A TRACTION POWER SUPPLY SYSTEM

Abstract: A traction supply system for a railway electrified with alternating current comprising a first supply system with a supply conductor (1), a return conductor (2, 3) and a number of first booster transformers (4) connected between the supply conductor and the return conductor, a second supply system with two feeding conductors, one of which is in phase (5) which and the other is in reverse phase (6) to the supply conductor, and a number of autotransformers (7, 9) arranged along the rail and connected between the first and the second supply system. The invention further comprises a number of second booster transformers (8a, 8b, 10) connected between the first and the second supply systems.

Description: A TRACTION POWER SUPPLY SYSTEM DESCRIPTION FIELD OF THE INVENTION The present invention relates to a traction supply system for an electrified railway comprising a first supply system with a supply conductor, a return conductor and a first number of booster transformers connected between the supply conductor and the return conductor, a second supply system with two feeding conductors, of which one is in reverse phase and the other is in phase with the supply conductor, and a number of autotrans- formers arranged along the railway and connected between the first and the second supply system. PRIOR ART It is well known to use booster transformers in traction supply systems to return currents in the ground to the rail or to a return conductor. The use of booster transformers in traction supply systems is closer described in the article "Kombinierte Strecken- speisung mit Auto- und Saugtransformatoren" by Torsten Schϋtte and Jens Thiede published in the journal "Elektrische Bahnen No. 7/2000, pages 249-253. Booster transformers in- tended for a couple of hundred kVA are placed with regular intervals along a supply conductor. For a supply system of 25 kV and 50 or 60 Hz an interval of about 3 km is sufficient and for a supply system of 15 kV and 16.7 Hz an interval of about 5 km is sufficient. The primary winding of each booster transformer is connected to the supply conductor over a gap in the supply conductor. A return conductor is arranged in parallel with the rail and the secondary winding of each booster transformer is connected to the return conductor. The ratio between the number of turns in the windings is one, which means that the trace current is forced to pass through the return winding instead of in the rail or in the ground. This arrangement decreases the currents in the ground around the rail and thus the appearance of magnetic fields that otherwise might cause problems through current induction in telephone or signal conductors close to the railway. One drawback with arranging several booster transformers in series in the main circuit in this way is that they cause increased impedance in the system, which implies a power loss. As an alternative to booster transformers, autotransformers can be used and their usage in traction supply systems is also described in said article in the journal "Elektrische Bahnen". The winding of each autotransformer is connected between the supply conductor and a negative feeding conductor. The rail is con- nected to an intermediate point on the winding. The train takes the main part of its current from the two closest autotransformers, but a smaller part of the current is taken from more distant autotransformers. The total supply current is given by the current of the train divided by the transformation degree of the autotransformers which usually is two. This means that the negative feeding conductor is to have the same potential relative to the ground as the supply conductor but with reverse phase. The current is transformed from the circuit that contains the supply conductor and the rail to the circuit comprises the supply conductor and the negative feeding conductor. In the same way as for the booster transformer system the autotransformer removes undesired currents in the rail, but not as efficiently. In difference to the booster transformer system, which works by balancing currents, the autotransformer system works by balancing voltages. Since the autotransformers are not perfect for removing currents in the rail, the distance between them should not exceed the distance between the booster transformers in a conventional booster transformer system with more than a factor two. However, this is unnecessarily close regarding what is needed for the voltage transmission. Since an autotransformer is expensive compared to a booster transformer, a solution with autotransformers only is too expensive. In GB 9910423.0 a combination of autotransformers and booster transformers is proposed, which admits a larger distance between the autotransformers. This distance is sufficient for the voltage transmission. The traction supply system shown comprises two separate supply systems, on one hand a first supply system comprising a supply conductor, a return conductor and a number of booster transformers connected between the supply conductor and the return conductor, and on the other hand a second supply system comprising a negative and a positive feeding conductor. With a positive feeding conductor means that the voltage in the conductor is in phase with the supply con- ductor and with a negative feeding