A resonant modular multilevel direct current, DC, converter, r-MMDC as well as a method of operating such an r-MMDC.

Abstract

A resonant modular multilevel Direct Current, DC, converter, R-MMDC, arranged for transferring power from a primary side to a secondary side, the converter comprising a transformer connected between the primary side and the secondary side, a resonant circuit, provided at the primary side, and comprising an inductance and a resonant capacitance, wherein the resonant circuit is tuned to a resonant frequency, at least two upper arms, each connected to a positive terminal of a DC power source at the primary side, and at least two lower arms, each connected to a negative terminal of the DC power source at the primary side, wherein midpoints of the at least two upper arms and the at least two lower arms are connected to different terminals of the transformer.

Description

[0001] TITLE

[0002] A resonant modular multilevel Direct Current, DC, converter, R-MMDC as well as a method of operating such an R-MMDC.

[0003] TECHNICAL FIELD

[0004] The present disclosure generally relates to a resonant modular multilevel Direct Current, DC, converter, R-MMDC, and more specifically to a new circuit of such an R-MMDC that is more efficient.

[0005] BACKGROUND OF THE DISCLOSURE

[0006] Resonant modular multilevel DC converters, R-MMDCs, are a promising solution for power transfer in medium-voltage, MV, Direct Current, DC, applications. It is valued for its modularity, scalability, and capability to manage short-circuit faults at the MV terminal. The conventional R-MMDC topology is designed to handle MV levels that far exceed the blocking voltage of individual switching devices.

[0007] To achieve this, the R-MMDC employs four arms. Each arm is composed of numerous half-bridge submodules, HBSMs, each equipped with a large energy storage capacitor. While this design enables MV operation, it introduces significant challenges. The substantial size and weight of the energy storage capacitors contribute to a large footprint, low compactness, and heavy overall weight, which drive up costs not only for equipment manufacturing but also for installation.

[0008] Additionally, the R-MMDC suffers from hard-switching operation due to a DC current flowing through the arms. Unlike two-level resonant converters widely used in low-voltage applications, the R-MMDC cannot fully achieve soft-switching conditions, resulting in reduced energy efficiency.

[0009] Summary It would be advantageous to achieve a resonant modular multilevel Direct Current, DC, converter, R-MMDC, that is more efficient and more compact compared to the prior art.

[0010] In a first aspect of the present disclosure, there is provided A resonant modular multilevel Direct Current, DC, converter, R-MMDC, arranged for transferring power from a primary side to a secondary side, the converter comprising:

[0011] a transformer connected between the primary side and the secondary side;

[0012] a resonant circuit, provided at the primary side, and comprising an inductance and a resonant capacitance, wherein the resonant circuit is tuned to a resonant frequency;

[0013] at least two upper arms, each connected to a positive terminal of a DC power source at the primary side, and at least two lower arms, each connected to a negative terminal of the DC power source at the primary side, wherein midpoints of the at least two upper arms and the at least two lower arms are connected to different terminals of the transformer,

[0014] wherein each of the at least two upper arms and the at least two lower arms comprise a plurality of submodules, SMs, wherein each of the plurality of SMs comprises an energy storage capacitance a bypass switch and an enabling switch, wherein the bypass switch is arranged to bypass the corresponding energy storage capacitance and wherein the enabling switch is arranged to enable the corresponding energy storage capacitance,

[0015] wherein the resonant capacitance of the resonant circuit is distributed over multiple submodules, wherein the resonant capacitance is distributed over at least four submodules, one for each of the two upper arms and two lower arms.

[0016] The inventor has found that it is beneficial to ensure that the resonant capacitance of the resonant circuit is distributed over at least one of the SMs in each arm. This has multiple advantages. This means that at least one submodule of an arm comprises a resonant capacitance.

[0017] First, this will ensure that a biased cosine resonance is provided. The resonant current caused by the resonant circuit follows a cosine waveform superimposed with a Direct Current, DC, bias during the primary power transfer stages. This compared to prior art solutions in which the resonant current typically follows a sine waveform. In other words, by distributing the resonant capacitance over the different SMs, a roughly ninety-degree phase shift of the resonant current is obtained.

