Extra-high voltage flexible direct current converter valve low-pressure pressurization test method and system

Abstract

The invention discloses a low-pressure pressurization test method and system for an extra-high voltage flexible direct current converter valve. The charging stage of the converter valve comprises the following steps in sequence: adjusting a voltage regulating transformer to the lowest gear, switching on an alternating current side switch, and then gradually adjusting the voltage of the voltage regulating transformer to a value required by a test; a T4 pipe of the converter valve full-bridge sub-module is conducted; part of the sub-modules with the highest voltage are cut off in turn, and active charging is carried out; when the voltage of the sub-modules reaches a preset value and active charging is completed, T2 tubes of all cut half-bridge sub-modules are broken, T2 tubes and T4 tubes of all cut full-bridge sub-modules are broken, at the moment, the sum of the voltage of the two bridge arm full-bridge sub-modules and the voltage of one bridge arm half-bridge sub-module is larger than the voltage of a converter transformer valve side line, and a starting resistor bypass switch is turned on. The surge current in the whole charging stage is reduced, and the protection action of the input power supply is prevented from being influenced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of extra-high voltage flexible direct current transmission, in particular to an extra-high voltage flexible direct current converter valve low-voltage withstand voltage test method and system. BACKGROUND

[0002] Extra-high voltage flexible direct current transmission technology is a key technology for building a new type of power system. It can realize efficient transmission of large-scale renewable energy over a long distance, solve the problem of cross-regional power transmission, has the ability of self-supporting reactive power, low loss and multi-terminal networking, significantly improves the safety and stability of the power grid, actively suppresses fault current through full-controlled IGBT devices to avoid the commutation failure problem of traditional direct current transmission, supports black start of the power grid, and greatly improves the transmission efficiency and land utilization.

[0003] The low-voltage withstand voltage test of the converter valve is carried out after the body test of the converter transformer, the body and valve control system joint debugging test, the converter station control protection device test, and the pole control and valve control joint debugging test. The purpose is to verify the correctness of the primary wiring of the converter transformer and the converter valve. The existing technology related to the low-voltage withstand voltage test of the converter is simple and only targets the traditional extra-high voltage direct current converter.

[0004] The traditional direct current low-voltage withstand voltage test scheme is shown in FIG. 1. In each single valve of the converter valve, one thyristor is selected, and the other thyristors are short-circuited by a temporary short-circuit line. A 12-pulse rectifier connection mode is temporarily connected, the positive and negative polar voltages on the direct current side are led out, and according to the calculation, the corresponding direct current load resistor is connected on the direct current side. Figure 1

[0005] (1) Disconnect the low-voltage side connection of the booster transformer, check whether the phase sequence and phase angle requirements of the test power source and the ignition voltage signal of the control and protection device are consistent.

[0006] (2) After checking and confirming that the synchronization signal is normal, disconnect the test power source, adjust the voltage regulating transformer to zero, and connect the low-voltage side connection of the booster transformer.

[0007] (3) Adjust the voltage regulating transformer, apply power to the converter transformer, unlock the converter valve, adjust the trigger angle, record the valve side voltage and waveform at each angle, and analyze whether the control connection of the synchronization loop and the trigger control loop is complete.

[0008] The topology and control method of the traditional extra-high voltage direct current converter and the flexible direct current converter valve are quite different. The extra-high voltage flexible direct current transmission project cannot refer to the existing low-voltage withstand voltage test method of the traditional extra-high voltage direct current converter. SUMMARY

[0009] ​To solve the technical problems existing in the prior art, the application provides a method and system for low-voltage pressure test of an extra-high voltage flexible AC transmission system (HVDC) valve, which places a start-up resistor bypass switch at the last step to reduce the impact current in the whole charging stage and avoid affecting the protection action of an input power supply.

[0010] To this end, the application adopts the following technical solution.

