Effect of the melt flow direction on the treeing process in polymeric insulation

Abstract

UDC 678:621.315.61 V. A. Volokhin, O. S. Gefle, and S. M. LebedevThis paper presents the results of a study of the effect of the residual mechanical stress formed in solid polymers at the stage of production on the initiation and growth of an electrical tree. It is shown that the time to tree initiation and the time to breakdown of polycarbonate samples can be determined from the results of investigation of treeing parameters.Introduction. One of the main factors responsible for the failure of high-voltage polymeric isolation in a strong electric field is the formation of incomplete-breakdown channels (trees) [1][2][3][4]. As a rule, such channels occur in local regions with the maximum electric-field intensity, for example, near electrode edges and polymer defects or in the presence of roughness on the electrode surface. The tree formation and growth depend on various factors: the measuring voltage, the electric-field inhomogeneity coefficient, the distance between the electrodes, the initial state of the samples, residual mechanical stresses, etc. [5,6]. The formation and distribution of residual mechanical stresses in insulation articles is due to the thermal shrinkage of the polymer, the rate of its cooling, the presence of embedded metal parts, the shape of the article, etc. In addition, the performance characteristics of insulation articles can affect not only technological parameters but also polymer processing methods. For example, in the production of articles by pressure die casting, the distribution of the residual mechanical stress influencing treeing can depend greatly on the polymer melt flow direction [7,8].The purpose of the present work was to study the effect of the melt flow direction on the space-time characteristics of treeing in polycarbonate in a highly inhomogeneous electric field.Experimental Technique. Samples with a point-plane electrode system were made of polycarbonate by pressure die casting in specially developed die molds. During the production of samples with various melt flow directions (Fig. 1), the technological parameters of processing of polycarbonate (melt temperature, excess pressure, and cooling rate) remained constant and the melt flow direction was varied by changing the arrangement of the molding channels in the die molds. Steel needles with a diameter of 1 mm and a point curvature radius r = (7.5 ± 0.5) μm were used as high-voltage electrodes. An aluminum foil 7 μm thick was glued to the lower surface of the samples to ensure contact to the grounded plate electrode. The distance between the electrodes was d = (9.3 ± 0.3) mm, and each set of samples contained not less than 20 pieces.One of the simplest and the most informative methods of determining the internal mechanical stress arising in articles produced from optically transparent polymeric dielectrics after hardening is the polarization optical method [9]. Fringe patterns of residual mechanical stress distribution in samples with various melt flow directions were recorded on the polarization optical setup shown sch...

Propagation of Electrical Trees under the Influence of Mechanical Stresses: A Short Review

“…The conclusions of [51] are in qualitative agreement with those of [43]. The conclusions of [51] were confirmed with polycarbonate samples in a later publication [52], where it was pointed out that the melt flow directions and the fringe patterns may influence the direction and propagation of electrical trees. An indirect evidence of the above is offered in [53], where an investigation of the electrical trees in outer and inner layers of different voltage rating XLPE cable insulation indicated that the growth rate of electrical trees in a 66 kV cable is much larger than that in the higher voltage rating cables.…”

Treeing resistance parameters of polymeric dielectrics

“…Properties of thermoplastic polymers can be changed in wide range not only depending on the type, chemical structure, and sub-molecular structure but also processing parameters [1][2][3][4][5][6][7]. When polymers are under a high temperature for a long time, the destruction or oxidation of macromolecules will take place.…”

“…In doing so, incorrect choice of the processing parameters can result in the deterioration of mechanical and electrical properties of polymeric products. For example, temperature, exposure time of a polymer under the melt and the cooling rate are the most significant factors at production of large-size insulation products by quasi-isothermal method [2,3]. At the processing of polymers by the injection molding such factors are temperature, pressure, and cooling rate [7].…”

“…Therefore the control of the processing parameters is necessary on the stage of the development of new processing technology or new types of products. It is especially important for HV insulators in which thermoplastic materials should possess not only the high mechanical strength but also the treeing resistance [2,3,6]. The study of the influence of the processing parameters on the treeing process parameters in polymeric insulation was the aim of this work.…”