A control method and device for a flexible direct current power transmission system

Abstract

This application discloses a control method and apparatus for a flexible DC transmission system, belonging to the field of flexible DC transmission technology. The control method for this flexible DC transmission system determines whether to perform primary frequency and voltage regulation during the transition from grid connection to islanding based on the current frequency and a first current AC voltage of the AC grid. When a second preset frequency condition is met, the system switches from grid connection mode to islanding mode, triggering secondary frequency and voltage regulation upon switching to islanding mode. Furthermore, during the transition from islanding to grid connection, the method determines whether to perform primary frequency and voltage regulation based on the second current AC voltage, current frequency, and current power of the AC grid to be connected, thus facilitating smooth control of the islanding-to-grid transition process after the islanding-to-grid enabling switch is activated. Therefore, smooth and reliable control of both the grid connection-to-islanding and islanding-to-grid transition processes can be achieved.

Description

Technical Field

[0001] This application relates to the field of flexible DC transmission technology, and in particular to a control method and apparatus for a flexible DC transmission system. Background Technology

[0002] Flexible DC transmission features flexible control, rapid dynamic response, and low harmonic content. It can quickly and independently adjust active and reactive power and has flexible control and coordination capabilities. It is widely regarded as one of the technical means for island power transmission, grid interconnection, reliable access to and effective utilization of new energy sources, and has broad application prospects.

[0003] However, after flexible DC transmission systems are applied to the power grid, changes in grid frequency and voltage render them ineffective as support systems, preventing them from fully realizing their performance potential. Furthermore, when the AC grid loses power, the flexible DC transmission system is prone to tripping due to significant disturbances during the transition from grid-connected to islanded operation, or overcurrent tripping during the transition from islanded to grid-connected operation. This results in poor reliability of the flexible DC transmission system's grid-connected-island switching control.

[0004] Therefore, it is necessary to provide a reliable control scheme for the islanding and switching of flexible DC transmission networks. Summary of the Invention

[0005] A control method and apparatus for a flexible DC transmission system are provided to achieve smooth and reliable control of the interconnection and islanding of flexible DC transmission networks.

[0006] Firstly, a control method for a flexible DC transmission system is provided, the method comprising:

[0007] When the flexible DC transmission system is in grid-connected mode, the primary frequency and voltage regulation of the grid-connected mode is determined based on the first preset frequency condition, the preset voltage condition, the current frequency of the current AC grid, and the first current AC voltage; wherein, the current AC grid is the AC grid connected to the flexible DC transmission system. If the current frequency meets the second preset frequency condition, the network mode will be switched to island mode, and the secondary frequency modulation function and the secondary voltage regulation function will be triggered simultaneously. In the case of the flexible DC transmission system in islanded mode, the primary frequency and voltage regulation situation of the islanded mode is determined based on the second current AC voltage, current frequency and current power of the AC grid to be connected. Deploy the island-to-network enabling pressure plate; Switch from islanded mode to networked mode, or keep islanded mode running and determine the target parameters for islanded to networked mode based on the current load parameters; The AC power grid to be connected to the grid will be switched on simultaneously.

[0008] In some embodiments, determining the primary frequency and voltage regulation of the grid connection mode based on a first preset frequency condition, a preset voltage condition, and the current frequency and first current AC voltage of the current AC power grid includes: If the current frequency meets the first preset frequency condition, a frequency modulation function is triggered, and / or if the first current AC voltage meets the preset voltage condition, a voltage regulation function is triggered.

[0009] In some embodiments, the primary frequency regulation function includes: adjusting the first active power output of the flexible DC transmission system according to the current frequency; wherein the first active power output is consistent with the first active power command issued by the dispatching authority within the primary frequency regulation dead zone.

[0010] In some embodiments, the primary voltage regulation function includes: adjusting the first reactive power output of the flexible DC transmission system according to the first current AC voltage; wherein the first reactive power output is consistent with the first reactive power command issued by the dispatching authority within the primary voltage regulation dead zone.

[0011] In some embodiments, the secondary frequency regulation function includes: adjusting the first active power command of the flexible DC transmission system according to the current active power of the current AC power grid, so that the first active power command is consistent with the current active power.

[0012] In some embodiments, the secondary voltage regulation function includes: adjusting the first reactive power command of the flexible DC transmission system according to the current reactive power of the current AC power grid, so that the first reactive power command is consistent with the current reactive power.

[0013] In some embodiments, the second preset frequency condition includes: the absolute value of the difference between the current frequency and the rated frequency of the current AC power grid is greater than the first preset frequency, or the absolute value of the difference between the current frequency and the rated frequency is greater than the second preset frequency; wherein the first preset frequency is greater than or equal to the second preset frequency.

