Multi-terminal flexible direct-current unified construction network type control structure and parameter valuing method

Abstract

The invention provides a multi-terminal flexible direct-current unified construction network type control structure and a parameter valuing method, the multi-terminal flexible direct-current unified construction network type control structure comprises three branch structures and a main branch structure, the main branch structure comprises common elements of three control structures, the three branch structures are in one-to-one correspondence with the three control structures, and the main branch structure is in one-to-one correspondence with the three control structures. Each branch structure comprises residual elements of the control structure corresponding to the branch structure except the common elements and a parameter controller; wherein the three control structures comprise a control structure when the MMC at the receiving end adopts direct current voltage control, a control structure when the MMC at the receiving end adopts active power control, and a control structure when the MMC is connected to an island system and voltage and frequency of an island need to be established. According to the invention, stable control and seamless mode switching of the MMC-MTDC system under different operation conditions can be realized.

Description

Technical Field

[0001] The present invention relates to the field of power grid technology, and in particular to a multi-terminal flexible direct current unified networking control structure and a parameter value taking method. Background Art

[0002] An important trend is that most onshore renewable energy bases are located in remote areas, far away from load centers, while offshore wind power bases continue to expand to deep sea areas. This geographical distribution has led to a significant increase in the distance between renewable energy power plants and the power grid, bringing unprecedented challenges to power transmission.

[0003] In this case, the multi-terminal high-voltage direct current (MMC-MTDC) transmission technology based on modular multilevel converters (MMC) is an effective way to connect renewable energy bases with the receiving AC power grid. This technology achieves the effective connection between the renewable energy base and the receiving AC power grid through the efficient conversion capability of MMC, significantly improving the flexibility and reliability of power transmission. The MMC-MTDC system not only has the advantages of large transmission capacity, low loss, and less line corridor occupancy, but also can effectively cope with the intermittent and uncertain nature of renewable energy generation, providing strong support for large-scale renewable energy grid connection.

[0004] However, with the continuous growth of renewable energy penetration, the stability problem of the traditional grid-connected control strategy of the MMC-MTDC system has become increasingly prominent. At the transmission end, when the MMC is connected to a weak grid with a relatively low short circuit, the grid synchronization technology based on the phase-locked loop is prone to cause system oscillation, affecting the stability of power transmission. At the receiving end, the inertia-free characteristics of the MMC lead to a decrease in the frequency stability of the grid, further exacerbating the stability risk of the system.

[0005] To solve the above problems, the grid-based control strategy has become a possible effective approach. Grid-based control simulates the operating characteristics of traditional synchronous generators to provide virtual inertia and damping for MMC, thereby enhancing the stability and frequency response capability of the system. However, there are many types of grid-based control strategies for MMC in MTDC systems, including Vf control, virtual synchronous generator (VSG) control, and power synchronization control (PSC). These strategies have significant differences in control structure, parameter setting, and dynamic response, making unified design and seamless mode switching extremely difficult.

[0006] During the system operation transition, such as in the event of a converter station failure or line interruption, the need for control mode change is particularly urgent. However, due to the incompatibility and differences between current network control strategies, control mode change may further aggravate system stability issues. Summary of the invention

[0007] In view of the defects in the prior art, the purpose of the present invention is to provide a multi-terminal flexible direct current unified networking control structure and parameter value selection method.

[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, wherein the main branch structure comprises common elements of the three control structures, the three branch structures correspond one to one with the three control structures, and each branch structure comprises 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 when the MMC converter station at the sending end is connected to the island system and needs to establish the voltage and frequency of the island.

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

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

[0012] -When the active power control parameter k p When it 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 it is not 0, the island system establishes voltage and frequency autonomously.

[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 adopted, the control characteristic is that the DC voltage is at a stable value;

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

[0017] -When fixed frequency control is adopted, the multi-terminal HVDC transmission system using modular multi-level is in an island operation state, and the control characteristic is that the system can autonomously establish voltage and frequency.

