Capacitance voltage bidirectional controllable MMC half-bridge circuit and control method thereof

Abstract

The application discloses a kind of capacitive voltage bidirectional controllable MMC half-bridge circuit and control method thereof, circuit includes switching module, capacitor module, detection control module and discharge module, by detecting capacitor voltage, when capacitor voltage exceeds certain range, change the working state of MMC half-bridge circuit module.When capacitor voltage is too high, change the current flow path, switch to other module work, while the voltage of capacitor is released through resistance.When the voltage of capacitor is too low, by switching the working state of MMC half-bridge module, make MMC half-bridge module work in charging state.By detecting the change of capacitor voltage and switching the working state of MMC half-bridge module in time, ensure that the voltage of capacitor does not exceed the specified range, realize the safe and stable operation of MMC half-bridge module.

Description

Technical Field

[0001] This invention relates to the field of power, and in particular to multilevel modular converters in power electronic converter circuits. Background Technology

[0002] With economic development and the rational allocation of energy resources, large-scale long-distance power transmission is required in the power system. The main transmission methods include ultra-high voltage (UHV) AC, UHV DC, and flexible low-frequency (LHF) transmission. From the perspectives of saving energy loss and transmitting high power, UHV DC and LHF transmission have significant advantages. Currently, several UHV DC transmission lines have been built in China. Flexible LHF transmission lines are mainly used in offshore wind power transmission projects. With the large-scale development and utilization of wind and other new energy sources, the technology for achieving low-frequency power transmission is gradually being researched and applied.

[0003] Modular multilevel converters (MMCs) used in flexible low-frequency power transmission systems are key components for converting power from 50Hz AC to DC to 20Hz AC. Currently used MMCs primarily consist of multiple modular half-bridge or full-bridge modules, with multiple identical half-bridge modules connected to the same bridge arm. Half-bridge circuits are simple and easy to control, therefore they are widely used in various applications, such as... Figure 1 As shown. T1 and T2 are Insulated Gate Bipolar Transistors (IGBTs), which are controllable IGBTs. D1 and D2 are power diodes connected in anti-parallel to the two IGBTs; they are uncontrollable devices and serve to provide freewheeling conduction. C0 is the module capacitor, which provides the output voltage.

[0004] Table 1 shows the working states of the MMC half-bridge module. Controlling the on or off states of T1 and T2 can change the working state of the half-bridge circuit and the output and state of the module.

[0005] Table 1. Operating Status of MMC Half-Bridge Module

[0006]

[0007] In modes 1 and 2, when current flows in from terminal A and out from terminal B, the module is in a charging state, and the capacitor voltage increases during charging. Conversely, in mode 2, when current flows in from terminal B, the capacitor is in a discharging state, and its voltage decreases. This change in capacitor voltage affects the voltage output from terminals A and B. Furthermore, if the capacitor voltage becomes too high, exceeding the IGBT's withstand voltage, it can cause the IGBT to break down. Therefore, to ensure the normal and safe operation of the MMC half-bridge module, the capacitor voltage needs to be maintained within a certain range.

[0008] As shown above, existing MMC half-bridge modules lack control and protection functions for capacitor voltage. When the capacitor is charging, the voltage increases to a certain level, and this increase cannot be suppressed. When the capacitor is discharging, the voltage decreases to a certain level, and this further decrease cannot be suppressed, causing the output voltage to exceed the allowable range and resulting in waveform distortion. When the voltage exceeds a certain range, the capacitor voltage becomes too high and may even exceed the breakdown voltages of the two IGBT devices and two diodes, causing device breakdown and damaging the MMC half-bridge module. When the capacitor voltage is low, the output voltage is low, and the output AC waveform is severely distorted. Summary of the Invention

[0009] To address the shortcomings of existing technologies, this invention provides a bidirectionally controllable MMC half-bridge circuit and its control method. When the capacitor voltage of the MMC half-bridge module increases or decreases to a certain level, the voltage of the capacitor is detected, and when the capacitor voltage exceeds a certain range, the operating state of the MMC half-bridge circuit is changed. By detecting changes in capacitor voltage and switching the operating state of the MMC half-bridge module in a timely manner, the voltage of the capacitor is ensured to remain within the specified range, thus achieving safe and stable operation of the MMC half-bridge module.

