A Coreless Current Probe for Multicore Cables

Abstract

To measure the current on each conductor in a three-phase three-core cable or a three-phase four-core cable without an armor or the armor that is made of nonmagnetic material, one has to remove its sheath/armor and applies a traditional device, such as a current transformer (CT) to each conductor, which is invasive and inconvenient. In this article, a Coreless current probe, composed of a tunnel-magnetoresistance (TMR)-based magnetic sensor array, is presented to nondestructively achieve current measurement for multicore cables. The conductor positions, incline angles, and current phasors of a multicore cable are simultaneously identified by solving a nonlinear least square (NLLS) problem. The current waveforms are reconstructed in real time through a real-valued matrix multiplication using the measured time-domain TMR sensor voltage signals. Both simulation and experiment are performed to validate the feasibility of the proposed method. The simulation study is first conducted and demonstrates that a maximum relative current amplitude error of less than 5% and a maximum phase error of lower than 4° can be achieved, and three-phase current waveforms with many harmonics are also successfully reconstructed by simulation. A prototype Coreless current probe is designed and fabricated. The experimental study involving fundamental current phasor measurement and current waveform reconstruction is subsequently performed on a three-phase three-core cable. The experimental results indicate that the maximum relative current amplitude error and phase error, respectively, have average values of 5.44% and 2.58°. Meanwhile, three-phase current waveforms with many harmonics generated by three types of appliances are also successfully reconstructed.