The application of the electromagnetic coreimperfection detector (EL CID) techniqueinvented by Sutton [19801] in the late 1970s is

now familiar to many manufacturers and users of largerotating electrical machines. Over the years a numberof initially puzzling phenomena have been observedin the results, particularly with respect to large diame-ter salient-pole machines, which are common inhydroelectric schemes. There have also been someinteresting features encountered in results from turbo-generators.

While all the phenomena observed have now beendefined and analysed by Ridley [20042], the purposeof this paper is to bring these matters into deeper focusso that EL CID may be used to greatest advantage.Some aspects may be already fully identified, but areincluded for completeness.

It is assumed that the basis of the technique is famil-iar, that is, in essence the stator under investigation forsoundness of its interlaminar insulation has a circum-ferential flux induced in it and any damage to the insu-lation allows a circulating fault current to flow (seeFig. 1). The magnetic fields from both the circumfer-ential flux and fault current are detected by a Chattockpotentiometer (also known as a Rogowski Coil), andby phase separation the magnitude of the fault defectmay be determined.

1. Magnitude of electromagneticinductionHistorically, the available excitation source during theearly development of EL CID was such that the mag-netic flux induced in the stator iron was about 4 percent of the normal operating value, and this hasbecome the standard. The EL CID results are approx-imately proportional to the level of flux induced.Therefore, it is acceptable to carry out the test at otherthan 4 per cent flux, but the results are then scaled asnecessary.

The degree of electromagnetic induction is deter-mined by setting the excitation volts per turn in accor-dance with well known electromagnetic theory [see

Appendix 2.1 of Ridley 20042]. The required value ofvolts per turn (Vt) is given approximately by [12.5 ×machine line voltage in kV] / [Stator winding seriesturns per phase], assuming that the combined spreadand chording factor [K] is 0.926. Vt is inversely pro-portional to K.

2. Form of excitation windingSeveral forms of excitation winding have been used(Fig. 2). In essence there are only two basic types. Onecomprises a bundle of conductors passed through thestator bore along the longitudinal axis (Fig. 1c). Theother consists of conductor turns, passing through thebore but wrapped closely around it (Fig. 1b).Electromagnetic field theory shows that for the firsttype, the magnetizing force [H] is constant, whetherinside or outside the iron core [see Appendix 5.1 ofRidley 20042). For the second configuration of theexcitation winding, it can be shown [see Appendix 5.2of Ridley 20042] that while the magnetizing forcewithin a homogeneous iron core is approximately con-stant, outside the core it is substantially zero.

But electrical machine cores are not homogeneous.They are built from a large number of segmental lam-inations, with inherently circumferential air gapsbetween segments, as well as longitudinal air gaps asa result of the interlaminar insulation and ventilationducts. In addition, the bore of the core is slotted to takethe stator winding. Thus, regardless of the form ofexcitation winding applied, the electromagnetic fieldis non-uniform. Moreover, the turns of the secondbasic type of excitation winding (Fig. 1b) are not usu-ally wound close to one another. This causes the mag-netizing force to vary across the core section.

In a practical situation, therefore, whatever the formof excitation winding, the resultant electromagneticfield comprises a main circumferential flux throughthe core, as well as leakage flux spilling into the airsurrounding it. Application of the sensor to the statorbore detects this leakage flux.

3. EL CID vector diagramInitially the relevant vector diagram, relating the elec-tromagnetic fields of the main flux and that producedby fault current (δ) in the laminations, was conceived

Hydropower & Dams Issue Four, 2005 87

A deeper insight into EL CIDG. K. Ridley, Consultant, UK

D. R. Bertenshaw, ADWEL International Ltd, UK EL CID is now a familiar technique for monitoring stator core condition, but in some circumstances,

peculiarities may arise in the result. Users will benefit from a full awareness of these aspects, and thispaper aims to bring them more fully into focus.

Fig. 2. Alternativeforms of excitationwinding.

