We received feedback from a new Technician who was scrapping out an old 5000 kva oil filled power transformer. It was over 40 years old and hadn’t been in service for 20 years. The primary voltage was 13,800 and the secondary ranged from 120 volts to 600 volts. These voltages made it an unusual transformer, but it was worth more for scrap copper and steel than for reliable reuse. In addition we had limited knowledge of the history of this 5 Mva transformer. The test results were barely acceptable, not good.
The young technician had never seen the inside of an old oil filled power transformer, especially such an unusual power transformer out of a foundry. The things he found interesting, a veteran oil transformer repair technician would take for granted. He was surprised how tightly the core and coils fit inside the tank and how the high and low voltage bushings connected to the transformer. He was amazed by the over 3” diameter round solid copper secondary bushings. He saw how the liquid level gauge and temperature gauge for a 5 Mva oil filled transformer worked. The level gauge inside the power transformer had a simple small rod with a cork at the end. The temperature gauge was just a well extending through the wall of the 5 Mva transformer with a temperature gauge installed in the well. The transformer had a basic tubing and pressure gauge system to add nitrogen or dry air. All very basic stuff, but once he saw all this stuff and the interior of the transformer, these basic things made a lot more sense. Good experience. If a picture is worth a 1000 words, then this simple experience was worth 10,000. Whether a 5000 Kva transformer, 10 Mva, 20 Mva or 50 Mva oil transformer, some of these transformer basics are the same.
MIDWEST was asked what we did with perfectly good obsolete or old used transformers. Bit of an oxymoron since obsolete might be understood to mean no longer any good. But the intent of the question was obvious. Transformers that are too old to be sold in the used transformer market might be maintained in MIDWEST’s pool of rental, temporary, and emergency transformers or they are just scrapped. Typically a rental transformer is just for temporary use. But we have had many transformers on rental for over a year. Especially larger Mva transformers. We have a few we didn’t scrap out but kept around solely because of the special voltages or size or physical configuration, to be used in those rare occasions when a manufacturing company, for example, is in trouble with delivery of a special replacement transformer. This usually happens when they had a non typical transformer suddenly fail. The difference between these special temporary transformers, which we sometimes call cling-ons, and our rental transformers is that we’d really like to get rid of the cling-ons. But about every time we think we’ll scrap out an obsolete old unit, we get a desperate call and it is the only thing that will work and the only one the customer can find. So it is resuscitated again and lives on. We do have fewer of these than ten years ago. We just scrapped out a 2000 Kva oil filled power transformer, 13,200 volts to a variable secondary of 120 volts to 600 volts. Weighed 18,000 pounds. A big old power transformer, specially built for a transformer power lab about 60 years ago. We finally got rid of it because we never rented it in two decades; And we did not have a good biography on the unit, although all our test results were good. The install cost as a rental, emergency, or temporary transformer would have been potentially huge because of the oversize and weight due to the many voltage taps. A potential customer would only need one functional voltage and would be paying a premium for a monster oil filled power transformer when they probably could find a unit with their specific voltage. Plus MIDWEST can not rent a transformer we no longer have full confidence in, even during an emergency. In the last month we designated over ten transformers to the scrap heap. Mainly because of a lack of full confidence in their reliability, even though many of them had good test results. MIDWEST knows electrical tests, on used oil filled and dry type power transformers, are not a perfect indication of the condition of the transformers. So MIDWEST looks for reasons to get rid of the old and the cling-ons.
