AC to ac mmc with reduced number of converter arms

Abstract

This disclosure relates to an AC-to-ACMC (2) configured to connect between a three-phase AC system (10a) and a single-phase AC system (10b). The MMC includes at least four converter arms (5) connected in a ring to allow a circulating current (I0) to circulate within the ring through each of the at least four converter arms. Each arm includes a plurality of converter units (4) connected in series. The MMC also includes phase terminals (x, y) arranged in a ring between the at least four converter arms, such that each of the at least four converter arms is separated from an adjacent converter arm on either side of the ring by at least one of the phase terminals. The phase terminals include corresponding terminals (x1, x2, x3) for each of the three phases of the three-phase AC system and corresponding terminals (y1, y2) for each of the positive and negative conductors of the single-phase AC system.

Description

Technical Field

[0001] This disclosure relates to MMCs connecting three-phase AC systems and single-phase AC systems. Background Technology

[0002] Modular multilevel converters (MMCs) are used in medium-voltage (MV) and high-voltage (HV) converter applications. An MMC consists of converter arms of series-connected (also known as cascaded) converter units, each unit including an energy storage device (typically a capacitor) and multiple semiconductor valves that form a full-bridge (also known as an H-bridge or bipolar) or half-bridge (also known as a unipolar) topology for the unit.

[0003] Direct converters with a binary star (also known as a binary Y or binary wye) topology in Figure 1a As shown, it is used, for example, in railway interlocking power grids for three-phase alternating current (AC) to single-phase alternating current (IAC) conversion (e.g., 3AC 50Hz to IAC 16.7Hz (or 60Hz to 25Hz) conversion) for low-frequency overhead contact systems, but this is not the most suitable solution when the input and output frequencies are equal (e.g., 50Hz to 50Hz). According to... Figure 1b An alternative solution could be an indirect conversion via a DC link, where an intermediate conversion step decouples the two AC systems and allows for fault-free switching between any input and output frequencies.

[0004] Figure 1a and Figure 1b The problem with known MMC topologies is that their main cost is allocated to their high-cost modular arms. Figure 1a The direct converter requires six converter arms, which are equivalent to three phase feet (one phase foot consists of two converter arms: an upper converter arm and a lower converter arm). Additionally, Figure 1b The indirect converter requires ten converter arms (equivalent to five phase pins), which significantly increases the total cost. Therefore, minimizing the number of converter arms as much as possible while maintaining overall performance may be crucial in reducing the overall cost of the converter and presenting a new techno-economically optimal topology.

[0005] For example, the increasing electrification of railway locomotives worldwide has driven the development and installation of efficient power electronic converters for handling railway power supplies. In many cases, in addition to the typical three-phase power supply, locomotives require single-phase voltage supplies with frequencies that are the same as or different from the main three-phase power grid. Therefore, specialized power electronic topologies are used to perform the necessary voltage / power conversion between the main power grid and the railway power supply system, which are considered to be mutually decoupled. Summary of the Invention

[0006] The purpose of this invention is to provide an AC to ACMMC with reduced complexity and cost.

[0007] According to one aspect of the invention, an AC-to-ACMC (MMC) configured to connect between a three-phase AC system and a single-phase AC system is provided. The MMC includes at least four converter arms connected in a ring to allow circulating current to circulate within the ring through each of the at least four converter arms. Each arm includes a plurality of converter units connected in series. The MMC also includes phase terminals arranged in a ring between the at least four converter arms, such that each of the at least four converter arms corresponds to any side of the ring and is separated from an adjacent converter arm by at least one of the phase terminals. The phase terminals include a corresponding terminal for each of the three phases of the three-phase AC system and a corresponding terminal for each of the positive and negative conductors of the single-phase AC system.

[0008] According to another aspect of the invention, an MMC device is provided, the MMC device including an embodiment of the MMC of the present disclosure connected between a three-phase and a single-phase AC system, and a first power transformer through which the three-phase AC system is connected to the MMC.

[0009] By arranging the converter arms in a ring, a direct three-phase to single-phase converter is obtained, which reduces the number of converter arms and lowers the cost and complexity of the MMC topology. To balance the DC capacitor voltage of the converter unit in the converter arm, a specific frequency and phase current I0 circulates in the ring formed by the converter arms through each arm of the series-connected converter unit.

