Design of Microcontroller Based Static VAR Compensator

The change in system voltage and frequency with changing load creates problem in power system stability. For the sake of system stability and reliability, the reactive power compensation is essential for flexible AC transmission system (FACTS). The variation of reactive power causes impact on the generating units, lines, circuit breakers, transformers, relays and isolators. However the reactive power can be compensated from the generating unit by changing the excitation of the generator. But if the total reactive power is feed only by the generation unit, it will lower the maximum real power capacity of generator. Moreover the additional current flow associated with reactive power can cause increased losses and excessive voltage sags. Therefore the local VAR compensator which is comprised of shunt reactors are used in transmission and distribution system for reactive power compensation. The conventional electro-mechanical reactive power compensator is not suitable for its slow switching speed which cannot commensurate with the rapid changing system load. The electronic reactive power compensator uses fast switching devices like thyristor which provides faster switching option than that of the conventional relay. The static VAR compensator (SVC) is the combination of thyristor switched capacitor (TSC) and thyristor controlled reactor (TCR) which is widely used in HVAC (High Voltage AC) and HVDC (High Voltage DC) systems. The design of the microcontroller based SVC with harmonics suppressor is discussed in this paper. Harmonics are generated which may unbalanced the system due to rapid switching of the connected reactor with the live transmission line. Therefore the harmonics suppressor is used in the proposed design. For precise switching of the thyristor, microcontroller is used which adds an significant feature of this designed SVC.