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. Unfortunately, the traditional transformer models available in Electromagnetic transient simulations program (EMTP-like) software packages are not capable of representing transformer behavior in a transient state, which includes high frequencies. Moreover, high frequency transformer models are often too complex or require confidential information on transformer geometry. However, in the design stage of insulation coordination it is particularly important to accurately calculate transmitted overvoltages through transformers. In the scope of this paper 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. Furthermore, the models’ capabilities to characterize a transformer at high frequencies are analyzed. 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.