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.