High Voltage Electric Utility Transmission
Detecting electrical faults on high voltage electrical transmission
lines is fairly easy and can be accomplished rapidly from a light aircraft.
However, even from short distances, accurate temperatures of electrical
faults are impossible to measure [quantify]. There are several problems
associated with temperature measurement from the air which include spot
size to target distance ratios, reflection of the objects surveyed,
having a sufficient load on the line at the time of the survey among
others. The spot size to target distance ratio is the number one problem
with temperature measurement. Specification writers have not yet realized
the seriousness of this problem and continue to ask for quantitative
data on fault areas. The fact is that infrared cameras that are in general
commercial use today cannot measure accurate temperatures on small objects
from distances of 50 feet...much less from reliably safe flying distances.
A one-inch (relative size of a transmission line splice) target cannot
be measured from that distance, plain and simple, although it can be
detected. These spot sizes are unmanageable and inaccurate on any target
that does not have a large homogeneous heat signature. The GRE is critical
to the measure of spatial resolution in aerial infrared thermography.
Nyquist's frequency theorem states that an object less than two times
the size of a sensor's GRE cannot be resolved for measurement, so a
3x3 pixel or GRE spot is needed for reliably obtaining measurements.
This shortcoming may be addressed by using more powerful lens to reduce
the GRE for a given distance, but then the sensor's FOV is then reduced,
limiting the area covered over a given period of time. So, if one is
using a small format IR camera (256x256 pixels) in a helicopter only
50 feet away from a 1 inch “hot spot”, it is impossible
to obtain accurate temperatures using a standard lens. The smallest
“hot spot” that could be accurately measured with one of
these imagers is over 2", even at that extreme short distance.
Also, from the air, using a more powerful lens does not work well because
vibration is more evident in the form of image 'shaking'. Image 'smearing'
may also occur due to an increase in the apparent speed of the sensor's
view across the ground. In the air, there are few substitutes for a
large pixel array, but even using large format detectors, one cannot
and should not profess to measure temperatures on very small objects.
These anomalies can be seen, and by comparing them to similarly loaded
phases or equipment, potential problem areas can be identified, saved
and marked on a map. For ‘good’ measurements, a ground verification
team should be used to inspect suspect hot spots from the ground (cloudy
nights are best) and verify the findings of the aerial IR survey. They
will be closer to the target and with a powerful lens on a stable surface,
much more accurate.
High Voltage Electric Utility Distribution Lines
Because they are smaller, lower to the ground and often run through
populated areas, high voltage electrical distribution lines are much
more difficult to see against all the thermal clutter on the ground
such as trees, street lights, people, animals, etc., than transmission
lines. Therefore they are best left to ground-based infrared thermographers.