Hexagonal MMC system pre-charging correction method, terminal and medium

Abstract

The present invention discloses a pre-charging correction method, terminal and medium for a hexagonal MMC system. Under the condition of unbalanced three-phase voltage of an AC power grid, the ratio of the rated value of the line voltage to the actual value of the line voltage is calculated according to the line voltage of the AC power grid sampled in real time, and the correction coefficient of the bridge arm current reference value is calculated by hysteresis control. The amplitude of each bridge arm current is controlled in the controllable charging stage, and the active power absorbed by each bridge arm is adjusted to balance the energy between each bridge arm. The present invention enables the hexagonal MMC system to use a closed-loop charging method to quickly and reliably charge the submodule to the rated value under the condition of unbalanced three-phase power grid, and at the same time, the balance of the energy of the bridge arm can be ensured, and the submodule has the advantages of good consistency and simple control.

Description

Technical Field

[0001] The invention relates to the field of power electronic conversion, and in particular to a pre-charging correction method, a terminal and a medium of a hexagonal MMC system under unbalanced three-phase voltage of a power grid. Background Art

[0002] Hexagonal MMC (H-MMC) is a new type of high-performance power electronic converter, which inherits almost all the advantages of traditional MMC, such as modular structure design, strong scalability, good fault tolerance, and less harmonics in output waveform. In addition, compared with BTB-MMC, hexagonal MMC also has its unique advantages, such as only 6 bridge arms are needed to realize direct AC / AC conversion of electric energy and good low-frequency characteristics. Therefore, hexagonal MMC is very suitable for medium / high voltage and high power applications such as wind power generation, motor drive, and low-frequency power transmission.

[0003] The premise for the normal operation of the hexagonal MMC is that all submodules have been charged to their rated values, otherwise forced startup will generate impact current and damage the device. The existing pre-charging strategies can be divided into two categories: open-loop pre-charging and closed-loop pre-charging. Compared with open-loop pre-charging, closed-loop pre-charging has the advantages of controllable charging current, good sub-module consistency, and short charging time. Therefore, it has more application prospects.

[0004] The existing closed-loop pre-charging strategy is discussed and designed when the grid voltage is in an ideal three-phase balanced state. However, the national standard GB / T12326-2008 stipulates that it is reasonable for the grid voltage to have a slight three-phase imbalance. In practice, it is difficult to achieve complete consistency of the three-phase load, and it is also common for the grid to have a three-phase voltage imbalance. If the hexagonal MMC is closed-loop pre-charged under this condition, the energy of each bridge arm will be unbalanced, the consistency of the submodules will deteriorate, and the control system may be disordered when switching to normal operating conditions. Summary of the invention

[0005] In view of the defects and shortcomings described in the background technology, the present invention provides a hexagonal MMC system pre-charging correction method, terminal and medium, which have the advantages of good sub-module consistency, average energy distribution of each bridge arm and good robustness.

[0006] In order to achieve the above object, the present invention adopts the following technical solution:

[0007] In a first aspect, a pre-charging correction method for a hexagonal MMC system is provided, which is applied to a hexagonal MMC system and comprises the following steps:

[0008] S1: disconnect the first three-phase contactor between the AC grid and the hexagonal MMC, and the second three-phase contactor between the hexagonal MMC and the three-phase AC equipment, detect the capacitor voltages of all full-bridge submodules in the hexagonal MMC and calculate their average value U c_ave ;

[0009] S2: Judge U c_ave Is it greater than the target value of the full-bridge submodule capacitor voltage? If so, execute step S3; if not, continue to execute step S1;

[0010] S3: Close the first three-phase contactor to detect the actual value of the AC grid line voltage under three-phase imbalance, which is recorded as U YX , U ZY , U XZ ;

[0011] S4: Calculate the rated value of the AC grid line voltage U g_ref with U YX , U ZY , U XZ The ratio of is respectively denoted as: k YX , k ZY , k XZ ;

