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.

 

[HamburgerHelper]

Waross,

The condition is described in the three leg section of this link. This isn't a nackfeed issue. Three legged cores always pass flux through the tank walls or oil. It just usually is not enough to matter.

https://generalpac.com/transformers/open-phase-condition-in-transformers

To me, what they should have done was specify a 4 leg core so that there would be a path for the zero sequence flux and it wouldn't path through the tank. Why they didn't start making 4 leg cores the company standard after the first incident is beyond me. Utilities get paid here on capital investment. It would not have been hard to get the cost of an extra leg included in the capital investment.

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

 

[waross]

Thanks HH.

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

 

[7anoter4]

Edison123 said:
higher impedance means longer limb and hence higher transformer. Is this correct?
In my opinion it is not.
The reactance of two windings [primary and secondary] transformer it is[approximate]:
X=2πfμoN^2/Lc*Lmt((d1+d2)/3+s) where d1 and d2 are the thickness of the primary respective secondary bobbin , s it is the space distance, N it is primary windings number of turns, Lmt average length of a turn and Lc bobbin height [or length].

 

[prc]

7anoter4 - That statement was from me. Edison only wanted to reconfirm. You are right and I am sorry I misled. It is the reverse ie higher impedance shorter limb and lower impedance higher limb. But frankly this will happen only for substantial change in impedance as impedance variation is always achieved by changing the number of turns. For large transformers limb height is fixed from transport limit considerations. Thanks for correcting me.

 

[prc]

HH- I think this problem of exorbitant tank heating will occur only with Ynyn connected distribution transformers and not with Dyn (universally used in India and we never heard of such a problem )or Ynyn units with a stabilizing winding. There is an excellent tutorial by Westinghouse engineers of 1978- Distribution Transformer Application considerations presented at Power System District engineers conference. I could not locate the link and don't know whether it can be posted here. Parts of it were later incorporated in C57.105 standard.
Them my query is- in India all sub transmission units ( 220~66 / 11 kV ) units are Ynyn without stabilizing tertiary but with 3 phase 3 limbed core. Here also we never heard about tank heating but sure there must be unbalanced loading ,but may be no operation on two phases. Or with solid grounding, with low earthing factors (Xo/X1) this will not happen?

 

[prc]

bacon4life - How this thermography was taken? From outside tank? The red patches are of shield? Then it looks bit strange for me. Usually the shields will be cooler than near by tank.Normally transformer designers follow two methods to mitigate the impact of leakage flux (extra load losses from eddy currents and hot spot temperatures in tank)- magnetic shielding where packets of silicon steel laminations fixed on tank(10-40 mm thick) covering the winding height + 1000 mm.The flux coming from top of winding enters the shield moves down and re-enter at bottom of winding. Since silicon steel provide a high permeability path with very low losses, leakage flux will not enter in to steel to create hot spots. Another way of creating magnetic path is providing shields (made of silicon steel laminations) at winding top to direct winding flux back in to core.
Second method is electro-magnetic shielding where conducting material(copper 5-6 mm thick) or aluminum ) is used to cover tank on areas where flux impinges. Flux will create an eddy current in copper sheet that will repel flux back in to the gap between tank and winding thereby shielding the tank surface. This will be effective where high MMF is developed on thank eg: high current leads, LV pockets of GSU etc.

 

[Mbrooke]

@PRC: Why was wye-wye chosen over delta wye? Soley because of the insulation reduction? Or other reasons.



Desperate question if anyone can answer it: How does impedance play out in a 3 winding delta-wye-wye 230/13.8/13.8kv? I'm seeing PSE&G doing this to limit fault current while using larger trafos and am considering the same.

 

[prc]

Mbrooke: Frankly I don't know. From Davidbeach I learnt that in US they have many different connections for distribution transformers. Probably most of the technology developed there and this may be an evolutionary issue. In India we have only Dyn11 for DTs.
You must get HV-LV1,HV-LV2 and LV1-Lv2 impedances ( from rating plate or test report) so that one can arrive at H,L1 & L2 impedances. In case you have only one or two, I can make a guesstimate provided you give me as much details as possible- application , rating plate etc.

 

[waross]

Impedance is first dependent on inductive reactance and resistance. Leakage flux is an additional factor.
Inductive reactance depends in part on the square of the number of turns, however the Volts per Hertz ratio must be observed.
Reducing the number of turns requires an adjustment to the core dimensions to adjust the acceptable V/Hz ratio.
The adjustment of the core along with changing the number of turns may in turn affect the length per turn of the windings.
Changing the number of turns and possibly the length per turn will affect the resistance.
Now add to this the additional factors such as leakage flux and changed winding diameters due to changed core dimensions and to allow for cooling passages.
Remember that the inductive reactance is governed by the square of the number of turns. As impedance is predominated by inductive reactance, the most effective way to change the impedance is often to change the number of turns. This will generally have an effect on the core dimensions and the winding resistance.
My old transformer book states that there is an economic "Sweet spot" for transformer impedance that is close to standard ratings.
Significant changes in impedance, either up or down generally incur added cost. (And often added weight)


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

 

[waross]

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

Manufacturers tend to favour the economic sweet spot first and impedance second. Standard ratings are probably close to the economic sweet spot.


