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.

• German DB developed a prototype of medium frequency transformer with 1 MVA permanent power for 15 kV 16 2/3 Hz system. Transformers of this power class are used in EMUs on local lines. The prototype has undergone stationary testing, integration into a vehicle for driving tests is planned (as of 1999).
• Alstom is going to continue the development begun by DB.
• There is also medium frequency technology for auxiliary power converters. This technology is however not considered here.

Weight and volume

The under-floor space for integrating the main transformer into an EMU is very limited. Small and light transformer technlogy is therefore an important requirement for decentralized traction.

Multi-system operability

The medium frequency transformer is inherently multi-system operable.

Energy efficiency

is a positive side effect.

Volume reduction

The small dimensions of medium frequency transformers are ideally fitted for EMUs because of limited build-in space.

Mass reduction

The advantages of a reduced transformer mass are numerous and range from advantages for car-body construction (less shear stress require less car-body stabilisation measures) to reduced wear.

Multi-system operability

The medium frequency transformer is inherently multi-system operable.

Medium frequency transformers cut energy consumption by increasing efficiency and reducing vehicle mass (all numbers refer to 16 2/3 Hz systems):

Efficiency effect

Efficiency of medium frequency transformers: > 94% (Efficiency of conventional transformers: ~ 92%)

Transformer efficiency (and with it the overall efficiency of power train) increased by 2 – 3%. It is assumed that the same efficiency gain can be obtained for higher power classes as well, e.g. high-speed trains or freight locomotives.

Efficiency gains in 50 Hz systems will be lower. Specific numbers are not available.

Mass effect

DB prototype about 40 – 50% lighter than conventional transformer.

Mass of conventional transformer in this power class: ~ 1 ton. Mass of trains in this power class: ~ 100 tons.

Typical mass reduction: ~ 0.5% of train mass.

	Traction	Brake energy recovery	Effect on train mass	Elasticity with regard to train mass	Effect on total energy consumption for traction
Suburban train	Electric	no	0,5 %	0,64	0,3 %
	(16,7 Hz)	yes	0,5 %	0,57	0,3 %

Mass effects in other power classes are difficult to assess as long as no prototypes exist. The case of suburban trains however shows that the additional effect through mass reduction is small compared to the efficiency effect.

Overall effect

Judging by ongoing R&D, medium frequency transformers may save up to 3 % in main line and up to 4 % in regional line vehicles (in 16 2/3 Hz systems)

Fleet-wide effect

MFT is only applicable to AC traction, and benefits are highest for 16,7 Hz systems. No data are available to determine whether application in 50 Hz systems is reasonable.

Estimated overall energy efficiency potential (in 16,7 Hz systems): 1 – 2 %.