2007 International Conference on Solid Dielectrics, Winchester, UK, July 8-13, 2007

The effect of cable terminations on dielectric response measurements

Bolarin Oyegoke', Andrew Thomas2, Tapan Saha2'School of Engineering Systems, Queensland University of Technology

Gardens Point, Brisbane, Australia2School of Information Technology and Electrical Engineering, University of Queensland

St Lucia, Brisbane, AustraliaE-mail: oyegokegitee.uq.edu.au

Abstract: This paper presents findings on dielectricresponse measurements on field aged and new mediumvoltage XLPE cables using a non-destructive frequencydomain technique. Emphasis is laid on the laboratoryand field acquired data interpretation. Results presentedin this paper shows possible error being introduced intothe judgement of water tree deterioration when theinfluence of the cable terminations is not properlyconsidered.

INTRODUCTION

Water tree degradation is a major problem for mediumvoltage XLPE cables. It is therefore necessary fordistribution companies to be able to access the level ofdegradation of their supply cables, to avoid catastrophicfailure and to plan efficient maintenance strategies. Aneffective way of accessing the level of water treedegradation is through non-destructive dielectricresponse measurements. One such measurementinvolves applying a frequency varying AC voltagesignal across the insulation and measuring theresponsive current, which is called Frequency DomainSpectroscopy (FDS). However, the interpretation of thefrequency varying dielectric response with respect to itsrelationship to the level of degradation within aninsulation is a complex task. This is made even morecomplicated by the possible effect of un-removableterminations on the dielectric response during fieldmeasurements. If these terminations contribute to thedielectric response of the cable in an excessive way,then it is easy to foresee a level of error beingintroduced into the judgement of the condition of acable.

Cable terminations influence the dielectric responsemeasured in a cable circuit. When measuring cables on-site e.g. cable made from low loss material PE or XLPE,accessories do in most cases limit the accuracy of themeasurement of the cable insulation. Therefore, it isimportant to know and understand the influence of theaccessories within the cable circuit.

In the earlier studies [1, 2] the dielectric response ofterminations made by different manufacturers has beenstudied and classified into two classes. These twoclasses are terminations exhibiting linear and non-linearresponse.

1-4244-0750-8/07/$20.00 ©2007 IEEE.

It has been found in previous work that stress coneterminations using geometrical field grading has a linearresponse. However, terminations using material field-grading are likely to have non-linear responses both inreal and imaginary part of the complex permittivity andalso in the conductivity. While the previous work isinformative and can be use as a guide it is not sure towhat extent this previous finding can be applied in oursituation and hence a need to perform additionalinvestigation on typical accessories in own network .

RESULTS

Field Measurement

The results of the dielectric response (DR)measurements performed on relatively new XLPEcables still in service are presented in Figures 1 and 2.The result presented in this figures are carried out at atest voltage of 200 V. It can be seen from both figuresthat the effect of a T-elbow connector can be neglectedfrom the DR measured data at frequencies higher than 1Hz. However, diagnostic interpretation rules based onDR measurement in low frequencies less than 1 Hzshould consider the contribution of a T-elbow connectorto the DR.

2Z5WWith T-elbow connector

-3 X.-4- Without T-elbow connector

-35

45

-5

-55

-6i5 L-3 2

log f (Hz)

Figure 1: Field measurement of the DR on newXLPE cable sample, CS12 phase LI, with andwithout T-elbow connectors. Measurementperformed using 200 V test voltage.

To eliminate the contribution of T-elbow connectors tothe DR it was decided to remove them in futuremeasurement. However, the contribution of the stressrelief cone to the measured DR still needs to beconsidered. An attempt to discriminate between the

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contribution of the stress relief cone to the measured DRis presented next.

-2.5X With T-elbow connector+- Without T-elbow connector

4

C-42 -4.5-

5)-5

-5.5-

6

65L3 0

log f (Hz)

Figure 2: Field measurement of DR on newXLPE cable CS12 phase L3 with and withoutT-elbow connectors. Measurement performedusing 200 V test voltage.

Laboratory Measurement

The results of a dielectric response (DR) measurementperformed on an old XLPE cable sample recoveredfrom the field is presented in Figure 3.

-1 No Guard 6kVL No Guard 9kV

2 <k f \ 7 0(~)NoGuard 12kV

-25

i5

Guard construction

A guard path is made by cutting through the insulationscreen. The guard path should ensure that a highimpedance between the guard electrode and themeasurement electrode remains. Two guardconstructions investigated in this paper are brieflydescribed below:

Guard 1: The insulation screen is removed to create aguard path of 5-10 mm as shown in Figure 4 below. Alayer of PTFE tape is then wound around the remaininginsulation screen and down onto the exposed XLPEsurface in the guard path, then up again over theinsulation screen. Another semi-conducting tape is thenwound from the geometric stress relief termination andback to cover the PTFE tape. The semi-conducting tapeand the insulation screen over-lapped slightly but do notmake contact as they are separated by PTFE tape.

Insulation screen Guar path Insulation screen

XLPE insulation

Figure 4: Guard construction (Guard 1).

Guard 2: The insulation screen is removed from the endof the cable to expose the underlaying XLPE insulationas shown in Figure 5.

4.5

-2 -1.5 -1 -0.5 0 0.5 1log f (Hz)

Figure 3: Laboratory measurement of the DRon an XLPE cable sample recovered from thefield installed with stress relief cone butwithout guard during measurement at 6, 9 and12 kV test voltage levels.

Figure 3 indicates that the sample under investigation isa good cable with linear response. Consequently, thestress relief cone on the cable under investigation can beclassified under a termination with linear response.However the observed response resembles what isknown as a leakage current (LC) type response inaccordance with a previous finding [3]. If there is nowater tree within the insulation of this cable then theobserved response is probably due to the stress reliefcone installed on the cable.

