Modular multilevel converter and voltage equalization control method

Abstract

The application discloses a modular multilevel converter and a voltage equalization control method, and relates to the electrical engineering field.A plurality of power device half-bridge sub-modules are cascaded on an alternating current side or a direct current side to form a cascade module, a plurality of power device half-bridge sub-modules in the cascade module share a same set of mechanical switches and thyristors, the number of devices such as the mechanical switches and the thyristors in the whole converter can be greatly reduced, and then the volume and the cost of the whole converter are reduced.The power device half-bridge sub-modules can realize voltage equalization in the cascade module and between the cascade modules without complex control;on the premise of reducing the cost and the volume, the voltage resistance limit and the switching loss are considered, and the whole converter is simply realized, so that the converter is widely applied.

Description

Technical Field

[0001] This invention relates to the field of electrical engineering, and in particular to a modular multilevel converter and a voltage equalization control method. Background Technology

[0002] Due to the challenges faced by power distribution networks in densely populated urban areas with high loads, such as a shortage of power supply corridors, limited grid capacity expansion, and the urgent need to optimize network losses, the current approach involves introducing DC technology into existing AC distribution network lines to upgrade them into flexible distribution networks. This effectively increases the DC power supply capacity in high-density, narrow-channel load areas. In medium-voltage distribution networks, MMCs (modular-multilevel converters) are generally used to connect medium-voltage DC and AC. MMCs offer advantages such as high efficiency, high power quality, and high reliability, but they also have disadvantages such as large footprint and high cost; currently, a 5MW converter can occupy up to 36m². 2 Medium-voltage distribution network MMCs typically use 1700VIGBT (Insulated Gate Bipolar Transistor) modules as the main power devices. The rated voltage of each submodule is approximately 800V, containing about 144 submodules, and requiring an additional 10-20% redundancy. Each submodule needs a DC capacitor in the mF range, which occupies more than 50% of the submodule's volume. Traditional MMCs are limited by the voltage withstand capability of the submodules, preventing a reduction in the number of submodules. The large number of submodules in the entire MMC results in a large overall size and high cost.

[0003] In existing technologies, there are two main solutions. The first solution uses a single high-voltage device. Currently, the highest withstand voltage of commercially available IGBTs in industry has reached 6500V. However, the cost of IGBTs at this voltage level is much higher than that of 1.7kV IGBTs, and the switching losses are also significant. Although this solves the withstand voltage limitation problem, it does not significantly reduce costs, and the switching losses in the entire MMC are also too high. The second solution is to use a series connection method. By connecting lower-voltage power devices, such as IGBTs or MOSFETs (metal-oxide-semiconductor field-effect transistors), in series, the higher voltage is carried out, and the total voltage is evenly distributed across the series-connected power devices. Although this solution solves the withstand voltage limitation problem, due to the influence of factors such as differences in drive delay and deviations in device manufacturing processes, there are switching delays among the series-connected devices. This results in an uneven voltage distribution across the series-connected devices, with some devices bearing higher voltages and potentially causing damage. It is difficult to achieve voltage equalization among the series-connected devices. Summary of the Invention

[0004] The purpose of this invention is to provide a modular multilevel converter and a voltage equalization control method. Through a compact layout, the overall size and cost of the converter are significantly reduced. Mechanical switches and thyristors only operate in case of faults, without affecting normal operation. Furthermore, the insulation voltage of mechanical switches and thyristors is easier to achieve with high voltages compared to power devices, without significantly impacting cost. The power device half-bridge sub-module can achieve voltage equalization of the converter without complex control processes. While reducing cost and size, this invention also addresses withstand voltage limitations and switching losses, facilitating the simple implementation of the entire converter and its widespread application.

[0005] To solve the above technical problems, the present invention provides a modular multilevel converter, comprising several cascaded modules. Each cascaded module includes a mechanical switch, a thyristor, and several power device half-bridge sub-modules. Each power device half-bridge sub-module includes a DC capacitor and two power devices connected in series. The DC capacitor and the circuit formed by the series connection of the two power devices are connected in parallel to form a parallel circuit.

[0006] Several power device half-bridge sub-modules are cascaded on the AC or DC side, and one end of the cascaded circuit is connected to the first end of the mechanical switch and the first end of the thyristor, respectively, and the other end of the cascaded circuit is connected to the second end of the mechanical switch and the second end of the thyristor, respectively.

[0007] Optionally, the AC sides of several of the power device half-bridge submodules are cascaded:

[0008] For any of the power device half-bridge sub-modules, the midpoint of the series connection of two of the power devices is connected to one end of the parallel circuit in a power device half-bridge sub-module cascaded with itself or the first end of the mechanical switch, and the other end of the parallel circuit is connected to the midpoint of the series connection of two of the power devices in another power device half-bridge sub-module cascaded with itself or the second end of the mechanical switch.

[0009] Optionally, several of the power device half-bridge submodules are cascaded on the DC side:

[0010] For any of the power device half-bridge submodules, the midpoint of the series connection of two of the power devices is connected to the first terminal or the second terminal of the mechanical switch, and one end of the parallel circuit is connected to the other end of the parallel circuit in a power device half-bridge submodule cascaded with itself.

[0011] Optionally, the cascade module further includes a heat sink and / or a control board, wherein the heat sink is disposed near the power device in the power device half-bridge submodule, and the control board is connected to the power device half-bridge submodule.

[0012] Optionally, the control board includes a local controller, a communication module, a driver chip, and a sampling chip. The local controller is connected to the central controller through the communication module. The input terminal of the driver chip is connected to the output terminal of the local controller, and the output terminal is connected to the control terminal of the power device in the power device half-bridge sub-module. The input terminal of the sampling chip is connected to the DC capacitor of each of the power device half-bridge sub-modules, and the output terminal is connected to the input terminal of the local controller.

[0013] To address the aforementioned technical problems, this invention also provides a voltage equalization control method, applicable to any cascade module in a modular multilevel converter as described above, comprising:

[0014] Sample the capacitor voltage of the DC capacitor in each of the power device half-bridge sub-modules;

[0015] Based on the capacitor voltage, determine whether each of the power device half-bridge sub-modules meets the voltage equalization condition;

[0016] If not, control the DC capacitor to charge or discharge so that the capacitor voltage of the DC capacitor in each power device half-bridge submodule remains balanced.

[0017] Optionally, when the DC side of the power device half-bridge submodule is cascaded and the power device half-bridge submodule is in a three-level operating mode, or when the AC side of the power device half-bridge submodule is cascaded, controlling the DC capacitor to charge or discharge includes:

[0018] The capacitor voltages of the DC capacitors in each of the power device half-bridge sub-modules are sorted.

[0019] When the current flowing through each of the power device half-bridge sub-modules is a positive current and the modular multilevel converter needs to insert a power device half-bridge sub-module, the power device sub-module with the lowest capacitor voltage is inserted.

[0020] When the current flowing through each of the power device half-bridge sub-modules is a positive current and the modular multilevel converter needs to remove the power device half-bridge sub-module, the power device sub-module with the highest capacitor voltage is removed.

