Modular multilevel converter with several cell group controllers per phase arm

Abstract

A method (200) of controlling a plurality of converter cells (240) in a phase leg of a modular multilevel converter, MMC, according to a master reference (210) is provided. The method comprises dividing (212) the master reference into a plurality of reference portions (220), and operating a plurality of modulators in parallel by using the plurality of converter cells grouped into a plurality of groups (250), each modulator controlling a respective one of the groups of converter cells according to one of the reference portions. A phase leg controlled according to such a method is also provided, as well as an MMC comprising at least one such phase leg, and a converter station comprising at least one such MMC.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the field of Modular Multilevel Converters, MMC, for power conversion. In particular, the present disclosure relates to control of phase arms in such MMCs. BACKGROUND

[0002] Due to the performance of Modular Multilevel Converters, MMC, in terms of, for example, reduced harmonics and increased efficiency, they are used to transfer electrical power between different types of power grids, for example between AC and DC grids. The harmonic performance of MMCs can be at least partly attributed to their use of a large number of converter cells (or submodules) in each phase arm (or phase valve), which allows the MMC to synthesize a voltage waveform that closely matches, for example, a sinusoidal voltage waveform.

[0003] However, due to the large number of converter cells (often in the hundreds or more in each phase arm), the control of MMCs is more complex compared to other converter topologies, such as older two-level converters. Balancing the converter cell voltages of all converter cells multiple times per second can require a control unit with a large amount of computational power to control all converter cells according to a main reference. This can also limit the overall performance of the MMC in case the available computational power is limited. SUMMARY

[0004] The present disclosure seeks to at least partly improve the problems discussed above. To achieve this, a method of controlling a plurality of converter cells in a phase arm of an MMC, a phase arm for an MMC, an MMC and a converter station are provided as defined in the independent claims. Further embodiments are provided in the dependent claims.

[0005] According to a first aspect of the present disclosure, a method of controlling a plurality of converter cells in a phase arm (or phase valve) of an MMC according to a main reference is provided. The method can comprise dividing the main reference into a plurality of reference portions. The method can further comprise operating a plurality of modulators in parallel by grouping the plurality of converter cells into a plurality of groups, wherein each modulator controls a respective one of the groups of converter cells according to one of the reference portions. The MMC can for example be a Voltage Source Controller (VSC).

[0006] In this context, grouping the converter cells into a plurality of groups can not necessarily require them to be physically grouped together. In other words, grouping the converter cells into groups can also be purely logical / abstract, such that the grouping can be changed at a later stage without requiring any or major physical changes to the phase arm, for example.

[0007] A "master reference" can for example be a control signal indicative of a voltage value that is to be tracked (e.g. synthesized) by suitably controlling the converter cells in a phase leg. Operating a phase leg in accordance with such a master reference is considered to mean that the output of the phase leg (i.e. the combined voltage of all its converter cells) substantially matches (or tracks) the master reference (e.g. at a certain moment in time, or also over time).

[0008] A "modulator" can for example be a device or control function (implemented in hardware, software or a combination thereof) that takes a reference portion as input and controls a group of converter cells such that the combined output voltage of the group of converter cells coincides with (e.g. synthesizes) the received reference portion. As will be described in more detail herein, such control of a converter cell can comprise the modulator sending one or more command signals to the converter cell such that the converter cell can insert or bypass its storage capacitor into / from the current path between the terminals of the converter cell.

[0009] A "reference portion" can for example be a partial contribution to the master reference such that if each modulator is able to control its group of converter cells to track the reference portion, the combined state of all converter cells in the phase leg will still coincide with the master reference.

[0010] Operating modulators in parallel is considered herein to mean that for example two modulators can simultaneously focus on their respective tasks, i.e. such that each modulator individually (in other words, independently of what the other modulator is doing) causes or controls its respective group of converter cells to output a combined voltage, e.g. in accordance with a reference portion. In this context, it is noted that "independently of" or "individually" does not necessarily prohibit any exchange of information between the modulators, whatever it may be. It is envisaged that even if there is for example a synchronization information or similar some exchange between the modulators, their operation is still considered to be in parallel as long as the modulators simultaneously try to control their respective group of converter cells in accordance with their respective reference portion.

