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This column focuses on electrical inspection methods and technologies
that are performed while the electrical system remains
energized. Although no-outage inspections can be very valuable
tools, always remember to comply with proper safely guidelines
when conducting energized, on-line inspections.
It felt like a fine spring morning in eastern Oregon as
my colleague and I rolled up the power plant driveway,
although the date on my Blackberry indicated mid-February.
After some friendly greetings and hand-shaking with
maintenance personnel, we signed in at the control room and
viewed the not too lengthy or boring safety video. Then it was
time to don our hard hats and head toward the substation all
half-million volts of it.
It was dry and sunny as our work boots crunched against the
coarse substation gravel rocks. The massive bushings and insulators
did not appear to have much, if any, condensation from the
night before as we gazed across an acre of GSU transformers,
SF6 dead-tank breakers and other equipment. Ideal conditions
for a corona image survey. After powering up our new corona
camera and making a few minor adjustments, we began doing
what no other person had ever done before in this substation
we were viewing an invisible phenomenon known as corona.
What we and plant personnel saw during the next few hours
that morning would better our understanding of high voltage
engineering in a manner that no text book possibly could.
We saw corona from insulators that required cleaning,
sharp edges from hardware connections, poor bus work corona
suppression, and dangerous insulator corona caused by cooling
tower residue deposits. As the prevailing wind puffed another
cloud from the cooling tower onto the westernmost insulators, we could hear the corona crackling intensify as the camera revealed
near flashover conditions. When it sounds like this, I tell
my guys to get out of the substation, our customer stated. Not a
bad idea, I thought. Next, we followed our customer to a corner
of the substation were we examined a flash-damaged insulator
laying in the gravel, then observed the additional weather sheds
added recently to the in-service insulators nearest the cooling
tower. For the most part, the increased surface creepage of the
modified insulators were doing their job by eliminating most of
the corona. However, as can be seen in Figure 1, one insulator
was clearly showing dangerous corona that consistently occurred
from a defect located half way down the insulator. Additionally,
we found severe corona activity occurring from the center
phase GSU transformer bushing as shown in Figure 2. We left
the power plant feeling that the customer greatly appreciated
the value of our new technology and would use the knowledge
gained that day to reduce future flashover likelihood.
One of the greatest difficulties when working with electricity
is that it cannot be seen. The technician must generally
determine its presence based upon meter deflection or
simple visual or audible warning signals.
Perhaps even more difficult to appreciate is the concept
of electrical fields surrounding conductors or insulators in
air. A visual representation of the electrical field is shown
in Figure 3, which illustrates the voltage gradient surrounding
an insulator. Medium- and high-voltage equipment is carefully designed so that electrical fields are contained.
If the electrical field is compromised by foreign materials,
poor design, or other factors, corona will occur. Corona is
responsible for the generation of ultraviolet radiation, ozone,
acids, heat, mechanical-erosion through ion bombardment,
electric power loss, and electromagnetic interference of radio
communications. Corona by-products destroy insulation and
lead to catastrophic failure while remaining invisible to the
eye. Corona can occur due to:
- poor component or system design
- improper installation or workmanship
- temporary contamination by icing, salt sprays near highways
or oceans, fog, agricultural chemicals, and other
chemicals
- damage due to storm wind or lightening
- damage due to vandalism
- cumulative damage due to corona activity caused by
weather conditions
- cumulative damage due to contamination from nearby
sources
Until recent technology breakthroughs, ultrasonic detection
was one of the only methods capable for detecting
corona. This technology is limited, especially when scanning
high voltage equipment from a distance since only the relative
direction of the problem source can be determined.
Today, a relatively new technology allows invisible corona to be visible
through a special imaging device. The handheld camera seen in Figure 4 clearly
displays the ionized air created by harmful corona activity by indirectly detecting
the associated ultraviolet radiation. The camera user can change the color
of the corona activity to provide better contrast against varying backgrounds,
and the camera records the corona image to allow subsequent transfer of the
image to a PC for report generation. This highly effective technology is quickly
becoming an indispensable tool for the technician. In addition to the obvious
maintenance applications for all types of medium- and high-voltage equipment,
the corona camera can also be used for:
- supplementing partial discharge and ultrasonic surveys
- acceptance testing of new or modified installations
- product quality assurance
- design verification
- field and shop high potential testing
- general research
We can now even have a much greater understanding
of equipment insulator and conductor behavior when conducting
routine high potential testing. Figure 5 displays the
absence of corona while performing an ac high potential
test of a medium-voltage circuit breaker, indicating good
insulator and conductor condition, while Figure 6 shows
unwanted corona generating from inadequate clearance of
the energized test conductor during the same test. It is apparent
that the breaker was good but our temporary shop
test setup was not.
The future use of corona imaging promises to provide
many new applications and many associated new surprises,
but one thing is certain we will never think of corona in
the same manner, for now we can see it.
Download a pdf version of this Corona Imaging Article.
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