Partial discharges (PD) have been recognized as a harmful ageing process for electrical insulation at the last century when the high voltage technology was introduced for the generation and transmission of electrical power. Since that time numerous papers and books appeared, dealing with the physics and recognition of partial discharges. First industrial PD tests of HV apparatus were introduced at the beginning of 1940. The method applied was based on NEMA 107, which specifies the measurement of ratio influence voltages (RIV) expressed in terms of µV. One disadvantage of this method is, however, that the RIV level is weighted according to the acoustical noise impression of the human ear, which is not correlated to the PD activity. Therefore, the IEC Technical Committee No.42 decided the issue of a separate standard on electrical PD measurement associated with the PD quantity apparent charge, which is expressed in terms of pC.
Top13.2 Partial Discharges Occurrence
Partial discharges are defined in IEC 60270 (2000) as: “Localized electrical discharges that only partially bridge the insulation between conductors and which can or cannot occur adjacent to a conductor”. Partial discharges are in general a consequence of local stress concentrations in the insulation or on the surface of the insulation. Generally, such discharges appear as pulses having duration of much less than 1 µs. From a physical point of view self-sustaining electron avalanches may happen only in gaseous dielectrics. Consequently, typical discharge types occurring in ambient air, such as glow, streamer and leader discharges, may also happen in gaseous inclusions due to imperfections in solid and liquid dielectrics. The pulse charge of glow discharges is in the order of few pC. Streamer discharges may create pulse charges ranging from about 10 pC up to some 100 pC. A transition from streamer to leader discharges may occur if the pulse charge exceeds few 1000 pC. In principle, there are three types of PD-related failure that can occur in power cable system. These partial discharges can be classified depend on the origin or location that result in field enhancement situation and produce partial discharge activities in cable systems. These types of PD are:
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Discharge result from internal cavity in the insulation media.
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Surface discharge along interfaces.
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Discharge in the form of electrical treeing in insulation.
(Figure 1) shows the three types of condition in power cable system that can result in discharge activities.
Figure 1. Types of PD-related failures that can occur in power cable system
A partial discharge occurs often within gas filled voids in solid or impregnated insulation or from sharp protrusions giving a field enhancement in gaseous, liquid or solid insulation systems of high voltage equipment. If the local field exceeds a certain limit determining the onset voltage and a seeding electron are present, and then an electron avalanche will result. This avalanche will for a partial discharge stop, either from the barrier effect of the cavity walls or from space charge effects when propagating in a gas or liquid. This process is very localized and transient in nature, with a typical duration of microseconds or less.
However, although of very short duration, the high electron energies in the discharge can interact with any solid or liquid dielectric materials in the immediate vicinity and cause bond breaking of molecules, changes in the chemical properties and ablation of the material.
It is important to recall that it is not the real local discharge we measure. It is the charge, this discharge induces on the nearby electrodes terminals or even more complicated: the wave propagation from this induced charge to the detector. In the higher frequency regime the concept of discrete components are no longer valid, and the concept of dipoles and wave excitation much be taken into use.