[0021] When the current flowing through each of the power device half-bridge sub-modules is a negative current and the modular multilevel converter needs to insert a power device half-bridge sub-module, the power device sub-module with the highest capacitor voltage is inserted.

[0022] When the current flowing through each of the power device half-bridge sub-modules is a negative current and the modular multilevel converter needs to remove the power device half-bridge sub-module, the power device sub-module with the lowest capacitor voltage is removed.

[0023] Optionally, determining whether each of the power device half-bridge submodules meets the voltage equalization condition based on the capacitor voltage includes:

[0024] Determine whether the difference in the DC capacitor voltage of the two power device half-bridge sub-modules is greater than a first preset threshold.

[0025] If so, then the power device half-bridge submodule is determined to not meet the voltage equalization requirement;

[0026] If not, then the power device half-bridge submodule is determined to satisfy voltage equalization.

[0027] Optionally, when the cascaded module includes two power device half-bridge sub-modules, the DC sides of the two power device half-bridge sub-modules are cascaded, and the power device half-bridge sub-modules are in a two-level operating mode, controlling the charging or discharging of the DC capacitor includes:

[0028] When the voltage across the DC capacitor is greater than the voltage across the other DC capacitor, the drive signal of the power device of the half-bridge submodule corresponding to the DC capacitor is delayed on the falling edge or advanced on the rising edge, and / or the drive signal of the power device of the other half-bridge submodule is delayed on the rising edge or advanced on the falling edge.

[0029] When the voltage across the DC capacitor is less than the voltage across the other DC capacitor, the drive signal of the power device of the half-bridge submodule corresponding to the DC capacitor is delayed on the rising edge or advanced on the falling edge, and / or the drive signal of the power device of the other half-bridge submodule is delayed on the falling edge or advanced on the rising edge.

[0030] Optionally, when the cascaded module includes two power device half-bridge sub-modules, the DC sides of the two power device half-bridge sub-modules are cascaded, and the power device half-bridge sub-modules are in a two-level operating mode, controlling the charging or discharging of the DC capacitor includes:

[0031] Adjust the switching mode of the power devices in the power device half-bridge submodule to switch the power device half-bridge submodule to a three-level operating mode;

[0032] After operating in three-level mode for several cycles until the difference in the DC capacitor voltage of the two power device half-bridge submodules is less than the second preset threshold, the switching mode of the power device half-bridge submodule is adjusted again to switch the power device half-bridge submodule to two-level mode.

[0033] This invention provides a modular multilevel converter comprising several cascaded modules. Each cascaded module includes mechanical switches, thyristors, and several power device half-bridge sub-modules. By cascading multiple power device half-bridge sub-modules on the AC or DC side, a cascaded module is formed. The power device half-bridge sub-modules in this cascaded module share the same set of mechanical switches and thyristors, significantly reducing the number of mechanical switches and thyristors in the entire converter. This compact layout greatly reduces the overall size and cost of the converter. Although higher voltage-level devices are required for the mechanical switches and thyristors, they only operate during faults and do not affect normal operation. Furthermore, the insulation voltage of mechanical switches and thyristors is easier to achieve than that of power devices, without significantly impacting cost. The power device half-bridge sub-modules directly reuse the structure of existing converters, achieving voltage equalization without complex control processes. While reducing cost and size, it also considers withstand voltage limitations and switching losses, facilitating the simple implementation of the entire converter and its widespread application.

[0034] This application also provides a voltage equalization control method, which has the same beneficial effects as the above-mentioned modular multilevel converter. Attached Figure Description

[0035] To more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the prior art and embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0036] Figure 1 This invention provides a schematic diagram of the structure of a modular multilevel converter;

[0037] Figure 2 This invention provides a schematic diagram of the structure of a cascaded module employing AC side cascading;

[0038] Figure 3 This invention provides a schematic diagram of the structure of a cascaded module using DC-side cascading;

[0039] Figure 4 A schematic diagram of a cascade module and its control system using AC side cascading provided by the present invention;

[0040] Figure 5 A schematic diagram of a cascaded module and its control system using DC-side cascading provided by the present invention;

[0041] Figure 6This is a schematic diagram of another cascaded module using DC-side cascading provided by the present invention;

[0042] Figure 7 A schematic flowchart of a pressure equalization control method provided by the present invention;

[0043] Figure 8 A schematic flowchart of a voltage equalization control method for a cascaded module using DC-side cascading provided by the present invention;

[0044] Figure 9 A schematic diagram of hysteresis comparison control provided by the present invention;

[0045] Figure 10 A schematic diagram of the signal waveform of a cascaded module provided by the present invention;

[0046] Figure 11 A schematic diagram of the current flow direction of a cascaded module provided by the present invention;

[0047] Figure 12 A schematic diagram of the current flow direction of another cascaded module provided by the present invention;

[0048] Figure 13 This invention provides a schematic diagram of signal generation for edge compensation duration.

[0049] Figure 14 This is a schematic flowchart of a voltage equalization control method for another cascaded module using DC-side cascading provided by the present invention. Detailed Implementation

[0050] The core of this invention is to provide a modular multilevel converter and a voltage equalization control method. Through a compact layout, the overall size and cost of the converter are significantly reduced. Mechanical switches and thyristors only operate in case of faults, without affecting normal operation. Furthermore, the insulation voltage of mechanical switches and thyristors is easier to achieve with high voltages compared to power devices, without excessively impacting cost. The power device half-bridge sub-module can achieve voltage equalization of the converter without complex control processes. While reducing cost and size, it also addresses withstand voltage limitations and switching losses, facilitating the simple implementation of the entire converter and its widespread application.

[0051] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0052] Please refer to Figure 1 , Figure 1 This is a schematic diagram of a modular multilevel converter provided by the present invention; please refer to... Figure 2 , Figure 2 This is a schematic diagram of a cascaded module using AC side cascading provided by the present invention; please refer to... Figure 3 , Figure 3 This invention provides a structural schematic diagram of a cascaded module using DC-side cascading. To solve the above-mentioned technical problems, this invention provides a modular multilevel converter, including several cascaded modules. Each cascaded module includes a mechanical switch Q, a thyristor K, and several power device half-bridge sub-modules 1. Each power device half-bridge sub-module 1 includes a DC capacitor and two power devices connected in series. The DC capacitor and the circuit after the two power devices are connected in series are connected in parallel to form a parallel circuit.

[0053] Several power device half-bridge sub-modules 1 are cascaded on the AC or DC side, and one end of the cascaded circuit is connected to the first terminal of mechanical switch Q and the first terminal of thyristor K respectively, and the other end of the cascaded circuit is connected to the second terminal of mechanical switch Q and the second terminal of thyristor K respectively.

