Wind farm step-up transformers - excessive gas problem 8

[ters]

Please ignore my post just below this one, it is a duplicate - accidently pressed submit before finishing Subject line.

A small wind farm of five units connects to a 44kV utility line. The facility is operating for about 1 year and experiences high level of gasses at turbine step up transformers 34.5kV/690V. Turbines are 2.2MW operate at 690V, step up transformers are 2.5MVA, while the main 44/34.5kV transformer is 11MVA.

Here is a long list of fact and things which were measured, analyzed or tried:

Gassing only occurs at all 2.5MVA units while the main 11MVA transformer is fine.

2.5MVA transformers are connected to full power converters, AC-DC-AC.

The oil gas analysis (done a number of times) indicates an increased level of hydrogen, ethane, methane and acetylene, which suggests presence of partial discharge but it is unclear what may be causing it. Content of gasses increases over time ad similar rate.

The entire 34.kV collector, which is a total ~20km long is undeground cable. There is also several km of 44kV cable before the facility connects to the utility OH line.

Gas level is unrelated to the collector topology and geography – there are two 2.5 transformers close to the main substation, and one shows a much higher (2 times) level of gas than the other, and the same applies for other two units which are >10km from the sub.

Total harmonic distortion is measured and found to be in the range of 2-3% at full load, which should not be causing gassing as the full load on a wind farm rarely occurs. Harmonic content is much higher at low load but that should not matter much in terms of gassing.

The owner ordered a replacement transformer which was made a bit larger, 2.6MVA and for significantly higher V/Hz ratio then the original ones, but after several months in operation that transformer developed the same problem. The harmonic factor for this transformer remained 1, as it was for the original unit.

Various P-Q analyzers were used to measure V, I, P, Q and TDH over a longer period of time and capture events, but nothing particularly significant was captured.

Except it was identified that when utility is switching a large 30MVA capacitor bank some 30km away on the same feeder, an overvoltage spike in the range of 150% and lasting about half cycle reaches the 2.5MVA transformers, but the capacitor is being switched relatively infrequently, the average is probably less than one per day.

Turbine converter reactive capability is higher than transformer rating (2.7MVA vs 2.5MVA) and under certain operating conditions units may operate with a low power factor, but transformer apparent power still very rarely exceeds 2.5MVA and may only occur for a short period of time when only one or two units are operating at low kW output. In which cased they will be ordered by an automatic controller to import a larger amount of VARs from the grid (the facility operates at fixed power factor mode).

Various sources indicate that many wind facilities experience somewhat increased level of transformer gassing, but that this phenomena is still poorly understood and science is still to explain it. There are various hypotheses why this is occurring and mine would be that this problem may have something to do with the fact that wind turbines are the most unstable generators by far in terms of power output and event not that many loads are as unstable. They are not as bad as arc furnaces but have a lot of similarities except changes are slower.

However, transformer gassing at this facility has already reached very alarming levels, and there must be something else in addition to that phenomena which affects many other facilties. But we seem to be running out of ideas what to try next. Any comments would be greatly welcomed!

Than you for your time to read this long story .

 

[ters]

We are aware of other wind farms experiencing this problem but looks like the extent varies widely. In some other cases the gassing level seems to be increased but not alarming. Looks like there is no clear scientific explanation as why this is happening and various theories are floating around. And while cases may differ, it still must have something to do with the nature of wind power - extremely unstable. There may be some direct relationship between ever changing power output and the hypothesis about gassing being induced by core static charges I mentioned in the previous post, but I cannot seem to understand its mechanism.

 

[Marmite]

The phenomenon of a core building up an electrostatic charge is well understood, and is not related just to wind farm transformers. Any conducting parts of a transformer will build up a charge unless they are solidly bonded to earth. The core is earthed at a single point only, to avoid circulating currents. The bond has to ensure good electrical contact, otherwise arcing may occur which generates gases. Sometimes the core-earth bond is internal, sometimes external to allow disconnection for testing of the core insulation to earth.
A transformer sees only voltage and current. Voltage and current are the same quantities regardless of the nature of the energy source driving the turbine.
Regards
Marmite

 

[Skogsgurra]

"Voltage and current are the same quantities regardless of the nature of the energy source driving the turbine"

Exactly the oppsite is true. A wind turbine with a PWM inverter between generator and transformer produces lots of ill effects that you do not have if the generator were feeding the transformer directly.

Gunnar Englund

http://www.gke.org

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

 

[jghrist]

Could the problem be related to capacitance of the 34.5 kV cables and resonance with the transformer reactance? Extensive 34.5 kV cable systems are not common (at least in the USA) except for wind farms. Maybe this is the source of the correlation of the problem with wind farms.

