Method, device and equipment for calculating short-circuit current level of direct-current bus of multi-terminal flexible direct-current back-to-back system

Abstract

The invention relates to a DC bus short-circuit current level calculation method, device and equipment of a multi-terminal flexible DC back-to-back system. The method comprises the following steps: acquiring system data, electrical quantity data and parameter data; valve side system short-circuit impedance, first valve side short-circuit impedance and first impedance data are obtained through calculation according to the system data, the parameter data and the electrical quantity data; obtaining a first manufacturing deviation and a second manufacturing deviation, and updating the first valve side short-circuit impedance according to the first manufacturing deviation to obtain a second valve side short-circuit impedance; updating the first impedance data according to the second manufacturing deviation to obtain second impedance data; calculating according to the system data, the parameter data, the valve side system short-circuit impedance, the second valve side short-circuit impedance and the second impedance data to obtain an AC short-circuit current component; calculating a discharge current according to the endurance capacity type and the electrical quantity data; calculating short-circuit current according to the discharge current and the alternating-current short-circuit current component; and calculating the short-circuit current level according to the short-circuit current.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of short-circuit current of multi-terminal flexible power system, and particularly relates to a DC bus short-circuit current level calculation method, device and equipment of a multi-terminal flexible DC back-to-back system. BACKGROUND

[0002] The multi-terminal flexible DC back-to-back system is an important technical means for improving power flow regulation and power system stability and flexibility of a load center. The multi-terminal flexible DC back-to-back system is directly connected in parallel on the DC side by a plurality of modular multilevel converters (MMCs), as shown in FIG. 1. When the equipment of the multi-terminal flexible DC back-to-back system is selected, it is very important to check the short-circuit current level of each main equipment in the flexible DC back-to-back system. If the short-circuit current level is not reasonably designed, the equipment will be damaged under severe operating conditions, and even serious safety accidents will occur. For the multi-terminal flexible DC back-to-back system, since a plurality of converters are directly connected in parallel on the DC side, when an inter-pole short circuit occurs on the DC bus, a very large short-circuit current will flow on the DC bus. If the designed short-circuit current level of the DC bus is not reasonable, a serious power safety accident will occur once a fault occurs. Figure 5 The topology structure of the existing modular multilevel converter is shown in FIG. 2. The modular multilevel converter includes A, B and C three phases, each phase includes two bridge arms of upper and lower, a total of 6 bridge arms, and each phase is connected to the AC side by a bridge arm inductor in the middle. In each bridge arm, N power modules are cascaded, and each power module mainly includes a power device and a DC capacitor.

[0003] Figure 6 At present, the overcurrent level of the modular multilevel converter is mainly obtained by simulation research. By setting faults under different operating conditions, design requirements are proposed according to the simulation results. In addition to the large simulation workload, when the simulation modeling accuracy has a large error, the accuracy of the simulation results cannot be evaluated, especially the simulation results are more dependent on the fault occurrence time, control protection strategy, etc. In many cases, the maximum DC bus short-circuit current level cannot be obtained by simulation, and there is a lack of theoretical guidance and comparison. There is no other related technical literature on the calculation method of the DC bus short-circuit current of the flexible DC back-to-back system and the modular multilevel converter. The above-mentioned overcurrent level of the modular multilevel converter is obtained by simulation results, and the fault occurrence time, control protection strategy, etc. have a greater impact on the simulation results. Therefore, in general cases, the maximum DC bus short-circuit current level cannot be obtained by simulation, and there is a lack of theoretical calculation guidance.

[0004] At present, the overcurrent level of the modular multilevel converter is mainly obtained by simulation research. By setting faults under different operating conditions, design requirements are proposed according to the simulation results. In addition to the large simulation workload, when the simulation modeling accuracy has a large error, the accuracy of the simulation results cannot be evaluated, especially the simulation results are more dependent on the fault occurrence time, control protection strategy, etc. In many cases, the maximum DC bus short-circuit current level cannot be obtained by simulation, and there is a lack of theoretical guidance and comparison. There is no other related technical literature on the calculation method of the DC bus short-circuit current of the flexible DC back-to-back system and the modular multilevel converter. The above-mentioned overcurrent level of the modular multilevel converter is obtained by simulation results, and the fault occurrence time, control protection strategy, etc. have a greater impact on the simulation results. Therefore, in general cases, the maximum DC bus short-circuit current level cannot be obtained by simulation, and there is a lack of theoretical calculation guidance.

[0005] ​Therefore, it has important practical significance to propose a DC bus short-circuit current level calculation method of a multi-terminal flexible DC back-to-back system, which can quickly determine the DC bus short-circuit current level and provide a theoretical reference for simulation results. SUMMARY

[0006] The application provides a DC bus short-circuit current level calculation method, device and equipment of a multi-terminal flexible DC back-to-back system, to solve the technical problem that the overcurrent level of an existing modular multilevel converter is obtained through simulation, but the simulation workload is large and the modeling accuracy cannot be guaranteed, resulting in uncertainty of the accuracy of the obtained overcurrent level.

[0007] To achieve the above-mentioned purpose, the application provides the following technical solutions:

[0008] On the one hand, a DC bus short-circuit current level calculation method of a multi-terminal flexible DC back-to-back system is provided, which is applied to a multi-terminal flexible DC back-to-back system, and the multi-terminal flexible DC back-to-back system includes a multi-terminal flexible power transmission module. Each flexible power transmission module includes an MMC, a transformer connected to the MMC, and an AC system connected to the transformer. The DC bus short-circuit current level calculation method includes the following steps:

[0009] Obtain system data of each AC system, electrical quantity data of each MMC, and parameter data of each transformer;

[0010] According to the system data, the parameter data and the electrical quantity data of each flexible power transmission module, the valve side system short-circuit impedance of the transformer, the first valve side short-circuit impedance and the first impedance data of the bridge arm reactor in the MMC are calculated.

