DC insulation tests of W7-X coils at CEA, Saclay

Abstract

The Wendelstein 7-X (W7-X) is an advanced optimized stellarator under construction at the Greifswald branch of the Max-Planck-Institut für Plasmaphysik. Its magnet system consists of 70 superconducting coils, i.e. five types of 10 non-planar (NPC) and two types of 10 planar (PC) individuals each. In case of an emergency shut-down like in succession of a quench, the voltage across a NPC may raise up to 450 V according to the specification. Since all the 10 coils of a type are connected in series, the total voltage could thus reach 4.5 kV. Therefore, the coils had to be provided with a corresponding electrical insulation. Its test voltage levels were fixed at 10.4 kV and 7.2 kV, respectively, for the NPCs and PCs. The coil acceptance tests take place in an ad hoc facility at CEA, Saclay where a large variety of trials are performed, including the verification of the electric insulation quality. Several DC high voltage tests (DHVT) are carried out to assess the proper manufacture of the ground insulation which stretches across the winding pack, conductor terminals, interlayer joints, and the numerous quench detection wires including their connections. The DHVTs are performed under different conditions: before cool down at room temperature and atmospheric pressure, then at cryogenic temperature and, finally, under vacuum at room temperature. Since the coils are necessarily connected to the test facility, the corresponding components like wires, plugs, or connections are also energized by the nominal voltage. Thus a flaw in the external test bed may invalidate the outcome of the experiment. It is therefore very important to determine whether the origin of a current leak is in the coil or in the test facility. To do so, it is necessary to have deep knowledge of the whole arrangement. The paper summarizes the experimental observations, data analysis, and consequent improvements which were implemented at CEA test facility.