CN112865568B - Voltage-sharing control method for optimizing average switching frequency of MMC (Modular multilevel converter) - Google Patents

Voltage-sharing control method for optimizing average switching frequency of MMC (Modular multilevel converter) Download PDF

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CN112865568B
CN112865568B CN202110120826.1A CN202110120826A CN112865568B CN 112865568 B CN112865568 B CN 112865568B CN 202110120826 A CN202110120826 A CN 202110120826A CN 112865568 B CN112865568 B CN 112865568B
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mmc
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CN112865568A (en
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罗永捷
宋勇辉
贾赞
李奇芬
蒲羿
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Chongqing University
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/02—Conversion of ac power input into dc power output without possibility of reversal
    • H02M7/04—Conversion of ac power input into dc power output without possibility of reversal by static converters
    • H02M7/12—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/21—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M7/217—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M7/219—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only in a bridge configuration
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/483—Converters with outputs that each can have more than two voltages levels
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/53—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M7/537—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
    • H02M7/5387—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration
    • H02M7/53871—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current

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  • Power Engineering (AREA)
  • Inverter Devices (AREA)

Abstract

The invention discloses a voltage-sharing control method for optimizing the average switching frequency of an MMC (modular multilevel converter), which takes a submodule rotation number optimization method as a core, a mark input group and an input group are reordered to form a rearranged sequence group, submodules are selected from the rearranged sequence group to be set to be removed, and the submodules which are not selected and set to be removed in the mark input group are set to be input, so that the condition that the voltage of individual submodules of the input group is higher or lower than that of individual submodules of the removal group is effectively avoided, unnecessary submodule rotation switching is eliminated, the voltage equalization effect can be ensured, and the switching frequency of the MMC system can be reduced. On the basis of the optimization method of the number of the sub-module rotations, the invention determines whether the optimization program of the number of the sub-module rotations is needed under the actual operation condition by adding the preposed judgment, and can further reduce the switching frequency.

Description

Voltage-sharing control method for optimizing average switching frequency of MMC (Modular multilevel converter)
Technical Field
The invention belongs to the technical field of power transmission and distribution of a power system, and particularly relates to a voltage-sharing control method for optimizing the average switching frequency of an MMC (modular multilevel converter).
Background
Modular Multilevel Converters (MMC) are different from two-level and three-level converters, and a mode of cascading a plurality of submodules is adopted, so that the modular multilevel converter has the advantages of high expansibility, high output waveform quality, low loss and the like, and gradually becomes a research hotspot in the field of high-voltage direct-current power transmission. With the continuous improvement of the capacity and the voltage grade, the number of the sub-modules and the power semiconductor devices in the high-voltage large-capacity MMC system is also increased rapidly, so that the problem of the operation loss of the converter valve is increasingly highlighted.
At present, MMC-HVDC systems which are put into operation at home and abroad widely adopt nearest level approximation modulation (NLM) and sequencing voltage-sharing control strategies. The method has the advantages of simplicity, convenience, easy use and high reliability, and is particularly suitable for the application field of high-voltage large-capacity MMC with high level number. Under the traditional sorting and voltage-sharing strategy, the submodule in the MMC bridge arm is quickly switched according to the reference voltage change of the bridge arm and the current direction of the bridge arm, and the balance control of capacitance energy is realized through the charge and discharge of a capacitor. In this way, the small capacitance transient energy difference during normal operation of the MMC can cause high-frequency switching of a large number of sub-modules, which generates high switching loss, and restricts the application of the method to a certain extent. In order to solve the problems, scholars at home and abroad provide a series of improved voltage-sharing methods, which comprise various voltage-sharing strategies of introducing a retention factor, an energy balance factor and a double retention factor, setting a maximum voltage deviation threshold of a submodule capacitor voltage, setting upper and lower limits of the submodule capacitor voltage and the like.
