Boxue Du

Recurrent plot analysis of leakage current for monitoring outdoor insulator performance

Outdoor insulators are widely accepted in power industry to maintain electrical insulation ranging from distribution to transmission lines. Such increasing development leads to a lack of appropriate diagnostic tools for assessing the performance of insulator in service, particularly in contaminated conditions. In order to monitor operating performance of contaminated outdoor insulators, a recurrent plot technique is proposed to analyze the leakage current passing through the insulator surface. Contamination tests were conducted in a laboratory by employing heavy salt fog with the deposition of non-soluble contamination. The leakage current was decomposed into different frequency components by using a wavelet transform technique. The temporal sequence of the extracted component was extended to m-dimensional phase space by using a phase-space reconstructed method. The recurrent plot is obtained to show that the topological structure of the high-frequency components is prominent to identify non-linear properties of discharge activities. Based on the analysis of the high-frequency components, the quantitative indicators of recurrent plot are obtained to reflect the underlying mechanism of flashover process. The results obtained indicate that the recurrent plot technique gives visual recurrent patterns of discharge activities for monitoring outdoor insulator performance. The dynamic behaviors on the insulator surface are graphically illustrated on the rectangular block structures with higher density of points. The structure changes indicate the switches of discharge states during the flashover process.

Recurrent plot analysis of leakage current on flashover performance of rime-iced composite insulator

Due to the wide application of composite insulators in the power industry, the insulator performance is challenged by various environments. To determine the flashover performance of rime-iced composite insulator, laboratory investigation was carried out in an artificial climate chamber to simulate different rime-ice morphology on the insulator surface. The configuration and characteristics of the rime-ice were demonstrated to establish the relationship between the rime-ice parameters and the flashover performance. In accordance with the discharge phenomena, the transition of leakage current (LC) until the flashover was analyzed by using a recurrent plot approach. After extracting the high frequency components by using a wavelet transform technique, the LC just before the flashover was extended to m dimensional phase space based on a phase space reconstructed method. The recurrent plot was obtained to reveal the non-linear characteristics of LC for identifying the dynamic behaviors on the insulator surface. It is shown that the propagation and properties of the discharges can be graphically projected on the topological structure of recurrent plot as a function of the rime-ice parameters. The process and underlying mechanism of flashover performance of rime-iced composite insulator can be visually reflected by the recurrent plot and the quantitative indicators of LC.

Gamma-ray Irradiation Inhibiting Surface Charge Accumulation on Polyethylene

Polymer insulating materials installed in electrical and electronic devices are widely used in spacecraft and nuclear power stations where the polymers are exposed to energetic radioactive rays. The chemical structure can be changed by radiation induced reactions that possibly gives rise to the alteration of characteristics of surface charge, which can enhance the local electrical field and cause surface discharge that damages the insulation. For the safe use of the material, its necessary to gain a good understanding of radiation effect on surface charge accumulation. In this paper, polyethylene was employed as test sample which was previously irradiated in air up to 100 kGy and then up to 1000 kGy with dose rate of 10 kGy / h by using a 60Co gamma-source. The sample surface was corona charged by applying a series of dc stresses on needle electrode that was set 6 mm above the surface. An electrostatic probe was designed to measure the surface charge density immediately after the charging process. Obtained results show the dependence of surface charge distribution upon the radiation dose varying as a function of the polarity of charging particles. It is suggested that surface charge accumulation depends upon the behavior of localized surface states, which is altered by radiation-induced degradation and cross-linking reactions of the chemical structure.

Effects of atmospheric pressure on DC resistance to tracking of polymer insulating materials

Polymer insulating materials are required for use in high altitude regions. For the use of polymer insulating materials in such regions, it must be confirmed whether the resistance to tracking at high altitudes is different from that at altitudes near the sea level. In this paper, an artificial atmospheric chamber was set up to investigate the resistance to tracking on a polymer insulating surface at DC voltage application under a reduced pressure. The tests were carried out in accordance with IEC 60112. The test results showed that the dependences of the resistance to tracking for polymer insulating materials upon the decrease in ambient pressure could be classified into three types. One type is for paper-based phenolic laminate and polycarbonate, where the resistance increases with the decreasing ambient pressure; the second is for polybutylene terephthalate where the resistance decreases with the decreasing ambient pressure; the last type is for epoxy resin where the resistance is almost independent of the decrease in the pressure. A Gaussian wavelet analysis was applied to discharge currents to discuss correlations among the resistance to tracking, the discharge energy level and the ambient pressure. It was found that the data of resistance to tracking with some of the materials obtained at altitudes near sea level are inadequate for use in a high-altitude environment. Problems may occur with respect to the safety and reliability of polymer insulating materials when the DC resistance to tracking decreases under low-pressure conditions.

Environmental factors affecting DC resistance to tracking of polyethylene

This paper discusses the environmental factors affecting DC resistance to tracking of polyethylene. The voltage, as determined under the conditions specified in the following, which will cause failure with the application of 50 drops of electrolyte, is used as a measure of the susceptibility of the material to tracking, it is called the CTI (contaminant tracking index). A tracking test was set up to investigate dc and ac tracking resistance of epoxy resin, phenolic resin and polycarbonate by IEC 112. The test solution was with 0.1 % ammonium chloride NH/sub 4/Cl in deionized water, giving a resistivity of approximately 4/spl Omega/m at 23/spl deg/C. A droplet was applied at intervals to keep up the discharge between the two electrodes. Epoxy and phenolic resins are tracking type materials.

