Stator Core Interlamination Faults and their Detection by Electromagnetic Means

UoM administered thesis: Phd

  • Authors:
  • David Bertenshaw

Abstract

Condition monitoring of large electrical machines, especially power stationgenerators, is now an integral part of their operation to maintain reliability. This thesisstudies the use of electromagnetic test (EMT) methods for the stator cores of largegenerators to detect interlamination insulation faults before they propagate to a level thatcan lead to machine failure. While the EMT has been long established, its competencecompared to reference thermal tests remains anecdotal. The work seeks to establish confidence in this correlation, research certain phenomena causing potential false test results, and prove the test sensitivity of short buried faults. This is important since it is also shown that the offline EMT is the only cost-effective condition monitoring method for large stator cores. The various EMT systems in use and the dominant EL CID system are reviewed. It is shown that despite their apparently uncorrelated metrics, there is a close equivalence between the differing fault power and current thresholds. The reference thermal high flux stator core test is assumed to be correlated to the EL CID test, but without evidence. Field research showed that there is quite a strong statistical correlation between the two tests, in accordance with the expectations. The studies also revealed the unrecognised impact of lamination segment joints on the high flux test excitation requirements.The basic theory of EL CID is analysed to determine the tests dependence on coreloss and fault inductance, showing that core loss can substantially attenuate severe faultsignal detection. The demodulation of the fault signal is also analysed to determine itssusceptibility to harmonics generated by variable permeability and hysteresis. A method isdeveloped to estimate low flux density Rayleigh hysteresis from commercial steelspecification data, which showed that harmonic errors will remain minor for normal steels.The impact of local variations in core loss and permeability are also analysed, and showncapable of generating substantial false error signals, verified by field experience. Theirprofile is identified and mitigation test strategies proposed. It is also shown that multiplesubstantial faults when axially aligned can interact and attenuate each other, with theinfluence extending around the whole circumference due to lamination eddy currents. Acompensating interpretation algorithm is proposed.Previous research had developed 3D FE models of faults buried in the core teeth andyoke to determine their detection sensitivity, but without experimental verification. A newexperimental technique is developed to introduce such faults, with correction for theeffects of intrusion. It is shown that the original models suffered from inaccurate faultcurrent computation, and did not include the effect of core packet air gaps. A method ofcompensating for measured fault current is developed, together with new FE models toestimate the impact of packet air gaps. After correction for the problems, it was shown thatthe combined FE models electromagnetic detection correlated closely with experiment,enabling a reliable determination of the EL CID sensitivity to short buried faults

Details

Original languageEnglish
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Award date31 Dec 2015