Impedance-based Stability Analysis in an Offshore Multi-Energy System

Abstract

In the forthcoming decades, an increasing share of energy is projected to be generated by offshore wind power plants (WPPs). Adverse interactions involving multi-converter systems and passive network elements could lead to subsynchronous oscillations (SSOs), possibly damaging grid components or otherwise interrupting the power flow. Secondly, the development of analytical models for converter-dominated grids is hindered due to nonlinear dynamics and proprietary specifications of converters. Therefore, the aim of this thesis was to investigate SSO phenomena in converter-dominated offshore systems with the measurement-based impedance stability method.

This thesis considered a multi-energy offshore system. At the offshore side, WPPs and ELs are located, totalling 2~GW and 500~MW, respectively. They are interconnected to the onshore grid by a bipolar DC link. Four control strategies have been integrated into the wind turbine (WT) converters; standard grid-following (GFL) and three grid-forming (GFM) strategies: droop, virtual synchronous generator (VSG) and synchronverter.

Subsequently, the impedance-based stability method was utilised. This method entails obtaining the impedance response by performing impedance scans. The next steps dictate the calculation of the return-ratio matrix and eigenloci, required for application of the generalised Nyquist stability criterion (GNSC). In order to improve the accuracy of the stability assessment, a frequency domain vector fitting algorithm is adopted, which provides continuous transfer functions. Thereby, stability indicators such as sensitivity peak, oscillatory frequency and damping ratio can be inferred. One drawback of the algorithm is the possible occurrence of spurious poles, which might result in incorrect stability conclusions, especially when fitting nonlinear eigenloci functions.

This thesis sought to contribute to the assessment and quantification of SSOs. To this end, it was studied how operating conditions based on parametric differences influence SSOs. It highlighted that critical oscillatory modes (OMs) could be extracted and quantified and that the offshore system is responsive to SSOs due to the presence of parallel-connected converters and passive cable elements. Furthermore, it revealed that interchanging GFL with GFM improves the oscillatory stability to a sufficient level. Thereby, reduced sensitivity and improved damping were observed for short offshore cable distance and WT underloading. Importantly, it was shown that 50/50 deployment ratio of GFL/GFM yields the best oscillatory performance and activation of EL saw additional enhancement upon the baseline. Altogether, it was illustrated that the impedance-based stability method enables insightful analyses of the oscillatory performance for converter-dominated systems.