DC TRACTION POWER SUPPLY

Abstract: A DC traction power supply system comprises a first supply conductor P for supplying power to vehicles on a first track, a second supply conductor N for supplying power to vehicles on a second track parallel to the first track, and a return conductor R. The first and second supply conductors (P, N) are supplied with equal and opposite voltages from common feeding stations (5, 6). An optional "DC autotransformer" (7) comprises energy storage means (12) rapidly switchable from one of the supply conductors (P, N) to the other in order to redistribute power.

Description: DC TRACTION POWER SUPPLY Background to the invention The present invention relates to a direct current electricity supply system for traction. In known DC traction supply systems, power is supplied via a catenary or third rail at a voltage from a few hundred volts up to 3 kV. Power is returned either via the rails acting as a return conductor or via an additional conductor at a negative voltage. Where parallel tracks are provided for vehicles running in opposite directions, the supply systems for the tracks can be connected in parallel or provided separately. Summary of the invention It is an object of the invention to double the effective feeding voltage of a DC traction supply system without requiring a separate negative feeder. Accordingly, the invention provides a direct current electricity supply system for two parallel vehicular tracks, comprising common means for supplying a first supply conductor for a first of said tracks with a positive voltage and also for supplying a second supply conductor for a second of said tracks with a negative voltage substantially equal in magnitude to said positive voltage, and at least one return conductor substantially at ground. The common supply means comprises feeding stations at which voltage is supplied symmetrically to the first and second supply conductors . The at least one return connector may comprise at least one rail of each vehicular track and optionally paralleling conductors may be connected to the return conductors to reduce their effective resistance. The invention may incorporate the circuitry for DC-fed systems described in Swedish Patent Application No. 9803519.9 which mimics the action of an autotransformer by receiving energy from a first supply conductor, storing the energy and then transferring it to a second supply conductor. Energy storing means may comprise at least one inductor and/or at least one capacitor. For example, a capacitor and/or an inductor may be rapidly switched so as to be alternately connected firstly between the first supply conductor and a return conductor and then between the return conductor and the second supply conductor. Alternatively, the DC sides of two inverters may be connected to the first and second supply conductors respectively, the AC sides of the inventors being connected to two windings of a common transformer. This doubles the transmission voltage and dramatically reduces the return current and the resulting electromagnetic and electrolytic/corrosive interference. Such circuitry is referred to herein as a "DC autotransformer" . The power to be redistributed by the DC autotransformer in the system of the present invention is only the difference between the power consumed on the two parallel tracks. This contrasts with the system described in SE 9803519.9 where the DC autotransformer must redistribute the entire power consumed between catenary and ground into the catenary/negative feeder loop. Brief Description of the Drawings The present invention will now be described in more detail, by way of example only, with respect to the accompanying drawings, in which: - Figure 1 is a schematic circuit diagram of a traction supply system according to one embodiment of the invention; Figure 2 is a schematic circuit diagram of a traction supply system according to an alternative embodiment; and Figures 3 to 8 show alternative systems, all exemplified by Figure 2, in more detail. Detailed description of the preferred embodiments Figure 1 schematically shows a DC traction power supply in which a first supply conductor P is supplied with a positive voltage and a second supply conductor N is supplied with a negative voltage substantially equal in magnitude to the positive voltage. The positive supply conductor P supplies power to vehicles running along one track and the negative supply conductor N supplies power to vehicles running in the opposite direction along a parallel track. Power is returned via a return conductor R which may be constituted by the rails along which the vehicles run and preferably paralleling conductors connected thereto. Symmetrical feeding stations 5, 6 supply power to the two supply conductors. It will be appreciated that the vehicles themselves serve to redistribute power between the two tracks and the more vehicles run, the less local power differences will occur. Thus, where traffic is heavy in both directions, the system shown in Figure 1 can be used without further modification, the remaining load difference being borne by the feeding stations . Figure 2 shows an alternative system comprising a DC autotransformer 7 serving to redistribute power differences between the two tracks. The DC autotransformer comprises energy storing means 12 , switching means 13 for switching the energy storing means between a connections to the first and second supply conductors P, N and a control device 14 for controlling the switching means 13 to operate at a frequency preferably greater than 300 Hz. Figure 3 shows one implementation of the DC autotransformer 7. An inductor 15 is connected to the return