Arrangement and method for transforming a voltage

Technical Field

[0001] The invention relates to an arrangement comprising a direct current, DC, traction power network adapted for supplying electrical energy to a rail system, an energy recuperation device adapted for returning surplus electrical energy of the DC traction power network into an AC traction network and/or an energy storage system adapted for storing surplus electrical energy of the DC traction power network and for feeding back the stored electrical energy into the DC traction power network. The invention further relates to a respective method for transforming a voltage, comprising the DC traction power network and the energy recuperation device and/or the energy storage system.

Background Art

[0002] DC traction power networks, also referred to as DC traction grids, are

known from prior art as DC electricity grids for supplying direct current to electrified rail system, for example to rail networks, trains, tramways and the like. Traditionally, three phase alternating current from an AC traction network is converted by power transformers, rotary transformers, static inverters, directional diode rectifiers and/or thyristors into a voltage and a direct current required by the trains.

[0003] Whenever a train brakes, its kinetic energy is converted into electrical energy and returned on a power line of the DC traction power network. For feeding the electrical energy back into the AC traction network thyristor inverters or self-commutating semiconductors like IGBTs have been installed as energy recuperation devices to operate in parallel to said traditional diode rectifiers. This way, the DC traction power network is fed with direct current from the AC traction network, and in a bidirectional way direct current resulting from the braking processes of the train is fed back from the DC traction power network into the AC traction network. In addition or alternatively, energy storage systems can be used for temporarily storing such surplus electrical energy resulting from said braking processes.

[0003a] EP 2 693 598 A1 describes that power of electrical sub-stations that

power a network of catenaries and the braking energy that the trains transmit to the network of catenaries are used for the mass power supply of equipment for charging vehicle batteries from the catenary, from an energy storage device, or from both, according to the power required by the equipment, the power stored in the device, and the power of the catenary, as long as the catenary power required by the trains is supplied.

[0003b] EP 2 255 992 A1 describes an electric railway power-supply system not requiring vast area for installation, excellent in rapid charge-discharge characteristic, and low in manufacturing cost.

[0004] However, DC traction power networks operate at different DC voltages.

Thus, different types of energy recuperation devices and energy storage systems are required in order to match the DC voltage of the DC traction power network. In particular, today ^' s energy recuperation devices and energy storage systems are characterized by a fixed i.e. single voltage for operating said device. Developing and designing an energy recuperation device and/or an energy storage system to operate at a different voltages requires costly and time-consuming research and development activities. Thus, there is a need to deploy a single energy recuperation device and/or an energy storage system model with various DC traction networks having different DC operating voltages.

Summary of invention

[0005] Therefore, it is an object of the invention to provide a solution for

connecting an energy recuperation device and/or an energy storage system characterized by a fixed operating voltage to DC traction power network having in contrast a different operating voltage. [0006] The object of the invention is solved by the features of the independent claims. Preferred embodiments are described in the dependent claims.

[0007] Thus, the object is solved by an arrangement comprising a direct current, DC, traction power network adapted for supplying electrical energy to a rail system, a transformer, and an energy recuperation device adapted for returning surplus electrical energy of the DC traction power network into an AC traction network and/or an energy storage system adapted for storing surplus electrical energy of the DC traction power network and for feeding back the stored electrical energy into the DC traction power network, whereby the transformer is electrically connected in series between the DC traction power network and the energy recuperation device and/or the energy storage system, and/or, comprising the energy recuperation device electrically connected in series between the DC traction power network and the transformer.

[0008] It is therefore a key point of the invention to provide a transformer between the energy recuperation device and the AC traction network and/or between the DC traction power network and the energy recuperation device respectively the energy storage system. Advantageously, energy recuperation devices respectively energy storage systems having a fixed operating voltage can be employed with DC traction power networks having different operating voltages. Such way the invention provides a possibility for using existing energy recuperation devices respectively energy storage systems with a wide variety of DC traction power networks, and thus allows for a very flexible and cost-efficient use of said devices. In other words, the invention allows for a single "standard" market solution of the energy recuperation device and/or the energy storage system that fit universally for DC traction power networks having different DC voltages.