[0011] As will be described later herein, a transition from the conventional use of a single modulator (which needs to control all converter cells in a phase leg) to the use of multiple modulators operating in parallel (each controlling a respective group of converter cells) can for example reduce the time required to control all converter cells in a phase leg to a desired state. As a result, the performance of the MMC can be improved due to the overall delay of the control being reduced.

[0012] In some embodiments, the master reference can be a voltage value, and the master reference can be divided such that the sum of the reference portions corresponds to (i.e. is equal to or at least approximately equal to) the voltage value of the master reference.

[0013] In some embodiments, each modulator can control a group of its respective converter cells according to a sorting criterion. As already mentioned above, using multiple modulators operating in parallel to sort the converter cells in a phase arm (rather than using a single modulator) can improve the performance of the MMC.

[0014] In some embodiments, such a sorting criterion can include increasing or decreasing the voltage of the converter cells.

[0015] In some embodiments, the method can include each modulator balancing the voltage of its respective group of converter cells based on the sorting. For example, the modulators can be operated such that the converter cell having the lowest voltage (and / or the lowest energy level) is charged first during a charging cycle of the phase arm, and such that the converter cell having the highest voltage (and / or the highest energy level) is discharged first during a discharging cycle of the phase arm. By properly balancing the cell voltages, such that the cells are charged and discharged at the right time, the efficiency of the power conversion performed by the MMC can be improved, for example.

[0016] In some embodiments, a group of converter cells controlled by one of the modulators can include a first type of converter cell, and another group of converter cells controlled by another one of the modulators can include a second type of converter cell different from the first type.

[0017] According to a second aspect of the present disclosure, a phase arm (or phase valve) for an MMC is provided. The phase arm can include a plurality of modulators (such as described above) and a plurality of converter cells that can be grouped into a plurality of groups of converter cells (also such as described above). Each modulator can be configured to receive one reference portion of a main reference, and to control a respective one of the groups of converter cells according to the received reference portion independently of and in parallel with any other modulator in the phase arm.

[0018] In some embodiments, the phase arm can be further configured to receive the main reference, to divide the received main reference into a plurality of reference portions, and to provide one of the reference portions to each modulator.

[0019] In some embodiments, a group of converter cells controlled by one of the modulators can include a first type of converter cell, and another group controlled by another one of the modulators can include a second type of converter cell different from the first type.

[0020] In some embodiments, the first type can be a half-bridge (HB) converter cell type and the second type can be a full-bridge (FB) converter cell type.

[0021] According to a third aspect of the disclosure, a modular multilevel converter (MMC) is provided. The MMC can comprise at least one phase leg as described herein with reference to the first aspect and / or the second aspect.

[0022] According to a fourth aspect of the disclosure, a converter station is provided. The converter station can comprise at least one MMC as described herein with reference to the third aspect. In some embodiments, the station can be an HVDC converter station.

[0023] The disclosure relates to all possible combinations of the features recited in the claims. The objects and features described according to the first aspect can be combined with or replaced by the objects and features described according to the second aspect, the third aspect and / or the fourth aspect, or vice versa.

[0024] Other objects and advantages of embodiments of the disclosure will be described below by way of example embodiments. BRIEF DESCRIPTION OF DRAWINGS

[0025] The exemplary embodiments will be described below with reference to the accompanying drawings, in which:

[0026] Figure 1a schematically illustrates a modular multilevel converter MMC comprising a phase leg according to one or more example embodiments of the disclosure;

[0027] Figure 1b schematically illustrates a converter station comprising at least one MMC according to one or more example embodiments of the disclosure, and

[0028] Figure 2 schematically illustrates a strategy (method) of controlling a phase leg according to one or more example embodiments of the disclosure.

