Bridge arm energy balance control method of modular multilevel converter

Abstract

The invention belongs to the technical field of power electronic system control, and particularly relates to a bridge arm energy balance control method of a modular multi-level converter, which comprises the following steps: generating a direct current bus current reference value for representing the direct current side energy regulation requirement of the modular multi-level converter; in combination with the energy state of each bridge leg, generating a bridge leg current reference value corresponding to each bridge leg by using a bridge leg current reference value distribution strategy; generating mutually independent bridge leg common-mode voltage modulation signals of the bridge legs; based on a common-mode modulation signal energy rectification distribution strategy, performing energy rectification type distribution in the bridge legs, and generating common-mode voltage modulation signals corresponding to the upper bridge arm and the lower bridge arm; the differential mode voltage modulation signals are combined with differential mode voltage modulation signals generated by alternating current side control to generate bridge arm voltage modulation signals corresponding to the bridge arms; trigger pulses of all sub-modules in the bridge arm of the modular multilevel converter are generated through a modulation method, and controlled adjustment of the energy of the bridge arm is achieved.

Description

Technical Field

[0001] This invention belongs to the field of power electronic system control technology, and specifically relates to a method for bridge arm energy equalization control of a modular multilevel converter. Background Technology

[0002] Modular multilevel converters (MMCs) are currently the most competitive converter topology in fields such as offshore wind power and flexible DC transmission, possessing significant scientific research and engineering application value. The MMC's arm structure, based on several capacitor modules, gives it unique energy utilization potential compared to traditional voltage source converters (VSCs). For example, in the field of offshore wind power transmission, existing technologies propose using MMC energy utilization to support the inertia of the onshore power grid, thus suppressing grid frequency variations. In the field of multi-terminal flexible DC transmission, existing technologies propose multi-AC network interconnection schemes based on MMC interconnected converter energy coordination to achieve buffering and flexible control of power flow between different networks. For instance, in a three-phase system, the MMC consists of three phases and six arms. According to the MMC modulation principle, the sum of the voltages of all sub-modules within each arm must be at least greater than or equal to the target voltage required by the modulation signal; otherwise, the system will experience overmodulation, increasing AC side harmonics and degrading power quality. Based on the stable control of the total energy of all sub-modules within the bridge arm by the MMC, ensuring the energy balance of each bridge arm is a necessary prerequisite for maintaining the stable and safe operation of the system.

[0003] In existing technologies, enabling a Multi-Phase Controller (MMC) to simultaneously possess DC voltage control capabilities and independent total energy regulation capabilities essentially involves constructing a control loop to simultaneously control both differential-mode and common-mode modulation signals. The differential-mode signal relates to the AC-side output voltage, while the common-mode signal relates to the DC-side output voltage. Current methods directly apply a single common-mode modulation signal from the DC-side control loop to all three bridge legs simultaneously, using it to generate trigger pulses for subsequent modulation modules. On one hand, when it's difficult to ensure perfect symmetry in the bridge leg structure and that the initial energy values ​​of the bridge legs are not identical, using the same common-mode modulation signal for all three bridge legs will result in the MMC lacking independent energy regulation capabilities for the bridge legs, leading to an imbalance in the energy of each bridge leg. On the other hand, the common-mode signal only specifies the sum of the voltages of the upper and lower arms within a single bridge leg, failing to flexibly adjust the energy distribution relationship between the upper and lower arms and thus failing to ensure energy balance among the arms.

[0004] Therefore, there is an urgent need for a bridge arm energy balancing control method for modular multilevel converters, which can maintain the energy balance of each of the three-phase bridge legs under DC voltage regulation and MMC total energy regulation, and can achieve bridge arm energy balancing by dynamically allocating a common-mode modulation signal between the upper and lower bridge arms within a bridge leg. Summary of the Invention

[0005] The purpose of this invention is to provide a method for controlling the energy balance of the bridge arm of a modular multilevel converter, characterized by the following steps:

[0006] Based on the deviation between the measured DC bus voltage and the target DC bus voltage, a reference value for the DC bus current is generated to characterize the DC-side energy regulation requirements of the modular multilevel converter.

