Hybrid MMC based on Si / SiC intensive redundancy module double-end sharing and operation strategy

Abstract

The invention provides a hybrid MMC based on Si / SiC intensive redundancy module double-end sharing and an operation strategy. The hybrid MMC comprises a three-phase circuit topology, an AC side output port, a first DC side energy storage capacitor and a second DC side energy storage capacitor. The three-phase circuit topology comprises three same single-phase topologies, and each single-phase topology comprises an upper bridge arm, a filter inductor, a multiplexing redundancy sub-module, a lower bridge arm and a filter inductor; the multiplexing redundancy sub-module comprises a first switch tube, a second switch tube, a third switch tube, a fourth switch tube, a fifth switch tube, a sixth switch tube and a direct current capacitor. By introducing the multiplexing redundancy sub-module, the upper bridge arm and the lower bridge arm are respectively subjected to redundancy configuration, so that the configuration number of the redundancy sub-module is reduced, the configuration of devices such as a radiator and a capacitor is also reduced, and the configuration cost of the MMC can be effectively optimized.

Description

Technical Field

[0001] The present application relates to the technical field of power systems, and in particular to the field of multi-level converter control. Background Art

[0002] In recent years, a certain region is vigorously developing clean energy such as photovoltaics and wind power to build a new power system, which puts higher requirements on power conversion devices. Modular multilevel converter (MMC) is widely used in distributed power grid connection, DC transmission, power quality management and other scenarios due to its high degree of modularity, good output waveform quality, and strong scalability. Compared with traditional two-level converters, MMC adopts a sub-module series structure, which increases the flexibility of output level and control, but requires more power devices, capacitors and heat dissipation equipment, resulting in an increase in MMC cost and failure risk. In the medium-voltage flexible power distribution scenario, due to the increase in land costs and grid construction density, higher requirements are placed on the operating efficiency, configuration cost and overall reliability of the medium-voltage MMC.

[0003] The power switching devices in traditional MMC mainly use silicon (Si)-based insulated-gate bipolar transistors (IGBT). Due to the physical limitations of Si-based devices, it is difficult to optimize the operating efficiency of the device. The third-generation wide bandgap semiconductor devices represented by silicon carbide semiconductor field-effect transistors (SiC MOSFET, Metal-Oxide Semiconductor Field-Effect Transistor,) have the advantages of high switching frequency and low switching loss. Applying them to medium-voltage MMC can significantly improve the operating efficiency of the device. However, the price of SiC MOSFET devices is 5 to 10 times that of Si IGBT devices of the same specification, and there are too many power switching devices in MMC. The large-scale use of SiCMOSFET to replace Si IGBT will increase the cost of the device.

[0004] In addition, the large number of sub-modules used in MMC will lead to a decrease in the overall reliability of the device, and the failure of any sub-module may cause the system to operate abnormally. Configuring redundant sub-modules is the most common method to improve the reliability of MMC. When an MMC sub-module fails, the faulty sub-module is removed and put into operation, and the redundant sub-module replaces the faulty sub-module and puts it into normal operation, which can effectively improve the reliability of MMC operation. However, traditional redundant sub-modules have problems such as single function, low utilization, and high configuration cost, and cannot balance the comprehensive requirements of MMC for reliability, operating efficiency, and device cost. Summary of the invention