[0018] Second, by distributing the resonance capacitance over one or more SMs, a capacitance may be provided in the SMs that is much smaller compared to the original storage capacitance. This is explained as follows. The distributed capacitance is connected in series with the energy storage capacitance. Such a series connected may be resembled, or embodied, using a single capacitor. The single capacitor is thus an equivalent capacitor for providing the combined capacitance of the energy storage capacitance and the distributed resonance capacitance.

[0019] The distributed resonance capacitance is much smaller compared to the relatively large energy storage capacitance. One of the advantages is then that the equivalent capacitor is much smaller compared to the relatively large energy storage capacitance.

[0020] In an example, said SMs comprise Half-Bridge submodules, HBSMs, or Full-Bridge submodules, or multi-level submodules.

[0021] The submodules may be homogenous in that all submodules are of a same type. The submodules may also be heterogeneous in that the submodules may be of a different type.

[0022] In an example, each of the SMs comprises the corresponding energy storage capacitance connected in series with a resonant capacitance, wherein the resonant capacitance forms a part of the distributed resonant capacitance.

[0023] At least two possibilities may thus exist. The first is directed to an equivalent capacitor that embodies the series connection of the energy storage capacitance with the resonance capacitance. The second is directed to actual series connection of capacitors, one capacitor embodies the energy storage capacitance and one capacitor embodies the distributed resonance capacitance.

[0024] In an example, each of the resonant capacitance in a submodule comprises 0,05 - 0,2 times the energy storage capacitance of that same corresponding submodule. In a further example, each of the SMs comprises equivalent capacitances having a value that is equivalent to a series combination of the energy storage capacitance and the distributed resonance capacitance.

[0025] In a further example, the R-MMDC comprises: a controller arranged for controlling the bypass switches and the enabling switches of the plurality of SMs of the at least two upper arms and the at least two lower arms such that a resonant current caused by the resonant circuit follows a cosine based waveform.

[0026] In another example, the controller is further arranged for controlling the bypass switches and the enabling switches of the plurality of SMs of the at least two upper arms and the at least two lower arms such that a DC bias caused by the resonant circuit counteracts a DC current flowing through the at least two upper arms and the at least two lower arms, the DC current caused by the energy storage capacitances comprised by the plurality of SMs of the at least two upper arms and the at least two lower arms.

[0027] In yet another example, the R-MMDC further comprises:

[0028] an input inductor connected between the at least two upper arms and the positive terminal of the DC power source for decoupling passive components at the DC power source.

[0029] The input inductor may be useful for preventing external passive components that are connected to the terminal of the DC power source to contribute in the internal biased cosine resonance that is accomplished by the R-MMDC in accordance with the first aspect of the present disclosure.

[0030] In a second aspect of the present disclosure, there is provided a method of operating a resonant modular multilevel Direct Current, DC, converter, R-MMDC, in accordance with any of the previous examples, wherein the method comprises the step of:

[0031] transferring, by the R-MMDC, power from the primary side to the secondary side.

[0032] It is noted that the advantages as explained with respect to the first aspect of the present disclosure, being the R-MMDC, are also applicable to the second aspect of the present disclosure, being the method of operating such an R-MMDC.

[0033] In an example, each of the SMs comprises the corresponding energy storage capacitance connected in series with a resonant capacitance, wherein the resonant capacitance forms a part of the distributed resonant capacitance. In a further example, each of the resonant capacitance is between 0,05 - 0,2 times the energy storage capacitance. The submodules may be Half-Bridge submodules, Full-Bridge submodules or multilevel submodules.

[0034] In a further example, each of the SMs comprises equivalent capacitances having a value that is equivalent to a series combination of the energy storage capacitance and the distributed resonance capacitance.

[0035] In another example, the R-MMDC comprises a controller and wherein the method comprises the step of:

[0036] controlling, by the controller, the bypass switches and the enabling switches of the plurality of SMs of the at least two upper arms and the at least two lower arms such that a resonant current caused by the resonant circuit follows a cosine based waveform.

