three-phase unit. As an example of the single-phase cores in the ungrounded distribution networks, the VTs can be connected phase-to-phase rather than phase-to-ground. As such, even two VTs are adequate to measure three phase voltages. However, upon the deenergization of one or two phases, the magnetizing inductances of the VTs are connected in series with the capacitance-to-ground of the open phases. If the VT saturates and enters ferroresonance, the secondary voltage will be distorted, the fundamental voltage output may be reduced and phase shifted, and hence the
magnitude of the measured zero sequence voltage 3Vo is decreased. Therefore, the failure of the protective relays to trip is likely. Such a ferroresonance scenario is also valid for a three-phase transformer unit with the delta winding connection.