Bridge arm multiplexing MMC topology based on improved full-bridge sub-module and control method

Abstract

The application relates to the technical field of high-voltage direct current transmission, in particular to a bridge arm multiplexing MMC topology based on an improved full-bridge sub-module and a control method, which comprises a parallel three-phase alternating current system and a direct current side voltage stabilizing circuit, each phase alternating current circuit in the three-phase alternating current system comprises an alternating current power supply, an alternating current side filter inductor, an upper switch circuit, a lower switch circuit and a multiplexing bridge arm; the multiplexing bridge arm comprises a plurality of improved full-bridge sub-modules connected in series, and adjacent improved full-bridge sub-modules are connected in the form of connecting the positive output port through the negative output port. The upper switch circuit and the lower switch circuit are controlled, the multiplexing bridge arm can be converted into an upper bridge arm or a lower bridge arm at different times, the use quantity of the MMC sub-modules is greatly reduced in the mode of time-sharing conversion of the multiplexing bridge arm, the volume and weight of the MMC are further reduced, the damping of the fault loop is increased, and the MMC converter direct current side fault rapid and smooth crossing is ensured.

Description

Technical Field

[0001] This invention relates to the field of high voltage direct current transmission technology, and in particular to a bridge arm multiplexing MMC topology and control method based on an improved full-bridge submodule. Background Technology

[0002] With the continuous penetration of renewable energy sources, especially the rapid development of offshore wind power generation technology, DC transmission technology based on voltage source converters has broad application prospects in large-scale power transmission and grid-connected operation of new energy sources. Modular multilevel converters (MMC), as the latest voltage source converter technology, have become the preferred solution for DC transmission converters due to their advantages such as independent control of active and reactive power, highly modular structure, high portability, and good harmonic characteristics. They are widely used in scenarios of grid-connected operation of new energy sources.

[0003] However, MMC converters are generally three-phase six-arm structures, with each arm containing a large number of half-bridge sub-modules. This consumes a large number of switching devices and passive components, resulting in high cost, large size, and heavy weight for MMC converters. At the same time, the large number of sub-module circuits require a large number of driver and sampling modules, which further increases the cost of MMC and the design difficulty of MMC control systems. In addition, MMC converters based on half-bridge sub-modules do not have the ability to ride through DC-side short-circuit faults. Existing engineering applications often use the method of adding DC circuit breakers and DC power dissipation devices, but this method has a long response time and high cost.

[0004] Existing improved MMC topologies have made significant contributions to reducing the cost and size of converters. However, these topologies are all based on three-phase six-arm bridges and still use a large number of switching devices and passive components, which cannot effectively reduce the cost, size, and weight of the converter. Existing three-phase three-arm MMC topologies use half-bridge sub-modules and do not have the ability to ride through DC-side faults. The cost of equipping them with DC circuit breakers and DC energy dissipation devices is high, making it difficult to guarantee the reliability and economy of the power transmission system. Therefore, there is an urgent need for an MMC converter that can significantly reduce the number of switching devices and passive components while having the ability to ride through DC-side short-circuit faults. Summary of the Invention

[0005] The purpose of this invention is to provide a bridge arm multiplexing MMC topology and control method based on an improved full-bridge submodule, so as to effectively reduce the cost, size and weight of the MMC converter, while enabling the DC transmission system to have fault ride-through capability.

[0006] To address the above technical problems, this invention provides a bridge arm multiplexing MMC topology and control method based on an improved full-bridge submodule.

[0007] In a first aspect, the present invention provides a bridge arm multiplexing MMC topology based on an improved full-bridge submodule, comprising:

[0008] A parallel three-phase AC system and a DC-side voltage regulator circuit are provided, with each end of the DC-side voltage regulator circuit connected to a port to form a DC-side port. The three-phase AC system includes an a-phase AC circuit, a b-phase AC circuit, and a c-phase AC circuit. Each phase AC circuit includes an AC power supply, an AC-side filter inductor, an upper switch circuit, a lower switch circuit, and a multiplexed bridge arm. The multiplexed bridge arm includes several improved full-bridge sub-modules connected in series. Adjacent improved full-bridge sub-modules are connected by connecting their negative output port to their positive output port.

[0009] The improved full-bridge submodule includes a first switch, a second switch, a third switch, a first diode, a second diode, a third diode, a fourth diode, a capacitor, and a resistor. The first switch and the first diode are connected in anti-parallel, the second switch and the second diode are connected in anti-parallel, and the third switch and the fourth diode are connected in anti-parallel. The emitter of the first switch and the collector of the second switch are connected in series and then in parallel with the capacitor. One end of the capacitor is connected to the collector of the first switch, and the other end is connected to the emitter of the second switch. The collector of the first switch is connected to the cathode of the third diode through the resistor, and the anode of the third diode is connected to the collector of the third switch.

