Full-bridge boost-buck converter cell for mmc

Abstract

The present disclosure relates to a full-bridge converter cell (4). The cell comprises a boost-buck (BB) arrangement (4b) comprising a plurality of semiconductor switches (Sx, Sy), the cell further comprising a bipolar arrangement (4a) arranged as a junction between the BB arrangement and terminals (A, B) of the cell. The bipolar arrangement comprises a plurality of semiconductor switches. The BB arrangement is configured to operate such that when power flows into the cell, power moves from the terminals through an inductor (L f ) to a main energy storage (C m ), and when power flows out of the cell, power moves from the main energy storage (C m ) through the inductor to the terminals. The bipolar arrangement is configured to enable the BB arrangement to operate irrespective of the polarity of the terminals of the cell.

Description

Technical Field

[0001] This disclosure relates to a full-bridge converter unit for a modular multilevel converter (MMC). Background Technology

[0002] One or more MMCs with multiple series (also known as chained) converter units have become a popular choice for medium-voltage (MV) and high-voltage (HV) grid-connected converters due to their enhanced modularity, scalability, excellent harmonic performance, and reduced losses. The trend towards smaller footprint and more compact designs has been a recent focus.

[0003] To reduce the potential integration of unit capacitors and / or energy storage devices within the unit, WO2016 / 150466A1 discloses a half-bridge (HB) configuration with a DC-DC stage connected to a main energy storage device, which can be a battery, supercapacitor, or ordinary capacitor. A filter inductor is also required to control the current / power flow between the main energy storage device and the filter energy storage device. Some of the switches switch near the fundamental frequency to insert or bypass units, while the DC-DC stage switches at higher frequencies (>1kHz) to reduce the size of the filter elements. The disadvantages of this unit include the need for two different types of switches (i.e., switches that switch near the fundamental switching frequency and the DC-DC stage that switches at a higher switching frequency) and a complex control architecture (i.e., a classification algorithm with a central control architecture for operating the switches at the fundamental frequency and a local unit-level controller for operating the DC-DC stage switches). Summary of the Invention

[0004] Traditional MMCs are composed of half-bridge (HB) or full-bridge (FB, i.e., bipolar) units, depending on the application. Since each HB or FB unit is a so-called buck converter, the unit's DC voltage must always be higher than the resulting output voltage; otherwise, the diodes would be forward biased and the unit would function as a diode rectifier. Of course, with harmonic injection, overmodulation up to, for example, 1.27 pu can be permitted. For simplicity, only linear modulation is considered here. For high-voltage DC (HVDC), static synchronous compensators (STATCOM), and other MMC applications (e.g., static frequency, railway power converters, etc.), the net DC energy flowing into the unit's energy storage device per fundamental frequency cycle can be zero. However, there may be fluctuating energy (e.g., 50 or 100 Hz, depending on the topology or operation) that needs to be stored in the unit's energy storage device. The unit capacitor can then be rated to ensure linear modulation across all operating points, taking into account the fluctuating energy from the converter arms. The expression for calculating the unit's energy storage device (here, the capacitor) can be:

[0005]

[0006] in,

[0007] ·C cell It is the required unit capacitor.

[0008] ·E arm，pk-pk It is the peak-to-peak arm ripple energy calculated based on the converter arm current and voltage waveforms.

[0009] ·N is the number of converter units per converter arm, and

[0010] ·U max and U min It is the unit voltage value at the maximum and minimum fluctuation points obtained from the system design, taking into account all operating points of the MMC.

[0011] Typically, traditional MMC cells are considered to have a 10% peak-to-peak cell voltage fluctuation. As a result, 90% of the cell's energy storage is unused, making the cell unnecessarily large.

[0012] It is now recognized that, through embodiments of the present invention, greater energy storage capacity (i.e., unit DC voltage) can be used to handle DC voltage fluctuations, thereby significantly reducing the size (both in terms of physical footprint and capacity) of each unit's energy storage device. For example, if 90% of the unit capacitor's energy is utilized, a reduction of up to 80% in the unit capacitor can be achieved.