[0010] It should be noted that any feature of any of these aspects may be applied to any other aspect, as appropriate. Similarly, any advantage of any of these aspects may be applied to any of the other aspects. Further objects, features, and advantages of the appended embodiments will become apparent from the following detailed disclosure, from the appended dependent claims, and from the accompanying drawings.

[0011] Generally, all terms used in the claims should be interpreted according to their ordinary meaning in the technical field, unless expressly defined herein. All references to “a / an / element, device, component, apparatus, step, etc.” should be interpreted as referring to at least one instance of that element, device, component, apparatus, step, etc., unless expressly stated otherwise. Unless expressly stated otherwise, the steps of any method disclosed herein need not be performed in the exact order disclosed. The use of “first,” “second,” etc., for different features / components of this disclosure is intended only to distinguish the feature / component from other similar features / components, and not to assign any order or hierarchy to the features / components. Attached Figure Description

[0012] Embodiments will be described by way of example with reference to the accompanying drawings, in which:

[0013] Figure 1a and Figure 1b This is a schematic diagram of different MMC topologies in existing technologies.

[0014] Figure 2a This is a schematic circuit diagram of an MMC topology for a three-phase to single-phase converter according to some embodiments of the present invention.

[0015] Figure 2b This is a schematic circuit diagram of an MMC topology for a three-phase to single-phase converter according to some embodiments of the present invention, wherein the single-phase system is connected via a transformer.

[0016] Figure 2c This is for a three-phase to single-phase converter (e.g., as per some embodiments of the invention) Figure 2a or Figure 2b A schematic circuit diagram of the MMC topology shown in the figure, in which the current path is schematically illustrated.

[0017] Figure 3 This is a schematic circuit diagram of another MMC topology for a three-phase to single-phase converter according to some embodiments of the present invention.

[0018] Figure 4 This is a schematic circuit diagram of another MMC topology for a three-phase to single-phase converter according to some embodiments of the present invention.

[0019] Figure 5 This is a schematic circuit diagram of a full-bridge converter unit of an MMC according to some embodiments of the present invention. Detailed Implementation

[0020] Embodiments will now be described more fully below with reference to the accompanying drawings, in which some embodiments are illustrated. However, many other embodiments of different forms are possible within the scope of this 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 this disclosure to those skilled in the art. Throughout the description, the same numerals refer to the same elements.

[0021] Figure 2a An MMC device 1, including an AC-to-AC converter 10a (e.g., a distribution network) and a single-phase AC system 10b (e.g., a railway power grid for powering at least one train 20), is shown. Figure 2a In the embodiment shown, the single-phase system 10b is grounded and directly connected to the MMC2, i.e., without via a transformer. On the other hand, on the three-phase side, the MMC2 is connected to the three-phase system 10a via a first transformer 3a (e.g., a standard three-phase, VV, or Scott transformer).

[0022] Observing the three-phase side of MMC2, the three-phase system 10a includes the grid side (primary side) of the first transformer 3a with a grid side voltage U. g1 The first phase, with grid-side voltage U g2 The second phase and having grid-side voltage U g3 The third phase, thus causing the converter-side voltage U on the converter side (secondary side) of the first transformer 3a. c1 U c2 and U c3 Each of the first, second, and third phases of the three-phase system 10a is connected on the converter side of the first transformer 3a to the corresponding first phase terminal x1, second phase terminal x2, and third phase terminal x3 in the MMC2.

[0023] Observing the single-phase side of MMC2, the single-phase system 10b has, for example, a single-phase voltage U of a railway power grid. train and current I train Single-phase voltage U train Between the positive and negative conductors of single-phase system 10b. The positive conductor is connected to the positive terminal y1 of MMC2, and the negative conductor is connected to the negative terminal y2 of MMC. It should be noted that the polarity of single-phase system 10b can be... Figure 2a The polarities shown are opposite, causing the positive terminal y1 and the negative terminal y2 to change positions relative to each other. For example... Figure 2a As shown, single-phase system 10b can be grounded, which facilitates direct connection of single-phase system 10b to MMC2 without the need for an intermediate transformer.

