LPOF Cables

EMP 31(11) #7619

Electric Power Components and Systems, 31:1101–1114, 2003Copyright c© Taylor & Francis Inc.ISSN: 1532-5008 print/1532-5016 onlineDOI: 10.1080/15325000390243300

Deterioration of LPOF Cable Insulation Due toIngress of SF6 Gas

MOHAMMAD I. QURESHIABDULAZIZ A. AL-SULAIMANElectrical Engineering DepartmentKing Saud UniversityRiyadh, Saudi Arabia

Contamination of oil in low-pressure, oil-filled, paper-insulated, high-voltagecables connected to the high-pressure chambers of gas-insulated switchgear,due to the leakage of SF6 gas, have often been reported by electric powerutilities. As a consequence this contamination can impair not only the dielectricbehavior of oil and oil-paper composite but also the dielectric integrity ofadjoining termination. This experimental investigation explores the impact oftrace amounts of SF6 gas in oil on its dielectric strength under the powerfrequency and lightning impulses. The results show that minor leakages of SF6gas improve the power frequency dielectric strength of oil. However, longercontact of gas with oil can significantly reduce it both under the power frequencyas well as the lightning impulses of both polarities. Vaporization of liquid isfound to cause pre-breakdown discharges which lead to its final breakdown.

Keywords electric cables, gas contamination

1. Introduction

Despite the advent of extruded polymeric insulation, paper-insulated oil-impreg-nated cables are still in widespread use worldwide because of their excellent long-term reliability, since (1) physical and chemical stability of cellulose renders ithighly resistant to deterioration in the absence of oxygen at temperatures evenabove 100◦ C, (2) they are highly resistant to damage from ionic discharge (corona),which readily destroys XLPE insulation, and (3) capacity for absorption of oil intohollow cellulose fibers and their interstices and onto the surface of the paper createsa homogeneous dielectric in which oil is segregated into thin films or minute pockets,the strength of which increases as their dimensions are reduced [1].

Manuscript received in final form on 23 July 2002.The authors thankfully acknowledge the financial and technical support given by

the Research Grants Committee, Deanship of Scientific Research, King Saud University,through their research grant No. 5/418. Thanks are also extended to Mr. M. I. Al-Asseriand K. S. Al-Motairi for carrying out the experimental work, and to Mr. A. R. F. Karimand Mr. T. Farooq for typing this manuscript.

Address correspondence to A. A. Al-Sulaiman, Electrical Engineering Dept., KingSaud University, P.O. Box 800, Riyadh 11421, Saudi Arabia. E-mail: [email protected]

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1102 M. I. Qureshi and A. A. Al-Sulaiman

Low-pressure oil-filled (LPOF) paper-insulated cables form an important linkin the transport of electric energy in the Arabian Gulf countries, at both HV aswell as EHV profiles. These utilities as well as others worldwide have occasionallyexperienced power outages on account of leakage of SF6 gas from gas-insulatedswitchgear (GIS) into oil-filled cables through their high-voltage terminations [2, 3].The weakest point in the GIS termination is the rubber seal as shown in Figure 1.The chamber of SF6 switchgear and the oil-filled cable side operate at differentpressure levels.

Since the gas is at high pressure, therefore at the instant of leakage, it will getinto oil and oil-paper composite. Slow leakage of SF6 into cable oil generally goesundetected for extended periods of time without appreciable changes in pressure.Utility engineers generally replace/replenish the oil in the cable network as soonas the concentration of SF6 gas or residual gas pressure in oil attains a certainpreset level during the service. These set values are based on the recommendationsof manufacturers; however, the details and scientific merit of these recommendedset values is generally not known. If the leakage is extensive, the gas can be trappedat the sealing end and experience a breakdown. However, the slow ingress of gasin the cable system will not lead to immediate breakdown but may cause adverseeffects that need to be explored. In particular, the dielectric loss factor, breakdownstrength of both the oil and impregnated paper, besides the chemical reactions ofSF6 and its by-products need to be addressed. These properties, if found to begetting deteriorated, can be detrimental to the life of these cables.

The leakage in the termination as elucidated above can occur in three forms(modes) and can be modeled as shown in Figure 2. In the first case, if the oil-gasinterface seal is ruptured, the gas can suddenly be trapped at the sealing end bypushing the oil away into the cable system. Consequently, the dry sealing end shallface a serious breakdown. In the second case the gas can permeate through themolecular mattress of the rubber sealing ring and come in gentle contact with theoil in the cable system. The oil has a strong tendency to absorb gas; therefore, inthe long term the oil molecules can get doped up with the SF6 gas molecules andthus pollute the fluid network. In the third case, pinhole formation in the sealingring due to aging can lead to the ingress of SF6 in oil in the form of a stream ofbubbles that may transit deep along the length of the cable.

