On-Line Partial Discharge Testing - WMEA

1 OnOn-- line Partial line Partial Discharge TestingDischarge TestingSkip HicksDirector -New Business DevelopmentWhat Is Partial Discharge ?What Is Partial Discharge ?What Is Partial Discharge ?zAccording the the International ElectrotechnicalCommission (IEC) International Standard 60270, Section published in 2000, the definition of Partial Discharge is: Localized electrical Discharge that only partially bridges the insulation between conductors and which can or cannot occur adjacent to a conductor. IEC 60270 Section NotesIEC 60270 Section NotesIEC 60270 Section Notesz Partial discharges are in general a consequence of local electrical stress concentrations in the insulation or on the surface of the z Corona is a form of Partial Discharge that occurs in gaseous media around conductors which are remote from solid or liquid z Partial discharges are often accompanied by emission of sound, light, heat.

2 And chemical Paper-Insulated Lead CablePaperPaper--Insulated Lead CableInsulated Lead CableExtruded MV Cable ConstructionExtruded MV Cable ConstructionExtruded MV Cable ConstructionEPR Cable ConstructionEPR Cable ConstructionEPR Cable ConstructionJacket____Jacket____SC*_____ SC*_____Strand Screen_*Strand Screen_*__Shield__Shield*Insulation*Insu lationConductorConductorEPR Insulation in Cable AccessoriesEPR Insulation in Cable AccessoriesEPR Insulation in Cable AccessorieszMost Pre-Molded Cable Terminations, Stress Cones and other Cable Accessories are manufactured from an EPR compound because of it s great resistance to treeing and the effects of corona Discharge around terminations and of Cable FailuresTypes of Cable FailuresTypes of Cable FailureszLaminar Insulation FailureszVoltage Stress Related Failures in Extruded InsulationzCable Accessory FailureLaminar Cable Insulation FailureLaminar Cable Insulation FailureLaminar Cable Insulation FailurePicture Provided By Pirelli WireLaminar Cable Insulation FailureLaminar Cable Insulation FailureLaminar Cable Insulation FailurePicture Provided By Pirelli WireTape removed showing electrical treeingVoltage Stress Related Failures in Extruded InsulationVoltage Stress Related Failures in Voltage Stress Related Failures

3 In Extruded InsulationExtruded InsulationzOverloads, defects or weak spots in insulation lead to chemical breakdown of the insulation, producing thermal runaway leading to decreased Insulation Resistance, increased leakage current and , manufacturing or workmanship results in voids or contamination which produce Partial Discharge and ultimate Trees Form and convert to Electrical Trees and lead to ultimate Stress Related Failures in Extruded InsulationVoltage Stress Related Failures in Voltage Stress Related Failures in Extruded InsulationExtruded Insulation[2]zWater migration into the insulation leads to water trees. Water Trees lead to decreased insulation resistance and very slight increase in leakage current. Water Trees do not directly produce Trees lead to electrical treeswhich produce Partial Discharge and ultimate failure.

4 [5]Water TreesWater TreesWater TreesWater TreeingWater TreeingWater TreeingEPR InsulationPicture Provided By General CableTRXLPE InsulationPicture Provided By General CableBowtie TreesBowtie TreesBowtie Trees[9]Bowtie Tree in XLPEB owtie Tree in EPRW ater Tree ConversionWater Tree ConversionWater Tree Conversion[2]zVoltage surges occur on systems due to lightning and/or high electric field caused by these surges result in power dissipation in the water channels existing in insulation sufficient to raise the temperature of the water and increase pressure in the water tree phenomena generates cavities sufficient to support Partial TreeingWater TreeingWater TreeingHMWPE InsulationPicture Provided By General CableXLPE InsulationPicture Provided By General CableWater Tree ConversionWater Tree ConversionWater Tree ConversionzPD Inception voltage is exceeded by lightning and other surges causing a PD in drops to below PD Extinction voltage and PD occurrence at which PD occurs causes the size and geometry of the insulation void to change.

5 Lowering the PD inception and extinction PD extinction voltage drops below system nominal voltage, PD s are continuous leading to electrical tree formation and ultimate insulation Tree ConversionWater Tree ConversionWater Tree Conversion500010000150002000025000300003 50004000045000 Impulse 1 Impulse 2 Impulse 3 Impulse 4 Impulse 57200 VInsulation FailureInsulation FailureInsulation FailurePicture Provided By Pirelli WireFailures in Cable AccessoriesFailures in Cable AccessoriesFailures in Cable AccessorieszThe majority of failures on distribution systems occur in terminations, splices and other cable mechanisms reported in Cable Accessories are predominantly involve Partial Discharge Deterioration caused by voids, contaminants and workmanship Partial Discharge ProcessThe Partial Discharge ProcessACOUSTICSACOUSTICSHEAT/CHEMICAL HEAT/CHEMICAL REACTIONSREACTIONSLIGHTLIGHTEM WAVESEM WAVESIEEE Standard 400-2001 Notes:IEEE Standard 400 IEEE Standard 400--2001 Notes:2001 Notes:z Partial Discharge measurement is an important method of assessing the quality of the insulation of power cable z A Partial Discharge is an electrical Discharge (formation of a streamer or arc) that does not bridge the entire space between two electrodes.

