.IntroductionThe conventional manner of measuring tan 6 of cable does not take account of the contribution from the semiconducting layers and the contact resistance.We established the method to separate tan 6 component of insulation layer (tan Ape) from tan 6 versus frequency characteristics measured in cable configuration.Tan 6 pe gives very accurate information about oxidation in insulation layer. Tan 6pe takes a maximum value at near -20'C in the aged cables,which is corresponding to It BII peak in the dielectric dispersion of oxidized PE. These results will be the basis for a new method of the cable diagnosis.Additionally, the application of distorted wave voltage was shown to be effective for field measurement of tan 6frequency characteristics.2.Tan 6frequency characteristics in the XLPE cable Tan 6frequency characteristics from 60 Hz to 400 Hz are shown in Fig.1.Specimens are XLPE cables removed from 11 years service and brand new ones.It seems that tan 6 in new cable and wire-shielded cable increases linearly with frequency, though tan 6 in tape-shielded removed cable has a hump.The equivalent circuit shown in Fig.2 was proposed for XLPE cable taking account of the contact region between the shielding conductor and outer semiconducting layer. ( 1 ) Eq. (1) is obtained from Fig.2.where tan 6pe is nearly constant in the range of 60 -4OOHz.Eq.(l) gives the tan &-frequency characteristics with a hump at W =I/CsRs which agrees with the feature of tape-shielded removed cable.
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