Method and device for starting an ultra-high voltage flexible direct current system

Abstract

This application discloses a startup method and apparatus for an ultra-high voltage flexible DC system, belonging to the field of converter technology. The startup method includes: activating a first converter station and a second converter station, and closing the DC-side bypass switch of a second unlocking valve group; the second unlocking valve group is a zero-voltage unlocking valve group; closing the first AC switch of the first converter station and the second AC switch of the second converter station to normally charge the first unlocking valve group and perform DC-side short-circuit charging on the second unlocking valve group; after the first and second converter stations have completed charging and met preset unlocking conditions, unlocking the first unlocking valve group at a preset voltage, unlocking the second unlocking valve group at zero voltage, and disconnecting the DC-side bypass switch; reducing the voltage of the first unlocking valve group from the preset voltage to the target operating voltage, and increasing the voltage of the second unlocking valve group from zero voltage to the target operating voltage. This application can achieve voltage reduction startup of the entire ultra-high voltage flexible DC system.

Description

Technical Field

[0001] This application relates to the field of converter technology, specifically to a method and apparatus for starting up an ultra-high voltage flexible DC system. Background Technology

[0002] During operation, ultra-high voltage (UHV) flexible DC transmission systems often encounter risks such as DC line insulation damage or lightning strikes, necessitating step-down startup. To enhance the overload capacity of the flexible DC system and fully utilize the capabilities of insulated-gate bipolar transistors (IGBTs), the valve-side voltage of the UHV flexible DC system is typically designed to be relatively high to reduce steady-state operating current. However, this design results in the electrode voltage of the UHV flexible DC system after charging under conventional charging methods being higher than the voltage required for step-down operation, thus preventing the implementation of step-down operation throughout the entire process. Therefore, a step-down startup method for the entire process is urgently needed. Summary of the Invention

[0003] A method and apparatus for starting an ultra-high voltage flexible DC system are provided, enabling the entire process of voltage reduction starting of the ultra-high voltage flexible DC system.

[0004] In a first aspect, a startup method for an ultra-high voltage flexible DC system is provided. The ultra-high voltage flexible DC system includes a first converter station and a second converter station. Both the first converter station and the second converter station include high-end valve groups and low-end valve groups. The two high-end valve groups are first unlocking valve groups and the two low-end valve groups are second unlocking valve groups, or the two low-end valve groups are first unlocking valve groups and the two high-end valve groups are second unlocking valve groups. The method includes: The first and second converter stations are put into operation, and the DC side bypass switch of the second unlocking valve group is closed; wherein, the second unlocking valve group is a zero-pressure unlocking valve group; Close the first AC switch of the first converter station and the second AC switch of the second converter station to charge the first unlocking valve group normally and to perform DC side short-circuit charging on the second unlocking valve group. When the first and second converter stations have completed charging and met the preset unlocking conditions, the first unlocking valve group is unlocked according to the preset voltage, the second unlocking valve group is unlocked according to zero voltage and the DC side bypass switch is disconnected. The first unlocking valve assembly is depressurized from the preset voltage to the target operating voltage, and the second unlocking valve assembly is boosted from zero voltage to the target operating voltage.

[0005] In some embodiments, the converter valve of the second unlocking valve group is a full-half-bridge hybrid topology converter valve; performing DC-side short-circuit charging on the second unlocking valve group includes: All modules of the first unlocking valve group are locked, and the full-bridge module capacitors in the bridge arm are charged through the diodes of the full-and-half-bridge hybrid topology converter valve; After the full-bridge module successfully draws power, the full-bridge module is gradually disconnected until the charging voltage of the half-bridge module is the same as that of the full-bridge module. After all modules of the full-bridge hybrid topology converter valve have successfully gained power, the N modules with the highest voltage are disconnected, where N is a positive integer.

[0006] In some embodiments, the first unlocking valve assembly includes a first switching valve and a second switching valve; unlocking the first unlocking valve assembly according to a preset voltage includes: The first converter valve is unlocked according to the preset voltage, and the outer loop of the first converter valve is controlled by active power control during unlocking. The second converter valve is unlocked according to the preset voltage, and the outer loop of the second converter valve is controlled by DC voltage control during unlocking.

[0007] In some embodiments, reducing the voltage of the first unlocking valve assembly from a preset voltage to a target operating voltage includes: When the first converter valve completes unlocking and reaches the preset time difference, the voltage is reduced according to the preset voltage; the preset time difference is the unlocking time difference between the first converter valve and the second converter valve. After the second converter valve is unlocked, it begins to reduce the voltage according to the preset voltage to achieve synchronous voltage reduction of the first and second converter valves.

[0008] In some embodiments, synchronously reducing the pressure of the first switching valve and the second switching valve includes: The reference wave DC bias of the first converter valve is reduced to the target operating voltage at a first preset speed; The DC voltage reference value and reference wave DC bias of the second converter valve are reduced to the target operating voltage at a first preset speed.

[0009] In some embodiments, the second unlocking valve assembly includes a third switching valve and a fourth switching valve; unlocking the second unlocking valve assembly at zero voltage includes: The outer loop control mode of the third converter valve is switched from active power control to submodule voltage control; wherein, the voltage reference value of submodule voltage control is the rated submodule voltage, and the DC bias of the reference wave of submodule voltage control is zero. Switch the outer loop control mode of the fourth converter valve from DC voltage control to submodule voltage control.

[0010] In some embodiments, boosting the second unlocking valve assembly from zero voltage to a target operating voltage includes: After the third converter valve is unlocked for a second preset time, a pressure boosting command is sent to the third converter valve and the fourth converter valve. The third converter valve is controlled to pressurize according to the pressurization command after a preset time difference following a pressurization command. The fourth converter valve is controlled to increase pressure upon receiving a pressure increase command, so as to achieve synchronous pressure increase of the third and fourth converter valves.

[0011] In some embodiments, synchronously pressurizing the third and fourth switching valves includes: The reference wave DC bias of the third converter valve is boosted to the target operating voltage at the second preset speed, and after the reference wave DC bias of the third converter valve is greater than the first preset value, the outer loop control mode of the third converter valve is switched to active power control. The reference wave DC bias of the fourth converter valve is boosted to the target operating voltage at a second preset speed, and after the reference wave DC bias of the fourth converter valve is greater than the second preset value, the outer loop control mode of the fourth converter valve is switched to DC voltage control.

