Energy storage type MMC low-frequency voltage-sharing hybrid control method, medium and product

Abstract

The invention discloses an energy storage type MMC low-frequency voltage-sharing hybrid control method, and belongs to the technical field of MCC control, and the method comprises the steps: obtaining an energy fluctuation range which can be regulated and controlled by a super capacitor according to the highest cut-off voltage and the lowest cut-off voltage when the super capacitor works normally, and obtaining an energy fluctuation range which can be regulated and controlled by the super capacitor according to the fluctuation range of sub-module energy when the MMC is at low frequency; obtaining the minimum frequency of the MMC capable of being regulated and controlled by the super capacitor; if the operation frequency of the MMC is smaller than the lowest frequency of the MMC capable of being regulated and controlled by the super capacitor, balancing the suspension capacitor voltage of the sub-module by adopting a high-frequency injection control method and the super capacitor C, otherwise, balancing the suspension capacitor voltage of the sub-module by adopting the super capacitor; each bridge arm of the MMC comprises a bridge arm inductor and N sub-modules, each sub-module adopts a suspension capacitor structure, and a super-capacitor is connected in parallel to two ends of a suspension capacitor of each sub-module through a bidirectional DC / DC converter to form a distributed energy storage unit. According to the method, the problem that the capacitance voltage amplitude of the MMC sub-module fluctuates greatly under the low-frequency working condition is solved.

Description

Technical Field

[0001] The invention relates to an energy storage type MMC low-frequency voltage-equalizing hybrid control method, medium and product, belonging to the technical field of MMC control. Background Art

[0002] In recent years, with the continuous increase in renewable energy power generation capacity and the proportion of clean energy in total energy consumption, Modular Multilevel Converter (MMC) has the advantages of high modularity, high voltage level, and good output waveform. It has been widely used in many fields such as medium and high voltage motor drive and high voltage flexible DC transmission.

[0003] The sub-module (SM) of the modular multilevel converter MMC adopts a suspended capacitor structure. The energy interaction between the DC side and the AC side is accompanied by the charging and discharging of the sub-module capacitor, resulting in fluctuations in its capacitor voltage. When the MMC operates under lower frequency conditions, the charging and discharging time of the sub-module becomes longer, which aggravates the fluctuation of the capacitor voltage. The long-term deviation of the bridge arm voltage will cause the output of the system to be distorted, affecting the normal operation of the system. The energy storage module of the energy storage MMC can balance the sub-module capacitor voltage, but it increases the hardware cost; the high-frequency injection control method can suppress the fluctuation of the sub-module capacitor voltage without additionally increasing the hardware cost, but the bridge arm circulation is large, which increases the device current stress and system loss. Therefore, a more flexible and reliable energy storage MMC control method is needed under low-frequency conditions. Summary of the invention

[0004] The purpose of the present invention is to provide a low-frequency voltage-equalizing hybrid control method for energy storage MMC. By comprehensively utilizing a high-frequency injection method and an energy storage module of the energy storage MMC, a hybrid control method is used to equalize the suspended capacitor voltage of the sub-module under relatively low frequency conditions to solve the problem of large fluctuations in the capacitor voltage amplitude of the MMC sub-module under low-frequency conditions. Compared with the high-frequency injection method, the bridge arm current and the circulating current can be reduced, thereby reducing the device current stress and system loss.

[0005] In order to solve the above technical problems, the present invention is implemented by adopting the following technical solutions.

[0006] In a first aspect, the present invention provides an energy storage type MMC low-frequency voltage-balanced hybrid control method, comprising:

[0007] The energy fluctuation range that can be controlled by the supercapacitor is calculated based on the supercapacitor and the maximum cut-off voltage and the minimum cut-off voltage when the supercapacitor is working normally;

[0008] According to the energy fluctuation range that the supercapacitor can control and the fluctuation range of the submodule energy at low frequency of MMC, the lowest frequency of MMC that the supercapacitor can control is calculated;

[0009] If the MMC operating frequency is lower than the minimum frequency that the supercapacitor can regulate, the high-frequency injection control method and the supercapacitor C are used to balance the suspension capacitor voltage of the submodule;

[0010] If the MMC operating frequency is greater than or equal to the minimum frequency of the MMC that can be regulated by the supercapacitor, the supercapacitor is used to balance the suspension capacitor voltage of the submodule;

[0011] Among them, the MMC includes three phases, each phase includes an upper bridge arm and a lower bridge arm, each bridge arm includes a bridge arm inductor and N sub-modules connected in series with the bridge arm inductor, the sub-module adopts a suspended capacitor structure, and the supercapacitor is connected in parallel to the two ends of the suspended capacitor of the sub-module through a bidirectional DC / DC converter to form a distributed energy storage unit.