[0013] According to the present invention, in order to generate the desired unit output voltage waveform without considering large voltage fluctuations on the unit energy storage device, a so-called buck-boost (BB) operation in the converter unit is required. In some embodiments, the proposed unit structure can be viewed as a current source downstream of a conventional FB unit used as a voltage source unit (e.g., providing an H-bridge topology for achieving bipolarity).

[0014] According to one aspect of the invention, a full-bridge converter unit is provided. The unit includes a BB device comprising a plurality of converter valves, each valve including a semiconductor switch and an anti-parallel diode. The unit also includes a bipolar device configured as a junction between the BB device and terminals of the unit. The bipolar device includes a plurality of converter valves, each valve including a semiconductor switch and an anti-parallel diode. The BB device is configured to operate by switching its semiconductor switch such that when power flows into the unit, power moves from the terminals through an inductor of the BB device to a main energy storage unit of the BB device, thereby charging the main energy storage unit. The BB device is also configured to operate by switching its semiconductor switch such that when power flows out of the unit, power moves from the main energy storage unit through an inductor to a terminal, thereby discharging the main energy storage unit. The bipolar device is configured to enable the BB device to operate by switching its semiconductor switch, regardless of the polarity of the unit terminals.

[0015] According to another aspect of the present invention, an MMC comprising a plurality of converter arms is provided, each arm comprising a plurality of converter units according to the present disclosure connected in series.

[0016] According to another aspect of the invention, a method is provided performed by a control device for controlling a converter unit according to an embodiment of the present disclosure in an MMC. The method includes charging a main energy storage device by moving power from a terminal through an inductor to the main energy storage device, and discharging the main energy storage device by moving power from the main energy storage device through an inductor to a terminal.

[0017] Embodiments of the present invention provide a full-bridge buck-boost (FB-BB) cell for MMC topologies (commonly referred to as MMC or chain-linked converters). Compared to conventional full-bridge (FB) cells, embodiments of the proposed cell can achieve a reduction in cell capacitance of up to 80%, but may require doubling the semiconductor ratings. Since the energy per cell can be significantly reduced (>5x), cell shoot-through and bypass protection requirements can be relaxed. This allows for switches with lower energy handling capabilities, i.e., bonded-wire devices, to be used in medium- and high-voltage grid-connected applications, resulting in lower costs. The proposed cell becomes more compact as the switching frequency of the FB-BB cell increases. This can be advantageous, for example, for silicon carbide (SiC) switches. Each FB-BB cell can generate a requested reference voltage with only switching frequency harmonic components, unlike conventional FB cells which typically also have lower-order harmonic components. Therefore, even with fewer units per converter arm, there is generally no generation of low-order harmonics in the converter, making the unit suitable for applications such as medium-voltage DC (MVDC), high-voltage DC (HVDC), static synchronous compensators (STATCOM), and other MMC applications where the number of units per arm can be less than 10 (e.g., static frequency, railway power converters, etc.).

[0018] Furthermore, it should be noted that the proposed FB-BB unit is a current source unit. Therefore, arm reactors may not be necessary, or their value may be reduced, as each unit can have its own reactor to control the current at the control level, thereby reducing the footprint.

[0019] It should be noted that any feature of any aspect may be applied to any other aspect where appropriate. Similarly, any advantage of any aspect may be applied to any other aspect. Further objects, features, and advantages of the appended embodiments will become apparent from the following detailed disclosure, dependent claims, and accompanying drawings.

[0020] Generally, all terms used in the claims should be interpreted according to their ordinary meaning in the technical field, unless otherwise expressly defined herein. All references to “a / an / element, device, component, apparatus, step, etc.” are openly interpreted as referring to at least one instance of an element, device, component, apparatus, step, etc., unless otherwise expressly stated. Unless expressly stated, 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 features / components from other similar features / components, and not to assign any order or hierarchy to the features / components. Attached Figure Description

[0021] The embodiments will be described by way of example and with reference to the accompanying drawings, in which:

[0022] Figure 1 This is a schematic circuit diagram of an MMC according to an embodiment of the present invention.

