WO2001047742A1

WO2001047742A1 - A plant and a method for transmitting electric power

Info

Publication number: WO2001047742A1
Application number: PCT/SE2000/002523
Authority: WO
Inventors: Gunnar HAMMARSTRÖM
Original assignee: Balfour Beatty Plc.
Priority date: 1999-12-23
Filing date: 2000-12-14
Publication date: 2001-07-05
Also published as: AU2563201A; SE9904762L; SE9904762D0; SE517050C2

Abstract

In a method for operation of a plant for transmitting electric power from a feeder (2) carrying alternating voltage or direct voltage to a contact line (11) carrying direct voltage for feeding railway vehicles, the control of the stations (13-16) for feeding electric power between the feeder and the contact line is coordinated so that upon a state of the first station (14) deviating from the normal operation state this control is modified for a station (15) adjacent to this first station for assisting the first station to fulfil its task and deliver electric power asked for by railway vehicles.

Description

A PLANT AND A METHOD FOR TRANSMITTING ELECTRIC POWER

FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a plant for transmitting electric power from a feeder carrying alternating voltage or direct voltage to a contact line carrying direct voltage for feeding railway vehicles, which comprises a plurality of stations, mutually spaced along the extension of the contact line for feeding elec- trie power between the feeder and the contact line and having each at least one arrangement adapted to convert the voltage from the feeder into a voltage suitable for the contact line, as well as a method according to the preamble of the appended independent method claim. Railway vehicles means vehicles being bound to a certain way for example for making the energy supply of the vehicle functioning. For example trains and trolley buses belong to such vehicles.

The invention is not restricted to any particular voltage level of the feeder, but it can be mentioned that this is typically considerably higher than the level of the direct voltage to the contact line, and the feeder may for example belong to the existing high voltage transmission network and will have voltage levels between 30 and 500 kV, while the nominal level of the direct volt- age on the contact line is typically between 500 V and 3 kV, although the invention is not at all restricted to these levels.

For illuminating but not in any way restricting the invention the case of an alternating voltage of the feeder, which is usually a three-phase voltage, and an arrangement of a station in form of a rectifier having controllable semiconductor devices, such as thyristors, for converting alternating voltage into direct voltage will hereinafter be discussed. However, this discussion is also applicable to the case of a feeder carrying direct voltage.

The control of the different arrangements is under normal operation conditions designed while considering several factors, such as energy consumption and supply of sufficient power to the vehicles so that they are not delayed. Comparatively great voltage variations are usually allowed within railway feeding systems, typically from -30 % to +20 % around a nominal voltage level for the contact line, so that it is possible to control the output voltage from the station to a desired level through control of the power semiconductor devices. The arrangement is then controlled so that said output voltage is well under the voltage maximally allowed on the contact line, so that it gets possible for the plant to take care of the braking energy delivered by the train along the contact line. This refed energy may then be used by other trains and/or be fed back to the feeder through a feeding station.

A said arrangement may for different reasons drop out, for example since any component thereof gets broken, a lightening stroke or the like, and the demand is usually put on plants of this type that they have to be able to manage to have uninfluenced operation when a feeding station drops out, especially for subways. Would namely the operation be influenced in the way that the plant in such a state is not able to deliver sufficient electric power to the railway vehicles, these have to be driven slower, which would result in unacceptable delays. Exactly said demand is usually deciding for the number of stations along a certain extension of the contact line, i.e. for the distance between adjacent such stations. The more stations required for a certain length of the contact line the higher will of course the costs for the plant be. Often are also two or more said arrangements (rectifier) arranged in one and the same station, in which these arrangements are then dimensioned so that the station is able to deliver the amount of electric power it has as a task to deliver also if one of the arrangements would drop out. This means that the arrangements are clearly over- dimensioned during normal operation and may by that be considered to be unnecessary costly.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a plant of the type defined in the introduction, which is able to reduce the drawbacks mentioned above of such plants already known to a large extent.

This object is according to the invention obtained by providing a device of the type defined in the introduction with members for detecting the state of the arrangements of each station and means for co-ordinating the control of the arrangements of the stations, and that these means, when detecting a state deviating from the normal operation state of at least one arrangement of any, here called the first, of the stations, are adapted to modify the control of at least one arrangement of at least one station located adjacent to the first station so as to counteract the influence made by the deviating state of the first station upon the possibility of the plant to provide the contact line with electric power asked for by railway vehicles.

