Simplified Cluster Balance Control of Cascaded H-Bridge STATCOM under Unbalanced Grid

The cluster balance control schemes that are reported so far for a cascaded H-bridge multilevel STATCOM under unbalanced grid voltages, using either zero-sequence voltage or negative sequence current or combination of the both, are complex and laborious. The schemes require a lot of calculation in every time step of algorithm implementation and large mathematical derivations and corresponding analysis in support. In this paper, a simple and generalized cluster balance control scheme is proposed for star connected cascaded H-bridge STATCOM without requiring any additional calculations. In the proposed scheme, dc voltages of three phase clusters are controlled independently which result in fast dynamic response. Further, the cluster balance control schemes that are reported in literature cannot ensure equal dc voltage balance among the three phase clusters, i.e., there exists steady-state error in the dc the voltages of the phase clusters. Therefore, to avoid unstable operation of the multilevel STATCOM, individual dc voltage control of the H-bridge inverters of the phase clusters is reported with Proportional (P) controllers, resulting in unequal dc voltage across the H-bridge inverters. The proposed cluster balance controller realizes zero steady-state error in the dc voltages of the three phase clusters. This made it possible to realize equal dc voltage balance across the H-bridge inverters by using PI controllers in the individual dc voltage control. The effectiveness of the proposed scheme is validated on a 9-level, 11 kV, 2 MVA STATCOM in Matlab Simulink.