[0023] Figure 2 This is a schematic circuit diagram of an embodiment of the FB-BB converter unit according to the present invention.

[0024] Figure 3 This is a schematic circuit diagram of another embodiment of the FB-BB converter unit according to the present invention.

[0025] Figure 4a This is an explanation Figure 3 The schematic circuit diagram of the first stage of the embodiment shows only the components used in the positive half-cycle of the unit operation.

[0026] Figure 4b This is an explanation Figure 3 The schematic circuit diagram of the second level of the embodiment shows only the components used in the negative half-cycle of the unit operation. Detailed Implementation

[0027] Embodiments will now be described more fully below with reference to the accompanying drawings, in which some embodiments are illustrated. However, many other embodiments in various forms are also 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 specification, the same numerals refer to the same elements.

[0028] Figure 1 MMC 1 is shown, which herein takes the form of a three-phase AC to DC chained converter employing a dual-star (also known as dual-Y or dual-Y) topology. It should be noted that for some embodiments of the invention, HVDC or MVDC may be preferred applications, but for some other embodiments (e.g., STATCOM (typically employing delta (Δ) or Y-shaped (Y) topologies), static frequency AC-AC converters (typically employing three-phase to single-phase or three-phase to three-phase MMC applications), other applications may also be preferred.

[0029] MMC 1 includes multiple converter arms 3 (which may alternatively be referred to as legs, branches, or chain links), each of which includes multiple converter units 4 connected in series (which may alternatively be referred to as chain links or cascades). In the HVDC / MVDC example shown in the figure, a dual-star topology is used, wherein each phase 2 has an upper arm 3a connected to one of the DC terminals (here, the positive DC terminal DC+) and a lower arm 3b connected to the other of the DC terminals (here, the negative terminal DC-). Each of the respective phases 2 is connected to or configured to be connected to the corresponding phase of the AC grid at the respective AC terminals a, b, and c of the MMC. It should be noted that the FB-BB unit 4 of the present invention can be used in any MMC topology other than the dual-star topology illustrated in the figure (e.g., a delta topology, a Y-topology, or a combination thereof).

[0030] When MMC 1 has a DC side (e.g., for HVDC or MVDC applications), such as Figure 1 As shown, the DC terminals DC+ and DC- can be connected to an energy storage system, which may include, for example, one or more batteries or supercapacitors or combinations thereof, or be composed of one or more batteries or supercapacitors or combinations thereof.

[0031] The control device 10 of MMC 1 may include a central controller and a distributed controller for controlling the operation of MMC.

[0032] Figure 2 An embodiment of FB-BB unit 4 is illustrated. Unit 4 includes a BB device 4b (used as a current source) and a bipolar device 4a (used as a voltage source).

[0033] The bipolar device 4a serves as a connector between the BB device and terminals A and B of the unit 4, ensuring that regardless of the polarity of terminals A and B, the BB device 4b can allow energy to flow from terminals A and B and / or the secondary energy storage device C when power flows into the unit. f Through inductor L f Move to primary energy storage unit C m And it allows energy to flow from the primary energy storage device C when the power flows out of the unit. m Through inductor L f Move to terminals A and B and / or secondary energy storage device C f .exist Figure 2 In this embodiment, this is achieved by ensuring that the polarity of the BB device 4b is always the same regardless of whether terminal A has a positive polarity and terminal B has a negative polarity or terminal B has a positive polarity and terminal A has a negative polarity (i.e., point b always has a negative polarity and point a always has a positive polarity). For this purpose, Figure 2The bipolar device 4a of the embodiment includes four converter valves T (T1, T2, T3 and T4) forming an H-bridge topology, each converter valve including a unidirectional semiconductor switch S (S1, S2, S3 or S4) and an anti-parallel diode D (D1, D2, D3 or D4).

[0034] Figure 2 The BB device 4b of the embodiment includes a primary energy storage unit C. m This includes, for example, a capacitor device comprising at least one capacitor or supercapacitor, or a battery device comprising at least one battery. The primary energy storage element is illustrated herein as a main capacitor C. m The BB device also includes a secondary energy storage unit C. f This includes, for example, a capacitor device comprising at least one capacitor or supercapacitor, or a battery device comprising at least one battery. A secondary energy storage device is illustrated herein as a filter capacitor C. f The BB device also includes an inductor L. f (For example, a reactor), also referred to here as a filter inductor L f .

