25-09-2012, 12:13 PM
Superconducting Transformers
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Abstract
The paper provides an overview of power applications of High Temperature Superconductivity (HTS) with emphasis on superconducting transformers. Properties of available HTS materials are reviewed and modelling aspects briefly introduced. A conceptual design of a 240MVA HTS grid autotransformer is summarised and principal features discussed. The economics of parallel operation is addressed. Finally, details of a small HTS demonstrator built and tested at Southampton University are provided.
INTRODUCTION
Recent catastrophic black-outs in New York, London and Italy (August and September 2003) have exposed major vulnerabilities in existing generation, transmission and distribution systems. Severe underinvestment, aging technology and conservatism in the approach to innovation are being blamed and have created a situation where reliability of the entire system is under question. Resources need to be directed into technologies that have the potential to improve the integrity of the system; high temperature superconductivity has such potential. It also has a positive environmental impact by significantly reducing the losses as well as size and weight of the power devices.
PROPERTIES OF HTSMATERIALS
The announcement in April 1986, by Muller and Bednorz (IBM) of superconductivity in the perovskite structure Lanthanum-Barium-Copper oxide at 30K, was an important step towards a wider application of superconductivity [11]. This was followed by the discovery of Wu and co-workers in January 1988, of Y1Ba2Cu3O7-8 (YBCO or 123), with a transition temperature of 93K, bringing superconductivity above the boiling point of liquid nitrogen (77.4K at 1Atm), a cheap and widely available cryogen. There has since been much effort on the search for new materials, and the optimization of processes for production of thin films (<1μm), thick films (10-100μm), bulk materials, wires and tapes (Fig. 1) in single or multifilament composites. Many practical problems remain to be solved, but the potential for engineering application is clear.
240MVA GRID AUTOTRANSFORMER
A design feasibility study was conducted for a 240MVA high temperature superconducting grid auto-transformer [1]. The principal feature of the design is the removal of the copper windings and their replacement by HTS equivalents. These are only a fraction (less than 10%) of the bulk of the conventional windings. The inevitable result is windings of reduced mechanical strength, which will stand neither the radial bursting forces nor the axial compressive forces that occur during through-fault conditions without special strengthening structural features. The tap winding is kept outside the cryostat to avoid the heat which could otherwise flow into liquid nitrogen through a large number of connections. With the bulk of the ohmic losses (in common and series windings) removed, it is possible to cope with the core loss and remaining ohmic loss (in the tap windings) by forced gas cooling, leading to an oil-less design of transformer. This has a great advantage of reducing fire risk and environmental hazard from oil spillage. Furthermore, the need for an explosion-proof outer steel tank is removed – though some form of enclosure must be provided for weatherproofing and acoustic noise reduction.
HTS TRANSFORMER DEMONSTRATOR
A small 10kVA HTS demonstrator transformer was designed, built and tested at Southampton University [8, 9]. In order to limit the material cost of this small single-phase unit (predominantly the cost of the superconducting tape), it was decided that the nominal rating at 78K should be 10kVA and only one winding, the secondary, should be superconducting; this also had the benefit of allowing direct comparison of performance between conventional and superconducting windings.
CONCLUSIONS
High Temperature Superconductivity has great potential in electric power applications (generators, motors, fault current limiters, transformers, flywheels, cables, etc.) as losses and sizes of devices are significantly reduced. The technology is now mature and prototypes are considered. The ability to predict and reduce all ‘cold’ losses is crucial to show economic advantages of HTS designs.