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12.10 OFF-LINE PARTIAL DISCHARGE TEST

The off-line partial discharge test directly measures the pulse currents resulting from PD withina winding energized at rated line-to-ground voltage. Thus, any failure process that creates PDas a symptom can be detected with this method. The test is mainly relevant for form-woundstator windings rated at 2300 V and above. However, a variation of the test is also relevant for

random-wound stators intended for use on PWM-type inverter-fed drives.

12.10.1 Purpose and Theory

Many of the stator winding failure processes described in Chapter 8 had PD as a direct causeor a symptom of the process. When a partial discharge pulse occurs, there is a very fast flow ofelectrons from one side of the gas filled void to the other side. Since the electrons are movingclose to the speed of light across a small distance, the pulse has a very short duration,typically a few nanoseconds [12.18]. Since the electrons carry a charge, each individualdischarge creates a current pulse (i = dq/dt). In addition to the electron current flow, therewillbe a flow of positive ions (created when the electrons are ionized from the gas molecules)in the opposite direction. However, the ions are much more massive than the electrons and,consequently, move much slower. Since the transition time of the ions across the gas gap is

relativity long, the magnitude of the current pulse due to ions is very small and usuallyneglected.

Each PD pulse current originates in a specific part of a winding. The current will travel alongthe coil conductors, and since the surge impedance of a coil in a slot is approximately 30ohms, a voltage pulse will also be created, according to Ohms law. The current and voltagepulse flow away from the PD site, and some portion of the pulse current and voltage will travelto the stator winding terminals. A Fourier transform of a current pulse generates frequenciesup to several hundred megahertz [12.18]. Any device sensitive to high frequencies can detectthe PD pulse currents. In the off-line PD test, the most common means of detecting the PDcurrents is to use a high-voltage capacitor connected to the stator terminal. Typicalcapacitances are 80 pF to 1000 pF. The capacitor is a very high impedance to the high ACvoltage (needed to energize the winding sufficiently to create the PD in any voids that may be

present), while being a very low impedance to the high-frequency PD pulse currents. Theoutput of the high-voltage capacitor drives a resistive or inductivecapacitive load (Figure12.2). The PD pulse current that passes through the capacitor will create a voltage pulseacross the resistor or inductivecapacitive network, which can be displayed on an oscilloscope,frequency spectrum analyzer, or other display device. Older oscilloscopes had troubledisplaying the very short duration PD pulses on the screen. Thus, some types of detectors usean inductivecapacitive load since the PD current will then create an oscillating pulse at lowerfrequency, which can be easily viewed on oscilloscopes. The bandwidth of the detector is thefrequency range of the high-voltage-detection capacitor in combination with the resistive orinductivecapacitive network load. Early detectors were sensitive to the 10 kHz, 100 kHz, or 1MHz ranges. Modern detectors can be sensitive up to the several hundred megahertz range[12.18]. Every PD will create its own pulse. Some PD pulses are larger than others. Asdescribed in [12.19], in general, the magnitude of a particular PD pulse is proportional to the

size of the void in which the PD occurred. Consequently, the bigger the detected PD pulse, thelarger is the defect that caused the discharge. In contrast, smaller defects tend to producesmaller PD pulses. The attraction of the PD test is that one concentrates on the larger pulsesand ignores the smaller pulses. In contrast to the capacitance or power factor tip-up tests,which are a measure of the total PD activity (or the total void content), the PD test enables themeasurement of the biggest defects. Since failure is likely to originate at the biggest defectsand not at the smaller defects, the PD test can indicate the condition of the winding at its mostdeteriorated portion.

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12.10.2 Test Method

12.10.2 Test Method

Like the tip-up tests in Sections 12.6 and 12.9, the off-line PD test requires a power supply toenergize the winding to at least rated phase-to-ground voltage. Thus, for large generatorstators, a conventional or resonant transformer rated at 20 to 40 kVA may be needed. Inaddition, a low-noise 0.1 Hz (VLF) power supply could also be used (see Section 12.4.2). It isbest to perform the PD test at the machine terminals, energizing one phase at a time, with theother two phases grounded. PD tests can be measured from the switchgear, but it is thenimportant to measure the PD in a frequency range less than about 1 MHz. As discussed inSection 13.4, power cables tend to strongly attenuate the higher frequency components of thePD pulses as they travel from the stator winding to the detector at switchgear. Thus,unrealistically low PD signals will be measured at the switchgear if the detector primarilyoperatesat frequencies greater than 1 MHz.

In the off-line PD test, it is common to gradually raise the applied voltage while monitoring thePD pulses on an oscilloscope screen. The voltage at which the PD is first detected is called thedischarge inception voltage (DIV). The voltage then is raised to normal line-toground operatingvoltage. The winding should remain energized for 10 to 15 minutes at this voltage, and thenthe PD recorded (Figure 12.3), including the peak magnitude of the PD pulses (Qm). Thesoak time is needed since the PD tends to be higher in the first few minutes after the voltageis applied. Space charge effects cause this, together with the build-up of gas pressure in thevoid due to deterioration caused by the PD. The voltage is then gradually lowered and thevoltage at which the PD is no longer discernable is measured. This is the discharge extinctionvoltage (DEV). The DEV is usually lower than the DIV, and it is desirableto have the DIV and DEV as high as possible.

For machines rated at 6 kV or more, the maximum test voltage is normally the rated line-to-ground voltage. As will be discussed in Section 13.4, a test at this voltage will usually detectdeterioration years before an in-service failure is likely. For machines rated at 23004100 V, atest at rated voltage may not produce significant PD, even in severely deteriorated statorinsulation. This is because there may be insufficient electric stress within the defects toachieve the 3 kV/mm needed in atmospheric air to cause PD. Some users then per form thetest at the rated line-to-line voltage; i.e., a 4 kV motor would have 4 kV applied between thecopper and the ground. This is a small hipot test, and the stator owner should be aware thatsuch an overvoltage may lead to failure.

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Conventional PD analyzers cannot be used with IFDs or surge testers (Section 12.12). Therisetimes produced by such voltage sources can be as short as 50 ns, which is only ten times

longer than the PD pulses. These surges generate frequencies up to 5 MHz. At this frequency,the high-voltage capacitor in Figure 12.2 will have low impedance, resulting in a high-voltagesurge being applied to the oscilloscope or other recording device. This may destroy the input.Specialized sensors have been developed that can extract the PD from the 1000 times highervoltage surge [12.20].