New very low frequency methods for testing extruded cables

A novel digital instrument has been developed for loss-factor measurement. It is program controlled, easy to operate, and has a measurement resolution of tan δ-0.1 Hz=0.01%. The loss angle is determined by a range measurement of crossovers of current and voltage. At 0.1 Hz tan δ has a considerably larger dynamic than at 50 Hz; for this reason tan δ-0.1 Hz is especially suitable for diagnostics of the operational efficiency of plastic cables. Measurements on naturally aged cables showed that there is a definite correlation between increased tan δ-0.1 Hz and reduced breakdown voltage. The tan δ-0.1 Hz measurement carried out at a nominal voltage U₀ allows nondestructive evaluation if the cable has been damaged by long water trees. With a 0.1-Hz test at 3U ₀ for approximately 1 h, serious weak points can be detected in the plastic insulation