[0054] Specifically, several power device half-bridge submodules 1 are cascaded on either the AC or DC side using cascading technology. Multiple cascaded power device half-bridge submodules 1 form a cascaded module. Each cascaded module shares a set of mechanical switches Q and thyristors K; that is, each power device half-bridge submodule 1 within a cascaded module uses a single bypass protection device. This reduces the number of components used in the entire converter, achieving a reduction in system size. Depending on the cascading location, the cascading of power device half-bridge submodules 1 within a module can be divided into AC-side cascading and DC-side cascading. The driving of each power device half-bridge submodule 1 within the same cascaded module can be independent and separately controlled. Furthermore, because there is a DC capacitor in each power device half-bridge submodule 1, the voltage across the capacitor cannot change abruptly. Therefore, the voltage fluctuation of the entire power device half-bridge submodule 1 during converter operation is small, making it easier to achieve voltage equalization control within the converter.

[0055] It's easy to understand that integrating multiple power device half-bridge submodules 1 into a cascade module, with each cascade module acting as a modular multilevel converter valve, and sharing a set of mechanical switches Q, thyristors K, heat sinks, and control boards, significantly reduces the overall size and cost of the converter through this compact layout. Furthermore, considering that the withstand voltage across the entire cascade module needs to be increased after cascading multiple power device half-bridge submodules 1, this can be achieved simply by using higher voltage-level devices for the mechanical switches Q and thyristors K. Typically, to meet high-voltage requirements, devices with voltage levels such as 8500V can be selected for the mechanical switches Q and thyristors K. Other components in the cascade module can directly use existing voltage levels, thus keeping the overall valve losses essentially unchanged and preventing excessive additional losses in the converter.

[0056] It is understandable that in a converter, there are multiple power device half-bridge sub-modules 1. Some power device half-bridge sub-modules 1 can be cascaded through DC-side cascading, and some power device half-bridge sub-modules 1 can be cascaded through AC-side cascading; or all power device half-bridge sub-modules 1 can be cascaded directly through AC-side cascading. The cascading method between the various power device half-bridge sub-modules 1 in the converter can be set according to the actual situation and application requirements.

[0057] In practical applications, with Figure 2 and Figure 3 For example, power devices S1, S2, and capacitor C1 constitute a power device half-bridge submodule 1, and power devices S3, S4, and capacitor C2 constitute another power device half-bridge submodule 1. Two power device half-bridge submodules 1 can be integrated into a cascaded module. Calculations show that this solution can reduce the footprint of a 5MW MMC by approximately one-third. This invention reduces the number of submodules in an MMC through a method of cascading half-bridges within a module, thereby reducing the overall size and cost of the MMC system. The mechanical switch Q and thyristor K in the cascaded module only operate during faults and do not affect normal operation; the insulation voltage of the mechanical switch Q and thyristor K is relatively easy to achieve at high levels compared to power devices, without affecting cost. The mechanical switch Q and thyristor K serve as bypass protection, but other types of devices can also be used. This application does not impose any special restrictions on the type and specific implementation of the mechanical switch Q and thyristor K, or on other modules in the cascade module. They can be selected and adjusted according to different situations in actual applications. This application also does not impose any special restrictions on the type and specific implementation of the power devices and capacitors in the power device half-bridge sub-module 1.

[0058] This invention provides a modular multilevel converter, comprising several cascaded modules. Each cascaded module includes a mechanical switch Q, a thyristor K, and several power device half-bridge sub-modules 1. By cascading multiple power device half-bridge sub-modules 1 on the AC or DC side, a cascaded module is formed. The power device half-bridge sub-modules 1 in this cascaded module share the same set of mechanical switches Q and thyristors K, which greatly reduces the number of devices such as mechanical switches Q and thyristors K in the entire converter. Through this compact layout, the size and cost of the entire converter are significantly reduced. Although higher voltage level devices are required for mechanical switches Q and thyristors K, since mechanical switches Q and thyristors K only operate during faults, they will not cause losses to normal operation. At the same time, the insulation voltage of mechanical switches Q and thyristors K is easier to achieve than that of power devices, without significantly affecting the cost. The power device half-bridge submodule 1 directly reuses the structure in the existing converter, and can achieve voltage equalization of the converter without complex control process. While reducing cost and size, it takes into account the withstand voltage limit and switching loss, which is conducive to the simple implementation of the entire converter and facilitates the widespread application of the converter.

[0059] Based on the above embodiments: Please refer to Figure 4 , Figure 4 This is a schematic diagram of a cascaded module and its control system using AC side cascading, provided by the present invention; please refer to... Figure 5 , Figure 5 This is a schematic diagram of a cascaded module and its control system using DC-side cascading, provided by the present invention; please refer to... Figure 6 , Figure 6 This is a schematic diagram of another cascaded module using DC-side cascading provided by the present invention.

[0060] As an optional embodiment, several power device half-bridge submodules 1 are cascaded on their AC sides:

[0061] For any power device half-bridge submodule 1, the midpoint of the series connection of two power devices is connected to one end of the parallel circuit in another power device half-bridge submodule 1 cascaded with itself or the first end of the mechanical switch Q, and the other end of the parallel circuit is connected to the midpoint of the series connection of two power devices in another power device half-bridge submodule 1 cascaded with itself or the second end of the mechanical switch Q.

[0062] It is not difficult to understand that the cascading method of the power device half-bridge submodule 1 can be AC ​​side cascading, such as... Figure 2 and Figure 4 As shown, Figure 2 This is a schematic diagram of a circuit structure for cascading the AC sides of two power device half-bridge submodules 1. Figure 4This is a schematic diagram of a circuit structure for cascading the AC side of N power device half-bridge submodule 1. Power device S... 2N-1 Power devices S 2N and capacitor C N This constitutes a power device half-bridge submodule 1. The midpoint of the series connection between two power devices in one power device half-bridge submodule 1 is connected to one end of the parallel circuit in another power device half-bridge submodule 1. Several power device half-bridge submodules 1 are cascaded in this manner. The first end of the cascaded circuit is the midpoint of the series connection between two power devices in one power device half-bridge submodule 1, and the second end of the cascaded circuit is one end of the parallel circuit in another power device half-bridge submodule 1. At this time, the first end of the cascaded circuit is connected to the first end of the mechanical switch Q and the first end of the thyristor K, respectively, and the second end of the cascaded circuit is connected to the second end of the mechanical switch Q and the second end of the thyristor K, respectively.

[0063] It should be noted that when cascading on the AC side, the central controller 11 of the converter still treats the two cascaded power device half-bridges as two separate sub-modules for control, such as... Figure 4 and Figure 5 As shown, the central controller 11 communicates with the cascaded module via two optical fibers, one for transmitting and one for receiving. The central controller 11 sends the drive signals of the two power device half-bridge sub-modules 1 to the local controller 13 in the cascaded module. The local controller 13 in the cascaded module decodes the modulation signal sent by the central controller 11 and generates the corresponding drive signal to control the two power device half-bridge sub-modules 1. The local controller 13 in the cascaded module sends the operating information such as capacitor voltage, temperature, and fault signals of the two power device half-bridge sub-modules 1 to the central controller 11. Each power device in the cascaded module can be switched on and off according to the independent drive signal. Figure 4 This is a schematic diagram of N power device half-bridge submodules 1 cascaded on the AC side. The control method is similar to the control process of two power device half-bridge submodules 1, and will not be described again in this application.

