Partial Discharge Theory and Applications to …

1 Partial Discharge Theory and Applications to Electrical SystemsGabe Paoletti, Golubev, PhDDivision Application EngineerManager, R&D, Predictive DiagnosticsCutler-Hammer Engineering ServicesCutler-Hammer Engineering ServicesPennsauken, NJ Minnetonka, MNAbstract - Partial Discharge monitoring is an effectiveon-line predictive maintenance test for motors andgenerators at 4160 volt and above, as well as otherelectrical distribution equipment. The benefits of on-line testing allow for equipment analysis anddiagnostics during normal production. Correctiveactions can be planned and implemented, resulting inreduced unscheduled downtime. An understandingof the Theory related to Partial Discharge , and therelationship to early detection of insulationdeterioration is required to properly evaluate thispredictive maintenance tool. This paper will presenta Theory to promote the understanding of partialdischarge technology, as well as variousimplementation and measurement techniques thathave evolved in the industry.

2 Data interpretation andcorrective actions will be reviewed, in conjunctionwith comprehensive predictive maintenance practicesthat employ Partial Discharge testing and BACKGROUNDR eliable manufacturing operations will always beconcerned with process production programs to maintain electricalequipment for peak performance have beenrecommended and implemented at various plants [1].Detailed motor failure analysis has been completed;resulting in the identification of approximately 30% offailure causes being related to electrical failures [2]. Asummary of the IEEE transaction entitled: Report ofLarge Motor Reliability Survey of Industrial andCommercial Installations, Part I [3] included both theresults of an IEEE survey and an EPRI survey. The twosources of information proved extremely useful since theIEEE survey identified the Failure Contributor , and theEPRI survey identified the Percentage Failure byComponent.

3 The IEEE survey includes an objectiveopinion, whereas the EPRI survey includes actual failedcomponents. The summary of the electrically relatedcauses of the two studies is shown in Table 1, and will bereferred to, when discussing root cause failures related topartial Discharge test 1 Motor Electrical Failure CausesIEEE StudyEPRI StudyFailureContributor%FailedComponent% IEEE publication under development, IEEE P1434- Guide to Measurement of Partial Discharges inRotating Machinery [4] also identifies similar failurecauses for motor insulation systems. These includethermal, electrical, environmental and mechanicalstresses. These factors correlate to the two studies, sincethey result in the stator ground insulation and turninsulation failure (EPRI Study); as well as can beinterpreted as normal deterioration (IEEE Study).The next section provides a review of Partial dischargetheory.

4 It is interesting to note that over 25 years ago,large motor manufacturers recognized the need forpartial Discharge testing in the slot area between thewinding insulation and the iron [5]. The testing wascalled the Slot Discharge Test and involved applying atest voltage while observing the waveform on anoscilloscope. At that time only minimal Partial dischargemeasurement technology was available, thereforelimiting the wide spread use of such Partial DISCHAGE THEORYP artial Discharge Theory involves an analysis ofmaterials, electric fields, arcing characteristics, pulsewave propagation and attenuation, sensor spatialsensitivity, frequency response and calibration, noise anddata interpretation. It is obvious from the above thatmost plant engineers will not have the time, or availableenergy, to pursue such a course of at the 1999 IEEE IAS Pulp and Paper Industry Conference in Seattle, WA: IEEE 1999 - Personal use of this material is an effort to promote a better understanding of partialdischarge (PD), this paper attempts to provide simplifiedmodels and relate the characteristics of these models tothe interpretation of PD test , we will present a few technical concepts relating topartial discharges.

5 Partial Discharge can be described asan electrical pulse or Discharge in a gas-filled void or ona dielectric surface of a solid or liquid insulation pulse or Discharge only partially bridges the gapbetween phase insulation to ground, or phase to discharges might occur in any void between thecopper conductor and the grounded motor framereference. The voids may be located between the copperconductor and insulation wall, or internal to theinsulation itself, between the outer insulation wall andthe grounded frame, or along the surface of theinsulation. The pulses occur at high frequencies;therefore they attenuate quickly as they pass to discharges are effectively small arcs occurringwithin the insulation system, therefore deteriorating theinsulation, and can result in eventual complete possible locations of voids within the insulationsystem are illustrated in Figure other area of Partial Discharge , which can eventuallyresult, is insulation tracking.

