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In ungrounded neutral system, ferroresonance arising from the interaction between the nonlinear characteristics of the electromagnetic potential transformer (PT) core with system capacitance takes place frequently. Under different system capacitance, ferroresonance of different frequency may occur: fundamental frequency ferroresonance, subbarmonic ferroresonance and harmonic ferroresonance. In this paper, we use mathematical models and analytic techniques to the study of this phenomenon. And experimental study in the laboratory verifies the precision of the result analyzed. INDEX TERMS-Ferroresonane; ungrounded neutral system; Electromagnetic PT