Modular multilevel converter control method and system for mixed use of sub-modules with different voltage specifications

Abstract

The invention belongs to the field of modular multilevel converter control, and particularly relates to a modular multilevel converter control method and system for mixed use of sub-modules of different voltage specifications. According to the method, a sub-module with the minimum rated voltage serves as a reference sub-module, and the ratio of the rated working voltage of each sub-module to the rated working voltage of the reference sub-module serves as the equivalent input number of the sub-module; according to the instantaneous voltage value of each sub-module, the equivalent input number and the optimization coefficient, obtaining a converted voltage value of each sub-module; according to the bridge arm modulation voltage instruction value and the rated working voltage of the reference sub-module, obtaining the equivalent input number demand of the total sub-module; issuing an input instruction to the sub-modules in the bridge arm according to the equivalent input number demand of the total sub-modules; and when each input instruction is issued, subtracting the equivalent number of the input sub-modules from the current residual equivalent input number, and when the equivalent number is smaller than a corresponding set threshold value, issuing a cutting instruction to the residual sub-modules, and cutting the residual sub-modules.

Description

Technical Field

[0001] The present invention belongs to the field of modular multi-level converter control, and in particular relates to a modular different-level converter control method and system for mixed use of sub-modules with different voltage specifications. Background Art

[0002] At present, power electronic equipment is developing in the direction of integration, high voltage, large capacity and high efficiency, and has been widely used in scenarios such as ultra-high voltage flexible direct current transmission, offshore wind power flexible direct current transmission, and offshore wind power low-frequency transmission. High-voltage and large-capacity power electronic converters generally adopt modular multi-level structures, such as modular multi-level converters based on half-bridge submodules, modular multi-level converters based on a mixture of half-bridge and full-bridge submodules, and modular matrix converters based on full-bridge submodules. At present, in flexible AC and DC transmission projects, converter power semiconductors mainly use mature IGBTs, including press-fit IGBTs and welded IGBTs. However, with the rapid development of power electronics technology, power semiconductors with higher voltage specifications have gradually met the conditions for engineering application, such as 6.5kV IGCTs and 6.5kV IGBTs. The use of higher-specification power semiconductors can increase the operating voltage of submodules and reduce the number of submodules, thereby achieving compactness and lightness of the entire converter.

[0003] As the core device in the converter, the engineering advancement of new power semiconductors needs to follow cautious and rigorous steps. First, partial application replacement is carried out in the demonstration project. After the reliability of the project is verified, the converter project with 100% new devices is built. For example, on the bridge arm of a modular multilevel converter, most of the submodules are based on mature 4.5kV crimped IGBTs, and a small number of submodules use new 6.5kV power devices. In the conventional modular multilevel converters in the past, after the converter valve receives the bridge arm modulation voltage instruction issued by the control and protection system, it converts the bridge arm modulation voltage into the number of submodules put into use through the rated voltage or average voltage of the submodule, and then controls the switching of the submodules based on the sorting results of the submodule voltages and the charging and discharging direction of the bridge arm current. However, when two or more power device sub-modules with two or more voltage specifications are used in the converter, due to the different rated operating voltages of the sub-modules, after receiving the modulation voltage instruction of the bridge arm voltage, it is difficult for the converter to control the power device sub-modules with different voltage specifications so that the bridge arm AC voltage can respond correctly, and it is difficult to ensure that while achieving the correct response of the bridge arm AC voltage, the voltage balance of all power device sub-modules with the same or different voltage specifications in the bridge arm is maintained. Summary of the invention

[0004] The object of the present invention is to provide a modular multi-level converter control method and system for mixing sub-modules of different voltage specifications, which is used to solve the problem in the prior art that when the voltage specification power device sub-modules used in the converter have different rated operating voltages, it is difficult to control the sub-modules with different voltage specifications so that the bridge arm AC voltage can respond correctly, and it is difficult to maintain the voltage balance of all power device sub-modules with the same or different voltage specifications in the bridge arm.