[0010] To solve the aforementioned technical problem, the technical solution adopted by this invention is: a bidirectional controllable MMC half-bridge circuit for capacitor voltage, comprising a switching module, a capacitor module, a detection and control module, and a discharge module. The switching module includes an upper switch and a lower switch connected in series, each consisting of two anti-parallel switches. External interfaces A and B of the switching module are led out at the midpoint between the upper and lower switches and below the lower switch. The capacitor module includes a capacitor C connected in parallel with the switching module. The detection and control module includes a capacitor voltage detection module, a current direction detection module, a comprehensive comparison trigger pulse generation module, a half-bridge communication unit, and a bridge arm comprehensive control module. The capacitor voltage detection module is connected in parallel with capacitor C and is used to detect the voltage of capacitor C in real time. The current direction detection module is connected in series with the external interface of the switching module and is used to detect the direction of the current flowing into the switching module in real time. Both the capacitor voltage detection module and the current direction detection module... Connected to the integrated comparison trigger pulse generation module, the half-bridge communication unit is connected between the integrated comparison trigger pulse generation module and the bridge arm integrated control module. The discharge module includes a discharge switch and a discharge resistor, which are connected in series and then in parallel with the switch module. The signals detected by the capacitor voltage detection module and the current direction detection module are transmitted to the integrated comparison trigger pulse generation module. The integrated comparison trigger pulse generation module determines whether the detected signal exceeds the specified range. After comprehensive judgment, it communicates with the bridge arm integrated control module. The bridge arm integrated control module integrates the working status of all MMC half-bridge circuits in the same bridge arm, generates control commands for the MMC half-bridge circuit, and sends them to the integrated comparison trigger pulse generation module through the half-bridge communication unit. The integrated comparison trigger pulse generation module generates trigger signals for the switch module and the discharge switch according to the control commands. The switch module and the discharge switch act according to the trigger signals, thereby realizing bidirectional controllable capacitor voltage.

[0011] Furthermore, the capacitor C is connected in parallel with a resistor R2, which makes the capacitor voltage more gradual during charging and discharging.

[0012] Furthermore, the capacitor voltage detection module includes a resistor R1 and a microammeter, which are connected in series and then in parallel across the switching module.

[0013] Furthermore, the current direction detection module is a current measuring coil, which is connected in series with the external interface B of the switching module led out below the lower switching transistor.

[0014] Furthermore, the switching module is composed of switching transistors T1, T2, T3, and T4 connected in antiparallel and then in series. Among them, switching transistors T1 and T2 are connected in antiparallel to form the upper switching transistor, and switching transistors T3 and T4 are connected in antiparallel to form the lower switching transistor.

[0015] This invention also discloses a control method for an MMC half-bridge circuit with bidirectional controllable capacitor voltage, comprising the following steps:

[0016] S01) Construct the MMC half-bridge circuit as described above. This circuit has four operating modes: Mode 1, switches T1 and T2 are turned on, switches T3 and T4 are turned off, current flows in from terminal A and flows out from terminal B, and the output voltage U AB = Uc, where Uc is the voltage across capacitor C. The voltage across capacitor C is rising, and the entire MMC half-bridge circuit is in a charging state. In mode 2, switches T1 and T2 are on, and switches T3 and T4 are off. Current flows in from terminal B and out from terminal A, resulting in an output voltage U. AB = Uc, where Uc is the voltage across capacitor C. When the voltage across capacitor C is decreasing, the entire MMC half-bridge circuit is in a charging / discharging state. In mode 3, switches T1 and T2 are off, and switches T3 and T4 are on. Current flows in from terminal A and out from terminal B, resulting in an output voltage U. AB =0, the voltage of capacitor C remains constant, and the entire MMC half-bridge circuit is in the off state; in mode 4, switches T1 and T2 are off, switches T3 and T4 are on, current flows in from terminal B and flows out from terminal A, and the output voltage U AB =0, the voltage of capacitor C is constant, and the entire MMC half-bridge circuit is in the cut-off state.

[0017] S02) Real-time detection of the voltage Uc of capacitor C, and determination of whether the voltage Uc is within the range of [Umin, Umax]. Umin and Umax are the minimum and maximum values ​​of capacitor voltage Uc under normal working conditions, respectively. If so, the capacitor voltage is within the normal range, and the MMC half-bridge circuit does not need to be adjusted or is adjusted in coordination with other MMC half-bridge circuits.