[0024] The converter 2 includes a plurality of at least four converter arms 5 connected in a ring to allow current to circulate through all the converter arms 5 in the ring. The converter arms forming a ring means that each of the at least four arms 5 is connected in a ring, with two adjacent arms connected to that arm and one adjacent arm connected to either side of that arm. Each converter arm 5 includes a plurality of converter units 4 connected in series (also referred to as cascaded), preferably full-bridge (also referred to as bipolar or H-bridge) converter units, but in some embodiments, some or all of the units 4 in one, some, or all of the arms 5 may be half-bridge (unipolar) units. For example, arms 5d, 5e, and 5c may include or be composed of half-bridge units, while arms 5a and 5b will be composed of full-bridge units. A corresponding phase reactor 6 may be connected in the ring at each arm 5 and in series with each arm. Three-phase phase terminals x1, x2 and x3, as well as single-phase phase terminals y1 and y2, are also included in the ring and connected in a ring-like manner such that each of the at least four converter arms 5 corresponds to any side of the ring and is separated from its adjacent converter arm by at least one of the phase terminals, i.e., at least one of the phase terminals is arranged between every two adjacent arms 5 in the ring.

[0025] exist Figure 2a In this embodiment, at least four converter arms of the MMC2 are composed of five converter arms 5a, 5b, 5c, 5d, and 5e. Below, starting from the third phase terminal x3 and proceeding clockwise, the configurations are described... Figure 2a The embodiment shows a sequence of rings formed by the converter arms and phase terminals. However, it should be noted that if the same embodiment is viewed from the opposite direction, the same sequence will appear counterclockwise. Therefore, Figure 2a In this embodiment, the five converter arms and five phase terminals are arranged in a loop in the following sequence (clockwise or counterclockwise):

[0026] 1) The third phase terminal x3 of the three-phase AC system 10a,

[0027] 2) The first converter arm 5a of the five converter arms,

[0028] 3) Phase terminals x2 of the second phase of the three-phase AC system 10a,

[0029] 4) The second converter arm 5b of the five converter arms,

[0030] 5) Phase terminal x1 of the first phase of the three-phase AC system 10a,

[0031] 6) The third converter arm, 5c, of the five converter arms.

[0032] 7) The positive conductor phase terminal y1 of the single-phase AC system 10b (or the negative conductor phase terminal y2 if the polarity of the single-phase AC system is reversed).

[0033] 8) The fourth converter arm, 5d, of the five converter arms.

[0034] 9) The negative conductor phase terminal y2 of the single-phase AC system 10b (or the positive conductor phase terminal y1 if the polarity of the single-phase AC system is reversed).

[0035] 10) The fifth converter arm (5e) of the five converter arms.

[0036] Figure 2b Another embodiment of the MMC device 1 is shown. The MMC device 1 is as follows: Figure 2a As discussed, but here MMC2 is connected to single-phase system 10b via a second (single-phase) transformer 3b. Through the second transformer 3b, single-phase system 10b does not require grounding.

[0037] Figure 2c MMC2 (for example, is shown) Figure 2a and Figure 2b The current path in some embodiments of any MMC. The first converter arm 5a is connected between the third phase terminal x3 and the second phase terminal x2, resulting in a voltage U2 across it and a current I2 passing through it in the current path from the third phase via the first converter arm 5a to the second phase, as shown by the arrow, where U2 = U c3 -U c2 The second converter arm 5b is connected between the first phase terminal x1 and the second phase terminal x2, resulting in a voltage U1 across it and a current I1 flowing through it in the current path from the first phase via the second converter arm 5b to the second phase, as shown by the arrow, where U1 = U c1 -U C2 .

[0038] The fourth converter arm 5d is connected between the positive terminal y1 and the negative terminal y2, resulting in a voltage across the fourth converter arm 5d equal to the potential difference between the positive and negative terminals y1 and y2. However, the current I in the single-phase system 10b... train The current flows through the other four converter arms 5e, 5a, 5b, and 5c in the current path indicated by the arrows. This means that the first converter arm 5a and the second converter arm 5b carry the current of both the three-phase system 10a and the single-phase system 10b (which typically have different frequencies).

[0039] The third converter arm 5c and the fifth converter arm 5e can be considered as internal arms, and these internal arms are... Figure 2c Each of the embodiments has two ends (U1-U2-U) train The voltage is 1 / 2, but in the opposite direction as indicated by the arrow.

