Coordinated Voltage Control in Electric Power Systems

This thesis deals with various aspects of coordinated voltage control in electric power systems. It consists of an introduction and six papers. The introduction contains a summary of the thesis and tutorials on voltage control and the search methods that have been applied. The first part deals with the coordination of cascaded tap changers in radial distribution feeders. Poor coordination causes unnecessary operations of the tap changers and consequently unnecessary wear as well as poor voltage quality. A tuning rule for the conventional (local) controllers as well as two new (centralized) schemes are proposed: firstly, an optimal controller based on on-line optimization and secondly, a rule based controller based on a fuzzy inference system. Results from simulations based on load patterns recorded during different seasons indicate that when tuned according to our recommendation, tap changers perform some 10 % fewer operations compared to when the present tuning is in use. The additional reductions with the fuzzy-rule based and optimal controllers are some 36 % and 45 %, respectively. A prototype of the fuzzy-rule based controller has been installed and successfully tested in a distribution feeder in the south of Sweden. The prototype was also used to validate the simulation results presented in the papers. Another control interaction problem studied is a phenomenon of limit cycles due to load-tap changer interaction. The role of the OLTC deadband in creating a limit cycle phenomenon is addressed. Analytical insights into the behaviour of power systems with continuous and discrete tap models are given using eigenvalue analysis, the describing function method and time simulation. The conditions for occurrence of limit cycles and the identification of key parameters influencing their characteristics are other important aspects of this investigation. The second part of the thesis describes a coordinated system protection