Considering the sensitivity of nodal voltage to line reactance and variation of reactive power of power grid comprehensively, the voltage instability index is defined as the distance between the current values and maximum allowable voltage, with these as weighted values of each bus and proposed the resperention of vulnerable region.

A voltage stability index based on synchronized phasor measurements is presented for trans- mission systems.

The applied static voltage stability index is researched and the relation between this index and load scale factor(LSF)is obtained.

According to the criterion of real roots of quadratic equation in voltage equation,three new node voltage stability indexes,i.

Based on the analyzed voltage,a new voltage stability index(U/Y_(cr))~2 is used to analyse voltage stability of contingency condition.

Analysis of the probability of voltage instability based on Thevenin equivalent parameters;

Mid and longer-term voltage instability are usually caused by the continuing increase of the load, and is characterized by a period when the voltage drop down quite slowly.

In order to make up the deficiency of traditional analytical method used in the research of voltage stability problem, we need to know the probability information concerned with voltage instability.

With consideration of voltage stability,the branch-based voltage stability index was used for quick screening and ranking of the contingencies,and the Continuous Power Flow(CPF) method was used to calculate the transmission capability.