A partial discharge is an electrical discharge that only partially bridges the insulation between conductors. Such a discharge is generally considered to take place as a precursor to total insulation failure but may exist for a long period of time, possibly years, before total breakdown occurs. In some circumstances the existence of the discharge will modify the stress distribution so as to initially reduce the tendency to total breakdown. In time, however, total breakdown will always result, often because the discharge itself leads to chemical breakdown of the insulation which reduces its electrical strength. Clearly, in a healthy transformer under normal operating conditions the only acceptable level of partial discharge is nil. ‘Normal operating conditions’ means any non-fault condition which is likely to occur in operation, for example system over voltages which may occur following a reduction in system load until corrected by tap changer operation or operator intervention, where necessary. It should be noted also that since many electrical systems frequently experience continuous over voltages of up to 10% there should be no partial discharge present with this level of overvoltage.
Detection of partial discharge relies on the fact that in a transformer, these cause transient changes of voltage to earth at every available winding terminal. The actual charge transferred at the location of a partial discharge cannot be measured directly. The preferred measure of the intensity of a partial discharge is the apparent charge ‘q’ as defined in IEC Publication 270. The specified provisional acceptance values of apparent charge referred to above are based on practical partial discharge measurements made on transformers which have passed traditional power-frequency dielectric tests.
The measuring equipment is connected to the terminals by matched coaxial cables. The measuring impedance in its simplest form is the matching impedance of the cable, which may, in turn, be the input impedance of the measuring instrument. The signal-to-noise ratio of the complete measuring system may be improved by the use of tuned circuits, pulse transformers and amplifiers between the test terminals and the cable. The circuit must present fairly constant impedance to the test terminals over the frequency range used for the partial discharge measurements.
When measuring partial discharge between the line terminal of a winding and the earthed tank a measuring impedance Zm is connected between the bushing tapping and the earthed flange. Calibration of the measuring circuit is carried out by injecting a series of known charges at the calibration terminals from a calibration generator.
Figure 1 shows a measurement and calibration circuit of this type where the calibration generator consists of a pulse generator and a series capacitor C0 of approximately 50 pF. Where the calibration terminals present a capacitance much greater than C0 the injected charge will be:
q0 = U0 .C0
where U0 is the voltage step
Figure 2 illustrates an arrangement where a bushing tapping is not available and the measuring impedance, with protective spark gap, is connected to the LV terminal of a partial discharge-free HV coupling capacitor C, whose value is large compared with C0. There are two types of measuring instrument in use:
Precautions must be taken to eliminate interference from radio broadcast stations, spurious partial discharges from other sources in the surrounding area, the power supply source and the terminal bushings. These include the fitting of electrostatic shielding on the outside of the transformer and oscillographic monitoring of the test. If a transformer exhibits unacceptable partial discharge levels then, because visible traces of partial discharge are not usually found, attempts must be made to identify the source without removing the transformer from its tank.
It may be useful to consider the following possibilities:
(a) Partial discharge in the insulation system may be caused by insufficient drying or oil impregnation. Reprocessing or a period of rest, followed by repetition of the test, may therefore be effective.
(b) A particular partial discharge gives rise to different values of apparent charge at different terminals of the transformer and the comparison of simultaneous indications at different terminals may give information about the location of the partial discharge source.
(c) Acoustic or ultrasonic detection of the physical location of the source within the tank.
For more information you can refer to BS 171.