Multi-terminal hvdc unified power flow controller and parameter setting method

Abstract

The application provides a multi-terminal flexible direct unified network type control structure and parameter value method, which comprises three branch structures and one total branch structure, the total branch structure comprises common elements of three control structures, the three branch structures correspond to the three control structures one by one, each branch structure comprises residual elements of the control structure corresponding to the branch structure after removing the common elements and a parameter controller; wherein the three control structures comprise a control structure when a receiving end MMC adopts direct current voltage control, a control structure when the receiving end MMC adopts active power control, and a control structure when the MMC is connected to an island system and voltage and frequency of the island need to be established. The application can realize stable control and seamless mode switching of the MMC-MTDC system under different operating conditions.

Description

Technical Field

[0001] This invention relates to the field of power grid technology, specifically to a multi-terminal flexible DC unified grid control structure and parameter value method. Background Technology

[0002] A significant trend is that onshore renewable energy bases are mostly located in remote areas, far from load centers, while offshore wind power bases are continuously expanding into deeper waters. This geographical distribution has led to a substantial increase in the distance between renewable energy power plants and the power grid, posing unprecedented challenges to power transmission.

[0003] In this context, multi-terminal high-voltage direct current (MMC-MTDC) transmission technology based on modular multilevel converters (MMCs) is an effective method for connecting renewable energy bases with the receiving-end AC grid. This technology leverages the high-efficiency switching capabilities of MMCs to achieve an effective connection between renewable energy bases and the receiving-end AC grid, significantly improving the flexibility and reliability of transmission. The MMC-MTDC system not only boasts advantages such as large transmission capacity, low losses, and minimal line corridor occupation, but also effectively addresses the intermittency and uncertainty of renewable energy generation, providing strong support for large-scale renewable energy grid integration.

[0004] However, with the continued growth in renewable energy penetration, the stability issues of traditional grid-connected control strategies for MMC-MTDC systems are becoming increasingly prominent. At the transmission end, when the MMC is connected to a weak grid with low short-circuit ratios, phase-locked loop (PLL)-based grid synchronization technology can easily induce system oscillations, affecting transmission stability. At the receiving end, the lack of inertia in the MMC leads to reduced grid frequency stability, further exacerbating system stability risks.

[0005] To address the aforementioned issues, grid-based control strategies have emerged as a potentially effective approach. By simulating the operating characteristics of a traditional synchronous generator, grid-based control provides virtual inertia and damping to the MMC (Multi-Mode Synchronous Generator), thereby enhancing system stability and frequency response. However, current grid-based control strategies for MMCs in MTDC (Multi-Mode Synchronous Data Center) systems are diverse, including Vf control, Virtual Synchronous Generator (VSG) control, and Power Synchronous Control (PSC). These strategies differ significantly in control structure, parameter settings, and dynamic response, making unified design and seamless switching exceptionally difficult.

[0006] During system transitions, the need for control mode changes becomes particularly urgent in the event of unforeseen circumstances such as converter station failures or line outages. However, due to the incompatibility and differences between current network-based control strategies, control mode changes may further exacerbate system stability issues. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the purpose of this invention is to provide a multi-terminal flexible-vertical unified network control structure and a method for determining parameter values.

[0008] According to one aspect of the present invention, a multi-terminal flexible DC unified grid control structure is provided for controlling a multi-terminal flexible DC transmission system, comprising three branch structures and one main branch structure. The main branch structure includes common elements of the three control structures. The three branch structures correspond one-to-one with the three control structures. Each branch structure includes the remaining elements of the corresponding control structure after removing the common elements and a parameter controller.

[0009] Among them, the three control structures include: the control structure when the MMC converter station at the receiving end adopts DC voltage control, the control structure when the MMC converter station at the receiving end adopts active power control, and the control structure for voltage and frequency when the MMC converter station at the transmitting end is connected to an islanded system and islanding needs to be established.

[0010] Preferably, the parameter controller executes one or more of the following control strategies by adjusting the values ​​of the control parameters:

[0011] -When DC voltage control parameter k DC When the value is not 0, the main converter station of the MMC converter station achieves DC voltage control;

[0012] -When the active power control parameter k p When the value is not 0, the slave converter station of the MMC converter station realizes active power control;

[0013] -When the submodule voltage control parameter k SM When the value is not 0, the islanded system autonomously establishes the voltage and frequency.