On the basis of the research, plum blossom, etc. of the institute of electrical technology of the Chinese academy of sciences put forward a practical engineering voltage-sharing control strategy with low switching frequency, and the switching frequency of the MMC system is optimized by fixing the module alternation number of each control period. The method is simple and easy to implement, and the switching frequency can be quantitatively calculated. However, since the number of module rotation in each control cycle is fixed, the switching states of the submodules in the bridge arm cannot be dynamically adjusted under different conditions, many unnecessary submodules can be alternately switched, and the switching frequency still needs to be further optimized.
Disclosure of Invention
Aiming at the technical defects, the invention provides a voltage-sharing control method for optimizing the average switching frequency of an MMC (modular multilevel converter), which solves the technical problem that the prior art cannot dynamically adjust the fixed rotation number according to the switching state of sub-modules in a bridge arm under different conditions, so that unnecessary rotation switching of the sub-modules is caused.
In order to solve the technical problem, the invention provides a voltage-sharing control method for optimizing the average switching frequency of an MMC, which comprises the following steps:
sequencing all the capacitor voltages of the sub-modules, and dividing the sub-modules in the same bridge arm into an input group and a cut-off group;
judging whether the difference value delta nref of the numbers of the modulation output conduction modules of the adjacent periods is larger than 0; if the delta nref is larger than or equal to 0, the input group needs to be added with the sub-module of delta nref | to ensure necessary switching; if delta nref is less than 0, it indicates that the input group needs to reduce | delta nref | submodules to ensure necessary switching;
on the premise of ensuring the stable operation of the system, namely on the premise of ensuring the number of necessary switching submodules, the number of submodule turns for realizing the voltage-sharing effect is optimized according to the following mode:
when the delta nref is more than or equal to 0, selecting delta nref + N in the excision group according to the charge-discharge state of the MMCbThe submodules with the minimum or the maximum capacitor voltage are marked to form a mark input group, NbRepresents a fixed number of rotations; the tag input set and the input set are then reordered to form a reordered set, and N is selected from the reordered setbThe submodules with the largest or smallest capacitor voltage are set to be cut off, and the submodules which are not selected and set to be cut off in the mark input group are set to be input, so that the number of necessary switching submodules is ensured, and the optimization of the number of the submodule turns is realized;
when Δ nref < 0, N was selected in the cut-off group according to the charge-discharge state of MMCbThe submodules with the minimum or the maximum capacitor voltage are marked to form a mark input group, NbRepresents a fixed number of rotations; the marked drop groups are then reordered from the drop group to form reordered groups, and | Δ nref | + N is selected from the reordered groupsbAnd the submodules with the largest or smallest capacitor voltage are set to be cut off, and the submodules which are not selected and set to be cut off in the mark input group are set to be input, so that the number of necessary switching submodules is ensured, and the optimization of the number of the submodule turns is realized.
Preferably, before the number of the sub-module rotations is optimized, a pre-judgment is further performed:
if xmin(k+1)＝xmin(k)，xmax(k+1)＝xmax(k)，ΔUmax(k+1)<ΔUmax(k) If the three conditions are met simultaneously, the capacitor voltage tends to be converged, and only necessary switching is performed; if the three conditions cannot be met at the same time, optimizing the rotation number of the sub-modules;
wherein x ismin(k +1) represents the submodule number, x, in the present period in which the capacitor voltage is the smallestmax(k +1) represents the number of the submodule with the largest capacitance voltage in the period; x is the number ofmin(k) The sub-module number, x, representing the minimum capacitor voltage in the previous cyclemax(k +1) represents the submodule number with the largest capacitance voltage in the last period; delta Umax(k +1) represents the maximum capacitance voltage difference value in the period; delta Umax(k) Representing the maximum capacitor voltage difference of the last periodThe value is obtained.
Preferably, when Δ nref ≧ 0, before the excision group marker is input, the following determination is also made: judgment of NOFF≥Δnref+Nb,NON≥NbWhether the two are true at the same time; if yes, putting the mark of the resection group; if not, performing necessary switching;
when Δ nref < 0, before the excision set markers are applied, the following judgment is also made: judgment of NON≥Δnref+Nb,NOFF≥NbWhether the two are true at the same time; if yes, putting the mark of the resection group; if not, performing necessary switching;
wherein N isOFFRepresenting the number of sub-modules of the resection group in the same bridge arm in the previous period; n is a radical ofONAnd representing the number of sub-modules put into groups in the same bridge arm in the previous period.
