MMC capacitor voltage balance control method based on virtual pulsation upper and lower limits

Abstract

The invention discloses an MMC capacitor voltage balance control method based on virtual pulse upper and lower limits. The MMC capacitor voltage balance control method specifically comprises the following steps: 1, predicting a sub-module capacitor voltage at the beginning of a next control period; 2, predicting the average capacitance voltage at the beginning of the next control period; 3, calculating the upper limit Ucmax and the lower limit Ucmin of the voltage pulsation of the virtual capacitor; 4, counting the total number of the sub-modules of which the capacitor voltage predicted values are greater than Ucmax or less than Ucmin; 5, calculating the increment number of the sub-modules needing to be input in the current control period; 6, calculating the total number of the sub-modules of which the switching state actually needs to be adjusted in the current control period; and 7, setting a regulation and control strategy and selecting the corresponding sub-modules for switching on and switching off. According to the method, no additional sensor needs to be added, the hardware structure of the MMC system does not need to be changed, control is simple and easy to implement, and high economical efficiency and practicability are achieved.

Description

Technical Field

[0001] The invention belongs to the field of multi-level power electronic converters, and in particular relates to an MMC capacitor voltage balance control method based on virtual pulsation upper and lower limits. Background Art

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

[0003] The voltage pulsation of the submodule capacitor in the modular multilevel converter directly affects the operating performance of the system. A larger voltage pulsation of the submodule capacitor will affect the output power quality of the MMC AC side. In current projects, the voltage pulsation of the submodule capacitor is generally limited to within ±5% of the average capacitor voltage.

[0004] There are two common methods to meet the voltage pulsation limit of the submodule capacitor in the modular multilevel converter. One is to increase the capacitance of the submodule, 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 higher switching frequency, which in turn leads to higher switching losses and reduces the operating efficiency of the system. In addition, the larger switching losses have an adverse effect on the heat dissipation design of the MMC system and the operating life of the power devices. Summary of the invention

[0005] Purpose of the invention: In order to solve the problems existing in the above-mentioned prior art, the present invention provides an MMC capacitor voltage balance control method based on virtual pulsation upper and lower limits.

[0006] Technical solution: The present invention discloses a MMC capacitor voltage balance control method based on virtual pulsation upper and lower limits, which specifically includes the following steps:

[0007] Step 1: Predict the capacitor voltage U of each submodule at the beginning of the next control cycle smpre ;

[0008] Step 2: Predict the average capacitor voltage U of all submodules in the bridge arm at the beginning of the next control cycle avepre ;

[0009] Calculate the upper limit U of the voltage ripple of the virtual capacitor of the submodule in the three-phase MMC cmax and the lower limit U cmin ;

[0010] Step 4: Count the voltages that were in the input state and predicted in the previous control cycle. smpreGreater than U cmax or less than U cmin The total number of submodules N jh_on ; Statistics of the cut-off 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 need to exchange running status in the end, defined as the exchange factor N jh ;

[0011] Step 5: Calculate the number of submodules N required for the current control cycle in,new The number of submodules N put into use in the previous control cycle in,old The number of submodule increments between incre ;

[0012] Step 6: Based on N jh Calculate the total number of submodules N that actually need to adjust the switching status in the current control cycle adj ;

[0013] Step 7: Based on ΔN incre and N adj , set the control strategy and select the corresponding sub-module for input and removal.