[0005] In order to achieve the above object, the present invention provides a modular multi-level converter control method for mixing sub-modules with different voltage specifications, the method comprising: taking the sub-module with the smallest rated voltage in each bridge arm of the converter as the reference sub-module of the bridge arm, and taking the ratio of the rated operating voltage of each sub-module in the bridge arm to the reference sub-module as the equivalent number of the sub-module;

[0006] In each valve control cycle, the converted voltage value of each submodule is obtained according to the instantaneous voltage value of each submodule, the equivalent number and the set weight corresponding to each submodule;

[0007] According to the acquired value of the bridge arm modulation voltage instruction and the rated working voltage of the bridge arm reference submodule, the total submodule equivalent input number requirement corresponding to the bridge arm modulation voltage instruction is obtained; according to the total submodule equivalent input number requirement, the input instruction is issued to the submodules in the bridge arm with the converted voltage value; the total submodule equivalent input number requirement is used as the initial remaining equivalent input number, and each time an input instruction is issued, the current remaining equivalent input number is subtracted from the equivalent number of submodules put into operation this time to obtain a new remaining equivalent input number; when the remaining equivalent input number is less than the corresponding set threshold, the input instruction is stopped and the removal instruction is issued to the remaining submodules;

[0008] The corresponding set threshold is greater than 0.4 and less than or equal to 0.6.

[0009] Beneficial effect: The present invention provides a novel control method for a modular multi-level converter in which sub-modules of different voltage specifications are mixed; the method preliminarily uses the sub-module with the smallest rated voltage in each bridge arm of the converter as the reference sub-module of the bridge arm in the modular multi-level converter in which sub-modules of different voltage specifications are mixed, and uses the ratio of the rated operating voltage of each sub-module in the bridge arm to the reference sub-module as the equivalent number of the sub-module, that is, converting the power device sub-modules of various different voltage specifications into the number of reference sub-modules (i.e., the equivalent number) according to the voltage, which is equivalent to determining that the bridge arm modulation voltage demand that can be met by investing a power device sub-module of a certain voltage specification is equivalent to the bridge arm modulation voltage demand that can be met by investing how many reference sub-modules; after the converter receives the modulation voltage instruction of the bridge arm voltage, the number of reference sub-modules that need to be invested to meet the instruction converted by the modulation voltage instruction (i.e., the total sub-module equivalent investment number requirement) is used as the target, and according to the total sub-module equivalent investment number requirement, an investment instruction is issued to the sub-modules in the bridge arm with the converted voltage value, and the investment instruction is issued according to whether the issued investment instruction meets the small Whether to issue a removal instruction to the remaining submodules is judged under the condition of the set threshold, that is, the sum of the voltage values ​​of all equivalently invested reference submodules satisfies the bridge arm modulation voltage instruction value; and when it is judged that investing another submodule will exceed the demand of the modulation voltage instruction and not investing this submodule will not meet the demand of the modulation voltage instruction, if the degree to which the voltage of the current bridge arm does not meet the demand of the modulation voltage instruction when this submodule is not invested is small (that is, the remaining equivalent investment number is less than the corresponding set threshold of 0.4 to 0.6), then stop issuing the investment instruction, otherwise it means that the degree to which the voltage of the current bridge arm does not meet the demand of the modulation voltage instruction is large (that is, the remaining equivalent investment number is not less than the corresponding set threshold of 0.4 to 0.6), so continue to issue the investment instruction to invest this submodule to eliminate the part of the current bridge arm voltage that does not meet the demand of the modulation voltage instruction. In summary, it can be ensured that the sum of the voltage values ​​of various power device submodules with different voltage specifications actually invested meets the bridge arm modulation voltage instruction value as accurately as possible, so that the bridge arm AC voltage can respond correctly.

[0010] Furthermore, the method of issuing input instructions to the sub-modules in the bridge arm with converted voltage values ​​according to the requirement of the equivalent number of total sub-modules to be put into operation includes: when the current of the bridge arm corresponding to the bridge arm modulation voltage instruction is in the charging direction, issuing input instructions to the sub-modules in the bridge arm in the order of the converted voltage values ​​from small to large; when the current of the bridge arm corresponding to the bridge arm modulation voltage instruction is in the discharging direction, issuing input instructions in the order of the converted voltage values ​​from large to small.

[0011] Furthermore, the set weight corresponding to each submodule is an optimization coefficient set according to the switching state of the submodule in the previous control cycle and the current charging and discharging direction.