[0014] In some embodiments, determining the primary frequency and voltage regulation of the islanding mode based on the second current AC voltage, current frequency, and current power conditions of the AC grid to be connected to includes: The primary frequency regulation function is triggered based on the active load, the second active power command, and the current frequency of the AC grid to be connected, and / or the primary voltage regulation function is triggered based on the second current AC voltage and the second reactive power command of the AC grid to be connected.

[0015] In some embodiments, after triggering the secondary frequency modulation function and the secondary voltage regulation function, the method further includes: Switch the primary frequency regulation dead zone and primary AC voltage regulation dead zone of the network mode to the primary frequency regulation dead zone and primary AC voltage regulation dead zone of the island mode. After the island-to-network enabling power supply is put into operation, the following steps are also taken: the primary frequency modulation dead zone and primary AC voltage regulation dead zone in the island mode are switched to the primary frequency modulation dead zone and primary AC voltage regulation dead zone in the island-to-network enabling power supply mode.

[0016] In some embodiments, determining the target parameters for islanding to network migration based on current load parameters includes: Determine the target parameters for islanding to network conversion based on the actual load current and rated current.

[0017] Secondly, embodiments of this application also provide a control device for a flexible DC transmission system, the device comprising: The first determining module is used to determine the primary frequency and voltage regulation situation of the interconnection mode when the flexible DC transmission system is in the grid-connected mode, based on the first preset frequency conditions, preset voltage conditions, and the current frequency and first current AC voltage of the current AC grid; wherein the current AC grid is the AC grid connected to the flexible DC transmission system. The first switching module is used to switch the network mode to the island mode when the current frequency meets the second preset frequency conditions. The trigger module is used to trigger the secondary frequency modulation function and the secondary voltage regulation function; The second determining module is used to determine the primary frequency and voltage regulation situation of the islanded mode when the flexible DC transmission system is in islanded mode, based on the second current AC voltage, current frequency and current power of the AC grid to be connected. The input module is used to input the island-to-network enable pressure plate; The second switching module is used to switch from island mode to network mode; The third determining module is used to determine the target parameters for islanding to network transfer based on the current load parameters while maintaining the islanding mode operation. The closing module is used to synchronize the AC power grid to be connected to the grid.

[0018] Beneficial Effects: This application provides a control method and apparatus for a flexible DC transmission system. The control method includes: when the flexible DC transmission system is in grid-connected mode, determining the primary frequency and voltage regulation of the grid-connected mode based on a first preset frequency condition, a preset voltage condition, and the current frequency and first current AC voltage of the current AC grid; wherein the current AC grid is the AC grid connected to the flexible DC transmission system; when the current frequency meets a second preset frequency condition, switching the grid-connected mode to islanded mode and triggering secondary frequency regulation and secondary voltage regulation functions; when the flexible DC transmission system is in islanded mode, determining the primary frequency and voltage regulation of the islanded mode based on the second current AC voltage, current frequency, and current power of the AC grid to be connected; activating the island-to-grid enabling switch; switching the islanded mode to grid-connected mode, or maintaining islanded mode operation and determining the target parameters for island-to-grid conversion based on the current load parameters; and synchronizing the connection of the AC grid to be connected. The control method for the flexible DC transmission system provided in this application determines whether to perform primary frequency and voltage regulation during the transition from grid connection to islanding based on the current frequency and first current AC voltage of the AC grid. When a second preset frequency condition is met, the system switches from grid connection mode to islanding mode, triggering secondary frequency and voltage regulation upon switching to islanding mode. Furthermore, during the transition from islanding to grid connection, the method determines whether to perform primary frequency and voltage regulation based on the second current AC voltage, current frequency, and current power of the AC grid to be connected. This facilitates smooth control of the islanding-to-grid transition process after the islanding-to-grid enabling switch is activated. Therefore, smooth and reliable control of both the grid connection-to-islanding and islanding-to-grid transition processes can be achieved. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] To gain a more complete understanding of this application and its beneficial effects, the following description will be provided in conjunction with the accompanying drawings, wherein the same reference numerals in the following description denote the same parts.

[0021] Figure 1 This is a flowchart of a control method for switching a flexible DC transmission system from grid connection to islanding, provided in an embodiment of this application. Figure 2 This is a schematic diagram of a primary frequency modulation provided in an embodiment of this application; Figure 3 This is a schematic diagram of a single voltage regulation provided in an embodiment of this application; Figure 4 This is a flowchart of a control method for switching a flexible DC transmission system from islanded to network provided in an embodiment of this application; Figure 5 This is a schematic diagram of the control system for islanded switching of flexible DC power transmission network provided in the embodiments of this application; Figure 6 This is a schematic diagram of the control device for a flexible DC transmission system provided in the embodiments of this application. Detailed Implementation

[0022] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the protection scope of this application.