[0014] Preferably, the MMC converter station has one or more of the following converter station control characteristics under different control parameters:

[0015] - When DC voltage control is used, the control characteristic is that the DC voltage is at a stable value;

[0016] - When active power control is used, the control characteristic is that the active power is at a stable value;

[0017] - When using fixed frequency control, the modular multi-level multi-terminal high-voltage DC transmission system operates in an islanded state, and the control characteristics are that the system can autonomously establish voltage and frequency.

[0018] Preferably, the MMC converter station, under the parameter combination of the parameter controller, has one or more of the following combined control characteristics:

[0019] -When DC voltage control parameter k DCand active power control parameter k p When the value is not 0, the MMC converter station has DC voltage-active power droop control characteristics:

[0020] k DC (U dcref -U dc )+k p (P ref -P ac )=0 (1)

[0021] -When the active power control parameter k p and submodule voltage control parameter k SM When the value is not 0, the MMC converter station has active power-frequency droop control characteristics:

[0022] k SM (U SMav -U SM0 )+k p (P ref -P ac )=0 (2)

[0023] -When DC voltage control parameter k DC and submodule voltage control parameter k SM When the value is not 0, the MMC converter station has a DC voltage-frequency droop control characteristic:

[0024] k DC (U dcref -U dc )+k SM (U SMav -U SM0 )=0 (3)

[0025] k DC k represents the DC voltage control parameter. p k represents the active power control parameter. SM U represents the voltage control parameters of the submodule. dcref U represents the rated DC voltage. dc P represents the actual value of the DC voltage. ref P represents the rated active power. ac U represents the actual value of active power. SM0 U represents the rated voltage of the submodule. SMav This indicates the actual value of the submodule voltage.

[0026] Preferably, the MMC converter station can provide power support to the power grid under the active power-frequency droop control characteristics, specifically:

[0027] By drawing an analogy between the dynamic characteristics of the MMC converter station and the dynamic characteristics of a synchronous generator, we obtain:

[0028]

[0029] Substituting (4) into (2) yields:

[0030]

[0031] In the formula, ω0 represents the rated value of the AC frequency, ω MMC This represents the actual value of the AC frequency of the converter station, and K represents the response coefficient of the submodule voltage to changes in the AC frequency.

[0032] Based on equation (5), it is determined that when the system frequency changes, the MMC converter adjusts its output value P. ac To adapt to system frequency deviations, this adjustment is made to the primary frequency regulation characteristics of the equivalent synchronous generator, providing frequency support for the power grid.

[0033] Preferably, the MMC converter station, under the droop control characteristics of the DC voltage-frequency, provides inertial support to the grid by adjusting the DC voltage to track the grid frequency, specifically as follows:

[0034] Substituting (4) into (3) yields:

[0035]

[0036] Based on equation (6), the DC voltage-frequency droop control provides inertial support to adapt to frequency changes by adjusting the DC voltage, tracking grid frequency changes, and extracting energy stored in the DC capacitor.

[0037] Preferably, the MMC converter station is located at k DC k p and k SM When all values ​​are not zero, the control structure will operate with droop control in the form of DC voltage-active power-frequency droop control. The MMC converter station has all the characteristics of the three droop control modes, specifically:

[0038] k DC (U dcref -U dc )+k p (P ref -P ac )+k SM (U SMav -U SM0 )=0 (7)

[0039] When the power grid is in a stable operating state, the power grid frequency remains at the rated value, and the control mode is DC voltage-active power droop control.

[0040] When a grid fault causes a change in the grid frequency, frequency droop control is initiated to enable DC voltage and active power to track the changes in grid frequency, thus providing frequency support to the grid.

[0041] According to a second aspect of the present invention, a method for determining control parameters of a multi-terminal flexible-vertical unified network control structure is provided, comprising:

[0042] Under grid-connected operation, the multi-terminal flexible DC transmission system adopts DC voltage-active power droop control, and the value range of DC voltage coefficient and active power coefficient is derived based on the DC voltage deviation range.

[0043] In islanded operation, the multi-terminal flexible DC transmission system adopts DC voltage-frequency droop control, and the range of frequency coefficient values ​​is derived based on the islanded frequency fluctuation range.