[0010] In a further embodiment, the positive output port of the improved full-bridge submodule is the common terminal connection point of the first diode and the second switch, and the negative output port of the improved full-bridge submodule is the common terminal connection point of the third diode and the third switch.

[0011] The first and third switches are used to jointly control the improved full-bridge submodule to switch to the active state; the second and third switches are used to jointly control the improved full-bridge submodule to switch to the bypass state.

[0012] In a further implementation, when a short-circuit fault occurs on the DC side, the improved full-bridge submodule switches to a locked state, and the resistor is connected to the fault current path to charge the capacitor of the improved full-bridge submodule through the fault current. The expression for the fault current is:

[0013]

[0014] in,

[0015]

[0016]

[0017] R eq =2N·R s +R f

[0018] In the formula, i fc Indicates the fault current; τ represents the capacitor time constant of the improved full-bridge submodule; t represents time; I fc Indicates the magnitude of the single-phase bridge arm fault current at the initial moment of the fault; ω represents the fundamental angular frequency of the fault current; L represents the bridge arm inductance; C represents the bridge arm submodule capacitance; R eq R represents the equivalent resistance of the fault circuit. f Indicates short-circuit resistance; N indicates the number of bridge arm sub-modules; U C Indicates the capacitor voltage of the bridge arm submodule; R s This indicates the resistance value of the resistor in the improved full-bridge submodule.

[0019] In a further embodiment, the upper switching circuit includes a first switching device, a second switching device, and a first inductor, and the lower switching circuit includes a third switching device, a fourth switching device, and a second inductor.

[0020] The collector of the first switching device is connected to the positive terminal of the DC port, the emitter of the first switching device is connected to the collector of the second switching device, and the emitter of the second switching device is connected to one end of the first inductor.

[0021] The collector of the third switching device is connected to one end of the second inductor, the emitter of the third switching device is connected to the collector of the fourth switching device, and the emitter of the fourth switching device is connected to the negative terminal of the DC port.

[0022] In a further embodiment, the first switching device and the second switching device are complementaryly connected, and the third switching device and the fourth switching device are complementaryly connected, specifically as follows:

[0023] When the AC side voltage of the three-phase AC circuit enters the positive half-cycle, the first and third switching devices are turned on simultaneously, and the second and fourth switching devices are turned off simultaneously.

[0024] When the AC side voltage of the three-phase AC circuit enters the negative half-cycle, the first and third switching devices are simultaneously disconnected, while the second and fourth switching devices are simultaneously turned on.

[0025] In a further embodiment, one end of the AC-side filter inductor is connected to the common terminal of the upper switching circuit and the lower switching circuit connected in series, and the other end of the AC-side filter inductor is connected to the output port of the AC power supply.

[0026] The DC-side voltage regulator circuit includes a first voltage regulator capacitor and a second voltage regulator capacitor. The positive terminal of the first voltage regulator capacitor is connected to the positive terminal of the DC port, the negative terminal of the first voltage regulator capacitor is connected to the positive terminal of the second voltage regulator capacitor, and the negative terminal of the second voltage regulator capacitor is connected to the negative terminal of the DC port.

[0027] Secondly, the present invention provides a control method for a bridge arm multiplexing MMC topology based on an improved full-bridge submodule. Applying the aforementioned bridge arm multiplexing MMC topology based on an improved full-bridge submodule, the control method includes the following steps:

[0028] When the AC side voltage of the three-phase AC circuit enters the positive half-cycle, the first and third switching devices are controlled to turn on simultaneously, and the second and fourth switching devices are controlled to turn off simultaneously.

[0029] When the AC side voltage of the three-phase AC circuit enters the negative half-cycle, the first and third switching devices are simultaneously disconnected, and the second and fourth switching devices are simultaneously turned on.

[0030] In a further implementation, during the positive half-cycle of the AC side voltage of the three-phase AC circuit, one end of the multiplexed bridge arm is connected to the positive terminal of the DC port, and the other end of the multiplexed bridge arm is connected to the AC port. The expression for the modulated voltage of the multiplexed bridge arm is:

[0031]

[0032] During the negative half-cycle of the AC side voltage in a three-phase AC circuit, one end of the multiplexed bridge arm is connected to the AC port, and the other end of the multiplexed bridge arm is connected to the negative terminal of the DC port. The expression for the modulated voltage of the multiplexed bridge arm is:

[0033]

[0034] In the formula, v Fx Indicates the multiplexed bridge arm modulation voltage; V dc Indicates the voltage at the DC port; v x This represents the AC side voltage of a three-phase AC circuit.

