Low Voltage Prototype Design, Fabrication and Testing of Ultra-Fast Disconnector (UFD) for Hybrid DC CB

Direct current Circuit Breakers (DC CB) are vital components for DC grids and the Hybrid DC CB is of high interest because of fast operating speed and low on state losses. The speed of operation of hybrid DC CB is crucially determined by the speed of Ultra-Fast Disconnector (UFD). This article presents in some detail design, fabrication and testing of a low voltage UFD at a University laboratory. The UFD is designed to operate in 2ms and separates the contacts by 3.0mm, which is adequate to withstand a voltage level of 7kV in air. The contacts of the UFD are designed to carry a rated current of 100A. The speed requirements demand a driver capable of supplying a current pulse of few kA, which is achieved using capacitive storage. The experimental measurements show that the UFD is capable of opening and closing in around 1.8ms. Higher opening speeds are possible but bounce becomes pronounced. The paper also presents a method of magnetic braking which eliminates bounce and enables application of higher forces and the opening time of 1.3ms is demonstrated. The speed of UFD open-close-open cycle is dependent on the charging time of the UFD driver circuit which is quite long because of large stored energy. It is concluded that at least two complete driver circuits are necessary.