Optimal Control Strategy of Back-to-Back Converter Based on AC/DC Voltage Source Converter

The back-to-back converter is a converter system composed of two voltage source converters (VSC). Because VSC can regulate the output of active and reactive power, the back-to-back converter has a good dynamic and static performance. However, not all passive loads are three-phase symmetrical. The existing control strategy may lead to asymmetric output voltage when back-to-back converter is used to supply unbalance load. Usually, an inner loop d/q decoupling controller, a constant DC voltage controller of the rectifier side, and a constant AC voltage controller of the inverter side are established. In this article, in order to improve the rectifier side, fuzzy online self-tuning is used to increase PI parameters on the outer voltage loop. PI control is replaced by PR control on the inner current loop. The improved rectifier control can reduce the overshoot and the output of harmonics. For the inverter side, the three-phase voltage will be imbalanced. Therefore, using PR control, which can track a sinusoidal signal without steady-state error, the control strategy of inverter is changed into "outer loop constant voltage and PR control-inner loop current d/q decoupling control." This strategy can solve the unbalance problem of three-phase AC output. The simulation results show that the control strategy can solve the problem of asymmetric output voltage of the inverter side effectively.