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A PLL-based Repetitive controller for a Single-Phase Grid-connected NPC Inverter

The repetitive controller stores and compensates for repeated errors in memory, eliminating periodic disturbances in the system. Therefore, it is an effective control method to compensate for steady state errors. Repetitive control has an integer number of fixed memories, which is the ratio of the reference signal frequency to the fixed sampling frequency. Unlike PI controllers or PR controllers, which have limitations in compensating harmonics due to the limitation of physical bandwidth, the repeat controller can theoretically compensate the selected frequency completely. Repetitive control has an integer number of fixed memories, which is the ratio of the reference signal frequency to the fixed sampling frequency. However, fluctuations in the grid frequency cause fluctuations in the reference frequency, it becomes difficult to keep the number of memories as an integer which causes a rapid deterioration in control performance. Therefore, even a small system frequency fluctuation can cause severe control performance degradation. To solve this problem, the proposed PLL-based repetitive controller using the system phase angle information estimated by the PLL method can adapt to system frequency fluctuations with a fixed sampling frequency. The proposed controller can effectively improve output current THD when fluctuation occurs in the grid frequency, which is verified through simulation.