500 NATIONAL POWER SYSTEMS CONFERENCE, NPSC 2002 Practices of Insulating Materials in Instrument

A.Masood, M.U.Zuberi, M.S.Alam, E.Husain, M.Y.Khan

I. INTRODUCTION TABLE I Type of Test Applied on The study of insulating material selection, behavior and Type Test CT PT performance are important aspects to be considered for Short Time Current Test Yes No designing any electrical instrument, machine or device. Temperature Rise Test Yes Yes Apart from conductors, forms the backbone of Lightning Impulse Voltage Withstand Yes Yes any electrical system. Though insulators form the Test backbone, they are also the weakest link in the system. H.V.Power Frequency Wet Withstand Yes Yes Hence, maximum care and attention is needed while Test choosing insulating materials for a given application so Routine Test that it gives the desired performance under worst working conditions. This paper addresses the problem of selection H.V. Power Frequency Dry Withstand Yes Yes criteria, test specifications and material treatment to justify Test on Primary Winding the use of various dielectrics/insulating materials used for H.V. Power Frequency Dry Withstand Yes Yes insulating high voltage instrument transformers i.e. CTs Test on Secondary Winding and PTs. Over-Voltage Inter-turn Test While in use, insulating materials are subjected to Partial Discharge Test Yes No various electrical, mechanical, thermal stresses and partial Yes Yes discharges. Therefore, criteria for selection of these materials is, that, they must withstand these stresses The temperature rise of winding when subjected to without or with such rate of deterioration such that their continuous thermal current or voltage as applicable to performance is not affected throughout the life expectancy instrument at the rated frequency and of the equipment, which is considered approximately 25- current/voltage output determines the class of insulation or 30 years.[1] dielectric surrounding the winding. Having determined the class For instrument transformers, selection and tests for of insulation or dielectric, the consideration is based on the insulating materials are major considerations. The mechanical/electromagnetic stress generated in windings and in governing standards of the is IS: 2705 , the dielectrics surrounding it, due to dynamic short time current IEC:185 and for the is IS:3156, IEC: condition. Mechanical stress may occur during normal handling 186. The requirements of are during manufacture, transportation and erection in case of CT specified in terms of service conditions, ratings and special and PT. features such as limiting dimensions etc. Performance is Besides, mechanical stress, the dielectric has also to evaluated by various type tests, routine tests and some withstand electrical stress during lightning impulse and power optional tests for checking special features not generally frequency voltage applications under wet as well as dry covered by type test or routine tests conditions. Partial discharge test is an indicator of i) presence of The various types of tests carried out on both CTs and air pocket in dielectric and/or ii) loose electric contact and/or iii) PTs affecting their dielectric medium are given in Table I. external corona discharge. This is subject to the condition that the properties of insulating material are in conformance to the specifications/ requirements. ______One basic requirement for all materials used in oil immersed transformers is that they should be compatible with insulating 1. A.Masood is a Lecturer in Electrical Engineering Department, AMU, Aligarh. [email protected] oil and should not react with or deteriorate the oil. Out of the 2. M.U.Zuberi is a Senior Lecturer in Electrical Engineering Department, three broad categories of insulation, namely, solid, liquid and AMU, Aligarh. [email protected] gaseous; solid and liquid insulation materials have been 3. M.S.Alam is a M.Tech student in Electrical Engineering Department, discussed. AMU, Aligarh, [email protected] 4. E.Husain is a Professor in Electrical Engineering Department, AMU, Tests for different insulation components used in CT/PT are Aligarh. [email protected] performed according to Indian Standards as mentioned in Table 5. M.Y.Khan, is Managing Director, SCT Ltd. Ghaziabad., II. [email protected]

INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR 721302, DECEMBER 27-29, 2002 501

