WO2000073102A1

WO2000073102A1 - A device for generating a one-phase alternating voltage

Info

Publication number: WO2000073102A1
Application number: PCT/SE2000/001058
Authority: WO
Inventors: Bertil Klerfors; Lars Nilsson; Thorsten Schütte; Sture Lindahl
Original assignee: Abb Ab
Priority date: 1999-05-27
Filing date: 2000-05-25
Publication date: 2000-12-07
Also published as: AU5434200A; SE9901938L; SE9901938D0; SE513655C2

Abstract

A device for generating a one-phase alternating voltage symmetrical with respect to ground comprises an alternating voltage generator (1) having four phases (11-14) with a shift of 90 electrical degrees. Two first (11, 13) of the phases having a mutual phase shift of 180° are provided with a terminal (15, 16) each for delivering said one-phase alternating voltage thereon. Reactances (17, 18) are connected to the phases of the generator for obtaining that a load connected to the generator will be substantially balanced and phase compensated, so that the currents in the four phases of the generator will be substantially just as high and substantially in phase with the voltage of the respective phase.

Description

A device for generating a one-phase alternating voltage

FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a device for generating a one- phase alternating voltage symmetrical with respect to ground.

The invention relates to generation of such a one-phase alter- nating voltage symmetrical with respect to ground in general, without any restriction with respect to magnitude or frequency of the voltage, and such devices may have all types of applications, but the generation of a one-phase alternating voltage for railway vehicles will be discussed hereinafter for illuminating but not in anyway restricting the invention and the problem to be solved thereby.

Such a one-phase alternating voltage has so far mainly been generated according to any of the following two concepts. The first consists in utilising an electric one-phase machine in generator operation, which may be driven by for example a motor or a water turbine, but this is associated with some serious disadvantages. High currents are necessarily induced in the damping windings, which such a rotor has to be provided with, in operation of such a machine as a consequence of the asymmetry of the one-phase case, which complicates the construction of the rotor but primarily causes considerable power losses. A great problem of such one-phase machines is constituted by the mechanical load dominated by a pulsating moment giving the machine substantial extra strains. Another disadvantage of a one-phase machine is the bad utilisation of the stator. A second concept is to generate a three phase alternating voltage through a rotating machine and then convert this into a one- phase alternating voltage through a so-called static converter device, but also this solution has considerable disadvantages. One of these disadvantages is that such a static converter device is costly. Another disadvantage consists in that the power losses of such a static converter are substantial. Another disadvantage is that particular transformers are required for static converters, since they may not handle the voltages normally asked for for example within the railway field, and such transformers are associated with high costs.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a device of the type defined in the introduction, which find a remedy to a large extent to the inconveniences mentioned above of such devices already known.

This object is according to the invention obtained by providing such a device with an alternating voltage generator with four phases with a shift of 90 electrical degrees, that two first of said phases having a mutual phase shift of 180° are provided with a terminal each so as to deliver said one-phase alternating voltage thereon, and that the device also comprises reactances (17,18) connected to the phases of the generator for causing a load connected to the generator to be substantially balanced and phase compensated, so that the currents in the four phases of the generator will be substantially just as high and are substantially in phase with the voltage of the respective phase.

Through this design of the generator with reactances for balancing and compensation a number of advantages are obtained. One of these is that the stator may be fully utilised. Another advantage is that it is possible to obtain a full or at least a far reaching compensation through a suitable dimensioning of said reactor meaning that the generator rotates evenly and delivers none-pulsating power on the four phases thereof.

It has turned out that exactly this combination of a four phase generator and the interconnection of the different phases in the way defined in the analogy to the Steinmetz connection has considerable advantages with respect to the utilisation of other generators for direct generation of one-phase alternating volt- age. A utilisation of a two-phase generator with the phases connected to one said terminal each for the one-phase alternating voltage symmetrical to ground would for example result in very high power pulsation on the output of the device. A use of a three-phase alternating voltage generator and connection of two of the phases thereof to one said terminal each for the one- phase alternating voltage and connecting the third phase to any of the first phases through a reactance would mean that the two voltages tapped would have a phase shift of 120°, so that a theoretical midpoint between these phases gets a voltage with respect to ground. This can be handled by grounding the midpoint on the winding of a transformer connected to the generator, so that the voltage is forced to a phase shift of 180° with the midpoint on ground potential. However, this requires the utilisation of a costly transformer in combination with the generator.

