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General information
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Description
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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. |
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General criteria
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Status of development: test series |
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- 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.
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Time horizon for broad application: in > 10 years |
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Manufacturers estimate 2-3 year development time after kick-off. The medium frequency transformer could be produced in series some time between 2005 and 2010. To become a standard in new stock it would then take approx. another decade. |
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Expected technological development: highly dynamic |
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Eddy currents limited the DB prototype to 96,2 % efficiency (without power electronics). Replacing aluminium by copper parts, the transformer itself (without power electronics) could reach 98,5 % efficiency according to DB experts. This would raise efficiency of the whole component (including the required power electronics) to 94 %.
In long-term perspective power electronics might altogether replace the on-board transformer. |
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Motivation:
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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. |
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Benefits (other than environmental): big |
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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.
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Barriers: medium |
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Complexity
High amount of power electronics. Very complex compared to conventional transformer. Main problem: Ensure operational reliability despite complexity. Can be solved by redundant construction: individual cascade modules may be switched off if broken down. |
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Success factors:
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(no details available) |
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Applicability for railway segments: high |
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Type of traction: electric - AC
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Type of transportation: passenger - main lines, passenger - high speed, passenger - regional lines, passenger - suburban lines, freight
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Main benefits in 16 2/3 Hz systems but conceivable for all electric vehicles.
Ideal transformer solution for EMUs due to small dimensions. |
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Grade of diffusion into railway markets:
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Diffusion into relevant segment of fleet: 0 % |
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Share of newly purchased stock: 0 % |
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(no details available) |
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Market potential (railways): highly uncertain |
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(no details available) |
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Example:
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(no details available) |
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Environmental criteria
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Impacts on energy efficiency:
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Energy efficiency potential for single vehicle: 2 - 5% |
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Energy efficiency potential throughout fleet: 1 - 2% |
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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.
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Traction |
Brake energy
recovery |
Effect on efficiency of power
train |
Elasticity with regard to efficiency of power
train |
Effect on
total energy consumption for
traction |
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High speed
train |
Electric |
no |
2 – 3 % |
1,00 |
2 – 3
% |
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(16,7 Hz) |
yes |
1,11 |
2 – 3
% |
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Intercity
train |
Electric |
no |
1,00 |
2 – 3
% |
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(16,7 Hz) |
yes |
1,12 |
2 – 3
% |
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Regional
train |
Electric |
no |
1,00 |
2 – 3
% |
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(16,7 Hz) |
yes |
1,33 |
3 – 4 %
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Suburban
train |
Electric |
no |
1,00 |
2 – 3
% |
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(16,7 Hz) |
yes |
1,42 |
3 – 4
% |
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Freight |
Electric(16,7 Hz) |
no |
1,00 |
2 – 3
% |
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Range: |
2 – 4
% |
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.
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Traction |
Brake energy
recovery |
Effect on train
mass |
Elasticity with regard to train
mass |
Effect on
total energy consumption for
traction |
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Suburban
train |
Electric |
no |
0,5 % |
0,64 |
0,3
% |
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(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
%. |
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Other environmental impacts: neutral |
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(no details available) |
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Economic criteria
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Vehicle - fix costs: medium |
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(no details available) |
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Vehicle - running costs: significant reduction |
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Energy costs are appreciably reduced in 16,7 Hz systems. |
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Infrastructure - fix costs: none |
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(no details available) |
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Infrastructure - running costs: unchanged |
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(no details available) |
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Scale effects: medium |
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(no details available) |
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Amortisation: (no data) |
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(no details available) |
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Application outside railway sector (this technology is railway specific)
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Overall rating
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Overall potential: very promising |
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Time horizon: long-term |
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Medium-frequency transformers represent an interesting lane of development towards more efficient transformers, an issue especially relevant in 16,7 Hz systems. Profitability in 50 Hz systems has to be assessed carefully. Present states of development do not yet allow a decision as to whether medium-frequency or HTSC transformer technology is the more attractive option. |
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date created: 2002-10-09
