TRACTION POWER SUPPLY SYSTEMS

Abstract: An electricity supply system for traction comprises first and second supply conductors K, N supplied with first and second voltages respectively, a return conductor R, and means, such as current booster transformers T2, T2' with three windings, for rendering the current in one of the conductors K equal to the sum of the currents in the other two conductors N, R.

Description: TRACTION POWER SUPPLY SYSTEMS Background to the invention The present invention relates to an electricity supply system for traction. Where power demands are heavy, the power delivered by a conventional traction supply system may be insufficient and additional power transmission means are therefore required to be installed. The additional transmission means may comprise a high voltage supply system providing a voltage which is symmetrical with respect to ground and thus requiring two high voltage conductors. The high voltage conductors may be mounted on separate poles or on the catenary poles, an example of the latter system being described in "Fύhrung einer Bahnstromleitung am Oberleitungsgestange" Bauer, K-H and Kiessling, F, Elektrische Bahnen 78 (1980) H.lO, pp 257-260. This paper also mentions the technical and economic disadvantages of mounting the two high voltage (in this case ±55 kV) conductors. The use of booster transformers in traction power supplies is described in R.J. Hill, "Electric Railway Traction Part 3 - Traction Power Supplies", Power Engineering Journal, December 1994, pp 275-286. A booster transformer with a unity turns ratio has its primary connected into the catenary wire and its secondary connected into a return conductor provided in parallel with the rails. Traction return current is forced from the rails and earth to flow through the return conductor and transformer secondary to equalise the Ampere-turns in the core set up by the primary current. A low feeding impedance is achieved and earth currents and their associated interference are reduced. Summary of the invention It is an object of the invention to provide an additional power transmission means whilst obtaining the advantages of booster transformers . The present invention provides an electricity supply system for traction, comprising first and second supply conductors supplied with first and second voltages, respectively, a return conductor, and means for rendering the current in one of said conductors substantially equal to the sum of the currents in the other two conductors. The current equalising means may comprise at least one current booster transformer. In one embodiment of the invention, at least one pair of current booster transformers is provided, one winding of each transformer of the pair being connected in parallel into the return conductor and the other winding of each transformer being connected into a respective one of the supply conductors . In an alternative embodiment, at least one current booster transformer has one winding connected into each of said three conductors. The transformer with three windings generally has a return conductor winding having a number of turns equal to the number of turns of each of the other two windings and is referred to herein as a current adding booster transformer . The first supply conductor may be a catenary and the second supply conductor may be a high voltage line constituting an additional power transmission means. The first and second voltages may be substantially in phase, and the current in the return conductor may be substantially equal to the sum of the currents in the first and second conductors. Alternatively, the first and second voltages may be substantially 180° out of phase, and the current in the first conductor may be substantially equal to the sum of the currents in the second and return conductors. In an alternative embodiment, the first and second voltages are substantially equal in magnitude but out of phase, i.e. symmetrical about ground. The return conductor may not extend for the entire distance between autotransformers to which the first and second conductors are connected. Instead, the return conductor may only extend for a central portion, say one third of that distance and may be connected at its ends to the rails along which a vehicle runs. Where the second supply conductor is a high volcage conductor, it can be made to bypass a portion of the system which passes through a station, city, tunnel or the like, together with a branch of the return conductor or catenary. In that portion the catenary and local return currents may be made equal by a current booster transformer, and the currents in the two bypassing conductors may similarly be equalised. 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 trac ion power supply system in accordance with a first embodiment of the invention; Figure 2 is a schematic circuit diagram of a second embodiment of traction power supply system; Figures 3a and 3b show alternative arrangements for mounting the conductors on poles; Figure 4 is a schematic circuit diagram of a third embodiment of traction power supply system; Figure 5 is a schematic circuit diagram of a fourth embodiment of traction power supply system; Figures 6 and 7 are schematic circuit diagrams of bypass arrangements; and Figures 8 and 9 are schematic circuit diagrams of fifth and sixth embodiments respectively of a traction power supply system. Detailed description of the preferred embodiments Figure 1 shows a supply transformer Tl, which in this example is an autotransformer connected to the 132 kV, 16.7 Hz grid as used in Sweden. Taps on the autotransformer are connected to a catenary K, at an idling voltage of 16.5 kV, a high voltage line H at 66 kV and a common return conductor R connected to the rails at intervals. At intervals of typically 5 km the windings of a current adding booster transformer T2 are connected into each of the conductors K, H and R. The transformer T2 is wound such that the number of turns in the return conductor winding is equal to the sum of the number of turns of the other two windings . This forces the current in the return conductor R to be II + 12, the sum of the catenary current 12 and the high voltage current II. At