A virtual fluctuation upper and lower limit-based MMC capacitor voltage balance control method

Abstract

The application discloses a kind of MMC capacitor voltage balance control methods based on virtual pulsation upper and lower limit, specifically:1: predict the submodule capacitor voltage at the beginning of next control cycle;2: predict the average capacitor voltage at the beginning of next control cycle;3: calculate the virtual capacitor voltage pulsation upper limit U cmax And lower limit U cmin ;4: the total number of submodule is counted, which capacitor voltage prediction value is greater than U cmax Or less than U cmin ;5: calculate the number of submodules to be put in the current control cycle;6: calculate the total number of submodules that actually need to be adjusted in the current control cycle;7: set the control strategy to select the corresponding submodule for input and removal.The application does not need to increase additional sensors, does not need to change the hardware structure of MMC system, and is simple and easy to implement, with strong economy and practicality.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of multi-level power electronic converters, and particularly relates to an MMC capacitor voltage balance control method based on virtual fluctuation upper and lower limits. BACKGROUND

[0002] Modular Multilevel Converter (MMC) has been widely applied in high-voltage flexible DC transmission, renewable energy grid connection, medium-voltage distribution network, etc. due to its small AC output harmonics, flexible voltage level expansion, active and reactive power decoupling control, and highly modularized structure.

[0003] The size of the sub-module capacitor voltage fluctuation in the modular multilevel converter directly affects the operation performance of the system. A larger sub-module capacitor voltage fluctuation will affect the power quality of the MMC AC side output. Currently, the sub-module capacitor voltage fluctuation is generally limited within ±5% of the average capacitor voltage in engineering.

[0004] In order to meet the sub-module capacitor voltage fluctuation limitation in the modular multilevel converter, there are two conventional methods. One is to increase the capacitance value of the sub-module, but this method will increase the construction cost of the MMC converter station. The other is to use the sorting method, but the conventional sorting method will cause a larger switching frequency, which in turn leads to higher switching loss and reduces the operating efficiency of the system. In addition, the larger switching loss has an adverse effect on the heat dissipation design of the MMC system and the operating life of the power device. SUMMARY

[0005] The application provides an MMC capacitor voltage balance control method based on virtual fluctuation upper and lower limits to solve the problems in the prior art.

[0006] The application discloses an MMC capacitor voltage balance control method based on virtual fluctuation upper and lower limits, which specifically comprises the following steps:

[0007] Step 1: predicting the capacitor voltage U of each sub-module at the beginning of the next control period smpre ;

[0008] Step 2: predicting the average capacitor voltage U of all sub-modules in the bridge arm at the beginning of the next control period avepre ;

[0009] calculating the virtual capacitor voltage fluctuation upper limit U cmax and the lower limit U cmin of the sub-module in the three-phase MMC;

[0010] Step 4: counting the sub-modules in the put-in state and the predicted voltage U smpreGreater than U cmax or less than U cmin The total number of submodules N jh_on ;Statistics show that the system was in a cutoff state in the previous control cycle and the predicted voltage U smpre Greater than U cmax or less than U cmin The total number of submodules N jh_off ; Calculate the total number of submodules that ultimately need to exchange their running states, and define it as the exchange factor N. jh ;

[0011] Step 5: Calculate the number N of sub-modules required for the current control cycle. in,new The number N of sub-modules put into operation in the previous control cycle in,old The number of submodule increments ΔN between them incre ;

[0012] Step 6: Based on N jh Calculate the total number N of submodules whose switching states actually need to be adjusted in the current control cycle. adj ;

[0013] Step 7: Based on ΔN incre and N adj Set control strategies and select the corresponding sub-modules for input and output.

[0014] Furthermore, the specific steps include the following:

[0015] In step 1, the predicted capacitor voltage U of the submodule at the start of the next control cycle smpre It can be obtained from the following expression.

[0016]

[0017] Among them, U sm (k) represents the capacitor voltage sampled by the submodule in the current control cycle; S sm (k) represents the operating state of the submodule in the current control cycle. When the submodule is activated, S... sm (k) = 1, when the submodule is removed, S sm (k)=0;i arm (k) represents the bridge arm current sampled in the current control cycle; T ctrl C represents the control cycle; C is the capacitance value in the submodule.

[0018] Furthermore, in step 2, the predicted average capacitor voltage U of all sub-modules of the bridge arm in the next control cycle... avepre for

[0019]

[0020] Among them, Uave (k) represents the average capacitor voltage sampled in the current control cycle; N represents the total number of submodules in the bridge arm.

