Buchholz Alarm 1

[edfeeoc2]

in 400 MVA transformer we get every night first stage alarm at , we noticed that the alarm comes when the ambient temperature is low and unit load is low. we susbect air ingress during low temperature time when both oil and air temperature are at thier lowest value.

we performed DGA for the oil and the result was as follow

Gas ppm
H2 82
O2 5980
N2 91848
CO2 8199
C2H4 Ethylene 38
C2H6 Ethane 185

C2H2 Acetylene 0

C3H8 & C3H6
Propane & Propene 183

CH4 Methane 149

CO Carbon Monoxide 1337


we are searching for any source of air ingress through leaking gaskets on the oil pipes and coolers.

Any feed back experience please. any available procedure


Thanks

 

[ScottyUK]

Low conservator oil level can cause this problem if the level drops so far that the Buchholz relay becomes empty. Thermal expansion can cause the level change, especially on a load which cycles.

DGA levels are of far greater use when considered as a trend than as a spot value. That said, some of the values are unusually high - methane, ethane, hydrogen certainly are. What does the trend information tell you? anything rising rapidly? When was the oil last processed?


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If we learn from our mistakes I'm getting a great education!

 

[edfeeoc2]

Thanks for your good reply, i could sea during the alarm time air bubles crossing the BUCHLOZ sight glass towards the conservator tank, soon i will let you know DGA last year results.

i think the conservator is higher than the BUCHLOZ, the level of the conservator is from 50 to 60%.

we checked the upper limit for methane, ethane, hydrogen they are within limits,

any other reasons ?

thanks

 

[edfeeoc2]

here are the DGA before one year

Gas ppm
H2 8
O2 18987
N2 82141
CO2 2488
C2H4 Ethylene 10
C2H6 Ethane 47

C2H2 Acetylene 0

C3H8 & C3H6
Propane & Propene 46
CH4 Methane 37
CO Carbon Monoxide 383

thanks again

 

[prc]

From the DGA values the transformer seems OK.
Since the trip contact is not closing low oil level cannot be the reason.
Please check the silical breather and its breather pipe line for any blocking.Then such phenomena can occur.

 

[edfeeoc2]

thanks alot , i am going to check it now,

the conservator tank is equiped with air bag connected to the silicaal breather. fyi we used to vent the BUCHHLOZ we we get the first alarm then it dissapear and no trip happens.if we did not vent it with the first alarm, is it going to trip after sometime?

Thanks again

 

[edfeeoc2]

HI PRC,

we removed the oil cup at the inlet of the silica gel breather and replace the oil there was nothing abnormal,how can we check if the breather is blocked?

 

[prc]

edfeeoc2,
Trust the conservator -breather is an open breathing type and not with air cell.Some of the occasions that I noticed breather pipe blocking were 1) Compacting of gel in breather and thereby air blocking 2) Insects building up in oil cup inlet 3) Simple errors in erection-closing the pipe with a full dia gasket at one of the pipe joints etc.

I dont think the problem is due to air sucking by oil pump.If so it should have happened always.

As you may be knowing transformer oil can contain air up to 8-10 % of its volume.For every 10 C fall in oil temperature,oil volume shrinks by 0.8%.So if breather pipe is closed, during temperature drop,a vacuum will be created over the oil level in conservator which in turn allows the dissolved air in oil to escape out as bubbles.

To your question, the situation will not cause a trip.But it is not a good condition to allow to continue.Bubbling can happen between two winding discs which can trigger a voltgae breakdown. I have seen such failures.

Best way to check whether breathing is happening freely or not is to watch the oil cup during a temperature change.You must find air bubbling in or out through the oil seal.If it is not happening,check for air blocking anywhere in pipe by disassembling and blowing air.

 

[edfeeoc2]

Thanks PRC,

i would like to give some data about our transformer, it is equiped by 7 pumps and 7 horizintal coolers located above the transformer, the conservator tank contains air bag connected to the silical breather,

yesterday we noticed one of the coolers dropping very little oil, we isolated inlet and outlet valves the situation didnt improve much.

to put back again this cooler in service, is there any risk of big quantity of air to enter the transformer and triggering of BUCHOLZ?.

cuz i want to understand what you mean by (I dont think the problem is due to air sucking by oil pump.If so it should have happened always)
for my understanding , is there some points on the cooling system that its pressure varies from positive to vacuum pressure during low load and low ambient temperature?

