Electrical power supply system for an electrically propelled vehicle and methods of controlling such an electrical power supply system

An electrical power supply system for an electrically propelled vehicle provided with a traction unit and an electrical connector and moving along a circulation rail includes an external power supply zone having a supply line extending along the circulation rail for connection with the electrical connector, and an autonomous power supply zone, located after the external power supply zone along the circulation rail. The supply line includes a main section. The supply line includes a terminal section, extending along the circulation rail in the external power supply zone at least between a first end of the main section and the autonomous power supply zone, for connection with the electrical connector, and a diode, electrically connecting the first end of the main section and a second end of the terminal section and designed to let an electrical current pass through from the main section to the terminal section.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to an electrical power supply system for an electrically propelled vehicle. The invention also relates to a method of controlling an electrical power supply system of the above-mentioned type, while the vehicle is moving from an external power supply zone towards an autonomous power supply zone and vice versa.

BACKGROUND ART OF THE INVENTION

In the electrical alimentation of vehicle field, in particular for electrically propelled vehicles, it is known to provide the vehicle with an autonomous power supply device, such as a battery or some capacitors, an electric connection member, such as a pantograph, and a power supply bus which electrically connects the pantograph and the battery to an electric motor of the vehicle.

Thus, it is known to use an electrical power supply system, including an external power supply zone and an autonomous power supply zone. The vehicle is supplied in the external power supply zone by means of an external power supply infrastructure, such as a catenary line. The pantograph realizes the electrical contact between the catenary line and the power supply bus of the vehicle. Once the pantograph is connected to the catenary line, an electric current flows between the catenary line and power supply bus of the vehicle.

On the opposite, when it is in the autonomous power supply zone, the vehicle is supplied by means of the autonomous power supply device.

However, while the vehicle is moving from a power supply zone to the other one, in particular in the instant when the pantograph is connecting or disconnecting from the catenary line, an electric arc is generated between the catenary line and the pantograph, as the electric current flowing between them does not extinguish instantaneously. The electric arc generated between the catenary line and the pantograph should be avoided since it may provoke detriments of the catenary line and of the pantograph, as well as a remarkable electromagnetic noise.

As explained in WO-A-2011/147708, it is therefore preferred to equip the power supply bus of the vehicle with an electrical protection device, such as a diode, which allows the electric current to pass only from the pantograph to the power supply bus of the vehicle. In other words, the diode mounted on the power supply bus of the vehicle is continuously crossed by the electric current of the catenary line for the power supply.

This approach leads to some technical inconvenients. First, the diode needs to be continuously cooled down by a proper system placed in the vehicle, in order to keep the right operating temperature. Second, the diode blocks any current flowing from the power supply bus towards the catenary line, which means that no electrical braking regeneration can be performed.

SUMMARY OF THE INVENTION

One object of this invention is to remedy this drawback more particularly by proposing a novel electrical power supply system which definitively suppresses the electric arc generation but allows electrical braking regeneration.

To this end, the invention concerns an electrical power supply system for an electrically propelled vehicle provided with a traction unit and an electrical connection member and moving along a circulation rail, the system including:an external power supply zone, having a supply line extending along the circulation rail for connection with the electrical connection member, the supply line being connected to a power supply infrastructure,an autonomous power supply zone, located after the external power supply zone along the circulation rail, where the vehicle is supplied by means of an autonomous power supply device,
the supply line including a main section, provided with a first end,
the external power supply system being characterized in that the supply line includes moreover:a terminal section, extending along the circulation rail in the external power supply zone at least between the first end of the main section and the autonomous power supply zone, for connection with the electrical connection member and provided with a second end,a diode, electrically connecting the first end of the main section and the second end of the terminal section and designed to let passing through an electrical current from the main section to the terminal section.

Owing to the invention, the diode is mounted directly on the supply line and it is crossed by the electric current only when the electrical connection member is connected to the terminal section, which is for a limited time interval. Moreover, the electrically braking regeneration is allowed along the main section of the supply line. Such a power supply system does not need therefore a cooling system dedicated to the diode.

