Local and centralized control of multi-terminal DC grids for secure operation of combined AC/DC systems

High Voltage Direct Current (HVDC) transmission has become increasingly popular in the recent years, fueled by the shift to renewable energy sources and the gradual decommissioning of conventional power plants. The vast majority of HVDC connections in the world consists of point-to-point links. The next step envisaged is the extension to Multi-Terminal DC (MTDC) grids. However, several challenges have to be first addressed, one of which is the secure operation of the combined AC/DC systems that are going to arise. This thesis proposes local and centralized control schemes in order to improve the security and to enhance the dynamic behavior of such systems, by taking advantage of the flexibility of the AC/DC converters.
First, a hierarchical structure is proposed to control an MTDC grid, consisting of a primary, a secondary and a tertiary level. The primary level aims at adjusting the power of the Voltage Source Converters (VSC) connected to the MTDC grid in order to correct any power imbalances that might arise. The secondary level monitors the MTDC grid to alleviate system-wide violations and steer it towards a desired operating point. The role of the tertiary level is to minimize the losses in the MTDC grid and ensure its secure operation after the loss of any single component.
Next, the issue of frequency support among asynchronous AC systems is explored. Since more and more conventional power plants are decommissioned in favor of new power electronics-interfaced generation, AC frequency control may become critical in the future. Two decentralized control schemes are proposed in this thesis. The VSCs are equipped with the proposed controllers and react to important frequency deviation to either provide a pre-defined participation to frequency support or to keep frequency inside a desired range. In both cases, constraints imposed by the DC grid are respected.
Last but not least, a framework for the security assessment of combined AC/DC grid is outlined. The control flexibility offered by MTDC grids must be taken into consideration when investigating corrective control actions against a set of credible contingencies. The framework highlights the need for cooperation and exchange of relevant information between the TSOs connected to the same MTDC grid, in order to avoid conflicts between them and ensure the security of the combined AC/DC system.