Linked in Comments

7/28/2019 Linked in Comments

1/14

An interesting observation regarding CO & CO2 generation for a transformer

loaded upto maximum 50% of its capacity

The CO & CO2 has increased over a period of 4 months. CO by 300% in first 4 months & then

by 30% in next 4 months while CO2 increased by about 35% & then actually decreased by 18%.

I can understand the next 4 months it can be just a bad sample/analysis but still with 50% load itis hard to explain the increase in first 4 months. The load shows abot K factor of 5. Any

thoughts?Is it possible to have localized insulation heating due to harmonics without an incraese in top oil

temp?

Vijayan Thaithodan CO2 & CO is absorbed from atmoshere also, throu conservator in transformer with

out air bag and through leaks. In case ratio of CO2/CO is with in limits no action is required

Aniruddha Narawane Jorge: Unfortunately I do not have the details but the percentages. All

other gases are within limits including Hydrogen per the communication I have received.CO/CO2 will generally point out to cellulose due to overheating but in this case the load is max50% so only possibility I see is some sort of localized heating of insulation which may not

necessarily increase the top oil temperature above IEEE limits. My guess is either there could be

problem at a lead or at another spot or the harmonics are causing something. I am not convinced

about the second probability though.

Lawrence B (Larry) Rudolf A couple years ago, we experienced a significant rise in in CO & CO2 on a

pair of new N2 blanketed transformers when they were initially energized. In the first month, both gas

levels rose to IEEE condition 2 levels, then on to condition 3. All other gas levels remained normal.

Transformers were loaded at less than 50% nameplate rating, there was no significant top oil

temperature, the cooling system was operating properly and the gassing rate was not load dependent.The manufacturer claimed the excessive CO & CO2 levels were likely caused by localized heating in areas

where excessive insulation was applied in manufacturing. Monitoring the gas levels, CO2 & CO both

exceeded the thresholds for utilizing the CO2/CO ratio. Both units had a ration of less than 5, when this

ratio should normally be greater than 7. CO & CO2 levels eventually stabilized in the condition 3.

Problems with the cooling system can cause overheating and increased CO & CO2 levels, even during

low load conditions. If you haven't already, you should verify that your oil temperatures are normal and

the cooling system is working properly. We have discovered closed oil valves, cooling control problems

and defective fans and/or pumps when oil temperatures are high and load was low.

P Ramachandran Abnormal CO 2/CO ratios were seen where certain grades of densified wood ornitrile rubber sheets( under the internal body of transformer for vibration reduction) were used.

venkataratnam bodapati sir,may be excessive insulation(crepe paper, Kraft paper ) applied on leads for

meeting the high volatage clearence with less gap B/W live leads to tank walls.Due to this reason on the

suface of live leads heat is not transfeered properly to oil.insulating paper inner layers slowly burns and

produce CO and CO2.
http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=97724603http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=97724603http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=28715179http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=28715179http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=172031305http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=172031305http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=24657393http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=24657393http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=103064636http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=103064636http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=103064636http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=24657393http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=172031305http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=28715179http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=97724603


2/14

Michael Miller A few questions:

Is this a new unit with no DGA history?

Has any work been done inside the unit?

Is this the first time the unit has been loaded this much?

Is this unit in parallel with another unit and if so do their impedances match up?

Aniruddha Narawane Larry: Thanks for the information, the temperatures are well within limits. The

transformers are not placed near the wall or the cooling is not blocked-internally & externally. Also the

ratio of CO/CO2 are generally more prominently viewed in case of power transformers since there is no

difinitive standard for DGA in case of distribution transformers

Venkat: This could be a possibility but other similar transformers at different sites do not show this. The

insulation is standard for all these types( voltage & BIL class)

Michael:These are units with 3 dga samples taken after put into service with CO & CO2 gases being the

only issue, all others being stable. The units are not loaded over 50% of their capacity. The avg load has

been below 50%. These units are on a common bus in groups of 3 or 4. I feel that the only suspect is

some harmonics. Question is can these harmonics (up to a level of K5) start localized heating ofinsulation?

P.Ramchandran: The units are being manufactured with standard insulation board used as antivibration

pads & did not have issues so far in past.

Thank you all for sharing the views.