By modifying the control of at least one arrangement of at least one station adjacent to a station for which a deviating operation state occurs in the way mentioned, i.e. by changing the criterions according to which the adjacent station is normally controlled, this adjacent station may assist said first station to fulfil its task, i.e. deliver the electric power asked for by the railway vehicles, while reducing the drawbacks mentioned above to a large extent. The fact is that it is possible to arrange the stations at a greater mutual distance along the contact line and nevertheless manage to feed sufficient electric power to the contact line upon a dropout of one station. In the case of two or more said arrangements in a station and the dropout of one of these, a part of the power this station has to deliver may now through the modified control of the adjacent station be delivered by this adjacent station, so that the arrangements, such as the rectifiers, may be dimensioned somewhat smaller and by that costs be saved, without renounce the redundancy requirements. Thus, the plant according to the invention gives priority to providing the railway vehicles with power at the expense of other factors, such as energy consumption, which are normally considered when determining the control of the arrangements of the stations during normal conditions.

According to a preferred embodiment of the invention said means, at said detection of a deviating operation state of the first station, are adapted to modify the control of at least one arrangement of at least the two stations located at both sides thereof so as to counteract the influence of the deviating state of the first station upon the possibility of the plant to provide the contact line with electric power. This may be advantageous for avoiding negative side effects of the control of one single station being too strongly modified, such as a too unfavourable load distribution between the other feeding stations.

According to another preferred embodiment of the invention said means are adapted to carry out said modification of the control of the arrangement in question so that the output voltage delivered by the station to which it belongs to the contact line is increased with respect to the case of a normal operation state of the first station. Exactly in plants of this type comparatively great voltage variations are normally allowed, and since the output voltage from the stations is normally kept on a level well below the level maximally allowed by a suitable control of the arrangements, such as controllable power semiconductor de- vices thereof, so that the plant may take care of energy generated by trains when braking, there is accordingly a possibility to increase the output voltage of one or more stations located adjacent to the station having completely or partially failed and by that deliver extra power from that station at the expense of other factors, such as energy consumption. It is of course also con- ceivable to modify the control of the arrangement or the arrangements in question so that the voltage is for sure increased considerably with respect to the normal operation case, but is nevertheless chosen to lie on a level having such a margin to the voltage maximally allowed that braking energy may still be entirely or partially regenerated.

However, according to another preferred embodiment of the invention said means are adapted to control the arrangement having a modified control so that said output voltage gets a level being substantially just as high as the voltage maximally allowed on the contact line. By designing the plant so that the voltage is in this way increased to the level maximally allowed it will be possible to obtain the greatest possible distance between the stations and a so small dimensioning as possible of the ar- rangements of a station when having a plurality thereof in each station while fulfilling the demand of uninfluenced operation upon drop out of at least one arrangement, in which this takes place at the expense of the energy consumption of the plant. Increased voltage means that the voltage level will for any load current be higher than would the control of the arrangement be unchanged, and for all other load currents the voltage has to be at the least just as high as for an unchanged control.

According to another preferred embodiment of the invention said means are adapted to control the arrangements of all stations included in the plant except for the first station, so that the output voltage of these is increased to a first level, with respect to the output voltage intended to be normally delivered by the stations to the contact line in the normal operation state of the first station. By exposing all stations not failing for the same voltage increasing regulation as the station closest to the failing station in this way, a solution of the problem according to the invention being very simple and by that favourable from the cost point of view but nevertheless acceptable with respect to the function is obtained.

According to another preferred embodiment of the invention said means are adapted to control at least one arrangement of a second station adjacent to said first station so that the output voltage of the second is increased, to a first level, with respect to the output voltage the station is intended to normally deliver to the contact line in normal operation state of the first station, and at least one arrangement of the station located on the opposite side of the second station with respect to the first station so that the output voltage of that station is increased to a sec- ond level, which is lower than said first but higher than the output voltage this station is intended to deliver to the contact line when the first station is in the normal operation state. Negative side effects of the modified control, such as feeding of electric power from one station into another may be avoided through such a stepwise reduction of the voltage increase of stations with modified control.

According to another preferred embodiment of the invention the plant comprises members adapted to accomplish a galvanical interruption of the contact line between adjacent stations, and the plant comprises members which, when detecting one said deviating operation state of the first station, are adapted to control said members for galvanical interruption and establish a galvanic separation of the contact line between a station controlled to increase the output voltage thereof and an adjacent station, the control of which has not been modified. By galvanically separating a station having a modified control in the form of a higher voltage from an adjacent station without any modified control, i.e. which delivers a direct voltage of a lower level, there will be no risk that any electric power from the station having a modified control is fed into said adjacent station, so that the stations on the opposite side of the place for galvanic separation of the contact line with respect to the station having modified control will not in any way be influenced, neither by the modified control of one or more stations or the operation dropout of at least one arrangement of said first station.

