The calculation method of existing voltage sag vulnerable area is analyzed and studied and a kind of fast calculation method of the vulnerable area based on voltage curve characteristics of the sensitive load node is proposed.In case of single-phase ground fault on the line

the expression of the sensitive load node voltage is analyzed.On this basis

the consistency parameter and the monotonic parameter

which are composed of the elements of the node impedance matrix

is defined to determine whether the voltage curve is monotonic.The lines are divided into two types in accordance with whether the voltage curve is monotonic or not and different critical fault point calculation methods are used.The IEEE 39-node standard test system is taken as an example to analyze the vulnerable area of sensitive load nodes.The result

compared with the existing analytical methods

shows that the algorithm improves the calculation speed while ensuring the accuracy.