Control method of a direct current power transmission system and direct current power transmission system

Abstract

The application provides a control method of a direct current transmission system and the direct current transmission system. The control method comprises the following steps: obtaining a direct current voltage difference between a direct current voltage reference value of a sending end and a direct current voltage measurement value of the sending end; determining a direct current power reference value based on the direct current voltage difference, and taking the direct current power reference value as an active power limit of a converter of the sending end connected to the direct current transmission system; determining a smaller value between the direct current power reference value and a direct current power real-time value as a system power instruction value, and performing auxiliary valve control of the sending end based on the system power instruction value to perform reactive power control and harmonic elimination. The reactive power control comprises auxiliary valve outlet voltage control and angle control, the harmonic elimination comprises harmonic current elimination and harmonic voltage elimination, and the angle is a phase difference between the auxiliary valve outlet voltage and an alternating current voltage connected to the sending end. The control effect of the scheme is good, the control logic is simple, and the cost is low without adding an additional coordination control system.

Description

Technical Field

[0001] This application relates to the field of high voltage direct current transmission, and more specifically, to a control method for a direct current transmission system and a direct current transmission system. Background Technology

[0002] With the accelerated global energy structure transformation, the installed capacity of new energy power generation such as wind and solar power has continued to grow rapidly. However, these new energy power generation systems generally connect to the grid via power electronic converters, replacing traditional synchronous generators. This has led to an increasingly prominent problem of grid "hollowing out," mainly manifested in a decrease in the grid's equivalent rotational inertia, a reduction in short-circuit capacity, a weakening of voltage support capability, and changes in system damping characteristics. These problems seriously threaten the stability, security, and power quality of the power grid, especially in regional power grids with a high proportion of new energy penetration.

[0003] To address the issue of grid hollowing out, particularly by providing necessary reactive power support and voltage control, a proposed solution employs a "Line Commutated Converter (LCC) + Static Var Generator (SVG)" approach. However, this solution has some technical limitations. For instance, since the LCC and SVG are two independent devices, an additional coordination control system is required for reactive power control and harmonic cancellation, increasing system complexity and investment costs. Summary of the Invention

[0004] The main objective of this application is to provide a control method and a DC transmission system to solve the problems of high cost and complex control logic of the existing LCC+SVG scheme when performing reactive power control and harmonic cancellation.

[0005] To achieve the above objectives, according to one aspect of this application, a control method for a DC transmission system is provided. The DC transmission system includes a sending end and a receiving end, both of which include converters. Each converter includes a main valve and an auxiliary valve. The main valve is an IGCT direct-flow valve, and the auxiliary valve includes a full-bridge module and / or a half-bridge module. The control method includes: acquiring a DC voltage difference, wherein the DC voltage difference is the difference between a reference DC voltage value sent from the sending end and a measured DC voltage value sent from the sending end; determining a DC power reference value based on the DC voltage difference, and... The DC power reference value serves as the active power limit required for the converter at the sending end to connect to the DC transmission system; the real-time DC power value is obtained, and the smaller value between the DC power reference value and the real-time DC power value is determined as the system power command value. Based on the system power command value, the auxiliary valve at the sending end is controlled to perform reactive power control and harmonic cancellation. The reactive power control includes auxiliary valve outlet voltage control and angle control. The harmonic cancellation includes harmonic current cancellation and harmonic voltage cancellation. The angle is the phase difference between the auxiliary valve outlet voltage and the AC voltage connected to the sending end.

[0006] Optionally, determining the DC power reference value based on the DC voltage difference includes: performing a proportional-integral operation on the DC voltage difference to determine the DC power reference value.

[0007] Optionally, obtaining the real-time DC power value includes: determining a calculated DC power value based on the measured DC voltage value sent by the sending end and the measured DC current value sent by the sending end; and determining the real-time DC power value by multiplying the calculated DC power value by an AC voltage over-limit coefficient, wherein the AC voltage over-limit coefficient is used to limit the real-time DC power value from falling below the grid safety access limit.

[0008] Optionally, controlling the auxiliary valve at the sending end based on the system power command value to perform harmonic cancellation includes: performing a power balance calculation based on the system power command value to obtain a reference value for the valve-side AC current; acquiring the sampling voltage of the filter inductor at the sending end and performing deadbeat control on the sampling voltage of the filter inductor to obtain an actual value for the valve-side AC current; acquiring the difference between the reference value and the actual value of the valve-side AC current to obtain an AC current difference; and performing harmonic current cancellation based on the AC current difference.

[0009] Optionally, harmonic current cancellation based on the AC current difference includes: using a harmonic controller to extract harmonics from the AC current difference to obtain harmonic current; inverting the harmonic current to obtain an inverted harmonic current; and superimposing the inverted harmonic current onto the initial value of the auxiliary valve outlet current reference value to obtain an updated auxiliary valve outlet current reference value, so as to cancel the harmonic current generated by the main valve.

[0010] Optionally, reactive power control is performed on the auxiliary valve at the sending end based on the system power command value, including: using a reactive power controller to perform reactive power control on the system power command value to obtain a fundamental reference value for the auxiliary valve outlet voltage; performing Fourier decomposition on the actual value of the AC voltage connected to the sending end to obtain the fundamental frequency, and subtracting it from the actual value of the auxiliary valve outlet voltage to determine a harmonic reference value for the auxiliary valve outlet voltage; superimposing the harmonic reference value and the fundamental reference value for the auxiliary valve outlet voltage to obtain a reference value for the auxiliary valve outlet voltage; and performing reactive power control based on the reference value for the auxiliary valve outlet voltage.

[0011] Optionally, before performing Fourier decomposition on the actual value of the AC voltage connected to the sending end to obtain the fundamental frequency, and subtracting it from the actual value of the auxiliary valve outlet voltage to determine the harmonic reference value of the auxiliary valve outlet voltage, the method further includes: adding the actual value of the AC voltage connected to the sending end and the sampling voltage of the filter inductor of the sending end to obtain the actual value of the auxiliary valve outlet voltage.

[0012] Optionally, the method further includes: determining the ratio of the system power command value to the measured DC voltage value sent by the sending end as the reference value of the DC current sent by the sending end; performing proportional-integral calculation on the reference value of the DC current sent by the sending end to obtain the trigger angle of the main valve of the sending end; and using the trigger angle of the main valve to trigger control the main valve for active power control.

[0013] According to one aspect of this application, a DC transmission system is provided, comprising: a sending end and a receiving end, each of the sending end and the receiving end including a converter, the converter including a main valve and an auxiliary valve, the main valve being an IGCT direct-flow valve, and the auxiliary valve including a full-bridge module and / or a half-bridge module; and a controller connected to the sending end and the receiving end for executing any of the control methods of the DC transmission system described above.