[0005] To overcome the above technical deficiencies, the present application provides a hybrid MMC based on double - end sharing of Si / SiC intensive redundant modules and an operation strategy. To achieve the above object, the present application is implemented according to the following technical solutions: In the first aspect, the present application provides a hybrid MMC based on double - end sharing of Si / SiC intensive redundant modules, including: a three - phase circuit topology, an AC - side output port , a first DC - side energy - storage capacitor , and a second DC - side energy - storage capacitor ; The three - phase circuit topology includes three identical single - phase topologies. The single - phase topology includes an upper - arm bridge ARMp, an upper - arm filter inductor , a multiplexed redundant sub - module MRSM, a lower - arm bridge ARMn, and a lower - arm filter inductor ; The multiplexed redundant sub - module MRSM includes a first switching transistor , a second switching transistor , a third switching transistor , a fourth switching transistor , a fifth switching transistor , and a sixth switching transistor , as well as a DC capacitor . The multiplexed redundant sub - module MRSM is also provided with three external connection ports, which are respectively , , ports; The collector of the first switching transistor , the collector of the third switching transistor , and the drain of the fifth switching transistor are all connected to the positive pole of the DC capacitor . The emitter of the second switching transistor , the emitter of the fourth switching transistor , and the source of the sixth switching transistor are all connected to the negative pole of the DC capacitor . The emitter of the first switching transistor , the collector of the second switching transistor are both connected to the port ; The emitter of the third switching transistor , the collector of the fourth switching transistor are connected to the port . The source of the fifth switching transistor , the drain of the sixth switching transistor are both connected to the port ; The The port is connected to one end of the upper-bridge arm filtering inductor , and the port is connected to one end of the lower-bridge arm filtering inductor , and the port is connected to the AC-side output port ; Both the upper-bridge arm ARMp and the lower-bridge arm ARMn include multiple half-bridge sub-modules , and the multiple half-bridge sub-modules are connected in series. One end of the last half-bridge sub-module in the upper-bridge arm ARMp is connected to the other end of the upper-bridge arm filtering inductor , and one end of the first half-bridge sub-module in the lower-bridge arm ARMn is connected to the other end of the lower-bridge arm filtering inductor ; One end of the first half-bridge sub-module in the upper-bridge arm ARMp is connected to the positive electrode of the first DC-side energy storage capacitor ; The last half-bridge sub-module in the lower-bridge arm ARMn is connected to the negative electrode of the second DC-side energy storage capacitor , and the negative electrode of the first DC-side energy storage capacitor and the positive electrode of the second DC-side energy storage capacitor are both connected to the ground terminal.

[0006] Optionally, the first switching tube , the second switching tube , the third switching tube , the fourth switching tube are all Si IGBT tubes, and the fifth switching tube , the sixth switching tube are all SiC MOSFET tubes.

[0007] In a second aspect, the present application provides an operation strategy for a modular multilevel converter. For a hybrid MMC based on Si / SiC integrated redundant module dual-end sharing according to the first aspect, the operation strategy of the fault handling method includes a normal operation handling strategy, a first fault handling strategy, and a second fault handling strategy. Among them, the normal operation handling strategy includes: Calculating the number of levels of the stepped wave output by the modular multilevel converter; Judging whether the number of levels changes; If so, collecting the DC-side capacitor voltage values of the multiplexed redundant sub-modules, and the average DC-side current and voltage values of the upper and lower bridge arms; Judging whether the DC-side capacitor voltage value is less than the average DC-side capacitor voltage value; If so, judging whether the direction of the system circulating current is greater than zero; If so, the multiplexing redundant sub-module operates in the first working mode; If the system circulating current direction is less than or equal to zero, the multiplexing redundant sub-module operates in the second working mode.

[0008] Optionally, when the judgment result that the DC-side capacitor voltage value is greater than the average DC-side current voltage is negative, it includes: Judging whether the system circulating current direction is greater than zero; If so, the multiplexing redundant sub-module operates in the second working mode; If not, the multiplexing redundant sub-module operates in the first working mode; Optionally, the first fault handling strategy includes: When a sub-module fails in the upper and lower bridge arms; Determine the fault occurrence location and determine the current first number of sub-modules put into operation in the bridge arm corresponding to the fault occurrence location; Judge whether the current first number of sub-modules put into operation is equal to the initial configuration number; If so, update the current first number of sub-modules based on the initial configuration number, and operate the multiplexing redundant sub-module in the first working mode; If not, the multiplexing redundant sub-module operates according to the normal operation handling strategy.

[0009] Optionally, the second fault handling strategy includes: When one sub-module fails in each of the upper and lower bridge arms; Determine the current second number of sub-modules put into operation in the upper bridge arm; Judge whether the current second number of sub-modules put into operation is equal to the initial configuration number; If so, determine the current third number of sub-modules put into operation in the lower bridge arm; Judge whether the current third number of sub-modules put into operation is equal to the initial configuration number; If so, update the current second number and the current third number simultaneously based on the initial configuration number, and operate the multiplexing redundant sub-module in the first working mode.

[0010] Optionally, when the judgment result that the current second number of sub-modules put into operation is equal to the initial configuration number is negative, it includes: Determine the current third number of sub-modules put into operation in the lower bridge arm; Judge whether the current third number of sub-modules put into operation is equal to the initial configuration number; If so, update the current third number of modules put into operation based on the initial configured number, and operate the multiplexing redundant sub-module in the first working mode; If not, operate the multiplexing redundant sub-module according to the normal operation processing strategy.

[0011] Optionally, when the judgment result of determining whether the current third number of modules put into operation is equal to the initial configured number is negative, it includes: Update the current second number of modules put into operation based on the initial configured number, and operate the multiplexing redundant sub-module in the first working mode.

