Magnetic probing of concentric neutrals in energized underground power distribution cables

In this paper, we present the design of prototypes and experimental results from implementing these prototypes to demonstrate a novel online diagnostic technique for probing the integrity of concentric neutral wires (CNs) in underground power distribution cables. Our technique uses anisotropic magneto-resistive (AMR) magnetic sensors that rotate around the circumference of the cable and record the corresponding emanating magnetic field. This magnetic field is a function of the currents passing through the CNs (as well as the central conductor, CC), and can be used to detect signatures of damaged (non-conductive) CNs. We describe the implementation of the prototypes, including the radios that ensure electrical isolation between the data repository (a laptop computer), and the energized cable. We further show experimental results from applying the prototypes to detect fabricated faults on cables in our lab underground cable test-bed under various CN current conditions. The results show that we can detect breaks in CNs in with 1 A_RMS return-current per CN, and at least up to 95 feet from the failure side (the maximum length of the cables in our test-bed).