GB2343431A - AC traction power supply system - Google Patents

Abstract

The supply system for parallel vehicular tracks comprises a first supply conductor P for a first of said tracks, a second supply conductor N for a second of said tracks and a return conductor R. There is a predetermined phase lag, for example of 90{, 120{ or 180{ between voltage supplied to said first conductor P and voltage supplied to said second conductor N.

Description

AC TRACTION POWER SUPPLY BACKGROUND TO TEE INVENTION The present invention relates to an alternating current electricity supply system for traction.

In known autotransformer fed AC traction supply systems, the winding of an autotransformer is connected between a catenary and an auxiliary negative feeder, with the rails, acting as a return conductor, tied to an intermediate point of the winding. The train draws current from two adjacent autotransformers, the total supply current being half the train current in the most common case of symmetrical autotransformers. Rail currents flow through the autotransformer windings in order to maintain the Ampere-turn balance in the core. Autotransformers are expensive and, where parallel tracks are provided for trains running in opposite directions, each requires its own auxiliary negative feeder.

Where traction supply systems are fed from a three phase public network, steps must be taken to balance the single phase railway load on the public network. Usually, the catenary is split into sections each fed from a different phase. The sections must be galvanically separated by a short non-energised portion which slows down the trains and complicates their handling.

SUMMARY OF THE INVENTION It is an object of the invention to provide an AC traction supply system which simultaneously supplies a plurality of parallel tracks in a more efficient manner.

The present invention provides an alternating current electricity supply system for parallel vehicular tracks, comprising a first supply conductor for a first of said tracks, a second supply conductor for a second of said tracks and at least one return conductor, characterised by a predetermined phase lag between voltage supplied to said first conductor and voltage supplied to said second conductor. The return conductors are preferably constituted by at least one rail of each track along which the vehicles run.

In one embodiment of the invention, the predetermined phase lag is substantially 180 , voltage being supplied symmetrically to the first and second supply conductors at feeding stations. Autotransformers may then be connected between the first and second supply conductors, with the return conductor (s) tied to an intermediate point of each autotransformer winding, in order to balance the currents in the first and second supply conductors and minimise the current in the return conductor (s). Further improvements in current balancing are attained by using at least one current booster transformer to interconnect the first, second and return conductors. For example, a booster transformer may have three windings, each connected into one of the conductors, or two booster transformers may be provided, each having a first winding connected into a respective one of the supply conductors and a second winding, the two second windings being connected, in parallel, into the return conductor.

In a further embodiment, the first and second tracks are each provided with more than one supply conductor. This allows the provision of an auxiliary high voltage feeding conductor, for example at 66kV as well as a lower voltage supply conductor (eg-16. 5kV) on the first track, and corresponding conductors on the second track supplied with a voltage 180 out of phase from that supplied to the conductors on the first track. At least one autotransformer may be connected between the two high voltage auxiliary conductors, with the lower voltage conductors connected to appropriate intermediate points on the autotransformer winding.

An alternative embodiment of the invention addresses the problem of symmetrically loading a three phase feeding network by making the predetermined phase lag substantially 90 and providing means for converting the three phase feeding network into two phases separated by 90 . The converting means may for example comprise the known Scott, Leblanc or Stenkvist connections. Each track may optionally have a negative feeder, supplied by the phase converting means, and at least one autotransformer.

In another alternative embodiment, the predetermined phase lag is substantially 120 , the first and second supply conductors being divided into sections, breaks between two adjacent sections of the first supply conductor occurring approximately at the mid-point of a section of the second supply conductor and breaks between two adjacent sections of the second supply conductor occurring approximately at the mid-point of a section of the first supply conductor, the first and. second supply conductors at any point along the system being arranged to be connected to two different phases of a three phase feeding network. Thus a break between sections of each of the supply conductors is only required at every second feeding station. Conventional three phase transformers can be used at the feeding stations.

Advantageously, all three phases fed by the transformer follow the railway, the phase not being used for supply at any point being carried by at least one conductor parallel to the supply conductors.

In the embodiments of the invention that are fed from a three phase network, the or each return conductor is preferably arranged to be connected to a neutral point of the three phase network.

