Voltage equalization control method, device and equipment of MMC and medium

Abstract

Embodiments of the present application provide a voltage equalization control method, device and equipment for MMC, and a medium. The method comprises: determining, for any bridge arm in the main circuit of the MMC, the current absolute value, the current rated value, the modulation index of the bridge arm, and the capacitor voltage of each submodule. Then, according to the current absolute value, the current rated value, the modulation index of the bridge arm, and the capacitor voltage of each submodule, a corresponding dynamic threshold adjustment coefficient is determined. Next, the dynamic threshold adjustment coefficient is multiplied by a preset reference threshold value, and the result obtained by the multiplication is determined as a target voltage threshold value. If the target voltage difference between the maximum submodule voltage and the minimum submodule voltage in the bridge arm is greater than the target voltage threshold value, the switching state of each submodule in the bridge arm is sorted and switched to achieve capacitor voltage equalization. The method is used to achieve the effect of reducing the switching frequency and the loss.

Description

Technical Field

[0001] This application relates to the field of power system transmission and distribution technology, and in particular to a voltage equalization control method, device, equipment and medium for MMC. Background Technology

[0002] Modular multilevel converters (MMCs) have become core equipment in the field of high voltage direct current transmission due to their modular design, easy expansion of voltage levels, large transmission capacity, high efficiency, and good output power quality.

[0003] However, due to differences in the charging and discharging currents of each submodule and the non-uniformity of switching states, cumulative voltage imbalance inevitably occurs in the capacitors. To address this voltage imbalance problem, traditional voltage equalization control mainly employs algorithms based on real-time voltage sorting. Specifically, within each control cycle, the algorithm measures the capacitor voltage of all submodules in the bridge arm in real time and calculates the difference between its maximum and minimum values ​​(i.e., Once the difference is detected to be greater than zero, the capacitor voltages of all submodules in the bridge arm will be sorted immediately, and the submodules with the highest and lowest voltages will be switched first to force equalization based on the sorting results.

[0004] However, existing technologies result in high switching frequencies, which in turn lead to significant switching losses. Summary of the Invention

[0005] This application provides a voltage equalization control method, apparatus, equipment, and medium for MMC, in order to achieve the technical effect of reducing switching losses.

[0006] In a first aspect, embodiments of this application provide a method for equalizing voltage control of an MMC, comprising:

[0007] For any bridge arm in the MMC main circuit, determine the absolute value of the current, the rated current, the modulation index, and the capacitor voltage of each submodule of the bridge arm.

[0008] Based on the absolute value of the current, the rated current, the modulation index, and the capacitor voltage of each submodule, determine the corresponding dynamic threshold adjustment coefficient.

[0009] The dynamic threshold adjustment coefficient is multiplied by a preset reference threshold, and the result of the multiplication is determined as the target voltage threshold.

[0010] If the target voltage difference between the maximum and minimum submodule voltages within the bridge arm is greater than the target voltage threshold, the switching states of each submodule within the bridge arm are sorted and switched to achieve capacitor voltage balancing.

[0011] In one possible implementation, determining the corresponding dynamic threshold adjustment coefficient based on the absolute value of the current, the rated current, the modulation index, and the capacitor voltage of each submodule includes:

[0012] The degree of drastic change in operating conditions is determined based on the modulation index and the capacitor voltage of each submodule.

[0013] If the degree of change in the operating condition is less than or equal to a preset degree threshold, then the corresponding dynamic threshold adjustment coefficient is determined based on the absolute value of the current of the bridge arm, the rated current, the modulation index, and the capacitor voltage of each submodule.

[0014] In one possible implementation, determining the degree of drastic change in operating conditions based on the modulation index and the capacitor voltage of each submodule includes:

[0015] Calculate the voltage fluctuation rate of the bridge arm based on the capacitor voltage of each submodule;

[0016] The degree of drastic change in the operating condition is calculated based on the modulation index and the voltage fluctuation rate.

[0017] In one possible implementation, determining the corresponding dynamic threshold adjustment coefficient based on the absolute value of the current, the rated current, the modulation index, and the capacitor voltage of each submodule includes:

[0018] Calculate the normalized current amplitude based on the absolute value of the current and the rated value of the current;

[0019] The dynamic threshold adjustment coefficient is determined based on the normalized current amplitude, the modulation index, and the voltage fluctuation rate through a preset mapping relationship. The preset mapping relationship is constructed in advance based on expert experience and / or simulation analysis. The preset mapping relationship is used to represent the correspondence between the normalized current amplitude, the modulation index, the voltage fluctuation rate, and the dynamic threshold adjustment coefficient.

[0020] In one possible implementation, determining the dynamic threshold adjustment coefficient based on the normalized current amplitude, the modulation index, and the voltage fluctuation rate through a preset mapping relationship includes:

[0021] The normalized current amplitude, the modulation index, and the voltage fluctuation rate are respectively fuzzified to determine the first fuzzy set corresponding to the normalized current amplitude, the second fuzzy set corresponding to the modulation index, and the third fuzzy set corresponding to the voltage fluctuation rate.

[0022] Based on the first fuzzy set, the second fuzzy set, and the third fuzzy set, at least one initial dynamic threshold adjustment coefficient is determined through the preset mapping relationship. The preset mapping relationship includes the initial dynamic threshold adjustment coefficient corresponding to any value of the first fuzzy set, any value of the second fuzzy set, and any value of the third fuzzy set.

[0023] The dynamic threshold adjustment coefficient is generated by processing the at least one initial dynamic threshold adjustment coefficient using the centroid method.

[0024] In one possible implementation, the method further includes:

[0025] If the degree of change in the operating condition is greater than the preset degree threshold, then the preset voltage threshold is determined as the target voltage threshold.

[0026] If the target voltage difference is greater than the target voltage threshold, the switching states of each sub-module in the bridge arm are sorted and switched to achieve capacitor voltage balancing.

[0027] In one possible implementation, when the degree of change in the operating condition is less than or equal to the preset degree threshold, the determined target voltage threshold is negatively correlated with the normalized current amplitude; and / or, when the modulation index is greater than the preset modulation index threshold, the determined target voltage threshold is negatively correlated with the voltage fluctuation rate.

[0028] Secondly, embodiments of this application provide a voltage equalization control device for MMC, comprising:

[0029] The first determining module is used to determine the absolute value of the current, the rated current, the modulation index, and the capacitor voltage of each sub-module for any bridge arm in the MMC main circuit.

[0030] The second determining module is used to determine the corresponding dynamic threshold adjustment coefficient based on the absolute value of the current, the rated value of the current, the modulation index, and the capacitor voltage of each sub-module of the bridge arm.

[0031] The third determining module is used to multiply the dynamic threshold adjustment coefficient by a preset reference threshold, and determine the result of the multiplication as the target voltage threshold.