 

[Skogsgurra]

Yes. That is What I tried to convey in my 6 Mar 13 18:18 post.

Gunnar Englund

http://www.gke.org

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

 

[ters]

In this particular case, the hypothesis abut collector resonance was considered (and partly modeled) but it was too fuzzy to find any sort of logical explanation for it given the diversity of actual oil tests:

There is a turbine which is just 200m from the substation, it has its own feeder (not daisy chained with other units), its collector lenght is neglifeable but its transformer is still developing about the same amount of combustible gasses as for another turbine which is on a different feeder and daisy chained with 3 other units and located 14km away from the substation. While two other units neither very close or very far from the sub represent the best and the worst case...

In summary, if the turbine at 200m is producing gasses at amount of 100% (a relative number), then here is what other 3 turbines are showing:

Unit at 200m, it own feeder; gasses 100%
Unit at 14km, different feeder; gasses still about 100%
Unit at 3km, different feeder then for the above 2; gasses 50%
Unit at 4.5km, same feeder as for 3km; gasses 200%.

Can the collector resonance result in such rather random impact on individual transformers which seem to have nothing to do with individual feeders, lengths, a particular location in the daisy chain (seems not to matter at all) or anything else?

At the same time, the main 44/34.5kV transformer which is just 200m away from the am turbine develops very little gasses (but comes from a different manufacturer and is of different design).

 

[Jebb]

You had said one transformer was removed from service for internal inspection and the manufacturer's report was that they found "nothing wrong with it". Did they just untank it for a visual inspection and did they also take the windings apart? Maybe a more detailed inspection by a third party is in order. The manufacturer may not be so interested because it's only 5 units so it may be time to try another xfmr manufacturer. Are the other wind farms experiencing issues using transformers xfmrs from the same manufacturers? I've seen issues before with a reputable manufacturer having QC problems after moving operations to a different location. The xfmr passed the factory QC test but the xfmrs failed months or even years later. It was determined that it was an issue with a "winder" at the facility. Xfmr building is much as an art as it is a science so there may be some manufacturing issues with the current vendor.

 

[ters]

For simplicity reasons I described one facility with 5 units, but there are actually two similar facilitates at the same location, connecting to the same utility line (separate connections) experiencing very similar problem. In summary, there are nine 2.5MVA transformer all gassing more than what appears to be normal, but it varies - as indicated above the worst case producing 4 times more than the best.

There are also two very small station service padmounts (50kVA) coming from the same manufacturer also gassing. But 10m from them, two main step-ups to 44kV being 9 and 11MVA don’t produce any excessive gas, however they come from a different supplier.

I had no opportunity to see internals of the removed transformer, but pictures taken by other parties indicate that they just un-tanked it for a visual inspection without taking windings apart. There are also no details that fuse assemblies were inspected at all.

Various web sources talk about wind turbine transformers gassing but we don’t have enough empirical data to compare with this specific case. There are verbal reports that another facility, not far away for the one we are discussing, is also gassing but that one uses different turbines (another manufacturer), transformers are also made by another supplier and it is connecting to a different utility line. But we don’t know to which extent and if is comparable to this case. However, that facility uses high ratio transformers stepping up from 690V to 44kV (the collector is 44kV) which may have something to do with this phenomena, as in either case 34.5/0.7 and 44/0.7 the ratio is fairly high. Before wind and solar times, the industry did use some high ratio transfomers, but for generation this sort of ratio was less common at the medium voltage or any level.

There are also reports that some wind facility in the same region don’t bother doing oil test at all so they even many not know that they could have a similar problem.

Trying another transformer is already suggested so it is up to the owner to assess the risk vs. cost to do so.

 

[Jebb]

Have you thought about problems with the oil itself. Take a look at this article from NETA Link . It could be something to look into with a oil testing lab like Doble, SD Myers, etc.... The strange thermal cycling from the wind machines conbined with the oil type could be a possible source of gassing.

 

[prc]

I could not comprehend the failure analysis by IEEE engineer for wind turbine transformers.Not convincing. Can you give the title,author & the session/year of the IEEE meeting.
I rule out resonnance and overvoltage. The gas pattern correspond more to overheating. Are you noticing high oil temperature during full load running of transformer.There is possibilty due to harmonics.
In fact new IEC standard IEC60076-16 ed1.0-2011 Transformers for wind turbine applications stipulate special requirements to meet these service conditins.

 

[acog]

Gents,

The transformer pictured and many transformers at wind installations in the states have under oil HV isolation switches. These switches are in the same oil as the transformer core and windings. If the HV isolation switches have been operated under voltage (which may form part of the turbine maintenance routine), gasses will be generated in the oil. This is normal.