My other view is all the DGA is done as per the standards written for power tarnsformers with a greater

volume of oil. I believe there is no definitive comment regarding distribution transformers.

Preparing to transport large oil immersed transformers.

I am looking for written information on how to prepare to transport large oil immmersed powertransformers. If you have any written information, or know where to find it, could you pleaseshare with me?

Carlos G. Hi Jose, in general for large units my advice would be to do the following:

- Drain all the oil from the unit and store in appropriate oil storage and transport containers.

- If you can, perform a Sweep Frequency Response Analysis (SFRA) with the transformer fully drained

but still with all its accessories and bushings. This will be useful to detect any coil movement after the

transformer reaches its destination.- Remove all external accessories (i.e. radiators, conservator tank, bracing & supports, detachable

cooling headers, bushings, large boxes, etc). All these accessories should be packed separately,

preferably in wooden crates suitable for the distance you'd like the unit to travel.

- While removing all these accessories, blanking plates with gas-tight seals and gaskets should be used to

cover all openings and flanges of the main tank.

- Use dry nitrogen (less than 0.05% of moisture or impurities and -50oC of dew-point) to pressurise the

unit. Anything above atmospheric pressure would suffice, but it would be preferable not to exceed 3

p.s.i. of pressure.
http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=136155914http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=136155914http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=28715179http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=28715179http://www.linkedin.com/groups?viewMemberFeed=&gid=772397&memberID=60391486http://www.linkedin.com/groups?viewMemberFeed=&gid=772397&memberID=60391486http://www.linkedin.com/groups?viewMemberFeed=&gid=772397&memberID=60391486http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=28715179http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=136155914


3/14

- If available, it is ideal to attach an automatic pressure regulator with a spare (full) nitrogen bottle in

case there any small leaks, this guarantees the main tank will remain pressurised. This is particularly

useful for long trips or extended waits.

- In parallel with this, you would ideally plan the route where the transformer is to be transported in

order to insure all road permits and proper clearances are checked (you don't want to get to a bridge to

find that the transformer doesn't fit, or worse, actually hit something along the road).

- Also, if you have an impact recorder, now would be the time to fit it to the main tank.

- Upon arrival, repeat this process in reverse, very much in the same way you would assemble/install a

new unit.

If you need additional info, please don't hesitate to ask.

Good luck, Carlos

P R. Please refer to IEEE standard C57.150-2012 Guide for transportation of Transformers and Reactors

rated 10 MVA or higher

I have a question for the group. What might be the best welding test method to

verify the transformer has no leaks prior to shipping?

Please contact me direct - [email protected]

Amitkumar Singh Ryan,

Inspect all seals and gasket joints to insure that no leaks are occurring.

The simplest method for testing for leaks is by gas pressure. The gas space in the unit should bepressurized at 5 PSI with dry nitrogen. The gas pressure should be monitored for a period of

approximately 24 hours.

A change in pressure does not necessarily indicate a leak. Any temperature increase or decrease in the

transformer will result in a subsequent increase or decrease of the gas pressure in the unit.

Ambient temperatures and tank pressure should be monitored for a 24 hour period.

If there is a significant drop in pressure during the 24 hour period, without any accompanying significant

decrease in ambient temperature, the tank must be checked for leaks. Re-pressurize the tank at 5 PSI if

necessary.

Using a solution of liquid soap and soft water, brush all weld and threaded joints above the oil level, all

bushing gasket flanges, and all hand hold cover gaskets. Any leaks in the gas space above the liquid willbe shown in the form of soap bubbles

Varghese Ittan The method explained by Mr.Amithkumar is the one usually followed by large capacity

transformer makers.When we apply 5 psi pressure on top of conservator oil, the pressure developed at

the pressure relief device(PRV) may be more than the operating pressure of the PRV.So it is advisable to
http://www.linkedin.com/groups?viewMemberFeed=&gid=772397&memberID=24657393http://www.linkedin.com/groups?viewMemberFeed=&gid=772397&memberID=24657393http://www.linkedin.com/groups?viewMemberFeed=&gid=3134122&memberID=26398849http://www.linkedin.com/groups?viewMemberFeed=&gid=3134122&memberID=26398849http://www.linkedin.com/groups?viewMemberFeed=&gid=3134122&memberID=154923908http://www.linkedin.com/groups?viewMemberFeed=&gid=3134122&memberID=154923908http://www.linkedin.com/groups?viewMemberFeed=&gid=3134122&memberID=154923908http://www.linkedin.com/groups?viewMemberFeed=&gid=3134122&memberID=26398849http://www.linkedin.com/groups?viewMemberFeed=&gid=772397&memberID=24657393


4/14

remove the PRV and temporarily blank the opening before applying pressure.

