AC-to-AC MMC with reduced number of converter arms

Abstract

An AC-to-AC modular multilevel converter (MMC) is configured to be connected between a three-phase AC system and a single-phase AC system. The MMC includes a number of converter arms connected in a ring to allow a circulating current to be circulated in the ring through each of the converter arms. Each converter arm includes series-connected converter cells. Phase terminals are arranged in the ring between the converter arms such that each of the converter arms is separated from neighboring converter arms by at least one of the phase terminals. The phase terminals include respective terminals for each of a first phase, a second phase and a third phase of the three-phase AC system and respective terminals for each of a positive conductor and a negative conductor of the single-phase AC system.

Claims

1. An AC-to-AC modular multilevel converter (MMC) configured to be connected between a three-phase AC system and a single-phase AC system, the MMC comprising: a plurality of converter arms including a first converter arm, a second converter arm, a third converter arm, and a fourth converter arm, the converter arms connected in a ring to allow a circulating current to be circulated in the ring through each of the converter arms, each converter arm comprising a plurality of series-connected converter cells; and phase terminals arranged in the ring between the converter arms such that each of the converter arms is separated from neighboring converter arms by at least one of the phase terminals, wherein the phase terminals comprise respective terminals for each of a first phase, a second phase and a third phase of the three-phase AC system and respective terminals for each of a positive conductor and a negative conductor of the single-phase AC system.

2. The MMC of claim 1, wherein the converter arms and phase terminals are in the ring arranged in the sequence of: the phase terminal of the third phase of the three-phase AC system; the first converter arm; the phase terminal of the second phase of the three-phase AC system; the second converter arm; the phase terminal of the first phase of the three-phase AC system; the third converter arm; the phase terminal of one of the positive conductor or the negative conductor of the single-phase AC system; the fourth converter arm; and the phase terminal of the other of the positive conductor or the negative conductor of the single-phase AC system.

3. The MMC of claim 1, wherein the plurality of converter arms consists of four converter arms.

4. The MMC of claim 1, wherein the plurality of converter arms consists of five converter arms including a fifth converter arm.

5. The MMC of claim 4, wherein the converter arms and phase terminals are in the ring arranged in the sequence of: the phase terminal of the third phase of the three-phase AC system; the first converter arm; the phase terminal of the second phase of the three-phase AC system; the second converter arm; the phase terminal of the first phase of the three-phase AC system; the third converter arm; the phase terminal of one of the positive conductor or the negative conductor of the single-phase AC system; the fourth converter arm; the phase terminal of the other of the positive conductor or the negative conductor of the single-phase AC system; and the fifth converter arm.

6. The MMC of claim 4 wherein the converter arms and phase terminals are in the ring arranged in the sequence of: the phase terminal of one of the positive conductor or the negative conductor of the single-phase AC system; the first converter arm; the phase terminal of the second phase of the three-phase AC system; the second converter arm; the phase terminal of the other of the positive conductor or the negative conductor of the single-phase AC system; the third converter arm; the phase terminal of the first phase of the three-phase AC system; the fourth converter arm; the phase terminal of the third phase of the three-phase AC system; and the fifth converter arm.

7. The MMC of claim 1, wherein the three-phase AC system has a nominal frequency of 50 or 60 Hz.

8. The MMC of claim 1, wherein the single-phase AC system has a nominal frequency of 50, 60, 25, 16.7 or 50/3 Hz.

9. The MMC of claim 1, wherein the three-phase AC system has a nominal frequency of 60 Hz and the single-phase AC system has a nominal frequency of 25 Hz.

10. The MMC of claim 1, wherein the three-phase AC system has a nominal frequency of 50 Hz and the single-phase AC system has a nominal frequency of 16.7 or 50/3 Hz.

11. A system comprising: a three-phase AC system; a single-phase AC system; a first power transformer; and a modular multilevel converter (MMC) coupled between the three-phase AC system and the first power transformer, the MMC comprising: a plurality of converter arms including a first converter arm, a second converter arm, a third converter arm, and a fourth converter arm, the converter arms connected in a ring to allow a circulating current to be circulated in the ring through each of the converter arms, each converter arm comprising a plurality of series-connected converter cells; and phase terminals arranged in the ring between the converter arms such that each of the converter arms is separated from neighboring converter arms by at least one of the phase terminals, wherein the phase terminals comprise respective terminals for each of the three phases of the three-phase AC system and respective terminals for each of a positive conductor and a negative conductor of the single-phase AC system.