[0023] In the embodiments of this application, "at least one" refers to one or more; "multiple" refers to two or more. In the description of this application, the terms "first," "second," "third," etc., are used only for the purpose of distinguishing descriptions and should not be construed as indicating or implying relative importance, nor should they be construed as indicating or implying order.

[0024] References such as “one embodiment” or “some embodiments” as described in this specification mean that one or more embodiments of this application include a specific feature, structure, or characteristic described in connection with that embodiment. Therefore, the terms “comprising,” “including,” “having,” and variations thereof, as used in this specification, mean “including, but not limited to,” unless otherwise specifically emphasized.

[0025] It should be noted that in the embodiments of this application, "and / or" describes the relationship between associated objects, indicating that there can be three relationships. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. In addition, the character " / ", unless otherwise specified, generally indicates that the associated objects before and after it are in an "or" relationship.

[0026] It should be noted that in the embodiments of this application, "connection" can be understood as electrical connection. The connection between two electrical components can be a direct or indirect connection between the two electrical components. For example, the connection between A and B can be a direct connection between A and B, or an indirect connection between A and B through one or more other electrical components.

[0027] Figure 1This is a flowchart illustrating a control method for switching a flexible DC transmission system from grid connection to islanding, as provided in an embodiment of this application. This application provides a control method for a flexible DC transmission system, applicable to the smooth and reliable control of the switching between grid connection and islanding in a DC transmission control system. This method can be executed by a control device for the flexible DC transmission system, which can be implemented in software and / or hardware and can be configured in the processor or controller of the DC transmission control system. The control method for the flexible DC transmission system includes two control processes: switching from grid connection to islanding and switching from islanding to grid connection. See also... Figure 1 The control method for switching from a networked system to an isolated system includes the following steps: Step 110: When the flexible DC transmission system is in grid-connected mode, determine the primary frequency and voltage regulation of the grid-connected mode based on the first preset frequency condition, preset voltage condition, current frequency of the current AC grid, and the first current AC voltage.

[0028] The first-order frequency and voltage regulation scenarios include: performing first-order frequency and voltage regulation, not performing first-order frequency and voltage regulation, performing first-order frequency regulation but not first-order voltage regulation, and performing first-order voltage regulation but not first-order frequency regulation.

[0029] Specifically, frequency modulation is achieved by triggering a frequency modulation function, and voltage regulation is achieved by triggering a voltage regulation function.

[0030] Specifically, the current operating modes of flexible DC transmission systems include grid-connected mode and islanded mode. During the transition from grid-connected to islanded mode, in grid-connected mode (i.e., before the transition), based on the first preset frequency condition, preset voltage condition, and the current frequency of the AC power grid... The first current AC voltage Um is used to determine the primary frequency and voltage regulation status under the current operating mode (i.e., grid-connected mode) to determine whether primary frequency and / or primary voltage regulation is required. This helps to achieve smooth and reliable switching control of the flexible DC transmission system during the grid-to-island transition, thereby ensuring the stability of system operation and power quality.

[0031] In some embodiments, determining the primary frequency and voltage regulation of the grid connection mode based on a first preset frequency condition, a preset voltage condition, and the current frequency and first current AC voltage of the current AC power grid includes: at the current frequency If the first preset frequency condition is met, a frequency regulation function is triggered, and / or if the first current AC voltage meets the preset voltage condition, a voltage regulation function is triggered.

[0032] Among them, the current frequency The current AC voltage can be obtained in real time by a voltage sensor.

[0033] The first preset frequency condition includes: the absolute value of the difference between the current frequency and the rated frequency of the current AC power grid is greater than the first preset frequency. The specific value of the first preset frequency can be set according to actual conditions and is not specifically limited here.

[0034] The first preset voltage condition includes: the absolute value of the difference between the current AC voltage and the set value of the current AC voltage is greater than the first preset voltage. The specific value of the first preset voltage can be set according to actual conditions and is not specifically limited here.

[0035] In some embodiments, the primary frequency modulation function includes: adjusting the frequency according to the current frequency. Adjust the first active power output of the flexible DC transmission system; wherein, within the primary frequency regulation dead zone, the first active power output can be changed according to the first active power command through virtual synchronous machine control, proportional-integral control, etc.

[0036] The first active power instruction refers to the active power instruction issued by the dispatcher.

[0037] Figure 2 This is a schematic diagram of a primary frequency modulation provided in an embodiment of this application. (See attached diagram.) Figure 2 kf represents the coefficient between frequency and active power output; fdb+ represents the upper limit of the frequency dead zone, and fdb- represents the lower limit of the frequency dead zone. Specifically, during the transition of a flexible DC transmission system from grid connection to islanding, in grid connection mode (i.e., before the transition to islanding), the frequency is determined based on the current frequency of the AC power grid. If the first preset frequency condition is met, a frequency modulation function is triggered, based on the current frequency. By altering the primary active power output of the flexible DC transmission system, the primary active power command remains unchanged outside the primary frequency regulation dead zone in grid-connected mode, but the active power output changes, thus adjusting the system frequency to achieve primary frequency regulation. This facilitates smooth and reliable switching control of the flexible DC transmission system during the grid-to-island transition, thereby ensuring the stability of system operation and power quality.