Compared with the prior art, the invention has the beneficial effects that:
1. in the actual operation working condition, the voltage of an individual submodule of the input group is higher or lower than that of an individual submodule of the cutting group along with the charging or discharging process, the actual operation working condition is completely ignored by the fixed alternation method in the prior art, in an ideal state, the voltage of the submodule of the input group is higher or lower than that of the submodule of the cutting group, alternation is carried out by using the fixed alternation number, the voltage equalization effect is not facilitated, meanwhile, the switching frequency of the MMC system is increased, and the MMC system belongs to unnecessary submodule alternation switching. The method is improved aiming at the fixed alternation method in the prior art, the mark input group and the input group are reordered to form a reordered group, the sub-modules are selected from the reordered group and are set to be removed, the sub-modules which are not selected and set to be removed in the mark input group are set to be input, the condition that the voltage of the individual sub-modules of the input group is higher or lower than the voltage of the individual sub-modules of the removed group is effectively avoided, unnecessary sub-module alternation switching is eliminated, the voltage equalization effect can be ensured, and the switching frequency of an MMC system can be reduced.
2. The invention passes the judgment condition xmin(k+1)＝xmin(k)，xmax(k+1)＝xmax(k)，ΔUmax(k+1)<ΔUmax(k) If yes, the voltage difference delta U of the maximum capacitor of the sub-module after charging and discharging in one period can be judged according to the voltage sequence of the sub-modulemaxThe voltage of the sub-modules still tends to converge in a decreasing trend, and the voltage tends to converge, namely the voltage of the sub-modules approaches to a given voltage, so that the voltage of each sub-module is maintained near the given voltage, and the voltage-sharing effect is achieved. Only necessary alternation is needed, and on the premise of meeting the stable output of the MMC alternating current and direct current voltages, compared with the existing engineering voltage-sharing method, the switching frequency can be further effectively reduced.
3. And comparing and judging before the cut group marks are put into use to ensure the smooth operation of the program, and simultaneously ensuring the necessary number of the sub-modules to be switched by executing necessary switching when the sub-module alternate number optimization program cannot be carried out, thereby ensuring the stable operation of the system.
Drawings
FIG. 1 is a schematic of the topology of an MMC;
FIG. 2 is a control flow chart of a voltage-sharing control method for optimizing the average switching frequency of an MMC in the present embodiment;
fig. 3 is a sub-module rotation condition for performing necessary switching after the capacitor voltage sequencing logic of the control flow chart is judged in the present embodiment;
fig. 4 is a sub-module rotation condition of the control flow chart under the condition of necessary switching + optimized rotation 1 in the present embodiment;
fig. 5 is a sub-module rotation condition of the control flow chart under the necessary switching + optimized rotation 2 condition in the present embodiment.
Detailed Description
Referring to a circuit topology of the MMC, as shown in fig. 1, each bridge arm includes a plurality of sub-modules, each sub-module includes a capacitor, a diode, and an IGBT, and a sub-module voltage is a capacitor voltage. The voltage-sharing control method for optimizing the average switching frequency of the MMC takes the submodule rotation number optimization method as a core, eliminates unnecessary submodule rotation switching, can ensure the voltage balancing effect, and can reduce the switching frequency of the MMC system. On the basis of the optimization method of the number of the sub-module rotations, the invention determines whether the optimization program of the number of the sub-module rotations is needed under the actual operation condition by adding the preposed judgment, and can further reduce the switching frequency. The following will specifically describe with reference to the pre-judgment and the optimization of the number of sub-module rotations.
