Neutral line voltage control method and system for flexible DC power transmission system, and medium

Abstract

The invention discloses a neutral line voltage control method and system of a flexible direct current power transmission system and a medium, mainly relates to the technical field of voltage control, and is used for solving the problem that the safe and stable operation of the system is influenced because the potential of a neutral point is raised due to an existing capacitor voltage balancing strategy. Comprising the steps that a current loop control d-axis current reference value is obtained through a direct-current voltage controller; obtaining a current loop control q-axis current reference value through a reactive power controller; according to the alternating current side output voltage, a zero-sequence voltage component is obtained through calculation, and a zero-sequence voltage reference value is obtained; obtaining an amplitude limiting coefficient k based on an extreme learning machine neural network algorithm; based on the zero-sequence voltage reference value, the amplitude limiting coefficient k, a preset reference voltage positive-sequence component and a reference voltage negative-sequence component, calculating to obtain an output reference voltage of the converter; and the current loop controller obtains a d-axis current reference value, a q-axis current reference value and an output reference voltage of the converter, and further outputs a reference voltage of each bridge arm in the converter station.

Description

Technical Field

[0001] The present application relates to the technical field of neutral line voltage control, and particularly to a neutral line voltage control method, system and medium for a flexible DC transmission system. Background Art

[0002] Due to the reverse distribution characteristics of energy resources and load centers, and because high-voltage DC transmission technology has the advantage of long-distance and large-capacity power transmission, it has become an important option for resource optimization 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 new energy grid-connected power generation, and are one of the important applicable technologies to solve the uneven distribution of energy and realize the global energy Internet.

[0003] The high-voltage DC transmission system based on modular multilevel converters has the advantages of low switching frequency, ability to achieve active and reactive decoupled control, and no commutation failure problems, etc., and has been widely used. The multi-terminal flexible DC system based on modular multilevel converters (MMC) has a flexible operation mode and is an effective technical means to solve the problems of clean energy grid connection and consumption. For grounding faults occurring at different positions in the DC power grid, the capacitor voltage balancing strategy of the MMC sub-module will play a regulating role during the fault process to control the output voltage of the converter. At the same time, the fault current will flow through the metal return line network, but it will also cause the neutral point potential to rise, affecting the safe and stable operation of the system.

[0004] Therefore, there is an urgent need for a neutral line voltage control method for a flexible DC transmission system to solve the problem that the existing capacitor voltage balancing strategy of the MMC sub-module will cause the neutral point potential to rise, affecting the safe and stable operation of the system. Summary of the Invention

[0005] In view of the above deficiencies of the prior art, the present application provides a neutral line voltage control method, system and medium for a flexible DC transmission system to solve the problem that the existing capacitor voltage balancing strategy of the MMC sub-module will cause the neutral point potential to rise, affecting the safe and stable operation of the system.

[0006] In a first aspect, the present application provides a neutral line voltage control method for a flexible DC transmission system, the method comprising: 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 by 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. According to the output voltage on the AC side, the zero-sequence voltage component is calculated. 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 output voltage on the AC side is input into the extreme learning machine neural network algorithm to obtain the amplitude limiting coefficient k. Based on the zero-sequence voltage reference value, the amplitude 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 bridge arm in the converter station.

[0007] In an implementation manner of the present application, passing the difference between the output DC voltage and the DC voltage reference value through a proportional-integral regulator to obtain the d-axis current reference value for current loop control specifically includes: Through the formula: , calculate the d-axis current reference value ; Wherein, K up represents the proportional coefficient of the DC voltage controller, K ui represents the integral coefficient of the DC voltage controller, U dc represents the output DC voltage, U dcref represents the DC voltage reference value, and S represents a preset constant.

[0008] In an implementation manner of the present application, passing the difference between the output reactive power and the reactive power reference value through a proportional-integral regulator to obtain the q-axis current reference value for current loop control specifically includes: Through the formula: , calculate the q-axis current reference value ; Wherein, K Qp represents the proportional coefficient of the reactive power controller, K Qi represents the integral coefficient of the reactive power controller, Q represents the output reactive power, Q ref represents the reactive power reference value.