According to another preferred embodiment of the invention the device comprises means for regulating the capacitance or the inductance of at least one of said reactances. By providing such a possibility to regulate the reactances the device may ensure that a desired reactive compensation for delivering a power having small pulsations may be obtained independently of larger variations existing of the power output from the device.

According to another preferred embodiment of the invention at least one of said reactances is fixed and the device comprises a damping winding. This is a solution being favourable from the cost point of view for a device being used for a substantially constant power output, since it may be sufficient to have fixed reactances and that the variation around an average power for which these reactances are dimensioned is taken care of by such a damping winding, which may be made considerably smaller than the damping winding to be used for a conventional one-phase alternating voltage generator.

According to another preferred embodiment of the invention said two first phases are at one end thereof opposite to said terminals at a point in common connected to ground. It will by this be possible to achieve by simple means that the voltages generated on said terminals will be symmetrical with the respect to ground.

According to another preferred embodiment of the invention all four phases are at their ends opposite to said terminals at a point in common connected to ground. A four phase generator included in such a device will be simple to the construction.

According to another preferred embodiment of the invention the four phases are in pairs connected in series with the two first phases connected to the respective terminal through another, second phase each. An advantage of this embodiment is that the number of winding turns per phase is reduced by a factor of the square rot of two. A further advantage of this embodiment is that the windings of the two first phases may be provided with an insulation being only dimensioned for a voltage in the order of 71 % of the voltage delivered on said two terminals. Thus, in this case the two phases being "innermost", i.e. connected to ground, are considered to constitute said first phases, and these are considered to be connected to said terminals, although not directly, but through the second phases.

According to another preferred embodiment of the invention the device is adapted to be directly connected to a one-phase alter- nating voltage network through said two terminals, and this is possible thanks to no need of any transformer for ensuring that the voltage will be symmetrical with respect to ground. It is then possible to achieve a direct generation of a one-phase alternat- ing voltage to such a network for high voltages, such as higher than 10kV and for example between 20 and 100kV, if according to a preferred embodiment of the invention a high voltage generator of the type with winding of a cable is used. A high voltage generator of the type described in WO 97/45919 of the applicant is preferably utilised, which thanks to the use of a cable in the form of an electric conductor having an envelope being able to enclose the electric field created around the conductor, may be designed to generate high voltages, such as well in the order of that of the feeding lines running in parallel with a contact line for railway vehicles, such as for example in Sweden normally 66kV, i.e. a voltage of 132kV between the supply line connected to one terminal of the device and the return line connected to the other terminal. The feeding line may also be comprised in a so called auto transformer system, in which the contact line and possibly conductors connected in parallel therewith, for example on 16,5kV are the supply lines of the device and a so called negative feeder in an opposite phase thereto is the return line of the device. The supply line, the return line and the rail are connected to auto transformers located along the railway. For ex- ample a 2^*16,5 = 33kV-system is obtained. Thus, the invention is particularly interesting in connection with utilising such a generator, which may generate directly to the network, since the device enables omitting the transformer.

According to another preferred embodiment of the invention a generator is adapted to deliver a one-phase alternating voltage having a frequency of 50Hz, 60Hz, 25Hz or 16 2/3Hz to said terminals, which are usual frequencies of feeding lines for railway vehicles. The invention is particularly interesting when there is a desire to generate a one-phase alternating voltage having a frequency being lower than the frequency of the voltage on the public alternating high voltage transmission network for electric power, such as for example in Sweden, where the desired one- phase alternating voltage for feeding railway vehicles is 16 2/3Hz, while the transmission networks conduct voltages having a frequency of 50Hz. This depends on the fact that at such differing frequencies frequency converters are required, in the case that no separate device having a generator for delivering a one-phase alternating voltage is used for obtaining such a voltage, and by that a device of the type according to the invention is particularly interesting at such a differing frequency. In the case of the same frequency of the one-phase voltage as on the public transmission network, it will be possible to manage without any frequency converter, without the use of any device for separate generation of a one-phase alternating voltage.

According to another preferred embodiment of the invention a rotor of the alternating voltage generator is arranged to be driven to rotation by wind power. A utilisation of a wind power mill in a device of this type enables direct generation of a one- phase alternating voltage having a low frequency, since such a rotor has a low number of revolutions, and tremendously many poles thereof or a costly gearbox would be required at demands on higher frequencies. Rotating one-phase machines already known are not conceivable exactly in the case of wind power, since it is in wind power mills very unfavourable to have a pulsating power, since a low vibration level is required for avoiding that different parts of the wind power mill is shaken apart. The way according to the present invention to obtain a strongly reduction of power pulsations enables by that the utilisation of wind power for generating a one-phase voltage having a frequency, for which the wind power is particularly suited.

