Air-Gapped Current Transformer simulation and accuracy assessment

The implementation of smart grids makes it necessary to have reliable and efficient measuring devices. Current sensors used for metering purposes in the grid must be accurate enough to overcome new challenges that power networks face such as injection of undesirable Direct Current (DC) in the system. A Current Transformer (CT) is a transformer widely used to measure current in an electrical circuit. This paper describes the modelling and simulation of a CT with a variable air gap, provides different results of cases with different air gap ratios and evaluates the respective primary and secondary currents in each case according to the standard IEC for instrument transformers. Results show that if an air gap is present, the transformer only reached an accuracy class of 0.5 when the air gap ratio is 0.001.

I. Introduction

A CT is an elemental device in power networks, providing a way to isolate a current in the system either for protection or for measuring purposes in the grid. The main function of CTs is to deliver an accurate representation when a primary current is introduced, however this is not always possible. That is why the behaviour of CTs in power networks has been studied for several years, with a special interest in the saturation of CTs due to their magnetic properties. CT saturation may lead to an inaccurate representation of the primary current, which could include amplitude error and phase error. Both will cause incorrect operation of protection devices or measuring equipment.