Method, device and equipment for ac voltage control of flexible dc power transmission system and medium

Abstract

The present application relates to the technical field of direct current transmission, and discloses a method, device, equipment and medium for AC voltage control of a flexible direct current transmission system, which comprises the following steps: obtaining a grid-side voltage reference value, equivalent reactance, valve-side voltage actual value and valve-side current actual value of a converter transformer of the flexible direct current transmission system; calculating a valve-side voltage reference value according to the grid-side voltage reference value, the equivalent reactance and the valve-side current actual value; obtaining a valve-side current reference value according to the valve-side voltage actual value and the valve-side voltage reference value; obtaining a current inner loop output value according to the valve-side current reference value and the valve-side current actual value; and performing dq-axis inverse transformation on the current inner loop output value to obtain an abc-axis reference voltage. The present method can reduce the grid-side voltage measuring points and increase the stability of the system.

Description

Technical Field

[0001] This invention relates to the field of DC power transmission technology, and in particular to an AC voltage control method, apparatus, terminal equipment, and computer-readable storage medium for a flexible DC power transmission system. Background Technology

[0002] In recent years, my country's offshore wind power has developed rapidly, with its scale continuously expanding and gradually extending into deep-sea areas. For large-scale offshore wind turbines, DC transmission is more economical than AC transmission. Flexible DC transmission systems, with their flexible operation and passive operation capabilities, are the main technical means for transmitting offshore wind power.

[0003] Currently, in offshore wind power flexible DC transmission systems, offshore flexible DC converters provide stable AC voltage to wind farms through islanded control mode. In islanded control mode, flexible DC converters generally employ a constant AC side voltage (U / f) control method to control the voltage amplitude and frequency at the grid connection point. The control target can be either the grid-side voltage or the valve-side voltage of the converter transformer. With the continuous expansion of wind power scale, and to meet the backup requirements of converter transformers, offshore converter stations need to adopt a parallel connection method for multiple converter transformers. If the flexible DC converter adopts the converter transformer grid-side voltage control method, multiple voltage control measurement points need to be arranged on the grid side, leading to increased project costs. Furthermore, when the converter transformer is switched on or off, the voltage control measurement points may need to be replaced, causing disturbances to the system.

[0004] In summary, existing flexible DC converters adopt grid-side voltage control of the converter transformer. Switching of voltage control measurement points will cause disturbances to the system, and the increase in the number of voltage control measurement points will lead to an increase in project cost. Summary of the Invention

[0005] This invention provides an AC voltage control method, device, terminal equipment, and computer-readable storage medium for a flexible DC transmission system, which reduces the number of grid-side voltage measurement points and increases system stability.

[0006] In a first aspect, to solve the above-mentioned technical problems, the present invention provides an AC voltage control method for a flexible DC transmission system, comprising:

[0007] Obtain the reference value of grid-side voltage, equivalent reactance, actual value of valve-side voltage, and actual value of valve-side current of the converter transformer in the flexible DC transmission system;

[0008] The valve-side voltage reference value is calculated based on the grid-side voltage reference value, the equivalent reactance, and the actual valve-side current value.

[0009] Based on the actual value of the valve-side voltage and the reference value of the valve-side voltage, the reference value of the valve-side current is obtained;

[0010] The inner loop output value of the current is obtained based on the valve-side current reference value and the valve-side current actual value;

[0011] The inner current loop output value is subjected to an inverse dq-axis transformation to obtain the abc-axis reference voltage.

[0012] Preferably, calculating the valve-side voltage reference value based on the grid-side voltage reference value, the equivalent reactance, and the actual valve-side current value includes:

[0013] The grid-side voltage reference value is transformed along the dq axis to obtain the positive and negative dq axis components of the grid-side voltage reference value.

[0014] The actual value of the valve-side current is transformed along the dq axis to obtain the positive and negative sequence dq axis components of the valve-side current.

[0015] Based on the relationship between the valve-side voltage reference value and the grid-side voltage reference value, the positive and negative sequence dq-axis components of the valve-side voltage reference value are calculated according to the equivalent reactance, the positive and negative sequence dq-axis components of the grid-side voltage reference value, and the positive and negative sequence dq-axis components of the valve-side current.

