Stability Enhancement of Weakly-Connected Offshore Wind Power Plants by Wind Turbines with Grid-Forming Converter Control

Offshore Wind Power Plants (OF-WPPs) have undergone continued growth, innovation, and technological advances over the past few decades. The matured technology of Grid Following (GFL) converter control has been used in the Wind Turbine Generators (WTGs) of a Wind Power Plant (WPP) up until the present, offering control of active and reactive powers and flexibility during grid-faults and other system events. However, the limitations of the GFL converters in terms of their lack of frequency control, non-voltage-source behavior, and lack of inertial contribution, and the need for measurement-based synchronization via typically slow-operating Phase-Locked Loops (PLLs) have been noted as contributors to power system instability. Thus, with the rise of Inverter Based Resources (IBRs) in the grid, these conventional GFL converter control methods need to be updated with Grid Forming (GFM) converter control methods to enhance their response and system stability by providing inertial response, self-synchronization (without heavy reliance on measurements,) voltage-source behavior, and voltage support to name a few. As a growing generation source, OF-WPP s equipped with GFM control can also contribute to system stability by providing these enhanced responses.

GFM converter control technology is a novel technology with many promising aspects. However, the scientific field has little experience implementing this technology on an industrial scale in the transmission grid. In addition, the grid codes and standards are still evolving to adapt to the upcoming GFM-based generations. Within these evolving grid codes and standards, there are also some misaligned expectations between different Transmission System Operators (TSOs) and Original Equipment Manufacturerss (OEMs) as to what a GFM converter should deliver (functional specifications,) how it differs from the conventional GFL control method, and what its limitations are. To this end, this thesis discusses existing grid codes, technical documents, recommendations, and standards to redefine and re-classify the GFM functional specifications and testing methods to assess the GFM functionalities. This thesis also formulates GFM performance requirements and test methods for OF-WPPs. The thesis uses the aforementioned requirements to test different GFM control architectures for stability study and enhancement. A comparison of various representative GFM control methods is provided under the guidelines developed based on the review of several grid codes, technical documents, and standards. The short-listed GFM control methods are tested against various GFM functionalities. Methods for enhancing the stability of weakly connected GFM OF-WPPs are provided for various cases: (a) interoperability with GFM devices from other vendors with different control architecture, (b) methods for temporary islanding withstand and re-synchronization, and (c) interoperability with Synchronous Condensers (SCs). Further, additional studies on the converter impedance passivity characteristics and passivity enhancement methods are presented in the appendix.

Overall, the thesis features a detailed review and re-classification of the existing GFM functional specifications and suggests specific testing requirements for each functional specification. Furthermore, a comparison of GFM performance of the various control methods shows the applicability of well-known GFM control methods for weakly connected OF-WPPs. Novel stability enhancement methods are provided to improve the interoperability of multi-converter systems and mitigate power and frequency oscillation issues. The thesis also provides methods for the GFM OF-WPP to withstand temporary islanding and enable a smooth resynchronization to the power grid. Finally, methods are provided to mathematically quantify and improve the GFM impedance passivity. The contributions of this thesis are in terms of functional specification of GFM OF-WPPs, as well as in terms of their stability enhancement methods.