[0014] Further, the specific steps include:

[0015] The capacitor voltage prediction value U of the submodule at the beginning of the next control cycle in step 1 is smpre It can be obtained by the following expression

[0016]

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

[0018] Furthermore, the average capacitor voltage prediction value U of all submodules of the bridge arm in the next control cycle in step 2 is avepre for

[0019]

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

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

[0022]

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

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

[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:

[0029] If the bridge arm current i of the current control cycle arm >0, and the number of submodule increments ΔN incre = 0, in the cut-off state of NN in,old Among the sub-modules, select N with the lowest voltage. jh Sub-modules are put into operation. In addition, in the put-in state N in,old Among the sub-modules, the N with the highest voltage is cut off. jh submodules;

[0030] If the bridge arm current i of the current control cycle arm >0, and ΔN incre >0, in the NN in the resection state in,old Among the sub-modules, select N with the lowest voltage. adj submodules are put into operation. In addition, in the N in,old Among the sub-modules, the N with the highest voltage is cut off. jh submodules;

[0031] If the bridge arm current i of the current control cycle arm >0, and ΔN incre <0, in the NN in the resection statein,old Among the sub-modules, select N with the lowest voltage. jh Submodule is put into operation. In addition, in the N in,old Among the sub-modules, the N with the highest voltage is cut off. adj submodules;

[0032] If the bridge arm current i of the current control cycle arm <0, and ΔN incre = 0, in the cut-off state of NN in,old Among the sub-modules, select N with the highest voltage. jh submodules are put into operation. In addition, in the N in,old Among the sub-modules, the N with the lowest voltage is cut off. jh submodules;

[0033] If the bridge arm current i of the current control cycle arm <0, and ΔN incre >0, in the NN in the resection state in,old Among the sub-modules, select N with the highest voltage. adj submodules are put into operation. In addition, in the N in,old Among the sub-modules, the N with the lowest voltage is cut off. jh submodules;

[0034] If the bridge arm current i of the current control cycle arm <0, and ΔN incre <0, in the NN in the resection state in,old Among the sub-modules, select N with the highest voltage. jh submodules are put into operation. In addition, in the N in,old Among the sub-modules, the N with the lowest voltage is cut off. adj submodules.

[0035] Beneficial effects:

[0036] 1. The MMC capacitor voltage balancing control method based on virtual pulsation upper and lower limits proposed in the present invention effectively balances the capacitor voltages between submodules and effectively reduces the capacitor voltage pulsation of the submodules, achieving a capacitor voltage pulsation less than ±5% of the average value, thereby ensuring the operating performance of the MMC system.

[0037] 2. The MMC capacitor voltage balance control method based on virtual pulsation upper and lower limits proposed in the present invention significantly reduces the sub-module switching frequency compared to the traditional sorting-based control method, thereby reducing the MMC switching loss and improving the system operation efficiency.

[0038] 3. The MMC capacitor voltage balance control method based on virtual pulsation upper and lower limits proposed in the present invention does not require additional sensors or changes in the hardware structure of the modular multi-level converter system. The control is simple and easy to implement, and has strong economy and practicality. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 1 is a schematic diagram of a topological structure of a three-phase modular multilevel converter according to an embodiment of the present invention;

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

[0041] The accompanying drawings, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0042] The present invention aims to solve the problems of large capacitor voltage pulsation, high switching frequency and high switching loss in modular multi-level converter submodules, and proposes a MMC capacitor voltage balance control method based on virtual pulsation upper and lower limits. This method is used in three-phase MMC. The topology of the three-phase MMC is as follows: Figure 1 It consists of six bridge arms, each of which contains N identical submodules and a bridge arm inductor L s .

[0043] like Figure 2 As shown, a MMC capacitor voltage balance control method based on virtual pulsation upper and lower limits includes the following steps:

[0044] S1: Predict the capacitor voltage U of each submodule at the beginning of the next control cycle smpre ;

[0045] S2: Predict the average capacitor voltage U of all submodules in the bridge arm at the beginning of the next control cycle avepre ;

[0046] S3: Calculate the upper limit U of the voltage ripple of the virtual capacitor of the three-phase MMC neutron module cmax and the lower limit U cmin ;

[0047] S4: Statistics of the voltage U that was in the input state in the previous control cycle and was predicted smpre Greater than U cmax or less than U cmin The total number of submodules N jh_on ; Statistics of the cut-off state in the previous control cycle and the predicted voltage U smpre Greater than U cmax or less than U cminThe total number of submodules N jh_off ; Calculate the total number of submodules that need to exchange running status in the end, defined as the exchange factor N jh ;