[0016] Preferably, the relationship between the valve-side voltage reference value and the grid-side voltage reference value is as follows:

[0017]

[0018] in, The positive sequence d-axis component of the valve-side voltage reference value. The positive-sequence q-axis component of the valve-side voltage reference value. The negative d-axis component of the valve-side voltage reference value. The negative sequence q-axis component of the valve-side voltage reference value; X t It is the equivalent reactance; i dP This represents the positive sequence d-axis component of the valve-side current. i qP This represents the positive-sequence q-axis component of the valve-side current. i dN This represents the negative sequence d-axis component of the valve-side current. i qN Negative-sequence q-axis component of valve-side current; The positive sequence d-axis component of the grid-side voltage reference value. This represents the positive-sequence q-axis component of the grid-side voltage reference value. The negative sequence d-axis component of the grid-side voltage reference value. The negative-sequence q-axis component of the grid-side voltage reference value.

[0019] Preferably, obtaining the valve-side current reference value based on the actual valve-side voltage value and the valve-side voltage reference value includes:

[0020] Subtract the valve-side voltage reference value from the actual valve-side voltage value to obtain the intermediate voltage value;

[0021] The intermediate voltage value is processed by a preset proportional-integral controller to obtain a valve-side current reference value.

[0022] Preferably, obtaining the inner current loop output value based on the valve-side current reference value and the actual valve-side current value includes:

[0023] Subtract the valve-side current reference value from the valve-side current actual value to obtain the intermediate current value;

[0024] The intermediate current value is processed based on the proportional-integral controller, the actual value of the valve-side voltage, and the preset decoupling term to obtain the inner loop current output value.

[0025] Preferably, the step of performing an inverse dq-axis transformation on the inner current loop output value to obtain the abc-axis reference voltage includes:

[0026] Based on the pre-configured dq coordinate transformation reference angle, the positive sequence current inner loop output value and the negative sequence current inner loop output value are subjected to dq axis inverse transformation to obtain the positive sequence abc axis reference voltage and the negative sequence abc axis reference voltage, respectively; wherein, the current inner loop output value includes the positive sequence current inner loop output value and the negative sequence current inner loop output value;

[0027] The positive-sequence abc-axis reference voltage is added to the negative-sequence abc-axis reference voltage to obtain the abc-axis reference voltage.

[0028] Preferably, the reference angle for the dq coordinate transformation is:

[0029]

[0030] in, The system's rated frequency, For time.

[0031] In a second aspect, the present invention provides an AC voltage control device for a flexible DC transmission system, comprising:

[0032] The data acquisition module is used to acquire the grid-side voltage reference value, equivalent reactance, actual valve-side voltage value, and actual valve-side current value of the converter transformer in the flexible DC transmission system.

[0033] The voltage calculation module is used to calculate the valve-side voltage reference value based on the grid-side voltage reference value, the equivalent reactance, and the actual value of the valve-side current.

[0034] The current calculation module is used to obtain the valve-side current reference value based on the actual value of the valve-side voltage and the valve-side voltage reference value;

[0035] The inner loop output module is used to obtain the inner loop output value of the current based on the valve-side current reference value and the actual valve-side current value.

[0036] The reference voltage output module is used to perform an inverse dq-axis transformation on the inner current loop output value to obtain the abc-axis reference voltage.

[0037] Thirdly, the present invention also provides a terminal device, including a processor, a memory, and a computer program stored in the memory and configured to be executed by the processor, wherein the processor executes the computer program to implement the AC voltage control method for the flexible DC transmission system described in any one of the above.

[0038] Fourthly, the present invention also provides a computer-readable storage medium comprising a stored computer program, wherein, when the computer program is executed, it controls the device containing the computer-readable storage medium to perform the AC voltage control method for the flexible DC transmission system described in any one of the above.

