Narrator: In part one of this series we learned about corona and its effect on the power system.
Now we will learn about how corona can be measured.
Each year utilities invest billions of dollars to upgrade existing high-voltage transmission
lines and to build new ones.
As we discussed earlier, it is important that these lines be as free as possible of corona
and the noise and interference it causes.
For this reason all transmission line hardware must be tested to demonstrate it is corona
free before it can be deployed on the power system.
[Sounds of transmission line work]
Jeff Hildreth, Electrical Engineer, Bonneville Power Administration: This is a transmission
line spacer damper.
It’s used to ensure that the spacing of the wires that make up a transmission line
are proper.
And this may look like a simple device, but it’s actually precisely engineered.
And one of the criteria that it has to meet is that it be corona free.
Narrator: This is a space damper that must meet exact standards specified by the utility.
One such requirement is that it be corona free for a voltage 15 percent higher than
that of the line on which it will be used.
But how can we measure that to demonstrate whether it meets the requirement?
Such tests are usually carried out at a high voltage laboratory such as the Bonneville
Power Administration’s High Voltage Laboratory in Vancouver, Washington.
The hardware, in this case a spacer damper, is installed on a full scale transmission
line mockup in the laboratory.
It can then be energized with high voltage using a specialized test transformer.
Since corona emits light, sound and radio interference, that gives us at least three
ways of measuring it.
[Method 1: Noise]
This is a sound pressure level meter.
As you can see, it responds to the intensity of the sound.
Jeff Hildreth: Can you do that one more time, where you come down from 300 [kilovolts] and
come back up, please?
Pete Anderson, Electrical-Mechanical Test & Development Craftsman II, Bonneville Power
Administration: Alright, 300 kV.
[Method 2: Light]
Narrator: Since we can darken the inside of the high-voltage lab, we can see the corona
either directly or with the help of a night vision scope.
This is called the “visual corona method.”
Jeff Hildreth: Can you bring the voltage up to 400 kV and turn the lights out?
Narrator: One advantage of this method is that we can see the precise location that
is causing the corona, which makes solving the problem a lot easier.
One interesting thing you learn after watching corona for a while is that there are two basic
types of corona, positive and negative.
The negative, or “glow corona” doesn’t require as much voltage and isn’t as disruptive.
Positive, or “plume corona” generally requires more voltage and produces more noise,
light and radio interference.
[Method 3: Radio Interference]
A specialized AM radio can be used to quantify a corona source.
First, the laboratory must be free of other radio noise to ensure that what the radio
picks up is actually caused by the equipment being tested.
Then a voltage is applied and the radio noise can be measured and recorded in microvolts.
Industry standards specify acceptable levels of radio noise for a given type of hardware.
[Method 4: Daylight Corona Camera]
Jeff Hildreth: It detects ultraviolet light in a spectrum that corona emits.
One innovative new way to detect corona is with a “daylight corona camera.”
We know that corona emits light and that if it is dark enough we can see that light.
However, it is impractical to inspect a real power line in this way because of sunlight,
moonlight or manmade light sources that may mask the corona light.
It turns out that corona emits light in the ultraviolet spectrum that is not commonly
found in sunlight or manmade light.
The daylight corona camera takes advantage of this and is able to display a dot on the
image where it detects this ultraviolet light.
[Sounds of corona on transmission line]
As you can see, we have many options to determine how much corona there is, what type and where
it is coming from.
This information helps utility engineers to design, build, operate and maintain transmission
lines that are free of the nuisance of corona.