conductor means that the voltage is in reverse phase to the supply conductor. A number of autotransformers are connected between the first and the second supply systems. The return currents in both the supply systems are governed by the booster transformers, which are arranged to balance the currents in the systems. The second supply system has a booster transformer arranged in the space between two autotransformers along the railway. A number of such booster transformers are thus arranged in series. These booster transformers are connected between the positive and the negative feeding conductor. The current balance obtained thanks to all these booster transformers prevents passage of current from the supply conductor to the feeding conductors, which passage could lead to a part of the current taking the path through the ground, thereby causing disturbances. The booster transformers thus handle the return of the current and the autotransformers take care of the power transmission. In such a traction supply system, as many expensive autotransformers are therefore not needed. One problem with the booster transformers in the second supply system is that they contribute to increasing the impedance in the system and that they for certain conditions can be subjected to a very high load and then have to work very hard. SUMMARY OF THE INVENTION The object of the present invention is to indicate a traction supply system with two supply systems, which do not induce disturbances in telephone or signal conductors close to the railway and where the autotransformers do not have to work unneces- sarily hard. The object is achieved with the previously described traction supply system, which according to the invention is characterised in that it comprises a number of second booster transformers connected between the first and the second supply system. By connecting the booster transformers between the supply systems instead of between the feeding conductors in the second supply system one obtains the desired current balance between the feeding conductors and the supply conductor without having to connect the booster transformers in series. Since the booster transformers no longer are connected in series a lower impedance and thus decreased losses in the second supply system are obtained. According to a further embodiment of the invention each of the second booster transformers is connected to one of the autotransformers. By arranging a booster transformer for each autotransformer, two booster transformers are always working to maintain the current balance when a vehicle is in a section be- tween two autotransformers. Since two booster transformers share the work, they are only loaded by half as much as in the previously known traction supply systems, where a booster transformer is arranged in the middle of the section between the autotransformers. According to a further embodiment of the invention the second booster transformers are connected between the autotransformers and one of the first or the second supply system. The advantages mentioned above are obtained with both these methods to connect the second booster transformer. The embodi- ment, in which the second booster transformers are connected between the autotransformers and the first supply system has the advantage that conventional autotransformers, with one winding insulated for high voltages and one winding insulated for low voltages, can be used. Such conventional autotrans- formers are both less expensive and smaller than autotransformers with two windings insulated for high voltages. The embodiment where the second booster transformers are connected between the autotransformers and the second supply system has the advantage that the current intensity through the booster transformer becomes smaller. In this embodiment the booster transformer must have two windings adapted for high voltages, i.e. insulated to withstand high voltages. One way of obtaining such a booster transformer is to connect two conven- tional booster transformers "back-to-back", i.e. the low voltage windings of both booster transformers are connected in order for the same current to flow through all the four windings. In a further embodiment of the invention the autotransformers are arranged such that the voltage in the feeding conductor, which is in phase with the supply conductor, is higher than the voltage in the supply conductor. BRIEF DESCRIPTION OF THE DRAWINGS The present invention is now to be explained by different as examples described embodiments and with reference to the ap- pended drawings. Fig. 1 shows a circuit diagram for a traction supply system according to the invention. Fig. 2, 3 and 4 show a traction supply system according to a second, a third, and a fourth embodiment, respectively. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Figure 1 shows a traction supply system comprising a first supply system comprising a supply conductor 1 and a return conductor 2, 3. The return conductor can either be constituted by the rail 2, along which the vehicle travels, or be a separate re- turn conductor 3 arranged parallel to and connected to the rail 2. A number of booster transformers 4 are arranged on roughly regular distances along the rail. The booster transformers 4 have two windings, one of which is connected to the supply conductor and the other is connected to the return conductor, i.e. to the separate return conductor 3 and/or to the rail 2. These booster transformers 4 have as their assignment to balance the current \ , in the return conductor 2, 3 and the supply conductor 1 so that an equal amount of current runs in the return conductor as in the supply conductor. Thus one avoids that some of the return current goes through the ground thereby generating magnetic fields causing disturbances in the environment. As apparent in the figure the booster transformer 4 is arranged over a gap in the rail. The traction supply system further comprises a second supply system comprising a positive feeding conductor 5 and a nega- tive feeding conductor 6. With a positive feeding conductor is meant a conductor in which the voltage is in phase with the voltage in the supply conductor and with a negative feeding conductor is meant a conductor in which the voltage is in reverse phase to the voltage in the supply conductor. On essentially regular intervals along the rail, a number of autotransformers 7 are arranged which are connected between the first and the second supply system. The second supply system has as its task to manage the power transmission to the vehicle. The dis- tance between two adjacent autotransformers 7 is considerably longer than the distance between two adjacent booster transformers 4 in the first supply system. In order to see to it that the currents l 2 in the positive and in the negative feeding conductors are equal and to prevent that a current passes between the first and the second supply system, a number of booster transformers 8a, 8b are connected between the supply systems. These booster transformers have as their task to balance the current between the positive and the nega- tive feeding conductors and the supply conductor. It is sufficient to arrange one booster transformer 8a, 8b for each autotransformer in order to prevent that current leaks between the first and the second supply system. Each of the booster transformers 4 and 7 has two windings and the number of turns in the wind- ings are equal. In the first embodiment of the invention, which is shown in fig 1 , the booster transformers 8a, 8b are connected between the second supply system and the autotransformers 7. Each of the autotransformers 7 has a winding, one end of which is connected to the negative feeding conductor 6 through one of the windings of booster transformer 8a, 8b, and the other end of which is connected to the supply conductor on one hand and to the positive feeding conductor 5 on the other hand through the other winding of the booster transformer 8a, 8b. The middle point of the winding of the autotransformer 7 is connected to the return conductor 2, 3. The positive feeding conductor is thus connected to the supply conductor through the booster transformer 8a, 8b and they will therefore have the same voltage level. The negative feeding conductor is connected to the supply conductor through the autotransformer and can therefore be given a voltage that is higher than the voltage of the supply conductor. Many different voltage combinations are possible. For example the supply conductor, and thus also the positive feeding conductor, can have a voltage which is +15 kV. The negative feeding conductor can then, for example, have a voltage that is -25 kV or -15 kV. The frequency is preferably 16.7 Hz. Since the windings in the booster transformers 8a are connected to the positive and the negative feeding conductor, both windings must be insulated for high voltages. As an alternative to such a booster transformer one can use two conventional booster transformers connected back to back which comprises one high and one low voltage winding. The booster transformer 8b in fig 1 consists of two conventional booster transformers that are connected back to back. This means that the high voltage winding of the first transformer is connected to the positive feeding conductor and the high voltage winding of the second transformer is connected to the negative feeding conductor. The low voltage windings of the first and the second booster transformer are connected to each other. Thus, the same amount of current passes through all four windings. The second supply system can either feed a track, i.e. a first supply system as described above, or feed several parallel tracks, i.e. several supply systems of the same type as the first supply system. Figure 1 shows two parallel tracks, each comprising a supply conductor 1 , 1 ', a rail 2, 2', and a separate return conductor 3, 3'. Both the parallel tracks are fed in parallel from the same positive 5 and negative 6 feeding conductor through the same autotransformers 7. Figure 2 shows a second embodiment of the invention in which both the positive 5 and the negative 6 feeding conductor have a higher voltage than the supply conductor 1 . The winding of each autotransformer 9 has been extended and the positive feeding conductor 5 is no longer, through the booster transformer 8a, connected to the same point on the autotransformer 9 as the supply conductor 1 . They are instead connected to separate points on the autotransformer. In this embodiment, the voltage can, for instance, be +25 kV on the positive feeding conductor, -25 kV on the negative feeding conductor, and +15 kV on the supply conductor. The