Stability analysis of grid-connected voltage-source converters with unbalanced loads

Since the future power system needs to provide sufficient and high electric power quality to various loads, it is envisaged that grid-connected converters, such as compensators for voltage control like STATCOM, are going to play a critical role in its operation. Voltage-source converters (VSCs) are among the most common topology used in grid-connected converters. The VSCs can be also used for connecting alternative renewable energy source to grid. Basically, a grid-connected VSC is a three-phase three-wire converter in which a capacitor is connected to its dc side, while its ac-side voltage is usually controlled based on a pulse-width modulation (PWM) technique. When operating under unbalanced voltages and currents at the point of common coupling (PCC), a voltage ripple appears at the dc side. However, the interaction between the VSC dc-side voltage ripple and the VSC ac-side currents has not yet been considered precisely. In this paper, we propose a model describing the interaction between the VSC dc-side voltage ripple and the VSC ac-side currents under unbalanced voltages. Furthermore, the proposed model is used to determine in which conditions the stability of the VSC ac-side currents can be guaranteed even in the presence of the dc-side voltage ripple.