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Hybrid circuit breakers are a class of protection devices to facilitate fault current limitation and fast interruption in AC and DC power systems. They are one of the key enabling technologies of multi-terminal DC transmission and distribution systems. As the power rating of DC power systems increases, mechanical switches in the normal conduction path are chosen to provide low on-state loss in a hybrid circuit breaker topology. These mechanical switches require fast actuation to achieve sub- millisecond switching capability, for which only a few actuation mechanisms have been reported to be suitable. Besides electromagnetic repulsion methods like the Thomson coil, piezoelectric actuators also appear to be a good candidate. This paper provides a survey on potential mechanical switches for hybrid circuit breakers considering different actuation mechanisms. Performances of mechanical switches reported in the literature have been summarized and compared, such as response time, displacement, and required actuation energy. 

Fast mechanical disconnect switches (FMS) are an integral part of hybrid circuit breakers, which are proposed as protection devices to clear faults in medium voltage distribution systems. The proposed FMS is a vacuum switch that is operated by an amplified piezoelectric actuator. Such actuators enable unprecedented speed and contact separation in less than one millisecond. The limitations that come with such designs are the low contact separation of typically less than one millimeter in open position and low contact force in the order of 100 N in closed position. This requires a new design of the contacts to operate under such constraints. The geometry of the contacts must be carefully designed to minimize electrical resistance when closed and minimize electric field enhancement when open. The paper presents finite element analysis and experimental results with the aim of identifying the most suitable contact geometry for FMS. The experiments show that optimized contact geometries have up to 40% less resistance than the initial spherical geometry. 

Fast mechanical disconnect switches are an integral part of hybrid circuit breakers, which are proposed as protection devices to clear faults in medium voltage distribution systems. Compared to their conventional counterparts, hybrid circuit breakers can have the ability to limit the fault current, which could allow more interconnections between substations with advantages with respect to grid reliability and resiliency. Furthermore, they enable the integration of distributed generation such as small solar power installations without expensive changes to the grid infrastructure. The proposed design of an ultrafast mechanical disconnect switch operates in vacuum, carries continuous current similar to conventional vacuum interrupters, opens at current zero, features minimum moving mass, and has an open contact separation of less than a millimeter. The limited separation distance requires an optimized contact geometry to keep the electric field within safe limits, minimize the moving mass, and reduce contact resistance. This paper proposes different contact geometries and uses finite element analysis to compare the contact geometries with respect to maximum electric field, mass, and contact resistance. Reductions of mass by 50% and reduction of contact resistance of 10% have been achieved.