[0009] Whenever a train of the rail system brakes, kinetic energy resulting from the braking is converted into electrical energy and returned into the DC traction power network as surplus electrical energy. Said surplus electrical energy is then transformed by the transformer and returned into the AC traction network by the energy recuperation device and/or stored

respectively fed back by the energy storage system into the DC traction power network. Returning surplus electrical energy into the AC traction network comprises in the context of the application returning said surplus electrical energy into an AC medium voltage grid as well, for example directly or via a recuperation device transformer as described below.

[0010] In a preferred embodiment the arrangement comprises the energy storage system, whereby the energy storage system comprises an AC/DC converter having an AC side and a DC side, and the AC side is electrically connected to the transformer or comprising the energy recuperation device, whereby the energy storage system is electrically connected in series between the transformer and the energy recuperation device such that the AC side of the AC/DC converter is electrically connected to the transformer and the DC side is electrically connected to the energy recuperation device. According to a further preferred embodiment the arrangement comprises the energy recuperation device electrically connected in series between the DC traction power network and the transformer or comprises a DC/AC converter having a DC side connected to the DC traction power network and an AC side connected to the transformer and further comprises an AC/DC converter having an AC side connected to the transformer and a DC side connected to the energy recuperation device and/or the energy storage system such that the transformer is connected in series between the energy recuperation device or the DC/AC converter and the AC/DC converter.

[001 1] With the transformer an energy recuperation device and/or an energy storage system having a fixed operating voltage can be easily employed with a DC traction power network having in contract a fixed operating voltage. For example, with a transformer having a conversion ratio between its primary side and secondary side of 2 an energy recuperation device and/or an energy storage system designed to operate at a fixed DC voltage of 1 ,5kV can be electrically connected to a DC traction power network operating at a DC voltage of 3kV. Providing a double stage converter is advantageous as such double stage converter has an inherent energy storage capacity and thus increases the amount of energy returned into the DC power traction network. The DC/AC converter and the energy storage system are preferably provided as an energy storage system utilizing said energy storage capacity.

[0012] According to another preferred embodiment, the arrangement comprises the energy storage system, the energy storage system comprises a super capacitor and/or a battery connected to the DC side and/or comprises a plurality of energy storage systems connected in series and/or in parallel. Said super capacitor and/or a battery can be provided as any suitable capacitor respectively battery adapted for storing the surplus electrical energy of the rail transportation system. By providing such energy storage system an energy consumption of the rail system can be advantageously decreased up to 30 percent. Preferably, the energy storage system comprises a useable storage energy per cabinet respective device of 6MJ, 8MJ or 10MJ respectively a useable energy 16kWh, 20kWh or 25kWh. Connecting a plurality of energy storage systems in series and/or in parallel advantageously increases overall storage capacity and in turn further decreases energy consumption of the rail system.

[0013] In another preferred embodiment the arrangement comprises the energy recuperation device and a recuperation device transformer adapted for connecting to the AC traction network, whereby the recuperation device transformer is preferably provided as an autotransformer, comprises a plurality of energy recuperation devices connected in series and/or in parallel and/or the transformer is provided as a high, medium, line and/or multi-winding transformer. If the energy recuperation device is electrically connected in series between the DC traction power network and the transformer, the transformer is preferably provided in an alternative embodiment as recuperation device transformer. In another preferred embodiment the energy recuperation device is directly connected to the AC traction network.

[0014] Generally, the conversion ratio of the transformer and/or of the

recuperation device transformer may comprise a conversion ratio of a primary side to a secondary side of≥ 1.1. Thereby the primary side preferably faces towards the DC traction power network. According to an especially preferred embodiment the conversion ratio is≥ 2, > 3,≥ 4, > 6, > 8 or > 10. With a conversion ratio of 4, for example, a DC traction power network operating at a DC voltage of 3kV can be connected via the transformer to an energy recuperation device and/or an energy storage system operating at a DC voltage of 750V. According to further preferred embodiment the DC voltage corresponding to the DC traction power network is 1 ,5kV, 3kV or 6kV and/or a voltage corresponding to the energy recuperation device and/or the energy storage system is 0,75kV, 1 ,5kV, 3kV or 6kV.