[0012] S5: Calculate the average value of the full-bridge submodule capacitor voltage of the fourth bridge arm and the fifth bridge arm, the first bridge arm and the sixth bridge arm, and the second bridge arm and the third bridge arm, respectively, and record it as U 45_ave , U 16_ave , U 23_ave , will U c_ave Respectively with U 45_ave , U 16_ave , U 23_ave Subtract, the absolute value of the difference is used as the input of the hysteresis control, and the output is recorded as Δ YX , Δ ZY , Δ XZ ;

[0013] S6: Based on k YX , k ZY , k XZ and Δ YX , Δ ZY , Δ XZ The current reference value correction coefficients of the fourth bridge arm and the fifth bridge arm, the first bridge arm and the sixth bridge arm, the second bridge arm and the third bridge arm are calculated respectively, and are recorded as: K YX , K ZY , K XZ ;

[0014] S7: Based on the phase current reference value of the three phases X, Y, and Z of the AC power grid and K YX , K ZY , KXZ Calculate the current reference value i of the bridge arm x_ref ,i x_ref Indicates the current reference value of the xth bridge arm;

[0015] S8: i x_ref And the actual value of the bridge arm current i x The difference is sent to the quasi-PR regulator, and the output bridge arm voltage reference value U x_ini ; Use voltage equalization control to obtain the voltage reference value superposition U of the i-th full-bridge submodule of the x-th bridge arm xi_add , where i is 1, 2, ... N, N is the number of full-bridge submodules in the bridge arm; x_ini Divide by N, then add xi_add Add together to get the voltage reference value U of the i-th full-bridge submodule of the x-th bridge arm xi_ref , sent to the carrier phase shift modulator to output the switching signal of the hexagonal MMC;

[0016] S9: Judgement U c_ave ≥ U c_ref If yes, it indicates that the pre-charging is completed, otherwise continue to step S8.

[0017] Further, the hexagonal MMC system includes an AC power grid, a hexagonal MMC, a three-phase AC device, a current limiting resistor, a first three-phase contactor, and a second three-phase contactor;

[0018] The hexagonal MMC is composed of six identical bridge arms connected end to end to form a ring structure, each bridge arm is composed of an inductor L and N full-bridge sub-modules connected in series; the six vertices of the hexagonal MMC are denoted as z1, z2, z3, z4, z5 and z6 in a counterclockwise direction;

[0019] After the first three-phase contactor is connected in parallel with the current limiting resistor, one end is respectively connected to z1, z5 and z3 of the hexagonal MMC; the other end is respectively connected to the Z, Y and X phases of the AC power grid;

[0020] One end of the second three-phase contactor is respectively connected to z6, z2 and z4 of the hexagonal MMC; the other end is respectively connected to the R, S and T phases of the three-phase AC device.

[0021] Furthermore, in step S2, the target value U of the capacitor voltage of the full-bridge submodule is 目标 Calculated by the following formula:

[0022] U 目标 =A× sqrt(2)×U g_ref / 2 / N

[0023] Among them, U g_refis the rated value of the AC grid line voltage, sqrt() represents the square root function, N is the number of full-bridge sub-modules in each bridge arm of the hexagonal MMC, and the value range of A is 0.93 to 0.95.

[0024] Further, in step S5, when the input of the hysteresis control is greater than 0.6%U c_ref , the hysteresis control output is 0.2; when the hysteresis control input is less than 0.6%U c_ref , the output is 0, where U c_ref is the full-bridge submodule capacitor voltage rating.

[0025] Furthermore, in step S6, K YX , K ZY , K XZ They are respectively expressed as follows:

[0026] K YX = k YX + sign(U c_ave -U 45_ave ) ×Δ YX

[0027] K ZY = k ZY + sign(U c_ave -U 16_ave ) ×Δ ZY

[0028] K XZ = k XZ + sign(U c_ave -U 23_ave ) ×Δ XZ

[0029] Among them, sign () is the sign function.

