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.
Implementation and First Operational Experiences With the High Voltage In-Service-Tests on the Superconducting Magnet System of Wendelstein 7-X
A hypothetical ground fault in the superconducting (sc) magnet system of Wendelstein 7-X would shift the midpoint of the grounding system and therefore increase the voltage to ground during a fast discharge of the sc magnet system. As this voltage could endanger the high voltage integrity of the sc magnet system, an In-Service-Test system has been developed to monitor the insulation of the sc magnet system during operation especially after changing the loads of the magnets. The system, that is associated to the power supply system, has been designed to lift the electrical potential of non-planar coil system with respect to ground by 1.5 kV and to measure the resulting leak current while the magnets are operated. The leak current is used to evaluate any changes in the insulation resistance and to decide whether a fast discharge can still be done without endangering the sc magnet system. The design of the In-Service-Test was evaluated with computer simulations of the power supply and the magnet system, and a prototype that was tested at one coil group together with the corresponding power supply. In 2014, the system was finally installed for all five non-planar coil groups and commissioning took place for the first experiment campaign in 2015. Operated during the first experimental campaigns of Wendelstein 7-X from 2015 to 2017 the experiences gained led to modifications that already have been implemented and tested during the operation campaigns in 2018 and 2019. The paper describes the implementation of the In-Service-Test system and the associated tests for the commissioning as well as the first operational results and corresponding modifications.
Verification tests of critical bolted connections of the W7-X coils
Abstract The functionality of the most critical bolted connection between a coil and the central support structure of the Wendelstein 7-X (W7-X) magnet system was mechanically tested on full-scale mock-ups. The object of the test campaign was to verify the final design, in particular the load capability of the connection. Special attention was drawn to its individual components (bolts, sleeves, and spherical washers) and the assembly steps, here especially the positioning and welding of wedges. Furthermore, the instrumentation foreseen for bolts on W7-X was studied. A consecutive series of tests was performed using mock-ups equipped with modified components. Each test sequence included activities at ambient temperature (assembly, bolt preload, and check of the measurement system) and load tests in liquid nitrogen. To simulate the expected lifetime of W7-X, several thousand load cycles were executed. Additionally, high load tests were performed to investigate the limits of the connection. It was shown that the connection could take the nominal loads with a safety margin of about 1.2. Above this the bolts starts to deform plastically. A MoS2 lubricant layer introduced between the relevant sliding surfaces of the bolted connection significantly reduced stick-slip which was detected during previous test campaigns. This paper gives an overview on the functionality test and main conclusions of the experimental test campaign.