A test circuit and method for operating a valve segment of an MCC converter valve

Abstract

This invention belongs to the field of flexible DC technology and discloses an operational test circuit and method for an MCC converter valve section. By setting up a multi-tap step-down transformer, its high-voltage side is electrically connected to the SDM commutation bridge side of the MCC test valve section. Multiple taps on the low-voltage side are connected in parallel with the capacitors of each sub-module of the accompanying valve section via series diodes. The peak output voltage of each tap matches the rated voltage of the sub-module capacitor of the accompanying valve section, thus providing an independent energy flow path for the SDM commutation bridge sub-module. This circuit topology allows cascaded sub-modules and the commutation bridge module to be tested synchronously in the same test circuit, effectively overcoming the problem of existing technologies that can only test cascaded sub-modules individually. This improves the completeness and efficiency of MCC converter valve type testing and provides a reliable test verification method for the engineering application of MCC converter valves.

Description

Technical Field

[0001] This invention belongs to the field of flexible DC technology, and particularly relates to an operational test circuit and method for an MCC converter valve section. Background Technology

[0002] Flexible DC transmission (VSC-HVDC) is a new generation of high-voltage direct current transmission technology composed of fully controlled power electronic devices. It features independent control of active and reactive power, no commutation failure issues, the ability to supply power to passive networks, and excellent harmonic performance. It is widely used in distributed renewable energy grid connection, asynchronous interconnection of AC grids, and supplying power to weak or isolated networks. Regarding converter topologies, compared to earlier two-level and three-level converters, the modular multi-level converter (MMC) has become the mainstream solution for current flexible DC transmission projects due to its suitability for high-voltage, high-capacity transmission, low power device losses, and high waveform quality. In recent years, to further improve converter performance, those skilled in the art have proposed the modular commutated converter (MCC) topology. Compared to MMC, MCC can significantly reduce the amount of submodule capacitors by more than 75%, while also reducing the number of power semiconductor devices. Based on this, the integrated gate commutated thyristor (IGCT) with high surge current capability is applied to the MCC to form the IGCT-MCC scheme. By utilizing the surge current capability of the IGCT and the soft switching characteristics of the MCC, high overload operation can be achieved economically, providing strong grid support capability for the next generation of high-resilience DC transmission systems.

[0003] The converter valve in flexible DC transmission is a core component of the flexible DC transmission system, and its safety and reliability directly affect the operational reliability of the entire transmission project. Before the converter valve is officially put into operation, it must undergo comprehensive type testing, including insulation testing and operational testing. The valve section operational test aims to verify the operating characteristics of key components of the converter valve under actual operating conditions using standard test conditions. Currently, the commonly used operational test circuit topology consists of one auxiliary valve section, one valve section under test, multiple power supply circuits, and one load inductor. Both the auxiliary and under-test valve sections are composed of n sub-modules connected in series, and the two are connected through the load inductor. The power supply circuits are responsible for charging the valve sections at the initial stage of the test and replenishing the energy lost during the test. During the test, the power supply circuit first charges the capacitor of the test valve section module to its rated value. Then, the test valve section pulses through the test valve section to gradually raise its voltage to the rated value. Subsequently, all power modules are unlocked, and the test valve section and the tested valve section operate according to their respective modulation waves. By adjusting the amplitude and phase of the modulation waves, the voltage of the tested valve section module and the valve section current are made consistent with the actual operating conditions, thereby verifying the module's stress resistance capability. However, this test circuit topology can only test the cascaded sub-modules (SM) of the MCC valve section. The switching-direction sub-module (SDM) cannot be tested simultaneously in the same circuit.

[0004] It is evident that the existing test circuit for flexible DC converter valves cannot meet the synchronous testing requirements of the cascade submodule and the commutation bridge submodule in the MCC valve section, thus restricting the integrity of the MCC converter valve type test and the progress of its engineering application. Summary of the Invention

[0005] This invention provides an operational test circuit and method for an MCC converter valve section. This circuit can meet the synchronous testing requirements of the cascade submodule and the commutation bridge submodule in the MCC valve section, thus promoting the integrity of MCC converter valve type testing and the progress of its engineering application.