[0012] In some embodiments, disconnecting the DC-side bypass switch includes: After the DC side bypass switch is unlocked and operated for a first preset time, a preset number of harmonic voltages are injected into the reference wave DC bias of the second unlocking valve group.

[0013] Secondly, a starting device for an ultra-high voltage flexible DC system is also provided. The ultra-high voltage flexible DC system includes a first converter station and a second converter station. Both the first converter station and the second converter station include high-end valve groups and low-end valve groups. Among them, the two high-end valve groups are the first unlocking valve groups and the two low-end valve groups are the second unlocking valve groups, or the two low-end valve groups are the first unlocking valve groups and the two high-end valve groups are the second unlocking valve groups. The device includes: The input module is used to input the first converter station and the second converter station; A closing module is used to close the DC side bypass switch of the second unlocking valve group; wherein, the second unlocking valve group is a zero-pressure unlocking valve group; The charging module is used to close the first AC switch of the first converter station and the second AC switch of the second converter station to charge the first unlocking valve group normally and to perform DC-side short-circuit charging on the second unlocking valve group. The unlocking module is used to unlock the first unlocking valve group according to a preset voltage and the second unlocking valve group according to zero voltage and disconnect the DC side bypass switch when the first and second converter stations have completed charging and met the preset unlocking conditions. The boost / buck module is used to step down the first unlocking valve group from the preset voltage to the target operating voltage, and to boost the second unlocking valve group from zero voltage to the target operating voltage.

[0014] Beneficial Effects: This application provides a method and apparatus for starting an ultra-high voltage flexible DC system. The method includes: activating a first converter station and a second converter station, and closing the DC-side bypass switch of a second unlocking valve group; wherein the second unlocking valve group is a zero-voltage unlocking valve group; closing the first AC switch of the first converter station and the second AC switch of the second converter station to normally charge the first unlocking valve group and perform DC-side short-circuit charging on the second unlocking valve group; after the first and second converter stations have completed charging and met the preset unlocking conditions, unlocking the first unlocking valve group at a preset voltage, unlocking the second unlocking valve group at zero voltage and disconnecting the DC-side bypass switch; reducing the voltage of the first unlocking valve group from the preset voltage to the target operating voltage, and increasing the voltage of the second unlocking valve group from zero voltage to the target operating voltage. The method provided in this application group the high-end valve groups and low-end valve groups of the first converter station and the second converter station into a first unlocking valve group and a second unlocking valve group, and performs differentiated charging, differentiated unlocking, synchronous voltage reduction and synchronous voltage increase from zero on the first unlocking valve group and the second unlocking valve group, so as to realize the voltage reduction start-up of the entire process of the UHV flexible DC system. Attached Figure Description

[0015] 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 accompanying 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.

[0016] Figure 1 This is a schematic diagram of an AC / DC switch for an ultra-high voltage flexible DC transmission system provided in an embodiment of this application; Figure 2 This is a flowchart of a startup method for an ultra-high voltage flexible DC system provided in an embodiment of this application; Figure 3 This is a schematic diagram of the two-station pole connection switch of the ultra-high voltage flexible DC transmission system provided in the embodiments of this application; Figure 4 This is a schematic diagram of the DC side bypass switch (BPS) of the two-station zero-voltage unlocking valve combination in the UHV flexible DC transmission system provided in this application embodiment; Figure 5 This is a schematic diagram of the AC switching charging of two stations in an ultra-high voltage flexible DC transmission system provided in this application embodiment; Figure 6 This is a schematic diagram of the submodule voltage after the high and low valve groups of the two stations of the UHV flexible DC transmission system have been charged, provided in the embodiments of this application. Figure 7 This is a schematic diagram of the controller for an ultra-high voltage flexible DC transmission system provided in an embodiment of this application; Figure 8This is a waveform diagram of the voltage reduction start-up test of the UHV flexible DC transmission system provided in the embodiments of this application; Figure 9 This is a schematic diagram of the BPS (Blocking Power System) after startup of the UHV flexible DC transmission system provided in the embodiments of this application; Figure 10 This is a schematic diagram of the overall process of a startup method for an ultra-high voltage flexible DC system provided in an embodiment of this application; Figure 11 This is a schematic diagram of the starting device for an ultra-high voltage flexible DC system provided in an embodiment of this application. Detailed Implementation

[0017] 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 some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0018] In the description of this application, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, are used only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, features defined with "first" and "second" may explicitly or implicitly include one or more of the stated features. In the description of this application, "a plurality of" means two or more, unless otherwise explicitly specified.

[0019] "A and / or B" includes the following three combinations: A only, B only, and a combination of A and B.

[0020] The use of "applies to" or "configured to" in this application implies open and inclusive language, which does not exclude the applicability to or configuration to devices performing additional tasks or steps. Additionally, the use of "based on" implies openness and inclusivity, because processes, steps, calculations, or other actions "based on" one or more of the stated conditions or values ​​may in practice be based on additional conditions or values ​​beyond those stated.

[0021] In this application, the term "exemplary" is used to mean "used as an example, illustration, or description." Any embodiment described as "exemplary" in this application is not necessarily to be construed as being more preferred or advantageous than other embodiments. The following description is provided to enable any person skilled in the art to make and use this application. Details are set forth in the following description for purposes of explanation. It should be understood that those skilled in the art will recognize that this application can be made without using these specific details. In other instances, well-known structures and processes are not described in detail to avoid obscuring the description of this application with unnecessary detail. Therefore, this application is not intended to be limited to the embodiments shown, but is consistent with the broadest scope of the principles and features disclosed in this application.

[0022] The applicant's research revealed that with the rapid development and widespread application of new energy power generation technologies, the proportion of new energy power generation in the power system is gradually increasing. However, new energy power generation typically relies on power electronic equipment to connect to the grid, and these devices themselves are relatively weak in terms of disturbance resistance and support capabilities. As the proportion of new energy power generation continues to increase, the low inertia characteristic of the power system becomes more pronounced, posing a greater challenge to the stability of the power system. Flexible DC transmission technology based on modular multilevel converters (MMCs) has advantages such as independent control of active and reactive power and the ability to supply power to passive systems. It can stably connect high-proportion new energy or 100% pure new energy, becoming a key technology for solving the problem of large-scale new energy grid connection and transmission.