[0012] Furthermore, according to the instantaneous voltage value of each submodule, the equivalent number and the set weight corresponding to each submodule, the method of obtaining the converted voltage value of each submodule includes:

[0013] U sm_sort [i]=U sm [i] / k i *k pwm [i]

[0014] Among them, U sm_sort [i] is the converted voltage value of each submodule, U sm [i] is the instantaneous voltage value of each submodule, k i is the equivalent number of benchmark submodules, k pwm [i] is the optimization coefficient set according to the switching state of the submodule in the previous control cycle and the current charging and discharging direction.

[0015] Furthermore, the method of setting the optimization coefficient according to the switching state of the submodule at the previous moment and the current charging and discharging direction includes:

[0016] For the submodule that was put into use in the previous valve control cycle, and the current bridge arm current is in the charging direction, the optimization coefficient of the submodule is set to a set value less than 1;

[0017] For the submodule that was put into operation in the previous valve control cycle, and the current bridge arm current is in the discharge direction, the optimization coefficient of the submodule is set to a set value greater than 1;

[0018] The optimization coefficient is set to 1 in all other cases.

[0019] Furthermore, according to the acquired value of the bridge arm modulation voltage instruction and the rated operating voltage of the reference submodule, the method of obtaining the equivalent number of submodules to be put into use includes:

[0020] N in =V arm / V sm_rate

[0021] Among them, N in is the total number of equivalent inputs required for submodules, V arm is the value of the bridge arm modulation voltage instruction, V sm_rate is the rated operating voltage of the reference submodule.

[0022] Furthermore, the corresponding set thresholds are all 0.5.

[0023] The present invention also provides a modular multilevel converter control system for mixing submodules of different voltage specifications, including a processor for executing a computer program to implement the steps of a modular multilevel converter control method for mixing submodules of different voltage specifications.

[0024] The modular multi-level converter control system with mixed use of sub-modules of different voltage specifications can achieve the same beneficial effects as the above-mentioned modular multi-level converter control method with mixed use of sub-modules of different voltage specifications. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 An example diagram of the connection relationship of a modular multilevel converter MMC in an embodiment of a modular multilevel converter control method for mixed use of sub-modules with different voltage specifications of the present invention;

[0026] Figure 2 This is an example diagram of the connection relationship of the modular matrix converter M3C in the embodiment of the modular multi-level converter control method for mixed use of sub-modules with different voltage specifications of the present invention;

[0027] Figure 3 This is an example flow chart of a modular multilevel converter control method for mixing sub-modules of different voltage specifications in an embodiment of the modular multilevel converter control method for mixing sub-modules of different voltage specifications of the present invention. DETAILED DESCRIPTION

[0028] 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 with reference to the accompanying drawings and embodiments.

[0029] Embodiment of control method for modular multi-level converter with mixed use of sub-modules of different voltage specifications

[0030] The present embodiment provides a novel control method for a modular multi-level converter with mixed sub-modules of different voltage specifications. The method determines that the bridge arm modulation voltage demand that can be met by investing a power device sub-module of a certain voltage specification is equivalent to the bridge arm modulation voltage demand that can be met by investing a number of reference sub-modules; then, an investment instruction is issued with the number of reference sub-modules that need to be invested converted from the modulation voltage instruction as the target. When it is determined that investing another sub-module will exceed the demand of the modulation voltage instruction and not investing this sub-module will not meet the demand of the modulation voltage instruction, if the degree to which the voltage of the current bridge arm does not meet the demand of the modulation voltage instruction when this sub-module is not invested is small, the issuance of the investment instruction is stopped; otherwise, the investment instruction is continued to be issued to invest this sub-module, so as to eliminate the part of the voltage of the current bridge arm that does not meet the demand of the modulation voltage instruction, and ensure that the sum of the voltage values ​​of the power device sub-modules of different voltage specifications actually invested can meet the bridge arm modulation voltage instruction value as accurately as possible so that the bridge arm AC voltage can respond correctly.

[0031] The method specifically comprises: taking the submodule with the smallest rated voltage in each bridge arm of the converter as the reference submodule of the bridge arm, and taking the ratio of the rated operating voltage of each submodule in the bridge arm to the reference submodule as the equivalent number of the submodule;

[0032] In each valve control cycle, the converted voltage value of each submodule is obtained according to the instantaneous voltage value, equivalent number and set weight corresponding to each submodule; the set weight corresponding to each submodule is the optimization coefficient set according to the switching state of the submodule in the previous control cycle and the current charging and discharging direction.

