Capacitor Voltage Balancing Control Method, System and MMC of MMC Sub-module

Abstract

The present invention relates to the technical field of MMC sub-module balance control, and discloses a capacitance voltage balance control method, system and MMC for MMC sub-modules, including: within each valve control period, judging whether the arm is in the near-zero current risk area according to the direction of the arm current; if the arm is not in the near-zero current risk area, judging whether the arm is in the modulation wave extreme value risk area according to the arm modulation voltage command; if the arm is in the near-zero current risk area or the modulation wave extreme value risk area, adopting a first switching strategy; if the arm is not in the modulation wave extreme value risk area, adopting a second switching strategy. By frequently judging the risk area for switching and corresponding switching strategies, the present invention effectively reduces the switching frequency, eliminates the narrow pulse phenomenon of the sub-module, and at the same time, through the sorting switching strategy based on the dynamic additional factor, effectively reduces the module rotation probability, and limits the fluctuation range of the module voltage within the required range, ensuring the safety and stability of the MMC operation.

Description

Technical Field

[0001] The present invention relates to the technical field of MMC sub-module balance control, and particularly to a method and system for controlling the capacitor voltage balance of MMC sub-modules and an MMC. Background Art

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

[0003] The commonly used control idea of the capacitor voltage balance control method is the switching control based on the sorting and equalizing of the capacitor voltage. Although this control idea is simple and easy to implement in the sorting logic and has a good equalizing effect, when it is used in the occasion with a very high number of levels or a large number of sub-module capacitors, it not only requires a large amount of sorting calculation, but also has a high switching frequency and large losses. The switching frequency not only has an important impact on the losses of the MMC, that is, the higher the switching frequency, the greater the losses of the converter and the worse the economy of the DC power transmission system, and the high switching frequency and the narrow pulses that may exist with a very short conduction time will also damage the power devices.

[0004] At present, most of the optimizations of the existing methods focus on the sorting aspect, and the calculation amount of the voltage sorting is reduced by optimizing the sorting algorithm, but the problem of high switching frequency still cannot be solved. Although the switching frequency can be optimized by introducing a proportional coefficient, a large proportional coefficient needs to be set to achieve an ideal frequency optimization effect. A large proportional coefficient will cause an increase in the voltage fluctuation range of the module, and frequent switching is likely to occur in some regions. Repeated switching will not only increase the switching frequency, but also the 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 method and system for controlling the capacitor voltage balance of MMC sub-modules and an MMC, so as to solve the problems of high switching frequency and large losses of the existing methods for MMC, achieve the effect of reducing the switching frequency of the MMC, eliminating the narrow pulse phenomenon of the sub-modules, and improving the safety of the MMC operation.

[0006] In the first aspect, the present invention provides a method for controlling the capacitor voltage balance of MMC sub-modules, and the method includes:

[0007] In each valve control period, according to the direction of the arm current, it is judged whether the arm is in the near-zero current risk area;

[0008] If the arm is not in the near-zero current risk area, then according to the arm modulation voltage command, it is judged whether the arm is in the modulation wave extreme value risk area;

[0009] If the arm is in the near-zero current risk area or the modulation wave extreme value risk area, then the first switching strategy is adopted to perform switching control on each sub-module of the 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 input modules;

[0010] If the arm is not in the modulation wave extreme value risk area, then the second switching strategy is adopted to perform switching control on each sub-module of the arm, and the second switching strategy is a switching strategy based on a dynamic additional factor.

[0011] Further, the step of judging whether the arm is in the near-zero current risk area according to the direction of the arm current includes:

[0012] Judge whether the absolute value of the arm current is less than the current threshold, and whether the arm current is changing in the direction of zero. If so, the counter is incremented by 1; otherwise, the counter is decremented by 1;

[0013] Multiply the arm current by the arm current in the previous valve control period. If the product is negative, the counter is cleared; if not negative, the counter remains unchanged;

[0014] Judge whether the accumulated value of the counter is greater than the counting threshold. If so, it is judged that the arm is in the near-zero current risk area.

