Practices of Insulating Materials in Instrument Transformers
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.
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