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: Figure 1 is a schematic circuit diagram showing the general concept of the invention; Figures 2,3 and 4 are schematic circuit diagrams of embodiments of the invention in which the predetermined phase lag is substantially 180 ; Figure 5 is a schematic elevation of a system according to the embodiment of Figure 4; Figures 6 and 7 are schematic circuit diagrams of embodiments in which the predetermined phase lag is substantially 90 ; Figures 8 and 9 are schematic circuit diagrams of embodiments in which the predetermined phase lag is substantially 120 ; and Figure 10 is a schematic elevation of a system according to the embodiment of Figure 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The electricity supply system shown in Figure 1 comprises a first supply conductor P, usually a catenary for vehicles running along one track and a second supply conductor N, also usually a catenary, for vehicles running along a parallel track in the opposite direction. The phase lag between the voltages on conductors P and N is predetermined. Power is returned via a return conductor R comprising at least one rail of each of the tracks, preferably connected to at least one paralleling conductor.

If the predetermined phase lag is 180 it will be appreciated that the effective feeding voltage has been doubled, as in a conventional autotransformer-fed traction system, but at lower cost. In the conventional system, the autotransformers must redistribute the entire power flowing between catenary and ground into the catenary/negative feeder loop. In the invention, the power to be redistributed is only the difference between the power consumed on the two parallel tracks. Thus, when the predetermined phase lag is 180 and traffic is heavy in both directions, the system shown in Figure 1 can be used without further modification, any remaining load difference being borne by the feeding stations (not shown). Alternatively, autotransformers 1 can be provided as shown in Figure 2. These autotransformers can be more widely spaced and/or have a lower power rating than is conventional.

A further advantage is that most of the current flows in the catenaries or supply conductors rather than the return conductors and the two catenaries together form an almost ideal bifilar conductor with currents flowing in opposite directions near to each other, thus minimising reactance and electromagnetic interference.

As shown in Figure 3, current balancing can be improved by the use of a special booster transformer 2 having three windings, one connected into each of the first and second supply conductors P, N and a third winding connected into a paralleling return conductor R'which extends parallel to only approximately the central third of the rails between autotransformers 1. The current in the paralleling return conductor is forced to be the difference between the currents in the supply conductors by the"current adding"booster transformer 2. The arrangement shown is useful for minimising the earth current from the rails R near the mid-point between the autotransformers where it tends to be at its maximum.

Alternatively, the paralleling return conductor could extend all the way between the autotransformers ; more than one current adding booster transformer could be provided; and/or each current-adding booster transformer could be replaced by two standard booster transformers, one winding of each being connected into a respective supply conductor and the other two windings being connected in parallel into the return conductor. These alternative arrangements are not shown in the drawings but are adapted from the disclosure of co-pending British Patent Application No. 9821696.3.

Figure 4 shows an alternative system where the predetermined phase lag is still 180 and high voltage auxiliary feeders H are provided, one for feeding each track.

Each high voltage feeder is in phase with the supply conductor P, N of the track fed. As an example, the high voltage feeders H could be at 66kV and the supply conductors P, N at 16. 5kV, but other voltages compatible with systems currently in use are possible. High voltage autotransformers 3 are connected from one high voltage feeder to the other, with the supply and return conductors connected to appropriate points on the windings. Further autotransformers 1 are connected between the supply conductors as before.

Figure 5 schematically shows how the conductors of Figure 4 can be mounted on poles. The high voltage feeders H are suspended on suitable insulators I. Preferably, each high voltage feeder is mounted on the pole carrying the supply conductor with which it is out of phase, i. e. the two poles carry the high voltage feeders for each other's tracks, in order to minimise reactance and magnetic fields. The voltage values shown in Figures 4 and 5 are only exemplary.

Figure 6 shows an embodiment of the invention in which the predetermined phase lag is 90 , supply conductors U, V for the respective tracks being fed via transformers in Scott connections 4 from a three phase 50 or 60Hz public supply network X, Y, Z. The effective transmission voltage is/2 times the supply conductor voltage. The two phases U, V into which the three phases have been converted by the Scott connections, extend along the entire length of the railway, no galvanic separation being required. The trains can therefore run with no interruption in the power fed thereto.

The longer the railway line and the heavier the traffic, the more equally the load will be distributed on the three phases of the public network. As with the embodiments already described, the vehicles themselves serve to redistribute power between the two tracks.

Figure 6 also shows an optional current-adding booster transformer 2, connected in the same manner as that shown in Figure 3. The booster transformer 2 forces a current which is the vector difference between the currents in the supply conductors U, V into a return conductor R'connected in parallel with the rails R. The booster transformer 2 may well be necessary to avoid electromagnetic disturbances in the rails as there will always be a vector difference between the currents in the supply conductors.

The Scott connections could be substituted by Leblanc, Stenkvist or any other connections converting a three phase system into a 90 two phase system.

