A neutral line voltage control method, system, and dielectric in a flexible DC transmission system.

Abstract

This application discloses a neutral line voltage control method, system, and medium in a flexible DC transmission system, mainly relating to the field of voltage control technology. It addresses the problem that existing capacitor voltage balancing strategies lead to a rise in neutral point potential, affecting the safe and stable operation of the system. The method includes: obtaining a reference value for the d-axis current under current loop control via a DC voltage controller; obtaining a reference value for the q-axis current under current loop control via a reactive power controller; calculating the zero-sequence voltage component based on the AC side output voltage to obtain a zero-sequence voltage reference value; obtaining a limiting coefficient k based on an extreme learning machine neural network algorithm; calculating the converter's output reference voltage based on the zero-sequence voltage reference value, the limiting coefficient k, the preset positive-sequence component of the reference voltage, and the negative-sequence component of the reference voltage; and the current loop controller acquiring the d-axis current reference value, the q-axis current reference value, and the converter's output reference voltage, and then outputting the reference voltage for each arm within the converter station.

Description

Technical Field

[0001] This application relates to the field of neutral line voltage control technology, and in particular to a neutral line voltage control method, system and medium in a flexible DC transmission system. Background Technology

[0002] Because energy resources and load centers are distributed in opposite directions, and high-voltage direct current (HVDC) transmission technology has the advantage of long-distance, large-capacity power transmission, it has become an important choice for optimizing resource allocation. Compared with AC power grids, DC power grids have their own unique advantages. They are suitable for large-capacity, long-distance power transmission and grid-connected power generation of new energy sources. They are one of the important applicable technologies for solving uneven energy distribution and realizing the global energy internet.

[0003] High-voltage direct current (HVDC) transmission systems based on modular multilevel converters (MMCs) have been widely adopted due to their advantages such as lower switching frequency, decoupling of active and reactive power control, and absence of commutation failure issues. Multi-terminal flexible DC systems based on MMCs offer flexible operation and are an effective technical means to address the grid connection and consumption of clean energy. When ground faults occur at different locations in the DC grid, the MMC submodule capacitor voltage balancing strategy plays a regulatory role in controlling the converter's output voltage during the fault process. Simultaneously, the fault current flows through the metallic return network, which can also cause a rise in the neutral point potential, affecting the safe and stable operation of the system.

[0004] Therefore, a neutral line voltage control method is urgently needed in flexible DC transmission systems to address the problem that existing MMC submodule capacitor voltage balancing strategies cause a rise in the neutral point potential, affecting the safe and stable operation of the system. Summary of the Invention

[0005] To address the aforementioned shortcomings of the prior art, this application provides a neutral line voltage control method, system, and medium in a flexible DC transmission system, thereby solving the problem that existing MMC submodule capacitor voltage balancing strategies can cause a rise in neutral point potential, affecting the safe and stable operation of the system.

[0006] In a first aspect, this application provides a method for neutral line voltage control in a flexible DC transmission system, the method comprising:

[0007] The DC bus voltage of the converter station in the flexible DC transmission system is controlled to the DC voltage reference value by a DC voltage controller. The difference between the output DC voltage and the DC voltage reference value is passed through a proportional-integral regulator to obtain the d-axis current reference value for current loop control. The reactive power output of the converter station is stabilized at the reactive power reference value by a reactive power controller. The difference between the output reactive power and the reactive power reference value is passed through a proportional-integral regulator to obtain the q-axis current reference value for current loop control. The zero-sequence voltage component is calculated based on the AC side output voltage. The zero-sequence voltage component is input into an infinite impulse response filter to obtain the zero-sequence voltage reference value. The voltage modulation ratio of the AC side output voltage is input into an extreme learning machine neural network algorithm to obtain the limiting coefficient k. Based on the zero-sequence voltage reference value, the limiting coefficient k, the preset positive-sequence component of the reference voltage, and the negative-sequence component of the reference voltage, the output reference voltage of the converter is calculated. The current loop controller obtains the d-axis current reference value, the q-axis current reference value, and the output reference voltage of the converter, and then outputs the reference voltage of each arm in the converter station.