A voltage-sharing control method for optimizing an average switching frequency of an MMC, as shown in fig. 2, includes the following steps:
step 1: and sequencing the capacitor voltages of all the sub-modules, and dividing the sub-modules in the same bridge arm into an input group and a cut-off group. Referring to fig. 2(a), the numbers of the maximum value and the minimum value of the capacitor voltage recorded in the present period and the previous period are respectively: x is the number ofmax(k+1)、xmin(k+1)、xmax(k)、xmin(k) The maximum capacitor voltage difference values of the submodules in two adjacent periods are respectively as follows: delta Umax(k+1)、ΔUmax(k) In that respect Judging logic xmin(k+1)＝xmin(k)，xmax(k+1)＝xmax(k)，ΔUmax(k+1)<ΔUmax(k) When the three conditions are met simultaneously, only necessary switching is carried out, and only the switching number of the sub-modules with stable MMC alternating current and direct current voltage output is needed, so that the system can be stable and has a voltage-sharing effect; if not, performing necessary switching and optimized switching alternately, namely performing the alternation number optimization of the sub-modules.
Step 2: the average switching frequency is extremely low and is the lower limit of the MMC switching frequency, the MMC alternating current and direct current output voltage is only stable, and the logic judgment flow chart is shown in figure 2 (b). The specific switching condition is as follows: when bridge arm current iarm>And when 0, judging the difference value delta nref of the number of the modulation output conducting modules of the adjacent periods. If Δ nref>0, selecting the submodules with the minimum voltage of delta nref in the submodule cutting group to be set as the switching-in state. On the contrary, when Δ nref<And when 0 is needed, selecting the submodules with the largest voltage of delta nref from the submodules put into the group, and setting the submodules to be in the cutting-off state. When bridge arm current iarmWhen the value is less than or equal to 0, if Δ nref>0, selecting the submodule with the largest voltage in the cutting group to be set as the on state, and conversely, when the voltage is equal to the voltage of the submodule, setting the submodule to be in the on state<At 0, selecting the submodule with the smallest voltage of delta nref from the input group and setting the submodule to be in an off state.
And step 3: necessary switching + optimized alternate switching. The voltage-sharing method used in the existing engineering effectively reduces the average switching frequency of the MMC by fixing the number of module turns of each control period on the premise of ensuring the voltage-sharing effect. The method performs N besides the necessary sub-module rotation in each control periodbAnd the sub-modules are rotated to ensure the voltage balance of the sub-modules. But each period N is changed according to the working condition and the system parameterbThere is an unnecessary rotation in the fixed rotation number, which is not good for the capacitor voltage equalization and increases the switching frequency of the MMC. The optimized rotation method improves the voltage-sharing method on the basis, eliminates the corresponding unnecessary rotation condition, can dynamically adjust the rotation number according to the real-time working condition and the parameter change, and obviously reduces the average switching frequency of the MMC. The specific optimization strategy is as follows: the necessary switching + optimization rotation 1, and the logic judgment flow chart is shown in fig. 2 (c). When bridge arm current iarm>0. When Δ nref is not less than 0, NOFF、NONJudging N for the number of sub-modules of the cutting group and the putting group in the bridge arm of the previous periodOFF≥Δnref+Nb，NON≥NbWhen established, Δ nref + N was selected from the excision groupbMarking the module with the minimum voltage as input, and selecting N from the submodules of the actual input and the marked inputbThe module with the largest voltage is set to be switched off.
And 4, step 4: necessary switching + optimization rotation 1. When bridge arm current iarm>0. When Δ nref is not less than 0, N is judgedOFF≥Δnref+Nb，NON≥NbIf the voltage is not satisfied, selecting the modules with the minimum voltage of delta nref from the excision group as input;
and 5: necessary switching + optimization rotation 1. When bridge arm current iarm>0、Δnref<When 0, judge NON≥Δnref+Nb，NOFF≥NbWhen true, select N from the excision setbMarking the module with the minimum voltage as input, and selecting N from the submodules of the actual input and the marked inputbThe block with the largest voltage of + | Δ nref | is set to be off.
