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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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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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Protection Performance Overview and Validation in Low Strength Areas
Assessment of AC Protection Performance in Weakening Systems from Webcast on held 9th September 2021
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Protection Performance Overview and Validation in Low Strength Areas
The integration of AC connected wind farms into the existing power network has significantly changed the characteristics of the system and adequate small signal wind turbine models will be required to assess system stability considering the existence of difference converter technologies. While the small signal behaviour of Type 4 turbine has been extensively studied, the small signal impedance of doubly-fed induction generator (DFIG) based Type 3 turbine has not been well understood. The purpose of this study is to develop small signal admittance model of Type 3 turbine under different control modes and operating points. In this report the admittances of Type 3 turbine are then compared to the Type 4 turbine to highlight the main differences between the two. The small-signal admittances of both turbine types from the analytical models are verified against the measurements from time-domain models.
This report concentrates on the development of small-signal LCC converter and LCC HVDC system models. The development of the analytical impedance model intends to provide further insight into the impact of circuit and control structure/parameters on system impedance, and similarly highlight the factors that need to be considered when an LCC HVDC connection is defining its frequency-dependent impedance accurately ahead of a network scale small-signal interaction study being performed. The developed impedance models can then be used for assessing stability and interactions between different converters (e.g., MMC, LCC, wind farm, etc.).