WO2001026201A1

WO2001026201A1 - A plant for transmitting electric power and a method for reconstructing such a plant

Info

Publication number: WO2001026201A1
Application number: PCT/SE2000/001900
Authority: WO
Inventors: Mats Leijon; Bertil Berggren; Bengt FRANKÉN; Per-Anders LÖF
Original assignee: Abb Ab
Priority date: 1999-10-01
Filing date: 2000-10-02
Publication date: 2001-04-12
Also published as: AU7977500A; SE9903540D0

Abstract

A plant for transmitting electric power between two separate alternating current networks (11, 12) comprises a high voltage connection (2) interconnecting them and adapted for transmission of electric power by carrying a voltage exceeding 36kV as well as rotary converters (1, 3) connecting the respective alternating current network directly to the high voltage connection and designed to convert alternating current power between the respective network and the high voltage connection while changing the frequency of the alternating current.

Description

A plant for transmitting electric power and a method for reconstructing such a plant

FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a plant for transmitting electric power between two separate alternating current networks, which comprises a high voltage connection interconnecting them and adapted for transmission of electric power by carrying a voltage exceeding 36 kV as well as devices for connection of the re- spective network to one end each of the high voltage connection and adapted to convert alternating current power between the respective network and the high voltage connection while changing the frequency of the alternating current, as well as a method according to the preamble of the appended method claim.

The high voltage connection of such a plant may extend over hundreds of kilometers and form a network together with other high voltage connections. Accordingly, said network may be constituted by other high voltage connections. These are inter- connected by said devices for converting alternating current power. "High voltage connection" comprises here overhead lines as well as cables. The cable consists of a current-carrying electric conductor surrounded by an insulating layer, which may be formed by for example a polymer and then have the character of a so called PEX-cable. It is mostly desirable to have a high plant voltage (normally called system voltage) being as high as possible on the high voltage connection, since the maximum transmission capacity thereof increases therewith, and thereby fewer connections are required for transmitting a certain power when the plant voltage is higher and costs may thereby be saved. Furthermore, losses in the connection decrease when the voltage rises as a consequence of a lower alternating current in the high voltage connection. However, there are political laws and regulations which set limits for how high voltage a connection may have within a given geographical area. The maximum power of the transmission through the high voltage connection is determined by current and voltage. The limit may possibly be set by thermal restrictions resulting from the current flowing in the connection, the ambience temperature and the cooling capacity in the surrounding media.

There are limits for how long a connection may be made without the need of special measures to be taken for ensuring an effi- cient transmission. A high voltage connection may be simpler characterized by means of a series resistance, a series reactance and a shunt capacitance. An overhead line is dominated by a series reactance and a shunt capacitance, while a cable more than an overhead line is dominated by a high shunt ca- pacitance, and phase compensation in the form of costly capacitor banks, series capacitors and shunt reactors, respectively, is therefore needed for generating and consuming reactive power, respectively, so as to manage the transmission over long distances with a maintained voltage at high and low load conditions as well as the ability to transmit active power being high enough to the receiver station. This makes the transmission through direct current an interesting alternative when the transmission distances are very long and thereby the costs for such a phase compensating are high. Losses in using direct current will also be lower, since only active power exists. On the other hand the converter stations for converting alternating current to direct current and conversely through power semiconductors are expensive and the losses at such conversion are not negligible, and therefore the transmission distances have to be comparatively long for making direct current to be an attractive alterna- tive.

It may also be mentioned that cables are more restricted with respect to possible length when alternating current is transmitted, which means that direct current has to be selected where the transmission through cable is an obligation, such as for example between islands or islands and mainland, instead of transmission through alternating current, even if the distance is still so short that the necessary choice of direct current gets expensive. It may also be a strong desire of removing overhead lines over certain areas, for example from the environmental point of view, since they disturb and divide the landscape in a way not desired and so on, and instead dig cables into the ground. However, if the distances over which the electric power is to be transmitted are too long it is possible to stick to over- head lines if direct current is chosen, which however for other reasons may be economically disadvantageous.