[0006] To achieve the above objectives, the present invention employs the following technical content: An operational test circuit for an MCC converter valve section includes a test valve section, a load inductor, a power replenishment circuit, an MCC test valve section, a multi-tap step-down transformer, and an AC power supply. The MCC test valve section includes multiple cascaded sub-modules connected in series and multiple sets of SDM commutation bridge modules; The SDM commutation bridge side of the MCC test valve section is electrically connected to the high-voltage side of the multi-tap step-down transformer. The low-voltage side of the multi-tap step-down transformer is provided with multiple taps. Each tap is connected in parallel with the sub-module capacitor of the test valve section through a series diode. The peak output voltage of each tap on the low-voltage side matches the rated voltage of the sub-module capacitor of the test valve section. The energy replenishment circuit is connected in parallel across the capacitor of any submodule of the test valve section to replenish the circuit power loss. The AC power supply is connected to the input terminals of the MCC test valve section and the accompanying test valve section respectively through diodes, and is used to charge the sub-module capacitors of the MCC test valve section and the accompanying test valve section in the initial stage of the test. The load inductor is connected in series between the MCC test valve section and the auxiliary test valve section.

[0007] Furthermore, the MCC test valve section includes 4 sets of SDM commutation bridge sub-modules. The number of SDM commutation bridge sub-modules in each set is determined by rounding up the ratio of the valve section voltage to the withstand voltage value of a single SDM commutation bridge sub-module in the project.

[0008] Furthermore, the four sets of SDM commutation bridge sub-modules constitute two pairs of bridge arms, namely the first bridge arm pair and the second bridge arm pair. During test operation, the first bridge arm pair and the second bridge arm pair are alternately turned on to simulate power frequency commutation.

[0009] Furthermore, the test valve section is composed of multiple sub-modules connected in series, and the rated voltage of the capacitor of the sub-module of the test valve section is the same as the rated voltage of the cascaded sub-module in the MCC test valve section.

[0010] Furthermore, the number of taps on the low-voltage side of the multi-tap step-down transformer is the same as the number of sub-modules in the test valve section; each tap is connected to a sub-module capacitor to simulate the stepped voltage waveform of the MCC converter valve in actual engineering.

[0011] Furthermore, it also includes AC switches; The AC switch is connected in series at the output terminal of the AC power supply and is used to control the on / off state of the initial charging circuit.

[0012] Furthermore, the power replenishment circuit includes a controllable charging power supply and a filtering element; the output terminal of the controllable charging power supply is electrically connected to the two ends of the submodule capacitor in parallel via the filtering element. The controllable charging power supply is configured to dynamically adjust the output current to maintain a constant capacitor voltage based on the voltage drop of the submodule capacitor in the test valve section.

[0013] An operational testing method for an MCC converter valve section, based on the aforementioned operational testing circuit for the MCC converter valve section, includes: Close the AC power supply and use the AC power supply to perform uncontrolled rectified charging of the sub-module capacitors of the MCC test valve section and the accompanying test valve section through the diode; When the voltage of a submodule capacitor is detected to reach the preset control threshold, the number of submodules put into operation in the test valve section is gradually reduced to half of the initial number. The submodules participating in operation are dynamically switched based on the voltage sorting strategy until the voltage of all submodules reaches the rated value. Disconnect the AC power supply and connect the power replenishment circuit to maintain stable circuit voltage. Unlock the MCC test valve section and the auxiliary test valve section, control each group of SDM commutation bridge submodules to conduct alternately according to a preset cycle, and adjust the modulation amplitude and phase of the MCC test valve section and the auxiliary test valve section so that the submodule voltage and valve section current of the MCC test valve section reproduce the actual engineering operation conditions.

