on a distribution transformer which is to be located inside a power plant. The transmitted overvoltages calculated with the models in the EMTP-type software program are compared with measurements.