[0039] Compared with the prior art, the present invention has the following beneficial effects:

[0040] This invention directly controls the amplitude and frequency of the converter transformer's valve-side voltage, avoiding the problems of increased grid-side voltage control measurement points and measurement point switching caused by multiple converter transformers connected in parallel. Simultaneously, it compensates for the voltage drop generated by the converter transformer through a voltage compensation algorithm. Under different system operating conditions, it can indirectly control the grid-side voltage of the converter transformer to a given value through direct control of the converter transformer's valve-side voltage, thereby increasing system stability. Furthermore, this invention compensates for the voltage drop generated on the transformer through a voltage compensation control loop, solving the problem of grid-side voltage deviating from the rated value. Attached Figure Description

[0041] Figure 1 This is a schematic flowchart of the AC voltage control method for a flexible DC transmission system provided in the first embodiment of the present invention;

[0042] Figure 2 This is a topology diagram of an offshore converter station;

[0043] Figure 3 This is an AC voltage control block diagram provided in an embodiment of the present invention;

[0044] Figure 4 This is a schematic diagram of the AC voltage control device for a flexible DC transmission system provided in the second embodiment of the present invention. Detailed Implementation

[0045] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0046] Reference Figure 1 The first embodiment of the present invention provides an AC voltage control method for a flexible DC transmission system, comprising the following steps:

[0047] S11, obtain the grid-side voltage reference value, equivalent reactance, valve-side voltage actual value, and valve-side current actual value of the converter transformer in the flexible DC transmission system;

[0048] S12, calculate the valve-side voltage reference value based on the grid-side voltage reference value, the equivalent reactance, and the actual valve-side current value;

[0049] S13, Based on the actual value of the valve-side voltage and the reference value of the valve-side voltage, obtain the reference value of the valve-side current;

[0050] S14. Obtain the inner loop output value of the current based on the valve-side current reference value and the valve-side current actual value;

[0051] S15, perform an inverse dq-axis transformation on the inner current loop output value to obtain the abc-axis reference voltage.

[0052] To facilitate understanding of the present invention, Figure 2 The analysis will be based on the offshore converter station topology shown in the diagram. Figure 2 Multiple wind farms are connected to the grid-side AC bus via collector lines, and then connected to the valve-side AC bus via two or more converter transformer groups. The valve-side AC bus is connected to the flexible DC converter valve. Different converter transformers are designed according to requirements, and can adopt a two-winding connection or a three-winding connection.

[0053] It should be noted that if the converter transformer grid-side voltage is used as the control point, control measurement points need to be arranged on the grid side of all branch transformers, resulting in a large number of control measurement points. Furthermore, when a controlled branch is disconnected, the control measurement point needs to be switched to another branch transformer grid-side control point, causing disturbance to the system. If the converter transformer valve-side voltage is used as the control measurement point, the valve-side voltage transformer is arranged on the valve-side AC bus, requiring only one voltage control measurement point. Moreover, when a single branch is disconnected, there is no need to switch the control measurement point.

[0054] In step S11, it is necessary to obtain the grid-side voltage reference value, equivalent reactance, valve-side voltage actual value, and valve-side current actual value of the converter transformer of the flexible DC transmission system.

[0055] In step S12, calculating the valve-side voltage reference value based on the grid-side voltage reference value, the equivalent reactance, and the actual valve-side current value specifically includes:

[0056] The grid-side voltage reference value is transformed along the dq axis to obtain the positive and negative dq axis components of the grid-side voltage reference value.

[0057] The actual value of the valve-side current is transformed along the dq axis to obtain the positive and negative sequence dq axis components of the valve-side current.

[0058] Based on the relationship between the valve-side voltage reference value and the grid-side voltage reference value, the positive and negative sequence dq-axis components of the valve-side voltage reference value are calculated according to the equivalent reactance, the positive and negative sequence dq-axis components of the grid-side voltage reference value, and the positive and negative sequence dq-axis components of the valve-side current.

[0059] It should be noted that, according to Kirchhoff's laws, the initial relationship between the valve-side voltage reference value and the grid-side voltage reference value is shown in equation (1):

[0060]

[0061] By matching the real parts with the real parts and the imaginary parts with the imaginary parts in equation (1), and assuming that the current reference direction is from the converter to the wind farm, the specific relationship between the valve-side voltage reference value and the grid-side voltage reference value can be obtained as shown in equation (2):

[0062]

[0063] In equations (1) and (2), The positive sequence d-axis component of the valve-side voltage reference value. The positive-sequence q-axis component of the valve-side voltage reference value. The negative d-axis component of the valve-side voltage reference value. The negative sequence q-axis component of the valve-side voltage reference value; X t It is the equivalent reactance; i dP This represents the positive sequence d-axis component of the valve-side current. i qP This represents the positive-sequence q-axis component of the valve-side current. i dN This represents the negative sequence d-axis component of the valve-side current. i qN Negative-sequence q-axis component of valve-side current; The positive sequence d-axis component of the grid-side voltage reference value. This represents the positive-sequence q-axis component of the grid-side voltage reference value. The negative sequence d-axis component of the grid-side voltage reference value. The negative-sequence q-axis component of the grid-side voltage reference value.

