Switching overvoltages in motor circuits for petrochemical applications

The switching of HV motor circuits in the MW range can cause large overvoltages on motor terminals. The amplitude of the resulting voltage spikes depends on breaker and circuit characteristics. If the amplitude of switching overvoltages exceeds the BIL (basic insulation level) of the motor, overvoltage protection circuitry should be installed to prevent a motor insulation failure. The decision whether overvoltage protection circuitry (surge pack) must be installed requires a prediction of the overvoltage level. This paper deals with the simulation of switching overvoltages on large motors in petrochemical applications. Motor circuits with a unit transformer between switchgear and motor have been investigated to reveal circuit configurations with a potential risk of unacceptable overvoltages. The simulations have revealed a large impact of cabling and transformer data on motor overvoltages. In most of the cases a unit transformer connected between breaker board and motor will provide an overvoltage protection to the motor. But the transformer can be also a main cause of large overvoltages when connected to cables with a specific length and capacitance. The paper provides some simple rules to recognize such critical circuit configurations