2003 Annual Report Conference on Electrical Insulation and Dielectric Phenomena

Surface finish effects on partial discharge with embedded electrodes

Aleta T. Wilder', Robert Hebner', and Yicheng Wang2'Center for Electromechanics,The Universityof Texas, AustinTX, USA

2ElectricityDivision,National Institute of Standards and Technology, Gaithersburg MD, USA

Abstract: Partial discharge measurements have beenused to characterize the response of dielectric insulationmaterials exposed to ac voltages. Electrode surfacefinish can affect such partial discharge characteristics.This paper provides comparisons to the effect of varioussurface finishes on electrodes embedded in a hightemperature epoxy including roughness, electrodematerial work function, and thin coatings.

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Introduction

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Historically, an important purpose of partial dischargemeasurements in apparatus has. been to promote lifeextension by identifying key trends in changes in partialdischarge activity. The purpose of this work is toextend the success of those efforts to initial testing ofnew insulation systems in novel machines. This paperpresents the results of a comparative study of partialdischarge (PD) characteristics of brass electrodesembedded in an epoxy, where the electrodes have beensubjected to various surface finishes.

Although electrode surface finish is known to affectPD, more information is needed concerning effects inparticular insulation systems for engineering-purposes.The choice of surface finish is often a cost considerationin building high voltage equipment and machines.Concurrently, increasing PD extinction voltages andlowering power losses are considered important inreducing dielectric aging [I].

We chose flat electrodes presenting a reasonablearea of electrode to epoxy interface. Electrodes wereembedded to reduce effects of humidity and othercomplications of surface discharges. The interfacialdischarges in such configurations are expected to berepresentative of some embedded machine coils andmagnet wire where multiple turns are encased or potted.

In this study, PD inception and extinction levels aregiven as rms voltages at 50 Hz. Charge populationswere counted for 120 seconds. The average energy of aPD event is calculated as defined in [2]. The PDinception and extinction levels, however, are noted asthe rms voltage at which PD is first observable within120 seconds on voltage rise and at the rms voltage atwhich PD is no longer observed on voltage decrease,respectively. Some researchers prefer to define thesecharacteristics in terms of the presence of one PD eventper cycle.

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Experimental Conditions

Partial Discharge Instrumentation

The digital detection system used in this work has beenpreviously described [3]. The system has been shown tobe extremely sensitive. The partial discharge events aretimed using a zero-crossing detector. This is combinedwith a summing amplifier and digitizer. Pulses withamplitudes lower than 10 pC were not recorded. Toproduce the applied voltage, a sinusoidal signal from anSRS345 waveform digitizer is fed into a two to oneCalex noninverting linear amplifier in line with a,Trek20/20a amplifier. The system is currently limited to 520kV peak-to-peak voltages.

Sample Design

Samples are a simple parallel plate capacitor consistingof two brass electrodes potted in epoxy within a PVCring of 3.9 cm inner diameter and 3.5 cm in height. Theepoxy is an unfilled, two-part potting system meetingclass H operating requirements [4]. The hardener is ananhydride amine blend. Samples were soaked, gelled,and then cured at 125°C for four hours following themanufacturer's recommendations. No post-cure heat-treatment was carried out.

Both electrodes are 0.635 cm diameter flat discsmounted on central threaded posts of 0.32 cm diameter.These posts extend linearly outside of the PVC ring atmid-height. We attempted to follow samplerecommendations in [5] to produce a gap of 1.4:tO.1mm(55 mils) for these equivalent electrode diameters. Theedge of the disc was rounded, although not by aRogowski radius, so for these samples the field is onlyapproximately uniform from electrode center toelectrode edge.

Electrode Surface Finishes

Pairs of brass electrodes were produced with fivedifferent finishes. These finishes are: A-smooth, B-veryrough, C-stress grading layer, D-platinum sputtercoated, and E-polyimide coated. The smooth surface ofsample A was produced using wet SiC paper of 600 grit,followed by polishing with suspended colloidal aluminato <I micron on a felt lap. The very rough electrodesurfaces were produced by wet grinding with 320 grit

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SiC paper. Figure I gives a representativeopticalmicrograph of surface B.

