Power Transformers Blog – Expert Safety and Usage Info

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.  

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.