An alternative system with a 90 phase lag is shown in Figure 7. A Scott connection 4'feeds supply conductors U, V as well as negative feeders-U,-V on the respective tracks.

Thus the three phase supply is effectively converted into four phases. Each track is provided with autotransformers 6 connected between the supply conductor and negative feeder.

The transmission voltage is doubled and the load well balanced on the three phase network.

Figure 8 shows a system in which the predetermined phase lag is 120 , the two supply conductors at any point along the line being fed from two of the three phases A, B, C of a public network. At each feeding station, only one of the two supply conductors requires a galvanic separation and the feeding station comprises a simple three phase transformer 5 (only one side of which is shown) with its star point connected to the return conductor R. The number of separations is halved as compared with a conventional system.

The effective transmission voltage is/3 times the supply conductor voltage, with a corresponding increase in the load capable of being operated. This system gives highly symmetrical loading of the public network.

The system of Figure 8 may be modified as shown in Figure 9 by making the third phase follow the railway along its entire length. Thus the two track railway is a full, periodically transposed, three phase line, the phase not being used for supply functioning in the manner of a negative feeder. A three phase transformer 5 can be connected at any point to feed the system. Consumers of auxiliary power such as 7 can also be connected anywhere. A device 8 comprising star-connected windings can be used as a kind of autotransformer.

Figure 10 shows how the conductors of the system of Figure 9 could be mounted at the line X-X of Figure 9. The third phase A running in parallel with the supply phases B and C gives very low reactance and stray magnetic fields.

It will be appreciated that modifications to the specific embodiments described above, including combinations of those embodiments, which do not depart from the scope of the appended claims, will be apparent to those skilled in the art.

Claims (16)

1. An alternating current electricity supply system for parallel vehicular tracks, comprising a first supply conductor for a first of said tracks, a second supply conductor for a second of said tracks and at least one return conductor, characterised by a predetermined phase lag between voltage supplied to said first conductor and voltage supplied to said second conductor.

2. A system according to claim 1, wherein the at least one return conductor is constituted by at least one rail of each track along which the vehicles run.

3. A system according to claim 1 or 2, wherein the predetermined phase lag is substantially 180 , voltage being supplied symmetrically to the first and second supply conductors at feeding stations.

4. A system according to claim 3, wherein at least one autotransformer is connected between the first and second supply conductors, with the return conductor (s) tied to an intermediate point of the or each autotransformer winding.

5. A system according to claim 3 or 4, wherein the first and second tracks are each provided with an auxiliary high voltage feeding conductor supplied with a high voltage in phase with the voltage on the respective supply conductor.

6. A system according to claim 5, wherein at least one autotransformer is connected between the two high voltage auxiliary conductors, with the supply conductors connected to appropriate intermediate points on the at least one autotransformer winding.

7. A system according to claim 1 or 2, wherein the predetermined phase lag is substantially 90 and means is provided for converting a three phase feeding network into two phases separated by 90 .

8. A system according to claim 7, wherein the converting means comprises Scott or Leblanc connections.

9. A system according to claim 7 or 8, wherein each track has a negative feeder, supplied by the phase converting means, and at least one autotransformer connected between said negative feeder and the respective supply conductor.

10. A system according to claim 1 or 2, wherein the predetermined phase lag is substantially 120 , the first and second supply conductors being divided into sections, breaks between two adjacent sections of the first supply conductor occurring approximately at the mid-point of a section of the second supply conductor and breaks between two adjacent sections of the second supply conductor occurring approximately at the mid-point of a section of the first supply conductor, the first and second supply conductors at any point along the system being arranged to be connected to two different phases of a three phase feeding network.

11. A system according to claim 10, including feeding stations comprising three phase transformers.

12. A system according to claim 10 or 11, wherein three phases follow the railway, the phase not being used for supply at any point being carried by at least one conductor parallel to the supply conductors.

13. A system according to claim 12, including at least one device comprising three windings mutually connected at one end at a star point, the other end of each of said windings being connected to a respective one of said three phases and the star point being connected to the at least one return conductor, such that said device operates in the manner of an autotransformer.

14. A system according to any preceding claim, wherein at least one current booster transformer interconnects the first, second and return conductors.

15. A system according to claim 14, wherein the at least one booster transformer has three windings, each connected into one of the conductors.

16. A system according to claim 14, wherein at least one pair of booster transformers is provided, each transformer of the at least one pair having a first winding connected into a respective one of the supply conductors, and a second winding, the two second windings being connected, in parallel, into the return conductor.