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

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

[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 it is not 0, the MMC converter station has the active power-frequency droop control characteristic:

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

[0023] -When the DC voltage control parameter k DC and submodule voltage control parameter k SM When it 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 represents the DC voltage control parameter, k p represents the active power control parameter, k SM Indicates the submodule voltage control parameter, U dcref Indicates the DC voltage rating, U dc Indicates the actual value of DC voltage, P ref Indicates the active power rating, P ac Indicates the actual value of active power, U SM0 Indicates the rated voltage of the submodule, U SMav Indicates the actual value of the submodule voltage.

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

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

[0028]

[0029] Substituting (4) into (2) we obtain:

[0030]

[0031] Where ω0 represents the rated value of the AC frequency, ω MMC represents the actual value of the AC frequency of the converter station, and K represents the reaction coefficient of the submodule voltage to the change of the AC frequency;

[0032] Based on formula (5), when the system frequency changes, the MMC converter adjusts the output value P ac To adapt to system frequency deviation, the adjustment is equivalent to the primary frequency regulation characteristics of the synchronous generator to provide frequency support for the power grid.

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

[0034] Substituting (4) into (3) we obtain:

[0035]

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

[0037] Preferably, the MMC converter station is at k DC , k p and k SM When all are not 0, the control structure will operate in the droop control form of DC voltage-active power-frequency droop control, and 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 grid is in a stable operating state, the grid frequency is maintained at the rated value, and the control mode is DC voltage-active power droop control;

[0040] When the grid frequency changes due to a grid fault, frequency droop control is initiated to enable the DC voltage and active power to track the changes in the grid frequency and provide frequency support for the grid.

[0041] According to a second aspect of the present invention, a method for obtaining control parameters of a multi-terminal flexible direct current unified networking control structure is provided, comprising:

[0042] Under interconnected 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 according to the DC voltage deviation range;

[0043] Under island operation, the multi-terminal flexible DC transmission system adopts DC voltage-frequency droop control, and the value range of the frequency coefficient is derived according to the island frequency fluctuation range.

[0044] Preferably, in the interconnected operation, the multi-terminal flexible DC power transmission system adopts DC voltage-active power droop control, and the value ranges of DC voltage coefficient and active power coefficient are derived according to the DC voltage deviation range, including:

[0045] Substituting (4) into (7), we obtain:

[0046]

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

[0048] For the entire multi-terminal flexible DC transmission system, the variable power of all converter stations is superimposed to obtain:

[0049]

[0050] When the system is running stably, the system frequency change rate is 0, that is, Δω gi =0, the system is simplified to:

[0051]

[0052] For the interconnected system, the output power on the AC side is an uncertain value, the DC voltage is often set as the control variable, and the fluctuation range of the DC voltage is less than a%. Formula (10) is organized as:

[0053]

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

[0055] Preferably, in the island operation, the multi-terminal flexible DC power transmission system adopts DC voltage-frequency droop control, and the value range of the frequency coefficient is derived according to the island frequency fluctuation range, including:

[0056] For an island system, all power will flow into the system. At this time, the AC side power is a known quantity, and the frequency fluctuation in the system needs to be paid attention to. Formula (8) can be summarized as follows:

[0057]

[0058] The power flowing into the island system is the rated power, the power change is 0, and the frequency fluctuation is less than a%. Formula (12) is organized as:

[0059]

[0060] At this time, the island system is in the DC voltage-frequency droop control mode. By setting k DC and k SM Establish and stabilize the system's DC voltage and frequency.

[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 direct current unified meshing control structure in the embodiment of the present invention is a unified, compatible and efficient structure, which realizes stable control and seamless mode switching of the MMC-MTDC system under different operating conditions. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0064] Figure 1 It is a schematic diagram of a DC side control scheme under different control objectives in the prior art;

[0065] Figure 2 A unified networking control structure diagram in one embodiment of the present invention;

[0066] Figure 3 A schematic diagram of control characteristic analysis of a unified meshed control structure in a preferred embodiment of the present invention;

[0067] Figure 4 A multi-terminal flexible direct current transmission grid-connected system for simulation experiment in a specific embodiment of the present invention;

[0068] Figure 5 A data diagram of the output power of each converter station in a simulation experiment in a specific embodiment of the present invention;

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

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

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

[0072] Fig. 9 Parameters of the SEN1 converter station in a specific embodiment of the present invention. DETAILED DESCRIPTION