[0007] Based on the DC bus current reference value and combined with the energy state of each bridge leg, a bridge leg current reference value allocation strategy is used to generate the corresponding bridge leg current reference value for each bridge leg.

[0008] Based on the bridge leg current reference value corresponding to each bridge leg, generate a bridge leg common mode voltage modulation signal that is independent of each bridge leg;

[0009] For the common-mode voltage modulation signal of the bridge leg, combined with the energy state of the upper and lower bridge arms in the corresponding bridge leg, based on the common-mode modulation signal energy correction allocation strategy, energy correction allocation is performed inside the bridge leg to generate common-mode voltage modulation signals corresponding to the upper and lower bridge arms.

[0010] The common-mode voltage modulation signal of the upper and lower bridge arms is combined with the differential-mode voltage modulation signal generated by the AC side control to generate the bridge arm voltage modulation signal corresponding to each bridge arm.

[0011] Based on the bridge arm voltage modulation signal, trigger pulses for each sub-module within the bridge arm of the modular multilevel converter are generated through a modulation method, thereby achieving controlled regulation of the bridge arm energy.

[0012] The generated DC bus current reference value for characterizing the DC-side energy regulation requirements of the modular multilevel converter includes:

[0013] A DC bus current reference value is output using a DC voltage control loop; the DC voltage control loop is built based on a PI controller, and the measured DC voltage value of the MMC is input to the DC voltage control loop. DC voltage reference value of MMC The PI controller outputs a reference value for the DC bus current. .

[0014] The bridge leg current reference value allocation strategy includes:

[0015] Step S21: Obtain energy for each bridge leg In the formula, The energy of bridge leg A; The energy for bridge leg B; The energy of bridge leg C;

[0016] Step S22: Determine the reference value of DC bus current Symbol, if Execute step S23 to execute the discharge distribution mode; otherwise, execute step S24 to execute the charge distribution mode.

[0017] Step S23: Execute the discharge distribution mode, including:

[0018] Calculate the total energy of MMC : ;

[0019] Reference values ​​for current of each bridge leg are allocated according to the energy storage ratio. : In the formula ;

[0020] Step S24: Execute the charging distribution mode, including:

[0021] Calculate the sum of the reciprocals of the energy of the MMC bridge legs. : ;

[0022] The reference values ​​for the current of each bridge leg are allocated according to the reciprocal ratio of energy storage. : In the formula ;

[0023] Step S25: Output reference values ​​for the current of each bridge leg. In the formula .

[0024] The generation of independent common-mode voltage modulation signals for each bridge leg includes:

[0025] Each bridge leg adopts the same DC current loop structure, which is built based on a PI controller, and the reference current values ​​of each bridge leg are input to the DC current loop structure. and the measured current values ​​of each bridge leg ,according to and The deviation between them, the output bridge leg of the PI controller Additional voltage ; and thus obtain the bridge legs common-mode voltage modulation signal :

[0026] ;

[0027] In the formula, This is the DC voltage measurement value of the MMC;

[0028] The measured values ​​of the current of each bridge leg In the formula and They represent the bridge legs respectively. The current values ​​of the upper and lower bridge arms.

[0029] The common-mode modulation signal energy correction allocation strategy includes:

[0030] Step S41: Obtain the bridge legs Energy of the inner upper and lower bridge arms ;

[0031] Step S42: Set a small correction amount ;

[0032] Step S43: Calculate the bridge leg based on energy. Inner upper arm common-mode voltage modulation signal :

[0033] ;

[0034] Step S44: Calculate the bridge legs Inner lower arm common-mode voltage modulation signal ;

[0035] ;

[0036] Step S45: Output bridge legs Common-mode voltage modulation signals of inner upper and lower bridge arms .

[0037] The step of combining the common-mode voltage modulation signal of the upper and lower bridge arms with the differential-mode voltage modulation signal generated by the AC side control to generate the bridge arm voltage modulation signal corresponding to each bridge arm includes:

[0038] Calculate the bridge legs Inner upper and lower bridge arm voltage modulation signals and :

[0039] ,

[0040] In the formula, Indicates the output of the AC side control loop. Phase-difference mode voltage modulation signal.

