A DC Link Capacitor Voltages Control Strategy for a PWM Cascade STATCOM

This paper proposes a control strategy which allows fast and independent regulation of the multiple DC link voltages in a cascade PWM STATCOM. This is accomplished by allowing each H-bridge to exchange real power due to voltage and current components other than those at the line frequency. Each H-bridge is set to output a voltage which contains a set of orthogonal voltage components, such as harmonics or sub-harmonics of the line voltage, and to draw a current which, in principle, contains the same components as the voltage so that this may result in an exchange of real power, hence in a charge or discharge of the DC capacitor. This provides extra degree of freedoms when compared to methods controlling only the fundamental components, typically used in STATCOMs. The current component demands are set by a multiple DC voltage regulator system designed as a MIMO system. A proportional sinusoidal controller is used to keep a close track of the demanded current. The concepts underlying the developing of the control strategy are explained in detail and a criterion for the selection of the parameters of the various controllers involved is also given. Experimental results from a low power prototype demonstrate the effectiveness of the proposed strategy to control the reactive current, the overall DC link voltage and to maintain the balance of capacitor voltages