Fig. 1. Typical interlaminar defect with circulating current.

as a simple orthogonal relationship, comprisingPHASE [P] and QUAD [Q] vectors respectively,placed at 90º to each other. The PHASE value wasproportional to the excitation current [Ie]. QUAD wasproportional to the fault current (δ).

It was always understood that δ contains an in-phasecomponent, but this was considered negligible, com-pared with the quadrature component, since the faultpath through the fault, stator laminations and core-build (or key) bars is predominantly resistive.

Test results indicated, particularly at joints betweenthe sections in large diameter hydro generators, thatthe phase relationship was more complex. The rela-tionship, for zero fault current, was identified by acharacteristic line, cutting the PHASE axis at a valueother than zero. This is basically the magnetic poten-tial difference [mpd] which would be the constantPHASE value [Pe] if the permeability around the corewas constant, and there was no fault current. Pe is cal-culated as [Magnetising Ampere-turns / Number ofTeeth (or Slots)] = AT / S. Thus, the equation of thecharacteristic line is given by P = Pe + RQ, where R isthe slope of the line. Ridley designated this as the‘zero δ line’, applicable for no fault current (δ). ThePHASE value at zero QUAD value is modifiedbecause of the effect of core joints and the presence orotherwise of the machine rotor during the EL CID test[see Section 7.6 of Ridley 20042].

To develop a comprehensive vector diagram, anoth-er factor had to be recognized and introduced (seebelow).

4. Stator winding circulating currentThe observation was made, early in the experience ofapplying EL CID to hydro generators, that the PHASEvalue sometimes varied significantly and cyclicallyfrom slot to slot around the core, that is, circumferen-tially, which was contrary to expectation. This variationwas occasionally sufficient to give negative values. Dr.T. W. Preston recognized that this could not occur with-out the presence of an electrical current to drive the fluxin a direction opposite to that of the applied excitation.It followed from this that there must be current circu-lating in the stator winding, when present during the ELCID test. This proved to be possible when the statorwinding includes parallel circuits, and was verified byestablishing that the pattern of circulating current corre-sponded closely to that of the varying PHASE values(Fig. 3). It was further verified, on request to Dr.Preston, by a finite element analysis of a wound statorsubjected to circumferential magnetic flux.

The effect has been further verified in the field bothon turbo-generators and hydro generators. Bertenshawand Sutton [20043] describe how the effect wasinduced in a simple turbo-generator without parallelwindings, but where the phases were all shorted toearth for safety. In this case, when the excitation cablewas not placed axially central to the bore, the asym-metric induction caused current to flow between thephase windings, giving negative PHASE values in oneregion (Fig. 4). An analysis of the winding layout andcurrents showed that up to 0.75 A was flowing, whichwas able to explain the phenomena. As shown in Fig.4, the correct central orientation of the excitationwinding effectively eliminates this effect.

Bertenshaw and Sutton [20043] also showed in astudy of a large hydro generator with seven parallelsthat the inevitable effect of core splits is to inducesome circulating current in any windings present, as aresult of magnetic asymmetries in these regions.These values may only be deduced if the windingarrangement is known. However, in all cases, the pres-ence of the winding is to introduce a constant standingsignal on both the PHASE and (to a lesser extent) theQUAD values for the whole slot, and if this standingvalue is subtracted, localized core faults may still bedetermined.

5. Further development of the vectordiagramThe significant effect of the stator winding on EL CIDtest results identifies the test set-up as a transformerwith its secondary winding either open-circuited orshort-circuited. The basic vector diagram is obtainedfrom well known transformer theory, and involves areversal of the standard phase rotation to correspondto that assumed in the original work. It should also berecognized that, whereas current induced in a trans-former secondary winding has the opposite sense tothat of a primary winding, vectorially the two addwhen referred to the same base. Thus the EL CID sen-sor picks up circulating current and fault current sig-nals which are negative in relation to the excitationcurrent, but these have to be reversed when drawingthe vector diagram. Account must also be taken ofhow the reference PHASE vector is established. Thisis derived from either the PHASE component of theelectromagnetic field at the bore as detected by an air-cored coil, or from the excitation current obtained bya current transformer placed round the excitationcable. Each case requires separate consideration.