[0038] In some embodiments, the primary voltage regulation function includes: adjusting the first reactive power output of the flexible DC transmission system according to the first current AC voltage; wherein the first reactive power output is within the primary voltage regulation dead zone and can be changed according to the first reactive power command through virtual synchronous machine control, proportional-integral control, etc.

[0039] Among them, the first reactive power instruction refers to the reactive power instruction issued by the dispatcher; Figure 3 This is a schematic diagram of a single voltage regulation provided in an embodiment of this application. (See attached diagram.) Figure 3Ku represents the coefficient between AC voltage and reactive power output; Udb+ represents the upper limit of the voltage dead zone, Udb- represents the lower limit of the voltage dead zone, and Uset represents the voltage set by the operator. Specific values ​​can be set according to actual conditions and are not specifically limited here. Specifically, during the transition of a flexible DC transmission system from grid connection to islanding, in grid connection mode (i.e., before the transition), if the preset voltage conditions are met based on the first current AC voltage of the current AC grid, the primary voltage regulation function is triggered. Based on the first current AC voltage, the first reactive power command of the flexible DC transmission system is changed. This ensures that outside the primary voltage regulation dead zone of grid connection mode, the first reactive power command remains unchanged, but the reactive power output of the flexible DC transmission system changes, thereby adjusting the system AC voltage to achieve primary voltage regulation. This facilitates smooth and reliable switching control of the flexible DC transmission system during the transition from grid connection to islanding, thus ensuring system stability and power quality.

[0040] Step 120: If the current frequency meets the second preset frequency condition, switch the network mode to island mode and trigger the secondary frequency modulation function and the secondary voltage regulation function.

[0041] The second preset frequency condition is the trigger condition for switching from network to island mode.

[0042] Specifically, during the transition of a flexible DC transmission system from grid connection to islanded operation, the current frequency of the AC power grid is monitored in real time during the grid connection process (i.e., before the transition to islanded operation). and the output frequency of flexible DC transmission systems At the current frequency and the output frequency of flexible DC transmission systems Once the grid-to-island transition trigger condition (i.e., the second preset frequency condition) is met, the current operating mode of the flexible DC transmission system will be switched from grid mode to island mode. At the same time, the secondary frequency regulation function and the secondary voltage regulation function will be triggered to perform secondary frequency regulation and secondary voltage regulation. This will further facilitate the smooth and reliable switching control of the flexible DC transmission system during the grid-to-island transition process, thereby ensuring the stability of system operation and power quality.

[0043] In some embodiments, the second preset frequency condition includes: the current frequency. With the rated frequency of the current AC power grid The absolute value of the difference is greater than the first preset frequency setting. Or, the current frequency With the rated frequency of the current AC power grid The absolute value of the difference is greater than the second preset frequency setting. Among them, the first preset frequency setting value Greater than or equal to the second preset frequency setting .

[0044] Specifically, during the transition of a flexible DC transmission system from grid connection to islanded operation, the current frequency of the AC power grid is monitored in real time during the grid connection process (i.e., before the transition to islanded operation). and the rated frequency of the current AC power grid When the conditions for switching from network to island mode are met: At the same time, delay the first time Or, satisfy At that time, the second time was delayed. This will switch the current operation mode of the flexible DC transmission system from grid-connected mode to islanded mode. Among other things, and Let the frequency deviation be a fixed value, and satisfy the following: ; ; In some embodiments, the secondary frequency regulation function includes: adjusting the first active power command of the flexible DC transmission system according to the current active power of the current AC power grid, so that the first active power command is consistent with the current active power.

[0045] Specifically, after the flexible DC transmission system completes the switch from grid connection to islanded operation mode, the secondary frequency regulation function is triggered to perform secondary frequency regulation, thereby achieving smooth and reliable switching control of the flexible DC transmission system during the grid connection to islanded process, thus ensuring the stability of system operation and power quality.

[0046] In some embodiments, the secondary voltage regulation function includes: adjusting the first reactive power command of the flexible DC transmission system according to the current reactive power of the current AC power grid, so that the first reactive power command is consistent with the current reactive power.

[0047] Specifically, after the flexible DC transmission system completes the switch from grid connection to islanded operation mode, the secondary frequency regulation function and the secondary voltage regulation function are triggered to perform secondary voltage regulation, thereby achieving smooth and reliable switching control of the flexible DC transmission system during the grid connection to islanded process, thus ensuring the stability of system operation and power quality.