The DGA experts on this forum may like to comment on whether this is consistent with the gas results you have.

The above conclusion does not explain why the 50kVA pad mounts are also gassing though. Perhaps a partial discharge test is in order?

 

[ters]

Thank you all for replies.

Jebb, yes we did suggest trying different oil in one of the units but it is not up to the owner to decided.

Prc, I just googled a specific sentence from that document and found it also available as public here:

http://grouper.ieee.org/groups/tran.../distr/F12-TransformerGassing&CoreGrounds.pdf

Written by this fellow:

http://www.hvolt.com/hopkinson.htm

I also could not fully comprehend it since this is not an actual paper which elaborates details but just seems to be the summary of a presentation.

Acog, the actual transformers at this facility have internal fuses (two in series) but no disconnect switches in the oil. Individual turbines can be isolate only on the LV (690V) side or the entire strings can be isolated to 35kV but opening an interrupter in the substation.

 

[prc]

ters, Thank you for the presentation. It is a probable core grounding probem that may arise in case of a particular type of distribution transformer design( wound core shell type).This design is used up to 3MVA by some manufacturers and please check your transformer is as per this design. The issue is, in case of wound core HV winding will be facing the core widthwise and in case core is grounded at outer lamination there is chance of high voltage getting developed between laminations causing PD in oil film resulting in H2 generation. The gas pattern reported by you does not tally with that and hence this chance can be overuled. This is the same case with gassing from thermal stress in oil.

Normally manufacturers take care of above type of core grounding issues.Voltage ratio is not an issue as today there are step up transformers working with 22/1200 kV voltage ratio (China)

So I will stick to my earlier conclusion.Manufacturer can help in diagnosis. Try with another make step up transformer

Can the star /delta winding connnection with 5 limbed core can result in some abnormality? Why in this type of aplication star/delta is used as in normal generator transformers it is always delta/star?

 

[mopractice]

We are experiencing the same problems with step up transformers at wind farms. In one phase of the project we have 16 turbines and about half are gassing and half are not, the transformers are all identical. We have done extensive testing in the collector system, including high frequency, and we can confirm that our problem is no caused by resonance or other issues in the collector.

The gas results indicate partial discharge, however not all of our transformers are gassing. We have swapped gassing transformers to locations where there is no gassing taking place. The "good" transformer maintained low gas and the "bad" transformers continued to increase hydrogen levels - indicating that the problem is internal and not collector related.

Core grounding and charge build up sounds like a good explanation but we do not see gassing in all of the transformers???

Some of our units have over 20,000 ppm hydrogen, in a couple instances the hydrogen levels went down since the last test. This may seem dramatic but load tap changers and voltage regulators have higher hydrogen levels.

We continue to look for answers.

 

[prc]

mopractice, in your case only H2 or other gases too? Can you share typical values. I ruled out core grounding issue in this case as in addition to H2 there is appreciable amounts of Methane and Ethane confirming a high temperature fault at around 200-500 C.

 

[ters]

Prc, thank you for you posts again.

mopractice, it is good to know that I’m not suffering alone . Thank you really very much for your post. Very informative.

If I understand your post well, you had two project phases, so transformers were not made at the same time, maybe one year apart, but still came from the same supplier? If so, the fact that one phase transformers will gas no matter where you put them suggest that there might be a manufacturing flaw with that particular series which may possibly be related to items which area already mentioned by other fellows above, like fuses, clips or other components in the oil (switches, taps, etc). Those third party components may be different (different make or model) between phases even though transformers are otherwise the same. Please keep us posted if you nail it down and I will do the same.

But even if this hypothesis is correct, it still may have something to do with the unstable nature of wind generation, or maybe not given the in the distribution world the transformers with internal fuses and switches did not seem to be very common.

 

[ters]

Prc, re your star/delta question, for all wind installations I have seen so far, the star was on the low side which connects to the converter.

 

[mopractice]

This oil test result is from one of the gassing transformers, taken in Nov 2012.
First number is a "bad' transformer the second is from an identical transformer installed in the same time frame

Hydrogen 11291 41
Methane 588 4
Ethane 83 2
Ethylene 1 1
Acetylene 0 0
Carbon mon 517 537
Carbon dio 1040 802
Nitorgen 67833 75255
Oxygen 10631 14498

There are 31 transformers installed on this phase of the project, all from the same manufacturer
Two have greater than 20,000ppm Hydrogen
Five are betweeen 10k and 20k
Three are between 5k and 10k
Nine are between 1k and 5k
Eleven have less than 1000 pph hydrogen (most less than 100ppm)

Several transformers have had decreases in hydrogen since the prior test. For example one dropped from 21k to 8.3k in a 5 month perios. More samples are being taken this week so we will see...