Verify that the unusual connection of VTs shown in Figure P7.3 provides zero- sequence voltage during a ground fault on an ungrounded system for operation of the overvoltage relay 59G. (This connection used by a large power system for ground detection on ungrounded systems supplied from delta tertiaries.) Open-delta-connected VTs are used for three-phase voltage, and the desire was to use these with minimum additions. Figure 7.5a connections using three auxiliary VTs in broken delta had a tendency in their system to go into ferro- resonance, probably because the resistor was not effective with the high leak- age impedance of the auxiliaries. The scheme shown was evolved using the two existing VTs and adding a VT-connected phase to ground. This added VT is either a 60 Hz rated twice line-to-line voltage unit or a 25 Hz unit rated line- to-line so that they operate low on the saturation curve. No ferroresonance has been encountered with this scheme. FIGURE P7.3 P 115 V Bus 59G 66.4 V Ballast resistor VTS 5 Auxiliary VTS 9