[0011] Obtain the first manufacturing deviation of each transformer and the second manufacturing deviation of each bridge arm reactor. Update the first valve side short-circuit impedance of each transformer according to the first manufacturing deviation to obtain the second valve side short-circuit impedance of the transformer. Update the first impedance data of each bridge arm reactor according to the second manufacturing deviation to obtain the second impedance data of the bridge arm reactor.

[0012] According to the system data, the parameter data, the valve side system short-circuit impedance, the second valve side short-circuit impedance and the second impedance data of each flexible power transmission module, the AC short-circuit current component input into the MMC is calculated.

[0013] Obtain the tolerance type of the MMC in each flexible power transmission module. According to the tolerance type and the electrical quantity data of the MMC, the discharge current of the DC capacitor in the MMC is calculated.

[0014] calculating the short-circuit current of each MMC according to the short-circuit current component of the AC short-circuit current and the discharging current of the MMC, obtaining the short-circuit current of the DC bus when the MMC has a DC bus pole-to-pole short-circuit fault;

[0015] calculating the short-circuit current of each MMC according to the short-circuit current component of the AC short-circuit current and the discharging current of the MMC, obtaining the short-circuit current of the DC bus when the MMC has a DC bus pole-to-pole short-circuit fault;

[0016] Preferably, the system data includes system short-circuit impedance and system maximum operating voltage, the electrical quantity data includes bridge arm reactor inductance, DC capacitor capacitance value, DC voltage, power module quantity and power device nominal rated current, and the parameter data includes grid side rated voltage, valve side rated voltage, transformer capacity and short-circuit impedance percentage; calculating the valve side system short-circuit impedance of the transformer, the first valve side short-circuit impedance and the first impedance data of the bridge arm reactor in the MMC of each end according to the system data, the parameter data and the electrical quantity data of the flexible power transmission module of each end, the first impedance data of the bridge arm reactor including:

[0017] calculating the valve side system short-circuit impedance of the transformer according to the system short-circuit impedance, the grid side rated voltage and the valve side rated voltage;

[0018] calculating the first valve side short-circuit impedance of the transformer according to the valve side rated voltage, the transformer capacity and the short-circuit impedance percentage;

[0019] calculating the first impedance data of the bridge arm reactor according to the bridge arm reactor inductance.

[0020] Preferably, calculating the discharging current of the DC capacitor in the MMC according to the tolerance capability type and the electrical quantity data of the MMC of each end includes:

[0021] if the tolerance capability type is not considering the current-carrying tolerance capability of the power device, calculating the discharging current of the DC capacitor in the MMC according to the bridge arm reactor inductance, the DC capacitor capacitance value, the DC voltage and the power module quantity of each MMC;

[0022] if the tolerance capability type is considering the current-carrying tolerance capability of the power device, calculating the discharging current of the DC capacitor in the MMC according to the power device nominal rated current of each MMC.

[0023] Preferably, calculating the short-circuit current of each MMC according to the short-circuit current component of the AC short-circuit current and the discharging current of the MMC, obtaining the short-circuit current of the DC bus when the MMC has a DC bus pole-to-pole short-circuit fault includes:

[0024] The initial short-circuit current level corresponding to the DC bus flowing through the MMC at the end where the DC side outlet of the MMC occurs in the DC inter-pole short-circuit situation is obtained according to the short-circuit current calculation of each end of the MMC;

[0025] The initial short-circuit current level with the largest value is selected from the initial short-circuit current levels of the multiple ends as the short-circuit current level of the DC bus flowing through the multiple-end flexible DC back-to-back system.

[0026] Preferably, the DC bus short-circuit current level calculation method of the multiple-end flexible DC back-to-back system further comprises: obtaining the initial short-circuit current level of the MMC DC side outlet in the DC inter-pole short-circuit situation according to the short-circuit current of each end of the MMC by using a short-circuit level formula; and the short-circuit level formula is:

[0027]

[0028] In the formula, Ishort_i is the short-circuit current of the i-th MMC DC side outlet in the DC inter-pole short-circuit situation, N is the number of flexible power transmission modules, Ishort_j is the short-circuit current of the j-th MMC, and Idc_short_i is the initial short-circuit current level of the i-th MMC DC side outlet in the DC inter-pole short-circuit situation.

[0029] On the other hand, a DC bus short-circuit current level calculation device of a multiple-end flexible DC back-to-back system is provided, which is applied to a multiple-end flexible DC back-to-back system, and the multiple-end flexible DC back-to-back system comprises multiple-end flexible power transmission modules, each end of the flexible power transmission module comprises an MMC, a transformer connected with the MMC, and an alternating current system connected with the transformer, and the DC bus short-circuit current level calculation device comprises a data acquisition module, a data calculation module, a data update module, a current component calculation module, a discharge current calculation module, a short-circuit current calculation module, and a short-circuit current level calculation module.

[0030] The data acquisition module is used to acquire system data of each alternating current system, electrical quantity data of each MMC, and parameter data of each transformer.

[0031] The data calculation module is used to calculate, according to the system data, the parameter data, and the electrical quantity data of each end of the flexible power transmission module, a valve side system short-circuit impedance of the transformer at the end, a first valve side short-circuit impedance, and first impedance data of a bridge arm reactor in the MMC at the end.

