Switching overvoltages in motor circuits

The switching of HV motor circuits in the megawatt range can cause large ov ervoltages on motor terminals. The amplitude of the resulting voltage spike s depends on breaker type and circuit characteristics. If the amplitude of switching overvoltages exceeds the basic insulation level of the motor, ove rvoltage protection circuitry should be installed to prevent a motor insula tion failure. The decision whether overvoltage protection circuitry (surge pack) must be installed requires a prediction of the overvoltage level. Thi s paper deals with switching overvoltages in motor circuits with a unit tra nsformer between switchgear and motor. Overvoltages of motor circuits were simulated using specially developed simulation models. The results of three case studies revealed a large impact of cabling and transformer on motor o vervoltages. In most of the cases, a unit transformer connected between swi tchgear and motor will provide an overvoltage protection to the motor. Howe ver, the transformer can also be a main cause of large overvoltages when co nnected to cables of specific length and capacitance. The paper pro-tides s ome simple rules to recognize such critical configurations.