[0012] The present application has the following beneficial effects: The MRSM proposed in the present application, as a redundant sub-module, can also cooperate with the upper and lower arm SMs to output a high-frequency and low-frequency superimposed voltage during the normal operation of the MMC, improving the utilization rate of the redundant sub-module. In addition, the reliability improvement method proposed in the present invention does not require redundant configuration for the upper and lower arms respectively. While reducing the number of configured redundant sub-modules, it also reduces the configuration of devices such as radiators and capacitors, and can effectively optimize the configuration cost of the MMC.

[0013] In addition to the objectives, features, and advantages described above, the present application has other objectives, features, and advantages. The following will refer to the accompanying drawings for a further detailed description of the present application. Description of the Drawings

[0014] The drawings forming a part of the present application are used to provide a further understanding of the present application. The schematic embodiments of the present application and their descriptions are used to explain the present application and do not constitute an improper limitation to the present application. In the drawings: Figure 1 is a schematic diagram of the existing SiC-based device and Si-based device hybrid MMC topology structure of the present application; Figure 2 is a schematic diagram of a hybrid MMC topology structure based on multi-terminal sharing of intensive redundant modules provided by an embodiment of the present application; Figure 3 is a schematic diagram of the equivalent circuit of the multiplexing redundant sub-module and the upper and lower arms provided by an embodiment of the present application; Figure 4 is a schematic diagram of the process of mode switching and voltage stabilization strategy when the multiplexing redundant sub-module is in the normal operation processing strategy provided by an embodiment of the present application; Figure 5 is a schematic diagram of the process of the first fault handling strategy of the multiplexing redundant sub-module provided by an embodiment of the present application; Figure 6 is a schematic diagram of the process of the second fault handling strategy of the multiplexing redundant sub-module provided by an embodiment of the present application; Figure 7 It is a schematic diagram of the MMC voltage waveform when the redundant sub-module is in the normal operation processing strategy during the experimental simulation analysis of the embodiment of the present application; Figure 7 (a) is the output voltage of the upper arm schematic diagram of the waveform, Figure 7 and (b) is the output voltage of the upper arm schematic diagram of the waveform, Figure 7 and (c) is the output voltage of phase A schematic diagram of the waveform, Figure 7 and (d) is the output voltage of the MRSM on the upper arm schematic diagram of the waveform, Figure 7 and (e) is the voltage waveform of the DC-side capacitor of the upper arm schematic diagram of the waveform, Figure 7 and (f) is the voltage waveform of the DC-side capacitor of the MRSM schematic diagram of the waveform; Figure 8 It is a schematic diagram of the MMC voltage waveform when the redundant sub-module is in the first fault processing strategy during the experimental simulation analysis of the embodiment of the present application; Figure 8 (a) is the output voltage of the upper arm schematic diagram of the waveform, Figure 8 and (b) is the output voltage of the upper arm schematic diagram of the waveform, Figure 8 and (c) is the output voltage of phase A schematic diagram of the waveform, Figure 8 and (d) is the output voltage of the MRSM on the upper arm schematic diagram of the waveform, Figure 8 and (e) is the voltage waveform of the DC-side capacitor of the upper arm schematic diagram of the waveform, Figure 8 and (f) is the voltage waveform of the DC-side capacitor of the MRSM schematic diagram of the waveform; Figure 9 It is a schematic diagram of the MMC voltage waveform when the redundant sub-module is in the second fault processing strategy during the experimental simulation analysis of the embodiment of the present application; Figure 9 (a) is the output voltage of the upper arm schematic diagram of the waveform, Figure 9 and (b) is the output voltage of the upper arm schematic diagram of the waveform, Figure 9 and (c) is the output voltage of phase A schematic diagram of the waveform, Figure 9 and (d) is the output voltage of the MRSM on the upper arm schematic diagram of the waveform, Figure 9 and (e) is the voltage waveform of the DC-side capacitor of the upper arm schematic diagram of the waveform, Figure 9 and (f) is the voltage waveform of the DC-side capacitor of the MRSM Waveform schematic diagram. Specific implementation manner

[0015] The embodiments of the present application will be described in detail below with reference to the accompanying drawings. However, the present application can be implemented in many different ways defined and covered by the claims.

[0016] It should be noted that the terms "first", "second", "third", etc. in the claims, the description, and the drawings of the present application are used to distinguish similar objects and are not used to describe a specific order or sequence. Data used in this way can be interchanged under appropriate circumstances so that the embodiments of the present application described herein can be implemented in an order other than that shown or described herein. In addition, the terms "including", "having", and their variants are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or units does not necessarily have to be limited to those steps or units clearly listed, but may include other steps or units not clearly listed or inherent to these processes, methods, products, or devices.