[0008] In one implementation of this application, the difference between the output DC voltage and the DC voltage reference value is passed through a proportional-integral regulator to obtain the d-axis current reference value for current loop control, specifically including:

[0009] Through the formula:

[0010] Calculate the reference value of the d-axis current. ;

[0011] in, K up This represents the proportional gain of the DC voltage controller. K ui This represents the integral coefficient of the DC voltage controller. U dc Indicates the output DC voltage. U dcref This represents the DC voltage reference value, and S represents a preset constant.

[0012] In one implementation of this application, the difference between the output reactive power and the reactive power reference value is passed through a proportional-integral regulator to obtain the q-axis current reference value for current loop control, specifically including:

[0013] Through the formula:

[0014] Calculate the reference value of the q-axis current. ;

[0015] in, K Qp This represents the proportional gain of the reactive power controller. K Qi This represents the integral coefficient of the reactive power controller.Q Indicates the output reactive power. Q ref This indicates the reference value for reactive power.

[0016] In one implementation of this application, the zero-sequence voltage component is calculated based on the AC-side output voltage; the zero-sequence voltage component is input into an infinite impulse response filter to obtain a zero-sequence voltage reference value; the voltage modulation ratio of the AC-side output voltage is input into an extreme learning machine neural network algorithm to obtain a limiting coefficient k; based on the zero-sequence voltage reference value, the limiting coefficient k, the preset positive-sequence component of the reference voltage, and the negative-sequence component of the reference voltage, the output reference voltage of the converter is calculated, specifically including:

[0017] The zero-sequence voltage component is obtained by summing the three-phase voltages of the AC output voltage of the converter and multiplying by 1 / 3.

[0018] The voltage modulation ratio of the AC side output voltage is input into the trained extreme learning machine neural network algorithm to obtain the limiting coefficient k of the zero-sequence voltage component of the output.

[0019] The zero-sequence voltage component is input to the infinite impulse response filter, and then processed through a preset formula within the infinite impulse response filter:

[0020] The reference value of the zero-sequence voltage is calculated. ;

[0021] Where x() represents the zero-sequence voltage component of the input, r belongs to [0,M], and M represents the preset input order. This represents the r-th preset input coefficient. Let M represent the t-th preset output coefficient, and N represent the preset output order, where n is greater than or equal to M and N.

[0022] Through the formula:

[0023] Calculate the output reference voltage of the converter. ;

[0024] in, V refx + This represents the positive sequence component of the reference voltage. V refx - This represents the negative sequence component of the reference voltage. The value represents the reference value of the zero-sequence voltage, and k represents the limiting factor.

[0025] In one implementation of this application, before inputting the voltage modulation ratio of the AC side output voltage into the trained extreme learning machine neural network algorithm, the method includes:

[0026] Establish an Extreme Learning Machine (ELM) neural network, which includes an input layer, hidden layers, and an output layer.

[0027] The hidden layer includes H There are neurons, and the weights from the input layer to the hidden layer are... ω 11 ~ ω 2H The threshold for hidden layer nodes is b 1~ b H The weights from the hidden layer to the output layer are β 11 ~ β 2H ;

[0028] Historical voltage modulation ratio and historical limiting coefficient k Input the Extreme Learning Machine (ELM) neural network, adjust the corresponding weights or thresholds of the input layer, hidden layer, and output layer of the ELM neural network, and obtain the trained ELM neural network algorithm.

[0029] Secondly, this application provides a neutral line voltage control system for a flexible DC transmission system, the system comprising:

[0030] The DC voltage control module is used to control the DC bus voltage of the converter station in the flexible DC transmission system to the DC voltage reference value through the DC voltage controller. The difference between the output DC voltage and the DC voltage reference value is passed through the proportional-integral regulator to obtain the d-axis current reference value for current loop control.

[0031] The reactive power regulation module is used to stabilize the reactive power output of the converter station at the reactive power reference value through the reactive power controller, and to obtain the q-axis current reference value of the current loop control by passing the difference between the output reactive power and the reactive power reference value through the proportional-integral regulator.

[0032] The converter calculation module is used to calculate the zero-sequence voltage component based on the AC side output voltage; input the zero-sequence voltage component into an infinite impulse response filter to obtain the zero-sequence voltage reference value; input the voltage modulation ratio of the AC side output voltage into an extreme learning machine neural network algorithm to obtain the limiting coefficient k; and calculate the converter output reference voltage based on the zero-sequence voltage reference value, the limiting coefficient k, the preset positive-sequence component of the reference voltage, and the negative-sequence component of the reference voltage.

