Before you read my blog………
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Have you ever gotten a set of gas analysis results from your testing lab and had some questions about their diagnosis?
Why would you do that?
Maybe because you have seen some misdiagnosed problems in the past?
Maybe because you have to justify that shutdown of a critical piece of equipment for repair?
Maybe you just don’t really buy into what the test lab is telling you.
And..
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Thank you,
Bob
Bob
And Now to the current post of my blog…..
Sampling Procedures for Oil Screening Tests
In this post I will discuss sampling procedures for the oil screening test. First I want to mention that samples should ONLY be taken by experienced qualified personnel who are fully aware of the dangers involved in being around high voltage equipment. This is by no means an instructional manual on how to sample transformer oil. It is meant only to educate and so that you the reader can know whether or not the tech that you brought in to perform the sampling has done an adequate job. I cannot be responsible for any yahoo that reads this and thinks he or she can do it themselves. Again, please leave the sampling up to the experts. They know what they’re doing and how to do it without injury. Even then I have seen several accidents occur when the tech lost his focus.There are other liquids used in transformers but here we are only discussing those filled with mineral oil. Mineral oil being lighter than water, therefore the sample should be taken at the lowest point available on the transformer.
When first approaching the equipment the technician takes note of several things such as the weather, the location of possible sampling points, the overall conditions in the substation, the proximity of current carrying components, cleanliness, lighting, leaks etc. These conditions should be recorded and kept with the sample because they will figure in to the evaluation of the sample when testing is performed.
Weather- Is it rainy or sunny? If the tech isn’t willing to cash it in because of rain then use another company, chances are good the results will be fudged. For emergency purposes sampling can be performed in the rain but an uncontaminated sample is a lot less likely. Even if the substation is indoors rainy weather should be noted in case the sample containers got wet. Sampling in the rain for an outdoor substation is not recommended for other reasons as well. One never knows when a lightning strike may occur and if it does that’s not the time to be hanging out in a substation. It really doesn’t take an Einstein to know that the best time to take a sample of oil is during dry sunny weather.
Note the ambient temperature. Is it hot or cold? Cold weather can cause the oil to contract creating a vacuum in the tank. A good tech always takes note of the ambient temperature, then checks the temperature gauge on the equipment along with the red line on the gauge for the maximum temperature. Then the tech will carefully touch the tank with the back of his/her hand to confirm what the gauge says. In about 5% of the cases they don’t work. If there is no gauge present the back of the hand will have to do. The tech uses the back of his/her hand for a couple of reasons, 1. The skin on that part of your hand is more delicate and senses temperature better than the tough skin on the palm, 2. If there happens to be something like a grounding problem and the outside of the equipment is energized your muscles will contract. In that position your hand will not grasp anything that it could lock on to. The tech should be trained to use the right hand for this. Using the left could cause current to pass through the heart causing fibrillation and stopping the heart.
If the transformer is cool to the touch and the weather is cool as well it’s wise to check to see if the transformer is under a vacuum. Most of them have gauges that you can check. Don’t always believe what it says. Tap the gauge. Look at the tank, if it looks like it’s sucked in a bit then there could be a significant vacuum on it. There is usually a nipple under the vacuum gauge. First check to see if it’s dry because there could be water in it or debris that can be sucked in. While holding your finger over the nipple release the valve under the gauge to check for vacuum. If a vacuum does exist, get a tank of dry nitrogen along with a regulator and a clean clear plastic tube. Be careful how much that you add to the top airspace. Remember that if it’s very cold out the pressure will increase when it warms up. That could blow out some gaskets and cause leaks. Use your best judgment but in any case if it’s cold out never increase the pressure to any more than two lbs. preferably less. If you should try to drain a sample with a vacuum on the top airspace the unit could suck air in from the bottom valve causing air to bubble up through the tank Causing arcs and sparks (one of the functions of the oil is to contain the voltage). Worst case scenario could be an explosion. Also after a rain I have seen water collect in a valve if it has not been plugged. That water can get sucked in along with an air bubble and be immediately distributed through the system. Don’t think that this can’t happen because it has happened to me. I got as far away from that piece of equipment as possible until the water settled in the bottom of the tank. As an additional note don’t forget to check for sudden pressure change shutoff relays. These relays will kill the power at a sudden pressure change and can cause real problems if the transformer is powering equipment that runs on a continuous processing line (been there too!).
When the tech checks the redline on the temperature gauge, records it and resets it, he or she should also take note of leaks. Did the temperature rise so much that it pushed oil out through the gasketed portions? Does it look like the gaskets are still leaking? Check the oil level. First check the gauge then feel up the side of the tank for a temperature difference at about where the oil level should be. If it’s hot, is it below the top level of the cooling fins? Why? Where did the oil go?
Look for safety issues like limbs or plants growing across the power lines. Is there garbage in the substation? Most of these are fenced in, is there a lock or some method to restrict access to the substation? Are the cooling fins free or is there something restricting the airflow? How about the condition with respect to corrosion? Is the paint all faded, is the unit rusty? What kind of paint is on it? If it’s a metallic paint such as a silver or aluminum color you can bet the transformer runs about ten degrees hotter than if it were painted with just white enamel.
Check the valve. What type is it? How does it look? Is the packing leaking? Is it a brass valve? The reason for this is that in older days many manufacturers used a “cider press “ valve. Just like the ones used on your gas line at home. They were meant for minimal use. The proper method of opening that type of valve is to loosen the nut underneath, then gently tap upwards on it loosening the tapered portion. Then it can be easily turned about ¼ turn with a crescent wrench from the top. A lazy or less than knowledgeable tech will just try and turn it from the top. Most of the time it will work but all it takes is once for the taper to shear off and your stuck with a gushing stream of oil that you’re not prepared to stop. It’s always good to be prepared with a “C” clamp that can be fitted around the top and bottom of this type of valve for a temporary field repair should a break occur.
If the unit does not have a valve on the bottom look for access at other locations. Most transformers have gauges that can be removed or plates near the top. In this case the technician needs a siphoning device attached to a tube that can reach the bottom of the tank. This is one the second worst scenario that you can encounter because it puts you in proximity of the wires. A very close friend of mine found a small plug that he could remove near the top of the transformer so he could feed a small tube in to siphon a sample. Problem was that the incoming lines were carrying over 7200 volts. As he cranked his crescent wrench down on the plug, his head dropped along with the rest of his body. He came too close to the wires and the power arced across the gap and blew off the bottom of his ear. Luckily he survived to tell the tale. Another reason that it’s worth every dime to have an experienced technician draw the sample. Finally if the unit does not have a valve or any other access if the bushings are not located on the top of the unit an experienced tech can remove the top and siphon out the sample. Believe me this is much easier said than done.
All of that said we are just about ready to draw an oil sample from the transformer, but as I’m looking at my watch I have run out of time and will have to leave that for the next posting. Looking at the previous two pages makes me wonder how many of you thought that it was this easy just to get to the point of drawing the sample?
As always if you have any questions, comments, input, or output please email me at transformerbob@gmail.com.
The quote of the day is:
“I bet after seeing us, George Washington would sue us for calling him "father." Will Rogers
Oh Yeah! Don’t forget to contact me to get your Dissolved Gas Analysis evaluation program. I guarantee you it’s well worth it. It’s a small investment in your toolboxes.
Thank you,
Bob