[0048] In some embodiments, after triggering the secondary frequency modulation function and the secondary voltage regulation function, the method further includes: switching the primary frequency modulation dead zone and the primary AC voltage regulation dead zone in the network mode to the primary frequency modulation dead zone and the primary AC voltage regulation dead zone in the islanded mode, thereby making the frequency and AC voltage control more stable in the islanded control mode.

[0049] In summary, the overall control process for switching from a networked system to an isolated system includes: Step 1: Detect the current frequency of the AC power grid to which the flexible DC transmission system is connected. and the output frequency of flexible DC transmission systems .

[0050] Step 2: At the current frequency of the current AC power grid. When the first preset frequency condition is met, a frequency regulation function is triggered to perform a frequency regulation, and / or, when the first current AC voltage of the current AC power grid meets the preset voltage condition, a voltage regulation function is triggered to perform a voltage regulation.

[0051] Step 3: After detecting that the flexible DC transmission system meets the trigger conditions for switching from grid connection to islanding, switch the current operating mode of the flexible DC transmission system from grid connection mode to islanding mode.

[0052] Step 4: After the flexible DC transmission system completes the switch from grid connection to island operation mode, the secondary frequency regulation function and the secondary voltage regulation function are triggered.

[0053] Step 5: Based on the current operating mode of the flexible DC transmission system, automatically switch the primary frequency regulation dead zone and the AC voltage regulation dead zone.

[0054] Figure 4 This is a flowchart of a control method for islanding-to-network conversion of a flexible DC transmission system provided in the embodiments of this application. (See attached document.) Figure 4 The control method for switching from isolated islands to a network includes the following steps: Step 210: When the flexible DC transmission system is in islanded mode, determine the primary frequency and voltage regulation of the islanded mode based on the second current AC voltage, current frequency and current power of the AC grid to be connected.

[0055] Among them, the AC grid to be connected to the grid is the AC grid that the flexible DC transmission system will be connected to in islanded mode.

[0056] The second current AC voltage of the AC grid to be connected to the grid can be obtained in real time through a voltage sensor. The current power status of the AC grid to be connected to the grid includes active load status, active power command status, and reactive power command status.

[0057] Specifically, during the transition of a flexible DC transmission system from islanded to grid-connected operation, the primary frequency and voltage regulation status under the current operating mode (i.e., islanded mode) is determined based on the second current AC voltage and current power status of the AC grid to be connected in islanded mode (i.e., before the transition from islanded to grid-connected operation). This helps to determine whether primary frequency and / or primary voltage regulation is required, which is beneficial for achieving smooth and reliable switching control of the flexible DC transmission system during the transition from grid-connected to islanded operation, thereby ensuring the stability of system operation and power quality.

[0058] In some embodiments, determining the primary frequency and voltage regulation status of the islanding mode based on the second current AC voltage, current frequency, and current power status of the AC grid to be connected to the grid includes: triggering the primary frequency regulation function based on the active load, second active power command, and current frequency of the AC grid to be connected to the grid, and / or triggering the primary voltage regulation function based on the second current AC voltage and second reactive power command of the AC grid to be connected to the grid.

[0059] Among them, the second active power instruction of the AC grid to be connected to the grid refers to the active power instruction issued by the dispatch center, and the second reactive power instruction of the AC grid to be connected to the grid refers to the reactive power instruction issued by the dispatch center.

[0060] The active load (i.e., active power) of the AC grid to be connected can be obtained in real time by voltage transformers and current transformers. The second current AC voltage of the AC grid to be connected can be obtained in real time by voltage sensors.

[0061] Step 220: Activate the island-to-network enable switch.

[0062] Among them, the island-to-grid enabling pressure plate is a pressure plate used to put into or take out the flexible DC transmission system "island-to-grid". It can be a hard pressure plate (e.g., a physical connection piece) or a soft pressure plate (i.e., set by software program).

[0063] In some embodiments, after the islanding-to-network enabling switch is engaged, the method further includes: switching the primary frequency modulation dead zone and primary AC voltage regulation dead zone of the islanding mode to the primary frequency modulation dead zone and primary AC voltage regulation dead zone of the islanding-to-network enabling switch engagement mode.

[0064] Specifically, after the island-to-network enable switch is activated, the primary frequency regulation dead zone and primary AC voltage regulation dead zone of the island mode are automatically switched to the primary frequency regulation dead zone and primary AC voltage regulation dead zone of the island-to-network enable switch activation mode.

[0065] For example, the primary frequency modulation dead zone in this application embodiment includes the primary frequency modulation dead zone under network mode control. FM dead zone under islanded mode control And the frequency modulation dead zone after the island-to-network enabling switch is put into operation. Furthermore, these primary frequency modulation dead zones satisfy the following relationship: ; Step 230: Switch from island mode to network mode, or keep island mode running and determine the target parameters for island-to-network transition based on the current load parameters.

