Lightweight modular multilevel converter based on half-wave transformation

Abstract

The invention relates to the technical field of power electronics, in particular to a lightweight modular multilevel converter based on half-wave transformation, which comprises a first phase unit, a second phase unit and a third phase unit, the first phase unit, the second phase unit and the third phase unit are the same, and each phase unit comprises a group of cascade bridge arm structures, a group of device series full bridges and a single-phase transformer, the cascade bridge arm structure is used for converting the direct-current voltage into multi-level half-wave voltage; the device series full bridge is used for converting the multi-level half-wave voltage into an alternating current voltage so as to be connected with an alternating current power grid through the single-phase transformer. Therefore, through the collaborative design of the cascade bridge arm and the device series full bridge, the direct current voltage is firstly converted into the multi-level half-wave voltage and then is inverted into the alternating current to be output, and the problems of large consumption and high cost of MMC sub-modules and devices in the related technology are solved, so that the cost of the MMC is reduced, and the economical efficiency of the converter is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power electronics, and particularly relates to a light-weight modular multilevel converter based on half-wave conversion. BACKGROUND

[0002] With the increasing penetration of renewable energy such as wind power and photovoltaic power in the power grid, large-capacity long-distance transmission of large-scale renewable energy has become an increasingly prominent problem.

[0003] In the related art, a modular multilevel converter (MMC) has the advantages of high voltage quality, flexible control, and easy expansion, and has broad application prospects in the fields of flexible DC power transmission and offshore wind power grid connection.

[0004] However, the MMC topology in the related art contains a large number of sub-modules, and a large number of capacitors and power devices are required for the converter station, which has a significant volume, weight, and investment cost. With the development of large-scale new energy DC grid connection projects, the demand for light-weight converter platforms is increasingly urgent, represented by the project of sending out offshore wind power through flexible DC. The high cost and large weight and volume of MMC equipment have become increasingly prominent, which has limited the larger-scale application of MMC and needs to be solved urgently. SUMMARY

[0005] The present application provides a light-weight modular multilevel converter based on half-wave conversion to solve the problem of large quantity and high cost of MMC sub-modules and devices in the related art, thereby reducing the cost of MMC and improving the economic efficiency of the converter.

[0006] The first aspect of the present application provides a light-weight modular multilevel converter based on half-wave conversion, comprising: first to third phase units, the first to third phase units are the same, and each phase unit comprises a group of cascaded bridge arm structures, a group of device series full-bridges, and a single-phase transformer, wherein, The cascaded bridge arm structure is used to convert a direct-current voltage into a multi-level half-wave voltage; The device series full-bridge is used to convert the multi-level half-wave voltage into an alternating-current voltage to connect an alternating-current power grid through the single-phase transformer.

[0007] Optionally, the cascaded bridge arm structure comprises: A first bridge arm, one end of the first bridge arm is connected with a first power terminal of a corresponding phase unit; A first bridge arm inductor, one end of the first bridge arm inductor is connected with the other end of the first bridge arm; a second bridge arm inductor, one end of the second bridge arm inductor being connected with the other end of the first bridge arm inductor, and a connection node between the first bridge arm inductor and the second bridge arm inductor being a first internal power node; a second bridge arm, one end of the second bridge arm being connected with the other end of the second bridge arm inductor; a third bridge arm, one end of the third bridge arm being connected with the other end of the second bridge arm; a third bridge arm inductor, one end of the third bridge arm inductor being connected with the other end of the third bridge arm; a fourth bridge arm inductor, one end of the fourth bridge arm inductor being connected with the other end of the third bridge arm inductor, and a connection node between the third bridge arm inductor and the fourth bridge arm inductor being a second internal power node; a fourth bridge arm, one end of the fourth bridge arm being connected with the other end of the fourth bridge arm inductor, and the other end of the fourth bridge arm being connected with a second power terminal of a corresponding phase unit.

[0008] Optionally, each of the first to fourth bridge arms comprises: a first sub-module to an Nth sub-module, N being an integer, wherein, each sub-module comprises: a capacitor, a first transistor and a second transistor, one end of the first capacitor being connected with a collector of the first transistor, an emitter of the first transistor being connected with a collector of the second transistor, and an emitter of the second transistor being connected with the other end of the first capacitor, wherein, a connection node between the emitter of the first transistor and the collector of the second transistor constitutes a first connection point, and a connection node between the emitter of the second transistor and the other end of the first capacitor constitutes a second connection point.

