SUPPLEMENTARY DC CONTROL IN A TWO AREA AC/DC POWER SYSTEM

Abstract

In this thesis results of research are reported for a study of the dynamic performance of a two-area power system interconnected by AC and DC tie-lines. The system includes supplementary DC control interacting with the conventional AC area controls. Taking advantage of the fast controllability of DC links, supplementary control utilizing DC power modulation improves system damping, reduces unnecessary governing system operation and reduces inadvertent energy interchange between the two areas. DC power modulation is based on variations of AC system signals such as net power interchange deviation and/or area frequency deviation. A linear model was used for the power system to study small changes around a base reference state and only the power/frequency control aspects of the overall power system control problems were considered. The model used and the simulation of it are described in this thesis. Eigen-structure analysis of the state space model of the power system was used to evaluate system stability, with and without supplementary control. Control optimization was achieved by minimization of a performance index to adjust the values of the controller parameters. The performance index selected is a function of Area Control of Error (ACE) and is a good measure of control effectiveness. The results of the eigen-structure analysis and the control optimization are presented. Computer simulation was used to obtain power system dynamic responses to step load disturbances and to estimate optimal controller parameters. The simulation studies considered various AC and AC/DC interconnection types and control features of the two area interconnected power system. Simulation results are presented in the form of time responses graphs and in summary tables. It was found that the dynamic performance of the two-area power system could be significantly improved with the aid of the controls in the DC line.