Capacitor voltage balance control method and system of MMC (Modular Multilevel Converter) sub-module and MMC

Abstract

The invention relates to the technical field of MMC sub-module balance control, and discloses a capacitor voltage balance control method and system for an MMC sub-module, and an MMC, and the method comprises the steps: judging whether a bridge arm is in a near-zero current risk region or not according to the direction of the current of the bridge arm in each valve control period; if the bridge arm is not in the near-zero current risk area, judging whether the bridge arm is in a modulating wave extreme value risk area or not according to the bridge arm modulation voltage instruction; if the bridge arm is in a near-zero current risk area or a modulating wave extreme value risk area, a first switching strategy is adopted; and if the bridge arm is not in the modulating wave extreme value risk area, a second switching strategy is adopted. According to the invention, the switching frequency is effectively reduced and the narrow pulse phenomenon of the sub-module is eliminated through the judgment of the frequent switching risk area and the corresponding switching strategy, meanwhile, the module alternation probability is effectively reduced through the sorting switching strategy based on the dynamic additional factor, and the fluctuation range of the module voltage is limited within the required range, so that the switching efficiency is improved. And the operation safety and stability of the MMC are ensured.

Description

Technical Field

[0001] The present invention relates to the technical field of MMC submodule balancing control, and in particular to a capacitor voltage balancing control method and system of an MMC submodule and an MMC. Background Art

[0002] As a new type of multilevel converter topology, modular multilevel converter (MMC) has good scalability in modular structure and has good application potential in high voltage DC transmission, reactive power compensation and other occasions with high voltage and power level requirements. During the operation of MMC, the bridge arm current will charge or discharge the power module capacitor in the input state, causing the power module capacitor voltage to increase or decrease. In order to achieve the voltage balance of the power module capacitor in the bridge arm, the valve controller of MMC needs to perform capacitor voltage balance control and maintain the consistency of submodule voltage through the rotation of submodules.

[0003] The commonly used capacitor voltage balancing control method is based on the switching control of capacitor voltage sorting and voltage balancing. Although this control idea is simple and easy in sorting logic and has a good voltage balancing effect, when it is used in situations where the number of levels is very high or there are many sub-module capacitors, it not only requires a large amount of sorting calculations, but also has a high switching frequency and large losses. The switching frequency not only has an important impact on the loss of MMC, that is, the higher the switching frequency, the greater the converter loss and the worse the economic efficiency of the DC transmission system, but also the high switching frequency and the possible narrow pulses with a short conduction time will damage the power devices.

[0004] At present, the optimization of existing methods mostly focuses on sorting. The calculation amount of voltage sorting is reduced by optimizing the sorting algorithm, but it still cannot solve the problem of high switching frequency. Although the switching frequency can be optimized by introducing a proportional coefficient, this optimization method needs to set a larger proportional coefficient to achieve the ideal frequency optimization effect. A larger proportional coefficient will cause the module voltage fluctuation range to increase, and frequent switching is prone to occur in some areas. Repeated switching will not only increase the switching frequency, but also too narrow input pulses will damage the power devices. Summary of the invention

[0005] In order to solve the above technical problems, the present invention provides a capacitor voltage balancing control method, system and MMC of an MMC submodule, so as to solve the problems of high switching frequency and large loss of the MMC in the existing method, thereby achieving the effect of reducing the switching frequency of the MMC, eliminating the narrow pulse phenomenon of the submodule, and improving the safety of the MMC operation.

[0006] In a first aspect, the present invention provides a capacitor voltage balancing control method for an MMC submodule, the method comprising: In each valve control cycle, whether the bridge arm is in the near-zero current risk zone is determined based on the direction of the bridge arm current; If the bridge arm is not in the near-zero current risk zone, then determine whether the bridge arm is in the modulation wave extreme value risk zone according to the bridge arm modulation voltage instruction; If the bridge arm is in a near-zero current risk zone or a modulation wave extreme value risk zone, a first switching strategy is adopted to control the switching of each submodule of the bridge arm, wherein the first switching strategy is a switching strategy that prohibits module rotation and only responds to an increase or decrease in the number of modules put into operation; If the bridge arm is not in the extreme value risk area of ​​the modulation wave, a second switching strategy is adopted to control the switching of each submodule of the bridge arm, and the second switching strategy is a switching strategy based on a dynamic additional factor.

[0007] Furthermore, the step of judging whether the bridge arm is in a near-zero current risk zone according to the direction of the bridge arm current includes: Determine whether the absolute value of the bridge arm current is less than the current threshold and whether the bridge arm current is changing towards zero. If so, the counter is incremented by 1, otherwise, the counter is decremented by 1. Multiply the bridge arm current by the bridge arm current of the previous valve control cycle. If the product is a negative number, the counter is cleared; if it is not a negative number, the counter remains unchanged. It is determined whether the accumulated value of the counter is greater than the counting threshold. If so, it is determined that the bridge arm is in a near-zero current risk zone.

[0008] Furthermore, the step of judging whether the bridge arm is in the modulation wave extreme value risk zone according to the bridge arm modulation voltage instruction includes: It is determined whether the bridge arm modulation voltage instruction is within a preset range. If so, it is determined that the bridge arm is not in the modulation wave extreme value risk zone. Otherwise, it is determined that the bridge arm is in the modulation wave extreme value risk zone.