The stress-grading layer is a 30 volume % graphitepowder, 70 volume % epoxy layer that was cured andpolished smooth. This layer essentially consists ofmicron size conductive particles in a dielectric matrixand is semiconductive. The platinum layer on electrodesurface D was sputtered from a platinum target in anargon plasma and deposited on the room temperaturebrass electrode surface that had been previouslysmoothand clean. The polyimide coating of surface E wascured from a commercial liquid precursor. [6]

Results

Pronounced differences in partial discharge behaviorwere observed between samples with different surfacefinishes. Values for PD characteristics of samples notpreviously exposed to voltages are listed in Table I.

.Effect of Roughness

The rougher sample B has a broader PD distributionthan any of the other samples. An example PDdistribution recorded at 13.6kV is given in Figure 2. Bis the only sample that exhibited significant positive PD.

Figure I. Optical Micrograph of sample B (x80).

B also exhibits a high mean charge and a relatively highenergy per PD event. Although the difference betweeninception and extinction voltages is within 0.1 kV, bothcharacteristic voltages are significantly lower than those

. of sample A, which were above our system's ability tomeasure.

Effect of Stress-Grading Layer

The layer chosen to apply to sample C for this studydoes not improve the inception or extinction voltages.Although the mean PD charge is low, the differencebetween the two voltages is large. Once PD eventsinitiate in this sample, they may catalyze more events.This appears to be a noted affect of some fillers.

Effect of Platinum Coating

Initial measurements of sample D consisting ofplatinum-coated electrodes were considered of interestdue to a change in work function (-5.7 eV) from that ofbrass (-4.5 eV). Concurrently, the resistivity of pureplatinum (-I Ixl 0-8ohm-m) is twice that of brass. Forthis type of coating process, however, these features didnot improve the characteristic inception and extinctionvoltages first measured, and the mean charge per eventis the highest of all the samples studied.

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Charge (pC)

Figure 2. Charge populations for sample B at 13.6 kV.

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Table I. PDcharacteristics of samples upon initial voltage exposure.

Gap Inception Extinction Mean Charge (pC) Energy per PD eventSample (mm) Voltage (kV) Voltage (kV) @ Voltage (x1 0-6Joules) @ Voltage

A 1.24 >14.1

B 1.22 13.1 13.0 68 @ 13.3 kV 0.90 @ 13.3kV

C 1.22 12.6 11.7 22 @ 12.6 kV 0.28 @ 12.6 kV

D 1.19 12.0 11.9 93 @ 12.0 kV 1.12 @ 12.0kV

E 1.17 11.2 11.2 21 @ 11.4 kV 0.24 @ 11.4kV

Effect of Polyimide Coating

Polyimide is known to have excellent dielectricproperties. Sample E, however, demonstrates the lowestinception and extinction voltages during initialmeasurements of all the samples studied. This samplealso exhibits the lowest mean charge and energy per PDevent.

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Effect of Exposure on Subsequent DischargeBehavior

Typically samples demonstrated improved PDcharacteristics with time. Often several days lapsedbetween measurements. The samples were stored atroom temperature during these lapses. Not only did theinception and extinction voltages increase, but the meancharge and standard deviations of the chargedistributions decreased.

Some PD characteristics for sample E are listed inTable 2. The preparation of this sample is describedabove. The polyimide-coated electrodes have a gap of1.17 mm. There was a lapse of ten days between theinitial and final measurements. One set of interim mea-surements was made. The total exposure to both 50 Hzand 500 Hz sinusoidal voltage is on the order of fourhours.

