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.
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