For partially finding a remedy to these inconveniences plants of the type defined in the introduction have been proposed among others through W093/18566, which describes such a plant, in which the conversion of the alternating current from said network takes place while reducing the frequency of the alternating current. Accordingly, the frequency of the alternating current transmitted on the high voltage connection is reduced, which means that less reactive power is produced/consumed by the high voltage connection which hereby will be "free" for transmission of more active power, so that at a given length of the connection the maximum power transmittable therethrough is increased. As an alternative the length of the connection without any requirement of phase compensation may be increased. A combination of these is of course also possible. A lower fre- quency also results in lower losses, since the reactive current will be lower.

However, a disadvantage of this plant already known is that the frequency converter devices used are not able to handle voltages being as high as the voltages to be transmitted on the high voltage connection, i.e. exceeding 36 kV, but transformers have had to be arranged between the respective converter device and the high voltage connection. This means that the efficiency at the power transmission between the respective network and the high voltage connection will not be as high as desired.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a plant of the type defined in the introduction, which improves the possibilities to transmit electric power on a high voltage connection through alternating current in a cost efficient way with respect to what is possible through plants already known of the type defined in the introduction.

This object is according to the invention obtained by providing such a plant, in which each device comprises a rotary converter in the form of a rotary asynchronous converter having at the in- put thereof a first stator connected to the respective alternating current network with a first frequency, at the output thereof a second stator connected to the high voltage connection with an alternating current having a second frequency as well as rotor means adapted to rotate in dependence of the first and second frequencies, each of said stators comprises at least one winding being at least partially formed by a cable in the form of a flexible electric conductor having an envelope capable to confine the electric field generated around the conductor, and the two rotary converters are through the outputs thereof directly connected to the high voltage connection so as to this directly deliver the voltage to be carried by the high voltage connection. By the fact that no transformers are arranged between the respective converter and the high voltage connection, the efficiency of the transmission of electric power between the re- spective network and the high voltage connection may be kept on a very high level (in the order of 98-99%). By the use of converters of this type, which have recently become known through the patent application W097/45912 of the applicant, for converting an alternating current having a said second frequency on the high voltage connection, a change of the frequency of the alternating current may be carried out in the places where this has to be made while creating negligible losses in the frequency conversion . This depends on the fact that a cable of said type may be used at the high voltages, which are concerned here. Thus, the frequency conversion will not, for example where the lower frequency has to be converted into a higher frequency for feeding alternating current further to final consumers, give rise to any additional losses.

According to a preferred embodiment of the invention the rotary converters are designed to convert an alternating current on the high voltage connection having a said second frequency, the magnitude of which is adapted to the electric conditions of the plant. Electric conditions relate here to all types of parameters, such as the plant voltage on which the high voltage connection is intended to be put, the maximum power possible to transmit to the connection, the length of the connection, the physical design of the connection - overhead line or cable or the like. A determined frequency has always been selected in plants already known for the alternating current transmitted on the high voltage connection, but the present invention accordingly intends to select exactly the frequency being most preferred in each individual case while considering the electric conditions of the plant. The choice of magnitude of said second frequency will in the practice mean that a frequency lower than the frequency of an alternating current, which is fed through the plant to final con- sumers directly or indirectly connected to said network, which is usually 50 Hz or 60 Hz, will be selected. Thus, the invention makes a delivery of an alternating current having such a low frequency and high voltage possible through the converters di- rectly to the high voltage connection, in which at a given length the maximum transmittable power therethrough may be increased according to the discussion further above. Another consequence thereof is that the cables for the alternating current may be made considerably longer to make these an alternative where only an overhead line or cables for direct current were possible before, and according to another preferred embodiment of the invention the high voltage connection is along at least a part of the extension thereof formed by a cable, in which it is particularly advantageous to connect the two converters directly with a cable, so that a completely insulated system, converter- high voltage connection-converter, is formed. Thus, by selecting lower frequencies alternating current cables may be utilized where they are more advantageous than overhead lines and cables for direct current, but where they until now have not been used as a consequence of too long transmission distances, for example as sea cables between islands or islands and mainland or as land cable.

It is pointed out that each transmission of electric power has a frequency that at given electric conditions and costs for the equipment of the plant is an optimum at a given point of time, and it is not at all sure that a frequency being as low as possible has to be the best.