[0017] In order to more vividly illustrate the problems raised in the background art, as Figure 1 shown, the existing MMC topology using a hybrid of SiC-based devices and Si-based devices. Each phase of this topology consists of symmetric upper and lower bridge arms. Compared with the traditional MMC, one of the N half-bridge sub-modules (Sub-modular, SM) in each bridge arm is replaced by two full-bridge sub-modules (FullSub-modular, FSM). The SM consists of 2 Si IGBTs, and the FSM consists of 4 SiC MOSFETs. By changing the modulation strategy, most of the switching actions of this topology are transferred from Si-based devices to SiC-based devices, which can reduce the total operating loss of the device. However, compared with the traditional MMC, this topology adds a total of 12 FSMs composed of SiC MOSFETs, greatly increasing the overall cost of the device. And when configuring redundant sub-modules subsequently, it is also necessary to configure redundancy for the SM and FSM in each bridge arm respectively, further increasing the configuration cost of the device.

[0018] To solve the above problems, as Figure 2 shown, the present application proposes a hybrid MMC based on the dual-end sharing of Si / SiC intensive redundant modules, including: Three-phase circuit topology, AC side output port 、First DC side energy storage capacitor ,Second DC side energy storage capacitor The three-phase circuit topology includes three identical single-phase topologies, namely A, B, and C. The three phases share a DC bus port, and the DC bus voltage is ; The single-phase topology includes an upper bridge arm ARMp, an upper bridge arm filter inductor , a multiplexed redundant sub-module MRSM, a lower bridge arm ARMn, and a lower bridge arm filter inductor ; The multiplexed redundant sub-module MRSM includes a first switching transistor , a second switching transistor , a third switching transistor , a fourth switching transistor , a fifth switching transistor , and a sixth switching transistor , as well as a DC capacitor . The multiplexed redundant sub-module MRSM is also provided with three external connection ports, namely , , ports; The collector of the first switching transistor , the collector of the third switching transistor , and the drain of the fifth switching transistor are all connected to the positive pole of the DC capacitor . The emitter of the second switching transistor , the emitter of the fourth switching transistor , and the source of the sixth switching transistor are all connected to the negative pole of the DC capacitor . The emitter of the first switching transistor , and the collector of the second switching transistor are both connected to port ; The emitter of the third switching transistor , the collector of the fourth switching transistor are connected to port . The source of the fifth switching transistor , and the drain of the sixth switching transistor are both connected to port ; Port is connected to one end of the upper bridge arm filter inductor , Port is connected to one end of the lower bridge arm filter inductor , Port is connected to the AC side output port u oa ; Both the upper bridge arm ARMp and the lower bridge arm ARMn include multiple half-bridge sub-modules SM aN , and the multiple half-bridge sub-modules are connected in series. One end of the last half-bridge sub-module in the upper bridge arm ARMp is connected to the upper bridge arm filter inductor At the other end, one end of the first half-bridge sub-module in the lower arm ARMn is connected to the other end of the lower-arm filter inductor ; one end of the first half-bridge sub-module in the upper arm ARMp is connected to the positive electrode of the first DC-side energy storage capacitor ; the last half-bridge sub-module in the lower arm ARMn is connected to the negative electrode of the second DC-side energy storage capacitor ; the negative electrode of the first DC-side energy storage capacitor and the positive electrode of the second DC-side energy storage capacitor are both connected to the ground wire.

[0019] It should also be noted that the first switch tube , the second switch tube , the third switch tube , the fourth switch tube are all Si IGBT tubes, and the fifth switch tube , the sixth switch tube are all SiCMOSFET tubes.

[0020] Based on the above-mentioned hybrid MMC based on Si / SiC integrated redundant module double-end sharing, the operation of the redundant sub-module of the present application is divided into three operation modes according to the fault conditions of the upper and lower arm SMs: normal operation, one SM fault in the upper or lower arm (the first fault handling strategy), and one SM fault in each of the upper arm and the lower arm (the second fault handling strategy). Next, these three operation strategies will be introduced in detail respectively.

[0021] 1) Normal operation handling strategy: Under normal operation conditions, all N (initial configured quantity) SMs in the upper and lower arms of the MMC operate normally. At this time, the MRSM can cooperate with the upper and lower arms, and by combining the nearest level modulation (NLM) and pulse width modulation (PWM), most of the switching actions are concentrated on the SiC MOSFET devices ( , ) in the MRSM, taking advantage of the high switching frequency and low switching loss of the SiC MOSFET to improve the operation efficiency of the MMC.