[0012] Further, the super capacitor is connected in parallel to both ends of the suspension capacitor of the submodule through a bidirectional DC / DC converter, wherein the working mode of the super capacitor includes a discharge mode and a charge mode;

[0013] The discharge mode includes: if the suspension capacitor voltage of the submodule is less than a preset reference value, the bidirectional DC / DC converter operates in a boost mode, and the supercapacitor charges the suspension capacitor of the submodule;

[0014] The charging mode includes: if the voltage of the suspension capacitor of the submodule is greater than or equal to a preset reference value, the bidirectional DC / DC converter operates in a buck mode, and the excess energy in the suspension capacitor of the submodule is transferred to the supercapacitor.

[0015] Furthermore, the energy fluctuation range that can be regulated by the supercapacitor is calculated based on the supercapacitor and the highest cut-off voltage and the lowest cut-off voltage when the supercapacitor is working normally, and is expressed as:

[0016] ;

[0017] In the formula, It is the energy fluctuation range that can be controlled by all supercapacitors on a single bridge arm of MMC. is the supercapacitor value, , They are respectively the highest cut-off voltage and the lowest cut-off voltage when the supercapacitor is working normally. is the number of submodules on a single bridge arm of the MMC, .

[0018] Furthermore, the calculation of the lowest frequency of the MMC that can be regulated by the supercapacitor according to the energy fluctuation range that can be regulated by the supercapacitor and the fluctuation range of the submodule energy at the low frequency of the MMC includes:

[0019] When the energy fluctuation range that can be regulated by all supercapacitors on a single bridge arm of the MMC is equal to the energy change of the upper bridge arm at low frequency of the MMC, the lowest output angular frequency of the MMC that can be regulated by the supercapacitor is obtained;

[0020] According to the minimum output angular frequency of MMC that can be regulated by the supercapacitor, the minimum frequency of MMC that can be regulated by the supercapacitor is calculated.

[0021] Furthermore, the three phases include phase a, and the calculation expression of the fluctuation range of the submodule energy of the upper bridge arm is:

[0022] Δ W pa = ∫ t 1 t 2 P ap d t =− U dc I m 2 ω [ 1 − m 2 ( 1 + cos 2 φ ) 3 ] ;

[0023] In the formula, is the energy change of the upper bridge arm p of phase a at low frequency of MMC, , They are the start time and end time of half cycle of MMC at low frequency, is the output power of the upper bridge arm p of phase a, For DC side The output voltage, is the amplitude of output phase voltage and phase current, is the power factor angle, is the output angular frequency, To adjust the system, is the cosine function.

[0024] Furthermore, according to the minimum output angular frequency of the MMC that can be regulated by the supercapacitor, the minimum frequency of the MMC that can be regulated by the supercapacitor is calculated and expressed as:

[0025] ;

[0026] In the formula, It indicates the lowest frequency of MMC that can be adjusted by supercapacitor. Indicates the minimum output angular frequency of MMC that can be adjusted by the supercapacitor.

[0027] Furthermore, if the MMC operating frequency is lower than the minimum frequency of the MMC that can be regulated by the supercapacitor, a high-frequency injection control method and a supercapacitor C are used to balance the suspended capacitor voltage of the submodule, including:

[0028] Inject high-frequency signals including high-frequency common-mode voltage and high-frequency circulating current into the upper bridge arm and the lower bridge arm to obtain the output power of the bridge arm;

[0029] The bridge arm output power includes four parts: the first part is the low-frequency component before the high-frequency signal is injected, including the upper bridge arm power and the lower bridge arm power; the second and third parts are the high-frequency components directly related to the high-frequency signal, and the fourth part is the product of the high-frequency voltage and current;

[0030] Ignore the second and third parts, and decompose the high-frequency component in the fourth part according to the product and difference principle of the sine function to obtain a high-frequency component that is equal to and opposite to the first part, offsetting the low-frequency component in the first part. The output power of the bridge arm only has a high-frequency component.

