Diss . ETH No . 15342 Increased Transmission Capacity by Forced Symmetrization

The electrical power industry experiences nowadays a significant need in modern techniques for increasing the capability of power transmission systems. The reason for this lies in growing power flows, caused by ris¬ ing power consumption, and in the deregulation of the electrical market, where power flows should be more flexible. These factors demand more transmission capability from the existing networks. The conservative expansion of the high voltage grid is often not desirable or not possible, because the approval of new overhead transmission lines meets strong opposition in society. Furthermore, it takes a long time and is gener¬ ally a risky long-term financial investment. New technological solutions are sought to meet the capability needs under consideration of modern environmental requirements. The presented work considers possibilities for ensuring power transmis¬ sion through an AC transmission line with a damaged phase conductor. The disturbance of the symmetrical pattern of currents and voltages in the surrounding network can be eliminated by active measures for sym¬ metry. The utilization of two remaining healthy phases of a three-phase transmission line with a damaged phase in this way can be an economical way to enhance the system reliability. The present planning procedure of the high voltage networks mostly re¬ spects the (n-1) criterion. This means that the network should not be subject to any overload or voltage drop below a strictly given limit when any network element is disconnected. Based on statistics, single phaseto-ground faults are the most frequent faults in transmission systems,