[0073] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several variations and improvements may be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0074] In the MT-HVDC (modular multi-level high-voltage direct current transmission) system, the current grid-type control strategies are mostly designed for specific control objectives. Specifically, when the MMC converter station at the receiving end adopts DC voltage control, Figure 1 When the MMC converter station at the receiving end adopts active power control, the control structure is shown in Figure (a). Figure 1 When the MMC is connected to an island system, the voltage and frequency of the island need to be established. Figure 1 The control structure shown in Figure (c). Figure 1 It can be seen that Figure 1 The control structure shown in Figure (a) has a DC voltage rating of U dcref and the actual value of the DC voltage U dc After calculation, it is input into the PI regulator, and the actual value of the DC voltage U dc The calculation result is calculated with the output of the PI regulator and the actual value of the submodule voltage U SMav effect. Figure 1 The control structure shown in Figure (b) has an active power rating of P ref And the actual value of active power P ac After calculation, it is input into the PI regulator, and the actual value of the DC voltage U dc The calculation result is calculated with the output of the PI regulator and the actual value of the submodule voltage U SMav effect. Figure 1The control structure shown in Figure (c) has a submodule voltage rating of U SM0 and the actual value of the submodule voltage U SMav After calculation, it is input into the PI regulator, and the actual value of the DC voltage U dc The calculation result is calculated with the output of the PI regulator and the actual value of the submodule voltage U SMav The above three control structures do not have 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 direct current 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 to 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 It can be seen that the common elements are: the actual value of the DC voltage U dc The calculation result is calculated with the output of the PI regulator and the actual value of the submodule voltage U SMav effect.

[0077] The above embodiment provides a unified, compatible and efficient structure, by adjusting the different parameters of the controllers of the three branch structures (DC voltage control parameter k DC , active power control parameter k p and submodule voltage control parameter k SM ) can be designed to achieve a combination of different control objectives, thereby achieving more diversified control modes.

[0078] The unified meshing control structure in the above embodiment is used to analyze the control characteristics. By adjusting the values ​​of the parameters of the above parameter controller, the entire control structure can implement different control strategies. That is to say, under different control strategies, the MMC converter station has different control characteristics. Therefore, in a preferred embodiment of the present invention, the parameter controller realizes different converter station control characteristics by setting different unified meshing control parameters. Specifically:

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

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

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

[0082] That is to say, different converter station control characteristics include: when DC voltage control is adopted, the control characteristic is that the DC voltage is at a stable value; when active power control is adopted, the control characteristic is that the active power is at a stable value; when fixed frequency control is adopted, the system is in an island operation state, and the control characteristic is that the system can independently establish voltage and frequency.

[0083] In another preferred embodiment, the control parameters of the parameter controller are combined so that the MMC converter station has a combination control characteristic of DC voltage-active power droop control, DC voltage-frequency droop control, and active power-frequency droop control. Specifically, Figure 3 As shown:

[0084] When k DC and k p When it is not 0, the entire unified mesh control structure can realize DC voltage-active power droop control, so that the MMC converter station has DC voltage-active power droop control characteristics. This control is usually applied to multiple receiving-end MMC stations, which helps the receiving-end MMC stations to coordinately control the DC voltage and autonomously distribute active power.

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

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

[0087] When k p and k SM When it is not 0, the entire unified grid control structure can realize the droop control of active power and frequency, so that the MMC converter station has the droop control of active power and frequency. This control can be applied to the slave MMC station at the receiving end. Compared with only controlling active power, it can provide frequency support for the power grid.

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

[0089] Where k SM Indicates the submodule voltage control parameter, U SMav Indicates the actual average value of the submodule voltage, U SM0 Indicates the rated average value of the submodule voltage, k p Represents the active power control parameter, P ref Indicates the rated value of active power, P ac Indicates the actual value of the active power on the AC side.

[0090] By analogy between the dynamic characteristics of MMC and synchronous generator, we can get:

[0091]

[0092] Substituting (3) into (2) yields:

[0093]

[0094] Where ω0 represents the rated value of the AC frequency, ω MMC It represents the actual value of the AC frequency of the converter station, and K represents the reaction coefficient of the submodule voltage to the change of AC frequency.