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

 

[bacon4life]

prc- I added an approximate outline of the silicon steel tank wall sheilding. The bottom and left of the tank is warmer as indicated by the arrows where no sheilding exists. The radiators take up the top half of the photo so the upper half of the tank wall is not really visible. We specified a high loss factor, so I think the extent of the wall sheilding optimized to reduce losses more than to control for temperature. This is a wye-delta-wye autotransformer.

 

[Mbrooke]

@PRC: I do not have any actual 3 winding units on hand. Its a concept I just digging into as a possible means to serve more load while reusing existing equipment. 115kv/13.8/13.8kv at 100MVA is what I'm thinking of.

 

[controlsdude]

30 years ago when i had a job winding transformers i saw a pattern on the % of impedance of transformers low to high side physical locations.

If the % impedance was > 3-5 the low side winding was next to core and there was a fixed distance air/insulation barrier to the high side of transformer.

If the % impedance was <3 (down to 1%) part of the low side was next to core, with a solid physical insulation barrier then the high side was wound on top of the 1st low side winding. Then a physical barrier of insulation was put on high side, then a 2nd low side winding was wound on top of insulation barrier.

 

[prc]

bacon4life - Now I understand. What you have taken is a thermography from out side of transformer. I am afraid we cannot say what you have seen is any hot spots from leakage flux from winding. Normally the shielding will cover the winding top to bottom and it should not create any hot spots outside. It can be from say, high current leads too.

Mbrooke- There are several types of three winding transformers. The unit that you mentioned may be of two LVs of equal rating and equal impedance to HV. This is normally done when LV breaker level exceeds. In this arrangements the LVs will be put one above the other (axially split LVs) and the impedance between the LVs will be double of the LV to HV values. The LVs are loosely coupled and hence loading of one LV will not affect the terminal voltage of other LV -a requirement for such transformers used at Power stations as start up transformer.
When LVs are of different MVA ratings(but total same as HV) they will be arranged radially and the individual circuit impedance (not LV to HV) shall be inversely proportional to their ratings to get loading proportional to their nominal ratings.
Controlsdude: This is done to reduce the weight even with reduced impedances. In effect, instead of X kVA , you are putting two X/2 kVA windings in parallel . This arrangement is followed even in very large GSUs to limit transportation weight and height.

 

[Mbrooke]

@PRC- how do they compare? Say I have a 115/13.8kv 50MVA 10% Z transformer, how would the fault current/impedance appear on one secondary of a 115/13.8/13.8 100MVA unit?

 

[prc]

Mbrooke - Impedances for the 100/50-50 MVA 115/13.8-13.8 kV axially split (LV2 placed axially above LV1 with HV axially split one above the other)) transformer will be Impedance HV-LV1 =Impedance HV-LV2 =10 %on 50 MVA base
LV1-LV2 =20 % on 50 MVA base; Impedance HV/LV1+LV2 (ie both LVs simultaneously loaded) =11.5 % on 100 MVA base.
Why LV1-LV2 impedance 20%? When you apply a voltage (as %of rated LV voltage) to LV1,with LV2 shorted (that is what is meant by LV1-LV2 impedance) , the power transfer will be from LV1 to HV (lower half) and then from HV upper half ( it is in parallel to lower half) to LV2.So this is equal to two transformers in series and hence impedance doubles.

 

[Mbrooke]

Makes sense. Much thank you- pink star approved!


Before you part- are 3 windings typical in India? Or rare? Are these 3 winding units or 2 winding units? They look like 2 winding- but the diagram almost hints at 3:

https://youtu.be/E3Ar1dzrq1U?t=24

 

[prc]

The diagram shows YNyn ie two winding transformers. In India up to 100 MVA YNyn transformers are with out stabilizing winding. Above this rating, stabilizing delta tertiary is normal. In a way, you can term them as three winding units. Multi secondary transformers are used - In thermal power stations as start up transformers 6.6-6.6 kV or 11-11 kV. These are 50-100 MVA units. Another application is large steel mills where there are many large motors at 6.6 or 11 kV. So multi secondary transformers of 50-200 MVA with different voltages for secondary windings are common. But in transmission substations multi secondary units are not preferred though was common early days. Many units used to fail due to inappropriate application issues -like wrong selection of impedance pairs resulting in heavy fault currents in one secondary, unbalanced load sharing etc.

Recently thousands of three winding transformers (2-12 MVA) are used here in solar farms where two primaries accept power from two inverters stepping up to one HV 11 kV winding. Such large GSUs are used in hydro-power projects to reduce number of transformers esp in South America. In India we avoid such application.

A good tutorial on Multi-winding transformers is Chapter 5 of Transformer Engineering by L F Blume (1951). This is based on a famous AIEE paper of 1924 -Theory of three winding transformers by A.Boyajin,a GE,Pittsfield veteran.

 

[prc]

I missed another recent application of multi winding GSU. In combined cycle thermal power stations, normally there will be two GSUs one taking power from gas turbine generator and a smaller one from steam generator. To reduce footprint at yard, it is possible to combine these in to one with two primaries of different voltages and ratings with one secondary feeding power to grid. I was involved in making such a 570 MVA GSU some time back. There also selection of impedance pairs is very important to avoid surprises from reactive and active power sharing.

 

[Mbrooke]

Found the PDF- epic- thanks again!

https://www.ccaps.umn.edu/documents...rPoints/2018/EconomicsofTransformerDesign.pdf