Secure optimal operation and control of integrated AC/MTDC meshed grids
Offshore wind energy is seen as the most promising source of electricity generation for achieving the European renewable energy targets. A number of wind farms are planned and under installation to collect the huge potential of wind energy at farther distances in the North Sea. The number of HVDC links in the North Sea is expected to increase with the development of offshore installations in Round 3 of the UK offshore windfarm programme. The increasing number of HVDC links and high power transfer control requirements leads to the formation of Multi-Terminal HVDC (MTDC) grid systems, which have become possible due to the technical advancements of VSC based HVDC systems. Additionally, a meshed MTDC grid structure can also provide interconnections for power trade across the Europe, which can help in better utilisation of power from offshore installations and can also support the AC network in tackling wind power variation issues. However, the integration of the meshed MTDC grid with the existing AC grid has more challenges to overcome alongside the added advantages. One of the major challenge is to ensure the secure and optimal operation of the combined AC/MTDC grid considering stability requirements of the AC and DC grids in different operating conditions. The behaviour of the DC grid is governed by the fast acting controllers due to the high number of power electronic equipment unlike AC grid. In combined operation the response to a disturbance of two integrated grids can be different. The power balancing, co-ordination and dispatch requirements need to be identified, to implement appropriate controls and formulate a control structure for combined operation of two grids with different characteristics under normal and disturbance conditions. In this thesis, the basic principles of well-established three-layered AC grid control is employed to identify the power balancing, coordination and dispatch requirements of the DC grid. Appropriate control methods are proposed for primary, secondary and tertiary control layers in order to accomplish the identified requirements for the secure and optimal operation of combined AC/MTDC grids. Firstly, a comparison study is performed on different power balancing controls to find the most suitable control method for the primary control of the meshed DC grid. Secondly, the combined AC/DC grid power flow method is proposed to provide updated references of the VSC station in order to maintain coordinated power flow control under secondary control layers. Finally, security constraint optimization method for combined AC/DC grid is proposed for economic dispatch under the tertiary control layer of the three-layered hierarchal control. A number of case studies are performed to implement the proposed control methods on a combined AC/DC test case network. The performance of the proposed control methods is validated in a hierarchical control structure for secure and optimal operation integrated AC/MTDC grids.