WO2024077657A1 - Modular multi-level converter, converter valve system and sub-module networking method - Google Patents

Abstract

A modular multi-level converter, a converter valve system and a sub-module networking method. Communication optical fibers between sub-modules in a group and valve controllers are simplified, by means of optical switches, from the original multi-receiving multi-transmitting mode to a one-receiving one-transmitting mode, such that the control and monitoring of the sub-modules can be completed using a relatively small number of optical fibers between converter valve controllers and the sub-modules, which can save on costs and reduce the difficulty in laying on the basis of ensuring control synchronization. By means of arranging the optical switches, it is also possible to bypass a faulty sub-module in a timely manner, thereby avoiding the impact on information transmission during uplink or downlink communication. Sub-modules directly forward an instruction when receiving the instruction, thereby ensuring instruction reception synchronization. The mode of uploading information of the sub-modules themselves involves time-sharing delivery in the same direction, such that serial delivery without congestion can be realized.

Description

Modular multi-level converter, converter valve system and sub-module networking method Technical Field

The present invention belongs to the field of power transmission equipment control, and in particular relates to a modular multi-level converter, a converter valve system and a sub-module networking method.

Background technique

The main problem, development direction and challenge facing the power grid now is the "double high" power grid, which means a high proportion of clean energy and a high proportion of power electronic devices. This requires two major technologies as support. One is flexible AC and DC transmission technology. This is because the access of various distributed energy sources to the power grid also requires the power grid to have better friendliness and flexibility.

Flexible DC transmission refers to high-voltage direct current transmission (HVDC) based on voltage source converter (VSC), which is a new type of direct current transmission after AC transmission and conventional direct current transmission. Its structure is similar to that of high-voltage direct current transmission. The main equipment of flexible direct current transmission includes flexible direct current converter valve, high-voltage direct current circuit breaker, converter transformer, flexible direct current transmission control and protection system and direct current reactor/smoothing reactor. The converter station is the most important part of the flexible direct current transmission system.

Modular multilevel converter (MMC) is the core equipment of flexible DC transmission. The emergence of MMC technology has greatly promoted the development of flexible DC transmission technology. Currently, the MMC projects that have been put into operation have reached the level of 1000MVA/±320kV, and 3000MVA/±500kV and 5000MVA/±800kV MMC-HVDC projects are also under construction. However, with the continuous improvement of MMC voltage and capacity levels and the continuous expansion of application fields, MMC and its DC transmission technology are also facing various new challenges. With the continuous increase in the number of levels, the number of bridge arm submodules is also increasing. Taking 840MVA/±500kV converter station A as an example, the number of its single bridge arm submodules reaches 540, resulting in a large number of optical fiber accesses between the submodule interface chassis and the converter valve system, high engineering costs, and great implementation difficulties. A low-cost solution is urgently needed.

Summary of the invention

The object of the present invention is to provide a modular multilevel converter, a converter valve system and a submodule networking method, which are used to solve the problems of high laying difficulty and high cost caused by a large number of optical fibers between the valve control system and the bridge arm submodule in the prior art.

In order to achieve the above object, the present invention provides a modular multi-level converter, including a transceiver optical fiber and a plurality of converter units;

Each commutation unit includes a submodule, a corresponding optical switch and a corresponding controller;

Multiple commutation units are divided into multiple groups. The submodules included in all commutation units in the same group are connected in series communication through transceiver optical fibers and their corresponding optical switches to form a series line. The optical switches at both ends of the series line are used to communicate with the valve control module to receive instructions issued by the valve control module or send submodule information to the valve control module.

The optical switch is used to send the instruction to the corresponding sub-module when the issued instruction is transmitted to the corresponding conversion unit; and if the optical switch is not the most downstream optical switch in the series communication line, the optical switch is also used to directly forward the instruction to the optical switch corresponding to the next sub-module when the corresponding sub-module receives the instruction.

