Ferroresonance is a common nonlinear phenomenon in power systems. The measured data from field test show that there exists quasi-periodic ferroresonance in the power system. In addition

some of the sub-harmonic fer-roresonance overvoltage cannot be distinguished from the quasi-periodic ferroresonance overvoltage. In this paper

to distinguish different types of ferroresonance by a characteristic quantity

the five-year data of ferroresonance over-voltage

which were online monitored in a substation

are analyzed

and the phase space reconstruction method is adopted to acquire the reconstructed phase space of ferroresonance overvoltage corresponding to the voltage time se-ries. On the basis of the reconstructed phase space

a method for calculating the average gray value of the two-dimen-sional reconstruction attractor is proposed to quantitatively express the nonlinear characteristic of the ferroresonance overvoltage. Result shows that there is obvious difference among the average gray values of different reconstruction at-tractors. Therefore

the average gray value of reconstruction attractor can be used to distinguish sub-harmonic ferrores-onance and quasi-periodic ferroresonance

and it can also help to identify other types of ferroresonance overvoltage.