[0009] In an implementation manner of the present application, according to the AC-side output voltage, a zero-sequence voltage component is calculated and obtained; 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, a preset reference voltage positive-sequence component, and a reference voltage negative-sequence component, the output reference voltage of the converter is calculated, specifically including: The three-phase voltages of the AC-side output voltage of the converter are summed and multiplied by 1 / 3 to obtain the zero-sequence voltage component; The voltage modulation ratio of the AC-side output voltage is input into a trained extreme learning machine neural network algorithm to obtain the limiting coefficient k of the output zero-sequence voltage component; The zero-sequence voltage component is input into an infinite impulse response filter, and through a preset formula in the infinite impulse response filter: , the reference value of the zero-sequence voltage is calculated and obtained ; where x( ) represents the input zero-sequence voltage component, r belongs to [0, M], M represents the preset input order, represents the r-th preset input coefficient, represents the t-th preset output coefficient, N represents the preset output order, and n is greater than or equal to M and N; Through the formula: , the output reference voltage of the converter is calculated ; where, V refx + represents the reference voltage positive-sequence component, V refx - represents the reference voltage negative-sequence component, represents the reference value of the zero-sequence voltage, and k represents the limiting coefficient.

[0010] In an implementation manner of the present application, before the voltage modulation ratio of the AC-side output voltage is input into a trained extreme learning machine neural network algorithm, the method includes: An extreme learning machine neural network is established, and the extreme learning machine neural network includes an input layer, a hidden layer, and an output layer; where the hidden layer includes H neurons, the weight from the input layer to the hidden layer is ω 11 ~ ω 2H , the threshold of the hidden layer node is b 1~ b H, the weights from the hidden layer to the output layer are β 11 ~ β 2H ; The historical voltage modulation ratio and the historical limiting coefficient k , are input into the extreme learning machine neural network, and the weights or thresholds corresponding to the input layer, hidden layer, and output layer in the extreme learning machine neural network are adjusted to obtain the trained extreme learning machine neural network algorithm.

[0011] In a second aspect, the present application provides a neutral line voltage control system for a flexible DC transmission system. The system includes: A DC voltage control module for controlling the DC bus voltage of the converter station in the flexible DC transmission system to a DC voltage reference value through a DC voltage controller, and passing the difference between the output DC voltage and the DC voltage reference value through a proportional-integral regulator to obtain the d-axis current reference value of the current loop control; A reactive power regulation module for stabilizing the reactive power output by the converter station at a reactive power reference value through a reactive power controller, and passing the difference between the output reactive power and the reactive power reference value through a proportional-integral regulator to obtain the q-axis current reference value of the current loop control; A converter calculation module for calculating the zero-sequence voltage component based on the output voltage on the AC side; inputting the zero-sequence voltage component into an infinite impulse response filter to obtain the zero-sequence voltage reference value; inputting the voltage modulation ratio of the output voltage on the AC side into the extreme learning machine neural network algorithm to obtain the limiting coefficient k; calculating 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; A current loop controller module for obtaining the d-axis current reference value, the q-axis current reference value, and the output reference voltage of the converter, and then outputting the reference voltage of each bridge arm in the converter station.

[0012] In an implementation manner of the present application, the DC voltage control module includes a d-axis current calculation unit, for calculating the d-axis current reference value through the formula: , to calculate the d-axis current reference value ; where K up represents the proportional coefficient of the DC voltage controller, K ui represents the integral coefficient of the DC voltage controller, U dc represents the output DC voltage, U dcref represents the DC voltage reference value, and S represents a preset constant.

[0013] In an implementation manner of the present application, the reactive power regulation module includes a q-axis current calculation unit, for calculating the reference value of the q-axis current through the formula: , where ; Among them, K Qp represents the proportional coefficient of the reactive power controller, K Qi represents the integral coefficient of the reactive power controller, Q represents the output reactive power, Q ref represents the reference value of the reactive power.

[0014] In an implementation manner of the present application, the converter calculation module includes a converter voltage calculation unit, for summing the three-phase voltages of the output voltage on the AC side of the converter and multiplying by 1 / 3 to obtain the zero-sequence voltage component; inputting the voltage modulation ratio of the output voltage on the AC side into the trained extreme learning machine neural network algorithm to obtain the limiting coefficient k of the output zero-sequence voltage component; inputting the zero-sequence voltage component into an infinite impulse response filter, and calculating the reference value of the zero-sequence voltage through the preset formula in the infinite impulse response filter: , where ; Among them, x( ) represents the input zero-sequence voltage component, r belongs to [0, M], M represents the preset input order, represents the rth preset input coefficient, represents the tth preset output coefficient, N represents the preset output order, and n is greater than or equal to M and N; Calculating the output reference voltage of the converter through the formula: , where ; Among them, V refx + represents the positive-sequence component of the reference voltage, V refx - represents the negative-sequence component of the reference voltage, represents the reference value of the zero-sequence voltage, and k represents the limiting coefficient.