According to another preferred embodiment of the invention the rotary converters are adapted to convert an alternating current on the high voltage connection having a said second frequency being multiples of the frequency of an alternating current fed through the plant to final consumers connected directly or indi- rectly to said network, such as 1 /2, 1 /3, 1 /4, 1 /5 of the frequency last mentioned. The use of such multiples of 50 or 60 Hz may in some cases be advantageous.

According to another preferred embodiment of the invention said converter is designed to convert an alternating current on the high voltage connection having a said second frequency being lower than 50 Hz, advantageously lower than 30 Hz and preferably between 5-30 Hz, 5-20 Hz, 5-15 Hz or 10-20 Hz. Frequencies in this order are advantageous for obtaining the advantages already thoroughly discussed above.

According to another preferred embodiment of the invention the rotary converters are adapted to convert an alternating current on the high voltage connection having a said second frequency with a magnitude being dependent upon the length of the high voltage connection, so that the longer the connection the lower said second frequency, when everything else is equal. It may hereby be obtained that no phase compensation is needed or that the extent thereof is restricted despite the fact that the high voltage connection is or is made long.

According to another preferred embodiment of the invention the rotary converters are designed to convert an alternating current on the high voltage connection having a said second frequency with a magnitude being dependent upon the maximum transmit- table power of the plant through the high voltage connection, so that said second frequency gets lower when the demands on maximum transmittable power increase when everything else is equal. This means that, as mentioned above, more power may be transmitted through a plant if desired without any obligation to increase the number of connections for that sake.

According to another preferred embodiment of the invention the rotary converters are designed to convert an alternating current on the high voltage connection having a said second frequency with a magnitude depending upon the level of the plant voltage intended to be carried by the high voltage connection, and according to yet another preferred embodiment of the invention the rotary converters are designed to convert an alternating current on the high voltage connection having a said second frequency with a magnitude depending upon the inductance and the capacitance of the high voltage connection. By an adaption of the frequency to these parameters the need for phase compensation may be reduced and the maximum length of the high voltage connection may be increased without any requirement of phase compensation.

According to another preferred embodiment of the invention the cable of the converters comprises an insulation system comprising at least two semiconducting layers, in which each layer forms substantially an equipotential surface, and on the other a solid insulation arranged therebetween.

The invention also relates to a method for reconstructing an existing plant for transmission of electric power between two sepa- rate alternating current networks through a high voltage connection according to the appended dependent method claims. It will in this way be possible to apply the idea according to the invention to plants already existing of the type in question and utilize it therefor.

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

BRIEF DESCRIPTION OF THE DRAWINGS

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

In the drawings: Fig 1 illustrates very schematically a plant for transmitting electric power between two separate alternating current networks through a high voltage connection and shall serve to explain the fundamental idea of the invention,

Fig 2 is a simplified model of a high voltage line in a plant according to Fig 1 ,

Fig 3 is a very schematic view of an asynchronous rotary converter adapted to interconnect two lines (network and high voltage connection) having different frequencies in a plant according to the invention,

Fig 4 is a view illustrating the construction of a cable used for the windings in the converter illustrated in Fig 3, and

Fig 5 is a diagram illustrating how the maximum power that may be transmitted by a high voltage line in a plant for transmitting electric power is changed with the voltage for two different frequencies.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

A plant for transmitting electric power is schematically illustrated in Fig 1 , which plant has a conventional construction but differs in one important aspect, which will be discussed thoroughly further below, from conventional plants of this type. The more cor- rect name for such a plant is probably "system for transmitting electric power". The plant has a high voltage line 2, which may extend over many hundreds of kilometers and stations 1 , 3 in the form of rotary converters may be arranged therealong for connecting alternating current networks 1 1 , 12 with the high voltage line. It is then important that the voltage of the high voltage line 2 is kept at a high level, namely exceeding 36 kV, and preferably above 80 kV, so that the transmission losses may be kept on a low level. It is illustrated how the alternating current network may be connected to intermediate stations 4, in which then transformers not shown normally are adapted to transform the voltage down to a lower level before the alternating current reaches the intermediate stations. Further down- transformation may then take place to the final consumers 5, in which, however, in the practice more levels than those illustrated in Fig 1 may exist before the final consumers are reached. The high voltage line 2 may be an overhead line, a cable dug down into the ground or a sea cable. It is characterizing for the invention that the rotary converters 1 , 3 are connected through the outputs thereof directly to the high voltage line 2 without any intermediate transformer, and how this is possible will be explained further below.

