O.S. Gefle

The barrier effect in dielectrics: the role of interfaces in the breakdown of inhomogeneous dielectrics

The barrier effect in high voltage engineering is usually understood to mean an increase in the breakdown voltage of the insulation gap due to the use of additional insulation layer (a barrier) which is placed between two layers of the main dielectric. This effect has been known for more than 70 years and it is widely applied in high voltage engineering with a needle-plane configuration to increase the breakdown voltage. The noticeable progress in the barrier effect study was achieved over the last 15 years, when applying new composite polymeric materials with specific properties, the so-called nanomaterials, as the barrier materials. An analysis of existing models of the barrier effect is presented. The influences of various factors such as the barrier position in the gap, the ratio of permittivities and conductivities of the barrier and main dielectric, space charge and inhomogeneous polarization are demonstrated in this review.

The barrier effect in three-layer solid dielectrics in quasi-uniform electric field

In this paper, the experimental results of the study of the behaviour of three-layer solid dielectrics in high electric field are presented. The breakdown voltage and the breakdown channel forms depend on factors such as the position of the boundaries between layers in the gap, the ratio of permittivities of layer materials, and the homogeneity of barrier materials. It was found that polarization plays the main role in the barrier effect.

Tree-inception in PMMA with a barrier

The experimental results of a study of the tree-inception phenomenon for three-layer dielectrics in a divergent field are presented in this paper. It is shown that the tree-inception time depends on both the position of the high-permittivity barrier in the insulating gap and the ratio of the permittivities of the barrier material and main dielectric, and that it has a maximum at the optimal barrier position. It is found that the tree-inception length has a minimum value at this barrier position. Good agreement between the coefficient of the local field non-uniformity and the tree-inception time or the initial tree length was found.

Barrier effect in dielectrics

This paper presents results of the barrier effect study in gaseous and solid dielectrics in divergent and quasi-uniform electric fields. The influence of various factors such as the position of the barrier in gap, the permittivity and the conductivity change both on the breakdown voltage (V/sub b/) and breakdown time (/spl tau//sub b/) in the uniform field and on the tree-initiation time (/spl tau//sub i/) in the divergent field was investigated. Application of barriers with high permittivity may result in a significant increase in /spl tau//sub b/. An analysis of existing models of the barrier effect is presented.

Influence of high-permittivity barriers on PD activity in three-layer dielectrics

It is a well-known fact that the breakdown voltage of dielectrics in a divergent field may be increased by using an additional insulating layer (a barrier) placed at the optimal position between the electrodes. This paper is devoted to the investigation of the barrier effect in solid dielectrics in a quasi-uniform electric field, i.e. the study of the relationship between partial discharge (PD) activity and the barrier position in the insulating gap. It is established that the PD parameters depend on the barrier position in the gap. In particular, the ignition voltage of critical PDs is increased by more than 50% at the optimal barrier position compared to that for other barrier positions.

The breakdown study of three-layer solid dielectrics

The objective of this paper is to study the influence of the lengthening the path of the breakdown channel due to increasing the tangential component of the electric field vector at the interfaces on the breakdown voltage of the laminated dielectrics.

Application of new composite materials for the field control in solid dielectrics

Composite materials with required electrical properties are widely used in HV constructions such as HV cables, HV insulators, HV lead-ins, HV machines etc., for the electric field control. These materials should have reasonable rheological and technological characteristics and they should be processed by the same way that a basic insulation material (for example, should be processed by co-extrusion). This paper is devoted to the experimental study of the application of composite materials made on the basis of polymeric materials such as synthetic rubber and LDPE filled with different fillers. These new materials were applied as an additional dielectric layers (barriers) in solid dielectrics to increase the breakdown voltage and life time of insulation.

Conductive carbon nanotube-reinforced polymer composites and their characterization

New electrically conductive composites reinforced by single-walled carbon nanotubes were studied in this work. The main properties of conductive composites, which were developed, such as electrical (dielectric spectroscopy and volume resistivity), mechanical (elongation at break, Youngs modulus, yield strength), and rheological (melt flow index (MFI)) were evaluated. Loading single-walled carbon nanotubes into carbon black/polymer composites lowers the volume resistivity of such composites. The application of a small quantity of single-walled carbon nanotubes in carbon black/polymer composites allows reducing the carbon black content and improving the rheological and processing properties. The conductive polymer composites developed as part of this study can be processed by conventional processing methods such as injection molding or extrusion. Ternary conductive composites can be applied as materials with improved processing properties in electrical engineering.

Relationship Between the Breakdown Strength and Low-frequency Dispersion of the Complex Permittivity of Polymeric Dielectrics

The relationship between the low-frequency dispersion of the complex permittivity and the breakdown strength of polymeric dielectrics was studied in this work. It was found that the minimum of the voltage-time characteristic of polymeric dielectrics that were studied is related with the dispersion of the complex permittivity. This allows us to predict the dielectric strength of polymeric dielectrics by means of the dielectric spectroscopy method. Good agreement between the experimental results and the model predicted data was found.

Effect of the Mechanical Strain on the Treeing Phenomenon

The influence of the mechanical strain on the tree inception and growth in polymeric dielectrics such as polymethyl methacrylate, polystyrene and polycarbonate was studied in this work. The tree inception and growth process in transparent dielectrics was monitored by the optical method in white and polarized light in the divergent field under AC voltage. It has been found that the tree-inception time and the tree-growth time are dependent on a residual mechanical strain in the vicinity of the needle tip and in the sample volume. The tree inception-time may be increased by means of the control of the mechanical strain in the vicinity of the needle tip. Not only the tree-inception and growth time but also the tree shape depends on residual mechanical strain.