Varghese

P Ramachandran The leak test in a transformer is done at various stages. As per IEC 60076-1 ed3.0-

2011, it shall be with 30kPa over the normal oil head in transformer for 24 hrs for trfs >20 MVA or >72.5

kV. For lower ratings pressure test is for 8 hours(clause 11.8)

When tank is fabricated, it is done with water filling and equivalent pressure generated at tank bottom.

At factory and at site (if demanded by user) it is done by applying dry air pressure inside the air bag or

over the oil level in conservator.Another way is to presssurise oil inside the tank after closing valve to

conservator using oil flter machine so as to get equivalent pressure at tank bottom.This will avoid air

dissolving in to oil.

Need Help For Leaky Bushings Quick Fix Compound.

I have been asked to formulate a patching compound for leaky bushings and would like some

feedback as to what properties are required in such an application. For example arc resistance oradhesion to oily substrates

Keith Ellis Hello Andreas:

First are we discussing power transformer bushings or distribution transformer bushings?

If it is a power transformer bushing that is leaking it's oil it is time bomb waiting to go off and patching

the leak is not a viable solution. Replace the bushing as soon as possible!

For distribution bushings without capacitive graded active part, the oil leak is generally the transformer's

oil and stopping that type of leak is viable, short term option until a more viable long term solution can

be applied.

You will need to discuss glue compatibility with transformer oils with oil experts.

Ty Lutes I agree with Keith, for power transformers, you always want to replace a leaking bushing,

especially 69kV and above because of the voltage stress. Because transformer oil expands and contracts,

you could also see bushings with compromised gaskets be completely filled in the site glass. This is not a

good situation either, especially during heavy loading.Mike McCravey I agree with Keith and Ty on this. Biggest concern would be how the oil interacts with

your compound and if acts the same with different transformer oils. Another concern is will the

compound adhere and stop the leak without having to drop oil level down below the bushing.

Nick de Lange Hello Andreas,

I faced the same problem/question a view years ago. There are three types of bushing leakage, on the

top, the porcelain and the bottom gasket. I found for every leakage another compound to use. Till now


5/14

we used all these methods a few times but in nearly all cases these solutions were not a permanent

solution. Before you use a compound to stop the leaking, make sure that the bushing is still in good

shape. Just replace or repair is always better and sometimes even cheaper.

Leaking on the top (separate bushing oil): two compound (Hyosol company, provide the company with

the specifications of the oil) or bleu silicone (locktide). You have to be sure that no oil, moister or dirt is

left on the surface.

Broken porcelain: two compound epoxy (Hyosol company, provide the company with the information

about the surface, in this case broken porcelain). When the oil actually came out of the bushing I

strongly recommend to replace the bushing...

A leaking gasket on the bottom of the bushing: Al trough I strongly recommend just to replace the

gasket, there is a compound that may help to stop the leaking. This is Belzona two compound. The

surface must be completely clean, so it is better to lower the oil level just beneath bushing flange and

then clean the surface. Please contact if you need more detail information.

Is viscosity of the oil relates with dielectric strength? if it relates how?

Luke Parthemore I do not see how viscosity would directly correlate with dielectric strength as

measured by ASTM D877 or D1816. That is, are you comparing 2 new mineral oils with different

viscosities?

However, if you are asking about aged oil, natural ester, or other fluid that has its viscosity increase

while in the unit, then that could also have lowered dielectric readings depending on how the fluid is

aging or if the unit has a fault.

Sergey Korobeynikov Pulse dielectric strength of viscous liquids strongly depends on viscosity. E.G. at

voltage time to breakdown is proportional to viscosity

Luke Parthemore Are you considering switching to a more viscous liquid? Are you changing from oil to

natural ester or heavyweight hydrocarbon? These other fluids have higher viscosity than mineral oil but

have the same/similar dielectric breakdown values.