[0009] Optionally, the first connection point of the first sub-module of the first bridge arm is connected with a first power terminal of a corresponding phase unit, the second connection point of the first sub-module of the first bridge arm and the first connection point of the Nth sub-module are sequentially connected, and the second connection point of the Nth sub-module of the first bridge arm is connected with one end of the first bridge arm inductor; the first connection point of the first sub-module of the second bridge arm is connected with the other end of the first bridge arm inductor, the second connection point of the first sub-module of the second bridge arm and the first connection point of the Nth sub-module are sequentially connected, and the second connection point of the Nth sub-module of the second bridge arm is connected with the first connection point of the first sub-module of the third bridge arm; the second connection point of the first sub-module of the third bridge arm and the first connection point of the Nth sub-module are sequentially connected, the second connection point of the Nth sub-module of the third bridge arm is connected with one end of the third bridge arm inductor, and one end of the third bridge arm inductor is connected with one end of the fourth bridge arm inductor; The first connection point of the first sub-module of the fourth bridge arm is connected with the other end of the fourth bridge arm inductor, the first connection point of the second sub-module to the first connection point of the Nth sub-module of the fourth bridge arm are sequentially connected, and the second connection point of the Nth sub-module of the fourth bridge arm is connected with the second power terminal of the corresponding phase unit.

[0010] Optionally, the device in series full-bridge comprises first to fourth switches, wherein, One end of the first switch and one end of the third switch are connected with the first internal power node of the corresponding phase unit; The other end of the first switch is connected with one end of the second switch, one end of the third switch is connected with one end of the fourth switch, and the connection node between the other end of the first switch and one end of the second switch is the third power terminal of the corresponding phase unit, and the connection node between the other end of the third switch and one end of the fourth switch is the fourth power terminal of the corresponding phase unit. The other end of the second switch and the other end of the fourth switch are connected with the second internal power node of the corresponding phase unit.

[0011] Optionally, the first to fourth switches are composed of one bidirectional insulated gate bipolar transistor and multiple bidirectional thyristors in series.

[0012] Optionally, the third power terminal and the fourth power terminal of each phase unit are respectively connected with the two ends of the primary side of the single-phase transformer of the corresponding phase unit. Optionally, Optionally, the secondary sides of the single-phase transformers of the first to third phase units are connected in star type.

[0013] Optionally, for the first to third phase units, when the voltage between the third power terminal and the fourth power terminal of the phase unit is greater than 0, the first switch and the fourth switch are turned on, and the second switch and the third switch are turned off; when the voltage between the third power terminal and the fourth power terminal of the phase unit is less than 0, the first switch and the fourth switch are turned off, and the second switch and the third switch are turned on.

[0014] Optionally, the voltage and current of the first bridge arm and the fourth bridge arm are the same, and the voltage and current of the second bridge arm and the third bridge arm are the same.

[0015] Therefore, the embodiment of the present application comprises the same first to third phase units, and each phase unit comprises a set of cascaded bridge arm structures, a set of device series full bridges and a single-phase transformer; the cascaded bridge arm structures are used for converting a direct current voltage into a multi-level half-wave voltage; the device series full bridges are used for converting the multi-level half-wave voltage into an alternating current voltage to connect an alternating current grid through the single-phase transformer. Therefore, through the collaborative design of the cascaded bridge arm and the device series full bridge, the direct current voltage is first converted into a multi-level half-wave voltage and then inverted into an alternating current output, solving the problems of large MMC sub-module and device quantity and high cost in the related art, thereby reducing the cost of the MMC and improving the economic efficiency of the converter.

[0016] Additional aspects and advantages of the present application will be made apparent by the following description and the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0017] The above and / or additional aspects and advantages of the present application will become apparent and be readily appreciated from the following description, including the accompanying drawings, wherein: Figure 1 A schematic diagram of a light-weight modular multilevel converter based on half-wave conversion according to an embodiment of the present application; Figure 2 A schematic diagram of the working principle of a half-wave conversion light-weight MMC according to an embodiment of the present application; Figure 3 A schematic diagram of the simulation results of a half-wave conversion light-weight MMC according to an embodiment of the present application; Figure 4 A schematic diagram of the topology structure of a conventional MMC in the related art. DETAILED DESCRIPTION

[0018] The embodiments of the present application are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference signs represent the same or similar elements or elements having the same or similar functions throughout. The embodiments described below by referring to the accompanying drawings are exemplary and are intended to explain the present application, and cannot be understood as a limitation of the present application.