This investigation simulates the second and third modes of leakage of SF6 gasinto the matrix of the cable through its termination. In the first case (mode II),the slow leakage of SF6 gas into the composite insulation of cable was simulatedby maintaining the oil and impregnated Kraft paper samples in contact with SF6gas at a pressure of 2.0 bar for periods that varied up to 120 days. The effects ofdifferent concentrations of dissolved SF6 were investigated for dielectric propertiesof oil and oil-paper composite. In the second case (mode III) the SF6 gas maintainedat 2.0 bar pressure was bubbled through the cable oil at a constant bubble dischargerate for periods that varied up to 100 days in this case. In this case the effects ofdifferent concentrations of dissolved SF6 gas in oil on the power frequency andimpulse behavior of cable oil have been investigated.

2. Experimental Techniques

In practice, the GIS works at a pressure of ∼5.0 bar, whereas the oil in the cableremains at a pressure of 2–4 bar depending on the site profile of the cable layout.

Oil-Filled Paper Insulation 1103

(a)

(b)

Figure 1. (a) Schematic of a 132 kV LPOF cable’s GIS termination. (b) Details of GISchamber and cable sealing end connection.


Figure 2. Different modes of gas leakage from GIS termination to LPOF cable.

In average the differential pressure remains around 2.0 bar. To simulate the slowleakage of gas to the oil system (mode II), a test chamber which acted as abuffer tank, as shown in Figure 3, was designed in which oil samples as well asimpregnated Kraft paper samples were introduced. The gas circuit that proceedsthis chamber was kept closed in this case. This chamber was kept at SF6 gas pressureof 2.0 bar while maintained at room temperature. The oil under test, a semisyntheticcable oil, was acquired from a local power utility and was dehydrated and filteredthrough a sintered glass filter of 10–16 µm porosity. The cellulose paper subjectedto these tests is the same as the one used in the 132 kV-LPOF cables operatingin Riyadh region. Different samples of oil and impregnated paper were prepared


Figure 3. Experimental setup for doping oil and paper samples with SF6 gas for mode IIand mode III.

by letting them stand in contact with SF6 gas for periods of 10, 67, and 119 days,respectively. Impregnated Kraft cellulose-based paper samples were prepared earlierin a vacuum chamber. Dielectric loss factor (εr tan δ) was measured by a precisionpower frequency Tettex type 2821 Schering bridge at an electric stress of 10 V/µm.Breakdown of the oil was measured by an auto 90 kV oil tester using VDE electrodesfollowing the testing technique as per IEC-156. Each data point on the curvesindicates an average value, while the standard deviation does not exceed 5%. Thebreakdown measurements of paper samples were made according to IEC-243. Inthis case the paper samples were immersed in the same oil in which they wereimpregnated and tests were performed using a 2.5 kVA, 0–50 kV, 60 Hz noise-freetransformer equipped with a fast electronic tripping system. In this case, due tolarge scatter in breakdown values, the average values are plotted along with theerror bars to show the range of the minimum and maximum scatter.

To simulate (mode III) in the laboratory, the gas from SF6 cylinder was pumpedinto air-evacuated buffer tank maintained at a precise pressure of 2.0 bar. This buffertank was connected with a gas dehumidifier cum filter and a needle control valve asshown in Figure 3. By the dint of this needle control valve a precise gas dischargeof 0.5 liter per minute could be easily regulated. In this way a smooth stream ofuniform-sized gas bubbles eject smoothly into the oil reservoir. Using this techniquefive samples of oils doped with SF6 gas were prepared by bubbling them for 2, 10,20, 50, and 100 days, respectively. Preliminary tests showed here that gas contactperiod in days is almost directly related to gas concentration in oil.

