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Relationship between power factor and resistivity of oiland the power factor of impregnated paperE C. R. Scarfe B.Sc A.C.G.I. A.M.I.E.E. and R. G. Pulsford B.Sc A.R.C.S.

ynopsisAn investigation has been carried out into the effect of ionic and other additives on the power factor ofoil-impregnated paper, with the object of relating the power factor of the impregnated paper to the powerfactor of the impregnant. For power-factor measurements, an improved test cell was used which wasdesigned to operate at high stresses over a wide temperature range. It was observed that there was avariable relationship between the power factor and the volume resistivity of oils depending upon the typeof additive. There was no correlation between the power factor of impregnated paper and the power factorof the impregnant, but a definite relationship was found between the power factor of impregnated paperand the volume resistivity of the impregnant, this relationship being independent of the type of additive.

1 ntroductionIn order to produce oil-impregnated-paper dielectrics

with the low power factors required for e.h.v. cables, it isnecessary to avoid the use of materials that might increasepolar losses or ionic conduction. Water-washed wood-pulppaper with low power factor is selected, and the cleanliness ofthe oil impregnant is also normally judged by its power factor.

A considerable amount of data exists on the effects ofvarious contaminants on the power factor of hydrocarbonoils' 6 and on the power factor of impregnated paper.710Attempts have been made to relate the power factor of impreg-nated paper to the power factor of the impregnant, but thesehave usually been unsuccessful. Jt has also been observed thatthere is no relation between the power factor of oil extractedfrom oil-filled cables and the power factor of the cableseither directly after manufacture or after they have been inservice.6 This is often attributed to difficulties in samplingfrom cables, but the work reported here suggests that such arelationship probably exists only for certain limited types ofoil contaminants.

Our own investigation into the effect of oxidation productson the electrical properties of oils lead to the observationthat, whereas contaminants affected both the power factorand volume resistivity p there was no constant relationshipbetween power factor and volume resistivity. It has beenshown by Balsbaugh and Assaf, and confirmed by Feick,Eich and Olds,12 that the dielectric losses in oil are mainlyionic, and it is suggested that d.c. measurements may besubstituted for power-factor measurements. We have foundthat this applies only to a limited class of materials as shownin Fig. I, where power-factor measurements made on con-taminated oils are plotted against the equivalent volumeresistivities. The theoretical relationship between power factorand resistivity, assuming all losses are ionic, is given byptan S = 1-63 x I010 for an oil with a dielectric constantof 2-2 and a measuring frequency of 50c/s, where p =resis-tivity (ohmcm) and tan 8 = power factor for the low valuesunder consideration. This is represented in Fig. I by theline towards the lower left-hand corner. Only a few measure-ments fall near this line, and as might have been expected,they were of oils to which fatty acids had been added. Mostother substances affected power factor to a greater extentPaper 4606 S, first received 2nd December 1963 and in revised form26th February 1964. It was presented at the Conference on Dielectricand Insulating Materials,9thApril1964Mr Scarfe and Mr.Pulsford are with Associated Electrical IndustriesLtdPROC. IEE Vol. 112 No. 3 M RCH 965

than resistivity, and so, as shown in Fig. 1, the theoreticalrelationship did not apply.The reason for this variable, but controlled, relationshipwas not at first obvious, and variables such as moisturecontent and testing stress were examined. The d.c. measure-ments were normally carried out at a stress of 3-3kV/cmand the a.c. measurements at 6-6kV/cm, but varying thesehad very little effect on the power-factor/volume-resistivityrelationship. Moisture contents were varied between 5 and50 parts in 106,also without effect.

This variable relationship is undoubtedly due to thebehaviour of different types of contaminant when under anelectric stress.

It is known that ionic materials in oil cause high a.c. losses,as well as d.c. conductance, and it is also found that materialswhich are not ionic in character, but are of acolloidal nature,produce high a.c. losses owing to the motion in an alternatingfield of large charged particles. Materials of a colloidal naturecan be formed in oil by oxidation, and are also producedunder corona discharge.4 Lt is convenient to represent themexperimentally by a material such as bitumen which readilyforms a colloidal system simply by addition to oil.

Those materials that are largely colloidal, such as bitumen,cause high a.c. losses, but the colloidal particles are rapidlyswept away in a d.c. field by electrophoresis and hence do nothave such an appreciable effect on the volume resistivity(which is normally measured after 1 min electrification).Such materials would thus cause greater a.c. losses than wouldbe expected from d.c. measurements, and if plotted in Fig. 1the results would fall above the theoretical line. A carboxylicacid is capable of dissociating into ions, particularly in anelectric field, and the 1min volume-resistivity value andthe 50c/s power factor will both be a measure of the ioniclosses and these measurements will fall on or near thetheoretical line.

This power-factor/volume-resistivity relationship is ofinterest in itself as a guide to the type of material causinghigh dielectric losses in an oil, but of greater importance isthe effect on the power factor of oil-impregnated paper. Itwas predicted that those oils with power-factor/volume-resistivity relationships towards the upper right-hand cornerof Fig. 1 would not affect the power factor of impregnatedpaper because of the colloidal nature of the additives. Thecolloidal particles would be large compared with the poresize of the paper and so would be trapped in the paper,their effect being eliminated or considerably reduced. Ionicmaterials would be expected to increase the power factor of

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impregnated paper because they would continuously disso-ciate in the oil phase, and, owing to their small magnitude,would be less liable to become trapped in the paper fibres.It was thus expected that high power factors would be pro-duced when paper was impregnated with oils whose power-factor/volume-resistivity relationship occurred towards the

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