[0011] In a first aspect, the application provides a method for low-voltage pressure test of an extra-high voltage flexible AC transmission system (HVDC) valve. A test circuit used for low-voltage pressure test includes, in series, a test power supply, a voltage regulating transformer, a step-up transformer, an AC circuit breaker, a start-up resistor, a converter transformer, and a converter valve based on a hybrid of half-bridge sub-modules and full-bridge sub-modules. The charging stage of the converter valve is in the following order: The voltage regulating transformer is adjusted to the lowest gear, an AC side switch is closed, and then the voltage of the voltage regulating transformer is gradually adjusted to a required value for test; A T4 tube of a full-bridge sub-module of the converter valve is turned on; Part of the sub-modules with the highest voltage are successively cut off for active charging; When the voltage of the sub-modules reaches a preset value to complete the active charging, T2 tubes of all the cut-off half-bridge sub-modules and T2 and T4 tubes of all the cut-off full-bridge sub-modules are disconnected, at this time, the sum of the voltage of the two bridge arms of the full-bridge sub-modules and the voltage of the half-bridge sub-module of one bridge arm is greater than the voltage of the valve side line of the converter transformer, and the start-up resistor bypass switch is closed.

[0012] Further, N sub-modules are selected for each bridge arm of the converter valve to carry out low-voltage pressure test, and the remaining sub-modules are bypassed by short-circuit wires. In the uncontrolled charging stage, the peak value of the valve side line voltage charges M sub-modules, M = N when the converter valve is composed of half-bridge sub-modules, M = the number of full-bridge modules of two bridge arms + the number of half-bridge modules of one bridge arm when the converter valve is composed of hybrid full-bridge and half-bridge sub-modules, and M = 2N when the converter valve is composed of full-bridge sub-modules.

[0013] Further, the threshold voltage of each sub-module entering the active charging is set as V1, and the effective value of the corresponding valve side line voltage is V1 x M / N kV. The lowest voltage required for the primary side of the converter transformer is k x V1 x M / N kV according to the transformation ratio k of the converter transformer. kV; The voltage of the converter transformer side of the step-up transformer is selected based on the effective value of the valve side line voltage and the lowest voltage required for the primary side of the converter transformer, considering the voltage drop on the start-up resistor and the margin.

[0014] Further, the time t1 is calculated in advance when the voltage of the sub-module decreases from the preset value to V1 when the converter valve is unlocked, to ensure that the converter valve is unlocked instantaneously before the voltage of the sub-module decreases to V1.​

[0015] Further, the starting resistor and the converter transformer are both provided in the main circuit of the UHV flexible DC power transmission project.

[0016] Further, before the low-voltage withstand test, the following conditions need to be met: 1) the connection between the corresponding phase and neutral line of the converter transformer network side is completed; 2) the test of the converter transformer is completed and the test result is qualified; 3) the connection of the converter valve and the installation and debugging of the protection control system are completed; 4) the single valve inspection test of the converter valve equipment is completed, and the test procedure and the temporary setting of the protection outlet of the control protection system are completed; 5) the switches and the knife switch of the main circuit of the converter valve have the operation conditions; 6) the converter valve sub-modules not participating in the test are bypassed reliably; 7) the converter station is isolated from the AC system, and the DC side of the converter station is isolated and the knife is pulled out, in the isolated state.

[0017] Further, the starting resistor is composed of a plurality of modules connected in series, and one or more modules connected in series are selected to be led out, and the resistance value of the starting resistor is adjusted according to the test situation.

[0018] Further, the unlocking stage of the converter valve is: first disconnecting the AC circuit breaker, and then unlocking the converter valve through background sequential control, the converter valve is phase-locked according to the voltage of the first AC voltage transformer, and the converter valve adopts open-loop control; the consistency of the voltage phase sequence and phase of the first AC voltage transformer and the second AC voltage transformer is checked to realize the checking of the primary wiring of the converter transformer and the converter valve.

[0019] The first AC voltage transformer is arranged at the side of the step-up transformer of the AC circuit breaker, and the second AC voltage transformer is arranged at the side of the starting resistor of the AC circuit breaker.

