Power Feedforward Method for Passivity-based Grid-connected PV Inverter in Weak Grids

For the conventional two-stage grid-connected photovoltaic (PV) inverter, the second stage usually adopts the voltage-current double closed-loop control. When the output power of PV panels changes, it is necessary to change the voltage of DC bus to change the energy output of the second stage. However, limited by the bandwidth of the outer voltage loop in the second stage, the dynamic response speed of the system is not ideal. Scheming for this problem, according to the characteristics of two-stage grid-connected PV inverter, the double closed-loop control containing outer voltage loop and passivity-based inner current loop is adopted and the power feedforward control is added to this structure. That is, add the output power of PV panels in the first stage as a feedforward to the inner current loop in the second stage. Therefore, the reference value of the inner current loop is composed of the output of both the outer voltage loop and the power feedforward loop. Simulation and experimental results show that the proposed method can effectively improve the dynamic response speed of the second stage to the power changes in first stage and shorten the transient response process. Besides, the power feedforward control can reduce the steady-state error and the overshoot of DC-bus voltage while improving the robustness of the system.