12. The system of claim 11, wherein the single-phase AC system is grounded and the MMC is connected to the single-phase AC system without a second power transformer there between.

13. The system of claim 11, further comprising a second power transformer via which the single-phase AC system is connected to the MMC.

14. The system of claim 11, wherein the single-phase AC system is a single-phase railway system.

15. The system of claim 11, wherein the three-phase AC system has a nominal frequency of 60 Hz and the single-phase AC system has a nominal frequency of 25 Hz.

16. A method of operating an electrical system, the method comprising: applying a three-phase AC voltage to an AC-to-AC modular multilevel converter (MMC); and receiving a single-phase AC voltage from the MMC; wherein the MMC comprises: a plurality of converter arms including a first converter arm, a second converter arm, a third converter arm, and a fourth converter arm, the converter arms connected in a ring so that current is circulated in the ring through each of the converter arms, each converter arm comprising a plurality of series-connected converter cells; and phase terminals arranged in the ring between the converter arms such that each of the converter arms is separated from neighboring converter arms by at least one of the phase terminals, wherein the phase terminals comprise respective terminals receiving a first phase, a second phase and a third phase of the three-phase voltage and respective positive and negative terminals coupled receiving single-phase AC voltage.

17. The method of claim 16, wherein the three-phase AC voltage has a nominal frequency of 60 Hz and the single-phase AC voltage has a nominal frequency of 25 Hz.

18. The method of claim 16, further comprising using the single-phase voltage to power a locomotive vehicle.

19. The method of claim 16, wherein the plurality of converter arms consists of four converter arms and the converter arms and phase terminals are in the ring arranged in the sequence of: the phase terminal of the third phase of the three-phase AC voltage; the first converter arm; the phase terminal of the second phase of the three-phase AC voltage; the second converter arm; the phase terminal of the first phase of the three-phase AC voltage; the third converter arm; one of the positive terminal or the negative terminal receiving the single-phase AC voltage; the fourth converter arm; and the other of the positive terminal or the negative terminal receiving the single-phase AC voltage.

20. The method of claim 16, wherein the plurality of converter arms consists of five converter arms and the converter arms and phase terminals are in the ring arranged in the sequence of: the phase terminal of the third phase of the three-phase AC voltage; the first converter arm; the phase terminal of the second phase of the three-phase AC voltage; the second converter arm; the phase terminal of the first phase of the three-phase AC voltage; the third converter arm; one of the positive terminal or the negative terminal receiving the single-phase AC voltage; the fourth converter arm; the other of the positive terminal or the negative terminal receiving the single-phase AC voltage; and a fifth converter arm.

21. The method of claim 16, wherein the plurality of converter arms consists of five converter arms and the converter arms and phase terminals are in the ring arranged in the sequence of: one of the positive terminal or the negative terminal receiving the single-phase AC voltage; the first converter arm; the phase terminal of the second phase of the three-phase AC voltage; the second converter arm; the other of the positive terminal or the negative terminal receiving the single-phase AC voltage; the third converter arm; the phase terminal of the first phase of the three-phase AC voltage; the fourth converter arm; the phase terminal of the third phase of the three-phase AC voltage; and a fifth converter arm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments will be described, by way of example, with reference to the accompanying drawings, in which:

(2) FIGS. 1A and 1B are schematic illustrations of different MMC topologies of the prior art.

(3) FIG. 2A is a schematic circuit diagram of an MMC topology for a three-phase to single-phase converter, in accordance with some embodiments of the present invention.

(4) FIG. 2B is a schematic circuit diagram of an MMC topology for a three-phase to single-phase converter, with the single-phase system connected via a transformer, in accordance with some embodiments of the present invention.

(5) FIG. 2C is a schematic circuit diagram of an MMC topology for a three-phase to single-phase converter, e.g., as in FIG. 2A or 2B, in which current paths are schematically illustrated, in accordance with some embodiments of the present invention.

