IEEE Power & Energy Magazine - November/December 2016 - 20

of improper lubrication of the mechanical tripping mechanism and the severe cold weather. The investigation also
determined that both breakers actually did complete their
tripping operation, but not in the expected (or specified)
time, so there was no protection failure or breaker failure to
operate-maybe just a "not quite stuck" breaker.

Single Moose-to-Ground Fault
Moose are not generally an annoyance to power system equipment, except possibly to utility vehicles traveling the highways. However, one misty spring morning in the north woods,
a moose went out for his usual walk to graze for berries and
foliage around the peat bogs. Alarms sounded in the control
room. Initially a 69-kV local transmission line tripped, and
then a generator and local customer load was lost as well. The
subsequent line patrol and investigation found a section of
poles of that 69-kV line leaning severely to the point that the
lowest phase conductor got tangled up in the moose's antlers.
What had caused the poles to lean was a combination of gusting winds and soft soil due to recent heavy rain.

Snakes in a Substation
Furry mammals don't have exclusive rights to wreak havoc
with the electrical system. In the mid-1960s, a new 115-kV
substation was built to serve the growing load in the mid-Hudson Valley of New York. Around 2:30 a.m. one summer morning, the district substation foreman was awakened by a call
from the system operator. He was informed that there had been
a bus differential operation in that station and that power was
out to a major part of the city and surrounding area. The foreman requested that the system operator call out two substation
electricians and he would meet them at the station. Upon arriving at the station, the foreman found the electricians outside
the control building, but laughing so hard they could barely
stand up. It appeared that large king snake, while looking for a
warm place to bed down for the night, had managed to slither
into the cable ducts and then inside the control building. From
there it had followed the wiring from the cable tray down into
the relay and metering cabinets. What the electricians found
was the dead (fried?) snake, hanging nearly vertical, with a
death grip on a terminal post on the back of a relay, and the
charred shadow of the snake on the inside of the cabinet door.

The Forest Strikes Back
The popular media and some urban legends circulated via the
Internet expect us to believe that trees never posed a risk to the
bulk power system before 2003. Or, possibly there is something about a power line that causes trees to grow faster. It
isn't that the trees reach up and take a bite out of the line, but
just when you thought it was safe to go back into the forest...

The Christmas Tree Effect
In the early years of the New York Power Pool, in the 1970s,
there were a number of events of varying severity that
involved trees and overhead 345-kV transmission lines. One
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ieee power & energy magazine

of the tree-contact events resulted in an east/west separation
of the New York cross-state 345-kV system between Rochester and Syracuse; another resulted in separation between
northern New York and northeastern Ontario. What was actually happening was that high ambient temperatures on those
hot summer afternoons combined with the higher west-toeast power transfer toward southeastern New York and New
York City caused the conductor to sag more than anticipated.
The pool dispatchers made light of these events saying that
the high current in the lines was attracting the trees to the
transmission line. Investigations of several events identified
instances where different rating methodologies used by the
various owners of all or parts of the affected lines resulted
in inconsistencies in the ratings used among the owning and
operating parties. The results of the reviews of these events
resulted in the New York Power Pool members developing and
applying a uniform rating process for all transmission facilities, as well as increased routine inspection of the rights of
way and clearances.

A Tree Grows in Wyoming
The high plains of western Wyoming are not generally
thought of as heavily forested; the vegetation tends to be the
opposite, mostly scrub brush with occasional small evergreen shrubs. But there are some places where transmission
lines cross creeks that are usually dry-washes in summer,
and this is where the problem arises. One hot August in the
late 1990s, a 345-kV transmission line going west from Wyoming toward Utah and Idaho tripped on consecutive days.
The first time the line tripped, special protections operated
to isolate the problem area and secure the rest of the Western
Interconnection. The second day, the local operators recognized the problem and were able to take corrective action
and forestall the special protection operation. After each
event, the line was patrolled by air but nothing was found,
and the line was returned to service and by that time the conductor had cooled sufficiently. About a week later, the line
tripped again. This time, as the air patrol took off to investigate, they observed smoke on the horizon. Arriving at the
source of the smoke, the patrol found several scorched and
smoldering branches near the top of a tree growing out of a
dry wash. When the phase conductor cooled off, it was found
to be well above the top of the tree, but the investigation also
found that, when subjected to a combination of heavy loading, high temperature, and little wind, the conductor could
sag into the top of the tree.

A Double-Edged Sword
When a tree branch contacts an energized line, burns off,
and the line successfully recloses, transmission line maintenance personnel and system operators would sometimes
call this "live-line tree trimming." Ideally, this should never
happen in our post-2003 blackout era of mandatory vegetation management standards. However, back in the mid- to
late-1990s, there were several similar events in the Pacific
november/december 2016

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - November/December 2016