Clive Reed

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.

Effects of gas phase on tree initiation in polyethylene

A simple model was proposed by two of the authors for tree initiation in polyethylene,1»2 in which the repeated process of charge injection and extraction between the electrode and the dielectric is responsible for tree initiation. It is made either directly between the two or indirectly via a certain gas phase which is considered to exist inevitably between the two. The former process is related to an electronic barrier which electrons are forced to jump over or tunnel through, i.e., the work function of a metal minus electron affinity of a dielectric. This paper deals with the latter process. Two kinds of experiments were carried out. First, a hypodermic needle was inserted into a polyethylene slab so that any kind of gas might be introduced to the tip of the needle. The test specimen referred to as the treeing specimen in this paper was set in a stainless steel cell filled with slightly pressurized SF6. Gases such as air, N2, O2 and SF6 were introduced from an outer gas reservoir to the needle system. Tree initiation voltages, corona pulse height spectra and shapes of the generated trees in their early stage were investigated. Second, a sewing-needle was used facing a polyethylene slab grounded on its opposite side. This is called the free space specimen in the paper for convenience. Corona starting voltages and pulse height distribution characteristics were measured when the stainless steel cell was filled with either air, N2, O2 or SF6 at atmospheric pressure. It is concluded that the existence of a thin gas-layer between a metal electrode and a dielectric will affect the tree initiation voltage in a typical treeing experiment, but that it is not caused by any preceding gaseous discharge. A process may be involved that the work function of a dielectric at its surface is affected by a gas through adsorption or chemical reaction, resulting in the change in the probability of electron injection.

Advances in polymer dielectrics over the past 50 years

The article summarizes the development and progress of organic polymers as dielectric materials and HV insulation over the period 1963 to 2013, in the United States and elsewhere in the world, an area with which John Tanaka was associated throughout his career.

Call for nominations - 2016 Thomas Dakin distinguished technical contributions award

Every two years, the IEEE DEIS selects and individual for their outstanding, original technical contributions in advacing science and techonology of dielectrics and electrical insulation. This award, which was first given in 1978, is named in honor of the late Thomas W. Dakin, one of the most distinguished scientist in this field.

Self-healing in polymer nanodielectrics

Self-healing, using applied ac or dc stresses, is a technique effectively used in metalized polymer film capacitors to eliminate defects in the polymer film and achieve increased breakdown stresses and higher commercial operating stresses and energy densities. Novel approaches for achieving self-healing in nano dielectrics, by modification of the chemical structure of spherical nanoparticles, is described and supported by preliminary experimental results. Avenues for further enhancement of self-healing using sacrificial nano capsules is presented.

Dielectric properties of polyetherimide films

The study of the dielectric properties of this polyetherimide shows that several distinct mechanisms are the source of the dielectric losses, depending on temperature and frequency. Water, which plays the dominant role at low temperatures, can be easily removed in an oven at about 100°C. Above the glass transition (∼220 °C), there is a large relaxation peak due to the motion of the main chain; this is a polar polymer. At an intermediate value, a relaxation phenomenon appears whose amplitude decreases by a factor of 2 when it is first heated. The mechanism is not fully understood but it probably involves the terminations of the molecules. The solvent has only a weak effect on these properties, provided there is less than 1 % left in the resin. Though further measurements are needed, especially on dc behavior, it is already possible to consider the polymer as a suitable material for making high-energy density capacitors for use at high temperatures. Since Tg = 220°C, a temperature such as 170°C seems to be quite acceptable for the polymer, the losses being in a reasonable range (D < 0.0025).

Partial discharge inception and extinction studies in polymer film structures impregnated with compressed SF 6 gas

1. Good consistency has been achieved in the discharge and breakdown performance of six-inch size coils wound on a new, specially-designed winding apparatus. 2. Normal probability plots indicate a single mode of breakdowns, partly determined by the conductor edge finish and partly by chance residual particles. 3. The validity of using multiple-sample test coils has been demonstrated. 4. There has been no evidence for conditioning effects on discharge behavior in tests in air or SF6 gas using coils constructed with precision slit aluminum. 5. Discharge inception and extinction are affected by gas pressure in short-time tests; breakdown is not.