(6) FIG. 3 is a schematic circuit diagram of another MMC topology for a three-phase to single-phase converter, in accordance with some embodiments of the present invention.

(7) FIG. 4 is a schematic circuit diagram of another MMC topology for a three-phase to single-phase converter, in accordance with some embodiments of the present invention.

(8) FIG. 5 is a schematic circuit diagram of a full-bridge converter cell of an MMC, in accordance with some embodiments of the present invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

(9) Embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments are shown. However, other embodiments in many different forms are possible within the scope of the present disclosure. Rather, the following embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. Like numbers refer to like elements throughout the description.

(10) FIG. 2A illustrates an MMC arrangement 1 comprising an AC-to-AC MMC 2. The MMC 2 is a three-phase to single-phase converter connected between a three-phase AC system 10a, e.g., a distribution grid, and a single-phase AC system 10b, e.g., a railway grid for powering at least one train 20. In the embodiment illustrated in FIG. 2A, the single-phase system 10b is grounded and connected directly to the MMC 2, i.e. not via a transformer. On the three-phase side, on the other hand, the MMC 2 is connected to the three-phase system 10a via a first transformer 3a, e.g., a standard three-phase type, V-V type or Scott transformer.

(11) Looking at the three-phase side of the MMC 2, the three-phase system 10a comprises a first phase having a grid-side voltage U.sub.g1, a second phase having a grid-side voltage U.sub.g2 and a third phase having a grid-side voltage U.sub.g3 on the grid-side (primary side) of the first transformer 3a, resulting in converter-side voltages U.sub.c1, U.sub.c2 and U.sub.c3 on the converter-side (secondary side) of the first transformer 3a. Each of the first, second and third phases of the three-phase system 10a connects, on the converter-side of the first transformer 3a, to respective first, second and third phase terminals x.sub.1, x.sub.2 and x.sub.3 in the MMC 2.

(12) Looking at the single-phase side of the MMC 2, the single phase system 10b has a single-phase voltage U.sub.train and current I.sub.train, e.g., of a railway grid. The single-phase voltage U.sub.train is between a positive conductor and a negative conductor of the single-phase system 10b. The positive conductor is connected to a positive phase terminal y.sub.1 of the MMC 2, and the negative conductor is connected to a negative phase terminal y.sub.2 of the MMC. It should be noted that the polarity of the single-phase system 10b may be reversed to what is shown in FIG. 2A, resulting in the positive and negative phase terminals y.sub.1 and y.sub.2 changing places with each other. As illustrated in FIG. 2A, the single-phase system 10b may be grounded, facilitating connecting the single-phase system 10b directly to the MMC 2 without the need for a transformer there between.

(13) The converter 2 a plurality of at least four converter arms 5 connected in a ring for allowing a current to be circulated through all of the converter arms 5 in said ring. That the converter arms are forming a ring implies that each arm 5 of the at least four arms is connected in the ring with two neighboring arms of the at least four arms connected to it, one neighboring arm to either side of the arm. Each converter arm 5 comprises a plurality of series connected (also called cascaded) converter cells 4, preferably full-bridge (also called bi-polar or H-bridge) converter cells but in some embodiments some or all cells 4 of one, some or all arms 5 may be half-bridge (mono-polar) cells. For instance, arms 5d, 5e and 5c could comprise or consist of half-bridge cells, while arms 5a and 5b would consist of full-bridge cells. A respective phase reactor 6 may be connected in the ring at and in series with each arm 5. The three-phase phase terminals x.sub.1, x.sub.2 and x.sub.3 and the single-phase phase terminals y.sub.1 and y.sub.2 are also comprised in the ring, connected in the ring such that each of the at least four converter arms 5 is to either side in the ring separated from its neighboring converter arms by at least one of the phase terminals, i.e. between each two neighboring arms 5 in the ring there is at least one of the phase terminals arranged.