[0030] Furthermore, in step S7, the current reference value of each bridge arm is expressed as follows:

[0031] i 1_ref = i 6_ref = K ZY / 3×(i gy_ref -i gz_ref )

[0032] i 2_ref = i 3_ref = K XZ / 3×(i gz_ref -i gx_ref )

[0033] i 4_ref = i 5_ref = KYX / 3×(i gx_ref -i gy_ref )

[0034] Among them, i x_ref represents the current reference value of the x-th bridge arm, x is 1, 2, ... 6; i gx_ref 、i gy_ref 、i gz_ref They are the phase current reference values ​​of the three phases X, Y, and Z of the AC power grid, and their phases are the same as the three-phase voltage.

[0035] In a second aspect, an electronic terminal is provided, comprising:

[0036] a memory having a computer program stored thereon;

[0037] The processor is used to load and execute the computer program to implement the hexagonal MMC system pre-charging correction method as described above.

[0038] In a third aspect, a computer-readable storage medium is provided, on which a computer program is stored. When the computer program is executed by a processor, the pre-charging correction method of the hexagonal MMC system as described above is implemented.

[0039] The present invention proposes a hexagonal MMC system pre-charging correction method, terminal and medium, which have the following beneficial effects:

[0040] (1) Under the condition of unbalanced grid voltage, although the current amplitudes of each bridge arm are equal, the voltage amplitudes on the bridge arm are not equal, thus causing imbalance in the power absorbed by each bridge arm. The present invention can ensure that the energy emitted by the grid in the pre-charging stage is evenly distributed among the bridge arms of the hexagonal MMC by adjusting the amplitude of the reference current of each bridge arm in the controllable charging stage, and the sub-modules have good consistency;

[0041] (2) When switching to normal operating conditions after charging is completed, the good consistency of the sub-modules will also shorten the adjustment time, reduce the overshoot, and improve the stability of the control system. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0043] Figure 1 is a topological structure diagram of a hexagonal MMC system provided by an embodiment of the present invention;

[0044] Figure 2 is a flow chart of a pre-charging correction method for a hexagonal MMC system provided by an embodiment of the present invention;

[0045] Figure 3 is the AC grid line voltage waveform provided by the embodiment of the present invention;

[0046] Figure 4 is a waveform of the average value of the capacitor voltage of the six bridge arm submodules without adopting the correction method provided in the embodiment of the present invention;

[0047] Figure 5 is a waveform of the average value of the capacitor voltage of 6 bridge arm submodules using the correction method provided in an embodiment of the present invention;

[0048] Figure 6 are current waveforms of the first bridge arm, the third bridge arm and the fifth bridge arm using the correction method provided in an embodiment of the present invention;

[0049] Figure 7 It is the capacitor voltage waveform of the first three submodules of the first bridge arm using the correction method provided in the embodiment of the present invention. DETAILED DESCRIPTION

[0050] To make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be described in detail below. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other implementation methods obtained by ordinary technicians in this field without creative work belong to the scope of protection of the present invention.

[0051] It should be noted that, in the description of the present invention, the terms "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or order. In addition, in the description of the present invention, unless otherwise specified, the meaning of "plurality" refers to at least two. When an element is considered to be "connected" to another element, it can be directly connected to the other element or there may be an intermediate element at the same time.

