Control and operation of MVDC systems with grid forming capability for the future GB power system

本博士的目的是探索2025年后英国电力系统的新型MVDC系统的控制和运行，展示这两种技术结合可能带来的优势。特别是，本文将探讨在拥挤的地区，如苏格兰中央带或伦敦，用MVDC线路取代现有的交流中压线路的想法。这将增加电力传输容量，但同时，电网的灵活性。使用标准的转换器控制，可以控制MVDC线路连接点的交流电压，与交流线路相比，提高了网络容量。但若能增强该变流器的成网能力，则可进一步提高中压电网的可控性和稳定性。由于具有电网形成能力的MVDC转换器，可以更好地支持电力系统，在频率事件时提供惯性，更具支持性的电压控制，增加可再生能源的渗透，甚至提供黑启动能力。考虑到前面几点，这些博士目标旨在研究具有并网变流器的MVDC系统的控制挑战和解决方案：-研究可再生能源高渗透的网格区域中压交直流混合网络的运行和控制。众所周知，中压电网比高压电网更加网状。在电压和功率流控制方面，这可能是一个挑战。由于MVDC线路为电网增加了更多的灵活性，因此应该开发一种新的策略来实现最佳的潮流控制和电压分布。同时，随着新增可再生能源发电接入中压电网，应研究MVDC链路与可再生能源之间的控制协调问题。-研究提供辅助服务，例如惯性和频率支持，以及中压混合交流和直流的黑启动。历史上，中压直流网络的行为是被动的，但随着分布式可再生能源、电动汽车和中压直流系统的加入，中压网络可能具有确保电力系统稳定、控制和运行的新作用。中压电网提供稳定辅助服务是一个新课题，需要进行一些研究，如MVDC链路与可再生能源之间的控制协调，以提供惯性支持。同时，可以提供黑启动等新的服务，帮助系统从停电事故中更快地恢复。本文将考虑MVDC链路和可再生能源来研究黑启动先例。-研究交直流混合中压网络中的控制交互作用。随着中压网络变得更加活跃，电源转换器的扩散更有可能控制可再生电源之间的相互作用，MVDC线路和任何其他转换器接口系统（例如电动汽车充电站）可能出现。本文将研究潜在的控制相互作用，并将建议缓解技术，以避免这一问题。

The aim of this PhD is to explore the control and operation of new MVDC system enhanced with grid forming controllers for the GB power system beyond 2025 showing the advantages that the combination of these two technologies might bring. In particular, this thesis will explore the idea to replace an existing AC medium voltage line in a congested area, like the Scottish central belt or London, for a MVDC line. This will increase the power transmission capacity but, at the same time, the flexibility of the electrical network. Using standard converter controls, the AC voltage at the point of connection of the MVDC line can be controlled improving the network capacity compared to AC lines. But if this converter controlled is enhanced with grid forming capabilities, the controllability and stability of the medium voltage network can be increased further. Thanks to the MVDC converters with grid forming capabilities, the electrical system can be better supported providing inertia in case of a frequency event, more supportive voltage control, increasing the penetration of renewable power and even providing black start capabilities. Taking into consideration the previous points, these PhD goals are designed to study the control challenges and solutions of MVDC systems with grid forming converters:- Study the operation and control of hybrid medium voltage AC and DC networks in meshed areas with high penetration of renewable power. Medium voltage networks are known for being more meshed than high voltage networks. In terms of voltage and power flow control, this might suppose a challenge. As more flexibility is added into the grid thanks to the MVDC line, a novel strategy should be developed to achieve optimal power flow control and voltage profiles. At the same time, as new renewable generation is connected to the medium-voltage network control coordination between the MVDC link and the renewable power should be studied.- Study the provision of ancillary services such as inertia and frequency support and black start from a medium-voltage hybrid AC and DC. Historically, MVDC networks had a passive behaviour but with the addition of distributed renewable power, electric vehicles and MVDC systems, the medium voltage network might have a new role ensuring the power system stability, control and operation. The provision of stability ancillary services from the medium voltage network is new and require some study such as the control coordination between the MVDC link and the renewable energy sources for inertia support provision. At the same time, new services like black start can be provided, helping the system to recover quicker from a blackout rs. This thesis would study the black start precedent considering the MVDC link and the renewable power sources. - Study the control interactions in hybrid AC and DC medium voltage networks. As the medium voltage networks become more active with the proliferation of power converters is more likely that control interactions between the renewable electrical sources, MVDC lines and any other converter interfaced system (e.g. electrical vehicle charging station) might appear. This thesis will study the potential control interaction and will suggest mitigation techniques to avoid this problem.