[0033] The current loop controller module is used to obtain the d-axis current reference value, the q-axis current reference value, and the converter output reference voltage, and then output the reference voltage of each bridge arm in the converter station.

[0034] In one implementation of this application, the DC voltage control module includes a d-axis current calculation unit.

[0035] Used in the formula:

[0036] Calculate the reference value of the d-axis current. ;

[0037] in, K up This represents the proportional gain of the DC voltage controller. K ui This represents the integral coefficient of the DC voltage controller. U dc Indicates the output DC voltage. U dcref This represents the DC voltage reference value, and S represents a preset constant.

[0038] In one implementation of this application, the reactive power regulation module includes a q-axis current calculation unit.

[0039] Used in the formula:

[0040] Calculate the reference value of the q-axis current. ;

[0041] in, K Qp This represents the proportional gain of the reactive power controller. K Qi This represents the integral coefficient of the reactive power controller. Q Indicates the output reactive power. Q ref This indicates the reference value for reactive power.

[0042] In one implementation of this application, the converter calculation module includes a converter voltage calculation unit.

[0043] This is used to sum the three-phase voltages of the AC output voltage of the converter and multiply them by 1 / 3 to obtain the zero-sequence voltage component;

[0044] The voltage modulation ratio of the AC side output voltage is input into the trained extreme learning machine neural network algorithm to obtain the limiting coefficient k of the zero-sequence voltage component of the output.

[0045] The zero-sequence voltage component is input to the infinite impulse response filter, and then processed through a preset formula within the infinite impulse response filter:

[0046] The reference value of the zero-sequence voltage is calculated. ;

[0047] Where x() represents the zero-sequence voltage component of the input, r belongs to [0,M], and M represents the preset input order. This represents the r-th preset input coefficient. Let M represent the t-th preset output coefficient, and N represent the preset output order, where n is greater than or equal to M and N.

[0048] Through the formula:

[0049] Calculate the output reference voltage of the converter. ;

[0050] in, V refx + This represents the positive sequence component of the reference voltage. V refx - This represents the negative sequence component of the reference voltage. The value represents the reference value of the zero-sequence voltage, and k represents the limiting factor.

[0051] Thirdly, this application provides a non-volatile computer storage medium storing computer instructions thereon, which, when executed, implement a neutral line voltage control method for a flexible DC transmission system as described above.

[0052] Those skilled in the art will understand that this application has at least the following beneficial effects:

[0053] This application provides a neutral line voltage control method, system, and medium for a flexible DC transmission system. Overvoltage suppression on the DC side is achieved by superimposing a zero-sequence voltage (limited by a limiting factor k) onto the original controller output voltage. During the calculation of the zero-sequence voltage, anomalies are filtered out using an infinite impulse response filter. The limiting factor is selected using an extreme learning machine neural network algorithm. This scheme, by suppressing DC side overvoltage, further suppresses neutral point overvoltage (when a ground fault occurs on the DC side, the current injected into the fault point by the converter stations at both ends of the fault point increases rapidly. The fault current flows to ground through the fault point, into the grounding electrode through the earth, and back to the fault point through the metallic return line. The fault current generates a voltage drop across the inductive and resistive elements of the metallic return line network, causing a potential rise in the neutral point of each station. The DC side overvoltage is obtained by adding the converter output voltage and the neutral point potential. Therefore, suppressing DC side overvoltage can effectively suppress neutral line overvoltage), improving the system's safe and stable operation level. This solves the problem that existing MMC submodule capacitor voltage balancing strategies cause a rise in the neutral point potential, affecting the safe and stable operation of the system. Attached Figure Description

[0054] To more clearly illustrate the technical solution of the present invention, the accompanying drawings used in the description will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0055] Figure 1 This is a flowchart of a neutral line voltage control method for a flexible DC transmission system provided in an embodiment of this application.

