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Resistance Measurements and Interpretations Relating to Flashover
of Artificially Contaminated Ceramic Insulator Strings
Abstract
The flashover of insulator strings occurring at normal working voltages under contaminated/polluted conditions, obviously deserves serious consideration. Though much researchhas been gone into pollution-induced flashover phenomena but grey areas still exist in our knowledge.In the present experimental study the breakdown (flashover) voltages across gaps on insulatortop surfaces and gaps between sheds (on the underside of an insulator), also the flashoverstudies on a single unit and a 3-unit insulator strings were carried out. An attempt has been madeto correlate the values obtained for all the cases. From the present investigation it was found thatresistance measurement of individual units of a polluted 3-unit string before and after flashoverindicates that strongly differing resistances could be the cause of flashover of ceramic disc insulatorstrings.
Keywords: artificially contaminated/polluted insulator strings, resistance measurements,insulator flashover, breakdown voltages, ceramic insulators
1 Introduction
and more critical at higher transmission voltages. Ithas been often stated that the upper limit of open-airtransmission voltage is set by pollution performanceof insulator strings [ 3 w 5 1 . In the present work theThe performance of insulators used on overheadtransmission lines, overhead distribution lines, andin outdoor substations is a kev factor in determin- following studies have been initially made : (i) Breakingthe reliability of power delivery systems l1 '1.A reduction in the performance of outdoor insulatorsoccurring mainly due to the pollution being collectedon the insulating surfaces from air-borne depositsmay form a conducting or partially conductingsurface layer when it is wet. It is impractical inmany situations to prevent the formation of such alayer and consequently the flashover performance ofthe insulators must be designed high enough to withstandall types of anticipated voltage stresses despitethe presence of a polluted layer. This becomes moredown voltages (BDV) for lcm to 14 cm air gaps (herethe electrodes were fixed to a stand) (ii) BDVs forlcm to 14 cm gaps on the top surface of the insulator(14 cm being the maximum gap distance frommetal cap to the tip of the normal ceramic disc insulator)(iii) BDVs from pin to various ribs, followedby the study of flashover of a single unit and a 3-unitinsulators in a string under polluted conditions, theresults obtained are analyzed and presented.2093B. Subba Reddy et al. : Flashover of Artificially Contaminated Ceramic Insulator Strings
2 Experimental arrangement
The test arrangement consists of a synthetic (plastic)tent of 3 m x 3 m x 3 m, which is supported by ametallic frame. Viewing ports are available on bothsides of the tent, allowing near-distance observationof the insulator surface. From the center of the topmetallic frame, provision is made for a suspensionstring, consisting of six disc insulators, of which thetop three insulators are dummy. The other three insulatorsform the 3-unit test object. The pin of thebottommost insulator is connected to a conductorclamp and an aluminum tube of 2 cm diameter and2 m longth to simulate field conditions. The highvoltage lead consisting of a silicone rubber cable istaken through a 10 cm diameter and 1.5 m long FRPtube. During experiments on a single unit, the top 5units are shorted.Steam fog is generated by boiling water in vats;the steam input rate for fog generation is controlledby the power input to the heater. Inthe present work the steam input rate used is0.055 kg/hr/ni3 as per [6]. Power is supplied froma 50 Hz, 60 kV, 60 kVA transformer with a primaryvoltage of 230/440 V and with a rated continuouscurrent of 1 A.3 Experimental techniqueemployed
The experiments were carried out in ambient laboratoryconditions with an electrode configuration(stainless steel 3 mm dia with hemispherical endsfor studies of breakdown voltages on insulator topsurface and between pins to various ribs). The voltagewas raised gradually (approx.1 kV/sec) in stepstill the breakdown of the gap. For each gap spacingten observations were recorded. In the similarway the flashover experiments with a single insulatorand three insulators in a string were respectivelyperformed.Initially the insulators were coated with the pollution(40 g of Kaolin + Nacl in a 5 g/liter of distilledwater) by dipping method (IEC 507 N 1991)and dried naturally for about 48 hours. Then theinsulators were mounted as a string and wetted bythe steam. The uniformity of the polluted layer waschecked by using a layer conductivity meter [6N71fabricated and assembled in the laboratory, meetingthe requirements of the IEC standards