An arrangement and a method for providing e/ec- tric power to a vehicle TECHNICAL FIELD OF THE INVENTION AND PRIOR ART The present invention relates to an arrangement for providing electric power to a vehicle from an AC-supply line through a pantograph of the vehicle in contact with the supply line when the vehicle is moving therealong according to the preamble of the appended claim 1 as well as a method for providing electric power to such a vehicle.

The vehicle may be any vehicle receiving electric power through a line along which it is moving, but it is mostly a question of a railway vehicle, such as a locomotive for different types of trains, including trams.

The supply line normally carries a one-phase alternating voltage of 10-30 kV, but the invention is not restricted thereto.

The converter may be any known converter used for converting an alternating voltage into a direct voltage and conversely with the use of Pulse Width Modulation, and it is in the most simple case a two-level converter, but it may just as well have more levels of the voltage pulses on the alternating voltage side, such as for example three tevets in a HPC (Neutral Point) converter.

The word"equipment"is here used for all type of equipment on such a vehicle or in wagons or cars connected thereto driven by electric power : Such equipment may for example include an electric motor for driving the vehicle, for example an asynchro-

nous motor, and lightening, heating devices and so on needing electric power. However, it is pointed out that it is also possible that a part of this equipment receives electric power from an- other source, such as a battery, which may be charged through the supply line or when the vehicle is braking. In the case of for example an asynchronous motor a further converter, a so called machine converter, is arranged in the electric power flow path downstreams said converter for creating a three-phase alter- nating voltage used to drive the motor.

"Control members"as used here and in the claims is to be inter- preted broadly and comprises all types of means used for the control of electric power consumption on board of the vehicle or wagons connected thereto. Examples of such control members are members actuated by the driver of the vehicle for driving the vehicle (accelerating, keeping the speed, braking...) and differ- ent types of regulators actuated by hand or through sensors for heating the vehicle and/or wagons or cars connected thereto.

The feeding of electric power in such supply lines is carried out with an attempt to keep the power factor in a said vehicle close to 1 for reducing power losses. In a low load operation of the vehicle, i. e. when said power orders correspond to a power de- livered to said equipment being very low with respect to the maximum power that may be delivered from the arrangement thereto, the voltage on the opposite side of the transformer with respect to the pantograph will be nearly exactly the same as the voltage on the supply line. This results in a number of problems should the contact between the pantograph and the supply line be defective, which in the practice means that an insulating layer is present between the pantograph and the line. This means that these problems occur when the weather results in a risk of formation of ice on the supply line. In such a case the difference in voltage on the supply line and on the alternating voltage side of the converter, related to a primary side of the transformer, which is very small, will be applied as a driving

voltage across the ice layer. This means that there will be no voltage being high enough for driving a current through the ice during a period of time before and after a said zero crossing of the current. Accordingly, the current to the transformer and the converter may be distorted, and an arc may be created between the pantograph and the supply line resulting in an Emk, which in the case of a transformer being connected with a primary winding to the pantograph, will drive a direct current through the primary winding of the transformer, so that the transformer may be saturated resulting in a propagation of oscillations to the direct voltage on the direct voltage side of the converter in such a case. These oscillations may occur at frequencies used for the signal system of the railway infrastructure, so that devices for protecting said signal system may be tripped and switch said arrangement partly or entirely off. Furthermore, said control unit uses a measured value of the voltage on the supply line for the control of the converter and an ice layer on the supply line means interruptions of such a supply line voltage signal, so that the voltage signal obtained through measuring the voltage on the pantograph will instead correspond to a voltage generated by the converter resulting in severe problems to control the converter, so that the driving of the vehicle may be uncomfortable or even fail totally. There is also a risk of tripping other protection devices on board of the vehicle, such as a device protecting the equipment of the vehicle against a short- circuit on the secondary winding side of the tranformer in the case described above of the transformer located between the pantograph and the converter, since the ice layer on the supply line may have the same influence on parameters used by such a protection device as a real such short-circuit, so that the protection device may trip falsely and thereby unnecessarily disturb the operation of the vehicle.

It is pointed out that the power flow through said converter may go in both directions, so that power may flow from the vehicle to

the supply line when the vehicle is braking, and the problems mentioned and the invention applies to both cases.

SUMMARY OF THE INVENTION The object of the present invention is to provide an arrangement and a method of the type defined in the introduction at least re- ducing the problems discussed above.