Another object of the invention is to provide a method according to the preamble of the appended method claim, which is considerably more advantageous than such methods already known. This object is according to the invention obtained by providing a method according to the appended independent method claim. The advantages of such a method and of the methods according to the embodiments defined in the dependent method claims appear without any doubt from the discussion above.

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

BRIEF DESCRIPTION OF THE DRAWINGS

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

In the drawings:

Fig 1 is a schematic view illustrating an arrangement for rectifying an alternating voltage by means of controllable thyristors useful in a plant according to the invention,

Fig 2 illustrates schematically the construction of a plant of the type according to the invention,

Fig 3 shows the voltage U versus the current I on the direct voltage side of the arrangement according to Fig 1 , Fig 4 shows the power P being available for a railway vehicle utilising a plant of the type shown in Fig 2 versus the voltage U received by the vehicle,

Fig 5 is a view corresponding to Fig 3 of the voltage U versus the current I on the direct voltage side of different stations of a plant according to Fig 2 when one of the stations completely or partially ceases to function and other adjacent stations get a modified control according to a method according to the invention,

Fig 6 illustrates schematically the construction of a car receiving electric power from a contact line of the plant according to Fig 2, and

Fig 7 is a view corresponding to Fig 2 of a plant according to another embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE I NVENTION

It is schematically illustrated in Fig 1 what an arrangement 1 adapted to convert an alternating voltage from a feeder carrying a three-phase alternating voltage into a direct voltage to a contact line for feeding railway vehicles may look like. It is then essential for the invention that the arrangement 1 is able to convert the voltage into a voltage suitable for the contact line, and this is achieved by having controllable power semiconductor de- vices, in this case thyristors, as the rectifying members 3 of the arrangement, which makes it possible to change the output voltage from the arrangement by changing the control thereof, opposite to a rectifying arrangement based on diodes, in which no such control of the voltage is possible. A transformer 4 is adapted to transform the voltage of the alternating voltage line 2 to a level, which may be handled by the arrangement 1 . The voltage level of the feeder 2 is in fact usually considerably higher than the level of the voltage being suitable for the railway vehicles for keeping the transmission losses on the feeder at a low level. The different phases 5-7 of the alternating voltage downstream of the transformer are in a conventional way connected to midpoints of a phaseleg 8-10 each of the rectifying arrangement 1 . It is pointed out that each thyristor symbol shown in the figure may stand for a plurality of thyristors connected in series. The rectifying arrangement 9 "chops" the alter- nating voltage in a known way to a direct voltage having a smaller superposed ripple.

The construction of a conceivable plant of the type according to the invention is illustrated in Fig 2, and it is here illustrated how a plurality of stations 13-16 mutually spaced along a contact line 1 1 for feeding railway vehicles 12 for feeding electric power between the feeder 2 and the contact line 1 1 are each provided with two rectifying arrangements 1 . This is normally the case for making the respective station able to provide railway vehicles 12 with at least such a high electric power upon a dropout of one of the arrangements that no vehicle has to stop. Would in fact the voltage delivered to the contact line by a station fall under 30% of the nominal voltage of the contact line, the power will be too low for driving the vehicles at all. On the other side, the voltage may be allowed to move up to 20% above said nominal level. The nominal direct voltage is defined as the average over a period of the alternating voltage for the direct voltage for which the contact line is dimensioned, and around which the direct voltage is normally allowed to vary within said voltage range. It is pointed out that the distance between adjacent stations is usually several kilometres.

It is also illustrated that the plant comprises an operation central

17, which is adapted to receive information about the state of each station through detecting members 18 and also send con- trol orders to the stations for the operation of the rectifying arrangements.

Finally, it is illustrated how the plant has a ground current breaker 31 adapted to close would the rail return current from a vehicle get that high that the potential close to the rail exceeds a certain level, and by that generate a ground current for lowering the potential.