much longer distances, typically 50 km, feeding transformers T3 are arranged along the railway. In this example the transformers T3 are 66 kV/lβ.5 kV autotransformers . Increased power transmission is achieved if the high voltage line H is at a voltage significantly higher than that of the catenary K, the upper limit of the high voltage being determined by insulation and safety considerations. In Europe, single phase high voltage networks having a voltage symmetrical with respect to ground are used (2 x 66 kV = 132 kV in Sweden and Switzerland and 2 x 55 kV = 110 kV in Germany and Austria) . Thus it is convenient for the high voltage line H to be at 55 kV or 66 kV. Since only one high voltage line is required, costs and safety risks are reduced. The catenary voltage is in phase with the high voltage. If no high voltage is supplied to the conductor H (11=0) , the system will continue to function as a normal current booster transformer traction feeding system. Conversely, if the catenary K is not energised (12=0) the system will act as a high voltage supply system with booster transformers. In comparison to a conventional autotransformer system in which autotransformers typically rated at 10 MVA and having a number of turns corresponding to twice the traction voltage are installed about every 10 km along the railway line, the system shown in Figure 1 has a much lower transformer rating and a correspondingly reduced cost. The booster transformers T2 have a rated power of a few hundred kVA and a few hundred volts are dropped across each winding. Figure 2 shows an alternative arrangement in which the taps on the autotransformer Tl are placed so that the voltages on the catenary conductor K and high voltage conductor H are 180° out of phase. The roles of the catenary K and return conductor R are interchanged as compared with Figure 1, and the magnitude of the catenary current is II + 13 where II is the high voltage current and 13 the return current. An advantage of this system is that the transmission voltage is further increased and in this example is 66 + 16.5 = 82.5 kV . If no high voltage is supplied to the conductor H (11=0) , the system will continue to function as a normal current booster traction feeding system. Conversely, if the catenary K is not energised, the system will act as a high voltage supply system with booster transformers, the voltage available being that between the high voltage conductor and ground (in this example 66 kV) . Figures 3a and 3b show two possible configurations for mounting the three conductors K, H and R on a single pole, I being a high voltage insulator. It should be understood that the voltages, frequencies and transformer types of transformers Tl and T3 can be different from those mentioned above. Thus the transformers Tl and T3 could be full transformers and the frequency could be any of the frequencies relevant for traction. A particularly advantageous choice of voltage for the high voltage conductor H in Figure 2, when the catenary K is at 16.5 kV (nominally .15 kV) and the frequency is 16.7 Hz, is 27.5 kV (nominally 25 kV) . 25 kV is a standardized voltage for railway use and thus standard components can be connected to the high voltage conductor. The resulting transmission voltage is 44 kV (nominally 40 kV) which is adequate for many applications. For 25 kV public frequency electrified railways the symmetric 2 x 25 kV system is a convenient choice with 55 kV (nominally 50 kV) transmission voltage. Figure 4 shows an alternative system for autotransformer- fed lines. The return conductor R is constituted by the rails along which vehicles run. In conventional current booster transformer systems, the booster transformers are seldom connected into the rails, a separate return conductor usually being provided since this must carry the entire current between two adjacent feeding stations. In an autotransformer system according to the invention, by contrast, the return current is essentially due only to the locomotive (s) between two adjacent autotransformers A. In many cases, the interference suppressing ability of this simplified system will be satisfactory, saving the considerable cost of a separate return conductor. In Figure 4, a booster transformer T2 shows the principle of the invention, whilst a further booster transformer T2 ' shows how dangerous voltages between rail sections can be avoided by forming an intermediate rail section connected to a center tap of the booster transformer T2 ' in a conventional manner. A further adaptation of the system is shown in Figure 5. Autotransformers A feed symmetrical voltages to a catenary K and a negative feeder N. The earth current has its maximum at the midpoint between two autotransformers and is zero at the location of the autotransformers . Thus a return conductor R' is installed along only approximately the middle third of the distance between the two autotransformers . The return- current flows along the rails for approximately the outer thirds of this distance, with earth currents being kept small. The return conductor R' is interconnected with the catenary K and negative feeder N by a current adding booster transformer T2. As well as the return currents, the induced voltages are also kept low by this arrangement. The voltage shown in Figure 5, 2 x 16.5 kV, is only an example and the system will work with other voltages and, in principle, non- symmetric voltages. It is considered that the systems shown in Figures 2, 4 and 5 will approximately double the distance between the expensive autotransformers as compared with conventional autotransformer-fed systems. Figures 6 and 7 show systems that can be used as adjuncts to the systems of Figures 1 and 2 respectively when the electrified railway passes through a station, city, tunnel, or other point at which it is undesirable that the high voltage conductor H should be present. In Figure 6, the high voltage conductor and a branched local return conductor B are arranged in a bypassing configuration. In Figure 7, the high voltage conductor and a branched catenary B' bypass a remaining portion of the system. The local return conductor is essentially at ground potential in the case of the system