[0095] From (4), we can see that when the system frequency changes, the MMC converter station will adjust the output value P ac To adapt to system frequency deviation, this characteristic is similar to the primary frequency regulation characteristic of a synchronous generator and can provide frequency support for the power grid.

[0096] When k DC and k SM When it is not equal to 0, the entire unified grid control structure can realize the droop control of DC voltage-frequency, so that the MMC converter station has the droop control characteristics of 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 master MMC station at the receiving end.

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

[0098] By analyzing formula (5), it can be found that the system frequency has a linear relationship with the active output. By adjusting the output value P ac It can adapt to system frequency deviation and provide frequency support for the power grid. This feature is similar to the primary frequency regulation feature of synchronous generators.

[0099] Substituting (3) into (5) we get:

[0100]

[0101] From (6), it can be seen that the control strategy can adjust the DC voltage to track the grid frequency change and provide inertial support by extracting the energy stored in the DC capacitor to adapt to the frequency change. The energy stored in the DC capacitor is extracted to specifically refer to: the energy stored in the DC capacitor is related to the submodule voltage, that is, As can be seen from formula (4), the submodule voltage is proportional to the grid frequency. When the grid frequency decreases, the submodule voltage will decrease accordingly, and the energy stored in the submodule capacitor will decrease accordingly. Therefore, the energy stored in the DC capacitor will be extracted to increase the output power on the AC side and provide inertial support. Tracking grid frequency changes means: when the grid frequency changes, ω0-ω MMC Not 0, DC voltage U dc The size will change, and this change is the DC voltage tracking the grid frequency change. Adjusting DC voltage: The system DC voltage is obtained by summing up the voltages of individual submodules. When the system frequency changes and affects the size of the submodule voltage, the size of the overall system DC voltage will also change. The deviation of the DC voltage should not exceed the rated deviation value of the converter, so by adjusting the control parameter k DC , k SM Make the DC voltage stable within a reasonable range while reflecting system frequency changes.

[0102] In addition, without communication, other MMC stations can sense changes in grid frequency by sensing changes in DC voltage and further inform the renewable energy base. The power plant and other MMC stations can then adjust the active power flow to adapt to the frequency change and provide active power support.

[0103] In the above embodiments, the operation of the converter station is still divided into networked operation and island operation according to the existing mode, and the control mode of networked operation can be divided into master-slave control and droop control. The control characteristic of the converter station during island operation is to autonomously establish voltage and frequency. In the master-slave operation under networked conditions, the control characteristic for the master converter station is to control the DC voltage, and the control characteristic for the slave converter station is to control the active power. In the droop control under networked conditions, the control characteristic is the autonomous distribution of active power between converter stations. If the master-slave control and the droop control are frequency mapped, the control characteristic is DC voltage-frequency droop control, active power-frequency droop control, or DC voltage-active power-frequency droop control.

[0104] In addition, in another preferred embodiment of the invention, k DC , k p and k SMAll of them are not 0, and the MMC converter station can operate in the droop control of three control modes: DC voltage, active power and frequency. In this case, the MMC converter station has all the characteristics of the above three droop control methods.

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

[0106] When the grid is in a stable operating state, the grid frequency is maintained at the rated value. At this time, the control mode is mainly based on DC voltage-active power droop control. When the grid frequency changes due to a grid fault, the frequency droop control starts to work, allowing the DC voltage and active power to track the changes in the grid frequency, thereby providing frequency support for the grid.

[0107] In the above embodiments, the proposed unified grid-type control structure can realize flexible switching of different control objectives without changing the control structure, realize seamless switching of different control modes, and 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 ) selection and adjustment require an applicable range, otherwise the entire system will be in an abnormal operating state. Therefore, based on the same inventive concept, in other embodiments of the present invention, a control parameter selection method for a multi-terminal flexible DC unified grid control structure is provided, and the dynamic characteristics of the MMC converter station in the above embodiment are analyzed, and combined with different droop control characteristics, the value range of the system control parameters under the networked operation and islanded operation conditions is derived, specifically:

[0109] Under networked operation, the system adopts DC voltage-active power droop control, and the value range of DC voltage coefficient and active power coefficient is derived according to the DC voltage deviation range;

[0110] Under island operation, the system adopts DC voltage-frequency droop control, and the value range of the frequency coefficient is derived according to the island frequency fluctuation range.

