500 NATIONAL POWER SYSTEMS CONFERENCE, NPSC 2002 Practices of Insulating Materials in Instrument Transformers 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, insulator 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 transformer 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 current transformer is IS: 2705 , the dielectrics surrounding it, due to dynamic short time current IEC:185 and for the voltage transformer is IS:3156, IEC: condition. Mechanical stress may occur during normal handling 186. The requirements of instrument transformer 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 Transformer Oil 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