F.Bellina, P.Bettini, P.Campostrini, A.DeLorenzi, E.Gaio, A.Luchetta, A.Maschio, R.Piovan, V.Toigo, P.Zaccaria
Consorzio RFX, Corso Stati Uniti 4, 35127 Padova (Italy)
In the framework of the ITER EU Home Team design activity an R&D Task was launched for the development of vacuum circuit breakers (VCB) able to interrupt DC currents up to 60 kA to be used as components of the Fast Discharge Units for the Toroidal Field (TF) and Poloidal Field (PF) ITER circuits. In particular the specifications for the tests state that the VCBs should demonstrate their capability to interrupt currents up to 66 kA, with an i2t of 8.7 x109 A2s, and to withstand recovery voltages up to 24 kV their life will be experimentally determined under these operating conditions.
Consorzio RFX has been requested from the breaker manufacturer to perform the life tests in its own Power Supply facility the power supply system of the RFX Ohmic Heating (OH) circuit, though designed for a nominal DC current of 50 kA, could be utilised with some modifications to fulfil the requirements of the tests.
In effect the busbar system of RFX OH circuit was designed for a short circuit current of over 300 kA, taking into account possible fault conditions. As dummy load for the tests a spare part of the OH winding has been used, made by six independent coils assembled in one pancake, which will be connected in series to form an inductor with L = 6 mH, R = 2.8 m , capable of carrying 70 kA and withstanding 118 kV from terminals to ground.
For supplying the current six converter units (each one capable of 12.5 kA, 2 kV no-load voltage) will be used in parallel, adding two more units to the usual arrangement of the OH circuit. The artificial current zero is produced by discharging two counterpulse banks (C = 306 5F each) through two parallel branches (composed of one 45 5H reactor and one fast recovery diode each). In this way the required 80 kA current in the counterpulse network is obtained without increasing the charging voltage of the capacitor bank.
A subset of electrical measurement used during RFX operation will be used with proper modifications to monitor the system and to record the test data.
Finally a protection crowbar, driven both directly from a hard-wired system and by the genral protection system of RFX, will be operated in case of misfiring of the counterpulse network, of failure of the VCB to open, of failure of the VCB to interrupt the current or to sustain the recovery voltage.
The tests are now in progress and the paper will report their main results, together with the design criteria of the facility.