[0035] Primary energy storage device C m Secondary energy storage device C f and inductor L f They are all connected in parallel, meaning each is connected to each of the other two. Therefore, two current paths for current circulation are formed within the BB device, namely, through the secondary energy storage device C. f Inductor L f The current path to the front end of the first BB conductor valve Ty, and through the primary energy storage device C m Inductor L f And the back-end current path of the second BB converter valve Tx.

[0036] In unit 4, the unit voltage Uo is formed between its two terminals A and B, and the DC voltage Udc is applied to the primary energy storage device C. m The currents Iac, IL, and Idc, shown schematically in the diagram, have been assigned directions. However, note that when discussing current flow here, it refers to current in the intended positive direction.

[0037] Unit 4 is configured such that when power flows into the unit, the primary energy storage device C m Charging occurs in two steps. First, when the first BB semiconductor switch Sy switches to conduction (also known as an on or closed switch) and the second BB semiconductor switch Sx switches to non-conduction (also known as an off or open switch), current (allowed) flows from the secondary energy storage device C. fThe positive side flows to the inductor L through the first BB semiconductor switch Sy. f (For example, the current circulates counterclockwise in the front-end path of the BB device), thereby charging the inductor. Then, when both the first BB semiconductor switch Sy and the second BB semiconductor switch Sx are switched to the non-conductive state, current is (allowed) from the charged inductor L. f The energy flows to the primary energy storage device C through the second anti-parallel diode BB, Dx. m The positive side (e.g., the current circulates clockwise in the back-end current path of the BB device) charges the primary energy storage device.

[0038] Similarly, unit 4 is configured such that when power flows out of the unit, the primary energy storage device C m The discharge occurs in two steps. First, when the first BB semiconductor switch Sy switches to non-conductive and the second BB semiconductor switch Sx switches to conductive, current is flowing or is being allowed to flow from the primary energy storage device C. m The positive electrode side flows to the inductor L through the second BB semiconductor switch Sx. f (For example, the current circulates counterclockwise in the back-end current path of the BB device), thereby charging the inductor. Then, when both the first BB semiconductor switch Sy and the second BB semiconductor switch Sx are switched to non-conductive, current is (allowed) from the charging inductor L. f The energy flows through the first anti-parallel diode BBDY to the secondary energy storage device C. f The positive side (e.g., the current circulates clockwise in the front current path of the BB device) charges the secondary energy storage device.

[0039] The first valve Ty includes a first switch Sy and a first anti-parallel diode Dy, and the second valve Tx includes a second switch Sx and a second anti-parallel diode Dx. Figure 2 In one embodiment, the first and second converter valves Ty and Tx are connected in series with each other, wherein the first and second switches Sy and Sx have the same polarity, that is, they are able to conduct current in the same direction of series connection when switching to conduction.

[0040] Therefore, in the embodiments, the BB device includes a primary energy storage unit C. m Inductor L f and secondary energy storage device C f and the first and second converter valves Ty and Tx. Secondary energy storage device C f It is connected in series with the first converter valve Ty, and together with the first converter valve, it is connected with the inductor L. f Parallel connection, and primary energy storage device C m It is connected in series with the second converter valve Tx, and together with the second converter valve Tx, it is connected with the inductor L.f Parallel connection.