[0014] Furthermore, all SDM commutation bridge submodules are divided into first arm pairs and second arm pairs; the control of each group of SDM commutation bridge submodules to alternately conduct according to a preset cycle includes: The first bridge arm is controlled to conduct once every power frequency cycle, and the second bridge arm is controlled to conduct once every power frequency cycle. The drive signals of the two pairs of bridge arms are out of phase to simulate the working condition of AC current flowing into the cascaded module chain through the power frequency reversal of the commutation bridge.

[0015] Furthermore, in the sub-modules that dynamically switch to participate in operation based on the voltage sorting strategy, the voltage sorting strategy is specifically implemented as follows: The voltage values ​​of all sub-modules in the test valve section are collected in real time and sorted. The sub-modules with lower voltage are connected to the circuit first, while the sub-modules with higher voltage are bypassed to achieve a balanced increase in the voltage of each sub-module.

[0016] Compared with the prior art, the present invention has the following beneficial effects: This invention provides an operational test circuit for an MCC converter valve section. A multi-tap step-down transformer is used, with its high-voltage side electrically connected to the SDM commutation bridge side of the MCC test valve section. Multiple taps on the low-voltage side are connected in parallel with the capacitors of each sub-module in the accompanying valve section via series diodes. The peak output voltage of each tap matches the rated voltage of the sub-module capacitor in the accompanying valve section, thus providing an independent energy flow path for the SDM commutation bridge sub-module. During testing, AC power simultaneously charges both the MCC test valve section and the accompanying valve section via diodes. A supplementary energy circuit is connected in parallel across the sub-module capacitor in the accompanying valve section to compensate for circuit power losses. A load inductor is connected in series between the two valve sections to achieve current coupling. This circuit topology allows cascaded sub-modules and commutation bridge sub-modules to be tested synchronously in the same test circuit, effectively overcoming the limitation of existing technologies that can only test cascaded sub-modules individually. This improves the completeness and efficiency of MCC converter valve type testing, providing a reliable test verification method for the engineering application of MCC converter valves.

[0017] This invention also provides a method for operational testing of an MCC converter valve segment. Based on the aforementioned operational testing circuit for the MCC converter valve segment, this method, in the initial stage of the test, uses an AC power supply via diodes to perform uncontrolled rectified charging of the submodule capacitors of the MCC test valve segment and the accompanying valve segment. When the submodule capacitor voltage reaches a preset control threshold, the number of submodules in the accompanying valve segment is gradually reduced to half of the initial number. A voltage sorting strategy is used to dynamically switch the participating submodules, ensuring that the capacitor voltages of each submodule are uniformly raised to their rated values, avoiding the voltage imbalance problem that may occur with traditional pulse charging. Subsequently, the AC power supply is disconnected and a recharge circuit is activated to maintain stable circuit voltage. After unlocking both valve segments, each group of SDM commutation bridge submodules is controlled to alternately conduct according to a preset cycle. By adjusting the modulation amplitude and phase, the test conditions are reproduced to reflect the actual engineering operation state. This method, through an orderly charging control strategy and an alternating SDM conduction mechanism, achieves synchronous operation testing of the cascaded submodules of the MCC valve segment and the commutation bridge modules in the same circuit, ensuring that the test conditions are highly consistent with the actual engineering conditions, and improving the accuracy and reliability of the test results. Attached Figure Description

[0018] Figure 1 A schematic diagram of an MCC converter valve topology provided in an embodiment of the present invention; Figure 2 This is a schematic diagram of the operational test circuit topology of a conventional MCC converter valve section provided in an embodiment of the present invention; Figure 3 This is a schematic diagram of the operational test circuit topology of an MCC converter valve section provided in an embodiment of the present invention. Detailed Implementation

[0019] To make the technical problems solved, the technical solutions, and the beneficial effects of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0020] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0021] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0022] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0023] The technical terms involved in this invention are explained below: VSC-HVDC: Full name is Voltage Source Converter based High Voltage Direct Current, which is high voltage direct current transmission based on voltage source converter, or flexible direct current transmission for short.

[0024] MMC stands for Modular Multi-level Converter.

[0025] MCC stands for Modular Commutated Converter.

