Wye ungrounded primary Wye ungrounded secondary

[Mbrooke]

How would one describe the operating difficulties in such a connection? To be honest I have no idea, however considering that a transformer is none linear, as is inrush/magnetizing current, I can see a "neutral shift" on the primary that would effect the secondary voltage. Is this true even under load?

 

[waross]

The simple answer is that the phase voltages will be in inverse proportion to the loads. In the simple analysis a heavy single phase load may make the other phase voltages rise to approach 1.73 PU.
In real life, transformer non-linearity and in particular magnetic saturation will limit the voltage rise in a transformer operated at rated voltage to about 15% or 20%
Another factor is the core construction. A three legged core exhibits a phantom delta effect. The phantom delta will further limit the voltage rise.
You may still experience switching transient overvoltages on energization.

Bill
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"Why not the best?"
Jimmy Carter

 

[Mbrooke]

That 1.73 rise would only occur with phase to neutral loads, but remember that the secondary neutral is also ungrounded/unused so all loads will connected L-L.

 

[HCBFlash]

What's the reason for going with that wye secondary, especially ungrounded?

.


Me wrong? I'm just fine-tuning my sarcasm!

 

[GroovyGuy]

A wye-wye transformer is common with utility supplied transformers. I believe that a wye (HV) primary is less expensive to build, as opposed to a delta winding.
In my experience I have frequently seen a tertiary delta winding (in larger transformers). This extra winding gives the 3rd harmonics a place to run around in, so as to not distort the secondary voltage.
Per the Flash's comment above, is there a reason why the secondary is ungrounded? How large is this transformer?

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[waross]

To make it simple, consider two resistors in series in series across a voltage source.
The voltage across each resistor will be in the inverse ratio of the resistances.
Now consider two transformers in series across a voltage source. One transformer is fully loaded and the other transformer has no load.
The impedance of the loaded transformer will be much less than the impedance of the unloaded transformer.
The voltages across the transformers will tend to be in the inverse ratio of their impedances up to a point.
When the voltage across the unloaded transformer approaches the saturation point, the impedance of the unloaded transformer will drop and the voltage across the unloaded transformer will stabilize near the saturation voltage. The balance of the applied voltage will be seen across the loaded transformer.
This is a simplification.
In the real world, the effective X:R ratio of the loaded transformer (X:R ratio of the transformer plus the reflected X:R ratio of the load) will be different than the X:R ratio of the unloaded transformer. The division of the reactive current will probably be different than the division of the real current.
Of course when you apply these basics to a three phase transformer bank, things get more complicated.
And last but not least, if the transformer has a three legged core, the phantom delta effect tends to stabilize the voltages.

Further to GG's comment on distribution transformers, many but not all transformers intended for line to neutral distribution service are constructed with only one high voltage bushing.

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

 

[Mbrooke]

Waross, what I suspected. How would this reflect on the secondary voltages though? Any phase angle distortion or phase shifting? Picture 3 167kv 2 bushing pigs, 19,920 primary 277 volt secondary connected in Y-Y.