Three-Level Equilibrium Strategy of DC Voltage Balance Control for H-Bridge Cascaded Active Power Filter

The H-bridge cascaded active power filter (APF) has broad application prospect in the harmonic procession of high voltage field. Due to differences between H bridges, the imbalance of DC side voltages affects not only compensation effect, but also the safe and stable operation of APF. By derivation of the power exchange model between APF and the power grid, the voltage balance among phases is achieved by the five-order zero-sequence voltage, and the global voltage balance is achieved by the fundamental positive-sequence active current. By appending an active voltage vector in the AC side of each H-bridge, the voltage balance is achieved within the single phase. This three-level equilibrium strategy controls each DC side voltage at the given value eventually. At the same time, the cascaded APF compensates harmonic and reactive current and reduces THD of the grid current. The feasibility of the method is verified by simulation and experimental results under low voltage conditions and provides the basis for application of the APF to a higher voltage level.