[0021] Furthermore, in step 3, the upper limit of the virtual capacitor voltage ripple U cmax and lower limit U cmin for

[0022]

[0023] Where σ is the capacitor voltage ripple limitation requirement of the MMC submodule.

[0024] Furthermore, in step 4, the exchange factor N jh The calculation method is as follows

[0025] N jh =max(N) jh_on N jh_off ).

[0026] Furthermore, in step 6, N adj The expression is

[0027] N adj =|ΔN incre |+N jh .

[0028] Furthermore, the control strategy in step 7 is as follows:

[0029] If the bridge arm current i in the current control cycle arm >0, and the incremental number of sub-modules ΔN incre When = 0, NN is in the cut-off state in,old Among the sub-modules, select N with the lowest voltage. jh Each submodule is deployed; additionally, N modules are in the deployed state. in,old Among the submodules, the one with the highest cutoff voltage is N. jh Each submodule;

[0030] If the bridge arm current i in the current control cycle arm >0, and ΔN incre When >0, in the NN in the cut-off state in,old Among the sub-modules, select N with the lowest voltage. adj Each submodule is deployed; in addition, N modules are in the deployed state. in,old Among the submodules, the one with the highest cutoff voltage is N. jh Each submodule;

[0031] If the bridge arm current i in the current control cycle arm >0, and ΔN incre When <0, in the NN in the cut-off statein,old Among the sub-modules, select N with the lowest voltage. jh Submodule deployment; in addition, N modules are in the deployment state. in,old Among the submodules, the one with the highest cutoff voltage is N. adj Each submodule;

[0032] If the bridge arm current i in the current control cycle arm <0, and ΔN incre When = 0, NN is in the cut-off state in,old Among the sub-modules, select N with the highest voltage. jh Each submodule is deployed; in addition, N modules are in the deployed state. in,old In each submodule, the N module with the lowest cutoff voltage is selected. jh Each submodule;

[0033] If the bridge arm current i in the current control cycle arm <0, and ΔN incre When >0, in the NN in the cut-off state in,old Among the sub-modules, select N with the highest voltage. adj Each submodule is deployed; in addition, N modules are in the deployed state. in,old In each submodule, the N module with the lowest cutoff voltage is selected. jh Each submodule;

[0034] If the bridge arm current i in the current control cycle arm <0, and ΔN incre When <0, in the NN in the cut-off state in,old Among the sub-modules, select N with the highest voltage. jh Each submodule is deployed; in addition, N modules are in the deployed state. in,old In each submodule, the N module with the lowest cutoff voltage is selected. adj Sub-modules.

[0035] Beneficial effects:

[0036] 1. The MMC capacitor voltage balancing control method based on virtual ripple upper and lower limits proposed in this invention effectively balances the capacitor voltage between sub-modules and effectively reduces the capacitor voltage ripple of sub-modules, achieving a capacitor voltage ripple of less than ±5% of the average value, thus ensuring the operating performance of the MMC system. Compared with the traditional low-switching frequency capacitor voltage balancing method that tries to keep the sub-modules in the switching state of the previous control cycle as much as possible, this invention also effectively balances the capacitor voltage between sub-modules and reduces the capacitor voltage ripple of sub-modules, achieving a capacitor voltage ripple of less than ±5% of the average value.

[0037] 2. The MMC capacitor voltage balance control method based on virtual pulsation upper and lower limits proposed in this invention significantly reduces the switching frequency of sub-modules compared with the traditional sorting-based control method, thereby reducing MMC switching losses and improving system operating efficiency.

[0038] 3. The MMC capacitor voltage balancing control method based on the virtual upper and lower limits of the pulsation has no need to additionally increase sensors, has no need to change the hardware structure of the modular multilevel converter system, is simple to control and easy to implement, and has strong economy and practicality. BRIEF DESCRIPTION OF DRAWINGS

[0039] Figure 1 is a schematic diagram of a three-phase modular multilevel converter topology structure of an embodiment of the present application;

[0040] Figure 2 is a schematic diagram of overall control of an embodiment of the present application. DETAILED DESCRIPTION

[0041] The accompanying drawings, which form a part of the present application, are used to provide further understanding of the present application, and the illustrative embodiments of the present application and their description serve the purpose of explaining the present application, and do not constitute improper limitation of the present application.

[0042] The present application aims at the problems of large capacitor voltage pulsation, high switching frequency and high switching loss of the modular multilevel converter sub-module, and proposes a MMC capacitor voltage balancing control method based on virtual upper and lower limits of the pulsation. The method is used in a three-phase MMC, and the topology structure of the three-phase MMC is as shown in Figure 1 . The three-phase MMC is composed of six bridge arms, each of which contains N identical sub-modules and a bridge arm inductor L s .