[0052] Figure 1The figure shows a topological structure diagram of a hexagonal MMC system. In the present invention, the hexagonal MMC system is composed of an AC power grid, a hexagonal MMC, a three-phase AC device, a current limiting resistor, a three-phase contactor 1 (a first three-phase contactor) and a three-phase contactor 2 (a second three-phase contactor); the hexagonal MMC is composed of six identical bridge arms connected end to end to form a ring structure, and each bridge arm is composed of an inductor L and N full-bridge sub-modules connected in series; the six vertices of the hexagonal MMC are denoted as z1, z2, z3, z4, z5 and z6 in a counterclockwise direction; after the three-phase contactor 1 is connected in parallel with the current limiting resistor, one end is respectively connected to z1, z5 and z3 of the hexagonal MMC; the other end is respectively connected to the Z, Y and X three-phases of the AC power grid; one end of the three-phase contactor 2 is respectively connected to z6, z2 and z4 of the hexagonal MMC; the other end is respectively connected to the R, S and T three-phases of the three-phase AC device.

[0053] In the following embodiments, the rated voltage of the AC power grid is U g_ref is 10kV, the hexagonal MMC capacity is 5MVA, the number of full-bridge submodules N is 6, and the full-bridge submodule capacitor voltage rating is U c_ref The following is an example where the voltage is 2500V, the submodule capacitance C is 20mF, and the bridge arm inductance L is 10mH.

[0054] Based on this, the embodiment of the present invention provides a pre-charging correction method for a hexagonal MMC system under three-phase unbalance of the power grid, wherein steps S1 and S2 are uncontrolled charging stages, and steps S3 to S9 are controlled charging stages. Figure 2 As shown, the following steps are included:

[0055] S1: Disconnect three-phase contactor 1 and three-phase contactor 2, detect the capacitor voltages of all full-bridge submodules in the hexagonal MMC and calculate their average value U c_ave .

[0056] S2: Judge U c_ave Is it greater than the target value of the full-bridge sub-module capacitor voltage? If so, execute step S3; if not, continue to execute step S1.

[0057] Among them, the target value of the full-bridge submodule capacitor voltage is U 目标 Calculated by the following formula:

[0058] U 目标 =A× sqrt(2)×U g_ref / 2 / N

[0059] Among them, U g_ref is the rated value of the AC grid line voltage, sqrt() represents the square root function, N is the number of full-bridge sub-modules in each bridge arm of the hexagonal MMC, and the value range of A is 0.93 to 0.95. In the following embodiments, A is taken as 0.95 as an example for description.

[0060] S3: Close three-phase contactor 1 to detect the actual value of the AC grid line voltage under three-phase imbalance, which is recorded as U YX , U ZY , U XZ .

[0061] S4: Calculate the rated value of the AC grid line voltage U g_ref with U YX , U ZY , U XZ The ratio of is respectively denoted as: k YX , k ZY , k XZ , the calculation formula is:

[0062] k YX =U g_ref / U YX

[0063] k ZY =U g_ref / U ZY

[0064] k XZ =U g_ref / U XZ .

[0065] S5: Calculate the average value of the full-bridge submodule capacitor voltage of the fourth bridge arm (bridge arm 4) and the fifth bridge arm (bridge arm 5), the first bridge arm (bridge arm 1) and the sixth bridge arm (bridge arm 6), the second bridge arm (bridge arm 2) and the third bridge arm (bridge arm 3), respectively, and record it as U 45_ave , U 16_ave , U 23_ave , will U c_ave Respectively with U 45_ave , U 16_ave , U 23_ave Subtract, the absolute value of the difference is used as the input of the hysteresis control, and the output is recorded as Δ YX , Δ ZY , Δ XZ When the hysteresis control input is greater than 0.6%U c_ref , the hysteresis control output is 0.2; when the hysteresis control input is less than 0.6%U c_ref , the output is 0, where U c_ref is the full-bridge submodule capacitor voltage rating.

[0066] S6: Based on k YX , k ZY , k XZ and Δ YX , Δ ZY , Δ XZThe current reference value correction coefficients of the fourth bridge arm (bridge arm 4) and the fifth bridge arm (bridge arm 5), the first bridge arm (bridge arm 1) and the sixth bridge arm (bridge arm 6), the second bridge arm (bridge arm 2) and the third bridge arm (bridge arm 3) are calculated respectively and recorded as: K YX , K ZY , K XZ , which is expressed as follows:

[0067] K YX = k YX + sign(U c_ave -U 45_ave ) ×Δ YX

[0068] K ZY = k ZY + sign(U c_ave -U 16_ave ) ×Δ ZY

[0069] K XZ = k XZ + sign(U c_ave -U 23_ave ) ×Δ XZ

[0070] Among them, sign () is the sign function.

