On load flow control in electric power systems

by Herbig, Arnim

This dissertation deals with the control of active power ow, or load ow, in electric power systems. During the last few years, interest in the possibilities to control the active power ows in transmission systems has increased signi cantly. There is anumber of reasons for this, coming both from the application side that is, from power system operations and from the technological side, where advances in power electronics and related technologies have made new system components available. Load ow control is by nature a multi input multi output problem, since any change of load ow in one line will be complemented by changes in other lines. Strong cross coupling between controllable components is to be expected, and the possibility of adverse interactions between these components cannot be rejected straightaway. Interactions with dynamic phenomena in the power system are also a source of concern. Three controllable components are investigated in this thesis, namely the controlled series capacitor (CSC), the phase angle regulator (PAR), and the uni ed power ow controller (UPFC). Properties and characteristics of these devices are investigated and discussed. A simple control strategy is proposed. This strategy is then analyzed extensively. Mathematical methods and physical knowledge about the pertinent phenomena are combined, and it is shown that this control strategy can be used for a fairly general class of devices. Computer simulations of the controlled system provide insight into the system behavior in a system of reasonable size. The robustness and stability of the control system are discussed as are its limits. Further, the behavior of the control strategy in a system where the modeling allows for dynamic phenomena are is investigated with computer simulations. It is discussed under which circumstances the control action has bene cial or detrimental e ect on the system dynamics. Finally, a graphical approach for analyzing the e ect of controllers on the closed loop behavior of a system is discussed. The phase root locus technique is presented and its application to power systems illustrated. TRITA-EES-0001 ISSN 1100-1607 iii iv