Normal Life expectancy of the generator transformer and transmission transformer will be about 25 to 30 years. Transformer will have normal life expectancy when it loaded below the name plate ratings.The transformer can be loaded above the normal rating (name plate rating) without affecting the life of the transformer at low ambient temperatures, for cyclic loading on the transformer where the increased rate of life usage during high loading periods is compensated by the reduced rate of life usage during the low loading periods.
The life expectancy of the transformer depends on the several factors. The most significant in reducing the life of the transformer are extraordinary events such as:
- Short circuits
- Loading beyond the nameplate ratings
The ability of the transformer to sustain such events depends upon its design having the capacity to withstand the magnitude of the events to control the temperature generated within acceptable limits, to minimize the concentration of the moisture, oxygen and other gases present in the insulation. Deterioration of the cellulose-based insulation may not be itself cause any damage during normal operation but the short circuit or subsequent over-voltages can damage the aged insulation resulting in the severe damage to the transformer.
When a transformer is loaded beyond its name plate ratings:
- The temperature of the insulation of the windings, leads and oil could attain unacceptable limits, i.e, rapid ageing of the insulation occurs which significantly reduces the life of the transformer
- The addition of heating resulting from the eddy currents in a metal work produced by the leakage flux may damage local insulation or create bubbles in the adjacent oil
- Unacceptable flux densities may appear in the parts of the magnetic circuits due to the combination of the main and leakage fluxes
- Bushings current transformers and cable connections could be exposed to over-stressing or encroachment into design margins
- The expansion of the oil due to higher temperatures could cause oil spillage
In order to avoid unnecessary hazards, the loading of the oil immersed transformers should be within the prescribed standards.