It is illustrated in Fig 3 how the voltage U from a rectifying arrangement 1 of the stations normally is controlled depending upon the output current I through controlling the thyristors 3, and it is shown how the voltage is kept close to the nominal level 19, such as for example 1500 V both at low and high power outputs, so that there will be no risk that a vehicle will not receive the power it desires and there is also a margin up to the voltage level 20 maximally allowed, which here would be 1800 V, so that energy generated upon braking of the vehicle may be refed to the contact line. Thus, the rectifying arrangements are controlled so that they follow the voltage-current-characteristic shown in Fig 3, which may be considered to be a possible optimisation while considering the demands upon delivered power, energy consumption and low transmission losses in normal operation of the plant i.e. when all stations are intact. In plants of this type already known also a control of the rectifying arrangements of the different stations takes place also according to this characteristic would any arrangement temporarily drop out. The drawbacks thereof have been thoroughly penetrated further above.

It is shown in Fig 4 what may then happen in the case that a station is not able to deliver a voltage being sufficiently high, namely that a vehicle may not be driven any longer at voltages lower than U₀. The arrangements are for this reason overdimen- sioned and/or the stations are arranged at sufficiently small in- tervals to each other in the way described above for avoiding this.

However, the present invention makes it possible to avoid such overdimensioning and to make said intervals larger than have been the case before without waving the redundancy requirements in any way, and this is accomplished by applying other control criterions for the stations in an emergency operation case, more exactly by in such a case giving priority to provide the trains with power at the expense of other factors, such as energy consumption. This takes place in the following way:

It is assumed that at least one arrangement 1 drops out of a first 14 of the stations. The operation central 17 will become aware thereof through said detecting member, and the operation central decides in such a case^: to modify the control of at least one arrangement of at least one station adjacent to the first station 14 so as to counteract the influence of the deviating state of the first station upon the possibility of the plant to provide the contact line 1 1 with electric power asked for by railway vehicles. Normally all arrangements of at least the two stations 13, 15 located on both sides of the first station 14 are influenced, so that these receive a modified control according to this principle. This means in the practice that the controllable semiconductor de- vices 3 are controlled so that the output voltage from these stations is considerably increased with respect to a normal operation state of the first station. Thus, the normally optimum voltage-current-characteristic for these stations are not longer followed, but they do now instead aim at being able to deliver the power asked for by the railway vehicles in spite of the partial dropout of the first station 14, i.e. in the practise deliver a higher power than normally. It is then possible to increase the output voltage from these adjacent stations to a level being very close to the voltage level 20 maximally allowed, which for sure means that electric energy delivered by a vehicle 12 (see Fig 6) in the engine 21 thereof at braking possibly has to be "burned" away, for example through any large resistor 22 on the roof of the car, since the voltage on the contact line otherwise gets too high. This type of modified control may also mean that the rail return current gets that high that the ground current breaker 31 has to be closed more often than normally for taking care of this and generate a ground current.

There is of course an apparent risk that in the case the voltage of for example the station 15 is increased substantially during certain operation conditions the voltage of the contact line at the station 16 by that will be that high that the station 15 then will feed electric power into the station 16. Also more than the adjacent stations may receive a modified control in a said emergency operation state of the first station 14 for avoiding this, such as for example also the station 16, and it is then advantageous that the voltage level of the stations with modified control is reduced in the direction away from the first station 14, so that the rectifying arrangements of the station 16 are in this case controlled to increase the output voltage of this station to a sec- ond level, which is lower than the level of the voltage delivered by the station 15. This counteracts the risk for feeding power from one station into another. Said voltage levels of the stations 15 and 16 in this embodiment of the invention are schematically illustrated in Fig 5 through the lines 23 and 24, respectively. The voltage-current-characteristic these stations would have had when controlling them according to the normal criterions is also illustrated here through 25.

Another possibility to solving the problem of feeding power from a station with a modified control into an adjacent station 2 is illustrated in Fig 7. It is here shown how a member 26 for galvanic separation is arranged on the contact line 1 1 between adjacent stations. Such a member is preferably arranged between each of the stations of the plant, although only one is shown in Fig 7. If we now assume that the station 27 drops out and the station 28 in the way according to the invention gets a modified control meaning a higher output voltage therefrom, the breaker 26 may then be controlled to galvanically separate the stations 28 and 29 from each other, so that no electric power may pass through the contact line 1 1 from the station 28 to the station 29.

It is also illustrated in Fig 7 that it is well conceivable that the feeder 2 carries a direct voltage and the stations are designed to convert this direct voltage into a direct voltage of another level.

The nominal voltage of the contact line of a plant according to the invention is preferably between 500 V and 5 kV, and more particularly is the nominal voltage 600, 750, 1200, 1500 or 3000 V.