[0009] Furthermore, the step of adopting the first switching strategy to control the switching of each submodule of the bridge arm includes: According to the bridge arm modulation voltage instruction of the current valve control cycle and the bridge arm modulation voltage instruction of the previous valve control cycle, it is determined whether the number of submodules put into operation in the current valve control cycle is the same as the number of submodules put into operation in the previous valve control cycle; If they are the same, the sub-module input instructions of the previous valve control cycle are maintained unchanged. If they are different, based on the increase or decrease in the number of sub-modules put into operation in the current valve control cycle and the number of sub-modules put into operation in the previous valve control cycle, operations such as adding new modules that were not put into operation in the previous valve control cycle or removing sub-modules that were put into operation in the previous valve control cycle are executed.

[0010] Further, the step of performing the operation of adding a submodule that was not put into operation in the previous valve control cycle or removing a submodule that was put into operation in the previous valve control cycle according to the increase or decrease in the number of submodules put into operation in the current valve control cycle and the number of submodules put into operation in the previous valve control cycle includes: When the bridge arm current is in the charging direction, the highest data position of the submodule that was removed in the previous valve control cycle is set to 1, and the submodules of the bridge arm are arranged in ascending order based on the module voltage, and the corresponding number of submodules are selected from the arranged module sequence in sequence according to the bridge arm modulation voltage instruction for operation; When the bridge arm current is in the discharge direction, the highest data position of the sub-module that has been put into operation in the previous valve control cycle is set to 1, and the sub-modules of the bridge arm are arranged in ascending order based on the module voltage. According to the bridge arm modulation voltage instruction, the corresponding number of sub-modules are selected in reverse order from the arranged module sequence for operation.

[0011] Furthermore, the step of adopting the second switching strategy to control the switching of each submodule of the bridge arm includes: According to the direction of the bridge arm current and the module voltage of the submodule that has been put into operation in the previous valve control cycle, the dynamic additional factor of the submodule that has been put into operation is calculated, and according to the dynamic additional factor, the module voltage of the submodule that has been put into operation is updated; According to the module voltage, the sub-modules of the bridge arm are arranged in ascending order, and according to the bridge arm current direction and the bridge arm modulation voltage instruction, a corresponding number of sub-modules are selected from the arranged module sequence for operation.

[0012] Further, the step of calculating the dynamic additional factor of the put-in-place submodule according to the direction of the bridge arm current and the module voltage of the put-in-place submodule in the previous valve control cycle, and updating the module voltage of the put-in-place submodule according to the dynamic additional factor includes: When the bridge arm current is in the charging direction, the difference between the dynamic additional high voltage threshold and the module voltage of the submodule that has been put into use in the previous valve control cycle is calculated, and the first dynamic additional factor is determined according to the comparison relationship between the difference and the minimum threshold of the additional factor; Subtracting the corresponding first dynamic additional factor from the module voltage of the submodule that has been put into use to obtain an updated module voltage of the submodule that has been put into use; When the bridge arm current is in the discharge direction, the difference between the module voltage of the submodule that has been put into use in the previous valve control cycle and the dynamic additional low-voltage threshold is calculated, and the second dynamic additional factor is determined according to the comparison relationship between the difference and the minimum threshold of the additional factor; The module voltage of the submodule that has been put into operation is added to the corresponding second dynamic additional factor to obtain an updated module voltage of the submodule that has been put into operation.

[0013] Furthermore, the step of selecting a corresponding number of submodules from the arranged module sequence for operation according to the bridge arm current direction and the bridge arm modulation voltage instruction includes: When the bridge arm current is in the charging direction, according to the bridge arm modulation voltage instruction, a corresponding number of submodules are sequentially selected from the arranged module sequence for operation; When the bridge arm current is in the discharge direction, according to the bridge arm modulation voltage instruction, a corresponding number of sub-modules are selected from the arranged module sequence in reverse order to be put into operation.

[0014] In a second aspect, the present invention provides a capacitor voltage balancing control system for an MMC submodule, the system comprising: The first risk zone determination module is used to determine whether the bridge arm is in a near-zero current risk zone according to the direction of the bridge arm current in each valve control cycle; The second risk zone determination module is used to determine whether the bridge arm is in the modulation wave extreme value risk zone according to the bridge arm modulation voltage instruction if the bridge arm is not in the near-zero current risk zone; A first strategy execution module is used to adopt a first switching strategy to control the switching of each submodule of the bridge arm if the bridge arm is in a near-zero current risk zone or a modulation wave extreme value risk zone, wherein the first switching strategy is a switching strategy that prohibits module rotation and only responds to an increase or decrease in the number of modules put into operation; The second strategy execution module is used to adopt a second switching strategy to control the switching of each sub-module of the bridge arm if the bridge arm is not in the modulation wave extreme risk area, and the second switching strategy is a switching strategy based on a dynamic additional factor.

[0015] In a third aspect, an embodiment of the present invention further provides an MMC, wherein the MMC includes a plurality of bridge arms, each bridge arm includes a plurality of sub-modules, and each sub-module of the MMC is controlled by the method described above.

