A unified dynamic model and control for the voltage-sourced converter under unbalanced grid conditions

This paper introduces and develops i) a novel unified dynamic model, and ii) new control schemes for a grid-connected static energy conversion system that is based on the voltage-sourced converter (VSC) configuration. The model and the controls are applicable to both the conventional two-level and the three-level neutral point diode clamped (NPC) VSCs. The model is unified since it represents the dynamic behavior of the VSC with respect to both positive- and negative-sequence components, due to unbalanced grid conditions and disturbances. To develop the unified model, the concepts of 'generalized sinusoidal pulse-width modulation (SPWM)' and 'VSC sequence subsystems' are presented. Based on the negative-sequence subsystem control, a new control strategy, to either balance the ac line currents or to mitigate dc-bus voltage ripples, is introduced. Application of the developed model and the controls to a variable-speed wind-power unit is presented in the paper. Index Terms--Instantaneous symmetrical components, neutral point diode clamped (NPC) converter, sequence subsystems, unbalanced operation, voltage-sourced converter (VSC), wind power.