[0056] Figure 2 This is a schematic diagram of the internal structure of a neutral line voltage control system in a flexible DC transmission system provided in an embodiment of this application. Detailed Implementation

[0057] Those skilled in the art should understand that the embodiments described below are merely preferred embodiments of this disclosure and do not imply that this disclosure can only be implemented through these preferred embodiments. These preferred embodiments are merely used to explain the technical principles of this disclosure and are not intended to limit the scope of protection of this disclosure. Based on the preferred embodiments provided by this disclosure, all other embodiments obtained by those skilled in the art without creative effort should still fall within the scope of protection of this disclosure.

[0058] It should also be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus. Unless otherwise specified, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element.

[0059] The technical solutions proposed in the embodiments of this application will be described in detail below with reference to the accompanying drawings.

[0060] The embodiment provides a neutral line voltage control method for a flexible DC transmission system, such as Figure 1 As shown in the embodiments of this application, the method mainly includes the following steps:

[0061] Step 110: Control the DC bus voltage of the converter station in the flexible DC transmission system to the DC voltage reference value through the DC voltage controller, and pass the difference between the output DC voltage and the DC voltage reference value through the proportional-integral regulator to obtain the d-axis current reference value of the current loop control.

[0062] It should be noted that the DC voltage controller specifically refers to: the DC voltage controller controls the DC bus voltage of the receiving-end converter station to the DC voltage reference value, and the difference between the output DC voltage and the DC voltage reference value is passed through a proportional-integral regulator to obtain the d-axis current reference value for current loop control.

[0063] In some embodiments, the difference between the output DC voltage and the DC voltage reference value is processed by a proportional-integral regulator to obtain the d-axis current reference value for current loop control, specifically including:

[0064] Through the formula:

[0065] Calculate the reference value of the d-axis current. ;

[0066] in, K up This represents the proportional gain of the DC voltage controller. K ui This represents the integral coefficient of the DC voltage controller. U dc Indicates the output DC voltage. U dcref This represents the DC voltage reference value, and S represents a preset constant.

[0067] Step 120: Stabilize the reactive power output of the converter station at the reactive power reference value through the reactive power controller, and obtain the q-axis current reference value of the current loop control by passing the difference between the output reactive power and the reactive power reference value through the proportional-integral regulator.

[0068] It should be noted that the reactive power controller specifically refers to: the reactive power controller stabilizes the reactive power output of the converter station at the reactive power reference value, and the difference between the output reactive power and the reactive power reference value is passed through a proportional-integral regulator to obtain the q-axis current reference value for current loop control.

[0069] In some embodiments, the difference between the output reactive power and the reactive power reference value is passed through a proportional-integral regulator to obtain the q-axis current reference value for current loop control, specifically including:

[0070] Through the formula:

[0071] Calculate the reference value of the q-axis current. ;

[0072] in, K Qp This represents the proportional gain of the reactive power controller. K Qi This represents the integral coefficient of the reactive power controller. Q Indicates the output reactive power. Qref This indicates the reference value for reactive power.

[0073] Step 130: Calculate the zero-sequence voltage component based on the AC side output voltage; input the zero-sequence voltage component into an infinite impulse response filter to obtain the zero-sequence voltage reference value; input the voltage modulation ratio of the AC side output voltage into an extreme learning machine neural network algorithm to obtain the limiting coefficient k; calculate the converter output reference voltage based on the zero-sequence voltage reference value, the limiting coefficient k, the preset positive-sequence component of the reference voltage, and the negative-sequence component of the reference voltage.

[0074] As an example, this step can be specifically described as follows:

[0075] The zero-sequence voltage component is obtained by summing the three-phase voltages of the AC output voltage of the converter and multiplying by 1 / 3.

[0076] The voltage modulation ratio of the AC side output voltage is input into the trained extreme learning machine neural network algorithm to obtain the limiting coefficient k of the zero-sequence voltage component of the output.

[0077] The zero-sequence voltage component is input to the infinite impulse response filter, and then processed through a preset formula within the infinite impulse response filter:

[0078] The reference value of the zero-sequence voltage is calculated. ;

[0079] Where x() represents the zero-sequence voltage component of the input, r belongs to [0,M], and M represents the preset input order. This represents the r-th preset input coefficient. Let M represent the t-th preset output coefficient, and N represent the preset output order, where n is greater than or equal to M and N.