[0023] Ultra-high voltage flexible direct current transmission systems employ multiple voltage source converters in series, which can multiply the transmission capacity. With the continuous increase in the scale of newly built new energy power plants, the demand for transmitting new energy power using ultra-high voltage flexible direct current is becoming increasingly urgent. At present, the application research of ultra-high voltage flexible direct current is still in its initial stage, and some control modes and control methods need to be explored.

[0024] During operation, UHVDC flexible DC transmission systems often encounter risks such as DC line insulation damage or lightning strikes, necessitating voltage reduction for startup. To enhance the overload capacity of the flexible DC transmission system and fully utilize the capabilities of Insulated Gate Bipolar Transistors (IGBTs), the valve-side voltage of the UHVDC flexible DC system is typically designed to be relatively high to reduce steady-state operating current. However, this design results in the electrode voltage of the UHVDC flexible DC system after charging under conventional charging methods being higher than the voltage required for voltage reduction operation. Consequently, it is impossible to achieve voltage reduction operation throughout the entire process, posing a risk of insulation breakdown during the charging phase.

[0025] Therefore, there is an urgent need to propose a method for voltage reduction and startup of an ultra-high voltage flexible DC system throughout the entire process.

[0026] In view of this, the present application provides a method and apparatus for starting an ultra-high voltage flexible DC system. By grouping the high-end valve group and low-end valve group of the first converter station and the second converter station into a first unlocking valve group and a second unlocking valve group, and performing differentiated charging, differentiated unlocking, synchronous voltage reduction and synchronous voltage increase from zero on the first unlocking valve group and the second unlocking valve group, the voltage reduction start-up of the ultra-high voltage flexible DC system throughout the entire process is realized.

[0027] The startup method provided in this application is applied to an ultra-high voltage flexible DC transmission system (i.e., an ultra-high voltage flexible DC transmission system), which includes a first converter station and a second converter station. Exemplarily, the first converter station is a sending-end converter station, and the second converter station is a receiving-end converter station. Both the first and second converter stations include a high-end valve group and a low-end valve group. For example, the first converter station includes a first high-end valve group and a first low-end valve group (i.e., a first converter valve and a third converter valve), and the second converter station includes a second high-end valve group and a second low-end valve group (i.e., a second converter valve and a fourth converter valve). The first high-end valve group and the second high-end valve group are a first unlocking valve group, and the first low-end valve group and the second low-end valve group are a second unlocking valve group; or, the first low-end valve group and the second low-end valve group are a first unlocking valve group, and the first high-end valve group and the second high-end valve group are a second unlocking valve group.

[0028] In this embodiment, the first unlocking valve group is a conventional unlocking valve group, and the second unlocking valve group is a zero-pressure unlocking valve group. For example, the high-end valve groups of the first and second converter stations can be used as conventional unlocking valve groups, while the low-end valve groups of the first and second converter stations can be used as zero-pressure unlocking valve groups. Alternatively, the low-end valve groups of the first and second converter stations can be used as conventional unlocking valve groups, while the high-end valve groups of the first and second converter stations can be used as zero-pressure unlocking valve groups. The specific settings can be configured according to actual conditions and are not specifically limited here. For example, in this embodiment, the high-end valve groups of the first and second converter stations are used as conventional unlocking valve groups, and the low-end valve groups of the first and second converter stations are used as zero-pressure unlocking valve groups for illustration, which will not be repeated below.

[0029] Figure 1 This is a schematic diagram of an AC / DC switch for an ultra-high voltage flexible DC transmission system provided in an embodiment of this application. For example, taking the first converter station as an example, see [link / reference]. Figure 1The AC / DC switches of the first high-end valve group of the first converter station include: first anode outgoing line disconnector 201, first cathode outgoing line disconnector 202, first two-station pole line switch 209, first DC side bypass switch 205, and first DC side bypass switch 206; the AC / DC switches of the first low-end valve group of the first converter station include: second anode outgoing line disconnector 203, second cathode outgoing line disconnector 204, second DC side bypass switch 207, second DC side bypass switch 208, first grounding switch disconnector 210, and second grounding switch disconnector 211.

[0030] Figure 2 This is a flowchart illustrating a startup method for an ultra-high voltage flexible DC transmission system provided in an embodiment of this application. (See attached document.) Figure 1 The method includes the following steps: Step 110: Put the first converter station and the second converter station into operation, and close the DC side bypass switch of the second unlocking valve group.

[0031] Figure 3 This is a schematic diagram of the two-station pole connection switch of the ultra-high voltage flexible DC transmission system provided in this application embodiment. (See also...) Figure 3 The AC switches for the second high-end valve group of the second converter station include: the third anode outlet switch 301, the third cathode outlet switch 302, the second two-station pole line switch 309, the third DC side bypass switch 305, and the third DC side bypass switch 306; the AC / DC switches for the second low-end valve group of the second converter station include: the fourth anode outlet switch 303, the fourth cathode outlet switch 304, the fourth DC side bypass switch 307, the fourth DC side bypass switch 308, the third grounding switch switch 310, and the fourth grounding switch switch 311.

[0032] Specifically, in the switching state: the high- and low-end valve groups of the first and second converter stations are engaged and connected. At both the sending and receiving ends, one valve group from the high- and low-end valve groups is selected as the normal unlocking valve group, and the other valve group is selected as the zero-pressure unlocking valve group. The zero-pressure unlocking valve group is combined with the DC side bypass switch (BPS). For an example, see [link to example]. Figure 1 and Figure 3 When the poles are connected, the first two-station pole line switch 209, the second two-station pole line switch 309, the first grounding switch spacer 210, the second grounding switch spacer 211, the third grounding switch spacer 310, and the fourth grounding switch spacer 311 are closed.

[0033] When the high- and low-end valve groups are put into operation, the first anode Isolator (AI) 201, the first cathode Isolator (CI) 202, the second anode Isolator 203, the second cathode Isolator 204, the third anode Isolator 301, the third cathode Isolator 302, the fourth anode Isolator 303, and the fourth cathode Isolator 304 of the first and second converter stations are closed.

