Study of the transient voltage behaviour of the present ITER TF coil design for determination of the test voltages and procedures : Task FU06-CT 2003- 00119 (EFDA/03-1055). Final report

ITER is designed as an experimental tokamak fusion reactor in which the conditions for controlled fusion reactions will be created and maintained. The confinement and control of the reacting plasma is performed by superconducting magnets. Insulation faults are regarded from the ITER International Team as the most probable cause of fusion magnet failure. Considering the difficulties involved in the replacement of a toroidal field coil in the ITER magnet system and the different problems occurring during high voltage tests of the ITER model coils further improvements in several aspects of high voltage technology for the realisation of the ITER magnets are indispensable. One of these aspects is the consideration of the transient electrical behaviour because it is well known that fast changes of voltages (e. g. lightning and switching impulses) may cause a non linear voltage distribution on the coil turns and possibly excite resonances within a large coil. Such high voltage stress can cause local overloading and irreversible destruction of the insulation system. This report will present the calculation of the terminal voltages within the ITER TF coil system and the voltage stress of the three insulation types (ground, radial plate and conductor insulation) within an individual ITER TF coil for the fast discharge and two fault cases. An electrical network model for the ITER TF coil is developed and simulated with the code PSpice. The internal inductances and capacitances as well as the capacitances to ground for the establishment of this network model are determined. Skin and proximity effect as well as the damping caused by eddy currents in the stainless steel radial plates, in which the conductor is embedded, are calculated by the FEM code Maxwell. For the complete TF circuit, composed of 18 TF coils and 9 fast discharge units, an additional network model is set up and implemented with the code PSpice. Due to the large size of the individual ITER TF magnets the resonance frequency is lower than for the TF model coil. It was also determined that the three types of insulation within a single TF coil are stressed with a nonlinear voltage distribution under a fast discharge condition. The non linear voltage distribution is enhanced in case of fast excitations applied in consequence of ground faults. Therefore the test voltages have to be defined in consideration of the stresses caused by fast discharges and realistic fault cases to ensure a reliable operation during the foreseen ITER lifetime. Proposals for the high voltage test procedures will be discussed based on the calculated voltage stress and the experiences gained during the ITER TF Model Coil test. Several acceptance criteria are suggested and recommendations are added to avoid unacceptable stress during the high voltage tests.