Effects of gamma-ray irradiation on dielectric surface breakdown of polybutylene polymers

Polymer materials are required to be used in radiation environments of space and nuclear power stations as insulation materials. For the use of polymer materials in such regions, it must be confirmed whether the electrical performance under radiation environments is different from that without radiation. This paper describes the effects of gamma-ray irradiation on dielectric breakdown of polybutylene naphthalate and terephthalate by applying a DC pulse voltage. Both the polybutylene naphthalate and polybutylene terephthalate were irradiated in air up to 100 kGy and then up to 1000 kGy with dose rate of 10 kGy/h by using a 60Co gamma-source. The total dose of gamma-ray irradiation effects on the time to dielectric breakdown and the discharge quantity of discharge current were discussed. Obtained results show that with increasing the total dose of gamma-ray irradiation, the time to dielectric breakdown increased with polybutylene naphthalate, but decreased with polybutylene terephalate. With increasing the total dose of gamma-ray irradiation, the discharge quantity decreased with polybutylene naphthalate, but increased with polybutylene terephalate. The experimental results suggest that the chemical structure plays a main role in the result of the radiation reaction, which is related to cross-linking and degradation reactions. Dielectric properties are improved by the irradiation for polybutylene polymers which contain more combined phenyls in the main chain.

Effects of low pressure on tracking failure of printed circuit boards

In general, printed circuit boards (PCBs) used in power devices and system operation in low pressure environments are more susceptible to damage from surface discharges. Additionally, repetitive operation of power systems is an important factor leading to tracking failure. Whether the tracking phenomenon at low pressures is different from that at atmospheric pressure needs better understanding to ensure reliable applications. In this paper, studies of tracking failure of PCBs in a low-pressure chamber, are reported for an electrode configuration comprising a pointed end of track close to the side of a track. Test samples were made by printing these tracks onto a glass-cloth epoxy-resin laminate. Experiments were carried out using repetitive unipolar pulses to simulate voltage surges. The ambient pressure was reduced from atmospheric pressure to a very low pressure (0.6 kPa). The time to failure and the cumulative charge passing before failure were recorded as function of the pressure, temperature, pulse interval and insulation distance. The results obtained indicate that the low pressure enhances the breakdown endurance of PCBs and the breakdown phenomena at low pressure are different from those at atmospheric pressure. With the decrease of the pressure, the cumulative charge is increased but the tracking failure is delayed. At low pressures, increasing temperature has little effect on the cumulative charge, but has significant influence on the time to tracking failure.

Recurrence plot analysis of discharge currents in tracking tests of gamma-ray irradiated polymers

Due to the widespread use of polymeric insulating materials in radiation environments, there is an increasing demand to evaluate the radiation effects on the surface dielectric characteristics of polymeric insulating materials. This paper presents a recurrence plot approach to analyze the surface discharge of gamma-ray irradiated polymeric insulating materials based on the tracking test described in IEC60112. Because the resulting comparative tracking index has wide variation, an attempt has been made to evaluate the resistance to tracking more consistently. In this research, for different dosages of gamma-ray irradiation, the discharge currents are detected when the discharge occurs on the sample surface. Recurrence plots of the discharge currents are derived. It is found that the resistance to tracking could be projected on a map as a function of the dosage of irradiation. The recurrence plots are sensitive and give visual methods for identification of the dosage of irradiation effects on the resistance to tracking. Results obtained show that with the increase of dosage of irradiation, the resistance to tracking of polybutylene terephthalate decreases, but increases for polyethylene terephthalate.

Phenomena and mechanism of electrical tree in silicone rubber

In this paper, room temperature vulcanized (RTV) silicone rubber (SiR) was employed as test sample to investigate the tree aging phenomena and mechanism of silicone rubber under commercial voltage. The structures and growth characteristics of electrical tree in SiR were observed by using a digital camera. A new parameter, the treeing proportion is introduced to describe the electrical tree propagation characteristics. Obtained results show that the tree channel of SiR composed of silicone compounds is non-conductive which is quite different from the conductive or semi-conductive carbonized channel within PE and XLPE. There are four kinds of electrical tree (few branch, branch, bush and mixed configurations) in RTV SiR under the commercial voltage, in which bush tree and branch tree take up a great proportion, and the probability of the appearance of bush tree increase with the applied voltage. In addition, three phases (initiation, rapid propagating and stagnation) of treeing propagation process are presented. The electrical tree propagates rapidly right after tree initiation, and the poor conductivity of electrical tree may have a great effect on the appearance of stagnation.

Effects of interfacial pressure on tracking failure between XLPE and silicon rubber

XLPE power cables are increasingly used in power distribution system for their excellent mechanical and electrical performance. However, interface between XLPE and silicon rubber in cable joint remains one of the weakest part of the cable system, because aging and relaxation of silicon rubber may reduce the interfacial pressure and then cause interfacial discharge and tracking failure. Therefore, this paper investigates the effects of interfacial pressure on interface discharge and tracking failure at the interface. Test samples were made by pressing a piece of XLPE and silicon rubber together under different interfacial pressures. AC voltage was applied on a pair of needle-plate electrodes at the interface with the insulation distance of 10 mm. Both the initial discharge voltage and the time to tracking failure were recorded with variation of the interfacial pressure and the applied voltage. Meanwhile, the discharge light and the carbonized configuration were also captured by using a high-speed camera. In order to quantify the light and the carbonization, the methods of image processing and fractal dimension (FD) were employed to establish the relationship between the quantitative characteristics of tracking process and the interfacial pressures. The results show that both the initial discharge voltage and the time to tracking failure increase with the increase of interfacial pressure. With increasing the interfacial pressure, the FDs of discharge light and carbonization path show the decreasing tendency, which indicates that the higher interfacial pressure can effectively delay the interfacial discharges and the tracking failure.