A novel adaptive frequency extraction method for fast and accurate connection between inverters and microgrids

Renewable energy is being rapidly developed and extensively utilized, which are commonly interfaced to the microgrids through the inverters. Therefore, the proper and fast extraction of the synchronous phases, frequency and amplitudes of the sequence voltages have brought major requirements under unbalanced and distorted microgrid voltage. In this regard, this paper presents a novel adaptive frequency method to improve the instantaneous sequence components detection method, which can significantly improve the accuracy of positive and negative sequence voltages. Based on the symmetric component method and the virtual vector construction module, the positive and negative sequence voltages with errors are calculated. The harmonic suppression module filters out the harmonics and random noise in the sequence voltages with errors. Then, the real-time difference frequency phase information of the sequence voltages is obtained by the difference frequency adjustment. With the closed-loop control of the difference frequency phase information, the microgrid frequency can be quickly and accurately obtained. The frequency information is fed back to the virtual vector construction module and the harmonic suppression module to obtain the frequency, amplitudes and synchronous phases of the positive and negative sequence voltages for inverter control in unbalanced and distorted microgrids. When the microgrid frequency changes greatly, the frequency detection module of this method can improve the accuracy of obtaining the positive and negative sequence voltages. Simulation results show that the proposed method can quickly and accurately capture the real-time sequence voltages in the three-phase asymmetric and distorted environment, and the validity and robustness of the proposed method are verified.