According to further aspects of the invention which are advantageous but not compulsory, such an electrical power supply system might incorporate one or more of the following features taken in any admissible configuration:the first end of the main section and the second end of the terminal section are partially overlapped on a transition portion along the direction of the circulation rail and the supply line is designed for the diode to be short-circuited between the first end of the main section and the second end of the terminal section by the electrical connection member;the terminal section is isolated, except from the main section and the connection member, and it has a floating voltage;the said electric power supply includes a detecting unit designed to predict in time the position of the electrical connection member of the vehicle along the supply line;it includes:means for driving of an output voltage of the autonomous power supply device of the vehicle in relation to an output voltage of the supply line, when the connection member of the vehicle is connected to the terminal section from the external power supply zone,means for disconnecting of the connection member of the vehicle only from the terminal section of the supply line;the said electrical power supply system includes means for setting the output voltage of the autonomous power supply device higher than the output voltage of the supply line while the connection member of the vehicle is in the terminal section and the vehicle is in a traction mode, in which the traction unit of the vehicle transforms the power supply in kinetic energy;it includes:means for setting the output voltage of the autonomous power supply device lower than the output voltage of the supply line while the connection member of the vehicle is before the terminal section;means for setting the output voltage of the autonomous power supply device higher than the output voltage of the supply line while the connection member of the vehicle is in the terminal sectionwhen the vehicle is in a braking mode, in which the traction unit of the vehicle transforms the kinetic energy in electrical energy;it includes:means for connecting the connection member of the vehicle only to the terminal section of the supply line,means for driving of an output voltage of the autonomous power supply device of the vehicle in relation to an output voltage of the supply line, when the connection member of the vehicle is connected to the terminal section from the autonomous power supply zone;the said electrical power supply system includes means for setting the output voltage of the autonomous power supply device lower than the output voltage of the supply line while the connection member of the vehicle is in the terminal section and the vehicle is in a traction mode, in which the traction unit of the vehicle transforms the power supply in kinetic energy;it includes:means for setting the output voltage of the autonomous power supply device lower than the output voltage of the supply line while the connection member of the vehicle is before the main section;means for setting the output voltage of the autonomous power supply device higher than the output voltage of the supply line while the connection member of the vehicle is in the main section,when the vehicle is in braking mode, in which the traction unit of the vehicle transforms the kinetic energy in electrical energy.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

FIG. 1shows an electrically propelled vehicle1, such as a railway vehicle, for example a train or a tramway, which is designed to move along a circulation rail2. The vehicle1includes a chassis3, an electric motor4, a traction unit5, several railway wheels6, a power supply bus8and an external line9. Moreover, an autonomous power supply device10, a current sensor11and a control unit12are located in the vehicle1and are part of a supply system which is described below. Finally, the vehicle1includes an inductance13and an electrical connection member14.

The chassis3is supported with respect to the ground G by the wheels6, two of these wheels6being visible onFIG. 1. Therefore, the chassis2represents the electric mass, or the zero voltage reference, of the vehicle1.

The electric motor4is, for example, a reversible electric motor and it is compatible with electrical regenerative braking. In other words, the electric motor4absorbs electric energy while the vehicle1is in a traction mode and supplies electric energy when the vehicle is in a braking mode. Therefore, electric motor4is mechanically connected to the wheels6of the vehicle1and it is designed to transform the electric energy in a kinetic energy of the wheels6when it is in the traction mode and, on the opposite, to transform the kinetic energy of the wheels6in the electric energy when it is in the braking mode. The electric motor4is, for example, an alternating current triphase electric motor.

The traction unit5is connected to the electric motor4and is designed to supply or collect electric energy to or from the motor4. In particular, the traction unit5is designed to convert a direct electric current in an alternating electric current for the motor4during the traction mode.

On the opposite, during the braking mode, it is designed as well to supply electric energy coming from the motor4to the autonomous power supply10. In particular, the traction unit5is designed to convert an alternating electric current produced by the motor4in a direct electric current.

The traction unit5is, for example, a power inverter.

The power supply bus8electrically connects the traction unit5to the external line9and to the autonomous power supply device10. The power supply bus8is then designed to be powered up at a voltage potential which is supplied by the external line9, the device10or the traction unit5. In other words, the bus8is designed to transfer a direct current from and towards the traction unit5. The power supply bus8also connects the traction unit5and the device10to the chassis2of the vehicle, assuring earthing of any electric current.

The autonomous power supply device10is designed to supply the electrical energy to the traction unit5during the traction mode or to store the electrical energy supplied by the traction unit5during the braking mode or the electrical energy supplied through the external line9. In particular, the device10is able to produce an output voltage VAon the power supply bus8so that a direct current IAis generated and transferred to the traction unit5. The output voltage VAhas a variable value.

