Fault analysis and special tests of thyristor for RFX AC/DC converters

In the RFX nuclear fusion experiment, 20 AC/DC converter units, for a total installed power of over 400 MVA, are used to feed the machine magnets. Due to the special characteristics of the load, and in particular to the expected fault conditions, the converter bridges must withstand a high number (approximately 2000) of severe overloads during the life of the machine; in particular, the thyristors must carry a current two or three times larger than the nominal current without any damage. In addition, the thyristors must not explode, even in the case of a slightly higher surge current than the previous one, when the fuse does not blow quickly. As the manufacturer's data did not cover the thyristor performance under these specific requirements, new tests on the thyristors had to be carried out, and the results are discussed