How does an air cooled or ventilated electrical power transformer located outside in the rain survive? Why doesn’t the rain cause an electrical short? These are the occasional questions of someone who wants to replace an existing dry type or air cooled transformer with a larger power transformer, when the existing power transformer is located outside, exposed to the rain, wind and sometimes snow. It does look a little strange sometimes, but the general purpose air cooled dry type electrical power transformers are fitted with weather shields. They should be. If they are not, you will find out very quickly the first time it rains. The weather shields are metal shrouds, hoods, fastened over the upper vent openings. They prevent rain and snow from entering the top open vents of the transformer. This is true as long as the rain is falling vertical or possibly at a 45 degree angle. The rain or snow hits the weather shield and drips off rather than landing inside the metal enclosure of the air cooled power transformer. We do see these 50 kva, 75 kva, or even 150 kva air cooled transformers fail. This really does not happen very often. And when it does, it’s usually because of a very unusual storm. Possibly where the wind is so strong, the rain is being driven almost sideways. More often we see failures due to snow filling the inside of the metal enclosure due to swirling and drifting of snow about and eventually inside the enclosure. We have seen this happen with 5 kv and even 15 kv class transformers. We are not fans of medium voltage outdoor air cooled transformers. As long as the enclosure remains intact, there should be no problem. And as long as there isn’t some unusual weather condition, they should be okay. In our business the term “should be” makes us cringe. When folks use the term “should be,” we think “sooner or later.” It’s healthy to be paranoid around electrical power.
Categories: Uncategorized Tags: 15 Kv Transformers, 150 Air Cooled Transformer, 5 Kv Class Transformers, 50 Kva Transformer, 75 Kva Transformer, air cooled transformer, Electrical Power, electrical power transformer, electrical transformers, Large Power Transformer, Medium Voltage Outdoor Air Cooled Transformers, Metal Enclosure, Metal Shrouds, Replacement Dry Type Transformer, Weather Shields
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 had a curious question from an engineering student about large oil filled electrical power transformers. He couldn’t understand why a 20 Mva power transformer filled with oil didn’t burn when it had electricity actually flowing through it. It was his gut instinct that electricity and any flammable liquid didn’t mix and would result in an immediate explosion or fire. Just made sense to him. He associated electricity with sparks and arcs. In our world, his question seemed ridiculous because we work around 2000 kva, 10,000 kva and 30 Mva oil filled power transformers all the time and it never occurs to us that electricity and oil don’t mix. Actually a quick load check on oil filled transformers is to put your hand on the side of the tank and see how hot it is. Yup, first we check to be sure the transformer is grounded. But we don’t worry about an explosion or fire if the tank is so hot you can’t keep your hand on it. We do worry about other things when a transformer is that hot, but not fire and explosion. Oil has been used for decades in oil switches and oil circuit breakers along with large oil filled power transformers. It’s the coolant and was used to quench the arc when the oil circuit breaker opened to interrupt the circuit. This student saw a video on the internet of a large high voltage oil filled transformer exploding and couldn’t believe there was actually oil in it. We explained oil flash point and fire point. Discussed the need for a source of ignition and that, under normal conditions, there would be no arcing in a transformer. Talked about oil not getting anywhere near hot enough to vaporize into an ignitable mixture with air, oxygen. But he had fixed in his mind that electricity and oil do not mix. We guessed he needed an opportunity to work on an old outdoor 10,000 Kva oil filled power transformer in the middle of Winter. That would probably cure his misconception.