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Transformer impedance

Transformer impedance

Transformer impedance

Hi all, this is my first post ever.

I have recently graduated, so in a way, I'm still studying.

I'm trying to find some kind of transformer impedance "range" for different types of transformers, to use in short-circuit calculations.

It's my understanding that, you never get to know the "real" %Z of a transformer in the "project" stage, not until you get to see its nameplate, and by the time you actually get to see the nameplate, you're supposed to be in the "construction" stage, so that's one of the reasons I'm looking for some kind of "design ranges". But looking on the net for information, I've come up with some questions:

1. When you order a transformer; can you specify the impedance?
2. How do YOU get your information when you do short-circuit calculation and it's a new project?
3. In IEEE std c57.12.00 and c57.12.01, it says that you can find the typical impedance values from the "Product Standards", what are these?

I would also appreciate if you could tell me which IEEE standards have this information. I've found that the color books may have this info, but also the C57 series and the NEMA standards, or if I have to get it from manufacturers (if this is the case, how?, web info or direct asking?)

Sorry if there are any grammatical errors, the thing is I'm from Mexico, and sorry if I ask too many questions, but like I said, even if I have graduated I consider myself to still be studying.

Thanks in advance, I would really appreciate any information, or at least a north to do more research, seriously thanks. smile

RE: Transformer impedance

Thank you very much smile I'll check the info and get back at you.

RE: Transformer impedance

Never stop stuying

RE: Transformer impedance

Especially studying grammar and spelling. lol

If we learn from our mistakes I'm getting a great education!

RE: Transformer impedance

Transformer impedances vary in various markets and with different utilities.In many utilities it was evolved based on historical reasons.Initially transformer engineers were not aware of the importance of impedance and went for as low a value as possible to limit regulation.Later when system fault levels went up need for a minimum impedance to limit fault current became evident.In general US has lower impedance values than UK.In India it is somewhere in between.
An authentic source for transformer impedance values in different countries is IEC Technical Report IEC/TR 60909 -2Ed2.0-2008-11,Short Circuit Current in three phase AC systems -Part2 Data of Electrical equipment for SC calculations.Of course your local transformer manufacturer can guide you regarding local norms for transformer impedances.

RE: Transformer impedance

Transformer impedance is uslaly specified by the end user. ANSI calls for a standard tolerance of +/- 7.5% to allow fault current calculations.

RE: Transformer impedance

Around here if the utility will use the lowest impedance of any particular size transformer.  
If they have 200 transformer in stock with impedance 5.5% and 10 of impedance 3.5%  they will use (and tell you to use) 3.5%.  The theory the lower impedance tranformer has a chance of showing up on your job.   

RE: Transformer impedance

Thank you all for your quick answers smile

So in the end, the best source will always be the manufacturer.

Do they charge for this information?

RE: Transformer impedance

The vendor will not charge you for asking the transformer impedance values. With reference to section 5.8 of ANSI C57.12.00 "Preferred standard values of impedance are INCLUDED in the product standards for particular types of transformers"

When you specify the preferred impedance for a given transformer, it must balance the following criteria.
a.) The impedance must be high enough to limit short-circuit currents to an acceptable values.

b.) The impedance must be low enough to keep steady-state regulation to an acceptable values.



Murphy's Law - If anything can go wrong..it will.

RE: Transformer impedance

Thank you very much for the information smile.

RE: Transformer impedance

1)  You can specify the impedance.  The farther away from a "standard" impedance you specify, the higher the probable cost of the transformer.

2)  Start with industry "standard" impedances.  In the U.S., they are found in the C57.12.xx series of standards (the product standards).  Each of these standards covers a specific range of transformer types (i.e. C57.12.26: IEEE Standard for Pad-Mounted,Compartmental-Type, Self-Cooled, Three-Phase Distribution Transformers for Use With Separable Insulated High- Voltage Connectors (34 500 Grd Y/19 920 V and Below; 2500 kVA and Smaller) and C57.12.20:  Standard For Overhead Type Distribution Transformers, 500 kVA and Smaller: High Voltage, 34500 Volts and Below; Low Voltage, 7970/13800Y Volts and Below.  There are probably similar sets of standards in the IEC world.

3) Some of the product standards explicity mention impedances, but some of the small transformer types do not.  See C57.12.10 for typical transmission impedances (paragraph 4.6 & Table 10).


RE: Transformer impedance

Thank you very much!

I'll look for the info in those standards smile.

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