EP1124706A1

EP1124706A1 - A plant for feeding railway vehicles

Info

Publication number: EP1124706A1
Application number: EP99970360A
Authority: EP
Inventors: Thorsten Schütte; Bo Kindell; Gunnar HAMMARSTRÖM
Original assignee: Balfour Beatty PLC
Current assignee: Balfour Beatty PLC
Priority date: 1998-10-14
Filing date: 1999-10-14
Publication date: 2001-08-22
Also published as: AU1425600A; PL347892A1; WO2000021777A1

Abstract

In a plant for feeding electric power to railway vehicles with a contact line (2) and a rail (3) also a second line (4) put on another potential differing from the potential of the contact line is there. One of the three parts contact line, second line and rail is adapted to have an intermediate potential located between the potential of the two other parts. An arrangement (7) has in a given point from the intermediate potential part a connection thereto and a separate connection to each of the two others of said parts and means for storing electrical energy. A device is adapted to cause said means to receive electrical energy through a direct voltage between a first of said two other parts and the intermediate potential part and feed out electrical energy through a direct voltage between the second of said two other parts and the intermediate potential part.

Description

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Applicant: DAIMLERCHRYSLER AG

A PLANT FOR FEEDING RAILWAY VEHICLES

FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a plant for feeding electric power to railway vehicles having a contact line adapted to be in contact with the vehicle for feeding current thereto and return feeding of the current through a rail on ground potential on which the vehicle bears.

The invention relates to feeding of electric power through alternating voltage as well as direct voltage to said railway vehicles, i.e. locomotives.

The word "contact line" is here to be given a broad sense and is intended to comprise a contact line suspended above the rail as well as a contact bar or so called third rail beside or between the rail. With respect to the direct voltage the nominal voltage levels are between 500 V and 3 kV, in which 500-1 500 V are preferably used for tram-cars, subways and the like, both with the contact line above the rail and beside the rail. 1 ,5 kV and 3 kV are the most common voltages for full-scale railways and are as a rule supplied by means of a contact line above the rail. However, also higher voltages, for example up to 6 kV, are conceivable.

The main problem with which plants already known of this type are occupied with, independently of whether the contact line is intended to conduct an alternating voltage or direct voltage, is to try to keep the transmission losses along the extension of the railway on such a low level as possible, so that such a large part as possible of the power needed is consumed by the vehicle, i.e. the efficiency is as high as possible. Another problem is that the voltage has a tendency to fall along the contact line between different stations for feeding electric power to the contact line, so that the maximum power available to the vehicle is restricted as a consequence of a too low voltage level of the contact line where the vehicle is located.

It has been tried to solve these problems for example by arranging so called auto-transformer systems in the alternating voltage case for avoiding an arrangement of said feeding stations at comparatively short intervals. As a consequence of the way of functioning thereof, which is based on induction, the auto-transformers may not be used for direct voltage. Furthermore, the voltage levels technically suited for railway vehicles are considerably lower for direct voltage than for alternating voltage. This means short distances between said stations for feeding electric power for direct voltage and thereby a high number thereof, which contain costly components, such as transformers and rectifiers.

Another disadvantage of plants already known of this type is that considerable currents are led from the vehicle through the rail, and these currents take the way with the lowest resistance, which may cause large damage, for example of metal objects exposed to corrosion, disturb electricity centrals and the like.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a plant of the type defined in the introduction, which reduces the disadvantages mentioned above of plants already known in a cost effi- cient way. This object is according to the invention obtained by providing such a plant, which comprises a second line put on a second potential different from the potential of the contact line, that one of the three parts contact line, second line and rail is adapted to have an intermediate potential located between the potential of the two other parts, that the plant also comprises an arrangement having a connection to the intermediate potential part at a given point therealong and a separate connection to each of the two others of said parts and which comprises means for storing electrical energy and a device adapted to bring said means to receive electrical energy through a direct voltage between a first of said two other parts and the intermediate potential part and feed out electrical energy through a direct voltage between the other of said two other parts and the intermediate potential part.

_, By providing the plant with such an arrangement it will be possible to supply electrical energy to a railway vehicle through said means for increasing the voltage of the contact line at the position of the vehicle to a level between the level otherwise as- sumed by the voltage between the second line and the rail and the voltage otherwise assumed between the rail and the contact line, so that the voltage is kept at a higher level along the contact line and thereby the distance between successive such feeding stations may be made longer.

This means in its turn that the voltage of the contact line at the location of the vehicle will be higher, so that the power transferable thereto will be higher.

Another advantage of the plant according to the invention is that the current will on the contact line between said part and a vehicle alternatingly be received from the contact line on said opposite side of the point with respect to the vehicle and from the arrangement, so that the full current I will only flow in the contact line between said point and the vehicle and then back in the rail to this point, while on said opposite side of the point the contact line will only conduct about l/q, where q = (U_p+U_n)/U_p, with U_p= the contact line voltage and U_n = the voltage in the second line, we have for U_p = U_n q = 2 and the current will then be about 1/2, while the current in the rail will disappear on the opposite side of the point with respect to the vehicle. This means that the transmission losses in the contact line on said opposite side of the point with respect to the vehicle get considerably lower. A higher power may be transmitted, since the voltage level will at the same time be higher thanks to the arrangement of the sec- ond line. Furthermore, the return current in the rail is mainly removed on said opposite side of the point with respect to the vehicle, so that the risk of fault currents, corrosion damage and the like is substantially reduced. It is pointed out that "the point" does not mean any point in the usual sense thereof here, but the place or the position along the contact line/rail in which the arrangement is located with the connections thereof is intended. It is underlined that it is of course well possible that said means is charged and discharged in the opposite direction to above, which illustrates the case in which the vehicle delivers electric power, in electric breaking of the vehicle, and the present invention also comprises that case.

According to a preferred embodiment of the invention the device is adapted to select that part of said two other parts, which has a potential difference with respect to the intermediate potential part being mostly reduced with respect to the case of no supply of electric power to or no draining of electric power from any of the three parts through any vehicle, as said second part for feeding electrical energy between that part and the intermediate potential part. It is hereby ensured that the arrangement feeds electric power to exactly the place where the need thereof is the highest, i.e. where the voltage of the contact line has fallen the most as a consequence of draining of electric power therefrom through railway vehicles. According to another preferred embodiment of the invention the second line is a second contact line which along the same rail follows upon the contact line first mentioned with a dead portion therebetween, the two contact lines are put on potentials of dif- ferent magnitudes but of the same sign, so that the contact line having the lowest absolute value of the potential forms said intermediate potential part, and said device is adapted to provide the vehicle with electrical energy from the contact line it is located at through said means for keeping the potential of this contact line on a higher level at this connection than in absence of the arrangement. It is hereby possible to transfer electric power between two contact lines on different potentials, such as for example the contact lines meeting at a border between two countries, for example between the Netherlands which has 1 ,5 kV and Belgium which has 3 kV on the contact lines thereof. If a vehicle is on the Dutch side the voltage between the contact line there and the rail will sink, while the voltage between the contact line on the Belgian side and the contact on the Dutch side, which is the intermediate potential part, increases, and charges are thereby fed through the arrangement to the contact line on the Dutch side and thereby to the vehicle for increasing the voltage of the contact line where the vehicle is positioned. Electric power may in this way be transferred between the contact lines on different potentials so as to provide electrical energy where the need thereof is the highest. Accordingly, the feeding stations connected to the two contact lines may support each other.