[0031] Furthermore, if the MMC operating frequency is lower than the minimum frequency of the MMC that can be regulated by the supercapacitor, a high-frequency injection control method and supercapacitor C are used to balance the suspension capacitor voltage of the a-phase neutron module, including:

[0032] Inject high-frequency signals including high-frequency common-mode voltage and high-frequency circulating current into the upper and lower bridge arms of phase a, and obtain the bridge arm output power, expressed as;

[0033] ;

[0034] In the formula, , are the output powers of the upper bridge arm p and the lower bridge arm n of phase a respectively, is the DC side output voltage, Phase a AC side The output voltage, is the DC bus current, Phase a AC side Bus current, represents the high frequency common mode voltage, It means that only the high-frequency injection method is used to balance the high-frequency circulating current component under the suspended capacitor voltage of the a-phase neutron module;

[0035] According to the calculation method of the bridge arm output power, the difference in output power between the upper bridge arm p and the lower bridge arm n of phase a is calculated. , expressed as:

[0036] ;

[0037] By making the output power difference between the upper bridge arm p and the lower bridge arm n of phase a contain no low-frequency component, the voltage fluctuation of the suspension capacitor of the submodule can be suppressed and the suspension capacitor voltage of the submodule can be balanced;

[0038] So the high frequency common mode voltage and high frequency circulating current are set to:

[0039] ;

[0040] In the formula, Represents the high-frequency common-mode voltage amplitude, Represents the high-frequency circulating current amplitude, ω h is the high frequency common mode voltage angular frequency.

[0041] In a second aspect, the present invention provides a computer-readable storage medium having a computer program / instruction stored thereon, wherein the computer program / instruction, when executed by a processor, implements the steps of the method described in the first aspect.

[0042] In a third aspect, the present invention provides a computer program product, comprising a computer program / instruction, characterized in that the computer program / instruction, when executed by a processor, implements the steps of the method described in the first aspect.

[0043] Compared with the prior art, the present invention has the following beneficial effects:

[0044] 1. The present invention calculates the energy fluctuation range of the supercapacitor under the highest and lowest cut-off voltages during normal operation, and combines the fluctuation range of the submodule energy at the low frequency of the MMC to obtain the lowest frequency of the MMC that can be regulated by the supercapacitor. When the operating frequency of the MMC is relatively high, the fluctuation of the submodule capacitor voltage can be effectively suppressed by relying solely on the supercapacitor, thereby greatly reducing the demand for the supercapacitor capacitance. Compared with the traditional MMC system, it not only reduces the use of supercapacitors and hardware costs, but also improves the economy of the entire system;

[0045] 2. Under the condition of low operating frequency of MMC, the present invention effectively suppresses the fluctuation of sub-module suspension capacitor voltage by introducing high-frequency injection control method and synergistic effect of supercapacitor. Compared with the traditional MMC system, it reduces the size of bridge arm current and circulating current, which not only reduces the current stress and system loss of the device, but also improves the operation efficiency and stability of the system.

[0046] 3. The present invention connects the supercapacitor in parallel to the two ends of the suspension capacitor of the submodule through a bidirectional DC / DC converter, thereby realizing efficient management and balance of energy. In the discharge mode, when the suspension capacitor voltage of the submodule is lower than the preset reference value, the supercapacitor charges the submodule through the boost mode, effectively preventing the voltage from being too low; in the charging mode, when the suspension capacitor voltage of the submodule is too high, the bidirectional DC / DC converter works in the buck mode, transferring the excess energy to the supercapacitor, avoiding the system instability caused by the over-high voltage. This design of bidirectional energy flow ensures the stability of the submodule voltage and the reliability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 The figure is a flow chart of a low-frequency voltage-equalizing hybrid control method of an energy storage type MMC provided by an embodiment of the present invention;

[0048] Figure 2 The figure shows a schematic diagram of a three-phase topological structure of an MMC provided by an embodiment of the present invention;

[0049] Figure 3 The figure is a schematic diagram of a control structure of a suspension capacitor voltage using a super capacitor equalization submodule provided in an embodiment of the present invention;

[0050] Figure 4 It is a schematic diagram of the control structure of the high-frequency injection control method and the suspended capacitor voltage of the super capacitor C balancing submodule provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0051] The technical solution of the present invention is described in detail below through the accompanying drawings and specific embodiments. It should be understood that the embodiments of the present invention and the specific features in the embodiments are detailed descriptions of the technical solution of the present invention, rather than limitations on the technical solution of the present invention. The embodiments of the present invention and the technical features in the embodiments may be combined with each other unless there is a conflict.

[0052] The term "and / or" is only a description of the association relationship between related objects, indicating that there can be three relationships. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " generally indicates that the related objects are in an "or" relationship.

[0053] Example 1

[0054] like Figure 1 As shown, this embodiment proposes an energy storage type MMC low-frequency voltage-balanced hybrid control method, which is characterized by comprising:

[0055] Step 1: Calculate the energy fluctuation range that the supercapacitor can regulate based on the supercapacitor and the maximum cutoff voltage and the minimum cutoff voltage when the supercapacitor is working normally.

[0056] The energy storage capacity of a supercapacitor is determined by its maximum cut-off voltage and minimum cut-off voltage during normal operation. The maximum cut-off voltage and minimum cut-off voltage limit the voltage range in which the supercapacitor can be safely charged and discharged.