[0016] The present invention provides a capacitor voltage balancing control method, system and MMC of an MMC submodule. The present invention targets two areas, namely the bridge arm current near-zero area and the modulation wave extreme value area, which are prone to repeated switching of submodules, and designs an operation strategy that only prohibits module rotation and only responds to the increase or decrease in the number of modules in the area, which can effectively reduce the switching frequency of the MMC and completely eliminate the narrow pulse phenomenon of the submodule. At the same time, the present invention also introduces a dynamic additional factor related to the instantaneous value of the submodule in the rated voltage area where the module voltage changes fastest and is prone to switching and rotating the submodule. Through the sorting switching strategy based on the dynamic additional factor, not only can the probability of module rotation be reduced when the module voltage changes rapidly, but also the fluctuation range of the module voltage can be limited to the required range, further ensuring the safety and stability of the MMC operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 1 is a flow chart of a capacitor voltage balancing control method of an MMC submodule in an embodiment of the present invention; Figure 2 This is the topological diagram of MMC; Figure 3 This is a schematic diagram of the risk area of ​​frequent switching of the bridge arm current near zero in the MMC; Figure 4 This is a schematic diagram of the frequent switching process of a single module; Figure 5 is a schematic diagram of the extreme value region of the modulation wave in an embodiment of the present invention; Figure 6 1 is another flow chart of a capacitor voltage balancing control method of an MMC submodule in an embodiment of the present invention; Figure 7 It is a structural schematic diagram of a capacitor voltage balancing control system of an MMC submodule in an embodiment of the present invention. DETAILED DESCRIPTION

[0018] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0019] See also Figure 1 The first embodiment of the present invention provides a capacitor voltage balancing control method for an MMC submodule, which includes steps S10 to S40: Step S10, in each valve control cycle, judging whether the bridge arm is in a near-zero current risk zone according to the direction of the bridge arm current; Step S20, if the bridge arm is not in the near-zero current risk zone, judging whether the bridge arm is in the modulation wave extreme value risk zone according to the bridge arm modulation voltage instruction; Step S30, if the bridge arm is in a near-zero current risk zone or a modulation wave extreme value risk zone, a first switching strategy is adopted to control the switching of each submodule of the bridge arm, wherein the first switching strategy is a switching strategy that prohibits module rotation and only responds to an increase or decrease in the number of modules put into operation; Step S40: If the bridge arm is not in the modulation wave extreme value risk zone, a second switching strategy is adopted to control the switching of each submodule of the bridge arm, wherein the second switching strategy is a switching strategy based on a dynamic additional factor.

[0020] The present invention provides a method for balancing and controlling the submodules of an MMC, wherein the MMC is a modular multi-level converter, and its topology is as follows: Figure 2 As shown, the modular multilevel converter generally has three phase units, each of which consists of an upper bridge arm and a lower bridge arm, and each bridge arm consists of several submodules SM i It is connected in series with a bridge arm reactor, and the nearest level approximation modulation strategy is used to control the bridge arm output voltage. By dynamically allocating the number of power modules that need to be in the input state in the upper and lower bridge arms, the required converter output voltage can be obtained by making it close to the bridge arm modulation wave. Figure 2 u a 、u b and u c Represent the voltages of the three phase units, I arm It represents the bridge arm current, and its suffixes abc represent the three phase units respectively. The suffixes u and d represent the upper and lower bridge arms respectively. Each submodule is a power module. T1 and T2 represent transistors. The specific structure and description thereof can refer to the conventional topology of MMC, and will not be described one by one here.

[0021] In the present invention, for each valve control cycle of the MMC, firstly, whether the bridge arm is in the near-zero current risk zone is determined according to the direction of the bridge arm current. The specific steps include: Determine whether the absolute value of the bridge arm current is less than the current threshold and whether the bridge arm current is changing towards zero. If so, the counter is incremented by 1, otherwise, the counter is decremented by 1. Multiply the bridge arm current by the bridge arm current of the previous valve control cycle. If the product is a negative number, the counter is cleared; if it is not a negative number, the counter remains unchanged. It is determined whether the accumulated value of the counter is greater than the counting threshold. If so, it is determined that the bridge arm is in a near-zero current risk zone.

[0022] In this embodiment, the near-zero current risk area is as follows: Figure 3 As shown, it means that the modules that are expected to be put into operation before and after the bridge arm current passes through zero will have characteristics reversal, for example, the larger value (or smaller value) in the module before the bridge arm current passes through zero changes to the smaller value (or larger value) of the module voltage after the bridge arm current passes through zero. It is easy to have a risk area where the power module that has just been cut off is quickly put into operation again after the bridge arm current passes through zero, or the put-in module is quickly cut off again after the bridge arm current passes through zero. This frequent switching situation not only increases the switching frequency, but also the too narrow input pulse will damage the power device. The frequent switching process of a single module is shown in the figure. Figure 4As shown, in the discharge direction, submodules 1 to 10 are sorted from low to high according to the voltage. At this time, the submodules put into operation are 1, 6 to 10, a total of 6. When the bridge arm current is close to zero, the number of bridge arms put into operation is reduced from 6 to 5, and No. 1 with the lowest voltage will be cut off. When the number of bridge arms put into operation remains unchanged and the direction of the bridge arm current changes from discharge to charging, according to the principle of sorting and equalizing voltage, the submodules put into operation change from No. 6 to 10 to No. 1 and No. 6 to 9, that is, No. 1 submodule is frequently switched on and off.