[0064] Specifically, the cascading method of the power device half-bridge submodule 1 can be AC ​​side cascading. When each power device half-bridge submodule 1 is cascaded through AC side cascading, the central controller 11 of the converter still treats the two cascaded power device half-bridges as two separate submodules for control. The control method is similar to that of traditional converters, which is simple and convenient and beneficial to user experience. AC side cascading effectively reduces the size and cost of the entire converter.

[0065] As an optional embodiment, several power device half-bridge submodules 1 are cascaded on their DC sides:

[0066] For any power device half-bridge submodule 1, the midpoint of the series connection of two power devices is connected to the first terminal or the second terminal of the mechanical switch Q, and one end of the parallel circuit is connected to the other end of the parallel circuit in a power device half-bridge submodule 1 that is cascaded with itself.

[0067] It is easy to understand that the cascading method of power device half-bridge submodule 1 can be DC-side cascading. DC-side cascading of power device half-bridge submodule 1 typically involves cascading the DC sides of two power device half-bridge submodules 1. DC-side cascading of power device half-bridge submodule 1 simply involves directly connecting one side of the DC capacitor of the power device half-bridge submodule 1. Its circuit topology is as follows: Figure 3 , Figure 5 Or such as Figure 6 As shown, the cascade module consists of four power devices and their freewheeling diodes, two identical capacitors and corresponding voltage-equalizing resistors, a mechanical switch Q as a bypass switch, and a thyristor K.

[0068] It should be noted that when cascading on the DC side, the cascaded module obtained through DC-side connection has two operating modes: two-level and three-level. In the two-level operating mode, the cascaded module can be equivalent to a power device half-bridge sub-module 1 with doubled output voltage. Figure 6 For example, in this operating state, power devices S1 and S4 will be turned on simultaneously, power devices S2 and S3 will be turned on simultaneously, and C1 and C2 will be inserted and removed simultaneously. The entire cascaded module can only output 0 and 2*V. C Two voltage levels; when power devices S1 and S4 are on, power devices S2 and S3 are off. Current flows sequentially from the positive terminal of the cascade module through the on-state S1, capacitor C1, capacitor C2, and the on-state S4, then exits from the negative terminal. At this time, the output voltage of the cascade module is 2V. C Capacitors C2 and C1 are inserted simultaneously. When power devices S2 and S3 are turned on, power devices S1 and S4 are turned off. The current flows from the positive terminal of the cascade module through the turned-on S2 and S3 in sequence and then flows out from the negative terminal of the cascade module. At this time, the output voltage of the cascade module is 0, and capacitors C2 and C1 are simultaneously disconnected.

[0069] In three-level operation mode, the cascaded module can output 0, V C and 2*V CThree voltage levels are available, and C1 or C2 can be inserted individually through different switching processes of the power devices, thereby achieving voltage balance between the two capacitors inside the cascade module. During the regulation process, the MMC system still treats the two power device half-bridge sub-modules 1 as two independent modules for modulation. When power devices S1 and S4 are turned on and power devices S2 and S3 are turned off, the current flows from the positive terminal of the cascade module through the turned-on S1, capacitor C1, capacitor C2, and the turned-on S4 in sequence, and then flows out from the negative terminal of the cascade module. At this time, the output voltage of the cascade module is 2*V. C Capacitors C2 and C1 are inserted simultaneously. When power devices S2 and S4 are turned on and power devices S1 and S3 are turned off, the current flows from the positive terminal of the cascade module through the turned-on S2, capacitor C2, and turned-on S4, and then flows out from the negative terminal of the cascade module. At this time, the output voltage of the cascade module is V. C Only capacitor C2 is inserted; when power devices S1 and S3 are turned on and power devices S2 and S4 are turned off, the current flows from the positive terminal of the cascade module through the turned-on S1, capacitor C1, and turned-on S3 in sequence, and then flows out from the negative terminal of the cascade module. At this time, the output voltage of the cascade module is V. C Only capacitor C1 is inserted; when power devices S2 and S3 are turned on and power devices S1 and S4 are turned off, the current flows from the positive terminal of the cascade module through the turned-on S2 and the turned-on S3 and then out from the negative terminal of the cascade module. At this time, the output voltage of the cascade module is 0, and capacitors C2 and C1 are simultaneously disconnected.

[0070] It should be noted that V C This refers to the voltage across a DC capacitor. During the operation of the converter, the voltages across the DC capacitors corresponding to the various power devices in the half-bridge submodule 1 remain balanced, therefore, V can be used as the voltage across the capacitor. C This represents the voltage across the DC capacitor corresponding to any power device half-bridge submodule 1; Figure 6 The g1 and g2 shown are two drive signals provided by the MMC system to the half-bridge sub-module 1, which are considered as two independent power devices. When the cascade module is working, it decodes g1 and g2 to generate the corresponding drive signals S1-S4. The decoding logic of the cascade module under the three-level working state is shown in Table 1.

[0071] Table 1 Switching logic of cascaded modules under three-level operating conditions

[0072] 0 1 1 0 0 0 0 1 0 1 0 1 1 0 1 0 1 0 1 0 0 1 1 1

[0073] Specifically, the cascading method of the power device half-bridge submodule 1 can be DC-side cascading. When each power device half-bridge submodule 1 is cascaded through DC-side cascading, the converter has two operating modes: two-level and three-level. In the three-level operating mode, the central controller 11 still treats the two cascaded power device half-bridges as two separate submodules for control, and the control method is similar to that of a traditional converter. In the two-level operating mode, the two capacitors in a cascaded module can only be inserted or removed simultaneously. The entire control method is simple, convenient, and highly flexible, which is beneficial to the user experience. DC-side cascading effectively reduces the size and cost of the entire converter.

[0074] As an optional embodiment, the cascade module also includes a heat sink and / or a control board, the heat sink being located near the power devices in the power device half-bridge submodule, and the control board being connected to the power device half-bridge submodule 1.

[0075] Considering that the cascaded module requires not only a bypass protection device consisting of mechanical switch Q and thyristor K, but also communication with the central controller 11 of the entire converter to control the half-bridge submodules 1 of each power device, a control board is added to the cascaded module. This control board acts as a local controller 13 corresponding to the cascaded module, responsible for communicating with the host computer, i.e., the central controller 11 of the entire converter, and for acquiring and controlling parameters such as capacitor voltage of the corresponding local cascaded module, generating drive signals, and performing other control tasks. The heat sink and control board are both located within the corresponding local cascaded module. In this case, several half-bridge submodules 1 of power devices in a cascaded module share a single set of mechanical switch Q, thyristor K, heat sink, and control board. Mechanical switch Q is responsible for maintaining a closed state for an extended period to bypass the entire cascaded module in case of a fault. Thyristor K is responsible for sharing a portion of the short-circuit current during a short circuit. The heat sink is responsible for heat dissipation, reducing the temperature of the power devices. The control board works in conjunction with the central controller 11 to achieve functions such as controlling each half-bridge submodule 1 of the power device and acquiring parameters. This application does not impose any specific restrictions on the type and implementation of heat sinks and control boards. Other modules can also be added to the cascaded module according to the actual application.

