Two-Degree-of-Freedom Proportional Integral Controllers for Stability Enhancement of Power Electronic Converters in Weak Grids: Inverter and Rectifier Operating Modes

Future power generation plants will be largely based on renewable energy sources such as wind or photovoltaic power. These plants are connected to the grid through power electronic converters, which may present stability problems, specifically in weak grids. Consequently, numerous stabilities studies have been conducted. In these studies, converters work as inverters; however, in power electronic interfaced loads, energy storage systems or High-Voltage Direct Current (HVDC) links, power converters can also function as a rectifier. Stability studies focusing on the rectifier operation have received little attention in previous research. In this paper, the Voltage Source Converter (VSC) stability is analysed for both the inverter and rectifier modes, with particular focus on the influence of the Phase-Locked Loop (PLL) and the current controllers’ bandwidths. Additionally, a Two-Degree-of-Freedom Proportional Integral (2DOF-PI) controller is proposed to expand the stable operating range. The stability study is carried out using a small-signal model validated through PSCAD simulations. The results show that for inverter operations, a slow PLL and fast current controllers yield better performance, whereas for rectifier operation, a fast PLL and slow current controllers are recommended. Finally, a robustness study based on the H∞-norm is carried out to provide some tuning recommendations for the controller parameters, confirming the different behaviour in inverter and rectifier operation.