[0023] In order to solve the influence of the current near-zero area, there are currently two commonly used methods. One is to reduce the influence of the current near-zero area by setting a current direction change hysteresis loop, but the current direction hysteresis loop only translates the entire risk area forward and backward, and does not reduce the influence of the zero-crossing area at all. The other is to use the driving circuit to extend the pulse time of the narrow pulse to circumvent the narrow pulse problem, but this method neither reduces the switching frequency nor causes short-term distortion of the bridge arm output voltage. In order to solve this problem, in this embodiment, by analyzing the direction of the bridge arm current, it is accurately determined whether the bridge arm is in the near-zero current risk area, and when it is in the near-zero current risk area, a corresponding switching control strategy is adopted to overcome the influence of the current near-zero area.

[0024] In this embodiment, the determination of the near-zero current risk zone is based on the direction of the bridge arm current. First, the bridge arm current I arm Is it in the near-zero region but not crossing the zero point, that is, the absolute value of the current is less than the current threshold I set And the current changes towards zero, and the formula is: |I arm | set And d|I arm | / dt<0, where t represents time and current threshold I set The setting is based on the current peak value, and preferably can be set to 5% to 10% of the current peak value.

[0025] If these two conditions are met, the counter is incremented by 1. If not, the counter is decremented by 1. This step is based on the current sampling frequency for real-time judgment. Then the bridge arm current I arm The bridge arm current I arm_pres If the product is less than zero, it means that the current has just passed zero, and the counter is cleared, otherwise, the counter remains unchanged. When the counter value is greater than the counting threshold, it means that the bridge arm is in the near-zero current risk zone, and it is easy for the submodule to be frequently switched. The counting threshold here is determined based on the valve control cycle and the counting time. For example, when the counting time is 1ms, the counting threshold can be set to 20 when the valve control cycle is 50us.

[0026] ​When the bridge arm is in the near-zero current risk zone, in order to avoid frequent switching of submodules, in this embodiment, a switching strategy that prohibits module rotation and only responds to the increase or decrease in the number of modules put into operation is adopted to control the switching of the submodules of the bridge arm. The specific steps of the switching strategy include: According to the bridge arm modulation voltage instruction of the current valve control cycle and the bridge arm modulation voltage instruction of the previous valve control cycle, it is determined whether the number of submodules put into operation in the current valve control cycle is the same as the number of submodules put into operation in the previous valve control cycle; If they are the same, the sub-module input instructions of the previous valve control cycle are maintained unchanged. If they are different, based on the increase or decrease in the number of sub-modules put into operation in the current valve control cycle and the number of sub-modules put into operation in the previous valve control cycle, operations such as adding new modules that were not put into operation in the previous valve control cycle or removing sub-modules that were put into operation in the previous valve control cycle are executed.

[0027] In this embodiment, prohibiting module rotation and only responding to the increase or decrease of the number of modules put into operation means that when the number of sub-modules to be put into operation in the current valve control cycle is the same as that in the previous valve control cycle, the sub-module put-in instruction of the previous valve control cycle is maintained unchanged, wherein the number of sub-modules put into operation is issued by the bridge arm modulation voltage instruction, that is, the bridge arm modulation voltage instruction N m That is the number of sub-modules to be invested.

[0028] When the number of submodules to be put into operation in the current valve control cycle changes compared with the previous valve control cycle, if the number increases, then on the basis of the submodules already put into operation in the previous cycle, the newly put into operation submodules are selected from the modules that have not been put into operation; if the number decreases, then the submodules already put into operation in the previous cycle are removed, and the remaining submodules remain in the put-in state. In order to implement this switching strategy, in a preferred embodiment, a sorting algorithm based on the highest data position 1 is used to sort the submodules, and the specific sorting steps include: When the bridge arm current is in the charging direction, the highest data position of the submodule that was removed in the previous valve control cycle is set to 1, and the submodules of the bridge arm are arranged in ascending order based on the module voltage, and the corresponding number of submodules are selected from the arranged module sequence in sequence according to the bridge arm modulation voltage instruction for operation; When the bridge arm current is in the discharge direction, the highest data position of the sub-module that has been put into operation in the previous valve control cycle is set to 1, and the sub-modules of the bridge arm are arranged in ascending order based on the module voltage. According to the bridge arm modulation voltage instruction, the corresponding number of sub-modules are selected in reverse order from the arranged module sequence for operation.

[0029] In this embodiment, the sorting steps are different based on the direction of the bridge arm current. First, when the bridge arm current is in the charging direction, the highest data position of the sub-module that was cut off in the previous valve control cycle is set to 1. The data bit of the sub-module stores the voltage information of the sub-module. When the highest data position is 1, the voltage of the sub-module is actually set to a large value. It should be noted here that if the highest bit of the data bit of the sub-module is a sign bit, the next non-sign bit is selected to be set to 1.

[0030] Then, the submodules are arranged in ascending order according to the module voltage from low to high. It can be seen that since the voltage value of the submodule removed in the previous valve control cycle is set to a larger value, these submodules will be arranged at the end of the sequence when sorting. In other words, the submodules put into use in the previous valve control cycle are arranged at the front of the sequence. m When selecting submodules, you only need to select N with lower voltage from the sequence. m Just invest in each sub-module.

