Power and distribution transformers are essential devices in electricity supply. Their ratings can vary from small size distribution transformers of a few kVA up to very large power transformers of 1000 MVA or more. In terms of voltage ratings transformers can have operating voltages up to several hundreds of kilovolts. They represent a major asset of the power utility and any industrial plant. Failure of a transformer can be very costly and dangerous for other major equipment and personnel alike. The design and operation of any transformer must fulfil certain requirements in order to withstand the electric, thermal and mechanical stresses during its service life. Tests and maintenance of transformers according to the relevant standards are intended to ensure that a transformer passing them will give trouble-free service for many years under the conditions it is likely to experience after its installation.