[0041] In any MMC (Chained Converter), the net DC energy exchanged with the grid by the unit energy storage device in each fundamental cycle can be zero. A switching average model can be designed to understand the switching S ratings of the proposed unit structure. The unit output voltage (Uo) and current (Io) are known quantities, from which the primary energy storage voltage (Udc) and current (Idc) waveforms can be determined as follows:

[0042]

[0043]

[0044] The average value of the switching current during the switching cycle can be defined as:

[0045] I Sy (t)=I dc (t) (3)

[0046] I Sx (t)=I0(t) (4)

[0047] Therefore, the first BB switch Sy is rated at the peak value of the unit output current Io (i.e., the arm current), while the second BB switch Sx is rated at the peak value of the DC current Idc. The first and second BB switches Sy and Sx must each be able to block the peak value of the sum of Uab and Udc. Assuming the peak value of Uab is the same as the peak value of Udc, each of the first and second BB switches must be able to block a voltage twice the nominal unit DC voltage Udc. Switches S1, S2, S3, and S4 of the bipolar device 4a are rated at the peak value of the unit output current (i.e., the arm current Io) and must block the AC voltage Uac.

[0048] For the power flowing into the cell (i.e., charging the main capacitor Cm) (that is, from the cell's AC side to the main capacitor Cm), the first BB switch Sy switches to conduct and the second BB switch Sx switches to deconduct. When the first BB switch Sy switches to conduct, the inductor L... f By filter capacitor C f When charging, and when it switches to non-conductive mode, the inductor L... f The energy stored in the capacitor is transferred to the main capacitor Cm through the second BB diode Dx. During steady-state operation of the converter, the equation controlling the duty cycle of the first BB switch Sy and the inductor current IL can be defined as:

[0049]

[0050]

[0051] Similarly, for the power flowing out of the cell (i.e., discharging the main capacitor Cm) (that is, from the main capacitor Cm to the AC side of the cell), the second BB switch Sx switches to conduct while the first BB switch Sy remains non-conductive. When the second BB switch Sx switches to conduct, the inductor Lf is charged by the main capacitor Cm, and when it switches to non-conductive, the energy stored in the inductor Lf is transferred to the filter capacitor Cf through the first BB diode Dy. During steady-state operation of the converter, the equation controlling the duty cycle of the second BB switch Sx and the inductor current IL can be defined as:

[0052]

[0053]

[0054] It can be noted that the direction of the inductor current IL is reversed to reverse the flow of electricity.

[0055] In the bipolar device 4a, respectively, the first and second switches S1 and S4 switch to conduct (the second and third switches S2 and S3 switch to non-conductivity) to generate a positive reference voltage, while the second and third switches S2 and S3 switch to conduct (the first and second switches S1 and S4 switch to non-conductivity) to generate a negative reference voltage. These switches S1, S2, S3 and S4 typically operate under zero-voltage switching (ZVS).

[0056] Figure 3 Another embodiment of FB-BB unit 4 is illustrated. The above is about... Figure 2 The discussion of the embodiments also applies to [other examples]. Figure 3 In the relevant parts of the embodiments.

[0057] Similarly, bipolar device 4a serves as a connector between the BB device and terminals A and B of unit 4, ensuring that regardless of the polarity of terminals A and B, the BB device 4b can allow energy to flow from unit terminals A and B (or secondary energy storage device C) when power flows into the unit. f ) through inductor L f (Here, either the first inductor Lf1 or the second inductor Lf2) moves to the primary energy storage device C. m And it allows energy to flow from the primary energy storage device C when the power flows out of the unit. m Move to unit terminals A and B (or secondary energy storage unit C) f However, in Figure 3 In one embodiment, the BB device 4b includes two separate BB stages (first BB stage 4b1 and second BB stage 4b2) sharing the same main capacitor Cm. Therefore, the bipolar device 4a can be reduced to only two valves T1 and T2 for switching between the two BB stages 4b1 and 4b2.

[0058] During the positive half-cycle, such as Figure 4a As shown, the first switch S1 of the bipolar device is conductive (the second switch S2 is not conductive), thereby enabling the first BB stage of the bipolar device, including the first BB switch Sy1, the second BB switch Sx1, and the inductor Lf1, to operate positively so that power is supplied to the primary and secondary energy storage devices C. m and C f Between (i.e., in the primary energy storage device C) m (Move between terminals A and B of the unit).