[0026] IGCT stands for Integrated Gate Commutated Thyristor.

[0027] SM: Full name Sub-Module, specifically referring to the cascaded sub-modules in the MCC valve section.

[0028] SDM: Short for Switching-Direction sub-Module, it is a unique sub-module type in MCC topology that distinguishes it from cascaded SM, and is used to implement current commutation function.

[0029] As mentioned in the background, Tsinghua University proposed the MCC topology in 2023. Compared with MMC, MCC can significantly reduce the amount of capacitors used in submodules by more than 75%, while also reducing the number of power semiconductor devices. Building on this, the high-surge-current power semiconductor device—the integrated gate commutator thyristor—is applied to the MCC, further proposing the IGCT-MCC scheme suitable for next-generation high-toughness DC transmission systems. The specific MCC converter valve topology is as follows: Figure 1 As shown; Figure 1 Middle,U dc U is the DC bus voltage; sc_a U sc_b U sc_c H1, H2, H3, and H4 represent the phase voltages of phases A, B, and C, respectively; H1, H2, H3, and H4 represent the positions of the SDM modules on the four arms of the commutation bridge; u ref2 This represents the reference voltage signal. Utilizing the surge current capability of the IGCT and the soft-switching characteristics of the MCC, the IGCT-MCC can economically achieve high overload capacity, providing robust network support capabilities. For example... Figure 2 The diagram shows a commonly used operational test circuit topology, primarily consisting of one test valve section, one valve section under test, multiple power supply circuits, and one load inductor. SM1 to SMn represent the first to the nth submodules, respectively. Both the test valve section and the valve section under test are composed of n submodules (the number of n can be determined according to engineering test requirements, generally not exceeding 8) connected in series. Each submodule of the test valve section is connected to the power supply circuit, and the test valve section and the valve section under test are connected through the load inductor. The power supply circuit is mainly used to charge the test valve section and the valve section under test during the initial stage of the test and to replenish the electrical energy consumed by the power modules and the load during the test. This circuit topology has the following drawback: it can only test the cascaded submodules SM of the MCC valve section; the commutation bridge submodule SDM cannot be tested simultaneously.

[0030] To address the aforementioned issues, this embodiment provides an operational test circuit for the valve section of an MCC converter valve. This circuit allows for simultaneous operational tests of the cascaded submodule SM and the commutation bridge submodule SDM valve sections, improving test efficiency and reducing test time.

[0031] For example, this embodiment provides an operational test circuit for an MCC converter valve section, including: a test valve section, a load inductor, a power replenishment circuit, an MCC test valve section, a multi-tap step-down transformer, and an AC power supply; The MCC test valve section includes multiple cascaded sub-modules connected in series and multiple sets of SDM commutation bridge modules; The SDM commutation bridge side of the MCC test valve section is electrically connected to the high-voltage side of the multi-tap step-down transformer. The low-voltage side of the multi-tap step-down transformer is provided with multiple taps. Each tap is connected in parallel with the sub-module capacitor of the test valve section through a series diode. The peak output voltage of each tap on the low-voltage side matches the rated voltage of the sub-module capacitor of the test valve section. The energy replenishment circuit is connected in parallel across the capacitor of any submodule of the test valve section to replenish the circuit power loss. The AC power supply is connected to the input terminals of the MCC test valve section and the accompanying test valve section respectively through diodes, and is used to charge the sub-module capacitors of the MCC test valve section and the accompanying test valve section in the initial stage of the test. The load inductor is connected in series between the MCC test valve section and the auxiliary test valve section.

[0032] The detection device provided in this embodiment will be further explained below with reference to the accompanying drawings: like Figure 3 As shown, this embodiment provides an operational test circuit for an MCC converter valve section, the topology of which includes a test valve section, a load inductor, a power replenishment circuit, an MCC test valve section, a multi-tap step-down transformer, and an AC power supply.