[0080] Through the formula:

[0081] Calculate the output reference voltage of the converter. ;

[0082] Where Vrefx+ represents the positive-sequence component of the reference voltage, and Vrefx- represents the negative-sequence component of the reference voltage. The value represents the reference value of the zero-sequence voltage, and k represents the limiting factor.

[0083] One method for training an extreme learning machine neural network is as follows:

[0084] Establish an Extreme Learning Machine (ELM) neural network, which includes an input layer, hidden layers, and an output layer.

[0085] The hidden layer includes H There are neurons, and the weights from the input layer to the hidden layer are... ω 11 ~ ω 2H The threshold for hidden layer nodes is b 1~ b H The weights from the hidden layer to the output layer are β 11 ~ β 2H ;

[0086] Historical voltage modulation ratio and historical limiting coefficient k Input the Extreme Learning Machine (ELM) neural network, adjust the corresponding weights or thresholds of the input layer, hidden layer, and output layer of the ELM neural network, and obtain the trained ELM neural network algorithm.

[0087] Step 140: The current loop controller acquires the d-axis current reference value, the q-axis current reference value, and the converter output reference voltage, and then outputs the reference voltage of each bridge arm in the converter station.

[0088] It should be noted that this step involves directly inputting the d-axis current reference value, q-axis current reference value, and converter output reference voltage into the existing current loop controller to obtain the output result.

[0089] In addition, this application Figure 2 This application provides a neutral line voltage control system for a flexible DC transmission system. For example... Figure 2 As shown, the system provided in this application embodiment mainly includes:

[0090] The DC voltage control module 210 is used to control the DC bus voltage of the converter station in the flexible DC transmission system to the DC voltage reference value through the DC voltage controller, and to obtain the d-axis current reference value of the current loop control by passing the difference between the output DC voltage and the DC voltage reference value through the proportional-integral regulator.

[0091] The DC voltage control module 210 includes a d-axis current calculation unit.

[0092] Used in the formula:

[0093] Calculate the reference value of the d-axis current. ;

[0094] in, K up This represents the proportional gain of the DC voltage controller. K ui This represents the integral coefficient of the DC voltage controller. U dc Indicates the output DC voltage. U dcrefThis represents the DC voltage reference value, and S represents a preset constant.

[0095] The reactive power regulation module 220 is used to stabilize the reactive power output of the converter station at the reactive power reference value through the reactive power controller, and to obtain the q-axis current reference value of the current loop control by passing the difference between the output reactive power and the reactive power reference value through the proportional-integral regulator.

[0096] The reactive power regulation module 220 includes a q-axis current calculation unit.

[0097] Used in the formula:

[0098] Calculate the reference value of the q-axis current. ;

[0099] in, K Qp This represents the proportional gain of the reactive power controller. K Qi This represents the integral coefficient of the reactive power controller. Q Indicates the output reactive power. Q ref This indicates the reference value for reactive power.

[0100] The converter calculation module 230 is used to calculate the zero-sequence voltage component based on the AC side output voltage; input the zero-sequence voltage component into an infinite impulse response filter to obtain a zero-sequence voltage reference value; input the voltage modulation ratio of the AC side output voltage into an extreme learning machine neural network algorithm to obtain a limiting coefficient k; and calculate the output reference voltage of the converter based on the zero-sequence voltage reference value, the limiting coefficient k, the preset positive-sequence component of the reference voltage, and the negative-sequence component of the reference voltage.

[0101] The converter calculation module 230 includes a converter voltage calculation unit, used to sum the three-phase voltages of the AC side output voltage of the converter and multiply by 1 / 3 to obtain the zero-sequence voltage component; input the voltage modulation ratio of the AC side output voltage into a trained extreme learning machine neural network algorithm to obtain the limiting coefficient k of the output zero-sequence voltage component; input the zero-sequence voltage component and an infinite impulse response filter, and pass the filter through a preset formula:

[0102] The reference value of the zero-sequence voltage is calculated. Where x() represents the zero-sequence voltage component of the input, r belongs to [0, M], and M represents the preset input order. This represents the r-th preset input coefficient. Let n represent the t-th preset output coefficient, and N represent the preset output order, where n is greater than or equal to M and N.

