Cronje, Willem

Doorsamy, Wesley

Dupré, Luc

Mohamed Abouelyazied Abdallh, Ahmed

English

Ghent University Academic Bibliography

A Prior Selection of Measurement Modality for Eccentricity Detection in Synchro-nous Machines using a Crame´r-Rao Lower Bound TechniqueWesley Doorsamy1, Ahmed Abdallh2, Willem Cronje1, Luc Dupre21University of the Witwatersrand, Johannesburg, South Africa; 2Electrical Energy, Systems and Automation, Ghent University, BelgiumRotating electrical machines are indispensablecomponents in industry. These machines requirededicated systems to prevent catastrophic failures,especially when involved in critical industrial pro-cesses. Therefore, it is essential to use continuousonline monitoring systems that can diagnose inci-pient faults at an early stage and allow for remedialaction before development of severe fault mecha-nisms. A typical example is the synchronous ma-chine which is commonly utilized in power genera-tion schemes where early fault detection methodsare imperative. In synchronous generators, an air-gap eccentricity fault can deteriorate through a cas-cading effect and result in problems such as bea-ring failure amongst others [1].Numerous methods are available for the diagno-sis of an eccentricity fault in rotating machines [2].Each technique is based on measurements whichhave an associated accuracy. Several methods areavailable to detect eccentricity in synchronous ge-nerators. Examples of other methods are shaft vol-tage (see Fig. 1) and magnetic flux monitoring,which have been found to have exceptional faultdiagnostic capabilities [3] and are selected for thepresented investigation.In practice, the accuracy of the identification pro-cess depends on the measured quantity beingused. On the one hand, the measurement noisemay significantly influence the results. On the otherhand, the sensitivity of the measured quantity onthe eccentricity level may have a large impact onthe identification results. Therefore, the selectionof the optimal measurement shall ensure the accu-racy of the eccentricity fault detection. In additionto the effect of the accuracy and sensitivity of themeasurement quantity, the identification procedureis significantly influenced by the ambiguous know-ledge of some model parameters. In rotating elec-trical machines, one of the most uncertain para-meters is the one related to the magnetic materialproperties. The material properties of the magneticcore of rotating electrical machines are classical-ly obtained by performing some standard measu-rements. However, these standard measurementsmay not give the actual properties of the materialthat exists in the machine. Therefore, knowledge ofthe real magnetic material properties located in themachine is uncertain. The effect of such uncertain-ties on the accuracy of the eccentricity identificati-on process needs to be estimated.The research presented in this paper aims at selec-ting the best measurement modality for diagnosisof a static eccentricity fault in a synchronous gene-rator. The Crame´r-Rao lower bound (CRLB) tech-nique is adopted to estimate the error associatedwith shaft voltage and magnetic flux based measu-rement techniques. This method takes into accountthe effect of the measurement noise as well as theuncertainty in some model parameters. The tech-nique has been recently used for estimating the er-ror in the inverse problem solution of magnetic ma-terial characterization in different electromagneticdevices [4]. First, a numerical analysis is perfor-med and thereafter these results are then validatedexperimentally by carrying out real measurements.To the best of the authors’ knowledge, this type ofinvestigation has not yet been presented and willdefinitely enhance the accuracy of the eccentricityidentification procedure.For the numerical analysis, four numerical casestudies are performed using the CRLB techniquetogether with a finite element (FE) model of a two-pole synchronous generator. The first one is rela-ted to no-load (NL) conditions, while the other ca-se studies are done for different levels of a resisti-ve loading, i.e. 4, 5, 6 . Each of the case studiesare investigated for different eccentricity levels withand without model uncertainties. Some of these re-sults are given in Fig. 2. The results indicate thatthe shaft voltage is a much more suitable measu-rement modality to detect the rotor eccentricity faultthan the flux probe voltage, for the same measure-ment noise.Numerical results, when uncertainty in the magne-tic material is considered (not shown here), alsoindicate the superiority of the shaft voltage mea-surement modality. Additionally, it is found that theidentification error becomes negligible for the shaftvoltage measurement at saturation conditions. Thismeans that the accurate knowledge of the saturati-on state of the magnetic material of the stator corewill not affect the identification results.Validation of the FE model and the proposed tech-nique was carried out under laboratory conditi-ons. A miniature synchronous generator, designedto mimic large turbo-generators, was used, seeFig. 1. The generator is fitted with flux probe andshaft voltage monitoring systems. The simulatedflux probe and shaft voltage signals correspond tothe experimental measurements indicating the ef-fectiveness of the FE model. Furthermore, appli-cation of the CRLB technique on the experimen-1tal signal harmonics, measured under eccentrici-ty fault conditions, yield a great correspondence tothe numerical results thereby validating the propo-sed technique. Overall results of the work also indi-cate that the shaft voltages are more sensitive to aneccentricity fault and provide a significantly loweridentification error. Furthermore, it is shown thatthe shaft voltage based technique performs excep-tionally well past linear operating regions which isa desirable trait, especially in power generation ap-plications.References[1] J. L. Kohler, J. Sottile, and F. C. Trutt, “Condition-based maintenance of electrical machines,” IEEEIAC, 34th IAS Annual Meeting , Pheonix, AZ, 1999,pp. 205-211.[2] K. Gyftakis and J. Kappatou, “A Novel and Ef-fective method of static eccentricity detection inThree-Phase PSH Induction motors,” IEEE Trans.Energy Convers ., vol. 28, pp. 405-412, 2013.[3] W. Doorsamy, W. A. Cronje, and A. S. Meyer,“Multiple fault diagnosis on a synchronous 2 polegenerator using shaft and flux probe signals”, IE-EE ICIT , Cape Town, 2013, pp. 362-367.[4] A. Abdallh and L. Dupre´, “Magnetic materi-al characterization using an inverse problem ap-proach,” in Advanced Magnetic Materials , ISBN978-953-51-0637-1, InTech, 2012.Fig.1: Experimental synchronous generator withview of shaft riding brush (driven-end) used forcondition monitoring purposes.Fig.2: (a,b) Normalized values of the simulatedflux probe and shaft voltage responses, for diffe-rent eccentricity levels at no-load conditions. (c)The identification error of the two measurementmodalities, when no uncertain model parametersare considered.2

A prior selection of measurement modality for eccentricity detection in synchronous machines using a Cramér-Rao lower bound technique

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A prior selection of measurement modality for eccentricity detection in synchronous machines using a Cramér-Rao lower bound technique

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