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Title:
SYNCHRONISING TRACTION CONVERTERS
Document Type and Number:
WIPO Patent Application WO/2017/137324
Kind Code:
A1
Abstract:
A synchronisation system (28) for at least two pulse-width modulated electrical traction converters (14a, 14d) comprises a first controller (18a) adapted for switching a first traction converter (14a) with pulse width modulation based on a first carrier signal (32a); a second controller (18d) adapted for switching a second traction converter (14d) with pulse width modulation based on a second carrier signal (32b); and an electrical line (22) for transmitting a synchronisation signal (24) as electrical signal between the first controller (18a) and the second controller (18d); wherein the first controller (18a) is adapted for generating the synchronisation signal (24) based on a timing of the first carrier signal (32a); wherein the second controller (18d) is adapted for determining a carrier shift of the second carrier signal (32b) with respect to the first carrier signal (32a) based on the synchronisation signal (24a).

Inventors:
MERMOD, Frederic (Bodenächerstrasse 16, 5417 Untersiggenthal, 5417, CH)
BALDINGER, Marc (Rankstrasse 17, 5430 Wettingen, 5430, CH)
DAEHLER, Peter (Unterer Leeweg 5, 5236 Remigen, 5236, CH)
WICKI, Stefan (Ackerweg 9, 5702 Niederlenz, 5702, CH)
Application Number:
EP2017/052395
Publication Date:
August 17, 2017
Filing Date:
February 03, 2017
Export Citation:
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Assignee:
ABB SCHWEIZ AG (Brown Boveri Strasse 6, 5400 Baden, 5400, CH)
International Classes:
H02P5/74; H02M1/00
Foreign References:
US20110260656A12011-10-27
US5923550A1999-07-13
JP2008109727A2008-05-08
Attorney, Agent or Firm:
ABB PATENT ATTORNEYS, ASSOCIATION 154, C/O ABB SCHWEIZ AG, INTELLECTUAL PROPERTY (Brown Boveri Strasse 6, 5400 Baden, 5400, CH)
Download PDF:
Claims:
CLAIMS

1. A synchronisation system (28) for at least two pulse-width modulated electrical traction converters (14a, 14d), the system comprising:

a first controller (18a) adapted for switching a first traction converter (14a) with pulse width modulation based on a first carrier signal (32a);

a second controller (18d) adapted for switching a second traction converter (14d) with pulse width modulation based on a second carrier signal (32b);

an electrical line (22) for transmitting a synchronisation signal (24) as electrical signal between the first controller (18a) and the second controller (18d);

wherein the first controller (18a) is adapted for generating the synchronisation signal (24) based on a timing of the first carrier signal (32a);

wherein the second controller (18d) is adapted for determining a carrier shift of the second carrier signal (32b) with respect to the first carrier signal (32a) based on the synchronisation signal (24a)

wherein the first controller (18a) is located in a first wagon (12a) of a train (10) and the second controller (18d) is located in a second wagon (12b) of the train (10);

wherein the electrical line (22) runs through the first wagon (12a) and the second wagon (12b).

2. The synchronisation system (28) of claim 1,

wherein the electrical line (22) interconnecting the first controller (18a) and the second controller (18d) comprises at least one decoupleable interconnection (26) provided by a coupling of two wagons (12) of a train (10).

3. The synchronisation system (28) of one of the preceding claims,

wherein the electrical line (22) comprises two wires (40a, 40b) and the synchronisation signal (24) is transmitted via the two lines (40a, 40b) via a differential signal.

4. The synchronisation system (28) of one of the preceding claims, wherein the electrical line (22) is at least partially shielded.

5. The synchronisation system (28) of one of the preceding claims,

wherein the electrical line (28) is at least partially an external power supply line (16) of a train (10) and/or an internal power supply line of a train (10).

6. The synchronisation system (28) of one of the preceding claims, further comprising: a transmitter (46) for receiving the synchronisation signal (24) as optical signal from the first controller (18a) and for transforming the synchronisation signal (24) into an electrical signal; and/or

a first fibre optics line (48a) interconnecting the first controller (18a) and the transmitter (46).

7. The synchronisation system (28) of one of the preceding claims, further comprising: a receiver (50) for receiving the synchronisation signal (24) as electrical signal and for transforming the synchronisation signal (24) into an optical signal for the second controller (18d); and/or

a second fibre optics line (48b) interconnecting the second controller (18d) and the receiver (50).

