We have investigated the partial discharge (PD) due to electrical treeing degradation in low-density polyethylene (LDPE), ethylene - vinyl acetate copolymer (EVA) and ethylene - acrylic acid copolymer (EAA) by a computer-aided partial discharge measurement system which allowed us to obtain phase-resolved PD pulse data. The experimental results revealed that the PD magnitude was strongly affected by the instantaneous applied voltage and that the occurrence of a PD was determined by the time derivative of the applied voltage (). The PD pulse-sequence analysis revealed the following: (i) a PD occurs in a discharge path which consists of a tree trunk and branches extending from the trunk; (ii) in each discharge path at most one PD occurs per half cycle. Based on these facts, a model of PDs due to electrical treeing was proposed. The influences of applied voltage and frequency were investigated by applying a triangular voltage. The number and average magnitude of PDs increased linearly with applied voltage whereas the PD charge per cycle increased quadratically. These results are in good agreement with the model.

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Experimental ac partial discharge (PD) patterns are presented for a disc-void (area greater than length) with metal surfaces, one metal and one insulating surface, and with both surfaces insulating. These patterns indicate that the portion of the surface over which the charge is deposited (discharge area) plays an important role in producing the fluctuations in discharge magnitude commonly observed for such voids. A simulation model for PD patterns in voids is presented, which expressly includes the effects of the charge distribution left on the void surface by consecutive PDs. This model defines two factors that control the PD propagation. These are: a minimum field required to maintain the discharge within an existing surface path Ein and a minimum peripheral field required to extend the discharge path Ep. There is also one other factor, the occurrence probability that controls the incidence of the PD. It is shown that the model gives a variation in PD magnitude throughout the active region of phase without any stochastic factors, i.e. when only variations in the discharge areas are allowed for, but that in order to reproduce the typical (turtle-like) PD patterns observed for the disc-void with insulating interfaces the occurrence probability and surface conductivity have to be included. The current model is compared to previous models and the relationship of the model factors to the change of PD patterns with ageing is briefly discussed.

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In order to check our physical model for partial discharges (PDs) in a tree channel, the light emission from a PD in an actual tree was observed. The experimental results agreed with the model. By using the model, conditions for the formation of typical phase-amplitude-number (ϕ-q-n) pattern of PDs from treeing was discussed.

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When the electrode separation is long or the applied voltage is low, the tree cannot directly grow to the plane electrode, and extinction of partial discharge (PD) with the re-growth of new channels can be observed after a long time. Before PD extinction, the light emission of the PD changes from the whole tree structure to only the region near the needle tip, and the PD pattern also changes from wing-like to turtle-like. These phenomena are considered to be related to the increase of channel conductivity by PDs. Assuming the channel conductivity is higher in the region at tree tips or in the tree channels than near the needle tip, the simulation results coincide with the experimental results.

Articles you may be interested inStudy on thermal properties and crystallization behavior of electron beam irradiated ethylene vinyl acetate (EVA)/waste tyre dust (WTD) blends in the presence of polyethylene graft maleic anhydride (PEgMAH) AIP Conf.Thermally stimulated depolarization current studies of poly(ethylene oxide) and poly(ethylene oxide) complexed with potassium thiocyanate Thermally stimulated currents (TSC) from polyethylene and ethylene-vinyl-acetate copolymers with a variety of vinyl-acetate contents have been investigated using a TSC technique with x-ray irradiation. Five main TSC peaks, which we believe are due to trapped carriers, were observed in these polymers. The detrapping of carriers is closely related to molecular-chain motions, and trapping sites are assigned to physical defects such as cavities formed by local arrangements of molecular chains rather than carbonyl groups. One of the TSC peaks is associated with traps in the amorphous region, one with traps in the boundary region between the amorphous and the crystalline regions, i.e., in the folded region, and another with traps in the crystalline region. The TSC spectrum from the dipole of the vinyl-acetate unit under a bias field shows a TSC peak accompanied by a current inversion. The average effective dipole moment for the vinyl-acetate unit was determined to be about 1.8 D from a TSC analysis.PACS numbers: 72.20.Jv, 71.5S.Ht, 61.80.Cb a The DSC was measured at the heating rate of 10 K/min.