[0032] The control module is used to sort and switch the switching states of each submodule in the bridge arm if the target voltage difference between the maximum submodule voltage and the minimum submodule voltage in the bridge arm is greater than the target voltage threshold, so as to achieve capacitor voltage balancing.

[0033] In one possible implementation, the second determining module is specifically used for:

[0034] The degree of drastic change in operating conditions is determined based on the modulation index and the capacitor voltage of each submodule.

[0035] If the degree of change in the operating condition is less than or equal to a preset degree threshold, then the corresponding dynamic threshold adjustment coefficient is determined based on the absolute value of the current of the bridge arm, the rated current, the modulation index, and the capacitor voltage of each submodule.

[0036] In one possible implementation, the second determining module is specifically used for:

[0037] Calculate the voltage fluctuation rate of the bridge arm based on the capacitor voltage of each submodule;

[0038] The degree of drastic change in the operating condition is calculated based on the modulation index and the voltage fluctuation rate.

[0039] In one possible implementation, the second determining module is specifically used for:

[0040] Calculate the normalized current amplitude based on the absolute value of the current and the rated value of the current;

[0041] The dynamic threshold adjustment coefficient is determined based on the normalized current amplitude, the modulation index, and the voltage fluctuation rate through a preset mapping relationship. The preset mapping relationship is constructed in advance based on expert experience and / or simulation analysis. The preset mapping relationship is used to represent the correspondence between the normalized current amplitude, the modulation index, the voltage fluctuation rate, and the dynamic threshold adjustment coefficient.

[0042] In one possible implementation, the second determining module is specifically used for:

[0043] The normalized current amplitude, the modulation index, and the voltage fluctuation rate are respectively fuzzified to determine the first fuzzy set corresponding to the normalized current amplitude, the second fuzzy set corresponding to the modulation index, and the third fuzzy set corresponding to the voltage fluctuation rate.

[0044] Based on the first fuzzy set, the second fuzzy set, and the third fuzzy set, at least one initial dynamic threshold adjustment coefficient is determined through the preset mapping relationship. The preset mapping relationship includes the initial dynamic threshold adjustment coefficient corresponding to any value of the first fuzzy set, any value of the second fuzzy set, and any value of the third fuzzy set.

[0045] The dynamic threshold adjustment coefficient is generated by processing the at least one initial dynamic threshold adjustment coefficient using the centroid method.

[0046] In one possible implementation, the control module is further used for

[0047] If the degree of change in the operating condition is greater than the preset degree threshold, then the preset voltage threshold is determined as the target voltage threshold.

[0048] If the target voltage difference is greater than the target voltage threshold, the switching states of each sub-module in the bridge arm are sorted and switched to achieve capacitor voltage balancing.

[0049] In one possible implementation, when the degree of change in the operating condition is less than or equal to the preset degree threshold, the determined target voltage threshold is negatively correlated with the normalized current amplitude; and / or, when the modulation index is greater than the preset modulation index threshold, the determined target voltage threshold is negatively correlated with the voltage fluctuation rate.

[0050] Thirdly, embodiments of this application provide an electronic device, including: a memory and a processor;

[0051] The memory stores computer-executed instructions;

[0052] The processor executes computer execution instructions stored in the memory, causing the processor to perform the first aspect and / or various possible implementations of the first aspect as described above.

[0053] Fourthly, embodiments of this application provide a computer-readable storage medium storing computer-executable instructions, which, when executed by a processor, are used to implement the first aspect and / or various possible implementations of the first aspect.

[0054] Fifthly, embodiments of this application provide a computer program product, including a computer program that, when executed by a processor, implements the first aspect and / or various possible implementations of the first aspect.

[0055] The voltage equalization control method, apparatus, device, and medium for MMC provided in this application first calculate an adaptive target voltage threshold based on the absolute value of the current in the bridge arm, the rated current, the modulation index, and the capacitor voltage of each submodule. Sequencing and switching operations are triggered only when the target voltage difference between the largest and smallest submodule voltages exceeds this target voltage threshold. This changes the condition for switching action from the presence of deviation to exceeding a dynamic threshold, thereby significantly reducing the average switching frequency and switching losses while ensuring the core performance of capacitor voltage equalization. Attached Figure Description

[0056] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0057] Figure 1 This is a typical topology diagram of MMC-HVDC;

[0058] Figure 2 This is a diagram of the SM structure.

[0059] Figure 3 This is a schematic diagram of the SM working modes;

[0060] Figure 4 Flowchart of the voltage equalization control method for MMC provided in this application Figure 1 ;

[0061] Figure 5 Flowchart of the voltage equalization control method for MMC provided in this application Figure 2 ;

[0062] Figure 6 Flowchart of the voltage equalization control method for MMC provided in this application Figure 3 ;

[0063] Figure 7 The diagram shows the working curves and state changes of the upper and lower bridge arm sub-modules.

[0064] Figure 8 Reference waveform and output waveform;

[0065] Figure 9 The current waveforms are shown for negative-sequence and zero-sequence circulating current control.

[0066] Figure 10 The curve showing the voltage variation of the capacitor in the lower bridge arm submodule;

[0067] Figure 11 A schematic diagram of the voltage equalization control device for the MMC provided in this application;

[0068] Figure 12 A schematic diagram of the structure of the electronic device provided in this application.

[0069] The accompanying drawings illustrate specific embodiments of this application, which will be described in more detail below. These drawings and descriptions are not intended to limit the scope of the concept in any way, but rather to illustrate the concept of this application to those skilled in the art through reference to particular embodiments. Detailed Implementation

[0070] Exemplary embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings. When the following description relates to the drawings, unless otherwise indicated, the same numbers in different drawings denote the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with this application. Rather, they are merely examples of apparatuses and methods consistent with some aspects of this application as detailed in the appended claims.

[0071] It should be noted that the user information (including but not limited to user device information, user personal information, etc.) and data (including but not limited to data used for analysis, data stored, data displayed, etc.) involved in this application are all information and data authorized by the user or fully authorized by all parties. Furthermore, the collection, use and processing of the relevant data must comply with relevant laws, regulations and standards, and corresponding operation entry points are provided for users to choose to authorize or refuse.

[0072] First, the application scenarios involved in this application will be explained:

[0073] In recent years, the global energy structure transformation and the rapid development of clean energy have created an urgent demand for long-distance, high-efficiency, and large-capacity power transmission technologies. High Voltage Direct Current (HVDC) technology has attracted much attention due to its significant advantages in these aspects. Against this backdrop, Modular Transmission Modules (MMCs), with their modular design, easy expansion of voltage levels, large transmission capacity, high efficiency, and good output power quality, have become core equipment in the field of HVDC transmission.

[0074] The topology of an MMC consists of a large number of sub-modules connected in series, and its normal operation depends on the equalization of the capacitor voltages of each sub-module. Figure 1 This is a typical topology diagram of MMC-HVDC. (Example:) Figure 1 As shown, the AC side interface consists of a three-phase transformer and a three-phase circuit breaker, and the AC side three-phase voltage u a1 u b1 u c1 After passing through a three-phase transformer and a three-phase circuit breaker, the current is connected to the upper and lower bridge arm connection nodes of each phase of the converter, corresponding to the phase current i. a i b i c This flows into the converter bridge arm.