After the modular multi-level converter connects the submodules in each group in series through an optical switch, it establishes communication with the valve control module through a pair of transceiver optical fibers, thereby replacing the original method of each submodule communicating with the valve control module through its own transceiver optical fiber. Since the distance between the submodules is relatively short, the optical fiber length required for connecting the submodules in series is very short, which is negligible compared with the optical fiber length required from the submodule to the valve control module. Therefore, the converter is equivalent to shrinking the longer communication optical fiber between the submodules in each group and the valve control from the original multiple transmissions to 1 reception and 1 transmission, and only fewer optical fibers are needed to complete the control and monitoring of the submodules. Therefore, the demand for the number of long optical fibers between the submodules and the valve control modules can be greatly reduced, thereby achieving the effect of saving costs and reducing the difficulty of laying. When each submodule receives an instruction, it directly forwards the instruction, ensuring the synchronization of the received instructions.

Furthermore, in order to achieve unblocked serial transmission, all controllers of the commutation units in the same group are used to transmit the information of the corresponding sub-modules to the valve control module through the corresponding optical switches in time-sharing manner according to the set time intervals; the time-sharing transmission mode of each controller is the same transmission.

Furthermore, in order to avoid the influence of the failed submodule on the process of continuing to forward instructions or sending data, the optical switch is also used to bypass the failed submodule when the corresponding submodule loses power.

Furthermore, the content of the issued instruction includes an instruction part, which includes control instructions corresponding to each sub-module number; the controller is also used to number the corresponding sub-modules, and after successfully verifying the issued instruction, obtain the corresponding control instruction according to the corresponding sub-module number.

Such a controller setting enables the sub-modules to uniformly accept the same instructions in the case of serial connection, and obtain the part of the instruction that matches the sub-module according to the number, without the need to issue instructions specifically, so it is more suitable for the serial connection between sub-modules.

Furthermore, to avoid receiving instructions that do not match the submodule, the content of the issued instruction also includes a frame header part and a check part, and the controller is also used to verify the frame header and the check part. If the frame header and the check part are correct, it indicates that the issued instruction is successfully verified.

Furthermore, in order to ensure that there is enough idle time when sending uplink data, the number of packets of the conversion unit is determined according to the total number of submodules and the control period: if the control period is m and the uplink communication duration of a single submodule is p, then the number of packets q = int(m/p), where the function int() is a floor function.

The present invention also provides a converter valve system, comprising a valve control module, characterized in that it also comprises the above-mentioned modular multilevel converter; the valve control module is used to issue instructions or receive submodule self information sent by the modular multilevel converter. The converter valve system can achieve the same beneficial effects as the above-mentioned modular multilevel converter.

The present invention also provides a submodule networking method, the steps are as follows:

1) A corresponding optical switch is set at each submodule to form a commutation unit; the commutation units are grouped, and each submodule in each group is connected in series communication with its corresponding optical switch through a transceiver optical fiber to form a series line; the optical switches at both ends of the series line are connected in communication with the valve control module;

2) When the instruction issued by the valve control module is transmitted to the optical switch through the transceiver optical fiber, it is sent to the corresponding sub-module through the optical switch; except for the sub-module at the most downstream of the series communication line, when other sub-modules receive the instruction, they directly forward the instruction through the corresponding optical switch; each commutation unit obtains the corresponding instruction information according to the instruction received by the sub-module;

3) When the commutation units in the same group need to send their own information to each submodule, they send their own information to the valve control module in time-sharing through the corresponding optical switch and transceiver optical fiber according to the set time interval; the time-sharing sending mode of each submodule is to send information in the same direction.

The submodule networking method uses an optical switch to reduce the communication optical fiber between the submodules and the valve control in each group from multiple transmissions to one reception and one transmission, so that the valve control and submodule of the converter valve can pass through fewer optical fibers to complete the control and monitoring of the submodule, which can save costs and reduce the difficulty of laying. When each submodule receives an instruction, it directly forwards the instruction, ensuring the synchronization of the received instructions.