This object is according to the invention obtained by providing such an arrangement in which the control unit is adapted to use a reference current value having a reactive current component, at least when there is a risk of formation of ice on the supply line and said power orders received correspond to a substan- tially lower power delivered to said equipment than the maxi- mum power that may be delivered from the arrangement thereto, for controlling the converter to cause a reactive current to flow between the supply line and the vehicle.

The addition of such a reactive current component to the refer- ence current value used when controlling the converter results in a difference between the alternating voltage of the supply line and the voltage on the alternating voltage side of the converter, related to a primary side of the transformer, which is important at zero crossing of the alternating current flowing between the supply line and the vehicle during icy conditions. It is pointed out that when the alternating voltage side of the converter is connected to the secondary side of the transformer the ratio of the transformer will of course result in an always existing difference of the voltage on the supply line and on the alternating voltage side of the converter, but a said driving voltage is only obtained when there is a voltage difference when relating the voltage on the alternating voltage side of the converter to the primary side of the transformer. Such a difference is obtained by adding a reactive current component to said reference current value. This voltage difference is then applied

as a driving voltage across the insulating ice layer, and when it is sufficiently high the current will break through the insulation layer, so that the pantograph has a continuous contact with the line. Accordingly, the problems discussed above will not even occur. Although it is only necessary to add a reactive current component to the reference current value when there is a risk of formation of ice and at the same time said power orders are low in comparison with a maximum power order, it is pointed out that the invention is not restricted to the case of adding such a reac- tive current component only then. These are only minimum crite- ria, and the control unit may very well be adapted to use such a reactive current component always when there is a risk of for- mation of ice independently of the magnitude of said power or- ders or always when the power orders received correspond to a substantially lower power delivered to said equipment than the maximum power that may be delivered from the arrangement thereto irrespectively of the existence of any risk of formation of ice on the supply line for making the arrangement very simple.

The control of the converter is unchanged with respect to the traditional control except for a reactive power consumption at lower loads and risk of ice formation.

According to a preferred embodiment of the invention the ar- rangement further comprises means adapted to measure the temperature outside the vehicle and means adapted to compare temperature values so obtained with a temperature threshold value and when a measured temperature value is above said temperature threshold value send a signal to the control unit to use a reference current value without said reactive current com- ponent for the control of the converter. Said comparing means is adapted to have a temperature between +2°C and +7°C, pref- erably between +4°C and +6°C, as said temperature threshold value. It is advantageous to in this way refrain from using a re- active current component in the reference current value for the control of the converter when there is no need of using such a

component, so that reactive currents resulting in losses and not desired in the supply line may be avoided when not necessary.

According to another preferred embodiment of the invention the control unit is adapted to use a reference current value having a reactive current component for the control of the converter un- der the condition that said power orders received correspond to a substantially lower power delivered to said equipment than the maximum power that may be delivered from the arrangement thereto. According to another preferred embodiment of the in- vention said condition of a low power ordered for the control of the converter means that the power ordered shall be lower than 0.5, advantageously 0.3 and preferably 0. 2 times said maximum power. The difference between the voltage of the supply line and on the opposite side of the tranformer will be dependent on the load (the power ordered) so that there will be no need of any addition of a reactive current component to the current reference value when the power ordered is high enough. Thanks to the in- troduction of this condition the supply line will not be unneces- sarily subjected to reactive currents.

According to another preferred embodiment of the invention the control unit is adapted to use a reference current value having a reactive current component with a magnitude increasing the difference of the voltage of the supply line and the voltage on the alternating voltage side of the converter, related to a primary side of the transformer, at zero crossing of the alternating current from the supply line to the vehicle substantially with <BR> <BR> respect to an absence of this reactive current component, and the control unit is preferably adapted to use a said reactive current component with a magnitude resulting in a said voltage difference exceeding 1 kV at said zero crossing. An ice layer may hold a voltage of about 1 kV/mm, and mostly a voltage dif- ference with a magnitude in the order mentioned will be enough for ensuring that there will be a drastic reduction of"jumps"to only"micro jumps"in the contact between the supply line and

the pantograph and thereby in the current from the supply line to the vehicle.

