Precise Modeling of Chopped Lightning Impulse Test of Power Transformers

Abstract

Performing insulation tests on power transformers is one of the most essential topics in high-voltage laboratories. According to international standards like IEC 60,076, full and chopped wave Lightning Impulse (LI) voltages must be applied to high voltage transformers. Since the wave shape parameters of the applied impulses must comply with the constraints that are specified in these standards, the setting of the impulse generator and test circuit must be modified correctly by test engineers. An accurate computerized simulation method is very desirable to help them accomplish this duty quickly and enhance the outcome results. Therefore, this paper proposes a method that is capable of simulating both Full Wave (FW) and Chopped Wave (CW) impulses in the time domain accurately. In this method, the frequency response of the transformer is measured and then the Vector Fitting (VF) technique is used to obtain its admittance model. Additionally, the transformer's impulse test circuit elements are precisely modelled. Afterwards, by solving the equations of the equivalent circuit in the state-space system, the time domain signals of all state variables are calculated. Finally, the output impulse voltages are computed. Several practical tests are accomplished on two power transformers to ensure the correctness of the proposed method.