[0064] It should be noted that in calculating equivalent reactance... X t First, the equivalent reactance (per-unit value) of a single transformer is obtained. Then, using the rated capacity of the flexible DC converter as a benchmark, the equivalent reactance of the single transformer is converted to obtain the per-unit value of the equivalent reactance of the single transformer. Assume there are a total of k The converter transformer is in operation, and the equivalent impedance of the converter transformer unit is... .

[0065] In step S13, the valve-side current reference value is obtained based on the actual value of the valve-side voltage and the reference value of the valve-side voltage, specifically including:

[0066] Subtract the valve-side voltage reference value from the actual valve-side voltage value to obtain the intermediate voltage value;

[0067] The intermediate voltage value is processed by a preset proportional-integral controller to obtain a valve-side current reference value.

[0068] To facilitate understanding of the present invention, the following will be combined with Figure 3 The embodiments of the present invention will be described in more detail below.

[0069] Reference Figure 3 In one implementation, the valve-side voltage control employs dual closed-loop control of positive and negative sequence voltage and current. The valve-side voltage reference value includes the positive-sequence d-axis component, the positive-sequence q-axis component, the negative-sequence d-axis component, and the negative-sequence q-axis component. Correspondingly, the actual valve-side voltage value includes the positive-sequence d-axis component, the positive-sequence q-axis component, the negative-sequence d-axis component, and the negative-sequence q-axis component.

[0070] Furthermore, the valve-side current reference values ​​include positive-sequence d-axis valve-side current reference values, positive-sequence q-axis valve-side current reference values, negative-sequence d-axis valve-side current reference values, and negative-sequence q-axis valve-side current reference values. The inner current loop includes a PI control unit, a valve-side voltage feedforward unit, and a decoupling control unit; the proportional-integral controller (PI controller) is the PI control unit.

[0071] Specifically, the positive sequence d-axis component of the valve-side voltage reference value. Subtract the positive sequence d-axis component of the actual value of the valve-side voltage u dPAfter passing through the PI controller, the positive sequence d-axis valve-side current reference value is obtained. ;

[0072] Valve-side voltage reference value positive sequence q-axis component Subtract the positive sequence q-axis component of the actual valve-side voltage u qP After passing through the PI controller, the positive sequence q-axis valve-side current reference value is obtained. ;

[0073] Valve-side voltage reference value negative sequence d-axis component Subtract the negative sequence d-axis component of the actual value of the valve-side voltage u dN After passing through the PI controller, the negative sequence d-axis valve-side current reference value is obtained. ;

[0074] Valve-side voltage reference value negative sequence q-axis component Subtract the negative sequence q-axis component of the actual valve-side voltage value u qN After passing through the PI controller, the negative sequence q-axis valve side current reference value is obtained. .

[0075] In step S14, the inner loop output value of the current is obtained based on the valve-side current reference value and the actual valve-side current value, specifically including:

[0076] Subtract the valve-side current reference value from the valve-side current actual value to obtain the intermediate current value;

[0077] The intermediate current value is processed based on the proportional-integral controller, the actual value of the valve-side voltage, and the preset decoupling term to obtain the inner loop current output value.

[0078] Reference Figure 3 In the positive sequence current inner loop, the reference value of the positive sequence d-axis valve-side current is... Subtract the positive sequence d-axis component of the actual value of the valve-side current i dP Then it goes through a PI controller, and then a decoupling term is added. K d i qP The positive sequence d-axis component of the actual value of the valve-side voltage u dP The positive sequence d-axis current inner loop output value is obtained. Reference value of positive sequence q-axis valve side current Subtract the positive sequence q-axis component of the actual value of the valve-side current i qP Then it goes through a PI controller, and then a decoupling term is added. K di dP The positive sequence q-axis component of the actual value of the valve-side voltage u qP The positive-sequence q-axis current inner loop output value is obtained. .in, K d is the decoupling coefficient.