88 Hydropower & Dams Issue Four, 2005

Fig. 3. Circulatingcurrent effect onEL CID values.

Fig. 4. Circulatingcurrents in a smallturbo-generator.

5.1 Air-cored reference coilIn this case, the reference vector does not contain acomponent corresponding to the portion of excitationcurrent required to establish the core loss in the iron, asit produces no flux. The detected flux does contain anyexisting contribution from stator winding circulatingcurrent. The PHASE and QUAD axes are identified asfor an operating point where fault current is zero, asseen in Fig. 5. The vector diagram for that location isfor a particular value of circulating current. At anotherlocation with a different value of circulating current,the operating point will move in the direction of thecirculating current vector. This identifies the ‘zero δline’, which is thus theoretically justified. When thereis fault current [δ], the corresponding vector is addedin a direction perpendicular to the basic flux vector.This is not at right angles to the established PHASEaxis, thus the QUAD value comprises componentsfrom both the circulating current and the fault current.

Since the stator winding circulating current is con-stant for each particular slot, the PHASE value alongthat slot is not varied by circulating current, as it isfrom slot to slot. Hence, by removing the meanQUAD value at each slot, the maximum δ value isderived approximately from the total variation in theQUAD reading. The EL CID software has a facilityfor readily achieving this. If the δ value appears topoint to a defective region, it can be determined moreaccurately from localized PHASE and QUAD valuesobtained by use of one of the small hand-heldChattock sensors.

If there is no stator winding circulating current, theessential vector diagram is that of a transformer withan open-circuited secondary winding. In this case, thePHASE value is essentially constant and QUAD val-ues are nearly equal to δ.

5.2 PHASE reference from a currenttransformer round the excitation cableA superficial consideration of stator winding circulat-ing current indicates that there is no net value to beprovided by the excitation current, since the currentssum to zero. However, they are induced only when theexcitation is asymmetric, deliberately by the user orinevitably because of core construction. In this case,the coupling between excitation winding and genera-tor windings is not symmetric and a portion of the cir-culating current must be contributed by the excitationwinding (as would be expected on simple energy con-

siderations). Moreover, it is possible that there may besome net fault current also reflected in the excitationcurrent. In any case, the basic reference vector is theexcitation current [Ie], which includes an orthogonalcomponent of current required to establish the ironloss. The vector diagram in Fig. 6 is drawn on theassumption that the PHASE reference is establishedwhere there is neither local stator winding circulatingcurrent nor fault current. As before, the variation ofcirculating current moves the operating point in itsvectorial direction, which is again the ‘zero δ line’.The variation in the QUAD reading for a particularslot once again indicates the maximum δ value, whichmay merit closer examination.

5.3 The need to establish a zero δ locationFor clarity this has been assumed above, but it is notstrictly necessary. In general, it is desirable to mini-mize the QUAD value detected to maintain readingswithin usable graphical scale limits. In this context, itmay be desirable to reset the PHASE reference to theelectromagnetic conditions at some other particularlocation than that established initially. This is particu-larly the case at core joints, but care should be takenin case accuracy is diminished.

5.4 Circulating current directionCirculating current is induced in locations such as acore joint, where there is disturbance of the magneticfield, and flows in stator winding parallel circuits,around which it must largely balance out.

Consequently, there are regions of the stator wherecirculating current flows in the opposite sense. Whenintroduced into the vector diagram, this opposite sensecurrent causes the operating point to move into the4th, or even the 3rd quadrant, as has been observed inEL CID test results.

