Applying Same phases to a 3 phase transformer

[I_am_learning]

while reading about sequence networks, I realized this. Suppose I apply same phase to the primary of Star-Delta transformer, say instead of applying A,B, and C phase to the three terminals, I mistakenly apply A,A and A phase to all the terminals, <Neutral is applied appropriately>.
Then since the voltages applied comprises zero sequence voltage, it will also induce zero sequence voltage in the secondary, and since the delta loop provides path for zero sequence current, circulating currents should flow.
My guess is, it should be devastating to the transformer.
Am I right?
Has anybody done this mistakenly?
Thanks.

 

[Carl Pugh]

A transformer with a shell type core will operate at 1/3 rated voltage OK. Above 1/3 rated voltage the core will saturate and large currents will flow and there will be high flux leakage. At rated voltage it will be a race between the transformer tank/or case melting and the winding melting.

In transformer with a core type core (No external legs), it will be a race between the transformer tank/or case melting and the winding melting.

Above assuming that the circuit breaker or fuse doesn't open.

Currents will flow in transformer primary.
Current in transformer secondary will be low unless secondary is shorted.

 

[I_am_learning]

Thanks Carl Pugh,
I think,the issue you are referring arises in 3-limbed transformer which has no return path for magnetic flux. In normal operation, the three fluxes in 3 limbs cancels each other so no return is required. But when same phase is applied to all, they don't cancel but instead add up, and the total flux needs to return through the air medium, there by greatly increasing the reluctance.
This results in huge magnetizing currents being consumed.
But with transformer with 4 legs (is this what you call 'shell type'), there will be a return limb for the flux, but since its not adequately sized, if we apply full voltage, then due to saturation, both huge magnetizing current will be consumed (although less than previous case) and huge core loss in the limb will occur as well.

Really thanks for bringing those issue to light.
But my original curiosity was referring to this
As opposed to what you said, Currents should flow in the secondary as well if the secondary is delta connected.

 

[Carl Pugh]

You are correct.
The transformer secondaries are short circuited.
There should be a file attached, that shows how I came to this conclusion.

 

[alphysicist]

I can confirm that applying same phases to a 3 phase transformer can have detrimental effects on its operation. This is because the transformer is designed to operate with balanced three-phase voltages, and any deviation from this can cause imbalances in the current and voltage levels, leading to overheating and potential damage to the transformer.

In this scenario, applying A,A and A phase to all terminals of the Star-Delta transformer would result in an unbalanced voltage across the windings, as the three phases are not evenly distributed. This would lead to increased circulating currents, which can cause overheating and damage to the transformer.

While it is possible for someone to make this mistake, it is not a common occurrence as proper training and understanding of sequence networks is essential in operating and maintaining transformers. Any deviations from the standard operating procedures can result in significant consequences, as seen in this scenario.

In conclusion, it is crucial to always follow the correct procedures and guidelines when operating transformers to ensure their safe and efficient operation. Any deviations from these procedures can have severe consequences and should be avoided.