[0018] S03) If the voltage Uc is greater than Umax, continue to determine the direction of the circuit current. If the current direction is from B to A, the circuit enters the discharge state. The comprehensive comparison trigger pulse forming module sends an on signal to the discharge switch until the capacitor voltage Uc reaches the allowable range, then the discharge switch is turned off. If the current direction is from A to B, the MMC half-bridge circuit is in working mode 1. The comprehensive comparison trigger signal forming module makes a judgment after considering the capacitor voltage Uc and the direction of the circuit current. It determines that the half-bridge circuit needs to switch to working mode 3, sends off signals to switches T1 and T2, and sends on signals to switches T3 and T4 and the discharge switch. At the same time, it communicates with the comprehensive control module of the same bridge arm and determines which group switches the corresponding working state.

[0019] (S04) If the voltage Uc is less than Umin, continue to determine the direction of the circuit current. If the current direction is from B to A, the circuit is in working mode 2. After comprehensively comparing the trigger signal forming module with the capacitor voltage Uc and the direction of the circuit current, it is determined that the half-bridge circuit needs to switch to working mode 4. A turn-off signal is sent to the switching transistors T1 and T2, and an turn-on signal is sent to the switching transistors T3 and T4. At the same time, it communicates with the integrated control module of the same bridge arm to determine which group should switch the corresponding working state. If the current direction is from A to B, it means that the voltage has entered the charging state after falling below the minimum voltage. At this time, the MMC half-bridge circuit is in the charging state and there is no need to change the working state of the circuit.

[0020] Furthermore, for an MMC bridge arm composed of multiple MMC half-bridge circuits, the half-bridge communication unit of each half-bridge circuit communicates with the bridge arm integrated control module. When a bridge arm needs to switch its working state, it communicates with the bridge arm integrated control module through the communication units of each circuit. The bridge arm integrated control module then makes a comprehensive judgment and decides which group of circuits should switch their working states.

[0021] Furthermore, the bridge arm integrated control module makes the following comprehensive judgment: When the bridge arm integrated control module receives a request from a half-bridge circuit to switch its working state, other half-bridge circuits send their own switchable status to the bridge arm integrated control module. The bridge arm integrated control module integrates the status of all half-bridge circuits and selects a group of switchable half-bridge circuits for mutual switching.

[0022] Furthermore, when the half-bridge circuit is in operating mode 1 or mode 2, it is in a non-switchable state; when the half-bridge circuit is in operating mode 3 or mode 4, it is in a switchable state.

[0023] The beneficial effects of this invention are as follows: This invention detects the capacitor voltage and changes the operating state of the MMC half-bridge circuit module when the capacitor voltage exceeds a certain range. When the capacitor voltage is too high, the current flow path is changed, switching to other modules, while simultaneously releasing the capacitor voltage through a resistor. When the capacitor voltage is too low, the operating state of the MMC half-bridge module is switched to a charging state. By detecting changes in capacitor voltage and switching the operating state of the MMC half-bridge module in a timely manner, this invention ensures that the capacitor voltage does not exceed the specified range, achieving safe and stable operation of the MMC half-bridge module. Furthermore, this invention also connects a resistor in parallel with the capacitor branch, which makes the capacitor voltage more gradual during charging and discharging.

[0024] This invention enables bidirectional control of the capacitor voltage of the MMC half-bridge module, ensuring that the capacitor voltage of the MMC half-bridge module remains within a stable and reliable voltage range. This guarantees both the output voltage waveform quality of the MMC half-bridge module and its safe and reliable operation. Attached Figure Description

[0025] Figure 1 The schematic diagram of an existing MMC half-bridge circuit;

[0026] Figure 2 This is a schematic diagram of the MMC half-bridge circuit described in this invention;

[0027] Figure 3 This is a schematic diagram of the half-bridge circuit described in this invention in operating mode 1;

[0028] Figure 4 This is a schematic diagram of the half-bridge circuit described in this invention in operating mode 2;

[0029] Figure 5 This is a schematic diagram of the half-bridge circuit described in this invention in operating mode 3;

[0030] Figure 6 This is a schematic diagram of the half-bridge circuit described in this invention in operating mode 4;

[0031] Figure 7 A flowchart illustrating the process when the capacitor voltage exceeds the maximum allowable range;

[0032] Figure 8 A flowchart illustrating the process when the capacitor voltage is below the minimum allowable range;

[0033] Figure 9 This is a schematic diagram showing the communication between the communication units of each circuit in the same bridge arm and the integrated control module of the bridge arm.

