Development of automatic slow voltage control for large power systems

Automatic control of system voltages provides advantages in terms of security, economy and ease of system operation. In some European countries, voltage control is organized in a three levels hierarchical structure. The automated generation-based secondary voltage control has been implemented for more than a decade. It is based on the pilot point concept and all generators in a given region are operated in an “aligned” operation. In modern power systems, as the system gets loaded more and more, it is becoming increasingly difficult to divide a power system into regions or areas for voltage control purposes. In North America, discrete control devices like capacitor/reactor banks and LTC transformers are commonly used for voltage control. However, the problem of efficiently coordinating the switching of large amount of discrete devices in an area remains largely unsolved. Therefore, those devices are mostly controlled manually by operators.; This thesis proposes an automatic discrete slow voltage controller, which is motivated towards implementation in the western Oregon system in the Pacific Northwest. The proposed framework is, however, very general and is scalable to any large power system. The western Oregon system is far from generators and the main voltage control devices are capacitor banks, reactor banks and LTC transformers. Owing to the close proximity of discrete devices in this area, it becomes difficult to divide the system into voltage control sub-areas. Therefore the system is treated as one coupled system. Because of the discrete nature of the problem, an integer-programming formulation for coordinated switching of discrete control devices is proposed. The generator-controlled buses are also treated as discrete devices with a set of voltage reference taps. Effects of control devices are evaluated by adaptive local computations. Relative preferences among control actions are formulated into the switching cost function so that the objective of minimum switching can be easily handled. The control objective is to keep the voltages within constraints with minimum switching cost. A robust control strategy is proposed to make the control feasible and optimal for a set of power-flow cases that may arise from load forecasts and from uncertainty in state estimation models. A graph theory based algorithm is implemented for detecting circular VAR flows caused by improper setting of reactive devices. A penalty term is formulated into the cost function so that circular VAR flows are kept minimal.; Efficient algorithms have been developed for the cases in which only one or two switchings are allowed at each step. When more than two simultaneous switchings are allowed at a time, it becomes a nonlinear combinatorial optimization problem. An approximate algorithm has been proposed and implemented.; This formulation can be easily extended to handle different objective functions, such as loss minimization, reactive margin maximization, etc.