Multi-level photovoltaic inverter sub-module loss balance control method under capacitor aging

Abstract

The invention provides a multi-level photovoltaic inverter sub-module loss balance control method under capacitor aging. The method comprises the following steps: monitoring the current of a bridge arm in real time; monitoring the capacitor voltage of each sub-module of the bridge arm in real time; on the basis of a modulation method of the multi-level photovoltaic inverter and a capacitor voltage balance strategy, obtaining a sub-module on-off state; calculating the loss of each sub-module in the bridge arm by using the bridge arm current, the capacitor voltage and the switching state; the average loss of the bridge arms is obtained by using the loss of each sub-module; and calculating the total loss average value of each sub-module of the bridge arm and the loss difference between the sub-modules, thereby changing the sorting frequency in the capacitor voltage balance control of the sub-modules in the bridge arm, and realizing the balance of the loss of the same bridge arm of the multi-level photovoltaic inverter. Compared with a conventional method, the hardware cost of the modular multilevel converter does not need to be increased, the influence on the quality of output electric energy is small, and the control algorithm is simple.

Description

Technical Field

[0001] The invention belongs to the technical field of multi-level power electronic converters, and in particular relates to a loss balancing control method for submodules of a multi-level photovoltaic inverter under capacitor aging. Background Art

[0002] The Modular Multilevel Converter (MMC) consists of six bridge arms, each of which is formed by cascading submodules. Different from traditional two-level and three-level converters, MMC uses a step wave approach to a sine wave, which has the advantages of high waveform quality and strong fault handling capability, and has received widespread attention and application in energy storage applications.

[0003] In actual operation, the capacitors of the submodules in the bridge arm of the multi-level photovoltaic inverter will age to varying degrees. The aging of the capacitors in the submodules in the bridge arm will cause the input time of each submodule to change, which will lead to uneven distribution of power loss of each submodule in the same bridge arm, which will affect the life distribution of power devices in the MMC and reduce the service life of the MMC.

[0004] To solve the problem of regulating and balancing the loss of sub-modules of multi-level photovoltaic inverters under capacitor aging, conventional methods optimize the loss of modular multi-level converters by optimizing the topological structure of sub-modules, optimizing the capacitor voltage balancing strategy of each sub-module in the same bridge arm, or optimizing the modulation strategy of modular multi-level converters. However, the above methods have the disadvantages of increasing the control difficulty of modular multi-level converters and increasing the output voltage distortion rate of modular multi-level converters. Therefore, there are certain difficulties in practical application.

[0005] Based on this, a loss balancing control method for sub-modules of multi-level photovoltaic inverters under capacitor aging is proposed. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a multi-level photovoltaic inverter submodule loss balance control method under capacitor aging in view of the deficiencies of the above-mentioned prior art, and to achieve the balance of the loss of the same bridge arm of the multi-level photovoltaic inverter by changing the sorting frequency in the capacitor voltage balance control of each submodule of the same bridge arm. Compared with the conventional method, there is no need to increase the hardware cost of the modular multi-level converter, the output power quality has little impact, and the control algorithm design is relatively simple, so as to solve the problems raised in the above-mentioned background technology.

[0007] In order to solve the above technical problems, the technical solution adopted by the present invention is: a multi-level photovoltaic inverter submodule loss balance control method under capacitor aging, comprising the following steps:

[0008] S1. Based on the multi-level photovoltaic inverter bridge arm current model, real-time monitoring of each bridge arm current ink(t) (n=a,b,c; k=u,l);

[0009] S2, real-time monitoring of the real-time capacitor voltage value UCnk_j (j=1, 2, ..., N) of N submodules on a certain phase bridge arm;

[0010] S3, obtaining the switching state Snk_j of the submodule in the bridge arm based on the modular multi-level converter modulation method and the capacitor voltage balance strategy;

[0011] S4, calculating the conduction loss Pnk_j of the power devices of each submodule in the bridge arm;

[0012] S5, calculating the switching loss Ps_nk_j of each submodule in the bridge arm;

[0013] S6. Sum the conduction loss and switching loss of each submodule to obtain the loss Ploss_nk_j of each submodule;

[0014] S7, calculate and obtain the average loss value Pave of all submodules in the bridge arm;

[0015] S8. Calculate the difference between the loss of each submodule in the bridge arm and the average value, change the sorting frequency fcsa_nk_j of the submodule, and make the loss of all submodules in the bridge arm balanced.