[0020] In a second aspect, the application provides a low-voltage withstand test system for a UHV flexible converter valve, which comprises: The AC side switch unit is used to adjust the voltage regulating transformer to the lowest gear, close the AC side switch, and then gradually adjust the voltage of the voltage regulating transformer to the required value of the test; The IGBT conduction unit is used to conduct the T4 tube of the full-bridge sub-module of the converter valve; The active charging unit is used to cut off part of the sub-modules with the highest voltage in turn for active charging; The starting resistor bypass switch unit is used to disconnect the T2 tube of all cut-off half-bridge sub-modules and the T2 tube and T4 tube of all cut-off full-bridge sub-modules when the sub-module voltage reaches the preset value and the active charging is completed, at this time, the sum of the voltages of the two bridge arm full-bridge sub-modules and the voltage of the half-bridge sub-module of one bridge arm is greater than the voltage of the valve side line of the converter transformer, and the starting resistor bypass switch is closed.

[0021] Further, the extra-high voltage flexible DC valve low-voltage pressure test system further comprises an unlocking unit: the AC circuit breaker is first disconnected, and then the DC valve is instantaneously unlocked through background sequential control, the DC valve is phase-locked according to the voltage of the first AC voltage transformer, and the DC valve adopts open-loop control; the consistency of the voltage phase sequence and phase of the first AC voltage transformer and the second AC voltage transformer is checked to check the primary wiring of the DC transformer and the DC valve.

[0022] The application has the following beneficial effects: the application places the "start-up resistor bypass switch" at the last step of the charging stage of the DC valve, reduces the impact current of the whole charging stage, avoids affecting the protection action of the input power supply, and provides a reference for subsequent extra-high voltage flexible DC engineering to carry out low-voltage pressure test. BRIEF DESCRIPTION OF DRAWINGS

[0023] To more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed in the specific embodiments or prior art description. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0024] Figure 1 a traditional DC low-voltage pressure test wiring diagram; Figure 2 a wiring diagram of the extra-high voltage flexible DC valve low-voltage pressure test of the application; Figure 3 a flowchart of the extra-high voltage flexible DC valve low-voltage pressure test method of the application; Figure 4 a composition diagram of the extra-high voltage flexible DC valve low-voltage pressure test system of the application; Figure 2 In the figure, HBSM represents a half-bridge sub-module, and FBSM represents a full-bridge sub-module. DETAILED DESCRIPTION

[0025] The technical solutions of the present application will be described in detail below with reference to the drawings. Obviously, the described embodiments are some embodiments of the present application, not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0026] Embodiment 1 The embodiment provides an extra-high voltage flexible DC valve low-voltage pressure test method.

[0027] Before the extra-high voltage flexible DC valve is subjected to low-voltage pressure test, the following conditions need to be met: 1) The connection between the corresponding phase and neutral line of the converter transformer network side is completed; 2) The converter transformer test is completed, and the test results are qualified; 3) The installation and debugging of the converter valve wiring and protection control system are completed; 4) The converter valve equipment completes the inspection test of the single valve, and the control protection system completes the test procedure and temporary setting of the protection outlet; 5) The switches and knife switches related to the main circuit of the converter valve have the operating conditions; 6) The converter valve sub-modules not participating in the test are reliably bypassed; 7) The converter station is isolated from the AC system, the DC side of the converter station is isolated, and the knife is pulled open, in the isolated state.

[0028] As shown in Figure 2 , the test circuit used in the low-voltage voltage boosting test of the UHV flexible DC converter valve includes a test power supply, a voltage regulating transformer, a step-up transformer, an AC circuit breaker, an AC voltage transformer, a starting resistor, a converter transformer, and a converter valve based on a hybrid of half-bridge sub-modules and full-bridge sub-modules. The starting resistor and the converter transformer are both provided with the main circuit of the UHV flexible DC power transmission project.

[0029] (1) Test power supply The test power supply is preferably connected from the construction transformer to minimize the impact on the operating equipment. If the construction transformer capacity is insufficient, a generator power supply or a station transformer connection is considered.

[0030] (2) Voltage regulating transformer The voltage regulating transformer is specially configured for the test. By adjusting the voltage regulating voltage, on the one hand, the impact current at the moment of closing the AC circuit breaker can be reduced, and on the other hand, the AC voltage can be adjusted to meet the test requirements.