According to another preferred embodiment of the invention the control unit is adapted to use a reference current value having a reactive current component decreasing with an increase of said power ordered. Said voltage difference at zero crossing of the current may still be kept on a sufficiently high level when the power ordered increases, so that no unnecessarily high reactive current is flowing in the supply line.

According to another preferred embodiment of the invention the arrangement comprises a transformer which has a primary winding connected to said pantograph and the converter has an alternating volt- age side thereof connected to a secondary winding of the transformer and is adapted to convert the alternating voltage of said secondary winding into a direct voltage on a direct voltage side of the converter and the arrangement comprises means adapted to measure the di- rect voltage on the direct voltage side of the converter and de- liver information thereabout to the control unit, and the control unit is adapted to use a reference current value having a reac- tive current component when the measured direct voltage has alternating voltage components above a determined level for one or more determined frequencies. It has been found that the quality of the direct voltage on the direct voltage side of the converter is a very good measure of the performance of the converter control and that deficiencies of this direct voltage in- dicates a degraded contact between the supply line and the pantograph and thereby a need of an addition of a reactive cur- rent component to the reference current value used for the con- trol of the converter.

According to another preferred embodiment of the invention the arrangement is adapted to be arranged in each individual vehi- cle when a plurality of such vehicles are directly or indirectly connected to each other in a train and to operate independently

of such arrangements in the other vehicle or vehicles in the train. This results in a very simple and by that cost efficient con- struction.

The invention also relates to a method according to the ap- pended method claims. The results of carrying out such a method and the preferred features thereof appear without any doubt from the above description of different embodiments of an arrangement according to the invention.

Furthermore, the invention also relates to a computer program and a computer readable medium according to the correspond- ing appended claims. It is easily understood that the method ac- cording to the invention defined in the appended set of method claims is well suited to be carried out through program instruc- tions from a processor that may be influenced by a computer program provided with the program steps in question.

Further advantages and preferred characteristics of the inven- tion appear from the following description and the other depend- ent claims.

BRIEF DESCRIPTION OF THE DRAWINGS With reference to the appended drawings, below follows a spe- cific description of preferred embodiments of the invention cited as examples.

In the drawings : Fig 1 is a very schematic view illustrating the main features of an arrangement according to the invention, Fig 2 is a simplified view used to explain the control of a con- verter shown in Fig 1,

Figs 3 and 4 are diagrams illustrating the voltage and the cur- rent of the supply line versus time for an arrangement according to Fig 1 when there is no ice on the supply line, Figs 5 and 6 are views corresponding to Figs 3 and 4, respec- tively, under icy conditions, Fig 7 is a diagram showing the voltage of the supply line and the current between the supply line and the vehicle versus time for an arrangement according to Fig 1 under icy conditions and when a reactive current component is added to the reference current value used for the control of the converter, Fig 8 is a very simplified diagram showing the voltage on the supply line and that on the alternating voltage side of the converter, related to the primary winding of the transformer, ver- sus time when low power is ordered for the control of said con- verter but with an addition of a reactive current component to said reference current value, Fig 9 illustrates schematically the magnitude of the reactive cur- rent in the supply line versus real power flowing between the supply line and the vehicle when using an arrangement accord- ing to the present invention, Figs 10 and 11 are diagrams illustrating the power factor versus real power for the case of driving the vehicle and braking the vehicle, respectively, when using a control scheme as indicated in Fig 9, Fig 12 is a diagram illustrating the direct voltage on the direct voltage side of said converter versus frequency under icy condi- tions according to the prior art, and Fig 13 is a view corresponding to Fig 12 when adding a reactive current component to said reference current value for the control

of the converter during icy conditions according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION A railway vehicle as well as an arrangement for providing elec- tric power thereto are very schematically illustrated in Fig 1. The vehicle 1 is arranged to move along an AC-supply line 2, which may for example carry a one-phase alternating voltage of 15 kV and 16 2/3 Hz (Sweden) and 25 kV and 50 Hz (Denmark). The vehicle is provided with a so-called pantograph 3 in contact with the supply line when the vehicle is moving therealong. However, it is here shown that there exists a distance between the supply line and the pantograph as a consequence of an insulating ice layer 4 covering the supply line. Such a layer could have a thickness of about 1 mm.