[0044] Preferably, under the network operation, the multi-terminal flexible DC transmission system adopts DC voltage-active power droop control, and derives the value ranges of the DC voltage coefficient and active power coefficient based on the DC voltage deviation range, including:

[0045] Substituting (4) into (7) gives:

[0046]

[0047] Equation (8) shows the power variation of a single converter station;

[0048] For the entire multi-terminal flexible DC transmission system, by superimposing the varying power of all converter stations, we obtain:

[0049]

[0050] When the system is running stably, the rate of change of the system frequency is 0, i.e., Δω gi =0, at which point the system simplifies to:

[0051]

[0052] For networked systems, the AC output power is an uncertain value, the DC voltage is often set as a control variable, and the fluctuation range of the DC voltage is less than a%. Equation (10) can be rearranged as follows:

[0053]

[0054] Based on equation (11), k is set according to the power allocation ratio among the converter stations on each grid side. DC k pThe value of .

[0055] Preferably, under islanded operation, the multi-terminal flexible DC transmission system adopts DC voltage-frequency droop control, and the value range of the frequency coefficient is derived based on the islanded frequency fluctuation range, including:

[0056] For an islanded system, all power will flow into the system. At this time, the AC power is a known quantity, and we need to pay attention to the frequency fluctuations within the system. Equation (8) can be rearranged as follows:

[0057]

[0058] The power flowing into the islanded system is the rated power, the power change is 0, and the frequency fluctuation is less than a%. Equation (12) can be rearranged as follows:

[0059]

[0060] At this time, the islanded system is in DC voltage-frequency droop control mode, by setting k DC and k SM Establish and stabilize the DC voltage and frequency of the system.

[0061] Compared with the prior art, the embodiments of the present invention have at least one of the following beneficial effects:

[0062] The multi-terminal flexible-vertical unified network control structure in this embodiment of the invention is a unified, compatible and efficient structure that enables stable control and seamless switching of the MMC-MTDC system under different operating conditions. Attached Figure Description

[0063] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0064] Figure 1 Schematic diagrams of DC-side control schemes under different control objectives in existing technologies;

[0065] Figure 2 This is a diagram of a unified network control structure in one embodiment of the present invention;

[0066] Figure 3 This is a schematic diagram illustrating the control characteristics of a unified network control structure in a preferred embodiment of the present invention.

[0067] Figure 4 This is a simulated experimental multi-terminal flexible DC transmission grid-connected system in a specific embodiment of the present invention;

[0068] Figure 5 This is a data graph of the output power of each converter station in a simulation experiment according to a specific embodiment of the present invention;

[0069] Figure 6 This is a data graph of DC voltage in a specific embodiment of the present invention;

[0070] Figure 7 This is a data graph showing the power fluctuation of the receiving-end converter station in a specific embodiment of the present invention;

[0071] Figure 8 The parameters of the REC3 converter station are shown in a specific embodiment of the present invention;

[0072] Figure 9 These are the parameters of the SEN1 converter station in a specific embodiment of the present invention. Detailed Implementation

[0073] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention. These all fall within the scope of protection of the present invention.

[0074] In MT-HVDC (Modular Multilevel High Voltage Direct Current) transmission systems, current grid-based control strategies are mostly designed for specific control objectives. Specifically, when the MMC converter station at the receiving end adopts DC voltage control, the following strategies are employed: Figure 1 The control structure is shown in Figure (a). When the MMC converter station at the receiving end adopts active power control, the following is used: Figure 1 The control structure is shown in Figure (b). When the MMC is connected to an islanded system, the islanded voltage and frequency need to be established, using... Figure 1 The control structure is shown in Figure (c). From Figure 1 It can be seen from this that Figure 1 The control structure shown in Figure (a) has a DC voltage rating U. dcref and the actual value of DC voltage U dc After calculation, the value of the DC voltage U is input to the PI regulator. dc The calculation is performed using the output of the PI regulator, and the result is compared with the actual value U of the MMC submodule voltage. SMav effect. Figure 1 The control structure shown in Figure (b) has an active power rating P. ref and the actual value of active power P ac After calculation, the value of the DC voltage U is input into the PI regulator. dc The calculation is performed using the output of the PI regulator, and the result is compared with the actual value U of the MMC submodule voltage. SMav effect. Figure 1The control structure shown in Figure (c) has a submodule voltage rating U. SM0 Actual value of submodule voltage U SMav After calculation, the value of the DC voltage U is input into the PI regulator. dc The calculation is performed using the output of the PI regulator, and the result is compared with the actual value U of the MMC submodule voltage. SMav Function. None of the three control structures mentioned above is a unified network control strategy that can adapt to all control objectives.