[0015] Further, the step of judging whether the arm is in the modulation wave extreme value risk area according to the arm modulation voltage command includes:

[0016] Judge whether the arm modulation voltage command is within the preset range. If so, it is determined that the arm is not in the modulation wave extreme value risk area; otherwise, it is determined that the arm is in the modulation wave extreme value risk area.

[0017] Further, the step of adopting the first switching strategy to perform switching control on each sub-module of the arm includes:

[0018] According to the arm modulation voltage command in the current valve control period and the arm modulation voltage command in the previous valve control period, judge whether the number of input sub-modules in the current valve control period is the same as the number of input sub-modules in the previous valve control period;

[0019] If they are the same, the sub-module input instruction in the previous valve control period remains unchanged. If they are different, based on the increase or decrease in the number of sub-modules input in the current valve control period compared to the previous valve control period, perform operations of adding new sub-modules among those not input in the previous valve control period or removing sub-modules that were input in the previous valve control period.

[0020] Further, the step of performing operations of adding new sub-modules among those not input in the previous valve control period or removing sub-modules that were input in the previous valve control period based on the increase or decrease in the number of sub-modules input in the current valve control period compared to the previous valve control period includes:

[0021] When the arm current is in the charging direction, set the highest data bit of the sub-module removed in the previous valve control period to 1, sort the sub-modules of the arm in ascending order based on the module voltage, and sequentially select the corresponding number of sub-modules from the sorted module sequence for input operations according to the arm modulation voltage instruction;

[0022] When the arm current is in the discharging direction, set the highest data bit of the sub-module input in the previous valve control period to 1, sort the sub-modules of the arm in ascending order based on the module voltage, and inversely select the corresponding number of sub-modules from the sorted module sequence for input operations according to the arm modulation voltage instruction.

[0023] Further, the step of using the second switching strategy to perform switching control on the sub-modules of the arm includes:

[0024] According to the direction of the arm current and the module voltage of the sub-modules input in the previous valve control period, calculate the dynamic additional factor of the input sub-modules, and update the module voltage of the input sub-modules according to the dynamic additional factor;

[0025] Sort the sub-modules of the arm in ascending order based on the module voltage, and select the corresponding number of sub-modules from the sorted module sequence for input operations according to the arm current direction and the arm modulation voltage instruction.

[0026] Further, the step of calculating the dynamic additional factor of the input sub-modules according to the direction of the arm current and the module voltage of the sub-modules input in the previous valve control period, and updating the module voltage of the input sub-modules according to the dynamic additional factor includes:

[0027] When the arm current is in the charging direction, calculate the difference between the dynamic additional high voltage threshold and the module voltage of the sub-modules input in the previous valve control period, and determine the first dynamic additional factor according to the comparison relationship between the difference and the additional factor minimum threshold;

[0028] Subtract the corresponding first dynamic additional factor from the module voltage of the inserted sub-module to obtain the updated module voltage of the inserted sub-module;

[0029] When the arm current is in the discharging direction, calculate the difference between the module voltage of the inserted sub-module in the previous valve control period and the dynamic additional low voltage threshold, and determine the second dynamic additional factor according to the comparison relationship between the difference and the additional factor minimum threshold;

[0030] Add the module voltage of the inserted sub-module to the corresponding second dynamic additional factor to obtain the updated module voltage of the inserted sub-module.

[0031] Further, the step of selecting the corresponding number of sub-modules from the arranged module sequence for insertion operation according to the arm current direction and the arm modulation voltage command includes:

[0032] When the arm current is in the charging direction, sequentially select the corresponding number of sub-modules from the arranged module sequence for insertion operation according to the arm modulation voltage command;

[0033] When the arm current is in the discharging direction, inversely select the corresponding number of sub-modules from the arranged module sequence for insertion operation according to the arm modulation voltage command.