[0033] According to the acquired value of the bridge arm modulation voltage instruction and the rated working voltage of the bridge arm reference submodule, the total submodule equivalent input number requirement corresponding to the bridge arm modulation voltage instruction is obtained; according to the total submodule equivalent input number requirement, the input instruction is issued to the submodules in the bridge arm with the converted voltage value; the total submodule equivalent input number requirement is used as the initial remaining equivalent input number, and each time an input instruction is issued, the current remaining equivalent input number is subtracted from the equivalent number of submodules put into operation this time to obtain a new remaining equivalent input number; when the remaining equivalent input number is less than the corresponding set threshold, the input instruction is stopped and the removal instruction is issued to the remaining submodules;

[0034] The corresponding set threshold is greater than 0.4 and less than or equal to 0.6.

[0035] The above method uses the ratio of the rated working voltage value of the power device submodules of various voltage specifications to the rated working voltage value of the reference submodule as the equivalent number of the reference submodule, which is equivalent to using the equivalent number of the reference submodule to replace the actual number of each submodule, that is, the actual number of each submodule is normalized and expressed as the equivalent number of the reference submodule; then, by converting the voltage value of each submodule, the number of reference submodules converted by the modulation voltage instruction to meet the instruction is obtained; this number is used as the initial investment target, and the investment instruction is issued according to the set sorting result of the submodule converted voltage and the charging and discharging direction of the bridge arm current. When the remaining equivalent number of inputs is less than the set threshold, the investment instruction is stopped, and the remaining submodules are removed. The range of the set threshold is 0.4 to 0.6, which actually means that when another submodule is put into use, the demand for the modulation voltage instruction will exceed the demand for the modulation voltage instruction, and if this submodule is not put into use, the demand for the modulation voltage instruction will not be met. The set threshold can be flexibly set within the range according to the actual situation (that is, the set threshold can be flexibly set according to the degree to which the voltage of the current bridge arm does not meet the demand for the modulation voltage instruction), so that the sum of the voltage values ​​of the various power device submodules of different specifications actually put into use is closer to the demand for the modulation voltage instruction, so that the bridge arm AC voltage can respond correctly. In this embodiment, the corresponding set thresholds are all 0.5. That is, it is equivalent to rounding off, that is, if the gap between the demand for the bridge arm modulation voltage instruction that can be met by all the submodules currently put into use and the demand for the bridge arm modulation voltage instruction itself is less than the demand for the bridge arm modulation voltage instruction that can be met by putting in 0.5 reference submodules, then no more submodules will be put into use. In other embodiments, when the bridge arm current corresponding to the bridge arm modulation voltage instruction is in the charging direction and the bridge arm current corresponding to the bridge arm modulation voltage instruction is in the discharging direction, the corresponding set thresholds can be set to different values.

[0036] Specifically, the converter applicable to this embodiment is a modular multi-level converter in which sub-modules of different voltage specifications are mixed, including: Figure 1 The flexible DC transmission modular multilevel converter MMC shown in FIG. Figure 2 The modular structure converters such as the low-frequency transmission modular matrix converter M3C shown in the figure; the modular multilevel converter with mixed sub-modules of different voltage specifications uses sub-modules of two or more voltage specifications in the bridge arm, and the rated operating voltage of the sub-modules is recorded as V sm_rate1 、V sm_rate2 、V sm_rate3 ,…,V sm_rateM Etc. Figure 3 As shown, a submodule with the smallest rated voltage is selected in each bridge arm of the converter as the reference submodule of the bridge arm. The rated voltage of the reference submodule of the bridge arm can be recorded as V sm_rate, pre-calculate the rated operating voltage V of each voltage specification submodule sm_ratei and the reference submodule rated operating voltage V sm_rate Each submodule uses the calculated ratio as its equivalent number k i , k i =V sm_ratei / V sm_rate .

