Increased Transmission Capacity by Forced Symmetrization

This paper deals with one-phase faults on high voltage transmission lines and possible methods of utilization of the transmission capacity of the sound phases. If an AC power grid has interconnections with damaged or disconnected single phases, non-symmetrical currents arise in the system. These non-symmetrical currents must be eliminated to provide an acceptable and secure operation of the power grid. Normally, if one phase of a transmission line is damaged, the whole line section or all three phases are disconnected from the grid to ensure the symmetrical mode of operation of the grid. The investigation in this paper is devoted to the case, when only the faulty phase is taken out of operation or disconnected while the two sound phases are still connected to the grid and transfer power. Special symmetrization compensators provide the needed symmetrical conditions at line terminals. In this case the transmission capacity of two sound phases can be utilized for further grid operation. Electrical parameters of the affected line as well as effective thermal power rating of the line will change. Theses changes are dependent on the particular symmetrization technique. Two symmetrization methods have been tried and compared in this work. Modern power electronics devices and measurement technology provide the necessary hardware basis for the practical implementation of such symmetrical compensators. The presented solutions increase the system reliability significantly and can be seen as competitive solution for an extensive grid expansion on the existing congestion routes.