Step 6: must be usedTurn 1 is switched + optimized. When bridge arm current iarm>0、Δnref<When 0, judge NON≥Δnref+Nb，NOFF≥NbIf the voltage is not satisfied, selecting | delta nref | modules with the maximum voltage from the input group to be set as cut-off;
and 7: the necessary switching + optimization rotation 2, and the logic judgment flow chart is shown in figure 2 (d). When bridge arm current iarmWhen the value is less than or equal to 0 and delta nref is greater than or equal to 0, judging NOFF≥Δnref+Nb，NON≥NbWhen established, Δ nref + N was selected from the excision groupbMarking the submodules with the maximum voltage as inputs, and selecting N from the submodules with the actual inputs and the marked inputsbThe module with the smallest voltage is set to be cut off;
and 8: necessary switching + optimization rotation 2. When bridge arm current iarmWhen the value is less than or equal to 0 and delta nref is greater than or equal to 0, judging NOFF≥Δnref+Nb，NON≥NbIf the voltage is not satisfied, selecting the sub-modules with the largest voltage of delta nref from the excision group as input;
and step 9: necessary switching + optimization rotation 2. When bridge arm current iarm≤0、Δnref<When 0, judge NON≥Δnref+Nb，NOFF≥NbWhen true, select N from the excision setbMarking the module with the maximum voltage as input, and selecting N from the submodules of the actual input and the marked inputbThe block with the smallest voltage of + | Δ nref | is set to be cut off.
Step 10: necessary switching + optimization rotation 2. When bridge arm current iarm≤0、Δnref<When 0, judge NON≥Δnref+Nb，NOFF≥NbIf the voltage is not satisfied, the module with the minimum voltage of | Δ nref | is selected from the input submodules and is set to be cut off.
In this embodiment, as in the judgment logic in fig. 2(a), the numbers of the maximum and minimum values of the capacitor voltages in the current period and the previous period and the maximum capacitor voltage difference value of the bridge arm sub-module are recorded as follows: x is the number ofmin(k+1)、xmax(k+1)、xmin(k)、xmax(k)、ΔUmax(k+1)、ΔUmax(k) In that respect When x ismin(k+1)＝xmin(k)，xmax(k+1)＝xmax(k)，ΔUmax(k+1)<ΔUmax(k) When the three conditions are met simultaneously, only necessary switching is carried out. According to the judgment condition, the number of the submodule corresponding to the maximum value and the minimum value of the voltage of the capacitor in the bridge arm is the same as the number of the previous period, the voltage sequence of the submodule in the bridge arm can change after the submodule is charged (or discharged) in one period, and N is needed to be carried out in the traditional voltage-sharing method at the momentbThe sub-modules rotate. Suppose that bridge arm current i at this timearm>0, if the sub-module voltage sequence is not rotated, the voltage difference delta U of the maximum capacitor of the sub-module can be known through charging in one period according to the sub-module voltage sequencemaxIn a decreasing trend, the sub-module voltage still tends to be stable. Bridge arm current iarmThe case of ≦ 0 is similar. The fixed number of turns at this moment is not beneficial to the voltage-sharing effect, and the switching frequency of the MMC system is increased. And on the premise of meeting the requirement of stable output of the MMC alternating current and direct current voltages, the switching frequency can be effectively reduced compared with the voltage-sharing method in the prior engineering, the alternation condition of the sub-modules under the judgment condition is shown in figure 3, and the alternation result shows that the capacitance of the sub-modules is still close to convergence only by performing the necessary alternation under the condition.
