Distribution of switching surges in the line-end coils of cable-connected motors

When specifying the winding insulation of electrical machines it is necessary to know the electrical stresses to which the winding will be subjected. In the case of interturn insulation the most severe stresses are caused by steep-fronted voltage waves produced by circuit-breaker closure. To some extent such steepfronted waves are reduced in severity by the time they reach the machine terminals by distortion and attenuation in the cable connecting the machine to the circuit breaker. In the studies described the interturn voltages in the line-end coils of cable-connected machines are evaluated. The equivalent circuits for the machine winding, the cable, and the energising source are obtained and compounded into an equivalent circuit for the complete system. The interturn voltages in the line-end coils of the machine are obtained from solutions of this equivalent circuit. Studies are made with a number of different cable lengths and types, and for a number of energisation source representations. Single-pole and 3-pole simultaneous closure of the circuit breaker are considered. The use of series inductors at the circuit breaker, or shunt capacitors at the motor terminals, for reducing the severity of the interturn voltages is also investigated. The results obtained indicate that the type of cable insulation and the length of cable are both significant in influencing the magnitude of interturn voltages. Energisation source impedance has a large effect. Comparison of seriesinductance and shunt-capacitance methods of surge protection reveals that both methods are effective, but that both produce other unwanted effects.