According to another preferred embodiment of the invention said second line is arranged substantially in parallel with the contact line and put on a potential of opposite sign to the potential of the contact line and having a magnitude substantially just as high or higher than the potential of the contact line when substantially no electric power is supplied thereto or drained therefrom through any vehicle, along the same rail said contact line is followed by a second contact line having a dead portion between these contact lines, the second contact line is adapted to have a potential having substantially the same difference with respect to the rail as the potential difference between the contact line first mentioned and the second line when substantially no electric power is supplied to or drained from any of the contact lines through any vehicle, and the arrangement is also connected to the second contact line so as to provide the vehicle with electrical energy from the arrangement when a vehicle is located the- realong. It is hereby possible to control the power flow from the arrangement either to the first contact line or the second contact line depending upon where the consumption is the highest, so that the two contact lines in the practice are interconnected and the voltage between the rail and the contact line may through the arrangement be increased for the contact line at which a vehicle is presently located.

According to another preferred embodiment of the invention the potential difference with respect to the intermediate potential is greater for one of the two other parts than for a second of these parts when substantially no electric power is supplied to or drained from the contact line, through any vehicle, the arrangement is connected to said one part through a capacitor and an electric switch connected in parallel therewith, and a unit is arranged to control closing and opening of the switch so that the potential difference of the input of said one part to the arrange- ment and the intermediate potential part gets substantially equal to the potential difference between the latter and the second part. The arrangement which may be mentioned direct current auto-transformer, may be made simpler to the construction when the voltages between the intermediate potential part and each of the two other parts are equal.

According to another preferred embodiment of the invention the potential difference with respect to the intermediate potential is substantially twice as great for one of the two other parts as for the second of these parts when substantially no electric power is supplied to or drained from the contact line, through any vehicle, the arrangement is connected to said one part through a capacitor and it comprises a further said arrangement having one connection to the intermediate potential part and one connection to the input of said one part to the arrangement first mentioned. Two arrangements operating with the relation 1 : 1 between the voltage of the two parts with respect to the intermediate potential part thereof may be used in this way so as to form a 1 ^-arrangement. Accordingly, two arrangements simple to the construction may be used. A plant of this type is particularly well suited for contact lines on 1 ,5 kV, which then get a -3 kV-return line.

According to another preferred embodiment of the invention said means comprises at least one inductor and/or at least one ca- pacitor for storing electrical energy. The present inventors have understood that use of inductors or capacitors for storing electrical energy is perfectly suitable in a plant of this type and means that this may be made comparatively simple and thereby to a low cost, at the same time as the losses in the arrangement may be kept low. More exactly, the inductors store in fact magnetic energy, and when they are charged electrical energy is converted into magnetic energy which upon discharging of them is converted into electrical energy again. This process is also intended to be comprised by the patent claim definitions with re- spect to storing of electrical energy and this process will hereinafter in this disclosure simplified be referred to as storing of electrical energy.

According to another preferred embodiment of the invention the plant comprises at least one inductor adapted to be charged by electrical energy in said second position and deliver this energy to the contact line in the first position, and the switching device has one single switch adapted to connect the intermediate potential connection to one of said parts for obtaining the first po- sition in a first switching position and in a second switching position connect the intermediate potential connection to the con- nection to the other of said parts for obtaining the second position. By using an inductor in this way for storing electrical energy it is possible to manage with one single switch for switching the intermediate potential connection with respect to the two other connections, wherethrough costs may be saved and losses of the arrangement be kept low.

According to another preferred embodiment of the invention, which constitutes a further development of the embodiment last mentioned, the plant has an inductor for storing electrical energy connected between the intermediate potential connection and through which said single switch is connected to the intermediate potential connection. Hereby the inductor reduces the stresses put on the switch at the same time as the energy stored thereby in one position of the switch is delivered to the contact line and the vehicle in the opposite position of the switch.

According to another preferred embodiment of the invention, which constitutes a further development of the embodiment last mentioned, the plant comprises two capacitors, namely one arranged between the intermediate potential part and the first of the two other parts and one arranged between the second part and the intermediate potential part, and the capacitors are connected to the intermediate potential part on the opposite side of the inductor with respect to the switch. These capacitors are charged and discharged, respectively, in the two positions of the switch, in which they will primarily function as a filter and reduce the ripple on the voltage received by the contact line between said point and the vehicle.

According to another preferred embodiment of the invention the connections of the arrangement to the two parts located on opposite sides of the intermediate potential has an inductor each arranged closer to the first of the parts and the second part, re- spectively, than the connection of said switch to these parts, and the capacitor is arranged between the connection to the first part and the connection to the second part on the opposite side of the switch with respect to the inductors. In this embodiment the capacitor as well as the inductors will alternatingly store and deliver electrical energy, more exactly in one position of the switch the capacitor will be charged through one inductor at the same time as the other inductor delivers energy to the first part, and in the other switching position the capacitor will be discharged by means of the inductor last mentioned and deliver energy to the first part, at the same time as the inductor first mentioned receives energy from the second part.

According to another preferred embodiment of the invention said means comprises two inductors in the form of a transformer having two coils connected in series on one and the same iron core with adjacent terminals of the coils interconnected in one connection point, the intermediate potential part is connected to said connection point, an individual switch is arranged at that terminal of each coil that is arranged on the opposite side of the connection point and arranged to be able to connect this termi- nal either to the first of the parts or the connection thereof to the second part, and the switching device is adapted to control the two switches to continuously assume opposite positions, so that when one connects the coil thereof to the connection to the first part the other connects the coil thereof to the connection to the second part for obtaining said first position and conversely for obtaining the second position. Thanks to the common core the inductance of the transformer will be high and the ripple on the voltage received through the arrangement between said point along the contact line and the vehicle will thereby be reduced drastically.

According to another preferred embodiment of the invention the plant comprises at least one inverter connected with the direct voltage side thereof to one of the parts and with the alternating voltage side to a first winding of a transformer, it comprises a rectifying means connected to a second winding of said trans- former and to the second of the parts, and the inverter and the rectifying means are connected to each other through a portion of a line connected to the intermediate potential part. Electrical energy may in this way be fed from said one part to the other of the parts and thereby a vehicle located there with the possibility to control the direct voltage delivered between the intermediate potential part and the other of the parts to a desired level through the selection of the relation between the two windings, at the same time as it will be possible to finely adjust the direct voltage by modifying the control of the inverter.

According to another preferred embodiment of the invention, which constitutes a further development of the embodiment last mentioned, said rectifying means is formed by a second inverter having the direct voltage side thereof connected to the second of the parts and the alternating voltage side thereof connected to said second winding of the transformer. Feeding of electrical power in both directions through the inverters is hereby enabled.