[0075] The converter has a three-phase structure, with each phase consisting of upper and lower arms. Each arm contains N sub-modules (SM) and one arm inductor L. arm The DC-side interface consists of a DC support capacitor and the DC line impedance Z. dc Composition. The DC support capacitor is divided into upper and lower units (u dc1 u dc2 After being connected in series, they jointly support the total DC bus voltage u. dc This provides energy buffering and DC voltage stabilization for the converter. The top ends of the upper bridge arms of all phases are connected to the positive terminal of the DC bus, and the bottom ends of the lower bridge arms are connected to the negative terminal of the DC bus. The DC line impedance Z... dcConnected in series between the DC positive terminal and the external transmission line, it is used to characterize the resistance and inductance characteristics of the DC transmission line.

[0076] Among them, the three-phase transformer realizes electrical isolation and voltage matching between the AC system and the converter, and suppresses harmonic injection and fault impact on the system side; the three-phase circuit breaker serves as a protective element, which can quickly disconnect the AC side electrical connection during system faults or maintenance.

[0077] Furthermore, Figure 2 This is a diagram of the SM architecture. (Example:) Figure 2 As shown, the submodule adopts a half-bridge structure, consisting of two Insulated Gate Bipolar Transistor (IGBT) switches (S1 and S2) and a DC capacitor C. sub composition.

[0078] pass Figure 1 and Figure 2 As can be seen, the modular multilevel structure facilitates voltage level expansion, significantly increases power transmission capacity, reduces switching losses resulting in higher power conversion efficiency, and improves voltage output stability. Due to the series connection and modular structure of the submodules, the voltage output is more stable and has lower harmonic content. Even when some submodules fail, the system can still continue to operate, demonstrating good fault tolerance.

[0079] exist Figure 2 Based on this, the switching state of the submodule determines whether the capacitor is connected to the bridge arm circuit through control signals, and the SM operating mode will be determined by... Figure 3 To express.

[0080] Figure 3 This is a schematic diagram of the SM working modes, such as... Figure 3 As shown, the SM operating modes include two types: insertion mode and bypass mode. In the insertion mode, S1 is on and S2 is off; at this time, the capacitor voltage... It is connected to the bridge arm; in the bypass state, S1 is off and S2 is on. At this time, the capacitor is short-circuited and does not participate in voltage synthesis.

[0081] However, due to differences in the charging and discharging currents of each submodule and the unevenness of switching states, cumulative voltage imbalance inevitably occurs in the capacitors. This voltage imbalance directly leads to waveform distortion and increased harmonic content in the converter output voltage, and causes some submodules to be subjected to excessively high voltage stress for extended periods, thereby accelerating capacitor aging and even causing failures.

[0082] To address the aforementioned voltage imbalance problem, traditional voltage equalization control primarily employs algorithms based on real-time voltage sorting. Specifically, within each control cycle, the algorithm measures the capacitor voltage of all submodules in the bridge arm in real time and calculates the difference between its maximum and minimum values ​​(i.e., Once the difference is detected to be greater than zero, the capacitor voltages of all submodules in the bridge arm will be sorted immediately, and the submodules with the highest and lowest voltages will be switched first to force equalization based on the sorting results.

[0083] Next, we will explain in detail how the difference between the maximum and minimum values ​​in this bridge arm is calculated.

[0084] Assuming the number of inserted submodules in the upper and lower bridge arms are respectively and Then the bridge arm voltage can be expressed as:

[0085] (1)

[0086] (2)

[0087] in, For the upper bridge arm voltage, This is the voltage of the lower bridge arm. This refers to the instantaneous output voltage of a single submodule. The inductance value of the bridge arm inductor. The instantaneous current flowing through the bridge arm, This is the instantaneous voltage of the submodule capacitor. The on-state voltage drop of S1 when the submodule is in the insertion state. This is the on-state voltage drop of S2 when the submodule is in bypass state.

[0088] According to Kirchhoff's voltage law, the AC side output voltage... With DC side voltage satisfy:

[0089] (3)

[0090] Further derivation yields the modulation expression for the bridge arm voltage as follows:

[0091] (4)

[0092] in, The modulation index, This is the instantaneous phase angle of the AC voltage.

[0093] Furthermore, the dynamic change of the submodule capacitor voltage is determined by both the bridge arm current and the switching state. For the i-th submodule, its capacitor voltage dynamic equation is:

[0094] (5)

[0095] Among them, S i(t)∈{0,1} represents the switching state of this submodule (0 for bypass, 1 for insertion). The capacitance value of the capacitor supporting the submodule. Let be the instantaneous capacitor voltage of the i-th submodule.

[0096] Assuming bridge arm current Composed of DC and AC components, it can be represented as:

[0097] (6)

[0098] in, This is the DC side current. The amplitude of the AC side current. The power factor angle.

[0099] When the submodule is in the insertion state, the charging and discharging power of the capacitor. for:

[0100] (7)

[0101] Therefore, the integral expression for the capacitor voltage can be obtained:

[0102] (8)

[0103] in, It is a dummy variable for integration.

[0104] The difference between the maximum and minimum values ​​of the capacitor voltages of all submodules in the bridge arm. That is:

[0105] (9)

[0106] However, while this method is directly effective, it is essentially a zero-tolerance rapid correction mechanism, and the submodule switching frequency of the bridge arm depends on... The amplitude of the voltage means that even a tiny voltage deviation will trigger the switching action, resulting in a switching frequency of up to several kilohertz and huge switching losses.

[0107] Therefore, how to effectively maintain the voltage balance of the submodule capacitors while significantly reducing switching losses has become a core technical problem that urgently needs to be solved in this field.

[0108] Based on the above-mentioned technical problems, the technical concept of this application is as follows:

[0109] It can be approximated as a first-order differential equation:

[0110] (10)

[0111] As can be seen from the above formula... The accumulation rate is related to the following factors

[0112] 1. Current direction: When >0 (charging state), the inserted high-voltage sub-modules charge faster, exacerbating voltage imbalance;

[0113] 2. Modulation depth: Under high modulation index (M→1), the voltage fluctuation of the bridge arm increases, resulting in significant differences in the charging and discharging of sub-modules;

[0114] 3. Switching frequency: High-frequency switching can quickly balance the voltage, but at the cost of increased losses.

[0115] Based on this, the inventors realized that by acquiring the absolute value of the current in the bridge arm, the rated current, the modulation index, and the capacitor voltage of each submodule, a dynamic threshold adjustment coefficient can be comprehensively determined, and a target voltage threshold adapted to the operating conditions can be calculated. Therefore, only when the target voltage difference between the voltage of the largest and smallest submodules within the bridge arm exceeds this target voltage threshold will the switching states of each submodule within the bridge arm be sorted and switched to achieve voltage equalization; if the target voltage difference does not exceed the threshold, no intervention is performed. This method changes the existing conditions for triggering voltage equalization operations, thereby effectively reducing switching frequency and losses.