[0015] In a third aspect, the present application provides a non-volatile computer storage medium, on which computer instructions are stored, and when the computer instructions are executed, a neutral line voltage control method for a flexible DC transmission system as described in any one of the above is implemented.

[0016] Those skilled in the art can understand that the present application has at least the following beneficial effects: The present application provides a method, a system and a medium for controlling the neutral line voltage of a flexible DC transmission system. The overvoltage suppression of the DC side is achieved by superimposing the zero-sequence voltage limited by the limiting coefficient k on the output voltage of the original controller. During the calculation of the zero-sequence voltage, the abnormal zero-sequence voltage is filtered by an infinite impulse response filter, and the selection of the limiting coefficient is realized through the extreme learning machine neural network algorithm. The above scheme suppresses the overvoltage of the system neutral point by suppressing the overvoltage of the DC side (when a grounding 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 enters the ground through the fault point, flows into the grounding electrode through the earth, and is shunted back to the fault point through the metal return line. The fault current generates a voltage drop across the inductive components of the metal return line network, resulting in the potential rise of the neutral points of each station. The DC side overvoltage is obtained by adding the converter output voltage and the neutral point potential. Therefore, the suppression of the DC side overvoltage can effectively suppress the overvoltage of the neutral line), improves the safe and stable operation level of the system, and solves the problem that the existing MMC sub-module capacitor voltage equalization strategy will cause the neutral point potential to rise, affecting the safe and stable operation of the system. Description of the Drawings

[0017] In order to more clearly illustrate the technical solutions of the present invention, the drawings required for description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0018] Figure 1 It is a flowchart of a method for controlling the neutral line voltage of a flexible DC transmission system provided by an embodiment of the present application.

[0019] Figure 2 It is a schematic diagram of the internal structure of a system for controlling the neutral line voltage of a flexible DC transmission system provided by an embodiment of the present application. Detailed Embodiments

[0020] Those skilled in the art should understand that the embodiments described below are only the preferred embodiments of the present disclosure, and do not mean that the present disclosure can only be implemented through the preferred embodiments. The preferred embodiments are only used to explain the technical principles of the present disclosure, and are not used to limit the protection scope of the present disclosure. Based on the preferred embodiments provided by the present disclosure, all other embodiments obtained by those of ordinary skill in the art without creative efforts should still fall within the protection scope of the present disclosure.

[0021] It should also be noted that the term "including", "comprising" or any other variant thereof is intended to cover non-exclusive inclusion, such that a process, method, commodity or device including a series of elements not only includes those elements but also other elements not expressly listed, or elements inherent to such process, method, commodity or device. Without further limitation, an element defined by the statement "including one..." does not exclude the presence of additional identical elements in the process, method, commodity or device including the element.

[0022] The technical solutions proposed in the embodiments of the present application will be described in detail below with reference to the accompanying drawings.

[0023] The embodiment provides a method for controlling the neutral line voltage of a flexible DC transmission system, as Figure 1 shown. The method provided in the embodiment of the present application mainly includes the following steps: Step 110: Control the DC bus voltage of the converter station in the flexible DC transmission system to the DC voltage reference value through a DC voltage controller, and pass the difference between the output DC voltage and the DC voltage reference value through a proportional-integral regulator to obtain the d-axis current reference value for current loop control.

[0024] 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 passes the difference between the output DC voltage and the DC voltage reference value through a proportional-integral regulator to obtain the d-axis current reference value for current loop control.

[0025] In some embodiments, passing the difference between the output DC voltage and the DC voltage reference value through a proportional-integral regulator to obtain the d-axis current reference value for current loop control specifically includes: Through the formula: , calculate the d-axis current reference value ; where K up represents the proportional coefficient of the DC voltage controller, K ui represents the integral coefficient of the DC voltage controller, U dc represents the output DC voltage, U dcref represents the DC voltage reference value, and S represents a preset constant.