(14) In the embodiment of FIG. 2A, the at least four converter arms of the MMC 2 consist of five converter arms 5a, 5b, 5c, 5d and 5e. Below, the sequence in the ring formed by the converter arms and phase terminals of the embodiment of FIG. 2A is given in a clockwise direction starting from the third phase terminal x.sub.3. However, it should be noted that if the same embodiment is seen from the opposite direction, the same sequence would be in the counter clockwise direction. Thus, the five converter arms and the five phase terminals of the embodiment of FIG. 2A are in the ring arranged in the (clockwise or counter clockwise) sequence of:

(15) 1) the phase terminal x.sub.3 of the third phase of the three-phase AC system 10a,

(16) 2) the first converter arm 5a of the five converter arms,

(17) 3) the phase terminal x.sub.2 of the second phase of the three-phase AC system 10a,

(18) 4) the second converter arm 5b of the five converter arms,

(19) 5) the phase terminal x.sub.1 of the first phase of the three-phase AC system 10a,

(20) 6) the third converter arm 5c of the five converter arms,

(21) 7) the phase terminal y.sub.1 of the positive conductor of the single-phase AC system 10b (or the phase terminal y.sub.2 of the negative conductor if the polarity of the single-phase AC system is reversed),

(22) 8) the fourth converter arm 5d of the five converter arms,

(23) 9) the phase terminal y.sub.2 of the negative conductor of the single-phase AC system 10b (or the phase terminal y.sub.1 of the positive conductor if the polarity of the single-phase AC system is reversed), and

(24) 10) the fifth converter arm (5e) of the five converter arms.

(25) FIG. 2B illustrates another embodiment of the MMC arrangement 1. The MMC arrangement 1 is as discussed in relation to FIG. 2A but here the MMC 2 is connected to the single-phase system 10b via a second (single-phase) transformer 3b. By means of the second transformer 3b, the single-phase system 10b may not need to be grounded.

(26) FIG. 2C illustrates current paths within some embodiments of the MMC 2, e.g., of the MMC of any of the FIGS. 2A and 2B. The first converter arm 5a is connected between the third phase terminal x.sub.3 and the second phase terminal x.sub.2, resulting in a voltage U.sub.2 across it and a current I.sub.2 through it as indicated by the arrows in a current path from the third phase to the second phase via the first converter arm 5a, where U.sub.2=U.sub.c3−U.sub.c2. The second converter arm 5b is connected between the first phase terminal x.sub.1 and the second phase terminal x.sub.2, resulting in a voltage U.sub.1 across it and a current I.sub.1 through it as indicated by the arrows in a current path from the first phase to the second phase via the second converter arm 5b, where U.sub.1=U.sub.c1−U.sub.c2.

(27) The fourth converter arm 5d is connected between the positive phase terminal y.sub.1 and the negative phase terminal y.sub.2, resulting in a voltage across the fourth converter arm 5d equal to the difference in potential between the positive and negative phase terminals y.sub.1 and y.sub.2. The current I.sub.train of the single-phase system 10b flows however in a current path as indicated by the arrows through the other four converter arms 5e, 5a, 5b and 5c. This implies that the first and second converter arms 5a and 5b carry currents of both the three-phase system 10a and the single-phase system 10b, typically of different frequencies.

(28) The third converter arm 5c and the fifth converter arm 5e may be regarded as internal arms, which in the embodiment of FIG. 2C each has a voltage of (U.sub.1−U.sub.2−U.sub.train)/2 across it, but in opposite directions as indicated by the arrows.