8. The synchronisation system (28) of one of the preceding claims, further comprising: a third controller (18b) adapted for switching a third traction converter (14b) with pulse width modulation based on a third carrier signal;

a fibre optics line (20) for transmitting the synchronisation signal (24) as optical signal between the first controller (18a) and the third controller (18b).

9. The synchronisation system (28) of one of the preceding claims,

wherein the first controller (18a) and/or a transmitter of the synchronisation signal is adapted to generate the synchronisation signal (24) as rectangular pulse synchronised with a timing of the first carrier signal (32a) and/or second carrier signal (32b).

10. The synchronisation system (28) of one of the preceding claims, wherein the first controller (18a) is adapted for determining an operation status of other traction converters (14b, 14c, 14d) and for adjusting a timing of the synchronisation signal (24) based on the operation status.

11. A converter system (30) for an electrical train, the converter system comprising: the synchronisation system (28) of one of the preceding claims,

a first traction converter (14a) controlled by the first controller (18a),

a second traction converter (14d) controlled by the second controller (18d).

12. The converter system (30) of claim 11 ,

wherein the first and the second traction converters (14a to 14d) are active rectifiers for converting a voltage supplied to the train into a DC voltage.

13. A method of synchronising at least two pulse-width modulated electrical traction converters (14a, 14d), the method comprising:

generating a synchronisation signal (24) in a first controller (18a) of a first traction converter (14a), wherein the first controller (18a) is located in a first wagon (12a) of a train (10) and the synchronisation signal (24) is based on a timing of a carrier signal (32a) of the first converter (14a) used for modulating a switching signal (38) for switching the first converter (14a);

transmitting the synchronisation signal (24) as electrical signal via an electrical line (22) to a second controller (18d) of a second traction converter (14d), wherein the second controller (18d) is located in a second wagon (12b) of the train (10) and the electrical line (22) runs through the first wagon (12a) and the second wagon (12b);

time-shifting a carrier signal (32b) of the second converter (14d) based on the synchronisation signal (24).

14. A use of an electrical line (22) to transmit a synchronisation signal (24) for carrier signals (32a, 32b) of pulse-width modulated traction converters (14a, 14d) in a train (10) , wherein a first controller (18a) is located in a first wagon (12a) of the train (10) and a second controller (18d) is located in a second wagon (12b) of the train (10) and wherein the electrical line (22) runs through the first wagon (12a) and the second wagon (12b).

Description:
DESCRIPTION

Synchronising traction converters

FIELD OF THE INVENTION

The invention relates to the field of traction converters. In particular, the invention relates to a synchronisation system and a method for at least two pulse-width modulated electrical traction converters, a converter system for an electrical train, and a use of an electrical line for synchronising at least two traction converters.

BACKGROUND OF THE INVENTION

Electrical trains may be supplied via a catenary line with electrical voltage. This power is converted by a system of one or more electrical converters into a voltage supplied to electrical motors driving the train. As a rule, the number of used converters depends on the rated power to be processed.

Due to their switching behaviour, the converters introduce disturbances into the catenary line or more general the supplying grid. However, official standards limit the possible disturbances in both AC and DC grids for traction applications. Usually, it is a demanding task to comply with these limits.

When the converters are pulse- width modulated, a good strategy for achieving low disturbances in the grid is to provide even displacements of the carrier signals of all operating converters. This, however, is only possible if the carrier signals of different converters are synchronised. If some converters and their carrier signals are not synchronised, undesired low frequency disturbances may occur, which are based on clock inaccuracies between differently running clocks of the converters.

One way to avoid these low frequency disturbances may be to synchronise the carrier signals and hence the converter switching to a multiple of the line frequency. However, in this case, for higher frequencies (1 kHz ... 2kHz) ofthe carrier signal, the software task timing may have to be timely decoupled from the converter switching, since the line frequency is not exactly constant. This may create additional complexities and undesired control delays in fast control loops. It is known to synchronise controllers mounted nearby within the same wagon by fibre optics lines. However, if they are allocated distantly from each other in different wagons, synchronisation via fibres may not be a robust solution anymore, since the fibres may have to be conducted through a number of wagons, which also may be decoupled from each other. US 2011/0260656 Al relates to a load drive system for a vehicle, in which a carrier signal from a carrier signal generation circuit, used for controlling a first electrical converter, is sent to a carrier signal inverting circuit, which generates an inverted carrier signal used for controlling a second electrical converter.