[0031] When the bridge arm current is in the discharge direction, the highest data position of the submodules that have been put into use in the previous valve control cycle is set to 1, that is, the voltage values ​​of these previously put into use submodules are set to a large value, and then the submodules are arranged in ascending order from low to high module voltage. In this sequence, the submodules that have been put into use in the previous valve control cycle will be arranged at the end of the sequence. When the bridge arm current is in the discharge direction, the submodules with higher voltage will be selected for use. m When selecting a submodule, you only need to select the N with higher voltage from the sequence in reverse order. m In the actual operation of the bridge arm, each execution of the switching instruction requires re-sorting and selection. This embodiment ensures that the number of modules put into operation this time is increased or decreased compared with the previous cycle, and the above switching strategy can be implemented.

[0032] In addition to the situation where submodules are frequently switched in the near-zero current area, there is another risk area that may cause frequent switching of submodules during actual operation. This risk area is the extreme value risk area of ​​the modulation wave. That is, when the modulation wave is at its maximum or minimum value, submodules are frequently switched, such as Figure 5As shown, it can be seen that when the modulation wave reaches a maximum value (≥0.95pu) or a minimum value (≤0.05pu), there are very few modules to be selected for the new change in the number of modules. If module switching rotation occurs due to the difference in module voltage, when the modulation number changes, only the sub-modules that have just participated in the rotation can be selected as the new changed modules, which will cause the power module that has just been cut off due to the rotation to be quickly put into use again after the modulation number changes, or the module that has just been put into use to be quickly cut off again.

[0033] Based on the above reasons, in this embodiment, when it is determined that the bridge arm is not in the near-zero current risk zone, it is necessary to further determine whether the bridge arm is in the modulation wave extreme value risk zone. The specific determination steps include: It is determined whether the bridge arm modulation voltage instruction is within a preset range. If so, it is determined that the bridge arm is not in the modulation wave extreme value risk zone. Otherwise, it is determined that the bridge arm is in the modulation wave extreme value risk zone.

[0034] In this embodiment, the determination of the modulation wave extreme value risk zone is based on the bridge arm modulation voltage instruction. During the operation of the bridge arm, its modulation wave represents the relationship between time and the number of input modules, that is, the modulation wave and the number of input modules are positively correlated. Therefore, the bridge arm modulation voltage instruction (the number of submodules to be put into operation) can be used to determine whether the bridge arm is in the modulation wave extreme value risk zone. Specifically, according to the rated number of submodules of the bridge arm, a threshold range (N L , N H ), preferably N H and N L It can be set to 95% and 5% of the rated number respectively. L <N m <N H When N m ≥N H or N m ≤N L When , it is determined that the bridge arm is in the extreme value risk area of ​​the modulation wave.

[0035] When the bridge arm is in the extreme value risk zone of the modulation wave, the same switching strategy as that when the bridge arm is in the near-zero current risk zone is adopted, that is, the switching strategy of prohibiting module rotation and only responding to the increase or decrease in the number of modules put into operation is adopted, which will not be repeated here.

[0036] When the bridge arm is neither in the near-zero current risk zone nor in the modulation wave extreme value risk zone, the second switching strategy is adopted, that is, the switching strategy based on the dynamic additional factor. The specific steps include: According to the direction of the bridge arm current and the module voltage of the submodule that has been put into operation in the previous valve control cycle, the dynamic additional factor of the submodule that has been put into operation is calculated, and according to the dynamic additional factor, the module voltage of the submodule that has been put into operation is updated; According to the module voltage, the sub-modules of the bridge arm are arranged in ascending order, and according to the bridge arm current direction and the bridge arm modulation voltage instruction, a corresponding number of sub-modules are selected from the arranged module sequence for operation.

[0037] In this embodiment, for the rated voltage region where the module voltage changes fastest and easily leads to the switching rotation of submodules, a dynamic additional factor related to the instantaneous value of the submodule is introduced, the goal of which is to reduce the probability of module rotation when the module voltage changes rapidly, while limiting the fluctuation range of the module voltage to the required range. That is, by introducing a dynamic additional factor to the submodules that have been put into use in the previous valve control cycle, the module voltage of the submodule is updated, thereby reducing the rotation probability of these submodules. The specific steps include: When the bridge arm current is in the charging direction, the difference between the dynamic additional high voltage threshold and the module voltage of the submodule that has been put into use in the previous valve control cycle is calculated, and the first dynamic additional factor is determined according to the comparison relationship between the difference and the minimum threshold of the additional factor; Subtracting the corresponding first dynamic additional factor from the module voltage of the submodule that has been put into use to obtain an updated module voltage of the submodule that has been put into use; When the bridge arm current is in the discharge direction, the difference between the module voltage of the submodule that has been put into use in the previous valve control cycle and the dynamic additional low-voltage threshold is calculated, and the second dynamic additional factor is determined according to the comparison relationship between the difference and the minimum threshold of the additional factor; The module voltage of the submodule that has been put into operation is added to the corresponding second dynamic additional factor to obtain an updated module voltage of the submodule that has been put into operation.