TABLE II fresh insulating oil, it is desirable to have more of nephthenes or paraffins, less of aromatics and none of olefines. Such an MATERIAL APPLICABL APPLICATION E optimum balance is achieved by a carefully controlled refining STANDARD process. An oil is known as nephthenic base or paraffin base oil Liquid dielectric depending upon the dominance of nephthenes or IS:335 Liquid dielectric and paraffins.Nephthenic base oils are more in use as dielectrics in coolant colder countries since they do not consist of wax type Solid dielectric constituent which increases their viscosity at very low Electric Grade insulating paper i.Kraft insulating paper IS:9335(Pt- Layer Winding temperatures. Due to cost consideration, in a country like India of medium air permeability 3/Sec.1) insulation. having moderate climate, it is advisable to use paraffin base oil. Major Insulation Insulating paper being hygroscopic [5], the incipient moisture ii.Crepe Paper IS:9335(Pt- Additional strength to is dried in heated vacuum chambers. The degree of dryness is 3/Sec.1) insulation iii.Press Paper IS : 8570 Inter-wound core monitored by measurements of loss tangent on the major insulation , Former in insulation. When optimum values of measurements are obtained PT’s during the drying cycle, instrument transformers are Insulating Tapes impregnated with dried and degassed transformer oil or else the i.Cotton tapes IS:1923 Various taping design and operation of the equipment is limited by the purposes occurrence of partial discharges either at the interface or in ii.Self-adhesive polyester IS:7809 Taping purposes film tape requiring higher voids. It is possible that under electrical stress and working mechanical and condition, a slow change in material may lead to localized electrical strength variations in physical properties, generally towards their iii.PVC tape Banding of LT CT. degradation [4]. Bakelite Insulating wound core supports. Un -impregnated densified Core Coil support IS:335, gives various min./max. values of characteristics laminated wood in PT ensured by oil manufacturers. Test certificate is furnished along Insulated Copper with the supply stating the exact values. These sixteen Conductor and Cables characteristics are - appearance, density, kinematic viscosity, i. PVC Insulated Copper IS:1554 Control wiring/ interfacial tension, flash point, pour point, neutralization value, Cable leads ii.Enameled round IS:4800 LT winding in CT. corrosive sulphur, electric strength, tand, specific resistance, winding wire HT/LT windings in oxidation stability, ageing characteristics after accelerated PT ageing, presence of oxidation inhibitor, water content and SK iii.P.T.F.E.Insulated Leads for LT values. Copper Cable terminations. Hollow Insulator On receipt of oil at the works, few quality tests are performed, i .Quartz/ Silica porcelain IS:2099 Insulating washer in namely, electric strength (BDV), tand, specific resistance and IS:5621 primary terminals water content. If the test reveal the need of filtration, it is done ii .Alumina porcelain IS:2099 Main insulator along with degassing. When results are satisfactory, oil is IS:5621 isolating live part from impregnated in transformers under vacuum at 60°C to avoid dead part. Thermoset Polymer undue thermal stress. i. Epoxy Resin system Various components However, ageing is an unavoidable phenomenon and like terminal deterioration is a continuous process owing to several reasons. encapsulation. Ingress of moisture and oxygen from the atmosphere may take Insulation between LT place in service. Oxidation at high wo rking temperatures is and HT windings in dry type transformer. accelerated due to catalytic effects of metal ions of copper, aluminum and others. It gives rise to formation of acids, sludge, II. BRIEF DESCRIPTION OF INSULATING peroxide, etc. which affect the physical, electrical and insulating MATERIALS properties of oil. Therefore, periodical condition monitoring of oil is recommended by IS: 1866 under strict sampling procedure A. TRANSFORMER OIL as per IS: 6855. Thus oil, if maintained properly, can give Petroleum based mineral insulating oil is used in reliable service rendering the equipment-desired life expectancy. transformer as a dielectric medium as well as a coolant. Transformer oil [1] which belongs to Class A insulation B. INSULATING KRAFT/CREPE PAPER mainly consists of four generic classes of organic Insulating Kraft & Crepe Paper, Class Y insulation, are one of compounds, namely, paraffins, nephthenes, aromatics the earliest forms of composite dielectrics used in high voltage and olefines. Paraffins and nephthenes are saturated engineering since the end of the 19th century. Paper as such has hydrocarbons while aromatics and olefines are unsaturated very poor dielectric properties, but when impregnated with oil, hydrocarbons. For better stability of low viscosity type the properties of the composite dielectric improve considerably. Paper is a product of 'Cellulose’ obtained from soft wood pulp