[0032] The data updating module is configured to obtain a first manufacturing deviation of each transformer and a second manufacturing deviation of each bridge reactor, update the first valve-side short-circuit impedance of each transformer according to the first manufacturing deviation of the transformer to obtain a second valve-side short-circuit impedance of the transformer, and update the first impedance data of each bridge reactor according to the second manufacturing deviation of the bridge reactor to obtain second impedance data of the bridge reactor.

[0033] The current component calculation module is configured to calculate an AC short-circuit current component of an MMC at each end according to the system data, the parameter data, the valve-side system short-circuit impedance, the second valve-side short-circuit impedance and the second impedance data of the flexible power transmission module at the end.

[0034] The discharge current calculation module is configured to obtain a withstand capability type of the MMC in each end of the flexible power transmission module, and calculate a discharge current of a DC capacitor in the MMC at the end according to the withstand capability type and the electrical quantity data of the MMC at the end.

[0035] The short-circuit current calculation module is configured to calculate a short-circuit current of the MMC in the case of a DC bus pole short-circuit fault according to the discharge current of the MMC and the AC short-circuit current component.

[0036] The short-circuit current level calculation module is configured to calculate a short-circuit current level of a DC bus in the multi-terminal flexible DC back-to-back system according to the short-circuit current of each MMC.

[0037] Preferably, the system data includes a system short-circuit impedance and a system maximum operating voltage, the electrical quantity data includes a bridge reactor inductance, a DC capacitor capacitance, a DC voltage, a number of power modules and a nominal rated current of a power device, the parameter data includes a grid-side rated voltage, a valve-side rated voltage, a transformer capacity and a short-circuit impedance percentage, and the data calculation module includes a first short-circuit impedance calculation submodule, a second short-circuit impedance calculation submodule and an impedance calculation submodule.

[0038] The first short-circuit impedance calculation submodule is configured to calculate a valve-side system short-circuit impedance of the transformer according to the system short-circuit impedance, the grid-side rated voltage and the valve-side rated voltage.

[0039] The second short-circuit impedance calculation submodule is configured to calculate a first valve-side short-circuit impedance of the transformer according to the valve-side rated voltage, the transformer capacity and the short-circuit impedance percentage.

[0040] The impedance calculation submodule is configured to calculate first impedance data of the bridge reactor according to the bridge reactor inductance.

[0041] Preferably, the discharge current calculation module comprises a first calculation sub-module and a second calculation sub-module.

[0042] The first calculation sub-module is configured to calculate the discharge current of the DC capacitor in each MMC according to the bridge arm reactor inductance, the DC capacitor capacitance value, the DC voltage and the number of power modules of the MMC, when the withstand capability type is the power device current withstand capability not taken into account.

[0043] The second calculation sub-module is configured to calculate the discharge current of the DC capacitor in each MMC according to the nominal rated current of the power device of the MMC, when the withstand capability type is the power device current withstand capability taken into account.

[0044] Preferably, the short-circuit current level calculation module is further configured to calculate the initial short-circuit current level corresponding to the DC bus flowing through the MMC in the case of DC inter-pole short-circuit, when the DC side outlet of any one end of the MMC occurs in the DC inter-pole short-circuit condition, according to the short-circuit current of each end of the MMC; and select the initial short-circuit current level with the largest value from the initial short-circuit current levels of the multiple ends as the short-circuit current level flowing through the DC bus in the multi-terminal VSC-HVDC back-to-back system.

[0045] In another aspect, a terminal device is provided, comprising a processor and a memory.

[0046] The memory is configured to store program code and transmit the program code to the processor.

[0047] The processor is configured to execute the DC bus short-circuit current level calculation method of the multi-terminal VSC-HVDC back-to-back system according to the instructions in the program code.

[0048] The DC bus short-circuit current level calculation method, device and equipment of the multi-terminal flexible DC back-to-back system are applied to the multi-terminal flexible DC back-to-back system, the multi-terminal flexible DC back-to-back system comprises multi-terminal flexible power transmission modules, each flexible power transmission module comprises an MMC, a transformer connected with the MMC and an alternating current system connected with the transformer, the DC bus short-circuit current level calculation method comprises obtaining system data of each alternating current system, electrical quantity data of each MMC and parameter data of each transformer; the valve side system short-circuit impedance of the transformer, the first valve side short-circuit impedance of the transformer and the first impedance data of the bridge arm reactor in the MMC of the flexible power transmission module are obtained according to the system data, the parameter data and the electrical quantity data of each flexible power transmission module; the first manufacturing deviation of each transformer and the second manufacturing deviation of each bridge arm reactor are obtained, the first valve side short-circuit impedance of each transformer is updated according to the first manufacturing deviation of the transformer, and the second valve side short-circuit impedance of the transformer is obtained; the first impedance data of each bridge arm reactor is updated according to the second manufacturing deviation of the bridge arm reactor, and the second impedance data of the bridge arm reactor is obtained; the alternating current short-circuit current component input into the MMC of each flexible power transmission module is obtained according to the system data, the parameter data, the valve side system short-circuit impedance, the second valve side short-circuit impedance and the second impedance data of each flexible power transmission module; the discharge current of the DC capacitor in the MMC is obtained according to the type of the withstand capability of the MMC and the electrical quantity data of the MMC; the short-circuit current of the MMC when a DC bus interphase short-circuit fault occurs is obtained according to the discharge current of the MMC and the alternating current short-circuit current component; and the short-circuit current level flowing through the DC bus in the multi-terminal flexible DC back-to-back system is obtained according to the short-circuit current of each MMC.

[0049] It can be seen from the above technical solutions that the multi-terminal flexible DC back-to-back system DC bus short-circuit current level calculation method can quickly and effectively evaluate the DC bus short-circuit current level of the multi-terminal flexible DC back-to-back system, avoid setting a large number of simulation conditions for checking during simulation research, and avoid a large amount of repeated simulation work, thereby solving the technical problem that the overcurrent level of the existing modular multilevel converter is obtained through simulation, but the simulation workload is large and the modeling accuracy cannot be guaranteed, and the accuracy of the obtained overcurrent level cannot be determined.