[0048] S5: Calculate the number of submodules N required for the current control cycle in,new The number of submodules N put into use in the previous control cycle in,old The number of submodule increments between incre ;

[0049] S6: Based on N jh Calculate the total number of submodules N that actually need to adjust the switching status in the current control cycle adj ;

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

[0051] The capacitor voltage prediction value U of the S1 neutron module at the beginning of the next control cycle smpre It can be obtained by the following expression:

[0052]

[0053] Among them, U sm (k) is the capacitor voltage sampled by the submodule in the current control cycle; S sm (k) is the operating status of the submodule in the current control cycle. When the submodule is put into operation, S sm (k) = 1, when the submodule is removed, S sm (k) = 0; i arm (k) is the bridge arm current sampled in the current control cycle; T ctrl is the control period; C is the capacitance value of the capacitor in the submodule.

[0054] The average capacitor voltage prediction value U of all submodules in the bridge arm of S2 at the beginning of the next control cycle avepre for:

[0055]

[0056] Among them, U ave (k) is the average capacitor voltage sampled in the current control cycle; N is the total number of submodules in the bridge arm.

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

[0058]

[0059] Wherein, σ is the capacitor voltage ripple limit requirement of the MMC submodule, which is generally ±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 S6N adj The expression is:

[0063] N adj =|ΔN incre |+N jh .

[0064] The control strategy in S7 is:

[0065] ①i arm >0 and ΔN incre = 0, in the cut-off state of NN in,old Among the sub-modules, N with the lowest input voltage jh submodules, in addition, in the input state N in,old Among the sub-modules, the N with the highest voltage is cut off. jh submodules.

[0066] ②i arm >0 and ΔN incre >0, in the NN in the resection state in,old Among the sub-modules, N with the lowest input voltage adj submodules, in addition, in the N in,old Among the sub-modules, the N with the highest voltage is cut off. jh submodules.

[0067] ③i arm >0 and ΔN incre <0: In the NN in the resection state in,old Among the sub-modules, N with the lowest input voltage jh submodules, in addition, in the N in,old Among the sub-modules, the N with the highest voltage is cut off. adj submodules.

[0068] ④i arm <0, and ΔN incre =0: In the NN in the cut-off state in,old Among the sub-modules, N with the highest input voltage jh submodules; in addition, in the Nin,old Among the sub-modules, the N with the lowest voltage is cut off. jh submodules.

[0069] ⑤i arm <0 and ΔN incre >0: NN in resection state in,old Among the sub-modules, N with the highest input voltage adj submodules; in addition, in the N in,old Among the sub-modules, the N with the lowest voltage is cut off. jh submodules.

[0070] ⑥i arm <0 and ΔN incre <0: In the NN in the resection state in,old Among the sub-modules, N with the highest input voltage jh submodules; in addition, in the N in,old Among the sub-modules, the N with the lowest voltage is cut off. adj submodules.

[0071] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

Claims

1. A MMC capacitor voltage balance control method based on virtual pulsation upper and lower limits, characterized in that: The specific steps include: Step 1: Predict the capacitor voltage U of each submodule at the beginning of the next control cycle smpre ; Step 2: Predict the average capacitor voltage U of all submodules in the bridge arm at the beginning of the next control cycle avepre ; Step 3: Calculate the upper limit of the voltage ripple U of the virtual capacitor of the three-phase MMC neutron module cmax and the lower limit U cmin ; Step 4: Count the voltages that were in the input state and predicted in the previous control cycle. smpre Greater than U cmax or less than U cmin The total number of submodules N jh_on ; Statistics of the cut-off 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 need to exchange running status in the end, defined as the exchange factor N jh ; Step 5: Calculate the number of submodules N required for the current control cycle in,new The number of submodules N put into use in the previous control cycle in,old The number of submodule increments between incre ; Step 6: Based on N jh Calculate the total number of submodules N that actually need to adjust the switching status in the current control cycle adj ; Step 7: Based on ΔN incre and N adj , set the control strategy and select the corresponding sub-module for input and removal.

