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Substation — Welded/EHV
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Blue highlighted items are industry standard and most frequently ordered.
The use of the laboratory is based on the fact
that it is the surface voltage gradient that
causes corona. Although most systems con-
sist of 3 phase conductors and a ground
plane, it is a rather simple matter to duplicate
in the laboratory the conductor surface volt-
age gradient as it exists on any of these
phase conductors with a single conductor
and a ground plane.
The following formula and nomograms give
this three phase to single phase equivalency.
Because this conversion is possible, all EHV
testing is done single phase; and there is no
necessity for 3 phase testing with its high
cost in terms of equipment and space.
Since voltage gradient is the significant fac-
tor, the single phase test does not have to be
done at the full voltage of an operation
system. By setting up the test closer to the
ground plane, the operation voltage gradient
can be obtained with a lower test voltage.
There is a limit, however, below which the
height cannot be lowered lest corona onset
and flashover occur simultaneously.
Generally, the minimum test height should be
about 10 times the diameter of the test
Normally, the conductor surface voltage
gradient at the extinction of corona in the
laboratory is calculated using the accompa-
nying equations. However, for test setups
involving unusual conductor configurations,
the conductor gradient cannot be readily cal-
culated. In these cases, a gradient calibrator
may be used. This is a small sphere mount-
ed on the conductor. It has previously been
calibrated for each conductor size to estab-
lish the surface voltage gradient that starts
positive corona on the sphere. With it tests
can be duplicated in any number of laborato-
ries. The applied voltages and ground dis-
tances could all be different. But the voltage
gradient on the surface of the conductor
when the corona occurs on the sphere will
always be the same. The calibrator provides
a convenient bench mark for measuring the
corona performance of connectors.
In use, the sphere is mounted on the con-
ductor in a connector test setup. The voltage
is raised until there is a corona on the sphere.
We already know from previous calibration
what the voltage gradient on the surface of
the conductor is at this point.