This paper presents a novel inter-cluster direct current (DC) capacitor voltage balancing control scheme for the single-star configured modular multilevel cascaded converter (MMCC)-based static synchronous compensator (STATCOM) 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 phase limbs of converter. This leads to the imbalance of DC voltages of the sub-module capacitors across the MMCC phases, and consequently, the malfunction of converter. The proposed solution is to inject both negative-sequence current (NSC) and zero-sequence voltage (ZSV) into the phase limbs of MMCC. A quantification factor Q_F is used to achieve the sharing of inter-cluster active power between the NSC and ZSV injection methods. Accurate determination of the quantification factor has been presented. In addition to maintaining the DC voltages of sub-module capacitor across the MMCC phases balanced, it also prevents the overcurrent and overvoltage of converter by injecting NSC and ZSV with the right proportion. The control scheme is validated on a 3.54 kV 1.2 MVA power system using MMCC-based STATCOM with 3-level bridge cells as sub-modules. The results show that the proposed scheme provides superior effectiveness in eliminating the voltage imbalance of DC capacitor in the phase limb while maintaining low voltage and current ratings.