Revisions on Alpha Plane for Enhanced Sensitivity of Line Differential Protection

Line current differential protection based on the traditional alpha plane may block internal faults with outfeed when providing higher security for external faults under current transformer (CT) saturation. This letter proves analytically that the external faults under CT saturation plot away from the negative real-axis of the complex plane and accordingly proposes revisions of the alpha plane characteristics. The modified alpha plane enhances the sensitivity of line current differential protection by unblocking more space along the negative real-axis where internal faults with outfeed are plotted.