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Discussion

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Opportunities to change inception and extinctionvoltages by modifying electrode surfaces have beendemonstrated on samples not previously exposed toelectric stress. We must emphasize that these wereinitial measurements on new samples. Partial dischargetesting, and other factors, can and do change theobserved distribution of partial discharges. The sig-nificant differences among the charge deposited undervarious surface finishes of the electrode support theaxiom that initially surface finishes have an importantrole in PD behavior. Earlier work has shown theimportance of interfacial voids in determining partialdischarge behavior in epoxy [7]. While that workshowed some affect of early decrease in the dischargecharge, in the early data it was not consistent. [n

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addition, the early time was measured in hours so theeffects seen here may have been masked.

The simple model of a discharge ca singirreversible damage to the epoxy leading cumulat velyto failure is not consistent with much of the datameasured. As usual, a numberof processes are lik ly tobe competing. One mechanism that could rais theinception voltage is changes to the electrode su ace,itself. A discharge can lead to microscopic p asticdeformation in the metal of the electrode su ace,typically increasing the field strength for subse uentemission [8].

A second possible mechanism is that the e .ttedelectrons or the ac field may encourage f rthercrosslinking that could lower the loss tangent. Theepoxy curing process used to produce the mea uredsamples followed the manufacturer's recommenda ions.However, post-cure heat-treatment to maxi 'zeproperties was not carried out. There may be suffi ientunpolymerized molecular groups in this e oxyprocessed in this manner to permit mole ularrearrangements and consequent increase in the degr e ofcrosslinking.

A third possible mechanism is based on inter acialorientation incited by ac voltages,especially at elec rodeinterfaces where electric fields are pronounced. Theability of gaseous and liquid dielectrics to recon gureand restore dielectric integrity during low-Ieve PDevents is well known [1]. [n the case of a soliddielectric, such voltage conditioning may perman ntlyremove defects through molecular reorientation.

References

[I] R. Bartnikas, "Partial discharges: their mechanism, det~ction,and measurement." IEEE Trans.Dielectrics & Elec. Insu~ation,vol. 9 no.5, pp, 763-808, 2002.

"Standard test method for detection and measurement oftrtial

discharge (corona) pulses in evaluation of insulation sys ems."Annual Book of ASTM Standards: DI868-93, , STM[nternational, West Conshohocken, PA.

[3] Y. Wang, et aI., "Digital recording and analysis of partialdischarges in point-dielectric gaps." Proceeding.s of tlie Int.Symp. Electrical/n.mlation 1998, pp. 440-443.

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Table 2. Results of repeated measurementson sample E.

Frequency Inception Extinction Mean Charge (pC) Energy per PO event(Hz) Voltage (kV) Voltage (kV) @ Voltage (x10-6Joules) @ Voltage

Initial 50 11.2 11.2 21.0 @ 11.4 kV 0.24 @ 11.4 kV

Measurements 500 7.8 7.6 19.2 @ 7.8 kV 0.15 @ 7.8 kV

Final 50 14.1 14.0 11.0 @ 14.1 kV 0.15 @ 14.1 kV

Measurements 500 13.9 13.8 16.0 @ 13.9 kV 0.22 @ 139 kV

[4] Connpoxy@ FR-IOBO Technical Data Sheet. Cytec Industries.Inc., Olean. NY: www.conaO.com

[5] "Standard test method for voltage endurance of solid electricalinsulating materials subjected to partial discharges (corona) onthe surface." Annual Book of ASTM Standards: D2275-01.ASTM International, West Conshohocken. PA.

[6] Drop Dispense Polyimide PI-2579B. HD MicroSystems, Parlin.NJ:, www.hdmicrosvstems.com/orodlorocess.html

[7] J. Holboll and M. Henriksen, " Partial Discharge Patterns andSurface Deterioation in Voids in Filled and Unfilled Epoxy,"

Proceedings of the 1992 IEEE Symp. Elec. Insul., pp. 354-358.

[8] E. Forster, "The MetallLiquid Interface: the Charge InjectionProcess," IEEE Trans. Elec. Insul.. vol EI-19, 1984, PP. 524-528.

Author address

Aleta T. Wilder, Center for Electro-mechanics, TheUniversity of Texas at Austin, 1 University Station, MCR7000, Austin, TX, USA, 78712. Email:a.wilder@mail.utexas.edu.

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