[0037] In a further example, the method further comprises the step of:

[0038] controlling, by the controller, the bypass switches and the enabling switches of the plurality of SMs of the at least two upper arms and the at least two lower arms such that a DC bias caused by the resonant circuit counteracts a DC current flowing through the at least two upper arms and the at least two lower arms, the DC current caused by the energy storage capacitances comprised by the plurality of SMs of the at least two upper arms and the at least two lower arms.

[0039] In yet another example, the R-MMDC further comprises:

[0040] an input inductor connected between the at least two upper arms and the positive terminal of the DC power source for decoupling passive components at the DC power source.

[0041] In the appended figures, similar components and / or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label by a dash and a second label that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label.

[0042] The above and other aspects of the disclosure will be apparent from and elucidated with reference to the examples described hereinafter.

[0043] BRIEF DESCRIPTION OF THE DRAWINGS The disclosure will now be discussed with reference to the drawings, which show in:

[0044] Figure 1 depicts a topology of R-MMDC of the prior art.

[0045] Figure 2 depicts an example of the topology of R-MMDC in accordance with the present disclosure.

[0046] Figure 3 depicts the shifting from a sine shaped resonance current waveform to a cosine shaped resonance current waveform superimposed with a DC bias during the primary power transfer stages is shown.

[0047] Figure 4 depicts a combination of the capacitances in a SM.

[0048] Figure 5 depicts another example of the topology of R-MMDC in accordance with the present disclosure.

[0049] Figure 6 depicts examples of submodules in accordance with the present disclosure;

[0050] Figure 7 depicts an example of an R-MMDC in accordance with the present disclosure, showing both the primary and the secondary side;

[0051] Figure 8 shows an example of a control scheme in accordance with the present disclosure;

[0052] Figures 9 - 12 show typical waveforms corresponding to the operation of the R-MMDC in accordance with the present disclosure.

[0053] DETAILED DESCRIPTION OF THE DISCLOSURE

[0054] A more detailed description is made with reference to particular examples, some of which are illustrated in the appended drawings, such that the features of the present disclosure may be understood in more detail. It is noted that the drawings only illustrate typical examples and are therefore not to be considered to limit the scope of the subject matter of the claims. The drawings are incorporated for facilitating an understanding of the disclosure and are thus not necessarily drawn to scale. Advantages of the subject matter as claimed will become apparent to those skilled in the art upon reading the description in conjunction with the accompanying drawings.

[0055] The ensuing description above provides preferred exemplary embodiment(s) only, and is not intended to limit the scope, applicability, or configuration of the disclosure. Rather, the ensuing description of the preferred exemplary embodiment(s) will provide those skilled in the art with an enabling description for implementing a preferred exemplary embodiment of the disclosure, it being understood that various changes may be made in the function and arrangement of elements, including combinations of features from different embodiments, without departing from the scope of the disclosure.

[0056] Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise," "comprising," and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to." As used herein, the terms "connected," "coupled," or any variant thereof means any connection or coupling, either direct or indirect, between two or more elements; the coupling or connection between the elements can be physical, logical, electromagnetic, or a combination thereof.

[0057] Additionally, the words "herein," "above," "below," and words of similar import, when used in this application, refer to this application as a whole and not to any particular portions of this application. Where the context permits, words in the Detailed Description using the singular or plural number may also include the plural or singular number respectively. The word "or" in reference to a list of two or more items, covers all the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list.

[0058] These and other changes can be made to the technology considering the following detailed description. While the description describes certain examples of the technology, and describes the best mode contemplated, no matter how detailed the description appears, the technology can be practiced in many ways. Details of the system may vary considerably in its specific implementation, while still being encompassed by the technology disclosed herein.

[0059] In figure 1, the topology of R-MMDC of the prior art is shown.

[0060] The Resonant Modular Multilevel DC Converter, R-MMDC, is an advanced technology designed for efficient power transfer in medium-voltage, MV, DC applications. Its advantages include modularity, scalability, and robust fault management capabilities at the MV terminal. The R-MMDC topology consists of at least four arms: upper and lower arms for each phase (armu1, armd1, armu2, and armd2). Each arm is composed of numerous half-bridge submodules, SMs, which include large energy storage capacitors, CESM. These capacitors help stabilize the voltage but also present significant drawbacks.

