In order to explain the mechanism of forming oscillation source in free oscillation
an equivalent circuit method based on the energy method is proposed. The equivalent circuits of various parts of the power system are developed
and the equivalent circuit of the system is obtained by combining them together. According to the circuit theory
an oscillation source locating criterion calculated with integral is converted into a variety of criteria represented by phasors. Three criteria are proposed for locating the oscillation source of the excitation system. By converting the three criteria into criteria represented by phasors and combining them together
the mechanism of forming oscillation source in the excitation system is analyzed. The correctness and effectiveness of the methods are verified by simulation. The methods proposed can be used to locate the oscillation source in the excitation system online and analyze the mechanism of oscillation source formed by inappropriate excitation parameter setting. This work is supported by National Natural Science Foundation of China (No. 51037003).