[0038] In this embodiment, when the bridge arm current is in the charging direction, each submodule that was in operation in the previous cycle is traversed to obtain the module voltage of the submodule, and then the dynamic additional factor is determined according to the formula: K smi = max{U H —U smi ,ΔU min}, where U H To dynamically add a high voltage threshold, it can be set to 1.1pu~1.2pu of the module's rated operating voltage, ΔU min It is the minimum threshold of the additional factor, which can be set to 3% to 5% of the rated working voltage of the module. smi is the module voltage of the ith put-in-place submodule, and then the module voltage of the put-in-place submodule is updated according to the dynamic additional factor: U smi =Usmi —K smi .

[0039] It can be seen that among the sub-modules of the bridge arm, the module voltages of the sub-modules that were put into operation in the previous cycle are updated, while the module voltages of the sub-modules that were not put into operation are still the current module voltages. At this time, the sub-modules of the bridge arm are sorted in ascending order according to the module voltages, and N sub-modules with smaller voltages are selected from the arranged module sequence in sequence according to the bridge arm modulation voltage instruction. m sub-modules are put into operation and the other modules are removed.

[0040] When the bridge arm current is in the discharge direction, similar to the above steps, the dynamic additional factor is calculated by traversing the sub-modules that have been put into use in the previous cycle. The difference from the above steps is that the calculation formula is different, namely: K smi =max{U smi —U L ,ΔU min}, where U L To dynamically add a low voltage threshold, it can be set to 0.8pu~0.9pu of the rated working voltage of the module. And update the module voltage of the submodule that has been put into use: U smi =U smi +K smi Then, the submodules of the bridge arm are sorted in ascending order according to the module voltage, and according to the bridge arm modulation voltage instruction, the N submodules with larger voltage are selected in reverse order from the arranged module sequence. m sub-modules are put into operation and the other modules are removed.

[0041] Through the switching strategy of this embodiment, not only the probability of module rotation can be reduced when the module voltage changes rapidly, thereby optimizing the larger switching frequency, but also the fluctuation range of the module voltage can be limited to the required range.

[0042] Combine the following Figure 6 The overall process of the capacitor voltage balancing control method of the MMC submodule provided by the present invention is described. After the valve control cycle starts, the bridge arm modulation voltage instruction, the bridge arm current and the module voltage are obtained according to the preset sampling frequency. First, the bridge arm current is judged in the near-zero area and not crossing the zero point, that is, whether the bridge arm current satisfies the formula |I arm | set And d|I arm | / dt<0, if it is satisfied, the counter will increase by 1, otherwise it will decrease by 1. If the bridge arm current I arm The bridge arm current I arm_pres ​If the product is less than zero, it means that the current has just passed zero, and the counter is cleared, otherwise, the counter remains unchanged. When the counter value is greater than the counting threshold, it means that the bridge arm is in the near-zero current risk zone.

[0043] When the bridge arm is in the near-zero current risk zone, a switching strategy of prohibiting module rotation and only responding to the increase or decrease in the number of modules put into operation is adopted. When the bridge arm is not in the near-zero current risk zone, the comparison relationship between the number of modules to be put into operation in the bridge arm modulation voltage instruction and the preset range is used to determine whether the bridge arm is in the modulation wave extreme value risk zone. If it is in the modulation wave extreme value risk zone, a switching strategy of prohibiting module rotation and only responding to the increase or decrease in the number of modules put into operation is adopted. If it is not in the modulation wave extreme value risk zone, the corresponding dynamic additional factor calculation formula is selected according to the current direction of the bridge arm current, and the module voltage of the sub-module that has been put into operation in the previous cycle is updated, and then the sub-modules of the bridge arm are sorted in ascending order according to the module voltage, and based on the current direction, the corresponding sub-module is selected from the sequence for operation, and other modules are removed. Thus, the switching operation of the MMC sub-module in a valve control cycle is completed.

[0044] The present embodiment provides a capacitor voltage balancing control method for an MMC submodule. The present invention targets two areas, namely the bridge arm current near-zero area and the modulation wave extreme value area, which are prone to repeated switching of submodules. By designing a switching strategy that only prohibits module rotation in this area and only responds to the increase or decrease in the number of modules put into operation, the switching frequency of the MMC is effectively reduced and the narrow pulse phenomenon of the submodule is completely eliminated. At the same time, the present invention also introduces a dynamic additional factor related to the instantaneous value of the submodule in the rated voltage area where the module voltage changes fastest and thus easily leads to switching rotation of the submodule. Through the sorting switching strategy based on the dynamic additional factor, not only can the probability of module rotation be reduced when the module voltage changes rapidly, but the fluctuation range of the module voltage can also be limited to the required range, further ensuring the safety and stability of the MMC operation.

[0045] See also Figure 7 Based on the same inventive concept, a capacitor voltage balancing control system of an MMC submodule proposed in a second embodiment of the present invention includes: The first risk zone determination module 10 is used to determine whether the bridge arm is in a near-zero current risk zone according to the direction of the bridge arm current in each valve control cycle; The second risk zone determination module 20 is used to determine whether the bridge arm is in the modulation wave extreme value risk zone according to the bridge arm modulation voltage instruction if the bridge arm is not in the near-zero current risk zone; A first strategy execution module 30 is used to adopt a first switching strategy to control the switching of each submodule of the bridge arm if the bridge arm is in a near-zero current risk zone or a modulation wave extreme value risk zone, wherein the first switching strategy is a switching strategy that prohibits module rotation and only responds to an increase or decrease in the number of modules put into operation; The second strategy execution module 40 is used to adopt a second switching strategy to control the switching of each submodule of the bridge arm if the bridge arm is not in the modulation wave extreme value risk area, and the second switching strategy is a switching strategy based on a dynamic additional factor.

