Strange things happen at high voltages. Insulators become conductors, and the air itself can conduct electricity. Conductors may become resistors, creating heat and limiting current flow. Ball lighting and plasma balls may form due to carbon deposits. Power transformers are subject to all these maladies, leading to self-destruction. To prevent these abnormalities, the KVA Electric Company points out that regular maintenance is required on all power transformers.
Time Between Inspection
Time between inspection and maintenance is of critical importance. If left unmaintained for long periods, dirt builds up. Furthermore, bolts on the terminal lugs may loosen, causing arcing between the wires and the terminals. Ceramic insulators become brittle in the winter, and may crack in the summer due to thermal expansion. All this leads to transformer failure. The University of Toledo specifies a yearly check-up on transformers. The manufacturer of the transformer may specify a different time between checks, and the time limit specified cannot be exceeded.
When a technician inspects a transformer, she looks for several things. She removes all power from the transformer, so its turned off. She inspects the terminals, to see that all bolts are tight. If oil filled, she checks the oil level. If nitrogen cooled, she checks the internal gas pressure. She also looks for signs of cracked porcelain insulators, or for streaks of carbon, indicating arc formations.
If the technician finds any broken parts, they are replaced. For example; both the University of Toledo and the United States Department of the Interior (USDOI) specify looking for cracked insulators. A cracked insulator allows high voltage electricity to leak though, leading to arcing and shorts. Another area is to fix any holes in the squirrel cage. A hole allows a squirrel in, leading to shorts when the squirrel chews through insulation. The technician scrutinizes the transformer closely for any worn or broken parts, and replaces them accordingly.
Looking for broken parts is not enough. If dust is allowed to accumulate between the terminals, the dust conducts electricity leading to a short. A high voltage travels through dust easily, like a conductor. The USDOI recommends using a vacuum cleaner to remove all the dust. Also, the air filters must be cleaned if they are dirty.