Field Testing of Oil Filled Transformers Part 1

Field Testing of Oil Filled Transformers

Field tests performed on transformer oil consist of:

Dielectric Test

Specific Gravity*

Color*

Visual*

Acidity

Sniff test*

Interfacial Tension

*Note Specific Gravity, visual, sniff, and color are not really tests, only observations for later comparison.

Dielectric  testing is performed with a hypot. A sample of oil is poured into a cup that has two electrodes set at 1/10 of an inch apart.  A controlled gradually rising current (from 0 to 50Kv) is applied to the electrodes until an arc jumps across the gap. The meter then stops and the breakdown voltage is recorded. The unit is reset for another test.  The technician performs three such tests on a sample then averages the three to come up with a reading.  For most observers any average that comes out to 27Kv or better is sufficient. Any readings lower than that can indicate a problem in the insulating value of the oil. 

When an unacceptable dielectric test occurs the testing technician first has to look at several factors. First, what does a visual observation of the sample reveal? Is there dirt carbon or other contamination suspended in the sample? Look at the bottom of the sample is there any free water present?  Was the sample collected in a clean uncontaminated container? Was the drain valve properly flushed? Was care taken to make sure that the sample was not contaminated by the technician’s hands? Is the hypot working properly? Is the electrode gap set correctly? Was there any contamination left in the cup from a previous sample?  A lot of things can make a dielectric test go south so it’s always best to confirm the readings with another properly drawn sample.

If the second sample renders an unacceptable reading as well then go back to the previous paragraph and recheck all of the items listed.  If the visual shows particles either at the bottom or in suspension then you must try to identify what the particles are.  Carbon in a sample will settle over time and create a black covering over the bottom of the sample container.  When the oil stirs carbon will wisp up looking like smoke in the liquid.  By the way, in case you haven’t guessed your sample should be taken in a dry uncontaminated container that you can see through such as a clear plastic or glass bottle.  If the sample has particles in it try to identify what it is. Anytime you have oil with particles in it I would say it’s not good.  It can be deteriorated paper or wood from the inside of the transformer or it could be media material from a filtration or reclamation process. Most of the time paper or wood from the inside of the unit will not cause a breakdown in the dielectric properties of the oil. They are good insulators, that’s why they are used inside the transformer. Experts may disagree somewhat and speculate that the deterioration of the cellulose material in the oil created a byproduct of moisture in the oil so further testing could be required.

Free water in transformer oil is the easiest to spot. Oil and water don’t readily mix and if you look closely free water will show up as tiny beads in the bottom of the sample container.

Now it’s time for a little research.  First look at the unit. Is it a sealed tank or is it open to the air? If it’s sealed does it have a positive pressure on it or a vacuum? If it has a positive pressure find out if they put a nitrogen blanket on it or something else? If something else then what? If the tank has a vacuum or the pressure/vacuum gauge reads zero check to see if there’s any water sitting on the horizontal surfaces of the tank.  If yes has there been significant temperature fluctuation lately? Could the unit have heated up in the daytime and when it cooled down at night sucked in some water sitting on it? Open or closed has there been any precipitation lately? Has the unit been serviced in some way? What was it serviced for? How was it serviced? What equipment did they use? Was the oil filtered? Was it changed out? Sometimes filter paper can absorb enough moisture from the air to add it to the oil. Thorough research can, in most cases, uncover the reasons for an unacceptable test without having to perform further tests. However keep in mind that further testing needs to be done to confirm what you may have narrowed down as the source of the problem.  A Karl Fischer Test looking for parts per million of moisture should be done. If the Karl Fischer test shows the moisture to be within limits then there could be something in the oil like fibers from the filtering media or some other difficult to spot contamination in the oil. As I said above, a little research can usually dig out a problem like this.

A color comparison may give some clues as well. Transformer oil darkens very gradually over many years.  If the oil is one digit or ½ digit different from the last observation it could just be the opinion of the tester.  It may have been slightly lighter or darker than what the colorimeter slide was showing and was recorded according to the judgement of the testing technician.  If you can go back further and find earlier test results a tracking of the progression would be helpful.  If the oil is significantly lighter (by more than 1 digit) it probably indicates that the oil has been changed out. Significantly darker could indicate an overheating problem,  the oil may have been reclaimed with an oil reclamation unit, could have had some darker oil added to it, or there could be something going seriously wrong in the transformer.  First give the oil a sniff. What does it smell like? Could there be any foreign contaminants in it? Something soluble that would mix with the oil? Does it have a burnt smell? Check the temperature gauge and feel the tank with your hand to confirm it. Most temperature gauges have a recording needle that gets pushed to the maximum temperature reached that doesn’t recede when the current temperature recedes. What does that maximum figure read? Is it unusually high?  If so try to find out why. Was there an unusually high load placed on the unit for a period of time? Was the air restricted in some way so as to not be able to flow over the cooling fins? Feel the tank and the cooling fins to make sure that the oil is circulating through both.  Along with all of the observations a color change mandates a dissolved gas analysis to rule out several potential problems. 

I’ll continue on my next post and discuss acidity. Remember if you have questions email them to me at transformerbob@gmail.com. 

Looking forward to writing again soon.

PS Check out the website I’m setting up at Transformer Oil for Dummys.

Thanks,

Bob

The holy passion of friendship is so sweet and steady and loyal and enduring in nature that it will last through a whole lifetime, if not asked to lend money.

MARK TWAIN,