Frequency Behavior of an MMC Test Bench System

Quester, Matthias; Loku, Fisnik; Yellisetti, Viswaja; Moser, Albert

doi:10.1109/energycon48941.2020.9236498

Cited by 3 publications

(1 citation statement)

References 7 publications

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“…Depending on the available information, both Z MMC and Z grid could be calculated using a frequency sweep approach or provided by either the respective grid operator or converter manufacturer, so that stability analyses can be conducted without the need for converter-internal data. Whereas previous literature mainly focuses on the derivation of the MMCs' equivalent impedance Z MMCeither in an analytical way [6][7][8] or via using frequency sweep measurements [11,12]-the DC network and the corresponding Z grid are often modeled in a simplified way; for example, DC cables are assumed as simple Pi-section models, which are very limited with regard to modeling the frequency dependent impedance behavior [13]. Hence, the overall DC grid impedance Z grid relevant for stability analyses can only be derived for a small bandwidth of lower frequencies.…”

Section: Introductionmentioning

confidence: 99%

Equivalent Impedance Calculation Method for Control Stability Assessment in HVDC Grids

et al. 2021

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A major challenge in the development of multi-vendor HVDC networks are converter control interactions. While recent publications have reported interoperability issues such as persistent oscillations for first multi-vendor HVDC setups with AC-side coupling, multi-terminal HVDC networks are expected to face similar challenges. To investigate DC-side control interactions and mitigate possible interoperability issues, several methods based on the converters’ and DC network’s impedances have been proposed in literature. For DC network’s impedance modelling, most methods require detailed knowledge of all converters’ design and controls. However, in multi-vendor HVDC networks, converter control parameters are not expected to be shared due to proprietary reasons. Therefore, to facilitate impedance-based stability analyses in multi-vendor MTDC networks, methods that do not require the disclosure of the existing converter controls are needed. Here, detailed impedance measurements can be applied; however, they are time-consuming and require new measurement for a single configuration change. This paper proposes an equivalent impedance calculation method suitable for multi-vendor DC networks, which for available black-box models or converter impedance characteristics can be modularly applied for various network configurations, including different control settings and operating points, while significantly reducing the required time for obtaining an equivalent DC network impedance.

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Section: Introductionmentioning

confidence: 99%

Equivalent Impedance Calculation Method for Control Stability Assessment in HVDC Grids

et al. 2021

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Vector fitting algorithms for impedance modelling of converter based systems

Yellisetti

Seibert

Quester

et al. 2021

2021 56th International Universities Power Engineering Conference (UPEC)

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Investigating the Converter-Driven Stability of an Offshore HVDC System

et al. 2021

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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.

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Frequency Behavior of an MMC Test Bench System

Cited by 3 publications

References 7 publications

Equivalent Impedance Calculation Method for Control Stability Assessment in HVDC Grids

Equivalent Impedance Calculation Method for Control Stability Assessment in HVDC Grids

Vector fitting algorithms for impedance modelling of converter based systems

Investigating the Converter-Driven Stability of an Offshore HVDC System

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