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Spac er Cable Reduc es TreeCaused Cus t om er In t e r rup t ions
Jam es D. Bouford ,Senior Member, I EEE
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Tr e e s & I n t e r r u p t i o n sThe most common power-line constructionthroughout most of the country is bare wire oncrossarms. While proven to be adequatelysafe and reasonably reliable over the past onehundred and twenty-five years, thisconfiguration is susceptible to outagescaused by tree contact.
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Tr e e s & I n t e r r u p t i o n s
THROUGHOUT a major portion of the UnitedStates, trees contacting power lines are asignificant cause of customer interruptions,both momentary and sustained.At some utilities in the Northeast, treecontacts contribute to more than 25% of allcustomer interruptions.
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Tr e e s & I n t e r r u p t i o n s
Trimming trees, by removing branches over,under and around the electric delivery systemconductors, has been the typical way utilitieshave tried to improve their reliability results.The general public is becoming morevociferous in their opposition to any removal
of trees, or their branches.
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Spac er Cable
Shunned by those utilities that emphasizedlowest first cost construction, due to its 20 -30% higher material cost over an open barewire configuration, it has proven to be a longterm benefit to many smaller municipalelectric utilities in New England, where someof them have used this construction almost
universally.
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Spac er Cable
Overhead spacer cable construction consistsof non-shielded, non-tensioned, insulatedconductors, supported in a close triangularconfiguration by insulating spacers from ahigh strength messenger/neutral.
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C o n d u c t o r s
ConductorsInsulated w/ polyethyleneTreated as uninsulated15 kV; 75 mils of natural polyethylenecovered by 75 mils of track resistant highdensity polyethylene
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SpacersSpacers
made of polyethylene, the same material asthe insulation on the phase conductorsflexible polyethylene jar rubbers havebeen the most commonly used method ofholding the conductors to the spacers,spacers can now be obtained with
integral locking clamps.
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Three Phase and S ing le Phase Spac ers
Not t o Sca l e
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Messengerprovides the full support of the non-tensionedphase conductors,is a high-strength aluma-weld/aluminum (AWAC)conductor.also acting as the system neutral, for a system ofnon-shielded, insulated conductors, it is requiredto be well grounded at a more frequent interval
than that required for open wire overheadconstruction by the National Electrical SafetyCode.
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The use of non-shielded conductors alsorequires that trees still be trimmed, since theircontact with the insulation will create a chargedensity differential, and cause a chargecurrent flow.Some utilities have specified that a 15 milsemi-conducting strand shield be added tominimize the damaging effects of an increased
electric field at a tree contact point, therebyextending the life of the conductor even if treecontact is made
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Cos t Cu t t ing Effo r t s
Some utilities have tried to use materials ofdiffering dielectric constants in theconstruction of spacer cable systems.
In every case known, these experiments toreduce the overall cost of spacer cablesystems have ended in reduced life of thecomponents and increased outages due to
material failure.
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Higher F i r s t Cos t
The cost of spacer cable will be higher thanopen bare-wire construction, due to theinsulation on the conductors, the spacers, themore frequent grounding required and themore specialized hardware.A Midwest utility estimated an additional 25%
cost over an equivalent bare-wire system
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Summary
Benefits: (for utility experiencing a large number of tree caused interruptions)
improved reliability results,reduced O&M costs,improved customer satisfaction,reduced customer animosity for veg mngt program,
Requirements:
Acceptance of higher construction costs,the need for a fully integrated insulation system, andmuch more frequent grounding
Spacer cable construction will provide the utility with amost reliable, lower total cost system than open, bare-wire construction.
