Protection of utility motors during switching

Utility motors are the high voltage motors used in thermal power stations as auxiliaries, fans, drive water pumps and other auxiliary items. The steep fronted surge voltages such as switching over-voltages mainly originate at the terminals of circuit-breaker during opening as well as closing operations. Failures of many utility motors have been blamed on steep fronted surge voltage. The increasing use of circuit-breakers requires careful protection of high voltage motors from switching surge voltage caused by prestrike and restrike of the breaker contacts. The sloping or limiting of the wave-front provides the protection for turn to turn insulation by distributing the switching over voltage across many turns adjacent to the line end. It is possible by using the suitable protective devices such as capacitance, resistance-capacitance and MO varistor or combination of these devices. Impulse voltage of 225 kV, 0.1/50 μs is generated and fed to the distributed winding of 6.6 kV motor with protective devices and measured the line end coil and turn to turn voltages. This paper presents a computer simulation study of switching and impulse voltage using MATLAB software package, computer simulation is carried out for comparison and to decide the best device with its location for the protection of high voltage motor.