[0015] In yet a further preferred embodiment the arrangement comprises the

energy recuperation device, whereby the energy recuperation device comprises a converter adapted for converting a DC voltage into an AC voltage and comprising a plurality of inverter cells, and each inverter cell comprises a self-commutating inverter semiconductor device. The self- commutating inverter semiconductor device is preferably provided as an insulated-gate bipolar transistor device (IGBT), as a bi-mode insulated- gate bipolar transistor device (BIGT), as a reverse-conducting IGBT, as an integrated gate-commutated thyristor device (IGCT), and/or a silicon carbide based unipolar or bipolar switching device. Further preferably, the IGBT is provided as a bi-mode insulated-gate bipolar transistor device (BIGT). According to a further preferred embodiment the converter is provided as a H-bridge topology comprising a plurality inverter cells and/or is provided as a 4Q voltage source converter. More preferably, the converter is provided according to norm IEC 62501 : 2009.

[0016] According to another preferred embodiment the DC traction power network is provided as a railway electrification system preferably for providing electrical energy to the rail system, a train, a tram and/or a trolleybus. Preferably, the DC traction power is electrically connected to the rail system and thereby to the train, tram respectively trolleybus. Further preferably, the railway electrification system comprises a contact line and a rail, whereby the rail and/or the contact line are electrically connected in regular or irregular intervals along their extension to the DC traction power network. The rail and/or the contact line can be provided as any rail respectively contact line known from prior art, for example as flat-bottom steel rails respectively as an overhead line or overhead wire for

transmitting electrical energy to trams, trolleybuses and/or trains.

[0017] The object of the invention is further solved by a method for transforming a voltage, comprising a direct current, DC, traction power network adapted for supplying electrical energy to a rail system, and an energy recuperation device adapted for returning surplus electrical energy of the DC traction power network into an AC traction network and/or an energy storage system adapted for storing surplus electrical energy of the DC traction power network and for feeding back the stored electrical energy into the DC traction power network, and comprising the step of: transforming, by a transformer electrically connected in series between the DC traction power network and the energy recuperation device and/or the energy storage system, a first voltage corresponding to the DC traction power network into a second voltage corresponding to the energy recuperation device and/or to the energy storage system, or, whereby the energy recuperation device comprises a DC side electrically connected to the DC traction power network and an AC side, and comprising the step of: transforming, by a transformer electrically connected in series between the energy

recuperation device and the AC traction network (5), a first voltage corresponding to the AC side into a second voltage corresponding to the AC traction network.

[0018] Thus, as an exemplary, advantageous application, the energy

recuperation device and/or the energy storage system operating on a fixed, second voltage can be connected via the transformer in a very flexible manner to any DC traction power network operating on the first voltage. Preferably, a conversion ratio between the first voltage and/to the second voltage is≥ 2, > 3,≥ 4, > 6,≥ 8 or > 10. Further preferred the first voltage is 1 ,5kV, 3kV or 6kV and/or the second voltage is 0,75kV, 1 ,5kV, 6kV.

[0019] According to another preferred embodiment the method comprises an AC/DC converter having an AC side and a DC side and the energy storage system, whereby the AC/DC converter is electrically connected in series between the transformer and the energy storage system such that the AC side of the AC/DC converter is connected to the transformer and the DC side is connected to the energy storage system. In a further preferred embodiment the method comprises the energy recuperation device electrically connected in series between the DC traction power network and the transformer or comprising a DC/AC converter having a DC side connected to the DC traction power network and an AC side connected to the transformer and further comprising an AC/DC converter having an AC side connected to the transformer and a DC side connected to the energy recuperation device and/or the energy storage system such that the transformer is connected in series between the energy

recuperation device or the DC/AC converter and the AC/DC converter.