6 ZPartial discharges may occur in a a at the tip of a well-developed water The Partial Discharge ProcessThe Partial Discharge ProcessMicroscopic spaces ( voids ) may be formed in insulation systems due to water tree growth, aging, installation or manufacturing defects. Continued stress and overvoltagescan initiate PD in and other forms of energy released by PD cause erosion of the internal surface of the erosion forms channels that develop into so-called electrical trees in the System FailureContinued PD produces further erosion until the electrical tree bridges the Test for Partial Discharges?Why Test for Partial Discharges?Why Test for Partial Discharges?zPartial Discharge Testing can detect: Insulation defects that may have occurred during cable system installation or the manufacturing process Insulation deterioration due to normal service operating conditionszPartial Discharge Testing is a PREDICTIVE qualitative analysis tool that can warn of a potential upcoming system Partial Discharge Testing program can locate problem areas (cable, splices, and/or terminations) in a cable Discharge test results can be trended over traditional High Potential Testing , Partial Discharge Testing is not destructiveto an insulation system.

7 (Hipottesting is typically a go-no-go test --the cable either fails or it doesn t). In addition, even massive insulation defects in extruded dielectric insulation cannot be detected with according to the IEEE 400-2001 Partial Discharge REQUIREMENTSAEIC Partial Discharge AEIC Partial Discharge REQUIREMENTSREQUIREMENTSM anufacturers Maximum Permissible PartialDischarge (pC) at % of rated Voltage-Ground150%200%250%300%400%197353 055808019755203550--19825203550--1987555 510199655555 Historical Testing MethodsHistorical Testing MethodsHistorical Testing MethodszOff- line Testing Techniques High Potential Testing Power Factor/Dissipation Factor Testing Very Low Frequency Testing (VLF)zIn-Service Testing Techniques Ultrasonic PD Detection Partial Discharge TestingOff- line Testing TechniquesOffOff-- line Testing TechniquesLine Testing TechniqueszRequire de-energizationof the cable under test and in some cases, completely removed from the distribution the advantage of close control of the test voltage, and if necessary raising the voltages above the normal operation from system and using a special voltage also minimizes noise, reducing the need for filtering and intense data Potential Testing -DCHigh Potential Testing High Potential Testing --DCDCIEEE, Electric Power Research Institute (EPRI), Insulated Cable Engineers Association (ICEA), and Association of Edison Illuminating Companies (AEIC) all agree on the following.

8 DC High Potential Maintenance Testing on aged (XLP) cables can damage the cable resulting in premature failure High Voltage DC Tests continue to be useful tests to check systems before they are placed in service. When used as maintenance tests the possibility of damage to the cable should be considered. Cable Manufacturer s must be consulted to determine acceptable voltage levels and recommendations typically include to test at your own risk High Potential Testing -DCHigh Potential Testing High Potential Testing --DCDC IEEE 400-2001 standard: "..even massive insulation defects in extruded dielectric insulation cannot be detected with DC at the recommendedvoltage levels."In other words --not only is DC High Potential Testing likely destructive for field-aged extruded cables, but it may not tell you a darn Potential Testing -DCHigh Potential Testing High Potential Testing --DCDCWHAT S THE BOTTOM line SKIP?

9 ?????????QUIT DC HIPOT QUIT DC HIPOT TESTINGTESTINGP ower Factor/Dissipation Factor TestingPower Factor/Dissipation Factor TestingPower Factor/Dissipation Factor TestingzEffective in locating weaknesses in insulation and potential hazards before impending a Go-No-Go TestzTesting does not overstress the insulation and can determine if the insulation is slowly degrading through limited to relatively short lengths of effective in detecting localized faults as the length of cable Low Frequency (VLF) TestingVery Low Frequency (VLF) TestingVery Low Frequency (VLF) TestingzVLF -High Potential Testing VLF damagesthe insulation lessthan DC Testing and has the capability of locating potential failure sites. VLF has the advantage of portability with low energy requirements, which results in much smaller test Partial Discharge Testing Voltage is raised to above the PD inception voltage to cause PD to occur.

10 PD is then locatable using Time Domain PD Detection methods. Cable must be disconnected to Testing TechniquesInIn--Service Testing TechniquesService Testing TechniqueszHas the obvious advantage that the cable is not put at any additional risk from the has the advantage that the cable is not removed from service, leaving it energized as it is for normal operationzRemoves the potential for damage due to inappropriate switching and yields no system contingency problemsUltrasonic PD TestingUltrasonic PD TestingUltrasonic PD TestingzCan pinpoint a suspected problem if the cable or accessory is not directly buried or is at least physically accessiblezThe high-frequency ultrasonic components of PD are extremely short wave in nature, fairly directional and easy to isolate from background noisezProblem must be accessibleOn- line Partial Discharge Testing System for Cable SystemsOnOn-- line Partial Discharge Testing line Partial Discharge Testing System for Cable SystemsSystem for Cable SystemszThere is zero down timeassociated with On-Line PD Testing because the the test is performed at normal operating voltage.