The pantograph is connected to a so called main transformer 5 (a number of other transformers may be arranged in the electric system of the vehicle), which is used for obtaining galvanic separation between the pantograph and the electric system of the vehicle and for transforming the voltage on a primary wind- ing 6 thereof to another level on its secondary winding 7 and for making it possible to obtain a control of the power flow between the supply line and the vehicle through controlling a converter 8 connected to said secondary winding.

The converter 8 is adapted to convert the alternating voltage of said secondary winding to a direct voltage on a direct voltage side of the converter. It is illustrated in Fig 2 what such a con- verter in the most simple form may look like. This converter has only one phase leg 9 with two current valves 10,11 connected in series. The secondary winding of the transformer constituting the alternating voltage side of the converter is connected to a midpoint 12 between the two current valves. Each current valve

is constituted by at least one controllable semiconductor device 13, such as an IGBT (Insulated Gate Bipolar Transisior)-or a GTO (Gate Turn-off Thyristor) and a rectifying diode 14 con- nected in anti-parallel therewith. However, it is pointed out that the symbols used in Fig 2 may stand for a plurality of such con- trollable semiconductor devices and diodes connected in series, in which case the semiconductor devices of the same current valve will be controlled simultaneously so as to act as one single semiconductor device. The opposite ends of the phase leg 9 are connected to a positive 15 and a negative 16 pole of a direct voltage side of the converter, and the voltage between these two poles is held at a substantially constant level by a capacitor 17 connected therebetween. The arrangement also comprises a control unit 18 adapted to control the semiconductor devices 13 of the current valves to alternatively turn on and turn off for generating a train of pulses according to a Pulse Width Modula- tion pattern on said midpoint 12.

The control of the control unit takes place in accordance with power and current reference values determined on the basis of power orders received from one or more control members, such as members controlled by the driver of the vehicle. The direction and the magnitude of the power flow through the converter may be regulated by the way of controlling the converter through the control unit. This is well known to persons with skill in the art.

It is shown in Fig 1 how the direct voltage side of the converter 8 may be connected to a so called machine converter 19 con- vert the direct voltage into a three-phase alternating voltage used to drive an asynchronous motor 20 for driving the vehicle.

The machine converter 19 may also be connected to other equipment 21, such as lightening, heating devices and so on, on board of the vehicle, or this equipment may be connected to the direct voltage side of the converter 8 through a separate con- verter for this equipment. A control member 22 for controlling the machine converter 19 and thereby the motor 20 is schemati-

cally indicated. This control member also sends signals to the control unit 18 to be used when deciding the control scheme of the converter 8. The arrangement also comprises means 23 adapted to measure the voltage on the supply line indirectly by measuring the voltage on the pantograph and deliver information thereabout to the control unit 18, which is adapted to base the control of the converter 8 on this measured voltage. The equip- ment 21 is also adapted to send signals informing about the power need of the equipment to the control unit 18.

The arrangement described so far is already known. Figs 3 and 4 show the voltage U and the current 1, respectively, on the sup- ply line to the vehicle versus time when a vehicle having the ar- rangement just described moves along a supply line having no ice thereon. Figs 5 and 6 are diagrams corresponding to those shown in Figs 3 and 4 obtained when driving along a supply line having ice thereon. It is shown in Fig 6 how the current is clearly distorted due to the ice. The appearance of such a current de- pends on a too low driving voltage across the ice layer at zero crossings of the current resulting in the problems discussed in the introduction.

The new features of the arrangement shown in Fig 1 belonging to the present invention and through which these problems are solved will now be described. The control unit 18 in the ar- rangement according to the present invention is adapted to use a reference current value having a reactive current component, at least when there is a risk of formation of ice on the supply line and said power orders received correspond to a substan- tially lower power delivered to said equipment than the maxi- mum power that may be delivered from the arrangement thereto, for controlling the converter 8 to cause-a reactive current to flow between the supply line 2 and the vehicle 1. Fig 7 corresponds to a combination of Fig 5 and Fig 6, but a reactive current com- ponent is here added to said reference current value for the control of the converter according to the present invention. It is

shown how the current has now a nearly ideal shape, so that the primary winding of the transformer will not be saturated any longer and oscillations of the direct voltage on the direct voltage side of the converter 8 may be heavily reduced to a level not disturbing the signal system of the vehicle any longer.

