DC-Link Voltage Ripple Cancellation Strategies for Three-Phase Four-Wire PV Inverters Under Unbalanced Grid Voltages

This paper proposes dc-link voltage ripple cancellation strategies for three-phase four-wire PV inverters under unbalanced grid voltages. The proposed PV inverter consists of a front-end interleaved dc-dc converter and a rear-end three-phase four-wire dc-ac inverter. Under normal condition, the dc-dc converter will achieve the maximum power point tracking (MPPT) function to harvest the maximum power from the PV array. In addition, the inverter will transfer the PV energy to the grid. However, during unbalanced grid voltage scenarios, the double line frequency ripple will be occurred on the inverter output power. Under this situation, the zero-sequence current can be controlled to eliminate the power ripple. But there will be significant dc-link voltage ripple on the split capacitors due to the injection of the zero-sequence current. The simplest way is to increase the dc-link capacitance to suppress the voltage ripple. Unfortunately, the circuit size and cost will be increased. In view of this, a dc-link voltage ripple cancellation strategy is proposed and developed. With the proposed strategy, the dc-link voltage ripple will be effectively eliminated without adding extra circuit and components, whereas the strategy can be realized via the digital signal processor (DSP). Theoretical analysis and mathematical derivations of the proposed method will also be presented. Finally, a 5kVA prototype circuit with both simulation and experimental results verify the feasibility and performance of the proposed circuit and control strategy.