Polymer vs Porcelain SuspensionInsulator Equivalence -What To Use?

with an SML equal to or greaterthan this M & E value.

Section Length EquivalenceThe modular construction of the

Hi*Lite XL insulators can matchwithin 11⁄2" any section lengthdesired. Often the structure designhas progressed well before insula-tor selections are made, and there islittle room for variations in sectionlength. Ohio Brass Hi*Lite XLpolymer insulators are produced invariable section lengths to meet anyneed, so the section length ofporcelain previously used can bematched closely.

Electrical EquivalenceAfter the section length of the

Hi*Lite XL polymer insulator isselected, the electrical characteris-tics must be verified for suitability

Size difference is apparent as equalled-strength insulators are side-by-side.

The superiority of polymerinsulators over porcelain insulatorsleads to several differences incharacteristics and applicationcriteria for these two insulationsystems. Examined in this article arethe various criteria used in selectinginsulators and the development of asuitable method for selecting thecorrect size polymer insulator.

Types of EquivalenceSeveral different methods of

making equivalence comparisonscome to mind when comparingpolymer and porcelain insulators.The first broad category is physicalequivalences, including the strengthrating of the insulators, and thesection length (coupling length) ofthe porcelain insulator string andthe polymer insulator unit.

The second broad category iselectrical equivalences. Among theseare the leakage distance, the 60 Hzwet and dry flashover values, andthe positive and negative impulsecritical values.

Physical EquivalenceThe ultimate strength of porcelain

suspension insulators is character-ized by the porcelain mechanicaland electrical (M & E) value. Thisvalue is determined from a mechani-cal strength test performed onindividual suspension bells. Theapplied electrical stress tests theintegrity of the relatively thinporcelain section between the capand pin to indicate the load at whichmechanical failure initiates. For

Ohio Brass polymer suspensioninsulators, the equivalent measureis the specified mechanical load(SML). The SML test is a 90-secondtension withstand test at the SMLvalue. Ohio Brass polymer insula-tors are available in strength ratingsequal to or exceeding porcelaininsulator M & E strengths.

Typically, a porcelain suspensioninsulator is proof tested at 50percent of its M & E value. OhioBrass Hi*Lite XL polymer insulatorsare proof tested at a value of 50percent of the SML. This routinetest load (RTL) is performed onevery Ohio Brass Hi*Lite suspen-sion insulator.

Therefore, in selecting the properstrength Hi*Lite XL insulator,consider only the porcelain M & Erating which was previously usedand select a Hi*Lite XL insulator

bells with a leakage distance of172.5 inches. The Hi*Lite XLinsulator with equivalent sectionlength is Catalog No. 511212, whichhas a leakage distance of 192 inches.In this case the Hi*Lite XL insulatoroffers 11 percent more leakagedistance in the same section length.

Switching ImpulseTransient Performance

Due to improvements in breakertechnology, switching transientshave been greatly reduced over thepast 50 years. For example, in the1930’s, switching transients weremeasured as high as 5.5 per unit,while today’s breakers will limit thevoltages to three per unit or less.Therefore, for the 230 kV system,the maximum switching surgevoltage to which the insulatorswould be subjected is 563 kV.

Lightning Impulse VoltagesLightning impulse voltages are

the most challenging to insulateagainst. Even manufacturers’literature indicates that a majordesign criteria of high voltageinsulators is that they should notflashover under any conditionexcept lightning.

Because lightning is such anunpredictable phenomenon, fewutilities know the stroke currentsfor which they have designed. Theyonly know the impulse criticalpositive and negative values listedfor the porcelain insulator strings inuse.

Often overlooked is that porce-lain insulator strings frequentlyinclude one to two “insuranceunits”. These insurance units wereincluded in the string in the eventthat units were damaged duringshipment or installation or punc-tured in service.

For example, in the 230 kV case,the 15 bells would have positiveand negative impulse critical valuesof 1345 and 1360 kV, respectively.However, if two damaged insula-tors are assumed in the string, thesevalues are reduced to 1185 and 1190kV respectively. By contrast theHi*Lite XL insulator is not prone todamage; it is always electricallyintact. The positive and negativeimpulse critical voltages for theHi*Lite insulator are 1215 and 1233kV, respectively; in both cases theyare higher.

Therefore, the lightning impulseperformance of the equivalentsection length Hi*Lite XL insulatorwill be equal to or greater than theporcelain insulator string.

The low-profile, high-strength Hi*Liteinsulator.

with the system voltage. In per-forming this analysis, three voltagestresses are typically considered: 60Hz power frequency, switchingimpulse, and lightning impulsevoltage.

60 Hz Power FrequencyA major cause of transmission

line flashovers is contaminationbuild-up on the insulator string.These flashovers can occur atnominal system voltage.

Leakage distance is the primaryfactor which determines aninsulator’s performance undercontaminated conditions. When anOhio Brass Hi*Lite XL polymerinsulator is selected with a sectionlength equal to the porcelain stringsthat were previously used, thespecific leakage distance of theHi*Lite insulator will be greaterthan the specific leakage distance ofthe porcelain insulator (leakagedivided by dry arc).

For example, at 230 kV, onetypical porcelain string used is 15

( ). x √ 2 x 3.0

The simplest way to coordinateinsulation levels with expectedswitching surge voltages is byusing the 60 Hz wet and dry values.There is some debate as to whetherthe 60 Hz wet or dry value shouldbe used in the coordination. Wewill assume that switching willoccur during a storm when theinsulators are wet.

The 15 porcelain bells will have a60 Hz wet flashover of 600 kV,while the Hi*Lite insulator, CatalogNo. 511013, has a 60 Hz wet flash-over of 700 kV. In both cases, thevalues exceed the 563 kV expectedswitching surge voltage.

Therefore, a Hi*Lite XL insulatorchosen with section length equal toporcelain string length will haveequal or better switching impulsevoltage performance.

√ 3

230 kV

ConclusionAll of the various electrical

equivalences indicate that selectingan Ohio Brass Hi*Lite XL polymerinsulator with a section lengthequal to that previously used inporcelain insulators will result in anassembly with equivalent orsuperior electrical performance.

The following table summarizesthe selection process. SelectingHi*Lite XL polymer insulatorsequivalents based on section lengthis the appropriate criterion.

Table:1. Mechanical: Choose Hi*Lite XL specified mechanical load

(SML) equal to or greater than porcelain M &E.

2. Section Length: Choose Hi*Lite XL section length to (orgreater than) porcelain section length.

3. Leakage Distance: If step 2 is followed, Hi*Lite XL specificleakage distance will be greater than porce-lain.

4. Switching Surge: If step 2 is followed, Hi*Lite XL performance(60 Hz wet flash-over) will be greater thanporcelain.

5. Impulse (Lightning): If step 2 is followed and allowances made for“insurance units,” Hi*Lite XL values aregreater than porcelain.

Linemen prefer working with lightweight polymer insulators.

©Copyright 2004 Hubbell • 210 North Allen Street • Centralia, MO 65240

NOTE: Because Hubbell has a policy of continuous product improvement, we reserve the right to change design and specifications without notice.

EU1205-HR

Hi-Tension NewsReprinted from

9301-H

Fall/Wint. 1993

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