High Frequency Signal Propagation in Solid Dielectric Tape Shielded Power Cables

High frequency signal propagation in shielded power cable is important to many cable diagnostic techniques including time domain reflectometry, partial discharge location, etc. The signal propagation characteristics, including electromagnetic propagation velocity, cable characteristic impedance, and attenuation are well studied in power cables with concentric neutral wires or solid metallic shield. However, high frequency signal propagation in tape shielded cable is more complicated as conduction at the tape laps tends to decrease with cable aging, which forces more of shield current to spiral down the tape shield. The cable inductance increases with the fraction of the shield current in circumferential direction, which affects all the signal propagation characteristics. High frequency signal propagation characteristics of tape shielded power cable were studied using a custom manufactured 15 kV cable sample which has a PET tape intercalated with the copper tape shield to eliminate conduction at the overlap, which simulates the effects of corrosion in aged tape shielded cables. The inductance of the cable sample was computed as a function of fraction of shield current following a spiraling path along the cable, as were the signal propagation velocity, and the cable characteristic impedance, all of which are in reasonable agreements with measurements. Calculations and measurements of high frequency attenuation indicate that large eddy current and “transformer effect” losses in the ground shield semicon dominate once a substantial fraction of shield current spirals down the tape.