(29) The voltages and currents of the three-phase side may be described as:
U.sub.1(t)=U.sub.in.Math.cos(ω.sub.int)  (1)
U.sub.2(t)=U.sub.in.Math.cos(ω.sub.int+θ.sub.1)  (2)
I.sub.1(t)=I.sub.in.Math.cos(ω.sub.int−φ.sub.1)  (3)
I.sub.2(t)=I.sub.in.Math.cos(ω.sub.int+θ.sub.1−φ.sub.1)  (4)
The voltage and current of the single-phase side may be described as:
U.sub.train(t)=U.sub.out−cos(ω.sub.outt+θ.sub.2)  (5)
I.sub.train(t)=I.sub.out.Math.cos(ω.sub.outt+θ.sub.2−φ.sub.2)  (6)
U.sub.in is the amplitude of each of U.sub.1 and U.sub.2, I.sub.in is the amplitude of each of I.sub.1 and I.sub.2, U.sub.out is the amplitude of U.sub.train, I.sub.out is the amplitude of I.sub.train, ω.sub.in is the three-phase frequency and ω.sub.out is the frequency of the single-phase system. For given three-phase and single-phase voltages (usually pre-determined), a desired power transfer P and power factors at the three-phase and single-phase sides will determine the magnitudes of the three-phase and single-phase currents. Out of the five arms 5 of the converter 2, the first and second arms 5a and 5b are connected to the fixed three-phase voltages U.sub.1(t) and U.sub.2 (t), the fourth arm 5d is connected to the fixed single-phase voltage U.sub.train(t), leaving the two internal arms 5c and 5e that create the voltage [U.sub.1(t)−U.sub.2(t)−U.sub.train(t)]/2, with the polarity shown in FIG. 2C. For all the converter arms to be balanced, the three-phase and single-phase currents flow in the way depicted in FIG. 2C, and a circulating current I.sub.0 must flow through all the arms 5 of the converter 2. This circulating current I.sub.o has the form of:
I.sub.0(t)=I.sub.circ.Math.cos(ω.sub.int+ψ.sub.circ)  (7)
where I.sub.circ and θ.sub.circ are functions of U.sub.in, U.sub.out, P, θ.sub.1, φ.sub.1, and φ.sub.2.

(30) FIG. 3 illustrates a further embodiment of the suggested reduced direct MMC topology with an even further reduced number of converter arms 5, to four converter arms. In this embodiments, the two internal arms 5c and 5e of FIGS. 2A and 2B, each of which bearing a voltage of [U.sub.1(t)−U.sub.2 (t)−U.sub.train(t)]/2, have been merged into a single internal arm 5ce that bears a voltage of U.sub.1(t)−U.sub.2(t)−U.sub.train (t) located between the second and fourth arms 5b and 5d in the ring. This results in both the third phase terminal x.sub.3 and the negative phase terminal y.sub.2 (or positive phase terminal y.sub.1 if the polarity of the single-phase system 10b is reversed) being located in the ring between the two neighboring first and fourth arms 5a and 5d. Alternatively, the merged internal arm 5ce could be located between the first and fourth arms 5a and 5d, which would result in both the first phase terminal x.sub.1 and the positive phase terminal y.sub.1 (or negative phase terminal y.sub.2 if the polarity of the single-phase system 10b is reversed) being located in the ring between the two neighboring second and fourth arms 5b and 5d. It is noted that the circulating current I.sub.o is also in this embodiment circulated in the ring through all of the converter arms 5a, 5b, 5ce and 5d.

(31) Below, the sequence in the ring formed by the converter arms and phase terminals of the embodiment of FIG. 3 is given in a clockwise direction starting from the third phase terminal x.sub.3. However, it should be noted that if the same embodiment is seen from the opposite direction, the same sequence would be in the counter clockwise direction. Thus, the four converter arms and the five phase terminals of the embodiment of FIG. 2A are in the ring arranged in the (clockwise or counter clockwise) sequence of:

(32) 1) the phase terminal x.sub.3 of the third phase of the three-phase AC system 10a,

(33) 2) the first converter arm 5a of the four converter arms,

(34) 3) the phase terminal x.sub.2 of the second phase of the three-phase AC system 10a,

(35) 4) the second converter arm 5b of the four converter arms,

(36) 5) the phase terminal x.sub.1 of the first phase of the three-phase AC system 10a,

(37) 6) the third (merged internal) converter arm 5ce of the four converter arms,

(38) 7) the phase terminal y.sub.1 of the positive conductor of the single-phase AC system 10b or the phase terminal y.sub.2 of the negative conductor if the polarity of the single-phase AC system is reversed, i.e., the phase terminal of one of the positive conductor or the negative conductor of the single-phase AC system,

(39) 8) the fourth converter arm 5d of the four converter arms, and

(40) 9) the phase terminal y.sub.2 of the negative conductor of the single-phase AC system 10b or the phase terminal y.sub.1 of the positive conductor if the polarity of the single-phase AC system is reversed, i.e., the phase terminal of the other of the positive conductor or the negative conductor of the single-phase AC system.