[0035] In a further implementation, when a short-circuit fault occurs on the DC side, the first, second, and third switches are turned off to switch the improved full-bridge submodule to a locked state. The resistor is connected to the fault current path, and the capacitor of the improved full-bridge submodule is charged through the fault current. The expression for the fault current is:

[0036]

[0037] in,

[0038]

[0039]

[0040] R eq =2N·R s +R f

[0041] In the formula, i fc Indicates the fault current; τ represents the capacitor time constant of the improved full-bridge submodule; t represents time; I fc Indicates the magnitude of the single-phase bridge arm fault current at the initial moment of the fault; ω represents the fundamental angular frequency of the fault current; L represents the bridge arm inductance; C represents the bridge arm submodule capacitance; R eq R represents the equivalent resistance of the fault circuit. f Indicates short-circuit resistance; N indicates the number of bridge arm sub-modules; U C Indicates the capacitor voltage of the bridge arm submodule; R s This indicates the resistance value of the resistor in the improved full-bridge submodule.

[0042] In a further embodiment, the expression for the voltage value of the capacitor in the improved full-bridge submodule is:

[0043]

[0044] in,

[0045]

[0046] In the formula, t1 and t2 represent any two moments during the short-circuit fault stage; E R P represents dissipated energy. R This indicates the power of the resistor in the improved full-bridge submodule.

[0047] This invention provides a bridge arm multiplexing MMC topology and control method based on an improved full-bridge submodule. The bridge arm multiplexing MMC topology includes a parallel three-phase AC system and a DC-side voltage regulator circuit. Each phase of the three-phase AC system includes an AC power supply, an AC-side filter inductor, an upper switching circuit, a lower switching circuit, and a multiplexed bridge arm. The multiplexed bridge arm includes several improved full-bridge submodules connected in series. Compared with the prior art, the bridge arm multiplexing MMC topology proposed in this invention controls the upper and lower switching circuits to convert the multiplexed bridge arm into an upper or lower bridge arm in a time-division multiplexing manner. This significantly reduces the number of MMC submodules and the size and weight of the MMC. Simultaneously, when a DC-side fault occurs, the series resistor in the improved full-bridge submodule is used to connect the fault current path, increasing the damping of the fault loop and accelerating the clearing speed of the fault current, thereby ensuring a rapid and stable fault ride-through on the DC side of the MMC converter. Attached Figure Description

[0048] Figure 1 This is a schematic diagram of the bridge arm reuse MMC topology based on the improved full-bridge submodule provided in an embodiment of the present invention;

[0049] Figure 2(a) is a schematic diagram of the current flow direction of the improved full-bridge submodule provided in the embodiment of the present invention in the put-in state;

[0050] Figure 2(b) is a schematic diagram of the current flow direction of the improved full-bridge submodule provided in the embodiment of the present invention in the bypass state;

[0051] Figure 2(c) is a schematic diagram of the positive current flow direction of the improved full-bridge submodule provided in the embodiment of the present invention under DC side fault blocking state;

[0052] Figure 2(d) is a schematic diagram of the reverse current flow of the improved full-bridge submodule provided in the embodiment of the present invention under DC-side fault blocking state;

[0053] Figure 3 This is an equivalent circuit diagram of the bridge arm multiplexing MMC topology based on the improved full-bridge submodule when a short-circuit fault occurs on the DC side, provided in an embodiment of the present invention.

[0054] Figure 4 This is a schematic flowchart of the control method for the bridge arm reuse MMC topology based on the improved full-bridge submodule provided in this embodiment of the invention;

[0055] Figure 5(a) is a schematic diagram of the control method of the bridge arm multiplexing MMC topology based on the improved full-bridge submodule provided in the embodiment of the present invention under normal operation;

[0056] Figure 5(b) is a schematic diagram of the control method of the bridge arm multiplexing MMC topology based on the improved full-bridge submodule under fault conditions provided by the embodiment of the present invention;

[0057] Figure 6(a) is a schematic diagram of the three-phase AC voltage waveform provided in an embodiment of the present invention;

[0058] Figure 6(b) is a schematic diagram of the three-phase current waveform on the AC side provided in an embodiment of the present invention;

[0059] Figure 6(c) is a schematic diagram of the multiplexed bridge arm voltage waveform provided in an embodiment of the present invention;

[0060] Figure 6(d) is a schematic diagram of the multiplexed bridge arm current waveform provided in an embodiment of the present invention;

[0061] Figure 6(e) is a schematic diagram of the DC output voltage waveform considering DC fault conditions provided in an embodiment of the present invention;

[0062] Figure 6(f) is a schematic diagram of the DC output current waveform considering DC fault conditions provided in an embodiment of the present invention. Detailed Implementation

[0063] The embodiments of the present invention are described in detail below with reference to the accompanying drawings. The embodiments are given for illustrative purposes only and should not be construed as limiting the present invention. The accompanying drawings are for reference and illustration only and do not constitute a limitation on the scope of patent protection of the present invention, because many changes can be made to the present invention without departing from the spirit and scope of the present invention.