According to another preferred embodiment of the invention the plant comprises a plurality of arrangements arranged at different points along the rail between two adjacent stations of the plant for feeding electric power to the contact line for maintaining the voltage level of the contact line high along the extension thereof between said stations. It is in this way possible to increase the distance between the stations while observing a determined minimum level of the voltage between these stations, so that considerable costs for transformers, rectifiers and the like may be saved.

According to another preferred embodiment of the invention this second line is given a potential which is of substantially the same magnitude as the potential of the contact line when substantially no electrical power is supplied to or drained from the latter through any vehicle. This constitutes a simple way of re- alizing the invention, since the second line then has the same insulation level as the contact line.

According to another preferred embodiment of the invention the second line is given a potential which is of a higher magnitude than the potential of the contact line when substantially no electrical power is supplied to or drained from the latter through any vehicle. It will hereby be possible to raise the level of the voltage of the contact line at said point even more and thereby an increase of the distance between the feeding stations is primarily made possible. A higher power may at the same time be supplied to the vehicle. See the embodiment with said 1 :2-relation above.

According to another preferred embodiment of the invention the switching device is adapted to carry out switching between the first and the second position so that the intermediate potential part is connected to the connection of the arrangement to the second of the parts during longer periods of time than it is con- nected to the connection of the arrangement to the first part, so as to displace the potential of the intermediate potential part towards the potential of the connection to the second part and thereby increase the voltage between the connection to the first part and that one to the intermediate potential part. By such an intelligent, switching of the switching device the voltage delivered by the second part may be utilized to a maximum for increasing the voltage of the first part in the point in question and the advantages of the invention may thereby be accentuated.

According to another preferred embodiment of the invention the switching device is adapted to cause a switching between the first and second position with such a high frequency that the voltage on one hand between the connection of the arrangement to the first part and the connection thereof to the intermediate potential part and on the other between the connection of the arrangement to the second part and the connection thereof to the intermediate potential part are kept substantially unchanged at a determined supply of electric power between the vehicle and the control line. By selecting the switching frequency that high the voltage on the first part will be kept substantially con- stant on the raised level obtained by the plant according to the invention, so that power pulsations are avoided.

According to another preferred embodiment of the invention the switching device comprises at least one switch in the form of a controllable semiconductor switch for said switching. The switching may by utilising such switches be performed easily and reliably with a high speed required, advantageously with a frequency exceeding 300 Hz, and preferably above 1 kHz, and for such high frequencies it is advantageous to use IGBT's (In- sulated Gate Bipolar Transistor), but also other semiconductor switches, such as thyristors, MOSFETS and so on are conceivable.

According to another preferred embodiment of the invention the contact line and the second line are connected to a direct voltage potential each for feeding direct current to and from the vehicle. The advantages of the plant according to the invention are particularly great in the direct voltage case, since it makes it possible to also for direct voltage supply obtain a plant having a similar function to that of an auto-transformer system for alternating voltage feeding. The nominal voltage between a contact line and the rail is in the direct voltage case advantageously between 500 V and 6 kV and preferably 500 V - 3 kV.

According to a further preferred embodiment of the invention the contact line is adapted to feed an alternating current to or from the vehicle, and the switching device is adapted to cause an alternation between the first and the second position with a frequency being much higher than the frequency of the alternating voltage on the contact line. Said alternating voltage will hereby substantially "stay still" during a switching period of the switch- ing device, so that the plant will have substantially the same function as in the direct voltage case.

Further advantages as well as advantageous features of the in- vention appear from the following description and the other dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

With respect to the appended drawings, below follows a description of preferred embodiments of the invention cited as examples.

In the drawings:

Fig 1 illustrates schematically the main principle for the present invention,

Fig 2 illustrates also schematically but a little bit more in detail a plant according to a preferred embodiment of the invention,

Figs 3 and 4 are views corresponding to Fig 1 of plants according to other preferred embodiments of the invention,

Figs 5-7 are views corresponding to Fig 2 of plants according to preferred embodiments of the invention,

Figs 8-12 are schematical views of plants according to other preferred embodiments of the invention,

Figs 13 and 14 are views corresponding to Fig 2 of plants according to other preferred embodiments of the invention, and

Fig 15 is a very schematic view illustrating a plant according to a further preferred embodiment of the invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The principle for a plant according to the invention for feeding direct voltage to a locomotive 1 in the simplest possible way is illustrated schematically in Fig 1 . The direct voltage case will all the time be discussed hereinafter, but it is repeated that the invention is not restricted thereto or to the voltage levels mentioned in the examples described.

The plant has in a conventional way a contact line 2 arranged to be in contact with the vehicle for feeding direct current thereto and return feeding of the current through a rail 3 on ground potential on which the vehicle bears. In the case of electric braking the contact line is of course instead fed from the vehicle. The contact line has for example a no-load voltage of 3,3 kV, i.e. it is on this potential with respect to the rail in absence of any power output therefrom, and this voltage is nominally 3 kV. The plant also comprises a second line 4 in the form of a return line or a so-called "negative feeder", which has a no-load voltage of -3,3 kV. The contact line and the negative feeder 4 are connected to stations 5, 6 for feeding electric power thereto schematically illustrated and arranged along the rail at comparatively large mutual distances, such as for example some tens of kilometers. These stations are preferably connected to a high voltage network conducting an alternating voltage and have converters for converting the alternating voltage into direct voltage for the contact line or as an alternative alternating voltage with a possibly different level and/or frequency.

The plant according to the invention comprises an arrangement 7 schematically illustrated through some boxes, which is arranged at a point 8, i.e. in a place, along the rail with a connection 9 to the rail, a connection 1 0 to the contact line and a con- nection 1 1 to the second line. The arrangement has also means 12 for storing electrical energy and a switching device 13 adapted to switch the rail connection with respect to the two other connections alternatingly between two positions, so that these switching positions co-operate with the means 12 so that the vehicle receives electrical energy through the contact line in a first position from the arrangement 7 and in a second position through a station 6 feeding electrical energy to the contact line. A control device 14 schematically indicated is arranged to control the switching device to carry out said switchings with a high frequency, preferably > 300 Hz. The meaning of this in the prac- tice will now be explained by describing the first embodiment of the invention shown in Fig 2.

The arrangement has in this embodiment an inductor 15 connected to the rail connection 9 and through which one single switch 16, for example an IGBT with a diode connected in series, is connected to the rail connection and adapted to alternatingly connect it to the contact line connection 10 and the connection 1 1 to the second line. The arrangement has also two capacitors 17, 18, namely one arranged between the contact line and the rail connection and one arranged between the second line and the rail connection, in which these are connected to the rail connection on the opposite side of the inductor 15 with respect to the switch 16. For the sake of exemplifying it may be mentioned that the following data may be valid for this embodi- ment: L₁₅, = 5mH, C₁₇=C₁₈=5mF, f=300 Hz.