[0057] The present invention determines the energy fluctuation range that can be effectively regulated by the supercapacitor, clarifies the energy storage potential of the supercapacitor, and ensures that the control strategy is carried out within a safe voltage range.

[0058] Step 2: According to the energy fluctuation range that the supercapacitor can control and the fluctuation range of the submodule energy at the low frequency of MMC, calculate the lowest frequency of MMC that the supercapacitor can control.

[0059] When the MMC operates at a low frequency, the energy fluctuation range of the submodule will increase. The energy storage capacity of the supercapacitor needs to match the fluctuation range of the submodule energy when the MMC is at a low frequency. The present invention calculates that the supercapacitor can effectively regulate the minimum operating frequency of the MMC, which provides a basis for the selection of the control strategy, that is, determining which control method to adopt according to the operating frequency of the MMC.

[0060] Step 3: Select a method for balancing the floating capacitor voltage of the submodule according to the MMC operating frequency.

[0061] If the MMC operating frequency is lower than the minimum frequency of the MMC that can be regulated by the supercapacitor, a high-frequency injection control method and a supercapacitor C are used to balance the suspended capacitor voltage of the submodule.

[0062] When the MMC operating frequency is low, the submodule energy fluctuates greatly, and the voltage cannot be effectively balanced by relying solely on supercapacitors. The present invention introduces a high-frequency injection control method, which injects high-frequency components into the system to assist in adjusting the voltage, which can speed up the voltage regulation speed and work together with supercapacitors to achieve more accurate voltage balance. At the same time, the stability and voltage balance of the MMC are also guaranteed when operating at low frequencies.

[0063] If the MMC operating frequency is greater than or equal to the minimum frequency of the MMC that can be regulated by the supercapacitor, the supercapacitor is used to balance the suspension capacitor voltage of the submodule.

[0064] When the MMC operating frequency is high, the submodule energy fluctuation is relatively small, and the energy storage capacity of the supercapacitor is sufficient to cope with these fluctuations, so the supercapacitor can be directly used for voltage balancing. The present invention simplifies the control strategy and improves the efficiency and reliability of the system. At the same time, when operating at high frequency, it achieves fast and accurate voltage balancing while reducing the control complexity.

[0065] Among them, the MMC includes three phases a, b, and c, each phase includes an upper bridge arm and a lower bridge arm, each bridge arm includes a bridge arm inductor L and N sub-modules SM connected in series with the bridge arm inductor, the sub-module adopts a suspended capacitor structure, and the supercapacitor is connected in parallel to the two ends of the suspended capacitor of the sub-module through a bidirectional DC / DC converter to form a distributed energy storage unit.

[0066] In summary, the present invention can better balance the suspended capacitor voltage of the sub-module, and provide the use of supercapacitors to suppress the fluctuation of the sub-module capacitor voltage under relatively high system frequency conditions, which greatly reduces the capacitance requirement of the supercapacitor compared to traditional MMC; under relatively low frequency conditions, a hybrid control method is used, which can reduce the bridge arm current and circulating current compared to the high-frequency injection method, thereby reducing device current stress and system loss.

[0067] Example 2

[0068] Based on the same inventive concept as Example 1, this example introduces a low-frequency voltage-balanced hybrid control method for energy storage MMC. Figure 2 As shown, the MMC includes three phases, each phase includes an upper bridge arm and a lower bridge arm, each bridge arm includes a bridge arm inductor and N submodules connected in series with the bridge arm inductor, and the submodule adopts a suspended capacitor structure, a super capacitor A distributed energy storage unit is formed by connecting a bidirectional DC / DC converter in parallel to both ends of the suspended capacitor of the submodule.

[0069] The super capacitor is connected in parallel to both ends of the suspension capacitor of the submodule through a bidirectional DC / DC converter, wherein the working mode of the super capacitor includes a discharge mode and a charge mode;

[0070] The discharge mode includes: if the suspension capacitor voltage of the submodule is less than a preset reference value, the bidirectional DC / DC converter operates in a boost mode, and the supercapacitor charges the suspension capacitor of the submodule;

[0071] The charging mode includes: if the voltage of the suspension capacitor of the submodule is greater than or equal to a preset reference value, the bidirectional DC / DC converter operates in a buck mode, and the excess energy in the suspension capacitor of the submodule is transferred to the supercapacitor.

[0072] In this embodiment, the energy storage type MMC low-frequency voltage-balanced hybrid control method includes the following steps:

[0073] Step 1: Based on the supercapacitor and the highest cut-off voltage and the lowest cut-off voltage when the supercapacitor is working normally, calculate the energy fluctuation range that the supercapacitor can regulate.