[0041] The specific method for generating trigger pulses for each sub-module within the modular multilevel converter bridge arm via modulation is as follows: the trigger pulse signals for the sub-modules within the bridge arm are obtained based on a near-level modulation method.

[0042] The near-level modulation method is as follows:

[0043] Set bridge legs Number of inner upper and lower bridge arm sub-modules deployed and Only depends on the nominal voltage value of the submodule ;

[0044] The number of upper and lower bridge arm sub-modules deployed and The calculation method is as follows:

[0045] .

[0046] Another object of the present invention is to provide a computer device including a memory and a processor, wherein the memory stores a computer program, and when the processor runs the computer program stored in the memory, the processor executes the bridge arm energy equalization control method of the modular multilevel converter according to the present invention.

[0047] Another object of the present invention is to provide a computer-readable storage medium having a computer program stored thereon, wherein when the computer program is executed by a processor, the processor performs the bridge arm energy equalization control method of the modular multilevel converter according to the present invention.

[0048] The beneficial effects of this invention are as follows:

[0049] 1) The MMC arm energy balance control method described in this application can simultaneously achieve energy balance between the three-phase bridge legs and between the upper and lower arms within any bridge leg. This avoids operational failures caused by excessive energy deviation between MMC arms. Furthermore, it will improve the application quality of MMC arm energy utilization engineering in fields such as offshore wind power and flexible DC transmission.

[0050] 2) The core of the MMC arm energy balancing control method described in this application is the DC current reference value allocation strategy at the leg level and the common-mode voltage modulation signal allocation strategy at the arm level. The algorithm relies only on basic linear mathematical operations and has low computational resource requirements.

[0051] 3) The MMC arm energy balance control method described in this application relies entirely on the PI controller commonly used in traditional control systems. The control algorithm is highly implementable and has strong engineering applicability. Furthermore, all required electrical measurements are the same electrical quantities that must be collected in traditional energy control systems. Therefore, the method does not require any additional hardware measurement components or devices, resulting in a high cost advantage. Attached Figure Description

[0052] Figure 1 This is a schematic diagram of the bridge arm energy equalization control method for a modular multilevel converter according to the present invention;

[0053] Figure 2 This is a schematic diagram of the DC voltage control loop structure according to an embodiment of the present invention;

[0054] Figure 3This is a schematic diagram of the DC current control loop structure used for each bridge leg in an embodiment of the present invention;

[0055] Figure 4 This is a schematic diagram of the per-unit waveform of the total MMC energy in the simulation experiment results of this embodiment of the invention;

[0056] Figure 5 This is a schematic diagram of the voltage waveform of the inner arm of the MMC in the simulation experiment results of an embodiment of the present invention;

[0057] Figure 6 This is a data flow diagram illustrating a bridge arm energy equalization control method for a modular multilevel converter according to an embodiment of the present invention. Detailed Implementation

[0058] This invention provides a method for controlling the energy balance of the bridge arms of a modular multilevel converter. The invention will be further described in detail below with reference to the accompanying drawings.

[0059] like Figure 1 The embodiment of the present invention shown discloses a method for controlling the energy balance of a bridge arm of a modular multilevel converter, comprising the following steps:

[0060] Based on the deviation between the measured DC bus voltage and the target DC bus voltage, a reference value for the DC bus current is generated to characterize the DC-side energy regulation requirements of the modular multilevel converter.

[0061] Based on the DC bus current reference value and combined with the energy state of each bridge leg, a bridge leg current reference value allocation strategy is used to generate the corresponding bridge leg current reference value for each bridge leg.

[0062] Based on the bridge leg current reference value corresponding to each bridge leg, generate a bridge leg common mode voltage modulation signal that is independent of each bridge leg;

[0063] For the common-mode voltage modulation signal of the bridge leg, combined with the energy state of the upper and lower bridge arms in the corresponding bridge leg, based on the common-mode modulation signal energy correction allocation strategy, energy correction allocation is performed inside the bridge leg to generate common-mode voltage modulation signals corresponding to the upper and lower bridge arms.