[0075] Therefore, in one embodiment of the present invention, a multi-terminal flexible-vertical unified network control structure is provided, such as... Figure 2 As shown, the structure includes three branch structures and one main branch structure. The main branch structure includes common elements of the three control structures. The three branch structures correspond one-to-one with the three control structures. Each branch structure includes the remaining elements of the corresponding control structure after removing the common elements and a parameter controller.

[0076] from Figure 2 As can be seen from this, the common element is: the actual value of the DC voltage U. dc The calculation is performed using the output of the PI regulator, and the result is compared with the actual value U of the MMC submodule voltage. SMav effect.

[0077] The above embodiments provide a unified, compatible, and efficient structure by adjusting different parameters (DC voltage control parameter k) of the controller in the three branch structures. DC Active power control parameter k p and submodule voltage control parameter k SM By designing the values ​​of ), different combinations of control objectives can be achieved, thereby realizing more diversified control modes.

[0078] Following the unified grid-type control structure described in the above embodiments, control characteristic analysis is performed. By adjusting the parameter values ​​of the parameter controller, the entire control structure can achieve different control strategies. That is, under different control strategies, the MMC converter station exhibits different control characteristics. Therefore, in a preferred embodiment of the present invention, the parameter controller achieves different converter station control characteristics by setting different unified grid-type control parameters. Specifically:

[0079] When k DC When the value is not 0, the MMC main converter station can achieve DC voltage control;

[0080] When k p When the value is not 0, the MMC can achieve active power control from the converter station;

[0081] When k SMWhen the value is not 0, the islanded system can autonomously establish voltage and frequency.

[0082] In other words, the control characteristics of different converter stations include: when DC voltage control is used, the control characteristic is that the DC voltage is at a stable value; when active power control is used, the control characteristic is that the active power is at a stable value; when constant frequency control is used, the system is in an islanded operation state, and the control characteristic is that the system can autonomously establish voltage and frequency.

[0083] In another preferred embodiment, the control parameters of the parameter controller are combined to enable the MMC converter station to possess combined control characteristics of DC voltage-active power droop control, DC voltage-frequency droop control, and active power-frequency droop control. Specifically, as shown... Figure 3 As shown:

[0084] When k DC and k p When the value is not zero, the entire unified network control structure can achieve DC voltage-active power droop control, giving the MMC converter station DC voltage-active power droop control characteristics. This control is typically applied to multiple receiver MMC stations, helping them to coordinately control the DC voltage and autonomously allocate active power.

[0085] k DC (U dcref -U dc )+k p (P ref -P ac )=0 (1)

[0086] In the formula k DC k represents the DC voltage control parameter. p U represents the active power control parameter. dcref U represents the rated value of DC voltage. dc P represents the actual value of the DC voltage. ref P represents the rated value of active power. ac This represents the actual value of the active power on the AC side.

[0087] When k p and k SM When the value is not zero, the entire unified grid-type control structure can achieve droop control of active power and frequency, enabling MMC converter stations to have droop control of both active power and frequency. This control can be applied to slave MMC stations at the receiving end. Compared to controlling only active power, it can provide frequency support for the power grid.

[0088] k SM (U SMav -U SM0 )+k p (Pre fP ac )=0 (2)

[0089] In the formula k SM U represents the voltage control parameters of the submodule. SMav U represents the actual average value of the submodule voltage. SM0 k represents the rated average voltage of the submodule. p P represents the active power control parameter. ref P represents the rated value of active power. ac This represents the actual value of the active power on the AC side.

[0090] By drawing an analogy between the dynamic characteristics of MMC and the dynamic characteristics of synchronous generators, we can conclude that:

[0091]

[0092] Substituting (3) into (2), we get:

[0093]

[0094] In the formula, ω0 represents the rated value of the AC frequency, ω MMC This represents the actual value of the AC frequency of the converter station, and K represents the response coefficient of the submodule voltage to changes in the AC frequency.