[0014] The technical solution of this application utilizes a power loop. Based on the system power command value obtained from the power loop, reactive power control and harmonic cancellation are achieved through auxiliary valves. The control logic is simple, requiring no additional coordination control system, resulting in lower costs. Furthermore, it facilitates stable voltage control, smooth power transition, and intelligent power limiting. This not only enhances the safety and reliability of power electronic equipment but also improves power quality, optimizes the grid connection performance of new energy sources, reduces the impact on the power grid, and improves the overall operating efficiency and economy of the power system. Attached Figure Description

[0015] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings:

[0016] Figure 1 A schematic diagram of a bidirectional F3C DC transmission system according to an embodiment of this application is shown;

[0017] Figure 2 A schematic diagram of the receiving end of a specific bidirectional F3C DC transmission system according to an embodiment of this application is shown;

[0018] Figure 3 A schematic flowchart of a control method for a DC transmission system according to an embodiment of this application is shown;

[0019] Figure 4 A schematic diagram illustrating the implementation principle of a control method for a DC transmission system according to an embodiment of this application is shown.

[0020] Figure 5 A schematic diagram of a specific delivery end structure according to an embodiment of this application is shown. Detailed Implementation

[0021] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0022] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0023] It should be understood that when an element (such as a layer, film, region, or substrate) is described as being "on" another element, the element may be directly on the other element, or there may be an intermediate element present. Furthermore, in the specification and claims, when an element is described as being "connected" to another element, the element may be "directly connected" to the other element, or "connected" to the other element via a third element.

[0024] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. It is understood that "at least one" refers to one or more, and "multiple" refers to two or more. "At least a portion of an element" refers to part or all of an element.

[0025] It is understood that the term "connection" in the following embodiments should be understood as "electrical connection," "communication connection," etc., if the connected circuits, modules, units, etc., have electrical signal or data transmission with each other.

[0026] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent three cases: A exists, A and B exist simultaneously, and B exists. In addition, the character " / " in this document generally indicates that the related objects before and after it have an "or" relationship.

[0027] The terminology used in the description of the various embodiments herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used in the description of the various embodiments and the appended claims, the word "part" is also intended to include the plural form unless the context clearly indicates otherwise.

[0028] The embodiments of this application will now be described in detail with reference to the accompanying drawings. However, those skilled in the art will understand that many technical details have been provided in the embodiments of this application to facilitate a better understanding of the application. However, the technical solutions claimed in this application can be implemented even without these technical details and various variations and modifications based on the following embodiments.

[0029] As introduced in the background section, in order to address the problem of grid hollowing out, especially to provide necessary reactive power support, the LCC+SVG in related solutions has some technical limitations, specifically including:

[0030] 1) Complex aspects of coordinated control: As two independent control units, LCC and SVG require the design of complex coordinated control strategies to achieve reactive power control and harmonic cancellation.

[0031] 2) Regarding harmonic control of AC voltage: SVG can control the AC voltage level by rapidly adjusting reactive power, theoretically helping to improve voltage quality. However, SVG itself does not have the function of directly removing AC voltage harmonics. In the LCC+SVG combination, SVG is mainly used to compensate for the system's reactive power and maintain the voltage level, thus indirectly affecting the harmonic content. LCC, during operation, generates characteristic harmonics (such as the 6k±1st harmonic), which superimpose into the AC system, affecting voltage quality. Therefore, it is usually necessary to add a dedicated filtering device to the system, such as an LC filter or an active power filter (APF), to cancel or reduce the harmonics introduced by the LCC, ensuring that the harmonic content of the AC voltage is within the allowable range.

[0032] 3) Harmonic control of AC current: SVG can quickly respond to and compensate for reactive and harmonic currents in the system, helping to reduce the harmonic content of AC current. SVG effectively counteracts harmonic currents generated by nonlinear loads in AC systems by detecting the harmonic components of the AC current in real time and then generating corresponding compensation currents. However, the harmonic suppression capability of SVG is limited by its capacity and control strategy. Especially when load and grid conditions vary significantly, SVG struggles to accurately and fully compensate for all harmonic currents, resulting in a still relatively high harmonic distortion rate of the AC current.

[0033] 4) Adaptability: Under weak grid conditions, the LCC has poor operational stability, and the harmonic suppression effect of the SVG may also decrease, thus the overall adaptability of the solution is challenged.

[0034] Therefore, although the LCC+SVG scheme has certain effects in reactive power control, AC voltage harmonic control, and AC current harmonic control, it has inherent limitations, such as the complexity of overall system coordination and control, the characteristic harmonics generated by LCC, and the limited harmonic compensation capability of SVG.

[0035] Based on the above analysis, this application provides a control method and a DC transmission system. Furthermore, this control method is applied to a bidirectional F3C DC transmission system, also known as a fully controlled composite DC transmission system. The bidirectional F3C DC transmission system, such as… Figure 1 As shown, it includes a feed end and a receiving end, both of which include converters. Each converter includes a main valve and auxiliary valves. The main valve is an IGCT direct-flow valve, and the auxiliary valves include full-bridge modules and / or half-bridge modules.

[0036] It should be noted that the reference values ​​for the DC voltage supplied by the sending end, the measured values ​​for the DC voltage supplied by the sending end, the measured values ​​for the DC current supplied by the sending end, the reference values ​​for the AC current supplied by the valve side, the actual values ​​for the AC current supplied by the valve side, the reference values ​​for the auxiliary valve outlet current, the fundamental reference values ​​for the auxiliary valve outlet voltage, the actual values ​​for the AC voltage supplied by the sending end, the harmonic reference values ​​for the auxiliary valve outlet voltage, the reference values ​​for the auxiliary valve outlet voltage, the actual values ​​for the auxiliary valve outlet voltage, and the measured values ​​for the DC voltage supplied by the sending end are all used in this article.

[0037] It should be noted that the "measured values" mentioned in this article are parameters that can be directly measured, while the "actual values" cannot be directly measured and need to be determined indirectly through other parameters.

[0038] See the receiving-end structure of a specific bidirectional F3C DC transmission system. Figure 2 The main valve is an IGCT direct-flow valve, and the auxiliary valve includes multiple sub-modules SM. Each sub-module SM is a full-bridge structure, including an IGCT and a diode located on any of the bridge arms.

[0039] See Figure 3 The control method for a DC transmission system includes the following steps:

[0040] Step S301: Obtain the DC voltage difference value, which is the difference between the DC voltage reference value sent by the transmitting end and the DC voltage measurement value sent by the transmitting end.

[0041] Among them, the DC voltage reference value sent by the sending end is the DC side voltage reference value, and the DC voltage measurement value sent by the sending end is the DC side voltage measurement value.

[0042] High-precision voltage sensors can be installed on the DC bus of a bidirectional F3C DC transmission system to obtain the DC voltage measurement value sent from the sending end. These sensors typically include voltage divider resistor networks, optocouplers, Hall effect sensors, etc., and can withstand high-voltage environments and provide stable and reliable measurement signals.

[0043] The above-mentioned DC voltage reference value sent by the sending end is determined as follows: the DC voltage reference value sent by the sending end needs to be determined together with the DC system voltage operation requirements and the active power transmitted by the system. The calculation method can be obtained by dividing the system required transmission power by the system rated operating current.