Further advantages as well as advantageous features of the invention appear from the following description and the other de- pendent claims. BRIEF DESCRIPTION OF THE DRAWINGS

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

In the drawings:

Fig 1 is a schematic and simplified view illustrating the general construction of a device according to a first preferred embodiment of the invention used for generating a one-phase alternating voltage to a feeding line for feeding electric power through a contact line to railway vehicles,

Fig 2 is a view similar to fig 1 of a device according to a second preferred embodiment of the invention,

Fig3a and 3b are views similar to that of fig 2 of devices according to further preferred embodiments of the invention,

Fig 4 illustrates the appearance of the instantaneous power, which may be delivered by a two-phase generator for generating a one-phase alternating voltage being symmetrical to ground,

Fig 5 is a view corresponding to fig 4 for a four-phase generator of the type according to the invention,

Fig 6 illustrates how in steps (in one step and in two steps, respectively) reactive compensation may be switched in for a device according the invention at different levels of the active power delivered by the device, in which the load current is drawn in function of the active power, Fig 7 illustrates rotor losses of a four-phase generator of a device according to the invention in function of the active power delivered at the stepwise connection of reactive compensation according to fig 6 with respect to the case of no such reactive compensation,

Fig 8 is a schematic view illustrating a high voltage generator of the type with winding of a cable, which ad- vantageously is used as four-phase generator in the devices according to the invention, and

Fig 9 shows the construction of a cable used in a high voltage generator according to fig 8.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

It is schematically illustrated in fig 1 how a generator 1 of a device according to a first preferred embodiment of the invention delivers a one-phase alternating voltage being symmetrical with respect to ground to a feeding line 2 having two conductors 3, 4 with a voltage therebetween of 132kV (+66kV -(-66)kV), which runs to railway feeding transformers 6 having a grounded midpoint 7 and arranged along a railway 5, said transformers having a secondary winding 8 connected with one end thereof to a contact line 9 for feeding electric power to railway vehicles 10 and with the other end thereof to ground. The transformer 6 de- livers for example a voltage of 16,5kV to the conduct line 9. The point of the high voltage on the feeding line 2 is that the transmission losses will be lower.

It is illustrated what a very simple embodiment of the device ac- cording to the invention may look like, and this has an alternating voltage generator having four-phases 1 1 -14 with a shift of 90 electrical degrees. A phase is here defined as one or several windings of the stator of the generator, which generates a voltage of a certain phase position. Two first 1 1 , 13 of the phases having a mutual phase shift of 180° are provided with a terminal 15, 16 each for delivering said one-phase alternating voltage thereupon. The two second phases 12, 14 are through one reactance each in the form of a capacitance 17, 18 connected to one of said terminals each for obtaining a balancing of the load on the four phases of the generator. The dimensioning of the capacitancies 17 and 18 is chosen while considering the normal power output from the device, a damping winding not shown may possibly be arranged for taken care of variations of the power output around this average power. The four phases are in the point 19 in common connected to ground. The phases 12, 14 are accordingly phase shifted with respect to each other by 180° and furthermore phase shifted by 90 degrees with respect to the two first phases 1 1 , 13. By the fact that all phases have one end point connected to a point in common, which may be required, a one-phase voltage symmetrical with respect to ground may be tapped between the free end points of the two first phases. By connecting reactive components in this way between different phase outputs of the generator, it is achieved that the load on the generator will be phase compensated and balanced, i.e. the currents in the four phases of the generator will be just as high and in phase with the respective phase voltage.

The instantaneously power P generated by a device of this type is drawn versus the time t in the graph of fig 5. It appears that the power pulsations are comparatively small, especially com- pared with the very large power pulsations of a two-phase generator without reactive compensation in the instantaneous power P according to fig 4. It is pointed out that when dimensioning the reactances consideration is preferably also spent on the reactances in the lines from the generator, in which these in the form of air lines have primarily a series inductance and in the form of cables primarily a shunt capacitance. The two connections 15 and 16 may be considered to be located quite a distance from the phases of the very generator and reactances of intermediate conductor portions are part of the reactances utilised for said balancing and phase compensation.