[0034] Figure 4 This is a schematic diagram of the DC-side bypass switch (BPS) for the two-station zero-voltage unlocking valve combination in the UHV flexible DC transmission system provided in this application embodiment. (See also...) Figure 1 and Figure 4 Close the DC-side bypass switch of the zero-pressure unlocking valve group (i.e., the second unlocking valve group), for example, close the second DC-side bypass switch 207 and the fourth DC-side bypass switch 307. Since the DC-side bypass switch BPS is located at the DC-side port of the converter valve, its two sides are connected to the DC positive terminal outlet separator AI and the negative terminal outlet separator CI of the converter valve, respectively. When the valve group is engaged, both outlet separators AI and CI are closed and form a circuit with the closed BPS, thereby realizing short-circuit charging and zero-pressure unlocking of the converter valve. For example, the second DC-side bypass switch 207, the second anode outlet separator 203, the second DC-side bypass switch 208, and the second cathode outlet separator 204 are all closed to form a circuit, and the fourth DC-side bypass switch 307, the fourth anode outlet separator 303, the fourth DC-side bypass switch 308, and the fourth cathode outlet separator 304 are all closed to form a circuit.

[0035] Step 120: Close the first AC switch of the first converter station and the second AC switch of the second converter station to charge the first unlocking valve group normally and to perform DC side short-circuit charging on the second unlocking valve group.

[0036] Figure 5 This is a schematic diagram of the AC switching charging of two stations in an ultra-high voltage flexible DC transmission system provided in this application embodiment. (See also...) Figure 5 The system charges the conventional unlocking valve group (i.e., the first unlocking valve group) normally by closing the first AC switch 212 and the second AC switch 312, and performs DC-side short-circuit charging on the zero-voltage unlocking valve group (i.e., the second unlocking valve group). The first AC switch 212 comprises two AC switches, namely, first AC switch A 2121 and first AC switch B 2122; the second AC switch 312 comprises two AC switches, namely, second AC switch A 3121 and second AC switch B 3122.

[0037] In some embodiments, the converter valve of the second unlocking valve group is a full-half-bridge hybrid topology converter valve; DC-side short-circuit charging of the second unlocking valve group includes: locking all modules of the first unlocking valve group, charging the full-bridge module capacitor in the bridge arm through the diode of the full-half-bridge hybrid topology converter valve; after the full-bridge module successfully draws power, gradually disconnecting the full-bridge module until the charging voltage of the half-bridge module is the same as the voltage of the full-bridge module; after all modules of the full-half-bridge hybrid topology converter valve successfully draw power, disconnecting the N modules with the highest voltage, where N is a positive integer.

[0038] The short-circuit charging state is a charging state of a full-half-bridge hybrid topology converter valve when the DC side is short-circuited. Its charging control is realized in the valve control system, and its charging process includes the following three steps: Step 1, Uncontrolled charging: All modules are locked. The AC system charges the full-bridge submodule capacitors in the bridge arm through the diodes of the converter valve. The half-bridge modules cannot be charged.

[0039] Step 2, Controlled Charging 1: After the full-bridge module successfully draws power, the full-bridge module is gradually disconnected until the voltage of the half-bridge module is close to that of the full-bridge module.

[0040] Step 3, Controllable Charging 2: After all submodules successfully draw power, select and cut off the N submodules with the highest voltage, and slowly increase N until the DC capacitor voltage of all submodules is balanced and reaches near the rated value.

[0041] Step 130: After the first and second converter stations have completed charging and met the preset unlocking conditions, the first unlocking valve group is unlocked according to the preset voltage, the second unlocking valve group is unlocked according to zero voltage, and the DC side bypass switch is disconnected.

[0042] Figure 6 This is a schematic diagram of the submodule voltage after the high and low valve groups of the two stations of the UHV flexible DC transmission system have been charged, as provided in the embodiments of this application. Figure 6 In the diagram, curve L11 represents the DC port voltage curve of the first high-end valve group of the first converter station, curve L12 represents the DC port voltage curve of the first low-end valve group of the first converter station, curve L13 represents the average voltage curve of the sub-module of the first high-end valve group of the first converter station, curve L14 represents the average voltage curve of the sub-module of the first low-end valve group of the first converter station, curve L15 represents the unlocking flag of the first high-end valve group of the first converter station, and curve L16 represents the unlocking flag of the first low-end valve group of the first converter station.

[0043] Curve L21 is the DC port voltage curve of the second high-end valve group of the second converter station; curve L22 is the DC port voltage curve of the second low-end valve group of the second converter station; curve L23 is the average voltage curve of the submodule of the second high-end valve group of the second converter station; curve L24 is the average voltage curve of the submodule of the second low-end valve group of the second converter station; curve L25 is the unlocking flag of the second high-end valve group of the second converter station; and curve L26 is the unlocking flag of the second low-end valve group of the second converter station.

[0044] The preset unlocking conditions refer to: the port voltage of the zero-pressure unlocking valve group at both the sending and receiving ends is zero; the port voltage of the conventional unlocking valve group meets the preset voltage conditions; and the voltage of the submodule is close to the rated voltage. The submodules are submodules of the first and second valve groups, including full-bridge and half-bridge submodules. For example, see [link to relevant documentation]. Figure 6 Both the sending and receiving ends have completed charging and met the preset unlocking conditions, as shown by curves L12 and L22. The port voltage of the zero-voltage unlocking valve group at the sending end (i.e., the first converter station) and the receiving end (the second converter station) is 0kV. As shown by curves L11 and L22, the port voltage of the conventional unlocking valve group is 284.9kV. As shown by curves L13, L14, L23, and L24, the voltage of the sub-modules (where the sub-modules are the sub-modules of the first and second valve groups, including full-bridge and half-bridge) are close to the rated voltage of 2kV, meeting the preset unlocking conditions. In the ±800kV polarity system (i.e., the UHV flexible DC transmission system), the step-down operation generally selects 70% of the rated voltage, that is, the step-down operation at the polarity of 560kV. Obviously, if both valve groups are charged in the conventional way, the polarity at this time is 284.9*2=569.8kV, which is 10kV higher than the step-down operation voltage, and does not meet the requirement of step-down throughout the process. The starting method provided in this application embodiment ensures that during charging, the electrode voltage is only 284.9kV, which is the charging voltage of a single valve group, thus meeting the voltage reduction requirements.

[0045] The specific value of the preset voltage can be set according to the actual situation, and no specific limit is made here.

[0046] In some embodiments, the first unlocking valve group includes a first converter valve and a second converter valve; unlocking the first unlocking valve group according to a preset voltage includes: first unlocking the first converter valve according to the preset voltage, and controlling the outer loop of the first converter valve through active power control during unlocking; then unlocking the second converter valve according to the preset voltage, and controlling the outer loop of the second converter valve through DC voltage control during unlocking.

[0047] For example, the first converter valve is the first high-end valve group of the first converter station, and the second converter valve is the second high-end valve group of the second converter station.