[0044] In step S301, the DC voltage difference is obtained by comparing the DC voltage reference value sent by the sending end with the DC voltage measurement value sent by the sending end. This step allows the system to accurately sense the difference between the actual state and the desired state of the DC side voltage.

[0045] Step S302: Determine the DC power reference value based on the DC voltage difference, and use the DC power reference value as the active power limit required for the converter at the sending end to connect to the DC transmission system;

[0046] Using the DC power reference value as the active power limit required for the converter at the sending end to be connected to the DC transmission system has the following technical advantages:

[0047] By setting an active power limit on the DC power reference value, the power output of the sending-end converter can be smoothly transitioned when the output power of new energy sources fluctuates. Even if there is a sudden change in the power on the power supply side, the converter can quickly adjust its operating state to avoid large-scale power fluctuations, thereby reducing the impact on the power grid and improving the overall stability and efficiency of the system.

[0048] Setting a DC power reference value helps maintain a stable DC-side voltage. Since DC voltage is the foundation of energy storage and transmission in the entire system, controlling DC power effectively manages energy flow and ensures that the DC-side voltage remains within a preset safe range. This is crucial for preventing system failures caused by overvoltage or undervoltage.

[0049] In weak grid environments, the dynamic response capability and stability of the system are particularly critical. By setting a DC power reference limit, the converter can automatically adjust its power output when grid conditions are poor, avoiding grid collapse or converter instability caused by excessive power extraction or supply, thus enhancing the system's survivability and operational quality under complex grid conditions.

[0050] Using the DC power reference value as the limit for active power simplifies the overall control architecture. Compared to complex control schemes with multiple variables and multiple levels, the control logic based on the DC power reference value is clearer, easier to implement, and reduces the computational burden of the control algorithm and the hardware resource requirements.

[0051] In step S302, the DC power reference value is set as the active power limit, which is an optimized control based on the DC voltage difference. In this way, the system can adjust the power output in real time to ensure that the active power of the sending-end converter does not exceed the safe range and can adapt to changes in grid demand, achieving smooth power transition and effective management.

[0052] Step S303: Obtain the real-time value of DC power, and determine the smaller value between the DC power reference value and the real-time value of DC power as the system power command value. Based on the system power command value, perform auxiliary valve control at the sending end to perform reactive power control and harmonic cancellation. Reactive power control includes auxiliary valve outlet voltage control and angle control. Harmonic cancellation includes harmonic current cancellation and harmonic voltage cancellation. The angle is the phase difference between the auxiliary valve outlet voltage and the AC voltage connected to the sending end.

[0053] In steps S301, S302, and S303 above, the process of "obtaining the real-time value of DC power and determining the smaller value between the DC power reference value and the real-time value of DC power as the system power command value to form a power loop" is described.

[0054] The reason for determining the smaller of the DC power reference value and the real-time DC power value as the system power command value is to ensure the safe and stable operation of the system, prevent the contradiction of excessive power but inability of equipment to output power, avoid overload, and protect power electronic equipment from damage. This can be explained from the following aspects:

[0055] In power electronic converters such as the F3C, the DC power reference value is typically a target value set based on the system's optimal operating state or user requirements. However, during actual operation, the real-time DC power value may exceed the equipment's safe operating range due to grid conditions, equipment capacity limitations, or changes in the external environment. By selecting a smaller value as the command value, it can be ensured that the system does not exceed its power handling capacity, preventing overload and protecting power electronic equipment from damage.

[0056] The output power of new energy sources (such as wind and solar power) often exhibits randomness and fluctuation. If the system operates simply according to a fixed power reference value, a sudden increase in the output power of new energy sources may lead to instantaneous power excess in the system, causing voltage spikes or other instability phenomena in the power grid. By comparing the real-time power value with the reference value and taking the smaller value as the command value, the system can adjust in real time to adapt to the fluctuations in new energy sources, maintain power balance, and ensure the stability of the power grid.

[0057] Under weak grid conditions, the system is subject to greater disturbances and has higher requirements for power control. Selecting a smaller value as the command value can prevent the system from drawing or releasing too much power when poor grid conditions are detected, which helps maintain the voltage and frequency stability of the grid, reduces the impact on the grid, and ensures the reliable operation of F3C equipment under various grid conditions.

[0058] In power grids with a high proportion of renewable energy sources, power dispatch faces greater challenges. Setting the power command value to the smaller of the real-time and reference values ​​facilitates refined management of renewable energy sources, enabling the power system to better respond to dispatch demands, avoid power waste, and optimize energy utilization efficiency.

[0059] By adopting this strategy, the system can respond more quickly to changes in power demand, whether it's an increase or decrease in renewable energy output or fluctuations in grid load. This flexible and rapid response capability is crucial for improving the overall controllability and adaptability of the system.

[0060] In step S303, the smaller value between the DC power reference value and the real-time value is determined as the system power command value. This operation reflects the system's intelligent power management and limitation capabilities. It ensures that even with changes in grid conditions or fluctuations in renewable energy output, the system will not operate under overload conditions, effectively avoiding the risks of overload and equipment damage, while also enhancing operational stability under weak grid conditions. Based on the system power command value, auxiliary valve control at the sending end is performed for reactive power control and harmonic cancellation. This allows the system to maintain voltage stability while more effectively suppressing harmonic currents and harmonic voltages on the AC side, ensuring superior reactive power control, significantly improving power quality, reducing harmonic pollution to the grid, and enhancing the grid's capacity and system operating efficiency.

[0061] Based on the foregoing description, the control method for a DC transmission system in this application, including steps S301, S302, and S303, first designs a power loop. Then, based on the system power command value obtained from the power loop, it controls reactive power and harmonic cancellation through the auxiliary valve at the sending end. The control logic is simple, and no additional coordination control system is required, resulting in lower costs. Furthermore, it facilitates stable voltage control, smooth power transition, and intelligent power limiting. This not only improves the safety and reliability of power electronic equipment but also enhances power quality, optimizes the grid connection performance of new energy sources, reduces the impact on the power grid, and improves the overall operating efficiency and economy of the power system.

[0062] To achieve accurate determination of the DC power reference value, the DC power reference value is determined based on the DC voltage difference, including: performing proportional-integral calculations on the DC voltage difference to determine the DC power reference value. The specific implementation is as follows:

[0063] ;

[0064] Indicates the DC power reference value. This indicates the reference value of the DC voltage sent from the transmitting end. This indicates the measured DC voltage value sent from the transmitting end. This is the proportionality coefficient. The integral coefficient is... The time for integration.

[0065] Determining the DC power reference value by performing proportional-integral calculations on the DC voltage difference has the following technical advantages:

[0066] The proportional term of a PI controller can quickly respond to changes in DC voltage difference, providing immediate control, while the integral term helps to offset steady-state errors, ensuring accurate control over long periods. This combination of dynamic response and steady-state error cancellation helps maintain system stability and efficiency in rapidly changing renewable energy grid-connected environments.

[0067] The integral term is used to accumulate the deviation until the system reaches a zero-error state. This ensures that even in the presence of external disturbances or model uncertainties, the system can eventually adjust to the required DC power reference value and achieve high-precision control.