Furthermore, in order to avoid the influence of a failed submodule on the process of continuing to forward instructions or sending data, if a submodule loses power, the failed submodule is bypassed through a corresponding optical switch.

Furthermore, the content of the instruction sent by the valve control module includes a frame header part, an instruction part and a verification part, and the instruction part includes control instructions corresponding to the numbers of each submodule; the commutation unit numbers the submodule corresponding to itself, and after successfully verifying the issued instruction, obtains the corresponding control instruction according to the number;

The method of obtaining the corresponding control instruction according to its own number after successfully verifying the issued instruction includes: verifying the frame header and the check part. If the frame header and the check part are correct, it indicates that the issued instruction is successfully verified, and then obtaining the corresponding control instruction according to its own number.

This setting of issuing instructions and obtaining corresponding numbers enables the sub-modules to uniformly accept the same instructions in the case of serial connection, and obtain the part of the instruction that matches the sub-module according to the number, without issuing instructions specifically. Therefore, it is more suitable for the serial connection between sub-modules and the efficiency of issuing instructions is higher.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic structural diagram of a converter valve system in an embodiment of the converter valve system of the present invention;

FIG2 is a schematic diagram of the format of a downlink data frame of a command sent by a valve control module during downlink communication in an embodiment of a converter valve system of the present invention;

3 is a schematic diagram of the format of an uplink data frame in which the controller transmits the status of the submodule itself during uplink communication in an embodiment of the converter valve system of the present invention;

FIG4 is a schematic structural diagram of bypassing a lost electronic module through an optical switch corresponding to a submodule in an embodiment of a converter valve system of the present invention;

5 is a control logic block diagram of a power failure module corresponding to a light switch bypass in downlink communication in an embodiment of a submodule networking method of the present invention;

FIG6 is a control logic block diagram of a power-off module corresponding to an optical switch bypass during uplink communication in an embodiment of a submodule networking method of the present invention.

Detailed ways

In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments.

Flow change valve system embodiment:

This embodiment provides an implementation of a converter valve system, which includes a valve control module and a modular multilevel converter. Referring to FIG. 1 , the converter valve control module is the valve control module, and the rest is the modular multilevel converter.

The modular multilevel converter includes a transceiver optical fiber and a plurality of converter units; each converter unit includes a submodule, a corresponding optical switch and a corresponding controller, as shown in FIG1 , each dotted box represents a converter unit, and the converter units corresponding to n submodules constitute a group of converter units; the plurality of converter units are divided into a plurality of groups, and the submodules included in all converter units in the same group are connected in series communication through the transceiver optical fiber and the corresponding optical switch thereof, so as to constitute a series circuit; with reference to FIG1 , the optical module is used for photoelectric conversion, converting the optical signal into an electrical signal, and is regarded as a part of the submodule, and the specific connection sequence in this embodiment is: the optical switch corresponding to the submodule 1 - the submodule 1 - the optical switch corresponding to the submodule 2 - the submodule 2 -… - the optical switch corresponding to the submodule n - the submodule n.

Among them, according to the actual project, in principle, when grouping, it should be ensured that sufficient idle space is reserved when sending data uplink (in this embodiment, sending the submodule's own status information uplink); when the commutation units corresponding to the submodules are grouped, the number q of each group is determined according to the control period m and the uplink communication period p: if the control period is m, and the uplink communication duration of a single submodule is p, then the number of groups q = int (m/p), where the function int() is a rounding function; for example, in this embodiment, the control period is 50us, and the uplink communication period is 2.2us, then the number of each group q = int (50/2.2), where 50/2.2≈22.7, then q = 22. Here, the number q of each group of commutation units is the number n of the above-mentioned submodules, that is, n = 22, and the modular multilevel converter includes 22 submodules and their corresponding commutation units.

