An Electrical Contractor called MIDWEST with an emergency. A customer lost a 1500 kva dry type power transformer. The voltage was 13,200 to 480. Two of the fuses in the primary load break switch had blown. They checked things out and couldn’t find anything wrong, but wanted MIDWEST to test the transformer anyway. Meanwhile they checked out the secondary switchgear and a 480 volt to 208 volt dry type transformer in the same room as the switchgear. They found no smell and no smoke, so they figured they might get lucky, although we both knew that was not very probably. When 15kv, 100 amp fuses blow, a lot of energy went somewhere. The 1500 kva dry type transformer was a typical indoor unit substation type, with LAs, cooling fans and a fan control package. When we got there, the electrical contractor already had the panels off the switchgear and transformer, and everything properly grounded. They offered to help haul up the test equipment, which is always nice, but we said to hold off for a minute. It took 15 seconds to get our flashlight and then it took another 15 seconds to inspect the transformer and tell them the transformer had failed and they needed to get a replacement transformer or a rental transformer. Their skepticism was obvious, and for good reason. But we showed them the debris that had blown out the bottom of the center high voltage winding. Then had them look down through ventilation ports of the blown high voltage winding so they could see the blown out turns within the high voltage winding. They understood immediately. We ran a quick turns ratio test to document the failure. A case of seeing is believing.
MIDWEST Uncategorized 1500 KVA Transformer, dry type transformer, high voltage windings, rental transformers, replacement transformer, Switchgear, transformer
This MIDWEST blog is about a low voltage, high current transformer used in a hardening process. The transformer was physically located deep in the process equipment. It was very difficult for the owner to access the transformer for maintenance. They just could not maintain it during normal production schedules. The process operators were having more and more trouble controlling the output voltage and current. It got to the point they just could no longer regulated the output current as necessary to perform the metal hardening process. MIDWEST was contracted to remove and replace the transformer or repair it. Whatever could be done. When the transformer was being removed, the problem was found immediately. There were massive copper bus bars on the secondary side of the transformer to carry the high currents. One of the output bus bar bolted connections had failed. It was held by only four bolts and the bolted connection had become loose. The copper at the connection was discolored and distorted from overheating. The connection contact surfaces were all destroyed by being overheated. The damaged copper and damaged connection contact surfaces for this damaged transformer could not be repaired. The copper was damaged all the way into the winding. All the other connections were intact. The repair bill was over $50,000.00. This four bolt connection cost over $12,000.00 per bolt to repair. The transformer replacement was not caused by a transformer failure, but by a simple bolted connection failure. MIDWEST frequently finds dry type transformer failures or replacement transformer projects are caused by the most probable failure mode because the needed preventive maintenance was not performed, because it would have been relatively expensive compared to routine, specified, maintenance. Everyone understands the value of the maintenance dollar. But it does not make sense to continually spend hard to get maintenance dollars on preventing very low probability failure modes and totally ignore the most probably failure mode all together, when the most probable mode would be catastrophic and the low probability failure modes have little consequence. This is a nightmare for maintenance supervisors. It can be tough to sell critical maintenance procedures that are not found in standard maintenance specifications.
MIDWEST Uncategorized Copper Bus Bars, Damaged Transformer, dry type transformer, High Current Transformer, Low Voltage Transformer, Preventive Electrical Maintenance, Replace Transformer, replacement transformer, Transformer Replacement
A MIDWEST Thermographer found an interesting problem with a 208 volt, 3 phase, 75 kva dry type general purpose transformer. The Infrared Scan showed the enclosure of a Square D transformer was much warmer on one side compared to the middle and other side. It was an older transformer and had been in service for many years. An enclosure must re-emit enough heat from the air cooled transformer and the difference in heat from one area of the transformer to another must be substantial enough for the Thermographer to see the difference in heat pattern. But when you’ve scanned 1000s of these dry type transformers over 15 years, you have a pretty good idea of what’s normal or not. A load check revealed one phase was at 220 amps, another at about 115 amps, and the third phase 90 amps. They definitely had a large load imbalance on their old transformer. The maintenance man said they had been having nuisance tripping on the output breaker, but when he checked the load, there was less than 100 amps, 50% load. Unfortunately he only checked the load on one phase and got unlucky enough to pick the lightest loaded phase. But it was also very easy for our Thermographer to see the load imbalance on the circuit breakers on the load and line side of the transformer. It is amazing how tough these old air cooled transformers are. 10 kva to 500 kva, general purpose dry type transformers are found everywhere. Some are real dinosaurs, beyond old, but not obsolete because they are still running. Some over 70 years. A couple other things about these really old dry type transformers. They were so overbuilt that they can be very forgiving of overloading and they can be really loud.
