Treeing in mechanically strained h.v.-cable polymers using conducting polymer electrodes

An improved laboratory technique for the measurement of tree initiation voltages in polymeric insulation is described, which allows a closer approach to the conditions in extruded cable insulation. The potential importance of using divergent-field electrodes fabricated from polymeric cable screen compound is demonstrated by a reversal of the order of merit of two polymers when screen compound and aluminium are used as electrodes. Reductions in tree initiation voltage of up to 40% have been created by bending the test specimens to achieve mechanical strain levels, believed to be in the range 5¿8%. The percentage reduction in tree initiation voltage appears to correlate with mechanical stress rather than strain, showing the importance of the modulus of the polymer. Any problems due to strain at the flexible-insulation/electrode boundary of this new test specimen arrangement are likely to be relevant to practical cables.