Principle

Transformers have high losses at low frequencies. In 16 2/3 Hz railway systems transformer losses therefore considerably reduce overall traction efficiency. Modern power electronics allow for a more efficient and much lighter alternative: the medium frequency transformer operating at 400 – 800 Hz or comparable frequencies.

Technical details of prototype developed by DB AG

A cascade of partial IGBT inverters converts catenary current from 16 2/3 Hz to 400 Hz. Each cascade module consists of two four-quadrant-controls connected by a DC link. Each module supplies one coil of the transformer operating at 400 Hz. The secondary side of the latter consists of only one coil directly connected to the rectifier and the usual motor inverters. The load-side rectifier has to be adapted to 400 Hz. This is the only modification required.

The medium frequency transformer is inherently multi-system operable. In order to use different AC or DC supply, it suffices to modify the software controlling the catenary-side inverters.

The medium frequency transformer allows a backwards power flow needed for regenerative braking.

Table 1: Technical data of a typical medium frequency transformer:

Power	1 MVA
Short circuit voltage	18,9%
Efficiency	96,2 %
Frequency	400 Hz
Transmission ratio	1:1
Number of primary coils	16
Number of secondary coils	2
Mass	450 kg

Source: Kunz 1999

Fields of application

Railways: Traction and auxiliary transformers

Energy supply grids: A similar transformer concept is discussed for the distribution transformers in energy supply grids, but involved voltages and lay-out impede comparability.

Manufacturer

DB AG (prototype)

First studies at Alstom, Siemens etc.