[0068] PI parameters (proportional coefficient and integral time constant) can be optimized according to system characteristics and operating conditions. This adaptive capability enables the controller to adapt to different working environments, including common challenges of new energy grid connection such as weak grids and rapid power fluctuations.

[0069] In this embodiment, obtaining the real-time DC power value includes: determining a calculated DC power value based on the measured DC voltage and current values ​​sent from the transmitting end; and determining the real-time DC power value by multiplying the calculated DC power value by an AC voltage limit-crossing coefficient, whereby the AC voltage limit-crossing coefficient is used to limit the real-time DC power value from falling below the grid safety access limit. Because the output power of new energy sources such as wind and solar power is uncertain, the system needs to be able to quickly adapt to changing power demands. By calculating the calculated DC power value in real time, the system can immediately respond to changes in DC voltage and current, achieving rapid power adjustment.

[0070] Active power is constrained by an AC voltage over-limit factor, ensuring that power fluctuations in renewable energy scenarios do not lead to AC voltage fluctuations. This guarantees reliable connection to the AC grid after grid connection control on the grid side, ensuring that the AC voltage does not exceed safe operating limits. This helps maintain grid voltage stability, avoids overvoltage or undervoltage problems, and ensures the safety and reliability of the power system.

[0071] Furthermore, the introduction of the AC voltage over-limit factor, by limiting the AC voltage level at the sending end and in conjunction with the bypassing of the DC system or energy-consuming devices, can prevent overvoltage in the sending-end system from occurring when active power cannot be effectively supplied during a fault at the receiving end of the DC system. When grid voltage drops or other unfavorable conditions occur, excessive power absorption or release can render the converter unreliable. Both the converter and other AC grid compensation devices will suffer severe reactive power consumption due to the motor effect. The over-limit factor reduces this consumption, thereby reducing the risk of equipment damage, extending equipment lifespan, and lowering the system failure rate.

[0072] By monitoring DC voltage and current in real time, dynamically calculating DC power, and adjusting power based on AC voltage over-limit coefficients, the system enables more precise energy dispatch and management. It can intelligently adjust the output power of new energy sources according to power demand and grid conditions, optimizing energy utilization efficiency and reducing waste.

[0073] This control strategy is based on direct power calculation and limit adjustment, avoiding complex multivariate control strategies, simplifying control logic, reducing the complexity of control algorithms and hardware resource requirements, and facilitating engineering implementation and maintenance.

[0074] Therefore, by calculating DC power in real time and using AC voltage over-limit coefficients to restrict power values, it is possible to not only achieve dynamic power adjustment of the system, ensure AC voltage stability, and prevent equipment overload, but also enhance the grid's capacity to accommodate new energy sources, optimize energy dispatch, simplify control logic, and improve the system's adaptability to weak grids and overall stability. These advantages collectively promote the safer and more efficient integration of new energy sources into the grid, providing key technical support for building a smart and green power system.

[0075] See the following formula for details:

[0076] ,in, Indicates the system power command value. Indicates the DC power reference value. This represents the real-time value of DC power. This represents the calculated DC power value. It is the product of the measured DC voltage and the measured DC current output from the transmitting end. This represents the AC voltage over-limit coefficient.

[0077] In this embodiment of the application, the auxiliary valve control of the sending end based on the system power command value to perform harmonic cancellation includes: performing power balance calculation based on the system power command value to obtain a reference value of AC current on the valve side.

[0078] Power balance calculations based on system power command values ​​allow for the precise generation of reference values ​​for valve-side AC current, ensuring accurate current control, according to the current system operating status and desired power output. Specifically, the system power command values ​​are substituted into the AC and DC system power balance equations to obtain the valve-side AC current reference value. This power balance calculation ensures that the AC-side power flow matches the DC-side power command. This command-based power control enables smooth power transitions under different operating conditions, reducing the impact of power fluctuations on the power grid, and is particularly suitable for power management in renewable energy grid-connected scenarios.

[0079] The sampling voltage of the filter inductor at the sending end is obtained, and the sampling voltage of the filter inductor is controlled without deadbeat to obtain the actual value of the AC current on the valve side.

[0080] Deadbeat control can accurately predict and control the dynamic process of current, ensuring it perfectly matches the reference value at the end of each sampling period, thus achieving current tracking with zero steady-state error. This method significantly improves the accuracy and speed of current control, and is particularly suitable for rapidly changing operating conditions, such as fluctuations in renewable energy generation.

[0081] The difference between the reference value and the actual value of the AC current on the valve side is obtained to obtain the AC current difference; harmonic current cancellation is performed based on the AC current difference.

[0082] By comparing the reference value and the actual value of the AC current on the valve side, the harmonic components in the current can be detected. Based on this difference, harmonic current cancellation is performed to cancel the current harmonics generated by the main valve.

[0083] Under weak grid conditions, the phase and amplitude relationships of voltage and current become unstable, making them prone to harmonic problems. Through power balance calculations and deadbeat control, the system can control the current more precisely, enhancing operational stability and control effectiveness in weak grid environments.

[0084] In a specific embodiment, harmonic current cancellation based on the AC current difference includes: using a harmonic controller to extract harmonics from the AC current difference to obtain harmonic currents; inverting the harmonic currents to obtain inverse harmonic currents; and superimposing the inverse harmonic currents onto the initial value of the auxiliary valve outlet current reference value to obtain an updated auxiliary valve outlet current reference value, thereby canceling the harmonic currents generated by the main valve. See details. Figure 4 and Figure 5 , Figure 4 The AC current difference is input to the harmonic controller for harmonic extraction. Figure 5 The valve-side AC current Is is shown. The purpose of the phase reversal here is to cancel the current harmonics generated by the main valve.

[0085] See Figure 4Specifically, the harmonic controller is used to control the harmonic current cancellation of the AC current difference.

[0086] Specifically, a resonant controller is implanted at the fundamental and harmonic frequencies, and its transfer function is:

[0087] ;

[0088] For function values, The h-th harmonic angular frequency, , It provides adjustable gain. PCI offers infinite gain in a specific frequency band to compensate for steady-state tracking errors.

[0089] Harmonic controllers can accurately extract harmonic components from AC current differences, meaning they can identify harmonics of different frequencies and amplitudes, allowing for targeted compensation. This precise identification capability is a prerequisite for effectively performing harmonic current cancellation.

[0090] By inverting the extracted harmonic current and then superimposing it onto the initial value of the auxiliary valve outlet current reference, the auxiliary valve generates a current equal in magnitude but opposite in direction to the harmonic current of the main valve, achieving precise harmonic current cancellation. This method effectively reduces the harmonic content of AC current and improves power quality.

[0091] The harmonic controller can analyze and track harmonic changes in AC current in real time. Even when the load or power grid conditions change rapidly, it can respond quickly and adjust the current command of the auxiliary valve to ensure that the harmonic suppression effect is not affected.