[0016] As a further illustration of the present invention, the conduction loss Pnk_j of the power device of the bridge arm submodule in S4 is expressed as:

[0017] P nk_j (I avg ,I rms ,T)=V0I avg +R0I 2 rms

[0018] Where V0 and R0 are threshold voltage and on-resistance respectively, I avg and I rms is the average value of the bridge arm current and the root mean square value of the bridge arm current.

[0019] As a further illustration of the present invention, the switching loss P of each submodule in the bridge arm in S5 is s_nk_j The calculation formula is as follows:

[0020]

[0021] Among them, at each switching moment, E total is the total switching loss energy, determined by the RMS value of the bridge arm current.

[0022] As a further illustration of the present invention, the loss P of each submodule in the bridge arm in S6 is loss_nk_j The calculation formula is as follows:

[0023] P loss_nk_j =P cnk_j +P sw_nk_j .

[0024] As a further illustration of the present invention, P ave Calculated according to the following formula:

[0025]

[0026] The difference between the loss of each submodule in the bridge arm of S8 and the average value ΔP loss_nk_j The calculation formula is:

[0027] ΔP loss_nk_j =P loss_nk_j -P ave .

[0028] As a further explanation of the present invention, the ordering frequency f of the submodules in the bridge arm is changed in S8. csa_nk_j , to achieve the balance of the loss of the same bridge arm of the multi-level photovoltaic inverter, specifically:

[0029] If ΔP loss_nk_j >0, then reduce the sorting frequency;

[0030] If ΔP loss_nk_j <0, then increase the sorting frequency,

[0031] Finally, ΔP loss_nk_j =0, that is, P loss_nk_j =P ave .

[0032] Compared with the prior art, the present invention has the following advantages:

[0033] 1. The multi-level photovoltaic inverter submodule loss balance control method under capacitor aging in the present invention calculates the total loss average value of each submodule of the bridge arm and the loss difference between each submodule, and then changes the sorting frequency in the capacitor voltage balance control of each submodule in the bridge arm to achieve the balance of the loss of the same bridge arm of the multi-level photovoltaic inverter. There is no need to change the capacitor voltage balance strategy or modulation strategy of the modular multilevel converter, which has little impact on the output voltage waveform quality of the modular multilevel converter and the control logic design is simple and easy to implement. Through this control method, the photovoltaic power generation system can more effectively utilize solar energy resources, reduce dependence on traditional energy, and greatly reduce carbon emissions and environmental pollution.

[0034] 2. The multi-level photovoltaic inverter submodule loss balance control method under capacitor aging in the present invention only needs to change the sorting frequency of each submodule in the bridge arm, without changing the topological structure of the modular multi-level converter submodule, and will not increase the construction cost of the modular multi-level converter and is easy to implement in the existing modular multi-level converter system, so it has strong practicality. At the same time, the multi-level photovoltaic inverter submodule loss balance control method under capacitor aging can improve the power generation efficiency of the photovoltaic power generation system, reduce energy waste, and achieve sustainable use of energy. This not only helps to mitigate climate change, but also leaves a cleaner and more sustainable living environment for future generations. Therefore, this environmentally friendly and green technological innovation will bring more opportunities and benefits for sustainable development to society. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 is a schematic diagram of a topological structure of a three-phase modular multi-level converter according to an embodiment of the present invention;

[0036] Figure 2 is a schematic diagram of the submodule topology structure of an embodiment of the present invention;

[0037] Figure 3 It is a schematic diagram of the overall method flow of an embodiment of the present invention. DETAILED DESCRIPTION