When the switch 16 is in the second switching position it is indicated by dashing in Fig 2, i.e. the rail connection 9 is connected to the connection 1 1 to the second line, the inductor 15 stores then electrical energy received from the second line 4 at the same time as the capacitor 18 is discharged. The vehicle 1 receives in this position electrical energy through the contact line 2 from the feeding station 6. When the switch 16 is then switched into a first switching position illustrated through a con- tinuos line in Fig 2, i.e. connects the rail connection 9 to the contact line connection, the electrical energy stored in the in- ductor 15 in the earlier moment will be delivered through the contact line connection to the contact line while partially charging the capacitor 17 and charging the capacitor 1 8, so that the vehicle 1 in this switching position will be fed by the arrange- ment through a current flowing through the contact line connection 10, the contact line 2, the vehicle and back through the rail

3 and the rail connection 9. We assume that the voltage in the contact line where the locomotive is located would in absence of the arrangement 7 have fallen to 2,7 kV. However, the voltage between the second line 4 and the rail 3 in the point 8 is 3,3 kV. However, by performing said switching the voltage differences between the rail and the contact line on one hand and the rail and the second line on the other may be reduced. If more exactly the times in the different switching positions are made equal the rail connection 9 will in average over the time be on a potential in the middle between the potentials of the second line

4 and of the contact line 2. If the switchings are carried out with a sufficiently high frequency the voltage between the different connections 9, 10 and 1 1 will be substantially constant over the time, except for a certain ripple caused.

We assume that the times in the two positions are equally long. The locomotive 1 will then half the time be supplied from the contact line 2 (in the dashed position of the switch 16) and half the time from the arrangement and thereby the second line 4 on the opposite side of the point 8 with respect to the locomotive 1 . This will mean that on the opposite side 9 of the point 8 with respect to the side 20 for the locomotive a direct current of the intensity I/2 with a superposed ripple will flow in the contact line 2 through the filtering of the inductor 15 and the capacitors 17 and 18. The same is valid in the second line 4 on the side 19. A current with an intensity of I will all the time flow in the contact line 2 and the rail 3 between the point 8 and the locomotive 1 . Accordingly, a current of I/2 will in average flow in the contact line 2 and the second line 4 on the side 19 with respect to the point 8, i.e. to the right in Fig 2. Accordingly, losses in the plant will be reduced thanks to the lower current flowing in the lines on the side 19 with respect to the point 8, and a higher power may be transmitted to the loco- motive or engine, since the plant ensures that the voltage is kept on a higher level at the point 8 than otherwise would be the case (in the case just described 3 kV instead of 2,7 kV). Furthermore, a rail current will only flow between the locomotive and the point 8, so that the problems of such a current disap- pear in other parts of the rail.

It is schematically shown in Fig 2 how an additional arrangement 7' is arranged along the rail at another point 21 , and a number of such points may be arranged between stations for feeding electrical power to the plant from feeding networks for enabling a great distance between these stations without a drop of the voltage to too low levels along the contact line for that sake.

It is of course possible to give the second line 4 a higher voltage with respect to the rail 3 than the contact line. It is then possible to either choose a considerably higher voltage, so that the power transmitting ability is increased remarkably. The second line requires than a higher insulation level. Or it is chosen to give the second line 4 a voltage addition within the frame of the same insulation level as the contact line, so that the voltage relationship for example is -3.7/+3.3 kV. This would then mean that at equally long times of the switch 16 in the respective position the voltage of the contact line 2 at the point 8 according to the calculation example mentioned above would be (2.7+3.7)/2, i.e. 3.2 kV. However, it is well possible to control the switch 16 so that another relationship between the times in the respective switching position occurs, and if the rail connection 9 is connected to the connection 1 1 to the second line during longer periods of time than it is connected to the connection 10 to the contact line the potential of the rail connection will be displaced towards the potential of the connection to the second line and the voltage between the contact line and the rail will thereby be higher at said point 8 than the voltage between the second line and the rail, so that in the case just described for example the voltage between the contact line and the rail could be 3.4 kV, while it is 3.0 kV between the second line and the rail. The voltage level may in this way be kept high even better and the distance between the feeding stations or between the arrangements between two stations may be increased. Conversely, it is possible to restrict the voltage of the contact line to the maxi- mum value, for example 3.3 kV, by a switching choice of the switching times, so as to avoid a too high voltage close to the feeding station, at a low load/no load or at return feeding from braking trains.

The plant schematically illustrated in Fig 3 according to a second preferred embodiment of the invention differs from the one according to Fig 2 by a direct connection of the switch 16 to the rail 2 through the rail connection 9. The connection of the arrangement to the contact line 10 and the connection 1 1 thereof to the second line have an inductor 22, 23 each arranged closer to the contact line and the second line, respectively, than the connection of the switch to these connections. Furthermore, a capacitor 24 is arranged between the contact line connection and the connection to the second line on the opposite side of the switch with respect to the inductors. The switch 16 is here arranged to alternatingly connect the rail connection 9 to the contact line connection 10 and the connection 1 1 to the second line in the same way as for the embodiment according to Fig 2. This gives the same basic function and results as for the ar- rangement according to Fig 2. In the position of the switch 16 shown by the solid line in Fig 3 the charging of the capacitor 24 by means of the inductor 23 takes place at the same time as the inductor 22 delivers energy to the contact line. When the switch 16 is in the dashed position a discharging of the capacitor 24 on the other hand takes place by means of the inductor 22 at the same time as the inductor 23 receives energy from the second line 4. Thus, inductors 22, 23 and the capacitor 24 store and deliver electrical energy alternatingly for reducing the difference of the voltage between on one hand the contact line and the rail and on the other the second line and the rail in the way de- scribed above.

Fig 4 illustrates a further preferred embodiment of the invention, in which the arrangement comprises two inductors 25,26 in the form of a transformer 27 having two coils connected in series on one and the same iron core 28 with adjacent terminals of the coils connected to each other in a connection point 29. The rail connection 9 is connected to the connection point. Furthermore, an individual switch 30, 31 is arranged at that terminal 32 and 33, respectively, of each coil that is arranged on the opposite side of the connection point and adapted to be able to connect this terminal either to the connection 10 of the arrangement to the contact line or the connection 1 1 thereof to the second line. The switching device is adapted to control the two switches to assume opposite positions all the time, so that when one con- nects the coil thereof to the connection to the contact line the other one connects its coil to the connection to the second line, as illustrated in Fig 4. This means that in for example the position shown in Fig 4 a current may flow through the rail, the coil 26 and the switch 31 , while a second possible current path is through the rail connection, the coil 25 and the switch 30. The basic result according to the invention mentioned above may be obtained by at the same time alternatingly switch the two switches 30, 31 . It is an advantage of this embodiment with respect to the two others that only half the current flows through each switch, so that these may be dimensioned accordingly and be made less expensive. It is on the other hand necessary to use two switches. By the fact that the two coils 25, 26 are arranged on a common iron core a much higher inductance is obtained, so that the ripple of the voltage on the contact line be- tween the point 8 and the vehicle 1 gets smaller. The inductance may in this case for example be 40 mH, while it for example could be 5 mH for the inductor 15 according to Fig 2.

