AMPLIFICATION OF ELECTROMECHANICAL STRESSES BETWEEN CLOSELY PLACED WATER MICRO-INCURRENTS IN THE INSULATION OF HIGH-VOLTAGE CABLES UNDER THE INFLUENCE OF AN ALTERNATING ELECTRIC FIELD

Abstract

For crosslinked polyethylene insulation of high-voltage and ultra-high-voltage cables, a mathematical model of its electromechanical degradation near water micro-inclusions has been developed based on a combination of electrostatic and mechanical analysis. The finite element method has been used to simulate the distribution of field strength and Maxwell pressure near single defects of spherical and ellipsoidal shape, as well as pairs of closely located inclusions for 110 and 330 kV cables. It has been shown that the elongation of the defect along the field at a fixed maximum size of 50 μm increases the maximum field strength from 3 to 35 times, and the maximum Maxwell pressure in these cases is from 0.09 to 12.2 MPa, i.e. up to ~67% of the accepted tensile strength of crosslinked polyethylene. As a result of the study of the interaction of two closely spaced inclusions, it was found that the convergence of spherical defects at a distance of up to 2 μm increases the maximum pressure on the insulation to 2.82 MPa, and for elongated ellipsoids 50×8.3 μm at a mutual distance of 2 μm the maximum pressure reaches 34.9 MPa, which is almost twice the strength limit of the insulation. It is shown that the most critical for cable insulation are groups of closely spaced, elongated along the field of water micro-inclusions, near which local cyclic stresses due to field amplification become sufficient both for the instantaneous destruction of thin areas of the dielectric between the inclusions and for long-term fatigue damage to the insulation, which leads to the formation and growth of water triangulations even at permissible average electrical loads. References 10, figures 2.