Low-temperature superconducting (LTS) transformers were first proposed in the 1970s, and designed to operate at 6◦K to 14◦K (−268◦C to −260◦C). The invention of high temperature superconducting (HTS) materials increased the prospects for superconducting units designed to operate between 20◦K to 77◦K. A three-phase 630 kVA, 18.7 kVl−−l/420 Vl−−l demonstration transformer based on HTS winding technology is presently under test on the power grid.

Superconducting transformers have about half the weight of conventional oil-filled transformers, and they require less space due to their reduced size, which is important for urban locations. They are nonflammable and employ environmentally benign liquid nitrogen as the cooling medium.

But perhaps the key advantage is their capability for overcapacity operation, due in part to the low temperatures at which HTS windings operate. Heat is the principal enemy of the paper-oil electrical insulation system of conventional power transformers.

HTS transformers operate in the ultra cold range of 20◦K to 77◦K (−253◦C to −196◦C), where insulation materials will not degrade. They can operate up to twice rated power, and they have a low series impedance, improving voltage regulation.

Conventional transformers typically have ηpower = 99.3% to 99.7% for the 30 MVA class. HTS transformers have a higher efficiency, to the extent that the reduced loss in a HTS unit can more than pay for its initial capital cost over its lifetime.

HTS units have a similar construction to the liquid-filled conventional transformer: the magnetic core carries super conducting windings cooled by liquid nitrogen, which is the only safe and low-cost cryogen available in liquid form in the 20◦K to 77◦K temperature range.

The superconducting windings are manufactured either as wires or as flat tapes using BSCCO-2223 material. To date there are not many data available concerning the reliability of HTS units. Most publications concede that a superior, cost-effective HTS transformer technology might take two decades to become available.

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