In this embodiment, a logic judgment flow chart of necessary switching + optimization rotation 1 is shown in fig. 2 (c). N is a radical ofOFF、NONThe number of the sub-modules of the same bridge arm cutting group and the same bridge arm throwing group in the previous period is respectively. When bridge arm current iarm>0. When Δ nref is not less than 0, N is judgedOFF≥Δnref+Nb，NON≥NbWhen the method is established, the practical fixed alternate voltage-sharing method in the existing engineering selects delta nref + N in the excision groupbSelecting N in the input group according to the module input with the minimum capacitor voltagebCutting off the module with the maximum capacitor voltage, wherein the selected delta nref + N in the cut-off group exists according to different MMC operation conditions and system parametersbThe voltage of the sub-module capacitor is larger than N selected from the input groupbThe capacitor voltage of each sub-module. At this time, if the stator sub-modules are alternated, the voltage difference of the bridge arm sub-modules can be increased, which is not favorable for the voltage balancing effectMeanwhile, the switching frequency of the MMC system is increased, and the switching belongs to alternative switching of unnecessary submodules. The proposed voltage-sharing control method for optimizing the average switching frequency of the MMC selects delta nref + N from the cut-off groupbMarking the module with the minimum voltage as input, and selecting N from the submodules of the actual input and the marked inputbThe module with the largest voltage is set to be switched off. Effectively avoids the selection of delta nref + N in the excision groupbThe voltage of the sub-module capacitor is larger than N selected from the input groupbThe condition of capacitance and voltage of each sub-module eliminates the alternative switching of unnecessary sub-modules, and the switching frequency of the MMC system can be reduced.
When bridge arm current iarm>0、Δnref<When 0, judge NON≥Δnref+Nb，NOFF≥NbWhen the method is established, the practical fixed alternate voltage-sharing method of the existing engineering selects N in the excision groupbSelecting | delta nref | + N in the input group of the module with the minimum capacitor voltagebThe module with the largest capacitance voltage is cut off, and N selected from a cut-off group exists according to different MMC operating conditions and system parametersbThe sub-module capacitor voltage is greater than the selected | Δ nref | + N in the input groupbThe capacitor voltage of each sub-module. The submodule fixing and alternation at the moment is not beneficial to the voltage balancing effect of the bridge arm submodules, and meanwhile, the switching frequency of the MMC system is increased, so that unnecessary submodule alternation switching is realized. The proposed voltage-sharing control method for optimizing the average switching frequency of the MMC selects N from the cut-off groupbMarking the module with the minimum voltage as input, and selecting N from the submodules of the actual input and the marked inputbThe block with the largest voltage of + | Δ nref | is set to be off. Effectively avoids N selected from the excision groupbThe sub-module capacitor voltage is greater than the selected | Δ nref | + N in the input groupbThe condition of capacitance and voltage of each submodule eliminates the alternative switching of unnecessary submodules, and the switching frequency of the MMC system can be reduced. The alternation condition of the sub-modules under the judgment condition is shown in figure 4, compared with the practical fixed alternation voltage-sharing control method in the prior art, the optimized voltage-sharing control method provided by the invention effectively avoids the unnecessary alternation condition existing in the prior art, and further reduces the number of the alternation conditionThe switching frequency of the MMC is lowered.
In this embodiment, a logic judgment flow chart of necessary switching + optimization rotation 2 is shown in fig. 2 (d). N is a radical ofOFF、NONThe number of the sub-modules of the cut-off group and the put-in group in the same bridge arm in the previous period is respectively 1 through necessary switching and optimization alternation. When bridge arm current iarm≤0、Δnref>When 0, judge NOFF≥Δnref+Nb，NON≥NbWhen the method is established, the practical fixed alternate voltage-sharing method of the existing engineering selects | delta nref | + N in the excision groupbSelecting N in the input group according to the maximum module input of the capacitor voltagebThe module with the largest capacitance voltage is cut off, and | delta nref | + N selected from the cut-off group exists according to different MMC operation conditions and system parametersbThe capacitor voltage of each sub-module is less than N selected from the input groupbThe capacitor voltage of each sub-module. The submodule fixing and alternation at the moment is not beneficial to the voltage balancing effect of the bridge arm submodules, and meanwhile, the switching frequency of the MMC system is increased, so that unnecessary submodule alternation switching is realized. The proposed voltage-sharing control method for optimizing the average switching frequency of the MMC selects | delta nref | + N from the resection groupbMarking the module with the minimum voltage as input, and selecting N from the submodules of the actual input and the marked inputbThe module with the largest voltage is set to be switched off. Effectively avoids | delta nref | plus N selected from the excision groupbThe capacitor voltage of each sub-module is less than N selected from the input groupbThe condition of capacitance and voltage of each submodule eliminates the alternative switching of unnecessary submodules, and the switching frequency of the MMC system can be reduced.