[0034] In a second aspect, the present invention provides a capacitor voltage balancing control system for an MMC sub-module, the system includes:

[0035] A first risk area determination module, configured to judge whether the arm is in a near-zero current risk area according to the direction of the arm current in each valve control period;

[0036] A second risk area determination module, configured to judge whether the arm is in a modulation wave extreme value risk area according to the arm modulation voltage command if the arm is not in the near-zero current risk area;

[0037] A first strategy execution module, configured to adopt a first switching strategy to perform switching control on each sub-module of the arm if the arm is in the near-zero current risk area or the modulation wave extreme value risk area, and the first switching strategy is a switching strategy that prohibits module rotation and only responds to the increase or decrease of the number of inserted modules;

[0038] A second strategy execution module, configured to adopt a second switching strategy to perform switching control on each sub-module of the arm if the 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.

[0039] In a third aspect, an embodiment of the present invention further provides an MMC, the MMC includes a plurality of arms, each arm includes a plurality of sub-modules, and each sub-module of the MMC is controlled by the method described above.

[0040] The present invention provides a capacitance voltage balancing control method, system and MMC for MMC sub-modules. Aiming at the two regions where the arm current is near zero and the modulation wave reaches the extreme value, which are prone to repeated switching of sub-modules, the present invention designs an operation strategy in these regions that only prohibits module rotation and only responds to the increase or decrease in the number of modules put into operation. This can effectively reduce the switching frequency of the MMC and completely eliminate the narrow pulse phenomenon of sub-modules. 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 region where the module voltage changes fastest and is prone to sub-module switching rotation. Through the sorting and 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 within the required range, further ensuring the safety and stability of the MMC operation. Brief Description of the Drawings

[0041] Figure 1 is a schematic flow chart of the capacitance voltage balancing control method for MMC sub-modules in an embodiment of the present invention;

[0042] Figure 2 is a topological structure diagram of the MMC;

[0043] Figure 3 is a schematic diagram of the near-zero frequent switching risk area of the arm current in the MMC;

[0044] Figure 4 is a schematic diagram of the process of frequent switching of a single module;

[0045] Figure 5 is a schematic diagram of the modulation wave extreme value area in an embodiment of the present invention;

[0046] Figure 6 is another schematic flow chart of the capacitance voltage balancing control method for MMC sub-modules in an embodiment of the present invention;

[0047] Figure 7 is a schematic structure diagram of the capacitance voltage balancing control system for MMC sub-modules in an embodiment of the present invention. Detailed Embodiments

[0048] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0049] Please refer to Figure 1, a method for controlling the capacitor voltage balance of an MMC sub-module proposed in the first embodiment of the present invention, which includes steps S10 to S40:

[0050] Step S10, within each valve control period, determine whether the arm is in the near-zero current risk area according to the direction of the arm current;

[0051] Step S20, if the arm is not in the near-zero current risk area, determine whether the arm is in the modulation wave extreme value risk area according to the arm modulation voltage command;

[0052] Step S30, if the arm is in the near-zero current risk area or the modulation wave extreme value risk area, adopt the first switching strategy to perform switching control on each sub-module of the 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;

[0053] Step S40, if the arm is not in the modulation wave extreme value risk area, adopt the second switching strategy to perform switching control on each sub-module of the arm, and the second switching strategy is a switching strategy based on a dynamic additional factor.

[0054] The present invention provides a method for balancing control of the sub-modules of an MMC. The MMC is a modular multilevel converter, and its topological structure is as Figure 2 shown. A modular multilevel converter generally has 3 phase units. Each phase unit consists of 1 upper arm and 1 lower arm. Each arm is composed of several sub-modules SM i and 1 arm reactor connected in series. The nearest level approximation modulation strategy is used to control the arm output voltage. By dynamically allocating the number of power modules that need to be in the on state in the upper and lower arms, making it approximate the arm modulation wave, the required converter output voltage can be obtained. Among them, Figure 2 the u a , u b and u c respectively represent the voltages of the three phase units, and I arm represents the arm current. The suffixes abc respectively represent the three phase units, and the suffixes u and d respectively represent the upper and lower arms. Each sub-module is a power module. T1 and T2 represent transistors. The specific structure and its description can refer to the conventional topological structure of the MMC, which will not be elaborated here one by one.