[0048] Figure 7This is a schematic diagram of the controller for an ultra-high voltage flexible direct current transmission system provided in an embodiment of this application. (See also...) Figure 7 The controller of the UHV flexible DC transmission system includes a DC bias controller, an inner current controller, a submodule voltage outer loop controller, a bridge arm voltage calculation unit, and a DC voltage / active power outer loop controller.

[0049] Figure 8 This is a waveform diagram of the voltage reduction start-up test of the UHV flexible DC transmission system provided in the embodiments of this application. Figure 8 In the diagram, curves S11 and S12 are the anode voltage curves of the first converter station, curve S13 is the DC port voltage curve of the first high-end valve group of the first converter station, curve S14 is the DC port voltage curve of the first low-end valve group of the first converter station, curve S15 is the BPS closed position flag of the first high-end valve group of the first converter station, and curve S16 is the BPS closed position flag of the first low-end valve group of the first converter station.

[0050] Curves S21 and S22 are the anode voltage curves of the second converter station, curve S23 is the DC port voltage curve of the second high-end valve group of the second converter station, curve S24 is the DC port voltage curve of the second low-end valve group of the second converter station, curve S25 is the BPS closed position flag of the second high-end valve group of the second converter station, and curve S26 is the BPS closed position flag of the second low-end valve group of the second converter station.

[0051] Specifically, the process of unlocking the conventional unlocking valve assembly normally with a preset voltage is as follows: (See...) Figure 7 and Figure 8 The receiving end conventional unlocking valve group (i.e., the first switching valve) unlocks first, such as... Figure 8 As shown at time t1, the outer ring is as follows when unlocking. Figure 7 The DC voltage control shown has a DC voltage reference value U. dcref and reference wave DC bias U ref_PZ This is the real-time value of the DC port voltage of the first high-side valve group before unlocking. Unlocking is performed after the normal unlocking of the sending-end valve group (i.e., the second converter valve). Figure 8 As shown at time t2, the outer ring is as follows when unlocking. Figure 7 The active power control shown has a reference wave DC bias U. ref_PZ Real-time value U of the DC port voltage of the second high-end valve group before unlocking UC .

[0052] In some embodiments, the second unlocking valve group includes a third converter valve and a fourth converter valve; unlocking the second unlocking valve group at zero voltage includes: switching the outer loop control mode of the third converter valve from active power control to submodule voltage control; wherein the voltage reference value of the submodule voltage control is the rated submodule voltage, and the DC bias of the reference wave of the submodule voltage control is zero; and switching the outer loop control mode of the fourth converter valve from DC voltage control to submodule voltage control.

[0053] For example, the third converter valve is the first low-end valve group of the first converter station, and the fourth converter valve is the second low-end valve group of the second converter station.

[0054] Specifically, the implementation process of zero DC voltage unlocking of the zero-pressure unlocking valve group is as follows: (See...) Figure 7 and Figure 8 Switch the receiving-end DC voltage control outer loop to, as follows Figure 7 The submodule voltage control shown has a reference value of the rated submodule voltage U. avgref Reference wave DC bias U ref_PZ Set to 0; switch the sending-end active power control outer loop to, as follows: Figure 7 The submodule voltage control shown has a reference value of the rated submodule voltage U. avgref Reference wave DC bias U ref_PZ It is 0. For example... Figure 8 As shown, the unlocking times of the two valve groups (i.e., the third and fourth switching valves) are not sequential.

[0055] In some embodiments, disconnecting the DC-side bypass switch includes: after the DC-side bypass switch has been unlocked for a first preset time, applying a DC bias (i.e., a DC bias circuit, such as...) to the reference wave of the second unlocked valve group (i.e., the third converter valve and the fourth converter valve). Figure 7 U ref_PZ (Inject a preset number of harmonic voltages)

[0056] The specific value of the first preset duration can be set according to the actual situation, and no specific limit is made here.

[0057] For example, the preset number of times is 6, but it can also be set to other values. The specific number can be set according to the actual situation, and no specific limitation is made here.

[0058] Specifically, the process of unlocking the zero-voltage unlocking valve group and then opening the DC-side bypass switch (BPS) is as follows: After the zero-voltage unlocking valve groups at the sending and receiving ends have been running for a period of time, a 6th harmonic voltage is injected into the reference wave DC bias of the second unlocking valve group (i.e., the third and fourth converter valves), thereby forming a 6th harmonic current on the DC side, creating a zero-crossing point for the BPS to open. Figure 8 As shown, the two stations share BPS in no particular order.

[0059] Step 140: Reduce the voltage of the first unlocking valve group from the preset voltage to the target operating voltage, and increase the voltage of the second unlocking valve group from zero voltage to the target operating voltage.

[0060] The specific value of the target operating voltage depends on the actual situation and can be set according to the actual situation; no specific limit is made here.

[0061] Figure 9 This is a schematic diagram of the BPS (Bypass Switch) after startup of the UHV flexible DC transmission system provided in this application embodiment. After the conventional unlocking valve groups at the sending and receiving ends are unlocked, they synchronously decrease from the unlocking voltage (i.e., the preset voltage) to the target operating voltage. After the zero-voltage unlocking valve groups at the sending and receiving ends disconnect the DC side bypass switch (BPS), they synchronously increase from 0 voltage to the target operating voltage. The stable operating state of the BPS is as follows: Figure 9 As shown.

[0062] In some embodiments, reducing the voltage of the first unlocking valve group from a preset voltage to a target operating voltage includes: starting to reduce the voltage according to a preset voltage when the first switching valve completes unlocking and reaches a preset time difference; the preset time difference is the unlocking time difference between the first switching valve and the second switching valve; and starting to reduce the voltage according to a preset voltage after the second switching valve completes unlocking, so as to achieve synchronous voltage reduction of the first switching valve and the second switching valve.

[0063] The preset time difference is the inter-station communication time, that is, the communication time between the first converter station and the second converter station.

[0064] Specifically, the method for synchronously reducing the voltage from the unlock voltage to the target operating voltage at both the sending and receiving ends is as follows: Timing is coordinated to ensure that the sending and receiving ends operate in a synchronized manner. Figure 8 As shown at time t2, the pressure is reduced at approximately the same rate. Since the receiving end sends an unlock signal to the sending end after unlocking, and the sending end unlocks immediately upon receiving the unlock signal, the time difference between the unlocking of the sending and receiving ends is equal to the inter-station communication time. Therefore, the pressure reduction times of the conventional unlocking valve groups at the sending and receiving ends are as follows: the receiving end begins to reduce pressure after a delay of the inter-station communication time after unlocking, and the sending end reduces pressure immediately after unlocking. The pressure reduction speeds of the two stations are consistent, thereby achieving synchronous pressure reduction of the conventional unlocking valve groups at the sending and receiving ends.