[0079] Reference Figure 3 In the negative sequence current inner loop, the negative sequence d-axis valve-side current reference value Negative sequence q-axis valve side current reference value Given a zero-sequence negative sequence d-axis valve-side current reference value Subtract the negative sequence d-axis component of the actual value of the valve-side current i dN Then it goes through a PI controller, and then a decoupling term is added. K d i qN The negative sequence d-axis component of the actual value of the valve-side voltage u dN The negative sequence d-axis current inner loop output value is obtained. Negative sequence q-axis valve side current reference value Subtract the negative sequence q-axis component of the actual value of the valve-side current i qN Then it goes through a PI controller, and then a decoupling term is added. K d i dN The negative sequence q-axis component of the actual value of the valve-side voltage u qN The negative sequence q-axis current inner loop output value is obtained. .

[0080] In step S15, the inner current loop output value is subjected to an inverse dq-axis transformation to obtain the abc-axis reference voltage, specifically including:

[0081] Based on the pre-configured dq coordinate transformation reference angle, the positive sequence current inner loop output value and the negative sequence current inner loop output value are subjected to dq axis inverse transformation to obtain the positive sequence abc axis reference voltage and the negative sequence abc axis reference voltage, respectively; wherein, the current inner loop output value includes the positive sequence current inner loop output value and the negative sequence current inner loop output value;

[0082] The positive-sequence abc-axis reference voltage is added to the negative-sequence abc-axis reference voltage to obtain the abc-axis reference voltage.

[0083] Wherein, the reference angle for the given dq coordinate transformation is ,in, The system's rated frequency, For time.

[0084] Reference Figure 3 The positive-sequence current inner loop output value and the negative-sequence current inner loop output value are respectively transformed by the dq axis to obtain the positive and negative sequence reference voltages of the controller abc axis. , Controller abc axis positive and negative sequence reference voltage , After adding them together, we obtain the reference voltages for the abc axes. .

[0085] This invention directly controls the amplitude and frequency of the converter transformer's valve-side voltage, avoiding the problems of increased grid-side voltage control measurement points and measurement point switching caused by multiple converter transformers connected in parallel. Simultaneously, it compensates for the voltage drop generated by the converter transformer through a voltage compensation algorithm. Under different system operating conditions, it can indirectly control the grid-side voltage of the converter transformer to a given value through direct control of the converter transformer's valve-side voltage, thereby increasing system stability. Furthermore, this invention compensates for the voltage drop generated on the transformer through a voltage compensation control loop, solving the problem of grid-side voltage deviating from the rated value.

[0086] Reference Figure 4 The second embodiment of the present invention provides an AC voltage control device for a flexible DC transmission system, comprising:

[0087] The data acquisition module is used to acquire the grid-side voltage reference value, equivalent reactance, actual valve-side voltage value, and actual valve-side current value of the converter transformer in the flexible DC transmission system.

[0088] The voltage calculation module is used to calculate the valve-side voltage reference value based on the grid-side voltage reference value, the equivalent reactance, and the actual value of the valve-side current.

[0089] The current calculation module is used to obtain the valve-side current reference value based on the actual value of the valve-side voltage and the valve-side voltage reference value;

[0090] The inner loop output module is used to obtain the inner loop output value of the current based on the valve-side current reference value and the actual valve-side current value.

[0091] The reference voltage output module is used to perform an inverse dq-axis transformation on the inner current loop output value to obtain the abc-axis reference voltage.

[0092] It should be noted that the AC voltage control device for a flexible DC transmission system provided in this embodiment of the invention is used to execute all the process steps of the AC voltage control method for a flexible DC transmission system in the above embodiment. The working principles and beneficial effects of the two are one-to-one, so they will not be described again.

[0093] This invention also provides a terminal device. The terminal device includes a processor, a memory, and a computer program stored in the memory and executable on the processor, such as an AC voltage control program for a flexible DC transmission system. When the processor executes the computer program, it implements the steps described in the various embodiments of the AC voltage control method for flexible DC transmission systems, for example... Figure 1 The step S11 shown. Alternatively, when the processor executes the computer program, it implements the functions of each module / unit in the above-described device embodiments, such as the voltage calculation module.