The autonomous power supply device10is provided, for example, with a battery or some capacitors.

The current sensor11is designed to measure a direct current flowing in the external line9and to give an information whether it measures a current or not to the control unit12.

The control unit12is designed to receive the information about the current flowing in the external line9from the current sensor11. The control unit12is designed as well to drive and set the output voltage VAof the autonomous power supply device10, while this is supplying the direct current IAto the traction unit5during the traction mode and depending on the position of the pantograph14along the supply line16and the position and the direction of the vehicle1along the circulation rail2. The control unit12sets the output voltage VAby switching on or off one or several rheostat choppers, which are not shown in the figures. The setting operated by the control unit12on the output voltage VAis lower than 500 ms, generally included between 200 and 300 ms.

The inductance13is connected in series to the external line9and is designed to attenuate an inrush current through the external line9. Thus, the external line9is designed to electrically connect the electrical connection member14to the power supply bus8.

The electrical connection member14is, for example, a pantograph and it is placed on the roof of the vehicle1. The pantograph14is designed to be mechanically controlled by a driver of the vehicle1. In particular, the pantograph14can be raised up or dropped down by means of a mechanic unit15which is placed on the roof of the vehicle1and which is electrically controlled by the driver of the vehicle1.

When the pantograph14is raised up, it is designed to electrically connect a supply line16to the external line9of the vehicle1. The supply line16is external of the vehicle1and part of an electrical power supply system18.

The electrical power supply system18is designed to supply a direct electric current to the traction unit5by means of the autonomous power supply device10and the control unit12or the supply line16. The electrical power supply system18includes an external power supply zone20and an autonomous power supply zone22. According to the direction of the vehicle1along the circulation rail2, the zone20and22follow one another.

The external power supply zone20includes the supply line16and a power supply infrastructure24.

The supply line16extends along the circulation rail2for connection with the pantograph14and is connected to the power supply infrastructure24.

The infrastructure24is designed to supply with electric energy the supply line16. In particular, the infrastructure24is designed to produce an output voltage VLon the supply line16. The output voltage VLhas a standard value which is, for example, equal or close to 750 V. The output voltage VLis designed to generate a direct electric current ILonly when the pantograph14is connected to the supply line16. In other words, the direct current ILflows in the supply line16, is absorbed by the pantograph14and is transferred to the traction unit5, when the pantograph14is connected to the supply line16at the output voltage VL.

As shown onFIG. 2, the supply line16includes a main section26, a terminal section28and a diode30. Therefore, the main section26, the terminal section28and the diode30are located in the external power supply zone20.

The main section26extends along the circulation rail2for connection with the pantograph14and is provided with a first end32. The main section26is permanently powered up at the output voltage VLof the supply line16which is produced by the infrastructure24.

The terminal section28extends along the circulation rail2for connection with the pantograph14and is provided with a second end34. The terminal section28has a floating voltage and is isolated except for the main section26and the pantograph14. Thus, the main section26and the pantograph14represent the only voltage sources for the terminal section28.

The first end32of the main section26and the second end34of the terminal section28are partially overlapped along the direction of the circulation rail2, defining therefore a transition portion36in the external power supply line20.

The diode30is designed to electrically connect the main section26and the terminal section28and to let passing through any electrical current from the main section26to the terminal section28. In particular, the diode30is connected between the first end32of the main section26and the second end34of the terminal section28. Along the supply line16, moving from the main section26towards the terminal section28, since the ends32and34are overlapped, the diode30is arranged backwards.

Then, the diode30is arranged to be short circuited between the first end32of the main section26and the second end34of the terminal section28by the pantograph14.

The electrical power supply system18includes moreover a detecting unit38and an emitter unit40.

The emitter unit40is placed on the zone20at a known distance from the transition portion36and from the terminal section28. The emitter unit40is designed to emit a radio signal flagging its presence along the zone20.

The detecting unit38is located in the vehicle1and is designed to receive the radio signal from the emitter unit40. The detecting unit38is provided with a data of the distance between the emitter unit40and the transition portion36and with a data of the distance between the emitter unit40and the terminal section28. When the detecting unit38receives the radio signal from the emitter unit40, it is designed to compute the position, along the supply line16, of the pantograph14in relation to the emitter unit40and, therefore, in relation to the transition portion36and the terminal section28. Moreover, the detecting unit38is provided with a data of the speed of the vehicle1. As it is known per se, knowing the position of the pantograph14and the speed of the vehicle1, the detecting unit38is designed to predict in time the position of the pantograph14along the line16. In particular, the detecting unit38is able to predict the instants when the pantograph14enters or quits the transition portion36and the terminal section28.