[0029] In the drawings, like reference numerals will be used to refer to like elements unless otherwise indicated. The drawings are only schematic and they are provided to illustrate example embodiments of the application and to provide a conceptual understanding of the structures and arrangements of the subject matter. Unless explicitly indicated, the drawings are not necessarily to scale. In order to assist with understanding the present disclosure and, in general, to assist with understanding the various examples of the application, reference is made to the drawings wherein: DETAILED DESCRIPTION

[0030] REFERENCE Figure 1a , Figure 1band Figure 2 The concept of the present disclosure will now be described in more detail.

[0031] Figure 1a An example of an MMC 100 according to one example embodiment of the present disclosure is schematically illustrated. The MMC 100 can for example be a voltage source controller (VSC). The MMC 100 is arranged to convert power between an AC side 110 and a DC side comprising a first DC terminal 120 and a second DC terminal 122. The AC side 110 can comprise one or more phases 112-114 and the MMC 100 can comprise one phase leg for each phase of the AC side 110.

[0032] Each such phase leg comprises two phase arms. In the following, only a single phase 112 of the AC side 110 and the part of the MMC 100 connected to this phase 112 will be discussed for illustrative purposes.

[0033] For the phase 112, the MMC 100 comprises one phase leg comprising an upper phase arm 130 and a lower phase arm 131. The upper phase arm 130 is connected between the first DC terminal 120 and the phase 112, while the lower phase arm 131 is connected between the phase 112 and the second DC terminal 122. Although not explicitly described herein, it is of course envisaged that similar arrangements of phase legs and phase arms 132, 133, 134 and 135 can be used for the other phases 113 and 114 of the AC side 110.

[0034] The upper phase arm 130 comprises a plurality of converter cells 140-146 connected in series between the first DC terminal 120 and the phase 112. A converter cell is envisaged as a functional unit comprising one or more storage capacitors (or similar devices for storing charge) and means for inserting such storage capacitor into the current path between the terminals of the converter cell or bypassing such storage capacitor from the current path between the terminals of the converter cell in a controlled manner. Such insertion or bypassing of the cell storage capacitor can be implemented using for example IGBTs or similar switching devices arranged and controlled in a suitable manner within the converter cell as known to the skilled person. The converter cell can for example be of the half bridge (HB) type or of the full bridge (FB) type, wherein in the latter case the converter cell can also control in which polarity to insert the storage capacitor into the current path between the cell terminals.

[0035] Of course, other elements can also be included in the phase arm 130, such as one or more inductors, additional switches, breakers, resistors or similar. Such additional elements are not illustrated in Figure 1a and are not discussed herein.

[0036] In the (upper) phase leg 130, the plurality of converter cells 140-146 are grouped into a plurality of groups 150, 151, and 152. In the current example, group 150 includes converter cells 140 and 141, group 151 includes converter cells 142 and 143, and group 152 includes the remaining converter cells 144, 145, and 146. It should be emphasized that the current example is for illustrative purposes only, and that, for example, one phase leg can include hundreds or more converter cells, depending on, for example, the required power rating. Likewise, it is of course also contemplated that one phase leg can have more than three groups 150, 151, and 152, and / or that each group can individually include more than two or three converter cells.

[0037] As described earlier herein, a "group of converter cells" does not necessarily imply or require a physical grouping of converter cells. A group can instead be a logical grouping of converter cells. Using the MMC 100 as an illustrative example, it is contemplated that, for example, a group can instead include converter cells 140 and 146, and a group can include converter cells 142, 143, 144, and 145. In other words, it is not required that two converter cells be physically close or belong to the same physical entity in order for them to belong to the same group. Figure 1a