frequency can preferably be 16.7 Hz. In the same way as in the previous embodiment example several parallel tracks can be fed from the positive 5 and the negative 6 feeding conductor owing to a simple parallel connection. Figure 3 shows a third embodiment of the invention where booster transformers 10 are connected between the first supply system and the autotransformer 7. Each of the autotransformers 7 has a winding, one end of which is connected to the negative feeding conductor 6 and the other end of which is connected on one hand to the positive feeding conductor 5 and on the other hand to the supply conductor 1 through one of the windings of the booster transformer 10. The middle point of the winding of the autotransformer 7 is connected to the return conductor 2, 3 through the second winding of the booster transformer 10. The positive feeding conductor 5 is thus connected to the supply conductor through one of the windings of the booster transformer 10 and they will therefore be on the same level of volt- age. The negative feeding conductor 6 is connected to the supply conductor 1 through the autotransformer 7 and can therefore be given a voltage which is higher than the voltage of the supply conductor. The booster transformers 10 in this embodiment example are of a conventional type with one high and one low voltage winding. In the same way as in the previous example several tracks 1 , 2, 3 and 1 ', 2\ 3' can be fed from the positive 5 and the negative 6 feeding conductor. The feeding can either be carried out with a simple parallel connection or through a separate booster transformer 12. Figure 4 shows a fourth embodiment of the invention where both the positive 5 and the negative 6 feeding conductor have a higher voltage than the supply conductor 1 . The winding in each of the autotransformers 9 has been extended and the positive feeding conductor 5 is connected to a point on the autotrans- former 9 which is separate from the point to which the supply conductor 1 is connected through the booster transformer 10. In the same way as in the previous example several parallel tracks 1 , 2, 3 and 1 ', 2', 3' can be fed from the positive 5 and the negative 6 feeding conductor. The feeding can either be carried out with a simple parallel connection or through a separate booster transformer 12.

Claims: CLAIMS 1 . A traction supply system for a railway electrified with alternating current comprising - a first supply system with a supply conductor (1 ), a return conductor (2, 3) and a number of first booster transformers (4) connected between the supply conductor and the return conductor, a second supply system with two feeding conductors, one of which is in phase (5) with and the other is in reverse phase (6) to the supply conductor, and a number of autotransformers (7, 9) arranged along the railway and connected between the first and the second supply system, characterised in that it further comprises a number of second booster transformers (8a, 8b, 10) connected between the first and the second supply system. 2. A traction supply system according to claim 1 , characterised in that each of the second booster transformers (8a, 8b, 10) is connected to one of the autotransformers (7, 9). 3. A traction supply system according to claim 1 or 2, characterised in that the second booster transformers (8a, 8b, 10) are connected between the autotransformers (7, 9) and one of the first or the second supply system. 4. A traction supply system according to claim 3, characterised in that the second booster transformers (10) are connected between the autotransformers (7, 9) and the first supply system. 5. A traction supply system according to claim 4, characterised in that the second booster transformers (10) comprises two windings one of which is connected between the autotransformer (7, 9) and the supply conductor (1 ) and the second wind- ing is connected between the autotransformer and the return conductor (1 , 2). 6. A traction supply system according to claim 3, characterised in that the second booster transformers (8a, 8b) are connected between the autotransformers (7, 9) and the second supply system. 7. A traction supply system according to claim 6, characterised in that the second booster transformers (8a, 8b) comprises two windings, one of which is connected between the autotransformer (7, 9) and one of said supply conductors (5, 6) and the second winding is connected between the autotransformer and the second of said supply conductors. 8. A traction supply system according to claim 6 or 7, characterized in that at least one of the second booster transformers (8a, 8b) comprises two windings intended for high voltages. 9. A traction supply system according to claim 6 or 7, characterised in that at least one of the second booster transformers (8b) comprises two booster transformers arranged back to back. 10. A traction supply system according to one of the previous claims, characterised in that the autotransformers (9) are arranged so that the voltage in the feeding conductor, which is in phase with the supply conductor, is higher than the voltage in the supply conductor.

Inventors: SCHUETTE THORSTEN, SCHUETTE, THORSTEN
Applicants: BALFOUR BEATTY PLC, SCHUETTE THORSTEN, BALFOUR BEATTY PLC., SCHUETTE, THORSTEN
Application year: 2001
Main classification: B60M3
IPC codes: B60M3