Novel negative sequence current detection and control strategy for H-bridge three-level active power filter

The H-bridge three-level inverter is a potential topology, which can improve the quality of the output voltage waveforms and decrease the requirement of DC-link voltage. However, its control method is much more complex than the traditional two-level inverter due to the more DC-link numbers. In this paper the DC-link voltage stable conditions and the effects of fundamental sequence components to the inverter input power are described in details. Then a DC-link voltage control strategy is proposed. The DC-link voltage controller is composed of two parts, the first one is to maintain the DC-link average voltage unchanged, and the second is to achieve the DC-link voltage of three phase by controlling the output negative sequence current. To eliminate the ill effect of the load fundamental negative sequence current, a novel harmonic detection algorithm implemented in dual-dq axis is proposed. Simulation and experimental results demonstrate that the proposed control strategy behaves a good stability of DC-link voltage, even if the load current is unbalanced.