[0074] In this embodiment, the energy fluctuation range that can be regulated by the supercapacitor is calculated based on the supercapacitor and the highest cut-off voltage and the lowest cut-off voltage when the supercapacitor is working normally, and is expressed as:

[0075] (1);

[0076] In the formula, It is the energy fluctuation range that can be controlled by all supercapacitors on a single bridge arm of MMC. is the supercapacitor value, , They are respectively the highest cut-off voltage and the lowest cut-off voltage when the supercapacitor is working normally. is the number of submodules on a single bridge arm of the MMC, .

[0077] Step 2: According to the energy fluctuation range that the supercapacitor can control and the fluctuation range of the submodule energy at the low frequency of MMC, calculate the lowest frequency of MMC that the supercapacitor can control.

[0078] In this embodiment, according to the energy fluctuation range that can be regulated by the supercapacitor and the fluctuation range of the submodule energy at the low frequency of the MMC, the lowest frequency of the MMC that can be regulated by the supercapacitor is calculated, including:

[0079] When the energy fluctuation range that can be regulated by all supercapacitors on a single bridge arm of the MMC is equal to the energy change of the upper bridge arm at low frequency of the MMC, the lowest output angular frequency of the MMC that can be regulated by the supercapacitor is obtained;

[0080] According to the minimum output angular frequency of MMC that can be regulated by the supercapacitor, the minimum frequency of MMC that can be regulated by the supercapacitor is calculated.

[0081] In this embodiment, the calculation expression of the fluctuation range of the submodule energy of the upper bridge arm of the a phase is:

[0082] Δ W pa = ∫ t 1 t 2 P ap d t =− U dc I m 2 ω [ 1 − m 2 ( 1 + cos 2 φ ) 3 ] (2);

[0083] In the formula, is the energy change of the upper bridge arm p of phase a at low frequency of MMC, , They are the start time and end time of half cycle of MMC at low frequency, is the output power of the upper bridge arm p of phase a, For DC side The output voltage, is the amplitude of output phase voltage and phase current, is the power factor angle, is the output angular frequency, To adjust the system, is the cosine function.

[0084] In this embodiment, the lowest frequency of the MMC that can be regulated by the supercapacitor is calculated based on the lowest output angular frequency of the MMC that can be regulated by the supercapacitor, and is expressed as:

[0085] (3);

[0086] In the formula, and Indicates the lowest frequency and lowest angular frequency of MMC that can be adjusted by supercapacitors.

[0087] Step 3: Select a method for balancing the floating capacitor voltage of the submodule according to the MMC operating frequency.

[0088] In this embodiment, if the MMC operating frequency is lower than the minimum frequency of the MMC that can be regulated by the supercapacitor, a high-frequency injection control method and a supercapacitor C are used to balance the suspended capacitor voltage of the submodule, such as Figure 4As shown, including:

[0089] Inject high-frequency signals including high-frequency common-mode voltage and high-frequency circulating current into the upper bridge arm and the lower bridge arm to obtain the output power of the bridge arm;

[0090] The bridge arm output power includes four parts: the first part is the low-frequency component before the high-frequency signal is injected, including the upper bridge arm power and the lower bridge arm power; the second and third parts are the high-frequency components directly related to the high-frequency signal, and the fourth part is the product of the high-frequency voltage and current;

[0091] Ignore the second and third parts, and decompose the high-frequency component in the fourth part according to the product and difference principle of the sine function to obtain a high-frequency component that is equal to and opposite to the first part, offsetting the low-frequency component in the first part. The output power of the bridge arm only has a high-frequency component.

[0092] In this embodiment, if the MMC operating frequency is lower than the minimum frequency of the MMC that can be regulated by the supercapacitor, a high-frequency injection control method and a supercapacitor C are used to balance the suspension capacitor voltage of the a-phase neutron module, including:

[0093] Inject high-frequency signals including high-frequency common-mode voltage and high-frequency circulating current into the upper and lower bridge arms of phase a, and obtain the bridge arm output power, expressed as;

[0094] (4);

[0095] In the formula, , are the output powers of the upper bridge arm p and the lower bridge arm n of phase a respectively, is the DC side output voltage, Phase a AC side The output voltage, For DC side Bus current, Phase a AC side Bus current, represents the high frequency common mode voltage, It means that only the high-frequency injection method is used to balance the high-frequency circulating current component under the suspended capacitor voltage of the a-phase neutron module;

[0096] According to the calculation method of the bridge arm output power, the difference in output power between the upper bridge arm p and the lower bridge arm n of phase a is calculated. , expressed as:

[0097] (5);

[0098] By making the output power difference between the upper bridge arm p and the lower bridge arm n of phase a contain no low-frequency component, the voltage fluctuation of the suspension capacitor of the submodule can be suppressed and the suspension capacitor voltage of the submodule can be balanced;

[0099] So the high frequency common mode voltage and high frequency circulating current are set to:

[0100] (6);

[0101] In the formula, Represents the high-frequency common-mode voltage amplitude, Represents the high-frequency circulating current amplitude, ω h is the high frequency common mode voltage angular frequency.