[0071] S7: Based on the phase current reference value of the three phases X, Y, and Z of the AC power grid and K YX , K ZY , K XZ Calculate the current reference value i of the bridge arm x_ref ,i x_ref It represents the current reference value of the x-th bridge arm, and the calculation formula is as follows:

[0072] i 1_ref = i 6_ref = K ZY / 3×(i gy_ref -i gz_ref )

[0073] i 2_ref = i 3_ref = K XZ / 3×(i gz_ref -i gx_ref )

[0074] i 4_ref = i 5_ref = K YX / 3×(i gx_ref -i gy_ref )

[0075] Among them, i x_ref represents the current reference value of the x-th bridge arm, x is 1, 2, ... 6; i gx_ref 、i gy_ref 、i gz_ref They are the phase current reference values ​​of the three phases X, Y, and Z of the AC power grid, and their phases are the same as the three-phase voltage.

[0076] S8: i x_ref And the actual value of the bridge arm current i x The difference is sent to the quasi-PR regulator, and the output bridge arm voltage reference value U x_ini ; Use voltage equalization control to obtain the voltage reference value superposition U of the i-th full-bridge submodule of the x-th bridge arm xi_add , where i is 1, 2, ... N; U x_ini Divide by N, then add xi_add Add together to get the voltage reference value U of the i-th full-bridge submodule of the x-th bridge arm xi_ref , sent to the carrier phase shift modulator to output the switching signal of the hexagonal MMC;

[0077] The voltage reference value superposition value U of the ith full-bridge submodule of the xth bridge arm xi_add Calculated by the following formula:

[0078] U xi_add =K×(U x_ave – U xi ) ×i x_ref

[0079] Among them, U x_ave Indicates the average value of the full-bridge submodule capacitor voltage of the xth bridge arm; U xi It represents the actual value of the capacitor voltage of the i-th full-bridge submodule of the x-th bridge arm; K represents the proportional coefficient.

[0080] S9: Judgement U c_ave ≥ U c_ref If yes, it indicates that the pre-charging is completed, otherwise continue to step S8.

[0081] Figure 3 is the AC grid line voltage waveform. GB / T12326-2008 stipulates that when the 10kV grid is operating normally, the negative sequence voltage imbalance shall not exceed 2%, and shall not exceed 4% in a short time. In the simulation, the grid voltage is set to be the superposition of the rated voltage and the negative sequence voltage of 0.04pu, and the grid line voltage amplitude relationship is U YX =U XZ ZY .

[0082] Figure 4 ​This is the waveform of the average voltage of the capacitors of the six bridge arm submodules without the correction method. The capacitors of the six bridge arm submodules are obviously charged unevenly. When charging is completed, the average voltage of the full bridge submodule capacitors of bridge arm 1 and bridge arm 6 differs from that of other bridge arms by about 90V.

[0083] Figure 5 This is the waveform of the average voltage of the capacitors of the six bridge arm sub-modules using the correction method. The capacitors of the six bridge arm sub-modules show the same charging imbalance in the uncontrolled charging stage, but after entering the controlled charging stage, after an adjustment time of about 0.36s, the average voltage of the capacitors of the six bridge arm sub-modules can be close to consistent until the charging is completed.

[0084] Figure 6 The current waveforms of bridge arms 1, 3 and 5 using the correction method, when switched to the controllable charging stage, the amplitude of the current waveform of bridge arm 1 decreases, while the amplitudes of the current waveforms of bridge arms 3 and 5 increase. In this way, the charging power of each bridge arm is adjusted to make the energy of each bridge arm tend to be balanced.

