Transformer Polarity

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For most single phase transformers, polarity is unimportant, all that matters is that the turns ratio must be correct for the intended application (plus of course suitable power rating and design)

For a 3 phase transformer intended to work on its own, polarity is umimportant.

For a 3 phase transformer to work in parralell with one or more other transformers, then polarity is important. Transformers of differing polarity wont work in parralell, no matter what alterations may be made to the external connections.

A 3 phase transformer intended to supply utilisation voltages normally has only 4 output terminals, one for each phase, and a fourth terminal for the neutral, intended to supply 3 phase, 4 wire.
There will be 3 windings inside the transformer, with one end of each winding connected together internaly to give the neutral.
If two otherwise identical transformers are manufactured, one with the bottom ends of each winding connected together, and the other one with the top ends connected together, then these transformers are said to have opposite polarity.
Each of them will work just fine on its own and provide a standard 3 phase, 4 wire service, but they will never work together.

It would be possible to build a 3 phase transformer with each end of each low voltage winding connected to a terminal, 6 terminals in total.
Such a transformer could be connected with either polarity, by altering the external connections.
This is not usual in practice as it adds to costs.

All British Standard transformers are built with the same polarity, and I would presume that similar standards exist in other countries.
 
broadgage said:
All British Standard transformers are built with the same polarity, and I would presume that similar standards exist in other countries.
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In the US, the standards call for different polarities depending on the size of the transformer. Some day 'utilities' and 'industries' will live in harmony.;)
 
In addition to what has been icluded in the previous posts I would also like to add that paying attention to vector diagrams is extremely importance.
I have attached a couple of charts showing transformer vector diagrams as related to how transformer windings are configured. You may have a give phase sequence at the primary of transformers by that on the secondary may be different depending upon how the transformer windings are configured. You will need to pay attention to the nameplate diagram on the transformer.
Most of the time simply assuring that you have the same phase sequence is enough but knowing that there is the posibility of a phase shift is also important to remember.
 

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templdl said:
In addition to what has been icluded in the previous posts I would also like to add that paying attention to vector diagrams is extremely importance.
I have attached a couple of charts showing transformer vector diagrams as related to how transformer windings are configured. You may have a give phase sequence at the primary of transformers by that on the secondary may be different depending upon how the transformer windings are configured. You will need to pay attention to the nameplate diagram on the transformer.
Most of the time simply assuring that you have the same phase sequence is enough but knowing that there is the posibility of a phase shift is also important to remember.
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These are some additions tables for vector diagrams.
 

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don_resqcapt19 said:
I believe it is also important when connecting 3 single phase transformers as a 3 phase bank.
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It depends on the design of the single phase transformers.
If both ends of the lower voltage winding are brought out to insulated terminals/bushings then polarity is essientialy arbitary and of no importance. Apparently incorrect polarity of one transformer may be easily corrected by swapping over the two lower voltage connections.

If however one end of the lower voltage winding is internaly bonded or connected to the grounded frame or tank of the transformer, then this remedy is not possible since the grounded end of the winding must form the neutral.
Therefore three single phase transformers for use as a three phase bank MUST have the same polarity if one end of the winding is grounded internly.

In some cases, both the high voltage terminals are insulated for full line voltage, but one end of the lower voltage winding is grounded.
In such cases, swapping over the two high voltage connections will correct apparently incorrect polarity.

UK practice is normally that a single phase, two winding transformer will have four insulated connections, two for each winding, and all insulated for the maximum line voltage to ground.
Such a transformer can be connected with either polarity.

Contary to USA practice, even small 3 phase services in the UK are normally from a 3 phase transformer, rather than from 3 single phase transformers as is usual in the USA.
 
In my job it is extremely important if we are building an overhead 3 phase bank. Get an additive mixed in with a couple of subtractives and the voltages will be extreme for two legs. You can see the relationship if you draw it out. Say we have an open delta (with two pots) with the angle at 60 degrees and one leg center grounded. Everyone knows what the voltage should be. Now draw this out with the angle at 120 degrees. the voltage between the two ends will be somewhere around 400-420 volts. (side angle side)

the polarity will determine if the pots are going to give the correct voltages at the correct angles. If you switch the polarity, you change the "leg" 180 degrees. We can switch the polarity as broadgage mentioned by either swapping the high side or low side, I prefer the high side.
Like he stated, it usually isn't important unless you are banking.
 
Why?

Why?

Thanks for all the info above, I figured I would follow up with another question. Why? Why have each is there an advtange to one over the other that makes one superior? Is there any specific type required for different protective relaying schemes? Just trying to wrap my head around why not just standardize on one type?
 
Mike01 said:
Thanks for all the info above, I figured I would follow up with another question. Why? Why have each is there an advtange to one over the other that makes one superior? Is there any specific type required for different protective relaying schemes? Just trying to wrap my head around why not just standardize on one type?
Click to expand...

I think Mexico is all additive and Canada is all subtractive. We are in the middle. I'm not 100% sure, but I believe in the early 80's the industry standardized somewhat. Below 8660 volts and 200 kVa, additive. Above 8660 and 200 kVa, subtractive.
Our new distribution pots are additive, out station power transformers are subtractive.
The polarity is determined by the way the windings are around the core. If you know about the right hand rule, use this and "picture" the current movement through the core.
 
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