[0037] Taking the above rated working voltage as V sm_rate1 The submodule of is taken as the reference submodule as an example. In this case, V sm_rate =V sm_rate1 , then substitute into the above formula (i.e. formula k i =V sm_ratei / V sm_rate )available:

[0038] The rated working voltage is V sm_rate1 The equivalent number k1 of submodules is V sm_rate1 / V sm_rate (V sm_rate1 )=1;

[0039] The rated working voltage is V sm_rate2 The equivalent number k2 of submodules is V sm_rate2 / V sm_rate (V sm_rate1 );

[0040] The rated working voltage is V sm_rate3 The equivalent number k3 of submodules is V sm_rate3 / V sm_rate (V sm_rate1 );

[0041] The equivalent number of each sub-module can be obtained in the same way.

[0042] Afterwards, based on the acquired bridge arm modulation voltage instruction value and the rated operating voltage of the reference submodule, the equivalent number of total submodules corresponding to the instruction is obtained, that is, the value of the bridge arm modulation voltage instruction must be met by multiplying the rated operating voltage of the reference submodule by the equivalent number.

[0043] Each time a submodule is issued, the current equivalent number is subtracted from the equivalent number k of the submodules that are currently being invested. i , as the new remaining equivalent input number k i+1; When the remaining equivalent number of inputs is less than the corresponding set threshold, stop issuing input instructions and issue removal instructions to the remaining submodules, and remove the remaining submodules. In each valve control cycle, when issuing input instructions or removal instructions to each submodule, the input instruction will not be issued to a submodule because it was in the input state in the previous valve control cycle, nor will the removal instruction be issued to a submodule because it was in the removal state in the previous valve control cycle. In this embodiment, the submodule with the smallest rated voltage in each bridge arm of the converter is used as the reference submodule, and the corresponding set threshold is 0.5. Then, when the remaining equivalent number of inputs is less than 0.5, stop issuing input instructions and issue removal instructions to the remaining submodules, and remove the remaining submodules (that is, when the remaining equivalent number of inputs is greater than 0.5, continue to select submodules from the remaining submodules for input; when the remaining equivalent number of inputs is less than 0.5, it is considered that the modulation wave response is complete, and the remaining submodules are removed).

[0044] In this embodiment, the method of issuing input instructions to the sub-modules in the bridge arm with converted voltage values ​​according to the requirement of the equivalent number of total sub-modules to be put into operation includes: when the bridge arm current corresponding to the bridge arm modulation voltage instruction is in the charging direction, issuing input instructions to the sub-modules in the bridge arm in the order of the converted voltage values ​​from small to large; when the bridge arm current corresponding to the bridge arm modulation voltage instruction is in the discharging direction, issuing input instructions in the order of the converted voltage values ​​from large to small.

[0045] Specifically, according to the bridge arm current direction corresponding to the bridge arm modulation voltage instruction, determine the order in which the input instructions are issued for the converted voltage values ​​of the sub-modules in the bridge arm, that is, when issuing the input instructions, the following two situations are discussed:

[0046] 1) When the bridge arm current corresponding to the bridge arm modulation voltage instruction is in the charging direction, in order to achieve sub-module voltage balance, the sub-module with lower voltage is charged first, and the input instruction is issued to the sub-modules in the bridge arm in the order of their voltage values ​​from large to small;

[0047] 2) When the bridge arm current corresponding to the bridge arm modulation voltage instruction is in the discharge direction, in order to achieve sub-module voltage balance, the sub-module with higher voltage is discharged first, and the input instruction is issued to the sub-modules in the bridge arm in the order of their voltage values ​​from small to large.

[0048] In this embodiment, according to the instantaneous voltage value of each submodule, the equivalent number and the set weight corresponding to each submodule, the method of obtaining the converted voltage value of each submodule includes:

[0049] U sm_sort [i]=U sm [i] / k i *k pwm [i]

[0050] Among them, U sm_sort [i] is the converted voltage value of each submodule, U sm [i] is the instantaneous voltage value of each submodule, k i is the equivalent number of benchmark submodules, k pwm [i] is the optimization coefficient set according to the switching state of the submodule in the previous control cycle and the current charging and discharging direction.