[0022] The equivalent circuit of the MRSM cooperating with the upper and lower arms is shown in Figure 3 . Among them, the output voltage of the upper arm is , the output voltage of the lower arm is , and the output voltage of the MRSM in the upper arm is , the output voltage of the lower bridge arm of the MRSM is .

[0023] According to the different switching states of the devices in the MRSM, its operating states are shown in Table 1. According to the positive and negative polarities of the output voltage of the MRSM, the operating states 1 and 2 can be defined as the first working mode 1, and the operating states 3 and 4 can be defined as the second working mode 2. By adjusting the operating mode of the MRSM, the stability of the DC-side capacitor voltage can be maintained.

[0024] Table 1 MRSM Operating Modes ; The upper and lower bridge arms adopt NLM. In the first working mode 1, the number of sub-modules put into operation 、 can be expressed as: (1) In the formula, is the DC bus voltage of the MMC, is the DC-side capacitor voltage of each sub-module of the MMC, is the reference voltage of the AC-side output, is the floor function.

[0025] The middle MRSM adopts PWM. In mode 1, the PWM reference voltages of the upper and lower bridge arms of the MRSM can be expressed as: (2) In the formula, are the reference values of the output voltages of the upper and lower bridge arms of the MRSM when operating in the first working mode 1, respectively.

[0026] In the second working mode 2, the number of sub-modules put into operation of the upper and lower bridge arms can be expressed as: (3) In the formula, is the ceiling function.

[0027] In the second working mode 2, the PWM reference voltages of the upper and lower bridge arms of the MRSM can be expressed as: (4) In the formula, are the reference values of the output voltages of the upper and lower bridge arms of the MRSM when operating in the second working mode 2, respectively.

[0028] According to the requirements of DC capacitive voltage regulation, the MRSM can flexibly switch between two operating modes to maintain voltage stability. The change in the DC-side capacitor voltage value of the MRSM can be jointly determined by its operating mode and the system circulating current as follows: When holds, the MRSM operates in the first operating mode 1, which can control the increase of its DC-side capacitor voltage value, and operates in the second operating mode 2, which can control the decrease of its DC-side capacitor voltage value; when holds, the MRSM operates in the second operating mode 2, which can control the increase of its DC-side capacitor voltage value, and operates in the first operating mode 1, which can control the decrease of its DC-side capacitor voltage value.

[0029] For the specific switching process, as Figure 4 shown, first calculate the number of levels of the stepped wave output by the modular multilevel converter , and the calculation formula of is as follows: (5) After calculating the number of levels of the stepped wave k , determine whether the number of levels k has changed, that is, compare it with the previous number of levels. If it has changed, collect the DC-side capacitor voltage value of the MRSM, as well as the average DC-side capacitor voltage values U AVG of the upper and lower bridge arms. Then compare the DC-side capacitor voltage value of the MRSM with the average DC-side capacitor voltage values U AVG of the upper and lower bridge arms to determine whether the DC-side capacitor voltage value is less than the average DC-side capacitor voltage value U AVG . If it is less, at this time, it is necessary to judge whether the direction of the system circulating current is greater than zero. If it is greater than zero, the MRSM operates in the first operating mode 1, and the number of sub-modules , put into operation on the upper and lower bridge arms can be calculated by formula (1). If the direction of the system circulating current is less than or equal to zero at this time, it will operate in the second operating mode 2, and the number of sub-modules put into operation on the upper and lower bridge arms can be calculated by formula (3).

[0030] If the aforementioned DC-side capacitor voltage value is greater than or equal to the average DC-side capacitor voltage value U AVG , at this time, it is also necessary to judge the direction of the system circulating current Whether it is greater than zero. If so, the MRSM operates in the second working mode 2, and the number of sub-modules in the upper and lower bridge arms that are put into operation can be calculated by formula (3). If the system circulating circuit direction is less than or equal to zero at this time, the MRSM operates in the first working mode 1, and the number of sub-modules in the upper and lower bridge arms that are put into operation , can be calculated by formula (1).

[0031] If the aforementioned DC-side capacitor voltage value is greater than or equal to the average value of the DC-side capacitor voltage U AVG , it is also necessary to determine whether the system circulating current direction icir is greater than zero at this time. If so, the MRSM operates in the second working mode 2, and the number of sub-modules in the upper and lower bridge arms that are put into operation can be calculated by formula (3). If the system circulating circuit direction is less than or equal to zero at this time, the MRSM operates in the first working mode 1, and the number of sub-modules in the upper and lower bridge arms that are put into operation , can be calculated by formula (1).