To evaluate the breakdown characteristics under standard lightning impulsesof both polarities, an impulse test cell built according to ASTM-D3300-77 [4] wasused in which the gap could be precisely adjusted from 1 to 30 mm. Before fillingthe cell, it was rinsed with petroleum ether and then dried in oven maintained at110◦ C. The steel ball was removed each time the cell was washed/rinsed. Standardlightning impulses of both polarities were obtained by a single-stage generator thatwas built for this purpose. Its details are given elsewhere [5]. The measurementswere made using a multiple-level method as per IEC-60060-2 [6], and a breakdown


probability distributions curve for a given sample and gap spacing was determinedby applying a set of five shots of impulses at a fixed voltage level at intervals of30 s. If the impulses failed to produce a breakdown, then the voltage was raisedby 0.5 kV under positive polarity and 3 kV in case of negative polarity impulses,and the test run as described above was repeated for five consecutive shots. Thisprocedure was continued till 5 out of 5, that is, a 100% breakdown level (V100) wasattained. In this study the plotted results show 0% probability (p) at p = 0.01%,whereas 100% probability is shown at p = 99.9%. After each run of breakdowns,the cell was thoroughly cleaned and its needle electrode was replaced before a freshsample was refilled in the test cell.

3. Results and Discussion

3.1. Related to Mode (II)

Effect of presence of SF6 gas in cable oil. Figure 4 illustrates the variation of lossfactor as a function of temperature. It is clear that the loss factor increases withtemperature; however, the loss factor in SF6-doped samples of oils is lower than ina fresh cable oil sample. However, in the present case the electronegativity of SF6gas does modify the dependence of electric losses of oil at electric fields of up to

Figure 4. Variation of loss factor of oil as a function of temperature. Numbers on curvesindicate the days the sample was kept in contact with gas. F = Fresh oil sample.


10 V/µm, and imparts opposite and beneficial effects. The variation of dielectricloss–temperature characteristics for different concentrations of SF6 gas (contactdays in figure) exhibits a nonmonotonic trend, which is contrary to the well-knownconduction losses due to impurities in the bulk of oil. Although utmost care wastaken during the preparation and treatment of samples, yet the likelihood of ingressof trace amounts of atmospheric moisture cannot be ruled out. Its content, oncethe samples were dopped with SF6, could not be monitored. The nonmonotonicvariation in loss magnitude can be attributed to the presence of OH− and H+ ionsand their reaction with the SF6 gas molecules or their stress-decomposed radicals. Itis known that electric losses of contaminated oil are increased since the presence ofelectric stress oxidizes the contaminating impurities. The contamination of mineralcable oil with trace amounts of low-ionization potential impurities has been foundto increase the conduction losses manifolds [7]. Similarly, under the action of electricarcs the SF6 dissolved in oil is decomposed into CF4, SF4, SOF2, SOF4, S2F10, HF,SO2, etc. [8, 10]. However, under mode (II), it is clear that such by-products are notformed, rather electron-scavenging reactions take place that cause overall reductionin the magnitude of the conduction process.

Figure 5 shows the effect of power frequency breakdown voltage of oil samplesas a function of gas content. It is clear that oil breakdown strength decreases withincrease in SF6 content (10 days), whereafter it increases and attains its freshoil breakdown level. This shows that prolonged contact of SF6 gas with oil shallnot effect its breakdown strength in a considerable manner. It is known that thebreakdown process is initiated when a minuscule amount of oil in contact with theelectrode is vaporized to gaseous form, which under the action of a Laplacian fieldpropagates toward the opposite electrode in the form of filamentry channels knownas trees [9, 17]. If the oil has a gas-absorbing tendency, then the formation of thisgaseous phase becomes difficult, whereas in gas-evolving liquids, tree formation

Figure 5. Effect of SF6 gas content on the breakdown voltage of oil samples. F = Freshoil sample.


can occur fairly easily. The presence of SF6 gas in oil has been shown to makeit gas absorbing, and during this process it is itself consumed through energy-transfer reactions [10]. Mineral-based oils are generally gas evolving. The reductionin breakdown level in oil up to the S10 level can be attributed to the presence of aninsufficient amount of SF6 in oil. As in this case, inefficient charge transfer reactionswould let it behave like an impurity. However, a further increase in SF6 content letsthese reactions become mature, which make the oil gas absorbing.

Effect of presence of SF6 in impregnated paper. Figure 6 explains the variation ofloss factor of impregnated samples as a function of temperature for different SF6-doped samples. Unlike in the bulk of oil, the beneficial effect of the electronegativityof SF6 molecules in the paper film is confined only to their minimal presence (twodays contact as shown in the figure). But the introduction of further content affectsadversely on its leakage current performance. It can be attributed to the presence ofweak organic acids present in the porous cellulosic mattress of Kraft paper as wellas due to remnant moisture content in its fibers, which generate H+ and OH− ions.These are known to posses large propensity to react with SF6 molecules as well aswith their stress-decomposed products [8]. With the increase in temperature thiseffect is further elevated. On the other hand, the breakdown strength of impregnated

Figure 6. Variation of loss factor of impregnated paper samples as a function of temper-ature. Numbers on curves indicate the days the sample was kept in contact with gas. F =Fresh oil sample.


