Patent ID: 12212248

In the drawings, like reference numerals will be used for like elements unless stated otherwise. Unless explicitly stated to the contrary, the drawings show only such elements that are necessary to illustrate the example embodiments, while other elements, in the interest of clarity, may be omitted or merely suggested. As illustrated in the figures, the sizes of elements and regions may be exaggerated for illustrative purposes and, thus, are provided to illustrate the general structures of the embodiments.

DETAILED DESCRIPTION

With reference toFIGS.1a,1band2, the concept of the present disclosure will now be described in more detail.

FIG.1aillustrates schematically an example of an MMC100according to one example embodiment of the present disclosure. The MMC100may for example be a voltage source controller (VSC). The MMC100is arranged to convert power between an AC side110and a DC side (including a first DC terminal120and a second DC terminal122). The AC side110may include one or more phases112-114, and the MMC100may include one phase leg for each phase of the AC side110.

Each such phase leg includes two phase arms. In what follows, for illustrative purposes, only the single phase112of the AC side110will be discussed, together with the parts of the MMC100connected to this phase112.

For the phase112, the MMC100includes a phase leg including an upper phase arm130and a lower phase arm131. The upper phase arm130is connected between the first DC terminal120and the phase112, while the lower phase arm131is connected between the phase112and the second DC terminal122. Although not explicitly described here, it is of course envisaged that similar arrangements of phase legs and phase arms132,133,134and135are available also for the other phases113and114of the AC side110.

The upper phase arm130includes a plurality of converter cells140-146connected in series between the first DC terminal120and the phase112. A converter cell is envisaged as a functional unit including one or more storage capacitors (or similar devices for storing charge), and means for, in a controlled way, both insert or bypass such storage capacitor(s) into/from a current path between the terminals of the converter cell. As known to the skilled person, such inserting or bypassing of the cell storage capacitors may be achieved using e.g. IGBTs or similar switching devices arranged and controlled in a suitable manner within the converter cell. A converter cell may for example be of a half-bridge (HB) type or a full-bridge (FB) type, where, in the latter case, the converter cell may also control with what polarity the storage capacitor(s) is/are inserted in the current path between the cell terminals.

There may, of course, also be other elements included in the phase arm130, such as for example one or more inductances, additional switches, breakers, resistances, or similar. Such additional elements are not illustrated inFIG.1anor discussed here.

In the (upper) phase arm130, the plurality of converter cells140-146are grouped into a plurality of groups150,151and152. In the current example, the group150includes the converter cells140and141, the group151includes the converter cells142and143, while the group152includes the remaining converter cells144,145and146. It should be emphasized that the current example is for illustrative purposes only, and that e.g. a phase arm may include several hundreds of converter cells, or more, depending on e.g. a required power rating. Likewise, it is of course also envisaged that a phase arm may have many more groups than the three groups150,151and152, and or/that each group may include more than two or three converter cells each.

As described earlier herein, a “group of converter cells” does not necessarily imply or require a physical grouping of the converter cells. A group may instead be a logical grouping of the converter cells. Using the MMC100ofFIG.1aas an illustrative example, it may be envisaged that one group may for example instead include the converter cells140and146, and one group may include the converter cells142,143,144and145. Phrased differently, it is not required for two converter cells to be physically close, or belong to a same physical entity, for them to belong to a same group.