[0066] The current load parameters include the current load current and current arm current of the flexible DC transmission system.

[0067] Specifically, after the island-to-grid conversion enable switch is activated, preprocessing is performed using one of the following two methods: Method 1: Switch the flexible DC transmission system's operation mode from island mode to grid mode. Method 2: Maintain the flexible DC transmission system in island mode and set the island-to-grid conversion setpoint (i.e., the island-to-grid conversion target parameter) according to the current load parameters, which is beneficial for achieving stable and reliable control of the island-to-grid conversion.

[0068] In some embodiments, determining the target parameters for islanding to grid connection based on the current load parameters includes: determining the target parameters for islanding to grid connection based on the actual load current and the rated current.

[0069] Specifically, after the islanding-to-network enable switch is activated, the dead zone of the primary frequency regulation is adjusted to be less than the dead zone of the remote synchronous closing frequency; the islanding-to-network setting is set to... .in, satisfy: ; in, Indicates the rated current. This represents the actual load current.

[0070] Step 240: Synchronize the AC power grid to be connected to the grid.

[0071] Among them, the AC power grid to be connected to the grid is a remote AC power grid.

[0072] In summary, the overall control process for converting isolated systems to a network includes: Step 1: The flexible DC transmission system performs a frequency regulation based on the active load and active power command of the AC grid to be connected, and a voltage regulation based on the current AC voltage and reactive power command of the AC grid to be connected.

[0073] Step 2: Activate the island-to-network enable switch. After the island-to-network enable switch is activated, the frequency dead zone will be automatically switched once.

[0074] Step 3: Perform preprocessing using one of the following two methods: Method 1: Switch the operation mode of the flexible DC transmission system from island mode to grid mode; Method 2: Maintain the island operation mode of the flexible DC transmission system and set the island-to-grid switching setpoint according to the current load parameters.

[0075] Step 4: The remote AC power grid synchronizes the switching.

[0076] It is understood that the control method for switching between grid-connected and islanded modes provided in this application aims to achieve a smooth and reliable switching of the flexible DC transmission system between grid-connected and islanded modes, and to ensure the stability of system operation and power quality. This method integrates primary frequency regulation, secondary frequency regulation, primary voltage regulation, and secondary voltage regulation functions, and includes two major control processes: grid-to-island and island-to-grid. When switching from grid-connected to islanded, the AC grid frequency and system output frequency are first detected. Once the triggering conditions are met, the operating mode is switched, and the secondary frequency regulation and secondary voltage regulation functions are triggered synchronously. The frequency regulation and voltage regulation dead zones are automatically switched according to the operating mode. When switching from islanded to grid-connected, the enabling switch is first activated. The operating mode is switched and the secondary frequency regulation and voltage regulation functions are disabled, or the islanded mode is maintained and a preprocessing method for setting grid-connected settings is implemented. Then, the remote AC grid is synchronously closed. It is evident that this application, through clear process design and functional collaborative control, effectively improves the adaptability and control accuracy of the flexible DC transmission system during different operating mode transitions, ensuring power supply reliability while also considering both active and reactive power output.

[0077] Figure 5 This is a schematic block diagram of a control system for islanded switching in a flexible DC power transmission network, as provided in an embodiment of this application. For example, see [link to relevant documentation]. Figure 5 The control system 1 for the flexible DC transmission network's grid-to-island switching includes: a grid-to-island control module 2, an island-to-grid control module 3, a primary frequency regulation module 4, a secondary frequency regulation module 6, a primary voltage regulation module 5, and a secondary voltage regulation module 7. Specifically, the grid-to-island control module 2 switches the flexible DC transmission system's operating mode from grid-connected to islanded based on the current frequency of the AC grid and the output frequency of the flexible DC transmission system. The island-to-grid control module 3 switches the flexible DC transmission system's operating mode from islanded to grid-connected. The primary frequency regulation module 4 changes the active power command of the flexible DC transmission system based on the current frequency. The primary voltage regulation module 5 changes the reactive power command of the flexible DC transmission system based on the current AC voltage. The secondary frequency regulation module 6 changes the active power command of the flexible DC transmission system based on the current active power, ensuring the active power command matches the current active power. The secondary voltage regulation module 7 changes the reactive power command of the flexible DC transmission system based on the current reactive power, ensuring the reactive power command matches the current reactive power.