[0111] The above embodiment can obtain the selection range of the control parameters, which is convenient for parameter selection. When the converter station performs conversion under different operating conditions, it is easy to make the system faster and more stable by selecting values ​​within the value range. At the same time, after the value range is defined, it is convenient for the system to automatically adjust the parameter range.

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

[0113]

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

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

[0116]

[0117] When the system is running stably, the system frequency change rate is 0, that is, Δω gi =0, then the system can be simplified as:

[0118]

[0119] For the interconnected system, the output power on the AC side is an uncertain value, the DC voltage is often set as the control variable, and the fluctuation range of the DC voltage should be less than 5%, so (10) can be transformed into:

[0120]

[0121] Where P WF_max Indicates the maximum output of the wind farm, P WF_min Indicates the minimum output of the wind farm.

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

[0123] In another preferred embodiment, for an island system, all power will flow into the system. At this time, the AC side power is a known quantity, and the main concern is the frequency fluctuation in the system. Therefore, equation (8) can be transformed into:

[0124]

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

[0126]

[0127] Where U dc_max Indicates the maximum DC voltage, U dc_min Indicates the minimum DC voltage.

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

[0129] In order to verify the feasibility and effect of the multi-terminal flexible DC unified network control structure and parameter selection method in the above embodiment, the following is established in PSCAD / EMTDC: Figure 4 The simulation model of the system shown in FIG. 1 is used, and the system using the structure and method of the embodiment of the present invention is simulated as follows:

[0130] 1. Simulation Case 1: Study on DC voltage-active power droop control strategy under unified grid control

[0131] Initially, the system is in a stable operating state, converter stations REC2 and REC4 use DC voltage-active power droop control, converter stations REC3 and REC5 use active power control mode, and the rated power is 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 of the output power of the sending-end converter station. At the same time, the DC voltage will remain stable within a reasonable range. This reflects the DC voltage-active power droop characteristic.

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

[0134] In simulation case 2, all five converter stations adopt unified grid control under three control strategies. Initially, the system operates stably. At t=5s, the frequency of power 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 will increase, and the power flowing into REC2, REC4 and REC5 will decrease, which shows that more power will flow into grid 3 to provide grid frequency support, which reflects the active power-frequency droop characteristics.

[0136] 3. Simulation Case 3: Study on DC voltage-frequency droop control strategy under unified grid control

[0137] Based on simulation case 2, the influence of frequency fluctuation on DC voltage is explored.

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

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

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

[0141] Depend on Fig. 9 It can be seen that when the system changes from networking to islanding, the system will quickly adapt to the change in active power, and the system frequency and AC voltage will change accordingly. The AC voltage will quickly stabilize after the fluctuation, and the frequency will also stabilize as the active power tends to stabilize. At this time, the system can establish voltage and frequency autonomously.

[0142] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various modifications or variations within the scope of the claims, which does not affect the essence of the present invention. The above preferred features can be used in any combination without conflicting with each other.

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 three control structures, the three branch structures correspond to the three control structures one by one, and 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 when the MMC converter station at the sending end is connected to the island system and needs to establish the voltage and frequency of the island.

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

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

4. The multi-terminal flexible DC unified networking control structure according to claim 1 is characterized in that: The MMC converter station has one or more of the following combined control features under the parameter combination of the parameter controller: -When the DC voltage control parameter k DC and active power control parameter k p When it is not 0, the MMC converter station has the DC voltage-active power droop control characteristic: k DC (U dcref -U dc )+k p (P ref -P ac )=0 (1) -When the active power control parameter k p and submodule voltage control parameter k SM When it is not 0, the MMC converter station has the active power-frequency droop control characteristic: k SM (U SMav -U SM0 )+k p (P ref -P ac )=0 (2) -When the DC voltage control parameter k DC and submodule voltage control parameter k SM When it is not 0, the MMC converter station has a DC voltage-frequency droop control characteristic: k DC (IN dcref -IN dc )+k SM (IN SMav -IN SM0 )=0 (3) k DC represents the DC voltage control parameter, k p represents the active power control parameter, k SM Indicates the submodule voltage control parameter, U dcref Indicates the DC voltage rating, U dc Indicates the actual value of DC voltage, P ref Indicates the active power rating, P ac Indicates the actual value of active power, U SM0 Indicates the rated voltage of the submodule, U SMav Indicates the actual value of the submodule voltage.

