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

Siemens AG predicts the following efficiency improvements:

Source: Weigel 2000

Although these values refer to rated load, it is assumed that the relative improvements are approximately true for all loads.

This yields the following elasticity table:

Source: IZT

These values refer to 16,7 Hz systems. For 50 Hz systems transformer losses are generally smaller and the effect of HTSC transformers on total energy consumption will be lower by 1 – 2 points giving a range of roughly 6 – 9 %.

Mass effects

The following gives an estimate of the order of magnitude of the secondary effects due to the reduced mass of the HTSC transformer:

• High-speed train: The typical mass of a high-speed train is some 400 tons (ICE 2). According to Siemens the mass of a HTSC transformer of the high-speed power class is about 4 tons lower than for conventional transformer (cf. Benefits). This corresponds to 1 % mass reduction which translates into an almost negligible effect on total energy consumption of ~ 0,2 %.
• Regional train: The typical mass of a regional train will be approx. 100-200 tons. The mass reduction due to the HTSC transformer will be ~ 2,6 tons (cf. Benefits) corresponding to ~ 1-3 % which corresponds to a reduction of total energy consumption of 1-2 %.

Total effect

Taking into account both mass and efficiency effects, one gets the following energy efficiency potential:

• Main line and high-speed trains: 8 - 10 %
• Suburban and regional trains: 9 - 13 %
• Freight trains: 7 %

Standstill consumption

During standstill the power required for maintaining the system at operating temperature is in the order of 1,2 kW for a 1 MVA transformer. It is about three times higher if the main transformer is also used for auxiliaries during standstill. According to Siemens, energy costs for standstill will still be lower than for conventional transformers (cf. Infrastructure – running costs).

Cooling agent

Liquid nitrogen is a cooling agent with less environmental impact than the oil used for cooling conventional transformers.