Flashover andCharging Characteristics ofaLongSolid Insulator Exposed toacVoltage inVacuum

Inourprevious studies, witha comparatively short cylindrical insulator of10mm length, wehavefound thatthecharging process duetoacvoltage excitation in vacuum ischaracterized by threedifferent states; Initiation, Quasi-stable state andStable state. Inallofthe states, theinsulator surface acquires positive charge irrespective ofapplied voltage amplitude orphase. Also, we havereported thatthemagnitude ofthecharge decreases withtheroughness ofinsulator, andthatthis results intheimprovement ofsurface insulation strength. Toapplytheaboveresults forpractical insulators usedfor vacuum valvesofVCBs,we haveinvestigated the characteristics foralonger insulator upto50mm madeof borosilicate (Pyrex@). Asaresult, wehaveconfirmed that thecharging process isbasically thesameasthatobserved intheprevious studies. We havealsoconfirmed the effectiveness ofroughening thesurface onreducing the charge magnitude andincreasing theflashover voltage. I.INTRODUCTION Currently, circuit breakers usedinhigh voltage power transmission systems arepredominantly oftheGCB typethat usesSF6, whichhasanextremely higheffect onglobal warming. So,withtheincreasing awareness to this issue inrecent years, there isatrend ofseeking the reduction ofSF6usage. Ontheother hand, VCBshad beenusedmostly belowtherelatively moderate 80kV voltage range, butnowthere wouldbeademandfor higher voltage products iftheycould bedeveloped, since theywouldprovide anenvironmentally friendly alternative toGCBs Forthedevelopment ofhighvoltage vacuumvalves, itisimportant toimprove theelectrical insulation betweenthecontact points (electrodes), andthe insulation attheinternal surface oftheglass orceramic valve body. Hereweconsider thelatter problem, i.e. the improvement oftheinsulation along vacuumsurfaces. Generally, theelectrical charging ofsolid insulators is thought tobethetrigger ofdischarge invacuum(1,2). Forthis reason, inthepast wehaveinvestigated thedc andaccharging process ofrelatively short (less than10 mm length) cylindrical insulator test pieces, using probe measurements and simulations, and founda characteristic charging process undertheacvoltage excitation (3,4). Herewereport ontheresults obtained byapplying theexisting research findings tolonger test pieces, withthegoal ofeventual practical applications. Thusthetest pieces weremadeofPyrexR whichisalso usedinindustrial equipment, andthelength was increased to50mm,according tothecapabilities ofthe test equipments available tous.Wereport ontheresults forthecharging process, charge inhibition effects, and theimprovement ofinsulation strength.