[0019] The present application is based on the above problems, and provides a light-weight modular multilevel converter based on half-wave conversion. The light-weight modular multilevel converter based on half-wave conversion comprises first to third phase units which are the same, and each phase unit comprises a set of cascaded bridge arm structures, a set of device series full-bridges, and a single-phase transformer. The cascaded bridge arm structures are used to convert a direct current voltage into a multilevel half-wave voltage. The device series full-bridges are used to convert the multilevel half-wave voltage into an alternating current voltage, so as to connect an alternating current grid through the single-phase transformer. Thus, through the collaborative design of the cascaded bridge arm and the device series full-bridge, the direct current voltage is first converted into a multilevel half-wave voltage and then inverted into an alternating current output, thereby solving the problems of large MMC sub-module and device usage and high cost in the related art, reducing the cost of the MMC, and improving the economy of the converter.

[0020] Specifically, Figure 1 A schematic diagram of the light-weight modular multilevel converter based on half-wave conversion is provided.

[0021] As Figure 1 shown, the light-weight modular multilevel converter based on half-wave conversion comprises first to third phase units which are the same, and each phase unit comprises a set of cascaded bridge arm structures, a set of device series full-bridges, and a single-phase transformer, wherein The cascaded bridge arm structures are used to convert a direct current voltage into a multilevel half-wave voltage. The device series full-bridges are used to convert the multilevel half-wave voltage into an alternating current voltage, so as to connect an alternating current grid through the single-phase transformer.

[0022] Specifically, as Figure 1 shown, wherein the U, V, and W three-phase structures are completely the same, and thus the A phase is taken as an example; the half-wave conversion light-weight MMC (Modular multilevel converter) is composed of three identical phase units, each phase unit comprising a set of cascaded bridge arm structures, a set of device series full-bridges, and a single-phase transformer. The cascaded bridge arm converts a direct current bus voltage (U dc ) into a multilevel half-wave voltage (U PN ) with a sinusoidal envelope through a multilevel modulation technology, only needs to generate a half-wave voltage, and reduces the voltage bearing of each bridge arm to U dc / 2, thereby reducing the number of sub-modules by half; the device series full-bridge converts the half-wave voltage (U PN ) into a complete alternating current voltage (U O ) through polarity reversal control, and through the half-wave-full-wave conversion mechanism, the amplitude of the alternating current output voltage is doubled, and the current stress is further reduced. The cascaded bridge arm structure, the device series full-bridge, and the single-phase transformer cooperate to maintain the same level number and harmonic performance as the traditional MMC, reduce the capacitor energy storage by 50%, and reduce the device current stress by 50%, thereby balancing the light weight and reliability requirements.

[0023] Optionally, in some embodiments, the cascaded bridge arm structure includes: a first bridge arm, one end of which is connected to a first power terminal of a corresponding phase unit; a first bridge arm inductor, one end of which is connected to the other end of the first bridge arm; a second bridge arm inductor, one end of which is connected to the other end of the first bridge arm inductor, the connection node between the first and second bridge arm inductors being a first internal power node; a second bridge arm, one end of which is connected to the other end of the second bridge arm inductor; a third bridge arm, one end of which is connected to the other end of the second bridge arm; a third bridge arm inductor, one end of which is connected to the other end of the third bridge arm; a fourth bridge arm inductor, one end of which is connected to the other end of the third bridge arm inductor, the connection node between the third and fourth bridge arm inductors being a second internal power node; and a fourth bridge arm, one end of which is connected to the other end of the fourth bridge arm inductor, the other end of which is connected to a second power terminal of a corresponding phase unit.

[0024] Optionally, in some embodiments, each of the first to fourth bridge arms includes: a first submodule to an Nth submodule, where N is an integer. Each submodule includes: a capacitor, a first transistor, and a second transistor. One end of the first capacitor is connected to the collector of the first transistor, the emitter of the first transistor is connected to the collector of the second transistor, and the emitter of the second transistor is connected to the other end of the first capacitor. The connection node between the emitter of the first transistor and the collector of the second transistor forms a first connection point, and the connection node between the emitter of the second transistor and the other end of the first capacitor forms a second connection point.

[0025] Optionally, in some embodiments, the first connection point of the first sub-module of the first bridge arm is connected to the first power terminal of the corresponding phase unit, the second connection point of the first sub-module of the first bridge arm to the first connection point of the Nth sub-module are sequentially connected, and the second connection point of the Nth sub-module of the first bridge arm is connected to one end of the first bridge arm inductor; the first connection point of the first sub-module of the second bridge arm is connected to the other end of the first bridge arm inductor, the second connection point of the first sub-module of the second bridge arm to the first connection point of the Nth sub-module are sequentially connected, and the second connection point of the Nth sub-module of the second bridge arm is connected to the first connection point of the first sub-module of the third bridge arm; the second connection point of the first sub-module of the third bridge arm to the first connection point of the Nth sub-module are sequentially connected, the second connection point of the Nth sub-module of the third bridge arm is connected to one end of the third bridge arm inductor, and one end of the third bridge arm inductor is connected to one end of the fourth bridge arm inductor; the first connection point of the first sub-module of the fourth bridge arm is connected to the other end of the fourth bridge arm inductor, the second connection point of the first sub-module of the fourth bridge arm to the first connection point of the Nth sub-module are sequentially connected, and the second connection point of the Nth sub-module of the fourth bridge arm is connected to the second power terminal of the corresponding phase unit.