Georg Daemisch I do not really understand this discussion. It is really new to me, that there is a

relationship betweeen viscosity and dielectric strength. Nevertheless there are other things to consider.

We need naturally a certain dielectric strength of our insulation liquid and the design will be based on

that. On the other hand we need to transport the heat of the windings. That means lower viscosity. As in

all engineering we need to make a compromise between both things. If you have higher viscosity you

need bigger channals to move the liquid. Therefore, if you use liquids of higher viscosity, you must adopt

the design for that

Sergey Korobeynikov Georg, when I tell about relationship between viscosity and electric strength I

mean pulse electric strength mainly. It is well known fact that breakdown voltage depends on viscosity.

In 1966 Z. Krasucki in experiments with hexachlorobiphenil had shown that time to breakdown change

from 10 microseconds to 10 seconds when viscosity change six order of magnitude. The explanation of

this data is in bubble model of breakdown initiation. Pulse electric strength at time duration of voltage

action change several times.

I think that bubbles plays key role in AC breakdown initiation too.

As for as the heat transfer I agree with you


6/14

Xiao Yi The viscosity might influence the bubble dispersion. If you measure the BDV based on multiple

shots, longer interval time should be set aside for bubble dispersion in viscous liquids otherwise you

have reduced BDV.

Xiao Yi I agree with Sergey that viscosity would influence bubbling during streamer propagation. For

different liquids, the influence is different as reviewed in this paper

www.rle.mit.edu/cehv/documents/62-IEEEEIM.pdf

Sergey Korobeynikov Xiao, I have own program that perform to compute a lot of pulse breakdown

dependencies including viscosity dependence for different liquids...

Mel Wright No.

Viscosity has nothing to do with the Dielectric Strength of a insulating fluid.

Luc Loiselle Hi Mohamed. The dissolved decay products or DDP (ASTM D 6802) and the turbidity

(ASTM D 6181) increased as the oil is ageing. So the the viscosity (ASTM D 445) is increasing too. Perhaps

for new oil, it's something else as explained previously.

I haven't done tests yet on that issues with the breakdown voltage. Perhaps, what I can say : if DDP and

turbidity are increasing as free radicals are increasing too (I have done measurement by DPPH) I guess

the breakdown voltage is getting lower because of those unwanted decay products... Hope it help!

Michael Miller Changes in viscosity will effect the oils ability to transfer heat an thus cool the

transformer.

Mark Lashbrook Viscosity may not have a direct influence on breakdown for small gaps under AC

voltage, in clean oil. However if we consider oil contaminated with particles, such as cellulose fibres,

then the viscosity of the fluid effects how these particles move. There is experimental evidence to

suggest that a more viscous liquid maintains higher breakdown values when contaminated, since the

motion of particles is impeded, preventing them from lining up in the electric field. Since all

transformers will pollute the dielectric fluid, to a greater or lesser extent, a more viscous fluid may have

advantages in this sense. As with most things in real dielectric systems the answer is not simple and as

Georg has said there needs to be a balance between dielectric strength and thermal performance.

Hugh Leyton Hi Mel,

Thanks. . . . I can not see that there would be any direct relationship between viscosity and insulation

properties, although I do get Xiao's comment about bubbles. . ... .But surely, no bubbles should be

formed, unless something has been stressed too far . . . . .I would have thought insulation properties

would be tied to the actual oil material being used, not its viscosity.

It might just be coincidental that some of the better insulating oils have a similar viscosity.

I get the feeling that if Pulses can cause a problem, then it sounds to me as if these pulses are large

enough to cause partial breakdown of the oils insulation properties....If this is continued repeatedly,

then I can see a larger breakdown following . . . .But surely, that is because its voltage limits are being

exceeded. ?