502 NATIONAL POWER SYSTEMS CONFERENCE, NPSC 2002 of mainly pine or spruce found in Scandinavia[1]. E. UNIMPREGNATED DENSIFIED LAMINATED WOOD Cellulose is a complex carbohydrate, which forms the Wood based laminates, Class A insulation, are manufactured chief constituent of the cell walls of plants. The cell walls from selected veneers (mostly 2 mm thick) obtained from contain lots of contaminants like lignin and resins which various timbers. The veneers are dried and partially impregnated are removed by the sulphate process treatment and careful with neutral phenol formaldehyde and then densified under heat water washing. and pressure. Alternate veneers with grain orientation at right Normal thickness of paper available in the market are angles to each other are placed so as to obtain high mechanical 15, 20, 25, 50, 65, 75, 100, 125, 160, 200 or 250 micro- strength. meters. It forms an excellent insulation between primary In areas, which require higher mechanical and moderate and secondary windings. Additional strength is provided electric strength, this material is used extensively, for example, during taping process so as not to allow presence of air in core clamps and vertical supports. As its moisture absorption pockets, which are trapped between layers. This additional capacity is high, wood is assembled with core and coil etc. and strength is provided by use of crepe paper. Crepe paper is then dried, degassed and impregnated with transformer oil so made of Kraft insulating paper by a process, which imparts that moisture is replaced with the oil. Wood yields compressive irregular close crimps to it , thus increasing its thickness as strength as high as 185 MPa and electric strength as high as 60 well as extensibility in the machine direction[2]. Crepe kV/mm edge wise. paper normally available has nominal values for elongation break at 50%, 80%, 120% and 200% etc. with coarse, F. COTTON TAPE medium or fine creeping. Cotton tape, Class Y insulation, is manufactured from cotton On impregnating the paper with transformer oil after yarn, which is reasonably free from reps, slubs, knots and kinks drying and degassing, the voids in paper are filled-up with etc. Tapes are evenly woven. They are singed, brushed and oil expelling the air. Drying process of paper takes place in calendered. Its use is limited to low tension areas, like taping a warm vacuum chamber. In order to avoid damage to core etc. It is available in 20-30 mm width of 50 – 100 meter cellulose, slow drying is performed within a temperature of roll lengths. As its breaking load is approx. 10 Kg, it can 120° C on achieving vacuum of 0.01 Torr, oil is provide moderate clamping strength. Its pH value is between 6.0 impregnated at the reduced temperature of about 60° C. and 8.5, chloride content is 10 ppm (max.) and conductivity of In the composite insulation, the dielectric properties are aqueous extract is 50 micro-mhos/cm. Therefore, it should be improved considerably. Electric strength improves from 10 dried alongwith the assembly and then impregnated with resin in or 11 kV/mm to 60 or 70 kV/mm. tand, decreases from 0.1 dry type transformers or with oil in oil immersed transformers. or 0.2 to 0.004 to 0.006 at approximately 90°C. Standard values of relative permittivity, loss tangent and electric G. SELF ADHESIVE POLYESTER FILM TAPE strength of paper impregnated with oil at 20° C are 3.5 - This tape, Class B insulation, is available with backing of 3.9, 0.0026 – 0.003 and 60 - 70 kV/mm. polyester film in two thickness of 50 and 80 micro-meter with Above results are considerably affected by the presence non corrosive thermosetting adhesive coating. It provides of moisture and ‘Service electric stress’. To give trouble excellent service in areas requiring adhesive strength of 3.0 free life to equipment, it is advisable to process and N/cm as well as electric breakdown capability of 5 kV (for 50 impregnate the paper with transformer oil carefully and micro- meter thick tape)/8 kV (for 80 micro-meter thick tape). It keep the maximum 'Service stress' under 2 kV/mm. withstands temperature upto 130° C with ease. It is also oil resistant. The firmness of the tape increases on heating at 150° C. PRESS PAPER C for 1 hour or 130° C for 2 hours and remains so on cooling; it Press paper, Class E insulation, is made of insulating does not soften appreciably on subsequent re-heating. The initial Kraft paper. A large no. of thin papers having thickness of heating of the thermoset adhesive improves the solvent resistant the order of 30 micro-meter are pressed together by a hot and increases the softening temperature. press method in the desired form. A glossy finish is provided to the surface. In the absence of binding material, H. PVC TAPE electrical properties of these boards are near that of high PVC, Class Y insulation, is a polymer product of the quality paper. When impregnated with oil, its dielectric monomer Vinylchloride derived from ethylene. PVC, a polar properties become equivalent to those of insulating Kraft dielectric, is very hard, brittle and thermally unstable product. paper. Therefore, other substances are added to make it suitable for the use. Tand of PVC compounds is high, which restricts its use D. BAKELITE only for low voltage application. 'Working stress' is limited to 3 Bakelite, Class E insulation, is available in the form of a kV/mm[1]. It works safely upto 90° C beyond which it begins board. It is produced by polymerization of phenol and softening. It is available in various colors, various tape width 15 formaldehyde resins, [1] which are subsequently cross- mm to 30 mm and roll lengths of 25 meters to 50 meters. linked under pressure. When dried, degassed and impregnated with oil, its relative permittivity becomes 4.2 and electric strength becomes 40 - 50 kV/mm.

INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR 721302, DECEMBER 27-29, 2002 503

I. ENAMELED ROUND WINDING WIRE application, it is important that the cast or encapsulation should not contain voids and humidity and the material is desired to be High conductivity annealed round copper conductor has homogeneous. Accordingly, drying and degassing of the uniform covering of durable and flexible synthetic enamel individual components of the mixture is advised. It has excellent suitable for elevated temperatures [1]. The enamel mechanical properties as it withstands tensile stress of 70-80 (Terephthalate polyester based) has a smooth surface free N/mm2 (max.), compressive stress of 200 - 220 N/ mm2 (max.) from embedded particles of dust and other deleterious and bending stress of 110 - 120 N/ mm2 (max.). Besides this, material. This enamel , Class H insulation, shows no electric strength is 18 - 20 kV/mm, Tand is 0.02-0.03 at 23° C, cracks even at working temperatures upto 180° C. The which decreases to 0.08 - 0.10 at 100°C. wire is available in fine and medium covering from 0.02 mm upto 5.00 mm. Wire showing abrasion resistance from In any composite insulation system due care is taken that the load 5 N to 8 N on nominal conductor diameter from 0.8 basic ingredient of the insulation system do not react chemically mm to 2.5 mm makes it ideal to wind on a former or core and must not deteriorate under prolonged exposure to humid air either manually or by machine. Electric breakdown voltage solid long as the temperature rise is not beyond permissible of approximately 4 to 5 kV on medium covering wires limits. makes it safe to employ in suitable layer windings in high voltage applications in PTs and CTs. III. MAJOR INSULATION LAYING AND PROCESSING

