A practical test circuit for combined high direct voltage and switching impulse voltage multi-stress tests

Factory acceptance tests performed on High Voltage Direct Current (HVDC) transmission apparatus usually involve the application of high direct voltage and switching impulse voltage in separate withstand tests used to qualify the insulation system. However, the actual stress subjected to the equipment insulation system while in-service on the HVDC transmission system, is a switching surge transient waveform super-imposed on the operating direct voltage bias. In this paper, a methodology for integrating two separate commercial test systems in a practical test circuit for combined direct voltage and switching impulse voltage tests is presented. A circuit using an impulse generator and dc test system were used to generate 500kV switching impulse waveform superimposed on 500 kV dc bias at both positive and negative polarity. The selection of circuit's components was based on results from computer simulation as well as intermediate tests on portions of the overall test circuit. Measurement of the combined voltage waveform was verified using a sphere gap method.