Why are transformers connected additive or subtractive? 2

[PE56]

What are the reasons, applications, advantages/disadvantages of connecting transformers (power or control) with an additive or subtractive polarity? Pole-mounted transformers (tank-type) that are less than 200KVA are usually connected with an additive polarity, why?

 

[Skogsgurra]

Not sure what you mean? The chosen tap? + or - a few percent? Is that it?

If so, I think it is so obvious that it is difficult to answer that question. If you need more voltage, use '+'. If voltage is too high, use '-'.


Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

I believe that the polarity is a design choice. I am not sure of the reason for a particular polarity.

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

 

[magoo2]

It sounds like it was a pretty arbitrary decision. The qualifier with the kVA range is 8660 volts and below. See C57.12.00.

Utilities run into unbalance problems when they make up a three-phase transformer bank with one or two transformers below 200 kVA and the other(s) above that. Also, if you had to parallel two ratings that straddled the 200 kVA rating (like a 167 kVA and a 250 kVA), you'd get some hefty circulating currents unless you were aware of the polarity differences.

 

[WoodrowJWeen]

Why additive or subtractive? In the words of Tevye in Fiddler on the Roof, "That I can tell you in one word...Tradition".

 

[davidbeach]

Actually, it's because they never figured out how to make it multiplicative. ;-)

 

[Skogsgurra]

I still don't understand. Additive and subtractive? Power transformers? Not PTs or CTs.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[davidbeach]

Per ANSI standards, some small power transformers have additive polarity. Into the polarity on the primary = into the polarity on the secondary. Polarity marks are also on opposite ends of the windings rather than on the same end for subtractive polarity. Ignore the polarity marks, watch the wiring and all is well.

 

[waross]

In the NEMA world additive or subtractive polarity describes the relative position of the bushings.
An additive polarity transformer will have H1 and X1 located diagonally.
A subtractive polarity transformer will have X1 adjacent to H1.
Or;
Additive polarity: H1 on the left, X1 on the right.
Subtractive polarity: H1 on the left, X1 on the left.

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

 

[Skogsgurra]

I had no idea. But life has been OK all the same. It doesn't matter, does it?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[rcw retired EE]

Gunnar "It doesn't matter, does it?"

It does if you replace a single phase transformer in a mulit- transformer, three-phase, pole-mounted bank with the wrong polarity. If you use the same physical jumpers and connections the secondary polarity will be reversed.

When I was a summer intern at the utility, the linemen made fun of me for being an "ELECTRICAL ENGINEER!" and not knowing additive or subtractive polarity. They told me if I didn't understand that basic idea I was going to blow something up.

I'm with you- just pay attention to the polarity and all is right with the world.

 

[stevenal]

I agree with Waross' answer. Here is how the two words come about:
Run a jumper from the two adjacent bushings on each winding, so your transformer is connected as an auto. When your auto is energized, it will either add to the source voltage or subtract from it. For example, Waross stated that subtractive transformers have H1 and X1 on the left. So attach your jumper from H2 to X2 and apply a potential from X1 to X2. When you measure from X1 to H1, you will find it measures less than your applied voltage confirming the polarity is subtractive. With an additive transformer, your jumper will connect X2 to H1 instead.

It is true I never heard the terms used in engineering school where we used o-scopes to see polarity. More of a lineman, quick check with a jumper and voltmeter in the field type of concept.

 

[potteryshard]

I seem to recall reading somewhere that lo those many years ago when distribution transformer designs were first being standardized, that most manufacturers offered either additive or subtractice polarities, but that those manufacturers all had "preferred" or "standard" polarities.

Apparently, at the time that industry-wide standards were being discussed, more of manufacturers specializing in smaller units preferred one orientation, and more of the manufacturers supplying the larger units preferred the opposite orientation. To keep the bulk of the units delivered under the "new" standard the same as most users were familiar with, the polarity flip was adopted.

I don't know if there is any truth to this legend, but it does sound like a logical way such a bizarre standards artifact could be memorialized.

 

[JJHorak]

Can some of you more experienced people comment on whether I have mixed up additive and subtractive in the two bullets?

The two versions of the xfmr arise out of different approaches to making the winding polarity orientiation recognizable without having to look at the busihg polarity marks.
- If you want the positive polarity of the HV and LV windings to consistently be on a particular physical side of the xfmr (e.g., away from the mounting brackets), you want a subtractive polarity xfmr. This design will create inconsistency in whether positive polarity of each winding is to your left or to your right as you look at the xfmr bushings.
- If you want the positive polarity of the HV and LV windings to be consistently on your left (right?) as you look at the bushings, then you want additive polarity. This design will create positive polarity that is on somewhat opposite sides of the xfmr.

 

[stevenal]

Timesabroom,

I think you have the right idea. However, I think you want to face all the low voltage bushings away from the bracket so the connections can be easily made up. And this is how they come from the factory.

The second bullet makes sense if you look at the high side bushings from the opposite side of the low voltage bushings. Might be easier to look at them from the low voltage side because all bushings can easily be seen at once. H1 will be on the left in both cases per ANSI when viewed from the low side.

To correct my post above, if the common three bushing low side transformer is used, substitute X3 everywhere I used X2 above.

 

[PHovnanian]

I don't think it makes a difference from the transformer operation or BIL point of view. But once you have one type in a transformer bank with buss bars fabricated to suit those units, replacing one with the opposite polarity can create a mess.

 

[magoo2]

Sometimes a picture is worth a thousand words. Here's an excerpt from the ABB Distribution Transformer Guide.

 

[jghrist]

Sometimes a picture is worth a thousand words. Here's an excerpt from the ABB Distribution Transformer Guide.

But what this picture doesn't show is that the secondary bushing designations are different (for the same location) for additive and subtractive polarity. With either polarity, E_H2-H1 is in phase with E_X2-X1.

 

[magoo2]

Maybe the GE version will help since it shows the two different cases and the bushings.

 

[prc]

During the first few decades of transformer engineering, there was no uniform pratice for polarity and the terms used then were positive polarity ( today's subtractive polarity) and negative polarity ( additive polarity) When primary and secondary windings are wound in the same direction,subtractive polarity resulted and when they were wound in opposite direction, additive polarity resulted.In 1920-21 NEMA formulated present convention and later ANSI adopted it in toto.

The reason why maufacturers adopted subtractive polarity as standard was it resulted in lesser voltage between HV and LV windings compared to additive polarity.For small distribution transformers this was not critical due to very low voltages and hence the reason for additive polarity for such units.