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

Abstract

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.

Power Cable Joint Model: Based on Lumped Components and Cascaded Transmission Line Approach

“…The S-parameters are measured for the cable joint for the frequency range of 300 kHz-800 MHz. As discussed in [7], two propagation modes exist in the three-core power cable, namely shield-to-phase (SP) mode between conductors and earth screen and phase-to-phase (PP) mode between conductors. This paper focuses on the SP mode because it is the detectable signal mode at the earth screen of the cable.…”

“…The lumped model is also valid here because the frequency components at the joint refection are within megahertz range. The slight mismatch between the simulation and the measure-ment for the reflection from the adaptor might be because of the adaptor calibration procedure error [7].…”

“…The impedance is 25.8 Ω while the propagation velocity is 186 m/µs. Also the measurement coaxial cable and the adaptor are calibrated as discussed in [7].…”

“…For high frequency signals, the underground power cable can be modeled as a transmission line [4]. References [5]- [7] provide a lumped component model for RMU for high frequency phenomena. However, literature on models for a power cable joint for high frequency signals is relatively scarce.…”

Effect of ground return path on partial discharge signal propagation along single-core and three-core power cables

“…When earth is not considered, signals travel within the cable confined by earth screen. For single-core cables, the propagation refers to the "coaxial mode" (signal propagates within cable, between earth screen and conductor) [2,3] while for three-core cables with symmetrically arranged phase conductors, two distinct modes are found from applying modal analysis [4]. However, earth is part of the transmission line model.…”