How do I determine if a transformer is beyond the normal temp range and is at risk of failure?A 225 kva installed in 1996 is running hot. I removed the cover and using a digital thermal meter read approx 125 degrees F on each winding. 243 degrees F on the top metal (winding support) frame. It's got parallel 750's off the load side and would be an expensive and difficult change.
The temperatures you measured seem hot but might be acceptable for the type of materials used to insulate the coil inside the transformer. Only the designer of the transformer could tell if it's too hot or not. One rule of thumb : for non industrial electrical devices, the outside temperature of the device should remain at a temperature that lets you touch it without feeling pain. But one thing is sure: high temperature is not good because the transformer will age faster and it's less efficient (and therefore generates more heat). The transformer can overheat for 2 reasons : either your load is above the nominal rating or the transformer is not able to cool down as designed (usually because it's in an enclosure which doesn't let the heat get out). If you are indeed getting 750kva from it, then you are definitely at risk. That's more than 3 times the nominal rating. I would be surprised that it lasted for so long. If you can't reduce the load (best option), try to let more air flow freely around it by removing stuff around it. Last resort try cooling it down by forcing cool air to flow around it with a fan. But this can be tricky because the bottleneck of the heat exchange could be inside the transformer so the outside might get cooler and the inside could remain very hot.
The first think you need to do is determine what the transformer's ratings are. The fact that i's nominally a 225kVA unit is only part of the story - what kind of insulation does it have, and what is the overtermperature capability of that insulation? Second, you need to determine what kind of cooling the transformer has, and whether that cooling is working properly. Some transformers are cooled only by exposure to ambient air, while other transformers are equipped ;with fans to blow air across cooling fins. Measuring the temperature of the coils using an external thermometer isn't appropriate - the limitation on transformer is the temperature internal to the winding. Is the transformer equipped with an interman temperature sensor? If not, then you can only base you conclusion on loading. I don't know what you mean when you say parallel 750's off the load side. You will need to determine the loading on the transformer, and compare that with the full rating of the transformer including any adjustment for cooling and ambient air temperature.
