Enhancing the Frequency Adaptability of Periodic Current Controllers for Grid-Connected Power Converters

It is mandatory for grid-connected power converters to synchronize the feed-in currents with the grid. Moreover, the power converters should produce feed-in currents with low total harmonic distortions according to the demands, by employing advanced current controllers, e.g., Proportional Resonant (PR) and Repetitive Controllers (RC). The synchronization is actually to detect the instantaneous grid information (e.g., frequency and phase of the grid voltage) for the current control, which is commonly performed by a Phase-Locked-Loop (PLL) system. As a consequence, harmonics and deviations in the estimated frequency by the PLL could lead to current tracking performance degradation, especially for the periodic signal controllers (e.g., PR and RC) of high frequency-dependency. In this paper, the impacts of frequency deviations induced by the PLL and/or the grid disturbances on the selected current controllers are investigated by analyzing the frequency adaptability of these current controllers. Subsequently, strategies to enhance the frequency variation-immunity for the current controllers are proposed for the power converters to produce high quality feed-in currents even in the presence of frequency deviations. Experiments on a single-phase grid-connected inverter system are presented, which have verified the proposals and also the effectiveness of the frequency adaptive current controllers.