[0102] If the MMC operating frequency is greater than or equal to the minimum frequency that the supercapacitor can control, the supercapacitor is used to balance the suspension capacitor voltage of the submodule, such as Figure 3 shown.

[0103] In this embodiment, when the MMC operating frequency is lower than the lowest frequency of the MMC that can be controlled by the supercapacitor, a high-frequency injection control method and supercapacitor C are used to balance the suspended capacitor voltage of the a-phase neutron module. Compared with only using the high-frequency injection control method to balance the suspended capacitor voltage of the sub-module, this embodiment can reduce the bridge arm current and circulating current, reduce device current stress and system loss.

[0104] Among them, the system angular frequency under the suspension capacitor voltage of the a-phase neutron module using the high-frequency injection control method and the supercapacitor C is taken The system angular frequency under the suspended capacitor voltage of the equalized submodule using only the high-frequency injection control method ,and .

[0105] Taking the upper bridge arm of phase a as an example, according to formula (2), the change value of the bridge arm energy under the suspension capacitor voltage of the a-phase neutron module balanced by the high-frequency injection control method and supercapacitor C is The change value of the bridge arm energy under the suspended capacitor voltage of the balanced submodule using only the high-frequency injection control method , expressed as:

[0106] { Δ W pa1 =− U dc I m 2 ω 1 [ 1 − m 2 ( 1 + cos 2 φ ) 3 ] Δ W pa2 =− U dc I m 2 ω 2 [ 1 − m 2 ( 1 + cos 2 φ ) 3 ] (7);

[0107] because , so the fluctuation of the submodule capacitor voltage can be suppressed by the energy storage MMC control method. Therefore, ideally, the maximum energy value that can be adjusted by the supercapacitor is equal to .

[0108] When the system frequency is ω1, the high-frequency injection control method and the supercapacitor C are used to balance the suspension capacitor voltage of the a-phase neutron module. The energy range that can be regulated by using the high-frequency injection control method and the supercapacitor C to balance the suspension capacitor voltage of the a-phase neutron module is expressed as:

[0109] (8);

[0110] In this embodiment, let ,in, is the proportionality coefficient, due to , can be obtained ,so k ∈ [ 0 , 1 ] , we can further get:

[0111] (9);

[0112] In the formula, It represents the high-frequency circulating current component under the suspended capacitor voltage of the a-phase neutron module using the high-frequency injection control method and the supercapacitor C to balance it.

[0113] In this embodiment, the energy range that can be controlled by using only the high-frequency injection method to balance the suspended capacitor voltage of the a-phase neutron module is expressed as:

[0114] (10);

[0115] In the formula, It indicates that only the high-frequency injection method is used to balance the high-frequency circulating current component under the suspended capacitor voltage of the a-phase neutron module.

[0116] When the high-frequency common-mode voltage remains unchanged, the high-frequency injection control method and supercapacitor C are used to balance the high-frequency circulating current component under the suspended capacitor voltage of the a-phase neutron module. The high-frequency circulating current component under the suspended capacitor voltage of the a-phase neutron module is balanced by only high-frequency injection method The relationship is expressed as:

[0117] (11);

[0118] Therefore, the high-frequency circulating current component required to be injected by the energy storage type MMC low-frequency equalizing voltage hybrid control method proposed by the present invention is smaller than the high-frequency circulating current component required to be injected by the method used by the traditional MMC.

[0119] Example 3

[0120] Based on the same inventive concept as other embodiments, this embodiment is a computer-readable storage medium on which a computer program / instruction is stored. When the computer program / instruction is executed by a processor, the steps of the method described in Embodiment 1 or 2 are implemented.

[0121] Example 4

[0122] Based on the same inventive concept as other embodiments, this embodiment is a computer program product, including a computer program / instruction, characterized in that the computer program / instruction implements the steps of the method described in Embodiment 1 or 2 when executed by a processor.