[0061] The large CESM values increase the weight and volume of each submodule, leading to a bulky and heavy system with a substantial physical footprint. This impacts the overall compactness of the converter and drives up costs for manufacturing, transportation, and installation. Furthermore, the presence of DC current in the arms results in hard-switching conditions for the switching devices. Unlike two-level resonant converters commonly used in low-voltage applications, which benefit from full soft-switching to improve energy efficiency, the R-MMDC struggles to achieve these conditions consistently.

[0062] This hard-switching operation not only reduces the converter’s efficiency but also increases the thermal and electrical stress on components, potentially shortening their lifespan. Addressing these challenges is crucial to enhance the R-MMDC’s competitiveness for MV applications, particularly in terms of cost-efficiency, compact design, and operational reliability.

[0063] In figure 2, the topology of R-MMDC in accordance with the present disclosure is depicted.

[0064] Different from the R-MMDC of figure 1, the concentrated resonant capacitor (Crin Figure 1) is changed to the distributed resonant capacitors that are connected in series with the big energy storage capacitors in submodules as seen in the figure. Another option is that an equivalent capacitor is provided, i.e. a capacitor that is equivalent to the series connection of the large storage capacitor with the distributed resonance capacitance. This is shown in figure 4.

[0065] The distributed resonant capacitors of the inserted submodules are connected in series to form a lumped resonant capacitor, which resonates with the resonant inductor Llk, which could use the leakage inductor of the transformer. As a result, only a single-frequency resonance is generated, enabling efficient power transfer.

[0066] In the disclosed biased-cosine-resonant modular multilevel DC converter, BCR-MMDC, the resonance behavior of the distributed resonant capacitors in the inserted submodules of a specific arm is significantly influenced by the large DC current that flows through them mainly during half of the power transfer stage, either in the positive or negative power transfer stage. This half-stage DC current flowing through the distributed resonant capacitors shapes the resonant current, transforming it from a conventional sine waveform into a biased cosine waveform as can be seen in figure 3. This contrasts with the conventional R-MMDC in figure 1, where no DC current flows through the centralized resonant capacitor.

[0067] Further, in the figure 2, the input of the BCR-MMDC is connected in series with a de inductor Lf, which of importance in preventing external passive components that connected to the MV terminal participating in the internal biased cosine resonance. Such decoupling greatly simplify the design of the soft switching and control loops of BCR-MMDC. Another functionality of this de inductor Lf is to mitigate rising slope of the fault current when a solid short circuit occurs close the MV terminal, so that the control system of BCR-MMDC has the time to do the proper protection and control strategies.

[0068] Further, the BCR-MMDC of figure 2 comprises the energy storage capacitor and the distributed resonant capacitor. Also, the value of Lf should be significantly greater than resonant inductor Lik, such that Lf can be treated as the constant current source for the internal biased cosine resonance, effectively decoupling it from the other passive system connected to MV terminal. Moreover, it should also be sufficiently large to limit fault current rising slope within the acceptable bounds. This allows it to provide a stable DC bias.

[0069] The invention is now explained in other words.

[0070] To enable the bottom switches in submodules to achieve zero-voltage turn-on, it may be useful for the negative turn-off currents of the upper switches to transition into the corresponding positive current levels, while simultaneously reducing the turn-on current of the bottom switches to negative currents.

[0071] This ensures that zero-voltage turn-on is achieved, thereby minimizing power loss.

[0072] To achieve this zero-voltage turn-on, the conventional resonant capacitor Cr(figure 1) from the standard R-MMDC is distributed across each of the four arms, using at least submodule for each arm. This leads to the creation of a new submodule configuration, in which the resonant capacitor CRSM is connected in series with the energy storage capacitor CESM- This new structure alters the traditional sinusoidal resonant current waveform, transforming it into a biased-cosine waveform. This innovation is referred to as a biased-cosine-resonant MMDC. The change from a sine resonance to a biased-cosine resonance enables the upper switches to have positive turn-off currents, while the bottom switches can turn on with negative currents, achieving the zero-voltage turn-on required.

