A study of feedforward control on stability of grid-parallel inverter with various grid impedance

The Current Controlled Voltage Source Inverter (CCVSI) has attracted substantial research due to its common application for the integration of renewable energy resources into the main grid. The single loop current controller is commonly used in industry for CCVSI due to its simplicity, desirable stability margins and dynamic performance. For the single loop controller, the point of common coupling (PCC) voltage feedforward control is used to improve the transient and disturbance rejection performance. However, this solution exhibits one drawback that has not yet been adequately discussed in the literature: the poor stability caused by the feedforward control itself. It is proved that the feedforward voltage results in a “positive feedback control” which essentially degrades the stability of the system and its control performance. Since the gain of the “positive feedback control” depends on the grid impedance, it is necessary to study how the stability and control performance of the grid parallel inverter is affected by the feedforward control under various grid impedances. The system without feedforward control is also investigated due the fact that the grid impedance generally affects the system stability. The analytical study shows that two cases have similar performance under stiff grid, while the case without feedforward has superior stability and control performance under weak grid impedance. The simulation and experimental results also validate the outcome of the analytical study.