[0050] The multi-terminal flexible DC back-to-back system DC bus short-circuit current level calculation device can quickly and effectively evaluate the DC bus short-circuit current level of the multi-terminal flexible DC back-to-back system through the data acquisition module, the data calculation module, the data update module, the current component calculation module, the discharge current calculation module, the short-circuit current calculation module and the short-circuit current level calculation module, avoid setting a large number of simulation conditions for checking during simulation research, and avoid a large amount of repeated simulation work. BRIEF DESCRIPTION OF DRAWINGS

[0051] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the accompanying drawings needed to be used in the description of the embodiments or the prior art will be briefly introduced. Obviously, the accompanying drawings in the following description only represent some of the embodiments of the present application, and for those skilled in the art, other drawings can also be obtained without creative labor.

[0052] Figure 1 A flow chart of steps of the DC bus short-circuit current level calculation method of the multi-terminal flexible DC back-to-back system described in the embodiments of the present application;

[0053] Figure 2 A DC inter-pole fault position diagram of the multi-terminal flexible DC back-to-back system in the DC bus short-circuit current level calculation method of the multi-terminal flexible DC back-to-back system described in the embodiments of the present application;

[0054] Figure 3 A frame diagram of the DC bus short-circuit current level calculation device of the multi-terminal flexible DC back-to-back system described in the embodiments of the present application;

[0055] Figure 4 A schematic diagram of the terminal device described in the embodiments of the present application;

[0056] Figure 5 A topology diagram of the existing multi-terminal flexible DC back-to-back system;

[0057] Figure 6 A topology structure diagram of the modular multilevel converter. DETAILED DESCRIPTION

[0058] In order to make the purposes, features and advantages of the present application more obvious and easy to understand, the technical solutions in the embodiments of the present application will be described clearly and completely in the following description of the embodiments of the present application, and obviously, the following described embodiments are only some of the embodiments of the present application, but not all the 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.

[0059] In the description of the embodiments of the present application, the terms “first” and “second” are only used for description purpose, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined with “first” and “second” can explicitly or implicitly include one or more of the features. In the description of the embodiments of the present application, the meaning of “multiple” is two or more, unless otherwise specifically limited.

[0060] In the embodiments of the present application, unless specifically defined and limited otherwise, the terms "mount", "connect", "connect", "fix", and the like should be understood in a broad sense, for example, it can be fixedly connected, or it can be detachably connected, or it can be integrated; it can be mechanically connected, or it can be electrically connected; it can be directly connected, or it can be indirectly connected through an intermediate medium; it can be the internal communication of two elements or the interaction relationship of two elements. For those skilled in the art, the specific meaning of the above terms in the embodiments of the present application can be understood according to the specific circumstances.

[0061] The embodiments of the present application provide a DC bus short-circuit current level calculation method, device and equipment of a multi-terminal flexible DC back-to-back system, which solves the technical problem that the overcurrent level of the existing modular multilevel converter is obtained by simulation, but the simulation workload is large and the modeling accuracy cannot be guaranteed, resulting in that the accuracy of the obtained overcurrent level cannot be determined.

[0062] Embodiment one:

[0063] Figure 1 The step flow chart of the DC bus short-circuit current level calculation method of the multi-terminal flexible DC back-to-back system described in the embodiments of the present application, Figure 2 The DC inter-pole fault position diagram of the multi-terminal flexible DC back-to-back system in the DC bus short-circuit current level calculation method of the multi-terminal flexible DC back-to-back system described in the embodiments of the present application.

[0064] As Figure 2 shown, the embodiments of the present application provide a DC bus short-circuit current level calculation method of a multi-terminal flexible DC back-to-back system, which is applied to a multi-terminal flexible DC back-to-back system, and the multi-terminal flexible DC back-to-back system includes a multi-terminal flexible power transmission module. Each flexible power transmission module includes an MMC and a transformer connected to the MMC and an AC system connected to the transformer.

[0065] It should be noted that MMC represents a modular multilevel converter. When an inter-pole short-circuit fault occurs in the DC bus of the multi-terminal flexible DC back-to-back system, the short-circuit current level of the DC bus is the largest. In this embodiment, the DC bus short-circuit current level calculation method of the multi-terminal flexible DC back-to-back system takes a four-terminal flexible DC back-to-back system as an example. The position of the inter-pole fault can occur at the DC side outlet of the modular multilevel converter MMC of each flexible power transmission module, such as Flt1 to Flt4 in Figure 2 .

[0066] As Figure 1 shown, the embodiments of the present application provide a DC bus short-circuit current level calculation method of a multi-terminal flexible DC back-to-back system, which is applied to a multi-terminal flexible DC back-to-back system, and the multi-terminal flexible DC back-to-back system includes a multi-terminal flexible power transmission module. Each flexible power transmission module includes an MMC and a transformer connected to the MMC and an AC system connected to the transformer.

[0067] S1. Obtain system data of each AC system, electrical quantity data of each MMC, and parameter data of each transformer.

[0068] It should be noted that in step S1, the data of each end flexible power transmission module is obtained, including the system data of the AC system of the end flexible power transmission module, the electrical quantity data of the MMC, and the parameter data of the transformer, to provide data for subsequent steps. In this embodiment, the system data includes system short-circuit impedance Xsc1i and system maximum operating voltage U1i_max, the electrical quantity data includes bridge arm reactor inductance Li, DC capacitor capacitance C, DC voltage Udc, power module number M, and power device nominal current Inom, and the parameter data includes grid side rated voltage U1i, valve side rated voltage U2i, transformer capacity STi, and short-circuit impedance percentage φi. Wherein, i=1, 2, …, N, N is the number of modular multilevel converters in parallel on the DC side of the multi-end flexible power transmission module or the multi-end flexible DC back-to-back system, N≥3.