[0043] As shown in Figure 2 , a MMC capacitor voltage balancing control method based on virtual upper and lower limits of the pulsation comprises the following steps:

[0044] S1: predicting the capacitor voltage U smpre of each sub-module at the beginning of the next control period;

[0045] S2: predicting the average capacitor voltage U avepre of all sub-modules in the bridge arm at the beginning of the next control period;

[0046] S3: calculating the upper limit U cmax and the lower limit U cmin of the virtual capacitor voltage pulsation of the sub-module in the three-phase MMC;

[0047] S4: counting the total number N smpre of the sub-modules in the last control period which are in the input state and whose predicted voltage U cmax is greater than U cmin or smaller than U jh_on ; counting the total number N smpre of the sub-modules in the last control period which are in the cut-off state and whose predicted voltage U cmax is greater than U cminThe total number of sub-modules N jh_off ; the total number of sub-modules N jh ;

[0048] S5: Calculate the number of sub-modules N in,new to be put into the current control period in,old between the number of sub-modules N incre ;

[0049] S6: Based on N jh , calculate the total number of sub-modules N adj ;

[0050] S7: Based on ΔN incre and N adj , set the control strategy to select the corresponding sub-modules for input and removal.

[0051] The predicted value U smpre of the capacitor voltage of the sub-module at the start of the next control period in S1 can be obtained by the following expression:

[0052]

[0053] Wherein, U sm (k) is the sampled capacitor voltage of the sub-module in the current control period; S sm (k) is the running state of the sub-module in the current control period, S sm (k) = 1 when the sub-module is put into, and S sm (k) = 0 when the sub-module is removed; i arm (k) is the sampled bridge arm current in the current control period; T ctrl is the control period; and C is the capacitor value in the sub-module.

[0054] The predicted value U avepre of the average capacitor voltage of all sub-modules of the bridge arm at the start of the next control period in S2 is:

[0055]

[0056] Wherein, U ave (k) is the sampled average capacitor voltage in the current control period; and N is the total number of sub-modules in the bridge arm.

[0057] The upper limit U cmax and the lower limit U cmin of the virtual capacitor voltage ripple in S3 are:

[0058]

[0059] Where σ is the capacitor voltage ripple limit requirement of MMC sub-module, usually ±5% in engineering.

[0060] The exchange factor N in S4 jh The calculation method is:

[0061] N jh = max(N jh_on , N jh_off ).

[0062] The expression of N in S6 adj is:

[0063] N adj = |ΔN incre | + N jh .

[0064] The regulation strategy in S7 is:

[0065] ① i arm > 0 and ΔN incre = 0: among the N-N in,old sub-modules in the cut-off state, the N jh sub-modules with the lowest input voltage are put into operation, in addition, among the N in,old sub-modules in the operation state, the N jh sub-modules with the highest cut-off voltage are cut off.

[0066] ② i arm > 0 and ΔN incre > 0: among the N-N in,old sub-modules in the cut-off state, the N adj sub-modules with the lowest input voltage are put into operation, in addition, among the N in,old sub-modules in the operation state, the N jh sub-modules with the highest cut-off voltage are cut off.

[0067] ③ i arm > 0 and ΔN incre < 0: among the N-N in,old sub-modules in the cut-off state, the N jh sub-modules with the lowest input voltage are put into operation, in addition, among the N in,old sub-modules in the operation state, the N adj sub-modules with the highest cut-off voltage are cut off.

[0068] ④ i arm < 0 and ΔN incre = 0: among the N-N in,old sub-modules in the cut-off state, the N jh sub-modules with the highest input voltage are put into operation, in addition, among the Nin,old The N sub-modules with the lowest cut-off voltage are cut off. jh The N sub-modules with the lowest cut-off voltage are cut off.

[0069] ⑤i arm <0 and ΔN incre >0: In the N-N sub-modules in the cut-off state, the N sub-modules with the highest turn-on voltage are turned on; in addition, in the N sub-modules in the turn-on state, the N sub-modules with the lowest cut-off voltage are cut off. in,old The N sub-modules with the lowest cut-off voltage are cut off. adj The N sub-modules with the lowest cut-off voltage are cut off. in,old The N sub-modules with the lowest cut-off voltage are cut off. jh The N sub-modules with the lowest cut-off voltage are cut off.

[0070] ⑥i arm <0 and ΔN incre <0: In the N-N sub-modules in the cut-off state, the N sub-modules with the highest turn-on voltage are turned on; in addition, in the N sub-modules in the turn-on state, the N sub-modules with the lowest cut-off voltage are cut off. in,old The N sub-modules with the lowest cut-off voltage are cut off. jh The N sub-modules with the lowest cut-off voltage are cut off. in,old The N sub-modules with the lowest cut-off voltage are cut off. adj The N sub-modules with the lowest cut-off voltage are cut off.