When bridge arm current iarm≤0、Δnref<When 0, judge NON≥Δnref+Nb，NOFF≥NbWhen the method is established, the practical fixed alternate voltage-sharing method of the existing engineering selects N in the excision groupbSelecting | delta nref | + N in the input group of the module with the largest capacitor voltagebThe module with the minimum capacitor voltage is cut off, and N selected from the cut-off group exists according to different MMC operating conditions and system parametersbThe capacitor voltage of each sub-module is less than | delta nref | + N selected from the input groupbThe capacitor voltage of each sub-module. The submodule fixing and alternation at the moment is not beneficial to the voltage balancing effect of the bridge arm submodules, and the average switching frequency of the MMC system is increased by alternation, so that the method belongs to unnecessary submodule alternation switching. The proposed voltage-sharing control method for optimizing the average switching frequency of the MMC selects N from the cut-off groupbMarking the module with the maximum voltage as input, and selecting N from the submodules of the actual input and the marked inputbThe block with the smallest voltage of + | Δ nref | is set to be cut off. Effectively avoids N selected from the excision groupbThe capacitor voltage of each sub-module is less than | delta nref | + N selected from the input groupbThe condition of capacitance and voltage of each submodule eliminates the alternate switching of unnecessary submodules, and the average switching frequency of the MMC system can be reduced. The rotation condition of the sub-modules under the judgment condition is shown in fig. 5, compared with the practical fixed rotation voltage-sharing control method in the prior art, the optimized voltage-sharing control method provided by the invention can effectively avoid the unnecessary rotation condition in the prior art, and further reduce the switching frequency of the MMC.

Claims (7)

1. A voltage-sharing control method for optimizing MMC average switching frequency is characterized by comprising the following steps:
sequencing all the capacitor voltages of the sub-modules, and dividing the sub-modules in the same bridge arm into an input group and a cut-off group;
judging whether the difference value delta nref of the numbers of the modulation output conduction modules of the adjacent periods is larger than 0; if the delta nref is larger than or equal to 0, the input group needs to be added with the sub-module of delta nref | to ensure necessary switching; if delta nref is less than 0, it indicates that the input group needs to reduce | delta nref | submodules to ensure necessary switching;
on the premise of ensuring the stable operation of the system, namely on the premise of ensuring the number of necessary switching submodules, the number of submodule turns for realizing the voltage-sharing effect is optimized according to the following mode:
when the delta nref is more than or equal to 0, selecting delta nref + N in the excision group according to the charge-discharge state of the MMCbThe submodules with the minimum or the maximum capacitor voltage are marked to form a mark input group, NbRepresents a fixed number of rotations; then put the mark into group and put into groupReordering to form a reordered group and selecting N from said reordered groupbThe submodules with the largest or smallest capacitor voltage are set to be cut off, and the submodules which are not selected and set to be cut off in the mark input group are set to be input, so that the number of necessary switching submodules is ensured, and the optimization of the number of the submodule turns is realized;
when Δ nref < 0, N was selected in the cut-off group according to the charge-discharge state of MMCbThe submodules with the minimum or the maximum capacitor voltage are marked to form a mark input group, NbRepresents a fixed number of rotations; the marked drop groups are then reordered from the drop group to form reordered groups, and | Δ nref | + N is selected from the reordered groupsbAnd the submodules with the largest or smallest capacitor voltage are set to be cut off, and the submodules which are not selected and set to be cut off in the mark input group are set to be input, so that the number of necessary switching submodules is ensured, and the optimization of the number of the submodule turns is realized.