A model for the high voltage line 2 is schematically illustrated in Fig 2, from which it appears that it has a series resistance indicated at 6 and a series inductance indicated at 7, R and L, re- spectively, and a shunt capacitance C/2 indicated at 8 at each end, i.e. the total shunt capacitance of the high voltage line distributed with half on each line end. The series inductance results in a consumption of reactive power with a magnitude 2πfLI², while the shunt capacitances result in a generation of reactive power with the magnitude of 2πfCU², in which f is the frequency of the alternating current, I is the efficiency value of the phase current and U the efficiency value of the plant voltage. The generation of reactive power will in an overhead line be higher than the consumption thereof when idling or at low load and the opposite at high load, and at long lines it will be necessary to have some compensation equipment for compensating for the phase shift between the voltage and the current otherwise caused.

According to a preferred embodiment of the present invention the desired frequency of the alternating current to be delivered by the two converters on the high voltage line depending upon the electric conditions of the plant, i.e. the length of the line 2, the type of the line-cable or overhead line-the voltage the line is intended to be put on, the power to be transmitted through the line and so on, is selected for a plant (transmission system) schematically indicated in Fig 1 . When the plant is constructed it is then intended that the frequency for the alternating current of the high voltage line is selected once and for all by designing the rotary converters 1 , 3 in a suitable way, but it is of course also within the scope of the invention to modify the frequency should there be strong reasons therefore. A selection of the frequency depending upon said electric conditions means in the practice that a lower frequency than the one now usually used, i.e. 50 Hz or 60 Hz, will be utilized for the alternating current in the high voltage line 2. This means that the reactive power will be reduced, i.e. a smaller part of the line will be used for reactive power, and so make the line free to a larger extent for active power and the maximum power that may be transmitted at a given voltage is increased. The losses are also reduced as well as the dependence of the phase compensation at a given transmitted power and it is possible to make high voltage lines longer without any need of phase compensation or smaller and thereby less expensive equipment may be used for this. Accordingly, the lower frequency may be used for increasing the maximum power that may be transmitted and/or the maximum length of the line without any need of phase compensation.

An asynchronous rotary converter known through the patent application W097/45912 of the applicant is schematically illus- trated in Fig 3. This enables the conversion of the alternating current between the respective alternating current network 1 1 , 12 and the high voltage line 2 to an alternating current having another frequency without the need of a transformer for any side of the converter by utilizing a cable of the type shown in Fig 4 and described more in detail below. This converter comprises a first stator 1 5 connected to a first high voltage line 16, for ex- ample an alternating current network 1 1 , having a first frequency and a second stator 17 connected to a second high voltage line 18, for example the high voltage line 2, having a second frequency f₂ as well as rotor means 19 adapted to rotate de- pending upon the first and second frequencies. By a suitable choice of the pole number relationship the condition between the frequencies and f₂ may be adjusted.

It will in the practice be suitable to provide converters as above for fixed frequencies on the high voltage line 2, such as 12.5 Hz, 15 Hz, 20 Hz, 25 Hz and the like as multiples of 50/60 Hz, and even if the calculations indicate that for example 1 1 .7 Hz would be the optimum frequency for a given conductor, then the closest "shelf size" would be selected.

