A Novel Current Transformer Based on Virtual Air Gap and its Basic Measuring Characteristics

If current transformer (CT) is saturated in operation, it will lead to incorrect current measurements and may cause relay maloperation, which can greatly threaten the safety of power grid. This paper analyzed the basic measuring characteristics of the proposed novel current transformer based on virtual air gap. This method can successfully compensate the distorted secondary current and expand the measuring range. Based on the simplified magnetization curve model, its working state is divided into linear working state, partially saturated working state and saturated state. Furthermore, its basic measuring characteristics are deducted theoretically. The theory is verified by FEM simulation results and prototype experiments. The difference between this method and other methods is also discussed. Compared with CT, this method adopts partial-gapped magnetic core and magnetic field sensor (MFS) for current reconstruction. When it works in rated state, this method is like a normal CT. Once the primary current saturated the small section core, it will be like a virtual gap, making the whole core more difficult to saturate and allowing the measurement of leakage flux. It has the advantages of simple and compact structure and can be embedded into the electronic current transformer (ECT) system. In this sense, it can have a good application prospect. The main limitation is the requirement of high permeability material for accuracy performance.

I. Introduction

Current is an important monitoring quantity for equipment and power networks. Wide range and broadband current measuring and sensing technology is very important for the construction of smart grid.