[0020] In another preferred embodiment the DC/AC converter or the energy

recuperation device, the transformer and the second DC/AC converter are provided as a frequency transformer and/or the AC/DC converter is provided as single or double stage converter and/or comprises an additional DC/DC stage. Preferably, the method comprises the energy storage system, whereby the energy storage system comprises a super capacitor and/or a battery and/or comprising a plurality of energy storage systems connected in series and/or in parallel. In another preferred embodiment the method comprises the energy recuperation device and a recuperation device transformer adapted for connecting to the AC traction network, whereby the recuperation device transformer is preferably provided as an autotransformer, comprising a plurality of energy recuperation devices connected in series and/or in parallel and/or whereby the transformer is provided as a high, medium, line and/or multi-winding transformer.

[0021] Further embodiments and advantages of the method are apparent for the man skilled in the art from before described arrangement.

Brief description of drawings [0022] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

[0023] In the drawings:

[0024] Fig. 1 shows an arrangement according to a first preferred embodiment of the invention in a schematic view,

[0025] Fig. 2 shows an arrangement according to a third preferred embodiment of the invention in a schematic view,

[0026] Fig. 3 shows the arrangement of Fig. 2 in a more detailed view, and

[0027] Fig. 4 shows an arrangement according to a fourth preferred embodiment of the invention in a schematic view.

Description of embodiments

[0028] Fig. 1 shows an arrangement according to a first preferred embodiment of the invention in a schematic view, comprising a direct current, DC, traction power network 1 , a transformer 14 and an energy recuperation device 3. The transformer 14 is electrically connected in series between the DC traction power network 1 and the energy recuperation device 3. Thus, the transformer 14 provides galvanic insulation between the DC traction power network 1 and the energy recuperation device 3.

[0029] The DC traction power network 1 operating at a first voltage is adapted for supplying electrical energy to a rail system 4. In particular, the DC traction power network 1 is electrically connected to the rail system 4 and is provided as a railway electrification system 1 1 for providing electrical energy to a train 13, shown in Fig. 3, of said rail system 4 via a power line, rail or the like. The train 13 thus receives via the power line electrical energy. [0030] The energy recuperation device 3 is adapted for returning surplus electrical energy of the rail system 4 respectively of the DC traction power network 1 into a connected AC traction network 5. In particular, the energy recuperation device 3 returns surplus braking energy of the train 13 back to the AC traction network 5, thus reducing the total energy consumption of the rail system 4. During decelerating or braking, the train 13, or a tram or trolleybus, is feeding braking energy back into the DC traction power network 1. Thus, instead of dissipating unused braking energy using onboard resistors, the energy recuperation device 3 recuperates the surplus braking energy from the DC traction power network 1 by feeding the energy back to the AC traction network 5.

[0031] The energy recuperation device 3 comprises a converter, not shown, for converting a DC voltage to a AC voltage. In the present embodiment the converter is adapted for converting a DC voltage corresponding to the energy recuperation device 3 into an AC voltage corresponding to the AC traction network 5. Therefore, an AC side of the energy recuperation device 3 is connected to the AC traction network 5 and a DC side is connected to the transformer 14 operating at said second voltage. The converter comprises a plurality of inverter cells. Each inverter cell comprises a self-commutating inverter semiconductor device. In particular, each converter is provided as a H-bridge topology comprising the plurality inverter cells. Alternative configurations are possible as well, for example, as a 4Q voltage source converter and/or at least two half-bridges each comprising two self-commutating inverter semiconductor devices. The self- commutating inverter semiconductor devices are provided as IGBTs.

[0032] The transformer 14 has a conversion ratio between the first voltage and/to the second voltage of 2. Beside that other conversion ratios are possible, such as, for example, 3, 4, 6, 8 or > 10. As said before, the first voltage corresponds to the voltage of the DC traction power network 1 and the second voltage corresponds the voltage of the energy recuperation device 3. Thus, as in the present embodiment the DC traction power network 1 operates at 3kV DC, the transformer 14 transforms the first voltage of 3kV to the second voltage of 1 ,5kV for operating the energy recuperation device 3. In alternative embodiments the first voltage corresponding to the DC traction power network 1 can be 1 ,5kV or 6kV and, with said conversion ratio of 2, the second voltage corresponding to the energy recuperation device 3 is 0,75kV or 3kV, respectively.

