AC TRACTION POWER SUPPLY STATION

Abstract: An AC traction power supply station comprises an electric power source (6) at a voltage lower than a desired catenary voltage, said electric power source being connected to a transformer (5) which is arranged to connect an auxiliary feeding line (3) to a catenary (1). The power source (6) may comprise a public grid, a DC-linked frequency and phase converter, an AC generator, a reactive power source such as a shunt capacitor, or an energy storage means.

Description: AC TRACTION POWER SUPPLY STATION BACKGROUND TO THE INVENTION The present invention relates to an alternating current electricity supply system for traction. In known power supply stations for AC traction, a transformer is usually needed to link the power source, which may be a public net, a frequency converter, a generator or an energy storage device, to the catenary. In only a few cases, such as in some cases of hydropower generation, is the power source voltage the same as the catenary-rail voltage. As the catenary to ground voltages for AC traction systems are relatively high, the most commonly used being a nominal voltage of 15 kV at 16 2/3 Hz and a nominal voltage of 25 kV at 50 or 60 Hz, the transformer which has to be built for these voltages is expensive, its cost being a significant part of the whole power supply station cost. The transformer percentage of the supply station cost increases for decreasing power rating, being a limiting factor for realizing low power supply stations such as the compact frequency converters proposed in H.E. Mordt and J.O. Gjerde: Compact inverters for supply to railways, 7 th European Conference on Power Electronics and its Applications, 8-10 Sept. 1997, Trondhei , Norway, Vol .3 , pp.487-491. Due to the increasing power demands, the distance between power supply stations is tending to become too long for many railway lines, resulting in undervoltage for trains moving at a long distance from supply stations . Placing a full scale supply station between the existing ones becomes needlessly expensive, as only a relatively low additional power supply, " in the order of a few MW, would be sufficient to maintain the catenary voltage at an acceptable level . For supply stations of this size, the transformer cost may be prohibitive . In some cases, it is sufficient to supply reactive power to maintain the catenary voltage at an acceptable level. For smaller reactive power demands, the dimensioning of this equipment, comprising e.g. capacitors and thyristors, for the full catenary voltage is uneconomical. In some cases, it can be advantageous to link an energy storage means such as a battery, a flywheel or a superconductive magnetic energy storage system (SMES) in order to smooth load fluctuations. Again, an expensive step- up transformer to the catenary voltage is needed. Finally, the railway line may pass a location suitable for small scale hydropower or wind power generation, but without access to the public grid. Supply of the generated power directly to the railway system is an alternative in this case. It may also be more profitable to produce power directly for the railway. Again, however, the costs for the transformer needed to step up the voltage to the catenary voltage may be prohibitive. SUMMARY OF THE INVENTION It is an object of the invention to provide an AC traction power supply station which eliminates the need for a large separate transformer. From one aspect, the present invention provides an AC traction power supply station comprising an electric power source at a voltage lower than a desired catenary voltage, said electric power source being connected to a transformer which is arranged to connect an auxiliary feeding line to a catenary. The auxiliary feeding line may be a separate high-voltage bipolar feeding line, a negative feeder in an autotransformer feeding system, a single line high voltage line connected to the catenary and a return conductor in a railway feeding system according to GB 9821696.3 where current booster transformers and autotransformers are used, or the catenary of an adjacent track in a railway feeding system according to GB 9824214.2 where there is a predetermined phase lag between the voltages applied to the catenaries . The power source can be of many different kinds and it should be understood that the requirement for the power source to have a voltage lower than the desired catenary voltage is a reference to the voltage of the secondary, i.e. the load, side of the power source. Suitable power sources and methods of connecting them to said transformer include the following :- 1) Direct connection to a distribution voltage grid for railways running at public frequency (50 or 60 Hz) with or without phase balancing circuitry; 2) Connection to the public grid via a DC-linked frequency and phase converter, converting three phases on the public side to a single phase on the railway side, either at a lower frequency, e.g. 16 2/3 Hz or at public frequency. In both cases, the converter can be of the kind described in Mordt and Gjerde, using the advantages of IGBT transistors; 3) Direct connection of an AC generator for low frequency or public frequency (e.g. hydropower or wind power) , the generator being single phase or three phase in combination with e.g. Stein etz circuitry for phase balancing; 4) Connection of an AC generator via a frequency and phase converter, the generator frequency being decoupled from the railway frequency; 5) A combination of connection to the public network using a converter as in 2) and generation using a converter as in 4) through a common DC- link of both converters (this converter arrangement is described in SE 9802889-7) ; 6) Connection of reactive power, in the most simple case a shunt capacitor. If variable reactive power is 5 required a suitable arrangement such as that described in SE 9600849.5 can be used, but dimensioned for a lower voltage than the catenary voltage. 