[0040] The voltage and current on the three-phase side can be described as:

[0041] U1(t)=U in ·cos(ω in t) (1)

[0042] U2(t)=U in ·cos(ω in t+θ1) (2)

[0043]

[0044]

[0045] The voltage and current on a single-phase side can be described as follows:

[0046] U train (t)=U out ·cos(ω out t+θ2) (5)

[0047]

[0048] U in It is the magnitude of each of U1 and U2, I m It is the magnitude of each of I1 and I2, U out It's U train The amplitude, l out isI train The amplitude, ω in It is a three-phase frequency, and ω out This is the frequency of the single-phase system. For a given three-phase and single-phase voltage (usually predetermined), the desired power transfer P and power factor at the three-phase and single-phase sides will determine the amplitudes of the three-phase and single-phase currents. In addition to the five arms 5 of converter 2, the first arm 5a and the second arm 5b are connected to the fixed three-phase voltages U1(t) and U2(t), and the fourth arm 5d is connected to the fixed single-phase voltage U. train (t), the remaining voltage generated is [U1(t)-U2(t)-U train The two inner arms 5c and 5e of (t)] / 2, where the polarity is in Figure 2c As shown in the diagram. To balance the currents of all converter arms, the three-phase and single-phase currents are... Figure 2c The flow is as depicted, and the circulating current I0 must flow through all arms 5 of converter 2. This circulating current I0 has the following form:

[0049] I0(t)=I circ ·cos(ω in t+θ circ (7)

[0050] Among them I circ and θ circ It's U in Uo ut P, θ1, and function

[0051] Figure 3 Another embodiment of the proposed reduced direct MMC topology is shown, with a further reduced number of converter arms 5 (reduced to four converter arms). In this embodiment, each of them bears [U1(t)-U2(t)-U train The two inner arms 5c and 5e of Figure 2, which are the voltages of (t)] / 2, have been merged into a single unit located between the second arm 5b and the fourth arm 5d in the ring, bearing U1(t)-U2(t)-U train The voltage of (t) is borne by a single internal arm 5ce. This results in the third phase terminal x3 and the negative phase terminal y2 (or the positive phase terminal y, if the polarity of the single-phase system 10b is reversed) both located between two adjacent first arms 5a and fourth arms 5d in the ring. Alternatively, the merged internal arm 5ce can be located between the first arm 5a and the fourth arm 5d, which would result in the first phase terminal x1 and the positive phase terminal y1 (or the negative phase terminal y2 if the polarity of the single-phase system 10b is reversed) both located between two adjacent second arms 5b and fourth arms 5d in the ring. Note that the circulating current I0 also circulates in the ring through all converter arms 5a, 5b, 5ce and 5d in this embodiment.

[0052] Below, starting from the third phase terminal x3, the diagram is given in a clockwise direction. Figure 3 The embodiment shows a sequence of rings formed by the converter arms and phase terminals. However, it should be noted that if the same embodiment is viewed from the opposite direction, the same sequence will be in a counter-clockwise direction. Therefore, Figure 2a In this embodiment, the four converter arms and five phase terminals are arranged in a loop in the following sequence (clockwise or counterclockwise):

[0053] 1) The third phase terminal x3 of the three-phase AC system 10a,

[0054] 2) The first converter arm 5a of the four converter arms,

[0055] 3) Phase terminals x2 of the second phase of the three-phase AC system 10a,

[0056] 4) The second converter arm 5b of the four converter arms,

[0057] 5) Phase terminal x1 of the first phase of the three-phase AC system 10a,

[0058] 6) The third (inner) converter arm of the four converter arms, 5ce.

[0059] 7) The positive conductor phase terminal y1 of the single-phase AC system 10b (or the negative conductor phase terminal y2 if the polarity of the single-phase AC system is reversed).

[0060] 8) The fourth converter arm 5d of the four converter arms, and

[0061] 9) The negative conductor phase terminal y2 of the single-phase AC system 10b (or the positive conductor phase terminal y1 if the polarity of the single-phase AC system is reversed).