[0069] S2. According to the system data, parameter data and electrical quantity data of each end flexible power transmission module, the valve side system short-circuit impedance of the transformer, the first valve side short-circuit impedance and the first impedance data of the bridge arm reactor in the MMC are calculated.

[0070] It should be noted that step S2 is to calculate the valve side system short-circuit impedance Xsc2i of the transformer, the first valve side short-circuit impedance XT2i and the first impedance data of the bridge arm reactor in the MMC according to the system data, parameter data and electrical quantity data of each end flexible power transmission module in step S1, to provide data for subsequent steps. In this embodiment, since the short-circuit current during the short-circuit of the multi-end flexible DC back-to-back DC side is mainly considered in two parts, one part is the AC feed-in of the AC system to the back-to-back system before the AC circuit breaker is tripped, and the other part is the capacitor discharge feed-in of the DC capacitor in the modular multilevel converter (MMC) due to the short-circuit of the DC side. For AC feed-in, step S2 is used to first convert the system short-circuit impedance Xsc1i of the AC system to the valve side of the transformer, to obtain the valve side system short-circuit impedance Xsc2i converted to the valve side of the transformer. Similarly, the first valve side short-circuit impedance XT2i of the short-circuit impedance of the transformer is also converted to the valve side of the transformer.

[0071] S3. Obtain the first manufacturing deviation of each transformer and the second manufacturing deviation of each bridge arm reactor, update the first valve side short-circuit impedance of each transformer according to the first manufacturing deviation thereof, to obtain the second valve side short-circuit impedance of the transformer; update the first impedance data of each bridge arm reactor according to the second manufacturing deviation thereof, to obtain the second impedance data of the bridge arm reactor.

[0072] It should be noted that in step S3, firstly, the first manufacturing deviation m% of each transformer and the second manufacturing deviation n% of each bridge arm reactor are obtained, and secondly, the parameters of the corresponding device are updated according to the manufacturing deviation of the device, so as to provide data for the subsequent steps, and the updated data is used for short-circuit current level calculation, so that the short-circuit current level of the modular multilevel converter is higher. In the embodiment, the first valve side short-circuit impedance of each transformer is updated by using a short-circuit impedance updating formula according to the first manufacturing deviation of the transformer, so as to obtain the second valve side short-circuit impedance of the transformer; and the first impedance data of each bridge arm reactor is updated by using an impedance value updating formula according to the second manufacturing deviation of the bridge arm reactor, so as to obtain the second impedance data of the bridge arm reactor. The short-circuit impedance updating formula is: , wherein XT2_i is the second valve side short-circuit impedance of the i th transformer; and the impedance value updating formula is: , wherein XL_i is the second impedance data of the i th bridge arm reactor. The first impedance data = Li * π * 100.

[0073] S4. According to the system data, the parameter data, the valve side system short-circuit impedance, the second valve side short-circuit impedance and the second impedance data of each flexible power transmission module, the AC short-circuit current component input into the MMC at the end is calculated.

[0074] It should be noted that in step S4, the AC short-circuit current component input into the MMC at the end is calculated according to the system data and the parameter data obtained in step S1, the valve side system short-circuit impedance Xsc2i obtained in step S2, the second valve side short-circuit impedance XT2_i and the second impedance data XL_i obtained in step S3, so as to provide data for the subsequent step of calculating the short-circuit current.

[0075] In the embodiment, the DC bus short-circuit current level calculation method of the multi-terminal flexible DC back-to-back system further comprises: according to the system data, the parameter data, the valve side system short-circuit impedance, the second valve side short-circuit impedance and the second impedance data of each flexible power transmission module, the AC short-circuit current component input into the MMC at the end is calculated by using a short-circuit current component formula. The short-circuit current component formula is:

[0076]

[0077] , wherein Iac_i is the AC short-circuit current component of the i th AC system input into the corresponding MMC, U2i is the valve side rated voltage of the i th transformer, XT2_i is the second valve side short-circuit impedance of the i th transformer, Xsc2i is the valve side system short-circuit impedance of the i th transformer, XL_i is the second impedance data of the i th bridge arm reactor, U1i_max is the system maximum operating voltage of the i th AC system, and U1i is the grid side rated voltage of the i th transformer.

[0078] S5. Obtain the withstand capability type of the MMC in each end flexible power transmission module, and calculate the discharge current of the DC capacitor in the MMC according to the withstand capability type and the electrical quantity data of the MMC.

[0079] It should be noted that in step S5, the discharge current Idc_i of the DC capacitor in the MMC is calculated in different ways according to the electrical quantity data of the MMC obtained in step S1 and the withstand capability type, so as to provide data for subsequent calculation of short-circuit current. In the embodiment, the method for calculating the short-circuit current level of the DC bus of the multi-end flexible DC back-to-back system adopts the calculation obtained in step S5 with respect to the capacitor discharge feed-in.

[0080] S6. Calculate the short-circuit current of the MMC when the DC bus inter-pole short-circuit fault occurs according to the discharge current of the MMC and the AC short-circuit current component.