[0055] In the present invention, for each valve control period of the MMC, first determine whether the arm is in the near-zero current risk area according to the direction of the arm current. The specific steps include:

[0056] Determine whether the absolute value of the arm current is less than the current threshold and whether the arm current is changing in the direction of zero. If so, increment the counter by 1; otherwise, decrement the counter by 1;

[0057] Multiply the arm current by the arm current in the previous valve control period. If the product is negative, clear the counter; if not negative, keep the counter unchanged.

[0058] Determine whether the accumulated value of the counter is greater than the counting threshold. If so, determine that the arm is in the near-zero current risk area.

[0059] In this embodiment, the near-zero current risk area is as Figure 3 shown, which refers to the risk area where the characteristics of the modules expected to be put in will reverse during the period before and after the arm current passes through zero. For example, from the larger value (or smaller value) in the module before the arm current passes through zero to the smaller value (or larger value) of the module voltage after passing through zero. It is easy to have the risk that the power module just removed will be quickly put in again after the arm current passes through zero, or the put-in module will be quickly removed again after the arm current passes through zero. This frequent switching not only increases the switching frequency, but also the too-narrow put-in pulse will damage the power device. The single-module frequent switching process is as Figure 4 shown. When discharging, the sub-modules No. 1 to No. 10 are sorted in ascending order of voltage. At this time, the put-in sub-modules are a total of 6, namely No. 1, No. 6 to No. 10. When the arm current approaches zero, the number of put-in arms decreases from 6 to 5, then the lowest-voltage No. 1 will be removed. When the number of put-in arms remains unchanged and the direction of the arm current changes from discharging to charging, according to the sorting and voltage equalization principle, the put-in sub-modules change from No. 6 to No. 10 to No. 1, No. 6 to No. 9, that is, the No. 1 sub-module has frequent switching.

[0060] To solve the influence of the current near-zero region, there are currently two common methods. One is to reduce the influence of the current near-zero region by setting a current direction change hysteresis loop. However, the current direction hysteresis loop only translates the entire risk region back and forth, and does not reduce the influence of the zero-crossing region at all. The other method is to extend the pulse time of the narrow pulse through the drive circuit to avoid the narrow pulse problem. But this method neither reduces the switching frequency nor causes short-term distortion of the arm output voltage. To solve this problem, in this embodiment, by analyzing the direction of the arm current, it is accurately determined whether the arm is in the near-zero current risk area, and corresponding switching control strategies are adopted when in the near-zero current risk area to overcome the influence of the current near-zero region.

[0061] In this embodiment, the determination of the near-zero current risk area is based on the direction of the arm current. First, judge whether the arm current I arm in the current valve control period is in the near-zero region but has not passed through zero, that is, the absolute value of the current is less than the current threshold I set and the current is changing towards zero, and its formula is expressed as: |I arm | < I set and d|I arm| / dt < 0, where t represents time and the current threshold is I set It is set based on the peak current and preferably can be set to 5% - 10% of the peak current.

[0062] If these two conditions are met, the counter is incremented by 1. If not, the counter is decremented by 1. This step is determined in real time based on the current sampling frequency. Then, the arm current I within the current valve control period arm is multiplied by the arm current I arm_pres of the previous valve control period. If the product is less than zero, it indicates that the current has just passed through zero, and the counter is cleared. Otherwise, the counter remains unchanged. When the value of the counter is greater than the counting threshold, it means that the arm is in the near-zero current risk area and sub-module frequent switching is likely to occur. Here, the counting threshold is determined based on the valve control period and the counting time. For example, when a counting time of 1 ms is selected, the counting threshold can be set to 20 at a valve control period of 50 us.

[0063] When the arm is in the near-zero current risk area, to avoid frequent switching of sub-modules, in this embodiment, a switching control strategy that prohibits module rotation and only responds to the increase or decrease in the number of inserted modules is adopted for the sub-modules of the arm. The specific steps of this switching control strategy include:

[0064] Based on the arm modulation voltage command of the current valve control period and the arm modulation voltage command of the previous valve control period, determine whether the number of inserted sub-modules in the current valve control period is the same as that in the previous valve control period;

[0065] If they are the same, the sub-module insertion command of the previous valve control period remains unchanged. If they are different, according to the increase or decrease in the number of inserted sub-modules in the current valve control period compared to the previous valve control period, perform the operation of adding new sub-modules among those not inserted in the previous valve control period or removing sub-modules that were inserted in the previous valve control period.