[0064] refer to Figure 1 This invention provides a bridge arm multiplexing MMC topology based on an improved full-bridge submodule, comprising: a parallel three-phase AC system and a DC-side voltage regulator circuit, wherein the two ends of the DC-side voltage regulator circuit are respectively connected to a port to form a DC-side port; the three-phase AC system includes an a-phase AC circuit, a b-phase AC circuit, and a c-phase AC circuit, each phase AC circuit including an AC power supply V. x (x=a,b,c), AC side filter inductor L f The upper switch circuit 11, the lower switch circuit 12, and the multiplexed bridge arm 13, in this embodiment, x = a, b, c.

[0065] In one embodiment, the DC-side voltage regulator circuit includes a first voltage regulator capacitor C1 and a second voltage regulator capacitor C2. The positive terminal of the first voltage regulator capacitor C1 is connected to the positive terminal of the DC port, the negative terminal of the first voltage regulator capacitor C1 is connected to the positive terminal of the second voltage regulator capacitor C2, and the negative terminal of the second voltage regulator capacitor C2 is connected to the negative terminal of the DC port.

[0066] In one embodiment, the AC-side filter inductor L fOne end of the AC side filter inductor L is connected to the common terminal of the upper switch circuit 11 and the lower switch circuit 12 connected in series. f The other end is connected to the AC power supply V x Connect to the output port.

[0067] In one embodiment, the upper switching circuit and the lower switching circuit in each phase AC system are connected in series, and the upper switching circuit 11 includes a first switching device S. x1 Second switching device S x2 and the first inductor L x1 First switching device S x1 The collector of the first switching device S is connected to the positive terminal of the DC port. x1 The emitter and the second switching device S x2 collector connection, second switching device S x2 The emitter and the first inductor L x1 One end is connected; the lower switching circuit 12 includes a third switching device S. x3 Fourth switching device S x4 Second inductor L x2 The third switching device S x3 The collector and the second inductor L x2 One end is connected to the third switching device S x3 The emitter and the fourth switching device S x4 collector connection, fourth switching device S x4 The emitter is connected to the negative terminal of the DC port. In this embodiment, the first switching device S in the upper switching circuit... x1 The third switching device S in the lower switching circuit x3 Simultaneously on and simultaneously off; the second switching device S in the upper switching circuit... x2 The fourth switching device S in the lower switching circuit x4 Simultaneously on and simultaneously off; the first switching device S x1 and the second switching device S x2 Complementary conduction, the third switching device S x3 and the fourth switching device S x4 Complementary conduction, specifically:

[0068] When the AC side voltage of the three-phase AC circuit enters the positive half-cycle, the first switching device S x1 and the third switching device S x3 Simultaneously, the second switching device S is turned on. x2 and the fourth switching device S x4 Disconnect simultaneously;

[0069] When the AC side voltage of the three-phase AC circuit enters the negative half-cycle, the first switching device S x1 and the third switching device S x3 Simultaneously disconnected, the second switching device S x2 and the fourth switching device S x4 Simultaneous conduction.

[0070] In one embodiment, one end of the multiplexing bridge arm 13 is connected to the first switching device S. x1 and the second switching device S x2 The common terminal, the other end of the multiplexed bridge arm 13 is connected to the third switching device S x3 and the fourth switching device S x4 The multiplexed bridge arm 13 comprises several series-connected improved full-bridge submodules. Adjacent improved full-bridge submodules are connected by connecting their negative output ports to their positive output ports. In this embodiment, the improved full-bridge submodule includes a first switch T1, a second switch T2, a third switch T4, a first diode D1, a second diode D2, a third diode D3, a fourth diode D4, a capacitor C, and a resistor R. s It should be noted that, compared to the traditional full-bridge submodule structure, the improved full-bridge submodule provided in this embodiment of the invention omits the IGBT (T3) and adds a resistor R in series. s This enables bidirectional power flow in the bridge arm reuse MMC topology, reducing the number of improved full-bridge submodules and their size and weight.

[0071] In this configuration, the first switch T1 is connected in anti-parallel with the first diode D1, the second switch T2 is connected in anti-parallel with the second diode D2, and the third switch T4 is connected in anti-parallel with the fourth diode D4. The emitter of the first switch T1 and the collector of the second switch T2 are connected in series and then in parallel with a capacitor C. One end of the capacitor C is connected to the collector of the first switch T1, and the other end of the capacitor C is connected to the emitter of the second switch T2. The collector of the first switch T1 is connected to a resistor R. s The cathode of the third diode D3 is connected to the cathode of the third diode D3, and the anode of the third diode D3 is connected to the collector of the third switch T4; the positive output port of the improved full-bridge submodule is the common terminal connection point of the first diode D1 and the second switch T2, and the negative output port of the improved full-bridge submodule is the common terminal connection point of the third diode D3 and the third switch T4.

