Improving interruption performance of mechanical circuit breakers by controlling pre-current-zero wave shape

Mechanical circuit breakers(MCBs)are the limiting component for current injection HVDC circuit breakers.Improving their interruption performance reduces requirements for capacitance and inductance needed in the injection circuit and thus space use and costs.Higher performance can be achieved by creating a period of low current gradient before zero crossing in the MCB,e.g.by using a saturable inductor(SI).In this paper,the impact of duration and steepness during the low current-gradient phase is linked to arc parameters of the investigated model gas circuit breaker.It is shown in a scaled experimental setup that an optimum design of the SI can be derived from arc time constant and interruption limits for constant current gradients.This optimisation results in a considerable increase of interruption performance.The feasibility of implementing an SI in a full-scale HVDC circuit breaker is demonstrated using simulations.Using an improved injection scheme,the stresses for the MCB can be reduced significantly.Consequently,the injection circuit components can be scaled down,making the topology more economical.The reduced interruption requirements might also make it possible to use a single gas interrupter instead of a series connection of vacuum interrupters,reducing the complexity of the mechanical switch.