A voltage stability indicator based on rotor heating

Voltage stability hinges on the continuing control of the total system´s supply and demand of reactive power within the overall transmission, subtransmission and distribution networks which connects the power stations to the consumers. This is complicated by the elements interspersed within this extensive network that both supply and consume reactive power. Extensive collapses have followed disturbances more severe than considered at the planning stages, but there is a puzzling delay of minutes between the disturbance and collapse. By simulating the physical process the critical factor has been found to be the action of rotor overcurrent protection in curtailing rotating unit reactive power output. The puzzling delay has been solved by the mathematical modelling of the generator rotor´s thermal response, which proves that above rated rotor current it would take minutes to reach the rotor´s temperature limit. The paper describes the heat run tests from which the parameters for mathematical rotor thermal models for 500 MW and 660 MW generators have been obtained. The algorithm that has been devised is both simple and reliable and allows computer evaluation, in a fraction of a second, of the period for rotor temperature limit to be reached. When integrated into SCADA, this information would give an early warning and so provide a valuable tool for the control system voltage stability