[0071] It should be further noted that each technical feature in the foregoing specific embodiments can be combined in any suitable manner, without contradiction. In order to avoid unnecessary repetition, the present application will not describe various possible combinations again.

Claims

1. A method for controlling the voltage balance of an MMC capacitor based on virtual pulsation upper and lower limits, characterized in that, Specifically, the steps include the following: Step 1: Predict the capacitor voltage of each submodule at the start of the next control cycle. ; Step 2: Predict the average capacitor voltage of all submodules in the bridge arm at the start of the next control cycle. ; Step 3: Calculate the upper limit of virtual capacitor voltage ripple in the three-phase MMC submodule. and lower limit ; Step 4: Statistically analyze the data that were in operation during the previous control cycle and whose predicted voltage was... Greater than or less Total number of submodules ;Statistics show that the data was in a cutoff state during the previous control cycle and the predicted voltage Greater than or less Total number of submodules ; The total number of submodules whose running states ultimately need to be swapped is calculated and defined as the swap factor. ; Step 5: Calculate the number of sub-modules required for the current control cycle. The number of sub-modules deployed in the previous control cycle Incremental number of submodules between ; Step 6: Based on Calculate the total number of submodules whose switching status actually needs to be adjusted in the current control cycle. ; Step 7: Based on and Set control strategies and select the appropriate sub-modules for input and output; The upper limit of virtual capacitor voltage ripple in step 3 and lower limit for ； in, This refers to the capacitor voltage ripple limitation requirement of the MMC submodule.

2. The method for MMC capacitor voltage balance control based on virtual pulsation upper and lower limits according to claim 1, characterized in that, The predicted capacitor voltage value of the submodule at the start of the next control cycle in step 1 for ； in, The capacitor voltage sampled by the submodule in the current control cycle; This refers to the operating status of the submodule in the current control cycle. When the submodule is activated, When a submodule is removed, ; The bridge arm current sampled in the current control cycle; To control the cycle; This refers to the capacitance value in the submodule.

3. The method for MMC capacitor voltage balance control based on virtual pulsation upper and lower limits according to claim 1, characterized in that, In step 2, the predicted average capacitor voltage of all sub-modules of the bridge arm at the start of the next control cycle. for ； in, The average capacitor voltage sampled during the current control cycle; This represents the total number of submodules in the bridge arm. The bridge arm current sampled in the current control cycle. To control the cycle, This refers to the capacitance value in the submodule.

4. The MMC capacitor voltage balance control method based on virtual pulsation upper and lower limits according to claim 1, characterized in that, exchange factor in step 4 The calculation method is as follows 。 5. The method for MMC capacitor voltage balance control based on virtual pulsation upper and lower limits according to claim 1, characterized in that, In step 6 The expression is: 。 6. The method for MMC capacitor voltage balance control based on virtual pulsation upper and lower limits according to claim 1, characterized in that, The control strategy in step 7 is as follows: If the bridge arm current in the current control cycle And the incremental number of sub-modules invested At that time, in the state of resection Among the sub-modules, select the one with the lowest voltage. Each sub-module is deployed; in addition, it is in a deployed state. Among the submodules, the one with the highest cut-off voltage Each submodule; This represents the total number of submodules in the bridge arm. If the bridge arm current in the current control cycle ,and At that time, in the state of resection Among the sub-modules, select the one with the lowest voltage. Each sub-module has been deployed; in addition, those in the deployment state... Among the submodules, the one with the highest cut-off voltage Each submodule; If the bridge arm current in the current control cycle ,and At that time, in the state of resection Among the sub-modules, select the one with the lowest voltage. Submodules are deployed; additionally, those in the deployed state... Among the submodules, the one with the highest cut-off voltage Each submodule; If the bridge arm current in the current control cycle ,and At that time, in the state of resection Among the sub-modules, select the one with the highest voltage. Each sub-module has been deployed; in addition, those in the deployment state... Among the submodules, the one with the lowest cut-off voltage. Each submodule; If the bridge arm current in the current control cycle ,and At that time, in the state of resection Among the sub-modules, select the one with the highest voltage. Each sub-module has been deployed; in addition, those in the deployment state... Among the submodules, the one with the lowest cut-off voltage. Each submodule; If the bridge arm current in the current control cycle ,and At that time, in the state of resection Among the sub-modules, select the one with the highest voltage. Each sub-module has been deployed; in addition, those in the deployment state... Among the submodules, the one with the lowest cut-off voltage. Sub-modules.

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