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.