[0066] In this embodiment, prohibiting module rotation and only responding to the increase or decrease in the number of inserted modules means that when the number of sub-modules to be inserted in the current valve control period is the same as that in the previous valve control period, the sub-module insertion command of the previous valve control period remains unchanged. Among them, the number of inserted sub-modules is issued by the arm modulation voltage command, that is, the arm modulation voltage command N m is the number of sub-modules to be inserted.

[0067] When the number of sub - modules to be put into operation in the present valve - control cycle changes compared with the previous valve - control cycle, if the number increases, based on the sub - modules already put into operation in the previous cycle, select the newly added sub - modules to be put into operation from the non - put - in modules; if the number decreases, cut off some of the sub - modules already put into operation in the previous cycle, and the remaining sub - modules remain in the put - in state. To implement this switching strategy, in a preferred embodiment, a sorting algorithm based on the highest data position 1 is used to sort each sub - module. The specific sorting steps are as follows:

[0068] When the arm current is in the charging direction, set the highest data position of the sub - modules cut off in the previous valve - control cycle to 1, sort each sub - module of the arm in ascending order based on the module voltage, and according to the arm modulation voltage command, sequentially select the corresponding number of sub - modules from the sorted module sequence for the put - in operation;

[0069] When the arm current is in the discharging direction, set the highest data position of the sub - modules already put into operation in the previous valve - control cycle to 1, sort each sub - module of the arm in ascending order based on the module voltage, and according to the arm modulation voltage command, inversely select the corresponding number of sub - modules from the sorted module sequence for the put - in operation.

[0070] In this embodiment, based on the direction of the arm current, the sorting steps are also different. First, for the case where the arm current is in the charging direction, set the highest data position of the sub - modules cut off in the previous valve - control cycle to 1. The voltage information of the sub - module is stored in the data bit of the sub - module. When its highest data position is set to 1, in fact, the voltage of this sub - module is set to a very large value. It should be noted here that if the highest bit of the data bit of the sub - module is the sign bit, then select the next non - sign bit for setting to 1.

[0071] Then sort each sub - module in ascending order according to the module voltage from low to high. It can be seen that since the voltage values of the sub - modules cut off in the previous valve - control cycle are set to larger values, these sub - modules will be arranged at the end of the sequence during sorting. That is to say, the sub - modules put into operation in the previous valve - control cycle are arranged at the front of the sequence. When selecting sub - modules according to the arm modulation voltage command N m select only N m sub - modules with lower voltages from the sequence and put them into operation.

[0072] When the arm current is in the discharging direction, the highest data position of the sub-modules that have been put into operation in the previous valve control period is set to 1, that is, the voltage values of these previously put-in sub-modules are set to a very large value, and then the sub-modules are sorted in ascending order according to the module voltage from low to high. In this sequence, the sub-modules that have been put into operation in the previous valve control period will be arranged at the end of the sequence. When the arm current is in the discharging direction, the sub-modules with higher voltages will be selected for input. According to the arm modulation voltage command N m When selecting sub-modules, only need to inversely select N m sub-modules with higher voltages from the sequence for input. In the actual operation process of the arm, for each execution of the switching instruction, sorting and selection need to be performed again. Through this method of this embodiment, it can be ensured that whether the number of inputs this time increases or decreases compared with the previous cycle, the above switching strategy can be achieved.

[0073] In addition to the situation that sub-modules are prone to frequent switching in the near-zero current region, in the actual operation process, there will be another risk region that will cause sub-modules to switch frequently. This risk region is the modulation wave extreme value risk region, that is, when the modulation wave is at its maximum or minimum value, sub-modules are prone to frequent switching. As Figure 5 shown, it can be seen that when the modulation wave is at its maximum value (≥0.95 pu) or minimum value (≤0.05 pu), there are very few selectable modules for the newly added change in the number of modules. If module switching rotation occurs due to the voltage difference of the modules at this time, when the number of modulations changes, then only the modules that have just participated in the rotation can be selected as the newly added and changed modules, which will cause the power modules that have just been cut off due to rotation to be quickly put in again after the number of modulations changes, or the modules that have just been put in to be quickly cut off again.

