MICROPROCESSOR-BASED STATIC REACTIVE POWER COMPENSATORS FOR UNBALANCED LOADS

A static reactive power (var) compensator (SVC), which consists of thy ristor-controlled reactors (TCRs) and relay-controled capacitors (RCCs ), is designed and implemented in this paper to improve the power fact or and to reduce the negative-sequence currents of an unbalanced power system. Each phase of the SVC can be independently controlled to prov ide different reactive power compensation. At each sampling instant, t he loading conditions are measured by a three-phase voltage transducer and three single-phase active and reactive power transducers placed b etween any two lines at the load bus and sent to the SVC controller wh ich is constructed on a microprocessor board. Each phase susceptance o f the SVC can be obtained from a simple function of voltage and power signals. Then the SVC controller determines the number of capacitors n eeded and the firing angle of thyristors in each phase and sends the c orresponding control signals to the relay circuits and the thyristor f iring circuits. The method to determine the compensation susceptance i s very suitable for on-line control and easier than all the methods de clared in the literature. Test results under two unbalanced load condi tions show that the SVC can improve the power factor close to unity, g reatly reduce the negative-sequence currents, and maintain the load bu s voltages at a balanced condition. The compensation method described in this paper can be easily adapted to other systems.