(41) It is noted that the embodiments of FIGS. 2 and 3, having at least four converter arms, all have the following sequence in common:

(42) 1) the phase terminal x.sub.3 of the third phase of the three-phase AC system 10a,

(43) 2) the first converter arm 5a of the at least four converter arms,

(44) 3) the phase terminal x.sub.2 of the second phase of the three-phase AC system 10a,

(45) 4) the second converter arm 5b of the at least four converter arms,

(46) 5) the phase terminal x.sub.1 of the first phase of the three-phase AC system 10a,

(47) 6) the third converter arm 5c or 5ce of the at least four converter arms,

(48) 7) the phase terminal y.sub.1 of the positive conductor of the single-phase AC system 10b or the phase terminal y.sub.2 of the negative conductor if the polarity of the single-phase AC system is reversed, i.e., the phase terminal of one of the positive conductor or the negative conductor of the single-phase AC system,

(49) 8) the fourth converter arm 5d of the at least four converter arms, and

(50) 9) the phase terminal y.sub.2 of the negative conductor of the single-phase AC system 10b or the phase terminal y.sub.1 of the positive conductor if the polarity of the single-phase AC system is reversed, i.e., the phase terminal of the other of the positive conductor or the negative conductor of the single-phase AC system.

(51) FIG. 4 illustrates another embodiment of the MMC 2 having five converter arms 5a, 5b, 5c, 5d and 5e, in which the converter arms are arranged in the ring as in the embodiments of FIGS. 2A, 2B and 2C, but where some of the phase terminals x.sub.1, x.sub.2, x.sub.3, y.sub.1 and y.sub.2 are positioned differently in the ring, illustrating that the positioning of the phase terminals in the ring is not limiting to the invention.

(52) Below, the sequence in the ring formed by the converter arms and phase terminals of the embodiment of FIG. 4 is given in a clockwise direction starting from the phase terminal before the first converter arm 5a. However, it should be noted that if the same embodiment is seen from the opposite direction, the same sequence would be in the counter clockwise direction. Thus, the five converter arms and the five phase terminals of the embodiment of FIG. 4 are in the ring arranged in the (clockwise or counter clockwise) sequence of:

(53) 1) the phase terminal y.sub.1 or y.sub.2 of one of the positive or negative conductors of the single-phase AC system 10b,

(54) 2) the first converter arm 5a of the converter arms,

(55) 3) the phase terminal x.sub.2 of the second phase of the three-phase AC system 10a,

(56) 4) the second converter arm 5b of the converter arms,

(57) 5) the phase terminal y.sub.2 or y.sub.1 of the other of the positive or negative conductors of the single-phase AC system 10b,

(58) 6) the third converter arm 5c of the converter arms,

(59) 7) phase terminal x.sub.1 of the first phase of the three-phase AC system 10a,

(60) 8) the fourth converter arm 5d of the converter arms,

(61) 9) the phase terminal x.sub.3 of the third phase of the three-phase AC system 10a, and

(62) 10) the fifth converter arm 5e of the converter arms.

(63) Again, it is noted that a circulating current Io is circulated in the ring through all of the converter arms also in the embodiment of FIG. 4.

(64) FIG. 5 illustrates a converter cell 4, here a full-bridge converter cell comprising a full-bridge topology of four semiconductor switches S1, S2, S3 and S4 connected across an energy storage 50 and allowing the cell to be bi-polar. The energy storage may typically comprise at least one capacitor.

(65) Embodiments of the converter 2 may be used with any nominal voltages of the three-phase and/or single-phase AC systems 10a and 10b, e.g., high-voltage of at least 80 kV, but some embodiments may be especially useful with medium voltage AC systems 10a and/or 10b having a nominal voltage within the range of 1-80 kV, e.g., within the range of 15-30 kV.

(66) The three-phase and single-phase AC systems 10a and 10b may have any, same or different (but preferably different), nominal fundamental frequency. In case, e.g., of a three-phase national distribution grid 10a, the nominal frequency may be 50 or 60 Hz. In case, e.g., of a single-phase railway system 10b, the nominal frequency may be 25 Hz (which is standard in North America), or 16.7 or 50/3 Hz (which is standard in some European countries). The circulating current I.sub.o typically has the same nominal fundamental frequency as the three-phase system 10a, e.g., 50 or 60 Hz, to efficiently balance the arms 5.

(67) The present disclosure has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the present disclosure, as defined by the appended claims.