[0034] Figure 10 This is a schematic diagram showing the communication between the circuits of each bridge arm and the bridge arm integrated controller in the MMC. Detailed Implementation

[0035] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0036] Example 1

[0037] This embodiment discloses an MMC half-bridge circuit with bidirectional controllable capacitor voltage, such as... Figure 2 As shown, it mainly consists of four parts. The first part is the switching module, which is composed of four Insulated Gate Bipolar Transistor (IGBT) switching devices connected in anti-parallel and then in series. Specifically, the switching module is composed of switching transistors T1, T2, T3, and T4 connected in anti-parallel and then in series. Switching transistors T1 and T2 are connected in anti-parallel to form the upper switching transistor, and switching transistors T3 and T4 are connected in anti-parallel to form the lower switching transistor. An external interface terminal A is led out between the upper and lower switching transistors, and an external interface terminal B is led out below the lower switching transistor.

[0038] Part ② is the capacitor module, which is an RC circuit. Resistor R2 and capacitor C are both connected in parallel with the switching module. Capacitor C serves as the output module voltage, and resistor R2 can slow down the charging and discharging process of the capacitor. The resistance of R2 is tens of ohms.

[0039] Part ③ is the detection and control module, which includes a capacitor voltage detection module, a current direction detection module, a comprehensive comparison trigger pulse generation module, a half-bridge communication unit, and a bridge arm comprehensive control module. The capacitor voltage detection module is connected in parallel with capacitor C to detect the voltage of capacitor C in real time. The current direction detection module is connected in series with the external interface of the switch module to detect the direction of the current flowing into the switch module in real time. Both the capacitor voltage detection module and the current direction detection module are connected to the comprehensive comparison trigger pulse generation module. The half-bridge communication unit is connected between the comprehensive comparison trigger pulse generation module and the bridge arm comprehensive control module to realize communication between the comprehensive comparison trigger pulse generation module and the bridge arm comprehensive control module.

[0040] The detection and control module can detect the voltage of the capacitor and the direction of the current flowing into the MMC half-bridge module in real time, and determine whether it exceeds the specified range based on the detection parameters. After comprehensive judgment, it communicates with the bridge arm integrated control module, communicates with the half-bridge modules of the same bridge arm of other MMCs and compares them to form the switching trigger signals of 5 switching devices T1, T2, T3 and T4 and T5.

[0041] Part ④ is the capacitor voltage discharge module, which includes a discharge switch T5 and a discharge resistor r. The discharge switch T5 and the discharge resistor r are connected in series to form a branch that is connected in parallel with the switch module. r is typically a few ohms. When the capacitor is detected to exceed the maximum allowable voltage Umax, T5 is turned on, and the capacitor is discharged through the resistor r, so that the capacitor voltage returns to the allowable range.

[0042] Table 2 below shows the "Operating Status Table of MMC Half-Bridge Module with Bidirectional Controllable Capacitor Voltage". This half-bridge module has different operating states when the four IGBT switching devices are in different operating states.

[0043] Table 2. Operating Status of the MMC Half-Bridge Module with Bidirectional Capacitor Voltage Controllability

[0044]

[0045] The four working modes are described below:

[0046] like Figure 3 As shown, the MMC half-bridge circuit operates in the on-and-charge state, i.e., mode 1. The current flows from terminal A to terminal B. IGBT devices T1 and T2 are turned on, while IGBT devices T3 and T4 are turned off. The current flow path is: terminal A - device T2 - capacitor C and resistor R2 - terminal B. At this time, the output voltage U...AB =U c Since the circuit is in a charging state, the capacitor voltage is rising. When the MMC half-bridge module is in the active state, it cannot switch with other modules; therefore, the communication unit of this MMC half-bridge circuit with the bridge arm integrated control module is in a non-switchable state.