When the vehicle1is in the autonomous power supply zone22, it is the autonomous power supply device10which supplies the traction unit5in the direct current IA. In particular, the current sensor11measures no current on the external line9and pass this information to the control unit12. The control unit12, then, sets the output voltage VAof the device10at the standard value of voltage, at which is set the output voltage VL, so that the traction unit5is well supplied by the device10.

The operating of the electrical power supply system18will be described here-under for four different operating situations.

In a first operating situation shown inFIGS. 2 and 3, the vehicle1is in the traction mode which means that the traction unit5is supplied with energy to drive the vehicle1. It is located in the external power supply zone20. The pantograph14is raised up and connected only to the main section26of the supply line16. Moreover, the vehicle1is moving towards the terminal section28and, therefore, towards the autonomous power supply zone22. Since the vehicle1is the traction mode, the direct current ILis absorbed by the pantograph14.

Since the current sensor11measures a current flowing through the external line9, the control unit12sets the output voltage VAof the device10at a value VA1, which is lower than the output voltage VL. Thus, the device10is collecting current from the bus8to be charged.

The output voltage VLof the supply line16generates, as mentioned above, the direct current ILwhich is absorbed by the pantograph14and transferred through the external line9and the bus8to the traction unit5and the device10.

Since the pantograph14absorbs the direct current IL, the following portions of the supply line16which are subsequent to the pantograph14with respect to the motion of the vehicle1, such as a part of the main section26, the diode30and the terminal section28, are not crossed by the direct current IL.

In the motion of the vehicle1, the detecting unit38receives the radio signal from the emitter unit40and computes the instant when the pantograph14will quit the transition portion36.

In the instant when the pantograph14enters the transition portion36, the diode30is short circuited between the first end32of the main section26and the second end34of the terminal section28by the pantograph14.

Thus, the voltage of the terminal section28is the output voltage VLof the supply line16.

In the instant when the pantograph14quits the transition portion36, the diode30is no more short-circuited by the pantograph14. The pantograph14is now connected to the terminal section28of the supply line16and, since the output voltage VLis higher than the output voltage VA, the direct current ILof the supply line16pass through the diode30, flows in the terminal section28and is absorbed by the pantograph14.

However, in the instant when the pantograph14quits the transition portion36, the control unit12begins to vary the output voltage VAof the autonomous power supply device10. The control unit12sets the output voltage VAequal or higher than the output voltage VLof the supply line16, as shown inFIG. 3. In particular, the control unit12sets the output voltage VAat a value VA2, which is higher than the voltage VL.

This setting requires a time lower than 300 ms. Thus, while the output voltage VLof the supply line16is still higher than the output voltage VAof the device10, the direct current ILof the supply line16flows in the terminal section28and is absorbed by the pantograph14, as mentioned above.

On the opposite, when the output voltage VAof the device10is equal or higher than the output voltage VLof the supply line16, the direct current ILstops flowing. The output voltage VAof the autonomous power supply device10generates a direct current IAwhich flows towards lower voltage potentials, i.e. the traction unit5. Thanks to the diode30, the direct current IAdoes not flow through the pantograph14back to the supply line16and the discharge of the autonomous power supply device10is prevented. Then, the direct current IAis absorbed only by the traction unit5.

When the pantograph14quits the terminal section28of the supply line16and enters the autonomous power supply zone22, no current is flowing through the line16and the pantograph14. Therefore, no electric arc is generated between the supply line16and the pantograph14.

When the vehicle1is in the autonomous power supply zone22, the driver of the vehicle1commands the mechanic unit15to drop down the pantograph14.

In a second operating situation as shown inFIGS. 2 and 4, the vehicle1is in the braking mode which means that the traction unit5produces electrical power. It is located in the external power supply zone20. Moreover, the vehicle1is moving towards the terminal section28and, therefore, towards the autonomous power supply zone22. The pantograph14is raised up and connected to the main section26of the supply line16.

