On Control Loop Interaction and EMT Dynamics in Shaping the Network Frequency Perturbation Plot

It is essential that the inverter-interfaced generation sources provide grid-forming (GFM) capability and ancillary services conventionally provided by synchronous generators. Recognizing the pivotal role of GFM inverters in this context, the National Energy System Operator recently amended its grid code facilitating the establishment of an ancillary services market based on GFM plants. The network frequency perturbation (NFP) plot is recommended in the amended grid code as a compliance testing tool of a GFM plant. The interpretation of the NFP plot in the associated interpretation guide is based on the oversimplified small-signal model of GFM inverters, which only considers the low-frequency dynamics of the active power control (APC) loop. Although the resulting model and analysis is simple and intuitive with such considerations, it is not known how ignorance of the dynamic coupling between the APC and reactive power control loops, as well as the electromagnetic transient dynamics of GFM inverters may affect the interpretation of the NFP plot. To address this challenge, the accurate small-signal model of a typical GFM inverter is established in this article, based on which the NFP plot is developed and compared with its simplified version. The outcome of this work provides practitioners with insights into the frequency ranges and conditions under which the simplification of the NFP plot can or cannot be made for modeling and interpreting the NFP plot. Experimental results are provided to validate the theoretical analysis.