[0047] like Figure 4 The diagram shows the MMC half-bridge circuit operating in both the on and off states. In mode 2, the current flows from terminal B to terminal A. IGBTs T1 and T2 are turned on, while IGBTs T3 and T4 are turned off. The current path is: terminal B - capacitor C and resistor R2 - T1 - terminal A. At this time, the output voltage U... AB =Uc, therefore, it is in a discharging state at this time, and the capacitor voltage is decreasing. Because the MMC half-bridge module is in the engaged state at this time, the communication unit of this MMC half-bridge circuit and the bridge arm integrated control module are in a "non-switchable" state.

[0048] like Figure 5 As shown, the MMC half-bridge module operates in the off state, i.e., mode 3, with T1 and T2 in the off state and T3 and T4 in the on state. The current flows from terminal A to terminal B, and the current path is A port - T3 device - B port. The output voltage U at this time... AB =0.

[0049] Figure 6 The MMC half-bridge module is operating in the off state, i.e., mode 4, with T1 and T2 in the off state and T3 and T4 in the on state. The current direction is from terminal B to terminal A, and the current flow path is terminal B - device T4 - terminal A. The output voltage U AB =0.

[0050] For an MMC half-bridge module in the disconnected state, it can switch working states with other modules in the same bridge arm of the MMC, that is, the half-bridge module's "communication unit" communicates with the "bridge arm integrated control" module, which is the "switchable" state.

[0051] Figures 3 to 6 In the image, the arrow indicates the direction of current flow.

[0052] Example 2

[0053] This embodiment discloses a control method for an MMC half-bridge circuit with bidirectional controllable capacitor voltage. This method is based on the circuit described in Embodiment 1 and has the four working modes described in Embodiment 1.

[0054] The allowable range of the voltage of capacitor C is Umin - Umax. The comprehensive comparison trigger pulse formation module in the MMC half-bridge circuit can detect whether the capacitor voltage is within the allowable range. In this embodiment, by switching the switching states of 4 IGBTs, the capacitor voltage can be controlled between Umin - Umax, that is, the capacitor voltage is bidirectionally controllable. The control method for the bidirectional controllability of the capacitor voltage is as follows:

[0055] Figure 7 It is the working process when the capacitor voltage exceeds the maximum allowable range. The capacitor voltage detection module continuously detects the range of the capacitor voltage. If it does not exceed the maximum allowable voltage range Umax, the working state of the module is not changed. If it is detected that the capacitor voltage exceeds the allowable range, Uc > Umax, the direction of the current is judged through the current coil. If the direction of the current coil is from A to B, the capacitor is in the charging state, and the working states of other modules in the same arm of the MMC need to be switched with each other. This module is switched from mode 1 (T1 and T2 are turned on, T3 and T4 are turned off) to mode 3 (T1 and T2 are turned off, T3 and T4 are turned on), and at the same time, T5 is also turned on. After the capacitor voltage reaches the allowable range, T5 is turned off. If it is detected that the current direction is from the B end to the A end, it indicates that the capacitor is in the discharging state at this time, and the working state does not need to be switched, but T5 needs to be turned on to reduce the voltage of the capacitor until it reaches the allowable voltage range.

[0056] Figure 8 It is the working process when the capacitor voltage is lower than the minimum allowable voltage range. Similarly, the capacitor voltage detection module continuously detects the voltage range of the capacitor. If the capacitor voltage is not lower than the minimum allowable voltage range Umin, the working state of the module does not need to be changed. If it is detected that the capacitor voltage is lower than the minimum allowable voltage range Umin, that is, Uc < Umin, the direction of the module current needs to be judged. If the direction of the current is from the B end to the A end, the capacitor voltage will continue to decrease, and communication with other modules is required, and the working states of the MMC half-bridge modules are changed with each other. This MMC half-bridge module is switched from mode 2 (T1 and T2 are turned on, T3 and T4 are turned off) to mode 4 (T1 and T2 are turned off, T3 and T4 are turned on). If it is detected that the current direction is from the A end to the B end, it means that the voltage has just dropped below the minimum voltage and then entered the charging state. At this time, the MMC half-bridge module is in the charging state, and the working state of the module does not need to be changed.

[0057] Such as Figure 9The diagram shows the topology of one bridge arm of an MMC consisting of four (or more) half-bridge modules. The communication units of each module communicate with the integrated bridge arm control module. When a bridge arm needs to switch operating states, the communication units of each module communicate with the integrated bridge arm control module. The integrated bridge arm control module then makes a comprehensive judgment and decides which group of modules should switch operating states. Switching between different groups of modules is to maintain the voltage balance of the entire bridge arm capacitor. After changing the output voltage of this half-bridge module, U... AB When the voltage drops to 0, other bridge arms need to switch their operating states to maintain the balance and consistency of the voltage in this bridge arm.