A device according to a second preferred embodiment of the invention is schematically illustrated in fig 2, and it differs from the one illustrated in fig 1 by the fact that the two capacitances 17, 18 may here be regulated and that an inductance 20 which also may be regulated, is connected in parallel with the two second phases 12, 14 between the ground point 19 and the opposite end of the phase. By providing both inductances and capacitances in this way a desired phase shift and possibility to a reactive compensation adapted to a varying power output on the terminals 15, 16 is obtained.

A device according to a third preferred embodiment of the invention is illustrated in fig 3a, in which both first phases 1 1 , 13 of the four phase generator are connected to a ground point 19 in common with one end thereof and with its second end to the terminal 15, 16 in question for delivering the one-phase alternating voltage through a series connection with a second phase 12 and 14, respectively. Reactances for said balancing of the load on the four phases of the generator are present in the form of a capacitance 22 connected in parallel with the two first phases 1 1 , 13 between the connection thereof to the respective second phase_, and an inductance 23, 24 connected in parallel with the respective second winding 12 and 14, respectively. The two capacitances of previous embodiments could here be sum- marised by one single capacitance. In this embodiment the two first windings 1 1 , 13 also only need to have an insulation managing 71 % of the voltage with respect to the environment with respect to what the two second phases 12, 14 here and all phases of the embodiment according to fig 1 and 2 have to have. Thus, in the case mentioned above these two windings may be provided with a cable having an insulation being in- tended for 47kV, while the windings of the phases 12 and 14 are provided with a cable dimensioned for 66kV. Furthermore, the number of winding turns per phase is reduced by the factor 0,707.

An embodiment according to the same principle as the one shown in fig 3a is illustrated in fig 3b. This differs from the previous one only by the fact that capacitances 46, 47 are here connected in parallel with the second phases 12, 14 and the in- ductance 48 is instead connected in parallel with the series connection of the two first phases 1 1 , 13.

As already discussed further above, it is particularly advantageous to utilise a device according to the invention when gener- ating a one-phase alternating voltage through wind power. The generator of the rotor may in such a case be provided with for example 16 poles, which would then require a rotational speed of 2,08 revolutions/second (2,08Hz) or 24 poles with 1 ,39Hz, which are typical numbers of revolutions for large wind power mills. Thus, it would in such a case be possible to manage without a gearbox and obtain direct generation of a one-phase alternating voltage having a frequency of 16 2/3Hz. By the possibility to reduce one-phase power fluctuations of a device according to the invention to such a large extent as defined above, it is possible to utilise the generator in wind power connections, where only small vibrations may be allowed.

It is illustrated in fig 6 how it its possible to switch in reactive compensation stepwise in connection with a four phase gen- erator according to the invention, in which the load current I is shown in function of the active power P. The curve 25 shows the load current, while the line provided with squares show how a one-phase compensation takes place by switching in a reactance when the active power reaches a third of the rated power. The reactive compensation has a capacity of two thirds of the rated power of the generator. The line 27 provided with triangles shows how a reactive compensation takes place in two steps by connecting one or two reactive compensation devices when the active power delivered reaches 20% and 60%, respectively, of the rated power, which both have a capacity of 45% of the rated power of the generator.

It is illustrated in fig 7 how such a compensation in one and two steps, respectively, influences the rotor losses P_R of a four phase generator according to the invention in the case of a purely ohmic load. The rotor losses R are shown in function of the active power P delivered in relation to the rated power for the reference case 28 of no compensation and the cases of one step compensation 29 and two step compensation 30, respectively, according to fig 6. It appears that one step compensation reduces the rotor losses by about 1 1 % of the rotor losses at full power in the reference case, while the two step compensation reduces the rotor losses by about 4% of the rotor losses at full power in the reference case. Thus, already through a one step compensation a highly considerable reduction of the rotor losses is obtained.

