Quench detection of superconducting bus-line of LHD by fuzzy theorem

In the LHD (Large-scale Helical Device) system, superconducting coils are fed by superconducting bus-lines. For sound operation of the LHD, a reliable quench detection system of the superconducting bus-lines is essential. The first difficulty for the quench detection is the lack of the voltage monitoring leads closely running along the conductors. The conductors of the bus-lines are covered by thick thermal insulation pipe. Taking care for the electrical insulation, no voltage lead is inserted inside the pipe. Therefore, one has to locate the voltage leads outside the pipe. This situation will bring heavy voltage noise induced by the stray magnetic field. So, the authors have proposed a voltage detection system which utilizes a noise cancellation technique. In this system, the difference of the voltage of the plus polarity lead and that of the minus one is monitored. The magnetically induced noises of both leads almost cancel each other in the monitoring voltage. All the voltage signals are converted to digital ones using 16-bit A/D converters, and processed digitally. These digitized voltage signals and other signals, like the current and liquid He, levels are input to PCs. The PCs carry out the fuzzy computation and calculate the "dangerous rate" which stands for the state of the superconducting bus-lines. To obtain further fine adjustment to minimize the noise voltage, a digital differentiation technique is also introduced. Experiments of the proposed fuzzy SC-bus monitoring system were carried out in May 1998, with the LHD system of NIFS, Told, Gifu, Japan. These experimental results are shown in this paper.