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   Inverter unit for DC substations  evaluated  
By installing thyristor inverters in substations of DC systems, a feeding back of recovered braking energy into the public mains becomes a possibility. This can considerably increase recuperation rates in suburban or regional DC systems.
Technology field: Regenerative braking and energy management
close main section General information
  close sub-section Description


Due to high transmission losses, DC systems do not reach high recuperation rates except in very dense urban networks.

The situation may be somewhat improved by making electric substations “reversible”, meaning that they can also operate in the reverse, feeding energy from the catenary to the public mains. If substations are equipped with inverters, recovered energy can be fed back into the supply grid whenever no other train is running close enough to use the recovered energy.

Technical details

The architecture and power electronics of DC substations are almost identical in most mass transit systems and mainly based on diode rectifiers.

If an antiparallel inverter is added to the diode rectifier, the power flow of the substation can be reversed, i.e. energy can be fed back into the supply grid. The inverter is only activated when recovered energy is available. The inverter setpoints have to be controlled in such a way that a certain share of the regenerated energy is available to be used by other trains for acceleration.

So the priority for the use of recovered energy is given to other trains. Only if no other train can take up the energy, it is fed by the inverter unit into the supply grid.

close main section General criteria
  close sub-section Status of development: in use
    Inverter-enhanced substations are a mature technology that is already in use in some local DC systems, e.g. an inverter unit is run by Kölner Verkehrs-Betriebe AG (KVB), Cologne, Germany, in one of its substations.
  Time horizon for broad application: 5 - 10 years
    (no details available)
  Expected technological development: (no data)
    (no details available)
    Energy savings
  Benefits (other than environmental): none
    (no details available)
  Barriers: high


Investment costs for inverter unit are high. KVB Cologne estimates a payback time of ten years.

    Success factors:
    (no details available)
  Applicability for railway segments: low
    Type of traction:  electric - DC
    Type of transportation:  passenger - regional lines, passenger - suburban lines

The installation of an inverter unit will not be economic at all substations in a network. The following local conditions favour the use of inverters (cf. Moninger (no year)):

  • Long headway of trains (low train frequency)
  • Large distances between passenger stations
  • Slopes in track topography
    Grade of diffusion into railway markets:
  Diffusion into relevant segment of fleet: < 5%
  Share of newly purchased stock: not applicable
    Systemic measure.
  Market potential (railways): low
    High investment costs currently impede a wide-spread introduction.
    Kölner Verkehrs-Betriebe AG (KVB), Cologne, Germany
close main section Environmental criteria
  close sub-section Impacts on energy efficiency:
  Energy efficiency potential for single vehicle: 2 - 5%
  Energy efficiency potential throughout fleet: < 1%
    KVB made a contract with its energy supplier to refund the energy fed back into the supply grid on the basis of a flat rate. This flat rate covers an amount of energy corresponding to approx. 5% of the gross energy intake at the substation. Given that the system can operate economically only at some substations, the system-wide saving potential (in one particular network) will be somewhat lower.
  Other environmental impacts: neutral
    (no details available)
close main section Economic criteria
  close sub-section Vehicle - fix costs: none
    (no details available)
  Vehicle - running costs: significant reduction
    Since the price of regenerated energy fed back into the grid is usually lower than for the consumed energy, as much energy as possible should be used by other trains rather than fed back into the supply grid.
  Infrastructure - fix costs: high
    Given the high investment costs of such a system, a thorough assessment of the most profitable site for installation (cf. General criteria - Applicability for railway segments) has to be carried out.
  Infrastructure - running costs: increased
    Although the inverter unit increases the complexity of the substation, maintenance costs are not necessarily high. KVB Cologne have made positive experience with virtually no downtimes in three years of operation.
  Scale effects: medium
    In case of a more wide-spread use of inverter units for substations in mass transit systems, a certain drop in prices is to be expected.
  Amortisation: > 5 years
    (no details available)
no data available Application outside railway sector (this technology is railway specific)
close main section Overall rating
  close sub-section Overall potential: promising
  Time horizon: mid-term
    The installation of inverter units in substations of local DC systems can make a substantial contribution to energy efficiency if the substation is carefully chosen in order to maximise the saving potential. For present prices, payback is critical and may be up to ten years. A comparative economic assessment of the use of inverter enhanced substations has to be made with respect to other solutions (mainly stationary energy storage).
References / Links:  Moninger, Gunselmann 1998;  Piro (no year);  Moninger (no year)
Related projects:
Contact persons:
 date created: 2002-10-09
© UIC - International Union of Railways 2003
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