[0092] The harmonic controller in this embodiment is implemented in software. Compared to a hardware-implemented harmonic controller, it eliminates the need for additional physical hardware, reducing system complexity and cost, as well as simplifying maintenance and installation. By reducing harmonic content, the system can minimize additional losses caused by harmonics, such as cable heating, motor vibration, and noise, thereby improving overall energy conversion efficiency and equipment lifespan.

[0093] Under weak grid conditions, harmonic problems may be more severe. By actively extracting and canceling harmonic currents, the system can maintain stable operation without relying on grid strength, reduce interference to the grid, and improve the system's adaptability in complex grid environments.

[0094] In this embodiment of the application, reactive power control is performed by controlling the auxiliary valve at the sending end based on the system power command value, including:

[0095] A reactive power controller is used to control the system power command value to obtain the fundamental reference value of the auxiliary valve outlet voltage.

[0096] In practice, the system power command value is decomposed into active power and reactive power. This is achieved by mapping the system power command value to a dq coordinate system, where the d-axis corresponds to the active power command value and the q-axis corresponds to the reactive power command value. The deviation between the reactive power command value and the actual reactive power value is obtained and input to the reactive power controller for calculation, outputting the fundamental reference value of the auxiliary valve outlet voltage. Specifically, the reactive power controller is usually based on the proportional-integral (PI) control principle, or other control strategies such as proportional-complex-integral (PCI) or multi-resonant control (MRC). The goal of the reactive power controller is to adjust the output of the auxiliary valve to make the actual reactive power value of the system closer to the reactive power command value.

[0097] The reactive power controller can accurately calculate the required reactive power based on the system power command value and generate a fundamental reference value for the auxiliary valve outlet voltage. This method ensures that the reactive power output closely matches the system demand, improving the accuracy and efficiency of reactive power regulation.

[0098] The fundamental frequency is obtained by Fourier decomposition of the actual AC voltage input to the sending end, and the difference between the fundamental frequency and the actual voltage output of the auxiliary valve is used to determine the harmonic reference value of the auxiliary valve output voltage.

[0099] The auxiliary valve outlet voltage reference value is obtained by superimposing the auxiliary valve outlet voltage harmonic reference value and the auxiliary valve outlet voltage fundamental reference value.

[0100] By using Fourier decomposition, the fundamental frequency and other harmonics can be extracted from the actual value of the AC voltage. Comparing this with the actual value of the auxiliary valve outlet voltage, a harmonic reference value for the auxiliary valve outlet voltage can be determined and generated. Superimposing this harmonic reference value with the fundamental frequency reference value can effectively cancel harmonics in the AC voltage, improve power quality, and reduce negative impacts on the power grid.

[0101] Reactive power control is performed based on the reference value of the auxiliary valve outlet voltage.

[0102] Reactive power control based on the auxiliary valve outlet voltage reference value is specifically implemented as follows: the outlet voltage difference is obtained by subtracting the auxiliary valve outlet voltage reference value from the actual outlet voltage value of the auxiliary valve, and the PWM signal is adjusted according to the outlet voltage difference value, i.e., the auxiliary valve drive signal is adjusted, thereby using the auxiliary valve drive signal to drive the auxiliary valve to act. Specifically, by controlling the opening and closing of the switching devices in the auxiliary valve, the actual outlet voltage value of the auxiliary valve is tracked and controlled to match the auxiliary valve outlet voltage reference value, thereby achieving reactive power control.

[0103] This method uses software algorithms instead of hardware filters, enabling it to quickly respond to changes in system power command values ​​and fluctuations in harmonic components in AC voltage, thus achieving dynamic control of reactive power and power quality.

[0104] In some embodiments, before performing Fourier decomposition on the actual value of the AC voltage input to the sending end to obtain the fundamental frequency, and subtracting it from the actual value of the auxiliary valve outlet voltage to determine the harmonic reference value of the auxiliary valve outlet voltage, the method further includes:

[0105] The actual value of the AC voltage connected to the sending end is added to the sampled voltage of the filter inductor at the sending end to obtain the actual value of the auxiliary valve outlet voltage.

[0106] See Figure 5 The actual value of the AC voltage Uac input to the sending end is added to the sampling voltage of the filter inductor to obtain the actual value of the auxiliary valve outlet voltage Us.

[0107] By adding the actual AC voltage value to the sampled voltage of the filter inductor, a signal reflecting the true voltage condition at the auxiliary valve outlet can be obtained. The voltage of the filter inductor reflects the influence of transient processes in the AC system. Superimposing it with the AC voltage can fully present the actual voltage state at the auxiliary valve outlet, ensuring that subsequent control decisions are based on accurate information.

[0108] This step ensures that the reactive power controller and harmonic controller receive signals that are closest to the actual state, thereby making control decisions more accurate and improving the effectiveness of reactive power control and harmonic suppression.

[0109] In this embodiment of the application, the method further includes:

[0110] The ratio of the system power command value to the measured DC voltage value sent by the sending end is determined as the reference value of the DC current sent by the sending end;

[0111] The trigger angle of the main valve at the sending end is obtained by performing proportional-integral calculation on the DC current reference value sent from the sending end.

[0112] The main valve is triggered and controlled by the triggering angle of the main valve to control active power.

[0113] See the following formula for details: ,in, This is the reference value for the DC current sent from the transmitting end. This is the system power command value. This is the measured value of the DC voltage sent from the transmitting end.

[0114] A precise DC current reference value can be obtained by calculating the ratio of the system power command value to the measured DC voltage value sent from the transmitting end. This reference value is dynamically adjusted based on the actual system state and the desired power output, ensuring the accuracy and real-time performance of DC current control.

[0115] Proportional-integral (PI) operation is a closed-loop control method that can continuously correct errors and ensure the consistency between the DC current reference value and the actual DC current value, thereby maintaining the stability of the DC voltage.

[0116] PI control has a fast response characteristic, which can quickly react to changes in the DC current reference value and adjust the trigger angle of the main valve in a timely manner, ensuring that the system can adapt quickly when the power command value changes, reducing fluctuations and instability during the transition process.

[0117] The method also includes: obtaining a phase compensation function; and applying the phase compensation function through feedback control to compensate for the phase delay of the auxiliary valve outlet voltage.

[0118] See the following formula for details:

[0119] Introducing phase compensation function : ,in, For adjustable gain, , is the time constant.

[0120] The phase compensation function can accurately compensate for the phase delay in harmonic voltages injected into the AC power grid, which is crucial for the cancellation of characteristic harmonics. Through this function, the system can ensure that the phase relationship between voltage and current meets requirements, improving the stability and efficiency of system operation.

[0121] Phase compensation improves a system's response to dynamic changes, especially when dealing with rapidly changing signals or grid disturbances. By reducing phase delay, the system can react to changes more quickly, which is particularly important for high-frequency power electronic converters, facilitating real-time control and rapid regulation.

[0122] The method also includes: using machine learning or other optimization methods to learn from a large amount of operating data and the corresponding optimal parameters in order to dynamically adjust the parameters of PI controllers, harmonic controllers, etc.