But, at the end of the day, the oil is primarily there to conduct the heat away by convection, so to be

efficient at that, it needs to be fairly low viscosity, so as to move the heat away readily.
http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=59895048http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=59895048http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=59895048http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=59895048http://www.rle.mit.edu/cehv/documents/62-IEEEEIM.pdfhttp://www.rle.mit.edu/cehv/documents/62-IEEEEIM.pdfhttp://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=34243907http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=34243907http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=26850121http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=26850121http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=58436895http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=58436895http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=136155914http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=136155914http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=86995301http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=86995301http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=175483702http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=175483702http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=175483702http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=86995301http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=136155914http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=58436895http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=26850121http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=34243907http://www.rle.mit.edu/cehv/documents/62-IEEEEIM.pdfhttp://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=59895048http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=59895048


7/14

Perhaps some oils have a higher, what is the word, "specific heat capacity", to be able to carry more

heat than some other oils

Mel Wright Huge:

Your correct. Viscostiy has NOTHING to do with the Dielectric Strength but after the arc in oil has

occured then other things can occure depending on:

1) is the current shut off or continuious. What is the flow rate of the oil being or over arc location. If fluid

is silicon does it leave a filiment of silica glass due to no flow contidion. All of these have an effect ONLY

AFTER the inital breakdown.

Askarels (PCB fluids) and mineral oil (MO) have the lowest viscosities compared to Silicon, R-Temp, FR3

and but they still have very similar dielectric strengths.

Chemical structure and purity from particles and polar contaminates such as water or conductive

particles are the main issues affecting dielectric strength.

Low Energy Degradation Triangle

(LEDT)

Introduction

Power transformers remain one of the key components of any power network and althoughpassive in nature internally it hosts a dynamic environment of magnetic forces, chemical

reactions and electrical activity which has to maintain the finest equilibrium to ensure long

term sustainability.

The proposed model, Low Energy Degradation Triangle, is composed from the three

dissolved gases hydrogen, methane and carbon monoxide. These three dissolved gases

generally start to be formed from low energy degradation processes within the power

transformer. The three gases are plotted as a triangular plot on a XY plane allowing for

creation of specific regions and trends as the transformer insulation degrades under

different operating conditions. The nature of the model is that it is sensitive to both

degradation of the cellulose and oil insulation and the amount of energy that may be

present.

The Low Energy Degradation Triangle provides an early detection of a change in

transformer health from normal to defective state. This model is potentially effective when
http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=26850121http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=26850121http://www.linkedin.com/groups?viewMemberFeed=&gid=769857&memberID=26850121


8/14

applied to on-line dissolved gas samples were trending of dissolved gases play a key role in

detecting incipient changes in the level of insulation degradation. The advantage of this

method is that it allows for the identification of a change in transformer health status caused

by degradation mechanisms developing from low energy sources. The Low Energy

Degradation Triangle has been successfully applied to the GSU transformer fleet within a

large Power Utility where significant defective transformer health statuses have been

identified and highlighted as a warning for intense monitoring.

Figure 1: LEDT

Energy Triangle Concept

It is found that H2 increases with increasing energy. CH4 starts to develop early from oil

degradation. CH4 however, due to chemical bonding, at higher temperatures start to

decrease in percentage formation. CO starts to develop early from paper degradation. CO

also, due to chemical bonding, starts to decrease with higher temperatures.

In a power transformer due to the breakdown of oil and paper carbon oxides and

hydrocarbons are produced. Due to the paper breaking down at much lower temperatures

than the oil, the CO levels tend to be higher than the H2 and CH4 at these temperatures

(usually at operating temperatures below 110C). The hypothesis that these three dissolved

gases can provide some indication of the insulation degradation is tested. Thus the

combination of these three gases in a triangular plot enables the interdependent

relationship to exist. For lower temperatures the CO values are high and the H2 and CH4 are

low. For moderate temperatures both H2 and CH4 tend to be higher than CO. For high
http://1.bp.blogspot.com/-ruSK2BgnZSE/UZ25boYSX7I/AAAAAAAAABE/xdZxa4t9CWo/s1600/LEDT.jpg


9/14

temperatures H2 tends to be the dominant gas. As long as there is oil and paper insulation

with elevated energy levels (temperature) this theory should hold true due to the chemical

nature of cellulose and hydrocarbon oil.

The proposed three dissolved gases (hydrogen, methane and carbon monoxide) are plotted

as a triangular plot on an XY plane similar to Duval triangle [Moodley2]. These three

dissolved gases form the sides of the triangle and are represented as percentages having a

summation of 100%. Each side of the triangle has a zero starting point vertex reaching 100

% on the far side. Movement along the triangular plot is clockwise for each of the three

parameters. Figure 1 provides an indication of the general layout of the triangular plot where

for further reference the starting vertex on the left is denoted as point O, top apex is

denoted as point M and the right vertex is denoted as point H.

