The voltage phase angles at the buses in an electrical power transmission network have
always been of special interest to power system operators. It is well-known that active
power flow over a transmission line is nearly proportional to the sine of the angle
difference between the voltages at the two terminals of the transmission line. As many
of the planning and operational considerations in electrical power systems are directly
concerned with the active power flow, measuring voltage angle differences across the
transmission line has been of concern for many years