[0072] In this embodiment, the first switch T1 and the third switch T4 are used to jointly control the improved full-bridge submodule to switch to the active state; the second switch T2 and the third switch T4 are used to jointly control the improved full-bridge submodule to switch to the bypass state, specifically, as shown in Figures 2(a), 2(b), 2(c), and 2(d), u SM For the improved full-bridge submodule port voltage, the positive voltage direction is specified as positive at the top and negative at the bottom; i SM To ensure the port current of the improved full-bridge submodule, the current flowing into the submodule is specified to be positive. When the first switch T1 and the third switch T4 of the improved full-bridge submodule are simultaneously turned on, the improved full-bridge submodule is in the ON state; when the second switch T2 and the third switch T4 are simultaneously turned on, the improved full-bridge submodule is in the bypass state; when a fault occurs on the DC side, such as... Figure 3 As shown, D is the equivalent diode of the fault circuit, which is composed of multiple submodule diodes connected in series; since the equivalent capacitance of the fault circuit is only 1 / N of the capacitance of the submodule, therefore... Figure 3 The symbol marked C / N indicates that the voltage across it is NU. C The equivalent inductance of the fault circuit is twice the inductance of the bridge arm, therefore in Figure 3 The value is marked as 2L; the fault circuit resistance is NR. s When a fault occurs on the DC side, the IGBT switching transistors in the submodule are turned off, and the improved full-bridge submodule in the MMC converter multiplex arm is in a locked state. The operating modes of the improved full-bridge submodule are shown in Table 1.

[0073] Table 1

[0074]

[0075]

[0076] When a short-circuit fault occurs on the DC side, the first switch T1, the second switch T2, and the third switch T4 are turned off. At this time, the improved full-bridge submodule switches to a locked state, and the resistor is connected to the fault current path, increasing the damping of the short-circuit fault loop, causing the fault current to drop rapidly. The fault current then charges the capacitor of the improved full-bridge submodule. The expression for the fault current is:

[0077]

[0078] in,

[0079]

[0080]

[0081] R eq =2N·Rs +R f

[0082] In the formula, i fc Indicates the fault current; τ represents the capacitor time constant of the improved full-bridge submodule; t represents time; I fc Indicates the magnitude of the single-phase bridge arm fault current at the initial moment of the fault; ω represents the fundamental angular frequency of the fault current; L represents the bridge arm inductance; C represents the bridge arm submodule capacitance; R eq R represents the equivalent resistance of the fault circuit. f Indicates short-circuit resistance; N indicates the number of bridge arm sub-modules; U C Indicates the capacitor voltage of the bridge arm submodule; R s This indicates the resistance value of the resistor in the improved full-bridge submodule.

[0083] In this embodiment, the multiplexed bridge arm is composed of an improved full-bridge submodule connected in series. The improved full-bridge submodule reduces the number of switching devices used and increases the number of individual resistors R. s When a DC-side short-circuit fault occurs in the DC transmission system, the improved full-bridge submodule in the MMC converter multiplex arm enters a locked state. At this time, the resistor R connected in series in the submodule... s Integrating the fault current path increases the damping of the short-circuit fault loop, which helps to accelerate the clearing of the fault current and causes it to decrease rapidly. Furthermore, during the fault, the improved full-bridge submodule series resistor R... s It can consume some of the system's surplus energy, effectively reducing the rise amplitude of the submodule capacitor voltage, that is, effectively reducing the amplitude of the submodule capacitor overvoltage, and ensuring the smooth and rapid fault ride-through capability of the DC transmission system; at the same time, this embodiment adopts a three-phase three-bridge arm circuit structure, which reduces the number of MMC converter submodules by reusing the bridge arms, thereby further reducing the use of switching devices and passive components, and thus effectively reducing the construction cost and size and weight of the MMC converter.

[0084] This invention provides a bridge arm multiplexing MMC topology based on an improved full-bridge submodule, comprising a parallel three-phase AC system and a DC-side voltage regulator circuit. Each phase of the three-phase AC system includes an AC power supply, an AC-side filter inductor, an upper switch circuit, a lower switch circuit, and a multiplexed bridge arm. The multiplexed bridge arm includes several improved full-bridge submodules connected in series. Adjacent improved full-bridge submodules are connected by connecting their negative output ports to their positive output ports. Compared with existing technologies, the bridge arm multiplexing MMC topology proposed in this embodiment can realize bidirectional power flow. This embodiment significantly reduces the number of MMC submodules used by using time-sharing multiplexing of bridge arms, thereby further reducing the size and weight of the MMC. At the same time, when a fault occurs on the DC side, the improved full-bridge submodule is controlled to enter a locked state, and the series resistor in the improved full-bridge submodule is connected to the fault current path, which increases the damping of the fault loop, accelerates the clearing speed of the fault current, ensures that the DC side fault of the MMC converter can be quickly and smoothly overcome, improves the stability of the system, and has good economy and practicality.

