Z. L. Ma

Effect of ambient temperature on electrical treeing characteristics in silicone rubber

In this paper, electrical treeing was investigated in room temperature vulcanized (RTV) silicone rubber (SiR) over a range of ambient temperatures. An ac voltage with a frequency of 50 Hz was applied between a needle-plate electrode to initiate the electrical tree at different ambient temperatures. Both the structures and the growth characteristics were observed by using a digital microscope system. Obtained results show that the tree initiates from a single branch with a white gap. Four typical tree structures, namely branch, bush, pine branch and bush-pine mixed tree, were observed within the sample. The occurrence of tree structures changes with the increase of ambient temperature, in which branch tree takes up a great proportion at 30°C while bush tree becomes dominated as the temperature rises up to 90°C. Meanwhile, the cumulative inception probability within the same time decreases obviously with the increase of ambient temperature. The growth rate of the tree is closely related to the ambient temperature. It is suggested that the increase of vulcanization network density and elastic modulus with ambient temperature may have great influence on the treeing characteristics (including growth rate, fractal dimension and treeing proportion).

Hydrophobicity evaluation of silicone rubber insulator using PD-induced electromagnetic wave

Silicone rubber insulators are widely used as outdoor insulation due to their excellent surface performance. However, the decrease even the loss of hydrophobicity will take place under various environments, which causes the essential concern on the hydrophobicity evaluation. Dynamic dropping test (DDT) method is proposed as one of the evaluation methods. In this paper, based on DDT method the PD-induced electromagnetic wave is investigated for the hydrophobicity evaluation. An antenna is utilized to receive the high-frequency electromagnetic wave produced by the surface discharges, which are associated with the hydrophobicity variation. The electromagnetic signals are analyzed by a signal-processing system based on the wavelet-transform theory. The obtained 3-D images of surface discharges contain the relationship among the frequency, the time and the amplitude, which can reveal different ageing condition of the insulators. Based on the 3-D images, the fractal dimension (FD) and the lacunarity of 2-D contour line spectra are discussed to quantitatively give the complexity of surface discharges. It is found that the distribution range of discharge becomes wider and the frequency increases with the ageing time in the 3-D images, and the FD value shows the increasing tendency with the increase of ageing level. Therefore, the PD-induced electromagnetic wave can recognize the characteristics of the surface discharges, which can be utilized to evaluate the hydrophobicity of silicone rubber insulators.

Decay behavior of surface charge on gamma-ray irradiated epoxy resin

Epoxy resin (ER) laminate was employed as sample to investigate the effects of gamma-ray irradiation on dynamic behavior of surface charge decay. Before the test, the samples were irradiated in air up to 100 kGy and then up to 1000 kGy with dosage rate of 10 kGy / h by using a 60Co gamma-source. The experiment was carried out at room temperature with relative humidity of ~ 50%. The sample surface was corona charged by applying dc voltage of ± 4 kV between needle-plate electrodes. An electrostatic voltmeter was employed to measure the surface charge. Obtained results show that with the increase of the radiation dose, the dissipation time of surface charge tends to decrease, which indicates that the charge decay is accelerated by the irradiation. It is suggested that the charge decay behavior is dependent upon the trap characteristics and moisture absorption at surface layer that are altered by the radiation induced chemical reactions.

Effect of magnetic field on electrical treeing behavior in XLPE cable insulation

This paper presents study aimed at clarifying the effect of magnetic field on electrical tree propagation in cross-linked polyethylene (XLPE) cable insulation. By adding dicumyl peroxide (DCP) as cross-linking agent into low density polyethylene (LDPE), the test samples are made at cross-linking temperature (CT) ranged from 150 °C to 180 °C. The dimension of the sample is 20 mm × 20 mm × 4 mm. A pair of needle-plate electrode is employed to introduce electrical tree within the material. A DC magnetic field is placed perpendicular to the electric field, and the magnetic flux density (MFD) is set at 350 mT, 450 mT and 550 mT, respectively. By applying AC voltage between the electrodes, the tree is initiated from the needle tip and progressively propagates towards the grounded electrode. The morphology of the tree is captured by using a digital microscope, and the mean growth rate of tree length as well as tree width is calculated. Obtained results show that with the increase of the MFD, the tree morphology is varied from branch type to bush type. The mean growth rate is dependent upon the CT and is changed with the MFD. It is suggested that partial discharge activity within the tree channels is varied by the electro-magnetic force, and as a consequence the treeing behavior is altered.

Effects of low temperature on treeing phenomena of silicone rubber

In this paper, room temperature vulcanized (RTV) silicone rubber (SiR) was employed as test sample to investigate the relationship between electrical tree propagation characteristics and the low experiment temperature. Power frequency voltage was applied on the SiR specimens through the needle-plate electrode with the same radius of needle tips to initiate the electrical tree at different low experiment temperatures. Both the structures and growth characteristics of electrical tree in SiR were observed by using a digital camera and a microscope system. Obtained results show that electrical tree in RTV SiR is white gap tree channel which maybe composed of silicone compounds instead of carbonized channel in XLPE. Electrical tree in SiR all initiate from single branch, and the width of initiative single branch channel varies a lot with the electrical tree structure. The structure of at experiment temperature from 0 °C to -70 °C in RTV SiR can be classified into three categories, which are branch, bush and pine branch tree. The distribution of tree structures changes with the experiment temperature. At the temperature of -70 °C, pine branch tree take up a great proportion, however, bush tree becomes the dominant structure when the temperature rise up to -50 °C and -30 °C. All kinds of electrical tree grow rapidly in the first beginning of the treeing propagation, and this process lasts only a few minutes.