[0046] The technical features and technical effects of the capacitor voltage balancing control system of the MMC submodule proposed in the embodiment of the present invention are the same as the method proposed in the embodiment of the present invention, and are not described in detail here. Each module in the capacitor voltage balancing control system of the above-mentioned MMC submodule can be implemented in whole or in part by software, hardware and a combination thereof. The above-mentioned modules can be embedded in or independent of the processor in the computer device in the form of hardware, or can be stored in the memory in the computer device in the form of software, so that the processor can call and execute the operations corresponding to the above modules.

[0047] In addition, an embodiment of the present invention further provides an MMC, the MMC includes a plurality of bridge arms, each bridge arm includes a plurality of sub-modules, and each sub-module of the MMC is controlled by the method described above.

[0048] In summary, an embodiment of the present invention proposes a capacitor voltage balancing control method, system and MMC for an MMC sub-module. The method determines whether the bridge arm is in a near-zero current risk zone according to the direction of the bridge arm current in each valve control cycle; if the bridge arm is not in the near-zero current risk zone, then determines whether the bridge arm is in the modulation wave extreme risk zone according to the bridge arm modulation voltage instruction; if the bridge arm is in the near-zero current risk zone or the modulation wave extreme risk zone, then a first switching strategy is adopted to control the switching of each sub-module of the bridge arm, and the first switching strategy is a switching strategy that prohibits module rotation and only responds to the increase or decrease in the number of modules put into operation; if the bridge arm is not in the modulation wave extreme risk zone, then a second switching strategy is adopted to control the switching of each sub-module of the bridge arm, and the second switching strategy is a switching strategy based on a dynamic additional factor. The present invention targets two areas, namely the bridge arm current near-zero area and the modulation wave extreme value area, which are prone to repeated switching of sub-modules. By designing a switching strategy that only prohibits module rotation and only responds to the increase or decrease in the number of modules put into operation in these areas, the switching frequency of the MMC is effectively reduced and the phenomenon of sub-module narrow pulse is completely eliminated. At the same time, the present invention also introduces a dynamic additional factor related to the instantaneous value of the sub-module in the rated voltage area where the module voltage changes fastest and thus easily leads to the switching rotation of the sub-module. Through the sorting switching strategy based on the dynamic additional factor, not only can the probability of module rotation be reduced when the module voltage changes rapidly, but the fluctuation range of the module voltage can also be limited to the required range, further ensuring the safety and stability of the MMC operation.

[0049] Each embodiment in this specification is described in a progressive manner, and the same or similar parts of each embodiment can be directly referred to each other, and each embodiment focuses on the differences from other embodiments. In particular, for the system embodiment, since it is basically similar to the method embodiment, the description is relatively simple, and the relevant parts can be referred to the partial description of the method embodiment. It should be noted that the technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, all possible combinations of the technical features in the above-mentioned embodiments are not described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0050] The above-mentioned embodiments only express several preferred implementation modes of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in the technical field, several improvements and substitutions can be made without departing from the technical principles of the present invention, and these improvements and substitutions should also be regarded as the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be based on the protection scope of the claims.

Claims

1. A capacitor voltage balancing control method for an MMC submodule, characterized in that: include: In each valve control cycle, the direction of the bridge arm current is used to determine whether the bridge arm is in a near-zero current risk zone; If the bridge arm is not in the near-zero current risk zone, then determine whether the bridge arm is in the modulation wave extreme value risk zone according to the bridge arm modulation voltage instruction; If the bridge arm is in a near-zero current risk zone or a modulation wave extreme value risk zone, a first switching strategy is adopted to control the switching of each submodule of the bridge arm, wherein the first switching strategy is a switching strategy that prohibits module rotation and only responds to an increase or decrease in the number of modules put into operation; If the bridge arm is not in the extreme value risk area of ​​the modulation wave, a second switching strategy is adopted to control the switching of each submodule of the bridge arm, and the second switching strategy is a switching strategy based on a dynamic additional factor.

2. The capacitor voltage balancing control method of the MMC submodule according to claim 1, characterized in that: The step of judging whether the bridge arm is in a near-zero current risk zone according to the direction of the bridge arm current comprises: Determine whether the absolute value of the bridge arm current is less than the current threshold and whether the bridge arm current is changing towards zero. If so, the counter is incremented by 1, otherwise, the counter is decremented by 1. Multiply the bridge arm current by the bridge arm current of the previous valve control cycle. If the product is a negative number, the counter is cleared; if it is not a negative number, the counter remains unchanged. It is determined whether the accumulated value of the counter is greater than the counting threshold. If so, it is determined that the bridge arm is in a near-zero current risk zone.

3. The capacitor voltage balancing control method of the MMC submodule according to claim 1, characterized in that: The step of judging whether the bridge arm is in the modulation wave extreme value risk zone according to the bridge arm modulation voltage instruction comprises: It is determined whether the bridge arm modulation voltage instruction is within a preset range. If so, it is determined that the bridge arm is not in the modulation wave extreme value risk zone. Otherwise, it is determined that the bridge arm is in the modulation wave extreme value risk zone.

