IEEE Power & Energy Magazine - November/December 2016 - 21

Northwest where transmission lines contacted tree branches.
In the densely forested areas of northern California, Oregon,
and Washington, trees are often nearly as tall as the extrahigh-voltage (EHV), generally 230 kV and above, transmission line towers. In deference to environmental concerns,
particularly in the scenic wilderness and national forests,
and the difficulty mobilizing equipment into the area to control the vegetation, contact between tree and line becomes
inevitable. In addition, the hazards are not limited to the line
tripping and potential loss of load, but the contact could start
a wildfire, posing an even greater risk to public safety and
economic loss. Conversely, a wildfire started by a lightning
strike or human carelessness can endanger the transmission line, and the operating utilities may have to take one
(or more) transmission lines out of service for protection,
degrading overall system reliability as a result. Tree trimming, or vegetation management as we refer to it now, is as
much an art as it is a balancing act to maintain the reliability
of the transmission system while preserving the ecosystem.

Self-Inflicted Wounds
Sometimes an unexpected failure occurs. Whether this was
unanticipated in the design or just an oversight, engineers
must be aware of internal conditions that can successfully
challenge even the most robust systems. Perhaps a U.S.
comic strip (Walt Kelly's Pogo) said it better: "we have met
the enemy and he is us." The following range from the simple
to complex, but each demonstrates weakness in our processes.

A Clean Substation Is...
Over a period of several weeks, and with annoying regularity,
a major EHV transmission line would trip. Each time, system
protection engineers would review the operation, determine
that there was no fault present within the protection zones
for that facility (or anywhere nearby), and return it to service.
After several occurrences, the investigation determined that a
utility maintenance crew was in the station at the time of each
trip. The crew was performing routine housekeeping chores
that involved sweeping and mopping the floor in the control
building. On those several occasions, either the broom or mop
handle hit the door of a relay panel cabinet with sufficient
force to cause excessive vibration that, in turn, caused the line
protection to operate and trip the line.
A similar event occurred in another substation when a
cleaning crew hit a panel door and resulted in the tripping
of a 345-kV transmission line. To avoid similar events in
the future, it was determined that the simple solution would
be to remove the door. Sometime later, during an actual
fault on that line, the primary protection failed to operate
correctly. The cause was determined to be the accumulation of debris on contacts that prevented the tripping of one
of the breakers associated with the line. In other words,
the accumulation of dust and dirt on the contacts was the
result of the panel door having been previously removed
was the principal cause. The solution to avoid future occurnovember/december 2016

rences? Reinstall the door, of course, and both of the utilities involved in these events took additional steps in the
familiarization and training of housekeeping personnel
with a focus on being more careful around all operating
equipment installations.

Field Test Run Amok
Sometimes we learn more about the behavior of equipment
and the system when things don't go according to plan. In
1984, during the final acceptance field testing of the new
back-to-back HVdc interconnection between the Hydro-Quebec (HQ) main system and the 765-kV transmission line from
HQ's Chateauguay station to the New York Power Authority's
(NYPA) 765-kV station at Massena, a 765/120-kV transformer
failure resulted in a sequence of 38 commutation failures. (See
Figure 1.) The HQ and NYPA engineers determined that the
repeated commutation failures were caused by voltage reductions and swings on the Beauharnois hydroelectric generators
that were also connected to the 120-kV bus. Similar commutation failure events were also reviewed, and it was discovered that most of these were coincident with a 120-Hz resonant condition. As a result of the unplanned, and unexpected,
transformer failure, HQ and NYPA engineers were able to
design and implement specialized controls and procedures
to address the second harmonic, voltage and reactive power
control, and more robust HVdc controls. Over the next several
years, these enhancements were developed, tested in simulation, and then verified with additional field tests. All of the
parties involved in the commissioning testing and subsequent
improvements agreed that much more was gained from the
unexpected controls behavior from the test that didn't happen
as intended than was learned from confirmation of expected
outcomes of all of the preceding tests.

Natural Causes
We have already discussed the results of animal or vegetable impacts on the power system. The natural, physical
forces of nature can most formidable. Severe storms are
usually what come to mind, but droughts and earthquakes
have made their impact as well.

Shake, Rattle, and Roll
In 1994, there were two major disturbances on the West
Coast. The first occurred in January as a direct result
of an earthquake in the Los Angeles (Northridge) area.
The second was the result of a severe winter storm in
Idaho. Common to both events, and for most of that year,
the Pacific HVdc interconnection was operating at relatively low power levels, sometimes even flowing north as
the systems in the northwest were importing power from
southern California and Arizona. There were a number of
remedial action schemes (special protection systems) that
were installed to mitigate the excess generation condition
that would occur in the northwest when the Pacific Intertie tripped; in general, the expectation was that the flows
ieee power & energy magazine

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