The plant according to the invention according to Fig 5 com- prises a first capacitor 34, which is arranged at a point 35, i.e. a place, along the rail in such a way that it is by the terminals 36, 37 or plates connectable to the contact line 2, the rail 3 and the second line 4, respectively, in a way described below. A first connection line 38 goes in the point 35 to the second line, a second connection line 39 to the rail and a third connection line 40 to the contact line. These lines are through two switches 41 , 42 connectable to the terminals of the capacitor, and the switches are controlled by an arrangement 43 so that the terminals of the capacitor are alternatingly connected either in a po- sition a), which is shown by solid lines in Fig 5 and means that a first terminal 37 of the capacitor is connected to the second line 4 and the second terminal 36 of the capacitor is connected to the rail 3, or a position b), which corresponds to the dashed lines in Fig 5 and in which the first terminal 37 of the capacitor is connected to the rail 3 and the second terminal 36 thereof is connected to the contact line 2. The control arrangement 43 is then arranged to control the switches 41 , 42 with a high frequency, preferably > 300 Hz. This means that the following takes place:

When the capacitor is connected in the position a) according to Fig 5 it will be charged to the voltage prevailing between the rail and the second line, i.e. 3.3 kV, and a switching then takes place so that the negative terminal of the capacitor is connected to the rail and the positive terminal to the contact line. We assume that in absence of the capacitor 34 the voltage in the contact line where the locomotive 1 is located would have fallen to 2.7 kV. The difference between the capacitor voltage (3.3 kV) and the contact line voltage (2.7 kV) will mean that in the posi- tion b) a charge is flowing from the capacitor in the system contact line/rail so that the contact line voltage increases at the same time as the capacitor voltage decreases. If the capacitor is then switched back to the position a) it will be charged again. If this switching process is carried out with a frequency being much higher than the time constant C x R, in which C is the ca- pacitance of the capacitor and R is the resistance of the locomotive, a voltage equalization will take place, so that substantially the same voltage will be over the capacitor, between the contact line and the rail and between the second line and the rail, i.e. about 3 kV. If the times in the positions a) and b) are just as long, the locomotive 1 will half the time be supplied through the contact line 2 (in the position a)) and half the time from the capacitor and thereby the second line 4 on the opposite side of the point 35 with respect to the locomotive 1 .- This will mean that on the opposite side 4 of the point 35 with respect to the side 45 for the locomotive 1 a current with the intensity I will flow in the contact line 2 half the time and a 0-current will flow during half the time. The same is valid in the second line 4 on said side 44. A current with the intensity I will all the time flow in the contact line 2 and the rail between the point 35 and the lo- comotive 1 . Furthermore, on the side 44 of the point 35 only a rectangular alternating current with the same frequency as the switching frequency will flow in the rail 3. Accordingly, only a current=l/2 will in average flow in the contact line 2 and in the second line 4 on said opposite side 44 of the point 35, i.e. to the left in Fig .5.

It is illustrated a little bit more in detail in Fig 6 how a plant functioning according to the principle of Fig 5 may be constructed. Semiconductor switches 46 in the form of thyristors are here arranged as switches for switching the capacitor 34 between said two positions a) and b). Furthermore, it is here illustrated that two additional capacitors 47, 48 are connected on one hand between the first connection line 38 and the second connection line 39 and on the other between the second con- nection line 39 and the third connection line 40. The capacitance and the reactance of the contact line 2, the rail 3 and the second line 4 together with the capacitance of the two capacitors 47 and 48 will result in a damping of the variation of the currents in the contact line 2, the rail 3 and the negative feeder 4 on the opposite side of the point 35 with respect to the loco- motive 1 , so that in the practice a current with the intensity I/2 will on that side all the time flow in the contact line and the second line, while the current in the rail 3 will be 0.

Accordingly, thanks to the lower current flowing in the lines on said opposite side 44 the losses in the plant will be reduced, and a higher current may be transmitted to the locomotive 1 , by the fact that the plant ensures that the voltage is kept on a higher level at the point 35 than otherwise would be the case. Furthermore, a rail current will only flow between the locomotive and the point 35, so that the problems this would have disappear in other parts of the rail. It is schematically shown in Fig 6 how a further arrangement having capacitors and so on is arranged along the rail at another point 49, and a number of such points may be arranged between stations for feeding electric power to the plant from feeding networks so as to enable a large distance between these stations without any drop of the voltage to too low levels along the contact line for that sake.

A plant according to further preferred embodiments of the in- vention is. very schematically shown in Fig 7, in which it is intended that the second line 4 shall have a voltage with respect to the ground (the rail 3), which is substantially twice as high as the voltage between the contact line 2 and the rail 3. This plant has two first capacitors 34, 34', which are adapted to be con- nected in series between the second line and the rail 3 in the position a), which is shown in Fig 7, so as to be charged by a voltage 2U thereacross, i.e. U across each of them, and to be connected in parallel between the rail 3 and the contact line 2 in the position b) so as to be discharged in parallel to the contact line with the voltages U thereof substantially corresponding to the voltage between the contact line 2 and the rail 3. Accord- ingly, the further switches 50-52 shown are adapted to be controlled so that this is achieved. This plant will function in the corresponding way as the plant illustrated in Figs 5 and 6.

It is illustrated in Fig 8 how in a plant according to a preferred embodiment of the invention two contact lines on different voltage level with respect to the rail but both at the same side thereof may be interconnected. Thus, one contact line 2, the one with the lower potential, here forms the intermediate potential part. We assume that the contact line 2 runs in the Netherlands, where the voltage level in absence of a vehicle is substantially 1 .5 kV, while the second contact line 2' runs in Belgium, where the corresponding voltage level is 3 kV. The two contact lines are separated by a dead portion 33. An arrangement 7 accord- ing to the invention is connected with one connection to the respective contact line and the connection to the rail. The arrangement 7 may for example be designed in any of the ways shown in Figs 2-7 or Fig 13 or 14 in this application or in another way having a corresponding function. It is now imagined that the vehicle, as shown, is on the Dutch side, which means that the voltage of this contact line 2 tends to sink, and for example sinks to 1 .2 kV. This results then through the arrangement 7 in a transport of charges from the Belgian contact line 2' through the arrangement 7 to the Dutch contact line 2, so that the voltag.e thereof is somewhat increased. On the other hand, would the consumption be higher on the Belgian side (the 3 kV- side), the voltage will then there sink and thereby also the voltage between the contact line 2 (the intermediate potential part) and the contact line 2', so that then a transport of charges will take place through the arrangement 7 to the contact line 2'. This looks in the practice as if the contact line continues from the 1 .5 kV- to the 3 kV-area.

It is illustrated in Fig 9 how it is possible to proceed for inter- connecting two contact lines 2, 2', such as those according to

Fig 8, but with the presence of a said second line 4 in the form of a negative feeder, which is here assumed to be at -1 .5 kV, on the Dutch side. It will here be the voltage to the right between the negative feeder 4 and the contact line 2 which will be facing the voltage between the contact line 2' and the rail 3 on the other side. Thus, these two voltages are in the normal case about 3 kV. The power will through the arrangement 7 be fed to the side on which the potential of the contact line sinks mostly, i.e. where the consumption is the highest. Electrical energy is then fed from the other side to the arrangement 7. It is illus- trated in Fig 10 how it would be possible to interconnect two different contact lines 2, 2' being on different potentials, such as for example 1 .5 kV and 3 kV as above, with a continuous second line 7 in the form of a negative feeder, which here is assumed to be on a voltage level with respect to the rail of -3 kV. The ar- rangement 7 is assumed to be a 1 :2-arrangement, while the arrangement 7' is a 1 : 1 -arrangement.

