Transformer Impedance?

[mbrooke]

How does changing a transformer's impedance during ordering change its construction? If impedance was not a concern (totally disregarded) during design and manufacturing, what would it typically come out to be for a given kva/MVA rating?

 

[gar]

190301-2015 EST

mbrooke:

I am not involved in transformer design.

Your first question.

Concerning series impedance. This primarily consists of two components, R and L.

One can increase the amount iron, larger core area, and in turn reduce the amount of copper, or the resistive component.

Separately for a given core area one can adjust the material, amount of air gap, and the way the coils couple to the core. Increased air gap increases leakage flux and inductance, changes in core material change leakage flux. Bifilar windings reduce leakage flux, and therefore inductance. Windings on different parts of the core increase leakage flux, and thus inductance. Tape wound cores with no specific air gap produce low leakage flux, and low inductance.

As flux density increases leakage flux increases.

Can't answer your other question.

.

 

[xptpcrewx]

Transformer Impedance?

If impedance was not a concern (totally disregarded) during design and manufacturing, what would it typically come out to be for a given kva/MVA rating?

As for the second question....
Every manufacturer would be different and it would vary based on many design factors; i.e. core type, material, voltage, etc.

Not sure if this helps but some some power engineering handbooks list typical per-unit impedances for given MVA ratings...

 

[mbrooke]

As for the second question....
Every manufacturer would be different and it would vary based on many design factors; i.e. core type, material, voltage, etc.

Not sure if this helps but some some power engineering handbooks list typical per-unit impedances for given MVA ratings...

If it helps I'm inquiring about 40/50/60MVA units- with 40MVA being the base rating. I always seem them in 8-9% impedance and wonder if thats deliberate or just typical when all else comes first.

 

[mbrooke]

190301-2015 EST

mbrooke:

I am not involved in transformer design.

Your first question.

Concerning series impedance. This primarily consists of two components, R and L.

One can increase the amount iron, larger core area, and in turn reduce the amount of copper, or the resistive component.

How does a larger iron core reduce the copper from a theory perspective?

Separately for a given core area one can adjust the material, amount of air gap, and the way the coils couple to the core. Increased air gap increases leakage flux and inductance, changes in core material change leakage flux. Bifilar windings reduce leakage flux, and therefore inductance. Windings on different parts of the core increase leakage flux, and thus inductance. Tape wound cores with no specific air gap produce low leakage flux, and low inductance.

As flux density increases leakage flux increases.

Can't answer your other question.

Well, you did answer this one I had in my mind

 

[xptpcrewx]

If it helps I'm inquiring about 40/50/60MVA units- with 40MVA being the base rating. I always seem them in 8-9% impedance and wonder if thats deliberate or just typical when all else comes first.

Gotcha. I’m not an expert on transformer impedance by any means, but I do know there is an ANSI standard out there somewhere that sets impedance requirements for power transformers. This is probably why you are seeing 8-9% for this MVA base rating.

 

[mbrooke]

Gotcha. I’m not an expert on transformer impedance by any means, but I do know there is an ANSI standard out there somewhere that sets impedance requirements for power transformers. This is probably why you are seeing 8-9% for this MVA base rating.

I'd give anything to know what the 8-9% is based on.

 

[electrofelon]

How does changing a transformer's impedance during ordering change its construction? If impedance was not a concern (totally disregarded) during design and manufacturing, what would it typically come out to be for a given kva/MVA rating?

If I may add a relevant question to the discussion: Pole units seem to have much lower impedance than pads yes? OR is that not true and just because the Pole units I tend to see are smaller than the padmounts I see?

 

[xptpcrewx]

If I may add a relevant question to the discussion: Pole units seem to have much lower impedance than pads yes? OR is that not true and just because the Pole units I tend to see are smaller than the padmounts I see?

In general smaller kVA transformers tend to have lower %Z and pole mount transformers happen to have smaller kVA ratings than pad mount units.

 

[xptpcrewx]

If it helps I'm inquiring about 40/50/60MVA units- with 40MVA being the base rating. I always seem them in 8-9% impedance and wonder if thats deliberate or just typical when all else comes first.

Can you tell me what the HV BIL is for these units?

 

[gar]

190301-2356 EST

mbrooke:

Saturation of a magnetic core is a characteristic that as magnetic flux density, note the word density, increases after some point that it gradually takes more incremental change in magnetizing force to produce a change in flux density. Most materials for transformers have a somewhat soft characteristic. But there are some with a very square characteristic. Magnetic core saturation was discovered by Edison and his crew early in the development of an efficient dynamo, circa 1878-79.

