Design and Prototype of the High Voltage In-Service-Tests on the Superconducting Magnet System of Wendelstein 7-X
The stresses induced during the energizing and de-energizing process of the Wendelstein 7-X super conducting magnet system could cause cracks in the embedding of the winding packages and consequently a ground fault that would not be detected during normal operation as the operating voltage of the magnet is system is too low. With a ground fault present during a fast discharge, the middle point grounding needed to half the voltage could be rendered useless and one coil group could be stressed with the full voltage of up to 4 kV which would endanger the high voltage integrity of the whole superconducting magnet system. To prevent such undesirable occasions, the insulation of the superconducting magnet system shall be monitored by a high voltage In-Service-Test. With the In-Service-Test, a voltage of 2.5 kV can be applied during the operation of the magnets especially after significant changes in the coil current. The measured leak current can then be used to determine any changes in the quality of the insulation. In a first step, the feasibility of such a system has been evaluated with a simulation model of one coil group with Simplorer. The next step was to find a design which can be implemented in the given magnet power supply system and to build and test a prototype for final confirmation of the concept.
Feasibility Study of a High Voltage In-Service-Tests on the Superconducting Magnet System of Wendelstein 7-X
During the energizing and de-energizing process of the Wendelstein 7-X magnet system, the forces induced in the superconducting coils could lead to stresses in the insulation of the coils, the interconnecting bus system, the current leads and the mechanical support structure. Such stress could cause a ground fault that would not be detected during normal operation. In normal condition the voltage across a coil group is halved by a middle point grounding system. If a ground fault is present during a fast discharge, the coil group could be stressed with the full voltage of up to 4 kV which could endanger the high voltage integrity of the whole superconducting magnet system. To prevent such undesirable occasions, the insulation of the superconducting magnet system shall be monitored by a high voltage In-Service-Test. That means that a test voltage shall be applied during the operation of the magnets. Therefore, it is planned to temporarily lift the middle point of the grounding system by a high voltage source of 2.5 kV and to measure the leak current. To evaluate the feasibility of such a system, a simulation model of one coil group has been created in Simplorer. Considering detailed parameters of the coil group, the model shall support the evaluation of how the high voltage influences the superconducting magnet system and shall determine the leak currents with and without certain damages in the insulation. The paper describes the Simplorer model with its different simulation scenarios and summarizes the results and findings.
FEM analysis of coil support connections in the Wendelstein 7-X magnet system
Abstract The objective of the Wendelstein 7-X project is to test potential application of stellarator-type fusion reactor as a power plant. In this device the plasma channel is under the control of a magnetic field generated from and complicated magnet system, comprised of 70 superconducting coils symmetrically arranged in five identical modules. Each coil is connected to a central ring with two bolted connections which transfer the loads resulting from the electromagnetic field and gravity. The aim of this work was to analyse the mechanical behaviour of the connections by means of detailed 3D finite element models. All simulations were performed assuming elasto-plastic behaviour of the materials, and the assembly stresses and friction contacts between the different parts of the connections. The stress distribution, displacements and the forces acting on the bolts and welds were studied using standard methods. The results were subsequently used to optimise the design of the critical central support elements.