[0123] Specifically, machine learning or other optimization methods are used to learn from a large amount of operating data (such as load, speed, harmonic distortion rate THD, error signal, etc.) and the corresponding optimal parameters (obtained through simulation or expert experience) to achieve dynamic adjustment of the parameters of PI controllers, harmonic controllers, etc. For example, for a PI controller, the inputs are "current error e, error change rate ec, load current IL", and the outputs are "proportional coefficient Kp, integral time Ti"; for a harmonic controller (such as a repetitive controller), the inputs are "harmonic order, THD value, grid frequency fluctuation", and the outputs are "compensation gain, filter coefficient".

[0124] The method also includes: using control error as the optimization objective, iteratively adjusting parameters through gradient descent to minimize the objective value of the objective function, thereby optimizing the parameters of the PI controller. The control error includes tracking error and harmonic suppression error. For example, for a PI controller, the objective function is defined as follows: (Integral of the square of the error), by calculation , Update the parameters along the negative gradient direction.

[0125] To enable those skilled in the art to better understand the technical solution of this application, the implementation process of the control method of the DC transmission system of this application will be described in detail below with reference to specific embodiments.

[0126] See Figure 4 Specifically, the steps include the following:

[0127] 1) In order to achieve DC voltage stability, it is necessary to calculate the difference between the DC voltage reference value sent by the sending end and the DC voltage measurement value sent by the sending end, and use the PI dynamic adjustment method to generate a DC power reference value. This reference value serves as the active power limit required for the F3C sending end converter to be connected to the system.

[0128] The DC voltage reference value sent from the transmitting end As the primary control variable, the DC power reference value is calculated using the power balance equation. :

[0129] ;

[0130] Indicates the DC power reference value. This indicates the reference value of the DC voltage sent from the transmitting end. This indicates the measured DC voltage value sent from the transmitting end. This is the proportionality coefficient. The integral coefficient is... The time for integration.

[0131] 2) The power is calculated by measuring the DC voltage and DC current sent from the sending end, and then multiplied by the AC voltage over-limit coefficient to obtain the real-time value of the DC power input to the system.

[0132] 3) Select the smaller value between the DC power reference value and the real-time DC power value as the system power command value for the next step of calculation. See details: Among these, the AC voltage over-limit factor is ensured to always remain below the grid safety access limit. Indicates the system power command value. Indicates the DC power reference value. This represents the real-time value of DC power. This represents the calculated DC power value. It is the product of the measured DC voltage and the measured DC current output from the transmitting end. This represents the AC voltage over-limit coefficient.

[0133] 4) Limit the DC current measurement value sent from the transmitting end, and calculate the DC current reference value using the formula: ,in, This is the reference value for the DC current sent from the transmitting end. This is the system power command value. The measured DC voltage value is sent from the transmitting end. The trigger angle of the F3C main valve is obtained by using a proportional-integral algorithm on the DC current reference value.

[0134] 5) Next, the reference value of the AC current on the valve side is obtained by calculating the power balance equations of the AC and DC systems;

[0135] 6) By detecting the sampling voltage of the F3C filter inductor, the actual value of the AC current on the valve side is obtained through a deadbeat control algorithm;

[0136] 7) The difference between the reference value of the AC current on the valve side and the actual value of the AC current on the valve side is calculated, and the reference value of the auxiliary valve outlet current is generated by the harmonic controller. The switching of the auxiliary valve sub-module is controlled to inject a preset harmonic current, thereby filtering out AC current harmonics.

[0137] Specifically, a resonant controller is implanted at the fundamental and harmonic frequencies, and its transfer function is:

[0138] ; The h-th harmonic angular frequency, , It provides adjustable gain. PCI offers infinite gain in a specific frequency band to compensate for steady-state tracking errors.

[0139] 8) The actual value of the AC voltage input to the sending end is added to the sampling voltage of the filter inductor to obtain the actual value of the auxiliary valve outlet voltage;

[0140] 9) After performing Fourier decomposition on the actual value of the AC voltage connected to the sending end, the fundamental wave is obtained, and the difference between the fundamental wave and the actual value of the auxiliary valve outlet voltage is used to determine the reference value of the auxiliary valve outlet voltage harmonics.

[0141] 10) Use a reactive power controller to perform reactive power control on the system power command value to obtain the fundamental reference value of the auxiliary valve outlet voltage. Superimpose the harmonic reference value of the auxiliary valve outlet voltage and the fundamental reference value of the auxiliary valve outlet voltage to obtain the reference value of the auxiliary valve outlet voltage. Perform reactive power control based on the reference value of the auxiliary valve outlet voltage.

[0142] In this scheme, the F3C topology circuit connects the main valve and the DC system. A stable DC voltage is generated through constant voltage control on the inverter side, and constant power control is adopted on the rectifier side. The purpose is to adjust the current based on the power input of the AC system on the rectifier side and the resulting power control loop, thereby meeting the control requirements.

[0143] It has the following technical advantages:

[0144] 1. Active power is constrained by the AC voltage limit factor, thereby ensuring that power fluctuations do not cause AC voltage fluctuations in the context of new energy access, thus guaranteeing reliable access to the AC grid after grid control on the access side.

[0145] 2. A dual closed-loop cascaded control structure (outer loop for power, inner loop for current) is adopted to achieve stable control of DC voltage under fluctuating input power. The outer loop is the power loop, which mainly generates a reference value of power based on DC voltage fluctuations through a proportional-integral algorithm.

[0146] 3. Feedforward-Feedback Composite Control: Through grid voltage feedforward control, the multi-resonant feedforward function suppresses grid background harmonics (such as the 5th / 7th / 11th harmonics) and reduces the impact of voltage distortion at the PCC point. The feedback correction stage introduces a dynamic feedback function to compensate for phase delay and enhance adaptability to weak grids.

[0147] 4. Dynamically adjust the phase margin of the control loop to offset the phase delay caused by the LCL filter under weak power grid conditions, reduce the voltage distortion rate at the PCC point by 50%, and still operate stably under extremely weak power grid conditions with a short-circuit ratio (SCR) < 1.5.

[0148] 5. Voltage loop control can prevent overvoltage protection from triggering shutdown, achieving dynamic voltage limiting and maintaining continuous operation. Current control overcomes the shortcomings of insufficient PI bandwidth, deadbeat tracking control achieves harmonic suppression, and feedforward phase compensation algorithm can achieve effective current control, thereby preventing large-scale power fluctuations and providing better adaptability to weak AC power grids.

[0149] This application also provides a DC power transmission system, including:

[0150] The sending end and the receiving end both include converters. The converters include main valves and auxiliary valves. The main valves are IGCT direct-flow valves, and the auxiliary valves include full-bridge modules and / or half-bridge modules.

[0151] A controller, connected to both the sending and receiving ends, is used to execute control methods for any type of DC transmission system.

[0152] In this embodiment, the auxiliary valve includes a full-bridge module. Each arm of the full-bridge module includes an IGBT and a diode connected in parallel, and also includes a capacitor connected in parallel with the upper and lower arms. See details. Figure 2 The full-bridge module is SM.

