Partial discharge propagation in three-core cables with common earth screen

Online partial discharge (PD) detection and location in cable systems is a valuable tool for the estimation of the condition of the system. Different types of cables are encountered in medium-voltage (MV) connections and appropriate models must be constructed to predict PD signal propagation. For three-core power cables with a single common Earth screen a multiconductor transmission line (MTL) model is required. This paper presents an MTL model of a such a cable, including interpretation of the decoupled solution as convenient propagation modes. A complicating factor in the estimation of the transmission line parameters of these propagation modes is the presence of semiconducting layers. These layers have a significant influence on the transmission line parameters. Unfortunately, the dielectric properties of these layers are usually unknown for the frequency range of interest for PD diagnostics. It is shown that nonetheless the characteristic impedance and propagation velocity can be estimated using information available in most cable specifications. The estimated values are validated using pulse response measurements on a cable sample.