Inter-Cluster Voltage Balancing Control of Modular Multilevel Cascaded Converter under Unbalanced Grid Voltage

This paper presents a new inter-cluster DC capacitor voltage balancing scheme for the single-star configured modular multilevel cascaded converter (MMCC)-based static synchronous compensator (STATCOM) operating under unbalanced grid voltage. The negative sequence component of grid voltage at the point of common connection (PCC) causes unbalanced active power flow in the converter phase limbs. This causes MMCC phase limb DC capacitor voltage imbalance and consequent converter malfunction. The solution proposed is to inject both negative sequence current (NSC) and zero sequence voltage (ZSV) into the MMCC phase limbs. A quantification factor Q f is used to achieve this sharing of the inter-cluster active powers between the NSC and ZSV injection methods. Accurate determination of the quantification factor has been presented. In addition to the phase limb DC capacitor voltages been balanced, it also prevents converter overcurrent and overvoltage by injecting NSC and ZSV in the right proportion. The control scheme is validated on a 3.54 kV 1.2 MVA grid system using MMCC-STATCOM having 3-level bridge cells as sub-modules. The results show that the proposed method provides superior effectiveness in eliminating phase limb DC capacitor voltage imbalance while maintaining low voltage and current ratings.