[0076] Specifically, the cascade module can also be equipped with a heat sink and / or a control board. The control board corresponds to the cascade module and works with the central controller 11 to realize the control process of each power device half-bridge sub-module 1, which further improves the control process of the entire converter. The heat sink can avoid the failure or damage of the power devices due to overheating during operation, and further ensure the safety and reliability of the entire converter.

[0077] As an optional embodiment, the control board includes a local controller 13, a communication module 12, a driver chip 14, and a sampling chip 15. The local controller 13 is connected to the central controller 11 through the communication module 12. The input terminal of the driver chip 14 is connected to the output terminal of the local controller 13, and the output terminal is connected to the control terminal of the power device in the power device half-bridge sub-module 1. The input terminal of the sampling chip 15 is connected to the DC capacitor of each power device half-bridge sub-module 1, and the output terminal is connected to the input terminal of the local controller 13.

[0078] It is easy to understand that the control board needs to cooperate with the central controller 11 to achieve multiple functions. Therefore, the control board's multiple functions are achieved through the cooperation of the local controller 13, the communication module 12, the driver chip 14, and the sampling chip 15. The central controller 11 and the local controller 13 communicate with each other through the communication module 12. After receiving the signal output by the central controller 11 through the communication module 12, the local controller 13 decodes the received signal to obtain the duty cycle required to control the power device half-bridge sub-module 1. Then, the local controller 13 generates a PWM signal corresponding to the duty cycle, and the driver chip 14 generates a corresponding drive signal based on the PWM signal. The drive signal is connected to the control terminal of each power device to realize the control process of each power device half-bridge sub-module 1. At the same time, the local controller 13 samples the voltage, temperature, and device fault status and parameters of the DC capacitor in the power device half-bridge sub-module 1 through the sampling chip 15. The sampling results are then uploaded to the central controller 11 through the communication module 12, thereby realizing the communication between the cascaded module and the central controller 11 of the entire converter. This application does not make any special restrictions on the specific types and implementation methods of the local controller 13, communication module 12, driver chip 14 and sampling chip 15. Typically, fiber optic transceivers are soldered on the control board, and fiber optic cables can be directly plugged in to serve as the communication module 12 to realize communication between the central controller 11 and the local controller 13. The local controller 13 can be implemented using a controller of the type FPGA (Field Programmable Gate Array).

[0079] Specifically, the various functions of the control board can be realized through the cooperation of the local controller 13, communication module 12, driver chip 14 and sampling chip 15. The circuit structure is simple and easy to implement, and the control process is simple and effective. It further improves the structure of the cascaded module and is conducive to the complete realization of the entire converter.

[0080] Please refer to Figure 7 , Figure 7The present invention provides a schematic flowchart of a voltage equalization control method; to solve the above-mentioned technical problems, the present invention also provides a voltage equalization control method, applied to any cascade module in a modular multilevel converter as described above, comprising:

[0081] S11: Sample the capacitor voltage of the DC capacitor in the half-bridge submodule 1 of each power device;

[0082] S12: Determine whether the voltage equalization condition is met for each power device half-bridge submodule 1 based on the capacitor voltage;

[0083] S13: If not, control the DC capacitor to charge or discharge so that the capacitor voltage of the DC capacitor of each power device half-bridge submodule 1 remains balanced.

[0084] Considering that the DC capacitor voltages of each power device half-bridge submodule 1 need to be kept at a uniform voltage throughout the entire operation of the converter, but the capacitor voltages within each power device half-bridge submodule 1 are affected by factors such as circuit parameters and drive delay, uneven capacitor voltages may occur. Therefore, certain voltage equalization measures are needed to ensure that the capacitor voltages within each power device half-bridge submodule 1 remain balanced. Thus, during application, each cascaded module needs to monitor the DC capacitor voltages of each power device half-bridge submodule 1 in real time, and ensure that the DC capacitor voltages of each power device half-bridge submodule 1 in the cascaded module remain balanced through the charging or discharging process of the DC capacitors, so as to ensure the normal operation of the converter.

[0085] Specifically, during the operation of the converter, it is necessary to ensure that the DC capacitor voltage of each power device half-bridge sub-module 1 in the cascaded module remains balanced. This is necessary to achieve voltage division of the entire converter's DC voltage through all power device half-bridge sub-modules 1, thereby avoiding overvoltage of the capacitor voltage corresponding to the power device half-bridge sub-module 1 in the converter. This further ensures the normal operation of the converter and improves the safety and reliability of the entire converter.

[0086] As an optional embodiment, when the DC side of the power device half-bridge submodule 1 is cascaded and the power device half-bridge submodule 1 is in a three-level operating mode, or when the AC side of the power device half-bridge submodule 1 is cascaded, controlling the DC capacitor to charge or discharge includes:

[0087] Sort the DC capacitor voltages of each power device half-bridge submodule 1.

[0088] When the current flowing through each power device half-bridge sub-module is a positive current and the modular multilevel converter needs to insert a power device half-bridge sub-module, insert the power device sub-module with the lowest capacitor voltage.

[0089] When the current flowing through each power device half-bridge sub-module is a positive current and the modular multilevel converter needs to remove the power device half-bridge sub-module, remove the power device sub-module with the highest capacitor voltage.

[0090] When the current flowing through each power device half-bridge sub-module is negative and the modular multilevel converter needs to insert a power device half-bridge sub-module, insert the power device sub-module with the highest capacitor voltage.

[0091] When the current flowing through each power device half-bridge submodule is negative and the modular multilevel converter needs to remove a power device half-bridge submodule, remove the power device submodule with the lowest capacitor voltage.

[0092] It's easy to understand that when the various power device half-bridge submodules 1 in the cascaded module are cascaded on the AC side or on the DC side and operate in a three-level working mode, the central controller 11 will treat the cascaded power device modules as two independent submodules for voltage equalization modulation and control. Therefore, the capacitor voltage can be balanced directly by comparing the order of the capacitor voltages of the DC capacitors of each power device half-bridge submodule 1. After sampling and sorting the capacitor voltages of all power device half-bridge submodules 1, when the current flowing through the power device half-bridge submodule 1 is positive, the system further determines whether the converter needs to insert or remove a power device half-bridge submodule 1 based on the actual application. If it needs to insert a power device half-bridge submodule 1, then the power device half-bridge submodule with the lower capacitor voltage will be inserted. The DC capacitor corresponding to module 1 is inserted into the circuit for charging. If it is necessary to remove power device half-bridge sub-module 1, the DC capacitor corresponding to the power device half-bridge sub-module 1 with the higher capacitor voltage is removed from the circuit to discharge it. When the current flowing through power device half-bridge sub-module 1 is negative, the converter is further determined according to the actual application situation whether it needs to insert or remove power device half-bridge sub-module 1. If it needs to be inserted, the DC capacitor corresponding to the power device half-bridge sub-module 1 with the higher capacitor voltage is inserted into the circuit to discharge it. If it needs to be removed, the DC capacitor corresponding to the power device half-bridge sub-module 1 with the lower capacitor voltage is removed from the circuit to charge it. This achieves voltage balance of all sub-module capacitors within the bridge arm. For example, Figure 2 and Figure 3 As shown, current flowing into the P terminal is defined as positive current, and current flowing into the N terminal is defined as negative current.