In the phase arm130of the MMC100, there are also provided three modulators160,161and162, each configured to control a respective one of the groups150,151and152of converter cells. Using the modulator160as an example, the modulator160may control e.g. which of the converter cells140and142that are in an “inserted” or a “bypassed” state, respectively. Here, as described earlier herein, an “inserted” state means that the converter cell in question has its storage capacitor(s) inserted into a current path between its terminals, while a “bypassed” state means that the converter cell in question does not have its storage capacitor(s) inserted into the current path between its terminals (i.e. such that a current passing between the terminals of the converter cell does not pass through the one or more storage capacitor(s) of the converter cell). By controlling the states of the converter cells, the modulator160may therefore regulate how many storage capacitors that are inserted in total in the current path, and thereby e.g. also regulate a total voltage across its terminals. The decision on which converter cells that should be in an inserted or a bypassed state may for example be taken by the modulator160based on an instruction provided to the modulator160. Such an instruction may, as will be described later herein, include or be a reference part corresponding to e.g. a desired voltage output across the terminals of the group150of converter cells controlled by the modulator160. The modulator160may then decide how many of its converter cells in the group150that need to be in an inserted state to meet the reference part (e.g. the desired voltage). To make such a decision, the modulator160may first measure (or be provided with) a charge (or energy) level (e.g. a voltage) for the storage capacitors of its converter cells, and then decide which converter cells that should be in what state. The decision may also, or in combination therewith, be taken based on e.g. a direction of a current in the phase arm130. If the current in the phase arm130is in a direction which allows charging of the storage capacitors of the converter cells, the modulator160may decide (by controlling the states of its converter cells) which storage capacitors that are to be charged or not. Such a decision may also be taken based on a measurement (or provision) of a charge level of each storage capacitor, i.e. a converter cell voltage.

FIG.1billustrates schematically an embodiment of a converter station170according to the present disclosure.

The converter station170includes at least one MMC101,102,103and104. One or more of the MMCs101,102,103and104may be the MMC100described above with reference toFIG.1a. The MMCs101-104are connected on their AC sides to a plurality of AC grids180. The MMCs101-104are connected in series, and in a bipolar configuration, where the MMC101and the MMC104connects to the respective DC pole190and192of a DC grid. The converter station170is configured to transfer power between the AC grids180and the DC grid, and may benefit from the control strategy described above due to the inclusion of at least one MMC where at least one phase arm is controlled as also described herein. In some embodiments, the converter station170may for example be a HVDC converter station.

FIG.2illustrates schematically an embodiment of a control strategy200(as implemented e.g. in a method and/or device as described herein) for controlling a phase arm of an MMC according to the present disclosure.

In the strategy (or method)200, a main reference210includes e.g. a main reference voltage according to which a phase arm of an MMC is to be operated (i.e. controlled). In a step212, the main reference210is divided into several reference parts220_1,220_2, . . . ,220_N, where N is an integer number greater than one. Each reference part220_1,220_2, . . . ,220_N is then provided to a respective modulator260_N,260_2, . . . ,260_N of the phase arm of the MMC. For example, the modulator260_1may correspond to the modulator160in the phase arm130described herein with reference toFIG.1, the modulator260_2may correspond to the modulator161, and so on and so forth.

After having received its respective reference part, each modulator260_1,260_2, . . . ,260_N uses its received reference part to control a respective group250_1,250_2, . . . ,250_N of converter cells to which the modulator has been assigned. Each group250_1,250_2, . . . ,250_N includes 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_MN}, where Mjis an integer corresponding to a number of converter cells in group number j. As described earlier herein, the number and types of converter cells in each group may be different.

The modulators260_1,260_2, . . . ,260_N are operated in parallel, where each modulator260_1,260_2, . . . ,260_N operates (i.e. control its respective converter cells) in accordance with its respective reference part220_1,220_2, . . . ,220_N. As part of the parallel operation, each modulator260_1,260_2, . . . ,260_N individually controls its respective converter cells such that e.g. an output voltage of a group controlled by each modulator matches the reference part assigned to the modulator.

In a more specific example, the main reference210may for example be a voltage reference Vref, indicating a desired output voltage across the full phase arm. In the step212, the voltage reference Vrefis divided into reference parts V1, V2, . . . , VN, which are then provided to a respective one of the modulators260_1,260_2, . . . ,260_N. As an example, each reference part may correspond to one Nth of the main reference, such that V1=V2= . . . =VN=Vref/N. Such a construction may assume e.g. that there is an equal number of converter cells in each group of converter cells, i.e. that M1=M2= . . . =MN. If such is not the case, or if not suitable for other reasons, it may be envisaged instead that e.g. Vj=Vref*kj, where kjis a weighting factor which may e.g. depend on both N but also on and therefore be different for each or some of the groups and modulators. It may for example be such that the reference parts are constructed so that a sum of all Vj's corresponds to, i.e. equals or at least approximates, Vref

In this or other embodiments, the main reference (e.g. Vref) may vary over time (e.g. Vref(t)). The reference parts may then be constructed such that, at a time instance, the modulators may be operated in parallel according to their respective reference parts, such that at the same time instance the phase arm is operated in accordance with the main reference. Phrased differently, each reference part may then be constructed such that Vj(t) is a function of Vref(t). If, for example, V(t)=sin(a*t), where a is some constant, Vj(t) may equal e.g. V(t)/N, or V(t)*kj, as described above, or similar.

