A Magnetic Core Permeance Model for Inductive Power Harvesting

A current transformer (CT) is a preferred device to harvest power in the high-voltage utility grid to power monitoring equipment. Widespread CT use is foreseen in future smart grid deployment. The most common CT use involves a laminated transformer iron split core installation around a high-voltage cable with a particular number of turns to power a load or burden. Less common is a gapless core installation to improve reliability and prevent CT degradation by avoiding gap corrosion. Even laminated high resistivity cores still suffer from eddy current loss. The goal is to design a core able to deliver the desired power at optimum number of turns, minimized dimensions, and minimal loss. This article focuses on the core permeance as a complex parameter, on core size, losses or efficiency, load impedance optimization, flux level, power factor performance, and the relationship between these parameters. A similar size split core shows reduced loss compared to a gapless core. Lossy cores can still operate relatively efficiently at low flux levels indicating that the load current should preferably approach the short circuit current. Notwithstanding nanocrystalline and ferrite material advantages, silicon-doped laminated iron remains the preferred choice for 50/60 Hz inductive power harvesting.