WO2001063736A1

WO2001063736A1 - A device for generating a one-phase alternating voltage

Info

Publication number: WO2001063736A1
Application number: PCT/SE2001/000358
Authority: WO
Inventors: Bo Kindell
Original assignee: Balfour Beatty Plc
Priority date: 2000-02-21
Filing date: 2001-02-19
Publication date: 2001-08-30
Also published as: SE0000545D0; SE518596C2; SE0000545L

Abstract

A device for generating a one-phase alternating voltage comprises a first converter (17) connected to an alternating voltage source for rectifying an alternating voltage and a second converter (28) connected to the first converter through a direct voltage intermediate link (23) and adapted to convert the direct voltage to a one-phase alternating voltage. The direct voltage intermediate link has a symmetry midpoint (24) with respect to voltage symmetrically connected to the second converter. The second converter has only one phase leg (29) with current valves connected in series and an output for the one-phase alternating voltage connected to a midpoint between the current valves. A line connected to said midpoint output is connected to a grounded return feeder (33) for current to a load connectable to the one-phase alternating voltage and this return feeder is connected to said symmetry midpoint of the direct voltage intermediate link.

Description

A DEVICE FOR GENERA TING A ONE-PHASE ALTERNA TING VOLTAGE

FIELD OF THE INVENTION AN D PRIOR ART

The present invention relates to a device for generating a one- phase alternating voltage comprising a first converter connected to an alternating voltage source for rectifying an alternating voltage and a second converter connected to the first converter through a direct voltage intermediate link and adapted to convert the direct voltage into a one-phase alternating voltage, said direct voltage intermediate link having a symmetry midpoint, with respect to the voltage symmetrically connected to the second converter.

Such devices, which may be called converters and most often are called frequency converters, since the device enables a free selection of the frequency of the one-phase alternating voltage independently of the frequency of the alternating voltage from the alternating voltage source, may be used anywhere where there is a need of generating a one-phase alternating voltage, and for illuminating the invention, but accordingly not in any restrict it, two fields of use of such devices will hereinafter be de- scribed, but a number of other fields of use for such devices are conceivable within the scope of the invention.

A first such field of use consists in using such a device in a so called converter station, which are placed along a line for elec- trified track traffic, primarily railway lines, for converting voltage and current of the three-phase feeding line into one-phase hav- ing a suitable frequency and voltage of a one-phase feeding line for providing electric power to vehicles moving along the track. The alternating voltage side of the first converter is normally through the secondary winding of a transformer connected to the three phases of the three-phase feeding line, in which the transformers are usually arranged to accomplish a transformation of the alternating voltage down to a lower level than the one of the three-phase feeding line. The first converter produces a rectifying of the voltage and this direct voltage is then led to said second converter, which is controlled through pulse width modulation to deliver an alternating voltage formed by alternat- ingly positive and negative voltage pulses for obtaining an alternating voltage fed to a primary winding of a transformer, the secondary winding of which is connected to the feeding line for the vehicles and ground. The inverter (the second converter) of the one-phase side is then controlled to create an alternating voltage of a desired frequency, usually 16 2/3 or 25 Hz, while the three phase feeding line has a frequency of usually 50 or 60 Hz. Besides the fact that the two transformers accomplish a de- sired voltage adaption, one to the two converters and the direct voltage intermediate link and the other to the one-phase track feeding line, they also ensure galvanic separation of the one- phase and multiple-phase side.

Another field of use for a device defined in the introduction is in a vehicle being electrically driven, such as a railway car. Although these devices already known are well functioning it would of course be interesting to be able to simplify them to save costs.

