The theory of CT saturation detection based on virtual impedance in bus-bar differential protection

Abstract

The serious short circuit current may possibly lead to current transformer (CT) saturation. If the CT saturates in an external fault, the bus-bar differential protection may operate incorrectly. Most bus-bar differential relays use low-impedance criterion to detect CT saturation to avoid false tripping for a nearby external fault. It is very important research direction for accurately detecting CT saturation in bus-bar differential protection. Analyzing CT saturation detection methods of the current waveform, the variety of differential current, harmonic ratio, and so on, these methods are not best solutions for serious CT saturation. The paper proposed a new method for CT saturation detection. The method is to suppose a virtual impedance branch to the power bus bar, then to calculate the value changes in the virtual impedance with the transmission line RL integral model, to detect CT saturation when an external or internal fault occurs.