Effects of nano filler on treeing phenomena of silicone rubber nanocomposites

Polymer nanocomposite has recently drawn considerable attention because nanocomposites or nanostructured polymers have the potential of improving the electrical, mechanical, and thermal properties as compared to the neat polymers [1]. In this paper, the polymer nanocomposite was made by the mixed addition of SiO2 nanoparticle with the radio of 20 nm into room temperature vulcanized (RTV) silicone rubber (SiR), with the filler content from 0 to 5 wt% respectively. The typical needle-plate electrode was employed to investigate the relationship between electrical tree propagation characteristics and the content of nano filler with the ac voltage of 50 Hz. Both the structures and growth characteristics of electrical tree in SiR were observed by using a digital camera and a microscope system. Obtained results show that the distribution of tree structures is different between base SiR and its nanocomposites specimens. The growth speed varies a lot with the content of nano fillers, and there is an obvious improvement in tree initiation time as the increase of filler content. The possible reasons for the improvement in electrical tree growth and initiation time with the addition of nano fillers are discussed.

Effect of cross-linking temperature on charge decay behavior in XLPE

XLPE films were employed as test samples to investigate the effect of cross-linking temperature on space charge decay behavior. The samples were cross-linked with temperature ranged from 150 °C to 180 °C in thickness of 200 µm. Corona charging method was employed to introduce charges on sample surface, after which open-circuit decay test was carried out at 35 °C with relative humidity of ~ 45%. Surface potential was measured by using an electrostatic probe, and mean decay rate of the charge was calculated. In addition, fourier-transform infrared (FTIR) spectroscopy was used to analyze the characteristic functional groups of byproducts that were generated in the cross-linking process within the samples. Obtained results show that with the increase of cross-linking temperature, mean decay rate of both negative charge and positive charge initially decreases then appears to increase. The decay behavior remarkably depends upon the polarity of charged particles. It is proposed that the charge decay dynamics is related to the content of cross-linking byproducts, namely acetophenone, cumyl alcohol and α-methylstyrene, which is varied as a function of the cross-linking temperature.

Dependence of electrical treeing behavior on cross-linking temperature of XLPE

In this paper, XLPE laminate was employed as test sample to investigate the effect of cross-linking temperature on characteristics of electrical treeing. The samples were cross-linked at the temperature ranged from 150 °C to 220 °C by adding dicumyl peroxide (DCP) as cross-linking agent into low density polyethylene (LDPE). A pair of needle-plate electrodes was employed to initiate the tree by applying ac voltage between the electrodes. The experiment was carried out at 30 °C, 60 °C and 90 °C, respectively. A digital microscope was employed to observe the growth process of electrical trees. The profile of crystal grains were captured by using a polarizing microscope. Obtained results show the dependence of characteristics of both tree initiation and propagation upon cross-linking temperature of XLPE. It is suggested that the treeing behavior is influenced by both crystallinity and crystal grain morphology which are varied as a function of the cross-linking temperature.

Effects of frequency on treeing phenomena in Silicone rubber

The use of Silicone rubber (SiR) is increasing as an insulating material of cable joint in power transmission line in recent years for its various excellent characteristics. However, few attentions has been focused on the influence of frequency on electrical tree characteristics. In this paper, room temperature vulcanized silicone rubber was employed as test sample to investigate the relationship between electrical tree propagation characteristics and the frequency of applied voltage. Electrical trees have been grown over a frequency range from 50 Hz to 10 kHz through the typical needle-plate electrode, both the structures and growth characteristics of electrical tree were observed by using a digital camera and a microscope system. The fractal dimension of electrical tree is calculated using a simple box-counting technique. Obtained results show that both the tree structures and the accumulative probability of tree inception vary a lot with the increase of frequency. In detail, the frequency reduces the time to initiation, accelerates tree growth rate and reduces the time to breakdown.

Hydrophobicity evaluation of polymer insulator using surface discharge characteristics

With the spread application of polymer insulators in UHV and EHV transmission lines, the decrease of hydrophobicity becomes one of prominent factors to affect the stability and reliability of power system. Recently, dynamic dropping test (DDT) has been employed to hydrophobicity evaluation, which has proven the occurrence of different discharge phenomena in relation to the hydrophobicity variation. However, limited analysis of discharge characteristics is carried out for the hydrophobicity evaluation by using DDT. In this paper, based on DDT method, the characterization of surface discharges, including the discharge magnitude, pulse number, adjacent discharge magnitude and adjacent time interval, have been analyzed to for the hydrophobicity evaluation. It is found that with the decrease of hydrophobicity, these characteristics of surface discharges apparently show the increasing tendency. The results obtained can indicate that the characterization of surface discharge in DDT method can be utilized to the hydrophobicity evaluation.