?Open Delta? VT's are two VT's connected phase-to-phase, usually A-B and B-C with the secondary outputs connected the same way with "B" phase grounded. This arrangement gives you readings for all three phase-to-phase voltages but no phase-neutral or phase to ground readings. It is usually set up for 120V output. Almost all metering and relaying works great, except phase-ground readings.
A ?Broken Delta? VT arrangement uses three VT's connected phase to ground (Wye) on the primary and in delta on the secondary, except one corner of the delta is broken open and a connected to a loading resistor and voltage relay. If the primary voltages are all matched phase-neutral, the secondary delta connections add the voltage to zero and no voltage is on the relay. A phase-ground fault unbalances the delta and puts a voltage on the relay.
Phase-phase VT's in open delta are very common in ANSI equipment. A big advantage is they are not as susceptible to ferro-resonance as wye connected VT's. They also take less space in the switchgear and are 30% cheaper.
A disadvantage with Open Delta is there are no phase-ground voltage measurements, which is only a disadvantage if directional relaying or distance relaying is needed. Some directional and voltage based ground fault detection relays don't work without the phase to ground input. Since most industrial systems MV are high resistance grounded and ground fault protection is provided by overcurrent, there isn?t a need for the phase-ground measurements.