[0033] The MCC test valve section consists of multiple cascaded sub-modules connected in series (SM). a1 SM aN-1 SM aN The system consists of multiple sets of SDM (SDM1, SDM2, SDM3, SDM4) commutation bridge submodules. In a preferred embodiment, the MCC test valve section includes four sets of SDM commutation bridge modules. The number of SDM commutation bridge modules in each set is determined by rounding up the ratio of the valve section voltage to the withstand voltage of a single SDM commutation bridge module in the engineering process. These four sets of SDM commutation bridge modules form two pairs of bridge arms, namely the first bridge arm pair and the second bridge arm pair. During test operation, the first bridge arm pair and the second bridge arm pair are alternately connected to simulate the power frequency commutation process, thereby realistically replicating the commutation operation state of the MCC converter valve in actual engineering.

[0034] Specifically, the SDM commutation bridge side of the MCC test valve section is electrically connected to the high-voltage side of the multi-tap step-down transformer. The low-voltage side of the multi-tap step-down transformer has multiple taps, each connected in parallel with a submodule capacitor of the test valve section via a series diode. The peak output voltage of each tap on the low-voltage side matches the rated voltage of the submodule capacitor of the test valve section. In one specific embodiment, the number of taps on the low-voltage side of the multi-tap step-down transformer is the same as the number of submodules in the test valve section, with each tap corresponding to a submodule capacitor. With this configuration, the voltage generated by the SDM commutation bridge submodule during operation is stepped down by the multi-tap step-down transformer and fed into the corresponding submodule capacitors of the test valve section, thereby simulating the stepped voltage waveform of the MCC converter valve in actual engineering. This provides an independent energy flow path for the SDM commutation bridge submodule, enabling it to conduct synchronous operational tests with cascaded submodules in the same test circuit.

[0035] Furthermore, the test valve section consists of multiple sub-modules connected in series. The rated voltage of the capacitor in the sub-module of the test valve section is the same as the rated voltage of the cascaded sub-module in the MCC test valve section, so as to ensure the matching of voltage levels between the two valve sections during the test and the accuracy of the test results.

[0036] Furthermore, a power replenishment circuit is connected in parallel across the capacitor of any submodule in the test valve section to compensate for circuit power losses. Specifically, the power replenishment circuit includes a controllable charging power supply and a filter element. The output of the controllable charging power supply is electrically connected to the capacitor in parallel via the filter element. During the test, the controllable charging power supply dynamically adjusts its output current to maintain a constant capacitor voltage based on the voltage drop of the submodule capacitor in the test valve section. This compensates for voltage attenuation caused by power module switching losses and load inductance losses, ensuring voltage stability of the test circuit during long-term continuous operation.

[0037] Furthermore, the AC power supply is connected to the input terminals of the MCC test valve section and the auxiliary test valve section via diodes, respectively, to perform uncontrolled rectified charging of the submodule capacitors of the MCC test valve section and the auxiliary test valve section during the initial stage of the test. An AC switch is connected in series at the output terminal of the AC power supply to control the on / off state of the initial charging circuit. A load inductor is connected in series between the MCC test valve section and the auxiliary test valve section to achieve current coupling between the two valve sections, simulating load conditions in actual engineering.

[0038] Based on the above-described operational test circuit, this embodiment also provides an operational test method for an MCC converter valve section, such as... Figure 3As shown, the MCC converter valve includes three-phase series-connected bridge arms. Each phase bridge arm consists of a cascaded modular multilevel submodule and an IGCT commutation bridge connected in parallel. AC current flows into the cascaded module chain after being flipped at power frequency by the commutation bridge. The three-phase cascaded module chain forms a DC circuit, and DC current flows into the three-phase cascaded module chain from the DC port. The DC filter circuit consists of the three-phase cascaded module chain bridge arms and a DC filter. Harmonic currents on the DC side of the MCC flow into the DC filter circuit from the cascaded module chain.

[0039] The specific steps of the operational test method for the MCC converter valve section include: Step 1: Close the AC power switch and use the AC power to perform uncontrolled rectified charging of the sub-module capacitors of the MCC test valve section and the accompanying test valve section through the diode.