[0026] Optionally, in some embodiments, the device is connected in series with a full bridge, including: a first to a fourth switch, wherein one end of the first switch and the third switch are connected to the first internal power node of the corresponding phase unit; the other end of the first switch is connected to one end of the second switch, one end of the third switch is connected to one end of the fourth switch, and the connection node between the other end of the first switch and the first end of the second switch is the third power terminal of the corresponding phase unit, and the connection node between the other end of the third switch and the first end of the fourth switch is the fourth power terminal of the corresponding phase unit; the other ends of the second switch and the other ends of the fourth switch are both connected to the second internal power node of the corresponding phase unit.

[0027] Among them, such as Figure 1 As shown in the figure, bridge arm 1 is the first bridge arm of this embodiment, bridge arm 2 is the second bridge arm of this embodiment, bridge arm 3 is the third bridge arm of this embodiment, and bridge arm 4 is the fourth bridge arm of this embodiment; the bridge arm inductance L in the figure... b1 This refers to the first bridge arm inductor in this embodiment of the application, specifically the bridge arm inductor L in the figure. b2 This refers to the second bridge arm inductor in this embodiment of the application, specifically the bridge arm inductor L in the figure. b3 This refers to the third bridge arm inductor in this embodiment of the application, specifically the bridge arm inductor L in the figure. b4 This refers to the fourth bridge arm inductor in this embodiment; X1 in the figure is the first power terminal in this embodiment, X2 in the figure is the second power terminal in this embodiment, X3 in the figure is the third power terminal in this embodiment, and X4 in the figure is the fourth power terminal in this embodiment; P in the figure is the first internal power node in this embodiment, and N in the figure is the second internal power node in this embodiment; M1 in the figure is the first connection point in this embodiment, and M2 in the figure is the second connection point in this embodiment.

[0028] It is understandable that, such as Figure 1 As shown, the cascaded bridge arm structure consists of bridge arm inductors L b1 Bridge arm 1, bridge arm inductor L b2 Bridge arm 2, bridge arm 3, bridge arm inductor L b3 Bridge arm inductor L b4The bridge arm 1, 2, 3, and 4 are connected in series with bridge arm 4. The full-bridge circuit consists of four series-connected units Q1, Q2, Q3, and Q4. Each phase unit has four power terminals X1, X2, X3, and X4, and also includes two internal power nodes P and N. Bridge arms 1, 2, 3, and 4 are formed by connecting sub-modules represented by half-bridge circuits in series. Each sub-module has two connection points M1 and M2. The M2 connection point of the previous sub-module is connected to the M1 connection point of the next sub-module. The M1 connection point of the first sub-module of bridge arm 1 is connected to the power terminal X1, and the M2 connection point of the last sub-module of bridge arm 1 is connected to the bridge arm inductor L. b1 At the upper end, bridge arm inductor L b1 Lower end and bridge arm inductance L b2 The upper end connects to the internal power node P, and the bridge arm inductor L b2 The lower end is connected to the M1 connection point of the first submodule of bridge arm 2, the M2 connection point of the last submodule of bridge arm 2 is connected to the M1 connection point of the first submodule of bridge arm 3, and the M2 connection point of the last submodule of bridge arm 3 is connected to the bridge arm inductor L. b3 At the upper end, bridge arm inductor L b3 Lower end and bridge arm inductance L b4 The upper end is connected to the internal power node N, and the bridge arm inductor L b4 The lower end is connected to the M1 connection point of the first sub-module of bridge arm 4, and the M2 connection point of the last sub-module of bridge arm 4 is connected to the power terminal X2.

[0029] Optionally, in some embodiments, the first to fourth switches are each composed of a bidirectional insulated-gate bipolar transistor and multiple bidirectional thyristors connected in series.

[0030] In the figure, Q1 is the first switch of this application embodiment, Q2 is the second switch of this application embodiment, Q3 is the third switch of this application embodiment, and Q4 is the fourth switch of this application embodiment.

[0031] It is understandable that, such as Figure 1 As shown, the series units Q1, Q2, Q3, and Q4 are composed of a bidirectional insulated gate bipolar transistor (IGBT) and several bidirectional thyristors connected in series, with connection points K1 and K2. The internal power node P is connected to the K1 connection point of the series units Q1 and Q3, and the internal power node N is connected to the K2 connection point of the series units Q2 and Q4. The K2 connection point of the series unit Q1 is connected to the K1 connection point of Q2, and the K2 connection point of the series unit Q3 is connected to the K1 connection point of Q4, which is also connected to the power terminal X3.