[0116] The technical solution of this application and how the technical solution of this application solves the above-mentioned technical problems are described in detail below with specific embodiments. These specific embodiments can be combined with each other, and the same or similar concepts or processes may not be described again in some embodiments. The embodiments of this application will now be described with reference to the accompanying drawings.

[0117] Figure 4 Flowchart of the voltage equalization control method for MMC provided in this application Figure 1 ,like Figure 4 As shown, the method includes:

[0118] S41. For any bridge arm in the MMC main circuit, determine the absolute value of the current, the rated current, the modulation index, and the capacitor voltage of each submodule.

[0119] The MMC main circuit refers to a power circuit entity that is composed of multiple sub-modules connected through a specific topology and is capable of high-voltage power conversion. The bridge arm is the basic building block in the MMC main circuit. Each phase circuit contains two bridge arms, upper and lower, and each bridge arm is composed of several sub-modules connected in series with a bridge arm inductor.

[0120] It should be understood that the structure of the MMC main circuit can be referenced. Figure 1 As shown, it will not be elaborated further here.

[0121] The absolute value of the current refers to the absolute value of the instantaneous current flowing through the bridge arm. It reflects the magnitude of the current in that bridge arm at the current moment and is used to determine whether the submodule capacitor is charging or discharging, as well as the charging and discharging intensity. For example, when the absolute value of the current is positive and large, the connected submodule capacitor will charge rapidly.

[0122] The rated current is a preset fixed parameter used to indicate the maximum safe current value that the bridge arm can withstand under long-term continuous operation.

[0123] The modulation index is used to determine how many sub-modules are needed to synthesize the required AC voltage for the bridge arm. The higher the value of the modulation index, the higher the voltage output of the bridge arm.

[0124] In practical applications, the instantaneous current value of the bridge arm can be measured by a current sensor, and its absolute value can be calculated; the modulation index sent by the system-level controller can be obtained; the voltage of each submodule capacitor can be measured by a voltage sensor; and the current rating value, which is a device inherent parameter, can be obtained and stored in the controller in advance.

[0125] S42. Determine the corresponding dynamic threshold adjustment coefficient based on the absolute value of the current in the bridge arm, the rated current, the modulation index, and the capacitor voltage of each submodule.

[0126] Among them, the dynamic threshold adjustment coefficient is a scaling factor that changes dynamically according to real-time operating conditions. Its value determines the strictness of the threshold used to determine whether to trigger the equalization action.

[0127] In one possible implementation, it can be first determined whether the MMC main circuit is in a transient state. Only when it is determined that the MMC main circuit is in a steady state can the corresponding dynamic threshold adjustment coefficient be determined based on the absolute value of the current of the bridge arm, the current rating, the modulation index, and the capacitor voltage of each submodule.

[0128] It should be understood that the specific implementation process and principles of this method will be explained later. Figure 5 The embodiments shown are described in detail here, and will not be repeated here.

[0129] In another possible implementation, the normalized current amplitude can be calculated based on the absolute current value and the rated current value. Then, the voltage fluctuation rate of the bridge arm is calculated based on the capacitor voltage of each submodule. Finally, a dynamic threshold adjustment coefficient is determined through a preset mapping relationship based on the normalized current amplitude, modulation index, and voltage fluctuation rate.

[0130] The preset mapping relationship is constructed in advance based on expert experience and / or simulation analysis. The preset mapping relationship is used to represent the correspondence between the normalized current amplitude, modulation index, voltage fluctuation rate and dynamic threshold adjustment coefficient.

[0131] The normalized current amplitude can be calculated using the following formula:

[0132] I norm =∣i arm | / I rated (11)

[0133] Among them, I norm For the normalized current amplitude, |i arm | is the absolute value of the current, I rated This is the rated current.

[0134] Voltage fluctuation rate is used to represent the stability and balance of voltage within the bridge arm. First, the average value of the capacitor voltage of all sub-modules in the bridge arm is calculated. Then, the standard deviation of each capacitor voltage relative to the average value is calculated. Finally, the standard deviation is divided by the average value, and the resulting value is determined as the voltage fluctuation rate.

[0135] Optionally, the domain of discourse for the normalized current amplitude is 0, 1.2, the domain of discourse for the modulation index M is 0.5, 1.0, and the domain of discourse for the voltage fluctuation rate δV is 0%, 20%.

[0136] Optionally, target voltage threshold It can be represented as:

[0137] (12)

[0138] The function f(.) must satisfy the following constraints:

[0139] 1. Monotonicity: |i arm When | increases, Reduce to speed up the switchover;

[0140] 2. Nonlinear compensation: Under high M, It must be negatively correlated with δV;

[0141] 3. Boundary constraints: Vth(t)∈[Vmin,Vmax], to avoid control failure caused by threshold exceeding the limit.

[0142] In other words, when the degree of change in operating conditions is less than or equal to a preset degree threshold, the determined target voltage threshold is negatively correlated with the normalized current amplitude. And / or, when the modulation index is greater than a preset modulation index threshold, the determined target voltage threshold is negatively correlated with the voltage fluctuation rate.

[0143] To achieve the above objectives, a fuzzy logic controller (FLC) is used to map the operating parameters to dynamic thresholds.

[0144] Specifically, the normalized current amplitude, modulation index, and voltage fluctuation rate are fuzzified to determine the first fuzzy set corresponding to the normalized current amplitude, the second fuzzy set corresponding to the modulation index, and the third fuzzy set corresponding to the voltage fluctuation rate. Then, based on the first, second, and third fuzzy sets, at least one initial dynamic threshold adjustment coefficient is determined through a preset mapping relationship. Finally, the at least one initial dynamic threshold adjustment coefficient is processed using the centroid method to generate the dynamic threshold adjustment coefficient.

[0145] The preset mapping relationship includes the initial dynamic threshold adjustment coefficient corresponding to any value of the first fuzzy set, any value of the second fuzzy set, and any value of the third fuzzy set.

[0146] Optionally, triangular and trapezoidal membership functions are used to cover the fuzzy sets of normalized current amplitude, modulation index, and voltage fluctuation rate:

[0147] The first fuzzy set: The fuzzy set is {low (L), medium (M), high (H)};

[0148] The second fuzzy set: The fuzzy set is {low (L), medium (M), high (H)};

[0149] The third fuzzy set: The fuzzy set is {small (S), medium (M), large (B)}.

[0150] For example, the above-mentioned preset mapping relationship can be represented by Table 1 below.

[0151] Table 1 Preset Mapping Relationships

[0152]

[0153] Among them, Table 1 This is the dynamic threshold adjustment coefficient.

