VT connection

[P1lts]

I have a scenario where I have an 11kV board with a bus bar connected 11/0.11kV V.T.

The V.T. Is wired Y-Y(g) I.e. Only the secondary neutral is earthed.

My query is this:--

What will the secondary voltages be if the primary sees a loss of one phase. I.e. Assume that the Red phase 11kV bus bar lost supply, what would the secondary voltages of the V.T. Be?

Thanks for any help.

 

[waross]

It depends;
What do you mean by earthed. Sometimes the terms earthed and neutral are used interchangeably but when you are considering transformer connections there is a difference.
Is this a single phase transformer or a three phase transformer.
Is this a metering transformer or a larger transformer?
If the transformer is three phase, does it have a three legged core? A three legged core has an effect called a "phantom' delta. The phantom delta will stabilize the secondary voltages in the case of a floating primary neutral connection.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[P1lts]

Bill

The transformer is a protection VT on an 11kV sw/bd.the secondary star point is solidly connected to earth.
It is a 3ph device wound on 1 core but I am cannot confirm the core construction but likely to be 3legs.

Thanks

 

[inpran]

P1lts, then only R phase voltage will not be there for you in the secondary. Y - B phase volatge will read 110 V, Y - R will read 63.5 V and B - R will read 63.5 V.

 

[waross]

Simple case:
Normal voltage phase to neutral;- 11kV x 1.73 = 6351 Volts.
With the loss of one phase, the other two phases become a series circuit across 11kV.
If the burdens are balanced, each winding will see 1/2 voltage or 5.5kV. The secondary voltages will be 110V x (5500/6351)= 95.3 Volts (86.6%).
For minor burden unbalances the secondaries will share 11kV in inverse proportion to the burdens.
For greater burden unbalances saturation will limit the maximum voltage of the burden with the higher voltage.
The lost phase will show zero volts.
Grounding is important for safety. Grounding does not affect the voltages. A neutral connection is important. A ground connection may also connect the neutral, but not always, depending on the circuit details.
This assumes equal X:R ratios of the burdens. If the power factors or X:R ratios of the burdens is not equal the solution becomes more complicated.
With one phase lost the circuit becomes a single phase circuit and both energized secondaries will be at the same phase angle, (or at 180 degrees if you prefer).
With a three legged core, if the primary neutral is connected to the primary neutral the loss of one phase will result in a blown fuse in the primary or a destroyed transformer due to the effect of the phantom neutral.
Even if the 11kV system source does not have a grounded neutral point, system capacitance to ground will usually form an artificial neutral with sufficient capacity to destroy a metering transformer, so a primary ground connection may be considered as a primary neutral connection.
Quick check: If the system charging current is much above the rated primary current of the metering transformer a primary neutral connection may destroy the transformer.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[scottf]

If I'm reading the original post correctly, I think it can be assumed that there are 3 x single-phase VTs connected Y-GY.

If 1 phase looses voltage (primary voltage = 0V), then the corresponding secondary phase goes to 0V.

Now, the question may be more complicated when you consider the supply transformer, how it is connected, and the impact of the loss of phase on that, but strictly from the VT's perspective, it's pretty simple.

 

[waross]

Hi scottf. I took it as a three phase transformer with a floating primary neutral.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[P1lts]

It is a 3 phase Tx. It is not 3 x 1 phase units. The primary neutral is floating, the secondary neutral is earthed solidly. The only loads connected to the secondary are 3off single phase under voltage relays.

Thanks for the help so far.

 

[scottf]

Even if the VT is a 3-phase unit, I'd be willing to bet it's 3 x 1-phase VTs in a common unit. I'm not aware of any true 3-phase VTs, i.e. not all on the same core.

 

[P1lts]

The reason I ask is that the VT supplies 3off under voltage relays connected in a 2oo3 tripping logic. The relays are set with a trip point if 80% nominal voltage. Therefore assuming a loss of a bus bar phase I was curious whether the UVR would operate.
It seems as if one relay will operate but the other two may see a reduced voltage level but not enough to operate. As the trip scheme is 2oo3 then it will probably not operate in this scenario.

Thanks

 

[waross]

I believe that you are correct. If the primaries are connected line to line, then the loss of one phase will drop the voltage on two secondaries to 50%.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[scottf]

But he said the primary of the VTs were connected "Y".

 

[waross]

Yes I know.
The point is that with a two out of three scheme, with a neutral connected wye primary, only one phase will be lost and a 2 of 3 scheme won't work.
With a floating primary wye, the loss of a phase will cause two voltages to drop to 86% and a 2 of 3 scheme set at 80% won't work.
With a delta primary on the VTs the loss of a phase will cause two phases to drop to 50% and the 2 of 3 scheme set at 80% will work.

Bill
--------------------
"Why not the best?"
Jimmy Carter