[0032] Optionally, in some embodiments, the third power terminal and the fourth power terminal of each phase unit are respectively connected to the two ends of the primary side of the single-phase transformer of the corresponding phase unit.

[0033] Optionally, in some embodiments, the secondary sides of the single-phase transformers of the first to third phase units are star-connected.

[0034] It is understandable that, such as Figure 1 As shown, the power terminals X3 and X4 of the three phase units are connected to the two ends of the primary side of the three single-phase transformers respectively. The secondary side of the three single-phase transformers adopts a star connection, that is, one end is a three-phase AC output of U, V and W, and the other end is a three-phase connection together.

[0035] Therefore, the embodiments of this application consist of three identical phase units, each phase unit including a set of cascaded bridge arm structures, a set of series-connected full-bridge devices, and a single-phase transformer. The cascaded bridge arm structure consists of a bridge arm inductor L. b1 Bridge arm 1, bridge arm inductor L b2 Bridge arm 2, bridge arm 3, bridge arm inductor L b3 Bridge arm inductor L b4 The cascaded bridge arm is connected in series with the other four arms. The full-bridge circuit consists of four sets of power devices connected in series. One side connects to the two output terminals of the cascaded bridge arm structure, and the other side is the AC output. The AC output of the three phase units is connected to the AC power grid through three single-phase transformers. Compared with traditional modular multilevel converters in related technologies, the proposed new topology can significantly reduce the number of sub-modules and capacitors, and improve the converter's economy.

[0036] Optionally, in some embodiments, for the first to third phase units, when the voltage between the third power terminal and the fourth power terminal of the phase unit is greater than 0, the first switch and the fourth switch are turned on, and the second switch and the third switch are turned off; when the voltage between the third power terminal and the fourth power terminal of the phase unit is less than 0, the first switch and the fourth switch are turned off, and the second switch and the third switch are turned on.

[0037] It is understandable that, such as Figure 2 and Figure 3 As shown, Figure 2 This is a schematic diagram illustrating the working principle of a lightweight modular multilevel converter (MMC) based on half-wave transform, according to one embodiment of this application. Figure 3 This is a schematic diagram illustrating the simulation results of a lightweight modular multilevel converter (MMC) based on half-wave transform, according to an embodiment of this application; the voltage U between the output terminals X3 and X4 of the phase unit is shown. O The waveform is a single-phase sine wave, when U OWhen the value is greater than 0, the series units Q1 and Q4 of the device are turned on, Q2 and Q3 are turned off, the internal power node P is connected to X3 and N is connected to X4, and the voltage U between P and N is... PN The waveform and U O The positive half-cycles are the same; when U O When the value is less than 0, the series units Q2 and Q3 of the device are turned on, Q1 and Q4 are turned off, and the internal power nodes P are connected to X4 and N are connected to X3. PN The waveform and U O Conversely, for U O The negative half-cycle voltage flips; therefore, U PN The waveform is U O The waveform is a half-wave sinusoidal voltage after rectification.

[0038] It should be noted that in this operating mode, the switching states of Q1, Q2, Q3, and Q4 only change in U. O Switching is performed near 0, so they only bear high voltage when static, but basically do not bear voltage when switching. There is no dynamic voltage equalization problem of series devices in other traditional technologies. Series connection of multiple devices can be easily achieved, and it also has high reliability.

[0039] Optionally, in some embodiments, the voltage and current of the first bridge arm and the fourth bridge arm are the same, and the voltage and current of the second bridge arm and the third bridge arm are the same.