[0154] The process of processing at least one initial dynamic threshold adjustment coefficient using the centroid method to generate the dynamic threshold adjustment coefficient can be achieved through the following steps: taking each initial dynamic threshold adjustment coefficient and its corresponding rule activation intensity (or membership degree) as a weighted element, and determining the weighted average center of all these elements as the dynamic threshold adjustment coefficient by calculating the weighted average center of all these elements.

[0155] S43. Multiply the dynamic threshold adjustment coefficient by the preset reference threshold, and determine the result of the multiplication as the target voltage threshold.

[0156] The preset reference threshold is a voltage constant set in advance according to the system design parameters, which is used to represent the maximum voltage deviation allowed under a certain reference or typical operating condition.

[0157] The target voltage threshold can be calculated using the following formula:

[0158] (13)

[0159] in, The target voltage threshold has a universe of discourse of 0.5 and 1.5. This is the baseline threshold.

[0160] S44. If the target voltage difference between the maximum and minimum submodule voltages within the bridge arm is greater than the target voltage threshold, the switching states of each submodule within the bridge arm are sorted and switched to achieve capacitor voltage balance.

[0161] Among them, sorting refers to determining the priority order of sub-modules to be put into operation, and switching refers to issuing an input or bypass switch command to the corresponding sub-module based on the sorting result and the number of sub-modules that need to be put into operation, so as to change the charging and discharging path of the capacitor and thus achieve voltage balance.

[0162] Based on the above embodiments, the switching frequency Defined as the average number of switches per unit time for a single submodule. For modules containing... The total number of switching operations for each submodule's bridge arm is equal to... The relationship is:

[0163] (14)

[0164] in, For the statistical period, Used to represent the i-th submodule at time t k The switching state, δ(S) i (t k )) indicates that submodule i is at time t k The switching event (0 for no switching, 1 for switching).

[0165] Assume that within the time interval Δt, ΔV unb (0) Increase to V th Then its accumulation rate is:

[0166] (15)

[0167] The time interval is:

[0168] (16)

[0169] The number of switching operations per unit time (i.e., the switching frequency) is:

[0170] (17)

[0171] When V th When fixed, f sw and Approximately satisfies:

[0172] (18)

[0173] However, in practical applications, V th Adjustments are needed based on operating conditions, therefore a more accurate dynamic model needs to be established.

[0174] To analyze the impact of load abrupt changes and modulation depth variations on voltage equalization, the static model needs to be extended to dynamic scenarios. Consider the following two typical operating conditions:

[0175] Operating Condition 1: Transient Operating Condition

[0176] Suppose the load current jumps from I1 to I2 at t=t0, and the change in the bridge arm current is:

[0177] (19)

[0178] At this point, the dynamic equation of the capacitor voltage needs to be solved piecewise, and the transient response of the voltage imbalance can be expressed as:

[0179] (20)

[0180] Operating Condition 2: Steady-State Operating Condition

[0181] When the modulation index switches from M1 to M2, the reference value of the bridge arm voltage changes, resulting in an increase in the number of inserted submodules, n. up (t) and n low (t) Dynamic adjustment. At this point, the charging and discharging rate of the capacitor voltage is strongly coupled with the submodule switching strategy, resulting in the following equation:

[0182] (twenty one)

[0183] To improve the accuracy of voltage equalization control of MMC under transient conditions, and to quickly suppress voltage fluctuations and reduce steady-state losses, it is possible to... Figure 5 The illustrated embodiment controls the MMC.

[0184] The voltage equalization control method for MMC provided in this application determines the absolute value of the current, the rated current, the modulation index, and the capacitor voltage of each submodule for any bridge arm in the MMC main circuit. Then, based on the absolute value of the current, the rated current, the modulation index, and the capacitor voltage of each submodule, a corresponding dynamic threshold adjustment coefficient is determined. Next, the dynamic threshold adjustment coefficient is multiplied by a preset reference threshold, and the result is determined as the target voltage threshold. If the target voltage difference between the maximum and minimum submodule voltages within the bridge arm is greater than the target voltage threshold, the switching states of each submodule within the bridge arm are sorted and switched to achieve capacitor voltage equalization. In this technical solution, no immediate response is made to any minor differences in the submodule voltages within the bridge arm. Instead, an adaptive target voltage threshold is first calculated based on the absolute value of the current, the rated current, the modulation index, and the capacitor voltage of each submodule. The sorting and switching operation is only triggered when the target voltage difference between the maximum and minimum submodule voltages exceeds this target voltage threshold. This changes the condition for switching action from the presence of deviation to exceeding the dynamic threshold, thereby significantly reducing the average switching frequency and switching losses while ensuring the core performance of capacitor voltage balance.

[0185] Figure 5 Flowchart of the voltage equalization control method for MMC provided in this application Figure 2 ,like Figure 5 As shown, S42 includes:

[0186] S51. Determine the degree of drastic change in operating conditions based on the modulation index and the capacitor voltage of each submodule.

[0187] In one possible implementation, the voltage fluctuation rate of the bridge arm is calculated based on the capacitor voltage of each submodule. Then, the degree of drastic change in operating conditions is calculated based on the modulation index and the voltage fluctuation rate.

[0188] The method for calculating the voltage fluctuation rate of the bridge arm based on the capacitor voltage of each submodule can be found in the relevant content in S42, and will not be repeated here.

[0189] The degree of drastic change in operating conditions can be calculated using the following formula:

[0190] (twenty two)

[0191] Wherein, α is a weighting coefficient, such as 0.5, 0.6, 0.7, etc., which can be preset based on empirical or experimental values. This application does not impose specific restrictions on this.

[0192] S52. If the degree of change in operating conditions is less than or equal to the preset degree threshold, the corresponding dynamic threshold adjustment coefficient shall be determined based on the absolute value of the current of the bridge arm, the rated value of the current, the modulation index, and the capacitor voltage of each submodule.

[0193] For example, the preset intensity threshold can be 20%, 25%, or 30%, which can be preset based on empirical or experimental values. This application embodiment does not impose specific limitations on this.

[0194] If the degree of change in operating conditions is less than or equal to the preset degree of change threshold, the MMC main circuit is determined to be in steady-state operating condition. If the degree of change in operating conditions is greater than the preset degree of change threshold, the MMC main circuit is determined to be in transient operating condition.

[0195] In one possible implementation, the normalized current amplitude is calculated based on the absolute value of the current and the rated current. Based on the normalized current amplitude, modulation index, and voltage fluctuation rate, a dynamic threshold adjustment coefficient is determined through a preset mapping relationship, which is pre-constructed based on expert experience and / or simulation analysis.

[0196] The preset mapping relationship is used to represent the correspondence between the normalized current amplitude, modulation index, voltage fluctuation rate, and dynamic threshold adjustment coefficient.

[0197] This implementation integrates three key dimensions reflecting load level, output command strength, and internal voltage stability. It makes comprehensive decisions through preset mapping rules (such as an expert experience base), enabling the generated dynamic threshold adjustment coefficient and subsequent target voltage threshold to fully and adaptively match the current complex operating conditions, thus overcoming the limitations of fixed thresholds.

