Summary of Ieee C37.110-2023 Guide

This paper summarizes the IEEE C37.110-2023 guide, which outlines the characteristics, behavior, and proper application of current transformers (CTs) in protective relaying systems. It explains how CTs operate under steady-state and fault conditions, and how factors such as core saturation, DC offset, and remanent flux can impact the CT’s secondary current accuracy and performance. Key topics include CT accuracy classes, excitation characteristics, knee-point and saturation voltages, and methods to evaluate CT performance under asymmetrical currents. The guide offers practical insights into CT ratio selection, burden calculations, and transient performance, including time-to-saturation. Application-specific considerations for electromechanical, static, and microprocessor relays are discussed, highlighting CT behavior across various protection schemes - such as generator, transformer, bus, and line protection. Additional topics include series and three-phase CT connections, auxiliary CTs, and bus configurations. Challenges posed by geomagnetic-induced currents (GICs) are also addressed. The guide also provides an overview of optical current sensors as emerging alternatives. By bridging fundamental concepts with CT application practices, the paper offers relay engineers essential guidance for reliable and secure CT performance in modern power systems.