Earth Faults During RFX Initial Fabrizio Bellina, Giuseppe Chitarin, Massimo Guamieri, Andrea Stella, Francesco Trevisan GRUPPO DI PADOVA PER RICERCHE SULLA FUSIONE Associazioni EURATOM-ENEA-CNR-Universita di Padsva Corso Stati Uniti. 4 - ABSTRACT RFX, the largest RFP machine, has air-core poloidal windings. A tree-shaped earthing geomeuy has been adopted for all the machine components, to avoid electrical loops. Nevertheless, during the first operation phase a number of accidental contacts occurre& which caused loops currents high enough to distort plasma equilibrium. These loops could be detected by means of RGM, a system designed to perform fast winding protection, but able to detect accidental earth currents as well. After careful analyses of the signals, these earth faults were always located and removed. The use of a compass resulted particularly useful in the occasion of a number of these faults, the others were detected by means of rogowski coil probes. 35020 Padova - Italy enclosed in insulating caps, while the OH coils have terminals in the core bore, far from the vacuum vessel. The Toroidal Field (TF) coil terminals (operating up to 3.5 kV to ground) are insulated from other components using insulating tiles. These design criteria are aimed to avoid the earth faults originating from high voltage points. The Torus Assembly, constituted by the vacuum Vessel, the stabilising Shell, the TF and the FS Coils, includes also a number of components, such as diagnostic equipment, pumping ports and ancillaries, cooling ducts, cable trays, which are more subjected to accidental contacts, due to the little distance among these components and to the frequent maintenance operations. To reduce the risk of accidental contacts visual inspection procedures have been adopted. 11. TORUS ASSEMBLY EARTH SYSTEM I. INTRODUCTION A. Supporting Structure RFX is the largest machine in the RFP confinement line, with 2 m and 0.5 m major and minor radii and expected plasma currents up to 2 MA. The machine has air-core poloidal field windings, able to produce a magnetic flux up to 15 Wb, and the magnetic field is relatively strong in all the region around the machine. The swing of the poloidal flux, that starts the plasma current, is required to occur in a relatively short times (15 to 50 ms), involving high time- derivatives of the magnetic field [ 1 ]. For all these reasons, currents can be induced in any conductive loop made up with machine components. Such unwanted effects, dangerous for plasma equilibrium, are prevented by means of a tree-shaped earthing geometry for all the machine components and most of the objects located around the machine which must be connected to earth. Besides that, a particular care has been given to the insulation of the coils operating at high voltage which are placed next to the vacuum vessel and the stabilising shell, where a large number of objects is assembled. The surface of both Ohmic Heating (OH) and Field Shaping (FS) coils, which operate up to 17.5 kV to ground, is varnished with semiconducting paint and grounded. The terminals of the FS coils are close to vacuum vessel ports, and are entirely The machine is provided with a Supporting Structure that supports both the OH coils and the Torus Assembly. The structure is made of an External Structure and an Internal Crown 121. Manuscript received October 14, 1993 0-7803- 1412-3$04.0001994IEEE A Fig I Scheme of the RFX Supporting Structure 345