[0085] In another embodiment, such as Figure 4 As shown, this embodiment of the invention provides a control method for a bridge arm multiplexing MMC topology based on an improved full-bridge submodule. Applying the aforementioned bridge arm multiplexing MMC topology based on an improved full-bridge submodule, the control method includes the following steps:

[0086] S1. When the AC side voltage of the three-phase AC circuit enters the positive half-cycle, control the first and third switching devices to turn on simultaneously, and control the second and fourth switching devices to turn off simultaneously.

[0087] S2. When the AC side voltage of the three-phase AC circuit enters the negative half-cycle, control the first and third switching devices to disconnect simultaneously, and control the second and fourth switching devices to turn on simultaneously.

[0088] As shown in Figures 5(a) and 5(b), during the positive half-cycle of the AC side voltage in the three-phase AC circuit, i.e. when the first switching device S... x1 and the third switching device S x3 When the circuit is on, one end of the multiplexed bridge arm is connected to the positive terminal of the DC port, and the other end of the multiplexed bridge arm is connected to the AC port. The expression for the modulated voltage of the multiplexed bridge arm is:

[0089]

[0090] During the negative half-cycle of the AC side voltage in the three-phase AC circuit, that is, when the second switching device S... x2 and the fourth switching device S x4When the circuit is on, one end of the multiplexed bridge arm is connected to the AC port, and the other end is connected to the negative terminal of the DC port. The expression for the modulated voltage of the multiplexed bridge arm is:

[0091]

[0092] In the formula, v Fx Indicates the multiplexed bridge arm modulation voltage; V dc Indicates the voltage at the DC port; v x This represents the AC side voltage of a three-phase AC circuit.

[0093] When a short-circuit fault occurs on the DC side, the first switch T1, the second switch T2, and the third switch T4 are turned off, causing the improved full-bridge submodule to switch to a latched state. The resistor R... s A fault current path is connected, and the capacitor of the improved full-bridge submodule is charged through the fault current. The expression for the fault current is:

[0094]

[0095] in,

[0096]

[0097]

[0098] R eq =2N·R s +R f

[0099] In the formula, i fc Indicates the fault current; τ represents the capacitor time constant of the improved full-bridge submodule; t represents time; I fc Indicates the magnitude of the single-phase bridge arm fault current at the initial moment of the fault; ω represents the fundamental angular frequency of the fault current; L represents the bridge arm inductance; C represents the bridge arm submodule capacitance; R eq R represents the equivalent resistance of the fault circuit. f Indicates short-circuit resistance; N indicates the number of bridge arm sub-modules; U C Indicates the capacitor voltage of the bridge arm submodule; R s This indicates the resistance value of the resistor in the improved full-bridge submodule.

[0100] According to the law of conservation of energy, the change in the voltage value of the capacitor in the improved full-bridge submodule can be expressed as follows:

[0101]

[0102] in,

[0103]

[0104] In the formula, t1 and t2 represent any two moments during the short-circuit fault stage; E R P represents dissipated energy; R This indicates the power of the resistor in the improved full-bridge submodule.

[0105] Specifically, when the three-phase AC side voltage v is detected x When entering the positive half-cycle, control the first switching device S to turn on. x1 and the third switching device S x3 Simultaneously control the shutdown of the second switching device S x2 and the fourth switching device S x4 When the three-phase AC side voltage v is detected x When entering the negative half-cycle, control the second switching device S to turn on. x2 and the fourth switching device S x4 Simultaneously, control the shutdown of the first switching device S x1 and the third switching device S x3 When a short-circuit fault is detected on the DC side, the improved full-bridge submodule enters a latched state. At this time, the first diode T1, the second switch T2, and the third switch T4 remain off. The MMC resumes normal operation after the fault is cleared. Simultaneously, when a short-circuit fault is detected on the DC side, the resistor R in the submodule... s By connecting to the fault current path, the damping of the short-circuit fault loop is increased, enabling the fault current to decrease rapidly. During the fault, the improved full-bridge submodule series resistor R... s It can consume some of the system's surplus energy, effectively reducing the rise in voltage of the submodule capacitor.

