PARTIAL DISCHARGE MEASUREMENTS
What is a 'partial discharge‘ Let us use the definition given in the International Standard of the IEC (International Electro technical Commission) related to partial discharge measurements, Partial discharge (PD) is a localized electrical discharge that only partially bridges the insulation between conductors and which may or may not occur adjacent to a conductor. This definition is supplemented by three notes, from which only notes 1 and2 shall be cited.
NOTE 1 – Partial discharges are in general a consequence of local electrical stress concentrations in the insulation or on the surface of the insulation. Generally such discharges appear as pulses of duration of much less than1 μs. More continuous forms may, however, occur, as for example the socalledpulse-less discharges in gaseous dielectrics. This kind of discharge will normally not be detected by the measurement methods described in this standard.
NOTE 2 – ‗Corona‘ is a form of partial discharge that occurs in gaseous media around conductors which are remote from solid or liquid insulation.‘ Corona‘ should not be used as a general term for all forms of PD.No further explanations are necessary to define this kind of phenomena: PDsare thus localized electrical discharges within any insulation system as applied in electrical apparatus, components or systems. In general PDs are restricted to a part of the dielectric materials used, and thus only partially bridging the electrodes between which the voltage is applied. The insulation may consist of solid, liquid or gaseous materials, or any combination of these. The term‘ partial discharge‘ includes a wide group of discharge phenomena:
(i) internal discharges occurring in voids or cavities within solid or liquid dielectrics;
(ii) surface discharges appearing at the boundary of different insulation materials;
(iii)corona discharges occurring in gaseous dielectrics in the presence of inhomogeneous fields;
(iv) Continuous impact of discharges in solid dielectrics forming discharge channels (treeing).
The significance of partial discharges on the life of insulation has long been recognized. Every discharge event causes a deterioration of the material by the energy impact of high energy electrons or accelerated ions, causing chemical transformations of many types. As will be shown later, the number of discharge events during a chosen time interval is strongly dependent on the kind of voltage applied and will be largest for a.c. voltages. It is also obvious that the actual deterioration is dependent upon the material used. Corona discharges in air will have no influence on the life expectancy of an overhead line; but PDs within a thermoplastic dielectric, e.g. PE, may cause breakdown within a few days.
It is still the aim of many investigations tolerate partial discharge to the lifetime of specified materials. Such a quantitatively defined relationship is, however, difficult to ensure. PD measurements have nevertheless gained great importance during the last four decades and large number publications are concerned either with the measuring techniques involved or with the deterioration effects of the insulation. The detection and measurement of discharges is based on the exchange of energy taking place during the discharge. These exchanges are manifested as:
(i) electrical pulse currents (with some exceptions, i.e. some types of glow discharges);
(ii) Dielectric losses;
(iii) e.m. radiation (light);
(iv) Sound (noise); increased gas pressure;
(v) Chemical reactions.
Therefore, discharge detection and measuring techniques may be based on the observation of any of the above phenomena. The oldest and simplest method relies on listening tothe acoustic noise from the discharge, the ‗hissing test‘. The sensitivity is, however, often low and difficulties arise in distinguishing between discharges and extraneous noise sources, particularly when tests are carried out on factory premises. It is also well known that the energy released by PD will increase the dissipation factor; a measurement of tan υ in dependency of voltage applied displays an ‗ionization knee‘, a bending of the otherwise straight dependency.
This knee, however, is blurred and not pronounced, even with an appreciable amount of PD, as the additional losses generated in very localized sections can be very small in comparison to the volume losses resulting from polarization processes.
The use of optical techniques is limited to discharges within transparent media and thus not applicable in most cases. Only modern acoustical detection methods utilizing ultrasonic transducers can successfully be used to localize the discharges. These very specialized methods are not treated here. The most frequently used and successful detection methods are the electrical ones, to which the new IEC Standard is also related. These methods aim to separate the impulse currents linked with partial discharges from another phenomena. The adequate application of different PD detectors which became now quite well defined and standardized presupposes fundamental knowledge about the electrical phenomena within the test samples and the test circuits. Thus an attempt is made to introduce the reader to the basics of these techniques without full treatment, which would-be too extensive. Not treated here, however, are non-electrical methods for PD detection.
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