[0059] During the negative half-cycle, such as Figure 4b As shown, the second switch S2 of the bipolar device is conductive (the first switch S1 is not conductive), thereby enabling the second bipolar stage, including the first bipolar switch Sy2, the second bipolar switch Sx2, and the inductor Lf2, to operate positively so that power is supplied to the primary and secondary energy storage devices C. m and C f It moves between (i.e., between the main energy storage device Cm and the terminals A and B of the unit).

[0060] In both bidirectional device 4a and BB device 4b, and for any embodiment of the invention, each converter valve T includes a unidirectional semiconductor switch S (which, depending on the application for which MMC 1 is used, includes, for example, Si or SiC (where SiC may be preferred for the relatively high switching frequency of embodiments of the invention) insulated gate bipolar transistor (IGBT), integrated gate commutated thyristor (IGCT), metal oxide semiconductor field-effect transistor (MOSFET), or dual-mode insulated gate transistor (BiGT)), and an anti-parallel diode D (connected to the switch S, but connected in anti-parallel by having opposite polarity, i.e. by being able to conduct in opposite directions).

[0061] In some embodiments of the present invention, the plurality of converter valves of the bipolar device 4a includes four converter valves T1, T2, T3, and T4 forming an H-bridge topology, for example in Figure 2 In the embodiments described above.

[0062] In some other embodiments of the invention, the bipolar device 4a is configured to alternately excite the first or second BB stage 4b1 or 4b2 of the BB device 4b, for example in Figure 3 In the embodiments described above.

[0063] In some other embodiments of the invention, each of the plurality of semiconductor switches Sx and Sy of the BB device 4b is configured for a switching frequency of at least 1 kHz (e.g., at least 5 or 10 kHz).

[0064] In some other embodiments of the invention, each of the plurality of semiconductor switches Sx and Sy in the BB device 4b comprises silicon carbide (SiC) or silicon (Si) semiconductor material, or a combination thereof. In some embodiments, silicon carbide is preferred due to its ability to handle high switching frequencies.

[0065] In some other embodiments of the invention, each of the plurality of semiconductor switches S1, S2, S3 and S4, and / or Sx and Sy of the BB device and / or bipolar device comprises an insulated-gate bipolar transistor (IGBT), an integrated gate-commutated thyristor (IGCT), a dual-mode insulated-gate transistor (BiGT), or a metal-oxide-semiconductor field-effect transistor (MOSFET). These are typical examples of semiconductor switching devices, but any other device may also be used in embodiments of the invention.

[0066] In some other embodiments of the present invention, the primary energy storage device C m and / or secondary energy storage device C f It includes capacitors, supercapacitors, or batteries, or is composed of capacitors, supercapacitors, or batteries.

[0067] In some other embodiments of the present invention, the MMC is in the form of a Y-shaped, double Y-shaped, triple Y-shaped, or Δ topology, or a combination thereof.

[0068] In some other embodiments of the invention, the MMC is configured to operate as a STATCOM, an HVDC or MVDC converter, or a railway interlocking power grid.

[0069] In some other embodiments of the invention, the MMC1 has a DC side including a positive DC terminal DC+ and a negative DC terminal DC-. In some embodiments, the positive and negative DC terminals are connected to an energy storage system, which includes, for example, one or more batteries or supercapacitors or combinations thereof, or is composed of one or more batteries or supercapacitors or combinations thereof.

[0070] According to a more general embodiment of the invention, a full-bridge converter unit 4 is provided. The unit includes a buck-boost (BB) device 4b comprising a plurality of semiconductor switches Sx and Sy. The unit also includes a bipolar device 4a configured as a junction between the BB device of the unit and terminals A and B. The bipolar device includes a plurality of semiconductor switches S1-S4. The BB device is configured to operate by switching its semiconductor switches such that when power flows into the unit, power is moved from the terminal (or secondary energy storage unit Cf) through an inductor Lf to the primary energy storage unit Cm, and when power flows out of the unit, power is moved from the primary energy storage unit Cm through an inductor to the terminal (or secondary energy storage unit). The bipolar device 4a is configured by switching its semiconductor switches such that the BB device 4b can operate independently of the polarity of the unit terminals.

