The paper concerns the protection of long high-voltage transmission lines on which the power-swing conditions are complicated by appreciable line-charging currents. Phase-comparison protection only is considered, depending on the difference in phase between the line currents at the two ends of the line. The problem involved is the study of the power-swing conditions and the setting of them on a phase-comparison diagram on which the protection relay characteristic is already drawn.The application of phase-comparison principles is here developed to a stage which suggests their wider use than at present, and limitations are defined in terms of line constants, including line length and voltage.The diagrams obtained show conclusively which load and power-swing conditions appear in the “trip” section of the phase-comparison field, and a simple envelope is derived mathematically and drawn to include all load and power-swing characteristics of the line. It is shown that the relay settings are determined by the no-load and light-load conditions, and not by the power-swing conditions, which are a problem of impedance protection relays. Although complicated to construct, power-swing curves are already available on the impedance diagram 1. The provision of an easily constructed phase-comparison power-swing diagram should assist in applying phase-comparison protection to long lines.