[0040] Step Two: When the submodule capacitor voltage reaches the preset control threshold, the number of submodules in the test valve section gradually decreases to half of the initial number. During this process, the participating submodules are dynamically switched based on a voltage sorting strategy until all submodule voltages reach their rated values. The voltage sorting strategy is implemented as follows: The voltage values ​​of all submodules in the test valve section are collected in real time and sorted. Submodules with lower voltages are prioritized for operation, while submodules with higher voltages are bypassed. This achieves a balanced voltage increase for each submodule, avoiding uneven charging caused by differences in submodule parameters. It should be noted that the control threshold in this step is the minimum voltage required for the module control board to operate normally, typically ranging from 300 to 400V. In practical applications, the specific value of this control threshold can be determined based on the actual operating conditions.

[0041] Step 3: Disconnect the AC power switch and connect the power replenishment circuit to maintain stable circuit voltage.

[0042] Step 4: Unlock the MCC test valve section and the auxiliary test valve section, and control each group of SDM commutator bridge submodules to alternately conduct according to a preset cycle. Specifically, control the first bridge arm pair and the second bridge arm pair to alternately conduct once every power frequency cycle, simulating the power frequency reversal process of the commutator bridge in actual engineering. At the same time, adjust the modulation amplitude and phase of the MCC test valve section and the auxiliary test valve section so that the submodule voltage and valve section current of the MCC test valve section reproduce the actual engineering operating conditions, thereby achieving the purpose of verifying the stress resistance capability of the MCC converter valve module.

[0043] In summary, compared with existing technologies, this invention provides an operational test circuit and method for an MCC converter valve section, which has the following advantages: This invention introduces a multi-tap step-down transformer to couple the SDM commutation bridge side with the sub-module capacitors of the test valve section, creating an independent energy flow path for the SDM commutation bridge sub-modules. This fundamentally solves the technical deficiency of traditional operational test circuits that cannot synchronously test cascaded sub-modules and commutation bridge sub-modules. During the charging phase, a dynamic switching strategy based on voltage sequencing replaces traditional pulse charging, ensuring that the capacitor voltages of each sub-module are raised to their rated values ​​evenly, avoiding charging imbalances caused by parameter differences. During the operation phase, two pairs of SDM bridge arms alternately conduct according to the power frequency cycle, realistically simulating the power frequency switching process of the commutation bridge in actual engineering of the MCC converter valve, ensuring a high degree of consistency between the test conditions and the actual operating state. The energy replenishment circuit uses a controllable charging power supply to dynamically adjust the output according to voltage drops, ensuring voltage stability during long-term continuous testing. This invention expands the MCC valve section type test from only covering cascaded sub-modules to fully covering both cascaded sub-modules and commutation bridge modules, significantly improving the completeness and efficiency of the test, and providing a reliable test verification method for the engineering application of MCC converter valves.

[0044] The above embodiments are merely one of the implementation methods for achieving the technical solution of the present invention. The scope of protection claimed by the present invention is not limited to this embodiment, but also includes any variations, substitutions and other implementation methods that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention.

Claims

1. A test circuit for the operation of an MCC converter valve section, characterized in that, This includes the test valve section, load inductance, energy replenishment circuit, MCC test valve section, multi-tap step-down transformer, and AC power supply; The MCC test valve section includes multiple cascaded sub-modules connected in series and multiple sets of SDM commutation bridge modules; The SDM commutation bridge side of the MCC test valve section is electrically connected to the high-voltage side of the multi-tap step-down transformer. The low-voltage side of the multi-tap step-down transformer is provided with multiple taps. Each tap is connected in parallel with the sub-module capacitor of the test valve section through a series diode. The peak output voltage of each tap on the low-voltage side matches the rated voltage of the sub-module capacitor of the test valve section. The energy replenishment circuit is connected in parallel across the capacitor of any submodule of the test valve section to replenish the circuit power loss. The AC power supply is connected to the input terminals of the MCC test valve section and the accompanying test valve section respectively through diodes, and is used to charge the sub-module capacitors of the MCC test valve section and the accompanying test valve section in the initial stage of the test. The load inductor is connected in series between the MCC test valve section and the auxiliary test valve section.