As already mentioned above the invention is particularly advantageous when using a generator enabling direct generation of a one-phase alternating voltage of the magnitude to be utilised, usually the voltage required on a feeding line to a contact line for transmitting electric power to railway vehicles or to the contact line, and such a generator for the high voltage case is already known through WO 97/45919 of the applicant, and the general construction thereof is very schematically shown in fig 8. The rotor 31 is shown very schematically with two rotor poles 32, 33 (it may in the practise have more) and the stator 34 have elements 35, in which a magnetic circuit is formed. The stator is in a conventional way composed by a laminated core of electric sheet successively composed by plates of sector-shape. A num- ber of teeth 37 extend from a back portion 36 of the core located radially outermost gradually inwardly towards the rotor. A corre- sponding number of slots 38 are arranged between the teeth. The slots receive a winding of layers of cables 39 running substantially axially and arranged radially outside each other. The cables 39 comprise an inner conductor 40 consisting of a plurality of strands 45 (se fig 9) and an insulation 41 arranged outside thereof. By using cables of the type described further below in such a generator high voltages may be generated directly without any insulation problems. This is achieved by designing the cable preferably as illustrated in fig 9.

This cable has an inner electric conductor 40 with an envelope 41 able to confine the electric field created around the conductor. The inner electric conductor 40 is flexible, and the envelope 41 forms an insulation system, which comprises an insulation 42 formed by a solid insulation material, preferably a material on polymeric basis, and outside the insulation an outer layer 43 having an electrical conductivity being higher than that of the insulation so that the outer layer through connection to ground or otherwise a comparatively low potential will be able to on one hand operate to equalise potential and on the other primarily enclose the electric field created as a consequence of said electric conductor 40 interiourly of the outer layer 43. Furthermore, the outer layer should have a resistivity being sufficient for minimising the electric losses in the outer layer. The insula- tion system comprises also an inner layer 44, which has said at least one electric conductor 40 arranged interiourly thereof and has an electrical conductivity being lower than that of the electric conductor but sufficient for making the inner layer to operate for equalising potential and by that act equalising with respect to the electric field outside the inner layer. Thus, such a cable is of the type corresponding to cables having a solid extruded insulation and today being used within power distribution, for example so called PEX-cables or cables with EPR-insulation. The term "solid insulation material" used means that the winding has to be without any liquid or gaseous insulation, for example in the form of oil. The insulation is instead formed by a polymeric material. Also the inner and outer layers are formed by a polymeric material, although a semiconducting one. The insulation 43 may be made of a solid thermal plastic material, such as low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP), polybuthylene (PB), polymethylpenten (PMP), cross-linked polyethylene (XLPE) or rubber such as ethylene- propylene rubber (EPR) or silicone rubber. With respect to the resistivity of the inner layer and the outer layer this should be within the range 10^"6Ωcm - 100 kΩcm, suitably 10^~3 - 1000Ωcm, preferably 1 -500Ωcm. The inner and the outer layer have advantageously a resistance which per length meter of the conductor/insulation system is within the range 50μΩ - 5MΩ.

The electric load or stress on the insulation system is reduced as a consequence of the fact that the inner and outer layers of the semiconducting materials around the insulation will tend to form substantially equipotential surfaces and the electric field in the insulation will in this way be distributed comparatively homogeneously over the thickness of the insulation.

The adherence between the insulation material and the inner and the outer semiconducting layers has to be uniform over substantially the entire interface thereof, so that no hollow spaces, pores or the like may be created. This is of course particularly important in high voltage applications, and a cable of this type has preferably an insulation system adapted for high voltage, suitably over 10kV, especially over 36kV and preferably over 72,5kV. Electrical and thermal stresses occurring at such high voltages put high demands on the insulation material. It is known that so called partial discharges, PD, in general is a se- vere problem for the insulation material in high voltage applications. Would hollow spaces, pores or the like be formed in an insulating material, inner corona discharges may occur att high electrical voltages, whereby the insulation material is gradually degraded and the result could be electrical breakdown through the insulation. This could result in a severe breakdown of the reactor.

It is advantageous that the inner and outer layers and the solid insulation have substantially the same thermal properties for avoiding the generation of such hollow spaces or pores, in which it is particularly important that they have substantially the same coefficient of thermal expansion, so that a perfect adherence between the different layers may be maintained during temperature changes thereof and the cable expands and contracts uniformously as a monolithic body upon temperature changes without any destruction or degradation of the interfaces. The insulation layer is for example a PEX-cable of cross-linked low- density polyethylene and the semiconducting layers of polyethylene with dust and metal particles admixed. Volume changes as a consequence of temperature changes are absorbed entirely as changes of the radius of the cable, and thanks to the comparatively small difference of the coefficients of thermal expansion of the layers with respect to the elasticity of these material, the radial expansion of the cable may take place while avoiding that the layers will get loose from each other.