[0038] ​In the phase arm 130 of the MMC 100, three modulators 160, 161, and 162 are also provided, each configured to control a corresponding group of converter units 150, 151, and 152. Using modulator 160 as an example, modulator 160 can control, for example, which of converter units 140 and 142 are in an “insertion” or “bypass” state, respectively. As described earlier herein, an “insertion” state means that the converter unit in question inserts its storage capacitor into the current path between its terminals, while a “bypass” state means that the converter unit in question does not insert its storage capacitor into the current path between its terminals (i.e., such that the current flowing between the terminals of the converter unit does not flow through one or more storage capacitors of the converter unit). By controlling the state of the converter units, modulator 160 can therefore adjust the total number of storage capacitors inserted in that current path, and thereby, for example, also adjust the total voltage across its terminals. For example, modulator 160 can make a decision on which converter units should be in an insertion or bypass state based on instructions provided to modulator 160. As will be described later herein, such instructions may include or correspond to a reference portion of a desired voltage, for example, across the terminals of a group 150 of converter units controlled by modulator 160. Modulator 160 can then determine how many of its converter units in group 150 need to be in an insertion state to satisfy that reference portion (e.g., the desired voltage). To make such a decision, modulator 160 may first measure (or be provided with) the charge (or energy) level (e.g., voltage) of the storage capacitors of its converter units, and then determine which converter units should be in what state. This decision may also be made based on, for example, the direction of the current in phase arm 130, or in combination with it. If the current in phase arm 130 is in a direction that allows charging of the storage capacitors of the converter units, modulator 160 can determine (by controlling the state of its converter units) which storage capacitors will be charged or not charged. Such a decision may also be made based on the measurement (or provision) of the charge level (i.e., the converter unit voltage) of each storage capacitor.

[0039] Figure 1b An embodiment of the converter station 170 according to the present disclosure is illustrated schematically.

[0040] The converter station 170 includes at least one MMC 101, 102, 103, and 104. One or more of MMC 101, 102, 103, and 104 may be as described above. Figure 1aThe described MMC 100. The MMCs 101-104 are connected at their AC side to a plurality of AC grids 180. The MMCs 101-104 are connected in series and in a bipolar configuration, with MMC 101 and MMC 104 being connected to respective DC poles 190 and 192 of a DC grid. The converter station 170 is configured to transfer power between the AC grids 180 and the DC grid, and can benefit from the control strategies described above due to comprising at least one MMC, wherein at least one phase arm is also controlled as described herein. In some embodiments, the converter station 170 can for example be an HVDC converter station.

[0041] Figure 2 One embodiment of a control strategy 200 (as implemented for example in a method and / or apparatus as described herein) for controlling a phase arm of an MMC according to the present disclosure is schematically illustrated.

[0042] In the strategy (or method) 200, a master reference 210 comprises for example a master reference voltage according to which the phase arm of the MMC is operated (i.e. controlled). In step 212, the master reference 210 is divided into a number of reference portions 220_1, 220_2,..., 220_N, where N is an integer greater than 1. Each reference portion 220_1, 220_2,..., 220_N is then provided to a respective modulator 260_1, 260_2,..., 260_N of the phase arm of the MMC. For example, modulator 260_1 can correspond to modulator 160 in phase arm 130 described herein with reference to Fig. 1, modulator 260_2 can correspond to modulator 161, and so on.

[0043] After having received its respective reference portion, each modulator 260_1, 260_2,..., 260_N uses its received reference portion to control a respective one of the groups 250_1, 250_2,..., 250_N of converter cells to which the modulator has been assigned. Each group 250_1, 250_2,..., 250_N comprises a respective plurality of converter cells {240_1_1, 240_1_2,..., 240_1_M1; 240_2_1, 240_2_2,..., 240_2_M2;... ; 240_N_1, 240_N_2,..., 240_N_M N}, where M j is an integer corresponding to the number of converter cells in group number j. As described earlier herein, the number and type of converter cells in each group can be different.

[0044] The modulators 260_1, 260_2,..., 260_N are operated in parallel, with each modulator 260_1, 260_2,..., 260_N operating (i.e. controlling its respective converter unit) in accordance with its respective reference portion 220_1, 220_2,..., 220_N. As part of being operated in parallel, each modulator 260_1, 260_2,..., 260_N individually controls its respective converter unit such that, for example, the output voltage controlled by one group of each modulator matches the reference portion assigned to that modulator.

