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

Abstract

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.

An Experimental Approach for Condition Monitoring of Magnetic Cores With Grain-Oriented Electrical Steels

“…Quality of the magnetic cores are determined by electrical and magnetic properties of the magnetic materials, coating of the laminations which determine the inter-laminar resistance between the laminations, clamping pressure, manufacturing processes, etc. Key amongst these are manufacturing processes which have direct impacts on properties of the materials and hence normal operation of the magnetic cores and related devices [1][2][3][4][5][6][7].…”

“…ILF leads to circulating eddy currents between the defective laminations, which cause hot spot and extra localised power loss at the defective zone. A few faults may not create high ILF currents; but with several faults, the induced fault currents could be large and cause excessive local heating in the damaged area [5][6]. Though a large number of ILF could result in catastrophic breakdown, the machine can still be in operation with a small number of ILF, but with higher power loss and hence lower efficiency.…”

“…ILF detection and condition monitoring of the magnetic cores have been an active area of research and interesting topic for designers and manufacturers of the electrical steels and laminated magnetic cores. In this regards, practical methods have been introduced and successfully employed to evaluate ILF of rotating machines [4][5][6][7][8][9][10][11][12] and transformers [13][14][15][16], and other magnetic devices with laminated cores. An overall review of these methods is performed in [1].…”

Interlaminar Insulation Faults Detection and Quality Assessment of Magnetic Cores Using Flux Injection Probe

“…Typically fault current loops are formed between the shorted laminations and fault points which are perpendicular to the direction of the flux. Interlaminar fault, which lead to inter-laminar fault current, is one of the most serious concerns of the manufacturers and customers of the electrical steels [5][6][7][8][9][10][11]. Fig 1-a shows a perspective view of a transformer limb with two possible interlaminar faults in the top step.…”

“…Eddy current distribution in the laminations without fault and inter-laminar fault current in presence of the faults, from cross section view of eddy current loss caused by the inter-laminar fault depends on the position of the fault points, fault current loop and number of shorted laminations. A few faults may not create a high inter-laminar fault current; but with several faults in the core the induced inter-laminar fault currents could be large and cause excessive local heating in the damaged area [10].…”