High voltage tests of the ITER toroidal field model coil insulation system

A superconducting Toroidal Field Model Coil (TFMC) was developed by the European Home Team within the framework of the R&D program of the International Thermonuclear Experimental Reactor (ITER) and is presently tested in the TOSKA facility of the Forschungszentrum Karlsruhe. To withstand the Lorentz forces the 98 turns of the ITER TFMC are divided into 10 pancakes, embedded in five stainless steel (SS) radial plates and enclosed in a grounded SS case. To minimize the voltage stress of the conductor insulation the potentials of all five inner pancake joints are connected to their own radial plates. This insulation concept offers the possibility to test three kinds of insulation types: the conductor insulation between the conductor and the surrounding radial plates, the radial plate insulation between two adjacent radial plates, and the ground insulation between all radial plates connected with the conductor and the grounded case. The rated terminal to terminal voltage is 9.8 kV for a symmetrical grounding. Based on the rated voltage, the insulation design and the manufacturers' specifications, a DC and AC test program is executed after each set of defined insulation stresses on every type of insulation at room and cryogenic temperatures. In addition, partial discharge measurements are performed as a nondestructive diagnostic tool for the observation of the status of electrical insulation. It is well known from conventional high-voltage engineering that large electrical systems must be designed to withstand internal overvoltages caused by fast rise times of the applied voltage. Therefore calculations were performed, both in time and frequency domains, to describe the transient electrical behavior of the ITER TFMC. To prove the insulation strength and to verify the calculations, high voltage impulse tests with fast rise times (9.8 kV peak) and low voltage high frequency tests are performed.