MIDWEST Uncategorized air cooled transformer, dry type transformer, generual purpose transformer, Infrared Scanning, load imbalance, square d transformer
Here is another MIDWEST transformer horror story. This involves a 1000 kva indoor dry type power transformer, 13.2 Kv to 120/208 volts. The first time we saw this transformer, we thought it was just a spare new transformer core and coil being stored in one of the customer’s equipment rooms. It looked just like a replacement transformer because the core and coil were not in a metal enclosure. It was out in the wide open area of the room with no protective enclosure or barriers, nothing to protect it. To our great surprise, we soon realized that this old transformer was energized. The hmmm and the exposed wires connected to it were a big tip. One had to walk around the transformer to get to one corner of the room. Beside electrical conduits, there were other overhead pipes in the room, plus communication cables. There were electrical panels and non electrical equipment in the room. The electrician said he was told it was okay because it was a locked door and nobody was suppose to go in there. He told his boss that he thought it was very dangerous, but he was told it met code. He thought the whole thing was nuts. We asked where the key was secured so that unqualified personnel could not enter the room, which they called a vault. The key was hung on a nail over a nearby doorway going into a mechanical equipment room. This was considered secure because nobody knew what the key was for, except the ‘right’ people. We explained the extreme shock hazard and arc flash hazard to anyone, even qualified personnel, entering the room and the fact that unqualified personnel may acquire access to the room because of the key location. In plain English, this set up was crazy. To this day, nothing may have changed, because someone decreed “it met code.”
MIDWEST Uncategorized dry type transformer, electrical panels, old transformer, replacement transformer
Two quick qualifiers: One, switchgear manufacturers get it right most of the time. And two, it often takes a pair of fresh eyes to spot the obvious. Now that some perspective has been established we can continue with impunity.
An educational facility had a 13.8 kv primary dry type transformer located in a basement substation. The transformer panels (the skin) had just been pulled per a routine scheduled shutdown testing and maintenance procedure. What our techs found staggers the imagination, the scene echoing with “what were they thinking?” terminology. No, it was not Al Capone’s hidden treasure or Indiana Jones’s arc of the covenant. But the word arc is getting close.
Lying on the concrete floor, like an extension cord, were the standard 15 kv non-shielded switchgear cables. The cables ran from the primary switch compartment to the transformer compartment and had a white chalk like substance running the length of the cable where it made contact with the floor. The chalk like substance was the result of corona (ionization of nitrogen in the air) induced by the intense electrical field between the cables and ground, the concrete floor.
A chemical cocktail soon forms as the corona goes about its business and over time arcing takes place causing carbon tracking (dark carbon deposits) and eventual destruction of the cable or an electrical fault. The cables were replaced and were properly supported in open air with proper clearances. A certain disaster was avoided.
MIDWEST General Transformer Information cable fault, carbon tracking, corona, dark carbon deposits, dry type transformer, switchgear cables
Frequently electricians and plant engineers ask how long dry type transformers last. The easy answer is, “Until they fail.” But that answer might result in a ‘click’ at the other end of the phone line. In this case we are talking about general purpose dry type air cooled transformers, 208, 240, and 480 volts. These transformers are located everywhere in manufacturing facilities. For most people they are just metal enclosures that are warm in the winter and make a funny humming sound. We were asked if they were good for 25 years or more.
The biggest enemy of air cooled dry type transformers are roof leaks and fork trucks. The transformers are good for many more than 25 years. But don’t let them get wet, dirty, hit, or buried under “stuff.” They do not like water or heat. If the ventilation is blocked, they may overheat and that will shorten their reliable life expectancy. So, if your general purpose, dry type, air cooled transformer is kept dry, clean, and well ventilated, it may last 30, 40, even 50 years. It should last until you retire, which has become the newest criteria for many electricians and plant engineers. Also, these transformers look innocent, but they are very dangerous, fatal, if they blow up or if something gets stuck inside the enclosure.
We are talking about transformers that might be 1000 kva with a primary voltage of 4160 volts or 13,200 volts and a secondary voltage of 480 or 208. They are in a metal enclosure with vents to allow air to circulate and cool the transformer. These transformers are indoor and usually found in locked electrical rooms. We see these transformers 60 years old and still humming away.
MIDWEST General Transformer Information dry type transformer, transformer life expectancy