[0078] Figure 6 This is a schematic block diagram of a control device for a flexible DC transmission system provided in an embodiment of this application. This application also provides a control device for a flexible DC transmission system, see below. Figure 6The control device 10 of the flexible DC transmission system further includes: a first determining module 11, used to determine the primary frequency and voltage regulation of the grid-connected mode when the flexible DC transmission system is in grid-connected mode, based on a first preset frequency condition, a preset voltage condition, and the current frequency and first current AC voltage of the current AC grid; wherein the current AC grid is the AC grid connected to the flexible DC transmission system; a first switching module 12, used to switch the grid-connected mode to islanded mode when the current frequency meets the second preset frequency condition; and a triggering module 13, used to trigger the secondary frequency regulation function and the second... The system includes: a second voltage regulation function; a second determination module 14, used to determine the primary frequency and voltage regulation of the islanded mode based on the current AC voltage, frequency, and power of the AC grid to be connected when the flexible DC transmission system is in islanded mode; an activation module 15, used to activate the island-to-grid enabling pressure plate; a second switching module 16, used to switch the islanded mode to grid-connected mode; a third determination module 17, used to determine the target parameters for island-to-grid conversion based on the current load parameters while maintaining islanded mode operation; and a closing module 18, used to synchronously close the AC grid to be connected.

[0079] The technical solution of this application provides a control device for a flexible DC transmission system. During the transition from grid connection to islanding in the flexible DC transmission system, it determines whether to perform primary frequency and voltage regulation based on the current frequency and a first current AC voltage of the current AC grid. When a second preset frequency condition is met, it switches from grid connection mode to islanding mode, and triggers secondary frequency and voltage regulation upon switching to islanding mode. Furthermore, during the transition from islanding to grid connection, it determines whether to perform primary frequency and voltage regulation based on the second current AC voltage, current frequency, and current power of the AC grid to be connected. This facilitates smooth control of the islanding-to-grid transition process after the islanding-to-grid enabling switch is activated. Therefore, smooth and reliable control of both the grid connection-to-islanding and islanding-to-grid transition processes can be achieved.

[0080] In some embodiments, the first determining module 11 is further configured to: trigger a frequency modulation function when the current frequency meets a first preset frequency condition, and / or trigger a voltage regulation function when the first current AC voltage meets a preset voltage condition.

[0081] In some embodiments, the primary frequency regulation function includes: adjusting the first active power output of the flexible DC transmission system according to the current frequency; wherein the first active power output is consistent with the first active power command issued by the dispatching authority within the primary frequency regulation dead zone.

[0082] In some embodiments, the primary voltage regulation function includes: adjusting the first reactive power output of the flexible DC transmission system according to the first current AC voltage; wherein the first reactive power output is consistent with the first reactive power command issued by the dispatching authority within the primary voltage regulation dead zone.

[0083] In some embodiments, the secondary frequency regulation function includes: adjusting the first active power command of the flexible DC transmission system according to the current active power of the current AC power grid, so that the first active power command is consistent with the current active power.

[0084] In some embodiments, the secondary voltage regulation function includes: adjusting the first reactive power command of the flexible DC transmission system according to the current reactive power of the current AC power grid, so that the first reactive power command is consistent with the current reactive power.

[0085] In some embodiments, the second preset frequency condition includes: the absolute value of the difference between the current frequency and the rated frequency of the current AC power grid is greater than the first preset frequency, or the absolute value of the difference between the current frequency and the rated frequency is greater than the second preset frequency; wherein the first preset frequency is greater than or equal to the second preset frequency.

[0086] In some embodiments, the second determining module 14 is further configured to: trigger a primary frequency regulation function based on the active load, the second active power command, and the current frequency of the AC grid to be connected to the grid, and / or trigger a primary voltage regulation function based on the second current AC voltage and the second reactive power command of the AC grid to be connected to the grid.

[0087] In some embodiments, after triggering the secondary frequency modulation function and the secondary voltage regulation function, the method further includes: switching the primary frequency modulation dead zone and the primary AC voltage regulation dead zone in the network mode to the primary frequency modulation dead zone and the primary AC voltage regulation dead zone in the island mode. After the island-to-network enabling power supply is put into operation, the following steps are also taken: the primary frequency modulation dead zone and primary AC voltage regulation dead zone in the island mode are switched to the primary frequency modulation dead zone and primary AC voltage regulation dead zone in the island-to-network enabling power supply mode.

[0088] In some embodiments, the third determining module 17 is further configured to: determine the target parameters for islanding to network transfer based on the actual load current and the rated current.

[0089] In the above embodiments, the descriptions of each embodiment have different focuses. For parts not described in detail in a certain embodiment, please refer to the relevant descriptions in other embodiments.

[0090] The above are merely preferred embodiments of this application and are not intended to limit this application in any way. Although this application has disclosed preferred embodiments as above, it is not intended to limit this application. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the technical solution of this application. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of this application without departing from the scope of the technical solution of this application shall still fall within the scope of the technical solution of this application.

