An Improved DC-Link Voltage Control Strategy of SAPF under Unbalanced Loads

The shunt active power filter (SAPF) based on three-leg voltage source converter is widely used to compensate the harmonic currents caused by nonlinear loads of grid. However, when the nonlinear loads are unbalanced, the grid currents compensated by SAPF are still unbalanced and distorted even an accurate current detection method is adopted. The reason is that when SAPF compensates the unbalanced loads of grid, the DC-link voltage will contain the second-order harmonic which has influence on the d-axis reference of SAPF's current control loop. Then the output of the SAPF will contain extra third-order harmonic and fundamental negative sequence currents, which make grid currents distorted and unbalanced. In this paper, in order to overcome the problem of poor filtering and dynamic performance by using traditional PI control for DC-link voltage, a MAF-PI controller which combines a moving average filter (MAF), a phase-lead compensator (PLC) and a PI control into DC-link voltage control loop is proposed. This strategy realizes the accurate harmonic currents compensation and excellent DC-link voltage dynamic response of SAPF for unbalanced loads. The effectiveness of the proposed strategy is verified through simulation results.