Induced Voltages in single-phased Three-phase Xfmr Core 1

[dpc]

"Hypothetical" question:

Three-phase core-type transformer - assume five-legged core.

If only one primary phase winding is energized, how much voltage could be induced in the "unenergized" primary and secondary windings?

Thanks,

Dumb Dave

 

[magoo2]

I think at most you get about half voltage in the other windings.

Here's my reasoning. Consider rectangular wound cores. A 5-legged core would have 4 of these cores placed side to side.

First consider energizing the center phase winding. This means the winding goes around the window formed by 2 innermost rectangular cores. These cores also make up half the magnetic circuit for the other 2 windings. So you get 1 pu flux on the core for the center winding. This means that the other 2 windings get 0.5 pu flux, which leads to half voltage.

Next consider energizing an outer winding. As before, this leads to 0.5 pu voltage to the adjacent center winding. The other outer leg would have essentially no voltage since it has no flux, but would probably pick up some induced voltage from the center winding. Nothing big, close to 0.

 

[unclebob]

After revisiting Magnetic Circuits, the flux generated would be divided by 4, in each of the "un-energized legs".
Since reluctance is the same in the core, the voltages would be divided by 4, thus 0.25 pu.

Waiting for other answers...

 

[dpc]

I'm thinking that with a five-legged core, the induced voltage in the other phases should be quite a bit less than for a three-legged core. Hard to believe I haven't dealt with this issue before in allthe preceding years I've been doing this stuff. Always more to learn.

 

[SteveFehr]

I'm inclined to agree with unclebob, except to add that it depends on which leg has voltage, and the added length of top/bottom core to travel through will mean that some legs may induce more than 25% voltage while some less. Would be in the area of 25% for each, though.

 

[waross]

I would expect that any load on the de-energized windings would divert the flux to the unloaded and/or outside legs and drop the induced voltage considerably.


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

 

[dpc]

Bill,

Yes, that's the idea I keep coming back to.

But I've since learned that the feeder had a wye-delta transformer on it, so that changes the thinking a bit.

We were trying explain a low voltage condition after two phases were lost on the feeder. The low voltage was on one the "de-energized" phases.

Thanks,

Dave

 

[waross]

Was the primary wye point on the wye/delta bank tied to the system neutral? I know that it is bad practice but it is done sometimes.
Also, some utilities close the wye point to neutral connection when energizing with fused cutouts to minimize switching surges. The action of the wye delta bank is quite different depending on the wye point connection.

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

 

[prc]

Answer to dpc's question- 50 % of applied voltage.This is different from 3 phase 3 limbed case.In 5 limbed core,while exciting any limb,the flux will have two equal paths nearby unlike the case in 3 limbed core.But caution,if you measure with a low voltage, say 400 V in a 400 kV winding, you may not get exact 50 % and there can be distortions.

 

[dpc]

Bill,

Yes the wye of the wye-delta is probably grounded and tied to the neutral.

 

[waross]

Loss of two phases on a wye delta transformer bank with the primary neutral connected.
With the delta secondary, the energized secondary will be developing full voltage. The two unenergized transformer windings will be in series across the energized winding. With equal loading, the unenergized windings will each back feed 50% voltage to the primary lines. If there is a connected load, the voltages will be in inverse proportion to the loads.
Note: RUS, formerly REA recommends that the primary neutral of a wye delta bank be left floating.

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

 

[dpc]

Thanks to all. It's been educational.

Bill,

That's basically what I was guessing for the wye-delta after I thought about it. It's just odd that I've never run across this problem before.

Cheers,

Dave

 

[electricpete]

First step is to figure out what voltage (magnitude and phase is applied to what winding. 2nd step is to figure out how the magnetic circuit reacts.

I may be mistaken, but I think the responses in this thread have missed the first step. I don't think it's easy to deenergize 2/3 primary phases.

If the primary is delta, then you have the phase-to-phase voltage applied accross parallel combination of one phase winding and two series phase windings. All windings have voltage applied (either full voltage or half voltage)

If the primary is wye, then you have phase to phase voltage applied accross two series phase windings. 2/3 phase windings have voltage applied.

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[dpc]

electricpete,

The wye primary was fed by a 4-wire primary where two fuses blew upstream. So it was energized line-to-neutral on one phase of the primary.