[0031] (3) Step-up transformer The step-up transformer is specially configured for the test, which boosts the test power supply to meet the requirement of charging the sub-module voltage to the preset value in the charging stage of the converter valve, and lays the foundation for reliable unlocking of the converter valve.

[0032] (4) AC circuit breaker The AC circuit breaker is specially configured for the test. When the AC circuit breaker is closed, the converter valve starts charging. When the converter valve completes the active charging (the sub-module voltage reaches the preset value), the AC circuit breaker is pulled open, and the passive phase checking of the converter valve is carried out. At this time, since the converter valve is unlocked under the condition that the AC circuit breaker is pulled open, the impact on the AC system is avoided, and the energy input from the AC system to the converter valve is also avoided, ensuring the safety of the converter valve sub-modules.

[0033] (5) Voltage transformer The AC voltage transformers are arranged on both sides of the AC circuit breaker. When the converter valve is instantaneously unlocked, the converter valve is phase-locked according to the voltage of the first AC voltage transformer PT1 and controlled by open loop, and the checking of the primary connection of the converter transformer and the converter valve can be realized by checking the phase sequence and phase of the first AC voltage transformer PT1 and the second AC voltage transformer PT2. The first AC voltage transformer PT1 is arranged on the side of the step-up transformer of the AC circuit breaker, and the second AC voltage transformer PT2 is arranged on the side of the starting resistor of the AC circuit breaker.

[0034] (6) Starting resistor The starting resistor is provided in the main circuit of the UHV flexible HVDC project. Since the voltage and current of the low-voltage voltage test are much lower than the rated working condition value, the resistance value of the starting resistor is relatively large, which can easily lead to the difficulty of the sub-module voltage reaching the preset value in the charging stage. Since the starting resistor is usually composed of multiple modules in series, one or several modules can be selected to be connected in series to reduce the resistance value of the starting resistor, and the resistance value of the starting resistor can be adjusted according to the test situation. In addition, the energy of the starting resistor in the charging stage needs to be checked to avoid the energy exceeding the design value and causing the resistor to burn out.

[0035] (7) Converter transformer The converter transformer is provided in the main circuit of the UHV flexible HVDC project.

[0036] (8) Converter valve A small part (N) of the sub-modules of each bridge arm of the converter valve is selected to carry out the low-voltage voltage test, and the remaining sub-modules are bypassed by short-circuiting. In the uncontrolled charging stage, the line voltage peak value charges M sub-modules (M=N when all the sub-modules are half-bridge sub-modules, M=the number of full-bridge modules of two bridge arms+the number of half-bridge modules of one bridge arm when the half-bridge sub-modules are mixed, and M=2N when all the sub-modules are half-bridge sub-modules), and the threshold voltage of each sub-module entering the active charging is V1. Thus, the corresponding valve-side line voltage effective value is V1×M / 1.414 kV. According to the transformation ratio k of the converter transformer, the minimum voltage required for the primary side of the converter transformer is k×V1×M / 1.414 kV. On the basis of the above valve-side line voltage effective value and the minimum voltage required for the primary side of the converter transformer, the voltage on the starting resistor is considered, and the voltage on the converter side of the step-up transformer is selected.

[0037] The above-mentioned low-voltage voltage test method of the UHV HVDC converter valve is composed of a charging stage and an unlocking stage, as shown in Figure 3 .

[0038] The charging stage of the converter valve has four stages of closing the AC side switch, turning on the T4 tube of the full-bridge sub-module, closing the start-up resistor bypass switch, and actively charging. The four stages are prone to generate impulse current, which may affect the protection action of the input power supply. In order to prevent the above problems, the charging strategy of the converter valve in the low-voltage withstand test needs to be optimized and designed.