[0103] Through the formula:

[0104] Calculate the output reference voltage of the converter. ;in, V refx + This represents the positive sequence component of the reference voltage. V refx - This represents the negative sequence component of the reference voltage. The value represents the reference value of the zero-sequence voltage, and k represents the limiting factor.

[0105] The current loop controller module 240 is used to acquire the d-axis current reference value, the q-axis current reference value, and the converter output reference voltage, and then output the reference voltage of each bridge arm in the converter station.

[0106] In addition, this application embodiment also provides a non-volatile computer storage medium storing executable instructions, which, when executed, implement the neutral line voltage control method of a flexible DC transmission system as described above.

[0107] The technical solutions of this disclosure have been described in conjunction with the preceding embodiments. However, it will be readily understood by those skilled in the art that the scope of protection of this disclosure is not limited to these specific embodiments. Without departing from the technical principles of this disclosure, those skilled in the art can disassemble and combine the technical solutions in the above embodiments, and can also make equivalent changes or substitutions to the relevant technical features. Any changes, equivalent substitutions, improvements, etc., made within the technical concept and / or technical principles of this disclosure will fall within the scope of protection of this disclosure.

Claims

1. A neutral line voltage control method for a flexible DC transmission system, characterized in that, The method includes: The DC bus voltage of the converter station in the flexible DC transmission system is controlled to the DC voltage reference value by the DC voltage controller. The difference between the output DC voltage and the DC voltage reference value is passed through the proportional-integral regulator to obtain the d-axis current reference value of the current loop control. The reactive power output of the converter station is stabilized at the reactive power reference value by the reactive power controller. The difference between the output reactive power and the reactive power reference value is passed through the proportional-integral regulator to obtain the q-axis current reference value for the current loop control. Based on the AC side output voltage, the zero-sequence voltage component is calculated; the zero-sequence voltage component is input into an infinite impulse response filter to obtain a zero-sequence voltage reference value; the voltage modulation ratio of the AC side output voltage is input into an extreme learning machine neural network algorithm to obtain a limiting coefficient k; based on the zero-sequence voltage reference value, the limiting coefficient k, the preset positive-sequence component of the reference voltage, and the negative-sequence component of the reference voltage, the converter output reference voltage is calculated; specifically including: The zero-sequence voltage component is obtained by summing the three-phase voltages of the AC output voltage of the converter and multiplying by 1 / 3. The voltage modulation ratio of the AC side output voltage is input into the trained extreme learning machine neural network algorithm to obtain the limiting coefficient k of the zero-sequence voltage component of the output. The zero-sequence voltage component is input to the infinite impulse response filter, and then processed through a preset formula within the infinite impulse response filter: The reference value of the zero-sequence voltage is calculated. ; Where x() represents the zero-sequence voltage component of the input, r belongs to [0,M], and M represents the preset input order. This represents the r-th preset input coefficient. Let M represent the t-th preset output coefficient, and N represent the preset output order, where n is greater than or equal to M and N. Through the formula: Calculate the output reference voltage of the converter. ; in, V refx + This represents the positive sequence component of the reference voltage. V refx - This represents the negative sequence component of the reference voltage. The reference value for zero-sequence voltage is represented by k, which represents the limiting factor. The current loop controller acquires the d-axis current reference value, the q-axis current reference value, and the converter output reference voltage, and then outputs the reference voltage of each bridge arm in the converter station.

2. The neutral line voltage control method for a flexible DC transmission system according to claim 1, characterized in that, The difference between the output DC voltage and the DC voltage reference value is processed by a proportional-integral regulator to obtain the d-axis current reference value for current loop control, specifically including: Through the formula: Calculate the reference value of the d-axis current. ; in, K up This represents the proportional coefficient of the DC voltage controller. K ui This represents the integral coefficient of the DC voltage controller. U dc Indicates the output DC voltage. U dcref This represents the DC voltage reference value, and S represents a preset constant.

3. The neutral line voltage control method for a flexible DC transmission system according to claim 1, characterized in that, The difference between the output reactive power and the reactive power reference value is processed by a proportional-integral controller to obtain the q-axis current reference value for current loop control, specifically including: Through the formula: Calculate the reference value of the q-axis current. ; in, K Qp This represents the proportional gain of the reactive power controller. K Qi This represents the integral coefficient of the reactive power controller. Q Indicates the output reactive power. Q ref This indicates the reference value for reactive power.

