Tap changer oil samplig
Tap changer oil samplig
(OP)
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.






RE: Tap changer oil samplig
RE: Tap changer oil samplig
RE: Tap changer oil samplig
RE: Tap changer oil samplig
RE: Tap changer oil samplig
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?
RE: Tap changer oil samplig
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RE: Tap changer oil samplig
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.
RE: Tap changer oil samplig
But if the oil is from main tank, then we have to worry.
RE: Tap changer oil samplig
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
RE: Tap changer oil samplig
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.
RE: Tap changer oil samplig
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
RE: Tap changer oil samplig
RE: Tap changer oil samplig
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.
RE: Tap changer oil samplig
RE: Tap changer oil samplig
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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RE: Tap changer oil samplig
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.
RE: Tap changer oil samplig
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RE: Tap changer oil samplig