Research on an Improved Voltage Feedforward Path of Grid-Connected Inverter Coping with Complex Stability Issues in Weak Grid

Phase-locked loop (PLL) and the grid voltage feedforward (GVF) are commonly adopted in grid-connected inverters. However, both of these two links may have impact on the stability of the system in weak grid. From the perspective of impedance, considering PLL and the GVF is equivalent to two additional admittances YPLL(s) and Yff(s) in parallel on top of the original output admittance. The output admittance Bode plots considering the PLL and grid voltage feed-forward are given in Fig.A1, which shows that the output characteristics in the middle and low frequency bands are influenced by the two factors, resulting in a joint-action frequency band. (Figure presented.) As the grid impedance increases, the admittance intersection frequency point enters the GVF-dominant frequency range of firstly. Accordingly, the primary consideration is to improve the GVF by introducing weighting coefficient to ensure the stability of the inverter as well as maintain a certain level of harmonic suppression capability of the system. With the increasing of the grid impedance, the bandwidth of the current loop decreases rapidly, approaching the frequency range influenced by the PLL bandwidth. This results in coupling between the current loop and the PLL. The introduction of the weighting coefficient into the GVF can enhance the bandwidth of the current loop, thereby improving the poor robustness induced by PLL. However, for weaker grids with smaller SCR, the presence of the PLL would bring about a less or even negative phase margin, leading to the system unstable. Consequently, the improving effect of the weighting-coefficient-based GVF is limited in this case. Therefore, as shown in Fig.A2, a voltage feedforward path is added to improve the phase introduced by the phase-locked loop, allowing the system to adapt to a wider range of varying grid impedance. Specifically, an all-pass filter (APF) is added in this path, and then phase compensation is applied to it, which is followed by the design of its parameters at last. (Figure presented.) Finally, a prototype was constructed, and experimental waveforms verified the theoretical analysis mentioned above. The results indicate that, with the addition of the improved voltage feedforward path to the grid-connected inverter, the system can maintain a certain level of robustness even when the grid impedance varies significantly. The contribution of this article lies in following aspects: (1) The frequency-band division considering dual unstable factors is carried out. Then, the three regions including the PLL-dominant region, joint-action region and GVF-dominant region are acquired. (2) It is found that, the GVF with weighted coefficient has the merit of enhancing the robustness issue caused by the PLL to a certain extent. While, the role of improvement is somewhat limited since the phase margin is not big enough. (3) In joint-action frequency region, on the basis of the weighted coefficient, the improved feedforward path is ultimately introduced to ensure a good stability of the system in weak grid. © 2024 China Machine Press. All rights reserved.