6. Curvature of the EL CID traceThis factor, demonstrated in Fig. 7, was readily elimi-nated by moving a nearby salient-pole away from thecore area under investigation. A finite element analy-sis established that salient-poles, when present, wouldalways have such an effect [for details see Section5.10 of Ridley 20042]. The magnitude of the effect isnot always of significance, but it is an erroneous indi-cation of deterioration in insulation condition towardsthe end of a core. It should, therefore, be recognizedand eliminated.

Hydropower & Dams Issue Four, 2005 89

Fig. 5. EL CID vector diagram taking account of circulatingcurrent with PHASE reference set to the leakage flux.

Fig. 6. EL CIDvector diagramtaking account ofcirculating currentwith PHASEreference set to theexcitation current.

7. EL CID trace axis slopeIf the single bundle of excitation conductors, intendedto be along the longitudinal axis of the core (see Fig.1c), is offset and/or misaligned, the result is shown bya finite element analysis to be a slope of the EL CIDtrace axis. This distortion of the results (see Fig. 8)again raises a query regarding the core condition.

8. EL CID trace tailsFig. 8 shows EL CID results for which the trace notonly slopes for the reasons described above, but alsohas tails, which again indicates superficially core enddeterioration. Such deterioration is possible, of course,especially if filing of the ends of the stator slots hasbeen undertaken with a view to protecting the statorwinding from possible damage by sharp corners.

In the early experience of the EL CID technique,however, it was observed that the location of a trans-verse portion of the excitation winding, of the formshown in Fig. 1c, caused this type of distortion whenthe spacing from the core end was less than about 1 m.In the case of turbo-generators, this spacing is usuallyenforced by the relatively long stator winding over-hang. For hydro generators, it can be quite difficult toachieve.

Application of electromagnetic theory to the case ofa current-carrying conductor located above an ironsurface [see Appendix 5.6 of Ridley 20042] shows thejustification for applying a spacing of at least 1 m. The

analysis also showed an unexpected result. The formof the flux density distribution induced in the iron canbe seen in Fig. 9. The surprising feature is that direct-ly beneath the cable, the flux density is zero, whateverthe spacing of the conductor from the iron. Thisexplains the deep valley in the value of EL CID resultsin the neighbourhood of the cable. An understandingof this phenomenon helps to locate the position of theexcitation cable, if it has not been recorded [see Fig.20 in Ridley 20042].

9. Correlation of EL CID and HFRTresultsA major concern of users of the EL CID technique isthe degree of reliability with which EL CID resultscan be interpreted. Some users have found very highvalues, indicating a serious core interlaminar insula-tion condition, yet no indication of a hot-spot devel-oping. It is now clear that there have been a number ofpossible reasons for this; most of which can be elimi-nated by virtue of the understanding reported here.

A question still remains as to the value to place onthe final value after taking account of all the modify-ing factors. Previous to the availability of EL CID, thequantitative method of assessing core condition wasthe high flux ring test (HFRT). Correlation betweenresults obtained by both methods for a given machinehas been sought by a world-wide questionnaire underthe auspices of CIGRE, as reported by Ridley [20024].Nevertheless, the result could only be given as a cor-relation band, rather than an exact relationship [seeAppendix 4.1 of Ridley 20042].

An experimental study on the relative effectivenessof HFRT and EL CID test in a buried fault describedby Bertenshaw et al [20045] shows that the relativeeffectiveness of the HFRT falls off rapidly if the defectis buried. A serious fault on a slot base (reading 520mA of QUAD) was buried under a simulated winding.The HFRT value (at 0.8T rms) after nearly 2 hours oftesting only reached a 6°C rise above core body tem-perature, despite rising above 37°C locally when mea-sured by buried thermocouples. This shows that theHFRT is likely to give misleading re-assurance whenthe faults are buried in the core or under windings, andcould explain much of the lack of visible symptoms offaults. However, the EL CID detection of buried faultsdown a slot base is only modestly attenuated, and cor-related satisfactorily at 7°C/100 mA.