[0123] In summary, the present invention calculates the energy fluctuation range of the supercapacitor at the highest and lowest cut-off voltages during normal operation, and combines the fluctuation range of the submodule energy at the low frequency of the MMC to obtain the lowest frequency of the MMC that can be regulated by the supercapacitor, so that when the operating frequency of the MMC is relatively high, the fluctuation of the submodule capacitor voltage can be effectively suppressed by relying solely on the supercapacitor, thereby greatly reducing the demand for the supercapacitor capacitance. Compared with the traditional MMC system, not only the use of supercapacitors is reduced, the hardware cost is reduced, but also the economy of the entire system is improved;

[0124] Under the condition of low operating frequency of MMC, the present invention effectively suppresses the fluctuation of submodule suspension capacitor voltage by introducing high-frequency injection control method and synergistic effect of supercapacitor. Compared with the traditional MMC system, the size of bridge arm current and circulating current is reduced, which not only reduces the current stress of the device and system loss, but also improves the operating efficiency and stability of the system.

[0125] The present invention connects the supercapacitor in parallel to the two ends of the suspension capacitor of the submodule through a bidirectional DC / DC converter, thereby realizing efficient management and balance of energy. In the discharge mode, when the suspension capacitor voltage of the submodule is lower than the preset reference value, the supercapacitor charges the submodule through the boost mode, effectively preventing the voltage from being too low; in the charging mode, when the suspension capacitor voltage of the submodule is too high, the bidirectional DC / DC converter works in the buck mode, transferring the excess energy to the supercapacitor, avoiding the system instability caused by the over-high voltage. This design of bidirectional energy flow ensures the stability of the submodule voltage and the reliability of the system.

[0126] It will be appreciated by those skilled in the art that embodiments of the present invention may be provided as methods, systems, or computer program products. Therefore, the present invention may take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Furthermore, the present invention may take 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 code.

[0127] The present invention is described with reference to flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to embodiments of the present invention. It should be understood that each process and / or block in the flowchart and / or block diagram, as well as the combination of processes and / or blocks 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 produce 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.

[0128] 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.

[0129] 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 A step that specifies a function in one or more boxes.

[0130] The embodiments of the present invention are described above in conjunction with the accompanying drawings, but the present invention is not limited to the above-mentioned specific implementation methods. The above-mentioned specific implementation methods are merely illustrative and not restrictive. Under the enlightenment of the present invention, ordinary technicians in this field can also make many forms without departing from the scope of protection of the purpose of the present invention and the claims, which all fall within the protection of the present invention.

Claims

1. A low-frequency voltage-equalizing hybrid control method for energy storage MMC, characterized in that: include: The energy fluctuation range that can be controlled by the supercapacitor is calculated based on the supercapacitor and the maximum cut-off voltage and the minimum cut-off voltage when the supercapacitor is working normally; According to the energy fluctuation range that the supercapacitor can control and the fluctuation range of the submodule energy at low frequency of MMC, the lowest frequency of MMC that the supercapacitor can control is calculated; If the MMC operating frequency is lower than the minimum frequency that the supercapacitor can regulate, the high-frequency injection control method and the supercapacitor C are used to balance the suspension capacitor voltage of the submodule; If the MMC operating frequency is greater than or equal to the minimum frequency of the MMC that can be regulated by the supercapacitor, the supercapacitor is used to balance the suspension capacitor voltage of the submodule; Among them, the MMC includes three phases, each phase includes an upper bridge arm and a lower bridge arm, each bridge arm includes a bridge arm inductor and N sub-modules connected in series with the bridge arm inductor, the sub-module adopts a suspended capacitor structure, and the supercapacitor is connected in parallel to the two ends of the suspended capacitor of the sub-module through a bidirectional DC / DC converter to form a distributed energy storage unit.

2. The energy storage type MMC low frequency voltage-balanced hybrid control method according to claim 1 is characterized in that: The super capacitor is connected in parallel to both ends of the suspension capacitor of the submodule through a bidirectional DC / DC converter, wherein the working mode of the super capacitor includes a discharge mode and a charge mode; The discharge mode includes: if the suspension capacitor voltage of the submodule is less than a preset reference value, the bidirectional DC / DC converter operates in a boost mode, and the supercapacitor charges the suspension capacitor of the submodule; The charging mode includes: if the voltage of the suspension capacitor of the submodule is greater than or equal to a preset reference value, the bidirectional DC / DC converter operates in a buck mode, and the excess energy in the suspension capacitor of the submodule is transferred to the supercapacitor.

3. The energy storage type MMC low-frequency voltage-balanced hybrid control method according to claim 1 is characterized in that: The energy fluctuation range that can be regulated by the supercapacitor is calculated based on the supercapacitor and the highest cut-off voltage and the lowest cut-off voltage when the supercapacitor is working normally, and is expressed as: ； In the formula, It is the energy fluctuation range that can be controlled by all supercapacitors on a single bridge arm of MMC. is the supercapacitor value, , They are respectively the highest cut-off voltage and the lowest cut-off voltage when the supercapacitor is working normally. is the number of submodules on a single bridge arm of the MMC, .