SUMMARY OF THE I NVENTION

The object of the present invention is to provide a device of the type defined in the introduction, which is simpler and primarily producible to a lower cost than that kind of devices already known, without renouncing the good function of the device. This object is according to the invention obtained by providing the second converter with at least one phase leg with current valves connected in series in the form of switching members and rectifying members connected in parallel therewith and an output for the one-phase alternating voltage connected to a midpoint between said current valves, and a line connected to said midpoint output is connected to a grounded return feeder for current to a load connectable to said one-phase alternating voltage and this return feeder is connected to said symmetry midpoint with respect to the voltage of the direct voltage intermediate link. By the fact that the second converter, which functions as inverter, only has one phase leg the direct voltage intermediate link will through the connection of the grounded re- turn feeder to the midpoint thereof be symmetrical with respect to ground and not jump as would be the case when having two or more phase legs of the inverter. Thus, the second converter (the inverter) may be adapted to be connected with the alternating voltage side thereof without any intermediate transformer to a load, which means that considerable costs may be saved by saving one transformer. However, one transformer between the first converter and the alternating voltage source (usually an alternating voltage network) is needed for obtaining galvanic separation of the alternating voltage source on one hand and the load connected to the one-phase alternating voltage on the other. The first converter is therefore preferably adapted to have the alternating voltage side thereof connected to the alternating voltage source through a transformer by being connected to the secondary winding thereof.

The number of phases of the alternating voltage of the alternating voltage source may be arbitrary, but according to a preferred embodiment of the invention they are three. The voltage of a three-phase feeding line may normally vary quite a lot, while in most cases, such as in feeding of railway vehicles, the one- phase side has less variations, so that it is wise to keep the transformer of the alternating voltage side of the first converter when removing one transformer.

According to a preferred embodiment of the invention the device comprises means for controlling the first converter to generate a voltage with equal phase and the same frequency as the one- phase alternating voltage on the alternating voltage side of the second converter on the alternating voltage side thereof, and the outputs of the alternating voltage side of the first converter are connected to said symmetry midpoint with respect to the voltage of the direct voltage intermediate link. This is advantageous, since the connection of said return feeder to the symmetry midpoint of the direct voltage intermediate link results in a load on the direct voltage intermediate link in the form of a current from the return feeder having the same frequency as the one-phase alternating voltage, and by the fact that the first converter is controlled in said way a current will occur from the alternating voltage side of the first converter to said symmetry midpoint of the intermediate link, and this current will have the same frequency as the current from the return feeder to the symmetry midpoint of the direct voltage intermediate link and it may thereby compensate the current from the return feeder, so that it is prevented that this passes the capacitors of the direct voltage intermediate link and these are by that spared. A voltage of equal phase means that in the case of for example three connections to the alternating voltage side of the first converter, i.e. when the alternating voltage source has three phases, an alternating voltage is then coming out from the first converter on the alternating voltage side thereof in each connection with the same frequency as the one-phase alternating voltage and in phase with the alternating voltage of the two other connections.

According to another preferred embodiment of the invention the three different secondary windings of the transformer connected to the different phase inputs of the first converter are Y-con- nected with respect to each other. Thus, the neutral conductor in common will then be connected to the symmetry midpoint of the direct voltage intermediate link and cause said compensating of the current from the return feeder. The first converter is then preferably controlled to deliver a current in said neutral con- ductor, which has substantially the same magnitude as the current from the return feeder.

According to another preferred embodiment of the invention the primary windings of the transformer connected to the different phases of the alternating voltage source are Δ-connected to each other. No inconveniences as a consequence of the current flowing through the secondary windings of the transformer to the symmetry midpoint of the direct voltage intermediate link will then occur, since the current component generated thereby in the primary winding of the transformer will then only circulate around in the Δ-connection and will not come out to the phases of the alternating voltage source, i.e. normally the three-phase feeding line.

According to another preferred embodiment of the invention the frequency of the alternating voltage from the alternating voltage source is 50 or 60 Hz, while the second converter advantageously is adapted to generate a one-phase alternating voltage having a frequency of 16 2/3 or 25 Hz.

According to another preferred embodiment of the invention the first converter is adapted to deliver a direct voltage of 20-60 kV on said direct voltage intermediate link, which makes it possible to obtain the voltage level normally asked for on one-phase rail- way feeding lines, since the peak value of the one-phase alternating voltage will then be substantially half of said direct voltage, and by that for example a direct voltage of 50 kV is required for obtaining a peak value of the one-phase alternating voltage of 25 kV, which results in a one-phase alternating volt- age of about 17 kV, which is often a suitable value. This is valid for the frequencies 50 and 16 2/3 Hz as above, while the one- phase alternating voltage is preferably about 12 kV for the frequencies 60 and 25 Hz.