4. The neutral line voltage control method for a flexible DC transmission system according to claim 1, characterized in that, Before inputting the voltage modulation ratio of the AC side output voltage into the trained extreme learning machine neural network algorithm, the method includes: Establish an Extreme Learning Machine (ELM) neural network, which includes an input layer, hidden layers, and an output layer. The hidden layer includes H There are neurons, and the weights from the input layer to the hidden layer are... ω 11 ~ ω 2H The threshold for hidden layer nodes is b 1~ b H The weights from the hidden layer to the output layer are β 11 ~ β 2H ; Historical voltage modulation ratio and historical limiting coefficient k Input the Extreme Learning Machine (ELM) neural network, adjust the corresponding weights or thresholds of the input layer, hidden layer, and output layer of the ELM neural network, and obtain the trained ELM neural network algorithm.

5. A neutral line voltage control system for a flexible DC transmission system, characterized in that, The system includes: The DC voltage control module is used to control the DC bus voltage of the converter station in the flexible DC transmission system to the DC voltage reference value through the DC voltage controller. The difference between the output DC voltage and the DC voltage reference value is passed through the proportional-integral regulator to obtain the d-axis current reference value for current loop control. The reactive power regulation module is used to stabilize the reactive power output of the converter station at the reactive power reference value through the reactive power controller, and to obtain the q-axis current reference value of the current loop control by passing the difference between the output reactive power and the reactive power reference value through the proportional-integral regulator. The converter calculation module is used to calculate the zero-sequence voltage component based on the AC side output voltage; input the zero-sequence voltage component into an infinite impulse response filter to obtain the zero-sequence voltage reference value; input the voltage modulation ratio of the AC side output voltage into an extreme learning machine neural network algorithm to obtain the limiting coefficient k; and calculate the converter output reference voltage based on the zero-sequence voltage reference value, the limiting coefficient k, the preset positive-sequence component of the reference voltage, and the negative-sequence component of the reference voltage. The converter calculation module includes a converter voltage calculation unit. This is used to sum the three-phase voltages of the AC output voltage of the converter and multiply them by 1 / 3 to obtain the zero-sequence voltage component; The voltage modulation ratio of the AC side output voltage is input into the trained extreme learning machine neural network algorithm to obtain the limiting coefficient k of the zero-sequence voltage component of the output. The zero-sequence voltage component is input to the infinite impulse response filter, and then processed through a preset formula within the infinite impulse response filter: The reference value of the zero-sequence voltage is calculated. ; Where x() represents the zero-sequence voltage component of the input, r belongs to [0,M], and M represents the preset input order. This represents the r-th preset input coefficient. Let M represent the t-th preset output coefficient, and N represent the preset output order, where n is greater than or equal to M and N. Through the formula: Calculate the output reference voltage of the converter. ; in, V refx + This represents the positive sequence component of the reference voltage. V refx - This represents the negative sequence component of the reference voltage. The reference value for zero-sequence voltage is represented by k, which represents the limiting factor. The current loop controller module is used to obtain the d-axis current reference value, the q-axis current reference value, and the converter output reference voltage, and then output the reference voltage of each bridge arm in the converter station.

6. The neutral line voltage control system for the flexible DC transmission system according to claim 5, characterized in that, The DC voltage control module includes a d-axis current calculation unit. Used in the formula: Calculate the reference value of the d-axis current. ; in, K up This represents the proportional coefficient of the DC voltage controller. K ui This represents the integral coefficient of the DC voltage controller. U dc Indicates the output DC voltage. U dcref This represents the DC voltage reference value, and S represents a preset constant.

7. The neutral line voltage control system for the flexible DC transmission system according to claim 5, characterized in that, The reactive power regulation module includes a q-axis current calculation unit. Used in the formula: Calculate the reference value of the q-axis current. ; in, K Qp This represents the proportional gain of the reactive power controller. K Qi This represents the integral coefficient of the reactive power controller. Q Indicates the output reactive power. Q ref This indicates the reference value for reactive power.

8. A non-volatile computer storage medium, characterized in that, It stores computer instructions, which, when executed, implement a neutral line voltage control method for a flexible DC transmission system as described in any one of claims 1-4.

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