Design and Investigation of Hybrid HV-DC Circuit Breaker based on SF6-MCB

Abstract

In the course of the increased generation of renewable energy, large amounts of energies must be transferred over long distances due to the remote location of the generation and to compensate the fluctuations in the generation of renewable energies. In this context, high voltage DC (HVDC) transmission gained increased attention due to the lower transmission losses over long distances compared to AC transmission and due to its applicability for sub-sea cable transmission for e.g. offshore windfarms, where AC transmission is limited to short distances. At the same time, the introduction of voltage source converters (VSC) basing on IGBTs, especially of modular multilevel converters (MMC), allows the creation of multi-terminal HVDC grids with low losses. However, the ability to interrupt fault currents and disconnect faulty parts of a grid is a key requirement for large multi-terminal grids. Classical AC circuit breaker (AC-CB) cannot be used due to the lack of a natural zero current crossing and due to the requirement of a low interruption time to protect the over-current-sensitive VSC from the fast increasing fault currents. Accordingly, new DC circuit breaker (DCCB) have to be developed. To avoid high on-state losses of pure semiconductor based DC-CB and long opening times of pure mechanical DC-CB, hybrid DC-CB are developed. The challenge is to find fast and reliable ways to interrupt fault currents with relatively low costs. Therefore, new approaches have to be investigated and new methods for designing DC-CBs must be developed. The aim of this thesis is to investigate different approaches, find new approaches and improve design methods. Different existing DC-CB topologies have been designed, optimized and compared. Their strengths and shortcomings have been analyzed and new optimization procedures have been developed, which especially focus on the properties and limitations of mechanical switches. Also, a new DC-CB topology has been developed, which focuses on the reduction of component numbers by adapting the interruption procedure to the fault current and reusing the components for multiple purposes. In addition, tranvsient voltages, which can result in an interruption failure, have been investigated and a corresponding model and solution for the optimization has been developed to improve the reliability of DC-CBs. Finally, new methods for reducing the fault current to zero have been investigated, which result in a higher performance and lower component numbers.