Transformer Impedance? 12

[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?


40/50/60MVA units are what I have in mind but really this applies to any power transformer.

 

[waross]

Fewer turns will drop both the resistance and the inductive reactance. This will probably require a greater cross section of core.
That will take away some of the saving in resistance.

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

 

[dpc]

The impedance is basically the leakage reactance - flux that does not link both windings. Transformer designers should weigh in, but it can be modified to some degree by winding techniques, wire type, and spacing. I think the BIL is a big factor in the impedance. The higher the BIL, the greater the leakage reactance. ANSI C57 has standard impedances based on the voltage level of the primary (high side).

 

[bacon4life]

DPC is right on.
Core shape (# of legs or shell form) and core steel material also impact leakage reactance. The order of windings also impacts leakage. For example transformer A would likely have lower leakage impedance than B:
A)core, LV LTC winding, LV winding, HV winding, HV DETC winding
B)core, LV winding, LV LTC winding, HV DETC winding, HV winding
For 3 winding transformers, the impedance to the tertiary is highly customizable based on the order of the winding.

When I asked a transformer manufacture which factors in my transformer specification could be relaxed to save cost, they seem to regard adjusting the impedance up or down by a couple of percent as a negligable-cost design contraint.

 

[Mbrooke]

Can I ask about a CWI ballast or neon transformer being put into perspective in terms of leakage reactance? To my understanding these can be operated short circuited, sometimes indefinitely.

 

[Mbrooke]

DETC winding? Sorry lol

 

[prc]

The transformer impedance (for large ratings this is almost same as % reactance, % resistance being very small) of a transformer depends on the number of turns in winding(vary as square of number of turns)and physical size of HV & LV windings(width and height) and the insulation radial gap between windings (depends on HV voltage ie BIL) During the first decades of transformer production in 19th century, engineers were trying to achieve as low a impedance as possible to minimize the voltage regulation. Later engineers understood that impedance has a beneficial effect- it gives protection during line short circuits( by reducing the short circuit current flowing through windings -it is(100/%impedance ) times the rated current of transformer).

For a particular MVA and HV voltage there is an optimum % impedance where the cost of transformer is minimum. When we increase or decrease from this value cost increases. But there is a minimum value beyond which it is extremely difficult to achieve lower values. When impedance goes up, transformer becomes a copper machine ie turns are increased, copper weight goes up, core weight comes down, copper loss goes up, core loss comes down-over all transformer weight comes down, height goes up. With lower impedance, transformer becomes core machine ie turns are less, core weight more, copper less, core loss more, copper loss less -, overall height reduced but becomes heavier. For easy memory -high impedance a slender, tall lady,low impedance fat, heavy and short lady.

Usually impedance varies between 2-5 % for distribution transformers and 6-25 % for Power Transformers. There are exceptions- for power transformers used in Power Lab (for short circuit testing)impedance will be low 1-2 % at 500-800 MVA , 30-60 % for small transformers used with power electronics ( traction transformers used inside rail coaches)

When transformer winding is provided with taps, physical radial position of regulating winding with respect to core and other windings is done such a way to get constant % impedance at all taps, increasing % impedance with increased tap numbers or decreasing % impedance with increasing tap numbers. This can become critical when parallel operation with existing transformers is required. Buyer should inform manufacturer the pattern of impedance variation of existing transformer with tap changing.

 

[cuky2000]

Below is an illustration showing some of the impacts changing the Impedance in power transformers

 

[Mbrooke]

prc & Cucky: Exactly what I was looking for!


Lastly, how does resistance and reactance change as impedance changes? Is voltage regulation linear? I'd assume more copper means higher R and the core being smaller leads to less X but I'm sure there is way more too it.

 

[prc]

Impedance (pu)= square root of (resistance squared +reactance squared ) pu resistance = copper loss(kW)/Transformer rating (kVA);reactance is calculated based on parameters mentioned earlier.

Voltage regulation = IR cos phi + IX sin phi (pu) Cos phi=power factor of load.

 

[edison123]

Very interesting thread. Thanks guys.

prc - IIRR, higher impedance means longer limb and hence higher transformer. Is this correct?

Muthu

http://www.edison.co.in

 

[Mbrooke]

Thanks everyone! This thread has been beyond words a HUGE help


@Cuky or anyone for that matter: is flux leakage that much of an issue? How much does it add to the price to mitigate overheating? And what is done exactly to mitigate it?

 

[waross]

Impedance is the result not the cause. When either the resistance or the reactance change then the impedance changes.


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

 

[HamburgerHelper]

I have seen more than one refinery with series reactances to limit fault current on low impedance transformers. Smooth move on that one.


I have also seen a plant that had breakers to short out series reactances when the plant wasn't being fed from two sources. That one is a little bit more forgivable but load break switches could have been used instead of full sized breakers.

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If you can't explain it to a six year old, you don't understand it yourself.

