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This paper presents a three-level three-phase diode clamped inverter that uses an auxiliary circuit to equalize its DC-bus capacitor voltages. The proposed converter is used in a static compensator (STATCOM) and a controller is designed for this new STATCOM. A state space model of this compensator connected to a grid is obtained first. Three single input single output (SISO) robust controllers are designed to control the StatCom terminal voltage, to regulate the DC-bus voltage and to equalize the capacitor voltages. The effectiveness of the control strategy is guaranteed by the fact that each of the SISO controllers is provided with appropriate nonlinear terms that compensate the interactions between the STATCOM sub-modules. The STATCOM performance is evaluated. The results demonstrate that it is capable to maintain its terminal voltage and its DC-bus voltage at their reference values while equalizing the capacitor voltages despite significant variations of grid parameters.