[0040] It is understandable that, such as Figure 2 As shown, the function of the cascaded bridge arm structure is to convert the constant DC bus voltage U between X1 and X2. dc Transform into U PN The half-wave sinusoidal voltage. Similar to the traditional MMC, the four arms of each phase unit in the half-wave transform lightweight MMC can be equivalent to four controlled voltage sources. The voltage of arms 1 to 4 is defined as U. b1 U b2 U b3 U b4 The waveforms of the voltages in the four bridge arms are as follows: Figure 2 As shown, to achieve the above-mentioned power conversion objective, the voltages of the four bridge arms should satisfy the following relationship, including that the sum of the voltages of the four bridge arms is U. dc To maintain a constant DC bus voltage, i.e.: ; And the sum of the voltages of bridge arms 2 and 3 is U. PN ,Right now: ; Furthermore, to ensure the balance of pressure resistance of each bridge arm, U b2 U b3 It should be distributed equally, that is: ; And U b1 U b4 It should also be distributed evenly. Combining the voltage distribution mentioned above, the calculation formula is as follows: ; The above expression also represents the configuration of the reference voltages for bridge arms 1 to 4. Based on the working principle, this embodiment also requires voltage balance control of the floating capacitors in the four bridge arms. This balance control includes three levels: the balance of the total energy of the four bridge arms, the balance of energy between bridge arms, and the balance of the capacitor voltages of each submodule within each bridge arm. The expressions for the currents of the four bridge arms are as follows, where each bridge arm current includes a portion i related to the output current. P and circulation i c The calculation formula is: ; The total energy balance of the bridge arms is equivalent to the sum of the DC power of the four bridge arms being 0, that is: ; Substituting the expressions for bridge arm current and bridge arm voltage, the total energy balance of the bridge arms is equivalent to: ; Given a fixed AC side voltage and current The value is fixed, therefore it can be controlled by controlling the circulating current i. c The DC component in the circuit is used to achieve total energy balance control. Considering that the voltage and current of bridge arms 1 and 4 are exactly the same, and the voltage and current of bridge arms 2 and 3 are exactly the same, the energy balance between the bridge arms is equivalent to the energy balance of bridge arms 1 and 2. Calculations show that: ; Based on AC side output and total energy balance control, i P i c The DC component in the circuit has been determined, therefore it can be controlled by controlling the circulating current i. c The second harmonic component in the modulation scheme enables energy balance control between bridge arms. The balance of capacitor voltage in each submodule within each bridge arm depends on the modulation method used. When using carrier phase-shift modulation, capacitor voltage balance control can be achieved by fine-tuning the reference voltage of each module. When using carrier stacking or nearest-level approximation modulation, capacitor voltage balance control can be achieved by adjusting the order of access and bypass of each submodule.

[0041] It should be noted that the sub-modules of the cascaded bridge arm can be selected as follows: Figure 1The half-bridge circuit structure shown can also be replaced by other circuit structures such as a full-bridge structure or a diode-clamped three-level circuit. Depending on the application scenario and DC bus voltage, the power electronic switching devices in this embodiment can be selected from different types and levels of devices, such as silicon MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors), IGCTs (Integrated Gate Commutated Thyristors), and wide-bandgap semiconductor switching devices made of silicon carbide and gallium nitride. The structure, placement, and connections of each component in this embodiment can be improved and equivalently transformed based on the embodiments of this application.

[0042] Therefore, the lightweight modular multilevel converter based on half-wave conversion proposed in this application consists of a cascaded bridge arm structure and a series-connected full-bridge. The cascaded bridge arm structure is composed of sub-modules, represented by half-bridge circuits, and bridge arm inductors connected in series, converting DC voltage into a multi-level half-wave voltage. The series-connected full-bridge consists of four sets of series-connected power electronic switching devices, converting the multi-level half-wave voltage into AC output. It withstands high voltage in static conditions, but during switching operation, it withstands at most the voltage of a single sub-module capacitor in the cascaded bridge arm, thus avoiding the dynamic voltage equalization problem inherent in traditional series-connected devices. By adopting this structure, the capacitor usage can be reduced by 50% compared to traditional MMCs, thereby significantly reducing the cost of MMCs and improving system economy.

[0043] After introducing the lightweight modular multilevel converter based on half-wave transform according to the embodiments of this application, a brief comparison is made between the new topology proposed in this embodiment and the traditional MMC topology.

[0044] Specifically, such as Figure 4 As shown, Figure 4 This is a schematic diagram of the traditional MMC topology in related technologies; the load-bearing capacity of each cascaded bridge arm in the traditional MMC topology is U. dc There are no series-connected components, and the AC output phase voltage range is -U. dc / 2~U dc / 2. In this application embodiment, the bearing capacity of each cascaded bridge arm of the half-wave transform MMC topology is U. dc / 2, the pressure of each device in series is U dc The AC side output phase voltage range is -U dc ~U dcAssuming that the number of submodules in each cascaded bridge arm of a conventional MMC topology is N, then the number of submodules in each cascaded bridge arm of the topology in this embodiment is N / 2, and the total number of devices required for each device series unit is N. Considering that the AC voltage amplitude output by the half-wave converter MMC topology in this embodiment is twice that of the conventional MMC, the phase current amplitude of the half-wave converter MMC topology is half that of the conventional MMC when the converter capacity is the same.