At least the two stators of the converter according to Fig 3 have at least a winding being at least partially formed by a cable in the form of a flexible electric conductor having an envelope capable of confining the electric field resulting around the con- ductor. By utilizing such a cable for said winding, it will be possible to adapt the converter to handle the high voltages carried by the high voltage line, i.e. the two transformers, one before and one after the respective converter may be saved. The cable 9, which is illustrated in Fig 4, has an inner flexible electric con- ductor 20 and an envelope 21 , which forms an insulation system comprising an insulation 22 formed by a solid insulation material, preferable a polymer based material, and outside the insulation an outer layer 23 having an electric conductivity being higher than the insulation so that the outer layer 23 by con- nection to ground or otherwise comparatively low potential is at one hand able to function potential equalizing and on the other mainly enclose the electric field created around said electric line 20, inside the outer layer. Furthermore, the outer layer will have a resistivity being sufficient for minimizing electric losses in the outer layer. The insulation system comprises furthermore an in- ner layer 24, which has said at least one electric conductor 20 arranged inside thereof and has an electric conductivity being lower than the one of the electric conductor but sufficient for making the inner layer to function potential equalizing and thereby equalizing with respect to the electric field outside the inner layer. Such a cable is accordingly of the type corresponding to cables having a solid extruded insulation today used within the power distribution, for example so called PEX-cables and cables with EPR-insulation. The term "solid insulation mate- rial" used means that the winding is without any liquid or gaseous insulation thereof, for example in the form of oil. Instead the insulation is intended to be achieved out of a polymeric material. Also the inner and outer layers are formed by a polymeric material, though a semiconducting one. The insulation 22 may be a solid thermo plastic material, such as low density polyethene (LDPE), high density polyethene (HDPE), polypropylene (PP), polybuthylene (PB), polymethylpentene (PMP), interconnected polyethylene (XLPE) or rubber, such as ethylene-propylene-rub- ber (EPR) or silicon rubber. With respect to the resistivity of the inner layer and the outer layer this has to lie within the range of 10^"6Ω cm - 100 k Ω cm, suitably 10^"3 - 1000 Ω cm, preferably 1 - 500 Ω cm. For the inner and outer layer a resistance being within the region 50 μ Ω - 5 M Ω is advantageous.

The electric load on the insulation system decreases as a consequence of the fact that the inner and outer layers of semiconducting material around the insulation will tend to form substantially equipotential surfaces and the electrical field in the insulation will in this way be distributed comparatively uniformly over the thickness of the insulation.

It is particularly advantageous to combine the two converters with a high voltage line in the form of a cable, since a completely insulated system converter-line-converter is obtained therethrough. It is illustrated in Fig 5 how the maximum transmittable power P of a high voltage line in a plant of the type according to the invention is changed with the voltage U for normal frequency A and for a lower frequency B. The calculations have for a given line been made with a normal frequency of 50 Hz and a lower frequency of 15 Hz. It appears here that the difference with respect to maximum transmittable power gets substantial and it is, as already mentioned, possible to chose not to transmit more power but instead make the line in question longer.

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

It is pointed out that said first frequency may be different for the respective converter, so that the two alternating current networks may carry an alternating current having different frequen- cies.

Claims

1 . A plant for transmitting electric power between two separate alternating current networks (1 1 , 12), which comprises a high voltage connection (2) interconnecting them and adapted for transmission of electric power by carrying a voltage exceeding 36 kV as well as devices (1 , 3) for connection of the respective network to one end each of the high voltage connection and adapted to convert alternating current power between the re- spective network and the high voltage connection while changing the frequency of the alternating current, characterized in that each device comprises a rotary converter in the form of a rotary asynchronous converter having at the input thereof a first stator (15) connected to the respective alternating current net- work (16) with a first frequency, at the output thereof a second stator (17) connected to the high voltage connection (2, 18) with an alternating current having a second frequency as well as rotor means (19) adapted to rotate in dependence of the first and second frequencies, that each of said stators (15, 17) comprises at least one winding being at least partially formed by a cable (9) in the form of a flexible electric conductor (20) having an envelope (21 ) capable to confine the electric field generated around the conductor, and that the two rotary converters are through the outputs thereof directly connected to the high volt- age connection (2, 18) so as to this directly deliver the voltage to be carried by the high voltage connection.

2. A plant according to claim 1 , characterized in that the rotary converters (1 , 3) are designed to convert an alternating current on the high voltage connection (2, 18) having a said second frequency, the magnitude of which is adapted to the electric conditions of the plant.

3. A plant according to claim 1 , characterized in that the rotary converters (1 , 3) are designed to convert an alternating current on the high voltage connection (2, 18) having a said second fre- quency being lower than the frequency of an alternating current, which through the plant is fed to final consumers (5) connected directly or indirectly to said network.

4. A plant according to claim 3, characterized in that the rotary converters (1 , 3) are designed to convert an alternating current on the high voltage connection (2, 18) having a said second frequency being substantially lower than the frequency of an alternating current, which through the plant is fed to final consumers (5) connected directly or indirectly to said network.