Greatest efficiency is obtained by designing a transformer to go slightly into saturation during a portion of each half cycle.

The integration of voltage vs time determines maximum flux density.

One needs to keep the shunt inductance of a transformer high to minimize magnetizing current. This does not directly relate to the series leakage inductance and mostly the shunt inductance is is ignored.

If you increase core area for a given coil, and excitation voltage is kept constant, then peak flux density is less. Thus, if the criteria for the transformer design is to work to a certain peak flux density, then as core area is increased less turns are required and copper resistance goes down.

6% is possibly to help reduce short circuit current. Ask some of the utility people.

.

 

[Sahib]

II always seem them in 8-9% impedance and wonder if thats deliberate or just typical when all else comes first.

Perhaps it is deliberate to reduce fault current.

 

[mbrooke]

If I may add a relevant question to the discussion: Pole units seem to have much lower impedance than pads yes? OR is that not true and just because the Pole units I tend to see are smaller than the padmounts I see?

I'll look at some pad specs, but pole pigs I know have very low impedance- something like 1.7% is very typical.

 

[mbrooke]

Can you tell me what the HV BIL is for these units?

450kv BIL for the 115kv side, 200kv BIL for the 34.5kv side- typical in what I see. 13.8kv is 125kv BIL if I remember right.

 

[mbrooke]

Perhaps it is deliberate to reduce fault current.

Perhaps- but why not 10% or 12.5%? I've seen 14.5% Z specd to control fault current on a 13.8kv bus. Then again 8% Z is better then 2% Z so you might be right.

 

[mbrooke]

190301-2356 EST

mbrooke:

Saturation of a magnetic core is a characteristic that as magnetic flux density, note the word density, increases after some point that it gradually takes more incremental change in magnetizing force to produce a change in flux density. Most materials for transformers have a somewhat soft characteristic. But there are some with a very square characteristic. Magnetic core saturation was discovered by Edison and his crew early in the development of an efficient dynamo, circa 1878-79.

Greatest efficiency is obtained by designing a transformer to go slightly into saturation during a portion of each half cycle.

The integration of voltage vs time determines maximum flux density.

One needs to keep the shunt inductance of a transformer high to minimize magnetizing current. This does not directly relate to the series leakage inductance and mostly the shunt inductance is is ignored.

If you increase core area for a given coil, and excitation voltage is kept constant, then peak flux density is less. Thus, if the criteria for the transformer design is to work to a certain peak flux density, then as core area is increased less turns are required and copper resistance goes down.

6% is possibly to help reduce short circuit current. Ask some of the utility people.

.

Makes sense. Does this effect the volt/hz ratio by chance? In that most transformers can tolerate +10% over voltage with a 1% frequency deviation without saturating the core excessively.

 

[xptpcrewx]

Transformer Impedance?

450kv BIL for the 115kv side, 200kv BIL for the 34.5kv side- typical in what I see. 13.8kv is 125kv BIL if I remember right.

If you look at ANSI/IEEE C57.12.10 it shows that, for 450 BIL, the impedance shall be 8.5% (without LTC) and 9.0% (with LTC) by default unless the user specifies another value. Additionally, the manufacturer must agree and the user should perform a system study to determine the proper value.

Why 8.5-9.0%. by default? Not exactly sure as the standard does not provide insight for the requirement. My guess would be: A performance acceptable, economical and practical design impedance that can be achieved for that BIL.

 

[mbrooke]

If you look at ANSI/IEEE C57.12.10 it shows that, for 450 BIL, the impedance shall be 8.5% (without LTC) and 9.0% (with LTC) by default unless the user specifies another value. Additionally, the manufacturer must agree and the user should perform a system study to determine the proper value.

Why 8.5-9.0%. by default? Not exactly sure as the standard does not provide insight for the requirement. My guess would be: A performance acceptable, economical and practical design impedance that can be achieved for that BIL.

Most likely.


Big help btw

 

[xptpcrewx]

Transformer Impedance?

Most likely.
Big help btw

One other comment... ANSI/IEEE C57.12.10 Table 3 lists the %Z based on HV BIL; and BIL is based on the weakest part of the effective insulation system (insulating medium/material and separation distance).

To achieve a higher BIL rating in a transformer would require that the HV winding be physically spaced-out further from the lower voltage windings. As the insulting medium/material and separation distance increases so does the leakage impedance, since the windings are not as tightly coupled (more stay flux = less flux linkage = higher leakage reactance).

I bet 8.0-9.5% is the ballpark figure associated with most standard designs.

Anyone else have a better explanation?

 

[mivey]

My guess would be: A performance acceptable, economical and practical design impedance that can be achieved for that BIL.

Yes.