Consideration of the factors involved in an HFRTindicates that its results are dependent on the generalambient conditions, which include much more thanambient temperature. Other factors are the attitude ofthe core when tested, the location of the core in itsnormal operational situation or otherwise, the exis-

90 Hydropower & Dams Issue Four, 2005

Fig. 7. EL CIDtrace curvaturecaused by the PoleProximity effect.

Fig. 8. EL CIDresults exhibitingtrace axis slopeand significantdeviation at oneend of the core.

Fig. 9. Theoretical variation of flux density in an iron surfacebelow a conductor.

tence of air currents, core size, and the duration of thetest. With regard to the last item, different personnelhave quite different standards.

It is recommended that a study is undertaken to iden-tify the effect of these ambient conditions on HFRTresults, so that correlation with EL CID results can beimproved, thus enhancing the reliability with whichthe latter may be used. Nevertheless, the EL CID tech-nique has been extensively applied and found to be auseful tool for monitoring core condition both for nor-mal service and when under repair.

10. ConclusionAlthough a number of phenomena in the EL CIDresults have been observed, they are now well under-stood, which allows their effect to be taken intoaccount. The understanding gained permits use of ELCID in the same relatively simple manner as first pro-posed, and the results are consequently acceptable asa pointer to the core condition. ◊

References1. Sutton, J., “EL CID: an easier way to test stator cores”,

Electrical Review, Vol. 207, No. 1, 1980.2. Ridley, G. K., “EL CID application and analysis”, Ed: 2,

ADWEL International Ltd; 2004.3. Bertenshaw, D. R and Sutton, J, “Application of the EL

CID Test with Circulating Currents in Stator Windings”,Inductica 2004, Berlin, Germany; June 2004.

4. Ridley, G. K., “Hydro-generator EL CID results referred toHigh Flux Ring Test results”, Proceedings, Session 2002 ofCIGRE, Paper 11-201; August 2002.

5. Bertenshaw, D. R et al, “Computational modelling of statorcore faults in large hydro-generators and turbo-generators”,Hydro 2004, Porto, Portugal; October 2004.

Hydropower & Dams Issue Four, 2005 91

Eur. Ing. G.K. Ridley was a designer of large electricalrotating machines, mainly hydro generators, for nearly 40years with BTH, Rugby, UK, and its successors, concludinghis career as Design Manager. He has been a registeredengineer with FEANI since 1990. He became a CharteredEngineer in 1960, subsequently achieving the status of FIEE(1970) and FIMechE (1992). He is also a Senior Member ofthe IEEE. He has been an active member of CIGRE since1992, having undertaken worldwide surveys of bearingdesign and practice for large vertical hydro generators, andalso the comparison of EL CID and High Flux Ring Tests(HFRT) results. He was invited to present a ‘preferredsubject’ paper on the latter topic at the biennial meeting ofthe CIGRE Session in Paris in 2002. Mr Ridley has had 37papers published and in 2000 he authored a book entitled‘EL CID - Application and Analysis’, the second edition ofwhich was published in 2004. Since nominal retirement in1994, he has served as an independent consultant on hydrogenerators, with a special interest in EL CID result analysis.

11 Hoskyn Close, Hillmorton, Rugby, Warwickshire CV214LA, UK.

Eur. Ing. David Bertenshaw graduated with a B.Sc. (Eng)degree in Electrical Engineering from Imperial College,London University, in 1971. He was elected as MIEE in1980, as FIEE in 1998, and is a Chartered Eur. Ing. He hasspent many years in the development of lighting and controlsystems, and communications systems, as R&D andEngineering Director. He joined ADWEL International, UK,in 1998 and he now directs the engineering and commercialmanagement of the company’s stator core test systems. Hehas authored and presented 20 conference papers, is theinventor on six international patents and a member ofnational standards committees relating to EMC.

ADWEL International, Park House, Greenhill Crescent,Watford, Herts WD18 8PH, UK.

G.K. Ridley

D. Bertenshaw