The stations are preferably designed for a nominal power being higher than 500 kW and lower than 40 MW.

The invention is of course not in any way restricted to the preferred embodiments described above, but many possibilities to modifications thereof would be apparent to a person skilled in the art without departing from the basic idea of the invention as defined in the appended claims.

It is apparent that one of these possibilities is to arrange members for galvanic separation of a plant having arrangements in the form of rectifiers.

Claims

1 . A plant for transmitting electric power from a feeder (2) carrying alternating voltage or direct voltage to a contact line (1 1 ) carrying direct voltage 'for feeding railway vehicles (12), which comprises a plurality of stations (13-16, 27-29) mutually spaced along the extension of the contact line for feeding electric power between the feeder and the contact line and having each at least one arrangement (1 ) adapted to convert the voltage from the feeder into a voltage suitable for the contact line, characterized in that it also comprises members (18) for detecting the state of the arrangements of each station and means (17) for co-ordinating the control of the arrangements of the stations, and that these means, when detecting a state deviating from the normal operation state of at least one arrangement of any, here called the first (14, 27), of the stations, are adapted to modify the control of at least one arrangement of at least one station (13, 15, 28) located adjacent to the first station so as to counteract the influence made by the deviating state of the first station upon the possibility of the plant to provide the contact line with electric power asked for by railway vehicles.

2. A plant according to claim 1 , characterized in that said means, at said detection of a deviating operation state of the first station (14), are adapted to modify the control of at least one arrangement of at least the two stations (1 3, 15) located at both sides thereof so as to counteract the influence of the deviating state of the first station upon the possibility of the plant to provide the contact line with electric power.

3. A plant according to claim 1 or 2, characterized in that said means (17), at said detection of said deviating operation state of the first station (14), is adapted to modify the control of at least one arrangement of a plurality of stations (15, 16) arranged consecutively along the contact line (1 1 ) so as to 1 o

counteract the influence of the deviating state of the first station upon the possibility of the plant to provide the contact line with electric power.

4. A plant according to any of the preceding claims, characterized in that said means (17) is adapted to determine that a deviating operation state of a station (14) exists when the capacity thereof to deliver electric power to the contact line is substantially reduced with respect to the normal capacity of that station.

5. A plant according to claim 4, characterized in that said means (17) are adapted to determine that a deviating operation state of a station (14) exists when a said arrangement (1 ) of the station is out of function.

6. A plant according to any of claims 1 -5, characterized in that said means (17) are adapted to carry out said modification of the control of the arrangement (1 ) in question so that the output voltage delivered by the station (13, 15, 28) to which it belongs to the contact line_; is increased with respect to a normal operation state of the first station (14, 27).

7. A plant according to claim 6, characterized in that said means are adapted to control the arrangement (1 ) by a modified control so that said output voltage receives a level being substantially just as high as the voltage (20) maximally allowed for the contact line.

8. A plant according to claim 3 and 6, characterized in that said means (17) are adapted to control the arrangements of all stations (13, 15, 16) included in the plant except for the first station (14), so that the output voltage of these is increased to a first level, with respect to the output voltage in- tended to be normally delivered by the stations to the contact line in the normal operation state of the first station.

. A plant according to claim 3 and 6, characterized in that said means (17) are adapted to control at least one arrangement of a second station (15) arranged adjacent to said first station (14) so that the output voltage of the second station is increased, to a first level (23), with respect to the output voltage which the station is intended to normally deliver to the contact line in the normal operation state of the first station, and at least one arrangement of the station (16) lo- cated on the opposite side of the second station with respect to the first station, so that the output voltage of that station is increased to a second level (24), which is lower than said first but higher than the output voltage intended to be delivered by this station to the contact line when the first station is in the normal operation state.

10. A plant according to claim 9, characterized in that said means are adapted to modify the control of a plurality of stations (13, 15, 16) located on both sides of said first station (14), so that the output voltage of these is increased with respect to the case of a normal operation state of the first station with an amount falling in the direction away from the first station.

1 1 . A plant according to any of claims 6-10, characterized in that it comprises members (26) adapted to accomplish a galvanical interruption of the contact line (1 1 ) between adjacent stations, and that the plant comprises members (30) which, when detecting one said deviating operation state of the first station, are adapted to control said members for galvanical interruption and establish a galvanic separation of the contact line between a station (28) controlled to increase the output voltage thereof and an adjacent station (29), the control of which has not been modified.

