RF transmission-line methods for in-service probing of concentric neutral wires in underground power distribution cables

Failure of concentric neutral (CN) wires in an underground power distribution cable can drastically reduce its useful life. New on-line approaches are needed to assess the deterioration of the CNs and to prevent unscheduled service disruptions. In this paper, we present new methods for probing the integrity of CN wires in underground power distribution cables using RF-waves. The primary advantage of our techniques compared to traditional CN diagnostics is the easy of applying our methods to energized underground power distribution cables. This paper briefly describes two methods for transmitting RF-waves through underground cables: a surface RF-wave, also called the Goubau Wave (GW), guided by all the CNs within the cable, and an RF-wave coupled to a two-wire transmission line composed of adjacent CNs. We focus on the use of GW to probe for CN degradation, due to good coupling of the wave to the underground cable, We show experimental results demonstrating the change in the signature of the transmitted GW signal through a section of degraded (broken) CNs.