[0065] In some embodiments, synchronously reducing the voltage of the first converter valve and the second converter valve includes: reducing the DC bias of the reference wave of the first converter valve to the target operating voltage at a first preset speed; and reducing the DC voltage reference value and the DC bias of the reference wave of the second converter valve to the target operating voltage at a first preset speed.

[0066] The specific value of the first preset speed can be set according to the actual situation, and no specific limit is made here.

[0067] Specifically, the voltage reduction process of the conventional unlocking valve group at the sending and receiving ends is as follows: the DC bias of the reference wave of the conventional unlocking valve group at the sending end is set according to a predetermined speed U. RAMP The voltage is reduced to the target operating voltage U of the valve assembly. REFDC The DC voltage reference value and reference wave DC bias of the conventional unlocking valve group at the receiving end are adjusted at the same speed U. RAMP The voltage is reduced to the target operating voltage U of the valve assembly. REFDC .

[0068] In some embodiments, boosting the second unlocking valve group from zero voltage to the target operating voltage includes: sending a boosting command to the third and fourth switching valves after the third switching valve is unlocked for a second preset time; controlling the third switching valve to boost the voltage according to the boosting command after receiving the boosting command and delaying for a preset time difference; and controlling the fourth switching valve to boost the voltage after receiving the boosting command, so as to achieve synchronous boosting of the third and fourth switching valves.

[0069] The specific value of the second preset duration can be set according to the actual situation, and no specific limit is made here.

[0070] Specifically, the process of synchronously increasing the voltage from 0 to the target operating voltage at both the sending and receiving ends is as follows: Timing coordination is performed. The sending-end zero-pressure unlocking valve group sends a unified voltage increase command. That is, after unlocking for a certain period, the sending-end zero-pressure unlocking valve group sends a voltage increase command to both its own valve group and the receiving-end zero-pressure unlocking valve group. Upon receiving the voltage increase command, the receiving-end zero-pressure unlocking valve group begins voltage increase after a delay of one inter-station communication time. Upon receiving the voltage increase command from the sending end via inter-station communication, the receiving-end zero-pressure unlocking valve group immediately increases the voltage. The voltage increase speeds at both ends are consistent, thus achieving synchronous voltage increase at both the sending and receiving ends. Figure 8 The zero-pressure unlocking valve group synchronously pressurizes at time t3.

[0071] In some embodiments, synchronously boosting the voltage of the third and fourth converter valves includes: boosting the DC bias of the reference wave of the third converter valve to the target operating voltage at a second preset rate, and switching the outer loop control mode of the third converter valve to active power control after the DC bias of the reference wave of the third converter valve is greater than a first preset value; boosting the DC bias of the reference wave of the fourth converter valve to the target operating voltage at a second preset rate, and switching the outer loop control mode of the fourth converter valve to DC voltage control after the DC bias of the reference wave of the fourth converter valve is greater than a second preset value.

[0072] The first and second preset values ​​are voltage values, and the specific values ​​can be set according to the actual situation. No specific limitations are made here.

[0073] Specifically, the process of pressurizing the zero-pressure unlocking valve group at both the sending and receiving ends is as follows: the reference wave DC bias of the conventional unlocking valve group at the sending end is adjusted according to a predetermined speed U. RAMPThe voltage is increased to the target operating voltage U of the valve assembly. REFDC The outer loop switches to active power control after the DC bias exceeds a certain value; the reference wave DC bias of the receiving end's conventional unlocking valve group follows the same speed U. RAMP The voltage is increased to the target operating voltage U of the valve assembly. REFDC The outer loop switches to DC voltage control after the DC bias exceeds a certain value, with the DC voltage reference value U. ucref DC bias U with reference wave ref_PZ same.

[0074] It is understood that the UHV flexible DC system startup method provided in this application groupes the high-end and low-end valve groups of the first and second converter stations into a first unlocking valve group and a second unlocking valve group, and performs differentiated charging, differentiated unlocking, synchronous voltage reduction, and synchronous voltage boosting from zero on the first and second unlocking valve groups. This achieves voltage reduction startup of the entire UHV flexible DC system, meeting the voltage reduction operation requirements of the UHV flexible DC transmission system. The reason for voltage reduction startup in this application embodiment is that the line insulation is damaged and cannot operate at high voltage. During the charging process in this application embodiment, one valve group is a 0-voltage valve group, so that the DC electrode voltage during charging is only the charging DC voltage of a single valve group. After unlocking, as the 0-voltage valve group boosts voltage (from 0 to the target value), the normally unlocked valve group simultaneously reduces voltage (from the charging voltage to the target value). Thus, during the unlocking process, the DC electrode voltage will not exceed the target value, achieving voltage reduction throughout the entire process. For example, in related technologies, the high and low valve groups are simultaneously AC charged, and the DC port voltage of the high and low valve groups is the initial peak voltage, such as 660kV (i.e., the DC side voltage after charging a single valve group is 330kV). However, for an 800kV UHVDC system, the reduced-voltage start-up requires a DC voltage of 70%, i.e., 560kV. This results in a voltage of 660kV after charging being much higher than 560kV. In the embodiment of this application, the first unlocking valve group is AC charged with a DC port voltage of 330kV, and the second unlocking valve group is short-circuited charged with a DC port voltage of 0kV. Thus, after the high and low valve groups are fully charged, the DC port voltage is lower than 560kV.

[0075] Figure 10 This is a schematic diagram of the overall process of a startup method for an ultra-high voltage flexible DC system provided in an embodiment of this application. For example, see [link to relevant documentation]. Figure 10 The overall process of starting up this ultra-high voltage flexible DC system includes the following steps: Step S101: Connect the two valve groups and connect the poles.

[0076] Step S102: Select either the high-end or low-end valve group as the regular unlocking valve group, and the other valve group as the zero-pressure unlocking valve group. The zero-pressure unlocking valve group is combined with the DC side bypass switch BPS.

[0077] Step S103: Close the AC switch of the two valve groups to charge the conventional unlocking valve group normally, and short-circuit the DC side of the zero-voltage unlocking valve group for charging.

