Toshinao Takeda

Space charge behavior in XLPE cable insulation under 0.2-1.2 MV/cm dc fields

This report deals with space charge behavior in PE (polyethylene) under dc fields. Direct observation of time-dependent space charge profiles in 3-mm thick XLPE (crosslinked low-density polyethylene) cable insulation under dc electric fields was performed using the pulsed electroacoustic method. Stable hetero charges were formed when the field was as low as 0.2 MV/cm, and intermittent generation of packet shaped space charges and their propagation through the insulation were observed when the field was as high as 0.7 MV/cm. These phenomena were reproduced in sheet specimens of XLPE and LDPE (low-density polyethylene). It was found that hetero charges resulted from heat treatment of the XLPE specimen containing antioxidant and acetophenone, which is one of the crosslinking by-products, suggesting dissociation of the antioxidant through solvation at high temperature by acetophenone. The packet charges were easily detected when acetophenone was diffused into the LDPE specimen. However, uniformity of acetophenone distribution prevented the packet charge generation. It is suggested on the basis of several experimental results that local ionization of impurities in the insulation through solvation by acetophenone takes place assisted by high field and leads to the packet charge generation. A numerical simulation was carried out based on the above model.

Origins of thermally stimulated current in polyethersulfone

In order to understand the dominant carrier species in electrical conduction in polyethersulfone (PES), thermally stimulated current (TSC) measurements were carried out under various conditions. It was found that PES has two TSC peaks in the temperature range from 20 to 220/spl deg/C, consisting of an /spl alpha/ peak at /spl sim/210/spl deg/C and a /spl beta/ peak whose peak temperature moves towards a higher temperature with an increasing poling temperature. Even when the sample was not poled, PES shows a spontaneous current with its peak at /spl sim/210/spl deg/C. Both the spontaneous current peak and the /spl alpha/ peak were found to disappear when the sample had been heated to 230/spl deg/C, keeping the external circuit closed. From such similarity of appearance and disappearance between the two peaks, it is concluded that they have the same origin. From the change in the spatial distribution of space charges inside the sample measured simultaneously with the TSC measurements, positive charges, probably due to K/sup +/ ions, existed nonuniformly in the sample from the beginning, and are thought to be responsible for the two peaks. From a similar study, the /spl beta/ peak is considered to be due to polarization of ionic space charges during the poling process.

Dominant conduction carriers in polyethersulfone

With the development of modern technology, the working condition for insulating materials has become even severer. Electrical insulating materials are demanded to have even higher reliability. Good thermal endurance has also become very important in recent years. Polyethersulfone (PES) is a new engineering thermoplastic material which can be used continuously at 180/spl deg/C thanks to its high glass transition temperature of 225/spl deg/C. It also has the highest-class resistance for chemicals among amorphous polymers. Therefore, PES is expected to be applied in electrical insulation as well as in various other industrial fields. In this paper, dominant conduction carriers in polyethersulfone are discussed through the experimental results obtained mainly by the thermally stimulated discharging current (TSC) and dc conduction current.

Development of all solid compact connection system for high voltage equipment - effect of thin air layer on breakdown strength of solid - solid interface

In order to reduce construction cost of substations and to become more environmentally benign, we have been proposing the All Solid Insulated Substation as a next generation substation, in which the high voltage section like busbars and transformer windings are insulated by the solid insulating materials such as epoxy resin and silicone rubber. In the All Solid Insulated Substation, compact connection systems between each piece of power equipment have to be developed for the reduction of the volume and area of the substation. From these viewpoints, this paper introduces the concept of the All Solid Insulated Substation and describes basic experimental investigation for the development of the compact connection system.

Interpretation of the relation between residual charge and water trees in XLPE cables

Crosslinked polyethylene (XLPE) insulated cables working in water are known to be easily degraded by forming water trees. This is one of the most serious concerns among users of XLPE cables. The possibility whether measurement of residual charge is usable as a tool to detect water trees in XLPE is discussed. Two types of samples were used; one is an XLPE cable with water trees and the other is an XLPE sheet with water trees grown by AC voltage application. The sheet sample was used since we thought that we could easily check the growth of water trees and could also use a generator of a small capacity, which enabled precise experiments. The results obtained are summarized as follows. (1) Residual charge was not measured in the sheet sample because of its small electrode area. (2) Residual charge was detected in the cable sample. (3) A clear frequency dependence of the residual charge was not observed. A model of the residual charge measurement is proposed, where the charges accumulated in a water tree are oscillated by AC electric field and are diffused through the water tree path with a high conductivity. The frequency dependence of the residual charge is discussed by dividing it into the contribution of the electric field distribution, that of the detrapping process, and that of the carrier transportation process into the electrode. As a result, it is confirmed that the weak frequency dependence of the residual charge is reasonable.

Correlation between space charge distribution under DC voltage and dielectric breakdown properties in XLPE under impulse voltage superposed onto DC voltage

Describes breakdown tests under DC long term application measuring space charge in XLPE. The time transition of space charge showed that dielectric breakdown cannot be explained by only the local maximum electric field in the bulk, but should be considered as the deterioration of insulating material caused by the increase of carrier density. In order to make a rational insulating design method for DC XLPE cables, space charge distribution immediately before dielectric breakdown under impulse voltage superposed DC voltage is studied. Correlation between the space charge distribution immediately before dielectric breakdown by DC impulse superposed voltage and the dielectric breakdown strength are discussed in this report.

Space charge evolution in XLPE cable insulation under high electric fields

The authors improved the conventional pulse electro-acoustic method to measure the space charge evolution in XLPE cable insulation with applying very high voltage. Using this technique, the space charge distribution was measured applying an electric field of over 1 MV/cm to 3-mm thick XLPE cable insulation. Under high fields, positive charge packets appeared intermittently in the vicinity of the anode and moved towards the cathode. This behavior is supposed to be affected by the change in temperature and material composition. This report deals with the results of space charge measurement at different temperatures, as well as with different material compositions.

A study of the method for evaluating dielectric materials using space charge measurement

This report deals with a study of the method for evaluating DC performance of dielectric materials using space charge and DC leakage current measurement. Two types of sheet specimens were prepared for space charge measurement, that is, simple specimens and sandwiched specimens. The simple specimens gave some information on space charge behavior. On the other hand, the sandwiched specimen made it easy to compare properties of two materials on space charge. Film specimens were used for dc leakage current measurement. Insulating materials may be evaluated in view of space charge behavior using this method.

Space charge behavior and electrical breakdown phenomena of acrylate-graft-polyethylenes

The space charge distribution and the impulse breakdown phenomena of acrylate-graft-polyethylenes, like acrylic acid and n-butylacrylate, were investigated. Heterocharges generated in low-density polyethylene were decreased with the introduction of acrylic acid and changed to homocharge formations with the increase of graft ratio. On the other hand, the space charge accumulation in n-butylacrylate grafted polyethylene remained unchanged for the whole range of the graft ratio. The impulse breakdown strength of the acrylate-graft-polyethylene was increased and changed according to the graft ratio.

Low-temperature TSC peak of polyethersulfone

Through the measurements of TSC and space charge distribution, it is concluded that the low-temperature /spl beta/ peak in TSC and the conduction current in the amorphous thermoplastic polymer polyethersulfone (PES) is mainly due to impurity ions.