Immittance-based black-box model identification via vector fitting methods for offshore wind power plant components

During an offshore wind power plant (OF-WPP) development, various equipment vendors such as STATCOMs and synchronous condensers provide a full-scale black-boxed model of their components to prevent their intellectual property and to avoid unwanted changes in the internal control structure and parameters. Such models are utilized for various time-domain studies; however, insights into small-signal and converter-driven stability miss out due to the lack of small-signal models. In order to perform a small-signal stability study of the entire OF-WPP with the possibility of extracting stability information from the black-box model components, advanced methods such as vector fitting are required. Vector fitting estimates the state-space model of the black-boxed system component from which the overall state-space model of the OF-WPP can be built, thus facilitating an overall small-signal stability analysis, which can provide insights into the source of system instability via modal analysis. This paper presents the application of vector-fitting methods on various OF-WPP components from their immittance scans to estimate the small-signal model of the component. A validation of the vector fit results against EMT-based and frequency domain model (FDM) based immittance scans are provided to demonstrate the accuracy of the proposed method. The results show that crucial model information can be extracted from the immittance scans of the black-boxed component models without the vendors needing to provide IP-protected internal model parameters explicitly. The extracted model information can be utilized for the overall small-signal stability analysis of the WPP.