The non-linear electrical behavior of zinc oxide, used to practical advantage in surge arresters on distribution power systems, has been replicated in epoxy resin, which has been filled with an appropriate volume fraction of this intrinsically semi-conducting material. Comparison has been made with epoxy resin containing a similar volume fraction of aluminum oxide in order to establish that the non-linearity observed in zinc oxide filled resin is a function of the zinc oxide itself and not simply an artifact resulting from the presence of any filler and the interfacial phenomena thereby arising. Previous work in this field, in both thermo-setting and thermo-plastic insulation has concentrated on dc voltage application. This work, in addition to extending the range of host materials to include epoxy resin using dc, has also explored the ac behavior. The results demonstrate that the inclusion of zinc oxide in epoxy resin is capable of producing non-linear conductivity when the filler volume fraction exceeds the percolation threshold ( /spl sim/ 15%).

Nonlinear dc and ac conductivity in electrically insulating composites

The addition of a nonlinear filler in the form of particulate zinc oxide to insulating materials has been found to produce a marked nonlinear conductivity in the resulting composite. While retaining its excellent insulating properties at low electric fields, this composite material acquires a measure of conductivity at high fields. The paper investigates the influence of this phenomenon on a number of characteristics of insulating materials, specifically, bulk conductivity, surface charge dissipation, electrical treeing and tracking. Models have been developed for simulation of the effects of adding the nonlinear filler to polyester resin. A good correlation exists between the theoretical models for surface charge dissipation and treeing resistance, modified to incorporate the nonlinear characteristic, and experimental data derived from polyester resin containing a range of concentrations of zinc oxide. For tracking, a contrary result was obtained which has been attributed to changes in the mechanical characteristics of the filled material. An improvement in the dissipation of surface charge and treeing resistance has been demonstrated without loss of the material's insulating properties under normal electric fields.

Improvement in electrical insulators by nonlinear fillers

The use of ferroelectric fillers in polymers can allow the formulation of a composite material which possesses enhanced AC currents at elevated electrical field strengths. This non-linear characteristic can be utilised as a stress relieving mechanism in applications with non-uniform electric fields. The enhanced characteristics are achieved through electrical field dependent permittivity mechanisms, which are occurring within the composite material. These stress relieving materials have distinct advantages over previous stress relieving polymer composites, using conductive or semi-conductive fillers, however, stress relief is only realisable under AC fields. The ferroelectric material used in these investigations is barium titanate, which has been used as a filler in an acrylic base resin system. The experimental results show how both the AC and DC conductivities are influenced by the electrical field strength, at a variety of filler concentrations. The research has also looked into how the AC and DC characteristics change as the degree of cure of the resin system is increased and how the particle size of the barium titanate influences the behaviour of the material.

Non-linear ferroelectric composite dielectric materials

Nonlinear composite dielectric materials can be formed through the inclusion of appropriate fillers in polymers. Depending on the type of filler used, the composite material can then possess either an electrical field-dependent conductivity (FDC) or an electrical field-dependent permittivity (FDP). These nonlinear properties can then be used for the purposes of electrical stress relief in insulation systems with divergent electric fields, reducing the intensity of highly stressed regions. FDC polymer composites are formed through the inclusion of narrow-band gap semiconductive fillers in a polymer matrix and can be used for electrical stress relief in both AC and DC applications. FDP materials on the other hand use ferroelectric fillers in polymers; however, the stress-relieving properties of these materials are only realisable under AC fields. The paper reports on the electrical characterisation of both of these stress-relieving materials, with the criteria required for them to function. The materials that have been studied in these investigations take the form of zinc oxide in epoxy resin and barium titanate in an acrylic resin, respectively. Improvements in thermal conductivity are also reported as a consequence of the introduction of these fillers.

Nonlinear fillers in electrical insulating materials

A new perception of interface charge accumulation in HVDC cable joints was inspired by novel measurement methods of PEA and Q(t) and quantum chemical calculations.

Space charge accumulation in double-layer dielectric systems—measurement methods and quantum chemical calculations

The very low conductivities of modern insulating materials do not permit the dissipation of accumulated space charge. Space charge can therefore persist both within the bulk of an insulator and on its surface for long periods of time. This can lead to regions of intense electric field in the surrounding dielectric, producing localised breakdown resulting in the erosion of surfaces and penetration of the bulk by electrical trees. A nonlinear characteristic has been produced in the conductivity of polyester resin by the introduction of an appropriate quantity of particulate ZnO. The critical filler/resin ratio has been determined above which the insulation resistance is high at low fields but decreases as the electric field increases. Alternative fillers, such as Al/sub 2/O/sub 3/ and SiC, have been incorporated into the resin for comparative purposes and to elucidate the conduction mechanisms. The enhanced conductivity at high fields in ZnO-filled polyester permits the dissipation of accumulated space charge at the extremities of electric tree channels as they impinge on barriers of this material, thereby reducing the local electric field. As a of this phenomenon, tree growth and penetration are inhibited, leading to extended lifetimes.

D.W. Auckland

Papers

Nonlinear fillers in electrical insulation