6 This usually occurs on theinsulation surface. These discharges can bridge thepotential gradient between the applied voltage andground by cracks or contaminated paths on the insulationsurface. This is illustrated in Figure above can be illustrated by development of asimplified model of the Partial discharges occurringwithin the insulation Insulation System ModelA simplified model of an insulation system can berepresented by a capacitance and resistance in parallel[6]. This is the concept employed in the use of powerfactor testing of insulation systems. The leakage currentis split between the resistive and capacitive paths. Thepower factor is the cosine of the phase angle between thetotal leakage current and the resistive component ofleakage current [5].The above model is also used for attenuator circuits inelectronics [7].

7 Signal attenuation results in reducing theamplitude of the electrical signal. This underlies theproblem with Partial Discharge detection. The insulationmedium, which is being exposed to the partialdischarges, acts to attenuate the signal, thereforeweakening this damaging signal which we are trying toidentify at our sensor locations. In addition, theattenuated Partial Discharge signal can be masked bysources of electrical noise, which shall be reviewed laterin this above concept of the insulation system being aneffective attenuator circuit gives rise to critical detectionissues, such as: Sensor locations and sensitivity Measurement system response to attenuated signals Noise detection and eliminationCopper ConductorVoid betweencopper andinsulationVoid internalto insulationVoid betweeninsulation andIron CoreIron CoreFigure 1 PD within Insulation SystemCopper ConductorIron CoreContaminatedInsulation cracksContaminatedinsulationsurfaceSurfa cedischargethrough airFigure 2 Surface Partial DischargesSimplifiedInsulationModelFigur e 3 simplified Insulation Model and Model for an Electronic AttenuatorInsulationB.

8 Partial Discharge Void ModelSimplified models of the area of the void have beendescribed as consisting of capacitors only [8]. Areview of the progressive failure mode of these voidsindicates an additional resistive component in parallelwith the capacitive component. An electricalequipment design handbook [9] states: Discharges oncestarted usually increase in magnitude with stressed time,but discharges can become short circuited bysemiconducting films inside the void and discharging isterminated. The referenced semiconducting films canalso consist of carbonization of the organic insulationmaterial within the void due to the arcing the model of the Partial Discharge void issimilar to that of the insulation medium failure modes have indicated a drop in partialdischarge intensity shortly prior to complete would occur when the internal arcing hadcarbonized to the point where the resistive component ofthe model was low enough to prevent a build-up ofvoltage across the void.

9 This new low resistivecomponent would also allow higher current flows, andadditional heating and resultant insulation damage. Theabove model, including the resistive componentcorrelates to the actual failure mode of a partialdischarge void, with the resistive component passingmore leakage current as the Partial discharges increasewith time. One form of this resistive component isvisible tracking on the surface of insulation. Anexplanation of tracking, and how surface partialdischarges are related to the development of trackingfollow [5]: Tracking damage has been traced entirely tothe locally intense heat caused by leakage currents flow thru any contaminated moisturefilm on the bridging insulating surface. As long as thisfilm is fairly broad and continuous, the heat associatedwith the leakage current is spread over a wide area andis dissipated.

10 However, heating promotes filmevaporation. This causes the film to break up into smallpools or islands. Each break in the film tends tointerrupt a segment of the leakage current, causing a tinyarc. Even though the arc is small, severe local heatingresults. The intense heat of the leakage current arc issufficient to cause a molecular and chemical breakdownof the underlying insulation. On organic materials, afrequent by-product of arcing is carbon. The above tiny arc along the insulation surface can berepresented by Partial Discharge activity. Figure 5illustrates the failure mode of deteriorated insulationrelated to the intensity of Partial 6 illustrates a circuit breaker bushing which asprogressive tracking highlighted for presentationpurposes. At the point near eventual failure, the trackingand resistive component of the insulation have increasedto the point where Partial discharges have been reduced,since the tiny arcs have caused the carbonization andtracking, therefore providing a direct path for currentflow.