SIMULATION OF STRING INSULATORS FORDETERMINATION OF VOLTAGE DISTRIBUTION ANDSTRING EFFICIENCYAim:To determine voltage distribution and string efficiency of suspension insulator with andwithout guard ring.Apparatus required:SN!ApparatusT"pe#ua$tit" T%e!r": A string of suspension insulators consists of a number of porcelain discs connected inseriesthroughmetalliclinks. Fig. 1(i)showsstringofsuspensioninsulators. Theporcelainportion of each disc is in between two metal links as shown in Fig. 1 (ii). Therefore, each discformsacapacitor asshowninFig. 1(iii). Thisisknownasmutual capacitanceor self!capacitance. "owever, in actual practice, capacitance also e#ists between metal fitting of eachdiscandtower or earth. Thisisknownasshunt capacitance1. $uetoshunt capacitance,charging current is not the same through all the discs of the string %&ee Fig. 1 (iii)'. Therefore,voltage across each disc will be different. (bviously, the disc nearest to the line conductor willhave the ma#imum voltage. Thus referring to Fig. 1 (iii), )1 will be much more than )* or )+.Fig. 1 &tring of &uspension insulatorsThe following points may be noted regarding the potential distribution over a string ofsuspension insulators,(i) The voltage impressed on a string of suspension insulators does not distribute itself uniformlyacross the individual discs due to the presence of shunt capacitance.(ii) The disc nearest to the conductor has ma#imum voltage across it. As we move towards theross!arm, the voltage across each disc goes on decreasing.(iii) Theunit nearest to theconductorisunder ma#imumelectrical stressand is likely tobepunctured. Therefore, means must be provided to e-uali.e the potential across each unit. (iv) /f the voltage impressed across the string were d.c, then voltage across each unit would bethe same. /t is because insulator capacitances are ineffective for d.c.String Efciency:As stated above, the voltage applied across the string of suspension insulators is not uniformlydistributedacrossvariousunitsordiscs. Thediscnearest totheconductorhasmuchhigherpotential than the other discs. This une-ual potential distribution is undesirable and is usuallye#pressed in terms of string efficiency.The ratio of voltage across the whole string to the product of number of discs and the voltageacross the disc nearest to the conductor is known as string efficiency i.e.,conductor nearest to disc across )oltage 0 n string the across )oltage1 efficiency &tringwhere n 1 number of discs in the string.&tring efficiency is an important consideration since it decides the potential distribution along thestring. The greater the string efficiency, the more uniform is the voltage distribution. Thus 1223string efficiency is an ideal case for which the voltage across each disc will be e#actly the same.Althoughit is impossibletoachieve1223stringefficiency, yet efforts shouldbemadetoimprove it as close to this value as possible.Met%!ds !& Impr!'i$( Stri$( E&&i)ie$)":/t hasbeenseenabovethat potential distributioninastringofsuspensioninsulatorsisnotuniform. The ma#imum voltage appears across the insulator nearest to the line conductor anddecreases progressively as the cross arm is approached. /f the insulation of the highest stressedinsulator(i.e.nearest toconductor) breaksdown orflash over takes place,thebreakdown ofother units will takeplace in succession. This necessitates e-uali.ing the potentialacross thevariousunitsofthestringi.e. toimprovethestringefficiency. Thevariousmethodsforthispurpose are,*i+ B" usi$( ,!$(er )r!ss-arms. The value of string efficiency depends upon the value of 4 i.e.,ratio of shunt capacitance to mutual capacitance. The lesser the value of 4, the greater is thestring efficiency and more uniform is the voltage distribution. The value of 4 can be decreasedbyreducing the shunt capacitance. /n order to reduce shunt capacitance, the distance ofconductor fromtower must beincreasedi.e., longer cross!arms shouldbeused. "owever,limitations of cost andstrengthof tower donot allowtheuseof verylongcross!arms. /npractice, 4 1 251 is the limit that can be achieved by this method.*ii+ B" (radi$( t%e i$su,at!rs. /n this method, insulators of different dimensions are so chosenthat each has a different capacitance. The insulators are capacitance graded i.e. theyareassembled in the string in such a way that the top unit has the minimum capacitance, increasingprogressively as the bottom unit (i.e., nearest to conductor) is reached. &ince voltage is inverselyproportional to capacitance, this method tends to e-uali.e the potential distribution across theunitsinthestring. Thismethodhasthedisadvantagethat alargenumberofdifferent!si.edinsulators are re-uired. "owever, good results can be obtained by using standard insulators formost of the string and larger units for that near to the line conductor.*iii+ B" usi$( a (uard ri$(. Fig. * &tring /nsulators with 6uard ringThe potential across each unit in a string can be e-uali.ed by using a guard ring which is a metalring electrically connected to the conductor and surrounding the bottom insulator as shown in theFig. *. The guard ring introduces capacitance between metal fittings and the line conductor. Theguard rings contoured in such a way that shunt capacitance currents i1, i* etc. are e-ual to metalfittinglinecapacitancecurrentsi71, i7*etc. Theresult isthat samechargingcurrent / flowsthrough each unit of string. onse-uently, there will be uniform potential distribution across theunits.Cir)uit Dia(ram:Fig +, 8ithout 6uard 9ingFig :, 8ith 6uard 9ing.r!)edure:/it%!ut Guard Ri$(:1. onnect the circuit as per the Fig. +. From one of the variac output terminals connect toterminals &1 and other variac output terminal to 6 as shown in Fig. +.*. Applyvoltagefromthevariacacrossthestringinstepsof*2) startingfrom+2) to112).+. ;easure the voltage across &1 and &*(which is to be noted as