Transmission-line estimations by combined power circle diagrams

Power circle diagrams drawn for either sending or receiving ends of a transmission line do not have a common centre for the voltage circles, nor are the power axes within the same semicircle. There is also a possibility of confusion over the sign of reactive power when conditions at both ends of the line are estimated by means of separate diagrams. A quadrant diagram with current taken as reference, which complies with the convention of the American Institute of Electrical Engineers for reactive power, is shown to be a satisfactory base on which a combined send-receive power circle diagram may be constructed. If such a diagram is built as a calculating board, all sending-and receiving-end quantities may be simply read with sufficient accuracy for the estimation of system load conditions. Examples of the use of the calculating board are given.