For example if hydrogen is 40 ppm, methane 10 ppm and carbon monoxide is 150 ppm the

sum is 200 ppm which is equivalent to 100%. The composition of hydrogen is 20%,

methane 5% and carbon monoxide 75%. This is plotted as point X in figure 1.

Hydrogen levels increase steadily over the fault range implying that the level of energy also

increases with increasing hydrogen. With increasing fault energy, and depending on the

involvement of oil and paper, the levels of methane and carbon monoxide also increase but

then start to decrease for higher energy levels. For medium energy conditions, all three

parameters are in the range 30-40% thus the point of intersection is located in the region at

the centre of the triangle. Low energy conditions mean low levels of methane and hydrogenwith some carbon monoxide thus the values are focused on the lower left side of the

triangle. Extremely high energy conditions as a result of arcing conditions usually imply high

levels of hydrogen with decreased levels of methane and carbon monoxide such that the

intersect points are focused on the lower right hand side of the triangle. More specific

conditions and locations are explored further with empirical evidence.

Let me know what you think!

I will present more detail and findings in further blog updates.

Case Study

Background:


10/14

A 700 MVA GSU transformer indicated increasing gassing trends before shut down in 2004

for an internal inspection. Prior to this incident the transformer was located with a different

generating unit and was taken off line due to an increase in acetylene levels from June

2002. The core and core clamps megger tested down to earth with the visual inspection

revealing insulation damaged between the core and core clamp feet. The transformer was

repaired and was commissioned in the new location (unit) on 23 May 2003. Shortly

thereafter, the dissolved gas analysis of oil samples taken on 27 June 2003 indicated sharp

movement towards elevated degradation energy levels as depicted on the Low-Energy

Triangle plotted in figure 1 Subsequent oil samples indicated a worsening state for the

transformer.

On 16 May 2004 the transformer was taken off line for an internal inspection. During the

internal inspection it was found that the blue phase HV exit lead conductors had burnt away.Upon further inspection it was also found that the LV winding exit leads showed signs of

severe overheating.

LEDT:From the LEDT in figure 1 it can be seen that the oil samples for the year 2000 and 2001

were within the normal region. The first trigger was received on 26 July 2002 where

subsequent samples were focused on this region. With the onset of acetylene levels the

transformer was taken out and then repaired.

Figure 1: LEDT Case Study 1 [Moodley1]
http://1.bp.blogspot.com/-xDp4ZSrwt1g/UacgM365PjI/AAAAAAAAABU/_ym8OiXpHV4/s1600/Case+Study+1+LEDT.JPG


11/14

After the repair, the reinstallation oil sample taken on the 23 May 2003 was within the

normal region. Although apparently repaired after the 2002 incident, the LEDT once again

indicated, within a few weeks of being returned to service on the 27 June 2003, that the

transformer was again in a defective state. Samples taken on the 8 July 2003, 7 October2003 and 5 November 2003 indicated a progressing defective state. Between December

2003 and May 2004 the LEDT indicated a steady increase in the level of degradation. The

slightly erratic path to failure is probably related to the variability of the analysis of the

dissolved gases under changing conditions of ambient temperature and transformer

loading.

Influence of Geomagnetic StormsDuring the investigative process the impact of geomagnetic storms on transformer condition

was attempted due to the evidence that GIC induce circulating currents in powertransformers giving rise to low energy thermal faults. The proposal that the LEDT was

sensitive to the effects of low energy degradation was also tested in this regard.

After reinstallation on 23 May 2003, the first solar storm was recorded on the 29/30 May

2003. The next oil sample recorded on the 27 June 2003 was in the defective region.

Subsequent samples taken on the 8 July 2003, 7 October 2003 and 5 November 2003 were

all preceded by solar storms as presented in figure 1. It is found that the solar storm events

correlated very closely to the changes in dissolved gases as presented in figure 1.

Reference[Moodley1] Moodley, N., Gaunt, C. T., Developing a Power Transformer Low Energy

Degradation Assessment Triangle, IEEE PES Power Africa Conference and Exposition, Johannesburg,

South Africa, 9-13 July 2012


12/14

Ferroresonance Occurrence In Power Transformer (photo by SNC Manufacturing Co. Inc.)