[0095] As shown in (4), when the system frequency changes, the MMC converter will adjust the output value P. ac To adapt to system frequency deviations, this characteristic is similar to the primary frequency regulation characteristic of a synchronous generator, which can provide frequency support for the power grid.

[0096] When k DC and k SM When the voltage is not equal to zero, the entire unified grid-type control structure can achieve DC voltage-frequency droop control, giving the MMC converter station droop control characteristics for both active power and frequency. This droop control provides inertial support by adjusting the DC voltage to track the grid frequency. It can be applied to the main MMC station at the receiving end.

[0097] k DC (U dcref -U dc )+k SM (U SMav -U SM0 )=0 (5)

[0098] Analysis of equation (5) reveals a linear relationship between system frequency and active power output. By adjusting the output value P... ac It can adapt to system frequency deviations, providing frequency support for the power grid. This characteristic is similar to the primary frequency regulation characteristic of a synchronous generator.

[0099] Substituting (3) into (5) yields:

[0100]

[0101] As shown in (6), this control strategy can track grid frequency changes by adjusting the DC voltage and provide inertial support by extracting energy stored in the DC capacitor to adapt to frequency changes. Specifically, extracting energy stored in the DC capacitor means that the energy stored in the DC capacitor is related to the submodule voltage, i.e. As shown in equation (4), the submodule voltage is proportional to the grid frequency. When the grid frequency decreases, the submodule voltage decreases accordingly, and the energy stored in the submodule capacitor decreases accordingly. Therefore, the energy stored in the DC capacitor is extracted to increase the AC output power and provide inertial support. Tracking the grid frequency change means that when the grid frequency changes, ω0-ω MMC The DC voltage U is not zero. dc The magnitude will change, meaning the DC voltage tracks changes in the grid frequency. Adjusting the DC voltage: The system DC voltage is obtained by summing the voltages of individual submodules. When changes in system frequency affect the submodule voltages, the overall system DC voltage will also change. The DC voltage deviation should not exceed the converter's rated deviation value; therefore, this is achieved by adjusting the control parameter k. DC k SM It can stabilize the DC voltage within a reasonable range while still reflecting changes in the system frequency.

[0102] Furthermore, without the need for communication, other MMC stations can sense changes in grid frequency by detecting variations in DC voltage and further notify renewable energy bases. Power plants and other MMC stations can then adjust active power flow to adapt to frequency changes and provide active power support.

[0103] In the above embodiments, the converter station operation follows the existing model, still divided into grid-connected operation and islanded operation. The control mode for grid-connected operation can be further divided into master-slave control and droop control. In islanded operation, the converter station's control characteristic is the autonomous establishment of voltage and frequency. In grid-connected master-slave operation, the control characteristic for the master converter station is controlling DC voltage, and for the slave converter station, it is controlling active power. In grid-connected droop control, the control characteristic is the autonomous allocation of active power among converter stations. If master-slave control and droop control are frequency-mapped, the control characteristics are DC voltage-frequency droop control, active power-frequency droop control, or DC voltage-active power-frequency droop control.

[0104] Furthermore, in yet another preferred embodiment of the invention, k is employed. DC k p and k SMSince none of the values ​​are zero, the MMC converter station can operate with droop control in three control modes: DC voltage, active power, and frequency. In this case, the MMC converter station possesses all the characteristics of the three droop control methods mentioned above.

[0105] k DC (U dcref -U dc )+k p (P ref -P ac )+k SM (U SMav -U SM0 )=0 (7)

[0106] When the power grid is operating stably, the grid frequency remains at its rated value. In this case, the control mode mainly relies on DC voltage-active power droop control. When a grid fault causes a change in the grid frequency, frequency droop control comes into play, enabling the DC voltage and active power to track the changes in grid frequency, thereby providing frequency support to the grid.

[0107] In the above embodiments, the proposed unified grid-type control structure can achieve flexible switching of different control objectives without changing the control structure, and achieve seamless switching of different control modes, which can enhance the stability of the MMC-MTDC transmission system during grid-connected operation.