J. HOLLOW INSULATOR Since partial discharge has been prescribed as a routine test (Table I), on all CTs and PTs of 132 kV and above rating, the Ceramics, Class C insulation, also known as porcelain in processing of insulation has to be done carefully to avoid any one of its forms, are widely used for insulators and occurrence of discharge, which may affect the life as well as the bushings in outdoor applications owing to their non - accuracy of measurements. hygroscopic nature and high mechanical strength. It constitutes of clay, aluminum oxide, feldspar and quartz. The primary winding is insulated with electrical grade kraft Ceramics [1] requiring high mechanical strength and low insulation paper. The insulation is laid over the region dielectric losses contain alumina. Tand values of porcelain enveloping the secondary cores and over the straight portion is high i.e. 20x10-3 at 20° C, it increases with temperature leading to the primary terminals. In the region around the and becomes 40x10-3 at 50° C, thereby, increasing the secondary cores, a shield is provided to protect the secondary dielectric losses. Breakdown strength is also less compared from the high voltage system. Grading of insulation by using to other materials, i.e. 20 kV/mm but remains stable upto metallic screens with shaped contours ensures a homogeneous 120° C. radial stress distribution, besides regulating the stress distribution axially in the air porcelain system as well. K. EPOXY RESIN After laying of the insulation, the coil is conditioned under The word epoxy refers to a compound containing more high humidity in a humidity chamber with a relative humidity of than one epoxide group per molecule, i.e., an oxygen atom about 90%. The changes in electrical properties (capacitance C, united with two carbon atoms joined already in some other insulation resistance R, and tand) are measured with time and way. Resin is a substance obtained from plant, it is also the process is continued till these properties become practically prepared by chemical processing of inorganic materials. constant. Figure 1 shows the measurement of C, R and tand for Epoxy Resins, Class F insulation, being thermosetting a typical sample. plastics exhibit hardening quality in their molecules and so addition of plasticizer, filler, accelerator and coloring agent As soon as the conditioning is over, the coil is transferred to a make it a distinguished flexible cast[1]. Long-term thermo- heated vacuum chamber connected to a sophisticated vacuum mechanical properties play an important role in the system to achieve a very high degree of vacuum during selection of casting systems for dry type transformers and processing. The degree of dryness is monitored by switchgear applications. Most commonly used epoxy resin measurements of C, R and tand. Figure 2 shows the variation of (solvent free and medium viscosity) is bisphenol-A (CY- the measured quantities during drying. When optimum values of 205). For hardening, acid anhydride (HY-905) is used. measurements are obtained during the drying cycle, the They are blended at 60-80° C under vacuum to facilitate insulating paper is impregnated with dried and degassed degassing. Pre-dried filler of consistent quality is then transformer oil. added under vacuum followed by addition of premix of plasticizer and accelerator also under vacuum prior to Finally, we keep in mind that no universal law is available to casting operation. The complete mix is cast in pre-heated interpret and understand the insulating material behavior; each moulds (containing the metal parts to be encapsulated) at one will fix the rule according to the stress condition[3-4]. 70 - 100° C under vacuum of 5 m bar. Cure is effected for 8 hours at 80° C and 8 hours for 130°C. In high voltage

504 NATIONAL POWER SYSTEMS CONFERENCE, NPSC 2002

IV. REFERENCES

[1] Design Manual of Current Transformers and Potential Transformers, 10000000 SCT Limited, Ghaziabad. [2] A.K.Shukla, B.E.Ramachandran, Raghuveera Rao, “Ageing studies on Kraft insulation paper under thermal and mechanical stresses”, 6th International Seminar on Elect. & Electronic insulating materials and 100000 systems, Hydrabad, India , pp IV-B-15-19, November 2000. [3] Nikolay Kolev, T.Genovski, F.Topalis, “Analysis And Investigation of Electrical Breakdown Critical Areas Of High Voltage Current 1000 Transformers”, 12 th International Symposium on High Voltage Engg. Vol.4, Bangalore, India, pp 809-812, August 2001. [4] C.Mayoux, "Degradation of Insulating Materials under Electrical 10 Stress," IEEE Trans. on Dielectrics & Electrical Insulation, Vol. 7, No. 5, Tangent delta pp 590-601, October 2000. [5] Ekram Husain, Aejaz Masood & Mujib Ullah Zuberi " Insulating 0.1 Materials -A Study of Futuristic Trend ", Sixth International Seminar on Electrical and Electronic Insulating Materials and Systems, (INSULEC-

2000) Hyderabad, lndia, pp-P-IV-45-51,23-24 November, 2000. Capacitance ( pF) , Resistance( Mohm), 0.001 0 2 6 10 14 18 22 26 30 Number of hours of drying Capacitance Resistance Tangent delta

10000 Fig. 2. Drying characteristics of coil sample

1000

100

10

1 ohm), Tangent delta

Capacitance(pF) , Resistance (M 0.1 0 4 8 12 16 20 24 28 32 Number of hours Capacitance Resistance tangent delta

Fig.1.Conditioning of coil sample at 92% relative humidity and at 32oC.