[0026] Step 120: Stabilize the reactive power output by the converter station at the reactive power reference value through a reactive power controller, and pass the difference between the output reactive power and the reactive power reference value through a proportional-integral regulator to obtain the q-axis current reference value for current loop control.

[0027] It should be noted that the reactive power controller specifically refers to: the reactive power controller stabilizes the reactive power output by the converter station at the reactive power reference value, and passes the difference between the output reactive power and the reactive power reference value through a proportional-integral regulator to obtain the q-axis current reference value for current loop control.

[0028] In some embodiments, passing the difference between the output reactive power and the reactive power reference value through a proportional-integral regulator to obtain the q-axis current reference value for current loop control specifically includes: Through the formula: , calculate the q-axis current reference value ; Wherein, K Qp represents the proportional coefficient of the reactive power controller, K Qi represents the integral coefficient of the reactive power controller, Q represents the output reactive power, Q ref represents the reactive power reference value.

[0029] Step 130: Calculate and obtain the zero-sequence voltage component according to 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 the extreme learning machine neural network algorithm to obtain the amplitude limiting coefficient k; calculate and obtain the output reference voltage of the converter based on the zero-sequence voltage reference value, the amplitude limiting coefficient k, the preset positive-sequence component of the reference voltage, and the negative-sequence component of the reference voltage.

[0030] As an example, this step can be specifically: 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 the trained extreme learning machine neural network algorithm to obtain the amplitude limiting coefficient k of the output zero-sequence voltage component; Input the zero-sequence voltage component into the infinite impulse response filter, and through the preset formula in the infinite impulse response filter: , calculate and obtain the reference value of the zero-sequence voltage ; Wherein, x( ) represents the input zero-sequence voltage component, r belongs to [0, M], M represents the preset input order, represents the r-th preset input coefficient, represents the t-th preset output coefficient, N represents the preset output order, and n is greater than or equal to M and N; Through the formula: , calculate the output reference voltage of the converter ; Wherein, Vrefx+ represents the positive sequence component of the reference voltage, and Vrefx- represents the negative sequence component of the reference voltage. represents the reference value of the zero-sequence voltage, and k represents the limiting coefficient.

[0031] Wherein, the method for training the extreme learning machine neural network can be: Establish an extreme learning machine neural network, and the extreme learning machine neural network includes an input layer, a hidden layer, and an output layer; Wherein, the hidden layer includes H neurons, and the weight from the input layer to the hidden layer is ω 11 ~ ω 2H , and the threshold of the hidden layer node is b 1~ b H , and the weight from the hidden layer to the output layer is β 11 ~ β 2H ; Input the historical voltage modulation ratio and the historical limiting coefficient k into the extreme learning machine neural network, and adjust the weights or thresholds corresponding to the input layer, hidden layer, and output layer in the extreme learning machine neural network to obtain a trained extreme learning machine neural network algorithm.

[0032] Step 140: The current loop controller obtains the d-axis current reference value, 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.

[0033] It should be noted that in this step, the d-axis current reference value, q-axis current reference value, and the output reference voltage of the converter are directly input into the existing current loop controller to obtain the output result.

[0034] In addition, this application Figure 2 is a neutral line voltage control system for a flexible DC transmission system provided by an embodiment of this application. As Figure 2 shown, the system provided by the embodiment of this application mainly includes: 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 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 for current loop control.

[0035] The DC voltage control module 210 includes a d-axis current calculation unit, which is used to use the formula: , calculate the d-axis current reference value ; Among them, K up represents the proportional coefficient of the DC voltage controller, K ui represents the integral coefficient of the DC voltage controller, U dc represents the output DC voltage, U dcref represents the DC voltage reference value, and S represents a preset constant.

[0036] The reactive power regulation module 220 is used to stabilize the reactive power output by the converter station at the reactive power reference value through the reactive power controller, and pass the difference between the output reactive power and the reactive power reference value through a proportional-integral regulator to obtain the current loop control q-axis current reference value.

[0037] The reactive power regulation module 220 includes a q-axis current calculation unit, which is used to calculate the q-axis current reference value through the formula: , calculate the q-axis current reference value ; Among them, K Qp represents the proportional coefficient of the reactive power controller, K Qi represents the integral coefficient of the reactive power controller, Q represents the output reactive power, Q ref represents the reactive power reference value.