[0073] The BCR-MMDC operates in two distinct resonant stages. During the positive stage, submodules in armu2and armd1are engaged, with their corresponding resonant capacitors connected in series. This forms an equivalent resonant capacitor. Similarly, the armd1stage contributes to the formation of an equivalent capacitor. The energy storage capacitors CESM can be treated as constant voltage sources during these stages.

[0074] Conversely, during the negative stage, armu1and armd2are active while the other arms are bypassed, creating symmetrical equivalent resonant capacitors.

[0075] Reference is not also made to figures 9 - 12 of the present application. BCR-MMDC may primarily exhibit three distinct operating modes, dependent on the relationship between the resonant frequency frand switching frequency fs

[0076] Mode 1 (fr<fs): During this mode, the BCR-MMDC achieves zero-voltage turn-on with the resonant waveform functioning as a cosine function with a DC bias. This produces a resonant waveform that peaks at the end of the positive stage, ensuring zero-voltage turn-on for all switches of the submodules.

[0077] Mode 2 (fs<= fr<2fs) Here, the resonant period extends beyond the switching frequency range but maintains the same cosine-shaped AC current with a DC bias. This also ensures zero-voltage turn-on for all switches of the submodules.

[0078] Mode 3 (fr= 2fs): This mode is unique because the resonance completes fully within a half-cycle. The AC current waveform follows a full-period biased-cosine waveform rather than a sine waveform. However, during this mode, the upper switch of SMs achieves zero-voltage turn-on, while the bottom switch not.

[0079] Additionally, a special mode (fr<< 2fs) exists for scenarios where the resonant period is much longer than the switching period. In this case, the waveform resembles a triangular pattern, with all switches achieving zero-voltage turn-on.

[0080] BCR-MMDC introduces a new level of system design flexibility while minimizing power losses by achieving zero-voltage turn-on in an efficient manner. This is further supported by introducing a derivative circuit, DCR-MMDC, based on the duality principle of inductor integration. The proposed technique integrates resonant capacitors CRSM and CESM in series into an equivalent smaller capacitor. This simplification reduces submodule component size, weight, and complexity while maintaining the system's efficiency.

[0081] In summary, the BCR-MMDC’s biased-cosine resonance and the integration into DCR-MMDC — enable efficient soft-switching by modifying traditional resonant waveforms and simplifying submodule design. They enhance the operational flexibility of MMDC configurations, reduce system complexity, and maintain high performance while ensuring system stability and efficient power conversion.

[0082] In figure 3, the shifting from a sine shaped resonance waveform to a cosine shaped resonance waveform superimposed with a DC bias during the primary power transfer stages is shown.

[0083] Here it is shown that armu1inserts the submodule resonant capacitors during the negative stage, allowing DC current to flow through them during this negative stage, while armd1inserts the submodule resonant capacitors during the positive stage, allowing DC current flow through these capacitors in that positive stage.

[0084] Therefore, the combination of the DC current and the cosine shaped waveform is essential to achieve full soft switching for all the switches in submodules. First, the DC bias in the resonant current cancels the DC component flowing through the arms, and thereby mitigates its negative impact. Second, the cosine waveform of the resonant current ensures the right current direction flowing into submodules during the switching instant to achieve soft-switching.

[0085] Figure 4 depicts an integration of the capacitances into one capacitance. Based on the duality principle, the capacitor integration technique can be applied on DCR-MMDC, i.e., the series-connected CRSMIJ and Ces / wij in the submodule can be integrated together and implemented by an equivalent capacitor CeqsMij.

[0086] Since the integrated capacitor CeqSMij, will be smaller than either C SM> or CESMij, this capacitor integration will simplify the implement and significantly reduce the weight and volume of the submodule capacitors.

[0087] Figure 5 depicts an example of the topology of R-MMDC in accordance with the present disclosure. Herein, the resonance inductances are distributed over the arms, wherein they are positioned, for example, after the SMs of the arms. Therefore, each arm of BCR-MMDC contains an arm inductor. This may be beneficial for a better protection of the arm from a short circuit fault. The operation of the BCR-MMDC does not change expect that both

[0088]

[0089] and arm inductors Larm1~ Larm4 will contribute the resonant inductor.