According to particularly preferred embodiments of the invention said return feeder is formed by a railway rail and the alternating voltage side of the second converter is formed by a one-phase railway feeding line for electrically driven vehicles, respectively.

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

BRIEF DESCRIPTION OF THE DRAWING

With reference to the appended drawing, below follows a specific description of a preferred embodiment of the invention cited as an example.

In the drawing:

Fig 1 is a schematic view of a device according to a preferred embodiment of the invention,

Fig 2 is a somewhat more detailed view of a part of the device according to Fig 1 .

DETAILED DESCRI PTION OF A PREFERRED EMBODIMENT OF THE INVENTION

A device for converting current and voltage between a first three-phase side 1 and a second one-phase side 2 is illustrated in Fig 1 . Such a device is suited for being a part of a converter station for delivering electric power from a three-phase line to a one-phase line for feeding vehicles electrically driven. A device has on the first side a transformer 3, the three primary windings phase line 10. The primary windings 4-6 are also mutually Δ- connected. The secondary windings 1 1 -13 of the transformer are through an inductor 14-16 each connected to a first converter 17, more exactly to a midpoint each of a phase leg each of this converter formed by current valves 18 connected in series (see Fig 2) in the form of switching members 19 and rectifying mem- bers 20 connected in anti-parallel therewith. Each such current valve has preferably a number of such switching members connected in series and rectifying members connected in anti-par- allel therewith, usually diodes, and the switching members, which are normally constituted by IGBT's, are adapted to be controlled simultaneously, so that they function as one single switching member, which is indicated by the summarizing symbols at the bottom in Fig 2. Each phaseleg is of three-level type and is connected to the two opposite poles 21 , 22 of a direct voltage intermediate link 23 and to the symmetry midpoint 24 thereof. The direct voltage intermediate link is in a conventional way formed by two capacitors 25, 26 connected in series and each connected between a pole 22 and 21 , respectively, and the symmetry midpoint 24. Furthermore, it has a filter 27 adapted to reduce and preferably remove harmonics generated through the function of the converter 17 and the second converter 28 arranged on the other side of the direct voltage intermediate link. The second converter has only one phase leg 29, on the output 30 of which a one-phase alternating voltage is generated and may through a smoothing inductor 31 be fed to a load 32. The one-phase side has also a return feeder 33, which in the case of a one-phase railway feeding line is constituted by the rail, and this is grounded, as well as a harmonic filter 34.

The return feeder 33 is connected to the symmetry midpoint 24 of the direct voltage intermediate link, so that this will always be on ground and not jump thanks to one single phase leg of the second converter 28 (the inverter) and the absolute value of the potentials of the two poles 21 , 22 of the direct voltage link will thereby always be substantially the same. Furthermore, the secondary windings 1 1 -13 of the transformer are Y-connected and connected through the neutral conductor 35 to the symmetry midpoint 24 of the direct voltage intermedi- ate link.

It is pointed out that in Fig 2 only the two converters and the direct voltage intermediate link have been shown, while the connections to the symmetry midpoint of the direct voltage interme- diate link through the return feeder and the neutral conductor are omitted.

The function of the device or frequency converter is as follows: The first converter 17 is preferably controlled in a conventional way through controlling the switching members thereof to deliver a direct voltage to the direct voltage intermediate link, while the switching members in their turn ensure that the direct voltage of the direct voltage intermediate link is converted into a three- phase symmetric 50 or 60 Hz-voltage out towards the secondary windings of the transformer. The first converter is also controlled through a control member 36 schematically indicated to deliver a voltage of equal phase having the same frequency as the frequency of the one-phase alternating voltage out to the respective secondary winding, i.e. for example a voltage having a fre- quency of for example 16 2/3 Hz in each secondary winding being in phase with the corresponding voltage in the other secondary windings. By the fact that the secondary windings are Y connected the currents so obtained will be added to each other and flow in the neutral conductor 35 between the transformer and the symmetry midpoint 24 of the intermediate link. This current will compensate the current produced in the return feeder 33 to the symmetry midpoint 24 of the intermediate link, so that this return current will not put any load on the capacitors of the intermediate link and results in disturbances. Furthermore, the current generated in the primary windings 4-6 of the transformer as a consequence of said current of equal phase in the secondary windings will not cause any inconveniences, but this current will only circulate around in the Δ-con- nection of the primary winding and not come out to the phases 7-9 of the three-phase line.