[0108] Of course, for the above control parameters (DC voltage control parameter k) DC Active power control parameter k p and submodule voltage control parameter k SM The selection and adjustment of parameters require a suitable range; otherwise, the entire system will be in an abnormal operating state. Therefore, based on the same inventive concept, other embodiments of this invention provide a method for determining the control parameters of a multi-terminal flexible-DC unified network control structure. By analyzing the dynamic characteristics of the MMC converter station in the above embodiments and combining different droop control characteristics, the range of system control parameters under network operation and islanded operation conditions is derived, specifically:

[0109] Under network operation, the system adopts DC voltage-active power droop control, and the value range of DC voltage coefficient and active power coefficient is derived based on the DC voltage deviation range.

[0110] In islanded operation, the system adopts DC voltage-frequency droop control, and the range of values ​​for the frequency coefficient is derived based on the islanded frequency fluctuation range.

[0111] The above embodiments provide a selection range for the control parameters, facilitating parameter value selection. When the converter station switches between different operating conditions, selecting values ​​within the specified range makes it easier for the system to stabilize more quickly. Furthermore, a defined value range allows the system to automatically adjust the parameter range.

[0112] In a preferred embodiment, substituting (3) into (7) yields:

[0113]

[0114] In the formula ΔP g Indicates power fluctuation, ΔU dc Indicates DC voltage fluctuation, Δω g H represents the system frequency fluctuation, and H represents the submodule transfer coefficient.

[0115] Equation (8) shows the power variation of a single converter station. For the entire system, the varying power can be superimposed to obtain:

[0116]

[0117] When the system is running stably, the rate of change of the system frequency is 0, i.e., Δω gi =0, at which point the system can be simplified to:

[0118]

[0119] For networked systems, the AC output power is an uncertain value, the DC voltage is often set as a control variable, and the fluctuation range of the DC voltage should be less than 5%. Therefore, (10) can be simplified to:

[0120]

[0121] In the formula P WF_max P represents the maximum output of the wind farm. WF_min This indicates the minimum output of the wind field.

[0122] At this point, k can be reasonably set by adjusting the power distribution ratio between the converter stations on each grid side. DC k p The value of .

[0123] In another preferred embodiment, for an islanded system, all power will flow into the system. In this case, the AC power is a known quantity, and the main concern is the frequency fluctuation within the system. Therefore, equation (8) can be simplified to:

[0124]

[0125] The power flowing into the islanded system is the rated power, the power change is 0, and the frequency fluctuation should be less than 5%. Therefore, equation (12) can be simplified to:

[0126]

[0127] In the formula U dc_max U represents the maximum DC voltage. dc_min This indicates the minimum DC voltage.

[0128] At this point, the islanded system is in DC voltage-frequency droop control mode. By appropriately setting k... DC and k SM To establish and stabilize the DC voltage and frequency of the system.

[0129] To verify the feasibility and effectiveness of the multi-terminal flexible-vertical unified mesh control structure and parameter value method described in the above embodiments, a system was established in PSCAD / EMTDC as follows: Figure 4 The simulation model of the system is shown, and the system using the structure and method of the embodiments of the present invention was simulated as follows:

[0130] 1. Simulation Case 1: Research on DC Voltage-Active Power Salient Control Strategy under Unified Network Control

[0131] Initially, the system is in a stable operating state. Converter stations REC2 and REC4 use DC voltage-active power droop control, while converter stations REC3 and REC5 use active power control mode, with a rated power of 100MW. At t=5s, the output power of the sending-end converter station decreases from 500MW to 400MW.

[0132] Depend on Figure 5 and Figure 6 It can be seen that the active power of REC3 and REC5 remains unchanged, while the power of REC2 and REC4 will change to adapt to the reduction in the output power of the sending-end converter station. Meanwhile, the DC voltage will remain stable within a reasonable range. This demonstrates the DC voltage-active power droop characteristic.

[0133] 2. Simulation Case 2: Research on Active Power-Frequency Droop Control Strategy under Unified Network Control

[0134] In simulation case 2, all five converter stations employ a unified grid-type control system under three control strategies. Initially, the system operates stably. At t = 5s, the frequency of grid 3 fluctuates, decreasing from 50Hz to 49.7Hz.