It is illustrated in Fig 1 1 how it is possible to obtain a 1 ^ -relationship of the arrangement 7, when the voltage between the second line (negative feeder) 4 and the rail 3 is higher than the voltage between the rail 3 and the contact line 2. The arrangement 7 is in this case connected to the second line 4 through a capacitor 54 and an electric switch 55 connected in parallel thereto. A unit 56 is adapted to control closing and opening of the switch. 55, so that the potential difference of the connection 57 to the arrangement and the rail gets substantially identical to the potential difference between the latter and the contact line 2. The voltage of the return line could for example be 3.7 kV in a normal 3 kV-system, which gives a corresponding addition with respect to a voltage drop allowed on the return line. The switch may then be controlled so that the voltage in the second line 4 will always be 3.3 kV as long as the return line has a higher voltage value. Should the voltage sink below 3.3 kV the switch is controlled to be "totally open". However, it would also be con- ceivable to proceed as illustrated in Fig 12, i.e. chose a substantially higher voltage on the return line 4, for example -2U, in which the voltage of the contact line with respect to the rail is +U. This is particularly suitable for 1 .5 kV-systems which will then have a -3 kV-return line. It is then possible to use a further arrangement 7' instead of the switch in a cascode connection according to Fig 12. The arrangement 7' will hereby be switched to -2U, -U and 0, while the arrangement 7 will be switched to - U, 0 and +U.

It is illustrated in Fig 13 what an arrangement 7 may look like in a plant according to another preferred embodiment of the invention. This arrangement has two inverters 58, 59, one of which has the direct voltage side thereof connected to the contact line 2 and the alternating voltage side connected to a winding 60 of a transformer 61 . The second inverter has the direct voltage side thereof connected to the second line 4 and the alternating voltage side thereof connected to a second winding 62 of the transformer. 63 illustrates the iron core of the transformer. By suitable control of the two inverters 58 and 59 electrical energy may be transmitted from one of the lines 2 and 4 to the other of them. The direct voltage sides of the two inverters are also connected to the rail 3. The relation between the number of winding turns of the windings 60 and 62 decides the voltage difference between the voltages U₁ and U₂. A fine adjustment may be carried out through the very control of the inverters.

A simplified embodiment of the arrangement 7 of the plant according to Fig 13 is illustrated in Fig 14, where one of the inverters 58 is changed for a diode rectifier 64. It is here illustrated in some more detail what the inverter may look like. The symbols 65 stand for example for thyristors. Energy may in this embodiment only be fed from the second line 4 through the arrangement 7 to the contact line 2. The direct voltage between the second line 4 and the rail 3 is converted in the inverter 59 to a high frequency alternating voltage, for example having a fre- quency of about 300 Hz. It is schematically illustrated in Fig 15 how arrangements 7 according to the invention are arranged along a line for feeding railway vehicles with a contact line 2 and a return line 4. Furthermore, it is illustrated how feeding stations 67 for feeding electrical energy to the contact line and the second line are arranged along the railway feeding line. These stations contain a rectifier 68, which through a transformer 69 is connected to an alternating voltage network having usually a considerably higher voltage, for example 130 kV. The rectifier 68 may be a so-called thyristor rectifier. The rectifier 68 is controlled according to a determined characteristic. The unit 66 arranged to control the arrangements 7 is adapted to control them according to a characteristic being similar to or substantially coinciding with the control characteristic for the rectifier 68 of the feeding stations. This means then that the arrangements 7 will function as extra feeding stations. Thus, the arrangements 7 are either so controlled that a characteristic being the same as for the rectifiers 68 of the surrounding stations is obtained so as to obtain a uniform load distribution between the different arrangements 7 and the stations, or according to another station-similar characteristic so as to obtain another load distribution, for example as a consequence of different rated power of the feeding stations, at the same time as the properties of the rectifying system are for the rest maintained for the system contact line-vehicle-return line and the feeding points thereof. This means a plurality of advantages, such as that there will be no risk of circulating currents. The railway vehicle feeding system will act as an old system without the arrangement, but having a higher capacity. No control superior to the control of the individual arrangements 7 will be necessary.

When realizing the invention different ways may be selected for the direct voltage case. When a new path is electrified according to the invention the self-evident solution will be to use feeding stations feeding both the contact line and the return line on opposite potentials, for example +/- 3 kV, feeding the return line with a considerably higher potential than the contact line, or as mentioned above design the stations to give the return line a potential having a somewhat higher value than the contact line. When reconstructing/upgrading plants already existing, there are several possibilities; the feeding stations are either upgraded, for example through supplementing a +3 kV-station by a -3 kV station in the same place, or the arrangements placed at said point have to do "the job" themselves by the fact that arrangements being close to for example a +3 kV-feeding station "shovels" charges over to the -3 kV-return line. The arrangements are symmetrical and may also work in the opposite direction at braking return feeding. This requires of course a higher power installed and/or a higher number of arrangements and has to be considered economically in relation to an up-grading of the feeding stations. Another possibility consists in reusing the feeding stations already existing in a transition to the plant according to the invention by letting every second station change polarity, so that they are alternatingly feeding in into the contact line and the return line. The railway line is in many cases before the up-grading already equipped with a reinforcing line connected in parallel with the contact line. It may then be easily connected to be the return line on the opposite potential, having either the same voltage, a somewhat higher voltage which is not changing the insulation level or a considerably higher voltage. The insulators have in the case last mentioned to be changed and the insulation distances to be dimensioned for this voltage.

The invention is of course not in any way restricted to the pre- ferred embodiments described above, but many possibilities to modifications thereof will be apparent to a man with ordinary skill in the art without departing from the basic idea of the invention as defined in the claims.

It would for example be possible to apply the invention on the case of feeding an alternating voltage to railway vehicles, such as for example a one-phase voltage having one of the frequencies usual for feeding railways 16 2/3, 25, 50 or 60 Hz, in which it is then necessary that the switching frequency for switching between the switching positions mentioned above are much higher than the frequency of the alternating voltage.

Furthermore, for example in the embodiment according to Fig 2 a further inductor may be arranged in the connections to the contact line and the second line between the respective capaci- tor and the line for reducing the ripple on the voltage obtained on the contact line. However, a larger L could possibly increase the switching losses. Another possibility to reduce the ripple is to increase the switching frequency, but the switching loss will then increase instead.

An additional way to reduce the ripple is instead of an arrangement according to the invention to arrange a plurality of smaller arrangements in parallel with a phase shift in the switching process so selected that the ripple is approximately minimized.

It would also be possible, as mentioned in the British patent application GB 9824215.2 of the applicant to provide the feeding lines for two tracks extending in parallel with a line each for feeding electrical energy to vehicles on these tracks, with oppo- site potentials, so that these feeding lines form the contact line and the second line, respectively, of a plant function according to the principles of the present invention.