7) Connection of energy storage means and associated equipment such as (a) battery and AC/DC converter, (b) 10 flywheel with generator/motor and AC/AC converter or (c) SMES and AC/DC converter. The use of batteries and flywheels in a railway power supply context is described in Klaus Reiner: Einsatzmόglichkeiten fur Energiespeicher im elektrischen Bahnbetrieb, Elektrische Bahnen 10/1993, pp 15 331-335 and the use of SMES (superconducting magnetic energy storage) in J.F. Karner, H.W. Lorenzen, R.M. Schroder: Kleine supraleitende Hochleistungsenergiespeicher Pilotanlage und technische Anwendungen, ETG Fachbericht 54 from VDE-Kongress 1994, pp. 139-156. 20 All the connections mentioned above can be combined which each other in parallel and/or by using more than one tap and/or separate winding of the transformer. The direction of active power flow in the connections mentioned above is always towards the railway in the pure 25 generation cases 3) and 4) , generally towards the railway with the temporary exception of regenerative braking in cases 1), 2) and 5) and alternating in direction for case 7) (charging/discharging) . From another aspect, the present invention provides an 30 electricity supply system for traction, comprising a catenary, a return conductor, an auxiliary feeding line and a power supply station as defined above. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which: - Fig.l is a schematic circuit diagram showing an embodiment of the invention comprising autotransforτners; Fig.2 is a schematic circuit diagram showing an embodiment of the invention for feeding from a separate high voltage line; Figs.3a, 3b, 3c and 3d are schematic circuit diagrams of different connections of transformer windings; Fig.4 is a schematic circuit diagram showing different types of power sources connected in parallel; and Fig.5 is a schematic circuit diagram of an embodiment in which a railway is supplied by three phases, all of which follow the track. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig.l is a schematic circuit diagram showing the general concept of the invention as applied to a railway supply system comprising a catenary 1, rail and/or other current return 2, a feeder 3 and autotransfor ers 5. The current supplied to the locomotive 4 is redistributed from the catenary/return circuit to the catenary/feeder circuit in case of the feeder having opposite phase compared to the catenary (negative feeder) and to the feeder/return circuit in case of the feeder having the same phase as the catenary and a higher voltage with respect to ground than the catenary. The autotransformer is equipped with an additional winding 7 connecting a power source 6 to the railway supply system. The railway supply system can be a conventional autotransformer system, an autotransformer system according to GB 9824214.4 which uses the catenary of a second track as negative feeder, symbolized by the additional locomotive 4a or a further supply system (not shown) according to GB 9821696.3 where current balance between catenary, feeder and return is forced by the use of either special current booster transformers having three windings or pairs of parallel-connected current booster transformers. Fig.2 is a schematic circuit diagram showing an embodiment of the invention for feeding from a separate high voltage line 11 using a feeding transformer 8 with a secondary winding 9 connected to the catenary 1 and the return 2, a primary winding 10 connected to a high voltage line 11 (e.g. 132 kV, 16 2/3 Hz in Sweden) and with an additional winding 7 connecting a power source 6 to the railway supply system. The use of autotransformers 5 in embodiments according to Fig.l and the use of full transformers 8 in embodiments according to Fig.2 is usual but in principle, both transformer solutions can be used in both cases. Figs.3a, 3b, 3c and 3d are schematic circuit diagrams of different connections of transformer windings according to the invention. In all four cases, an autotransformer 5 as described in Fig.l is used as an example and has connections to a catenary 1, a (ground) return 2 and a (negative) feeder 3. In Fig.3a the power source 6 is connected between ground 2 and an additional tap 13, resulting in a voltage which is asymmetrical with respect to ground. In Fig.3b, the power source 6 is connected by two additional taps 13a and 13b, yielding a symmetrical voltage with respect to ground. In Fig.3c, the power source 6 is connected to a separate winding 7, allowing complete galvanic separation between source and railway supply system. In Fig.3d, two power sources 6 and 6a are connected to a separate windings 7, 7a respectively. Fig.4 is a schematic circuit diagram showing different types of power sources connected according to the invention. In this hypothetical example, a plurality of different sources is shown, connected in parallel to a separate winding 7 of the autotransformer 5. A generator 14 or a public three phase network 15 can be connected directly to the winding 7. The generator could be single phase or three phase, in the latter case using symmetrizing circuitry as the Steinmetz circuitry. Such symmetrizing circuitry is also advantageous when connecting the public network 15 directly as above. The