Abstract
Offshore wind farms are increasingly built in the North Sea and the number of HVDC systems transmitting the wind power to shore increases as well. To connect offshore wind farms to adjacent AC transmission systems, onshore and offshore modular multilevel converters transform the transmitted power from AC to DC and vice versa. Additionally, modern wind farms mainly use wind turbines connected to the offshore point of common coupling via voltage source converters. However, converters and their control systems can cause unwanted interactions, referred to as converter-driven stability problems. The resulting instabilities can be predicted by applying an impedance-based analysis in the frequency domain. Considering that the converter models and system data are often confidential and cannot be exchanged in real systems, this paper proposes an enhanced impedance measurement method suitable for black-box applications to investigate the interactions. A frequency response analysis identifies coupling currents depending on the control system. The currents are subsequently added to the impedance models to achieve higher accuracy. The proposed method is applied to assess an offshore HVDC system’s converter-driven stability, using impedance measurements of laboratory converters and a wind turbine converter controller replica. The results show that the onshore modular multilevel converter interacts with AC grids of moderate short-circuit ratios. However, no interactions are identified between the offshore converter and the connected wind farm.
Highlights
With the increasing number of High-Voltage Direct Current (HVDC) links integrated into the Alternating Current (AC) transmission system and converters, power electronics relying on complex control systems play a more dominant role in system stability
Using the scaled ModularMultilevel Converters (MMCs) TB and the Wind Turbine (WT) Voltage Source Converters (VSCs) controller impedances, the converterdriven stability of a wind farm connected to the AC onshore grid by an HVDC link is assessed
The results indicate that couplings between frequencies and their impact are highly dependent on the converters’ control system
Summary
With the increasing number of High-Voltage Direct Current (HVDC) links integrated into the Alternating Current (AC) transmission system and converters, power electronics relying on complex control systems play a more dominant role in system stability. Stability analyses often utilize analytical impedance models derived based on a white-box approach that implies that the converter structure and the control system’s block diagrams are known [14,21,22,23] This approach offers excellent insight into the cause of interactions and can be used to investigate the impact of specific controllers and parameters on stability [24]. This paper presents an approach to assess the stability of a system, where no detailed information of a converter control system is provided and where the system’s models (e.g., converter and grid impedance) are provided by different entities.
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