[0198] Specifically, the normalized current amplitude, modulation index, and voltage fluctuation rate are fuzzified to determine a first fuzzy set corresponding to the normalized current amplitude, a second fuzzy set corresponding to the modulation index, and a third fuzzy set corresponding to the voltage fluctuation rate. Then, based on the first, second, and third fuzzy sets, at least one initial dynamic threshold adjustment coefficient is determined through a preset mapping relationship. This preset mapping relationship includes the initial dynamic threshold adjustment coefficient corresponding to any value from the first, second, and third fuzzy sets. Finally, the at least one initial dynamic threshold adjustment coefficient is processed using the centroid method to generate the dynamic threshold adjustment coefficient.

[0199] It should be understood that this implementation method can refer to the relevant content in S42, and will not be repeated here.

[0200] In practical applications, the above process simulates the comprehensive judgment of human experts, improving the stability and robustness of the output dynamic threshold adjustment coefficient.

[0201] Optionally, in some embodiments, if the degree of change in operating conditions is greater than a preset degree threshold, then the preset voltage threshold is determined as the target voltage threshold. If the target voltage difference is greater than the target voltage threshold, then the switching states of each submodule within the bridge arm are sorted and switched to achieve capacitor voltage balancing.

[0202] For example, the preset voltage threshold can be 10V, 15V, 20V, etc., which is the maximum voltage threshold allowed by the MMC main circuit under transient conditions. It can be preset based on empirical or experimental values. This application embodiment does not impose specific limitations on this.

[0203] First, the degree of drastic change in operating conditions is determined based on the modulation index and the capacitor voltages of each submodule, thus identifying whether the current operating condition is transient or steady-state. In transient conditions, a preset, lower fixed voltage threshold (i.e., the preset voltage threshold) is directly set as the target voltage threshold. This means that a stricter standard (i.e., a smaller allowable voltage difference) is needed to trigger the voltage equalization operation, thereby ensuring that voltage imbalance is strongly suppressed with a higher switching frequency during transient processes where dynamic changes are drastic and voltage divergence may occur, prioritizing the transient stability and safety of the system. In steady-state conditions, a dynamic threshold adjustment coefficient is dynamically determined based on the absolute value of the current, the rated current, the modulation index, and the capacitor voltages of each submodule, thereby dynamically confirming the target voltage threshold and improving the rationality and specificity of the control process.

[0204] In conclusion, Figure 6 Flowchart of the voltage equalization control method for MMC provided in this application Figure 3 ,like Figure 6 As shown, the voltage equalization control method of this MMC includes the following steps:

[0205] S61. Calculate the degree of drastic change in operating conditions based on the voltage fluctuation rate and modulation index of the bridge arm.

[0206] S62. Determine whether the degree of drastic change in operating conditions exceeds the preset drastic threshold.

[0207] If yes, proceed to S63; otherwise, proceed to S65.

[0208] S63, Determine to enter the transient phase control.

[0209] S64. Set the preset voltage threshold as the target voltage threshold. Then, execute S68.

[0210] S65. Determine whether to enter the steady-state control phase.

[0211] S66. Determine the dynamic threshold adjustment coefficient based on the normalized current amplitude, modulation index, and voltage fluctuation rate.

[0212] S67. Multiply the dynamic threshold adjustment coefficient by the preset reference threshold, and determine the result of the multiplication as the target voltage threshold.

[0213] S68. If the target voltage difference between the maximum and minimum submodule voltages within the bridge arm is greater than the target voltage threshold, the switching states of each submodule within the bridge arm are sorted and switched to achieve capacitor voltage balance.

[0214] In summary, this application analyzes the established dynamic coupling model of switching frequency and voltage imbalance, revealing the quantitative relationship between the target voltage threshold and operating parameters. Furthermore, an adaptive threshold adjustment mechanism is designed, combining fuzzy logic with a time-sharing compensation strategy to dynamically optimize the target voltage threshold, thereby balancing switching losses and voltage equalization accuracy. This scheme obtains the absolute current value, rated current value, modulation index, and capacitor voltage of each submodule of the bridge arm, and determines the dynamic threshold adjustment coefficient based on this, achieving online adaptive adjustment of the target voltage threshold and overcoming the static limitations of traditional fixed thresholds. By determining the severity of operating condition changes based on the modulation index and capacitor voltage of each submodule, a two-stage control logic is designed. When the severity of operating condition changes exceeds a preset severity threshold, a low fixed threshold is used to quickly equalize the voltage; when the severity of operating condition changes is less than or equal to the preset severity threshold, the dynamic threshold adjustment coefficient based on fuzzy logic is restored to reduce losses. Furthermore, by analyzing the impact of voltage imbalance on capacitor voltage fluctuations, an optimization criterion for submodule capacitor parameters considering unbalanced states is proposed, providing a theoretical basis for system design in low switching frequency scenarios and improving economic efficiency.

[0215] This application combines fuzzy logic with operating parameters determined by the absolute value of current, rated current, modulation index, and capacitor voltage of each submodule to achieve online adaptive adjustment of the target voltage threshold, breaking through the static limitations of traditional fixed thresholds. By determining the degree of drastic change in operating conditions based on the modulation index and capacitor voltage of each submodule, and designing two-stage control logic, a low fixed threshold is used to quickly balance the voltage during periods of load abrupt changes, while a dynamic threshold adjustment mechanism is restored during steady-state periods to reduce losses. By analyzing the impact of voltage imbalance on capacitor voltage fluctuations, an optimization criterion for submodule capacitor parameters considering unbalanced states is proposed, improving the economic efficiency of system design.

[0216] A stability analysis was performed on the scheme provided in this application, ΔV unb The dynamic equation is:

[0217] (twenty three)

[0218] Where β is the control gain.

[0219] The Lyapunov function is chosen as follows: Its derivative is:

[0220] (twenty four)

[0221] By adjusting V th Make This ensures the asymptotic stability of the system.

[0222] To verify the effectiveness of the voltage equalization control proposed in this application, a 4-SM single-phase MMC simulation model was built on the Matlab / Simulink platform for simulation verification. Specifically, the number of conducting submodules and their operating status in the upper and lower bridge arms were monitored, including the number of conducting submodules in the upper and lower bridge arms and the conducting status of each bridge arm. Different conducting states of the upper and lower bridge arms enabled the tracking of the reference waveform.

[0223] Figure 7 This shows the working curves and state change diagrams of the upper and lower bridge arm sub-modules. For example... Figure 7 As shown, the horizontal axis of (a) and (b) represents time, and the vertical axis of (a) represents the number of submodules turned on (n), which is an integer from 0 to 4, representing the number of submodules in the insertion state of the upper arm (red curve) and lower arm (blue curve) at a certain moment. The vertical axis of (b) represents the submodule switching state (S). i The value can be 0 or 1, where Si=1 indicates that the submodule is in the insertion state (capacitor connected to the bridge arm), S i =0 indicates that the submodule is in bypass state (capacitor is short-circuited). Different colored curves represent different submodules in the bridge arm, such as... Figure 7 The green, orange, purple, and magenta curves in the diagram correspond to different sub-modules.

