Transformer Oil Acidity and Interfacial Tension

Transformer Oil Acidity and Interfacial Tension

Continuing on we have to look at the acidity level and interfacial tension of the oil. These readings should always be compared together. As oil oxidizes over time it begins to create acids within the solution. This process can be greatly accelerated by heat. In normal conditions it takes many years for the acid levels to reach a point where servicing is needed. Most transformer oil has an inhibitor added to slow down the oxidation process thereby extending the life of the equipment. Chemically what happens is that the oil molecules have spaces reserved in them that oxygen atoms will fit into perfectly. The inhibitor molecules fill up those spaces (although not as perfectly as an oxygen atom) until they are forced out at a later date by oxygen atoms. When this happens the acids formed begin to attack the tensile strength of the cellulose materials (wood and paper) in the transformer making them brittle and subject to cracking due to constant vibration. As a byproduct the formation of acids produces a tarry like substance in the oil that we call sludge. The best way to visualize sludge is to think of it as asphalt. It will deposit on the horizontal surfaces of the transformer and harden causing a furthering of the brittleness. So sludge in a transformer is a direct result of the acid formation in oil.
Acidity levels are measured on KOH per gm. KOH is potassium hydroxide and is used to neutralize the acid. The test technician will put enough drops of KOH into a measured mixture of oil and isopropyl alcohol to turn the sample pink and hold. Then he/she calculating the amount of KOH used in a set formula to render a neutralization number. This is a pretty straightforward test. There is some variability in that the technicians’ judgment is employed in determining the correct shade of pink but it’s so simple that most technicians can come up with reliable results easily and there aren’t many things that can throw off the test result. An extremely dark color can also make testing difficult.
The interfacial tension test is a test that must be done by an experienced technician to get valid results. It takes quite a bit of experience to generate reproducible IFT tests. If it’s not something that the tech does on a regular basis you would want them to run it at least three times. Interfacial tension is reported in units of force or dynes per centimeter. The technician must be knowledgeable, have steady hands, use clean equipment and have a clean sample. To better understand how delicate the test is the IFT of clean distilled water should be somewhere around 75 to 80 dynes per centimeter. That is the force that it takes to penetrate the surface of the water. That’s why some insects can “float” on top of the water without getting wet. They aren’t heavy enough to penetrate the surface.
In performing the test a sample of uncontaminated distilled water is poured into a beaker sitting on a platform mounted on the IFT instrument. The water needs to cover a platinum ring suspended on the apparatus that protruded into the beaker. The technician must then carefully pour some of the oil into the beaker slowly so that it sits upon the water. Normally about ½ to ¾ inch of water and about ¼ inch of oil. The platform is then loosened and the ring is carefully placed at the top of the water level just at the interface of the water and oil. The ring is then released from a locking device and it should float motionless in the mixture. The platform under beaker is then slowly lowered using a screw knob by the technician’s left hand. At the same time the technician exerts upward force on the ring by turning another screw knob in his right hand. Care must be taken to assure even pulling in each direction. What will happen is that the ring will get dragged down and resist breaking the surface of the water into the oil. When the ring does break the surface it will jump into the oil. At that point the technician stops and makes note of the reading on the dial of the instrument. This test requires coordination and a very delicate hand. If the readings don’t make sense then it should be repeated for accuracy. There is a plethora of reasons why an IFT test can go wrong.  The ring and beaker must be clean and free of any residue of soap. Soap will break down the interfacial tension of the water to prove this carefully place a needle on a sample of distilled water on a clean beaker. It sits on top of the water and will not sink if carefully placed. Add a drop of soapy water and it will immediately sink to the bottom. The soap has broken down the interfacial tension of the water. The ring must be cleaned and all residue removed before the test. Normally a dip in alcohol will clean it adequately.
Most of the time comparing the two will confirm a correctly performed test.  
If the neutralization number is between 0.00 and 0.10 the IFT should be between 30.0 and 45.0
If the NN is between 0.05 and 0.10 the IFT should be between 27.1 and 29.9
If the NN is between 0.11 and 0.15 the IFT should be between 24.0 and 27.0
If the NN is between 0.16 and 0.40 the IFT should be between 18.0 and 23.9
If the NN is between 0.41 and 0.65 the IFT should be between 14.0 and 17.9
If the NN is between 0.66 and 1.50 the IFT should be between 9.0 and 13.9
If the NN is greater than 1.51 its safe to say you have a problem.

What do you do if they are not in the above ranges? Same as above, we go back to the history of the transformer. If the NN is low and the IFT is low was the oil changed in the transformer? Look at the color was there a lightening from the last test?  If the color hasn’t changed was the oil filtered in some way? Possibly a “cold” oil reclamation process? Did you repeat the tests? How do they compare from the previous data if any?
What if the NN number is too high for the IFT? Check for a marked color change. Has the unit been overheating? If it is overheating is the oil at the proper level or is it low? If it’s low why? Is there a leak?
Make sure the sampling apparatus is clean, uncontaminated and that the technician knows what he is doing when he draws the sample. (Sampling will be discussed in a future post)
Finally confirm that you have the right sample. It’s easy to mix them up.

Specific Gravity is not really a test. It’s merely recording the number and comparing it to past observances. The specific gravity device used to measure most oil samples is not a precise instrument and the number that you get from it is a ballpark. The specific Gravity of water is 1.0. I have seen uncontaminated transformer oil anywhere between 0.865 to 0.890. If the oil is grossly contaminated with PCB (well over 1.0, usually about 1.5) it will be above the 0.890 mark. In many cases you will be able to detect PCB contamination just by sniffing a sample. A few manufacturers dipped the windings in PCB for testing (the test facility was indoors) back in earlier days. Those transformers have high specific gravities accompanied by a strong PCB smell.
Visual test is an observation looking for particulate matter like filter media residue or degrading cellulose material. If the sample has any free water it can be seen at the bottom of the sample. As mentioned in the other tests above if you see something in a sample that is not supposed to be there you have to begin an investigation of 1. What is it? 2. How did it get there. Again most times a second sample should be taken. If it looks like cellulose further more extensive testing should be done. In the case of water if the second sample shows it Karl Fischer tests should be performed to determine how much is in the oil and windings. GE made a great nomograph precisely for that purpose. You can find it in many places on the internet.
The sniff test is just that. The technician has to sniff the sample and determine if there is any unusual odor present. A good nose can detect arcing, overheating, PCB’s or other problems. As with the above tests always put things together and compare all of the other tests when you get an unusual result. It will help lead you to potential problems. Sometimes it will force you to perform other more expensive tests but if you saved the equipment it’s mostly worth it.

I’m working on a spreadsheet that can analyze oil test results for you  to post on my website Transformer Oil for Dummys.  Remember if you have questions email them to me at transformerbob@gmail.com. 

Regards,

Bob

“You can always count on Americans to do the right thing—after they’ve tried everything else.” – Winston Churchill