A comparison between STATCOMs using PWM voltage control and hysteresis current control (HCC)

The Voltage Source Inverter (VSI) has now become the fundamental building block component in Flexible AC Transmission Systems (FACTS). Among the various applications of a VSI, the Static Synchronous Compensator (STATCOM) is the most popular one. It injects a set of three-phase balanced sinusoidal currents with controllable magnitude and phase angle, into the transmission line to regulate the transmission line voltage or to compensate for the reactive power. Under the condition that no external source/sink is available on the DC side, it should also absorb a small amount of active power from the transmission line to maintain the DC bus voltage constant and compensate for any real power losses within the VSI. In this process, the control method of the VSI is one of the key factors to influence the performance of the STATCOM. This thesis investigates the effect of two modulation schemes on the performance of a STATCOM. The first is the Pulse Width Modulation (PWM) voltage control and the second Hysteresis Current Control (HCC). The performance of the two schemes under steady-state and transient conditions are assessed by means of simulations using EMTPRV. The results presented in the thesis justify the fact that PWM voltage control has become the de-facto modulation technique for STATCOM applications despite well known limitation of generation of low voltage harmonics with large magnitudes when operating at low switching frequencies