Ferroresonance occurs when l ine capacitance resonateswith the magnetizing reactance of acorewhile it goes in and out of saturation.

Ferroresonance is usually associated with potential transformers, which areinstrument

transformersthat are used to develop voltages used by relays; however, it can also occur with

power transformers under special circumstances.

Ferroresonance is another occurrence that can cause equipment damage; fortunately, it is preventable

by simply avoiding certain types of transformer connections with the types of circumstances that enable

it to occur.

Because these connections are routinely avoided in practice, ferroresonance is not encounteredvery often and there isnt much information about it in the literature.
http://electrical-engineering-portal.com/voltage-instrument-transformer-vt-introductionhttp://electrical-engineering-portal.com/voltage-instrument-transformer-vt-introductionhttp://electrical-engineering-portal.com/voltage-instrument-transformer-vt-introductionhttp://electrical-engineering-portal.com/voltage-instrument-transformer-vt-introductionhttp://electrical-engineering-portal.com/voltage-instrument-transformer-vt-introductionhttp://electrical-engineering-portal.com/voltage-instrument-transformer-vt-introduction


13/14

Ferroresonance is worthy of mention, however, because it can utterl y destroy a transformer.

Figure 1 - A situation where ferroresonance may occur. The cable capacitances form three

parallel L-C elements that are in series with each other and the source voltages.

The necessary conditions for ferroresonance are established in the system shown in F igure 1.

In the example shown in Figure 1, the -connected tert iary windingof a largethree-winding

substation transformersupplies a distribution type station-service transformer with a Grd.-Y

primary winding. The supply lines to the station-service transformers are through a set ofshielded cables. If the cable runs are fairly long, a significant amount of phase-to-ground

capacitance may exist.

When atransformer coreoperates near saturation, the B-H curve is highly nonlinear, and the effective

permeability of the core can take on a range of values that vary with the changes in flux density.

Each of the inductances shown as L1, L2, and L3, will have instantaneous inductance values

that are proportional to the effective permeability of the core at any given instant in time. These

inductances form parallel L-C circuits that are in series with one another and in series with thesource voltage.
http://electrical-engineering-portal.com/delta-star-transformer-connection-overviewhttp://electrical-engineering-portal.com/delta-star-transformer-connection-overviewhttp://electrical-engineering-portal.com/delta-star-transformer-connection-overviewhttp://electrical-engineering-portal.com/delta-star-transformer-connection-overviewhttp://electrical-engineering-portal.com/when-transformer-has-a-stomach-pain-and-wants-tohttp://electrical-engineering-portal.com/when-transformer-has-a-stomach-pain-and-wants-tohttp://electrical-engineering-portal.com/when-transformer-has-a-stomach-pain-and-wants-tohttp://electrical-engineering-portal.com/when-transformer-has-a-stomach-pain-and-wants-tohttp://electrical-engineering-portal.com/delta-star-transformer-connection-overviewhttp://electrical-engineering-portal.com/delta-star-transformer-connection-overview


14/14

Since L1, L2, and L3 are constantly varying along with the effective permeability of the core, it

is almost certain that a series resonant condition will exist at least part of the time during every

cycle.

When a series resonance exists, even for a brief period, this causes very large voltages across the L-C

elements. These voltages are capable of destroying the transformer and any other equipmentconnected to it.

The nonlinear nature of this problem makes mathematical analysis virtually impossible, but thephenomenon has been observed both in the field and experimentally, and the voltages have beenmeasured and recorded.

In the example above, the conditions for ferroresonance can be disrupted by the simple expedientof -connectedsecondary windingto the station service transformer.

The -connected windingassures that the vector sum of the voltages of all three phases add to

zero, stabilizing the neutral point of the Y-connected primary windingand preventing excessivevoltage across the windings. The presence of a -connected secondary winding will essentially

snuf f out ferroresonance in this circuit.
http://electrical-engineering-portal.com/power-transformer-construction-windingshttp://electrical-engineering-portal.com/power-transformer-construction-windingshttp://electrical-engineering-portal.com/power-transformer-construction-windingshttp://electrical-engineering-portal.com/power-transformer-construction-windings

Linked in Comments

Documents

Transcript of Linked in Comments