Inductive sensor concept for partial discharge monitoring of power cables based on a coil with a magnetic flux concentrator that is less susceptible to saturation effects

Inhalt:
One suitable method for online condition monitoring of power cables is the measurement of partial discharges (PD). Common sensors for detecting PD are high frequency current transformers (HFCT). Installing an HFCT around the entire cable, rather than just the shield, provides high sensitivity but requires a solution for saturation of the ferromagnetic core due to high rated currents. Saturation of the core leads to a significant reduction in measurement sensitivity for high-frequency PD pulses. While air core coils, such as Rogowski coils, generally do not have this problem, the PD measure-ment sensitivity of air core coils is considerably lower. This publication presents an innovative sensor concept for PD measurement that combines the advantages of HFCTs and air core coils. The concept is based on constructing the secondary winding with a half-ring ferromagnetic core. A ferrite with a high relative permeability, as used for HFCTs, is used for the half-ring so that the magnetic flux is concentrated by the material. This novel sensor concept is modelled and analyzed with an equivalent circuit diagram as well as a numerical field calculation. The models are validated with measurements with a prototype of the sensor. Overall, a meas-uring sensitivity comparable to that of HFCTs with toroidal cores is achieved with this concept. At the same time, the core material is not saturated at typical rated currents in the medium voltage networks, so that the linearity of the PD measurement is maintained.