5. A multi-terminal flexible DC unified networking control structure according to claim 4, characterized in that: The MMC converter station can provide power support for the power grid under the active power-frequency droop control characteristics, specifically: By analogy between the dynamic characteristics of the MMC converter station and the dynamic characteristics of the synchronous generator, we can obtain: Substituting (4) into (2) we obtain: Where ω0 represents the rated value of the AC frequency, ω MMC represents the actual value of the AC frequency of the converter station, and K represents the reaction coefficient of the submodule voltage to the change of the AC frequency; Based on formula (5), when the system frequency changes, the MMC converter adjusts the output value P ac To adapt to system frequency deviation, the adjustment is equivalent to the primary frequency regulation characteristics of the synchronous generator to provide frequency support for the power grid.

6. A multi-terminal flexible DC unified networking control structure according to claim 5, characterized in that: The MMC converter station provides inertia support for the power grid by adjusting the DC voltage to track the power grid frequency under the DC voltage-frequency droop control characteristics, specifically: Substituting (4) into (3) we obtain: Based on formula (6), the DC voltage-frequency droop control provides inertial support by adjusting the DC voltage, tracking the grid frequency change and extracting the energy stored in the DC capacitor to adapt to the frequency change.

7. A multi-terminal flexible DC unified networking control structure according to claim 4, characterized in that: The MMC converter station is at k DC , k p and k SM When all are not 0, the control structure will operate in the droop control form of DC voltage-active power-frequency droop control, and the MMC converter station has all the characteristics of the three droop control modes, specifically: k DC (IN dcref -IN dc )+k p (P ref -P ac )+k SM (IN SMav -IN SM0 )=0 (7) When the grid is in a stable operating state, the grid frequency is maintained at the rated value, and the control mode is DC voltage-active power droop control; When the grid frequency changes due to a grid fault, frequency droop control is initiated to enable the DC voltage and active power to track the changes in the grid frequency and provide frequency support for the grid.

8. A method for obtaining control parameters of a multi-terminal flexible DC unified networking control structure, characterized in that: include: Under interconnected 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 according to the DC voltage deviation range; Under island operation, the multi-terminal flexible DC transmission system adopts DC voltage-frequency droop control, and the value range of the frequency coefficient is derived according to the island frequency fluctuation range.

9. The method for obtaining control parameters of a multi-terminal flexible DC unified networking control structure according to claim 8, characterized in that: Under the interconnected operation, the multi-terminal flexible DC transmission system adopts DC voltage-active power droop control, and the value ranges of DC voltage coefficient and active power coefficient are derived according to the DC voltage deviation range, including: Substituting (4) into (7), we obtain: Equation (8) shows the power variation of a single converter station; For the entire multi-terminal flexible DC transmission system, the variable power of all converter stations is superimposed to obtain: When the system is running stably, the system frequency change rate is 0, that is, Δω gi =0, the system is simplified to: For the interconnected system, the output power on the AC side is an uncertain value, the DC voltage is often set as the control variable, and the fluctuation range of the DC voltage is less than a%. Formula (10) is organized as: Based on formula (11), k is set according to the power allocation ratio between the grid-side converter stations. DC , k p The value of .

10. The method for obtaining control parameters of a multi-terminal flexible DC unified networking control structure according to claim 8, characterized in that: In the island operation, the multi-terminal flexible DC transmission system adopts DC voltage-frequency droop control, and the value range of the frequency coefficient is derived according to the island frequency fluctuation range, including: For an island system, all power will flow into the system. At this time, the AC side power is a known quantity, and the frequency fluctuation in the system needs to be paid attention to. Formula (8) can be summarized as follows: The power flowing into the island system is the rated power, the power change is 0, and the frequency fluctuation is less than a%. Formula (12) is organized as: At this time, the island system is in the DC voltage-frequency droop control mode. By setting k DC and k SM Establish and stabilize the system's DC voltage and frequency.

Images