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A three-phase unbalanced system will reduce mechanical and electrical energy interaction and thus, to some extent, inhibit subsynchronous resonance (SSR). Based on this principle, a compensation method using a single-phase static synchronous series compensator (SSSC) is proposed. In this method, only one phase employs an SSSC to replace part of the fixed series capacitor, while the other two phases have fixed series compensation (FSC). By controlling the SSSC, the three-phase reactance at power frequency is balanced, while at sub-synchronous frequencies, the system is unbalanced and thus inhibits SSO. A single-phase SSSC model based on a cascaded H bridge topology is built for time-domain simulations. The damping effect and the minimum degree of compensation required from the SSSC are analyzed by transient simulations. Simulation results show the method can effectively restrain SSO and has little effect on the three-phase voltage unbalance factor. Compared with employing SSSC in all three phases, this method is more economical.