2 VTs vs 3 VTs

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vinod

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I have one technical doubt regarding our VT (voltage transformer) system which we are using in all our substations.
Our PT system is called Two VT system which is normally using in Metal Clad switchegars.(Sabic and Aramco too)
Do you know why we are going for this two VT systems (open delta) instead of three VT system.
 
Open delta vs two vt system

Open delta vs two vt system

Dears,

Is the open delta and two PT system is same?
I hope there is some different in the scheme,
Open delta involves three PTs and opens at one end,
but in two PT systems involves two PT and three wires.
Am I correct or not?Pls confirm me,
 
The reason for using 2 pts in an open-delta configuration is usually one of cost. If the loading on the pt circuit is less then 57.7% of what it would be for using 3 pts then you can use the 2 pt set-up. It still provides you with all three phases so you do not sacrafice any metering or relay protection.
 
?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.
 
rcwilson said:
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.
Click to expand...

Why will the directional or distance relaying not work in this case?
 
mull982 said:
Why will the directional or distance relaying not work in this case?
Click to expand...

Most directional and distance relays need a voltage to ground reference to compare with the phase current nfr groudn faults. Some can use a zero sequence polarising current from CT's on the neutral of the transformer, but most need phase-neutral voltages for accurate calculation of fault direction and distance.

The relay needs to see a zero sequence voltage to compare to the zero sequence current to find out which direction the current is flowing.

Precise Phase-ground voltages are needed for this type of calculation, whether it is done digitally or with analog circuits or windings, coils and phae shifting circuits.

The voltages provided by the Open Delta do not have enough zero sequence component.
 
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