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.”
One common concern with pad mount transformers is in the area of oil sampling. Companies with large pad mount transformers may sample and test the oil in the transformers annually to determine the reliability of the transformer. The condition of the oil reflects the current state of the transformer. Now here’s where it gets tricky. Large pad mount transformers are designed with two cabinet doors. One door exposes the high voltage cables and bushings and the other door exposes the secondary, low voltage, side. The sample valve can be located in either compartment but most of the time the valve is located in the secondary cable side. Years ago it was not uncommon for an intrepid technician with nerves of steel to pull an oil sample from an energized pad mount transformer, the valve being located at the bottom of the transformer while hot cable lugs were located only a couple feet above the oil valve. With safety becoming a paramount concern in industry today, it is no longer prudent nor standard practice to pull an oil sample on an energized pad mount transformer. The hot lugs being within short reach of the technician create a shock hazard. And the secondary side of a transformer is one of the most dangerous arc flash hazards there is. Oil sampling is now performed only during scheduled shutdown of the transformer.
However, there still remains somewhat of a challenge when it comes to pulling an oil sample from a pad mount transformer. The secondary compartment of pad mount transformers can be a pretty fully house. There may be as many as six cables attached to each secondary bushing, making it very difficult to access the sampling valve through that jungle of vines. One way around this, literally, and we’re finding it used more and more in industry, is extending the sample value system to a small secure box on the exterior of the transformer enclosure. The valve would then be enclosed in a newly created box with a locked access door mounted on the exterior wall of the cabinet. In this way the transformer can be sampled at any time, while energized, without exposing the technician to shock or arc blast hazards.