conductor R. A switch 16, for example an insulated gate bipolar transistor (IGBT) with a diode connected in series, is arranged to connect the inductor 15 alternately to the first and second supply conductors P, N. A capacitor 17 is connected between the first supply conductor P and the return conductor side of the inductor 15 and a capacitor 18 is connected between the second supply conductor N and the return conductor side of the inductor 15. In a particular example, the inductor 15 is 5 mH, the capacitors 17, 18 are both 5 mF and the switching frequency is 300 Hz. Because the inductor 15 is rapidly switched between connections to either the first supply conductor P or the second supply conductor N, it receives energy from whichever of the two supply conductors is more lightly loaded, stores the energy and passes it to the more heavily loaded supply conductor. If the switching frequency is sufficiently high then any voltage drop in the supply conductors P, N caused by loading will be shared equally between the two supply conductors. Thus the distance between adjacent feeding stations can be increased if one or more DC autotransformers are present between the feeding stations. Figure 4 shows an alternative DC autotransformer in which the switch 16 is arranged to connect the return conductor R alternately to an inductor 22 connected to the first supply conductor P and an inductor 23 connected to the second supply conductor N. A capacitor 24 is connected across the switched ends of the inductors 22, 23. The circuit of Figure 4 redistributes energy in a similar manner to that of Figure 3. The capacitor 24 is either charged by whichever inductor 22, 23 is disconnected from the return conductor R, or discharged by the inductor which is connected to the return conductor R, depending on the relative loading of the first and second supply conductors P, N. Figure 5 shows a further alternative DC autotransformer in which the return conductor R is connected to the interconnection 29 of two series-connected coils 25, 26 of a transformer 27 having a core 28. Switches 30, 31 are arranged to connect the transformer 27 across the supply conductors P, N in alternate directions (i.e. the switches are controlled so that either both are in the positions shown by the solid lines or both are in the positions shown by the dotted lines) . Two current paths are established at any time, each comprising one of the switches 30, 31, its associated coil 25, 26 respectively and the return conductor R. Only half of the current flows through each switch and the switches can therefore be smaller and cheaper than those of the embodiments shown in Figures 3 and 4. The use of an iron cored transformer, having an inductance of say 40 H, decreases the voltage ripple on the supply conductors P, N. Figure 6 shows yet another embodiment of DC autotransformer in which, depending on the state of switches 41, 42, a capacitor 34 is either connected between the positive conductor P and the return conductor R, or between the return conductor R and the negative conductor N. The electronic switches 41, 42 are operated at a frequency preferably greater than 300 Hz by a control device 45 generating a suitable pulse train. As shown in Figure 7, the switches 41, 42 may be formed from antiparallel connected thyristors (triacs) 46 as shown, or alternatively from insulated gate bipolar transistors. Switching transients are damped by smoothing capacitors 47, 48 and by the line impedance. Figure 8 shows still another embodiment of DC autotransformer comprising two inverters 58, 59, the DC sides of which are connected to the first and second supply conductors P, N respectively. The AC sides of the inverters are connected to respective windings 60, 62 of a transformer 61 having a core 63. By suitable control of the inverters 58, 59 electrical energy can be transmitted from one of the supply conductors P, N to the other of them. Since some power redistribution is carried out by the vehicles themselves, DC autotransformers in the systems of Figures 2 to 8 can have an increased spacing and/or a decreased power rating compared with the system described in SE 9803519.9. The present invention is particularly suitable for heavily loaded railways with few branching points .

Claims: CLAIMS 1. A direct current electricity supply system for two parallel vehicular tracks, comprising common means for supplying a first supply conductor for a first of said tracks with a positive voltage and also for supplying a second supply conductor for a second of said tracks with a negative voltage substantially equal in magnitude to said positive voltage, and at least one return conductor substantially at ground. 2. A system according to claim 1, wherein said at least one return conductor comprises at least one rail of each vehicular track. 3. A system according to claim 1 or 2, wherein at least one paralleling conductor is connected to the at least one return conductor. 4. A system according to claim 1, 2 or 3, comprising means for rapidly switching energy storage means alternately between a connection to the first supply conductor and a connection to the second supply conductor. 5. A system according to claim 4, wherein said energy storage means comprises at least one inductor. 6. A system according to claim 5, wherein said at least one inductor comprises a transformer. 7. A system according to claim 6, wherein said transformer is connected to the first and second supply conductors by means of respective inverters . 8. A system according to claim 4 or 5 , wherein said energy storage means comprises at least one capacitor.

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