[0064] The common-mode voltage modulation signal of the upper and lower bridge arms is combined with the differential-mode voltage modulation signal generated by the AC side control to generate the bridge arm voltage modulation signal corresponding to each bridge arm.

[0065] Based on the bridge arm voltage modulation signal, trigger pulses for each sub-module within the bridge arm of the modular multilevel converter are generated through a modulation method, thereby achieving controlled regulation of the bridge arm energy.

[0066] In a specific embodiment, the data stream of a bridge arm energy equalization control method for a modular multilevel converter disclosed in this invention is as follows: Figure 6 As shown, the specific process is as follows:

[0067] Step 1: The DC voltage control loop outputs a reference value for the DC bus current;

[0068] Step 2: Based on the bridge leg current reference value allocation strategy, obtain the current reference value for each bridge leg;

[0069] Step 3: Based on the bridge leg current reference value, each bridge leg obtains its common-mode voltage modulation signal through the DC current control loop;

[0070] Step 4: Each bridge leg obtains the common-mode voltage modulation signal of its corresponding upper and lower bridge arms through the common-mode modulation signal energy correction and allocation strategy.

[0071] Step 5: Based on the differential-mode voltage modulation signal and the common-mode voltage modulation signal of each bridge arm, the voltage modulation signal of each bridge arm is obtained using the modulation signal calculation method of each bridge arm;

[0072] Step 6: Obtain the trigger pulses of each submodule within the MMC bridge arm using a conventional near-level modulation method.

[0073] In this embodiment, two problems exist in the existing technology for achieving bridge arm energy balance control:

[0074] Question 1: The bridge arm energy balancing method based on the circulating current control between the upper and lower bridge arms of MMC requires the additional construction of a control loop for the circulating current between the upper and lower bridge arms, resulting in a more complex control system design.

[0075] Question 2: To meet the energy balance requirements between bridge legs, phase-to-phase zero-sequence component control is used to achieve energy balance between bridge legs. This requires the introduction of an additional control loop, which leads to a more complex control system design, and also requires an additional zero-sequence component extraction step.

[0076] To address the limitation of existing technologies in achieving energy balance among the legs and arms within each leg of an MMC (Multi-Modifier Controller) that simultaneously possesses DC voltage control and total energy control capabilities, this invention proposes an MMC arm energy balancing control method. This method achieves precise energy control of the MMC arms through an explicit control loop. Without any additional control loops, it utilizes only two simple linear algorithms: a leg current reference value allocation strategy and a common-mode modulation signal energy correction allocation strategy. This simultaneously achieves energy balance between different MMC legs and between upper and lower arms within each leg. No additional control loops, measurement components, or hardware devices are required.

[0077] The following section provides a detailed explanation of each step in the bridge arm energy equalization control method for the modular multilevel converter.

[0078] Step S1: Based on the deviation between the measured DC bus voltage value and the target DC bus voltage value, generate a reference value for the DC bus current to characterize the DC-side energy regulation requirements of the modular multilevel converter;

[0079] The generated DC bus current reference value for characterizing the DC-side energy regulation requirements of the modular multilevel converter includes:

[0080] A DC bus current reference value is output using a DC voltage control loop; the DC voltage control loop is built based on a PI controller, and the measured DC voltage value of the MMC is input to the DC voltage control loop. DC voltage reference value of MMC The PI controller outputs a reference value for the DC bus current. .

[0081] In this embodiment, the DC voltage control loop is built based on a PI controller. The structure of the DC voltage control loop in this embodiment is as follows: Figure 2 As shown, in the formula , These are the measured and reference values ​​of the MMC's DC voltage, respectively. In this stage, the PI controller will output the reference value of the DC bus current. .

[0082] In step S1, the DC voltage control loop outputs a DC bus current reference value. The system first measures the DC bus voltage and then compares it with the set target DC voltage. The DC voltage control loop provides a DC bus current reference value to indicate the magnitude and direction of the current that should be injected or absorbed on the DC side, which is equivalent to the total energy regulation command of the entire MMC at the current moment.