[0032] 2) The first fault handling strategy (one SM in the upper / lower bridge arm fails): When one SM in the upper bridge arm or the lower bridge arm fails, the MRSM can play a role in supporting the level for the faulty bridge arm, ensuring the stable operation of the MMC without affecting the output quality of the MMC.

[0033] At this time, it is first necessary to determine the fault location, that is, to determine the specific bridge arm where the fault occurs, and then confirm the current first number of sub-modules put into operation in the bridge arm corresponding to the fault location. Then, it is judged whether the current first number of sub-modules put into operation is equal to the initial configuration number. The initial configuration number can be understood as configuring a certain number of sub-modules for each bridge arm at the beginning considering various factors of the system. If the current first number of sub-modules put into operation is equal to the initial configuration number, the current first number of sub-modules put into operation is updated according to the initial configuration number, and the MRSM operates in the first working mode 1. If not, it means that the current first number of sub-modules put into operation is less than the initial configuration number, and the MRSM is operated according to the aforementioned normal operation handling strategy.

[0034] For a more vivid understanding of the first fault handling strategy, as Figure 5 shown, this application takes the occurrence of an upper bridge arm fault as an example to make the following detailed description: After detecting the occurrence of the fault, it is confirmed that the fault occurs in the upper bridge arm, and then the current first working number of the upper bridge arm sub-modules is judged After that, fault protection detection is carried out, that is, it is judged whether the current first working quantity of the upper-arm sub-modules is equal to the initial configured quantity. If After that, fault protection detection is carried out, that is, it is judged whether the current first working quantity of the upper-arm sub-modules is equal to the initial configured quantity. If , it is necessary to correct the number of SMs put into the upper arm, and let , and at the same time let the MRSM work in Mode 1 to ensure the balance of the DC bus voltage. The same is true for faults in the lower arm.

[0035] It should be noted that Figure 5 At the same time, a more detailed fault handling process is also shown, but the essence is the same as the handling process when a fault occurs in the aforementioned upper arm. Therefore, for the sake of brevity, other solutions will not be described in detail one by one.

[0036] 3) The second fault handling strategy (one SM in each of the upper / lower arms has a fault): When one SM in each of the upper and lower arms has a fault, the MRSM can simultaneously play a role in supporting the voltage level for the upper and lower arms to ensure the normal operation of the MMC.

[0037] After detecting that one SM in each of the upper and lower arms has a fault, additional fault protection measures need to be added. The specific handling process is as Figure 6 shown. First, determine the current second working quantity of the sub-modules in the upper arm , and then judge whether the second quantity is equal to the initial configured quantity N. If so, then immediately determine the current third working quantity of the sub-modules in the lower arm , and then judge whether the third quantity is equal to the initial configured quantity N. If it is equal, then based on the initial configured quantity N, update the current second working quantity and the current third working quantity , and let , , and run the MRSM in the first working mode 1. If at this time the current third working quantity of the sub-modules in the lower arm is not equal to the initial configured quantity N, then based on the initial configured quantity N, update the current second working quantity , that is, let , keep the current third working quantity unchanged, and run the MRSM in the first working mode 1.

[0038] If the above-mentioned current second working quantity is not equal to the initial configured quantity N , then determine the current third working quantity of the sub-modules in the lower arm , determine the third quantity of the current input work whether it is equal to the initial configured quantity N , if it is equal, based on the initial configured quantity N , update the third quantity of the current input work , that is, make , the second quantity of the current input work remain unchanged, and run the MRSM in the first working mode 1. If it is not equal, run the MRSM according to the normal operation processing strategy.

[0039] It should be noted that for the case where the current input work quantity of the sub-module mentioned above in this application is not equal to the initial configured quantity, there is only one case where the current input work quantity of the sub-module is less than the initial configured quantity. And the redundant sub-module of this application can only handle the above two fault cases. If the number of faulty sub-modules in the upper and lower bridge arms exceeds one, the redundant sub-module needs to be reconfigured, which does not belong to the specific situation targeted by this application.

[0040] Experimental simulation analysis To verify the feasibility of the operation strategy of the modular multilevel converter proposed in this application, a simulation experiment platform was built in MATLAB / Simulink software, and the simulation parameters are shown in Table 2: Table 2 Main simulation parameters ; Taking the upper bridge arm of phase A as an example, the simulation waveforms of the strategy proposed in the present invention under different operating conditions are as follows.