[0093] Specifically, when the various power device half-bridge sub-modules 1 in the cascaded module operate in a three-level working mode, either cascaded on the AC side or cascaded on the DC side, the capacitor voltage can be balanced directly by comparing the order of the capacitor voltages of the DC capacitors in each power device half-bridge sub-module 1. Furthermore, it determines which DC capacitors need to be charged or discharged based on the different current conditions, further improving the voltage equalization control process of the cascaded module. The entire control process is simple, convenient, and easy to implement, facilitating the easy application of the entire converter and further improving the voltage equalization control process of the entire converter.

[0094] As an optional embodiment, determining whether each power device half-bridge submodule 1 meets the voltage equalization condition based on the capacitor voltage includes:

[0095] Determine whether the difference in the capacitor voltage of the DC capacitor of the two power device half-bridge submodule 1 is greater than a first preset threshold.

[0096] If so, then the power device half-bridge submodule 1 is determined to not satisfy voltage equalization.

[0097] If not, then the power device half-bridge submodule 1 is determined to satisfy voltage equalization.

[0098] When the cascaded modules operate in a two-level mode with DC-side cascading, power devices S1 and S4 will only be turned on synchronously, as will power devices S2 and S3. C1 and C2 can only be inserted and removed simultaneously during operation. The two capacitors within the entire cascaded module are treated as a single unit for voltage prioritization, resulting in simultaneous discharge and charging, making voltage equalization impossible. Therefore, the voltage of the two capacitors cannot be adjusted individually. However, this situation still leads to internal capacitor voltage imbalance. Therefore, it is necessary to determine the relationship between the voltage difference of the DC capacitors in the two power device half-bridge sub-modules 1 and a first preset threshold. If the voltage difference is too large, it indicates that the DC capacitors in the two power device half-bridge sub-modules 1 do not meet the voltage equalization requirement and further adjustment is needed. If the voltage difference does not exceed the first preset threshold, it indicates that the DC capacitors in the two power device half-bridge sub-modules 1 meet the voltage equalization requirement, and the converter can operate normally.

[0099] Specifically, when the cascaded modules are cascaded on the DC side and operate in a two-level working mode, it is not possible to directly determine whether the power device half-bridge sub-module 1 meets the voltage equalization condition by sorting the capacitor voltages. Since the DC side is usually cascaded with two power device half-bridge sub-modules 1, the process of determining whether the power device half-bridge sub-module 1 meets the voltage equalization condition can be achieved by determining the voltage difference between the two power device half-bridge sub-modules 1, thus further improving the process of determining whether the power device half-bridge sub-module 1 meets the voltage equalization condition.

[0100] Please refer to Figure 8 , Figure 8 This is a flowchart illustrating a voltage equalization control method for a cascaded module using DC-side cascading provided by the present invention; please refer to... Figure 9 , Figure 9 This invention provides a schematic diagram of hysteresis comparison control; as an optional embodiment, when the cascaded module includes two power device half-bridge sub-modules 1, the DC sides of the two power device half-bridge sub-modules 1 are cascaded and the power device half-bridge sub-modules 1 are in a two-level operating mode, controlling the DC capacitor to charge or discharge includes:

[0101] When the voltage across the DC capacitor is greater than the voltage across the other DC capacitor, the drive signal of the power device in the half-bridge sub-module 1 corresponding to the DC capacitor is delayed on the falling edge or advanced on the rising edge, and / or the drive signal of the power device in the other half-bridge sub-module 1 is delayed on the rising edge or advanced on the falling edge.

[0102] When the voltage across the DC capacitor is less than the voltage across the other DC capacitor, the drive signal of the power device in the half-bridge submodule 1 corresponding to the DC capacitor is delayed on the rising edge or advanced on the falling edge, and / or the drive signal of the power device in the other half-bridge submodule 1 is delayed on the falling edge or advanced on the rising edge.

[0103] It's easy to understand that edge compensation can be used to achieve voltage equalization of the capacitors in the half-bridge submodule 1 of the cascaded power devices. For example... Figure 6 The cascaded module shown, its edge compensation control logic and specific process are as follows: Figure 8 As shown, the capacitor voltage V of the two power devices half-bridge submodule 1 is sampled. C1 and V C2 V C1 The voltage across capacitor C1 and V C2 The voltage across capacitor C2 is used to determine whether the voltage difference between the two exceeds a set first preset threshold V. thH If the voltage difference is greater than the first preset threshold, a certain edge compensation Δt is added to the drive signal of the power device to realize the charging or discharging process of capacitor C1 and capacitor C2, until the voltage difference is less than the set hysteresis threshold, that is, the second preset threshold V. thL V thH >V thL The hysteresis comparison control diagram is shown below. Figure 9 As shown in Table 2, there are multiple options for adding edge compensation. Edge compensation can be added to all drive signals S1-S4, and compensation can be added to either rising or falling edges. Edge advance compensation and delay compensation can be performed. The effects of different edge compensations are shown in Table 2.

[0104] Table 2. Effects of different edge compensation methods

[0105]

[0106]

[0107] Specifically, please refer to Figure 10 , Figure 11 and Figure 12 , Figure 10 A schematic diagram of the signal waveform of a cascaded module provided by the present invention; Figure 11 A schematic diagram of the current flow direction of a cascaded module provided by the present invention; Figure 12 This is a schematic diagram of the current flow direction of another cascaded module provided by the present invention; please refer to... Figure 13 , Figure 13 This invention provides a schematic diagram of signal generation for edge compensation duration; taking the rising edge delay compensation of the drive signal of power device S1 as an example, when V C2 When the voltage is too high, a drive delay of Δt can be added to the rising edge of the drive signal of the power device S1, such as... Figure 10 As shown in the shaded area, during the delay process, power device S4 is already turned on while power device S1 remains off. At this time, if the current flowing through the cascaded module is positive, such as... Figure 11 As shown, capacitors C1 and C2 are inserted into the circuit together for charging; the bolded portion in the diagram represents the current flowing through them. However, if the current flowing through the cascaded module is negative, such as... Figure 12 As shown, capacitor C2 is discharged separately through the anti-parallel diodes of power devices S4 and S2, enabling individual adjustment of the capacitor voltage of C2. The bolded portion in the figure represents the current flowing through it. The edge compensation time Δt can be a manually set constant or a dynamic compensation value obtained through PI closed-loop control. Figure 13 As shown, by V C1 -V C2 This difference is compared with the reference value 0 to obtain the error value. The error value is then passed through the PI controller to obtain the required compensation amount Δt. According to Table 2, this compensation amount is then added to the rising or falling edge of the corresponding drive signal.