[0085] Figure 7 The capacitor voltage waveforms of the first three submodules of bridge arm 1 using the correction method are almost identical, and have good consistency.

[0086] The above embodiment provides a hexagonal MMC system pre-charging correction method, which has the following beneficial effects:

[0087] (1) Under the condition of unbalanced grid voltage, although the current amplitudes of each bridge arm are equal, the voltage amplitudes on the bridge arm are not equal, thus causing imbalance in the power absorbed by each bridge arm. The present invention can ensure that the energy emitted by the grid in the pre-charging stage is evenly distributed among the bridge arms of the hexagonal MMC by adjusting the amplitude of the reference current of each bridge arm in the controllable charging stage, and the sub-modules have good consistency;

[0088] (2) When switching to normal operating conditions after charging is completed, the good consistency of the sub-modules will also shorten the adjustment time, reduce the overshoot, and improve the stability of the control system.

[0089] An embodiment of the present invention further provides an electronic terminal, including:

[0090] a memory having a computer program stored thereon;

[0091] The processor is used to load and execute the computer program to implement the hexagonal MMC system pre-charging correction method as described above.

[0092] The embodiment of the present invention further provides a computer-readable storage medium on which a computer program is stored. When the computer program is executed by a processor, the pre-charging correction method for the hexagonal MMC system as described above is implemented.

[0093] It can be understood that the same or similar parts of the above embodiments can be referenced to each other, and the contents not described in detail in some embodiments can refer to the same or similar contents in other embodiments.

[0094] Those skilled in the art will appreciate that the embodiments of the present application may be provided as methods, systems, or computer program products. Therefore, the present application may adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Moreover, the present application may adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program codes.

[0095] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0096] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0097] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0098] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. A pre-charging correction method for a hexagonal MMC system, characterized in that: Applied to the hexagonal MMC system, including the following steps: S1: disconnect the first three-phase contactor between the AC grid and the hexagonal MMC, and the second three-phase contactor between the hexagonal MMC and the three-phase AC equipment, detect the capacitor voltages of all full-bridge submodules in the hexagonal MMC and calculate their average value U c_ave ; S2: Judge U c_ave Is it greater than the target value of the full-bridge submodule capacitor voltage? If so, execute step S3; if not, continue to execute step S1; S3: Close the first three-phase contactor to detect the actual value of the AC grid line voltage under three-phase imbalance, which is recorded as U YX , U ZY , U XZ ; S4: Calculate the rated value of the AC grid line voltage U g_ref with U YX , U ZY , U XZ The ratio of is respectively denoted as: k YX , k ZY , k XZ ; S5: Calculate the average value of the full-bridge submodule capacitor voltage of the fourth bridge arm and the fifth bridge arm, the first bridge arm and the sixth bridge arm, and the second bridge arm and the third bridge arm, respectively, and record it as U 45_ave , U 16_ave , U 23_ave , will U c_ave Respectively with U 45_ave , U 16_ave , U 23_ave Subtract, the absolute value of the difference is used as the input of the hysteresis control, and the output is recorded as Δ YX , Δ ZY , Δ XZ ; S6: Based on k YX , k ZY , k XZ and Δ YX , Δ ZY , Δ XZ The current reference value correction coefficients of the fourth bridge arm and the fifth bridge arm, the first bridge arm and the sixth bridge arm, the second bridge arm and the third bridge arm are calculated respectively, and are recorded as: K YX , K ZY , K XZ ; S7: Based on the phase current reference value of the three phases X, Y, and Z of the AC power grid and K YX , K ZY , K XZ Calculate the current reference value i of the bridge arm x_ref ,i x_ref Indicates the current reference value of the xth bridge arm; S8: i x_ref and the actual value of the bridge arm current i x The difference is sent to the quasi-PR regulator, and the output bridge arm voltage reference value U x_ini ; Use voltage equalization control to obtain the voltage reference value superposition U of the i-th full-bridge submodule of the x-th bridge arm xi_add , where i is 1, 2, ... N, N is the number of full-bridge submodules in the bridge arm; x_ini Divide by N, then add xi_add Add together to get the voltage reference value U of the i-th full-bridge submodule of the x-th bridge arm xi_ref , sent to the carrier phase shift modulator to output the switching signal of the hexagonal MMC; S9: Judgement U c_ave ≥ U c_ref If yes, it indicates that the pre-charging is completed, otherwise continue to step S8.