[0038] The converter calculation module 230 is used to calculate the zero-sequence voltage component according to the output voltage on the AC side; 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 output voltage on the AC side into the extreme learning machine neural network algorithm to obtain the limiting coefficient k; calculate the output reference voltage of the converter based on the zero-sequence voltage reference value, the limiting coefficient k, the preset reference voltage positive-sequence component, and the reference voltage negative-sequence component.

[0039] The converter calculation module 230 includes a converter voltage calculation unit, which is used to sum the three-phase voltages of the output voltage on the AC side of the converter and multiply by 1 / 3 to obtain the zero-sequence voltage component; input the voltage modulation ratio of the output voltage on the AC side into the 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 into an infinite impulse response filter, and calculate the reference value of the zero-sequence voltage through the preset formula in the infinite impulse response filter: , calculate the reference value of the zero-sequence voltage ; where \(x( )\) represents the input zero-sequence voltage component, \(r\in[0,M]\), \(M\) represents the preset input order, represents the \(r\)th preset input coefficient, represents the \(t\)th preset output coefficient, \(N\) represents the preset output order, and \(n\geq M\) and \(N\).

[0040] Through the formula: , calculate the output reference voltage of the converter ; where, V refx + represents the positive-sequence component of the reference voltage, V refx - represents the negative-sequence component of the reference voltage, represents the reference value of the zero-sequence voltage, and \(k\) represents the limiting coefficient.

[0041] The current-loop controller module 240 is used to obtain the d-axis current reference value, the q-axis current reference value, and the output reference voltage of the converter, and then output the reference voltage of each arm in the converter station.

[0042] In addition, the embodiment of the present application also provides a non-volatile computer storage medium, on which executable instructions are stored, and when the executable instructions are executed, the neutral line voltage control method of a flexible DC transmission system as described above is implemented.

[0043] So far, the technical solutions of the present disclosure have been described in combination with multiple foregoing embodiments. However, it is easy for those skilled in the art to understand that the protection scope of the present disclosure is not limited to these specific embodiments. Without departing from the technical principle of the present disclosure, those skilled in the art can split and combine the technical solutions in the above-mentioned various embodiments, and can also make equivalent changes or replacements to the relevant technical features. Any changes, equivalent replacements, improvements, etc. made within the technical concept and / or technical principle of the present disclosure will fall within the protection scope of the present disclosure.

Claims

1. A method for controlling neutral line voltage of a flexible direct current transmission system, characterized in that: The method comprises: The DC bus voltage of the converter station in the flexible DC transmission system is controlled to be a DC voltage reference value by a DC voltage controller, and the difference between the output DC voltage and the DC voltage reference value is passed through a proportional-integral regulator to obtain a d-axis current reference value for current loop control; The reactive power controller is used to stabilize the reactive power output of the converter station at a 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 a q-axis current reference value for current loop control; According to the output voltage on the AC side, a 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 output voltage on the AC side 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, a preset reference voltage positive-sequence component, and a reference voltage negative-sequence component, 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 bridge arm in the converter station.

2. The method for controlling neutral line voltage of a flexible direct current transmission system according to claim 1, characterized in that: 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 of the current loop control, specifically including: By formula: , calculate the d-axis current reference value ; in, K up represents the DC voltage controller proportional coefficient, K ui represents the integral coefficient of the DC voltage controller, U dc Indicates the output DC voltage, U dcref represents the DC voltage reference value, and S represents the preset constant.

3. The method for controlling neutral line voltage of a flexible direct current transmission system according to claim 1, characterized in that: 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 of the current loop control, which specifically includes: By formula: , calculate the q-axis current reference value ; in, K Qp represents the reactive power controller proportional coefficient, K Qi represents the integral coefficient of the reactive power controller, Q Represents the output reactive power, Q ref Indicates the reactive power reference value.