[0081] It should be noted that step S6 is to calculate the short-circuit current of the DC bus when the DC bus inter-pole short-circuit fault occurs in a single modular multilevel converter MMC in the multi-end flexible DC back-to-back system by using a short-circuit current formula according to the AC short-circuit current component calculated in step S4 and the discharge current calculated in step S5. The short-circuit current formula is: , wherein Ishort_i is the short-circuit current of the DC side outlet of the i-th MMC when the DC inter-pole short-circuit fault occurs, Idc_i is the discharge current of the DC capacitor in the i-th MMC, and Iac_i is the AC short-circuit current component of the i-th MMC.

[0082] S7. Calculate the short-circuit current level flowing through the DC bus in the multi-end flexible DC back-to-back system according to the short-circuit current of each MMC.

[0083] It should be noted that in step S7, the short-circuit current level flowing through the DC bus in the multi-end flexible DC back-to-back system is calculated according to the short-circuit current of each MMC calculated in step S6. In the embodiment, the short-circuit current level of the DC bus is the algebraic sum of the short-circuit currents of all the modular multilevel converters MMCs in the multi-end flexible DC back-to-back system except the faulty modular multilevel converter MMC. In actual situations, the parameters of each modular multilevel converter MMC, transformer and AC system in the multi-end flexible DC back-to-back system can be inconsistent, and the initial short-circuit current level Idc_short_i of each modular multilevel converter MMC is also different, so the short-circuit current level of the DC bus of the multi-end flexible DC back-to-back system needs to take the maximum value of the initial short-circuit current level Idc_short_i of the DC bus inter-pole short-circuit of any modular multilevel converter MMC as the short-circuit current level flowing through the DC bus in the multi-end flexible DC back-to-back system.

[0084] In the embodiments of the present application, compared with the simulation waveform evaluation, if the running conditions and fault time are set unreasonably during the simulation research, the short-circuit current level obtained by the simulation research will be significantly smaller, which will have a significant impact on the equipment selection. The DC bus short-circuit current level calculation method of the multi-terminal flexible DC back-to-back system can quickly and effectively evaluate the DC bus short-circuit current level of the multi-terminal flexible DC back-to-back system, avoid setting a large number of simulation conditions for checking during the simulation research, and avoid a large amount of repeated simulation work.

[0085] It should be noted that the DC bus short-circuit current level calculation method of the multi-terminal flexible DC back-to-back system has important practical significance. The result is not dependent on the system operating condition change and fault parameter setting, and the DC bus short-circuit current level can be quickly and effectively determined. At the same time, the simulation result can provide a theoretical reference for comparison and checking, and a large amount of simulation checking work can be avoided. In the embodiments, the DC bus short-circuit current level calculation method of the multi-terminal flexible DC back-to-back system is also applicable to two-terminal back-to-back systems, single MMC inverter grid-connected systems and other power systems.

[0086] The application provides a DC bus short-circuit current level calculation method of a multi-terminal flexible DC back-to-back system, which is applied to the multi-terminal flexible DC back-to-back system, and the multi-terminal flexible DC back-to-back system comprises a multi-terminal flexible power transmission module, each flexible power transmission module comprises an MMC, a transformer connected with the MMC, and an alternating current system connected with the transformer, and the DC bus short-circuit current level calculation method comprises the following steps: obtaining system data of each alternating current system, electrical quantity data of each MMC, and parameter data of each transformer; calculating the valve side system short-circuit impedance of the transformer, the first valve side short-circuit impedance, and the first impedance data of the bridge arm reactor in the MMC of each flexible power transmission module according to the system data, the parameter data, and the electrical quantity data of each flexible power transmission module; obtaining the first manufacturing deviation of each transformer and the second manufacturing deviation of each bridge arm reactor, updating the first valve side short-circuit impedance of each transformer according to the first manufacturing deviation of the transformer, to obtain the second valve side short-circuit impedance of the transformer; updating the first impedance data of each bridge arm reactor according to the second manufacturing deviation of the bridge arm reactor, to obtain the second impedance data of the bridge arm reactor; calculating the alternating current short-circuit current component input into the MMC of each flexible power transmission module according to the system data, the parameter data, the valve side system short-circuit impedance, the second valve side short-circuit impedance, and the second impedance data of each flexible power transmission module; obtaining the withstand capability type of the MMC in each flexible power transmission module, and calculating the discharge current of the DC capacitor in the MMC according to the withstand capability type and the electrical quantity data of the MMC; calculating the short-circuit current of the MMC in the case of a DC bus inter-pole short-circuit fault according to the discharge current of the MMC and the alternating current short-circuit current component; and calculating the short-circuit current level flowing through the DC bus in the multi-terminal flexible DC back-to-back system according to the short-circuit current of each MMC. The DC bus short-circuit current level calculation method of the multi-terminal flexible DC back-to-back system can quickly and effectively evaluate the DC bus short-circuit current level of the multi-terminal flexible DC back-to-back system, avoid setting a large number of simulation conditions for checking during simulation research, avoid a large amount of repeated simulation work, and solve the technical problem that the overcurrent level of the existing modular multilevel converter is obtained through simulation, but the simulation workload is large and the modeling precision cannot be guaranteed, so that the accuracy of the obtained overcurrent level cannot be determined.

[0087] In an embodiment of the application, the calculation of the valve side system short-circuit impedance of the transformer, the first valve side short-circuit impedance, and the first impedance data of the bridge arm reactor in the MMC of each flexible power transmission module according to the system data, the parameter data, and the electrical quantity data of each flexible power transmission module comprises the following steps:

[0088] The valve side system short-circuit impedance of the transformer is calculated according to the system short-circuit impedance, the grid side rated voltage, and the valve side rated voltage;

[0089] The first valve side short-circuit impedance of the transformer is calculated according to the valve side rated voltage, the transformer capacity, and the short-circuit impedance percentage;

[0090] According to the bridge arm reactor inductance calculation, the first impedance data of the bridge arm reactor is obtained.