[0135] Depend on Figure 7 It can be seen that when the frequency of grid 3 decreases, the power flow direction of the system will change. The power flowing into REC3 increases, while the power flowing into REC2, REC4 and REC5 decreases. This shows that more power will flow into grid 3 to provide grid frequency support, which reflects the active power-frequency droop characteristic.

[0136] 3. Simulation Case 3: Research on DC Voltage-Frequency Droop Control Strategy under Unified Network Control

[0137] Based on simulation case 2, we explore the impact of frequency fluctuations on DC voltage.

[0138] Depend on Figure 8 It can be seen that when the frequency fluctuates, the submodule voltage and DC voltage will change with the frequency fluctuation, which reflects the DC voltage-frequency droop characteristic.

[0139] 4. Simulation Case 4: Research on Network-to-Islanding Strategy under Unified Network Control Structure

[0140] In simulation case 4, the system is initially in a stable operating state. At t=5s, the sending-end converter station SEN1 will switch from grid-connected operation to islanded operation, and the active power will decrease from 500MW to 400MW.

[0141] Depend on Figure 9 It is known that when the system transitions from a networked to an islanded system, the system will quickly adapt to changes in active power, and the system frequency and AC voltage will change accordingly. The AC voltage will stabilize rapidly after fluctuations, and the frequency will also stabilize as the active power stabilizes. At this point, the system can autonomously establish its voltage and frequency.

[0142] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention. The above preferred features can be used in any combination without conflict.

Claims

1. A multi-terminal flexible DC unified grid control structure for controlling a multi-terminal flexible DC transmission system, characterized in that, It includes three branch structures and one main branch structure. The main branch structure includes common elements of the three control structures. The three branch structures correspond one-to-one with the three control structures. Each branch structure includes the remaining elements of the corresponding control structure after removing the common elements and a parameter controller. Among them, the three control structures include: the control structure when the MMC converter station at the receiving end adopts DC voltage control, the control structure when the MMC converter station at the receiving end adopts active power control, and the control structure for voltage and frequency when the MMC converter station at the transmitting end is connected to an islanded system and islanding needs to be established. The MMC converter station, under the parameter combination of the parameter controller, has one or more of the following combined control characteristics: -When DC voltage control parameters k DC and active power control parameters k p When the value is not 0, the MMC converter station has DC voltage-active power droop control characteristics: （1） -When active power control parameters k p and sub-module voltage control parameters k SM When the value is not 0, the MMC converter station has active power-frequency droop control characteristics: （2） -When DC voltage control parameters k DC and sub-module voltage control parameters k SM When the value is not 0, the MMC converter station has a DC voltage-frequency droop control characteristic: （3） k DC Indicates DC voltage control parameters. k p This represents the active power control parameters. k SM This indicates the voltage control parameters of the submodule. U dcref Indicates the rated DC voltage. U dc This represents the actual value of the DC voltage. P ref Indicates the rated active power. P ac This represents the actual value of active power. U SM0 Indicates the rated voltage of the submodule. U SMav This indicates the actual value of the submodule voltage; The MMC converter station, under the active power-frequency droop control characteristics, can provide power support to the grid, specifically as follows: By drawing an analogy between the dynamic characteristics of the MMC converter station and the dynamic characteristics of a synchronous generator, we obtain: （4） Substituting (4) into (2) yields: （5） In the formula Indicates the rated value of the AC frequency. This represents the actual value of the AC frequency of the converter station. K This represents the response coefficient of the submodule voltage to changes in AC frequency; Based on equation (5), it is determined that when the system frequency changes, the MMC converter adjusts its output value. P ac To adapt to system frequency deviations, this adjustment is made to the primary frequency regulation characteristics of the equivalent synchronous generator, providing frequency support for the power grid.

2. The multi-terminal flexible straight unified network control structure according to claim 1, characterized in that, The parameter controller, by adjusting the values ​​of the control parameters, executes one or more of the following control strategies: -When DC voltage control parameters k DC When the value is not 0, the main converter station of the MMC converter station achieves DC voltage control; -When active power control parameters k p When the value is not 0, the slave converter station of the MMC converter station realizes active power control; -When the submodule voltage control parameters k SM When the value is not 0, the islanded system autonomously establishes the voltage and frequency.

