Implementation of a Cascade D-STATCOM under Unbalanced Conditions

This paper presents a control strategy of star-connected cascade distribution static synchronous compensators (D-STATCOMs) operating under unbalanced conditions. By equivalent transformation, an accurate abc-frame mathematical model of the cascade D-STATCOM with unknown disturbances and parametric perturbation is proposed. Then a novel robust control with L-2-gain performance based on the combination with repetitive control and passivity-based control is used to realize the reference current tracking control, where repetitive control is used to compensate the periodic disturbances, and the passivity-based control is used to guarantee the convergence of repetitive control and asymptotic tracking of the control objectives, as well as compensate the influence of aperiodic disturbances on the control performance. The dc voltage control consists of three layers, in which the first layer is realized by injecting a positive-sequence active current, the second by injecting a zero-sequence voltage, and the third based on the active power exchange among H-bridge modules. The effectiveness and performance of this control strategy is verified by simulation and has been tested on a practical installation and operation of +/- 5 MVAR, 10-kV cascade D-STATCOM.