Since the vehicle1is in the braking mode, the traction unit5produces a voltage VBand the power supply bus8is powered up at this potential. Thus, a direct current IBis generated in the bus8and is used to charge the autonomous power supply device10and/or is absorbed by the power supply infrastructure24through the pantograph14and the supply line16. In fact, an electric current may flow from the pantograph14to the supply line16since the pantograph14is in contact with the main section26where there is no diode. The voltage VBon the power supply bus8has a value VB2which is higher than the output voltage VLof the supply line16.

In the motion of the vehicle1, the detecting unit38receives the radio signal from the emitter unit40and computes the instants when the pantograph14will enter and quit the transition portion36.

Then, before the pantograph14enters the transition portion36, the control unit12begins to vary the voltage VBon the power supply bus8. In particular, the control unit12sets the voltage VBat a value VB1, which is lower than the output voltage VLof the supply line16, by switching on the rheostat choppers.

Since the output voltage VBis lower than the output voltage VL, the direct current ILflows from the supply line16through the pantograph14to the traction unit5. In other words, during and small before the transition portion36, even in the braking mode, the traction unit5is supplied with the direct current ILof the supply line16, so that the bus8is powered up at the voltage VL.

For all the time while the pantograph14is moving along the transition portion36, the control unit12keeps the voltage VBlower than the output voltage VLof the supply line16, so that the bus8is powered up at the voltage VL.

As described above, moving along the transition portion36, the diode30is short circuited by the pantograph14. In the instant when the pantograph14quits the transition portion36, the diode30is no more short circuited by the pantograph14and the direct current ILof the supply line16pass through the diode30, flows in the terminal section28and is absorbed by the pantograph14.

However, in the instant when the pantograph14quits the transition portion36, the control unit12begins to vary the voltage VBon the power supply bus8. In particular, the control unit12sets the voltage VBat the value VB2, which is higher than the output voltage VLof the supply line16, by means of the rheostat choppers.

When the output voltage VLof the supply line16is still higher than the voltage VBon the bus8, the direct current ILof the supply line16flows through diode30and the terminal section28and is absorbed by the pantograph14.

On the opposite, when the voltage VBon the bus8is finally higher than the output voltage VLof the supply line16, the direct current ILstops flowing. The traction unit5supplies the rheostat choppers with the direct current IBgenerated by the electrical generative braking process of the motor4. Because of the diode30, the direct current IBfrom the traction unit5does not flow through the pantograph14back to the supply line16.

When the pantograph14quits the terminal section28of the supply line16and enters the autonomous power supply zone22, no current is flowing through the line16and the pantograph14. Therefore, no electric arc is generated between the supply line16and the pantograph14.

When the vehicle1is in the autonomous power supply zone22, the driver of the vehicle1commands the mechanic unit15to drop down the pantograph14.

In a third operating situation shown inFIGS. 5, 6 and 7, the vehicle1is in the traction mode and located in the autonomous power supply zone22. Moreover, the vehicle1is moving towards the terminal section28of the supply line16and, therefore, towards the external power supply zone20. The pantograph14is dropped down since the traction unit5of the vehicle1is supplied by the autonomous power supply device10with the direct current IA. As the current sensor11measures no current in the external line9and the real value of the output voltage VLof the supply line16is not known, the output voltage VAof the autonomous power supply device10is set by the control unit12at a value VA3which is a high voltage, for example, equal to 900V. Thus, the voltage VA3allows improving efficiency of the power supply system18.

Since the vehicle1is approaching the external power supply zone20, the driver of the vehicle1commands the mechanic unit15to raise up the pantograph14. In the motion of the vehicle1, the detecting unit38receives the radio signal from the emitter unit40and computes the instant when the pantograph14will be in contact with the terminal section28.

Once pantograph14is connected to the terminal section28of the supply line16, the output voltage VA, which is at the value VA3, can be higher or lower than the output voltage VL.

If the voltage VAis higher than the voltage VL, no electric arc is generated between the pantograph14and the supply line16. Then, before the pantograph14enters the transition portion36, the control unit12begins to vary the output voltage VA. In particular, the control unit12sets the output voltage VAat the value VA1which is lower than the output voltage VLof the supply line16.

