Preprint Article Version 1 Preserved in Portico This version is not peer-reviewed

Investigating the Converter-Driven Stability of an Offshore HVDC System

Version 1 : Received: 17 March 2021 / Approved: 18 March 2021 / Online: 18 March 2021 (12:31:45 CET)

A peer-reviewed article of this Preprint also exists.

Quester, M.; Loku, F.; Azzati, O.E.; Noris, L.; Yang, Y.; Moser, A. Investigating the Converter-Driven Stability of an Offshore HVDC System. Energies 2021, 14, 2341. Quester, M.; Loku, F.; Azzati, O.E.; Noris, L.; Yang, Y.; Moser, A. Investigating the Converter-Driven Stability of an Offshore HVDC System. Energies 2021, 14, 2341.

Journal reference: Energies 2021, 14, 2341
DOI: 10.3390/en14082341

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. Besides, 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. 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’s interactions with AC grids of moderate short-circuit ratios can lead to instabilities. The offshore system analysis reveals that considering the offshore grid topology is crucial for assessing interactions between the wind turbine controllers and the offshore modular multilevel converter. It is shown that different stability margins result from varying offshore grid layouts.

Keywords

Converter-driven stability; HVDC; Offshore, MMC, Converter, Replica, impedance-based stability analysis, Interactions.

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