[0108] Specifically, edge compensation can be used to perform voltage equalization control on cascaded modules operating in two-level mode on the DC side. By using edge compensation, the capacitors in the cascaded modules operating in two-level mode can be individually adjusted to control the power devices in the cascaded modules, such as the half-bridge sub-module 1, to meet voltage balance, thereby further improving the voltage equalization control process of the entire converter.

[0109] Please refer to Figure 14 , Figure 14 This is a flowchart illustrating a voltage equalization control method for another cascaded module using DC-side cascading provided by the present invention. As an optional embodiment, when the cascaded module includes two power device half-bridge sub-modules 1, the two power device half-bridge sub-modules 1 are cascaded on the DC side and the power device half-bridge sub-modules 1 are in a two-level operating mode, controlling the DC capacitor to charge or discharge includes:

[0110] Adjust the switching mode of the power devices in the power device half-bridge submodule 1 so that the power device half-bridge submodule 1 switches to a three-level operating mode;

[0111] After operating in three-level mode for several cycles until the difference in the DC capacitor voltage of the two power device half-bridge submodule 1 is less than the second preset threshold, the switching mode of the power device half-bridge submodule 1 is adjusted again to switch the power device half-bridge submodule 1 to two-level mode.

[0112] Considering that the cascaded modules in three-level operating mode can achieve voltage equalization when cascaded on the DC side, voltage equalization of the cascaded modules in two-level operating mode can be achieved by briefly switching the operating mode. Under normal operating conditions, since the two capacitors of the cascaded module in two-level operating mode are inserted and removed synchronously, the cascaded modules operating in two-level mode will be sorted according to the sum of the voltages of the cascaded capacitors C1 and C2. Therefore, when the voltage difference between capacitors C1 and C2 is detected to exceed the set first preset threshold V, the voltage equalization can be achieved. thH At times, such as Figure 14 As shown, the cascade module switches its operating mode to a three-level operating mode. It then obtains the corresponding switching signals by sorting the voltages of capacitors C1 and C2. Following the switching rules in Table 1 for the cascade module in three-level operating mode, it distributes the drive signals to control the individual charging or discharging of each capacitor in the cascade module. In other words, according to the above embodiment, the cascade module, cascaded on the DC side and operating in three-level mode, is subjected to voltage equalization regulation. After the cascade module operates in three-level mode for n cycles, it checks the voltages of capacitors C1 and C2 again. If the voltage difference is less than the set hysteresis comparison threshold, i.e., the second preset threshold V... thL Then, it proves that the power device half-bridge submodule 1 in the cascaded module has met the voltage equalization condition, and the cascaded module returns to the two-level working mode and continues to work normally; if the voltage difference is not less than the second preset threshold V thL Then it continues to operate in three-level mode for n cycles until the voltage difference between C1 and C2 is less than V. thL .

[0113] Specifically, considering that both the two-level and three-level operating modes of the cascaded modules adopt the DC-side cascading method and have the same circuit structure, and that the cascaded modules in the three-level operating mode can directly achieve voltage equalization control through the sorting of capacitor voltages, the operating mode can be briefly switched. The cascaded modules can be adjusted to meet the voltage equalization condition using the three-level operating mode before switching back to the two-level operating mode. This directly uses a single voltage equalization control process to complete the voltage equalization control process of the cascaded modules under different conditions, further simplifying the voltage equalization control process of the entire converter and facilitating the simple implementation of the entire converter.

[0114] For an introduction to the voltage equalization control method provided by this invention, please refer to the above-described embodiment of the modular multilevel converter; this invention will not be repeated here.

[0115] To address the aforementioned technical problems, this invention also provides a voltage equalization control system, applicable to any cascade module in a modular multilevel converter as described above, comprising:

[0116] The sampling unit is used to sample the capacitor voltage of the DC capacitor of each power device half-bridge submodule 1;

[0117] The judgment unit is used to determine whether each power device half-bridge submodule 1 meets the voltage equalization condition based on the capacitor voltage; if not, it triggers the control unit.

[0118] The control unit is used to control the charging or discharging of the DC capacitors so that the capacitor voltages of the DC capacitors in each power device half-bridge submodule 1 are kept balanced.

[0119] For an introduction to the pressure equalization control system provided by this invention, please refer to the above-described embodiments of the pressure equalization control method; further details of this invention will not be repeated here.

[0120] To address the aforementioned technical problems, the present invention also provides an electronic device, comprising:

[0121] Memory, used to store computer programs;

[0122] A processor for implementing the steps of the voltage equalization control method as described above.

[0123] The processor may include one or more processing cores, such as a quad-core processor or an octa-core processor. The processor can be implemented using at least one hardware form of ARM (Advanced RISC Machines), DSP (Digital Signal Processor), FPGA (Field-Programmable Gate Array), or PLA (Programmable Logic Array). The processor may also include a main processor and a coprocessor. The main processor, also known as the central processing unit, is used to process data in the wake-up state; the coprocessor is a low-power processor used to process data in the standby state. In some embodiments, the processor may integrate a GPU (graphics processing unit), which is responsible for rendering and drawing the content to be displayed on the screen. In some embodiments, the processor may also include an AI (Artificial Intelligence) processor, which is used to handle computational operations related to machine learning.

[0124] The memory may include one or more computer-readable storage media, which may be non-transitory. The memory may also include high-speed random access memory and non-volatile memory, such as one or more disk storage devices or flash memory devices. In this embodiment, the memory is used to store at least the following computer program, which, after being loaded and executed by the processor, is capable of implementing the relevant steps of the voltage equalization control method disclosed in any of the foregoing embodiments. In addition, the resources stored in the memory may also include an operating system and data, and the storage method may be temporary or permanent storage. The operating system may include Windows, Unix, Linux, etc. The data may include, but is not limited to, data related to the voltage equalization control method.

[0125] In some embodiments, the electronic device may further include a display screen, input / output interfaces, communication interfaces, a power supply, and a communication bus.

[0126] Those skilled in the art will understand that the foregoing structure does not constitute a limitation on the electronic device and may include more or fewer components than those shown in the figure.

[0127] For an introduction to the electronic device provided by this invention, please refer to the above-described embodiment of the voltage equalization control method; further details of this invention will not be repeated here.

[0128] To address the aforementioned technical problems, the present invention also provides a computer-readable storage medium storing a computer program, which, when executed by a processor, implements the steps of the aforementioned voltage equalization control method.

[0129] It is understood that if the methods in the above embodiments are implemented as software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of this application, in essence, or the part that contributes to the prior art, or all or part of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and executes all or part of the steps of the methods described in the various embodiments of this application. Specifically, the computer-readable storage medium may include, but is not limited to, any type of disk, including floppy disks, optical disks, and portable hard drives, or any type of media or device suitable for storing instructions or data, etc., and this application does not make any special limitations here.

[0130] For an introduction to the computer-readable storage medium provided by the present invention, please refer to the above-described embodiments of the voltage equalization control method; the present invention will not be described in detail here.