US 5 923 550 A relates to interference reduction by a harmonic phase shift in multiple PWM controlled rectifiers, which are part of a system supplying traction motors of cars of a train.

DESCRIPTION OF THE INVENTION

It is an objective of the invention to simplify the synchronisation between traction converters in a train and to provide a simple and easy to install synchronisation system.

This objective is achieved by the subject-matter of the independent claims. Further exemplary embodiments are evident from the dependent claims and the following description.

A first aspect of the invention relates to a synchronisation system for at least two pulse- width modulated electrical traction converters. A traction converter may be a converter that is used for converting electrical power that is used for driving a train. For example, a traction converter may comprise a rectifier and/or inverter, which may rectify electrical power from a distribution grid and/or which may invert a DC power from a DC link. In the end, the electrical power may be supplied to one or more electrical motors driving the train.

According to an embodiment of the invention, the synchronisation system comprises a first controller adapted for switching a first traction converter with pulse width modulation based on a first carrier signal; a second controller adapted for switching a second traction converter with pulse width modulation based on a second carrier signal; and an electrical line for transmitting a synchronisation signal as electrical signal between the first controller and the second controller; wherein the first controller is adapted for generating the synchronisation signal based on a timing of the first carrier signal; and wherein the second controller is adapted for determining a carrier shift of the second carrier signal with respect to the first carrier signal based on the synchronisation signal. Both the traction converters may be pulse-width modulated, i.e. may comprise a controller that is adapted for generating a switching signal based on a carrier signal. The carrier signal, which, for example, may be a triangular signal, may be compared with a reference signal (for example a reference voltage) and the switching signal may be generated based on a crossing of the carrier signal with the reference signal.

For generating as less harmonics as possible, the carrier signals of the controllers are synchronised with a synchronisation signal, which is generated by the first controller, transmitted by an electrical line to the second controller and used by the second controller for synchronisation. For example, the synchronisation signal may be a pulsed signal, and the timing of the pulses may determine a timing of the second carrier signal.

The timing of a carrier signal may be defined by a time point of a zero-crossing of the carrier signal. The carrier shift of a carrier signal may be a time interval from a zero-crossing of the carrier signal to another regular signal, such as a clock signal of a controller. For example, the first controller may generate a pulse for the synchronisation signal at a zero- crossing and the second controller may set carrier shift such that the pulse is received at a zero-crossing of the second carrier signal. It may be possible, that the first controller and/or second controller add a time shift to the timing of the synchronisation signal to compensate for signal delay times.

The first controller may be seen as the master controller for synchronisation, the second controller as slave controller. It has to be understood that the system may comprise more than one slave controllers.

It may be seen as a gist of the invention that the synchronisation signal is not transmitted via a fibre optics line, which is usually used for synchronising controllers near converters, since fibre optics lines are not disturbed by electromagnetic fields emitted near electrical converters, but by an electrical line, which may be integrated more easily into a train. For example, it may be easier and more robust to use wires in the train instead of fibre optics.

The controller may be part of the respective converter. For example, the controller may comprise electronics for controlling the respective controller, such as FPGA devices and/or may be integrated into a housing of the respective converter.

According to an embodiment of the invention, the first controller (and the first traction converter) is located in a first wagon of a train and the second controller (and the second traction converter) is located in another or a second wagon of the train. The electrical line may run through the first wagon and the second wagon. In other words, the electrical line may interconnect traction converters in two different wagons. It may be possible that the electrical line runs through, inside or outside further wagons between the first and second wagon.

According to an embodiment of the invention, the electrical line interconnecting the first controller and the second controller comprises at least one decoupleable interconnection provided by a coupling of two wagons of a train. The first and second wagon housing the first and second controllers may be decoupled from each other. In this case, also the electrical line may de decoupled and later easily coupled, for example with corresponding plugs.

It has to be understood that the electrical line may have a length of more than 10 m. According to an embodiment of the invention, the electrical line comprises two wires and the synchronisation signal is transmitted via the two lines via a differential signal. The signal on the second wire may be the inverse signal of the first wire. In such a way, electromagnetic disturbance may not strongly disturb the synchronisation signal, which may be the difference of the two signals on the two wires.

