Aiming at the problem that the voltage fluctuation of single capacitor on DC side of the cascaded H-bridge static synchronous compensator（STATCOM）causes unbalance of the phase and the total voltage

a coordinated control strategy of three controllers was proposed. In this strategy

the DC-side single-capacitor voltage controller is of voltage loop

and the single-capacitor voltage is compared with the reference voltage for PI adjustment to achieve voltage balance control of the H-bridge unit capacitor. The controller for DC-side voltage and reactive current is of voltage and current double-loop control

and the square sum of the total capacitance voltage is compared with the reference voltage for PI adjustment to achieve balance control of the total capacitance voltage. Moreover

a simple method for obtaining the reactive reference current was proposed to effectively improve the reliability of the system. Simulation and experiment on the cascaded H-bridge seven-level STATCOM showed that the proposed control strategy could keep the DC-sidecapacitor voltage within the allowable range with good dynamic performance and robustness when the system is insteady state and the load changes suddenly

and could make the phases of the grid-side voltage and currentapproximately same. Ultimately

balance control of the DC-side capacitor voltage was achieved

which verifies the effectiveness of the proposed control strategy.