Non-Intrusive Online Time Domain Reflectometry Technique for Power Cables

Commercially available Time Domain Reflectometry technique relies on direct injection of the probe signal into the power cable conductors. This requires disconnecting the cable conductors from the bus resulting in undesirable downtime of the load centres. Couplers have been proposed for galvanic isolation between the power and instrumentation circuits in online reflectometry systems for live cables. However, couplers weaken and distort the injected signal reducing the sensitivity of the system, besides being bulky and intrusive. Another problem in online reflectometry is the unwanted travel of the probe signal upstream of the cable and its reflections. These are the reasons for the limited commercial availability of online reflectometry products. This article presents a novel Online Time Domain reflectometry scheme for detecting local partial damages in live electric cables. The scheme proposes the use of a clamp-on type inductive coupler along with a specially designed narrow-band pulse to reduce signal distortion. An innovative application of the quarter wave transformer principle is proposed to maximize signal travel downstream of the cable and thereby eliminate the echoes generated by the undesired reflections from upstream. The proposed scheme is non-intrusive and easy to install on in-service cables. The scheme is experimentally validated on energized cables in a real distribution panel.