3. The multi-terminal flexible straight unified network control structure according to claim 2, characterized in that, The MMC converter station, under different control parameters, has one or more of the following converter station control characteristics: - When DC voltage control is used, the control characteristic is that the DC voltage is at a stable value; - When active power control is used, the control characteristic is that the active power is at a stable value; - When using fixed frequency control, the modular multi-level multi-terminal high-voltage DC transmission system operates in an islanded state, and the control characteristics are that the system can autonomously establish voltage and frequency.

4. The multi-terminal flexible straight unified network control structure according to claim 1, characterized in that, The MMC converter station, under the droop control characteristics of the DC voltage-frequency, provides inertial support to the grid by adjusting the DC voltage to track the grid frequency, specifically as follows: Substituting (4) into (3) yields: (6) Based on equation (6), the DC voltage-frequency droop control provides inertial support to adapt to frequency changes by adjusting the DC voltage, tracking grid frequency changes, and extracting energy stored in the DC capacitor.

5. The multi-terminal flexible straight unified network control structure according to claim 1, characterized in that, The MMC converter station k DC , k p and k SM When all values ​​are non-zero, the control structure will operate with droop control in the form of DC voltage-active power-frequency droop control. The MMC converter station has all the characteristics of the three droop control modes, specifically: (7) When the power grid is in a stable operating state, the power grid frequency remains at the rated value, and the control mode is DC voltage-active power droop control. When a grid fault causes a change in the grid frequency, frequency droop control is initiated to enable DC voltage and active power to track the changes in grid frequency, thus providing frequency support to the grid.

6. A method for determining control parameters in a multi-terminal flexible-vertical unified network control structure, characterized in that, include: Under grid-connected operation, the multi-terminal flexible DC transmission system adopts DC voltage-active power droop control, and the value range of DC voltage coefficient and active power coefficient is derived based on the DC voltage deviation range. Under islanded operation, the multi-terminal flexible DC transmission system adopts DC voltage-frequency droop control, and the value range of the frequency coefficient is derived based on the islanded frequency fluctuation range. The feature is that, under the network operation, the multi-terminal flexible DC transmission system adopts DC voltage-active power droop control, and derives the value ranges of the DC voltage coefficient and active power coefficient based on the DC voltage deviation range, including: Substituting (4) into (7) gives: (8) Equation (8) shows the power variation of a single converter station; In the formula Indicates power fluctuation, k DC Indicates DC voltage control parameters. k p This represents the active power control parameters. Indicates DC voltage fluctuation. H Indicates the coefficients passed from the submodule. k SM This indicates the voltage control parameters of the submodule. Indicates system frequency fluctuation. U SM0 Indicates the rated voltage of the submodule. Indicates the rated value of the AC frequency. K This represents the response coefficient of the submodule voltage to changes in AC frequency; For the entire multi-terminal flexible DC transmission system, by superimposing the varying power of all converter stations, we obtain: (9) When the system is running stably, the system frequency change rate is 0, that is... At this point, the system simplifies to: (10) For networked systems, the AC output power is an uncertain value, the DC voltage is often set as a control variable, and the fluctuation range of the DC voltage is less than a%. Equation (10) can be rearranged as follows: (11) In the formula P WF_max Indicates the maximum output of the wind farm. P WF_min This indicates the minimum output of the wind farm; Based on equation (11), the power allocation ratio between each grid-side converter station is set. k DC , k p The value of .

7. The method for determining control parameters of a multi-terminal flexible-vertical unified network control structure according to claim 6, characterized in that, Under islanded operation, the multi-terminal flexible DC transmission system adopts DC voltage-frequency droop control. The range of frequency coefficient values ​​is derived based on the islanded frequency fluctuation range, including: For an islanded system, all power will flow into the system. At this time, the AC power is a known quantity, and we need to pay attention to the frequency fluctuations within the system. Equation (8) can be rearranged as follows: (12) The power flowing into the islanded system is the rated power, the power change is 0, and the frequency fluctuation is less than a%. Equation (12) can be rearranged as follows: (13) In the formula U dc_max This indicates the maximum DC voltage. U dc_min Indicates the minimum DC voltage; At this time, the islanded system is in DC voltage-frequency droop control mode, by setting... k DC and k SM Establish and stabilize the DC voltage and frequency of the system.

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