[0083] Step S2: Based on the DC bus current reference value and the energy state of each bridge leg, use the bridge leg current reference value allocation strategy to generate the bridge leg current reference value corresponding to each bridge leg.

[0084] The bridge leg current reference value allocation strategy includes:

[0085] Step S21: Obtain energy for each bridge leg In the formula, The energy of bridge leg A; The energy for bridge leg B; The energy of bridge leg C;

[0086] Step S22: Determine the reference value of DC bus current Symbol, if Execute step S23 to execute the discharge distribution mode; otherwise, execute step S24 to execute the charge distribution mode.

[0087] Step S23: Execute the discharge distribution mode, including:

[0088] Calculate the total energy of MMC : ;

[0089] Reference values ​​for current of each bridge leg are allocated according to the energy storage ratio. : In the formula ;

[0090] Step S24: Execute the charging distribution mode, including:

[0091] Calculate the sum of the reciprocals of the energy of the MMC bridge legs. : ;

[0092] The reference values ​​for the current of each bridge leg are allocated according to the reciprocal ratio of energy storage. : In the formula ;

[0093] Step S25: Output reference values ​​for the current of each bridge leg. In the formula .

[0094] In this embodiment, the bridge leg current reference value allocation strategy is shown in Table 1:

[0095]

[0096] In this embodiment, the implementation process of the bridge leg current reference value allocation strategy includes:

[0097] Obtain the energy of each bridge leg in (1) ;

[0098] In (2), determine the DC bus current reference value output in step 1. The sign case. If If so, execute (3) discharge distribution mode; otherwise, execute (4) charge distribution mode.

[0099] In (3) Based on The proportional allocation is calculated using the following formula: In the formula Total MMC Energy .

[0100] The significance of this formula is that when the energy of the bridge leg accounts for a higher proportion of the total energy of the MMC, it is allocated to release a larger proportion of the discharge current, forcing the energy of the bridge leg to decrease, thereby ensuring that the energy of each bridge leg remains balanced. This means that under the bridge leg current reference value allocation strategy, the sum of the current reference values ​​of each bridge arm will equal [the sum of the current reference values ​​of each bridge arm]. This indicates that the total current reference value borne by each bridge leg after allocation will be the same as the original input signal, ensuring... The overall current control effect will not be affected.

[0101] In (4) Based on The reciprocal relation distribution is calculated using the following formula: In the formula The sum of the reciprocals of the energy of the MMC bridge legs The significance of this formula is that the lower the proportion of bridge leg energy in the total MMC energy, the larger proportion of charging current is allocated to it, forcing an increase in bridge leg energy, thereby ensuring that the energy of each bridge leg remains balanced. This also ensures... , making The overall current control effect for the three bridge legs will not be affected.

[0102] In step S2, based on the bridge leg current reference value allocation strategy, the "total current task" is reasonably allocated to the three bridge legs according to the current energy state of each bridge leg.

[0103] Step S3: Based on the bridge leg current reference value corresponding to each bridge leg, generate a bridge leg common mode voltage modulation signal that is independent of each bridge leg;

[0104] The generation of independent common-mode voltage modulation signals for each bridge leg includes:

[0105] Each bridge leg adopts the same DC current loop structure, which is built based on a PI controller, and the reference current values ​​of each bridge leg are input to the DC current loop structure. and the measured current values ​​of each bridge leg ,according to and The deviation between them, the output bridge leg of the PI controller Additional voltage ; and thus obtain the bridge legs common-mode voltage modulation signal :

[0106] ;

[0107] In the formula, This is the DC voltage measurement value of the MMC;

[0108] The measured values ​​of the current of each bridge leg In the formula and They represent the bridge legs respectively. The current values ​​of the upper and lower bridge arms.

