Advanced Computing

Conference 6'^ International on

Advanced Computing

&

Co munication Technologies (rcAccr-2oL2)

Volume1

2012 3" Nou"mber,

Editors Prof.(Dr.)R.K.Choudhary Er. Ravi KumarSachdeva

6- Intemational Conference on Advanced Computing & CommunicationTechnologies

(ICACCT--2012),ISBN:

9?8-93-82062-6'l:1

Studyof SurfacePotentialandElectric Stress on TractionInsulators SubbaReddyB. Indian Instituteof Science, Bangalore

Tanaya Chaudhuri NIT Silchar. Assam

Monali Madhulita NIT Rourkela, Orissa Abstract-4ptima I perform ance of traction insulators is essential to ensure reliability of railway traction electrification system. They are exposed to environmental degradation and vandalism due to the overhead erection in railway lines. The likely damages that can take place are cracks in the sheds, or shorting of sheds due to formation of conducting dust and moisture layer on the surface. Fault in one shed increases the stress on the healthy sheds, resulting in an undesirable deviation of the electric field and potential profiles of the insulator from the normal ones,which may also lead to a flashover across the healthy portions. Surface charge simulation method (SCSM) is used to find the potential and electric field for the most widely used traction insulators in the country, and the stress increment is studied. The simulation results obtained for both healthy and fault-imposed insulators are presented, The experimental results of dry and wet withstand tests are presented along with the leakage current performance for the traction insulators studied.

which hampersthe withstand capacityof the insulator. For better functioning of traction insulators, stress variationsshouldbe analysed,using the electric field and potential profiles in normal and faulty conditions. Presentlythereis no dataavailablefor the electric stress and potentialdistribution for the taction insulatorsused in the country. Hence a detailed simulation and experimental study is attempted in the present investigation. II. NUMERICAL CoMPUTATIoN

Keywords: crack fault; shorted sheds; surtace charge simulation method; surface field; surface potential; traction insularors. I. INTRoDUCTIoN

Electric railways are widespreadthroughoutthe world and are based on either direct current or altematingcurent supplies.In overheadelectrification systems,the supplyof electricityis maintainedthrough an overheadsystemof suspendedcablesknown as the catenary with its arxiliaries and a pantograph arrangement[1]. Tractioninsulatorsplay an important role as far as power supply to the trainsis concemed. They are usedto provide rigid supportfor the overhead catenary wire [2]. The proper functioning of traction insulators is essentialto maintain the reliability of power supplyto the electrictrains. They may malfirnctionbecauseof regularexposure to mechanicalvibrations,vandalismand environmental effects. They may also crumble due to increment of electric stress,further leadingto failure/flashoverof the insulator. A fracture/crackonce formed on the sruface ofthe insulatorspropagates radially andgraduallyleads to mechanicaVinsulationfailure of the insulators. Further the accumulationof dust particlesandmoisture forms a conductingpathwayon the surfaceof the sheds 3lI

Over the years various numericalmethodshave been employedto solve complex physical systems. Numerical methods are advantageousover analytical methodsas they can largely reducecomplexitiesin computationalalgorithm. Various numerical methods have been adoptedfor computationof potential and electric field for normal and faulty ceramicdisc type insulatorin a string[3-5]. Apart from domain based methods like Finite Difference Method (FDM), Finite Element Method (FEIO, andboundarybasedmethodssuchas Boundary Element Method (BEIO, SurfaceCharge Simulation Method (SCSM),Charge SimulationMethod (CSIO, some other existing methods are the Monte Carlo Method (Mcluf), the Moment Method (MM) and the Methodof lmages(MI). The problem under study is of open geometry (boundary) type. Hence boundary based methods as CSM and SCSM would be very suitable.However,a serious disadvantageof CSM is that boundary conditionsare satisfiedonly at selectedpoints on the boundary and hence field along the conductor and dielectric boundary show significant variation in both angleandmagrritude.Hence,SCSMhas been selected for study.TheSCSMcode[4-6] developedwas suitably modified and employedfor the presentinvestigation. The Galerkin's method with piecewise linear interpolationfunction is employedowing to its higher accuracy[6].

6'h International Conference on Advanced Computing & Communication Technologies (ICACCT-2012), ISBN: 978-93-82062-67-7

when crack is initiated and Fig.10 and Fig.ll when crack has propagated deeper.It is observed that the surfacepotential and gradientat the fault location i.e., at the position of crack, has increased,when comparedto its healthy form. Furthermore,the increase is more whenthe crack hasgone deeper.

III. SnraurarroN REsuLrs Simulations are carried out with faults imposed on the insulator. To generate fault, either some sheds were intentionally shorted or a crack/fracture was initiated on the ceramic portion. Table I lists the five traction insulators considered for study. A.

