ANALYSIS AND ELIMINATION OF CAPACITORS SWITCHING TRANSIENT OVERVOLTAGES IN POWER FACTOR IMPROVEMENT OF INDUCTION MOTORS

Abstract

Induction motors are being used in more applications than ever before due to their versatility, dependability, ease of control and economy. The wide use of these motors has created a burden on the sub-transmission and distribution network. Some of the known problems are voltage dips, high starting current, and a low lagging power factor especially at light loads. Shunt capacitor banks are generally used to improve their low lagging power factor. Improvement of power factor can reduce power costs, release electrical capacity of the distribution system, raise the voltage level, and reduce the system losses. The switching of shunt capacitor banks at utility substations and on distribution feeders creates voltage and current transients in the power system which may be damaging to power system equipment. Transient overvoltages due to the energizing of capacitor banks are the most common source of overvoltages on many power systems. the transient occurs because of the difference between the system voltage and the voltage on the capacitor. A basic characteristic of capacitors is that the voltage across them cannot change instantaneously. If a capacitor is at zero voltage and system voltage is applied to it, the system voltage will be pulled down to nearly zero momentarily. There will then be a capacitor inrush current as the capacitor charges. The voltage on the capacitor will then recovers and overshoot the system voltage by roughly the same amount that it dropped, and then oscillate around the fundamental system voltage. The capacitor voltage will continue to oscillate with the oscillation gradually getting damped out, usually within a cycle depending on the system resistance. The magnitude of the transient and its characteristic oscillation frequency will depend on the characteristics of the electric power system. There are some methods of reducing capacitor switching transients such as pre-insertion resisters, fixed inductor, pre-insertion inductor and zero voltage (synchronous) closing control.

This thesis studies the effects of the transient overvoltages and currents that occur during switching shunt capacitor banks to improve the induction motors power factor, on the performance of the motors. Also the thesis presents a proof, discussion and implementation of the zero voltage (synchronous) closing control technique using phototriac optocouplers. The thesis includes simulation and experimental results for the mentioned technique. Good agreement between the simulation and experimental results has been found. The thesis proves that random switching of power factor improvement capacitor banks results in transient overvoltages and very high inrush currents on the induction motor, these transient overvoltages cause very high overshoot or undershoot into the motor torque, which may result in mechanical damages in the motor shaft. Also it proves the validity and effectiveness of the zero voltage closing technique in eliminating the capacitor switching transients through many study cases at different operating conditions of the motor.