In this embodiment, the optical switches at both ends of the series line are used to communicate with the valve control module to receive instructions issued by the valve control module or send submodule information to the valve control module; as shown in Figure 1, the optical switch corresponding to submodule 1 and the optical switch corresponding to submodule n are connected to the valve control module through an interface (the interface structure is not shown in the figure). In this embodiment, through the structure of the above-mentioned converter, 22 submodules can be connected to the valve control through only one pair of receiving and transmitting optical fibers and their corresponding optical switches, without the need to set up a receiving and transmitting optical fiber connected to the valve control for each submodule, thereby simplifying the connection method and reducing the connection cost.

In this embodiment, the valve control module is used to issue instructions or receive sub-module information sent by the modular multilevel converter; the optical switch of the modular multilevel converter is used to send the instruction to the corresponding sub-module when the issued instruction is transmitted to the corresponding conversion unit; and if the optical switch is not the optical switch at the most downstream of the series communication line, the optical switch is also used to directly forward the instruction to the optical switch corresponding to the next sub-module without delay when the corresponding sub-module receives the instruction. Since the speed of optical signal transmission is relatively fast, the time consumption of the transmission process can be ignored, and the forwarding method can ensure that the time deviation of each module receiving the instruction is only the photoelectric conversion time (<1us), thereby ensuring the synchronization of receiving instructions.

The instructions issued by the valve control module are in accordance with the frame format in Figure 2, and the instruction content includes a frame header part, an instruction part and a check part, where the instruction part includes control instructions corresponding to each submodule number. In this embodiment, the instruction content is embodied as: frame header + 22 submodule instructions (arranged in order) + check, the valve control system sends the instruction data in this format, and communicates with the modular multilevel converter through the optical switch and the transceiver optical fiber; in other embodiments, the instruction parts corresponding to each submodule number can also be sorted in other ways, not necessarily in order. Correspondingly, the controller in each commutation unit is used to number the corresponding submodule according to the dial switch or FLASH built-in, and after successfully checking the instructions received by the submodule, obtain the corresponding control instructions according to the corresponding submodule number. Among them, the process of the controller checking the instruction is: verifying the frame header and the check part, if the frame header and the check part are correct, the received instruction is successfully checked. Since each group of commutation units in this embodiment includes 22 modules, which exceeds the upper limit of the dip switch, the modules will be numbered through the built-in FLASH, and the controller in each commutation unit obtains the control instruction according to the corresponding submodule number, such as the control instruction obtained by submodule 1 is the module 1 instruction in Figure 2. During the downlink communication process, the controller corresponding to each commutation unit is synchronized according to the downlink communication frame completion flag to ensure the synchronization of the submodules in the commutation unit.

The controller of the commutation unit is also used to upload the status of the submodule corresponding to the commutation unit to the valve control module according to the uplink communication frame format shown in Figure 3; since the submodule status needs to be uploaded in series, it needs to be uploaded in time. In this embodiment, each group of 22 submodules, then each controller controls the corresponding commutation unit to upload its own status data at an interval of 2.27us, wherein the commutation unit corresponding to submodule 1 sends data at time 0, the commutation unit corresponding to submodule 2 sends data at time 2.27us, ... The commutation unit corresponding to submodule 22 sends data at time 47.67us (21*2.27); the uploaded data frame format is set according to Figure 3, and the data frame content sent by each commutation unit includes: frame header + status of the corresponding submodule + check, such as frame header + module q status + check; it should be noted that in order to avoid transmission congestion caused by different upload directions, the time-sharing upload mode of each controller is the same upward upload, then the overall data frame effect of time-sharing upload can be reflected in the form of the entire uplink data frame in Figure 3.

