New Criteria of Voltage Stability Margin for the Purpose of Load Shedding

Voltage instability is a phenomenon which often contributes to the development of power system disturbances. While increasing load admittance, bus voltage decreases to such a degree that the apparent power (V²Y) does not increase. As a result, voltage collapses with all consequences resulting from it. During the large-scale power system disturbance, the last line of defense is the load shedding at the stations, where the stability margin becomes dangerously low. To do that, there is a need to use automatic devices which process local signals, detect the decreased margin, and activate the load shedding. As a criteria of operating such devices, the voltage level is adopted. The disadvantage of such an approach results from the fact that the relations between the voltage level and the stability limit depend very much on the load power factor. To overcome this weakness, this paper presents a criterion which is directly based on the definition of voltage stability. It calculates the derivative of apparent power against the admittance (dS/dY). It may be easily accomplished, because both the power and the admittance are measurable, and the changes of load occurs continuously, as a result of switching on and off the impedances, and/or actions of the transformer on load tap-changing devices.

I. Introduction

Disturbance developments are caused by the instability of the system. The IEEE/CIGRE Task Force defined the notion stability in the following way: “Power system stability is an ability of an electric power system, for a given initial operating condition, to regain a state of operating equilibrium after being subjected to a physical disturbance, with most system variables bounded so that practically the entire system remains intact.” [1] Several modes of stability were defined.

Rotor angle stability: which may be a small–disturbance angle stability or transient stability.

Frequency stability: which may be short term or long term.

Voltage stability: which may be large disturbance voltage stability or small disturbance voltage stability, and both of them may be short term or long term in nature.