State space analysis and duty cycle control of a switched reactance based center-point-clamped reactive power compensator

A new approach for static reactive power compensation has been presented in this paper. Conventional thyristor controlled Static VAr Compensators (SVC) have inherent disadvantages like slow response times and poor harmonic performance as these FACTS devices are based on slow switching power electronics devices. Alternately, Pulse Width Modulated (PWM) dc-ac inverters and direct ac-ac converter structures offer higher bandwidth and push the spectral content to higher switching frequencies that are easier to filter. However, the application space of this approach is limited by the low voltage blocking capability of power devices employed in these converters. A center-point-clamped ac-ac direct power converter has been reported recently in literature which operates on the principle of neutral-point-clamped dc-ac inverter. By clamping the grid voltage to its mid-point, the center-point-clamped converter structure reduces voltage stress on the bi-directional switches by 50%. Compared to the conventional two-level and multilevel dc-ac inverters, the proposed compensator based on direct ac-ac conversion has a simpler structure and control. The operating principle as well as dynamic analysis for the proposed VAr compensation approach has been presented in the paper. A feedback controller has been designed for closed loop control. Simulation results presented in the paper verify that proposed converter offers better control of reactive power, retrofit capability, and reduced voltage stress on the bi-directional switches. Furthermore, it has been shown that leading and lagging reactive compensation can be accomplished with a smooth control of the reactance through duty cycle modulation.