Supervisor:
Zhe Chen

Co-Supervisor:
Birgitte Bak Jensen, Shuju Hu

Assessment Committee:
Zhenyu Yang (Chair)
Oriol Gomis-Bellmunt, Technical University of Catalonia
Elena Gaio, National Research Council

Moderator:
Zhenyu Yang

Abstract:

High Voltage Direct Current (HVDC) transmission system is a promising solution for integration of large-scale Renewable Energy Sources (RES) into electrical power systems. The new generation of Power Electronic Converter (PEC), called Voltage Source Converter (VSC), provides power systems with robust control of voltage and power. Also, there is no restriction on multiple infeed and multi-terminal connections in the HVDC grid based on the VSCs. With development of HVDC system technology, Modular Multilevel Converters (MMC) are introduced that have a superior performance compared to conventional VSC systems. However, DC protection has always been a great challenge in such a new structure of power transmission system i.e., the multi-terminal HVDC grids based on the MMCs.

One of the main objectives of this project is to propose a protection coordination in multi-terminal HVDC grids based on Half Bridge (HB)-MMCs. Such a system-level coordination for HB-MMC-based HVDC grids provides the system with a regulation under the DC fault condition to determine whether an MMC should be disconnected from the grid or not, based on its unique conditions including the MMC position and DC Fault-Ride-Through (FRT) scenarios. Also, another objective of this study is to shed light on DC protection design criteria for the HB-MMC-based HVDC grids considering the system transient stability under DC fault conditions. Each multi-terminal VSC-HVDC grid should end up with a unique DC protection based on its own requirements including the characteristics of the embedded AC grid. But as discussed before, the fastest DC protection is usually suggested in the literature, regardless of the characteristics of the embedded AC grid. Therefore, the impacts of the grid characteristics along with the transient stability margin on DC protection of the VSC-HVDC system have comprehensively been investigated in this thesis.

Considering the mentioned objectives, this study proposes a new protection coordination for the multi-terminal HB-MMC-based HVDC systems under DC fault conditions. The proposed protection coordination determines whether an MMC should stay connected or should be tripped, regarding the MMC position and the DC Circuit Breaker's (DCCB) operation. The proposed method depends on DCCB's operation as DC link protection and HB-MMC converters in the multi-terminal HVDC grids. Moreover, this study discusses the impacts of different grid parameters including HVDC power, the SCR and inertia time constant on the HVDC system stability through the proposed mathematical expressions. The study also investigates the impacts of the grid components including AC-side and DC-side reactors on the stability of the VSC-HVDC systems. Then, the thesis discusses the appropriate design of the DC protection for the multi-terminal HVDC grids considering the system transient stability. The study also discusses the appropriate selection of DCCB type considering the transient stability of the embedded AC grid. The performances of DC protections using different types of DCCBs have been evaluated and investigated along with different AC and DC grid characteristics.