Sometimes simple oil filled power transformer repairs can be made very complicated by seemingly small hidden details. Sometimes the circumstances that create the complication evolved over time. Or, in some cases, it was just poor engineering. In this case the new transformer, switchgear, and substation structure were pieced out to different engineers and contractors, 20 years past. Not a good idea. Here’s an example of a job made hard by the lack of foresight and design coordination An electric utility had a 20 Mva power transformer with side mount secondary bushings, 15 Kv. The bushings connected to a 15 Kv enclosed busway that ran from the transformer to the 15 kv switchgear inside the building. The bushings were side mounted in a throat for connection to the busway. One of the bushings had a small leak from the crown gasket. But the oil was pooling and dripping slowly from the throat enclosure. This would normally not be a very complicated repair, even though the leaking bushing would have to be removed to replace the gasket. Access to the bushing connections inside the transformer were very easy. Access for proper oil handling equipment for work on a 115 kv high voltage transformer was easy. But the bushings were not top mounted. They were side mounted and the bushings had to be removed 24 inches in order to get them out of the transformer side throat. Usually you just remove the immediate flexible connections to the bushings and one section of busway and have at it. In this case, because of the configuration and supports for the busway, three sections of busway, including a 90, would have to be disassembled. And this was huge bus for a 20 Mva transformer. It was as if the busway was erected first and then the transformer slid in place to connect to the bus. Then overhead structure installed and more infrastructure install adjacent to the transformer such that you couldn’t move or pick the transformer, even if you wanted to, in order to save time. No thought was given to access or service of the secondary bushings. So the Utility cleaned up the oil and redefined the leak as a weep, to be monitored. They couldn’t handle a 48 hour outage to do the repair. Whether this was a 10 Mva, 20 Mva, or 30 Mva oil filled power transformer, the simple leak repair would be a monstrous job. But sooner or later the old power transformer will have to be repaired.
Anyone in our business has seen transformers with concrete pads no longer level. MIDWEST frequently sees padmount transformers off kilter because the concrete pad has shifted, usually because the soil in one area has washed out. It occurs far less frequently, but sometimes we see the concrete pad for a large outdoor power transformer has settled on one side, causing the transformer to no longer be level. Slight settling might not pose a problem. But we frequently, using Infrared Scanning, find old oil filled outdoor power transformers that are not cooling properly. This occurs when an old electrical power transformer has long cooling tubes. On some transformers, the oil level may only fill 2/3 of the upper heads for the cooling tubes. If the transformer is slightly out of level, some of the outer cooling tubes on the transformer may tilt high enough that the oil will no longer reach the head and will not circulate. These cooling tubes will actually look cold when viewed with Infrared Thermography.
If you find your transformer is not level or is actually not circulating properly, do not attempt to correct this while the transformer is energized. This is a very bad idea. You are too close if anything goes wrong. And there are things that can go wrong with oil filled transformers that you can’t even imagine. Transformers are very heavy, especially old obsolete transformers. They can get very unhappy and the equipment they are connected to, can get very unhappy and make a big mess, if you try to level them energized. It’s tempting, because it looks so easy. But, if something goes wrong, it’s hard to get out of the way.
MIDWEST was called on an emergency after a contractor tried to level a 3750 kva, 25 kv to 480 volt transformer, that was connect to service bus. The secondary bus faulted at the transformer throat connection and pretty much destroyed everything, transformer and bus. What seemed like a good idea one moment, turned into a catastrophe the next. Again, MIDWEST recommends thinking “consequences, not probability.”
During a highly technical conversation about the life expectance of old and new electrical transformers, MIDWEST was asked by the Consulting Team what the most common failure mode for outdoor oil filled power transformers was. The discussion involved 1000 kva to 10 Mva power distribution transformers typically found in the outdoor substations of manufacturing plants. We were discussing Failure Mode and Effects Analysis (FMEA) and its value for reliability optimization. The consultant was using military data for failure mode and frequency of occurrence. There was poor connectivity between that data and our world of big old oil transformers used by industry. It was even less appropriate to use historical military data on the new oil transformers used today to replace those old tubs. The whole thing was rather ridiculous as every one struggled to find legitimacy in the analysis. The solution came with a twist from a reliable and too often forgotten source.
MIDWEST’s senior field service technicians and service engineers were asked what the most common failure mode was, based on their experience, experience that exceeded 100 years. Their one word answer was, “Raccoons.” After the technical minds recovered and realized the answer was more than just a little jab and a lot hilarious, they realized it was true. So now they asked what should be done to lower the probability of this critical failure mode. The answer, “No raccoons.” Problem solved.