Integrated scheme for VAR compensation and harmonic control using active filter

Abstract

The goal of this thesis is to obtain a simple and easily implement able scheme for VAR compensation and harmonic reduction in a power system. The basic idea of compensation in a power system having specified voltage is analysed. The requirement of a current source compensator is explained. A basic current source compensator is designed to meet the requirement. A current source capable of supplying reactive power is composed of an inverter supplied by a dc current source. For proper operation the inverter switches are provided short periods, termed the overlapping guard band, during which all the switches are closed in order to prevent the dc current from being interrupted. The desired compensation current is produced by pulse width modulation of a high frequency carrier by the extracted template. A lowpass filter is designed to stop the high frequency carrier but pass the compensation current of the inverter's output. A damping resistor is incorporated in the output filter. The dc current source is obtained by rectifying the supply voltage and connecting a dc inductor in series. A transformer is necessary for isolation and for stepping down the voltage to a suitable level. The transformer volume is reduced by designing it for operation at a high frequency. An additional line frequency rectifier and high frequency inverter are needed to supply the transformer. An analytical model of the compensator based on differential equations is provided. Results of compensation of various linear and nonlinear loads and combinations are given using the analytical model and two different circuit simulation softwares. Agreement of these results is evidence that the compensator is correctly designed both theoretically and practically. Comparison with a Voltage Source Compensator working under the same conditions shows that the Current Source Compensator performs at least as well. A practical design is achievable if the converter losses can be reduced. Reduction of converter losses by many methods is standard practice but only elimination of the intermediate high frequency stage has been done as an option. All previous schemes are complicated and not generalized and none of them will be simpler or more readily irnplementable than the scheme reported in this work.