Stability of Frequency Regulation with a Grid Following Inverter

Abstract

With increasing integration of intermittent renewable generation, such as wind and solar power, additional frequency regulation is required in power systems. This is driven by generation fluctuations and reduced system inertia, as fossil fuel generation is phased out. Frequency regulation is partially provided through ancillary services, including frequency containment reserve for normal operation (FCR-N) and emerging services such as dynamic fast frequency reserve (dynamic FFR) and synthetic inertia, which aim to mitigate rapid frequency deviations. Most renewable energy sources are today connected through grid-following (GFL) inverters, which could provide these services. However, active power injection for frequency support can introduce local frequency disturbances that may destabilize the inverter. The stability of the inverter depends on the resulting frequency deviation, grid strength, and the inverter’s frequency filter. This thesis investigates these stability challenges for a GFL inverter by evaluating the stability limits of regulating strength and synthetic inertia, as well as the performance of the frequency ancillary services at Chalmers University’s power system laboratory. The stability limits of FCR-N operation were evaluated both during grid-connected and islanded operation with a synchronous generator (SG), while synthetic inertia stability limits were assessed only during grid-connected operation. Grid strength was varied through changes in the impedance between the inverter and the local grid connection. The stability limits and performance of FCR-N, dynamic FFR, and synthetic inertia were investigated for different frequency measurement methods and frequency filters with varying time constants. The results showed that a GFL inverter could provide frequency services while maintaining stable operation for most combinations of filters, filter time constants, and grid conditions, including islanded operation with an SG and grid-connected operation with varying grid impedance. Laboratory experiments demonstrated improved stability margins when a lead-lag filter was used to attenuate high-frequency components in the active power response. During islanded operation, the inverter could independently provide sufficient frequency regulation to maintain stable grid operation. For dynamic FFR, the inverter mitigated both the rate of change of frequency and the frequency nadir. Compared to synthetic inertia, dynamic FFR reduced the frequency nadir more effectively but required higher energy capacity for the same peak power output.