Using bus impedance and bus P-Q curve for voltage stability control

Voltage control and voltage stability problems are not new in the operation of electric power networks but are now receiving special attention in many countries because of a greater need for improving efficiency of usage of transmission capacity in the competitive electric energy market. Practically, the idea of P-V and Q-V curves is used to determine the maximal reactive margin at load buses to avoid voltage collapse. In this paper, the idea of bus P-Q curve is proposed for the assessment of such a reactive margin. The P-Q curve for bus load is created on the basis of bus impedance. To assess on-line the bus voltage stability margin the measurements of the bus voltage and bus current are needed. First of all, the mathematical background of the proposed idea is given. Then numerical analysis of the example power system is presented. The bus reactive margins calculated on the basis of the bus impedance are compared with the relevant margins obtained from Q-V curve