[0109] In this embodiment, each bridge leg adopts the same DC current loop structure. The DC current control loop structure adopted by each bridge leg in this embodiment of the invention is as follows: Figure 3 As shown ( Figure 3 The content in this document represents only one bridge leg. (Bridge leg current measurement value) In the formula and They represent the bridge legs respectively. The current values ​​of the upper and lower bridge arms within the bridge. According to... and The deviation between them, the output bridge leg of the PI controller Additional voltage This leads to the bridge legs. common-mode voltage modulation signal :

[0110]

[0111] In the formula, This is the DC voltage measurement value of the MMC.

[0112] In step S3, a DC current control loop is established for each bridge leg: the actual current flowing through the bridge leg is measured in real time; it is compared with the current reference value of the bridge leg; based on the deviation, a required additional voltage is calculated to form a common-mode voltage modulation signal specific to the bridge leg.

[0113] Step S4: For the common-mode voltage modulation signal of the bridge leg, combined with the energy state of the upper and lower bridge arms in the corresponding bridge leg, based on the common-mode modulation signal energy correction allocation strategy, energy correction allocation is performed inside the bridge leg to generate common-mode voltage modulation signals corresponding to the upper and lower bridge arms.

[0114] The common-mode modulation signal energy correction allocation strategy includes:

[0115] Step S41: Obtain the bridge legs Energy of the inner upper and lower bridge arms ;

[0116] Step S42: Set a small correction amount ;

[0117] Step S43: Calculate the bridge leg based on energy. Inner upper arm common-mode voltage modulation signal :

[0118] ;

[0119] Step S44: Calculate the bridge legs Inner lower arm common-mode voltage modulation signal ;

[0120] ;

[0121] Step S45: Output bridge legs Common-mode voltage modulation signals of inner upper and lower bridge arms .

[0122] In this embodiment, the common-mode modulation signal energy correction allocation strategy is shown in Table 2.

[0123]

[0124] In this embodiment, the specific implementation process of the common-mode modulation signal energy correction allocation strategy is as follows:

[0125] Based on the obtained bridge legs Energy of the inner upper and lower bridge arms Within (2), based on the bridge leg Inner lower arm energy Calculate the common-mode voltage modulation signal of the upper arm. :

[0126]

[0127] In the formula, This indicates that a small correction amount is set to prevent calculation errors caused by zero energy. Furthermore, based on... The bridge legs can be calculated. Inner lower arm common-mode voltage modulation signal :

[0128]

[0129] The significance of this method lies in the bridge legs Inner upper arm energy Energy higher than the lower bridge arm At that time, more bridge leg common-mode voltage modulation signals will be applied. The energy is allocated to the lower axle arm, allowing it to recover its energy. The reverse is also true.

[0130] In step S4, the voltage distribution difference will change the charging and discharging state of the upper and lower bridge arms, eventually causing the energy of the upper and lower bridge arms to gradually become consistent, thus promoting energy balance of the bridge arms.

[0131] Step S5: Combine the common-mode voltage modulation signal of the upper and lower bridge arms with the differential-mode voltage modulation signal generated by the AC side control to generate the bridge arm voltage modulation signal corresponding to each bridge arm;

[0132] The step of combining the common-mode voltage modulation signal of the upper and lower bridge arms with the differential-mode voltage modulation signal generated by the AC side control to generate the bridge arm voltage modulation signal corresponding to each bridge arm includes:

[0133] Calculate the bridge legs Inner upper and lower bridge arm voltage modulation signals and :

[0134] ,

[0135] In the formula, Indicates the output of the AC side control loop. Phase-difference mode voltage modulation signal.

[0136] In step S5, the bridge legs are calculated. Inner upper and lower bridge arm voltage modulation signals and This allows us to obtain the voltage target that each bridge arm should ultimately achieve.

[0137] Step S6: Based on the bridge arm voltage modulation signal, trigger pulses for each sub-module within the modular multilevel converter bridge arm are generated using a modulation method to achieve controlled regulation of the bridge arm energy.

[0138] The step of combining the common-mode voltage modulation signal of the upper and lower bridge arms with the differential-mode voltage modulation signal generated by the AC side control to generate the bridge arm voltage modulation signal corresponding to each bridge arm includes:

[0139] Calculate the bridge legs Inner upper and lower bridge arm voltage modulation signals and :

[0140] ,

[0141] In the formula, Indicates the output of the AC side control loop. Phase-difference mode voltage modulation signal.