4. The energy storage type MMC low frequency voltage-balanced hybrid control method according to claim 3 is characterized in that: The calculation of the lowest frequency of the MMC that can be regulated by the supercapacitor according to the energy fluctuation range that can be regulated by the supercapacitor and the fluctuation range of the submodule energy at the low frequency of the MMC includes: When the energy fluctuation range that can be regulated by all supercapacitors on a single bridge arm of the MMC is equal to the energy change of the upper bridge arm at low frequency of the MMC, the lowest output angular frequency of the MMC that can be regulated by the supercapacitor is obtained; According to the minimum output angular frequency of MMC that can be regulated by the supercapacitor, the minimum frequency of MMC that can be regulated by the supercapacitor is calculated.

5. The energy storage type MMC low frequency voltage-balanced hybrid control method according to claim 4 is characterized in that: The three phases include phase a, and the calculation expression of the fluctuation range of the submodule energy of the upper bridge arm is: ； In the formula, is the energy change of the upper bridge arm p of phase a at low frequency of MMC, , They are the start time and end time of half cycle of MMC at low frequency, is the output power of the upper bridge arm p of phase a, For DC side The output voltage, is the amplitude of the output phase voltage and phase current, is the power factor angle, is the output angular frequency, To adjust the system, is the cosine function.

6. The energy storage type MMC low frequency voltage-balanced hybrid control method according to claim 4 is characterized in that: According to the minimum output angular frequency of the MMC that can be regulated by the supercapacitor, the minimum frequency of the MMC that can be regulated by the supercapacitor is calculated and expressed as: ； In the formula, It indicates the lowest frequency of MMC that can be adjusted by supercapacitor. Indicates the minimum output angular frequency of MMC that can be adjusted by the supercapacitor.

7. The energy storage type MMC low frequency voltage-balanced hybrid control method according to claim 1, characterized in that: If the MMC operating frequency is lower than the minimum frequency that the supercapacitor can regulate, a high-frequency injection control method and a supercapacitor C are used to balance the suspended capacitor voltage of the submodule, including: Inject high-frequency signals including high-frequency common-mode voltage and high-frequency circulating current into the upper bridge arm and the lower bridge arm to obtain the output power of the bridge arm; The bridge arm output power includes four parts: the first part is the low-frequency component before the high-frequency signal is injected, including the upper bridge arm power and the lower bridge arm power; the second and third parts are the high-frequency components directly related to the high-frequency signal, and the fourth part is the product of the high-frequency voltage and current; Ignore the second and third parts, and decompose the high-frequency component in the fourth part according to the product and difference principle of the sine function to obtain a high-frequency component that is equal to and opposite to the first part, offsetting the low-frequency component in the first part. The output power of the bridge arm only has a high-frequency component.

8. The energy storage type MMC low frequency voltage-balanced hybrid control method according to claim 7 is characterized in that: If the MMC operating frequency is lower than the minimum frequency of the MMC that can be regulated by the supercapacitor, a high-frequency injection control method and supercapacitor C are used to balance the suspension capacitor voltage of the a-phase neutron module, including: The high-frequency signal injected into the upper bridge arm and the lower bridge arm of phase a includes high-frequency common-mode voltage and high-frequency circulating current. The output power of the bridge arm is expressed as; ； In the formula, , are the output powers of the upper bridge arm p and the lower bridge arm n of phase a respectively, is the DC output voltage, Phase a AC side The output voltage, is the DC bus current, Phase a AC side Bus current, represents the high frequency common mode voltage, It means that only the high-frequency injection method is used to balance the high-frequency circulating current component under the suspended capacitor voltage of the a-phase neutron module; According to the calculation method of the bridge arm output power, the difference in output power between the upper bridge arm p and the lower bridge arm n of phase a is calculated as , expressed as: ； By making the output power difference between the upper bridge arm p and the lower bridge arm n of phase a contain no low-frequency component, the voltage fluctuation of the suspension capacitor of the submodule can be suppressed and the suspension capacitor voltage of the submodule can be balanced; So the high frequency common mode voltage and high frequency circulating current are set to: ； In the formula, Represents the high-frequency common-mode voltage amplitude, Represents the high-frequency circulating current amplitude, ω h is the high frequency common mode voltage angular frequency.

9. A computer-readable storage medium having a computer program / instruction stored thereon, characterized in that: When the computer program / instructions are executed by a processor, the steps of the method according to any one of claims 1 to 8 are implemented.

10. A computer program product comprising a computer program / instructions, characterized in that When the computer program / instructions are executed by a processor, the steps of the method according to any one of claims 1 to 8 are implemented.

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