[0045] Furthermore, based on this result, although the total number of submodules in the half-wave converter MMC topology is the same as that of the traditional MMC under the same DC bus voltage conditions, the fundamental current amplitude flowing through each bridge arm is half that of the traditional MMC. Therefore, the total energy storage of the capacitor in the half-wave converter MMC topology is 50% of that in the traditional MMC topology, which can significantly reduce the cost and footprint of the converter. Simultaneously, under the above conditions, the current stress of a single power device in the half-wave converter MMC topology is also half that of the traditional MMC topology. Therefore, although the half-wave converter MMC topology has a larger total number of devices than the traditional MMC topology due to the addition of a series full-bridge device, its total device volt-ampere capacity is the same as that of the traditional MMC. Table 1 compares the economics of the traditional MMC and the half-wave converter lightweight MMC topology. The cost comparison between the traditional MMC topology based on half-bridge submodules and the half-wave converter MMC topology is shown in Table 1. Table 1

[0046] Furthermore, in the lightweight half-wave converter MMC of this application embodiment, the series-connected units only bear high voltage in the static state and basically do not bear voltage during switching. Therefore, the dynamic voltage equalization problem of series-connected devices in traditional series-connected converters in related technologies does not exist. The series-connected voltage equalization circuit is simple, the series connection difficulty is low, and the series connection of multiple devices can be easily implemented, resulting in high reliability. Under the aforementioned module quantity ratio, the lightweight half-wave converter MMC of this application embodiment has the same number of voltage levels as a traditional MMC, and the harmonic performance of both is consistent. In addition, the lightweight half-wave converter MMC of this application embodiment has a voltage range of 0 to U... dc It can work normally within the specified range, unlike some other topologies that reduce the number of submodules and components but can only work normally within a specific AC / DC voltage ratio range.

[0047] Therefore, the embodiments of this application significantly reduce the amount of capacitor used in the converter, improving the economy of the converter; the device series unit switches hardly bear voltage when switching, the device series connection is easy and the reliability is high; and the characteristics of the embodiments of this application are consistent with the traditional MMC when working normally.

[0048] According to the lightweight modular multilevel converter based on half-wave conversion proposed in this application, the embodiments of this application include identical first to third phase units, and each phase unit includes a set of cascaded bridge arm structures, a set of device series full-bridges, and a single-phase transformer. The cascaded bridge arm structures are used to convert DC voltage into multilevel half-wave voltage; the device series full-bridge is used to convert the multilevel half-wave voltage into AC voltage, so as to connect to the AC power grid through the single-phase transformer. Thus, through the coordinated design of the cascaded bridge arms and the device series full-bridge, the DC voltage is first converted into a multilevel half-wave voltage and then inverted into AC output, solving the problems of large quantity and high cost of MMC submodules and devices in related technologies, thereby reducing the cost of MMC and improving the economics of the converter.

[0049] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0050] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "N" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0051] Any process or method described in the flowchart or otherwise herein can be understood as representing a module, segment, or portion of code comprising one or more N executable instructions for implementing custom logic functions or processes, and the scope of the preferred embodiments of this application includes additional implementations in which functions may be performed not in the order shown or discussed, including substantially simultaneously or in reverse order depending on the functions involved, as should be understood by those skilled in the art to which embodiments of this application pertain.

[0052] It should be understood that the various parts of this application can be implemented using hardware, software, firmware, or a combination thereof. In the above embodiments, the N steps or methods can be implemented using software or firmware stored in memory and executed by a suitable instruction execution system. For example, if implemented in hardware as in another embodiment, it can be implemented using any one or a combination of the following techniques known in the art: discrete logic circuits having logic gates for implementing logical functions on data signals, application-specific integrated circuits (ASICs) having suitable combinational logic gates, programmable gate arrays (PGAs), field-programmable gate arrays (FPGAs), etc.

[0053] Those skilled in the art will understand that all or part of the steps of the methods in the above embodiments can be implemented by a program instructing related hardware. The program can be stored in a computer-readable storage medium, and when executed, the program includes one or a combination of the steps of the method embodiments.

Claims

1. A lightweight modular multilevel converter based on half-wave transform, characterized in that, include: The first to third phase units are identical, and each phase unit includes a set of cascaded bridge arm structures, a set of series-connected full-bridge devices, and a single-phase transformer. The cascaded bridge arm structure is used to convert DC voltage into multi-level half-wave voltage; The device is connected in series with a full bridge to convert the multi-level half-wave voltage into an AC voltage, which is then connected to the AC power grid via the single-phase transformer.