During actual operation, submodule failures are generally drive failures, overvoltage failures, power failures, bypass malfunctions and communication failures. If one or more submodules fail, the faulty submodule will be bypassed for 3-5ms. After bypassing, the submodule will discharge through the discharge circuit until the submodule capacitor is exhausted and loses power. On this basis, in order to isolate the impact of the faulty submodule, the optical switch is also used to bypass the failed submodule after the corresponding submodule loses power. As shown in Figure 4, when submodule 1 is a failed submodule, submodule 1 is bypassed through the optical switch corresponding to submodule 1. At this time, submodule 1 can no longer upload data through the corresponding optical switch. The overall uploaded data is only the redundant part of the lost submodule 1. The instructions originally required to be transmitted to submodule 1 are also directly forwarded to the next optical switch by the corresponding optical switch, so it will not affect the normal information transmission of other conversion units. During the switching time period of the optical switch, other powered modules will have link interruption. At this time, other powered modules will execute the cut-off instruction to themselves during the switching time of the optical switch. Taking the half-bridge sub-module as an example, when the cut-off instruction is executed on itself, the IGBT corresponding to the module is turned on, which is equivalent to bypassing the sub-module itself, so that the capacitor of the sub-module will not continue to charge, thereby preventing the sub-module from overvoltage.

The equipment related to the valve control of the converter valve is generally arranged in the valve control room, while the submodule is generally arranged in the valve hall. The communication link is generally more than 100 meters, which is expensive and difficult to lay. The converter valve system in this embodiment reasonably groups the submodules, and after the submodules in each group are connected in series through the optical switch, communication is established with the valve control module through a pair of transceiver optical fibers, thereby replacing the original method of each submodule communicating with the valve control module through its own transceiver optical fiber. Since the distance between the submodules is relatively close, the optical fiber length required for connecting the submodules in series is very short, which is negligible compared with the optical fiber length required from the submodule to the valve control module. Therefore, the communication optical fiber between the submodules in the group and the valve control is reduced from the original 22 transmissions and 22 to 1 reception and 1 transmission, which greatly reduces the cost. According to the components as serial networking units, the cost of a single optical switch is about 200 yuan (there is still room for price reduction), and a component uses a pair of long optical fibers, which costs about 2,000 yuan. The number of valve control bridge arm interface cabinets can be reduced to 1. According to the above scheme, a single project can save nearly one million yuan in costs.

Modular multilevel converter embodiment:

This embodiment provides a technical solution for a modular multilevel converter. Since the structure, specific principle and usage of the modular multilevel converter have been described in detail in the above-mentioned converter valve system embodiment, they will not be repeated here.

Submodule networking method embodiment:

This embodiment provides a technical solution for a submodule networking method, and the specific steps are as follows:

1) A corresponding optical switch is set at each submodule to form a commutation unit; the commutation units are grouped, and each submodule in each group is serially connected to the valve control module through the transceiver optical fiber and its corresponding optical switch.

According to the actual project, when the commutation units are grouped, the number q of each group is determined according to the control period m and the uplink communication period p: if the control period is m and the uplink communication duration of a single submodule is p, then the number of groups q = int(m/p), where the function int() is a rounding function; for example, in this embodiment, the control period is 50us and the uplink communication period is 2.2us, then the number of each group q = int(50/2.2), where 50/2.2≈22.7, then q = 22. Here, the number q of each group of commutation units is the number n of the above-mentioned submodules, that is, n = 22, and the group includes 22 submodules and their corresponding commutation units.

In combination with the specific structure in the above-mentioned converter valve system embodiment, in this embodiment, the specific connection sequence during serial connection is: valve control module-optical switch corresponding to submodule 1-submodule 1-optical switch corresponding to submodule 2-submodule 2-…-optical switch corresponding to submodule n-submodule n-valve control module.

2) When the instruction issued by the valve control module is transmitted to the optical switch through the transceiver optical fiber, it is sent to the corresponding sub-module through the optical switch; except for the last sub-module connected in series, when other sub-modules receive the instruction, they directly forward the instruction through the corresponding optical switch; each commutation unit verifies the instruction received by the sub-module and obtains the corresponding instruction information.

The content of the instruction issued by the valve control module includes a frame header part, an instruction part and a verification part. The instruction part here includes control instructions corresponding to the numbers of each sub-module; therefore, the commutation unit numbers the corresponding sub-module according to the dip switch or FLASH built-in, and obtains the corresponding control instruction according to the number after successfully verifying the received instruction; since each group of commutation units in this embodiment includes 22 modules, which exceeds the upper limit of the dip switch, the modules will be numbered through the built-in FLASH.

