AST8800 Partial Discharge Detection Option
 Conduct pass/fail partial discharge testing quickly and easily in a
manufacturing environment
 Provide customers with high quality wound products
 Ensure insulation longevity in variable speed drive applications
 Upgrade existing Windows based AST8800s in the field and maintain
Master file compatibility
What is the Partial Discharge Test?
Partial Discharge (PD) also known as Corona is a very small electrical discharge
caused by the ionization of oxygen in the presence of a high voltage electrical
field. The ionized oxygen or ozone chemically reacts with many insulation
materials and over a long period of time is corrosive to the insulation causing
premature failure in the winding insulation system.
Partial discharge has long been a known failure mode in high voltage motors and
generators. Motors and generators which pass a high voltage breakdown
insulation test will usually fail prematurely if they can not pass a partial discharge
test. In contrast, low voltage motors traditionally do not encounter voltages which
generate ozone except in the case of relatively rare line transients. The primary
concern with line transients is breakdown. Line transients are not frequent
enough to cause a serious PD failure concern. So testing for the presence of PD
in lower voltage motors is usually considered unnecessary.
But, partial discharge has become a major concern in motors operating at
nominally low voltages (i.e. under 600 volts) due to the effects of variable speed
inverter drive systems. Variable speed drives are notorious for causing frequent
over voltage transients which far exceed the nominal operating voltage. These
frequent transients can exceed the Corona Inception Voltage (CIV) and therefore
generate ozone damaging the insulation of low voltage motors slowly over time.
Eventually the insulation will degrade to the point where a hard arc occurs
causing a catastrophic insulation failure.
The PD test is beneficial in motor testing even when variable speed drives are
not used. The PD test can often detect missing or damaged slot and phase
insulation which may reduce the CIV. The lack of insulation can lead to
premature failure during normal operation. The strong magnetic fields cause the
magnet wire to flex at 50 or 60 Hz. This mechanical flexing causes wire insulation
to wear when rubbing against adjacent wire and the laminated core. The
damaged wire insulation eventually fails causing a ground fault or a turn–to-turn
or phase-to-phase failure.
Fig. 1 Low voltage AC traction motor stator with turn to turn insulation failure.
High voltage hipot and impulse tests have long been used to test for catastrophic
insulation failure of the ground, phase and wire insulation of low voltage motors.
These tests can also be used in combination with a PD detector to generate and
identify the presence of PD. The transients caused by inverter drives are best
simulated by the high voltage impulse test rather than the hipot test. The high
voltage transient applied by the impulse test stresses the phase and turn
insulation in the same manner as the inverter induced transients.
When a high voltage impulse is applied to a winding and the CIV is exceeded,
low energy, wide bandwidth electrical discharges are generated. It is beneficial
and possible to detect these discharges or the absence of them as a means of
ensuring that the insulation system of a motor will have a long life when used in a
variable speed drive application.
Partial Discharge technology has been pioneered primarily by the predictive
maintenance industry which uses it to detect the onset of premature insulation
failure in high voltage windings. Manufacturers of smaller motors find it difficult to
use the technology developed for the maintenance industry. They operate in a
higher volume production environment. Tests are often performed by low skill
employees. Test hardware must be simple to use. Test data must be output as a
pass/fail result.
How Can I Use the Partial Discharge Test?
Baker has incorporated a method for detecting the presence of PD quickly and
effectively in a motor manufacturing environment. Modifications to the traditional
impulse test hardware circuit allow the high frequency partial discharge energy to
be captured and digitized along with the damped sinusoidal impulse response.
The digitizing circuit samples the impulse waveform at high speed. Many
sampled data points are acquired so that the high frequency components can be
properly displayed and analyzed.
Software algorithms separate the high frequency component from the low
frequency impulse waveform. The low frequency waveform is displayed and
analyzed using Baker’s patented Error Area Ratio analysis method. The high
frequency PD data is displayed and analyzed using a summation calculation to
quantify the PD data as a single value. This value is calibrated to convert it to
Coulombs. A user adjustable time window is used to set the start time and end
time of the data included in the PD summation. A pass/fail threshold value sets
the level above which the PD level becomes unacceptable.
Fig. 2 Typical Surge/PD programming screen after successful programming. Note the high
frequency corona data shown on the time axis below the impulse peak.
Most users find it beneficial to conduct two sets of impulse tests. The commonly
used breakdown test is usually performed at a higher test voltage than the PD
test. The PD impulse test is usually performed near the minimum desired CIV.
Tests performed near the CIV allow for a PD pass/fail threshold setting fairly
close to zero.
During production testing, an operator will see a Pass or Fail result. The digitized
Corona data can be viewed or printed after each test. The number of Corona
failures is tallied in the Statistics file displayed on the front screen. Corona test
values are included in the raw test data stored to the xml results database.
Fig. 3 Typical results screen in which the measured Corona value exceeds the limit.