[0074] For the above reasons, in this embodiment, when it is determined that the arm is not in the near-zero current risk region, it is also necessary to further determine whether the arm is in the modulation wave extreme value risk region. The specific determination steps include:

[0075] Judge whether the arm modulation voltage command is within the preset range. If so, it is determined that the arm is not in the modulation wave extreme value risk region; otherwise, it is determined that the arm is in the modulation wave extreme value risk region.

[0076] In this embodiment, the determination of the modulation wave extreme value risk region is based on the arm modulation voltage command. During the operation of the arm, its modulation wave represents the relationship between time and the number of inputs, that is, the modulation wave is positively correlated with the number of inputs. Therefore, it is possible to determine whether the arm is in the modulation wave extreme value risk region through the arm modulation voltage command (the number of sub-modules to be put in). Specifically, according to the rated number of sub-modules of the arm, a threshold range (N L ,NH ), preferably N H and N L can be set to 95% and 5% of the rated number respectively. When N L <N m <N H it is determined that the bridge arm is not in the risk area of the modulation wave extreme value. When N m ≥N H or N m ≤N L it is determined that the bridge arm is in the risk area of the modulation wave extreme value.

[0077] When the bridge arm is in the risk area of the modulation wave extreme value, the same switching strategy as when judging that the bridge arm is in the near-zero current risk area is adopted, that is, the switching strategy that only responds to the increase and decrease of the number of inserted modules and prohibits module rotation will not be repeated here.

[0078] When the bridge arm is neither in the near-zero current risk area nor in the modulation wave extreme value risk area, the second switching strategy is adopted, that is, the switching strategy based on the dynamic additional factor. The specific steps include:

[0079] Calculate the dynamic additional factor of the inserted sub-modules according to the direction of the bridge arm current and the module voltages of the sub-modules inserted in the previous valve control period, and update the module voltages of the inserted sub-modules according to the dynamic additional factor;

[0080] Arrange the sub-modules of the bridge arm in ascending order according to the module voltages, and select the corresponding number of sub-modules from the arranged module sequence for insertion operation according to the bridge arm current direction and the bridge arm modulation voltage command.

[0081] In this embodiment, for the rated voltage area where the module voltage changes fastest and is prone to cause sub-module switching rotation, a dynamic additional factor related to the instantaneous value of the sub-module is introduced. Its purpose is to reduce the module rotation probability when the module voltage changes rapidly, and at the same time limit the fluctuation range of the module voltage within the required range. That is, by introducing a dynamic additional factor to the sub-modules inserted in the previous valve control period, the module voltages of the sub-modules are updated, so as to reduce the rotation probability of these sub-modules. The specific steps include:

[0082] When the bridge arm current is in the charging direction, calculate the difference between the dynamic additional high voltage threshold and the module voltages of the sub-modules inserted in the previous valve control period, and determine the first dynamic additional factor according to the comparison relationship between the difference and the additional factor minimum threshold;

[0083] Subtract the corresponding first dynamic additional factor from the module voltages of the inserted sub-modules to obtain the updated module voltages of the inserted sub-modules;

[0084] When the arm current is in the discharging direction, calculate the difference between the module voltage of the sub-modules that have been put into operation in the previous valve control period and the dynamic additional low voltage threshold, and determine the second dynamic additional factor according to the comparison relationship between the difference and the minimum threshold of the additional factor;

[0085] Add the module voltage of the sub-modules that have been put into operation to the corresponding second dynamic additional factor to obtain the updated module voltage of the sub-modules that have been put into operation.