2. The lightweight modular multilevel converter based on half-wave transform according to claim 1, characterized in that, The cascaded bridge arm structure includes: The first bridge arm, one end of which is connected to the first power terminal of the corresponding phase unit; First bridge arm inductor, one end of the first bridge arm inductor is connected to the other end of the first bridge arm; The second bridge arm inductor has one end connected to the other end of the first bridge arm inductor, and the connection node between the first bridge arm inductor and the second bridge arm inductor is the first internal power node. The second bridge arm, one end of which is connected to the other end of the second bridge arm inductor; The third bridge arm, one end of which is connected to the other end of the second bridge arm; The third bridge arm inductor, one end of which is connected to the other end of the third bridge arm; The fourth bridge arm inductor has one end connected to the other end of the third bridge arm inductor, and the connection node between the third bridge arm inductor and the fourth bridge arm inductor is the second internal power node. The fourth bridge arm has one end connected to the other end of the fourth bridge arm inductor, and the other end connected to the second power terminal of the corresponding phase unit.

3. The lightweight modular multilevel converter based on half-wave transform according to claim 2, characterized in that, The first through fourth bridge arms each include: the first submodule to the Nth submodule, where N is an integer, where... Each submodule includes: a capacitor, a first transistor, and a second transistor. One end of the first capacitor is connected to the collector of the first transistor, the emitter of the first transistor is connected to the collector of the second transistor, and the emitter of the second transistor is connected to the other end of the first capacitor. The connection node between the emitter of the first transistor and the collector of the second transistor forms a first connection point, and the connection node between the emitter of the second transistor and the other end of the first capacitor forms a second connection point.

4. The lightweight modular multilevel converter based on half-wave transform according to claim 3, characterized in that, The first connection point of the first sub-module of the first bridge arm is connected to the first power terminal of the corresponding phase unit. The second connection point of the first sub-module of the first bridge arm to the first connection point of the Nth sub-module are connected in sequence. The second connection point of the Nth sub-module of the first bridge arm is connected to one end of the inductor of the first bridge arm. The first connection point of the first sub-module of the second bridge arm is connected to the other end of the inductor of the first bridge arm. The second connection point of the first sub-module of the second bridge arm to the first connection point of the Nth sub-module are connected in sequence. The second connection point of the Nth sub-module of the second bridge arm is connected to the first connection point of the first sub-module of the third bridge arm. The second connection point of the first sub-module of the third bridge arm to the first connection point of the Nth sub-module are connected in sequence. The second connection point of the Nth sub-module of the third bridge arm is connected to one end of the inductor of the third bridge arm. One end of the inductor of the third bridge arm is connected to one end of the inductor of the fourth bridge arm. The first connection point of the first sub-module of the fourth bridge arm is connected to the other end of the inductor of the fourth bridge arm. The second connection point of the first sub-module of the fourth bridge arm to the first connection point of the Nth sub-module are connected in sequence. The second connection point of the Nth sub-module of the fourth bridge arm is connected to the second power terminal of the corresponding phase unit.

5. The lightweight modular multilevel converter based on half-wave transform according to claim 3, characterized in that, The device is connected in series with a full bridge, comprising: first to fourth switches, wherein, One end of the first switch and the third switch are connected to the first internal power node of the corresponding phase unit. The other end of the first switch is connected to one end of the second switch, one end of the third switch is connected to one end of the fourth switch, and the connection node between the other end of the first switch and one end of the second switch is the third power terminal of the corresponding phase unit, and the connection node between the other end of the third switch and one end of the fourth switch is the fourth power terminal of the corresponding phase unit. The other end of the second switch and the other end of the fourth switch are both connected to the second internal power node of the corresponding phase unit.

6. The lightweight modular multilevel converter based on half-wave transform according to claim 5, characterized in that, The first to fourth switches are each composed of a bidirectional insulated gate bipolar transistor and multiple bidirectional thyristors connected in series.

7. The lightweight modular multilevel converter based on half-wave transform according to claim 5, characterized in that, The third and fourth power terminals of each phase unit are respectively connected to the two ends of the primary side of the single-phase transformer of the corresponding phase unit.

8. The lightweight modular multilevel converter based on half-wave transform according to claim 5, characterized in that, The secondary sides of the single-phase transformers in the first to third phase units are connected in a star configuration.

9. The lightweight modular multilevel converter based on half-wave transform according to claim 5, characterized in that, For the first to third phase units, when the voltage between the third power terminal and the fourth power terminal of the phase unit is greater than 0, the first switch and the fourth switch are turned on, and the second switch and the third switch are turned off. When the voltage between the third power terminal and the fourth power terminal of the phase unit is less than 0, the first switch and the fourth switch are turned off, and the second switch and the third switch are turned on.

10. The lightweight modular multilevel converter based on half-wave transform according to claim 2, characterized in that, The voltage and current of the first bridge arm and the fourth bridge arm are the same, and the voltage and current of the second bridge arm and the third bridge arm are the same.

Images