Claims

A plant for feeding electric power to railway vehicles having a contact line (2, 2') adapted to be in contact with the vehicle (1 ) for feeding current thereto and return feeding of the current through a rail (3) on ground potential on which the vehicle bears, characterized in that it comprises a second line (2', 4) put on a second potential different from the potential of the contact line, that one of the three parts contact line, second line and rail is adapted to have an intermediate potential located between the potential of the two other parts, that the plant also comprises an arrangement (7) having a connection to the intermediate potential part at a given point therealong and a separate connection to each of the two others of said parts and which comprises means (12) for storing electrical energy and a device (13) adapted to bring said means (12) to receive electrical energy through a direct voltage between a first of said two other parts and the intermediate potential part and feed out electric energy through a direct voltage between the other of said two other parts and the intermediate potential part.

2. A plant according to claim 1 , characterized in that the device (12) is adapted to select that part of said two other parts (2, 2'), which has a potential difference with respect to the intermediate potential part (2) being mostly reduced with respect to the case of no supply of electric power to or no draining of electric power from any of the three parts through any vehicle, as said second part for feeding electrical energy between that part and the intermediate potential part (2).

3. A plant according to claim 1 or 2, characterized in that the device (13) is a switching device adapted to switch the connection of the intermediate potential part with respect to the two other connections alternatingly between two positions, so that these switching positions co-operate with said means (12) so that the vehicle receives electrical energy through the contact line in the first position from the arrangement and in second position through a station feeding the contact line with electrical energy, so that the potential of the contact line is kept on a higher level at this connection than in absence of the arrange- ment and thereby the losses in the contact line on the opposite side of the connection with respect to the vehicle are reduced.

4. A plant according to claim 3, characterized in that said second line (4) is arranged substantially parallel to the contact line (2) and put on a potential of opposite sign to the potential of the contact line and having a magnitude substantially just as high or higher than the potential of the contact line when substantially no electric power is supplied thereto or drained therefrom through any vehicle.

5. A plant according to any of the preceding claims, characterized in that the second line is a second contact line (2') which along the same rail follows upon the contact line (2) first mentioned with a dead portion therebetween, that the two contact lines are put on potentials of different magnitudes but of the same sign, so that the contact line (2) having the lowest absolute value of the potential forms said intermediate potential part, and that said device is adapted to provide the vehicle with electrical energy from the contact line it is located at through said means for. keeping the potential of this contact line on a higher level at this connection than in absence of the arrangement.

6. A plant according to claim 4, characterized in that along the same rail said contact line (2) is followed by a second contact line (2') having a dead portion between these contact lines, that the second contact line is adapted to have a potential having substantially the same difference with respect to the rail (3) as the potential difference between the contact line (2) first mentioned and the second line (4) when substantially no electric power is supplied to or drained from any of the contact lines through any vehicle, and that the arrangement (7) is also con- nected to the second contact line so as to provide the vehicle with electrical energy from the arrangement when a vehicle is located therealong.

7. A plant according to claim 1 , characterized in that the potential difference with respect to the intermediate potential is greater for one (4) of the two other parts than for a second of these parts when substantially no electric power is supplied to or drained from the contact line, through any vehicle, that the arrangement (7) is connected to said one part through a capacitor (54) and an electric switch (55) connected in parallel therewith, and that a unit (56) is arranged to control closing and opening of the switch so that the potential difference of the input (57) of said one part to the arrangement and the intermediate potential part (3) gets substantially equal to the potential difference between the latter and the second part (2).

8. A plant according to claim 1 , characterized in that the potential difference with respect to the intermediate potential is sub- stantially twice as great for one (4) of the two other parts as for the second (2) of these parts when substantially no electric power is supplied to or drained from the contact line, through any vehicle, that the arrangement (7) is connected to said one part through a capacitor (54) and that it comprises a further said arrangement (7') having one connection to the intermediate potential part and one connection to the input of said one part to the arrangement first mentioned.

9. A plant according to any of the preceding claims, character- ized in that said means comprises at least one inductor (15, 22,

23, 25, 26) and/or at least one capacitor (17, 1 8, 24) for storing electrical energy.

10. A plant according to claim 3 or claim 3 and any of the other preceding claims, characterized in that said means (12) are adapted to be charged by electrical energy in said second posi- tion and deliver at least a part thereof to the contact line (4) in the first position.

1 1. A plant according to claim 3 or claim 3 and any of the other preceding claims, characterized in that it comprises at least one inductor (15) adapted to be charged by electrical energy in said second position and deliver this energy to the contact line in the first position, and that the switching device has one single switch (16) adapted to connect the intermediate potential con- nection (9) to one of said parts (10) for obtaining the first position in a first switching position and in a second switching position connect the intermediate potential connection to the connection (1 1 ) to the other of said parts for obtaining the second position.

12. A plant according to claim 1 1 , characterized in that it comprises at least one capacitor (17, 18, 24) arranged between two of said connections of the arrangement.

13. A plant according to claim 1 1 or 12, characterized in that it has an inductor (15) for storing electrical energy connected to the intermediate potential connection (9) and through which said single switch (16) is connected to the intermediate potential connection.

14. A plant according to claims 12 and 13, characterized in that it comprises two capacitors (17, 18), namely one arranged between the intermediate potential part and a first of the two other parts and one arranged between the other part and the interme- diate potential part, and that the capacitors are connected to the intermediate potential part (9) on the opposite side of the inductor (15) with respect to the switch (16).

15. A plant according to claim 12, characterized in that the connections (10) of the arrangement to the two parts located on opposite sides of the intermediate potential has an inductor (22, 23) each arranged closer to the first of the parts and the second part, respectively, than the connection of said switch (16) to these parts, and that the capacitor (24) is arranged between the connection to the first part and the connection to the second part on the opposite side of the switch with respect to the inductors.

16. A plant according to claim 3 or claim 3 and any of the other preceding claims, characterized in that said means comprises two inductors (25, 26) in the form of a transformer (27) having two coils connected in series on one and the same iron core (28) with adjacent terminals of the coils interconnected in one connection point (29), that the intermediate potential part (9) is connected to said connection point, that an individual switch (30, 31 ) is arranged at that terminal (32, 33) of each coil that is arranged on the opposite side of the connection point and arranged to be able to connect this terminal either to the first of the parts or the connection thereof to the second part, and that the switching device is adapted to control the two switches to continuously assume opposite positions, so that when one connects the coil thereof to the connection to the first part the other connects the coil thereof to the connection to the second part for obtaining said first position and conversely for obtaining the second position.

17. A plant according to claim 1 , characterized in that it comprises at least one inverter (58,59) connected with the direct voltage side thereof to one of the parts and with the alternating voltage side to a first winding (62) of a transformer (61 ), that it comprises a rectifying means (58, 64) connected to a second winding (60) of said transformer and to the second of the parts, and that the inverter and the rectifying means are connected to each other through a portion of a line connected to the intermediate potential part.

18. A plant according to claim 17, characterized in that said rectifying means (64) is a diode bridge.

19. A plant according to claim 17, characterized in that said rectifying means is formed by a second inverter (58) having the direct voltage side thereof connected to the second of the parts and the alternating voltage side connected to said second winding of the transformer.

