Impedance-based stability analysis of MW-sized inverter-based resources connected to weak grids considering phase delay compensation

Abstract

The operation of inverter-based resources connected to weak grids poses stability challenges. For megawatt-sized converters, the limitations imposed in terms of switching frequency result in non-negligible delays introduced by the digital implementation of the control system. These delays impact both system design and small-signal stability limits. To allow the proper characterization of the study, an investigation of bottom-up design approaches and best practices is conducted for selecting per-unit parameters. An in-depth analytical impedance-based modeling of the resource is performed to account for the proper representation of components and control system in the synchronous-reference frame, including control and modulation delays that result in axis coupling. A phase delay compensation technique is proposed and implemented within a modified current controller. The impedance-based stability analysis is performed through the application of the generalized Nyquist criterion to assess the maximum power that the resource can deliver during weak grid operations. The analysis shows that the proposed phase delay compensation technique effectively extends the small-signal stability limit of the inverter-based resource. The methodology is validated through the state-space eigenvalue analysis, and a set of EMT simulations. Finally, results are validated using a control hardware-in-the-loop experimental setup, which employs a real-time FPGA-based simulation platform and an actual converter controller.