According to an embodiment of the invention, the electrical line is at least partially shielded or vice versa, partially not shielded. However, it also may be possible to shield the electrical line to protect the signal transmission from electromagnetic disturbances. In particular, the electrical line may be shielded near or inside the housing of a traction converter.

According to an embodiment of the invention, the electrical line is at least partially an external power supply line of a train and/or an internal power supply line of a train. It may not be necessary that an additional electrical line is provided in the train. Already existing electrical lines may be used for transmitting the signal, such as already existing power lines. This line may be an external power supply line, such as a catenary or overhead line, and/or an internal power supply line, such as a battery supply line.

According to an embodiment of the invention, the synchronisation system further comprises a transmitter for receiving the synchronisation signal as optical signal from the first controller and for transforming the synchronisation signal into an electrical signal and a first fibre optics line interconnecting the first controller and the transmitter. For example, the first fibre optics line may run from the first controller outside the housing of the converter (where electromagnetic disturbances are lower). There, the optical synchronisation signal may be transformed into an electrical synchronisation signal and may be introduced into the electrical line (which, for example, may be a power supply line). According to an embodiment of the invention, the synchronisation system further comprises a receiver for receiving the synchronisation signal as electrical signal and for transforming the synchronisation signal into an optical signal for the second controller and a second fibre optics line interconnecting the second controller and the receiver. For example, the electrical synchronisation signal (which may be taken from a power supply line) may be transformed into an electrical signal outside the housing of a converter and may be transmitted via the second fibre optics line into the housing to the second controller.

According to an embodiment of the invention, the synchronisation system further comprises a third controller adapted for switching a third traction converter with pulse width modulation based on a third carrier signal and a fibre optics line for transmitting the synchronisation signal as optical signal between the first controller and the third controller. It also may be possible that further "slave" controllers also may be synchronised solely via optical synchronisation signals, without an intermediate electrical line.

According to an embodiment of the invention, the first controller and/or a transmitter of the synchronisation signal is adapted to generate the synchronisation signal as rectangular pulse synchronised with a timing of the second carrier signal. The synchronisation signal may have two steep edges, which edges may have a timing corresponding to the timing of the first and/or second carrier signal.

According to an embodiment of the invention, the first controller is adapted for determining an operation status of other traction converters and for adjusting a timing of the synchronisation signal based on the operation status. For example, the first "master" controller may receive a signal from a central controller that one of the other converters has stopped or has started to operate. In this case, the carrier shifts of the carrier signals of the "slaver" controllers (in particular the second carrier signal) may have to be changed to reduce harmonics introduced the distribution grid. I.e. the first controller may change the synchronisation signal based on the operation status of other converters.

For example, the information of the operation status may be distributed via a common field bus (such as CANopen). In case of a converter being out of service, the carrier shifts of the other converters may be automatically adapted in order to get the optimal interference spectrum for each mode of operation.

A further aspect of the invention relates to a converter system for an electrical train, which comprises the synchronisation system as described in the above and in the following, the first traction converter controlled by the first controller and the second traction converter controlled by the second controller. For example, the first and the second traction converters are active rectifiers for converting a voltage supplied to the train into a DC voltage.

A further aspect of the invention relates to a method of synchronising at least two pulse- width modulated electrical traction converters. The method may be performed by the controllers of the traction converters of the synchronisation system.

According to an embodiment of the invention, the method comprises: generating a synchronisation signal in a first controller of a first traction converter, wherein the synchronisation signal is based on a timing of a carrier signal of the first converter used for modulating a switching signal for switching the first converter; transmitting the synchronisation signal as electrical signal via an electrical line to a second controller of a second traction converter; and time-shifting a carrier signal of the second converter based on the synchronisation signal.

A further aspect of the invention relates to a use of an electrical line to transmit a synchronisation signal for carrier signals of pulse- width modulated traction converters in a train. The usage of an electrical line instead of a fibre optics line bridging most of the distance (such as more than 80% of the distance) between the two controllers may simplify the installation of the synchronisation system in one or more wagons of a train.

It has to be understood that features of the converter system, the method and the use as described in the above and in the following may be features of the synchronisation system as described in the above and in the following, and vice versa.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject-matter of the invention will be explained in more detail in the following text with reference to exemplary embodiments which are illustrated in the attached drawings.