[0062] Note that Figure 2 and [the other figure] have at least four converter arms. Figure 3 All the embodiments commonly have the following sequence:

[0063] 1) The third phase terminal x3 of the three-phase AC system 10a,

[0064] 2) The first converter arm 5a of at least four converter arms,

[0065] 3) Phase terminals x2 of the second phase of the three-phase AC system 10a,

[0066] 4) At least the second converter arm 5b of the four converter arms,

[0067] 5) Phase terminal x1 of the first phase of the three-phase AC system 10a,

[0068] 6) At least the third converter arm of the four converter arms, either 5c or 5ce.

[0069] 7) The positive conductor phase terminal y1 of the single-phase AC system 10b (or the negative conductor phase terminal y2 if the polarity of the single-phase AC system is reversed).

[0070] 8) At least the fourth converter arm 5d of the four converter arms, and

[0071] 9) The negative conductor phase terminal y2 of the single-phase AC system 10b (or the positive conductor phase terminal y1 if the polarity of the single-phase AC system is reversed).

[0072] Figure 4 Another embodiment of the MMC2 with five converter arms 5a, 5b, 5c, 5d, and 5e is shown, wherein the converter arms are as follows: Figure 2a , 2b As in embodiment 2c, the phase terminals are arranged in a ring, but some of the phase terminals x1, x2, x3, y1, and y2 are positioned differently in the ring, demonstrating that the positioning of the phase terminals in the ring does not limit the invention.

[0073] Below, starting from the phase terminal before the first switching arm 5a, the sequence is given in a clockwise direction. Figure 4 The embodiment shows a sequence of rings formed by the converter arms and phase terminals. However, it should be noted that if the same embodiment is viewed from the opposite direction, the same sequence will be in a counter-clockwise direction. Therefore, Figure 4 In this embodiment, the five converter arms and five phase terminals are arranged in a loop in the following sequence (clockwise or counterclockwise):

[0074] 1) One of the phase terminals y1 or y2 of the positive or negative conductor in a single-phase AC system 10b.

[0075] 2) The first converter arm 5a in the converter arm,

[0076] 3) Phase terminals x2 of the second phase of the three-phase AC system 10a,

[0077] 4) The second converter arm 5b in the converter arm,

[0078] 5) The other phase terminal y2 or y1 of the positive or negative conductor in the single-phase AC system 10b,

[0079] 6) The third converter arm 5c in the converter arm,

[0080] 7) Phase terminal x1 of the first phase of the three-phase AC system 10a,

[0081] 8) The fourth converter arm 5d in the converter arm,

[0082] 9) The third phase terminal x3 of the three-phase AC system 10a, and

[0083] 10) The fifth converter arm 5e in the converter arm.

[0084] Again, note that the circulating current I0 is also... Figure 4 In one embodiment, all converter arms are circulated in a loop.

[0085] Figure 5 The converter unit 4 is shown, here a full-bridge converter unit, which includes a full-bridge topology with four semiconductor switches S1, S2, S3 and S4 connected across the energy storage device 50, and allows the unit to be bipolar. The energy storage device typically includes at least one capacitor.

[0086] Embodiments of converter 2 can be used at any nominal voltage (e.g., at least 80 kV high voltage) in three-phase and / or single-phase AC systems 10a and 10b, but some embodiments may be particularly useful in the case of medium-voltage AC systems 10a and / or 10b with nominal voltages in the range of 1 to 80 kV (e.g., in the range of 15 to 30 kV).

[0087] Three-phase and single-phase AC systems 10a and 10b can have any same or different (but preferably different) nominal fundamental frequency. For example, in the case of a three-phase national distribution network 10a, the nominal frequency can be 50 or 60 Hz. For example, in the case of a single-phase railway system 10b, the nominal frequency can 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 I0 typically has the same nominal fundamental frequency (e.g., 50 or 60 Hz) as the three-phase system 10a to effectively balance arm 5.

[0088] The present disclosure has been described above primarily with reference to several embodiments. However, as will be readily understood by those skilled in the art, other embodiments besides those disclosed above are also possible within the scope of the present disclosure as defined by the appended claims.

