EL CID results interpretation in the presence of substantial core faults

Monitoring the health of large turbine-generators is now an integral part of their operation. For the stator core the EL CID test has proved a fast, reliable and valuable means of measuring the interlamination insulation integrity, with a simple to understand measure of acceptance. However there are some special cases where this simple interpretation using just the QUAD value can be inadequate, in particular when there is a large existing fault in the same core packet. Since the total magnetic potential (m.m.f.) around the core must always equal the excitation current, as this is the only external current, core slots away from the faulted region in the same packet will all acquire small Quad Recovery signals in opposite polarity to the fault. Normally this is inconsequential, however when large faults occur it can become a consideration in interpreting the EL CID test results. This paper describes the construction of computer FEA models that demonstrate the distribution and uniformity of the effect, which thus enables corrections to be applied. In a case study this effect reduced the amplitude of another smaller fault in the same circumferential region, which led to it going unnoticed until the main fault had been repaired. From this work, guidance is given on interpretation of EL CID results in the case of substantial or multiple core faults in the same region.