[0086] In this embodiment, when the arm current is in the charging direction, traverse each sub-module that was put into operation in the previous cycle, obtain its module voltage, and then determine its dynamic additional factor according to the formula: K smi = max{U H —U smi ,ΔU min}, where U H is the dynamic additional high voltage threshold, which can be set to 1.1 pu - 1.2 pu of the rated working voltage of the module, ΔU min is the minimum threshold of the additional factor, which can be set to 3% - 5% of the rated working voltage of the module, and U smi is the module voltage of the i-th sub-module that has been put into operation. Then, update the module voltage of the sub-modules that have been put into operation according to the dynamic additional factor: U smi =U smi —K smi .

[0087] It can be seen that among the sub-modules of the arm, for those sub-modules that were put into operation in the previous cycle, their module voltages are updated, while for those sub-modules that were not put into operation, they remain the current module voltages. At this time, sort the sub-modules of the arm in ascending order according to the module voltage, and according to the arm modulation voltage command, sequentially select N m sub-modules with smaller voltages from the arranged module sequence for input operation, and cut off other modules.

[0088] When the arm current is in the discharging direction, similar to the above steps, calculate its dynamic additional factor by traversing the sub-modules that have been put into operation in the previous cycle. The difference from the above steps is that its calculation formula is different, that is: K smi =max{U smi —U L ,ΔU min}, where in the formula, U L is the dynamic additional low voltage threshold, which can be set to 0.8 pu - 0.9 pu of the rated working voltage of the module. And update the module voltage of the sub-modules that have been put into operation: U smi =U smi +K smiThen, the sub - modules of each arm are sorted in ascending order according to the module voltage, and according to the arm modulation voltage command, N sub - modules with larger voltages are selected in reverse order from the arranged module sequence for input operation, and the other modules are cut off. m sub - modules are put into operation, and the other modules are cut off.

[0089] Through the switching strategy of this embodiment, not only can the probability of module rotation be reduced when the module voltage changes rapidly, thus optimizing a relatively large switching frequency, but also the fluctuation range of the module voltage can be limited within the required range.

[0090] The following combines Figure 6 to illustrate the overall process of the capacitor voltage equalization control method for the MMC sub - modules provided by the present invention. After the start of this valve control period, the arm modulation voltage command, arm current, and module voltage are obtained according to the preset sampling frequency. First, it is judged whether the arm current is in the near - zero region and has not passed through zero, that is, it is judged whether the arm current satisfies the formula |I arm |<I set and d|I arm | / dt < 0. If it is satisfied, the counter is incremented by 1, otherwise it is decremented by 1. If the arm current I arm in the current valve control period is multiplied by the arm current I arm_pres in the previous valve control period, and if the product is less than zero, it means that the current has just passed through zero, then the counter is cleared, otherwise, the counter remains unchanged. When the value of the counter is greater than the counting threshold, it indicates that the arm is in the near - zero current risk area.

[0091] When the arm is in the near - zero current risk area, a switching strategy that prohibits module rotation and only responds to the increase or decrease of the number of modules put into operation is adopted. When the arm is not in the near - zero current risk area, according to the comparison relationship between the number of modules to be put into operation in the arm modulation voltage command and the preset range, it is judged whether the arm is in the modulation wave extreme value risk area. If it is in the modulation wave extreme value risk area, a switching strategy that prohibits module rotation and only responds to the increase or decrease of the number of modules put into operation is adopted. If it is not in the modulation wave extreme value risk area, according to the current direction of the arm current, the corresponding dynamic additional factor calculation formula is selected, and the module voltages of the sub - modules that have been put into operation in the previous period are updated. Then, the sub - modules of each arm are sorted in ascending order according to the module voltage, and based on the current direction, the corresponding sub - modules are selected from the sequence for input operation, and the other modules are cut off. Thus, the switching operation of the MMC sub - modules within one valve control period is completed.

[0092] A method for controlling the capacitor voltage balance of an MMC sub-module provided in this embodiment. In view of the two regions where the arm current is near zero and the modulation wave is at its extreme value, which are likely to cause repeated switching of sub-modules, the present invention designs 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 regions, effectively reducing the switching frequency of the MMC and completely eliminating the narrow pulse phenomenon of sub-modules. 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 region where the module voltage changes fastest and is likely to cause switching rotation of sub-modules. Through the sorting and 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 within the required range, further ensuring the safety and stability of the MMC operation.