Fig 8 schematically illustrates how the invention displaces the voltage UT on the alternating voltage side of the converter 8 related to the primary side of the transformer with respect to that Us on the supply line for low powers ordered, so that there will be a substantial driving vo) tage Ui applied across the ice layer 4 at the zero crossing of the current. I is the current from the supply line. In this case the power factor is rather low with nearly no real power transferred.

The arrangement according to the invention comprises means 24 adapted to measure the temperature outside the vehicle and means 25 adapted to compare temperature values so obtained with a temperature threshold value and when a measured tem- perature value is above said temperature threshold value send a signal to the control unit 18 to use a reference current value without said reactive current component for the control of the converter avoiding that a reactive current is flowing in the supply line when there is no need therefor. Said temperature threshold value is preferably set to be between +2°C and +7°C, preferably between +4°C and +6°C, for instance +50C.

The control unit 18 is preferably also adapted to use a reference current value having a reactive current component for the con- trol of the converter 8 under the condition that said power orders received correspond to a substantially lower power delivered to said equipment than the maximum power that may be delivered from the arrangement thereto. This is schematically shown in Fig 9, in which it is shown for a supply line carrying 25 kV of a frequency of 50 Hz how the reactive current component changes with the real power P delivered from the arrangement to the

equipment of the vehicle. The reactive current component is substantially constant for power orders being nearly zero to a certain value Pi as of which the reactive current decreases con- tinuously with increasing power ordered to be zero at a power order ? 2. Pi is here 200 kW and P2 500 kW, whereas the maxi- mum power that may be delivered from the arrangement to the vehicle is about 3 MW, i. e. 6 times higher than P2. Accordingly, a reactive current component is only added to the reference cur- rent value for the control of the converter 8 under low load con- ditions, so that the maximum current through the arrangement will not be influenced by the control scheme according to the in- vention.

Figs 10 and 11 illustrates the development of the power factor cos (p, (p being the angle between the current from the supply line to the vehicle and the voltage on the supply line, versus the real power P for the control of the converter 8 carried out while add- ing a reactive current component in accordance with Fig 9 for the case of driving operation (Fig 10) and braking operation (Fig 11) of the motor 20. It is shown how the power factor will be nearly 1 as soon as the power flow through the converter 8 reaches any substantial value.

The arrangement according to the present invention also com- prises means 26 adapted to measure the direct voltage on the direct voltage side of the converter 8 and deliver information thereabout to the control unit 18, which is adapted to use a ref- erence current value having a reactive current component when the measured direct voltage has alternating voltage components above a determined level for one or more determined frequen- cies. Such alternating voltage components will indicate a poor contact between the supply line and the pantograph and may re- sult in tripping of a so-called LIWA (Line Interference Monitor) protecting the signal system of the vehicle. Peak values of the direct voltage Udc on the direct voltage side of the converter 8 versus the frequency f are shown in Fig 12 for a normal control

of the converter 8 without any addition of a reactive current component to the current reference value during icy conditions.

This diagram is for a 15 kV supply line with the frequency of 16 2/3 Hz. There is a peak in the direct voltage around 44 Hz, which is almost exactly the resonance frequency of the direct voltage link 15-17 of the converter. This peak has a value of about 40 V. The corresponding diagram for the case of an addi- tion of a reactive current component of 30 A to the current refer- ence value for controlling the converter 8 is illustrated in Fig 13.

It is shown how the oscillations of the direct voltage have de- creased considerably, and the peak around 44 Hz is now less than 12 V.

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

The control unit may for example be adapted to use a reference current value having a reactive current component for a prede- termined time period, for example at least during 0.5 hour, and preferably during at least 1,5 hours, as of a condition for such a control of the converter is detected to be satisfied. This means that when for example a risk of ice formation on the supply line has once been detected, a reactive current component will be added to the reference current value for the control of the con- verter during said predetermined time period, possibly under the condition that the power ordered is at the same time at a low level.

Although it has been shown in Fig 1 how the transformer is lo- cated in said series connection closer to the pantograph than the converter the invention is not restricted thereto, but the con- verter may just as well be located between the transformer and the pantograph.

In the case of two or more vehicles being connected directly or indirectly to each other in a train an arrangement according to the present invention may be arranged in each individual such vehicle and to operate independently of such arrangements in other vehicle or vehicles in the train.

It is also possible that the arrangement has more than one con- verter connected to a said transformer, and the control unit may then be adapted to use a reference current value having a reac- tive current component shared equally by the converters.