TABLE I: SPECIFICATIoNS oF TRACTI0N INSULATORSUSED FoR STUDY

Fault Induced Due to Sheds-shorting Insulator TYpe A

Insulator surface collects pollution over an extended period and becomes wet partially or completely, during dew, light rain, mist, fog or melting snow. The pollution layer becomes conductive and when energised, a surface leakage current flows. Due to non-uniform surfaceprofile, heat generationrate is nonuniform, leading to formation of dry bands. Voltage across insulator gets concentrated across dry bands, literally shorting major portions of the insulator. Wont cases ciul witness a complete flashover. This fault has been realisedby simulating shorting of shedsin Type C traction insulator. Type C is a bracket traction insulator consisting of six sheds. Three cases have been studied: shorting the top two sheds i.e, the two sheds nearest to ground conductor end, shorting the bottom two sheds i.e, the two sheds nearestto line conductor end, and shorting the middle two sheds. The insulator profiles for these faults are shown in Fig l, Fig 2, and Fig 3 respectively. The potential and electric stresseshave been studied against the profile of healthy insulator, and the comparison resultsare shown in Fig 4 and Fig 5. It is observedthat a sharp rise in potential and as well as surfacefield occurs at the location of fault in the insulator. The rise is more, in case of insulator with sheds nearestto line end shorted. This implies that the fault near the line end causes more stress on normal insulator when compared to fault initiated at the middle and ground end ofthe insulator. B.

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Fault Induced Due to CrqcH Fracture

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External factors like vandalism and mechanical vibrations can create cracks on the insulator surface, which with time, propagate radially and degrade insulation strength considerably. This fault has been realised by simulating two stagesof crack development. The fust stage refen to the initiation of a crack on Type C traction insulator. The second stage refers to the stage where it has deeply propagated inside the insulator. The profiles for these fault-imposed stagesare shown in Fig 6 and Fig 7 respectively. Comparison results for surface field and surface potential are given in Fig.8 and Fig. 9 respectively

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312

Study of SurfacePotential and Elecfic Stess on Traction Insulators

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Fig. 3: Type C Insulator Profile with Middle Two Sheds Shorted

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Fig 4: ComparisonofSurface PotentialofType C forNormal and Shed-shorting Fault

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Fig. 5: Comparison ofSurface Field ofType C for Normal and Shed-shortine Fault

313

6'h International Conference on Advanced Computing & Communication Technologies 0CACCT-2012), ISBN: 978'93'82062-67-7

IV. E)GERIMENTAL RESULTS

NormaVdry and wet withstand tests were performed on Type C and Type B traction insulators' The experimental setup is shown in Fig. 12. It consists of a main transformer, a regulating transformer, and a control panel. The High Voltage transformer is rated at 150 kV-2Al100 kV-3A/50 kV-6A, at 300 kVA. An input voltage of 400V is fed to the transformer by a two phaseac supply. The input curent rating is 750 A. The output of the test source is calibrated with a standard reference potential transformer model: 20321/Sl6b,

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VTO II55. The leakage current measurements were made using a Rigol make DS1042C,2-channel, and 40MHz, 400MSa/s digital storage oscilloscope across a 50C) resistor at the ground end. The leakage current measurements were monitored as different levels of voltageswere applied from 25 kV to 100 kV. Fig 13 and Fig 14 show the leakage crurent pulses for 100 kV and 70kV on Type C insulator for dry and wet conditions respectively. Fig 15 shows leakage current pulses for 75kV dry test. Fig 16 and Fig 17 show the leakage current pulses for 25 kV and 70 kV on Type B insulator for wet conditions. Type B is a stayarm traction insulator with sevennumber of sheds. These plots show that the leakage current pulses were almost insignificant dwing dry conditions. With the increasein voltage acrossthe insulator, the leakage current pulses increase in magrritude.

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Fig 12: Schematic Diagram ofExperimental setup

314

Study ofSurface Potential and Electric Stess on Traction Insulators

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Fig. 16: Performanceof Type B Insulator underWet Condition for 25kV

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Simulation study was conducted for five types of traction insulators presently being used in the country. The surface gadient and potential profiles of shorting the top two sheds i.e, the two sheds nearest to ground conductor end, shorting the bottom two sheds i.e, the two shed nezrest to line conductor end, and shorting the middle two sheds are presented against the healthy insulators.The stresswas seen to be maximum in case of shorting near line conductor end as compared to the shorting at the middle or near the ground end of the unit.Similarly the stresswas seento increasein caseof crack propagated deeply in the insulator body as compared to the initial progress. heliminary experimentation was carried out on traction insulators and the perfornance of leakage current magnitude was monitored for drylnormal and wet conditions. The field and potential data never existed for the haction insulators presently used in the country. Hence, it is believed that theseresults are presented for the first time and will be useful to the utility engineers.