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Rural
Electrification Magazine,
October 1995
Battling
Elements and Animals
by
John Marks, Technical Editor
Non-visible
insulator failure has plagued a number of utilities in recent
years. When insulators fail, leakage current jumps to grounded
assemblies sporadically, causing blinking lights, radio static
and TV interference for consumers. A few years ago, Cloverland
Electric Co-op in Dafter, Michigan, noticed a rapid increase in
customer complaints, along with a steady increase in recloser
operations. "We tried tightening all the pole line hardware,
trimming trees more frequently and increased line patrolling all
to no avail," says Dick Newland, area superintendent.
"Finally,
we obtained an insulator tester from Hi-Test Detection Instruments
that allowed linemen to 'meg' insulators while the line was energized.
We ultimately tested about 35,000 insulators, and found and corrected
more that 2,200 defective units." Then, he says, customer
complaints and service difficulties decreased dramatically, saving
the co-op $75,000 annually in trouble calls-an impressive savings
for a total investment of $8,000 in two instruments.
John
Farquhar of Hi-Test Detection Instruments says that while any
bell-type porcelain insulator may crack, those with pins and caps
made of aluminum (instead of malleable iron) are especially prone.
This is because aluminum and porcelain expand and contract at
very different rates. The resultant hairline cracks beneath the
caps aren't visible during routine inspections. Eventually, enough
heat builds up from leakage current that the insulator bell explodes.
Even before that happens, the partial discharge causes radio and
TV interference and, sometimes, pole fires.
Dick
Disselhorst, line superintendent at Missouri Rural Electric Co-op
in Palmyra, Mo., was so impressed with the tester that he ordered
one while it was being demonstrated. Five of the co-op's poles
had burned down because of faulty suspension insulators. "When
you see the pole turn black, you know you've got a problem,"
says Disselhorst. "Before this happens, we look at the square
washers and can see blackening (charring) around the washer. The
instrument pays for itself if it prevents one failure, such as
on a corner pole right out of a substation strung with 4/0 wire."
Disselhorst
says that his insulators, 95 percent of which have aluminum hubs,
have been going bad only after 15 years. Any insulator will fail
eventually, and we've got some that have been up there almost
60 years." Disselhorst adds that he finds the Hi-Test instrument
useful in salvaging good insulators from disconnected lines.
Jack
Roemer, operations manager at Cherryland Electric Co-op in Grawn,
Mich., reports that his co-op has used the Hi-Test instrument
to test porcelain suspension-type insulators on 11 distribution
feeders over the last year-and-a-half. "We replaced more
that 400 failed insulators," he says, "and our complaints
on blinking lights and radio interference have been reduced considerably.
However, it will take us another five to eight years to complete
the remaining 33 feeders. Meanwhile, we expect to see a steady
reduction in overall system line loss." Roemer found that
most of the failed insulators dated from 1977 to 1979, and all
were aluminum ball-and-pin.
Copyright
© 2000 HD Electric Company
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