[0090] When each arm of the Biased-Cosine Resonant Modular Multilevel Converter, BCR-MMDC, incorporates an arm inductor to enhance protection against short-circuit faults, the operations of the BCR-MMDC remain unchanged. However, the arm inductors — denoted as Larmithrough Larm4— will now also contribute to the resonant inductance. Assuming that all these arm inductors have identical inductance values, denoted as Larm, the equivalent operating circuit may be adjusted accordingly to account for their presence.

[0091] The updated equivalent circuit reflects the combined effect of the resonant inductance and the arm inductors, as shown in the revised equivalent diagram.

[0092] The inclusion of arm inductors introduces additional inductive effects, but they do not alter the core biased-cosine resonance behavior of the system. These inductors serve as part of the overall resonant inductor network, thereby contributing to the system’s ability to achieve resonance while maintaining protection from short-circuit faults. Essentially, the arm inductors supplement the resonant inductor without compromising the system's fundamental operating principle or the biased cosine resonance waveform.

[0093] Before turning to the subsequent features, some aspects of the present disclosure are discussed in more detail.

[0094] The Biased Cosine Resonance Modular Multilevel DC-DC Converter, BCR-MMDC, in accordance with the present disclosure introduces several features compared to the prior art Resonant Modular Multilevel DC-DC Converters, R-MMDCs.

[0095] These features address challenges such as soft-switching efficiency, resonance behavior, and fault current management while expanding operational flexibility and reducing the size of the capacitors.

[0096] Feature 1: Biased Cosine Resonance

[0097] A characteristic of the BCR-MMDC is that the resonant current follows a cosine waveform superimposed with a DC bias during primary power transfer stages. In contrast, conventional R-MMDCs utilize a sine waveform. This unique waveform enables soft-switching of all submodule switches. The DC bias effectively cancels the DC component flowing through the arms, mitigating its negative impact. Simultaneously, the cosine waveform ensures the correct current direction at switching instants, essential for achieving soft-switching.

[0098] Feature 2: Distributed Resonant Capacitance

[0099] Unlike conventional R-MMDCs that use centralized resonant capacitors, the BCR-MMDC employs distributed resonant capacitance / capacitors provided in the submodules. These capacitors may form a lumped resonant capacitor in resonance with a resonance inductance, for example the leakage inductor, generating singlefrequency resonance for efficient power transfer.

[0100] The distributed capacitors are influenced by a DC current during either the positive or negative power transfer stages, shaping the resonant current into the biased cosine waveform discussed earlier. This configuration allows dynamic adjustment of resonant capacitance by varying the number of inserted submodules, broadening the voltage gain range and extending the soft-switching range. Additionally, integrating the resonant and energy storage capacitors reduces submodule size and weight.

[0101] Feature 3: DC Inductor for Resonance Decoupling and Fault Control A DC inductor may be placed in series with the BCR-MMDC's input. This inductor decouples external passive components from the internal biased cosine resonance, simplifying the soft-switching design and control loops. Moreover, this inductor mitigates the rising slope of fault currents during short circuits near the MV terminal, providing the control system adequate time to implement protective measures.

[0102] Feature 4: Soft-Switching Control Strategy

[0103] The control strategy combines voltage gain regulation and soft-switching improvements. It detects the turn-on current of switches to achieve zero-voltage switching and may adjust the resonant frequency by modifying the number of inserted submodules. This enables precise voltage gain and resonant frequency modulation. A coordinated mapping block translates control variables into modulation signals such as duty cycle, resonant frequency, and switching period, further enhancing flexibility and efficiency.

[0104] Figure 6 depicts examples of submodules in accordance with the present disclosure. Figure 6 presents potential submodule structures within the BCR-MMDC, encompassing half-bridge submodules, full-bridge submodules, and multilevel submodules, among others. Any combination submodule of these configurations, as well as other submodules currently used in R-MMDC, will falls within the protective scope of the present disclosure, as long as it can realize biased cosine resonance by inserting at least one submodule in each arm comprising a distributed capacitance of the resonance converter.

