A Simple Zero-Sequence-Voltage-Based Cluster Voltage Balancing Control and the Negative Sequence Current Compensation Region Identification for Star-Connected Cascaded H-Bridge STATCOM

In some application circumstances, both positive and negative sequence reactive power compensations are desired from the star-connected cascaded H-bridge static synchronous compensator (CHB STATCOM). However, the imbalanced phase currents/voltages will cause uneven active power flow among three clusters, which makes the cluster voltage balance more challenging. In this scenario, only zero sequence voltage is available to balance the cluster voltages. Although the zero sequence voltage can be calculated in all operating conditions based on the equilibrium of the active power for each phase cluster, the calculation is definitely complicated due to the zero sequence voltage expressed in the form of trigonometric function in the abc or dq0 frame. In this paper, the zero sequence voltage is represented in a dc form in the dq frame through introducing a virtually orthogonal zero sequence voltage. As a result, a linear relationship between the active powers and the zero sequence voltage is established, based on which, a simple zero sequence voltage calculation method is proposed with a small calculation burden. Furthermore, the accurate compensation region for the negative sequence current injection is identified by establishing the relationship between the maximum converter voltage and the negative sequence currents taking into consideration the initial phase angle of the negative sequence currents, while still keeping the cluster voltage balanced by using the zero sequence voltage control. The effectiveness of the proposed control and the compensation region are verified by the simulation and experimental results on a 400 V/±7.5 kVAr star-connected CHB STATCOM.