Comparisons between a Series and a shunt FACTS for tapping and power flow control in half-wavelength transmission lines

The half-wavelength transmission principle is an attractive solution for transmitting bulk power over long distances, particularly if the distances are in the range of 2300 to 2700 km, in case of 60-Hz power system frequency, or 2800 to 3200 km, in case of 50-Hz power system frequency. These long lines have to be compensated with series and shunt reactors and capacitors in such a way to tune the electrical length of the line to be equivalent to a little more than half the wavelength at the power system frequency. This paper presents two solutions to provide simultaneously power flow control through the half-wavelength line and also to derive a portion of this power to or from a local power system, without mischaracterizing its half-wavelength behaviour. It will be demonstrated that power flow control together with power tapping can be performed optimally by means of a series FACTS if the local system to be interconnected to the half-wavelength line is located in a range up to about 500 km from the sending or receiving end of the line. Contrarily, a shunt FACTS is more effective to perform both functionalities in case of applications in the central section of the line. Simulation results from a frequency dependent model of transmission line and the complete models of the series and the shunt FACTS devices were obtained in the PSCAD/EMTDC to demonstrate the feasibility of the proposed systems.