Contactless Current Measurement for Enclosed Multiconductor Systems Based on Sensor Array

How to measure ac currents in a bundle of inaccessible, enclosed conductors has been a challenging problem but with many potential applications. This paper presents a method to solve the problem using an array of magnetic field sensors. The proposed method consists of two novel ideas. The first idea is to use an off-site calibration method to establish sensor parameters including sensing position and angle, which provides more accurate sensor parameter estimation than their nominal values. The second idea is to “measure” (i.e., calculate) conductor currents and positions based on the sensed magnetic fields and preestablished sensor parameters. This process is simplified since the sensor information is obtained by the first idea. Both calibration and measurement tasks are formulated as nonlinear least square problems and solved efficiently. The proposed method is demonstrated for the cases involving ideal three-conductor system and practical residential service cable enclosed in a plastic conduit. The method has the potential for contactless current measurement of Romex cables and overhead distribution lines.

I. Introduction

Current sensing is one of the basic measurement techniques for modern power systems, ranging from residential, industrial to utility practices [1]. How to measure currents in a bundle of conductors enclosed in a structure has been a challenging task but with great potential applications. For example, Romex cable has two or three conductors enclosed in a plastic cover. It has been difficult to measure the currents of these conductors without isolating the conductors first. A method to estimate currents for such cables using external sensors was patented [2]. Contactless measurement of overhead line currents encounter similar challenges. In this case, magnetic fields measured by an array of sensors are used to estimate currents of distribution feeders [3]. Recently, the need to monitor home energy use by home owners has motivated the research on measuring currents in three conductors enclosed in a plastic conduit [4], as a key enabling technique for nonintrusive load monitoring (NILM) [5].