High voltage investigations for ITER coils

The superconducting ITER magnets will be excited with high voltage during operation and fast discharge. Because the coils are complex systems the internal voltage distribution can differ to a large extent from the ideal linear voltage distribution. Hence, the determination of the transient voltage distribution is important for a proper insulation co-ordination and will provide a necessary basis for the verification of the individual insulation design and the choice of test voltages and waveforms. Transient calculations for the ITER TF coil system have been performed for fast discharge and fault scenarios to define test voltages for ITER TF. Even the investigation of the TF coil fast discharge without a fault showed that additions of the ITER documentation are recommended. Calculations of the transient electrical behaviour have also been started for two PF coils. nductor and radial plate insulation of the ITER TF Model Coil were exposed at room temperature to test voltages derived from the results of the calculations for the TF coils. Breakdown appeared during the highest ac voltage step. Increasing of the partial discharge activity before the first breakdown was measured. A fault location was identified in the ground insulation by burnout.