Compensation of an Air-Gapped Current Transformer Considering the Hysteresis Characteristics of the Core

An air-gapped current transformer (CT) has been used to reduce a remanent flux in the core, particularly in the case of auto-reclosure. However, it cause larger transient, ratio and phase errors than the iron-cored CT because of the small magnetizing inductance. This paper presents a compensation algorithm for the air-gapped CT using the hysteresis characteristics of the core during the fault conditions including auto-reclosure as well as in the steady-state. The core flux is calculated from the measured secondary current of the air-gapped CT and inserted into hysteresis loop to estimate the exciting current. Finally, the correct current is estimated by adding the measured secondary current to the estimated exciting current. Various test results clearly indicate that the proposed compensation algorithm can improve the accuracy of the air-gapped CT significantly and reduce core cross-section of the air-gapped CT significantly.