Active Power Return Suppression Strategy of Three-Phase Cascaded Multilevel Photovoltaic Inverter Based on Harmonic Compensation

At present, the grid connection guidelines require large and medium-sized photovoltaic (PV) power stations to have certain low voltage ride through (LVRT) capability, that is, the inverter should actively support the grid voltage in case of grid failure. However, three-phase isolated cascaded H-bridge (CHB) PV inverter uses isolated DC-DC converter with unidirectional energy flow, so the active power return problem cannot be avoided. During asymmetric fault, the returned active power will accumulate at the DC side capacitor, so the capacitor voltage will pump up, eventually causing the inverter to trigger overvoltage protection and be off-grid, which does not meet LVRT standard. Especially, when inter-phase short circuit fault occurs, the active power return problem is particularly serious and prominent. To deal with this issue, this paper proposes a multi-harmonic compensation strategy based on adaptive zero-sequence voltage, which is able to reduce the overmodulation area, and further decrease the active return operation range of the three-phase common dc-bus CHB PV inverter under inter-phase short circuit fault condition. The effectiveness and feasibility of the proposed control strategy are verified by the experimental results.