Abstract
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.
Highlights
State-of-the-art point-to-point HVDC links, mainly utilized for the integration of renewable energy sources and as interconnectors between asynchronous AC transmission systems, are envisaged to develop into multi-terminal DC (MTDC) networks to increase the operational flexibility and the reliability in case of faults, and provide a secure power supply [1,2,3]
universal line model (ULM) can generally be represented by simple Pi-sections, consisting of a sin RL branch or frequency dependent (FD) Pi-sections, consisting of cascaded Pi-sectio with several parallel RL branches
From the impedance-based stability analysis using the bode diagrams in Figure 12 it is determined that regardless of the operating points of the other MMCs, the system becomes unstable if the reference active power for MMC 4 is set to Pref,4 = −0.6 p.u. or lower
Summary
State-of-the-art point-to-point HVDC links, mainly utilized for the integration of renewable energy sources and as interconnectors between asynchronous AC transmission systems, are envisaged to develop into multi-terminal DC (MTDC) networks to increase the operational flexibility and the reliability in case of faults, and provide a secure power supply [1,2,3]. Provided that the DC-side impedance behavior of the MMCs is given for several control modes and operating points, no further details of converters are required to accurately calculate the equivalent impedance behavior of a DC grid as seen from the DC terminals of any MMC of interest These impedances can be obtained from the vendors or by using an appropriate frequency sweep method (as applied in this paper). Fast-Interaction converter-driven stability range, that is defined to be in several hundred of Hz and possibly reach kHz [4] Given these appropriately derived impedances, the proposed method allows a fast impedance-based analysis for a multitude of possible system topologies, control modes, and operating points, such that HVDC network stability can be investigated comprehensively and impact factors and worst-case operating scenarios can be derived, allowing the application of methods to enhance the interoperability and stability.
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