Partial discharge is not most common tests. However these may provide very useful information in some of the cases. This test occur in oil filled transformer due to presence of voids or cavities in the solid insulation, conducting particles in paper or oil, wet fibers and gas bubbles in oil, sharp conductors or electrode edges against papers. The partial discharges also occur due to poor processing, ingress of moisture, trapped air due to incorrect oil filling, long term degradation of the insulation and design defects. For enhanced reliability of transformer in transmission network, on site PD measurement and location has become of increasing interest. On-site PD detection by electrical technique is associated with considerable problems due to radiated and coupled interference by surrounding conducting objects, noise and radio signals. Through some types of PD signals are radily identified but still interpretation of PD test results requires great ideal of experience and experimentation.
Electrical detection also requires a PD free HV test source, a coupling capacitor and outage time for the transformer under test. However this technique is time consuming, costly and extremely disturbed by electrical noise.
Acoustic methods have been proved to be more sensitive for PD detection at site. Acoustic techniques have the advantage that can be used on energized transformer for both detection and location of PD and these methods are not susceptible to interference from outside sources when properly applied.
The preferable frequency bandwidth for acoustic detection is between 50 kHZ to 150 kHZ to avoid external interference and noise. It is advisable to conduct PD measurements at several locations on the transformer with acoustic detector to estimate the total discharge effects and also to determine the location of PD inside the transformer.
The online or off-line PD test can be used to directly measure the pulse currents generated by PD within a machine winding. The machine-winding insulation has voids that are fi lled 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 specifi c 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 fl uctuations (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 ratedgreater than 4 kV, PD can be a sign of deterioration involving external surfaces (slot or end turn) or of delamination internal to the ground wall. PDcurrent 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 highfrequency 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 fi rst 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 suffi cient 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 winding 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 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 a t 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 groundwall, or phase-to-phase insulation.