[0105] Figure 7 depicts an example of an R-MMDC in accordance with the present disclosure, showing both the primary and the secondary side.

[0106] Figure 7 presents the possible single-phase configurations based on the present disclosure of BCR-MMDC, where the Power Bridge 1 may use the distributed resonance capacitance, the transformer can design the magnetizing inductance with a very large value realize the series resonance converter, SRC, operation, or with a relatively low value to realize the LLC resonance converter operation, LLC; the Power Bridge 2 can have multiple options, wherein the submodules may comprise the resonance capacitance or not.

[0107] It is noted that the present disclosure also encompasses a three-phase solution.

[0108] Figure 8 shows an example of a control scheme in accordance with the present disclosure.

[0109] Figure 8 illustrates the diagram of the soft-switching strategy for the disclosed BCR-MMDC, which consists of three main control blocks: a voltage gain control block, a soft-switching control block, and a coordinated-mapping block. These blocks work together to ensure optimal system performance under varying operational conditions.

[0110] First, a scenario with a wide operation gain range is discussed before turning to figure 8. In this scenario, the voltage gain control block compares the low-voltage terminal voltage to a reference voltage. The resulting error is sent to the voltage controller — typically a PI controller — to generate the duty cycle D. The soft-switching control block monitors the turn-on current of Q ij and compares it with a reference current, representing the minimum current needed to achieve zero-voltage turn-on. The resulting error is sent to the soft-switching controller to generate the switching period Ts. Meanwhile, the coordinated-mapping block takes the duty cycle D and the switching period Tdand sends them to an optimizer to determine the optimal resonant frequency fr. The optimization solves a problem with constraints to minimize power loss while maintaining a fixed voltage gain and ensuring zero-voltage turn-on. The determined resonant frequency is then converted into the number of inserted submodules, Nin, using a rounding integer function. These parameters — D, Nm, and Ts— are subsequently used to modulate the driving signals for the submodules in the MMDC.

[0111] Another example is proposed for scenarios where achieving zero de current ripple on the input terminal is necessary, as shown in Figure 8. In this case, the voltage gain control block generates a sequence of duty cycles (Ds). Similar to the previous embodiment, an optimizer calculates the optimal resonant frequency fr, which is later converted into the number of inserted submodules. The soft-switching control block continues to generate the switching period. Additionally, the duty cycles of the driving signals for all submodule switches are maintained at a fixed value of 0.5. The resulting parameters — Ds, Nm, and Ts— are then used to modulate the driving signals for the submodules, ensuring zero de current ripple at the input terminal.

[0112] As noted above, particular terminology used when describing certain features or aspects of the technology should not be taken to imply that the terminology is being redefined herein to be restricted to any specific characteristics, features, or aspects of the technology with which that terminology is associated. In general, the terms used in the following claims should not be construed to limit the technology to the specific examples disclosed in the specification, unless the Detailed Description section explicitly defines such terms. Accordingly, the actual scope of the technology encompasses not only the disclosed examples, but also all equivalent ways of practicing or implementing the technology under the claims.

Claims

CLAIMS1. A resonant modular multilevel Direct Current, DC, converter, R-MMDC, arranged for transferring power from a primary side to a secondary side, the converter comprising:a transformer connected between the primary side and the secondary side;a resonant circuit, provided at the primary side, and comprising an inductance and a resonant capacitance, wherein the resonant circuit is tuned to a resonant frequency;at least two upper arms, each connected to a positive terminal of a DC power source at the primary side, and at least two lower arms, each connected to a negative terminal of the DC power source at the primary side, wherein midpoints of the at least two upper arms and the at least two lower arms are connected to different terminals of the transformer,wherein each of the at least two upper arms and the at least two lower arms comprise a plurality of submodules, SMs, wherein each of the plurality of SMs comprises an energy storage capacitance a bypass switch and an enabling switch, wherein the bypass switch is arranged to bypass the corresponding energy storage capacitance and wherein the enabling switch is arranged to enable the corresponding energy storage capacitance,wherein the resonant capacitance of the resonant circuit is distributed over multiple submodules, the resonant capacitance being distributed over at least four submodules, one for each of the two upper arms and two lower arms.