[0041] (1) Normal operation: When the MMC operates normally, the simulation waveform of the output voltage of phase A is as Figure 7 shown. Figure 7 (a) is the output voltage of the upper bridge arm of phase A, and the MRSM can cooperate with 8 SMs in the upper bridge arm to output a 9-level voltage with high and low frequency superposition; Figure 7 (b) is the output stepped wave voltage of the upper bridge arm of phase A, which can adjust the number of levels according to the need of voltage stabilization; Figure 7 (c) is the output voltage of phase A, and the total harmonic distortion (THD) is 1.09%; Figure 7 (d) is the output voltage of the MRSM in the upper bridge arm, concentrating most of the switching actions on the SiC MOSFET device; Figure 7 (e) is the DC side capacitor voltage of the upper bridge arm of phase A, which can be stabilized near 1250V; Figure 7 (f) is the DC side capacitor voltage of the MRSM, which can be stabilized near 1250V.

[0042] (2)One SM fault in the upper / lower arm: When there is one SM fault in the upper arm of the MMC, the simulated waveform of the A-phase output voltage is as Figure 8 shown. Figure 8 The meanings of the figures in Figure 7 are the same. After one SM fault in the upper arm is removed, the MRSM can undertake the role of supporting the output voltage level of the upper arm. While ensuring its own high-frequency PWM voltage output, it outputs a 9-level voltage together with the 7 normal-running SMs in the upper arm. After the upper-arm fault, due to the asymmetry between the upper and lower arms, the voltage fluctuations of the DC-side capacitors of the upper arm and the MRSM increase slightly. However, through the voltage-stabilizing strategy proposed in the present invention, it can still be controlled within ±5%, meeting the DC-side capacitor voltage stabilization standard.

[0043] (3)One SM fault in each of the upper and lower arms: When there is one SM fault in each of the upper and lower arms of the MMC, the simulated waveform of the A-phase output voltage is as Figure 9 shown. Figure 9 The meanings of the figures in Figure 8 are the same. After one SM fault in each of the upper and lower arms is removed, the MRSM can use the switching of the operating states shown in Table 1 to output a 9-level voltage together with the 7 normal-running SMs in the upper and lower arms. And after the fault occurs, through the voltage-stabilizing strategy proposed in the present invention, the voltage fluctuations of the DC-side capacitors of the upper arm and the MRSM can be stabilized within ±5% of the rated value, meeting the DC-side capacitor voltage stabilization standard.

[0044] In summary, for the topology structure and method proposed in this application, by configuring the multiplexing redundant sub-module (MRSM), the operating efficiency of the MMC is effectively improved; and after a sub-module (SM) fault occurs in the upper and lower arms until the fault is repaired, the MRSM can ensure the normal operation of the MMC. The MRSM adopts a construction method of mixing Si IGBT and SiC MOSFET devices. By means of the modulation method, most of the switching actions are concentrated on the SiC MOSFET, effectively utilizing its advantages of high switching frequency and low switching loss, and effectively improving the reliability of the MMC. As a redundant sub-module, the MRSM can also cooperate with the upper and lower arm SMs to output high- and low-frequency superimposed voltages during the normal operation of the MMC, improving the utilization rate of the redundant sub-module. In addition, the reliability improvement method proposed in the present invention does not require redundant configuration for the upper and lower arms respectively, reducing the number of configured redundant sub-modules and also reducing the configuration of devices such as radiators and capacitors, and can effectively optimize the configuration cost of the MMC.

[0045] The above are only the preferred embodiments of the present application and are not intended to limit the present application. For those skilled in the art, various modifications and variations can be made to the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included within the protection scope of the present application.

