MMC Impedance Modelling and Interaction of Converters in Close Proximity

Authors: Yin Chen; Lie Xu; Agustí Egea-Àlvarez; Benjamin Marshall; Md Habibur Rahman; Adeuyi Daniel Oluwole

This article develops a small-signal impedance model of modular multilevel converters (MMCs) using the harmonic state-space (HSS) method and studies the stability in a multiple converter scenario. In order to simplify analysis of the coupling characteristics between different frequencies in MMCs, the proposed model is developed in the positive–negative–zero (PN0) sequence frame, where the zero-sequence current in a three-phase three-wire system is directly set to zero without introducing a complicated method. A simple 2 × 2 admittance matrix in the PN0-frame is extracted from the MMC small-signal model for ease of system stability analysis. Using the developed impedance model, the multi-infeed interaction factor (MIIF) measure is adopted to analyze the most significant interactions for multi-infeed converter systems to be prioritized. Different outer-loop controllers are adopted and compared in the analysis to illustrate the effect of different control modes on converter impedance and system stability. Analytical studies and time-domain simulation results are provided to validate the proposed model and stability analysis.

Published in: IEEE Journal of Emerging and Selected Topics in Power Electronics ( Volume: 9, Issue: 6, Dec. 2021)

Supported by the HVDC Centre as part of project: Stability assessment and mitigation HVDC converter interactions

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Impact of HVDC Convertor Technology Choice on Reinforcement within the GB Grid

HVDC ‘bootstraps’ are becoming a key option in the reinforcement of the transmission system. As the generation mix moves towards renewables, sources are becoming more dispersed and located further from load centres, requiring extensive transmission network upgrades to get the power to where it is needed. As a result of restrictions in space and consenting issues locating new circuits onshore, there has been a move to locate new circuits offshore. This requires long cable circuits, where HVDC technology must be used. This note explores how the choice of HVDC technology can affect the system integration challenge of the required upgrades to the transmission system. Download file below.

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