Among them, after successfully verifying the issued instructions, the method of obtaining the corresponding control instructions according to its own number includes: verifying the frame header and the check part. If the frame header and the check part are correct, the corresponding control instructions are obtained according to their own number. For example, the control instructions obtained by sub-module 1 are the module 1 instructions in Figure 2.

3) When the commutation units in the same group need to send their own information to each submodule, they send their own information to the valve control module in time division through the corresponding optical switch and transceiver optical fiber according to the set time interval.

Since each group of commutation units needs to upload the module status in series, it needs to be uploaded in time. In this embodiment, each group of 22 sub-modules uploads the status data of the sub-modules at an interval of 2.27us, wherein the commutation unit corresponding to sub-module 1 sends data at time 0, the commutation unit corresponding to sub-module 2 sends data at time 2.27us, ... the commutation unit corresponding to sub-module 22 sends data at time 47.67us (21*2.27); the uploaded data frame format is set according to Figure 3, and the data frame content sent by each commutation unit includes: frame header + status of the corresponding sub-module + check, such as frame header + module q status + check; it should be noted that in order to avoid transmission congestion caused by different upload directions, the time-sharing upload mode of each controller is the same upload, then the overall data frame effect of time-sharing upload can be reflected in the form of the entire uplink data frame in Figure 3.

During the actual operation, the failure of the submodule is generally a drive failure, an overvoltage failure, a power failure, a bypass malfunction, and a communication failure. If one or more of the submodules fails, the faulty submodule will be bypassed, and the bypass time is 3-5ms. After the bypass, the submodule will discharge through the discharge circuit until the submodule capacitor is exhausted and loses power; based on this, in order to isolate the influence of the faulty submodule, during the downlink communication process of 2) or the uplink communication process of 3), if the submodule loses power, the lost submodule is bypassed through the optical switch. Among them, Figure 5 is a control logic block diagram of the power-losing module corresponding to the optical switch bypass during downlink communication, and Figure 6 is a control logic block diagram of the power-losing module corresponding to the optical switch bypass during uplink communication. For example, when submodule 1 is a power-losing submodule, the optical switch corresponding to submodule 1 is bypassed, and submodule 1 can no longer upload data through the corresponding optical switch at this time. The overall uploaded data is only the redundant part of the lost submodule 1, and the instructions originally required to be transmitted to submodule 1 are also directly forwarded to the next optical switch by the corresponding optical switch, so it will not affect the normal information transmission of other commutation units. Since other powered modules will have link interruption during the switching time of the optical switch, other powered modules will execute the cut-off instruction to themselves during the switching time of the optical switch. Taking the half-bridge sub-module as an example, when the cut-off instruction is executed on itself, the IGBT corresponding to the module is turned on, which is equivalent to bypassing the sub-module itself, so that the capacitor of the sub-module will not continue to charge, thereby preventing the sub-module from overvoltage.

The characteristics of the present invention are: the communication optical fiber between the submodules and the valve control in the group is simplified from the original multiple receiving and multiple sending to 1 receiving and 1 sending through the optical switch, so that the valve control of the converter valve and the submodule can pass through fewer optical fibers to complete the control and monitoring of the submodule, and on the basis of ensuring the synchronization of control, it can save costs and reduce the difficulty of laying. And through the setting of the optical switch, the faulty submodule can be bypassed in time to avoid affecting the information transmission in the uplink or downlink communication process; each submodule directly forwards the instruction when receiving the instruction, ensuring the synchronization of the received instruction; the way of uploading its own information is time-sharing uploading in the same direction, which can realize unblocked serial uploading.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that the specific implementation modes of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be included in the scope of protection of the claims of the present invention.