Claims

1. A hybrid MMC based on dual - end sharing of Si / SiC intensive redundant modules, characterized in that, Including: Three-phase circuit topology, AC-side output port , first DC-side energy storage capacitor , second DC-side energy storage capacitor ; The three-phase circuit topology includes three identical single-phase topologies. Each single-phase topology includes an upper-bridge arm ARMp, an upper-bridge arm filter inductor , a multiplexing redundant sub-module MRSM, a lower-bridge arm ARMn, and a lower-bridge arm filter inductor ; The multiplexing redundant sub-module MRSM includes a first switching transistor , a second switching transistor , a third switching transistor , a fourth switching transistor , a fifth switching transistor and a sixth switching transistor , and a DC capacitor . The multiplexing redundant sub-module MRSM is further provided with three external connection ports, namely , , ports; The collector of the first switching transistor , the collector of the third switching transistor , and the drain of the fifth switching transistor are all connected to the positive electrode of the DC capacitor . The emitter of the second switching transistor , the emitter of the fourth switching transistor , and the source of the sixth switching transistor are all connected to the negative electrode of the DC capacitor . The emitter of the first switching transistor , and the collector of the second switching transistor are both connected to the port ; The emitter of the third switching transistor , the collector of the fourth switching transistor are connected to the port . The source of the fifth switching transistor , and the drain of the sixth switching transistor are both connected to the port ; The port is connected to one end of the upper bridge arm filtering inductor . The port is connected to one end of the lower bridge arm filtering inductor . The port is connected to the AC side output port ; The upper bridge arm ARMp and the lower bridge arm ARMn both include a plurality of half-bridge sub-modules , and the plurality of half-bridge sub-modules are connected in series. One end of the last half-bridge sub-module in the upper bridge arm ARMp is connected to the other end of the upper bridge arm filter inductor . One end of the first half-bridge sub-module in the lower bridge arm ARMn is connected to the other end of the lower bridge arm filter inductor ; One end of the first half-bridge sub-module in the upper bridge arm ARMp is connected to the positive electrode of the first DC-side energy storage capacitor ; The last half-bridge sub-module in the lower bridge arm ARMn is connected to the negative electrode of the second DC-side energy storage capacitor ; The negative electrode of the first DC-side energy storage capacitor and the positive electrode of the second DC-side energy storage capacitor are both connected to the ground terminal.

2. The hybrid MMC according to claim 1, wherein The first switching transistor , the second switching transistor , the third switching transistor , the fourth switching transistor are all Si IGBT transistors, and the fifth switching transistor , the sixth switching transistor are all SiC MOSFET transistors.

3. An operating strategy for a modular multilevel converter, characterized in that For a hybrid MMC based on dual - end sharing of Si / SiC intensive redundant modules according to claim 1, the operation strategy includes a normal operation processing strategy, a first fault processing strategy, and a second fault processing strategy. Among them, the normal operation processing strategy includes: Calculating the number of levels of the stepped wave output by the modular multilevel converter; Judging whether the number of levels changes; If so, collecting the DC - side capacitor voltage value of the multiplexing redundant sub - module and the average DC - side current and voltage values of the upper and lower bridge arms; Judging whether the DC - side capacitor voltage value is less than the average DC - side capacitor voltage; If so, judging whether the system circulating current direction is greater than zero; If so, the multiplexing redundant sub - module operates in the first working mode; If the system circulating current direction is less than or equal to zero, the multiplexing redundant sub - module operates in the second working mode.

4. The strategy according to claim 3, characterized in that, When the judgment result that the DC - side capacitor voltage value is greater than the average DC - side current and voltage is negative, it includes: Judging whether the system circulating current direction is greater than zero; If so, the multiplexing redundant sub - module operates in the second working mode; If not, the multiplexing redundant sub - module operates in the first working mode.

5. The strategy according to claim 3, wherein The first fault processing strategy includes: When one sub - module in the upper and lower bridge arms fails; Determining the fault location and the current first number of sub - modules put into work in the bridge arm corresponding to the fault location; Judging whether the current first number of sub - modules put into work is equal to the initial configuration number; If so, updating the current first number of sub - modules based on the initial configuration number and operating the multiplexing redundant sub - module in the first working mode; If not, the multiplexing redundant sub - module operates according to the normal operation processing strategy.

6. The strategy according to claim 3, wherein The second fault processing strategy includes: When one sub - module in each of the upper and lower bridge arms fails; Determining the current second number of sub - modules put into work in the upper bridge arm; Judging whether the current second number of sub - modules put into work is equal to the initial configuration number; If so, determining the current third number of sub - modules put into work in the lower bridge arm; Judging whether the current third number of sub - modules put into work is equal to the initial configuration number; If so, simultaneously updating the current second number and the current third number based on the initial configuration number and operating the multiplexing redundant sub - module in the first working mode.

7. The strategy according to claim 6, wherein When the judgment result that the current second number of sub - modules put into work is equal to the initial configuration number is negative, it includes: Determining the current third number of sub - modules put into work in the lower bridge arm; Judging whether the current third number of sub - modules put into work is equal to the initial configuration number; If so, updating the current third number based on the initial configuration number and operating the multiplexing redundant sub - module in the first working mode; If not, operating the multiplexing redundant sub - module according to the normal operation processing strategy.

8. The strategy according to claim 6, characterized in that, When the judgment result that the current third number of sub - modules put into work is equal to the initial configuration number is negative, it includes: Based on the initial configured quantity, update the current second quantity put into operation, and operate the multiplexing redundant sub-module in the first working mode.

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