Why measure potential with a three winding transformer?
There are a few reasons you might have multiple secondary windings on a VT. (syn. dual-secondary VT.)

The below are all examples of things I have personally worked with, specified, or built.

Dual purpose voltage measuring / auxiliary power VTs

The below example is from an 11kV switchboard.

The 110 VAC, 200 VA, Class B winding is supplying the metering bus for instruments throughout the switchboard - volt meters, kWh meters, protection relays, and the like. (Note Class B is an old standard - these days one would specify a metering class like "CL 0.5, 200 VA" to the IEC 60044 standard.)

The 152 VAC winding is doing something quite unusual - it's supplying a set of three-phase rectifiers, which in turn supply a 200 VDC closing bus. The 200 VDC closing bus powers up the closing solenoids (item 52S) for the 11kV oil circuit breakers. These solenoids are fairly heavy duty, hence the high voltage.

(These days, one would specify a spring charge motor at 110 VDC or 48 VDC, instead of this solenoid at 200 VDC.)

More practically, if you have a substation with no local LV power supply, you can use a VT secondary to get a small amount of power to run protection relays etc.



Separate outputs for metering and protection

Another reason to have dual secondaries is if your VT needs to supply both metering instruments, and protection relays.

Metering

The metering winding, which supplies kWh meters, needs to be very accurate. These VT's are what we use to measure electricity consumption for the accountants, and they demand an accurate measurement!

A typical "CLASS 0.5" metering winding will be rated for 0.5% accuracy when the voltage is between 80% and 120% of nominal voltage. Outside the 80-120% voltage range, the behaviour of a metering-class VT is undefined. (AS/IEC 60044.2:2007 section 12.2 refers.)

If the voltage is outside 80-120%, something is faulty, and accurately counting the cost of electricity is not our primary concern for the moment.

Protection

The protection winding has the opposite job. Protection relays generally work off rough measurements - a voltage measurement of 80-120% indicates everything is working normally. A voltage measurement of 20%, or 180%, indicates a serious undervoltage or overvoltage. So the protection winding doesn't need to be very accurate - but it does need to operate over a wide range of voltage.

A typical "CLASS 3P" protection winding would be accurate to 3% over a range of 5% - 190% the nominal voltage. (AS/IEC 60044.2:2007 section 13.2 refers.)

Typical specification

OUTDOOR, SINGLE PHASE VOLTAGE TRANSFORMER

WINDING 1:
33,000/√3 V : 110/√3 V
CLASS 0.5M
100 VA

WINDING 2:
33,000/√3 V : 110/√3 V
CLASS 3P
50 VA

BIL 36/70/200 kV
POLLUTION CLASS IV (HEAVY - 31mm/kV)
VOLTAGE FACTOR 1.2 CONTINUOUS / 1.9 FOR 30 SECONDS
TO AS/IEC 60044.2


This is a 33kV voltage transformer with two 110V secondaries.

The No.1 secondary is for metering, with accuracy of 0.5% over a voltage range of 80% - 120% nominal voltage. This accuracy will be maintained provided we put less than 100 VA of burden on the winding.

The No.2 secondary is for metering. This will have accuracy of 3% over a voltage range of 5% to 190% of nominal voltage. This accuracy will be maintained provided we put less than 50 VA of burden on the winding.

Note that the secondaries of a VT are dependent on each other, so overloading one winding will also affect the accuracy of the second winding.

To provide a star-connected winding and a "broken-delta" winding

In the example below, a set of three single-phase VT's have been connected to provide:


a set of three-phase 110/√3 V metering outputs, for metering.
a 110V residual output - the output from this is equal to the vector sum of the phase voltages: $$3V_0 = V_a + V_b + V_c$$

This is useful for neutral voltage displacement relays, which operate directly off this quantity 3V0.

This is also handy for old-school directional earth fault relays, which used to need this quantity for operation. These days, directional earth fault relays just measure Va, Vb, and Vc, then do the vector sum in software. So residual windings aren't as necessary as they used to be.

Finally, if you're worried about ferroresonance - as I was, in the example below - you can connect a power resistor across the residual winding to burn off any excess power. Putting such a damping resistor on a residual winding has the advantage that it only burns off power when it needs to. If you put damping resistors on a normal three-phase wye-connected VT, they burn off power all the time. (See Schneider Cahier Technique no. 190, Ferroresonance, for guidance on this topic, including selection of the resistance and power ratings.)






This is probably as complete a discussion of multiple-secondary VTs as you'll find anywhere. Questions and comments are welcome.