[0153] This application also provides a control device for a DC transmission system. It should be noted that the control device for the DC transmission system in this application can be used to execute the control device for the DC transmission system provided in this application. This device is used to implement the above embodiments and preferred embodiments; details already described will not be repeated. As used below, the term "module" can refer to a combination of software and / or hardware that implements a predetermined function. Although the device described in the following embodiments is preferably implemented in software, hardware implementation, or a combination of software and hardware, is also possible and contemplated.

[0154] The following describes the control device for a DC transmission system provided in the embodiments of this application. This device is used to implement the control method for the aforementioned DC transmission system. The DC transmission system includes a sending end and a receiving end, both of which include converters. Each converter includes a main valve and an auxiliary valve. The device includes:

[0155] The first acquisition unit is used to acquire the DC voltage difference, which is the difference between the DC voltage reference value sent by the sending end and the DC voltage measurement value sent by the sending end.

[0156] The first determining unit is used to determine the DC power reference value based on the DC voltage difference, and to use the DC power reference value as the active power limit required for the converter at the sending end to connect to the DC transmission system.

[0157] The processing unit is used to acquire the real-time value of DC power, and determine the smaller value between the DC power reference value and the real-time value of DC power as the system power command value. Based on the system power command value, it performs auxiliary valve control at the sending end to perform reactive power control and harmonic cancellation. The reactive power control includes auxiliary valve outlet voltage control and angle control. The harmonic cancellation includes harmonic current cancellation and harmonic voltage cancellation. The angle is the phase difference between the auxiliary valve outlet voltage and the AC voltage connected to the sending end.

[0158] For specific representations of voltage and current in the embodiments of this application, please refer to [link to relevant documentation]. Figure 5 The parameters are: AC voltage Uac connected to the sending end, AC current Is on the valve side, auxiliary valve outlet voltage Us, auxiliary valve outlet current Iss, DC current Idc sent out by the sending end, and DC voltage Udc sent out by the sending end.

[0159] The control device for the DC transmission system disclosed in this application, through a first acquisition unit, a first determination unit, and a processing unit, achieves the following: First, a power loop is designed; then, based on the system power command value obtained from the power loop, harmonic control of the AC voltage and AC current at the sending end is achieved through an auxiliary valve. This results in good harmonic suppression, simple control logic, and low cost as no additional components are required. Furthermore, it achieves stable voltage control, smooth power transition, and intelligent power limiting. This not only improves the safety and reliability of power electronic equipment but also enhances power quality, optimizes the grid connection performance of new energy sources, reduces the impact on the power grid, and improves the overall operating efficiency and economy of the power system.

[0160] Specifically, the first determining unit is used to perform proportional-integral calculations on the DC voltage difference to determine the DC power reference value.

[0161] As an optional solution, the processing unit includes a first determining module and a second determining module. The first determining module is used to determine the calculated DC power value based on the measured DC voltage value and the measured DC current value sent by the sending end. The second determining module is used to determine the real-time DC power value by multiplying the calculated DC power value by the AC voltage over-limit coefficient. The AC voltage over-limit coefficient is used to limit the real-time DC power value from falling below the grid safety access limit.

[0162] The processing unit also includes a calculation module, an acquisition and control module, a first acquisition module, and a control module. The calculation module is used to perform power balance calculations based on the system power command value to obtain the valve-side AC current reference value. The acquisition and control module is used to acquire the sampling voltage of the filter inductor at the sending end and perform deadbeat control on the sampling voltage of the filter inductor to obtain the actual value of the valve-side AC current. The first acquisition module is used to acquire the difference between the valve-side AC current reference value and the valve-side AC current actual value to obtain the AC current difference value. The cancellation module is used to perform harmonic current cancellation based on the AC current difference value.

[0163] The cancellation module includes an extraction submodule and a cancellation submodule. The extraction submodule is used to extract harmonics from the AC current difference using a harmonic controller to obtain the harmonic current. The cancellation submodule is used to invert the harmonic current to obtain an inverted harmonic current, and then superimpose the inverted harmonic current onto the initial value of the auxiliary valve outlet current reference value to obtain an updated auxiliary valve outlet current reference value, so as to cancel the harmonic current generated by the main valve.

[0164] The processing unit includes a first reactive power control module, a decomposition and determination module, a superposition module, and a second reactive power control module. The first reactive power control module is used to perform reactive power control on the system power command value using a reactive power controller to obtain the fundamental reference value of the auxiliary valve outlet voltage. The decomposition and determination module is used to perform Fourier decomposition on the actual value of the AC voltage connected to the sending end to obtain the fundamental frequency, and to determine the harmonic reference value of the auxiliary valve outlet voltage by subtracting it from the actual value of the auxiliary valve outlet voltage. The superposition module is used to superimpose the harmonic reference value of the auxiliary valve outlet voltage and the fundamental reference value of the auxiliary valve outlet voltage to obtain the reference value of the auxiliary valve outlet voltage. The second reactive power control module is used to perform reactive power control based on the reference value of the auxiliary valve outlet voltage.

[0165] The device also includes a second acquisition unit, which is used to add the actual value of the AC voltage connected to the sending end and the sampling voltage of the filter inductor at the sending end to obtain the actual value of the auxiliary valve outlet voltage before performing Fourier decomposition on the actual value of the AC voltage connected to the sending end to obtain the fundamental wave and subtracting it from the actual value of the auxiliary valve outlet voltage to determine the harmonic reference value of the auxiliary valve outlet voltage.

[0166] The device also includes a second determining unit, a calculation unit, and a trigger control unit. The second determining unit is used to determine the ratio of the system power command value and the measured DC voltage value sent by the sending end as the reference value of the DC current sent by the sending end. The calculation unit is used to perform proportional-integral calculation on the reference value of the DC current sent by the sending end to obtain the trigger angle of the main valve at the sending end. The trigger control unit is used to trigger the main valve using the trigger angle of the main valve to perform active power control.

[0167] The technical effects of the control method embodiment of the DC transmission system in this application can be applied to the control device embodiment of the DC transmission system in this application.

[0168] The control device of the DC transmission system includes a processor and a memory. The aforementioned first acquisition unit, etc., are all stored as program units in the memory, and the processor executes the aforementioned program units stored in the memory to achieve the corresponding functions. All of the above modules are located in the same processor; or, the above modules are located in different processors in any combination.

[0169] The processor contains a kernel, which retrieves the corresponding program unit from memory. One or more kernels can be configured. By adjusting the kernel parameters, the problems of high cost and complex control logic associated with the existing LCC+SVG scheme, which requires an additional coordination control system for reactive power control and harmonic cancellation, can be addressed.

[0170] The memory may include non-permanent memory in computer-readable media, such as random access memory (RAM) and / or non-volatile memory, such as read-only memory (ROM) or flash RAM, and the memory includes at least one memory chip.

[0171] This invention provides a computer-readable storage medium including a stored program, wherein the program controls the device containing the computer-readable storage medium to execute a control method for a DC transmission system when it is running.