[0058] Figure 10 As shown, four (or more) MMC half-bridge modules are connected in series to form one arm of the MMC. When circuit 4 detects that the capacitor voltage is too high, its communication unit communicates with the "bridge arm integrated control module" and sends a request to switch the operating state. In the normal state, the circuits of the same bridge arm of the MMC communicate with the "bridge arm integrated control module" and send a status indicating whether they can be switched: "switchable" or "not switchable". The "bridge arm integrated control module" then considers the status of other circuits in this MMC bridge arm and determines which group of half-bridge circuits 1 and 3 in the "switchable" state can switch between each other. Figure 10 In the process, both circuits 1 and 3 can be switched. The switch of circuit 1 to mode 1 is determined by the top-to-bottom order of circuits 1 and 3 in the same bridge arm.

[0059] After the "bridge arm integrated control module" makes a decision, it sends a switching command to the half-bridge module that needs to be switched. The module that needs to switch its working state then sends trigger signals to each IGBT device T1, T2, T3, T4 and T5 to realize the switching of working state.

[0060] The above description is merely the basic principle and preferred embodiment of the present invention. Improvements and substitutions made by those skilled in the art based on the present invention are within the scope of protection of the present invention.

Claims

1. A bidirectionally controllable MMC half-bridge circuit, characterized in that: The system includes a switching module, a capacitor module, a detection and control module, and a discharge module. The switching module consists of an upper and a lower switching transistor connected in series. Each upper and lower switching transistor is composed of two anti-parallel switching transistors. The upper switching transistor is formed by anti-parallel connection of transistors T1 and T2, and the lower switching transistor is formed by anti-parallel connection of transistors T3 and T4. External interfaces A and B of the switching module are led out from the middle of the upper and lower switching transistors and below the lower switching transistor. The capacitor module includes a capacitor C connected in parallel with the switching module. The detection and control module includes a capacitor voltage detection module, a current direction detection module, a comprehensive comparison trigger pulse generation module, a half-bridge communication unit, and a bridge arm comprehensive control module. The capacitor voltage detection module is connected in parallel with capacitor C to detect the voltage of capacitor C in real time. The current direction detection module is connected in series with the external interface of the switching module to detect the direction of the current flowing into the switching module in real time. Both the capacitor voltage detection module and the current direction detection module are connected in series with the external interface of the switching module. The integrated comparison trigger pulse generation module is connected to the bridge arm integrated control module. The half-bridge communication unit is connected between the integrated comparison trigger pulse generation module and the bridge arm integrated control module. The discharge module includes a discharge switch and a discharge resistor, which are connected in series and then in parallel with the switch module. The signals detected by the capacitor voltage detection module and the current direction detection module are transmitted to the integrated comparison trigger pulse generation module. The integrated comparison trigger pulse generation module determines whether the detected signal exceeds the specified range. After comprehensive judgment, it communicates with the bridge arm integrated control module. The bridge arm integrated control module integrates the working status of all MMC half-bridge circuits in the same bridge arm, generates control commands for the MMC half-bridge circuit, and sends them to the integrated comparison trigger pulse generation module through the half-bridge communication unit. The integrated comparison trigger pulse generation module generates trigger signals for the switch module and the discharge switch according to the control commands. The switch module and the discharge switch act according to the trigger signals, thereby realizing bidirectional capacitor voltage. This MMC half-bridge circuit has four operating modes: Mode 1: Switches T1 and T2 are on, and switches T3 and T4 are off. Current flows in from terminal A and out from terminal B. The output voltage UAB = Uc, where Uc is the voltage across capacitor C. The voltage across capacitor C is rising, and the entire MMC half-bridge circuit is in a charging state. Mode 2: Switches T1 and T2 are on, and switches T3 and T4 are off. Current flows in from terminal B and out from terminal A. The output voltage UAB = Uc is the voltage across capacitor C. When the voltage across capacitor C is decreasing, the entire MMC half-bridge circuit is in a discharge state. In mode 3, switches T1 and T2 are off, and switches T3 and T4 are on. Current flows in from terminal A and out from terminal B. The output voltage UAB = 0, the voltage across capacitor C remains constant, and the entire MMC half-bridge circuit is in a cut-off state. In mode 4, switches T1 and T2 are off, and switches T3 and T4 are on. Current flows in from terminal B and out from terminal A. The output voltage UAB = 0, the voltage across capacitor C remains constant, and the entire MMC half-bridge circuit is in a cut-off state. The voltage Uc of capacitor C is detected in real time to determine whether the voltage Uc is within the range of [Umin, Umax]. Umin and Umax are the minimum and maximum values ​​of capacitor voltage Uc under normal working conditions, respectively. If it is, the capacitor voltage is within the normal range, and the MMC half-bridge circuit does not need to be adjusted or is adjusted in conjunction with other MMC half-bridge circuits. If the voltage Uc is greater than Umax, the direction of the circuit current is further determined. If the current direction is from B to A, the MMC half-bridge circuit enters the discharge state. The comprehensive comparison trigger pulse forming module sends an on signal to the discharge switch until the capacitor voltage Uc reaches the allowable range, at which point the discharge switch is turned off. If the current direction is from A to B, the MMC half-bridge circuit is in working mode 1. The comprehensive comparison trigger signal forming module combines the capacitor voltage Uc and the direction of the circuit current to make a judgment. If it is determined that the MMC half-bridge circuit needs to switch to working mode 3, it sends off signals to switches T1 and T2, and on signals to switches T3 and T4 as well as the discharge switch. At the same time, it communicates with the comprehensive control module of the same bridge arm and determines which group should switch the corresponding working state. If the voltage Uc is less than Umin, the direction of the circuit current is further determined. If the current flows from end B to end A, the MMC half-bridge circuit is in operating mode 2. The trigger signal forming module makes a judgment after comprehensively comparing the capacitor voltage Uc and the direction of the circuit current. If it is determined that the MMC half-bridge circuit needs to switch to operating mode 4, it sends a turn-off signal to switches T1 and T2 and a turn-on signal to switches T3 and T4. At the same time, it communicates with the integrated control module of the same bridge arm to determine which group should switch the corresponding operating state. If the current flows from end A to end B, it means that the voltage has entered the charging state after falling below the minimum voltage. At this time, the MMC half-bridge circuit is in the charging state and there is no need to change the operating state of the MMC half-bridge circuit.