[0093] Please refer to Figure 7 , based on the same inventive concept, a capacitor voltage balance control system for an MMC sub-module proposed in the second embodiment of the present invention includes:

[0094] The first risk area determination module 10 is used to determine whether the arm is in the near-zero current risk area according to the direction of the arm current in each valve control period;

[0095] The second risk area determination module 20 is used to determine whether the arm is in the modulation wave extreme value risk area according to the arm modulation voltage command if the arm is not in the near-zero current risk area;

[0096] The first strategy execution module 30 is used to adopt the first switching strategy to control the switching of each sub-module of the arm if the arm is in the near-zero current risk area or the modulation wave extreme value risk area, 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;

[0097] The second strategy execution module 40 is used to adopt the second switching strategy to control the switching of each sub-module of the arm if the 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.

[0098] The technical features and technical effects of the capacitor voltage balance control system for the MMC sub-module proposed in the embodiment of the present invention are the same as those of the method proposed in the embodiment of the present invention, and will not be elaborated here. Each module in the above capacitor voltage balance control system for the MMC sub-module can be implemented in whole or in part by software, hardware, and their combination. The above-mentioned modules can be embedded in or independent of the processor in the computer device in the form of hardware, or stored in the memory of the computer device in the form of software, so that the processor can call and execute the operations corresponding to the above-mentioned modules.

[0099] In addition, an embodiment of the present invention further provides an MMC. The MMC includes a plurality of arms, each arm includes a plurality of sub-modules, and the sub-modules of the MMC are controlled by the method described above.

[0100] In summary, an embodiment of the present invention provides a method and system for controlling the capacitor voltage balance of an MMC sub-module and an MMC. In each valve control period, the method determines whether the arm is in the near-zero current risk area according to the direction of the arm current; if the arm is not in the near-zero current risk area, it determines whether the arm is in the modulation wave extreme value risk area according to the arm modulation voltage command; if the arm is in the near-zero current risk area or the modulation wave extreme value risk area, a first switching strategy is adopted to perform switching control on the sub-modules of the arm. 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 arm is not in the modulation wave extreme value risk area, a second switching strategy is adopted to perform switching control on the sub-modules of the arm. The second switching strategy is a switching strategy based on a dynamic additional factor. The present invention aims at two regions, namely the near-zero region of the arm current and the extreme value region of the modulation wave, which are likely to cause 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 this region, the switching frequency of the MMC is effectively reduced and the narrow pulse phenomenon of the sub-modules 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 region where the module voltage changes fastest and is likely to cause sub-module switching rotation. Through the sorting and switching strategy based on the dynamic additional factor, not only can the module rotation probability be reduced when the module voltage changes rapidly, but also the fluctuation range of the module voltage can be limited within the required range, further ensuring the safety and stability of the operation of the MMC.

[0101] Each embodiment in this specification is described in a progressive manner. For the parts that are the same or similar in each embodiment, reference can be made to each other. 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 reference can be made to the partial description of the method embodiment for the relevant parts. It should be noted that the technical features of the above embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered that the scope described in this specification.

[0102] The above-described embodiments merely represent several preferred embodiments of the present application. The description thereof is relatively specific and detailed, but it should not be construed as a limitation on the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the technical principles of the present invention, several improvements and substitutions can be made, 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 subject to the protection scope of the claims described above.

Claims

1. A capacitor voltage balancing control method for an MMC submodule, characterized in that: include: 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 zone of the modulation wave, 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; 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.

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 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.

5. 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.

6. The capacitor voltage balancing control method of the MMC submodule according to claim 5, 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.

7. The capacitor voltage balancing control method of the MMC submodule according to claim 5, 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.

8. 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 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 submodule input instruction of the previous valve control cycle is maintained unchanged; if they are different, according to the increase or decrease of the number of submodules input in the current valve control cycle and the number of submodules input in the previous valve control cycle, the operation of adding to the submodule that was not put into operation in the previous valve control cycle or removing from the submodule that was put into operation in the previous valve control cycle is performed; 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.

9. 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 7.

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