[0078] Step S104: After both valve groups are charged, the normal unlocking valve group is unlocked normally with a certain voltage, and the zero-voltage unlocking valve group is unlocked with 0 DC voltage. After unlocking, the DC side bypass switch BPS is switched off.

[0079] Step S105: After the conventional unlocking valve group is unlocked, the voltage is gradually reduced to the operating target value (i.e., the target operating voltage). After the zero-pressure unlocking valve group is unlocked, the voltage is gradually increased from 0 to the operating target value until the DC side voltage of both types of valve groups reaches the voltage reduction operating target value, thus realizing the full-process voltage reduction operation of the pole.

[0080] In summary, the UHV flexible DC system startup method provided in this application is a step-down startup method for UHV flexible DC systems, which enables the flexible DC system to start and operate with reduced voltage throughout the entire process, starting from the charging stage. This application can solve the problem that the UHV flexible DC transmission system cannot operate with reduced voltage throughout the entire process because the pole line voltage is greater than the target value for step-down operation during charging, thus avoiding insulation breakdown and achieving step-down startup of the UHV flexible DC transmission system.

[0081] Figure 11 This is a schematic block diagram of a starting device for an ultra-high voltage flexible DC system provided in an embodiment of this application. On the other hand, this embodiment provides a starting device for an ultra-high voltage flexible DC system; please refer to [link to relevant documentation]. Figure 11 The ultra-high voltage flexible DC system starting device 10 includes: an activation module 11 for activating the first converter station and the second converter station; a closing module 12 for closing the DC side bypass switch of the second unlocking valve group, wherein the second unlocking valve group is a zero-voltage unlocking valve group; a charging module 13 for closing the first AC switch of the first converter station and the second AC switch of the second converter station to normally charge the first unlocking valve group and perform DC side short-circuit charging on the second unlocking valve group; an unlocking module 14 for unlocking the first unlocking valve group at a preset voltage and unlocking the second unlocking valve group at zero voltage and disconnecting the DC side bypass switch when the first converter station and the second converter station have completed charging and met the preset unlocking conditions; and a boost / buck module 15 for stepping down the first unlocking valve group from the preset voltage to the target operating voltage and stepping up the second unlocking valve group from zero voltage to the target operating voltage.

[0082] The technical solution of this application provides a starting device for an ultra-high voltage flexible DC system. This device groups the high-end and low-end valve groups of the first and second converter stations into a first unlocking valve group and a second unlocking valve group, and performs differentiated charging, differentiated unlocking, synchronous voltage reduction, and synchronous voltage increase from zero on the first and second unlocking valve groups. This achieves voltage reduction starting of the entire ultra-high voltage flexible DC system. The reason for voltage reduction starting in this application embodiment is that the line insulation is damaged, preventing operation at high voltage. During the charging process in this application embodiment, one valve group is a 0-voltage valve group, ensuring that the DC electrode voltage is only the charging DC voltage of a single valve group during charging. After unlocking, while the 0-voltage valve group increases voltage (from 0 to the target value), the normally unlocked valve group simultaneously decreases voltage (from the charging voltage to the target value). Therefore, during unlocking, the DC electrode voltage will not exceed the target value, achieving voltage reduction throughout the entire process.

[0083] In some embodiments, the charging module 13 is further configured to: All modules of the first unlocking valve group are locked, and the full-bridge module capacitors in the bridge arm are charged through the diodes of the full-and-half-bridge hybrid topology converter valve; After the full-bridge module successfully draws power, the full-bridge module is gradually disconnected until the charging voltage of the half-bridge module is the same as that of the full-bridge module. After all modules of the full-bridge hybrid topology converter valve have successfully gained power, the N modules with the highest voltage are disconnected, where N is a positive integer.

[0084] In some embodiments, the first unlocking valve assembly includes a first switching valve and a second switching valve; the unlocking module 14 is further configured to: The first converter valve is unlocked according to the preset voltage, and the outer loop of the first converter valve is controlled by active power control during unlocking. The second converter valve is unlocked according to the preset voltage, and the outer loop of the second converter valve is controlled by DC voltage control during unlocking.

[0085] In some embodiments, the boost / buck module 15 is further configured to: When the first converter valve completes unlocking and reaches the preset time difference, the voltage is reduced according to the preset voltage; the preset time difference is the unlocking time difference between the first converter valve and the second converter valve. After the second converter valve is unlocked, it begins to reduce the voltage according to the preset voltage to achieve synchronous voltage reduction of the first and second converter valves.

[0086] In some embodiments, the boost / buck module 15 is further configured to: The reference wave DC bias of the first converter valve is reduced to the target operating voltage at a first preset speed; The DC voltage reference value and reference wave DC bias of the second converter valve are reduced to the target operating voltage at a first preset speed.

[0087] In some embodiments, the second unlocking valve assembly includes a third switching valve and a fourth switching valve; the unlocking module 14 is further configured to: The outer loop control mode of the third converter valve is switched from active power control to submodule voltage control; wherein, the voltage reference value of submodule voltage control is the rated submodule voltage, and the DC bias of the reference wave of submodule voltage control is zero. Switch the outer loop control mode of the fourth converter valve from DC voltage control to submodule voltage control.

[0088] In some embodiments, the boost / buck module 15 is further configured to: After the third converter valve is unlocked for a second preset time, a pressure boosting command is sent to the third converter valve and the fourth converter valve. The third converter valve is controlled to pressurize according to the pressurization command after a preset time difference following a pressurization command. The fourth converter valve is controlled to increase pressure upon receiving a pressure increase command, so as to achieve synchronous pressure increase of the third and fourth converter valves.

[0089] In some embodiments, the boost / buck module 15 is further configured to: The reference wave DC bias of the third converter valve is boosted to the target operating voltage at the second preset speed, and after the reference wave DC bias of the third converter valve is greater than the first preset value, the outer loop control mode of the third converter valve is switched to active power control. The reference wave DC bias of the fourth converter valve is boosted to the target operating voltage at a second preset speed, and after the reference wave DC bias of the fourth converter valve is greater than the second preset value, the outer loop control mode of the fourth converter valve is switched to DC voltage control.

[0090] In some embodiments, the unlocking module 14 is further configured to: After the DC side bypass switch is unlocked and operated for a first preset time, a preset number of harmonic voltages are injected into the reference wave DC bias of the second unlocking valve group.