[0091] It should be noted that the valve side system short circuit impedance of the transformer is obtained by using the first impedance conversion formula to calculate according to the system short circuit impedance, the network side rated voltage and the valve side rated voltage; the first valve side short circuit impedance of the transformer is obtained by using the second impedance conversion formula to calculate according to the valve side rated voltage, the transformer capacity and the short circuit impedance percentage. The first impedance conversion formula is:

[0092]

[0093] The second impedance conversion formula is:

[0094]

[0095] In the formula, Xsc2i is the valve side system short circuit impedance of the i th transformer, Xsc1i is the system short circuit impedance of the i th alternating current system, U2i is the valve side rated voltage of the i th transformer, U1i is the network side rated voltage of the i th transformer, STi is the transformer capacity of the i th transformer, φi is the short circuit impedance percentage of the i th transformer, and XT2i is the first valve side short circuit impedance of the i th transformer.

[0096] In an embodiment of the present application, the discharge current of the DC capacitor in the MMC is obtained according to the endurance capability type and the electrical quantity data of the MMC, and the discharge current of the DC capacitor in the MMC includes:

[0097] If the endurance capability type is not considering the current endurance capability of the power device, the discharge current of the DC capacitor in the MMC is obtained according to the bridge arm reactor inductance of each MMC, the DC capacitor capacitance value, the DC voltage and the number of power modules;

[0098] If the endurance capability type is considering the current endurance capability of the power device, the discharge current of the DC capacitor in the MMC is obtained according to the nominal rated current of the power device of each MMC.

[0099] It should be noted that the discharging current of the DC capacitor in each MMC is calculated by the first discharging current formula according to the bridge reactor inductance of each MMC, the DC capacitor capacitance, the DC voltage and the number of power modules. The discharging current of the DC capacitor in each MMC is calculated by the second discharging current formula according to the nominal rated current of the power device. In the embodiment, the discharging current of the DC capacitor in the MMC is calculated by the first discharging current formula without considering the current-carrying capacity of the power device. Generally, if the current-carrying capacity of the power device is considered, the DC capacitor discharging current value calculated by the first discharging current formula is larger and exceeds the overcurrent capacity of the power device. The maximum turn-off current of the power device is usually 2 times of the nominal rated current Inom. Therefore, if the current-carrying capacity of the power device is considered, the DC capacitor discharging current Idc_i can be calculated by the second discharging current formula.

[0100] In the embodiment of the application, the first discharging current formula is:

[0101]

[0102] The second discharging current formula is: ;

[0103] In the formula, Idc_i is the discharging current of the DC capacitor in the i th MMC, Udc is the DC voltage, Li is the bridge reactor inductance of the i th MMC, C is the DC capacitor capacitance, M is the number of power modules, and Inom is the nominal rated current of the power device.

[0104] In one embodiment of the application, the short-circuit current level flowing through the DC bus in the multi-terminal flexible DC back-to-back system is obtained according to the short-circuit current calculation of each MMC, including:

[0105] In the case of a DC interelectrode short circuit at the DC side outlet of any MMC, the initial short-circuit current level corresponding to the DC interelectrode short circuit flowing through the DC bus of the MMC is obtained according to the short-circuit current calculation of each MMC;

[0106] The largest initial short-circuit current level is selected from the initial short-circuit current levels of the multi-terminal as the short-circuit current level flowing through the DC bus in the multi-terminal flexible DC back-to-back system;

[0107] In the formula, the initial short-circuit current level of the MMC DC side outlet in the case of a DC interelectrode short circuit is calculated by the short-circuit level formula according to the short-circuit current of each MMC. The short-circuit level formula is:

[0108]

[0109] In the formula, Ishort_i is the short-circuit current of the DC side outlet of the i-th MMC when a DC inter-pole short circuit occurs, N is the number of terminals of the flexible power transmission module, Ishort_j is the short-circuit current of the j-th MMC, and Idc_short_i is the initial short-circuit current level of the DC side outlet of the i-th MMC when a DC inter-pole short circuit occurs.

[0110] It should be noted that in the multi-terminal flexible DC back-to-back system, if a DC inter-pole short circuit occurs at the DC side outlet of any one of the modular multilevel converters (MMCi, such as any one of MMC1-MMC4 in the embodiment) Figure 2 , the short-circuit current level of the DC bus is mainly determined by the remaining modular multilevel converters, and therefore the short-circuit current level flowing through the DC bus is calculated using the short-circuit level formula, in which i represents the number of the modular multilevel converter MMC at the DC side outlet where the DC inter-pole short circuit occurs, and j represents the number of any one of the modular multilevel converters MMC. In the embodiment, the short-circuit current level Idc_short flowing through the DC bus in the multi-terminal flexible DC back-to-back system is calculated using the short-circuit level formula .

[0111] Embodiment Two

[0112] Figure 3 A schematic diagram of the framework of the DC bus short-circuit current level calculation device of the multi-terminal flexible DC back-to-back system described in the embodiments of the present application.

[0113] As shown in Figure 3 , the embodiments of the present application provide a DC bus short-circuit current level calculation device for a multi-terminal flexible DC back-to-back system, which is applied to the multi-terminal flexible DC back-to-back system. The multi-terminal flexible DC back-to-back system includes a multi-terminal flexible power transmission module, each terminal flexible power transmission module includes an MMC, a transformer connected to the MMC, and an alternating current system connected to the transformer. The DC bus short-circuit current level calculation device includes a data acquisition module 10, a data calculation module 20, a data update module 30, a current component calculation module 40, a discharge current calculation module 50, a short-circuit current calculation module 60, and a short-circuit current level calculation module 70.