[0142] The near-level modulation method is as follows:

[0143] Set bridge legs Number of inner upper and lower bridge arm sub-modules deployed and Only depends on the nominal voltage value of the submodule ;

[0144] The number of upper and lower bridge arm sub-modules deployed and The calculation method is as follows:

[0145] .

[0146] In this embodiment, the trigger pulse signal of the submodule within the bridge arm is obtained based on a near-level modulation method. (Bridge leg) Number of inner upper and lower bridge arm sub-modules deployed and Only depends on the nominal voltage value of the submodule calculate:

[0147]

[0148] In step S6, the voltage target that each bridge arm should ultimately achieve, obtained in step S5, is converted into trigger pulses for each submodule within the specific MMC bridge arm, thereby achieving bridge arm energy balance.

[0149] Another embodiment of the present invention provides a computer device including a memory and a processor, wherein the memory stores a computer program, and when the processor runs the computer program stored in the memory, the processor executes the bridge arm energy equalization control method of the modular multilevel converter according to the present invention.

[0150] Another embodiment of the present invention provides a computer-readable storage medium having a computer program stored thereon, wherein when the computer program is executed by a processor, the processor performs the bridge arm energy equalization control method of the modular multilevel converter according to the present invention.

[0151] To verify the effectiveness of the bridge arm energy equalization control method for a modular multilevel converter disclosed in this invention, the following simulation experiment was conducted. Figure 4 and Figure 5 The simulation results of the MMC arm energy balance control method are presented. Figure 4 This is a schematic diagram of the normalized waveform of the total MMC energy in the simulation experiment results of this embodiment of the invention, showing the normalized total energy curves of the six arms of the MMC. Figure 5 This is a schematic diagram of the voltage waveforms of the MMC inner bridge arms in the simulation experiment results of an embodiment of the present invention, showing the voltage waveforms of the six corresponding bridge arms. Figure 4 and Figure 5 It can be seen that even if the total energy of the MMC is continuously adjusted, the method can always stably ensure that the voltage (i.e., the energy of the six bridge arms) remains balanced in real time. Furthermore, Figure 5The two sub-figures shown in the image, when magnified, display the bridge arm voltage waveforms from 3 s to 3.3 s and from 5 s to 5.3 s. It can be observed that the bridge arm voltage waveforms conform to the dynamic capacitance curve of the submodule under normal MMC operating conditions. Therefore, this experiment verifies the correctness and effectiveness of the bridge arm energy balancing control method of the modular multilevel converter.

Claims

1. A method for controlling the energy balance of a bridge arm in a modular multilevel converter, characterized in that, Includes the following steps: Based on the deviation between the measured DC bus voltage and the target DC bus voltage, a reference value for the DC bus current is generated to characterize the DC-side energy regulation requirements of the modular multilevel converter. Based on the DC bus current reference value and combined with the energy state of each bridge leg, a bridge leg current reference value allocation strategy is used to generate the corresponding bridge leg current reference value for each bridge leg. Based on the bridge leg current reference value corresponding to each bridge leg, generate a bridge leg common mode voltage modulation signal that is independent of each bridge leg; For the common-mode voltage modulation signal of the bridge leg, combined with the energy state of the upper and lower bridge arms in the corresponding bridge leg, based on the common-mode modulation signal energy correction allocation strategy, energy correction allocation is performed inside the bridge leg to generate common-mode voltage modulation signals corresponding to the upper and lower bridge arms. The common-mode voltage modulation signal of the upper and lower bridge arms is combined with the differential-mode voltage modulation signal generated by the AC side control to generate the bridge arm voltage modulation signal corresponding to each bridge arm. Based on the bridge arm voltage modulation signal, trigger pulses for each sub-module within the bridge arm of the modular multilevel converter are generated through a modulation method, thereby achieving controlled regulation of the bridge arm energy.

