Switching Surges in large power Industrial Motors

Large industrial motors are used for auxiliary operations in industrial establishments like cement plants, paper and pulp industries. For switching operations generally SF6 or medium voltage vacuum breakers are used. Depending upon the nature of duty, there are frequent switching-in and switching-off operations. Occasionally, there is likelihood of faults also in the motor winding due to insulation failure of winding coils. The connecting distances involved between the power supply point and the motors are generally of the order of kilometer due to the width and breadth of the above Industries. To meet the requirements of switching duties, the breakers are required to operate frequently. In most cases, the industries utilized medium voltage circuit breakers of vacuum or SF6 type at 6.6 KV and 11 KV class for switching duties. Switching operations cause steep fronted transients at the terminal of the breaker due to operation of vacuum or SF6 circuit breakers. These transients propagate along the length of cable and reach the motor terminal. Since, the steepness of surge is very high, a non-linear distribution occurs in the motor coil.