We had the Original High Voltage Step Up transformers at work for 31 years, life cycle by GE was figured to 25 years so dragged an extra 6 years of life out of basically oil filled explosive devices. As they age the organic materials inside them(Wood and paper of all things) breaks down in the oil bath, after a time that allows arcing to occur which transfers to accumulated gases of the oil breakdown due to the heat of arcing inside the normally oil filled casings. I walked around those 10,000 gallon oil filled time bombs for sixteen years taking gas level readings where was explained if saw more than a considered dangerous level to stay away from them. That number got adjusted UP several times until we as operators said enough and they went to remote equipment.
The lead time of large transformers as these is from 2 to 5 years dependent on the design, saw the effect of two failures at our dirt burner sites with the same conditions as we watched where those devices failed. Utilities today are scraping the bottom of the barrel, they are dealing with regulations exceeding capability to deal with while rates are held artificially down for the poor to be able to have electricity only to still fail to pay their bills.
Most do not understand power plants make no profits, they are expense only usually tied to a 25 year life cycle development loan. O&M as to wages/benefits/tooling, fuels, major overhauls and loan payments are all that are recovered during the years of operations, the profits go to the owning utility as to end of use rates. Power lines, poles, distribution transformers, distribution substations, linemen and equipment for repairs, even metering mechanisms are paid for by the consumer with their energy usage fee, power stations and line transmission lines towers & substations with all the manpower etc. are just the base number for the electricity per kwh. The nuke I worked at cost the company $.06/kwh, most dirt burners were similar, the CTG units were closer to $.30/kwh due to extent of maintenance per hour run time and fuel consumption. The Customer rate here is $.15/kwh.
These are the old(still installed) and new replacement transformers when we did that swap out. These changed 25,000VAC to 345,000VAC(closer to 360,000 for actual transmission)