[0051] The method of setting the optimization coefficient according to the switching state of the submodule at the previous moment and the current charging and discharging direction includes: for the submodule that was put into use in the previous valve control cycle, and the current bridge arm current is in the charging direction, the optimization coefficient of the submodule is set to a set value less than 1;

[0052] For the submodule that was put into operation in the previous valve control cycle, and the current bridge arm current is in the discharge direction, the optimization coefficient of the submodule is set to a set value greater than 1;

[0053] The optimization coefficient is set to 1 in other cases. Specifically, after obtaining the equivalent number of each submodule, the rated voltage of each submodule is first converted to obtain the converted voltage value of each submodule; the converted voltage value of each submodule is calculated according to the following formula:

[0054] U sm_sort [i]=U sm [i] / k i *k pwm [i]

[0055] Among them, U sm_sort [i] is the converted voltage value of each submodule (i.e. the converted voltage value of the i-th submodule), U sm [i] is the instantaneous voltage value of each submodule (i.e., the instantaneous voltage value of the i-th submodule), k i is the equivalent number of benchmark submodules, k pwm [i] is the optimization coefficient (i.e., the set weight corresponding to each sub-module).

[0056] With rated working voltage V sm_rate1 Taking the submodule of as an example, k1=1 of the submodule has been obtained through pre-calculation, so the converted voltage value of the submodule is specifically:

[0057] U sm_sort [1]=U sm [1]*k pwm [1]

[0058] The above optimization coefficient k pwmIt is a coefficient set according to the switching state of each submodule at the previous moment and the current charge and discharge direction. The setting of this optimization coefficient is divided into three cases for discussion:

[0059] A) For a submodule that was put into use in the previous control cycle, and the current arm current is in the charging direction, the optimization coefficient of the submodule is a set value less than 1;

[0060] B) For a submodule that was put into use in the previous control cycle, and the current arm current is in the discharge direction, the optimization coefficient of the submodule is a set value greater than 1;

[0061] C) The optimization factor is set to 1 in all other cases.

[0062] By setting the optimization coefficient k pwm That is, when the current arm current is in the charging direction, the weight of the submodule that has been put into operation in the previous valve control cycle is reduced. Since the submodules in the arm are issued with their voltage values ​​in a descending order when the arm current is in the charging direction, the weight reduction operation is equivalent to placing the submodule that has been put into operation at the previous moment as far back as possible in the order of issuing the input instruction, so that under the same voltage conditions, the submodule that has not been put into operation in the previous valve control cycle is given priority to be put into operation, rather than the submodule that has been put into operation in the previous valve control cycle, thereby avoiding frequent switching operations of the submodules as much as possible; the same applies when the current arm current is in the discharging direction.

[0063] In this embodiment, the method of obtaining the equivalent number of total submodules required for input according to the acquired bridge arm modulation voltage command value and the rated operating voltage value of the reference submodule includes:

[0064] N in =V arm / V sm_rate

[0065] Among them, N in is the total number of equivalent inputs required for submodules, V arm is the bridge arm modulation voltage command value, V sm_rate It is the rated operating voltage value of the reference submodule.

[0066] Specifically, after obtaining the bridge arm modulation voltage command value, the total submodule equivalent input number requirement is calculated; the submodule equivalent input number requirement is calculated according to the following formula:

[0067] N in =V arm / V sm_rate

[0068] Among them, N inis the total number of equivalent inputs required for submodules, V arm is the bridge arm modulation voltage command value, V sm_rate is the rated working voltage value of the reference submodule. The total submodule equivalent input number requirement is actually converted into the number of all submodules required for equivalent input by converting the bridge arm modulation voltage command value and dividing it by the rated working voltage value of the reference submodule.

[0069] Embodiment of modular multi-level converter control system with mixed use of sub-modules of different voltage specifications

[0070] The present embodiment provides a new modular multilevel converter control system for mixing sub-modules of different voltage specifications. The system includes a processor, in which executable program instructions are stored. The executable program instructions are used to implement a modular multilevel converter control method for mixing sub-modules of different voltage specifications as in the above-mentioned modular multilevel converter control method embodiment for mixing sub-modules of different voltage specifications.

[0071] Since the specific working mode and working principle of the modular multilevel converter control system for mixing sub-modules of different voltage specifications in this embodiment have been described in detail in the above-mentioned modular multilevel converter control method embodiment for mixing sub-modules of different voltage specifications, they will not be repeated here.

[0072] It should be understood that the above specific embodiments of the present invention are only used to illustrate or explain the principles of the present invention, but do not constitute a limitation to the present invention.

