Masayuki Nagao

High-field conduction and breakdown in insulating polymers. Present situation and future prospects

We review the results of electrical conduction and breakdown research reported mainly after 1980 in Japan, and comment on the present situation and future prospect of research work in this field. Effects of introduction of polar groups into polymers on electric strength F/sub b/ are summarized. Influences of copolymerization and modification of morphology and chemical structure on F/sub b/ of polyethylene are also reviewed. Finally, we show currently used experimental techniques for direct observation of space charge, prebreakdown current and localized heat generation. >

Dielectric properties of XLPE/Sio2 nanocomposites based on CIGRE WG D1.24 cooperative test results

A comprehensive experimental investigation of XLPE and its nanocomposite with fumed silica (SiO2) has been performed by CIGRE Working Group D1.24, in cooperative tests conducted by a number of members; covering materials characterization, real and imaginary permittivity, dc conductivity, space charge formation, dielectric breakdown strength, and partial discharge resistance. The research is unique, since all test samples were prepared by one source, and then evaluated by several expert members and their research organizations. The XLPE used for preparation of the nanocomposites was a standard commercial material used for extruded power cables. The improved XLPE samples, based on nanocomposite formulations with fumed silica, were prepared specifically for this study. Results of the different investigations are summarized in each section; conclusions are given. Overall, several important improvements over unfilled XLPE are confirmed, which augur well for future potential application in the field of extruded HV and EHV cables. Some differences/discrepancies in the data of participants are thought to be the result of instrumental and individual experimental technique differences.

Detection of Joule heating before dielectric breakdown in polyethylene films

Local Joule heating in polyethylene films was detected at room temperature before dielectric breakdown under DC field. When a high-temperature point appeared in a film, the conduction current increased with time. The final breakdown occurred at the point of the highest temperature. This concurrence of the breakdown point and the Joule heating point suggests that a thermal process takes part in the DC electrical breakdown of polyethylene films at room temperature and that there are weak points leading to the breakdown in the film where the current density is above the average. Almost the same results were observed for ethylene-vinyl acetate copolymer films, although the local Joule heating and the electrical breakdown occurred at lower electric fields than those for polyethylene films. >

Effects of nano-sized MgO-filler on electrical phenomena under DC voltage application in LDPE

A nanocomposite material, composed of nano-sized MgO-filler added to a low-density polyethylene (LDPE), was subjected to electrical property investigations under DC voltage application. The volume resistivity of LDPE, under 40-80 kV/mm DC field at 90 /spl deg/C, increases by the power of 10 due to the addition of only a few percent of nano-filler. The amount of space charge in LDPE under high electric field is reduced and the ratio of stress enhancement by space charge becomes less due to the addition of nano-sized MgO-filler. The DC breakdown strength of LDPE is increased by adding nano-sized MgO-filler. Based on these results, the mechanism where by the nano-sized MgO filler had improved DC characteristics of LDPE was discussed.

New approach to diagnostic method of water trees

A novel method for the detection of high-field AC conduction loss current containing harmonics in polymeric insulating materials is presented. The method is based on the use of the unbalanced operation of a current-comparator-type capacitance bridge and the deconvolution technique of signal processing. This technique was experimentally applied to the diagnosis of water trees in CV power cables by detecting the waveform of the high-field AC conduction current of the cables with and without water tree degradation. The distortion of the current waveform measured for the water-tree-degraded cable appeared to be larger than that observed for nondegraded cables. This result might be related to the nonlinear electric field dependence of the AC conduction loss current around water trees.<<ETX>>

Evaluation of ethylene-propylene rubber as electrical insulating material for a superconducting cable

Mechanical and electrical properties of ethylene propylene rubber (EPR) pieces were studied in comparison with polyethylene (PE) in the cryogenic temperature region to examine the possibility of its use as solid electrical insulating material in superconducting cables. The mechanical properties of EPR are preferable to those of PE mainly because of its low shrinkage. The electric strength and dielectric loss tangent of EPR are comparable with those of PE at liquid helium temperature. Based on the encouraging results, an extruded EPR-insulated cable was fabricated and its 15-m length was cooled down to liquid helium temperature successfully. Partial discharge experiments at that temperature showed good electrical characteristics. This is a breakthrough in terms of the electrical insulation design of cryogenic cables. >

High-field dielectric properties and AC dissipation current waveforms of polyethylene film

The authors have developed a new method for the detection of AC dissipation in dielectric materials under high AC field. Using this technique, high-field dielectric loss properties and their relation to AC dissipation current of low-density polyethylene (LDPE) films are measured. Near room temperature, tan delta of a LDPE film has small electric field dependence. In this temperature region, the dissipation current shows saturation around the voltage peaks in phase even with high-field application. At higher temperatures, however, the high-field tan delta tends to have a strong field dependence, and the AC dissipation current deforms and is nonlinear near the voltage peaks. The nonlinearity of the AC dissipation current decreases under the decrease of tan delta with the voltage application period. These results show that the large increase of tan delta in the high-field and high-temperature region is related to the loss due to DC-like carrier motion. >

A Model for Dielectric Breakdown in Plasma-Polymerized Styrene Thin Films

A new, simple breakdown model is presented as a result of experiments on plasma-polymerized styrene thin films. In this model, the current is controlled by Fowler-Nordheim tunnelling emission from the cathode, and bulk breakdown is governed by an impulse thermal process. The numerically-calculated values, disregarding the space charge, can be fitted to the dependence of the electric strength on temperature, field rise rate and prestress and are also consistent with all the experimental results. Also, steady-state avalanching is considered as a possible mechanism for bulk conduction. It is found that provided the mobility of positive charges is beyond a certain value, hardly any space charge is formed, giving a physical basis for neglecting the space charge. Also, impulse thermal breakdown can occur before a negative resistance appears. The parameters involved in the model are also discussed.

High repetition rate PEA system for in-situ space charge measurement during breakdown tests

In order to investigate transient space charge phenomena, it is essential that the space charge profile be observed at a high repetition rate. We have developed a new space charge measurement system using the pulsed electroacoustic (PEA) method, which can measure the space charge profiles every 10 /spl mu/s. It employs the most recent digitising oscilloscope model and a semiconductor switch. The effect of prestressing on impulse breakdown voltage of a low-density polyethylene sheet was investigated by using the new system. Experimental results suggest that positive charge injection was dominant immediately before the breakdown, and charge injection during the prestressing causes distortion of the electric field near the electrode, and enhances the subsequent charge injection due to the impulse voltage.

Simultaneous measurements of space charge distribution and external circuit current up to electrical breakdown in LDPE film

A space charge distribution and an external circuit current were simultaneously measured up to electrical breakdown in a 100 /spl mu/m thick low-density polyethylene (LDPE) film with a semi-conducting electrode under dc voltage by using pulse electro-acoustic (PEA) method. We estimated an electric field distribution and a conduction current from these experiments. An electric field distortion due to a positive charge accumulation was observed at room temperature near the breakdown field. At 60/spl deg/C and 90/spl deg/C, however, an electric field distortion due to negative charge was observed, but the electric field distortions were not so large even near the breakdown field. The estimated conduction current reached to about 10 mA/m/sup 2/ before the electrical breakdown in each measurements. The obtained results suggest that the LDPE film becomes quasiconductive for negative charge at the field near breakdown and that the electrical breakdown is affected by the conduction current rather than the maximum electric field within the sample in the LDPE film.