[0039] In order to reduce the impulse current in the whole charging stage, the "closing the start-up resistor bypass switch" is implemented in the last step. The optimized charging strategy is as follows: Close the AC side switch, and then gradually adjust the voltage of the voltage regulating transformer to the required value of the test. Turn on the T4 tube of the full-bridge sub-module of the converter valve. Alternately cut off the sub-modules with the highest voltage, and actively charge. When the sub-module voltage reaches the preset value to complete the active charging, the T2 tube of all cut-off half-bridge sub-modules and the T2 tube and T4 tube of all cut-off full-bridge sub-modules are disconnected. At this time, the sum of the voltage of the two bridge arms of the full-bridge sub-module and the voltage of the half-bridge sub-module of one bridge arm is greater than the valve side line voltage of the converter transformer. Close the start-up resistor bypass switch. The time t1 of the sub-module voltage from the preset value to V1 when the converter valve is unlocked needs to be calculated in advance to ensure that the converter valve completes the instantaneous unlocking before the sub-module voltage drops to V1.

[0040] The unlocking stage of the converter valve is: disconnecting the AC circuit breaker in sequence through background control, and then instantaneously unlocking the converter valve. When unlocking, the converter valve is phase-locked according to the voltage of the first AC voltage transformer, and the converter valve adopts open-loop control. By checking whether the voltage phase sequence and phase of the first AC voltage transformer and the second AC voltage transformer are consistent, the primary wiring of the converter transformer and the converter valve is checked.

[0041] The low-voltage withstand test method of the UHV flexible converter valve of the embodiment is simulated as follows.

[0042] The simulation parameters are as follows: 1. Voltage regulating transformer The rated voltage is 400V / 0-400V, the Y / Y wiring, and the capacity is 630kVA.

[0043] 2. Step-up transformer The rated voltage is 35kV / 0.4kV, the Dyn wiring, and the capacity is 630kVA.

[0044] 3. Start-up resistor The start-up resistor is 2166.66Ω.

[0045] 4. Converter transformer Table 1. Converter transformer parameter table

[0046] 5. Converter valve Each bridge arm selects 15 sub-modules (9 full-bridge, 6 half-bridge) to carry out low-voltage voltage boosting test, and the rest of the sub-modules are bypassed by short-circuit wires. The full-bridge sub-module is T4 tube at 500V, and 3 sub-modules are cut off in the active charging stage.

[0047] The simulation results are as follows: 1. Waveform in the charging stage It can be seen from the simulation results of the sub-module voltage, bridge arm current and current waveform at the 400V side in the starting charging process that the maximum moment of the impact current is when the AC circuit breaker is closed, at which time the peak current flowing through the 400V side of the construction transformer is about 1100A, the peak impact current flowing through the 400V side of the construction transformer is about 750A when the T4 tube of the full-bridge sub-module is closed, the peak impact current flowing through the 400V side of the construction transformer is about 300A after bypassing the last sub-module in the active voltage balancing stage, and the sub-module capacitor voltage is about 1.5kV in the steady state.

[0048] It can be seen from the simulation results of the starting resistance current, starting resistance current effective value and starting resistance energy waveform that the maximum moment of the impact current flowing through the starting resistance is when the AC circuit breaker is closed, at which time the peak current is 13A, the effective value is 9A, and the steady-state current effective value is about 0.2A.

[0049] 2. Waveform in the instantaneous unlocking stage It can be seen from the simulation results that the voltage phase sequence of PT1 and PT2 on both sides of the AC circuit breaker is consistent, thereby judging the correctness of the primary wiring of the converter transformer and the converter valve.

[0050] Embodiment 2 The embodiment provides a UHV flexible DC converter valve low-voltage voltage boosting test system for realizing the UHV flexible DC converter valve low-voltage voltage boosting test method in embodiment 1, as shown in the figure, which is composed of an AC side switch unit, an IGBT conduction unit, an active charging unit, a starting resistance bypass switch unit and an unlocking unit. Figure 4