2. The operational test circuit for an MCC converter valve section according to claim 1, characterized in that, The MCC test valve section includes 4 sets of SDM commutation bridge sub-modules. The number of SDM commutation bridge sub-modules in each set is determined by rounding up the ratio of the valve section voltage to the withstand voltage value of a single SDM commutation bridge sub-module in the project.

3. The operational test circuit for an MCC converter valve section according to claim 2, characterized in that, The four sets of SDM commutation bridge sub-modules constitute two pairs of bridge arms, namely the first bridge arm pair and the second bridge arm pair. During the test operation, the first bridge arm pair and the second bridge arm pair are alternately turned on to simulate power frequency commutation.

4. The operational test circuit for an MCC converter valve section according to claim 1, characterized in that, The test valve section is composed of multiple sub-modules connected in series, and the rated voltage of the capacitor of the sub-module in the test valve section is the same as the rated voltage of the cascaded sub-module in the MCC test valve section.

5. The operational test circuit for an MCC converter valve section according to claim 1, characterized in that, The number of taps on the low-voltage side of the multi-tap step-down transformer is the same as the number of sub-modules in the test valve section; each tap is connected to a sub-module capacitor to simulate the stepped voltage waveform of the MCC converter valve in actual engineering.

6. The operational test circuit for an MCC converter valve section according to claim 1, characterized in that, It also includes AC switches; The AC switch is connected in series at the output terminal of the AC power supply and is used to control the on / off state of the initial charging circuit.

7. The operational test circuit for an MCC converter valve section according to claim 1, characterized in that, The power replenishment circuit includes a controllable charging power supply and a filtering element; the output terminal of the controllable charging power supply is electrically connected to the two ends of the submodule capacitor in parallel via the filtering element. The controllable charging power supply is configured to dynamically adjust the output current to maintain a constant capacitor voltage based on the voltage drop of the submodule capacitor in the test valve section.

8. A method for operational testing of an MCC converter valve section, based on the operational testing circuit of the MCC converter valve section according to any one of claims 1-7, characterized in that, include: Close the AC power supply and use the AC power supply to perform uncontrolled rectified charging of the sub-module capacitors of the MCC test valve section and the accompanying test valve section through the diode; When the voltage of a submodule capacitor is detected to reach the preset control threshold, the number of submodules put into operation in the test valve section is gradually reduced to half of the initial number. The submodules participating in operation are dynamically switched based on the voltage sorting strategy until the voltage of all submodules reaches the rated value. Disconnect the AC power supply and connect the power replenishment circuit to maintain stable circuit voltage. Unlock the MCC test valve section and the auxiliary test valve section, control each group of SDM commutation bridge submodules to conduct alternately according to a preset cycle, and adjust the modulation amplitude and phase of the MCC test valve section and the auxiliary test valve section so that the submodule voltage and valve section current of the MCC test valve section reproduce the actual engineering operation conditions.

9. The operational testing method for an MCC converter valve section according to claim 8, characterized in that, All SDM commutation bridge submodules are divided into first bridge arm pairs and second bridge arm pairs; the control of each group of SDM commutation bridge modules to alternately conduct according to a preset cycle includes: The first bridge arm is controlled to conduct once every power frequency cycle, and the second bridge arm is controlled to conduct once every power frequency cycle. The drive signals of the two pairs of bridge arms are out of phase to simulate the working condition of AC current flowing into the cascaded module chain through the power frequency reversal of the commutation bridge.

10. The operational testing method for an MCC converter valve section according to claim 8, characterized in that, In the sub-modules that dynamically switch to participate in operation based on the voltage sorting strategy, the voltage sorting strategy is specifically implemented as follows: The voltage values ​​of all sub-modules in the test valve section are collected in real time and sorted. The sub-modules with lower voltage are connected to the circuit first, while the sub-modules with higher voltage are bypassed to achieve a balanced increase in the voltage of each sub-module.

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