Transformers

Conventional power transformers are very simple and reliable components which play a central role in electrical systems. Their efficiency, for large power ratings, is well above 99%. This excellent performance has been reached after more than a century of designing, manufacturing and operating experience. It must be noted however that, despite such high efficiency, the cost of losses is a high percentage of the total owning cost of the transformer during its lifetime. For large transformers the purchase price is less than 25 % of the total owning cost [1]. Significant savings could be made even with a higher purchase price, if losses can be reduced. Intangible but important benefits in terms of reduced CO2 emissions could also be obtained. It is estimated that, if losses could be reduced by 1/3 for 50 % of the existing transformers, a total saving of 25.6 TWh per year could be generated in the United States alone, corresponding to 1.5 107 ton of CO2 reduction [2]. Superconducting transformers have the potential to offer the following advantages with respect to their conventional counterparts: • Reduced losses which allow considerable saving in operational costs and reduced carbon footprint • Reduced size and weight which make SC transformers very attractive for upgrading existing substations and for transport applications. • Possible operation in overload conditions without increase in temperature. This avoids ageing of insulating materials thus improving reliability and providing longer transformer life. • Lower leakage reactance. This mitigates voltage drop across the transformer and reduces the need for tap changers. • Possible fault current limiting capability. This provides protection of all components during the fault without weakening the grid in normal operation. • Elimination of mineral oil for refrigeration. This avoids the risk of fire and reduces the environmental impact, especially for high voltage transformers. In the following sections superconducting transformers are investigated in detail. In particular in section 2 basic principles are reviewed and the impact of using superconducting materials is discussed with reference to several aspects. Technological issues for the manufacture of reliable SC transformers are examined in section 3. The state of development of SC transformers worldwide is also briefly reviewed. Finally in section 4 the design of a full-scale HTS transformer is carried out and admissible AC losses are calculated based on economic considerations in comparison with a conventional transformer of the same size.