Measurement of transmission line parameters of three-core power cables with common earth screen

In power cables fast transient signals arise because of partial discharges. These signals propagate to the cable ends where they can be detected for diagnostic purposes. To enable optimal detection sensitivity and to judge their severity the propagation parameters Z(c) (characteristic impedance) and g (propagation coefficient) need to be known. A three-core power cable with a single metallic earth screen around the assembly of the cores has multiple, coupled propagation modes with corresponding characteristic impedances and propagation coefficients. This paper presents a practical method to measure and analyse the cable parameters. The propagation modes are decoupled into a modal solution. The modal solution is interpreted in terms of convenient propagation modes: a shield-to-phase (SP) propagation mode between conductors and earth screen and two identical phase-to-phase (PP) modes between conductors. The measurement method, based on a pulse response measurement, to determine all transmission line parameters of the SP and PP modes is proposed and tested on a cable sample. The model is validated by predicting the time, shape and amplitude of multiple reflections in all modes resulting from an injected pulse.