[0172] This invention provides a processor for running a program, wherein the program executes a control method for a DC transmission system.

[0173] This invention provides a device including a processor, a memory, and a program stored in the memory and executable on the processor. When the processor executes the program, it implements the steps of a control method for a DC transmission system. The device described herein can be a server, PC, tablet, mobile phone, etc.

[0174] This application also provides a computer program product that, when executed on a data processing device, is adapted to perform the steps of an initialization control method for a DC transmission system.

[0175] It is obvious to those skilled in the art that the modules or steps of the present invention described above can be implemented using general-purpose computing devices. They can be centralized on a single computing device or distributed across a network of multiple computing devices. They can be implemented using computer-executable program code, and thus can be stored in a storage device for execution by a computing device. In some cases, the steps shown or described can be performed in a different order than those described herein, or they can be fabricated as separate integrated circuit modules, or multiple modules or steps can be fabricated as a single integrated circuit module. Thus, the present invention is not limited to any particular combination of hardware and software.

[0176] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0177] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0178] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0179] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0180] In a typical configuration, a computing device includes one or more processors (CPU), input / output interfaces, network interfaces, and memory.

[0181] Memory may include non-persistent memory in computer-readable media, such as random access memory (RAM) and / or non-volatile memory, such as read-only memory (ROM) or flash RAM. Memory is an example of computer-readable media.

[0182] Computer-readable media includes both permanent and non-permanent, removable and non-removable media that can store information using any method or technology. Information can be computer-readable instructions, data structures, modules of programs, or other data. Examples of computer storage media include, but are not limited to, phase-change memory (PRAM), static random access memory (SRAM), dynamic random access memory (DRAM), other types of random access memory (RAM), read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), flash memory or other memory technologies, CD-ROM, digital versatile optical disc (DVD) or other optical storage, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other non-transferable medium that can be used to store information accessible by a computing device. As defined herein, computer-readable media does not include transient computer-readable media, such as modulated data signals and carrier waves.

[0183] 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.

[0184] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0185] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A control method for a DC transmission system, characterized in that, The DC transmission system includes a sending end and a receiving end, both of which include converters. Each converter includes a main valve and an auxiliary valve. The main valve is an IGCT direct-flow valve, and the auxiliary valve includes a full-bridge module and / or a half-bridge module. The control method includes: Obtain the DC voltage difference value, which is the difference between the DC voltage reference value sent by the sending end and the DC voltage measurement value sent by the sending end; A DC power reference value is determined based on the DC voltage difference, and the DC power reference value is used as the active power limit required for the converter at the sending end to connect to the DC transmission system. The system acquires a real-time DC power value and determines the smaller of the DC power reference value and the real-time DC power value as the system power command value. Based on the system power command value, it performs auxiliary valve control at the sending end to perform reactive power control and harmonic cancellation. The reactive power control includes auxiliary valve outlet voltage control and angle control. The harmonic cancellation includes harmonic current cancellation and harmonic voltage cancellation. The angle is the phase difference between the auxiliary valve outlet voltage and the AC voltage connected to the sending end.

2. The control method for a DC transmission system according to claim 1, characterized in that, Determining the DC power reference value based on the DC voltage difference includes: The DC power reference value is determined by performing proportional-integral calculations on the DC voltage difference.

3. The control method for a DC transmission system according to claim 1, characterized in that, Obtain real-time DC power values, including: The calculated DC power value is determined based on the measured DC voltage and DC current values ​​sent from the sending end. The product of the calculated DC power value and the AC voltage over-limit coefficient is determined as the real-time DC power value. The AC voltage over-limit coefficient is used to limit the real-time DC power value from falling below the grid safety access limit.

4. The control method for a DC transmission system according to claim 1, characterized in that, Based on the system power command value, the auxiliary valve control at the sending end is performed to cancel harmonics, including: Based on the system power command value, a power balance calculation is performed to obtain the valve-side AC current reference value; The sampling voltage of the filter inductor at the sending end is obtained, and the sampling voltage of the filter inductor is controlled without deadbeat to obtain the actual value of the AC current on the valve side. The difference between the reference value of the valve-side AC current and the actual value of the valve-side AC current is obtained to obtain the AC current difference. Harmonic current cancellation is performed based on the difference in AC current.

5. The control method for a DC transmission system according to claim 4, characterized in that, Harmonic current cancellation based on the aforementioned AC current difference includes: The harmonic current is obtained by extracting harmonics from the AC current difference using a harmonic controller. The harmonic current is inverted to obtain an inverted harmonic current. The inverted harmonic current is then superimposed on the initial value of the auxiliary valve outlet current reference value to obtain an updated auxiliary valve outlet current reference value, thereby offsetting the harmonic current generated by the main valve.

6. The control method for a DC transmission system according to claim 1, characterized in that, Based on the system power command value, the auxiliary valve at the sending end is controlled to perform reactive power control, including: A reactive power controller is used to perform reactive power control on the system power command value to obtain the fundamental reference value of the auxiliary valve outlet voltage. The fundamental frequency is obtained by performing Fourier decomposition on the actual value of the AC voltage connected to the sending end, and the difference between the fundamental frequency and the actual value of the auxiliary valve outlet voltage is used to determine the reference value of the auxiliary valve outlet voltage harmonics. The auxiliary valve outlet voltage reference value is obtained by superimposing the harmonic reference value of the auxiliary valve outlet voltage and the fundamental reference value of the auxiliary valve outlet voltage. The reactive power control is performed based on the reference value of the auxiliary valve outlet voltage.

7. The control method for a DC transmission system according to claim 6, characterized in that, Before performing Fourier decomposition on the actual value of the AC voltage input to the sending end to obtain the fundamental frequency, and calculating the difference between this fundamental frequency and the actual value of the auxiliary valve outlet voltage to determine the harmonic reference value of the auxiliary valve outlet voltage, the method further includes: The actual value of the AC voltage connected to the sending end is added to the sampled voltage of the filter inductor at the sending end to obtain the actual value of the auxiliary valve outlet voltage.

8. The control method for a DC transmission system according to claim 1, characterized in that, The method further includes: The ratio of the system power command value to the measured DC voltage value sent by the sending end is determined as the DC current reference value sent by the sending end; The trigger angle of the main valve at the sending end is obtained by performing proportional-integral calculation on the DC current reference value sent from the sending end. The main valve is triggered and controlled by the triggering angle of the main valve to control active power.

9. A DC transmission system, characterized in that, include: The device includes a feed end and a receiving end, both of which include a converter. The converter includes a main valve and an auxiliary valve. The main valve is an IGCT direct-flow valve. The auxiliary valve includes a full-bridge module and / or a half-bridge module. A controller, connected to the sending end and the receiving end, is used to execute the control method of the DC transmission system according to any one of claims 1 to 8.

10. The DC transmission system according to claim 9, characterized in that, The auxiliary valve includes a full-bridge module, each arm of which includes an IGBT and a diode connected in parallel, and also includes a capacitor connected in parallel with the upper and lower arms.

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