Electromagnetic modelling and detection of buried stator core faults

Abstract

Interlamination insulation faults in the stator cores of large electrical machines can damage both winding insulation and stator core, thus confidence in electromagnetic test results is important. They may be validated by FE methods, however the 3D models required for short faults are computationally challenged by laminated structures, requiring approximations. A homogenised 3D FE model was used to model faults buried in the teeth and yoke of the core, with a new experimental methodology developed to calibrate fault currents. Limitations were identified in modelling just a core section due to images and the constraint of axial packet air gaps on fault flux dispersion. A system of transverse 2D FE models of the principal fault flux paths in the core were constructed to estimate the differential impact on fault signals by the air gap presence and applied to the 3D FE model. Together with corrections for images this gave close predictions of experimental results, supporting the validity of the model. The verified electromagnetic test results now permit assessment of the threat that a detected buried fault presents.

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].…”

Inter‐laminar fault analysis of magnetic cores with grain‐oriented electrical steels under harmonic distortion magnetisations

“…Specific power loss published in the data sheets of the material, however, do not count for the geometry of the magnetic cores, and degradation of the material due to manufacturing processes. Furthermore, it is well distinguished that low inter-laminar resistance in the clamped magnetic cores due to, for example, edge burr or damage on the surface coating has a significant impact on the local and overall power loss of the magnetic cores [11][12][13][14]. Therefore, designers of the electrical machines and transformers usually find considerable deviation between the Epstein frame results and overall power losses measured from the assembled cores.…”

Development of a new approach to core quality assessment of modern electrical machines

“…Core quality mainly depends on electrical and magnetic properties of the magnetic material, quality of the insulation material, which determines the inter-laminar resistance between the adjacent laminations, clamping pressure, magnetising condition and so on. Key amongst these are inter-laminar faults which have been identified as a major threat for normal operation of electrical machines and transformers [3][4][5][6][7][8][9].…”