Figure 7. Effect of SF6 gas content on the breakdown voltage of impregnated papersamples. F = Fresh oil sample.

paper increases almost by 20% due to a longer contact with SF6 gas, as illustratedin Figure 7. This again shows that partial discharge activity that is a precurserto the final breakdown event is suppressed due to the gas-absorbing character ofSF6-doped paper insulation.

The results presented so far reflect the gas content absorbed in the cable’sinsulant up to 120 days. However, it is to be noted that once the oil-paper compositegets fully saturated, then free gas molecules will be left migrating in the cable. Thesewill eventually end up in the form of bubbles at locations that are comparatively ata higher mechanical level in the cable network. If these locations are sealing endsor splices, where generally the high stresses prevail, then strong partial dischargeactivity and local arcing can take place. These activities, once initiated, can generatevery corrosive by-products that may cause rapid deterioration of paper insulation.

3.2. Related to Mode (III)

Effects of SF6 in oil on ac breakdown. Figure 8 illustrates the variation of theratio of power frequency breakdown voltage of SF6-doped oil samples (Vbp) to thebreakdown voltage of a clean oil sample (Vbc) as a function of the SF6 gas contactdays in oil. It is clear that addition of SF6 gas molecules up to concentrations thatare related to 10 days contact with oil causes a significant increase in the breakdownstrength, but beyond that level of concentration it decreases semilinearly with theincrease in gas concentration. A decline of 30% is noticed after 100 days of gascontact, and it is anticipated that prolonged contact will further deteriorate thischaracteristic of oil.


Figure 8. Variation of the ratio of [Vbp/Vbc] as a function of SF6 gas concentration inoil samples under the application of power frequency stresses. Numbers on curves showelectrode gap spacing.

Under the action of alternating fields, electrohydrodynamic (EHD) motion ofthe liquid sets in, which creates turbulence in the oil which can lead to formationof cavities. Similarly, particles in the bulk of the liquid and those residing on theelectrodes can get raised up while leaving cavities in their wake. These cavities be-come a source of initiation of discharge in the interelectrode gap [8, 11]. However,the presence of SF6 gas molecules in such cavities will suppress the discharge initi-ation processes because of their electronegativity and electron-trapping character.The second effect of EHD motion in oil can be in the form of cooling at the tip ofasperities in the electrodes from where the discharges are known to initiate due tovaporization of minuscule oil films that are in contact with asperities. Vaporizationof liquid at such sites is known to generate trees in the oil, which finally lead tobreakdown of oil. The presence of SF6 in oil has been found to increase the EHDmotion [12], thus enhancing the cooling effect and suppression of tree formation.


Contact of SF6 in oil samples exceeding 10 days shows a significant decline in Vbp

values. This could be attributed to the by-products of SF6 that are very corrosivein nature and cause a considerable increase in the leakage current of oil [10]. Theseby-products may be formed as a result of intense discharge activity in the vaporbubbles/cavities in the presence of excess amount of gas. Ali [13] has indeed re-ported a significant increase in partial discharge activity of SF6-doped oil samples.Which one of the two processes, viz., the cavitation and the vaporization, is theactual source of such effects, or whether it is that only one of them is operative inthe present situation, needs to be carefully looked into.

Under the action of short-duration unipolar impulses, the particles do notget sufficient time to lift off mechanically the electrodes to generate evacuouscavities. The role of EHD motion and its associated instabilities in the insulatingliquid, which occur as a result of a transient unipolar charge injection, have beeninvestigated by several researchers [14, 15]. Though the particulate contaminantswith diameters less than 1 mm in nonpolar liquids have been proved to act as chargecarriers [15], yet a significant time delay between the application of high-voltage(HV) impulse and the onset of EHD turbulence is reported. In the case of mineraloil and several nonpolar insulating liquids, it has been reported by Cross and Wang[14] that such time delays vary in order of milliseconds under the application ofvoltage steps of up to 105 volts. Therefore in the present experiments, under theapplication of 50 µs duration unipolar HV impulses the EHD motion cannot setin. Similarly, shockwaves are also found to propagate with the onset of dischargestreamers. However, any subsequent discharge activity that could arise as a resultof turbulence generated as a consequence of such shock waves is not possible undersuch short-duration impulses, and to the best of the authors’ knowledge has neverbeen reported in literature [16, 17]. Based on the preceding discussion, it is ourpremise that under the application of short-duration impulses the two sources ofdischarge initiation, viz., the vaporization and cavitation, can be isolated. Moreover,the determination of impulse breakdown strength is also important, since the cablenetwork is occasionally subjected to lightning surges during service.

