Numerical busbar differential protection using generalised alpha plane

Abstract

This study presents a digital differential busbar protection scheme based on generalised alpha plane approach which combines the benefits of percentage differential approach and two-restrain alpha plane approach. The proposed scheme utilises one cycle current transformer (CT) secondary current signals of all the bays connected to the busbar to map the operating points on a complex alpha plane. The proposed scheme has been evaluated by modelling an existing 400 kV Indian power generating station in PSCAD/EMTDC software package. The performance of the proposed scheme has been evaluated on large numbers of cases with wide variation in system and fault parameters. In order to verify authenticity of the proposed scheme, a laboratory prototype of the proposed busbar protection scheme has been developed. From the developed prototype, CT secondary current signals are captured during internal faults and external fault with CT saturation condition. Comparison between the simulation and prototype results clearly shows the effectiveness of the proposed scheme in terms of higher sensitivity during internal faults and better stability in case of external faults. The proposed protection scheme has a high response speed (around 5 ms) and hence can be considered on par with modern busbar protection schemes.