Claims

1. A control method for a modular multi-level converter with mixed use of sub-modules of different voltage specifications, characterized in that: include: The submodule with the smallest rated voltage in each bridge arm of the converter is respectively used as the reference submodule of the bridge arm, and the ratio of the rated operating voltage of each submodule in the bridge arm to the reference submodule is used as the equivalent number of the submodule; In each valve control cycle, the converted voltage value of each submodule is obtained according to the instantaneous voltage value of each submodule, the equivalent number and the set weight corresponding to each submodule; According to the acquired value of the bridge arm modulation voltage instruction and the rated working voltage of the bridge arm reference submodule, the total submodule equivalent input number requirement corresponding to the bridge arm modulation voltage instruction is obtained; according to the total submodule equivalent input number requirement, the input instruction is issued to the submodules in the bridge arm with the converted voltage value; the total submodule equivalent input number requirement is used as the initial remaining equivalent input number, and each time an input instruction is issued, the current remaining equivalent input number is subtracted from the equivalent number of submodules put into operation this time to obtain a new remaining equivalent input number; when the remaining equivalent input number is less than the corresponding set threshold, the input instruction is stopped and the removal instruction is issued to the remaining submodules; The corresponding set threshold is greater than 0.4 and less than or equal to 0.

6.

2. The modular multi-level converter control method for mixing sub-modules of different voltage specifications according to claim 1 is characterized in that: The method of issuing input instructions to the sub-modules in the bridge arm with the converted voltage value according to the equivalent input number requirement of the total sub-modules includes: when the current of the bridge arm corresponding to the bridge arm modulation voltage instruction is in the charging direction, issuing the input instructions to the sub-modules in the bridge arm in the order of the converted voltage values ​​from small to large; when the current of the bridge arm corresponding to the bridge arm modulation voltage instruction is in the discharging direction, issuing the input instructions in the order of the converted voltage values ​​from large to small.

3. The control method for a modular multi-level converter with mixed use of sub-modules of different voltage specifications according to claim 2, characterized in that: The set weight corresponding to each submodule is an optimization coefficient set according to the switching state of the submodule in the previous control cycle and the current charging and discharging direction.

4. The control method for a modular multi-level converter with mixed use of sub-modules of different voltage specifications according to claim 3 is characterized in that: According to the instantaneous voltage value of each submodule, the equivalent number and the set weight corresponding to each submodule, the method of obtaining the converted voltage value of each submodule includes: IN sm_sort [i]=U sm [i] / k i *k pwm [and] Among them, U sm_sort [i] is the converted voltage value of each submodule, U sm [i] is the instantaneous voltage value of each submodule, k i is the equivalent number of benchmark submodules, k pwm [i] is the optimization coefficient set according to the switching state of the submodule in the previous control cycle and the current charging and discharging direction.

5. The control method for a modular multi-level converter with mixed use of sub-modules of different voltage specifications according to claim 3 or 4, characterized in that: The method of setting the optimization coefficient according to the switching state of the submodule at the previous moment and the current charging and discharging direction includes: for the submodule that was put into use in the previous valve control cycle, and the current bridge arm current is in the charging direction, the optimization coefficient of the submodule is set to a set value less than 1; For the submodule that was put into operation in the previous valve control cycle, and the current bridge arm current is in the discharge direction, the optimization coefficient of the submodule is set to a set value greater than 1; The optimization coefficient is set to 1 in all other cases.

6. The control method for a modular multi-level converter with mixed use of sub-modules of different voltage specifications according to any one of claims 1 to 4, characterized in that: According to the acquired bridge arm modulation voltage command value and the rated operating voltage of the reference submodule, the method of obtaining the equivalent input number requirement of the total submodules includes: N in =V arm / V sm_rate Among them, N in is the total number of equivalent inputs required for submodules, V arm is the value of the bridge arm modulation voltage instruction, V sm_rate is the rated operating voltage of the reference submodule.

7. The control method for a modular multi-level converter with mixed use of sub-modules of different voltage specifications according to any one of claims 1 to 4, characterized in that: The corresponding thresholds are all set to 0.

5.

8. A modular multi-level converter control system with mixed use of sub-modules of different voltage specifications, comprising a processor, characterized in that: The processor is used to execute a computer program to implement the steps of the modular multi-level converter control method for mixing sub-modules with different voltage specifications as described in any one of claims 1 to 7.

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