Tap changer oil samplig

[npower0073]

On site we have a three winding step up transformer 440MVA, 400kV/15.75kV/15kV. Lately we made maintenacne on the tap changer (MR), changed the oil and put back in service. After 10 days we made ne oil analysis and after 40 days we made a second oil analysis. The results of the first analysis were OK, the results of the second showed onyl increased gas ppm (more than 100 ppm) and showed that very good oil dielectric strength. We suspect that there is arching inside the tap cahgner contacts. Can you please give some feed back on that issue? Next week we will make the third consequitive sampling.

 

[edison123]

MR makes one of the best tap changers. Did anyone fiddle with the spring contacts ?

 

[Zogzog]

Sampling error is possible, so a 3rd sample is the best thing to do. Can you post the DGA results?

 

[npower0073]

edison123 , we made work on the spring contacts of the diverter switch.

 

[edison123]

What sort of work was done on the spring contacts ? Were the springs replaced ? If so, were the new springs from MR ?

 

[npower0073]

The results from the oil sampling show very good dieelctric strength but increased concentration of gases.
Find below the resutls from two concequtive sampling

Hydrogen(H2) (ppm,w) 3414 25075
Oxygen(O2)(ppm,w) 8492 7452
Nitrogen(N2) (ppm,w) 120665 35969
Carbon monoxide (CO)(ppm,w) 14 148
Carbon dioxide(CO2) (ppm,w) 71 141
Methane (CH4) (ppm,w) 406 7887
Ethylene (C2H4) (ppm,w) 531 5405
Acetylene(C2H2) (ppm,w) 627 7352
Ethane(C2H6) (ppm,w) 368 1534
Propene(C3H6) (ppm,w) 68 1610
Propane(C3H8) (ppm,w) 0 434
Propyne(C3H4) (ppm,w) 8 455
Boutane (C4H10) (ppm,w) 0 0
C2H2 / C2H4 1,18 1,36
CH4 / H2 0,12 0,31
C2H6 / CH4 0,91 0,19
C2H4 / C2H6 1,44 3,52
CO2 / CO 5,22 0,95
C2H2 / C2H6 1,7 4,79
C2H4 / C3H6 7,81 3,36
Dissolved gas (ppm, v) 134664 93463

Can you pelase advise?

 

[electricpete]

This is normal for some types of tapchangers (called arcing types). I don't know if yours is one of those types.



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[subtech]

Most troubling are the Ethane, Methane, and Ethylene numbers. These are "heating gases", or more accurately stated, over heating gases.
Something in the TC is getting very hot.
Also of concern is the Hydrogen. This occurs with partial discharge or low level "corona".
Was the "changed" oil new, or from another transformer?
The nitrogen doesn't hurt anything, but the numbers are typical of oil that has been under a nitrogen blanket.
If the oil came from another transformer, this may partially explain the CO and CO2 numbers. CO2 occurs naturally in main tank oil and is a result of the natural degradation of cellulose insulation. CO in transformer oil typically signals very rapidly degrading cellulose insulation and is not natural.(over heating main tank oil that has a high moisture content drives increases in CO)
If this oil came from another transformer, you need to look carefully at that unit. If not, it very much looks like the oil from the main tank of the subject transformer is mixing with your tapchanger oil. (bad seals)
It seems to me a serious investigation of this unit is very much in order.

 

[prc]

When you say tap changer oil sample,it means the oil that you have taken out from diverter switch for sampling.Why you did DGA on it? Normally the only test done on this oil is BDV and moisture content.MR has indicated limiting values for these parameters.Arcing will be there in diverter switch whenever tap changing is done.So the type of readins that you got are quite normal.

But if the oil is from main tank, then we have to worry.

 

[defreitas]

In my opinin, when talking about an OLTC you are always going to have dissolved gasses, which are created every time that there is tap change.

As far as I can see it would be similar results if you did a DGA on an oil sample from an oil filled CB or RMU that have frequent operation, in which case as long as you know that the Dielectric Strength is ok, it is all good.

Have you noticed any carbon in the oil? That will be something that will be present in the OLTC oil a lot quicker than in the main tank.

Dan de Freitas

 

[npower0073]

subtech , the oil was new, not from another transformer but from new drums.

prc, i mean oil from the tap changer diverter switch tank. THe BDV is very good, 60kV and higher.

defreitas, I have not seen any carbon in the oil. BUT during the first 30 months of transformer operation there was no sampling point thus no sampling oil taken. During last outage we put new sampling pipes, made maintenance and installled the oil filter OF 100 of MR. When we took of the old oil it was COMPLETELY DARK and was very thick, like oils of machines.

 

[isquaredr]

npower0073
The oltc you talk about sounds like the combined selector / diverter type (maybe seperate and you are looking at the diverter tank), if this is the case every time a tap change occurs arcing takes place, the arcing is similar to an oil circuit breaker interupting fault current, this too generates combustable gases and carbon.
I would be worried if these gases were not present ie has the tap changer been stuck and not working correctly.

If the gases you talk about are in a seperate selector tank then I agree this is very bad and urgent investigation is required.
hope this helps

 

[npower0073]

isquaredr , The diverter switch is not separate from the tap changer, is separate from the main oil tank.

 

[subtech]

Different gases occur for different reasons.
The rapidly elevated Ethane, Methane, and Ethylene numbers signal something very hot in the compartment.
Normally operating circuit breakers (and tap changers) generate very little if any Ethane, Methane, or Ethylene.

 

[npower0073]

an isoaltin and earthing, we lokked and found air on the bucholz relay of tap chager compartment, also there was air when we vented. But the oil (probably due to the filter ) was almost clear.

 

[electricpete]

an isoaltin and earthing

Can you clarify. Are you saying you had a trip due to Bucholz relay and found gas.

Subtech and others may certainly be right that the dga alone indicates a problem. Without knowing the type of tapchanger or the normal range for this type, I personally would be hesitaant to make any conclusion for this type. Some tapchangers have resistive bridging circuits that I would assume are expected to generate heating gases.

Do you have other similar transformer tapchangers to compare to?

Also infrared scan might be interesting. Look for temperautre rise from main tank to tapchanger tank and from bottom to top of tapchanger tank. If possible compare to similar transformers, but if you have no similar transformers then as a rough thumbrule, you should see less than 5C difference on these differential temperares.

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[prc]

Whenever a tap changing is done, there will be arcing inside the diveter switch and gases also will be generated. Hence the buccholtz relay used with tap changer is a special one-it will have only trip contacts and not alarm contacts. ie it will operate only for a oil velocity and not for gas collection.

As I mentioned earlier,to my knowledge, MR recommends only BDV and moisture level for this oil and not DGA.DGA will always show all thermal gases and acetylene.When a serious fault occurs inside diverter switch,say stuck up resistor contacts or arcing to earth, then due to fault current, an oil surge will move to OLTC conservator activating the Buccholtz relay.If the pressure rise is too high it will rupture protective pressure relief device/diaphragm plate mounted over the diverter switch cover.

 

[arcflash99]

You could send your samples to TjH2B. They have special tests they can run for OLTC's, OCB's and Regulators. The test uses a different set of parameters to diagnose the oil since it will be exposed to arcing.

http://www.tjh2b.com/

 

[npower0073]

This week we made the tap cahnger inspection, it was almost new, no sign of overheating or whatever else. Seems we have to reconsider the need for gas analysis.