The continuously increasing demand for electric power and the economic access to remote renewable energy sources such as off-shore wind power or solar thermal generation in deserts have revived the interest in high-voltage direct current (HVDC) multiterminal systems (networks). A lot of work was done in this area, especially in the 1980s, but only two three-terminal systems were realized. Since then, HVDC technology has advanced considerably and, despite numerous technical challenges, the realization of large-scale HVDC networks is now seriously discussed and considered. For the acceptance and reliability of these networks, the availability of HVDC circuit breakers (CBs) will be critical, making them one of the key enabling technologies. Numerous ideas for HVDC breaker schemes have been published and patented, but no acceptable solution has been found to interrupt HVDC short-circuit currents. This paper aims to summarize the literature, especially that of the last two decades, on technology areas that are relevant to HVDC breakers. By comparing the mainly 20+ years old, state-of-the art HVDC CBs to the new HVDC technology, existing discrepancies become evident. Areas where additional research and development are needed are identified and proposed. for the couple of well-known applications are discussed.

Submodule overcurrent caused by DC pole-to-pole fault in modular multilevel converter HVDC (MMC-HVDC) system is one of the important research objects about its electrical characteristics. In this paper, the fault mechanism before and after the converter blocked was analyzed respectively and the circuit model for the analysis of submodule overcurrent was explored. The analytic equation for overcurrent calculation was deduced and a detailed analysis was also performed. The changes of submodule overcurrent stress with different circuit parameters were obtained and the key issues were also summed up. The results indicate that the submodule overcurrent is the AC system three-phase short-circuit current superposed the discharging current before the converter blocked, and the submodule overcurrent is the AC system three-phase short-circuit current superposed the valve reactor freewheeling current after the converter blocked. From the computation and simulation results, it is concluded that the analytical method is feasible and its calculation results are comparatively precise.