Current low power core testing using EL CID

Traditional test methods to provide information on the condition of insulation between core laminations of large rotating electrical machines require a substantial supply to excite the core at rated magnetic flux, temperature rises due to faults being detected by various means. The test is performed with the rotor removed utilising significant time and labour. Increasing use is now made by manufacturers and utilities of the alternative low flux EL CID (electromagnetic core imperfection detector) test. With low excitation power, faults are detected electromagnetically with the ability to detect faults behind windings. Taking much less time than the rated flux test, and with reduced safety constraints, it may be the only test option practicable in an outage. In some instances tests may be made without removal of the rotor, not feasible with a high power test. An outline of the theoretical and practical considerations for EL CID testing are discussed. EL CID is applicable to both turbogenerators and hydrogenerators, differences are mainly confirmed to test implementation and interpretation of spurious effects