Fig. 1 schematically shows a train with a converter system according to an embodiment of the invention.

Fig. 2 shows a diagram with signals used in the converter system of Fig. 1.

Fig. 3 schematically shows a synchronisation system according to an embodiment of the invention. The reference symbols used in the drawings, and their meanings, are listed in summary form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Fig. 1 shows a train 10 from above and from the side, which comprises a plurality of wagons 12. In a first wagon 12a, the train 10 comprises two traction converters 14a, 14b, which are adapted for converting an AC voltage from an overhead line 16 into a DC voltage supplied to a DC link, which furthermore inverted into an AC voltage used to drive electrical motors, which are adapted for rotating wheels of the train 10.

Analogously, the train 10 comprises a last, second wagon 12b, which comprises two further traction converters 14c, 14d, which may be designed analogously like the traction converters 14a, 14b.

Each of the converters 14a to 14d comprises a controller 18a, 18b, 18c, 18d. The controllers 18a to 18d are adapted for switching the converters 14a to 14d with pulse width modulation and for synchronising each other.

The controllers 18a, 18b (or 18c, 18d) within one wagon 12 are adapted to synchronise each other via synchronisation signals sent via a fibre optic line 20. Controllers 18a, 18d (or 18b, 18 c) within different wagons 12 are adapted to synchronise each other via synchronisation signals 24 sent via electrical lines 22.

It has to be noted that a first controller 18a in the first wagon 12a may be interconnected with a second controller 18d in the second wagon 12b via a first electrical line 22 and that a third controller 18b in the first wagon 12a may be interconnected with a fourth controller 18c in the second wagon 12b via a second electrical line 22. Furthermore, the first controller 18a and the third controller 18b may be interconnected via first fibre optics line 20 and the second controller 18d and the fourth controller 18c may be interconnected via a second fibre optics line 20. The controllers 18a to 18d and the lines 20, 22 (and optionally 16) may be seen as part of a synchronisation system 28. Together with the converters 14a to 14d, a converter system 30 adapted for synchronisation of remote converters is formed.

The electrical lines 22 may be a dedicated line for synchronisation, may be a power line used for distribution of electrical power inside the wagons (such as a battery net line or an auxiliary power supply line). Also, the overhead line 16 may be used for transmitting synchronisation signals between the controllers 18a to 18d. The electrical lines 22 may comprise decoupleable interconnections 26 (such as plugs) that may be decoupled, when two wagons 12 are decoupled.

It may be that one of the controllers, such as 18a, with respect to synchronisation is a master controller, while the other controllers, such as 18b to 18d, are slave controllers. The master controller 18a may generate the synchronisation signal 24 and may send it via the lines 20, 22 (and optionally 16) to the slave controllers. For example, the synchronisation signal 24 may be sent via the electrical line 22 to the controller 18d and/or via the fibre optics line 20 to the controller 18b. Furthermore, the synchronisation signal 24 may be sent from the controller 18b via the other electrical line 22 to the controller 18c. Additionally or alternatively, the synchronisation signal 24 may be sent from the controller 18d via the other fibre optics line 20 to the controller 18c.

Fig. 2 shows diagrams with signals that are used by the controllers 18a to 18d for controlling and switching the converters 14a to 14d.

For example, the controller 18a generates a carrier signal 32a, which may be a regular zig-zag-signal of a specific periodicity (and/or frequency) having specific regular zero- crossings 34 that are synchronised with an internal clock of the controller 18. The controller 18a receives or generates a reference signal 36 (such as a voltage reference), from which a switching signal 38 is derived based on the carrier signal 32a. For example, whenever the reference signal 36 crosses the carrier signal 32a, the switching signal may change from "+" to In this context, "+" and "-" may be used for switching switches of the converter 14a, for example for connecting a DC link of the converter 14a either in a positive way ("+") to an input or in a negative way ("-").

The other controllers 18b to 18d may control their converters 14b to 14d in an analogous way. For example, as shown in Fig. 2, the controller 18d may control the converter 14d with a further carrier signal 32b (which may have the same form, nearly the same periodicity, nearly the same frequency, etc.).

However, since the controller 18d may have a clock of its own (which may slightly deviate from the clock of the controller 18a) and/or since the carrier signals 32a, 32b are generated (without synchronisation) independently from each other in their controllers 18a, 18d, the carrier signals 32a, 32b may have a phase shift and or slightly deviating frequencies/periodicities.

