Evaluation of EL CID indications not associated with stator core interlaminar insulation faults

The integrity of the interlamination insulation in large stator cores is commonly tested with EL CID. Some recent tests on large turbo-generators resulted in apparent faults indicated by axially aligned bands of negative and positive EL CID Quad signals nearly 180° around the machine. However no other sign of core fault could be determined. Investigation showed the core had been systematically stacked in 180° arcs using two different lamination materials. EL CID is normally insensitive to variation in core loss and permeability, however it will respond to steels of varying characteristics placed in the same circumferential plane. This paper shows observed data and a new technique to analyze the loss energy vector was developed to demonstrate how these Quad signal artifacts can develop. From this an analytical model based on data from the steel was able to numerically predict the observed signals. By elimination of all other possible sources of these signals, this was concluded to be the root cause. The material property variances were significant only at the low 4% flux levels used in the EL CID test. At full service flux the variances reduced to acceptable levels so do not impact on the high flux test or machine reliability.