[0051] The starting resistance AC side switch unit is used to adjust the voltage regulator transformer to the lowest gear, close the AC side switch, and then gradually adjust the voltage of the voltage regulator transformer to the required value of the test; The IGBT conduction unit is used to conduct the T4 tube of the full-bridge sub-module of the converter valve; The active charging unit is used to cut off part of the sub-modules with the highest voltage in turn for active charging; ​The start-up resistor bypass switch unit is closed when the voltage of the sub-modules reaches the preset value and the active charging is completed, the T2 tubes of all the cut-off half-bridge sub-modules are disconnected, and the T2 tubes and T4 tubes of all the cut-off full-bridge sub-modules are disconnected, at this time, the sum of the voltage of the two bridge arms of the full-bridge sub-modules and the voltage of the half-bridge sub-module is greater than the valve side line voltage of the converter transformer, and the start-up resistor bypass switch is closed; The unlocking unit is controlled by the background sequence to disconnect the AC circuit breaker first, and then to instantaneously unlock the converter valve, when unlocking, the converter valve is phase-locked according to the voltage of the first AC voltage transformer, and the converter valve adopts open-loop control; the voltage phase sequence and phase of the first AC voltage transformer and the second AC voltage transformer are checked to realize the checking of the primary wiring of the converter transformer and the converter valve.

[0052] It should be noted that each unit in the above-mentioned UHV flexible DC converter valve low-voltage withstand voltage test system can be realized by software, hardware, or a combination thereof. The above-mentioned units can be embedded in or independent of the processor in the computer device in hardware form, or can be stored in the memory in the computer device in software form, so as to be called and executed by the processor. For specific limitations of the UHV flexible DC converter valve low-voltage withstand voltage test system, see the limitations of the UHV flexible DC converter valve low-voltage withstand voltage test method (i.e. embodiment 1) in the above, both of which have the same functions and effects, and will not be described here.

[0053] The above description of the embodiments is to facilitate the understanding and application of the present application by those skilled in the art. Those skilled in the art can easily make various modifications to the above embodiments, and apply the general principles described herein to other embodiments without creative labor. Therefore, the present application is not limited to the above embodiments, and the improvements and modifications of the present application made by those skilled in the art according to the disclosure of the present application should be within the scope of protection of the present application.

Claims

1. A low-voltage pressurization test method for a UHV flexible DC converter valve, wherein the test circuit used in the low-voltage pressurization test includes a test power supply, a voltage regulating transformer, a step-up transformer, an AC circuit breaker, a starting resistor, a converter transformer, and a converter valve based on a mixture of half-bridge submodules and full-bridge submodules connected in series. The method is characterized in that: The charging stages of the converter valve are as follows in order: Adjust the voltage regulating transformer to the lowest gear, close the AC side switch, and then gradually adjust the voltage of the voltage regulating transformer to the required value for the test; Conducting the T4 tube of the converter valve full bridge submodule; Cut off some submodules with the highest voltage in turn for active charging; When the submodule voltage reaches the preset value and active charging is completed, the T2 tubes of all removed half-bridge submodules and the T2 and T4 tubes of all removed full-bridge submodules are disconnected. At this time, the sum of the voltages of the two bridge arm full-bridge submodules and the voltage of one bridge arm half-bridge submodule is greater than the valve-side line voltage of the converter transformer, and the starting resistor bypass switch is closed.

2. The low-pressure pressurization test method for ultra-high voltage flexible direct current converter valve according to claim 1 is characterized in that: N submodules are selected from each bridge arm of the converter valve for low-voltage pressurization testing, and the remaining submodules are bypassed with short-circuit wiring. During the uncontrolled charging stage, the valve-side line voltage peak charges the M submodules. When the converter valve is composed entirely of half-bridge submodules, M=N. When the converter valve is composed of a mixture of full-bridge and half-bridge submodules, M=the number of full-bridge modules in the two bridge arms + the number of half-bridge modules in one bridge arm. When the converter valve is composed entirely of full-bridge submodules, M=2N.

3. The low-pressure pressurization test method for ultra-high voltage flexible direct current converter valve according to claim 2 is characterized in that: The threshold voltage for each submodule to enter active charging is set to V1, and the corresponding valve side line voltage effective value is V1×M / kV, according to the transformation ratio k of the converter transformer, the minimum voltage required on the primary side of the converter transformer is calculated as k×V1×M / kV; Based on the above valve-side line voltage effective value and the minimum voltage required on the primary side of the converter transformer, the voltage drop on the starting resistor and the margin are taken into consideration to select the converter transformer side voltage of the step-up transformer.