[0038] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0039] The present invention aims at the problem of regulating and balancing the loss of submodules of multi-level photovoltaic inverters under capacitor aging, wherein the topological structure of the three-phase MMC and the submodules is as follows Figure 1 , Figure 2 As shown,

[0040] The three-phase MMC consists of six bridge arms, each of which contains j (j=1, 2, ..., N) submodules (SM) with the same topology and a bridge arm inductor L. arm The submodule is a half-bridge structure, consisting of two diodes D1 and D2, two power switches T1 and T2, and a DC capacitor C.

[0041] like Figure 3As shown, based on the topological structure of the above-mentioned three-phase MMC and submodules, a technical solution is provided: a multi-level photovoltaic inverter submodule loss balance control method under capacitor aging, including: real-time monitoring of the current of the bridge arm; real-time monitoring of the capacitor voltage of each submodule of the bridge arm; obtaining the submodule switch state based on the modulation method of the multi-level photovoltaic inverter and the capacitor voltage balance strategy; using the bridge arm current, capacitor voltage and switch state to calculate the loss of each submodule in the bridge arm; using the loss of each submodule to obtain the average loss of the bridge arm; calculating the average value of the total loss of each submodule of the bridge arm and the loss difference between each submodule, thereby changing the sorting frequency of the submodules in the bridge arm, and realizing the balance of the loss of the same bridge arm of the multi-level photovoltaic inverter.

[0042] The specific steps include:

[0043] S1. Based on the multi-level photovoltaic inverter bridge arm current model, real-time monitoring of each bridge arm current ink(t) (n=a,b,c; k=u,l);

[0044] S2, real-time monitoring of the real-time capacitor voltage value UCnk_j (j=1, 2, ..., N) of N submodules on a certain phase bridge arm;

[0045] S3, obtaining the switching state Snk_j of the submodule in the bridge arm based on the modular multi-level converter modulation method and the capacitor voltage balance strategy;

[0046] S4, calculating the conduction loss Pnk_j of the power devices of each submodule in the bridge arm;

[0047] The conduction loss Pnk_j of the power device of the bridge arm submodule is expressed as:

[0048] P nk_j (I avg ,I rms ,T)=V0I avg +R0I 2 rms

[0049] Where V0 and R0 are threshold voltage and on-resistance respectively, I avg and I rms is the average value of the bridge arm current and the root mean square value of the bridge arm current.

[0050] S5, calculating the switching loss Ps_nk_j of each submodule in the bridge arm;

[0051] The switching loss P of each submodule in the bridge arm is s_nk_j The calculation formula is as follows:

[0052]

[0053] Among them, at each switching moment, E total is the total switching loss energy, determined by the RMS value of the bridge arm current.

[0054] S6. Sum the conduction loss and switching loss of each submodule to obtain the loss Ploss_nk_j of each submodule;

[0055] The loss of each submodule in the bridge arm is P loss_nk_j The calculation formula is as follows:

[0056] P loss_nk_j =P cnk_j +P sw_nk_j .

[0057] S7, calculate and obtain the average loss value Pave of all submodules in the bridge arm;

[0058] Calculated according to the following formula:

[0059]

[0060] S8. Calculate the difference between the loss of each submodule in the bridge arm and the average value, change the sorting frequency fcsa_nk_j of the submodule, and make the loss of all submodules in the bridge arm balanced.

[0061] The difference between the loss of each submodule in the bridge arm and the average value is ΔP loss_nk_j The calculation formula is:

[0062] ΔP loss_nk_j =P loss_nk_j -P ave .

[0063] Change the sorting frequency f of the submodules in the bridge arm csa_nk_j , to achieve the balance of the loss of the same bridge arm of the multi-level photovoltaic inverter, specifically:

[0064] If ΔP loss_nk_j >0, then reduce the sorting frequency;

[0065] If ΔP loss_nk_j <0, then increase the sorting frequency,

[0066] Finally, ΔP loss_nk_j =0, that is, P loss_nk_j =P ave .