12. A plant according to any of the preceding claims, characterized in that the feeder (2) is adapted to carry an alternating voltage and the arrangements (1 ) of the stations are adapted to convert alternating voltage into direct voltage.

13. A plant according to any of claims 1 -1 1 , characterized in that the feeder (2) is adapted to carry direct voltage and the arrangements (1 ) of the stations (27-29) are adapted to convert direct voltage into direct voltage of another level.

14. A plant according to any of the preceding claims, characterized in that each of the arrangements (1 ) of the stations has controllable power semiconductor devices (3) and members for controlling thereof.

15. A plant according to claim 12 and 14, characterized in that each of the arrangements (1 ) has controllable rectifying devices (3) and the members _:for controlling them are adapted to accomplish the control so that the arrangement converts an alternating voltage into a direct voltage having a magnitude that may be varied.

16. A plant according to claim 15, characterized in that said rectifying devices (3) are thyristors.

17. A method for operation of a plant for transmitting electric power from a feeder (2) carrying alternating voltage or direct voltage to a contact line (1 1 ) carrying direct voltage for feeding railway vehicles (12), said plant comprising a plural- ity of stations (1 3-16, 27^28) mutually spaced along the extension of the contact line for feeding electric power between the feeder and the contact line and having each at least one arrangement (1 ) adapted to convert the voltage from the feeder into a voltage suitable for the contact line, characterized in that the state of the arrangements of each station is detected, and that the control of the stations is co- ordinated so that when detecting a state of at least one of the arrangements of any, here called the first (14, 27), of the stations deviating from the normal operation state, the control of at least one arrangement of at least one station (13, 15, 28) located adjacent to the first station is modified for counteracting the influence of the deviating state of the first station upon the possibility of the plant to provide the contact line with electric power asked for by railway vehicles.

18. A method according to claim 17, characterized in that upon said detection of a deviating operation state of the first station (14) the control of at least one arrangement of at least the two stations (13, 15) located on both sides thereof is modified for counteracting the influence of the deviating state of the first station upon the possibility of the plant to provide the contact line with electric power.

19. A method according to claim 17 or 18, characterized in that upon said detection of said deviating operation state of the first station (14) the control of at least one arrangement (1 ) of a plurality of consecutive stations (15, 16) along the contact line is modified, so that the influence of the deviating state of the first station upon the possibility of the plant to provide the contact line with electric power is counteracted.

20. A method according to any of claims 17-19, characterized in that it is established that a deviating operation state exists for a station (14, 27) when the capacity of this station to deliver electric power to the contact line is substantially re- duced with respect to the capacity this station normally has.

21 . A method according to claim 20, characterized in that it is established that a deviating operation state of a station (14, 27) exists when a said arrangement (1 ) of the station is out of function.

22. A method according to any of claims (17-21 ), characterized in that the modification of the control of the arrangement in question is carried out so that the output voltage delivered by the station (13, 15, 16, 28) to which it belongs to the contact line is increased with respect to the case of a normal operation state of the first station (14).

23. A method according to claim 22, characterized in that the modified control of the arrangement is carried out so that said output voltage gets a level (23) being substantially just as high as the voltage (20) maximally allowed on the contact line.

24. A method according to claim 19 and 22, characterized in that the arrangements of alt stations (13, 15, 16) of the plant except of said first station (14) are controlled so that the output voltage of these is increased, to a first level (23), with respect to the output voltage intended to be normally delivered by the stations to the contact line in normal operation state of the first station.

25. A method according to claim 19 and 22, characterized in that at least one arrangement of a second station (15) arranged adjacent to said first station (14) is controlled so that the output voltage of the second station is increased, to a first level (23), with respect to the output voltage intended to be normally delivered by the station to the contact line in normal operation state of the first station, and that at least one arrangement of the station (16) located on the opposite side of the second station with respect to the first station is controlled so that the output voltage of that station is increased to a second level (24), which is lower than said first one but higher than the output voltage this station is intended to deliver to the contact line when the first station is in the normal operation state.

26. A method according to claim 25, characterized in that when detecting said deviating operation state of the first station (14) the control of a plurality of stations (13, 15, 16) located on both sides of the first station is modified, so that the out- put voltage thereof is increased with respect to the case of a normal operation state of the first station with an amount falling in the direction away from the first station .

27. A method according to any of claims 17-26, characterized in that when detecting a said deviating operation state of the first station (27) a galvanical interruption of the contact line between a station (28) controlled to increase its output voltage and an adjacent station (29), the control of which has not been modified, is accomplished for galvanically separate these stations from each other.