[0106] To verify the effectiveness of the bridge arm multiplexing MMC topology and its control method proposed in this embodiment, simulation verification was performed on the bridge arm multiplexing MMC topology and its control method based on the improved full-bridge submodule. Table 2 shows the simulation parameter settings, as shown below:

[0107] Table 2

[0108]

[0109]

[0110] Based on the simulation parameters shown in Table 2, the bridge arm multiplexing MMC topology and its control method based on the improved full-bridge submodule were simulated and verified. The simulation results are as follows: Figure 6(a) , 6(b)As shown in Figures 6(c), 6(d), 6(e), and 6(f), the simulation results demonstrate that the three-phase, three-arm bridge arm multiplexing method can effectively realize the traditional MMC converter function. Under DC short-circuit fault conditions, the improved full-bridge submodule is in a locked state, and the short-circuit current can quickly drop to 0 through the series resistance of the improved submodule, enabling rapid and stable passage through DC short-circuit faults. The simulation results verify the effectiveness and feasibility of the bridge arm multiplexing MMC topology and its control method based on the improved full-bridge submodule.

[0111] It should be noted that the sequence number of each process does not imply the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0112] For specific limitations regarding the control method of a bridge arm multiplexing MMC topology based on an improved full-bridge submodule, please refer to the above-described limitations of a bridge arm multiplexing MMC topology based on an improved full-bridge submodule, which will not be repeated here. Those skilled in the art will recognize that the various modules and steps described in conjunction with the embodiments disclosed in this application can be implemented in hardware, software, or a combination of both. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of this application.

[0113] This invention provides a control method for a bridge arm multiplexing MMC topology based on an improved full-bridge submodule. The control method controls the upper and lower switching circuits to time-division multiplex the multiplexed bridge arm to either the upper or lower bridge arm. This time-division multiplexing not only effectively reduces the construction cost and weight of the MMC converter but also significantly reduces the number of switching devices used. Furthermore, by adding a single resistor to the improved full-bridge submodule, a fault current path is connected through the added series resistor in the DC-side fault. This not only increases the damping of the fault loop and accelerates the clearing speed of the fault current but also effectively reduces the amplitude of the submodule capacitor overvoltage. This achieves the goal of rapid and stable fault ride-through on the DC side of the MMC converter, significantly improving system stability. The method has advantages such as simple control logic, low system hardware cost, and good system stability.

[0114] The embodiments described above are merely preferred embodiments of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various improvements and substitutions without departing from the technical principles of this invention, and these improvements and substitutions should also be considered within the scope of protection of this application. Therefore, the scope of protection of this patent application should be determined by the scope of the claims.

Claims

1. A bridge arm multiplexing MMC topology based on an improved full-bridge submodule, characterized in that, include: A parallel three-phase AC system and a DC-side voltage regulator circuit are provided, with each end of the DC-side voltage regulator circuit connected to a port to form a DC-side port. The three-phase AC system includes an a-phase AC circuit, a b-phase AC circuit, and a c-phase AC circuit. Each phase AC circuit includes an AC power supply, an AC-side filter inductor, an upper switch circuit, a lower switch circuit, and a multiplexed bridge arm. The multiplexed bridge arm includes several improved full-bridge sub-modules connected in series. Adjacent improved full-bridge sub-modules are connected by connecting their negative output port to their positive output port. The improved full-bridge submodule includes a first switch, a second switch, a third switch, a first diode, a second diode, a third diode, a fourth diode, a capacitor, and a resistor. The first switch and the first diode are connected in anti-parallel, the second switch and the second diode are connected in anti-parallel, and the third switch and the fourth diode are connected in anti-parallel. The emitter of the first switch and the collector of the second switch are connected in series and then in parallel with the capacitor. One end of the capacitor is connected to the collector of the first switch, and the other end is connected to the emitter of the second switch. The collector of the first switch is connected to the cathode of the third diode through the resistor, and the anode of the third diode is connected to the collector of the third switch. The positive output port of the improved full-bridge submodule is the common terminal connection point of the first diode and the second switch, and the negative output port of the improved full-bridge submodule is the common terminal connection point of the third diode and the third switch; wherein, the first switch and the third switch are used to jointly control the improved full-bridge submodule to switch to the active state; the second switch and the third switch are used to jointly control the improved full-bridge submodule to switch to the bypass state.

2. The bridge arm multiplexing MMC topology based on an improved full-bridge submodule as described in claim 1, characterized in that, When a short-circuit fault occurs on the DC side, the improved full-bridge submodule switches to a locked state, and the resistor is connected to the fault current path to charge the capacitor of the improved full-bridge submodule through the fault current. The expression for the fault current is: in, In the formula, Indicates fault current; The capacitor time constant of the improved full-bridge submodule is represented by t; t represents time. This indicates the magnitude of the fault current in a single-phase bridge arm at the initial moment of the fault. Indicates the fundamental angular frequency of the fault current; L represents the inductance value of the bridge arm; C represents the capacitance value of the bridge arm submodule; Indicates the equivalent resistance of the faulty circuit; Indicates the short-circuit resistance; N represents the number of bridge arm sub-modules; Indicates the capacitor voltage of the bridge arm submodule; This indicates the resistance value of the resistor in the improved full-bridge submodule.