2. The MMC half-bridge circuit with bidirectional controllable capacitor voltage according to claim 1, characterized in that: A capacitor C is connected in parallel with a resistor R2.

3. The MMC half-bridge circuit with bidirectional controllable capacitor voltage according to claim 1, characterized in that: The capacitor voltage detection module includes a resistor R1 and a microammeter, which are connected in series and then in parallel across the switching module.

4. The MMC half-bridge circuit with bidirectional controllable capacitor voltage according to claim 1, characterized in that: The current direction detection module is a current measuring coil, which is connected in series with the external interface B of the switch module led out below the lower switch tube.

5. The MMC half-bridge circuit with bidirectional controllable capacitor voltage according to claim 1, characterized in that: For an MMC bridge arm composed of multiple MMC half-bridge circuits, the half-bridge communication unit of each MMC half-bridge circuit communicates with the bridge arm integrated control module. When a bridge arm needs to switch its working state, it communicates with the bridge arm integrated control module through the half-bridge communication unit of each MMC half-bridge circuit. After comprehensive judgment, the bridge arm integrated control module decides which group of MMC half-bridge circuits should switch their working states.

6. The MMC half-bridge circuit with bidirectional controllable capacitor voltage according to claim 5, characterized in that: The process of the bridge arm integrated control module's comprehensive judgment is as follows: When the bridge arm integrated control module receives a request from an MMC half-bridge circuit to switch its working state, other MMC half-bridge circuits send their own switchable status to the bridge arm integrated control module. The bridge arm integrated control module integrates the status of all MMC half-bridge circuits and selects a group of switchable MMC half-bridge circuits for mutual switching.

7. The MMC half-bridge circuit with bidirectional controllable capacitor voltage according to claim 6, characterized in that: When the MMC half-bridge circuit is in operating mode 1 or mode 2, it is in a non-switchable state; when the MMC half-bridge circuit is in operating mode 3 or mode 4, it is in a switchable state.

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