Claims

1. A control method for a flexible DC transmission system, characterized in that, The method includes: When the flexible DC transmission system is in grid-connected mode, the primary frequency and voltage regulation of the grid-connected mode is determined based on the first preset frequency condition, the preset voltage condition, the current frequency of the current AC grid, and the first current AC voltage; wherein, the current AC grid is the AC grid connected to the flexible DC transmission system. When the current frequency meets the second preset frequency condition, the network mode is switched to island mode, and the secondary frequency modulation function and the secondary voltage regulation function are triggered simultaneously. In the case of the flexible DC transmission system being in islanded mode, the primary frequency and voltage regulation of the islanded mode are determined based on the second current AC voltage, current frequency, and current power of the AC grid to be connected. Deploy the island-to-network enabling pressure plate; Alternatively, switch the isolated mode to the networked mode, or keep the isolated mode running and determine the target parameters for switching from isolated to networked mode based on the current load parameters; The AC power grid to be connected to the grid will be synchronously closed.

2. The method according to claim 1, characterized in that, The step of determining the primary frequency and voltage regulation of the grid connection mode based on the first preset frequency condition, the preset voltage condition, and the current frequency and the first current AC voltage of the current AC power grid includes: If the current frequency meets the first preset frequency condition, a frequency adjustment function is triggered, and / or, if the first current AC voltage meets the preset voltage condition, a voltage adjustment function is triggered.

3. The method according to claim 2, characterized in that, The primary frequency regulation function includes: adjusting the first active power output of the flexible DC transmission system according to the current frequency; wherein, the first active power output changes according to the first active power command issued by the dispatcher within the primary frequency regulation dead zone.

4. The method according to claim 2, characterized in that, The primary voltage regulation function includes: adjusting the first reactive power output of the flexible DC transmission system according to the first current AC voltage; wherein, the first reactive power output changes according to the first reactive power command issued by the dispatcher within the primary voltage regulation dead zone.

5. The method according to claim 1, characterized in that, The secondary frequency regulation function includes: adjusting the first active power command of the flexible DC transmission system according to the current active power of the current AC power grid, so that the first active power command is consistent with the current active power.

6. The method according to claim 1, characterized in that, The secondary voltage regulation function includes: adjusting the first reactive power command of the flexible DC transmission system according to the current reactive power of the current AC power grid, so that the first reactive power command is consistent with the current reactive power.

7. The method according to claim 1, characterized in that, The second preset frequency condition includes: the absolute value of the difference between the current frequency and the rated frequency of the current AC power grid is greater than the first preset frequency, or the absolute value of the difference between the current frequency and the rated frequency is greater than the second preset frequency; wherein, the first preset frequency is greater than or equal to the second preset frequency.

8. The method according to claim 1, characterized in that, The step of determining the primary frequency and voltage regulation of the islanding mode based on the second current AC voltage, current frequency, and current power status of the AC power grid to be connected includes: Based on the active load, the second active power command, and the current frequency of the AC grid to be connected to the grid, a primary frequency regulation function is triggered, and / or, based on the second current AC voltage and the second reactive power command of the AC grid to be connected to the grid, a primary voltage regulation function is triggered.

9. The method according to claim 1, characterized in that, After triggering the secondary frequency modulation function and the secondary voltage regulation function, the following is also included: The primary frequency modulation dead zone and primary AC voltage regulation dead zone of the network mode are switched to the primary frequency modulation dead zone and primary AC voltage regulation dead zone of the island mode. After the island-to-network enabling switch is activated, the method further includes: switching the primary frequency modulation dead zone and primary AC voltage regulation dead zone of the island mode to the primary frequency modulation dead zone and primary AC voltage regulation dead zone of the island-to-network enabling switch activation mode.

10. The method according to claim 1, characterized in that, The step of determining the target parameters for islanding to network conversion based on the current load parameters includes: The target parameters for the island-to-network transition are determined based on the actual load current and the rated current.

11. A control device for a flexible DC transmission system, characterized in that, The device includes: The first determining module is used to determine the primary frequency and voltage regulation situation of the interconnected mode when the flexible DC transmission system is in the grid-connected mode, based on the first preset frequency condition, the preset voltage condition, the current frequency of the current AC grid, and the first current AC voltage; wherein, the current AC grid is the AC grid connected to the flexible DC transmission system. The first switching module is used to switch the network mode to island mode when the current frequency meets the second preset frequency condition; The trigger module is used to trigger the secondary frequency modulation function and the secondary voltage regulation function; The second determining module is used to determine the primary frequency and voltage regulation of the islanded mode when the flexible DC transmission system is in the islanded mode, based on the second current AC voltage, current frequency and current power of the AC grid to be connected. The input module is used to input the island-to-network enable pressure plate; The second switching module is used to switch the island mode to the network mode; The third determining module is used to determine the target parameters for islanding to network transfer based on the current load parameters while maintaining the islanding mode operation. The closing module is used to synchronize the AC power grid to be connected to the grid.

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