4. The low-pressure pressurization test method for ultra-high voltage flexible direct current converter valve according to claim 3 is characterized in that: It is necessary to precalculate the time t1 for the submodule voltage to drop from the preset value to V1 when the converter valve is unlocked to ensure that the converter valve is instantaneously unlocked before the submodule voltage drops to V1.

5. The low-pressure pressurization test method for ultra-high voltage flexible direct current converter valve according to claim 1 is characterized in that: The starting resistor and converter transformer are both provided in the main circuit of the ultra-high voltage flexible direct current transmission project.

6. The low-pressure pressurization test method for ultra-high voltage flexible direct current converter valve according to claim 1 is characterized in that: Before conducting a low-pressure pressurization test, the following conditions must be met: 1) All connections between the corresponding phases and neutral lines on the grid side of the converter transformer are completed; 2) The converter transformer test is completed and the test results are qualified; 3) The converter valve wiring and protection control system installation and commissioning are completed; 4) The converter valve equipment completes the inspection and test of the single valve, and the control and protection system completes the test procedures and the temporary setting of the protection outlet; 5) The switches and knife switches related to the main circuit of the converter valve are ready for operation; 6) The converter valve submodules not participating in the test are reliably bypassed; 7) The converter station is isolated from the AC system, and the DC side knife is pulled open and is in an isolated state.

7. The low-pressure pressurization test method for ultra-high voltage flexible direct current converter valve according to claim 1 is characterized in that: The starting resistor is composed of multiple modules connected in series. One or more modules are selected to be connected in series and the resistance of the starting resistor is adjusted according to the test situation.

8. The low-pressure pressurization test method for ultra-high voltage flexible direct current converter valve according to claim 1 is characterized in that: The unlocking phase of the converter valve is as follows: the AC circuit breaker is first disconnected through background sequence control, and then the converter valve is instantaneously unlocked. During unlocking, the converter valve is phase-locked according to the voltage of the first AC voltage transformer, and the converter valve adopts open-loop control. By checking whether the voltage phase sequence and phase of the first AC voltage transformer and the second AC voltage transformer are consistent, the primary wiring of the converter transformer and the converter valve is verified. The first AC voltage transformer is arranged on the step-up transformer side of the AC circuit breaker, and the second AC voltage transformer is arranged on the starting resistor side of the AC circuit breaker.

9. A low-voltage pressurization test system for ultra-high voltage flexible DC converter valves, wherein the test circuit used in the low-voltage pressurization test includes a test power supply, a voltage regulating transformer, a step-up transformer, an AC circuit breaker, a starting resistor, a converter transformer, and a converter valve based on a mixture of half-bridge submodules and full-bridge submodules connected in series. The system is characterized in that: include: Close the AC side switch unit: used to adjust the voltage regulating transformer to the lowest gear, close the AC side switch, and then gradually adjust the voltage of the voltage regulating transformer to the required value for the test; IGBT conduction unit: used to conduct the T4 tube of the converter valve full bridge submodule; Active charging unit: used to remove some submodules with the highest voltage in turn for active charging; Close the starting resistor bypass switch unit: When the submodule voltage reaches the preset value and completes active charging, disconnect the T2 tubes of all removed half-bridge submodules and disconnect the T2 tubes and T4 tubes of all removed full-bridge submodules. At this time, the sum of the voltage of the two bridge arm full-bridge submodules and the voltage of one bridge arm half-bridge submodule is greater than the valve-side line voltage of the converter transformer, and close the starting resistor bypass switch.

10. The low-pressure pressurization test system for ultra-high voltage flexible direct current converter valves according to claim 9 is characterized in that: The system also includes an unlocking unit that first disconnects the AC circuit breaker through background sequence control and then instantly unlocks the converter valve. During unlocking, the converter valve is phase-locked according to the voltage of the first AC voltage transformer, and the converter valve adopts open-loop control. The primary wiring of the converter transformer and the converter valve is verified by checking whether the voltage phase sequence and phase of the first AC voltage transformer and the second AC voltage transformer are consistent. The first AC voltage transformer is arranged on the step-up transformer side of the AC circuit breaker, and the second AC voltage transformer is arranged on the starting resistor side of the AC circuit breaker.

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