Internal Transient Over-Voltages in Large Fusion Coils

Magnets show a linear voltage distribution along the windings for slow changing currents. This simple behavior may be lost when fast transients occur, e.g. for fast discharge or earth fault events. Such transients may become critical with respect to the magnet lifetime if they are not considered in design and acceptance tests. The problem of internal transient over-voltages can become more essential for large magnets with high number of turns because the critical frequencies for transients are lowered due to the large number of capacitances and inductances of windings and winding packs. At KIT large fusion magnets such as POLO and the ITER Toroidal Field Model coil (TFMC) have been investigated in the large coil testing facility TOSKA. For these coils direct measurements of internal over-voltages were possible and could be compared to calculations of transient voltages. As a consequence a set of transient voltage calculations was executed at KIT for ITER coils. An overview of calculations and results for ITER TF and PF coils is given and consequences for high voltage testing are discussed.