[0224] exist Figure 7 Based on this, the upper and lower SMs are turned on sequentially to obtain the output waveform as shown below. Figure 8 As shown. Figure 8 For reference waveforms and output waveforms. For example... Figure 8 As shown, the reference voltage is a sine wave with an amplitude of 1V and a frequency of 50Hz; the single-phase output voltage has an amplitude of 2000V and a frequency of 50Hz. Comparison reveals that using multiple sub-modules of the MMC allows for better implementation of multi-level inverter circuits.

[0225] Furthermore, through Figure 9 as well as Figure 10 Monitor the capacitor voltages of the upper bridge arms PM1, PM2, PM3, PM4 and the lower bridge arms PM5, PM6, PM7, PM8 respectively. It should be understood that PM1, PM2, PM3, PM4, PM5, PM6, PM7, and PM8 refer to the submodule numbers. Figure 9The current waveforms are shown for negative-sequence and zero-sequence circulating current control. Figure 10 This is the voltage variation curve of the capacitor in the lower bridge arm submodule. (Example:) Figure 9 as well as Figure 10 It can be seen that the capacitor voltages of the eight PMs are basically the same, with very small errors.

[0226] Figure 11 A schematic diagram of the voltage equalization control device for the MMC provided in this application is shown below. Figure 4 As shown, the MMC voltage equalization control device 110 provided in this embodiment includes:

[0227] The first determining module 1101 is used to determine the absolute value of the current, the rated value of the current, the modulation index, and the capacitor voltage of each sub-module for any bridge arm in the MMC main circuit.

[0228] The second determining module 1102 is used to determine the corresponding dynamic threshold adjustment coefficient based on the absolute value of the current of the bridge arm, the rated value of the current, the modulation index, and the capacitor voltage of each sub-module.

[0229] The third determining module 1103 is used to multiply the dynamic threshold adjustment coefficient by the preset reference threshold and determine the result of the multiplication as the target voltage threshold.

[0230] The control module 1104 is used to sort and switch the switching states of each submodule in the bridge arm if the target voltage difference between the maximum submodule voltage and the minimum submodule voltage in the bridge arm is greater than the target voltage threshold, so as to achieve capacitor voltage balance.

[0231] In one possible implementation, the second determining module 1102 is specifically used for:

[0232] The degree of drastic change in operating conditions is determined based on the modulation index and the capacitor voltage of each submodule.

[0233] If the degree of change in operating conditions is less than or equal to the preset degree of change threshold, the corresponding dynamic threshold adjustment coefficient is determined based on the absolute value of the current of the bridge arm, the rated current, the modulation index, and the capacitor voltage of each submodule.

[0234] In one possible implementation, the second determining module 1102 is specifically used for:

[0235] Calculate the voltage fluctuation rate of the bridge arm based on the capacitor voltage of each submodule;

[0236] The degree of drastic change in operating conditions is calculated based on the modulation index and voltage fluctuation rate.

[0237] In one possible implementation, the second determining module 1102 is specifically used for:

[0238] Calculate the normalized current amplitude based on the absolute value of the current and the rated current value;

[0239] Based on the normalized current amplitude, modulation index, and voltage fluctuation rate, the dynamic threshold adjustment coefficient is determined through a preset mapping relationship. The preset mapping relationship is constructed in advance based on expert experience and / or simulation analysis. The preset mapping relationship is used to represent the correspondence between the normalized current amplitude, modulation index, voltage fluctuation rate, and dynamic threshold adjustment coefficient.

[0240] In one possible implementation, the second determining module 1102 is specifically used for:

[0241] The normalized current amplitude, modulation index, and voltage fluctuation rate are fuzzified respectively to determine the first fuzzy set corresponding to the normalized current amplitude, the second fuzzy set corresponding to the modulation index, and the third fuzzy set corresponding to the voltage fluctuation rate.

[0242] Based on the first fuzzy set, the second fuzzy set, and the third fuzzy set, at least one initial dynamic threshold adjustment coefficient is determined through a preset mapping relationship. The preset mapping relationship includes the initial dynamic threshold adjustment coefficient corresponding to any value of the first fuzzy set, any value of the second fuzzy set, and any value of the third fuzzy set.

[0243] The dynamic threshold adjustment coefficient is generated by processing at least one initial dynamic threshold adjustment coefficient using the centroid method.

[0244] In one possible implementation, the control module 1104 is further used for

[0245] If the degree of change in operating conditions is greater than the preset degree of change threshold, then the preset voltage threshold will be determined as the target voltage threshold.

[0246] If the target voltage difference is greater than the target voltage threshold, the switching states of each sub-module in the bridge arm are sorted and switched to achieve capacitor voltage balancing.

[0247] In one possible implementation, when the degree of drastic change in operating conditions is less than or equal to a preset degree of drastic change threshold, the determined target voltage threshold is negatively correlated with the normalized current amplitude; and / or, when the modulation index is greater than a preset modulation index threshold, the determined target voltage threshold is negatively correlated with the voltage fluctuation rate.

[0248] The MMC pressure equalization control device provided in this embodiment can execute the method provided in the above method embodiment. Its implementation principle and technical effect are similar, and will not be described in detail here.

[0249] Figure 12 A schematic diagram of the structure of the electronic device provided in this application. Figure 12As shown, the electronic device 120 provided in this embodiment includes at least one processor 1201 and a memory 1202. Optionally, the electronic device 120 further includes a communication component 1203. The processor 1201, the memory 1202, and the communication component 1203 are connected via a bus 1204.

[0250] In a specific implementation, at least one processor 1201 executes computer execution instructions stored in memory 1202, causing at least one processor 1201 to perform the above-described method.

[0251] The specific implementation process of processor 1201 can be found in the above method embodiments, and its implementation principle and technical effect are similar. It will not be repeated here.

[0252] In the above embodiments, it should be understood that the processor can be a Central Processing Unit (CPU), or other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), etc. The general-purpose processor can be a microprocessor or any conventional processor. The steps of the method disclosed in this invention can be directly implemented by a hardware processor, or implemented by a combination of hardware and software modules within the processor.

[0253] The memory may include random access memory (RAM) and may also include non-volatile memory (NVM), such as at least one disk storage device.

[0254] The bus can be an Industry Standard Architecture (ISA) bus, a Peripheral Component Interconnect (PCI) bus, or an Extended Industry Standard Architecture (EISA) bus, etc. Buses can be categorized as address buses, data buses, control buses, etc. For ease of illustration, the buses shown in the accompanying drawings are not limited to a single bus or a single type of bus.