4. The capacitor voltage balancing control method of the MMC submodule according to claim 1, characterized in that: The step of adopting the first switching strategy to control the switching of each submodule of the bridge arm includes: According to the bridge arm modulation voltage instruction of the current valve control cycle and the bridge arm modulation voltage instruction of the previous valve control cycle, it is determined whether the number of submodules put into operation in the current valve control cycle is the same as the number of submodules put into operation in the previous valve control cycle; If they are the same, the sub-module input instructions of the previous valve control cycle are maintained unchanged. If they are different, based on the increase or decrease in the number of sub-modules put into operation in the current valve control cycle and the number of sub-modules put into operation in the previous valve control cycle, operations such as adding new modules that were not put into operation in the previous valve control cycle or removing sub-modules that were put into operation in the previous valve control cycle are executed.

5. The capacitor voltage balancing control method of the MMC submodule according to claim 4, characterized in that: The step of adding a submodule that was not put into operation in the previous valve control cycle or removing a submodule that was put into operation in the previous valve control cycle according to the increase or decrease of the number of submodules put into operation in the current valve control cycle and the number of submodules put into operation in the previous valve control cycle comprises: When the bridge arm current is in the charging direction, the highest data position of the submodule that was removed in the previous valve control cycle is set to 1, and the submodules of the bridge arm are arranged in ascending order based on the module voltage, and the corresponding number of submodules are selected from the arranged module sequence in sequence according to the bridge arm modulation voltage instruction for operation; When the bridge arm current is in the discharge direction, the highest data position of the sub-module that has been put into operation in the previous valve control cycle is set to 1, and the sub-modules of the bridge arm are arranged in ascending order based on the module voltage. According to the bridge arm modulation voltage instruction, the corresponding number of sub-modules are selected in reverse order from the arranged module sequence for operation.

6. The capacitor voltage balancing control method of the MMC submodule according to claim 1, characterized in that: The step of adopting the second switching strategy to control the switching of each submodule of the bridge arm includes: According to the direction of the bridge arm current and the module voltage of the submodule that has been put into operation in the previous valve control cycle, the dynamic additional factor of the submodule that has been put into operation is calculated, and according to the dynamic additional factor, the module voltage of the submodule that has been put into operation is updated; According to the module voltage, the sub-modules of the bridge arm are arranged in ascending order, and according to the bridge arm current direction and the bridge arm modulation voltage instruction, a corresponding number of sub-modules are selected from the arranged module sequence for operation.

7. The capacitor voltage balancing control method of the MMC submodule according to claim 6, characterized in that: The step of calculating the dynamic additional factor of the put-in-place submodule according to the direction of the bridge arm current and the module voltage of the put-in-place submodule in the previous valve control cycle, and updating the module voltage of the put-in-place submodule according to the dynamic additional factor comprises: When the bridge arm current is in the charging direction, the difference between the dynamic additional high voltage threshold and the module voltage of the submodule that has been put into use in the previous valve control cycle is calculated, and the first dynamic additional factor is determined according to the comparison relationship between the difference and the minimum threshold of the additional factor; Subtracting the corresponding first dynamic additional factor from the module voltage of the submodule that has been put into use to obtain an updated module voltage of the submodule that has been put into use; When the bridge arm current is in the discharge direction, the difference between the module voltage of the submodule that has been put into use in the previous valve control cycle and the dynamic additional low-voltage threshold is calculated, and the second dynamic additional factor is determined according to the comparison relationship between the difference and the minimum threshold of the additional factor; The module voltage of the submodule that has been put into operation is added to the corresponding second dynamic additional factor to obtain an updated module voltage of the submodule that has been put into operation.

8. The capacitor voltage balancing control method of the MMC submodule according to claim 6, characterized in that: The step of selecting a corresponding number of submodules from the arranged module sequence for operation according to the bridge arm current direction and the bridge arm modulation voltage instruction comprises: When the bridge arm current is in the charging direction, according to the bridge arm modulation voltage instruction, a corresponding number of submodules are sequentially selected from the arranged module sequence for operation; When the bridge arm current is in the discharge direction, according to the bridge arm modulation voltage instruction, a corresponding number of sub-modules are selected from the arranged module sequence in reverse order to be put into operation.

9. A capacitor voltage balancing control system for an MMC submodule, characterized in that: include: The first risk zone determination module is used to determine whether the bridge arm is in a near-zero current risk zone according to the direction of the bridge arm current in each valve control cycle; The second risk zone determination module is used to determine whether the bridge arm is in the modulation wave extreme value risk zone according to the bridge arm modulation voltage instruction if the bridge arm is not in the near-zero current risk zone; A first strategy execution module is used to adopt a first switching strategy to control the switching of each submodule of the bridge arm if the bridge arm is in a near-zero current risk zone or a modulation wave extreme value risk zone, wherein the first switching strategy is a switching strategy that prohibits module rotation and only responds to an increase or decrease in the number of modules put into operation; The second strategy execution module is used to adopt a second switching strategy to control the switching of each sub-module of the bridge arm if the bridge arm is not in the modulation wave extreme risk area, and the second switching strategy is a switching strategy based on a dynamic additional factor.

10. An MMC, comprising a plurality of bridge arms, each bridge arm comprising a plurality of submodules, characterized in that: Each submodule of the MMC is controlled by the method according to any one of claims 1 to 8.

Images