[0091] 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.

[0092] The above provides a detailed description of the ultra-high voltage flexible DC system startup method and apparatus provided in the embodiments of this application. Specific examples have been used to illustrate the principles and implementation methods of this application. The description of the above embodiments is only for the purpose of helping to understand the method and core ideas of this application. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of this application. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. A method for starting an ultra-high voltage flexible DC transmission system, characterized in that, The ultra-high voltage flexible DC system includes a first converter station and a second converter station. Both the first converter station and the second converter station include high-end valve groups and low-end valve groups. Specifically, the two high-end valve groups are first unlocking valve groups and the two low-end valve groups are second unlocking valve groups, or the two low-end valve groups are first unlocking valve groups and the two high-end valve groups are second unlocking valve groups. The method includes: The first converter station and the second converter station are put into operation, and the DC side bypass switch of the second unlocking valve group is closed; wherein, the second unlocking valve group is a zero-pressure unlocking valve group; Close the first AC switch of the first converter station and the second AC switch of the second converter station to charge the first unlocking valve group normally and to perform DC-side short-circuit charging on the second unlocking valve group. When the first converter station and the second converter station have completed charging and met the preset unlocking conditions, the first unlocking valve group is unlocked according to the preset voltage, the second unlocking valve group is unlocked according to zero voltage, and the DC side bypass switch is disconnected. The first unlocking valve assembly is depressurized from the preset voltage to the target operating voltage, and the second unlocking valve assembly is boosted from zero voltage to the target operating voltage.

2. The method according to claim 1, characterized in that, The second unlocking valve group uses a hybrid full-bridge / half-bridge topology converter valve; the DC-side short-circuit charging of the second unlocking valve group includes: All modules of the first unlocking valve group are locked, and the full-bridge module capacitor in the bridge arm is charged through the diode of the full-and-half-bridge hybrid topology converter valve; After the full-bridge module successfully draws power, the full-bridge module is gradually disconnected until the charging voltage of the half-bridge module is the same as that of the full-bridge module. After all modules of the full-bridge hybrid topology converter valve have successfully gained power, the N modules with the highest voltage are disconnected, where N is a positive integer.

3. The method according to claim 1, characterized in that, The first unlocking valve assembly includes a first switching valve and a second switching valve; the step of unlocking the first unlocking valve assembly according to a preset voltage includes: The first converter valve is unlocked according to the preset voltage, and the outer loop of the first converter valve is controlled by active power control during unlocking. The second converter valve is unlocked according to the preset voltage, and the outer loop of the second converter valve is controlled by DC voltage control during unlocking.

4. The method according to claim 3, characterized in that, The step of reducing the voltage of the first unlocking valve assembly from the preset voltage to the target operating voltage includes: When the first converter valve completes unlocking and reaches a preset time difference, the voltage is reduced according to the preset voltage; the preset time difference is the unlocking time difference between the first converter valve and the second converter valve; After the second converter valve is unlocked, it begins to reduce the voltage according to the preset voltage to achieve synchronous voltage reduction of the first converter valve and the second converter valve.

5. The method according to claim 4, characterized in that, The synchronous pressure reduction of the first switching valve and the second switching valve includes: The reference wave DC bias of the first converter valve is reduced to the target operating voltage at a first preset speed; The DC voltage reference value and reference wave DC bias of the second converter valve are reduced to the target operating voltage at the first preset speed.

6. The method according to claim 1, characterized in that, The second unlocking valve assembly includes a third switching valve and a fourth switching valve; the step of unlocking the second unlocking valve assembly at zero voltage includes: The outer loop control mode of the third converter valve is switched from active power control to submodule voltage control; wherein, the voltage reference value of the submodule voltage control is the rated submodule voltage, and the DC bias of the reference wave of the submodule voltage control is zero. The outer loop control mode of the fourth converter valve is switched from DC voltage control to the submodule voltage control.

7. The method according to claim 6, characterized in that, The step of boosting the voltage of the second unlocking valve assembly from zero to the target operating voltage includes: After the third converter valve is unlocked for a second preset time, a pressure boosting command is sent to the third converter valve and the fourth converter valve; The third converter valve is controlled to increase the pressure according to the pressure increase command after a preset time difference after receiving the pressure increase command; The fourth converter valve is controlled to increase pressure upon receiving the pressure increase command, so as to achieve synchronous pressure increase of the third converter valve and the fourth converter valve.

8. The method according to claim 7, characterized in that, The synchronous pressurization of the third and fourth switching valves includes: The reference wave DC bias of the third converter valve is boosted to the target operating voltage at a second preset speed, and after the reference wave DC bias of the third converter valve is greater than the first preset value, the outer loop control mode of the third converter valve is switched to the active power control. The reference wave DC bias of the fourth converter valve is boosted to the target operating voltage at the second preset speed, and after the reference wave DC bias of the fourth converter valve is greater than the second preset value, the outer loop control mode of the fourth converter valve is switched to the DC voltage control.

9. The method according to claim 1, characterized in that, Disconnecting the DC-side bypass switch includes: After the DC-side bypass switch is unlocked and operated for a first preset time, a preset number of harmonic voltages are injected into the reference wave DC bias of the second unlocking valve group.

10. A starting device for an ultra-high voltage flexible DC system, characterized in that, The ultra-high voltage flexible DC system includes a first converter station and a second converter station. Both the first converter station and the second converter station include high-end valve groups and low-end valve groups. Specifically, the two high-end valve groups are first unlocking valve groups and the two low-end valve groups are second unlocking valve groups, or the two low-end valve groups are first unlocking valve groups and the two high-end valve groups are second unlocking valve groups. The device includes: The input module is used to input the first converter station and the second converter station; A closing module is used to close the DC side bypass switch of the second unlocking valve group; wherein, the second unlocking valve group is a zero-pressure unlocking valve group; The charging module is used to close the first AC switch of the first converter station and the second AC switch of the second converter station to charge the first unlocking valve group normally and to perform DC-side short-circuit charging on the second unlocking valve group. The unlocking module is used to unlock the first unlocking valve group according to a preset voltage and the second unlocking valve group according to zero voltage and disconnect the DC side bypass switch when the first converter station and the second converter station have completed charging and met the preset unlocking conditions. The step-up / step-down module is used to step down the first unlocking valve group from the preset voltage to the target operating voltage, and to step up the second unlocking valve group from zero voltage to the target operating voltage.

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