2. The voltage-sharing control method for optimizing the average switching frequency of the MMC according to claim 1, characterized in that: when bridge arm current iarmWhen the voltage is more than 0, the MMC is in the charging state, and when the delta nref is more than or equal to 0, the delta nref + N is selected from the excision groupbThe sub-blocks with the minimum capacitance voltage form a mark input group, and N is selected in the reordering groupbThe sub-module with the largest capacitor voltage is set to be cut off;
when bridge arm current iarmWhen the voltage is more than 0, the MMC is in a charging state, and when the delta nref is less than 0, N is selected from the excision groupbThe sub-blocks with the minimum capacitance voltage form a mark input group, and | Δ nref | + N is selected in the reordering groupbThe largest submodule of the capacitor voltage is set to be cut off.
3. The voltage-sharing control method for optimizing the average switching frequency of the MMC according to claim 1, characterized in that: when bridge arm current iarmWhen the voltage is less than 0, the MMC is in a discharge state, and when the delta nref is more than or equal to 0, the delta nref + N is selected from the excision groupbThe sub-modules with the largest capacitance voltage form a mark input group, and N is selected from the reordering groupbThe minimum submodule of capacitor voltage is set as a cutRemoving;
when bridge arm current iarmWhen < 0, MMC is in discharge state, and when Δ nref < 0, N is selected from the excision groupbThe sub-blocks with the largest capacitance voltage form the mark input group, and | Δ nref | + N is selected in the reordering groupbThe individual capacitor voltage minimum submodule is set to cut off.
4. The voltage-sharing control method for optimizing the average switching frequency of the MMC according to claim 1, characterized in that: before the rotation number of the sub-modules is optimized, the method also carries out the preposed judgment:
if xmin(k+1)＝xmin(k)，xmax(k+1)＝xmax(k)，ΔUmax(k+1)<ΔUmax(k) If the three conditions are met simultaneously, the capacitor voltage tends to be converged, and only necessary switching is performed; if the three conditions cannot be met at the same time, optimizing the rotation number of the sub-modules;
wherein x ismin(k +1) represents the submodule number, x, in the present period in which the capacitor voltage is the smallestmax(k +1) represents the number of the submodule with the largest capacitance voltage in the period; x is the number ofmin(k) Represents the sub-module number, x, with the minimum capacitor voltage in the previous cyclemax(k +1) represents the number of the submodule with the largest capacitance voltage in the last period; delta Umax(k +1) represents the maximum capacitance voltage difference value in the period; delta Umax(k) Representing the maximum capacitor voltage difference over the last period.
5. The voltage-sharing control method for optimizing the average switching frequency of the MMC according to claim 1, characterized in that: when Δ nref ≧ 0, before the excision group marker is put in, the following determination is made: judgment of NOFF≥Δnref+Nb,NON≥NbWhether the two are true at the same time; if yes, putting the mark of the resection group; if not, performing necessary switching;
when Δ nref < 0, before the excision set markers are applied, the following judgment is also made: judgment of NON≥Δnref+Nb,NOFF≥NbWhether the two are true at the same time; if yes, the method will cut off the group markRecording the investment; if not, performing necessary switching;
wherein N isOFFRepresenting the number of sub-modules of the resection group in the same bridge arm in the previous period; n is a radical ofONAnd representing the number of sub-modules put into groups in the same bridge arm in the previous period.
6. The voltage-sharing control method for optimizing the average switching frequency of the MMC according to claim 1 or 4, characterized in that: when bridge arm current iarmWhen the voltage is more than 0 and the voltage is more than or equal to 0, setting the minimum submodules of the capacitor voltage in the cutting group to be input so as to realize necessary switching;
when bridge arm current iarmAnd when the voltage is more than 0 and the voltage is less than 0, the required switching is realized by selecting | delta nref | maximum sub-modules of the capacitor voltage in the input group to be cut off.
7. The voltage-sharing control method for optimizing the average switching frequency of the MMC according to claim 1 or 4, characterized in that: when bridge arm current iarmWhen the voltage is less than 0 and the voltage is more than or equal to 0, setting the largest sub-modules of the capacitors with the voltage of delta nref in the cutting group as input to realize necessary switching;
when bridge arm current iarm< 0, and Δ nref < 0, the necessary switching is achieved by selecting | Δ nref | capacitance voltage minimum sub-modules in the input set to be switched off.
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