[0131] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the apparatus disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple; relevant parts can be referred to the method section.

[0132] It should also be noted that, in this specification, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0133] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A modular multilevel converter, characterized in that, It includes several cascaded modules, each including a mechanical switch, a thyristor, and several power device half-bridge sub-modules. Each power device half-bridge sub-module includes a DC capacitor and two power devices connected in series. The DC capacitor and the circuit formed by the series connection of the two power devices are connected in parallel to form a parallel circuit. Several power device half-bridge sub-modules are cascaded on the AC or DC side, and one end of the cascaded circuit is connected to the first end of the mechanical switch and the first end of the thyristor, respectively, and the other end of the cascaded circuit is connected to the second end of the mechanical switch and the second end of the thyristor, respectively. Cascading of several of the power device half-bridge submodules on the AC side: For any of the power device half-bridge sub-modules, the midpoint of the series connection of two of the power devices is connected to one end of the parallel circuit in a power device half-bridge sub-module cascaded with itself or the first end of the mechanical switch, and the other end of the parallel circuit is connected to the midpoint of the series connection of two of the power devices in another power device half-bridge sub-module cascaded with itself or the second end of the mechanical switch. Several of the power device half-bridge submodules are cascaded on the DC side: For any of the power device half-bridge submodules, the midpoint of the series connection of two of the power devices is connected to the first terminal or the second terminal of the mechanical switch, and one end of the parallel circuit is connected to the other end of the parallel circuit in a power device half-bridge submodule cascaded with itself. Among them, the capacitor voltage of the DC capacitor in each of the power device half-bridge sub-modules remains balanced; When the AC side of the power device half-bridge submodule is cascaded, or when the DC side of the power device half-bridge submodule is cascaded and the power device half-bridge submodule is in a three-level operating mode, the specific control process for maintaining the balanced capacitor voltage of the DC capacitor of each power device half-bridge submodule includes: The capacitor voltages of the DC capacitors in each of the power device half-bridge sub-modules are sorted. When the current flowing through each of the power device half-bridge sub-modules is a positive current and the modular multilevel converter needs to insert a power device half-bridge sub-module, the power device sub-module with the lowest capacitor voltage is inserted. When the current flowing through each of the power device half-bridge sub-modules is a positive current and the modular multilevel converter needs to remove the power device half-bridge sub-module, the power device sub-module with the highest capacitor voltage is removed. When the current flowing through each of the power device half-bridge sub-modules is a negative current and the modular multilevel converter needs to insert a power device half-bridge sub-module, the power device sub-module with the highest capacitor voltage is inserted. When the current flowing through each of the power device half-bridge sub-modules is a negative current and the modular multilevel converter needs to remove the power device half-bridge sub-module, the power device sub-module with the lowest capacitor voltage is removed.

2. The modular multilevel converter as described in claim 1, characterized in that, The cascaded module also includes a heat sink and / or a control board. The heat sink is located near the power device in the power device half-bridge submodule, and the control board is connected to the power device half-bridge submodule.

3. The modular multilevel converter as described in claim 2, characterized in that, The control board includes a local controller, a communication module, a driver chip, and a sampling chip. The local controller is connected to the central controller through the communication module. The input terminal of the driver chip is connected to the output terminal of the local controller, and the output terminal is connected to the control terminal of the power device in the power device half-bridge sub-module. The input terminal of the sampling chip is connected to the DC capacitor of each power device half-bridge sub-module, and the output terminal is connected to the input terminal of the local controller.

4. A pressure equalization control method, characterized in that, An application of any cascade module in a modular multilevel converter as described in any one of claims 1 to 3, comprising: Sample the capacitor voltage of the DC capacitor in each of the power device half-bridge sub-modules; Based on the capacitor voltage, determine whether each of the power device half-bridge sub-modules meets the voltage equalization condition; If not, control the DC capacitor to charge or discharge so that the capacitor voltage of the DC capacitor of each power device half-bridge submodule remains balanced; When the DC side of the power device half-bridge submodule is cascaded and the power device half-bridge submodule is in three-level operating mode, or when the AC side of the power device half-bridge submodule is cascaded, controlling the charging or discharging of the DC capacitor includes: The capacitor voltages of the DC capacitors in each of the power device half-bridge sub-modules are sorted. When the current flowing through each of the power device half-bridge sub-modules is a positive current and the modular multilevel converter needs to insert a power device half-bridge sub-module, the power device sub-module with the lowest capacitor voltage is inserted. When the current flowing through each of the power device half-bridge sub-modules is a positive current and the modular multilevel converter needs to remove the power device half-bridge sub-module, the power device sub-module with the highest capacitor voltage is removed. When the current flowing through each of the power device half-bridge sub-modules is a negative current and the modular multilevel converter needs to insert a power device half-bridge sub-module, the power device sub-module with the highest capacitor voltage is inserted. When the current flowing through each of the power device half-bridge sub-modules is a negative current and the modular multilevel converter needs to remove the power device half-bridge sub-module, the power device sub-module with the lowest capacitor voltage is removed.

5. The pressure equalization control method as described in claim 4, characterized in that, The step of determining whether each of the power device half-bridge submodules meets the voltage equalization condition based on the capacitor voltage includes: Determine whether the difference in the DC capacitor voltage of the two power device half-bridge sub-modules is greater than a first preset threshold. If so, then the power device half-bridge submodule is determined to not meet the voltage equalization requirement; If not, then the power device half-bridge submodule is determined to satisfy voltage equalization.

6. The pressure equalization control method as described in claim 5, characterized in that, When the cascaded module includes two power device half-bridge sub-modules, the DC sides of the two power device half-bridge sub-modules are cascaded, and the power device half-bridge sub-modules are in a two-level operating mode, controlling the charging or discharging of the DC capacitor includes: When the voltage across the DC capacitor is greater than the voltage across the other DC capacitor, the drive signal of the power device of the half-bridge submodule corresponding to the DC capacitor is delayed on the falling edge or advanced on the rising edge, and / or the drive signal of the power device of the other half-bridge submodule is delayed on the rising edge or advanced on the falling edge. When the voltage across the DC capacitor is less than the voltage across the other DC capacitor, the drive signal of the power device of the half-bridge submodule corresponding to the DC capacitor is delayed on the rising edge or advanced on the falling edge, and / or the drive signal of the power device of the other half-bridge submodule is delayed on the falling edge or advanced on the rising edge.

7. The pressure equalization control method as described in claim 5, characterized in that, When the cascaded module includes two power device half-bridge sub-modules, the DC sides of the two power device half-bridge sub-modules are cascaded, and the power device half-bridge sub-modules are in a two-level operating mode, controlling the charging or discharging of the DC capacitor includes: Adjust the switching mode of the power devices in the power device half-bridge submodule to switch the power device half-bridge submodule to a three-level operating mode; After operating in three-level mode for several cycles until the difference in the DC capacitor voltage of the two power device half-bridge submodules is less than the second preset threshold, the switching mode of the power device half-bridge submodule is adjusted again to switch the power device half-bridge submodule to two-level mode.

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