The most meaningful measure of insulation integrity is the AC voltage at
which significant, sustained partial discharges (or corona) occur. This is
called the Corona Onset Voltage (COV). In all cases, the COV shall be greater
than the operating voltage. Repeatable, distinct corona activity can be seen
in most insulating media at AC voltages considerably lower than in the DC
case. Voids, fractures, separations, delaminations and other defects can be
observed with AC testing which is not possible using any other method. Several
manufacturers of partial discharge test equipment have been successful at
detecting low-level corona (in the order of 0.1 to 1 picocoulomb (pC)) with
practical, cost- effective equipment. The more successful high-voltage
designers and manufacturers have used this equipment to:
(1) Understand basic corona phenomena.
(2) Compare different types of materials, components, subassemblies,
(3) Provide a consistent screening tool for research and development,
production and "health" checks of finished components and equipment.
Detailed procedures can and should be developed for the various required
tests and evaluations.
A block diagram of a more general method is shown in Figure 5-2 wherein a
component is subjected to a 60 Hz AC voltage, provided by a corona-free AC
power source. Partial discharges in the device-under-test are detected and
displayed. In more recently designed equipment, computer-aided pulse height
analyzers are used to characterize the corona "signatures" of the
There is considerable variation in the sensitivity of various corona test
sets due to inherent noise in the equipment and the method of corona pulse
detection. Where an oscilloscope is used for the display of corona pulses,
only an approximate CIV can be measured. Test equipment that applies pulse
height analyzers in conjunction with a computer can give a definite measure of
the corona intensity. This is expressed in picocoulombs per second when
averaged over a specified period of test time. AC and DC corona inception test
voltages should be specified from 25% to 100% above the working voltage. Since
the corona intensity increases at the second to ninth power with increasing
applied voltage, the intensity at the working voltage will be much less than
the established limit (1 to 5 pC) and, for practical purposes, this intensity
can be considered nil; i.e., the component or system is "corona free."
A generally used procedure is as follows:
(1) The device is placed in the test setup, and a simulation of a corona
pulse is introduced into the sample to allow accurate calibration of the
system's detection and display apparatus.
(2) The applied 60 Hz AC voltage is slowly increased until significant
corona activity is seen. (For most test equipment, 50 pC is the typical
threshold of inception; with filtering and operating in a shielded room, the
test equipment can reliably detect inception values considerably less; i.e.,
in the 1 to 10 pC region.)
(3) The test is then repeated at least once more, and if the value
correlates with the initial value, the inception voltage is thus recorded. The
corona extinction voltage (CEV), i.e., the AC voltage whereby the observed
corona extinguishes, is also measured and recorded.
(4) The above test, simple in concept and very easy to perform, is repeated
as necessary to allow periodic checks of the insulating media; e.g., CIV
levels at different processing points (or with differing materials) can be
used to pinpoint where voids, cracks, delaminations, etc., occur and thus
allow process and material improvements.