The cable has also to have a certain flexibility, and it is flexible down to a radius of curvature below 25 times the diameter of the cable so that bending may take place while ensuring a good adherence between the respective layers and the solid insula- tion. The cable is suitably flexible to a radius of curvature below 15 times the diameter of the cable, and preferably to a radius of curvature below 10 times the diameter of the cable. The E- modulus of the different layers of the insulation system should be substantially equal so as to not induce any unnecessary shearing stresses in the interfaces between the different layers, so that a reduction of the shearing stresses that may be created between the different layers when exerting the cable to powerful bending resulting in tension stresses on the outside of the bend and compression stresses on the inside of the bend may take place. The invention is of course not in any way restricted to the preferred embodiments described above, but many possibilities to modifications thereof would be apparent to a man with skill in the art without departing from the basic idea of the invention such as defined in the appended claims.

It would for example be well possible to connect reactances in the different embodiments in a different way, and it would also be possible to let some of them be fixed and provide a possibility to regulate some.

It is pointed out that "four phase generator" having a grounded midpoint in common in reality is to be put on equal level to a two phase generator with each phase provided with a grounded midpoint, so that it is also possible to consider the four phase generator according to the invention as a particular type of a two phase generator, and the patent claim definition is intended to also cover this.

Claims

1 . A device for generating a one-phase alternating voltage symmetrical with respect to ground, characterized in that it comprises an alternating voltage generator (1 ) with four phases (1 1 -14) with a shift of 90 electrical degrees, that two first (1 1 ,13) of said phases having a mutual phase shift of 180° are provided with a terminal (15,16) each so as to deliver said one-phase alternating voltage thereon, and that the device also comprises reactances (17,18) connected to the phases of the generator for causing a load connected to the generator to be substantially balanced and phase compensated, so that the currents in the four phases of the generator will be substantially just as high and are substantially in phase with the voltage of the respective phase.

2. A device according to claim 1 , characterized in that said reactances are present in the form of capacitances (17,18,22) and/or inductances (20,21 ,23,24).

3. A device according to claim 1 or 2, characterized in that it comprises means for regulating the capacitance or inductance of at least one of said reactances (17, 18,20,21 ).

4. A device according to any of claims 1 -3, characterized in that at least one of said reactances is fixed and the device comprises a damping winding.

5. A device according to any of claims 1 -4, characterized in that said two first phases (1 1 ,13) are at one end thereof opposite to said terminals connected at a point (19) in common to ground.

6. A device according to any of claims 1 -5, characterized in that all four phases (1 1 -14) are at their ends opposite to said terminals (15,16) at a point in common connected to ground (19).

7. A device according to claim 5, characterized in that the four phases are in pairs connected in series with the two first phases (1 1 ,13) connected to the respective terminal (15,16) through one of the other, second phases (12,14) each.

8. A device according to claim 6, characterized in that each second phase (12,14) is connected to said terminal (15,16) through a capacitance (17, 18).

9. A device according to claim 6 and possibly any of claims 2-5, and 8, characterized in that each first phase (1 1 ,13) com- prises a capacitance (17,18) connected in parallel therewith between the ground point (19) and said terminal (15,16).

10. A device according to claim 6 and any of claims 2-5 and 8 or 9, characterized in that it for each second phase (12, 14) comprises an inductance (20,21 ) connected in parallel therewith between said ground point (19) and the opposite end of the phase.

1 1 . A device according to claim 7, characterized in that it com- prises a capacitance connected in parallel with the two first phases (1 1 ,13) between the connection of the two first phases to the respective second phase, and that the second phases have between their both ends an inductance (23,24) each connected in parallel therewith.

12. A device according to claim 7, characterized in that the two first phases (1 1 , 13) have windings with an isolation dimensioned for a lower voltage than the windings of the two second phases (12, 14).

13. A device according to any of the preceding claims, characterized in that it is adapted to be directly connected to a one phase alternating voltage network (2) through said two terminals (15,16).

14. A device according to claim 13, characterized in that it is designed to deliver an electric power to railway vehicles (10).

15. A device according to claim 14, characterized in that said one phase alternating voltage network comprises a feeding line (2) connected to one of said terminals (15, 16) for feeding electric power to a contact line (9) for feeding one phase alternating voltage to railway vehicles (10).

16. A device according to any of claims 1 -15, characterized in that it is adapted to deliver a voltage with respect to ground being higher than 10kV, advantageously higher than 15kV and preferably between 20 and 100kV, to said terminals (15,16).