Claims

1. An AC-to-AC MMC (2) configured to connect between a three-phase AC system (10a) and a single-phase AC system (10b), said MMC comprising: At least four converter arms (5) are connected in a ring to allow a circulating current (I0) to circulate through each of the at least four converter arms in the ring, and each arm (5) includes a plurality of converter units (4) connected in series. as well as Phase terminals (x, y) are arranged in the ring between the at least four converter arms (5) such that each of the at least four converter arms corresponds to any side of the ring and is separated from the adjacent converter arm by at least one phase terminal, the phase terminals including a corresponding terminal (x1, x2, x3) for each of the three phases of the three-phase AC system (10a) and a corresponding terminal (y1, y2) for each of the positive and negative conductors of the single-phase AC system (10b).

2. The MMC according to claim 1, wherein the at least four converter arms (5a, 5b, 5c / 5ce, 5d) and phase terminals (x1, x2, x3, y1, y2) are arranged in a loop in the following sequence: -The phase terminal (x3) of the third phase of the three-phase AC system (10a), - The first converter arm (5a) of the at least four converter arms -The phase terminal (x2) of the second phase of the three-phase AC system (10a), - The second converter arm (5b) of the at least four converter arms -The phase terminal (x1) of the first phase of the three-phase AC system (10a), -The third converter arm (5c / 5ce) of the at least four converter arms -The phase terminal (y1 / y2) of one of the positive or negative conductors of the single-phase AC system (10b), -The fourth converter arm (5d) of the at least four converter arms, and - The phase terminal (y2 / y1) of the other of the positive or negative conductor of the single-phase AC system (10b).

3. The MMC according to claim 1, wherein the at least four converter arms (5) are composed of four converter arms (5a, 5b, 5ce, 5d).

4. The MMC according to claim 1, wherein the at least four converter arms (5) are composed of five converter arms (5a, 5b, 5c, 5d, 5e).

5. The MMC according to claim 4, wherein the converter arms (5a, 5b, 5c, 5d, 5e) and phase terminals (x1, x2, x3, y1, y2) are arranged in a loop in the following sequence: -The phase terminal (x3) of the third phase of the three-phase AC system (10a), -The first converter arm (5a) of the converter arm -The phase terminal (x2) of the second phase of the three-phase AC system (10a), -The second converter arm (5b) of the converter arm -The phase terminal (x1) of the first phase of the three-phase AC system (10a), -The third converter arm (5c) of the converter arm -The phase terminal (y1 / y2) of one of the positive or negative conductors of the single-phase AC system (10b), -The fourth converter arm (5d) of the converter arm -The phase terminal (y2 / y1) of the other of the positive or negative conductor of the single-phase AC system (10b), and - The fifth converter arm (5e) of the converter arm.

6. The MMC according to claim 4, wherein the converter arms (5a, 5b, 5c, 5d, 5e) and phase terminals (x1, x2, x3, y1, y2) are arranged in a loop in the following sequence: -The phase terminal (y1 / y2) of one of the positive or negative conductors of the single-phase AC system (10b), -The first converter arm (5a) of the converter arm -The phase terminal (x2) of the second phase of the three-phase AC system (10a), -The second converter arm (5b) of the converter arm -The phase terminal (y2 / y1) of the other of the positive or negative conductors in the single-phase AC system (10b), -The third converter arm (5c) of the converter arm -The phase terminal (x1) of the first phase of the three-phase AC system (10a), -The fourth converter arm (5d) of the converter arm -The phase terminal (x3) of the third phase of the three-phase AC system (10a), and - The fifth converter arm (5e) of the converter arm.

7. The MMC according to any one of claims 1 to 6, wherein the three-phase AC system (10a) has a nominal frequency of 50 Hz or 60 Hz.

8. The MMC according to any one of claims 1 to 6, wherein the single-phase AC system (10b) has a nominal frequency of 50 Hz, 60 Hz, 25 Hz, 16.7 Hz or 50 / 3 Hz.

9. An MMC device (1), comprising: According to any one of the preceding claims, the MMC (2) is connected between the three-phase AC system and the single-phase AC system (10a, 10b); and The first power transformer (3a) is used to connect the three-phase AC system (10a) to the MMC.

10. The MMC device according to claim 9, wherein the single-phase AC system (10b) is grounded and the MMC (2) is directly connected to the single-phase AC system, and there is no second power transformer (3b) between the MMC (2) and the single-phase AC system.

11. The MMC device according to claim 9, further comprising: The second power transformer (3b) is used to connect the single-phase AC system (10b) to the MMC (2).

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