Motors and Generators:Partial Discharge Test

Partial Discharge Test

One of the causes of machine insulation degradation and failure is due to partial discharge (PD) in the gas-filled voids in the stator insulation system. The online or off-line PD test can be used to directly measure the pulse cur- rents generated by PD within a machine winding. The machine-winding insulation has voids that are filled with air or gas and they are of varying sizes. The PD pulses are of very short duration (few nanoseconds) because the void cavities are small however some pulses may be larger than other PD pulses. The PD test is applicable to form-wound stator windings of machines rated at 2300 V and above. Each PD pulse current originating in specific part of the winding will travels along the coil conductors. The PD current pulses in turn generate voltage pulses because of the surge impedance of the coils in the slots. Any device sensitive to high frequency can detect the PD pulses and thereby these small charge fluctuations (in coulomb) are measured by testing the corresponding current variations. By this method the weak points of the insulation (location of the PD) are measured. However, this method does not provide information on the condition of insulation at those points where PD has not occurred. PD is an electrical discharge that only partially bridges the insulation between conductors. Especially for machines rated greater than 4 kV, PD can be a sign of deterioration involving external sur- faces (slot or end turn) or of delamination internal to the ground wall. PD current pulses can be measured in two ways: (1) off-line PD measurements and (2) online PD measurements.

The off-line PD test requires a power supply to energize the winding to at least rated phase-to-neutral voltage. It is best to perform this test one phase at a time with the other two phases grounded. The most common means of detecting the PD current pulses is to use a high-voltage capacitor connected to the stator terminals. The capacitor blocks the normal 60 HZ AC voltage applied to energize the windings while passing the high- frequency PD pulse currents. The output of the high-voltage capacitor is coupled to a resistive or inductive-capacitive load. The PD high-frequency current passing through the capacitor create a voltage pulse across the resistive or inductive-capacitive network which then can be displayed on an oscilloscope, or other display device. Every PD will create its own pulse and the magnitude of a particular PD pulse will be proportional to the size of void cavity.

During the off-line PD test, the applied voltage is gradually raised while monitoring the PD pulses on the oscilloscope at the machine terminals. The voltage at which the PD is first detected is known as PD inception voltage. The voltage is then raised to rated line-to-neutral operating voltage and held for 10 to 15 min while PD pulses are recorded. The voltage is then gradually reduced and the voltage at which the PD is no longer visible is recorded. This voltage is known as the PD extinction voltage. For motors rated 2300–4000 V, the phase-to-neutral voltage may not be sufficient to produce discharges, therefore some owners will perform the PD test at line-to-line voltage. It should be kept in mind that using line-to-line voltage for PD test exceeds the normal insulation voltage rating of the winding and could lead to wind- ing insulation failure. During off-line PD measurements, it may be possible to measure at the line end and neutral end of the individual circuits or phases with all other circuits grounded or alternatively with all circuits tested in parallel. This will provide an indication as to whether the PD is more pronounced at the line or neutral ends and whether the phase-to-phase insulation is a source of PD activity.

The off-line PD test as discussed above does not pinpoint the location of the PD discharges in the stator windings. To determine the location of the PD discharges in the stator, PD probe test is performed. Two special probes are available to help locate the site of PD in the stator. One probe is designed to detect the electromagnetic (radio frequency) energy and the other to detect acoustic energy. The electromagnetic probe is usually tuned to 5 MHz essen- tially making it a modified AM radio with an antenna mounted on a one end of the probe handle. The probe with the antenna end then can be moved around the stator windings to locate where the PD is occurring. The ultra- sonic probe is a directional microphone that picks up the acoustic pulses that are being generated by the movement of high-velocity electrons and ions due to PD in the stator windings. The ultrasonic sensor, used with appropriate safety precautions, can be useful for locating sites of higher PD activity at specific slots in the core of the machine. To perform PD probe test may require partially dismantling the machine, such as removing the rotor.

The online PD test is performed during normal operation when the machine is running at constant operating voltage. The PD monitor directly detects stator winding PD activity thereby including the effects of load, temperature, and voltage, which can provide important information as to the probable cause of the PD activity. The PD activity generates current pulses that are typically of very short duration and propagate throughout the stator windings. Each PD pulse comprises of frequencies ranging from DC to several hundred megahertz. The online PD test is similar to the off-line PD test in many respects and they require use of sensors, which can take the form of the following:

• Coupling capacitors at line terminals

• Radio frequency current transformer on ground wire

• Radio frequency current transformer on ground of insulation shield of supply cable

• Radio frequency current transformer on conductor between neutral of stator and grounding impedance

• Impedance across joint between machine frame and terminal box

The instrumentation used with these sensors can consist of the following:

• Radio noise meter for narrow band measurements between 100 kHz and several hundreds of megahertz. Each type of PD, such as slot discharge or end arm discharge may have its own unique frequency spectrum.

• Broadband measurement using an oscilloscope or pulse height analyzer to provide an indication as to the number, polarity, and phase position of the PD pulses. Polarity may indicate whether the PD is on a surface or if it is internal. Phase position may indicate whether the PD involves the ground wall, or phase-to-phase insulation.

During online measurements, the operator of the radio noise meter needs to identify RF signals from radio stations and sites of PD activity or sparking external to the machine so as to exclude these data from the analysis. Online PD measurements using broadband detection systems may make use of bandwidth, attenuation, and pulse travel time for noise reduction. Trending of individual machines based on periodic or continuous online PD measurements under identical operating conditions, or comparison between similar machines can indicate a need for off-line PD measurements to confirm and locate the probable source of the discharge activity.

Because of the complexity in which PD pulses propagate within machine windings and the profound effect of bandwidth upon the response of the PD detection instrumentation, it has not been possible to establish meaningful limits for the PD magnitudes, which are measured at the terminal of the machine. Comparisons may be possible between machines of the same design using the same sensors and detection instrumentation of identical bandwidth. For additional information, refer to IEEE standard 1434-2000, IEEE Trial-Use Guide to the Measurement of Partial Discharges in Rotating Machinery.

Slot Discharge Test

This test is performed to evaluate the coil surface grounding in the slot portion. The stator coil outer surfaces are painted with conducting varnish in order to make good electrical contact with the machine frame to prevent voids. However, at higher voltages, ionization can take place in the voids, resulting in insulation damage. The slot discharge test consists of applying approximately 7 kV AC and observing the wave form on an oscilloscope. This wave form is compared to a wave form of one coil side arcing to the slot at a single point. The slot discharge phenomena usually consist of high frequencies, such as 2500 Hz/s. The line disturbances are usually filtered out in order to obtain an accurate slot discharge phenomena.

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