[0045] In one more specific example, the main reference 210 can for example be a voltage reference V ref indicating a desired output voltage across the full phase leg. In step 212, the voltage reference V ref is divided into reference portions V1, V2,..., V N which are then provided to a respective one of the modulators 260_1, 260_2,..., 260_N. As one example, each reference portion can correspond to Nth of the main reference, such that V1= V2=... = V N = V ref / N. Such a structure can for example assume that there is an equal number of converter units in each group of converter units, i.e. M1= M2=... = M N If this is not the case, or if it is not appropriate for other reasons, it can instead be envisaged that, for example, V j = V ref *k j , where k j is a weighting factor which can for example depend both on N and on M j and thus be different for each or some of the groups and modulators. For example, it can be such that the reference portions are structured such that the sum of all V j ’ corresponds to (i.e. is equal to or at least approximates) V ref .

[0046] In this or other embodiments, the main reference (e.g. V ref ) can vary over time (e.g. V ref (t)). The reference portions can then be structured such that at one time instant, the modulators can be operated in parallel in accordance with their respective reference portions, such that at the same time instant, the phase leg is operated in accordance with the main reference. In other words, each reference portion can then be structured such that V j (t) is a function of V ref (t). For example, if V(t) = sin(a*t), where a is some constant, then V j(t) can be equal to, for example, V(t) / N, or V(t)*k j As described above, or similar.

[0047] The examples given above are of course only specific examples of how the reference part can be constructed, and it is envisaged that there are many other alternatives that will fall under the concept of the present disclosure.

[0048] A number of benefits of the concept of the present disclosure will now be described in more detail. Note that such benefits apply to all embodiments according to any aspect of the present disclosure described herein, if not explicitly stated to the contrary.

[0049] During operation of the MMC, the phase arms of the MMC can be continuously switched between being in a charging cycle (when the current in the phase arm moves in a first direction) and being in a discharging cycle (when the current in the phase arm moves in a second direction opposite the first direction). During the charging cycle, control of the phase arm can require determining which of the converter cells are to be in the inserted state, so that their storage capacitors can be charged. One control strategy can for example comprise measuring the cell voltages (i.e. the charge currently stored in the cell capacitors), and inserting the converter cell that currently has the lowest cell voltage (i.e. the converter cell corresponding to the lowest stored energy). To this end, it can be required to sort all converter cells according to their cell voltages. Such a sorting operation can comprise constructing a list of all voltages, and then sorting the list according to a sorting criterion, such as descending or ascending cell voltage. Likewise, during the discharging cycle, one control strategy can comprise inserting the converter cell that currently has the highest cell voltage (i.e. the converter cell corresponding to the highest stored energy). Such a strategy can thus also require a sorting operation.

[0050] Because the direction of the current in the phase arm can change several times per second, and because the update of the master reference value can be required more frequently, the list of cell voltages can be updated and resorted many times per second.

[0051] In a conventional MMC, where the converter cells of one phase arm are not controlled according to the present disclosure, a single modulator for that phase arm can be used, controlling all converter cells according to a single master reference. For modern MMCs, the number of converter cells in each arm can be large (e.g. comprising at least several hundreds or more converter cells), and thus the time required for controlling the phase arm for each instant can be large, as each sorting operation can require sorting of several hundreds or more items. This can increase the latency, and increase the demands on hardware and software in terms of, for example, required speed and memory consumption.

[0052] However, in an MMC utilizing the present disclosure, a task such as classification can be divided into a plurality of smaller tasks that can be performed in parallel, with each modulator independently classifying a much smaller list at a time. This can reduce latency and ease the requirements on, for example, hardware performance. In particular, since each modulator faces a simpler task, the individual modulators can be constructed less complexly than a conventional modulator responsible for controlling all of the converter cells of one phase arm.

[0053] An additional benefit can be that the converter cells of one group controlled by one modulator can be of a different type than the converter cells of one group controlled by another modulator. For example, one group can include full-bridge type converter cells, while another group can include half-bridge type converter cells. Since the modulators can each be constructed to handle and control one specific type of converter cell, the present disclosure can provide an improved way of handling a phase arm of mixed types. In a conventional MMC, a phase arm of mixed types would require a single modulator to be more complex, as it would have to account for differences between the different converter cell types in the phase arm it controls.