2. The bridge arm energy equalization control method for a modular multilevel converter according to claim 1, characterized in that, The generated DC bus current reference value for characterizing the DC-side energy regulation requirements of the modular multilevel converter includes: A DC bus current reference value is output using a DC voltage control loop; the DC voltage control loop is built based on a PI controller, and the measured DC voltage value of the MMC is input to the DC voltage control loop. DC voltage reference value of MMC The PI controller outputs a reference value for the DC bus current. .

3. The bridge arm energy equalization control method for a modular multilevel converter according to claim 1, characterized in that, The bridge leg current reference value allocation strategy includes: Step S21: Obtain energy for each bridge leg In the formula, The energy of bridge leg A; The energy for bridge leg B; The energy of bridge leg C; Step S22: Determine the reference value of DC bus current Symbol, if Execute step S23 to execute the discharge distribution mode; otherwise, execute step S24 to execute the charge distribution mode. Step S23: Execute the discharge distribution mode, including: Calculate the total energy of MMC : ; Reference values ​​for current of each bridge leg are allocated according to the energy storage ratio. : In the formula ; Step S24: Execute the charging distribution mode, including: Calculate the sum of the reciprocals of the energy of the MMC bridge legs. : ; The reference values ​​for the current of each bridge leg are allocated according to the reciprocal ratio of energy storage. : In the formula ; Step S25: Output reference values ​​for the current of each bridge leg. In the formula .

4. The bridge arm energy equalization control method for a modular multilevel converter according to claim 1, characterized in that, The generation of independent common-mode voltage modulation signals for each bridge leg includes: Each bridge leg adopts the same DC current loop structure, which is built based on a PI controller, and the reference current values ​​of each bridge leg are input to the DC current loop structure. and the measured current values ​​of each bridge leg ,according to and The deviation between them, the output bridge leg of the PI controller Additional voltage ; and thus obtain the bridge legs common-mode voltage modulation signal : ； In the formula, This is the DC voltage measurement value of the MMC; The measured values ​​of the current of each bridge leg In the formula and They represent the bridge legs respectively. The current values ​​of the upper and lower bridge arms.

5. The bridge arm energy equalization control method for a modular multilevel converter according to claim 1, characterized in that, The common-mode modulation signal energy correction allocation strategy includes: Step S41: Obtain the bridge legs Energy of the inner upper and lower bridge arms ; Step S42: Set a small correction amount ; Step S43: Calculate the bridge legs based on energy. Inner upper arm common-mode voltage modulation signal : ； Step S44: Calculate the bridge legs Inner lower arm common-mode voltage modulation signal ; ； Step S45: Output bridge legs Common-mode voltage modulation signals of inner upper and lower bridge arms .

6. The bridge arm energy equalization control method for a modular multilevel converter according to claim 1, characterized in that, The step of combining the common-mode voltage modulation signal of the upper and lower bridge arms with the differential-mode voltage modulation signal generated by the AC side control to generate the bridge arm voltage modulation signal corresponding to each bridge arm includes: Calculate the bridge legs Inner upper and lower bridge arm voltage modulation signals and : ， In the formula, Indicates the output of the AC side control loop. Phase-difference mode voltage modulation signal.

7. The bridge arm energy equalization control method for a modular multilevel converter according to claim 1, characterized in that, The specific method for generating trigger pulses for each sub-module within the modular multilevel converter bridge arm via modulation is as follows: the trigger pulse signals for the sub-modules within the bridge arm are obtained based on a near-level modulation method.

8. The bridge arm energy equalization control method for a modular multilevel converter according to claim 7, characterized in that, The near-level modulation method is as follows: Set bridge legs Number of inner upper and lower bridge arm sub-modules deployed and Only depends on the nominal voltage value of the submodule ; The number of upper and lower bridge arm sub-modules deployed and The calculation method is as follows: 。 9. A computer device, characterized in that, It includes a memory and a processor, wherein the memory stores a computer program, and when the processor runs the computer program stored in the memory, the processor executes the bridge arm energy equalization control method of the modular multilevel converter according to any one of claims 1 to 8.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by a processor, the processor performs the bridge arm energy equalization control method for a modular multilevel converter according to any one of claims 1 to 8.

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