[0067] This embodiment calculates the total loss average value of each submodule of the bridge arm and the loss difference between each submodule, and then changes the sorting frequency in the capacitor voltage balance control of each submodule in the bridge arm to achieve the balance of the loss of the same bridge arm of the multi-level photovoltaic inverter. Compared with the conventional method, there is no need to increase the construction cost of the modular multilevel converter, the output power quality has little impact and the control algorithm design is relatively simple. At the same time, there is no need to change the topological structure of the modular multilevel converter submodule, the construction cost of the modular multilevel converter will not be increased, and it is easy to implement in the existing modular multilevel converter system, which has strong practicality, and can improve the power generation efficiency of the photovoltaic power generation system, reduce energy waste, and achieve sustainable use of energy.

[0068] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0069] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for controlling loss balance of submodules of a multi-level photovoltaic inverter under capacitor aging, characterized in that: The following steps are involved: S1. Based on the multi-level photovoltaic inverter bridge arm current model, real-time monitoring of each bridge arm current ink(t) (n=a,b,c; k=u,l); S2, real-time monitoring of the real-time capacitor voltage value UCnk_j (j=1, 2, ..., N) of N submodules on a certain phase bridge arm; S3, obtaining the switching state Snk_j of the submodule in the bridge arm based on the modular multi-level converter modulation method and the capacitor voltage balance strategy; S4, calculating the conduction loss Pnk_j of the power devices of each submodule in the bridge arm; S5, calculating the switching loss Ps_nk_j of each submodule in the bridge arm; S6. Sum the conduction loss and switching loss of each submodule to obtain the loss Ploss_nk_j of each submodule; S7, calculate and obtain the average loss value Pave of all submodules in the bridge arm; S8. Calculate the difference between the loss of each submodule in the bridge arm and the average value, change the sorting frequency fcsa_nk_j of the submodule, and make the loss of all submodules in the bridge arm balanced.

2. The method for controlling loss balance of submodules of a multi-level photovoltaic inverter under capacitor aging according to claim 1, characterized in that: The conduction loss Pnk_j of the power device of the bridge arm submodule in S4 is expressed as: P nk_j (I avg ,I rms ,T)=V0I avg +R0I 2 rms Where V0 and R0 are threshold voltage and on-resistance respectively, I avg and I rms is the average value of the bridge arm current and the root mean square value of the bridge arm current.

3. The method for controlling loss balance of submodules of a multi-level photovoltaic inverter under capacitor aging according to claim 2, characterized in that: The switching loss P of each submodule in the bridge arm of S5 s_nk_j The calculation formula is as follows: Among them, at each switching moment, E total is the total switching loss energy, determined by the RMS value of the bridge arm current.

4. The method for controlling loss balance of submodules of a multi-level photovoltaic inverter under capacitor aging according to claim 3, characterized in that: The loss P of each submodule in the bridge arm of S6 loss_nk_j The calculation formula is as follows: P loss_nk_j =P cnk_j +P sw_nk_j 。 5. The method for controlling loss balance of submodules of a multi-level photovoltaic inverter under capacitor aging according to claim 4, characterized in that: P ave Calculated according to the following formula: The difference between the loss of each submodule in the bridge arm of S8 and the average value ΔP loss_nk_j The calculation formula is: ΔP loss_nk_j =P loss_nk_j -P ave 。 6. The method for controlling loss balance of submodules of a multi-level photovoltaic inverter under capacitor aging according to claim 5, characterized in that: The sorting frequency f of the submodules in the bridge arm is changed in S8 csa_nk_j , to achieve the balance of the loss of the same bridge arm of the multi-level photovoltaic inverter, specifically: If ΔP loss_nk_j >0, then reduce the sorting frequency; If ΔP loss_nk_j <0, then increase the sorting frequency, Finally, ΔP loss_nk_j =0, that is, P loss_nk_j =P ave .

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