Operation and protection of VSC-HVDC grids

Voltage source converter based high-voltage direct-current (VSC-HVDC) systems have shown their advantages over line commutated converter (LCC) based systems in renewable energy integrations, weak ac grid connections and passive network energisations. Worldwide applications of VSC-HVDC have created great potentials to build multi-terminal DC (MTDC) grids to further improve the efficiency and flexibility of power networks. However, technical challenges still exist in operating MTDC grids safely and reliably. Current flow controls and fault protections within MTDC grids have not been fully addressed and remain critical aspects that need to be further studied. This thesis focuses on conducting investigations on operations and protections of VSC based dc grids. Current flow controls in meshed MTDC grids are needed to avoid the overload of transmission lines. Analysis of different types of current flow controllers (CFCs) is conducted in this thesis. It is revealed that the half bridge CFCs (HB-CFCs) are of low cost and high flexibility. A level-shift modulation method, as well as a dual-loop control, is proposed to reduce the switching losses of the HB-CFC and improve its controll ability. To guide its ontroller design, small-signal models of HB-CFCs are derived. The function of the HB-CFC and the effectiveness of the proposed modulation method are verified through simulations Protection of dc faults is deemed to be of high cost since dc circuit breakers (DCCBs) are expensive. To cope with this issue, a new device, integrating of DCCBs and HB-CFCs, is proposed. The presented new device can significantly reduce the number of semiconductor devices while containing the functions of the two devices. Detailed analysis of the proposed device is conducted, and simulations are carried out in PSCAD/EMTDC to verify the analysis. Besides dc faults and grid-side ac faults, valve-side ac faults will induce severe consequences to half-bridge (HB) and full-bridge (FB) modular multilevel converters (MMCs), especially in bipolar HVDC systems. However, fault behaviour and protection methods have not been fully studied for such faults. To bridge this gap, the analysis of valve-side single-phase fault for MMC based HVDC systems are investigated in this thesis. Protection methods against such faults are proposed. For completeness, the effectiveness of the proposed methods is verified by simulations conducted in PSCAD/EMTDC. The study of this research is expected to contribute to the operation and protection of VSC-HVDC grids