Microprocessor-based static reactive power compensators for unbalanced loads

Abstract

A static reactive power (var) compensator (SVC), which consists of thyristor-controlled reactors (TCRs) and relay-controlled capacitors (RCCs), is designed and implemented in this paper to improve the power factor and to reduce the negative-sequence currents of an unbalanced power system. Each phase of the SVC can be independently controlled to provide different reactive power compensation. At each sampling instant, the loading conditions are measured by a three-phase voltage transducer and three single-phase active and reactive power transducers placed between any two lines at the load bus and sent to the SVC controller which is constructed on a microprocessor board. Each phase susceptance of the SVC can be obtained from a simple function of voltage and power signals. Then the SVC controller determines the number of capacitors needed and the firing angle of thyristors in each phase and sends the corresponding control signals to the relay circuits and the thyristor firing circuits. The method to determine the compensation susceptance is very suitable for on-line control and easier than all the methods declared in the literature. Test results under two unbalanced load conditions show that the SVC can improve the power factor close to unity, greatly reduce the negative-sequence currents, and maintain the load bus voltages at a balanced condition. The compensation method described in this paper can be easily adapted to other systems.