5. A plant according to claim 3 or 4, characterized in that the rotary converters (1 , 3) are designed to convert an alternating current on the high voltage connection (2, 18) having a said second frequency which is multiples of the frequency of an alternating current fed through the plant to final consumers (5) connected directly or indirectly to said network, such as 1 /2, 1/3, 1/4, 1/5 of the frequency last mentioned.

6. A plant according to claim 5, characterized in that said second frequency is multiples of 50 or 60 Hz.

7. A plant according to any of the preceding claims, characterized in that the rotary converters (1 , 3) are designed to convert an alternating current on the high voltage connection (2, 18) having a said second frequency which is lower than 50 Hz, advantageously lower than 30 Hz and preferably between 5-30 Hz, 5-20 Hz, 5-15 Hz or 10-20 Hz.

8. A plant according to claim 3, characterized in that the rotary converters (1 , 3) are designed to convert an alternating current on the high voltage connection (2, 18) having a said second frequency which is at least 10 Hz, advantageously at least 20 Hz and preferably at least 40 Hz lower than the frequency of an al- ternating current, which is fed through the plant to final consumers (5) connected directly or indirectly to said network.

9. A plant according to any of the preceding claims, characterized in that the rotary converters (1 , 3) are adapted to convert an alternating current on the high voltage connection (2, 18) having a said second frequency with a magnitude being dependent upon the length of the high voltage connection, so that the longer the connection the lower said second frequency, when everything else is equal.

10. A plant according to any of the preceding claims, characterized in that the rotary converters (1 , 3) are designed to convert an alternating current on the high voltage connection (2, 18) having a said second frequency with a magnitude being dependent upon the maximum transmittable power of the plant through the high voltage connection, so that said second frequency gets lower when the demands on maximum transmittable power increase when everything else is equal.

1 1 . A plant according to any of the preceding claims, charac- terized in that the rotary converters (1 , 3) are designed to convert an alternating current on the high voltage connection (2, 18) having a said second frequency with a magnitude being dependent upon the level of the plant voltage intended for the high voltage connection.

12. A plant according to any of the preceding claims, characterized in that the rotary converters (1 , 3) are designed to convert an alternating current on the high voltage connection (2, 18) having a said second frequency with a magnitude being de- pendent upon the inductance and the capacitance of the high voltage connection.

13. A plant according to any of the preceding claims, characterized in that the high voltage connection (2, 18) is at least along a part of the extension thereof formed by a cable.

14. A plant according to any of the preceding claims, characterized in that the rotary converters (1 , 3) are designed to deliver a predetermined second frequency on the outputs thereof at a determined first frequency each on their respective inputs.

15. A plant according to any of the preceding claims, characterized in that the cable of the converters comprises an insulating system comprising on one hand at least two semiconducting layers (23, 24), in which each layer forms substantially an equi- potential surface, and on the other a solid insulation (22) is arranged therebetween.

16. A plant according to claim 15, characterized in that at least one of said semiconducting layers (23, 24) has substantially the same coefficient of thermal expansion as the solid insulation (22).

17. A plant according to claim 16, characterized in that the potential of the inner (24) of said layers is substantially the same as the potential of the conductor (20).

18. A plant according to claim 16 or 17, characterized in that the outer (23) of said layers is designed to form substantially an equipotential surface surrounding the conductor.

19. A plant according to any of claims 15-18, characterized in that at least two of said layers (23, 24) have substantially the same coefficients of thermal expansion.

20. A method for reconstructing an existing plant for transmitting electric power between two separate alternating current networks (1 1 , 12) through a high voltage connection (2) adapted to carry a voltage exceeding 36 kV, characterized in that a rotary converter in the form of a rotary asynchronous converter having on the input thereof a first stator (15) is connected through this input to a respective alternating current network (16) having a- first frequency, on the output thereof a second stator (17) is connected through this output to the high voltage connection (18) having an alternating current with a second frequency as well as rotor means (19) adapted to rotate depending upon the first and second frequencies, in which each of said stators (15, 17) comprises at least a winding being at least partially formed by a cable (9) in the form of a flexible electric conductor (20) having an envelope (21 ) capable of confining the electric field generated around the conductor, so that the two rotary convert- ers are through the outputs thereof directly connected to the high voltage connection so as to this directly deliver the voltage it has to carry.