Power factor improvement of varying lagging load by PWM switched single capacitor

Abstract

In this thesis, an imp0l1ant overvIew has been gIven about the need for power factor improvement, the history of power factor correction by capacitors and the rcview of static power factor con.ection methods. A new method is proposed which improves the power factor automatically of varying lagging loads to unity using one single large shunt capacitor instead of using a bank of switching capacitors. Basically, this control scheme is a static power factor improvement method by continuous voltage or current control of a capacitor. In this work, the voltage across the capacitor is being changed by a bi-directional switch to control the magnitude of compensating capacitor current and thereby attaining unity power factor. The switching device is turned on-offby a pulse width modulated signal having moderately high frequency (several times of source frequency) to produce a current which is equal to the reactive component of load current in magnitude but is directcd in opposite direction. As a result line current will be in phase with source voltage. The control scheme is involved in detecting the power factor angle of the load and also the magnitude of the load current and then making the capacitor voltage is changed automatically according to the var compensation of the load to improve line power factor.. The scheme is simple in the sense that it uses only one static bi-directional switch controlled by an electronic control circuit. The electronic control circuit is also simple that uses only analog ICs and some discrete digital components.