2. An R-MMDC in accordance with claim 1, wherein said SMs comprise any of:Half-Bridge submodules, HBSMs;Full-bridge submodules, FBSMs;Multi-Level submodules, MLSMs.

3. A R-MMDC in accordance with any of the previous claims, wherein each of the SMs comprises the corresponding energy storage capacitance connected in series with a resonant capacitance, wherein the resonant capacitor forms a part of the distributed resonant capacitance.

4. A R-MMDC in accordance with any of the previous claims, wherein each of the resonant capacitances in the submodules are between 0,05 - 0,2 times the energy storage capacitance in the respective submodules.

5. A R-MMDC in accordance with any of the previous claims, wherein each of the SMs comprises equivalent capacitances having a value that is equivalent to a series combination of the energy storage capacitance and the distributed resonance capacitance.

6. A R-MMDC in accordance with any of the previous claims, wherein the R-MMDC comprises:a controller arranged for controlling the bypass switches and the enabling switches of the plurality of SMs of the at least two upper arms and the at least two lower arms such that a resonant current caused by the resonant circuit follows a cosine based waveform.

7. A R-MMDC in accordance with claim 6 wherein the controller is further arranged for:controlling the bypass switches and the enabling switches of the plurality of SMs of the at least two upper arms and the at least two lower arms such that a DC bias caused by the resonant circuit counteracts a DC current flowing through the at least two upper arms and the at least two lower arms, the DC current being caused by the energy storage capacitors comprised in the plurality of SMs of the at least two upper arms and the at least two lower arms.

8. A R-MMDC in accordance with any of the previous claims, wherein the R-MMDC further comprises:an input inductor connected between the at least two upper arms and the positive terminal of the DC power source to decouple the external passive components at the DC power source from the internal resonance.

9. A R-MMDC in accordance with any of the previous claims, wherein said inductance of the resonant circuit is distributed over the arms, thereby providing said inductance in series with said resonance capacitance.

10. A R-MMDC in accordance with any of the previous claims, wherein the R-MMDC comprises a controller arranged for:controlling said R-MMDC with a switching frequency lower than the resonance frequency;controlling said R-MMDC with a switching frequency higher than the resonance frequency, and / orcontrolling said R-MMDC with a switching frequency lower than twice of the resonance frequency.

11. A R-MMDC in accordance with any of the previous claims, wherein the R-MMDC comprises:a resonant circuit, provided at the secondary side, and comprising an inductance and a resonant capacitance, wherein the resonant circuit is tuned to a resonant frequency;at least two upper arms and at least two lower arms, wherein midpoints of the at least two upper arms and the at least two lower arms at the secondary side are connected to different terminals of the transformer at the secondary side,wherein each of the at least two upper arms and the at least two lower arms at the secondary side comprise a plurality of submodules, SMs, wherein each of the plurality of SMs comprises an energy storage capacitance a bypass switch and an enabling switch, wherein the bypass switch is arranged to bypass the corresponding energy storage capacitance and wherein the enabling switch is arranged to enable the corresponding energy storage capacitance,wherein the resonant capacitance of the resonant circuit is distributed over multiple submodules, wherein it is distributed over at least four submodules, one for each of the two upper arms and two lower arms.

12. A R-MMDC in accordance with any of the previous claims, wherein the R-MMDC comprises a controller arranged for any of:determining the number of submodules to activate, using said corresponding enabling switches, based on a terminal voltage / current and based on a current crossing;regulating modulation variables of said controller by deterring turn-on current of said switches in said corresponding submodules.

13. A method of operating a resonant modular multilevel Direct Current, DC, converter, R-MMDC, in accordance with any of the previous claims, wherein the method comprises the step of:transferring, by the R-MMDC, power from the primary side to the secondary side.

14. A method in accordance with claim 13, wherein the R-MMDC comprises a controller, said method comprising the steps of:determining, by the controller, the number of submodules to activate, using said corresponding enabling switches, based on a terminal voltage / current and based on a current crossing;regulating, by the controller, modulation variables of said controller by deterring turn-on current of said switches in said corresponding submodules

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