Digital control method of DC voltage for active power filters with neutral-point clamped three-level inverter

Due to the limitation of voltage capability and current capability of power devices, it is very difficult to handle nonlinear loads for the traditional active power filter (APF) with two-level inverter in high voltage high power grid. Three-level NPC inverter has been put into practical use for years especially in high voltage high power grid. This paper researches the APF with three-level inverter and presents a close-loop digital control algorithm of DC voltage for the APF. In dq two-phase rotating coordinate, this paper divides the control system of DC voltage into the outer-loop voltage controller and the inner-loop current controller. In the outer-loop controller, the mean value of DC voltage is detected by the adaptive filter and regulated by the PI controller using active command current. In the current-loop controller, this paper researches the issues of neutral-point voltage control from the space vector PWM (SVPWM) point of view in detail. Then this paper proposes a simple neutral-point voltage control method that requires the knowledge of the current direction in each phase and the neutral-point voltage ripple's direction. Based on that information, the small vectors that will move the neutral-point voltage in the direction opposite from the direction of unbalance can be selected. The experiment results illustrate that the performance of the proposed approach is satisfactory.