The strong impact characteristics of the impact loads is a serious threat to the security and stability of an isolated power system.As the existing evaluation index cannot accurately and effectively assess the instantaneous dynamic characteristics of the impact loads

a set of transient evaluation index is proposed based on the instantaneous power theory and key parameters from the source

which is used to assess the starting

loading

and shedding processes of the two types of the common used impact loads.The index can better represent the dynamic process and transient characteristics of the effect of the nonlinear loads to the isolated power system.By controlling the key parameters of the two impact loads to approach the unstable state

and the stability analysis of voltage

work angle and frequency is carried out.It is found that the difference of the instability factor between the two impact loads is the different types of impact power

which brings the frequency instability and voltage instability to the system

respectively

and cause the rotate speed drop and protection of exiter system in the source.It is indicated that the constant-power charging or improving the speed regulating system can be adopted for the load of energy storage device

while the soft-start or the reactive power regulator can be used for the motor load to enhance the system stability.