[0094] For example, the computer program may be divided into one or more modules / units, which are stored in the memory and executed by the processor to complete the present invention. The one or more modules / units may be a series of computer program instruction segments capable of performing a specific function, which describe the execution process of the computer program in the terminal device.

[0095] The terminal device may be a desktop computer, laptop, handheld computer, or smart tablet, etc. The terminal device may include, but is not limited to, a processor and memory. Those skilled in the art will understand that the above components are merely examples of terminal devices and do not constitute a limitation on the terminal device. It may include more or fewer components than described above, or a combination of certain components, or different components. For example, the terminal device may also include input / output devices, network access devices, buses, etc.

[0096] The processor can be a Central Processing Unit (CPU), or other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. A general-purpose processor can be a microprocessor or any conventional processor. The processor is the control center of the terminal device, connecting all parts of the terminal device via various interfaces and lines.

[0097] The memory can be used to store the computer programs and / or modules. The processor implements various functions of the terminal device by running or executing the computer programs and / or modules stored in the memory and by calling data stored in the memory. The memory may mainly include a program storage area and a data storage area. The program storage area may store the operating system, at least one application program required for a function (such as sound playback function, image playback function, etc.), etc.; the data storage area may store data created according to the use of the mobile phone (such as audio data, phonebook, etc.). In addition, the memory may include high-speed random access memory, and may also include non-volatile memory, such as hard disk, memory, plug-in hard disk, smart media card (SMC), secure digital (SD) card, flash card, at least one disk storage device, flash memory device, or other volatile solid-state storage device.

[0098] Wherein, if the modules / units integrated in the terminal device are implemented as software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, all or part of the processes in the methods of the above embodiments of the present invention can also be implemented by a computer program instructing related hardware. The computer program can be stored in a computer-readable storage medium, and when the computer program is executed by a processor, it can implement the steps of the various method embodiments described above. Wherein, the computer program includes computer program code, which can be in the form of source code, object code, executable file, or some intermediate form, etc. The computer-readable medium can include: any entity or device capable of carrying the computer program code, recording medium, USB flash drive, portable hard drive, magnetic disk, optical disk, computer memory, read-only memory (ROM), random access memory (RAM), electrical carrier signals, telecommunication signals, and software distribution media, etc. It should be noted that the content contained in the computer-readable medium can be appropriately added or removed according to the requirements of legislation and patent practice in the jurisdiction. For example, in some jurisdictions, according to legislation and patent practice, the computer-readable medium does not include electrical carrier signals and telecommunication signals.

[0099] It should be noted that the device embodiments described above are merely illustrative. The units described as separate components may or may not be physically separate, and the components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the modules can be selected to achieve the purpose of this embodiment according to actual needs. Furthermore, in the accompanying drawings of the device embodiments provided by this invention, the connection relationships between modules indicate that they have communication connections, which can be specifically implemented as one or more communication buses or signal lines. Those skilled in the art can understand and implement this without any creative effort.

[0100] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the scope of protection of the present invention. In particular, it should be noted that any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention for those skilled in the art.

Claims

1. A method of AC voltage control for a flexible HVDC power transmission system, characterized by, The method comprises the following steps: obtaining a grid-side voltage reference value, an equivalent reactance, an actual valve-side voltage value and an actual valve-side current value of a flexible HVDC converter transformer; calculating a valve-side voltage reference value according to the grid-side voltage reference value, the equivalent reactance and the actual valve-side current value; obtaining a valve-side current reference value according to the actual valve-side voltage value and the valve-side voltage reference value; obtaining a current inner loop output value according to the valve-side current reference value and the actual valve-side current value; performing dq-axis inverse transformation on the current inner loop output value to obtain an abc-axis reference voltage.

