Principle

The main transformer accounts for a substantial share of traction losses. This is especially true in 16,7 Hz systems. An innovative transformer concept using ceramic high-temperature superconductors instead of copper as winding material could reduce the transformer losses almost to zero.

HTSC

Superconduction (the loss-free electric conduction properties of some materials at very low temperatures) was discovered in 1911. The superconductors known then were metallic and required cooling down to –269° C which was achieved only by expensive liquid helium. In 1986, ceramic materials were discovered having superconductive properties at much higher temperatures of about -196 °C. This temperature can be achieved by liquid nitrogen cooling allowing for a considerable reduction of costs and complexity of superconductor cooling.

Possible applications of HTSC aim at

1. Optimisation of conventional equipment: motor, transformer, cable etc.
2. Development of innovative equipment: magnetic energy storage, current limiter etc.

The transformer prototype made by Siemens

Siemens AG has developed two prototypes of HTSC transformers (a 100 kVA model and a 1 MVA demonstrator) in order to show principal feasibility for railway-relevant power classes.

The coils are made from Bi-2223 (Bi2Sr2Ca2Cu3O10) conductor tapes of 3 mm width and 0,3 mm thickness. These filaments of ceramic superconductors are embedded into a pure Ag or AgMg matrix and a jacket acting as an insulator in normal operation and providing a defined circuit in case of quenching (i.e. breakdown of superconduction). The coils are located around an iron core. The operating temperature of the transformer is 67 K (-206 °C). This temperature is produced by a surrounding cryostat based on liquid nitrogen (LN2) cooling.

Figure XXX shows the layout of the 1 MVA demonstrator.

Einscannen aus Weigel 2000!

General
Nominal output	1000 kVA
Frequency	50 Hz
Voltage	25 kV / 2 x 1.4 kV
Current	40 A / 2 x 360 A
Core
Height / width	1080 / 622 mm
Cross-section	329.8 cm2
Induction	1.7 T
Cryostat
Length (inside)	1140 mm
Width / height (inside)	832 / 420 mm
Winding (Bi-2223)
Diameter (HV/LV)	304 / 228, 382 mm
Height	5000 mm

Source: Henning et al. 2000

Operational characteristics

Due to the time-consuming cool down process and the low permissible temperature gradient to ensure minimum material stresses, the HTSC transformer has to be kept at operating temperature even during standstill periods. For standstill of up to seven hours the thermal time constant is sufficient to maintain HTSC material at operating temperature. Beyond that the cooling system must be supplied either from catenary or from external supply.

Manufacturer

Siemens AG (in co-operation with DB AG) and others