[0255] This application also provides a computer program product, including a computer program that, when executed by a processor, implements the above-described method.

[0256] This application also provides a computer-readable storage medium storing computer-executable instructions, which, when executed by a processor, implement the above-described method.

[0257] The aforementioned readable storage medium can be implemented by any type of volatile or non-volatile storage device or a combination thereof, such as Static Random-Access Memory (SRAM), Electrically Erasable Programmable Read-Only Memory (EEPROM), Erasable Programmable Read-Only Memory (EPROM), Programmable Read-Only Memory (PROM), Read-Only Memory (ROM), magnetic storage, flash memory, magnetic disk, or optical disk. The readable storage medium can be any available medium accessible to a general-purpose or special-purpose computer.

[0258] An exemplary readable storage medium is coupled to a processor, enabling the processor to read information from and write information to the readable storage medium. Of course, the readable storage medium can also be a component of the processor. The processor and the readable storage medium can reside in an application-specific integrated circuit (ASIC). Alternatively, the processor and the readable storage medium can exist as discrete components in the device.

[0259] The division of units is merely a logical functional division; in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be indirect coupling or communication connection through some interfaces, devices, or units, and may be electrical, mechanical, or other forms.

[0260] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0261] In addition, the functional units in the various embodiments of the present invention can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit.

[0262] If a function is implemented as a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of this invention, or the part that contributes to the prior art, or a part of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute all or part of the steps of the methods of the various embodiments of this invention. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, ROM, RAM, magnetic disks, or optical disks.

[0263] Those skilled in the art will understand that all or part of the steps of the above-described method embodiments can be implemented by hardware related to program instructions. The aforementioned program can be stored in a computer-readable storage medium. When executed, the program performs the steps of the above-described method embodiments; and the aforementioned storage medium includes various media capable of storing program code, such as ROM, RAM, magnetic disks, or optical disks.

[0264] Finally, it should be noted that other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein, and is not limited to the precise structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. A method for equalizing voltage control in MMC, characterized in that, include: For any bridge arm in the main circuit of the modular multilevel converter (MMC), determine the absolute value of the current, the rated current, the modulation index, and the capacitor voltage of each submodule of the bridge arm. Based on the absolute value of the current, the rated current, the modulation index, and the capacitor voltage of each submodule, determine the corresponding dynamic threshold adjustment coefficient. The dynamic threshold adjustment coefficient is multiplied by a preset reference threshold, and the result of the multiplication is determined as the target voltage threshold. If the target voltage difference between the maximum and minimum submodule voltages within the bridge arm is greater than the target voltage threshold, the switching states of each submodule within the bridge arm are sorted and switched to achieve capacitor voltage balancing.

2. The method according to claim 1, characterized in that, The step of determining the corresponding dynamic threshold adjustment coefficient based on the absolute value of the current, the rated current, the modulation index, and the capacitor voltage of each submodule includes: The degree of drastic change in operating conditions is determined based on the modulation index and the capacitor voltage of each submodule. If the degree of change in the operating condition is less than or equal to a preset degree threshold, then the corresponding dynamic threshold adjustment coefficient is determined based on the absolute value of the current of the bridge arm, the rated current, the modulation index, and the capacitor voltage of each submodule.

3. The method according to claim 2, characterized in that, The determination of the degree of drastic change in operating conditions based on the modulation index and the capacitor voltage of each submodule includes: Calculate the voltage fluctuation rate of the bridge arm based on the capacitor voltage of each submodule; The degree of drastic change in the operating condition is calculated based on the modulation index and the voltage fluctuation rate.

4. The method according to claim 3, characterized in that, The step of determining the corresponding dynamic threshold adjustment coefficient based on the absolute value of the current, the rated current, the modulation index, and the capacitor voltage of each submodule includes: Calculate the normalized current amplitude based on the absolute value of the current and the rated value of the current; The dynamic threshold adjustment coefficient is determined based on the normalized current amplitude, the modulation index, and the voltage fluctuation rate through a preset mapping relationship. The preset mapping relationship is constructed in advance based on expert experience and / or simulation analysis. The preset mapping relationship is used to represent the correspondence between the normalized current amplitude, the modulation index, the voltage fluctuation rate, and the dynamic threshold adjustment coefficient.

5. The method according to claim 4, characterized in that, The step of determining the dynamic threshold adjustment coefficient based on the normalized current amplitude, the modulation index, and the voltage fluctuation rate through a preset mapping relationship includes: The normalized current amplitude, the modulation index, and the voltage fluctuation rate are respectively fuzzified to determine the first fuzzy set corresponding to the normalized current amplitude, the second fuzzy set corresponding to the modulation index, and the third fuzzy set corresponding to the voltage fluctuation rate. Based on the first fuzzy set, the second fuzzy set, and the third fuzzy set, at least one initial dynamic threshold adjustment coefficient is determined through the preset mapping relationship. The preset mapping relationship includes the initial dynamic threshold adjustment coefficient corresponding to any value of the first fuzzy set, any value of the second fuzzy set, and any value of the third fuzzy set. The dynamic threshold adjustment coefficient is generated by processing the at least one initial dynamic threshold adjustment coefficient using the centroid method.

6. The method according to claim 2, characterized in that, The method further includes: If the degree of change in the operating condition is greater than the preset degree threshold, then the preset voltage threshold is determined as the target voltage threshold. If the target voltage difference is greater than the target voltage threshold, the switching states of each sub-module in the bridge arm are sorted and switched to achieve capacitor voltage balancing.

7. The method according to claim 4 or 5, characterized in that, When the degree of change in the operating condition is less than or equal to the preset degree of change threshold, the determined target voltage threshold is negatively correlated with the normalized current amplitude; and / or, when the modulation index is greater than the preset modulation index threshold, the determined target voltage threshold is negatively correlated with the voltage fluctuation rate.

8. A voltage equalization control device for MMC, characterized in that, include: The first determining module is used to determine the absolute value of the current, the rated current, the modulation index, and the capacitor voltage of each sub-module for any bridge arm in the main circuit of the modular multilevel converter (MMC). The second determining module is used to determine the corresponding dynamic threshold adjustment coefficient based on the absolute value of the current, the rated value of the current, the modulation index, and the capacitor voltage of each sub-module of the bridge arm. The third determining module is used to multiply the dynamic threshold adjustment coefficient by a preset reference threshold, and determine the result of the multiplication as the target voltage threshold. The control module is used to sort and switch the switching states of each submodule in the bridge arm if the target voltage difference between the maximum submodule voltage and the minimum submodule voltage in the bridge arm is greater than the target voltage threshold, so as to achieve capacitor voltage balance.

9. An electronic device, characterized in that, include: Memory and processor; The memory stores computer-executed instructions; The processor executes computer execution instructions stored in the memory, causing the processor to perform the method as described in any one of claims 1-7.

10. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores computer-executable instructions, which, when executed by a processor, are used to implement the method as described in any one of claims 1-7.

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