17. A device according to any of the preceding claims, characterized in that a generator is designed to deliver a one phase alternating voltage having a frequency of 50Hz, 60Hz, 25Hz or 16 2/3Hz to said terminals (15, 16).

18. A device according to any of the preceding claims, characterized in that the generator is adapted to deliver a one phase alternating voltage having a frequency being lower than the frequency of the voltage on the public alternating high voltage transmission network for electric power to said terminals (15,16).

19. A device according to any of the preceding claims, characterized in that a rotor (31 ) of the alternating voltage genera- tor is adapted to be rotated by wind power.

20. A device according to any of the preceding claims, characterized in that the generator is a high voltage generator adapted to directly generate a high voltage on said terminals (15,16).

21 . A device according to claim 20, characterized in that the high voltage generator (1 ) is of the type with winding of a cable.

22. A device according to claim 21 , characterized in that the windings forming the phases (1 1 -14) of the generator are formed by a cable (39) in the form of a flexible electrical conductor (40) with an envelope (41 ) able to enclose the electric field created around the conductor.

23. A device according to claim 22, in which the envelope comprises an insulation system, characterized in that the insulation system comprises an insulation formed by a solid insulation material (42) and outside thereof an outer layer (43) having an electrical conductivity which is higher than the electrical conductivity of the insulation so that the outer layer may through connection to ground or otherwise comparatively low potential be able to on one hand operate for equalising potential and on the other substantially enclose the electrical field generated as a consequence of said electrical conductor

(40) inwardly of the outer layer.

24. A device according to claim 22 or 23, in which the envelope comprises an insulation system, characterized in that the in- sulation system comprises an insulation (42) formed by a solid insulation material and an inner layer (44) interiorly thereof, that said at least one electrical conductor is arranged interiorly of the inner layer and that the inner layer has an electrical conductivity being lower than the electrical conduc- tivity of the electrical conductor but sufficient for making the inner layer to operate to equalise potential and by that to equalise the electric field outside the inner layer.

25. A device according to claim 23 or 24, characterized in that the inner (44) and the outer (43) layers and the solid insulation (42) have substantially equal thermal properties.

26. A device according to any of claims 23-25, characterized in that the inner (44) and/or outer (43) layer comprises a semi- conducting material.

27. A device according to any of claims 23-26, characterized in that the inner layer (44) and/or the outer layer (43) has a resistivity in the range 10^"6Ωcm - 100kΩcm suitably 10^"3 - 1000Ωcm, preferably 1 -500Ωcm.

28. A device according to any of claims 23-27, characterized in that the inner layer (44) and/or the outer layer (43) has a resistance which per length meter of the conductor/insulation system is in the range 50μΩ - 5MΩ.

29. A device according to any of claims 23-28, characterized in that the solid insulation (42) and the inner layer (44) and/or the outer layer (43) are formed by polymeric material.

30. A device according to any of claims 23-29, characterized in that the inner layer (44) and/or the outer layer (43) and the solid insulation (42) are rigidly connected to each other over substantially the entire interface to ensure adherence also on flexing and temperature change.

31 . A device according to any of claims 23-30, characterized in that the solid insulation (42) and the inner layer (44) and/or the outer layer (43) are formed by materials having a high elasticity to maintain mutual adherence on strains during operation.

32. A device according to claim 31 , characterized in that the solid insulation (42) and the inner layer (44) and/or the outer layer (43) are of materials having substantially equal E- modulus.

33. A device according to any of claims 23-32, characterized in that the inner layer (44) and/or the outer layer (43) and the solid insulation (42) are formed by materials having substan- tially equal coefficients of thermal expansion.

34. A device according to any of claims 23-33, characterized in that the inner layer (44) is in electric contact with the at least one electric conductor (40).

35. A device according to claim 34, characterized in that said at least one electric conductor (40) comprises a number of strands (45) and that at least one strand of the electric conductor is at least in part uninsulated and arranged in electric contact with the inner layer (44).

36. A device according to any of claims 23-35, characterized in that the conductor ant its insulation system are designed for high voltage, suitably over 10kV, in particular over 36kV and preferably over 72,5kV.

37. A device according to any of the preceding claims, characterized in that it is adapted to be connected through the windings to a high voltage, suitably over 10kV, in particular over 36kV and preferably over 72,5kV.