[0054] The use of multiple independent modulators according to the present disclosure can also, for example, enable "on-the-fly" maintenance of the converter cells in a phase arm. For example, instead of having to shut down the entire arm (and thus most likely the entire MMC as well), it can be envisioned that converter cells in one group can be replaced or repaired (and / or the modulator controlling the converter cells in that group itself) while the converter cells in the other group controlled by the other modulator continue to operate. Likewise, the independent modulators can also enable the "on-the-fly" addition and removal of one or more modulators (and thus groups of converter cells).

[0055] In summary, the present disclosure provides an improved way of controlling an MMC. This is achieved by dividing the converter cells of one phase arm of the MMC into a number of groups (physically or logically), where each group of converter cells can be controlled by its own modulator, which can perform the task of controlling that group of converter cells according to one reference portion of a master reference independently of and in parallel with the other modulators (and, for example, to perform cell voltage balancing within the group). Thus, latency can be reduced. This can further allow an increased control bandwidth of the MMC, such that the MMC can provide, for example, positive damping and control in higher frequency harmonics. Additionally, the control software and / or hardware used for each individual modulator can be less complex, as each modulator is only required to solve a fraction of a complete task (such as classifying a large list of converter cells according to a classification criterion).

[0056] Although features and elements are described above in particular combinations, each feature or element can be used alone without the other features and elements or in various combinations with or without other features and elements.

[0057] In addition, variations to the disclosed embodiments can become apparent to those of ordinary skill in the art upon reading the foregoing description by virtue of the appended claims. In the claims, the words "including" and "comprising" do not exclude the other elements not listed in the claims. The words "a" or "an" preceding an element in the claims do not exclude the presence of two or more such elements. The implementation with the features of the independent claims which are mutually different in themselves are not meant as alternatives but can be combined together.

Claims

1. A method (200) for controlling a plurality of converter units (240) in the phase arm of a modular multilevel converter (MMC) according to a master reference (210), comprising: The main reference is divided (212) into multiple reference parts (220), and By grouping the plurality of converter units into multiple groups (250), multiple modulators operate in parallel, each modulator controlling a corresponding group of converter units within its group according to a reference section, each modulator controlling its corresponding group of converter units according to a classification criterion, and each modulator balancing the voltage of its corresponding group of converter units based on the classification. in, One group of converter units controlled by one of the modulators includes converter units of the first type, and another group of converter units controlled by another modulator in the modulators includes converter units of the second type, which are different from the first type. The primary reference is the voltage value. The reference portion is constructed, at least in part, based on the number of converter units in a group according to a weighting factor, such that the sum of the reference portions corresponds to the voltage value, and The classification criteria include increasing or decreasing the converter unit voltage.

2. A phase arm (130) for a modular multilevel converter (MMC) (100), wherein the phase arm includes a plurality of modulators (160, 161, 162) and a plurality of converter units (140-146) capable of being divided into a plurality of groups (150, 151, 152) to form converter units, and in, One group of converter units controlled by one of the modulators includes converter units of a first type, and another group of converter units controlled by another modulator includes converter units of a second type, different from the first type. Each modulator is configured to: receive a reference portion of a master reference, and independently of and in parallel with any other modulator in the phase arm, control a corresponding group of converter units according to the received reference portion; each modulator controls its corresponding group of converter units according to a classification criterion; and each modulator balances the voltage of its corresponding group of converter units based on the classification, wherein the classification criterion includes increasing or decreasing the converter unit voltage, and The phase arm is configured as follows: Receive the main reference as the voltage value; The received master reference is divided into multiple reference portions based at least in part on the number of converter units in a group according to a weighting factor, such that the sum of the reference portions corresponds to the voltage value. One of the reference portions is provided to each modulator.

3. The phase arm according to claim 2, wherein the first type is a half-bridge HB converter unit type, and the second type is a full-bridge FB converter unit type.

4. A modular multilevel converter (MMC) (100) comprising at least one phase arm (130) according to any one of claims 2 to 3.

5. A converter station (170) comprising at least one MMC (100-104) according to claim 4.

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