Considerable costs are saved through the inventional construction of the frequency converter by the fact that one transformer, the one on the one-phase side, could be removed, at the same time as also some energy losses, which otherwise would have been created in the transformer, may be eliminated.

The invention is of course not in any way restricted to the pre- ferred embodiment described above.

The frequency of the voltage on the one-phase side may for example very well be equal to the voltage on the alternating voltage side of the first converter. The number of the phases on the alternating voltage side of the first converter may also be any other than shown in the figures.

It is pointed out that although the title of the invention is "a device for generating a one-phase alternating voltage", the device will of course function in both directions, as conventional converters and could in fact just as well be called "converter between one-phase and multiple-phase alternating voltage systems" but the first title is chosen for pointing at the main object of the invention.

Claims

1 . A device for generating a one-phase alternating voltage comprising a first converter (17) connected to an alternating voltage source (10) for rectifying an alternating voltage and a second converter (28) connected to the first converter through a direct voltage intermediate link (23) and adapted to convert the direct voltage into a one-phase alternating voltage, said direct voltage intermediate link having a symmetry midpoint (24), with respect to the voltage symmetrically connected to the second converter, characterized in that the second converter has only one phase leg (29) with current valves (18) connected in series in the form of switching members (19) and rectifying members (20) connected in parallel therewith and an output (30) for the one-phase alternating voltage connected to a midpoint between said current valves, and that a line connected to said midpoint output (30) is connected to a grounded return feeder (33) for current to a load (32) connectable to said one-phase alternating voltage and this return feeder is connected to said symmetry midpoint (24) with respect to the voltage of the direct voltage intermediate link.

2. A device according to claim 1 , characterized in that the second converter (28) is adapted to be connected through the alter- nating voltages side thereof to a load (32) without any intermediate transformer.

3. A device according to claim 1 or 2, characterized in that the first converter (17) is adapted to have the alternating voltage side thereof connected to the alternating voltage source through a transformer (3) by being connected to the secondary winding (4-6) thereof.

4. A device according to any of claims 1 -3, characterized in that it comprises means (36) for controlling the first converter (17) to generate a voltage with equal phase and the same frequency as the one-phase alternating voltage on the alternating voltage side of the second converter (28) on the alternating voltage side thereof, and that the outputs of the alternating voltage side of the first converter are connected to said symmetry midpoint (24) with respect to the voltage of the direct voltage intermediate link (23).

5. A device according to any of claims 1 -4, characterized in that it comprises at least two phases connected to the alternating voltage side of the first converter (17).

6. A device according to claim 5, characterized in that it comprises three phases connected to the alternating voltage side of the first converter (17).

7. A device according to claims 3-6, characterized in that the different secondary windings (4-6) of the transformer (3) connected to the different phase inputs of the first converter (17) are Y-connected with respect to each other.

8. A device according to claim 7, characterized in that the primary windings (7-9) of the transformer (3) connected to the different phases of the alternating voltage source are Δ-connected to each other.

9. A device according to any of the preceding claims, characterized in that the frequency of the alternating voltage from the alternating voltage source is 50 or 60 Hz.

10. A device according to any of the preceding claims, characterized in that the second converter (28) is adapted to generate a one-phase alternating voltage having a frequency of 16 2/3 or 25 Hz.

1 1 . A device according to any of the preceding claims, characterized in that the first converter (17) is adapted to deliver a di- rect voltage of 20-60 kV on said direct voltage intermediate link (23).

12. A device according to any of the preceding claims, charac- terized in that said return feeder (33) is formed by a railway rail.

13. A device according to any of the preceding claims, characterized in that the alternating voltage side of the second converter (28) is formed by a one-phase line (30) for feeding vehi- cles electrically driven.

14. A device according to any of the preceding claims, characterized in that the alternating voltage source is formed by a three-phase feeding line (10).

15. A device according to claims 13 and 14, characterized in that it is adapted to be a part of a converter station for delivering electric power from the three-phase line (10) to the one- phase line (30) or conversely.