Coordinated Power Flow Control to Enhance Steady-State Security in Power Systems

Due to the rapid technological progress, the consumption of electric energy increases continuously. But the transmission systems are not extended to the same extent because building of new lines is difficult for environmental as well as political reasons. Hence, the systems are driven closer to their limits resulting in congestions and critical situations endangering the system security. Power Flow Control devices such as Flexible AC Transmission Systems (FACTS) provide the opportunity to influence power flows and voltages and therefore to enhance system security, e.g. by resolving congestions and improving the voltage profile. Even though the focus lies on Static Var Compensators (SVC), Thyristor-Controlled Series Compensators (TCSC) and Thyristor-Controlled Phase Shifting Transformers (TCPST), the developed methods can also be applied to any arbitrary controllable devices. In order to benefit from these devices, an appropriate control is necessary. In this thesis, an Optimal Power Flow problem is formulated and solved to find the optimal device settings. One of the objectives is to ensure N-1 security because if the stress on the power grid grows, failures of system components become more probable. When the system is not in an N-1 secure state, an outage of a single component may trigger cascading failures in the worst case resulting in a blackout. In order to take N-1 security into account in the Optimal Power Flow problem in an efficient way, a new Current Injection method is developed which accurately determines the line currents in case of an outage without having to carry out a full load flow calculation. As Power Flow Control devices have only influence on a limited area in their vicinity, it is not necessary to take the entire grid into account in the Optimal Power Flow calculations. Sensitivity analysis is used to v vi Abstract identify the area of influence of the considered devices and to set up the optimization problem for the limited area. Hence, the applicability of the developed control is independent of the size of the power system. If there are several devices placed in the same system, the areas assigned to these devices might overlap indicating mutual influences. Therefore, a coordination of the control entities is needed in order to avoid conflicting behavior of the devices raising the issue of Multi-Area Control. Here, the method based on Approximate Newton Directions is extended for the case of overlapping areas. In addition, it is taken into account that part of the grid might not be included in any of the areas. Finally, simulations for the UCTE system show the applicability of the developed control to realistic power systems.