Effects of SF6 in oil on impulse breakdown. Figure 9 depicts breakdown probabilitydistribution as a function of applied voltage in a fresh oil sample. Similar results areobtained for all the other oil samples and gap spacings investigated. It is clear thatunder the application of positive lightning impulses, the distribution is Gaussian,whereas it is non-Gaussian under negative polarity. It is clear that negative V50values are higher than positive V50 values, which emphasize the severity of theimpact of positive lightning surges on the oil-filled cable systems.

Figure 10 shows the ratio of [Vbp/Vbc] based on V50 values of breakdown underboth polarities of impulses as a function of SF6 gas concentration in oil samples.There is ∼15% reduction in [V50] of gas-polluted oil under negative impulses, whichtends toward saturation despite the increase in gas concentration. On the otherhand, the V50 values decrease with increase in gas contact time, which approaches∼30% in the case of a 100 days aged sample. It is anticipated that it will furtherdeteriorate with the increase in contact period.

It is also interesting to note that contact periods of up to 10 days show signif-icant decrease in V50 values under both polarities, whereas under power frequencyvoltage these samples showed ∼25% increase in Vb. This result clearly demonstratesthat cavity-based breakdown processes are not the ones which lead to breakdown


Figure 9. Breakdown probability distribution as a function of applied voltage, under theapplication of standard lightning impulses on fresh oil sample.

of oil, but certainly the vaporization of minuscule liquid films in contact with pro-trusions and asperities on high-voltage electrodes is the main controlling factor oftree formation in oil, which finally leads to its breakdown.

Further work on the effects of leakage of SF6 gas on the other dielectriccharacteristics of impregnated cellulose-based insulation of EHV cables is currentlyin progress in this laboratory.


Figure 10. Variation of [Vbp/Vbc] as a function of SF6 gas concentration in oil samplesunder the application of standard lightning high-voltage impulses.

4. Conclusions

Based on the presented experimental results of this ongoing research the followingconclusions are derived:

1. Under power frequency, the slow leakage of SF6 gas in the form of bubblescauses a slight increase in the breakdown of cable oil, which significantlydecreases due to longer contact of gas with oil.

2. Under the impact of lightning surges the dielectric strength of SF6-doped oilis impaired. The worst effects are imparted under the application of positivepolarity surges.

3. It is anticipated that even under longer periods of gas contact, provided thebubbles are not formed, the composite insulation of oil-filled cable shall notbe effected.

4. These results reject the theory of cavitation-induced breakdown of oil, but in-stead support the formation of pre-breakdown trees due to liquid vaporization.

References

[1] K. Bow, “Selected panel session: 1994 transmission and distribution conf. and exhi-bition,” IEEE Electrical Insulation Magazine, vol. 11, no. 3, pp. 44–51, 1995.


[2] M. Al-Huwaiti and A. P. Cela, “SF6 ingress in LPOF cable sealing ends,” Proceedingsof CIGRE (GCC) 5th Symposium on High Voltage O/H Line and UndergroundCables, Qatar, 1994, Paper No. 4.

[3] J. A. H. Jamal et al., “MEW-Kuwait experience in the maintenance and repair ofunderground filled cables,” Proceedings of CIGRE (GCC) 5th Symposium on HighVoltage O/H Line and Underground Cables, Qatar, 1994, Paper No. 6.

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[10] A. Denat and B. Gosse, “Influence of SF6 on the electrical behavior and gassing prop-erties of transformer oil,” IEEE Publication 81CH 1594-1, Proceedings of 7th Interna-tional Conference on Conduction and Breakdown of Dielectric Liquids, pp. 411–415,1981.

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[13] A. S. Ali, “Effect of dissolved SF6 on the performance of paper-oil cable insulation,”Electrotechnika, vol. 62, no. 3, pp. 40–42, 1991.

[14] J. D. Cross and H. T. Wang, “Interfacial instability: a new approach to transientEHD motion with unipolar injection,” IEEE Trans. on Electrical Insulation, vol. 28,no. 4, pp. 641–646, 1991.

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[17] A. Beroul et al., “Propagation and structure of streamers in liquid dielectrics,” IEEEElectrical Insulation Magazine, vol. 14, no. 2, pp. 6–17, 1998.

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