Claims (10)

1. A modular multi-level converter, characterized in that it comprises a transceiver optical fiber and a plurality of converter units;

   Each commutation unit includes a submodule, a corresponding optical switch and a corresponding controller;

   Multiple commutation units are divided into multiple groups. The submodules included in all commutation units in the same group are connected in series communication through transceiver optical fibers and their corresponding optical switches to form a series line. The optical switches at both ends of the series line are used to communicate with the valve control module to receive instructions issued by the valve control module or send submodule information to the valve control module.

   The optical switch is used to send the instruction to the corresponding sub-module when the issued instruction is transmitted to the corresponding conversion unit; and if the optical switch is not the most downstream optical switch in the series communication line, the optical switch is also used to directly forward the instruction to the optical switch corresponding to the next sub-module when the corresponding sub-module receives the instruction.
2. The modular multilevel converter according to claim 1 is characterized in that all controllers of the commutation units in the same group are used to send information of the corresponding submodules to the valve control module through the corresponding optical switches in a time-sharing manner according to a set time interval; and the time-sharing sending mode of each controller is the same upward sending.
3. The modular multilevel converter according to claim 1, characterized in that the optical switch is also used to bypass the sub-module when the corresponding sub-module loses power.
4. The modular multilevel converter according to any one of claims 1 to 3 is characterized in that the content of the issued instruction includes an instruction part, and the instruction part includes control instructions corresponding to each sub-module number respectively; the controller is also used to number the corresponding sub-modules, and after successfully verifying the issued instruction, obtain the corresponding control instruction according to the corresponding sub-module number.
5. The modular multilevel converter according to claim 4 is characterized in that the content of the issued instruction also includes a frame header part and a check part, and the controller is also used to verify the frame header and the check part. If the frame header and the check part are correct, it indicates that the issued instruction is successfully verified.
6. The modular multilevel converter according to any one of claims 1 to 3 is characterized in that the number of groups of the commutation unit is determined according to the total number of submodules and the control period: if the control period is m and the uplink communication duration of a single submodule is p, the number of groups q = int(m/p), wherein the function int() is a floor function.
7. A converter valve system, comprising a valve control module, characterized in that it also comprises a modular multilevel converter as described in any one of claims 1 to 6; the valve control module is used to issue instructions or receive sub-module self information sent by the modular multilevel converter.
8. A submodule networking method, characterized in that the steps are as follows:

   1) A corresponding optical switch is set at each submodule to form a commutation unit; the commutation units are grouped, and each submodule in each group is connected in series communication with its corresponding optical switch through a transceiver optical fiber to form a series line; the optical switches at both ends of the series line are connected in communication with the valve control module;

   2) When the instruction issued by the valve control module is transmitted to the optical switch through the transceiver optical fiber, it is sent to the corresponding sub-module through the optical switch; except for the sub-module at the most downstream of the series communication line, when other sub-modules receive the instruction, they directly forward the instruction through the corresponding optical switch; each commutation unit obtains the corresponding instruction information according to the instruction received by the sub-module;

   3) When the commutation units in the same group need to send their own information to each submodule, they send their own information to the valve control module in time-sharing through the corresponding optical switch and transceiver optical fiber according to the set time interval; the time-sharing sending mode of each submodule is to send information in the same direction.
9. The submodule networking method according to claim 8 is characterized in that if a submodule loses power, the lost submodule is bypassed through a corresponding optical switch.
10. The submodule networking method according to claim 8 or 9 is characterized in that the content of the instruction issued by the valve control module includes a frame header part, an instruction part and a verification part, and the instruction part includes control instructions corresponding to the numbers of each submodule; the commutation unit numbers the submodule corresponding to itself, and after successfully verifying the issued instruction, obtains the corresponding control instruction according to the number;

    The method of obtaining the corresponding control instruction according to its own number after successfully verifying the issued instruction includes: verifying the frame header and the check part. If the frame header and the check part are correct, it indicates that the issued instruction is successfully verified, and then obtaining the corresponding control instruction according to its own number.

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