The examples given above are of course only particular examples of how the reference parts may be constructed, and it is envisaged that many other alternatives, that would fall under the concept of the present disclosure, exist.

Several benefits of the concept of the present disclosure will now be described in more detail. It is noted that, if not explicitly stated to the contrary, such benefits apply to all embodiments according to any aspect of the present disclosure described herein.

During operation of an MMC, a phase arm of the MMC may continuously switch between being in a charging cycle (when 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 to the first direction). During the charging cycle, control of the phase arm may require determining which of the converter cells that are to be in an inserted state such that their storage capacitor(s) may be charged. One strategy of control may e.g. include measuring the cell voltages (i.e. the charge currently stored in the cell capacitors), and to insert the converter cells currently having the lowest cell voltages (i.e. the converter cells corresponding to the lowest stored energy). To do so, it may be required to sort all of the converter cells according to their cell voltages. Such a sorting operation may include constructing a list of all the voltages, and then sort the list according to a sorting criterion (such as e.g. a descending or increasing cell voltage). Likewise, during a discharging cycle, one strategy of control may include inserting the converter cells currently having the highest cell voltages (i.e. the converter cells corresponding to the highest stored energy). Such a strategy may thus also require a sorting operation.

As the direction of the current in the phase arm may change several times per second, and as an update of the main reference value may be required ever more so often, the list of cell voltages may be updated and resorted a large number of times per second.

In a conventional MMC, wherein the converter cells of a phase arm are not controlled in accordance with the present disclosure, all converter cells may be controlled according to a single main reference, using a single modulator for the phase arm. For modern MMCs, the number of converter cells in each arm may be large (e.g. including at least several hundred or more of converter cells), and the time needed for controlling the phase arm for each time instance may thus be large, as each sorting operation may require a sorting of several hundred items or more. This may increase latency and put an increased demand on both hardware and software in terms of e.g. required speed and memory consumption.

In an MMC making use of the present disclosure, however, the task of e.g. sorting may be divided into multiple smaller tasks, which may be executed in parallel where each modulator independently sorts a much smaller list each time. This may decrease latency and ease the requirements on e.g. hardware performance. Especially, as each modulator is faced with a simpler task, the individual modulators may be constructed less complex than a conventional modulator responsible for control of all of the converter cells of a phase arm.

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

The use of multiple independent modulators according to the present disclosure may also for example enable “on-the-fly” maintenance of converter cells in the phase arm. For example, instead of having to shut a whole arm down (and thereby most likely also shut down the full MMC), it may be envisaged that for example the converter cells in a single group (and/or the modulator itself controlling the converter cells in that group) may be replaced or repaired while the converter cells in the other groups, controlled by another modulator, continues in operation. Likewise, independent modulators may also enable “on-the-fly” addition and removal of one or more modulators (and thereby of groups of converter cells).

In summary, the present disclosure provides an improved way of controlling an MMC. This is achieved by, for a phase arm of the MMC, dividing (physically or logically) the converter cells of the phase arm into several groups, where each group of converter cells may be controlled by its own modulator which may, independent of and in parallel with the other modulators, execute a task of controlling the group of converter cells according to a reference part of a main reference (and to e.g. perform cell voltage balancing within the group). As a result, latency may be reduced. This may further allow to increase a control bandwidth of the MMC such that the MMC may provide e.g. positive damping and control in higher frequency harmonics. Additionally, the control software and/or hardware for each individual modulator may be made less complex, as each modulator is required only to solve part of a full task (such as e.g. sorting of a large list of converter cells according to a sorting criterion).

Although features and elements may be described above in particular combinations, each feature or element may be used alone without the other features and elements or in various combinations with or without other features and elements.

Additionally, variations to the disclosed embodiments may be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the words “comprising” and “including” does not exclude other elements, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be used to advantage.