of Figure 6 or at the catenary voltage in the case of Figure 7. The catenary current and the current in the non-bypassing return conductor are equalised by an ordinary current booster transformer T. In some cases, especially for shorter distances, this transformer T can be omitted. On the other hand, current control can alternatively or additionally be enhanced by inserting a current booster transformer into the bypass, linking the bypassing high voltage conductor with the branch B or B' . The embodiments of the invention shown in Figures 1, 2, 4 and 5 can be modified by substituting two ordinary current booster transformers for each current adding booster transformer. Figure 8 shows how this modification affects the system described above with respect to Figure 1 in which the catenary and high voltage currents are in phase. A first conventional current booster transformer T2a has windings connected into the high voltage conductor H and a first branch of the return conductor R, thus forcing the high voltage current II into this branch. A second branch of the return conductor carries current 12, the catenary current, by the action of a second conventional current booster transformer T2b . The first and second branches are connected in parallel and thus the current in the return conductor R is 11+12. Advantageously, disconnecters D can be used to disconnect the two branches of R, thus making the use of only one of the catenary K or high voltage conductor H easier in an emergency. Figure 9 shows the corresponding modification of the system of Figure 2 or 5 , in which the voltages on conductors K and H are of opposite phase. In this case a first booster transformer T2a forces current II into a first branch of the return conductor R and a second booster transformer T2b forces current 11+13 into a second branch of the return conductor. A current II flows in a closed loop formed from the two parallel connected branches, and the current in the return conductor R becomes the difference between 11+13 and II, i.e. 13 as desired. This system has the advantage that when conductors K and K are fed from autotransformers and their voltages are equal m magnitude but opposite m phase, conventional current booster transformers for that voltage can be used. Again, disconnecters D are provided for disconnecting the branches of the return conductor R m order to utilise only one of the catenary K or high voltage conductor H m an emergency. The current booster transformers T2 , T2a, T2b used m the invention could be oil -filled transformers, but more preferably they are formed as "dry" transformers in which the windings for high voltage and/or catenary are surrounded m turn by semiconducting, insulating and semiconducting layers, with the return conductor winding being formed as a coaxial screen on this winding or at least one of these windings Whilst the embodiments of the invention described above with respect to Figures 1, 2, 7 and 8 include a dedicated return conductor R, it should be understood that the return conductor could alternatively be constituted by the rails along which the vehicle runs It should also be pointed out that the catenaries, high voltage conductors and return conductors/rails of more than one track (e.g. two m a double- tracked line) can be connected to one autotransformer common to the tracks.

Claims: CLAIMS 1. An electricity supply system for traction, comprising first and second supply conductors supplied with first and second voltages respectively, a return conductor, and means for rendering the current in one of said conductors substantially equal to the sum of the currents in the other two conductors . 2. A system according to claim 1, wherein the current equalising means comprises at least one current booster transformer. 3. A system according to claim 2, wherein the current equalising means comprises at least one pair of current booster transformers, each transformer of the pair having one winding connected into one of two parallel branches of the return conductor, and the other winding of each transformer of the pair being connected into a respective one of the supply conductors. 4. A system according to claim 2, wherein said at least one current booster transformer has one winding connected into each of said three conductors. 5. A system according to any preceding claim, wherein the first supply conductor is a catenary and the second supply conductor is a high voltage line constituting an additional power transmission means. 6. A system according to claim 5, wherein the first and second voltages are substantially in phase, and the current in the return conductor is substantially equal to the sum of the currents in the first and second conductors. 7. A system according to claim 5, wherein the first and second voltages are substantially 180° out of phase, and the current in the first conductor is substantially equal to the sum of the currents in the second and return conductors. 8. A system according to any one of claims 1 to 4, wherein the first and second voltages are substantially symmetrical about ground. 9. A system according to claim 8, wherein the return conductor extends only over a central portion of the distance between autotransformers to which the first and second conductors are connected. 10. A system according to claim 9, wherein said portion is approximately one third of said distance. 11. A system according to any of claims 1 to 8 , wherein the return conductor is constituted by rails along which a vehicle runs . 12. A system according to claim 6, wherein a portion of the high voltage line and a branch of the return conductor bypass a remaining portion of the system. 13. A system according to claim 7, wherein a portion of the high voltage line and a branch of the catenary bypass a remaining portion of the system. 14. A system according to claim 12 or 13, wherein the currents in the remaining portion of the return conductor and catenary are equalised by means of a current booster transformer . 15. A system according to claim 12, 13 or 14, wherein the currents in the two bypassing conductors are equalised by means of a current booster transformer.

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