2. The method of controlling AC voltage of a flexible HVDC power transmission system according to claim 1, characterized in that, The calculation of the valve-side voltage reference value according to the grid-side voltage reference value, the equivalent reactance and the actual valve-side current value comprises the following steps: performing dq-axis transformation on the grid-side voltage reference value to obtain positive and negative sequence dq-axis components of the grid-side voltage reference value; performing dq-axis transformation on the actual valve-side current value to obtain positive and negative sequence dq-axis components of the actual valve-side current value; calculating positive and negative sequence dq-axis components of the valve-side voltage reference value according to the equivalent reactance, the positive and negative sequence dq-axis components of the grid-side voltage reference value and the positive and negative sequence dq-axis components of the actual valve-side current value based on the relationship between the valve-side voltage reference value and the grid-side voltage reference value.

3. The method of AC voltage control for a flexible HVDC system of claim 2, wherein, The relationship between the valve-side voltage reference value and the grid-side voltage reference value is as follows: wherein is a positive sequence d-axis component of the valve-side voltage reference value, is a positive sequence q-axis component of the valve-side voltage reference value, is a negative sequence d-axis component of the valve-side voltage reference value, is a negative sequence q-axis component of the valve-side voltage reference value; X t is an equivalent reactance; i dP is a positive sequence d-axis component of the valve-side current, i qP is a positive sequence q-axis component of the valve-side current, i dN is a negative sequence d-axis component of the valve-side current, i qN is a negative sequence q-axis component of the valve-side current; is a positive sequence d-axis component of the grid-side voltage reference value, is a positive sequence q-axis component of the grid-side voltage reference value, is a negative sequence d-axis component of the grid-side voltage reference value, is a negative sequence q-axis component of the grid-side voltage reference value.

4. The method of claim 1, wherein, The obtaining of the valve-side current reference value according to the actual valve-side voltage value and the valve-side voltage reference value comprises the following steps: subtracting the actual valve-side voltage value from the valve-side voltage reference value to obtain an intermediate voltage value; processing the intermediate voltage value according to a preset proportional-integral controller to obtain the valve-side current reference value.

5. The method of controlling AC voltage of a flexible HVDC system according to claim 4, characterized in that, The obtaining of the current inner loop output value according to the valve-side current reference value and the actual valve-side current value comprises the following steps: subtracting the actual valve-side current value from the valve-side current reference value to obtain an intermediate current value; processing the intermediate current value according to the proportional-integral controller, the actual valve-side voltage value and a preset decoupling term to obtain the current inner loop output value.

6. The method of AC voltage control for a flexible HVDC system of claim 1, wherein, The dq-axis inverse transformation of the current inner loop output value to obtain the abc-axis reference voltage comprises the following steps: performing dq-axis inverse transformation on positive and negative sequence current inner loop output values according to a pre-configured dq coordinate transformation reference angle to obtain positive and negative sequence abc-axis reference voltages respectively; wherein the current inner loop output value comprises the positive and negative sequence current inner loop output values; adding the positive sequence abc-axis reference voltage and the negative sequence abc-axis reference voltage to obtain the abc-axis reference voltage.

7. The method of controlling AC voltage of a flexible HVDC system according to claim 6, characterized in that, The dq coordinate transformation reference angle is as follows: wherein is the system rated frequency, is time.

8. An alternating voltage control device for a flexible HVDC power transmission system, characterized in that The method comprises the following steps: a data acquisition module is configured to obtain a grid-side voltage reference value, an equivalent reactance, an actual valve-side voltage value and an actual valve-side current value of a flexible HVDC converter transformer; a voltage calculation module is configured to calculate a valve-side voltage reference value according to the grid-side voltage reference value, the equivalent reactance and the actual valve-side current value; a current calculation module is configured to obtain a valve-side current reference value according to the actual valve-side voltage value and the valve-side voltage reference value; an inner loop current output module, configured to obtain a current inner loop output value according to the valve-side current reference value and the valve-side current actual value; a reference voltage output module, configured to perform dq-axis inverse transformation on the current inner loop output value to obtain an abc-axis reference voltage.

9. A terminal device, comprising: A computer readable storage medium stores a computer program, wherein the computer program, when executed by a processor, implements the method for controlling AC voltage of a flexible HVDC power transmission system according to any one of claims 1 to 7.

10. A computer-readable storage medium, characterized in that, The computer readable storage medium stores a computer program, wherein the computer program, when executed by a processor, implements the method for controlling AC voltage of a flexible HVDC power transmission system according to any one of claims 1 to 7.

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