4. The method for controlling neutral line voltage of a flexible direct current transmission system according to claim 1, characterized in that: According to the output voltage on the AC side, the zero-sequence voltage component is calculated; the zero-sequence voltage component is input into the infinite impulse response filter to obtain the zero-sequence voltage reference value; the voltage modulation ratio of the output voltage on the AC side is input into the extreme learning machine neural network algorithm to obtain the limiting coefficient k; Based on the zero-sequence voltage reference value, the limit coefficient k, the preset reference voltage positive sequence component, and the reference voltage negative sequence component, the output reference voltage of the converter is calculated and obtained, specifically including: Sum the three-phase voltages of the converter AC side output voltage and multiply 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 output zero-sequence voltage component; The zero-sequence voltage component and the input infinite impulse response filter are passed through the preset formula in the infinite impulse response filter: , calculate the reference value of zero-sequence voltage ; Where x( ) represents the input zero-sequence voltage component, r belongs to [0,M], M represents the preset input order, represents the rth preset input coefficient, represents the tth preset output coefficient, N represents the preset output order, and n is greater than or equal to M and N; By formula: , calculate the output reference voltage of the converter ; in, V refx + represents the positive sequence component of the reference voltage, V refx - represents the negative sequence component of the reference voltage, It represents the reference value of zero-sequence voltage, and k represents the limiting coefficient.

5. The method for controlling neutral line voltage of a flexible direct current transmission system according to claim 4, 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: Establishing an extreme learning machine neural network, wherein the extreme learning machine neural network includes an input layer, a hidden layer and an output layer; The hidden layer includes H neurons, and the weight from the input layer to the hidden layer is ω 11 ~ ω 2H , the threshold of the hidden layer node is b 1~ b H , the weight from the hidden layer to the output layer is β 11 ~ β 2H ; The historical voltage modulation ratio and the historical limiting coefficient k , input the extreme learning machine neural network, adjust the corresponding weights or thresholds of the input layer, hidden layer and output layer in the extreme learning machine neural network, and obtain the trained extreme learning machine neural network algorithm.

6. A neutral line voltage control system for a flexible direct current transmission system, characterized in that: The system comprises: A DC voltage control module is used to control the DC bus voltage of the converter station in the flexible DC transmission system to a DC voltage reference value through a DC voltage controller, and to obtain a d-axis current reference value for current loop control by passing the difference between the output DC voltage and the DC voltage reference value through a proportional-integral regulator; The reactive power regulation module is used to stabilize the reactive power outputted by the converter station at a reactive power reference value through a reactive power controller, and to obtain a q-axis current reference value for current loop control by passing the difference between the output reactive power and the reactive power reference value through a proportional-integral regulator; The converter calculation module is used to calculate and obtain the zero-sequence voltage component according to the AC side output voltage; input the zero-sequence voltage component into the infinite impulse response filter to obtain the zero-sequence voltage reference value; input the voltage modulation ratio of the AC side output voltage into the 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 reference voltage positive sequence component, and the reference voltage negative sequence component, calculate and obtain the output reference voltage of the converter; The current loop controller module is used to obtain the d-axis current reference value, the q-axis current reference value, and the output reference voltage of the converter, and then output the reference voltage of each bridge arm in the converter station.

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

8. The neutral line voltage control system of the flexible direct current transmission system according to claim 6, characterized in that: The reactive power regulation module includes a q-axis current calculation unit, Used by the formula: , calculate the q-axis current reference value ; in, K Qp represents the reactive power controller proportional coefficient, K Qi represents the integral coefficient of the reactive power controller, Q Represents the output reactive power, Q ref Indicates the reactive power reference value.

9. The neutral line voltage control system of the flexible direct current transmission system according to claim 6, characterized in that: The converter calculation module includes a converter voltage calculation unit, It is used to sum the three-phase voltage of the output voltage on the AC side of the converter and multiply it 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 output zero-sequence voltage component; The zero-sequence voltage component and the input infinite impulse response filter are passed through the preset formula in the infinite impulse response filter: , calculate the reference value of zero-sequence voltage ; Where x( ) represents the input zero-sequence voltage component, r belongs to [0,M], M represents the preset input order, represents the rth preset input coefficient, represents the tth preset output coefficient, N represents the preset output order, and n is greater than or equal to M and N; By formula: , calculate the output reference voltage of the converter ; in, V refx + represents the positive sequence component of the reference voltage, V refx - represents the negative sequence component of the reference voltage, It represents the reference value of zero-sequence voltage, and k represents the limiting coefficient.

10. A non-volatile computer storage medium, characterized in that: Computer instructions are stored thereon, and when the computer instructions are executed, a method for controlling the neutral line voltage of a flexible direct current transmission system as described in any one of claims 1 to 5 is implemented.

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