Three winding transformer

[pithpal]

Hi
I asked this question earlier but in others thread.
I recently got a demo CD from ETAP and there was this three winding transformer in which both secondary and tertiary had flows in opposite directions. and In primary ths power was being fed back to utility. May be I cannot perceieve but how two windings wound on same care and linking to one directional flux in primary and can have two different directions in secondary and tertiary. I would appreciate a word.
Regards,

 

[pithpal]

Hi Waross,
I hope u can drop a word.
Thanks

 

[waross]

Hi pithpal
Can you give me a little more information? Is the transformer energised from more than one source? What are the loads and voltages?
I'll repeat some simple stuff you already know, just so we're on the same page to start.
Basic transformer theory applies. When a flux is induced in a core, any conductors cut by the flux will have a voltage induced in them. That is each and every turn in every winding will have the same voltage induced in it.
By turn I mean one turn of a conductor around the core. By winding I mean a number of turns connected in series with the the two end connections available for use. It doesn't matter how the turns are connected in series. You can have as many windings on a transformer as you have physical space for. The voltage output of each winding will be the sum of the individual turns.
The restrictions are the voltage that the turns and windings are insulated for and the ampacity of the conductors used for the windings.
For the sake of this discussion I am ignoring special transformers such as voltage regulators, ballasts and current limiting transformers. I'm also assuming unity power factor.
Probably what's confusing you is the back EMF. As the flux increases in the core, it tends to oppose the force causing it. The back EMF in the primary winding opposes the applied EMF that is inducing it. By primary winding, I mean the one that the power is fed into.
The induced voltages in all the windings is in the same direction. The applied voltage and the resulting current is in the opposite direction. If you back feed power into another winding, the current in the primary winding will reduce to zero and then increase in the opposite direction.
Now current is flowing back into the line.
Another possible point of confussion is the nomenclature.
When someone says tertiary winding, many people tend to think, High voltage primary. Low voltage secondary, Tertiary voltage somewhere in between.
The other convention (Perhaps more precise) is; The primary winding is the winding that is energized by the source. The secondary winding is the one supplying the load.
If a transformer is tying two systems together, and power transfer can be in either direction, primary and secondary can get a little confusing. The transformer would probably be refrenced as the high voltage side and the low voltage side rather than primary and secondary. The current in each winding will reverse depending on which direction the power flow is. The flux will always be in the same direction and the current direction in the tertiary winding will be in the same direction in which ever or the other windings is acting as the secondary, or receiving power.


Let's try looking at a hypothetical industrial plant. (This plant has unity power factor and negligible harmonic distortion)
The incoming power is 34500 Volts.
The plant distributes power at 12000 Volts.
There is a legacy system at 4160 Volts.
There is a large co-generation installation. It is connected to the 12000 Volt system.
A transformer is designed and purchased with a 34500 Volt primary.
A 12000 volt secondary.
A 4160 volt tertiary.
The plant is running on the utility lines. the co-gen is off line.
The legacy system is drawing normal load.
The co-gen is started up and starts to pick up the load.
As the input power of the co-generator is increased by opening the steam valve, the generator starts to pick up the plant load. As the generator picks up more load, the current on the 12000 Volt winding of the transformer is reduced. The generator reaches the output level that equals the plant load. Now there is no 12000 Volt load on the transformer. The 12000 Volt winding current is zero, and the 34500 Volt current is the current required to support the 4160 Volt load on the tertiary winding. As the generator output continues to increase The curent in the 12000 Volt winding starts to increase, but in the opposite direction. Now the co-gen picks up the load on the 4160 Volt winding. When the output of the co-gen equals the plant load, and the 4160 load, the 34500 Volt current of the transformer is the magnetizing current. As the output of the co-gen is increased further, the co-gen supplies the magnetizing currents and the current on the 34500 Volt winding reverses and is now delivering power back into the line.
The curent in the 4160 volt winding has not changed direction, but the 34500 Volt current and the 12000 Volt currents have reversed direction depending on the direction of power flow.
Hope this helps.

 

[pithpal]

Thanks Waross. I will reply tomorrow after studying it properly and thanks again for spending some moments from your precious time.
Regards,

 

[Powerman83]

The simple answer might be that the polarity of the X and Y windings are different from each other. The polarity depends on the actual construction of the transformer for either WYE or delta configurations. If these windings are both load only secondaries it would make no differnce if the polarities were reversed. The magnetic aspects of the transformer would not be affected at all. It would all be about the connections.