A Control and Switching Method for Photovoltaic Inverter in Grid-Connected and Off-Grid Modes without Energy Storage

With the increasing application of renewable energy sources (RES), the randomness and volatility of RES power leads to severe power balancing issues, which may cause power quality issues like voltage deviation. Currently, the photovoltaic inverter may directly disconnect from the grid to solve the problem, which results in serious waste of power. Especially, in distributed networks, this may result in conflicts among different users. To address these issues, this paper proposes a control method and switching strategy for photovoltaic (PV) inverters under different operating conditions. The control method achieves the current-controlled operation when PV inverters connect to the grid, ensuring stable power output. When power quality issues arise on the grid side, the PV inverters operate in voltage-controlled mode, running in island mode to power the loads even without energy storage. Besides, a switching control strategy has been formulated to reduce the power impact and oscillations caused by the mode transition. Experimental verification has shown that the photovoltaic inverter can operate stably under both operating modes, with minimal power fluctuations during the switching process, proving the effectiveness of the proposed method.