 

[cuky2000]

Hi MBrook,

Q1) is flux leakage that much of an issue?
a-In most cases this leakage flux is not an issue.
b-This flux links the primary and secondary winding and is responsible for the self-reactance X1 & X2. This will create a voltage drop in both windings that limit the short circuit effect.
c-The IEEE Std C57.12.10 and the IEC Std 60076-5 provide guidance to determine standard impedance. (See enclosed Impedances Graphs)
Outside of the standard impedance range, the cost of the transformer most likely will increase.

Q.2) How much does it add to the price to mitigate overheating?
a) The flux that is not linked to the winding will do with the core, tank or any other magnetic steel part inducing emf and circulating current (Eddy current) that create losses in form of heat.
b) Stray Loss inside and outside the winding is another source of heat losses
c) This heat can be localized creating undesirable hotspots

Q.3) What is done exactly to mitigate it?:
a) Transformer manufacturer optimized design sophisticated finite element analysis computer software to simulate losses and guaranty data.
b) Transformer manufacturer manages the price risk of the volatility of copper/aluminum and steel at the time of procurement. Any uncertainty will add contingency cost.
c) Use high permeability-grade, domain-refined “H” grade, cold-rolled grain-oriented silicon steel (“M” grade steel is used in some applications). Annealing all core steel after slitting provides optimal loss performance.
d) During design and manufacturing some of the strategy used is:
Reduce magwire conductor size with suitable transpositions, Use of CTC (Continuously Transposed Cable), for Tie plates/Ebars use segment steel or use stainless., Tank & Clamp - magnetic core pack shielding, use high current busbar/bushings – shield magnetic structures w/ aluminum or copper shielding or use stainless steel.

 

[HamburgerHelper]

One place that I worked at had issues with distribution transformer shell heating on wye-wye transformers with loss of phase. You get zero sequence flux that passes through the shell, the outside of the transformer, and it will heat it up until it fails. One of the newbie engineers from an ivory league school with an average engineering program thought it worthwhile to model the transformer to try to determine how much time the operator had to isolate the transformer before it was damaged. I admired his optimism to solve the problem but groaned over his underestimation of the difficulty of the problem and how much time it would take for someone to do it right. The only way that it made sense to me is that he had a supervisor that knew he was bored and just wanted him out of his hair and let him go out on a snipe hunt.

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If you can't explain it to a six year old, you don't understand it yourself.

 

[waross]

HH
Are you sure that was not an overload issue?
Was this a three phase transformer with a three legged core? Phantom delta?
I ran a community of about 5000 customers on two phases with wye:wye connections on single phase transformers for a long time with no issues.
We lost one phase of an undersea cable and we were on a paupers budget.
As I remember it was over a year until a surplus undersea cable within our budget was located.
Actually we did eventually service a generator rotor (preventative, not a failure) due to overheating from the unbalanced loading.

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

 

[HamburgerHelper]

Yes, it was a three leg core. The shell (not the core) creates a one turn delta tertiary. What was happening, was that a phase would be lost on the distribution system because the field techs would just use whatever on hand to repair the distribution lines. Some of the bolts and whatnot were not the same metal as the conductor and galvanic corrosion would rust up and form bad connections that would be worked loose by constant heating and cooling until the line fell. This was a systemic problem that was all over the system and could not be fixed easily. When this happened, all the transformers on a string would be damaged. Millions of dollars in distribution transformers. No one fixed the problem even though it was well known because no one would have seen the problem more than once in their entire career. The utility only lost a whole string of transformers every 20-25 years so it was not a problem that was begging to be fixed even if it was costly. The reason that a solution was being pushed for was that it was getting to be so bad that if some of the longer strings failed, they wouldn't be able to ready replace all those transformers. Some of the longer strings I think had like 20+ transformers. The solution that was come to was to put communication devices out on the strings so that lose of phase could be monitored. I proposed using IR cameras to try to find all the connections in the system that were weak to try to be proactive on fixing those lines before they fell and they went with doing that too. I was really hoping that they could just pull in all the customer smart meter data to determine if a a phase had been loss but they didn't have smart meters everywhere in their system yet. I too thought they might have been able to just listen to customer complaint calls as well because there had to be a bunch of customers on these long strings that would noticed when their 3 phase equipment (likely air conditioners) just quit working. The window the manufacturer gave us for how long it took our transformers to fail was around 2-3 hours.

------------------------------------------------------------------------------------------
If you can't explain it to a six year old, you don't understand it yourself.

 

[bacon4life]

One way manufacturers deal with leakage flux is by attaching layers of core steel type material to the tank wall. During a heat run test, the extent of the sheilding is quite evident as seen in the attached thermal image. The sheilding begins about 0.6 meters in from the sides and bottom of the transformer. At the bottom left corner without shielding, the tank wall was about 15 C hotter that areas without stray flux.

 

[waross]

HH Thanks for the info.
I was faced with wye:delta connections. In that case the delta would back feed the missing phase. Combine that with fuse protected distribution circuits, a work outage every Sunday and single phase switching (one fuse at a time to restore power) and the problems were endless. I suspect that you may have had back-feed overload issues as well as leakage flux. We may never know exactly.

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