[0033] Fig. 2 shows a second preferred embodiment, comprising both the energy recuperation device 3 and the energy storage system 6. The energy storage system 6 comprises a super capacitor and/or a battery 12 and a DC/AC converter 17, as described below. As can be seen, the energy recuperation device 3 and the energy storage system 6 are both connected to the transformer 14. Thus it is possible to return surplus electrical energy of the rail transportation system 4 into the AC traction network 5 and to feed back electrical energy into the DC traction power network 1.

[0034] Fig. 3 shows the embodiment of Fig. 2 in a more detailed view. A train 13 is shown as part of the rail system 4, electrically connected via a rail electrification system i.e. via a rail and contact line connected to the DC traction power network 1. An AC medium voltage (MV) grid 15 feeds electrical energy via a MV transformer 15a to the AC traction network 5 and via a feeding converter 15b to the DC traction power network 1. This way an AC voltage of the AC MV grid 15 is transformed into voltage suitable for powering the train 13, converted from AC to DC by the feeding converter 15b and fed into the DC traction power network 1.

[0035] The transformer 14 is connected in series between an AC/DC converter 16 and the DC/AC converter 17. The AC/DC converter 16 has an AC side connected to a primary side of the transformer 14 and a DC side connected to the DC traction power network 1. The DC/AC converter 17 has a DC side connected to the energy recuperation device 3 and the super capacitor and/or the battery 12, and has an AC side connected to a secondary side of the transformer 14. The transformer 14 is provided as a high, medium or line frequency transformer 14 and comprises said conversion ratio between its first side and/to second side.

[0036] The energy recuperation device 3 is either directly connected to the AC traction network 5, as indicated by the dashed line on the left showing various configurations, or connected a recuperation device transformer 18 to the AC traction network 5 or the AC MV grid 15. The recuperation device transformer 18 is provided as an autotransformer or as a (standard) transformer as shown in the middle. While not shown, a plurality of energy recuperation devices 3 can be provided connected in series and/or in parallel. Further, the DC/AC converter 17 can be provided as single or double stage converter and/or may comprise an additional DC/DC stage.

[0037] Fig. 4 shows a further embodiment in a schematic view, wherein the

energy recuperation device 3, comprising a DC to AC converter, is electrically connected in series between the DC traction power network 1 and the transformer 14. The transformer 14, as shown on the right side in Fig. 4, is this way connected in series between the energy recuperation device 3 and the energy storage system 6. The recuperation device transformer 18 can be provided as well, being connected with its primary side between the transformer 14 and the AC/DC converter 17 or between the energy recuperation device 3 and the transformer 14, and connected with its secondary side to the AC traction network 5 or the AC MV grid 15, as indicated by the dashed lines in the middle.

[0038] In addition or alternatively, the transformer 14 can be provided instead of the recuperation device transformer 18 such that the transformer 14 is connected in series between the energy recuperation device 3 and the AC traction network 5 or the AC MV grid 15, as indicated by the dashed line on the left side. This way the transformer 14 transforms a first voltage corresponding to an AC side of the energy recuperation device 3 into a second voltage corresponding to the AC traction network 5 respectively to the AC MV grid 15. A combination of before described alternatives are possible as well, as shown in Fig. 4 with said dashed lines.

[0039] While the invention has been illustrated and described in detail in the

drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to be disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting scope.

Reference signs list

1 DC traction power network

3 energy recuperation device

4 rail system

5 AC traction network

6 energy storage system

1 1 railway electrification system

12 super capacitor, battery

13 train

14 transformer

15 AC MV grid

15a MV transformer

15b feeding converter

16 AC/DC converter

17 DC/AC converter

18 recuperation device transformer