Welding behavior of vacuum interrupters equipped with CuCr contact material caused by making and breaking operations under short circuit current interruption

Abstract

Contact materials based on copper-chromium (CuCr) are widely used in vacuum interrupters for medium voltage applications. Copper-chromium materials with a chromium content between 25 and up to 50 wt.% are selected for the majority of vacuum interrupter applications. For short circuit current making and breaking operations in particular, the material used in the majority of transverse magnetic field (TMF) contact systems worldwide is based on CuCr-25...30 (25 to 30wt.% Cr and 75 to 70wt.% Cu). This study was carried out on standard contact material with a chromium content of 25wt.% and the TMF contact system. A huge number of making, and some breaking, operations under short circuit current conditions were performed on standard vacuum interrupters, as well as materials with micro added elements and chemical compounds to reduce the welding breaking forces. In addition, different contact material production technologies were tested and compared. The tests show that the weld strength decreases according both to the selected production technology, and the composition of the contact material itself. The welding forces, especially after the making operation, were measured by quasi-static tensile test (via slow motion movement of the test set-up), and under dynamic conditions such as the opening operation of the vacuum circuit breaker. Both tests were carried out at ambient temperatures. The statistical effect of the development of the welding force from operation to operation is presented. Characteristics of the fractured surfaces and at the cross section were investigated after a number of making and breaking operations by using a light microscope, scanning electron microscopy (SEM), and energy dispersive X-ray (EDX).