generator 14 or the public network can also be connected to the winding 7 by means of a DC-linked AC/AC converter 17, usually but not necessarily connected to the public network by a transformer 16. At least one generator 14, the public three phase network 15 and the winding 7 are connected by AC/DC converters 19 sharing a common DC-link 20. Another source is a reactive power source, here symbolized as a variable capacitance 18. This may comprise capacitors, reactors and power electronics such as the equipment described in SE 9600849-5. Still another source is an energy storage means, here symbolized as a battery 21, connected to the winding 7 via an AC/DC converter 19. Instead of a battery, other energy storage means, such as generators with flywheels or SMES devices can be used. In all of the cases mentioned above, the direct connection between the power source and the winding 7 may, for reasons such as galvanic separation, be replaced by a transformer (not shown) , this transformer being made for a voltage much lower than the catenary voltage and thus less expensive than a transformer for that voltage which would be necessary in the absence of the transformer 5 according to the invention. As the public network connected to the railway system according to the invention may be weak, the converter 17 can limit the side effects on the public network by better filtering of single phase pulsation and overtones and limiting the maximum power drawn from the public network compared to full size converters. In this regard reference is again made to the advantages of modern IGBT technology described by Mordt and Gjerde. Fig.5 is a schematic circuit diagram of an embodiment in which the railway is supplied by all three phases following the track according to GB 9824214.2. Two tracks, symbolized by the use of two locomotives 4 and 4a are supplied from catenaries 1 and 3 using two of the three phases and the return 2. The third phase follows the track on the feeder 22, periodically being connected with the catenaries 1 and 3 by "three phase autotransformers" 23 made of star connected windings . A power source 6 is connected to the transformer, for example using delta connected windings 4. As in the single phase case, taps on the main winding may be used in place of separate windings. The transformers 5,8 and 23 used in this invention could be conventional oil-filled transformers, but in alternative embodiments they are formed as "dry" transformers in which at least the windings for the catenary and/or the feeder are surrounded in turn by semiconducting, insulating and semiconducting layers according to WO 97/45847. It will be appreciated that modifications to the specific embodiments described above, which do not depart from the scope of the appended claims, will be apparent to those skilled in the art.

Claims: CLAIMS 1. An AC traction power supply station comprising an electric power source at a voltage lower than a desired catenary voltage, said electric power source being connected to a transformer which is arranged to connect an auxiliary feeding line to a catenary. 2. A power supply station according to claim 1, wherein said transformer is an autotransformer . 3. A power supply station according to claim 1 or 2 , wherein the power source is connected to a separate winding of said transformer provided for the purpose of connecting the power source to said transformer. 4. A power supply station according to claim 1 or 2, wherein the power source is connected to at least one intermediate point of a winding of said transformer. 5. A power supply station according to any preceding claim, wherein the power source comprises a connection to a public electricity supply network. 6. A power supply station according to claim 1, 2, 3 or 4, wherein the power source comprises an AC generator. 7. A power supply station according to any preceding claim, wherein the power source comprises a DC-linked frequency and phase convertor. 8. A power supply station according to claim 1, 2, 3 or 4, wherein the power source supplies reactive power only. 9. A power supply station according to claim 1, 2, 3 or 4, wherein the power source comprises energy storage means . 10. A power supply station according to claim 9, wherein the power source comprises a battery. 11. A power supply station according to claim 9, wherein the power source comprises a flywheel. 12. A power supply station according to claim 9, wherein the power source comprises a superconducting magnetic energy storage device. 13. A power supply station according to any preceding claim, comprising a plurality of power sources connected to said transformer in parallel. 14. An electricity supply system for traction, comprising a catenary, a return conductor, an auxiliary feeding line and a power supply station as claimed in any preceding claim. 15. A system as claimed in claim 14, wherein said auxiliary feeding line is a high-voltage bipolar feeding line . 16. A system as claimed in claim 14, wherein said auxiliary feeding line is a negative feeder and said transformer is an autotransformer . 17. A system as claimed in claim 14, wherein said catenary supplies power to a first track, said auxiliary feeding line is a further catenary supplying power to a second track adjacent said first track, and a predetermined phase lag exists between the voltages on the catenaries of said first and second tracks.

Inventors: HAMMARSTROEM GUNNAR, HAMMARSTROEM, GUNNAR
Applicants: DAIMLER CHRYSLER AG, HAMMARSTROEM GUNNAR, DAIMLERCHRYSLER AG, HAMMARSTROEM, GUNNAR
Application year: 1999
Main classification: B60M3
IPC codes: B60M3