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In view of the increasing use of variable frequency drive equipment, the active and reactive demand characteristics of a power grid with a high percentage of variable frequency loads under transient state are analyzed. Due to the fact that a large number of variable frequency motors in power grid cannot be modeled one by one, an aggregation motor model is needed to approximate them. In this paper, a variable frequency motor model is built on the MATLAB/Simulink simulation platform, and the influence of frequency converter on induction motors aggregation is analyzed. Then, an induction motors aggregation method is used to aggregate the variable frequency motors. The transient simulation of the power grid is carried out when the induction motors and the variable frequency motors respectively occupy a high proportion. The simulation results show that variable frequency motors show different load characteristics from the conventional induction motors, and a large number of them can make the system suffer the active impact after the fault recovery process. [ABSTRACT FROM AUTHOR]