[0071] 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. A converter unit (4) for a modular multilevel converter (MMC) (1), the unit comprising: A boost / buck converter (4b) comprising a plurality of converter valves (Tx, Ty), each valve comprising a semiconductor switch (Sx, Sy) and an anti-parallel diode (Dx, Dy); and A bipolar device (4a) is configured as a junction between the buck-boost device (4b) of the unit and the terminals (A, B) and includes a plurality of converter valves (T1, T2, T3, T4), each valve including a semiconductor switch (S1, S2, S3, S4) and an anti-parallel diode (D1, D2, D3, D4). The step-up / step-down device (4b) is configured to operate by switching its semiconductor switches, such that: When power flows into the unit, the power moves from the terminals (A, B) through the inductor (Lf) of the buck-boost device (4b) to the main energy storage unit (Cm) of the buck-boost device (4b) to charge the main energy storage unit, and When power flows out of the unit, power moves from the main energy storage device (Cm) through the inductor (Lf) to the terminals (A, B) to discharge the main energy storage device; The bipolar device (4a) is configured to enable the buck-boost device (4b) to operate regardless of the polarity of the unit terminals (A, B) by switching its semiconductor switch; and The main energy storage device (Cm), the secondary energy storage device (Cf) of the step-up / step-down device (4b) and the inductor (Lf) are connected in parallel with each other, and each of the main energy storage device (Cm) and the secondary energy storage device (Cf) is composed of at least one capacitor or supercapacitor.

2. The unit according to claim 1, wherein, The bipolar device (4a) includes multiple converter valves comprising four converter valves (T1, T2, T3, T4) forming an H-bridge topology.

3. The unit according to claim 1, wherein, The bipolar device (4a) is configured to alternately stimulate the first or second step-up or step-down stage (4b1, 4b2) of the step-up or step-down device (4b).

4. The unit according to any one of the preceding claims, wherein, Each of the multiple semiconductor switches (Sx, Sy) of the buck-boost device (4b) is configured for a switching frequency of at least 1 kHz.

5. The unit according to any one of claims 1-3, wherein, Each of the multiple semiconductor switches (Sx, Sy) in the buck-boost device (4b) comprises silicon carbide (SiC) or silicon-Si semiconductor material, or a combination thereof.

6. The unit according to claim 5, wherein, Each of the multiple semiconductor switches (Sx, Sy) in the buck-boost device (4b) comprises silicon carbide (SiC).

7. The unit according to any one of claims 1-3, wherein, Each of the multiple semiconductor switches (S1, S2, S3, S4, Sx, Sy) of the buck-boost device (4b) and / or the bipolar device includes an insulated gate bipolar transistor (IGBT), an integrated gate commutated thyristor (IGCT), a dual-mode insulated gate transistor (BiGT), or a metal-oxide-semiconductor field-effect transistor (MOSFET).

8. A modular multilevel converter (MMC) (1) comprising a plurality of converter arms (3), each arm comprising a plurality of converter units (4) connected in series according to any one of the preceding claims.

9. The MMC according to claim 8, wherein, The MMC has a Y-shaped, double Y-shaped, triple Y-shaped, or Δ topology, or a combination thereof.

10. The MMC according to any one of claims 8 to 9, wherein, The MMC is configured to operate as a STATCOM, an HVDC or MVDC converter, or a railway interlocking power grid.

11. The MMC according to any one of claims 8 to 9, wherein, The MMC (1) has a DC side including a positive DC terminal (DC+) and a negative DC terminal (DC-).

12. The MMC according to claim 11, wherein, The positive DC terminal and the negative DC terminal are connected to an energy storage system, which includes one or more batteries or supercapacitors, or combinations thereof, or is composed of one or more batteries or supercapacitors, or combinations thereof.

13. A method performed by a control device (10) for controlling a converter unit (4) in a modular multilevel converter (MMC) (1) according to any one of claims 1 to 7, the method comprising: The main energy storage device (Cm) is charged by moving electricity from the terminals (A, B) through the inductor (Lf) to the main energy storage device (Cm); as well as The main energy storage device (Cm) is discharged by moving power from the main energy storage device (Cm) through the inductor (Lf) to the terminals (A, B).

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