At this point an attempt is made to quantify thecontribution of the stress relief cone to the magnitudeand shape of the measured dielectric response byinstalling a guard on both terminals of the sample underinvestigation.

PTFE tape

Insulation screen Conducting tape

XLPE insulation|

Figure 5: Guard construction (Guard 2).

A layer of PTFE tape is then wound around theremaining insulation screen and down onto the exposedXLPE surface, then up again over the insulation screen.Aluminium conducting tape is then wound from thegeometric stress relief termination and back to cover thePTFE tape. The conducting tape and the insulationscreen over-lapped slightly but do not make contact asthey are separated by PTFE tape.

The DR due to these guard constructions is presented inFigure 6. In addition, a similar measurement withoutguard is presented also. The result of the measurementsat 6 kV test voltage level are presented for both guardconstructions. This is because at a voltage higher than 6

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kV surface discharges is observed, especially with theguard 1 construction.

-1 85r

the knife cut and guard construction is presented inFigure 8.

No Guard- Guard 1O Guard 2

-4

-4.5

-2 15 -1 -0.5 0 0.5 1log f (Hz)

Figure 6: Laboratory measurement of theeffect of stress relief termination on anXLPE cable sample recovered from the fieldwith and without guard during measurementat 6 kV test voltage.

-3.2

-34

2 -1.5 -1 -0.5log f (Hz)

Figure 7: DR of a newMeasurement without guards.

Independent of guard construction, it can be seen fromFigure 6, that the dielectric response of a field agedcable with guard is lower than the similar measuredresponse without the guard. This indicates thatwhichever guard arrangement is used one may stillcome to the same conclusion that the presence of a

stress relief cone in the cable circuit increases themagnitude and changes the shape of the measureddielectric response of an aged XLPE cable.Consequently, an improper conclusion can be drawnabout the status of the cable insulation if thecharacteristic of the termination within the cable circuitis unknown. Having said this it is worth mentioning thefact that with Guard 1 the field strength in the guardpath may become so high that surface partial dischargesmay be generated at voltages below the cable designvoltage. This could also explain why the response due toGuard 1 (Figure 6) is slightly higher than a similarresponse with Guard 2.

Knife cut and non-linearity ofDR

Damage in the form of a knife cut and its influence on

the DR is studied using the guard construction, Guard 2,described above. However, this study is conducted on a

new 185 mm sq XLPE cable and the measured DR isshown in Figure 7, where non-linearity can be observedat frequencies less than 1 Hz. This non-linearity couldbe to a non-linear surface conduction mechanism ratherthan degradation since this cable has never been inservice. Further more, results in Figure 3 indicate thatthe geometrical stress relief cone used in the presentstudy has a linear response though with higher losscompared to the XLPE insulation.

A knife cut about 0.5mm dip and 100mm long was

introduced into XLPE insulation surface at one end ofthe cable in order to simulate the effect of poor

workmanship on termination. Guard 2 was constructedon the terminals. Measured DR on the new cable after

I52 -1.5 -1 -0.5 0 0.5 1

log f (Hz)

Figure 8: DR of a new XLPE cable.Measurement with guards and knife cut.

As can be seen the response is higher at 9 and 12 kVthan similar measurement without guard in Figure 7. Inaddition, the non-linearity is strongly represented at allfrequency points.

The DR of Figure 8 can be attributed to poor

workmanship in the form of a knife cut on thetermination. In fact, the knife cut led to partialdischarges at 6 kV with strong partial discharge (PD)activities at 9 and 12 kV test voltage levels.

A non-linear characteristic of some terminations issimilar to the response from water tree degradation, andoften makes it difficult to differentiate between watertree within XLPE insulation and accessories. Asillustrated in Figure 8, poor installation of cableterminations in the form of a knife cut can also cause

the DR to behave non-linearly with an increase in testvoltage. An interesting thing in Figure 8 is that the slopeof the response is practically the same at all voltagelevels. This implies that the conductivity of the knife cutchannel is changing with the applied voltage. The DRpresented in Figure 8 can be used to discriminate theeffect of poor workmanship on the termination from a

defect due to water trees.

701

0 0.5 1

XLPE cable.

-2.2

-2.4

-ZE-o

-N .2.Em0

c-0m

0)0

SUMMARY

T-elbow connectors and or stress relief cones in thecable circuit can contribute to the DR ofXLPE cables ina similar way as water trees. The effect of the T-elbowconnectors can be eliminated by removing them duringthe diagnostic testing. A stress relief cone has linearresponse but with a loss that is higher than that ofXLPEinsulation. A knife cut during termination installationcan cause the DR to be non-linear with applied voltagein a similar way as water trees within XLPE cables.However, the slope of the response is practically thesame at all voltage levels.

REFERENCES

[1] A. Avellan, P. Werelius and R. Eriksson 2000" Frequency Domain Response of Medium VoltageXLPE Cable Terminations and Its Influence on CableDiagnostics", Conference record of the IEEEInternational Symposium on Electrical Insulation(ISEI), pp. 105-108, Anaheim, CA USA.

[2] S. Hvidsten, P. werelius and J. Christensen2001" Evaluation of On-site Dielectric ResponseMethods for Non-Destructive Testing of water treedMV XLPE Cables", IEE Conference Publication No.482, 2001, paper 1.48 .

[3] P. Werelius, P. Tharning, R. Eriksson, B.Holmgren, and U. Gafvert 2001" DielectricSpectroscopy for Diagnosis of Water Tree Deteriorationin XLPE cables", IEEE Transactions on Dielectrics andElectrical Insulation, vol. 8 No. 1, 2001, pp. 27-42.

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