If the voltage VAis lower than the voltage VL, an inrush current flows in the terminal section28and is absorbed by the pantograph14. The inductance13acts as a filter and attenuates the inrush current. The output voltage VAin the power supply bus8increases rapidly towards the voltage value of the output voltage VL. By means of the rheostat choppers, the control unit12sets the output voltage VAat the value VA1so that the direct current ILis absorbed by the pantograph14. In the case of a voltage peak of the output voltage VAin the power supply bus8which exceeds a fixed threshold VT, for example 1100V, as shown in dotted line onFIG. 7, the rheostats choppers acts as a crowbar circuit in order to regulate the output voltage VAat the value VA1.

When the output voltage VLof the supply line16became higher than the output voltage VAof the device10and the pantograph14has not yet entered the transition portion36, a direct current ILof the supply line16passes the diode30and is absorbed by the pantograph14. In other words, the traction unit5of the vehicle1is supplied by the supply line16.

When the pantograph14enters the transition portion36, the diode30is short circuited by the pantograph14and the diode30is passing the direct current ILof the supply line16.

When the pantograph14quits the transition portion36and enters the main section26, the output voltage VLof the supply line16is always higher than the output voltage VAof the device10and the vehicle1is supplied by the supply line16.

In a fourth operating situation shown inFIGS. 5, 8 and 9, the vehicle1is in the braking mode and located in the autonomous power supply zone22. Moreover, the vehicle1is moving towards the terminal section28of the supply line16and, therefore, towards the external power supply zone20. The pantograph14is dropped down and the real value of the output voltage VLof the supply line16is not known. Since the vehicle1is in the braking mode, the traction unit5produces the voltage VBand the power supply bus8is powered up at this potential. Thus, a direct current IBis generated on the bus8and is used to charge the autonomous power supply device10. The voltage VBon the power supply bus8has a value VB3which is a high voltage, for example, equal to 900V. Since the vehicle1is approaching the external power supply zone20, the driver of the vehicle1commands the mechanic unit15to raise up the pantograph14. In the motion of the vehicle1, the detecting unit38receives the radio signal from the emitter unit40and computes the instant when the pantograph14will be in contact with the terminal section28.

When the pantograph gets in contact with the terminal section28of the supply line16, no electric arc is generated between the pantograph14and the supply line16,

Once pantograph14is connected to the terminal section28, of the supply line16, the output voltage VB, which is at the value VB3, can be higher or lower than the output voltage VL.

If the voltage VBis higher than the voltage VL, no electric arc is generated between the pantograph14and the supply line16and the diode30prevents the direct current IBfrom flowing back to the supply line16. Then, before the pantograph14enters the transition portion36, the control unit12begins to vary the voltage VB. In particular, the control unit12sets the voltage VBat the value VB1, which is lower than the output voltage VLof the supply line16, by means of the rheostat choppers. When the pantograph14enters the transition portion36, the output voltage VLof the supply line16is higher than the voltage VBon the bus8and the diode30is short circuited by the pantograph14. The direct current ILof the supply line16is absorbed by the pantograph14. In other words, even in the braking mode, the traction unit5of the vehicle1is supplied by the supply line16. In the instant when the pantograph14quits the transition portion36, the control unit12begins to vary the voltage VBon the power supply bus8. In particular, the control unit12sets the voltage VBat the value VB2, which is higher than the output voltage VLof the supply line16, by means of the rheostat choppers. When the voltage VBon the bus8is higher than the output voltage VLof the supply line16, the direct current ILstops flowing. The traction unit5generates the direct current IBby means of the electrical generative braking process of the motor4.

If the voltage VBis lower than the voltage VL, the inrush current flows in the terminal section28and is absorbed by the pantograph14. The inductance13acts as a filter and attenuates the inrush current. The output voltage VBin the power supply bus8increases rapidly towards the voltage value of the output voltage VL. By means of the rheostat choppers, the control unit12sets the output voltage VBat the value VB1so that the direct current ILis absorbed by the pantograph14. When the pantograph14enters the transition portion36, the output voltage VLof the supply line16is higher than the voltage VBon the bus8and the diode30is short circuited by the pantograph14. The direct current ILof the supply line16is absorbed by the pantograph14.

In the instant when the pantograph14quits the transition portion36, the control unit12begins to vary the voltage VBon the power supply bus8. In particular, the control unit12sets the voltage VBat the value VB2, which is higher than the output voltage VLof the supply line16, by means of the rheostat choppers.

In all the operating situations, when the diode is crossed by the current IL, that lasts for a short time. The diode30, then, does not need any cooling system or adapted precaution device. The operating situations and the embodiments mentioned here-above can be combined in order to generate new embodiments and operating situations of the invention.