2. The hexagonal MMC system pre-charging correction method according to claim 1, characterized in that: The hexagonal MMC system includes an AC power grid, a hexagonal MMC, a three-phase AC device, a current limiting resistor, a first three-phase contactor and a second three-phase contactor; The hexagonal MMC is composed of six identical bridge arms connected end to end to form a ring structure, each bridge arm is composed of an inductor L and N full-bridge sub-modules connected in series; the six vertices of the hexagonal MMC are denoted as z1, z2, z3, z4, z5 and z6 in a counterclockwise direction; After the first three-phase contactor is connected in parallel with the current limiting resistor, one end is respectively connected to z1, z5 and z3 of the hexagonal MMC; the other end is respectively connected to the Z, Y and X phases of the AC power grid; One end of the second three-phase contactor is respectively connected to z6, z2 and z4 of the hexagonal MMC; the other end is respectively connected to the R, S and T phases of the three-phase AC device.

3. The hexagonal MMC system pre-charging correction method according to claim 1, characterized in that: The target value U of the capacitor voltage of the full-bridge submodule in step S2 目标 Calculated by the following formula: The 目标 =A× sqrt(2)×U g_ref / 2 / N; Among them, U g_ref is the rated value of the AC grid line voltage, sqrt() represents the square root function, N is the number of full-bridge sub-modules in each bridge arm of the hexagonal MMC, and the value range of A is 0.93 to 0.

95.

4. The hexagonal MMC system pre-charging correction method according to claim 1, characterized in that: In step S5, when the input of the hysteresis control is greater than 0.6%U c_ref , the hysteresis control output is 0.2; when the hysteresis control input is less than 0.6%U c_ref , the output is 0, where U c_ref is the full-bridge submodule capacitor voltage rating.

5. The hexagonal MMC system pre-charging correction method according to claim 1, characterized in that: In step S6, K YX , K ZY , K XZ They are respectively expressed as follows: K YX = k YX + sign(U c_ave - U 45_ave ) ×Δ YX； K ZY = k ZY + sign(U c_ave - U 16_ave ) ×Δ ZY； K XZ = k XZ + sign(U c_ave - U 23_ave ) ×Δ XZ； Among them, sign () is the sign function.

6. The hexagonal MMC system pre-charging correction method according to claim 1, characterized in that: In step S7, the current reference value of each bridge arm is expressed as follows: i 1_ref = i 6_ref = K ZY / 3×(i gy_ref - i gz_ref ); i 2_ref = i 3_ref = K XZ / 3×(i gz_ref - i gx_ref ); i 4_ref = i 5_ref = K YX / 3×(i gx_ref - i gy_ref ); Among them, i x_ref represents the current reference value of the x-th bridge arm, x is 1, 2, ... 6; i gx_ref 、i gy_ref 、i gz_ref They are the phase current reference values ​​of the three phases X, Y, and Z of the AC power grid, and their phases are the same as the three-phase voltages.

7. An electronic terminal, characterized in that: include: a memory having a computer program stored thereon; A processor, configured to load and execute the computer program to implement the hexagonal MMC system pre-charging correction method according to any one of claims 1 to 6.

8. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the hexagonal MMC system pre-charging correction method according to any one of claims 1 to 6 is implemented.

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