3. The bridge arm multiplexing MMC topology based on an improved full-bridge submodule as described in claim 1, characterized in that: The upper switching circuit includes a first switching device, a second switching device, and a first inductor; the lower switching circuit includes a third switching device, a fourth switching device, and a second inductor. The collector of the first switching device is connected to the positive terminal of the DC port, the emitter of the first switching device is connected to the collector of the second switching device, and the emitter of the second switching device is connected to one end of the first inductor. The collector of the third switching device is connected to one end of the second inductor, the emitter of the third switching device is connected to the collector of the fourth switching device, and the emitter of the fourth switching device is connected to the negative terminal of the DC port.

4. The bridge arm multiplexing MMC topology based on an improved full-bridge submodule as described in claim 3, characterized in that, The first and second switching devices are complementaryly connected, and the third and fourth switching devices are complementaryly connected, specifically as follows: When the AC side voltage of the three-phase AC circuit enters the positive half-cycle, the first and third switching devices are turned on simultaneously, and the second and fourth switching devices are turned off simultaneously. When the AC side voltage of the three-phase AC circuit enters the negative half-cycle, the first and third switching devices are simultaneously disconnected, while the second and fourth switching devices are simultaneously turned on.

5. The bridge arm multiplexing MMC topology based on an improved full-bridge submodule as described in claim 1, characterized in that: One end of the AC-side filter inductor is connected to the common terminal of the upper and lower switching circuits connected in series, and the other end of the AC-side filter inductor is connected to the output port of the AC power supply. The DC-side voltage regulator circuit includes a first voltage regulator capacitor and a second voltage regulator capacitor. The positive terminal of the first voltage regulator capacitor is connected to the positive terminal of the DC port, the negative terminal of the first voltage regulator capacitor is connected to the positive terminal of the second voltage regulator capacitor, and the negative terminal of the second voltage regulator capacitor is connected to the negative terminal of the DC port.

6. A control method for a bridge arm multiplexing MMC topology based on an improved full-bridge submodule, characterized in that, Applying the bridge arm multiplexing MMC topology based on any one of claims 1 to 5, the control method includes the following steps: When the AC side voltage of the three-phase AC circuit enters the positive half-cycle, the first and third switching devices are controlled to turn on simultaneously, and the second and fourth switching devices are controlled to turn off simultaneously. When the AC side voltage of the three-phase AC circuit enters the negative half-cycle, the first and third switching devices are simultaneously disconnected, and the second and fourth switching devices are simultaneously turned on.

7. The control method for a bridge arm multiplexing MMC topology based on an improved full-bridge submodule as described in claim 6, characterized in that, During the positive half-cycle of the AC side voltage in a three-phase AC circuit, one end of the multiplexed bridge arm is connected to the positive terminal of the DC port, and the other end of the multiplexed bridge arm is connected to the AC port. The expression for the modulated voltage of the multiplexed bridge arm is: During the negative half-cycle of the AC side voltage in a three-phase AC circuit, one end of the multiplexed bridge arm is connected to the AC port, and the other end of the multiplexed bridge arm is connected to the negative terminal of the DC port. The expression for the modulated voltage of the multiplexed bridge arm is: In the formula, Indicates the multiplexed bridge arm modulation voltage; Indicates the voltage at the DC port; This represents the AC side voltage of a three-phase AC circuit.

8. The control method for a bridge arm multiplexing MMC topology based on an improved full-bridge submodule as described in claim 7, characterized in that, When a short-circuit fault occurs on the DC side, the first, second, and third switches are turned off to switch the improved full-bridge submodule to a locked state. The resistor is connected to the fault current path, and the capacitor of the improved full-bridge submodule is charged through the fault current. The expression for the fault current is: in, In the formula, Indicates fault current; The capacitor time constant of the improved full-bridge submodule is represented by t; t represents time. This indicates the magnitude of the fault current in a single-phase bridge arm at the initial moment of the fault. Indicates the fundamental angular frequency of the fault current; L represents the inductance value of the bridge arm; C represents the capacitance value of the bridge arm submodule; Indicates the equivalent resistance of the faulty circuit; Indicates the short-circuit resistance; N represents the number of bridge arm sub-modules; Indicates the capacitor voltage of the bridge arm submodule; This indicates the resistance value of the resistor in the improved full-bridge submodule.

9. The control method for a bridge arm multiplexing MMC topology based on an improved full-bridge submodule as described in claim 8, characterized in that, The expression for the voltage value of the capacitor in the improved full-bridge submodule is: in, In the formula, , Represents any two moments during the short-circuit fault phase; This represents dissipated energy; This indicates the power of the resistor in the improved full-bridge submodule.

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