Modern Solutions to Stabilize Numerical Differential Relays for Current Transformer Saturation during External Faults

Modern numerical relays use improved measuring and stabilizing methods and can therefore tolerate current transformer (CT) saturation to a large extend. Dimensioning of CTs for short time to saturation however requires precise consideration of the transient CT behaviour in the first milliseconds after fault inception. Therefore, traditional calculation and dimensioning guidelines have to be reviewed and completed accordingly. Initially the paper discusses CT dimensioning and specifications that need to be considered for modern numerical relays. Typical CT dimensioning rules are given, based on theoretical calculation and test series. Then, the paper will discuss modern algorithms for fast detection of CT saturation and methods of adaptive measurement and stabilization that are available today in some modern numerical relays