2. The MMC capacitor voltage balance control method based on virtual pulsation upper and lower limits according to claim 1 is characterized in that: The capacitor voltage prediction value U of the submodule at the beginning of the next control cycle in step 1 is smpre for Among them, U sm (k) is the capacitor voltage sampled by the submodule in the current control cycle; S sm (k) is the operating status of the submodule in the current control cycle. When the submodule is put into operation, S sm (k) = 1, when the submodule is removed, S sm (k) = 0; i arm (k) is the bridge arm current sampled in the current control cycle; T ctrl is the control period; C is the capacitance value of the capacitor in the submodule.

3. The MMC capacitor voltage balance control method based on virtual pulsation upper and lower limits according to claim 1 is characterized in that: The average capacitor voltage prediction value U of all submodules of the bridge arm at the beginning of the next control cycle in step 2 avepre for Among them, U ave (k) is the average capacitor voltage sampled in the current control cycle; N is the total number of submodules in the bridge arm, i arm (k) is the bridge arm current sampled in the current control cycle, T ctrl is the control period, and C is the capacitance value of the capacitor in the submodule.

4. The MMC capacitor voltage balance control method based on virtual pulsation upper and lower limits according to claim 1 is characterized in that: The virtual capacitor voltage pulsation upper limit U in step 3 cmax and the lower limit U cmin for Where σ is the capacitor voltage ripple limit requirement of the MMC submodule.

5. The MMC capacitor voltage balance control method based on virtual pulsation upper and lower limits according to claim 1 is characterized in that: The exchange factor N in step 4 jh The calculation method is N jh =max(N jh_on ,N jh_off )。 6. The MMC capacitor voltage balance control method based on virtual pulsation upper and lower limits according to claim 1 is characterized in that: In step 6, N adj The expression is: N adj =|ΔN incre |+N jh 。 7. The MMC capacitor voltage balance control method based on virtual pulsation upper and lower limits according to claim 1 is characterized in that: The control strategy in step 7 is: If the bridge arm current i of the current control cycle arm >0, and the number of submodule increments ΔN incre = 0, in the cut-off state of NN in,old Among the sub-modules, select N with the lowest voltage. jh Sub-modules are put into operation. In addition, in the put-in state N in,old Among the sub-modules, the N with the highest voltage is cut off. jh submodules; N is the total number of submodules in the bridge arm; If the bridge arm current i of the current control cycle arm >0, and ΔN incre >0, in the NN in the resection state in,old Among the sub-modules, select N with the lowest voltage. adj submodules are put into operation. In addition, in the N in,old Among the sub-modules, the N with the highest voltage is cut off. jh submodules; If the bridge arm current i of the current control cycle arm >0, and ΔN incre <0, in the NN in the resection state in,old Among the sub-modules, select N with the lowest voltage. jh Submodule is put into operation. In addition, in the N in,old Among the sub-modules, the N with the highest voltage is cut off. adj submodules; If the bridge arm current i of the current control cycle arm <0, and ΔN incre = 0, in the cut-off state of NN in,old Among the sub-modules, select N with the highest voltage. jh submodules are put into operation. In addition, in the N in,old Among the sub-modules, the N with the lowest voltage is cut off. jh submodules; If the bridge arm current i of the current control cycle arm <0, and ΔN incre >0, in the NN in the resection state in,old Among the sub-modules, select N with the highest voltage. adj submodules are put into operation. In addition, in the N in,old Among the sub-modules, the N with the lowest voltage is cut off. jh submodules; If the bridge arm current i of the current control cycle arm <0, and ΔN incre <0, in the NN in the resection state in,old Among the sub-modules, select N with the highest voltage. jh submodules are put into operation. In addition, in the N in,old Among the sub-modules, the N with the lowest voltage is cut off. adj submodules.

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