To synchronise the carrier signals 32a, 32b, the controller 18a may send periodically a synchronisation signal 24 to the controller 18d via the electrical line 22. The synchronisation signal 24 may have a rectangular form and a starting (or alternatively ending) edge of the synchronisation signal 24 may be synchronised with the zero-crossing 34 of the first carrier signal 32a. When receiving the synchronisation signal 24, the second control 18d may synchronise the zero-crossing 34 of its carrier signal 32b with the starting (or alternatively ending) edge of the synchronisation signal 24.

The zero-crossing 34 of the first carrier signal 32a may be seen as the timing of the first carrier signal 32a, the starting (or alternatively ending) edge of the synchronisation signal 24 may be seen as the timing of the synchronisation signal 24 and/or the zero-crossing 34 of the second carrier signal 32b may be seen as the timing of the second carrier signal 32b. In such a way, the first controller 18a may generate a synchronisation signal 24 with a timing based on the timing of the first carrier signal 32a and the second controller 18b may synchronise the timing of the second carrier signal 32b with the timing of the synchronisation signal 24.

It has to be noted that either the first (master) controller 18a or the second (slave) controller 18d may add or subtract a timing offset to the synchronisation signal 24 to compensate for signal running times between the controllers 18a, 18d and/or processing times inside the controllers 18a, 18d.

Furthermore, the first (master) controller 18a may control relative carrier shifts between different converters 14a to 14d, by adding or subtracting a timing offset to the timing of the synchronisation signal 24. For example, the master controller 18a may receive information about the operation status of the other converters 14b to 14d and may be adapted to carrier shifts between different converters 14a to 14d in such a way that harmonics generated in the overhead line 15 are minimized. It may be possible that the master controller 18a adapts the carrier shift of one converter 14b with another timing offset as the one for a further converter 14c.

Fig. 3 shows a converter system 30 with a synchronisation system 28, which is based on a two-wire electrical line 22 with two wires 40a, 40b. This makes it possible to transmit the electrical synchronisation signal 24 as differential signal (for example as a first signal on a first wire 40a and a reversed second signal on a second wire 40b).

The two wires 40a, 4b may be terminated with a resistance/impedance 44 to adjust the impedance of the electrical line 22.

However, it also may be possible that the electrical synchronisation signal 24 is sent via only one wire. Furthermore, the electrical line 22 may be shielded with an electromagnetic shielding 42, at least on the vicinity of sources of strong electromagnetic radiation (such as converter supply lines, etc.). It also may be possible that the electrical line 22 is at least partially not shielded.

The controller 18a (or 18b) may comprise a transmitter 46, which is adapted to modulate the synchronisation signal 24 as electrical signal in the electrical line 22. The transmitter 46 may be interconnected with (other parts of) the controller 18a (or 18b) via a fibre optics line 48a and/or may be adapted to transduce an optical synchronisation signal into an electrical synchronisation signal. For example, the transmitter 46 may be placed outside of an electrical shielding of the converter 14a (or 14b).

On the other hand, the controller 18c (or 18d) may comprise a receiver 50, which is adapted to demodulate the electrical synchronisation signal 24. The receiver 50 may be interconnected with (other parts of) the controller 18c (or 18d) via a fibre optics line 48b and/or may be adapted to transduce an electrical synchronisation signal into an optical synchronisation signal. For example, the transmitter 46 may be placed outside of an electrical shielding of the converter 14c (or 14d).

As an example, the fibre optics lines 48a and/or 48b may be connected to an optical CAN bus interface of the controller 18a, 18b, 18c, 18d and the transmitter 46 and/or receiver 50 may be CAN bus transducers between an optical and an electrical CAN bus.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art and practising the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or controller or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope. LIST OF REFERENCE SYMBOLS

10 train

12, 12a, 12b wagon

12a first wagon

12b second wagon

14a, 14b, 14c, 14d traction converter

16 overhead line

18a, 18b, 18c, 18d controllers

20 fibre optics line

22 electrical line

24 synchronisation signal

26 interconnection

28 synchronisation system

30 converter system

32a, 32b carrier signal

34 zero-crossing

36 reference signal

38 switching signal

40a, 40b wire

42 shielding

44 resistance

46 transmitter

48a, 48b fibre optics line

50 receiver