Thanks again for your support

 

[prc]

In case pumps are sucking in air, the excess air after saturating the oil should be appearing in relay through out the day irespective of load /ambient condition.Since gas is seen only at low load with low ambient temperature,situation as I expressed can result in gas.

You can stop the leak in cooler using some epoxy compound and then put it in to use.In such cooler banks there will always be a spare cooler so that you can use transformer without one cooler also.

Since you said transformer is with air cell there cannot be any free breathing.You have to check first if there is any air outside the air cell.Normally it should be full of oil.Please check the plain window sight glass in conservator to check the oil level out side the conservator.

 

[edfeeoc2]

we removed the silica gel from the breather and today early morning again alarm. the level scale on the conservator tank showing 50 to 60 % which i believe the level of the air bag inside the conservator.is it ok to operate without silica gel for one day for test?
we always operate at this level also the other unit is the same, i will look for this plain window

if the air bag has hole, is it going to block the breathing? cuz i couldn't see any bubbles in the oil cup after removing the silica gel and connect again for one complete day.

 

[prc]

I suggest that you please go through the instruction manual supplied by the manufacturer and find out the constructional details of conservator and air cell.Some times it is a baloon (air cell) floating inside conservator.Oil will have no contact with air.Breather will be connected to inside of ballon.Outside of ballon will be full with oil.No air there.There will be two oil level indicators.One magnetic to show the air ballon level.Another oil sight window,which shall be full on all occasions.

Another alternative is a rubber diaphragm at the middle of conservator.In all these cases air must move in and out through.As per your statement it is not happening.Please look carefully why it is not happening.Someplace, it is bypassing breather?

Anyway the physics can be only as I explained-I hope so. Please investigate carefully and will be happy if you inform success.

 

[prc]

Some years back I had an experience with transformers installed at Texas in a big wind farm.In that area, ambient can suddenly change up 30 C within 30 minutes due to hail stroms.Trf buccholtz operated (both alarm and trip) The problem was air cell was blocking the out let pipe from Buccholtz to conservator( a design error)during the sudden rush of oil from conservator to main tank due to oil contraction.Then oil in relay pipe contracts and relay was getting emptied.In your case situation is not exactly same.I am mentioning this to show how many ways things can go wrong.

 

[edfeeoc2]

in fact we checked the conservator tank trere is only the level indicator connected to the bottom of the air cell,currently we stop some cooling fans at low load to maintain oil temperature above 45C not to get BUCHOOLZ alarm.

 

[ScottyUK]

prc,

"Since the trip contact is not closing low oil level cannot be the reason."

I think you are too quick to dismiss the obvious.

Normally the alarm is connected to the upper contact because this is where the gas accumulates. The lower float with the surge plate is used for the trip, and therefore the upper float will detect a low oil level by detecting the presence of gas (air in this case) in the Buchholz before the transformer trips. If you ignore the alarm and let more oil leak out, or if the temperature drops a little further, then the transformer will trip when the lower float drops. I strongly suspect this is what is happening here - seen it too often not to recognise the symptoms!

Conservator level indicators are often worse than useless, especially the pad cell type with a remote indicator when used in conjunction with a bladder in the tank. When oil level is abnormally low the bladder rests on the indicator mechanism giving a false indication. The best level indicators are sightglass types.


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If we learn from our mistakes I'm getting a great education!

 

[prc]

Scotty, I was only reiterating what you had said earlier!

You had said that trip will work when the relay is empty of oil.In this case trip is not operating so I concluded that low oil level may not be the reason.In the example I quoted from Texas,both alarm and trip operated.The air cell blocked the relay pipe end in the conservator due to sudden surge of oil back in to transformer tank. Oil in the pipe was emptied due to contraction of oil with temperature.

In India we always provide a plain oil sight window opposite to magnetic oil level guage to monitor the condition of air cell and also to see whether there is any air leakage in to conservator.On healthy condition, oil sight window should show full always.

 

[ScottyUK]

Ooops! Should have explained myself more clearly!


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If we learn from our mistakes I'm getting a great education!

 

[edfeeoc2]

when the alarm comes we vent the BUCHHOLZ then the alarm reset for three to 5 times during the low temp period.

 

[kitloke77]

Based on the two sets of test results, the rate of increase of gases within a year is about 300percent. Does this normally happen? Is this plainly ingress of air or it is fault gases, why not collect the buchholz relay gas for lab test?