20. A plant according to claim 4, characterized in that the second line is put on a potential of an opposite sign to the potential of the contact line and of a magnitude substantially n times the potential of the contact line when substantially no electric power is supplied to or drained from the latter, through any vehicle, n being an integer > 1 , and at least one capacitor (34, 34') and an arrangement (41-43, 46, 50-52) adapted to control the terminals of the capacitor so as to in a given point (35) along the rail alternatingly connect either a) a first terminal (37) of the capacitor to the second line and the second terminal (36) of the capacitor to the rail or b) the first terminal of the capacitor to the rail and the second terminal of the capacitor to the contact line so that when electric power is fed to the vehicle in the position according to a) the capacitor is charged through the second line and in the position according to b) the capacitor is charged through the contact line for keeping the potential of the contact line on a higher level at said point than in absence of the capacitor and thereby reduce losses in the contact line on the opposite side (44) of the point (35) with respect to the vehicle (1 ).

21 . A plant according to claim 20, characterized in that the control arrangement is adapted to switch the terminals of the capacitor (34, 34') between the position a) and b) with such a high frequency that the voltage of the capacitor is maintained substantially unchanged for a determined supply of electric power between the vehicle (1 ) and the contact line (2).

22. A plant according to claim 21 , characterized in that said frequency is much higher than R X C, in which R is the resistance of the vehicle and C is the capacitance of the capacitor (34, 34').

23. A plant according to any of claims 20-22, characterized in that it comprises two second capacitors (47, 48), one (47) of which is connected by the terminals thereof between a first connection line (38) leading to the second line (4) and a second connection line (39) leading to the rail (3) and a second (48) is connected by the terminals thereof between the second connection line and a third connection line (40) leading to the contact line (2), and that the control arrangement comprises switches (46) adapted to alternatingly connect to different ones of said connection lines for obtaining the position a) or b).

24. A plant according to any of claims 20-23, characterized in that the control arrangement has switches (46) in the form of controllable semiconductor switches for switching the terminals of the capacitor (34, 34') between the position a) and b).

25. A plant according to any of claims 20-24, characterized in that n > 2 and that it has n capacitors having substantially the same capacitance and controlled by the control arrangement between the positions a) and b) in such a way that these n capacitors in the position a) are connected in series between the second line (4) and the rail (3) and in the position b) are connected in parallel between the rail (3) and the contact line (2) so that when feeding electric power to the vehicle the n capacitors will be charged in series by a voltage substantially n x U there- across in the position a) and discharged parallel to a voltage of substantially U across each of them in the position b), in which U corresponds to the potential difference between the contact line and the rail.

26. A plant according to any of claims 20-25, characterized in that the contact line and the second line (4) are connected to a direct voltage potential each for feeding direct current to or from the vehicle.

27. A plant according to claim 26, characterized in that the contact line (2) is put on positive potential and the second line (4) on negative potential.

28. A plant according to any of claims 1 -27, characterized in that it comprises a plurality of arrangements (7, 7') arranged at different points along the rail (3) between two adjacent stations (5, 6) of the plant for feeding electric power to the contact line (2) for maintaining the voltage level of the contact line high along the extension thereof between said stations.

29. A plant according to claim 3 or claim 3 and any of the other preceding claims, characterized in that the switching device (13) is arranged to carry out the switching between the two po- sitions so that these two positions will be assumed alternatingly during periods of time of substantially the same length.

30. A plant according to any of claims 1 -29, characterized in that the second line (4) is given a potential being of substan- tially the same magnitude as the potential of the contact line (2) when substantially no electric power is supplied to or drained from the latter through any vehicle.

31 . A plant according to any of claims 1 -29, characterized in that the second line (4) is given a potential having a higher magnitude than the potential of the contact line (2) when substantially no electric power is supplied to or drained from the latter through any vehicle.

32. A plant according to claim 3 or claim 3 and any of the other preceding claims, characterized in that the switching device (13) is adapted to carry out the switching between the first and the second position so that the intermediate potential part (9) is connected to the connection (1 1 ) of the arrangement to the second of the parts during longer periods of time than it is connected to the connection (10) of the arrangement to the first part, so as to displace the potential of the intermediate potential part towards the potential of the connection to the second part and thereby increase the voltage between the connection to the first part and that one to the intermediate potential part.

33. A plant according to claim 3 or claim 3 and any of the other preceding claims, characterized in that the switching device (13) is arranged to cause a switching between the first and second position with such a high frequency that the voltage on one hand between the connection (10) of the arrangement to the first part and the connection thereof to the intermediate potential part and on the other between the connection (1 1 ) of the arrangement to the second part and the connection (9) thereof to intermediate potential part are kept substantially unchanged at a determined supply of electric power between the vehicle and the control line.

34. A plant according to any of the preceding claims, characterized in that the first part is the contact line, the second part the second line and the intermediate potential part the rail.

35. A plant according to claim 3 or claim 3 and any of the other preceding claims, characterized in that the switching device comprises at least one switch (16, 30, 31 ) in the form of a con- trollable semiconductor switch for said switching.

36. A plant according to claim 35, characterized in that the switch (16, 30, 31 ) is a thyristor.

37. A plant according to claim 35, characterized in that the switch (16, 30, 31 ) is an IGBT.

38. A plant according to any of claims 1 -37, characterized in that the contact line (2) and the second line (4) are connected to a direct voltage potential each for feeding direct current to or from the vehicle.

39. A plant according to claim 38, characterized in that the contact line (2) is put on positive potential and the second line (4) on negative potential or conversely.

40. A plant according to claim 38 or 39, characterized in that the voltage between the contact line (2) and the rail (3) is between 500 V and 6 kV, preferably between 500 V and 3 ^'kV.

41 . A plant according to claim 3 or claim 3 and any of the other preceding claims, characterized in that the contact line (2) is adapted to feed an alternating current to or from the vehicle, and that the switching device (13) is adapted to cause an alternation between the first and second position with a frequency being much higher than the frequency of the alternating voltage on the contact line.

42. A plant according to claim 41 , characterized in that said alternating voltage has a frequency of 16 2/3 or 25 HZ.

43. A plant according to claim 3 or claim 3 and any of the other preceding claims, characterized in that the switching device (13) is adapted to cause a switching between the first and second position with a frequency being advantageously higher than 300 Hz, preferably higher than 1 kHz.

44. A plant according to claim 3 or claim 3 and any of the other preceding claims, characterized in that a plurality of said arrangements (7) are connected in parallel to each other to the contact line (2), the rail (3) and the second line (4) at said determined point (8) and the switching device thereof are adapted to switch between the first and second position with a mutual phase shift for smoothing out ripple on the voltage fed to the vehicle.

45. A plant according to any of claims 1-44, characterized in that it comprises two tracks running parallel to each other having a line each for feeding electric energy to vehicles thereon and having opposite potentials, so that one of the feeding lines forms said contact line and the other feeding line forms said second line.

46. A plant according to any of the preceding claims, characterized in that it comprises members (66) adapted to control said arrangement (7) according to a characteristic being similar to or coinciding with the characteristic according to which stations (67) surrounding the arrangement for feeding electrical energy to one or more of said parts are controlled.

47. A plant according to claim 46, characterized in that said stations have thyristor rectifiers (68) controlled according to said characteristic for converting alternating voltage into direct voltage.