Specifying generator transformers

As break downs of production transformers are costly and often result in long outages it is essential for reliability of operation that the transformers are manufactured to be robust. The aim of this M.Sc. thesis is to investigate if deviations should be made from the standard IEC 60076 – Power Transformers in order to describe a robust large power transformer for procurement. The investigation has been conducted in three stages. In the first stage a definition of the term robust was developed in consultation with the client Vattenfall Vattenkraft. In the second stage six parameters, called key parameters, which significantly affect transformer robustness were identified. The identification was based on an analysis of transformer construction, a description of occurring faults and an analysis of statistics. In the third and final stage the standard was evaluated regarding requirements on the six key parameters. The evaluation was primarily conducted by means of interviewing persons with relevant knowledge of the evaluated aspects. The identified key parameters are: Temperature, regarding mainly hot spot temperature rises; Type and properties of the insulation; Quality and performance of the manufacturing facility; Ability to withstand external short circuit; Loss evaluation, since the magnitude of the losses dictates the forces developed during transients, and finally impurities within the insulation. It has been found that, in order to describe a robust transformer, deviations from the standard should be made in the specification for procurement. A total of 22 recommended deviations have been presented including the following: The hot spot temperature at the core surface should be specified and verified by calculation and test. The rated voltage for insulation sizing should be chosen higher than the standard suggests. The ability to withstand external short circuit should be validated by calculation. The loss evaluation should be specified, a value of 11,000 euro/kW for no load losses is recommended. The transformer tank lid should be welded in place in order to ensure a tight seal over time. The investigation was limited to the internal parts of large production power transformers with a rated apparent power of at least 20 MVA and a rated voltage of 40 – 400 kV. OLTCs and transformer bushings are not covered