[0114] The data acquisition module 10 is configured to acquire system data of each alternating current system, electrical quantity data of each MMC, and parameter data of each transformer.

[0115] The data calculation module 20 is configured to calculate valve side system short-circuit impedance of the terminal transformer, first valve side short-circuit impedance, and first impedance data of the bridge arm reactor in the terminal MMC according to the system data, parameter data, and electrical quantity data of each terminal flexible power transmission module.

[0116] The data updating module 30 is configured to obtain the first manufacturing deviation of each transformer and the second manufacturing deviation of each bridge arm reactor, update the first valve side short circuit impedance of each transformer according to the first manufacturing deviation of the transformer to obtain the second valve side short circuit impedance of the transformer, and update the first impedance data of each bridge arm reactor according to the second manufacturing deviation of the bridge arm reactor to obtain the second impedance data of the bridge arm reactor.

[0117] The current component calculation module 40 is configured to calculate the AC short circuit current component input into the MMC at each end according to the system data, the parameter data, the valve side system short circuit impedance, the second valve side short circuit impedance and the second impedance data of the flexible power transmission module at each end.

[0118] The discharge current calculation module 50 is configured to obtain the withstand capability type of the MMC in each flexible power transmission module, and calculate the discharge current of the DC capacitor in the MMC according to the withstand capability type and the electrical quantity data of the MMC at each end.

[0119] The short circuit current calculation module 60 is configured to calculate the short circuit current of the MMC in the DC bus interline short circuit fault according to the discharge current of the MMC and the AC short circuit current component.

[0120] The short circuit current level calculation module 70 is configured to calculate the short circuit current level of the DC bus in the multi-terminal flexible DC back-to-back system according to the short circuit current of the MMC at each end.

[0121] It should be noted that the content of the modules in the device of the second embodiment has been described in the content of the steps of the method of the first embodiment, and the content of the modules of the DC bus short circuit current level calculation device of the multi-terminal flexible DC back-to-back system will not be described repeatedly in this embodiment. In this embodiment, the DC bus short circuit current level calculation device of the multi-terminal flexible DC back-to-back system can quickly and effectively evaluate the DC bus short circuit current level of the multi-terminal flexible DC back-to-back system through the data acquisition module, the data calculation module, the data updating module, the current component calculation module, the discharge current calculation module, the short circuit current calculation module and the short circuit current level calculation module, avoid setting a large number of simulation conditions for checking during simulation research, and avoid a large amount of repeated simulation work.

[0122] In the embodiments of the present application, the system data includes the system short circuit impedance and the system maximum operating voltage, the electrical quantity data includes the bridge arm reactor inductance, the DC capacitor capacitance value, the DC voltage, the number of power modules and the nominal rated current of the power device, the parameter data includes the grid side rated voltage, the valve side rated voltage, the transformer capacity and the short circuit impedance percentage, and the data calculation module 20 includes the first short circuit impedance calculation submodule, the second short circuit impedance calculation submodule and the impedance calculation submodule.

[0123] The first short-circuit impedance calculation submodule is configured to calculate the valve-side system short-circuit impedance of the transformer according to the system short-circuit impedance, the grid-side rated voltage and the valve-side rated voltage.

[0124] The second short-circuit impedance calculation submodule is configured to calculate the first valve-side short-circuit impedance of the transformer according to the valve-side rated voltage, the transformer capacity and the short-circuit impedance percentage.

[0125] The impedance calculation submodule is configured to calculate the first impedance data of the bridge arm reactor according to the bridge arm reactor inductance.

[0126] In the embodiment of the application, the discharge current calculation module 50 comprises a first calculation submodule and a second calculation submodule.

[0127] The first calculation submodule is configured to calculate the discharge current of the DC capacitor in each MMC according to the bridge arm reactor inductance of each MMC, the DC capacitor capacitance value, the DC voltage and the number of power modules, when the withstand capability type is the current withstand capability of the power device.

[0128] The second calculation submodule is configured to calculate the discharge current of the DC capacitor in each MMC according to the nominal rated current of the power device of each MMC, when the withstand capability type is the current withstand capability of the power device.

[0129] In the embodiment of the application, the short-circuit current level calculation module 70 is further configured to calculate the initial short-circuit current level corresponding to the inter-pole short-circuit of each MMC according to the short-circuit current of each MMC, when the inter-pole short-circuit occurs at the DC side outlet of any MMC, and select the initial short-circuit current level with the largest value from the initial short-circuit current levels of the multi-terminal as the short-circuit current level flowing through the DC bus in the multi-terminal flexible DC back-to-back system.

[0130] Embodiment three:

[0131] Figure 4 A schematic diagram of the terminal device is shown in the embodiment of the application.

[0132] As shown in Figure 4 The terminal device provided by the embodiment of the application comprises a processor and a memory.

[0133] The memory is configured to store program code and transmit the program code to the processor.

[0134] The processor is configured to execute the DC bus short-circuit current level calculation method of the multi-terminal flexible DC back-to-back system according to the instructions in the program code.

[0135] It should be noted that the processor is configured to execute the steps of the method for calculating the DC bus short-circuit current level of a multi-terminal VSC-HVDC system according to the instructions in the program code. Alternatively, the processor executes the computer program to realize the functions of the modules / units in the above system / device embodiments.

[0136] For example, the computer program can be divided into one or more modules / units, one or more modules / units are stored in the memory and executed by the processor to complete the present application. One or more modules / units can be a series of computer program instruction segments capable of completing a specific function, which is used to describe the execution process of the computer program in the terminal device.

[0137] The terminal device can be a desktop computer, a notebook computer, a palm computer, a cloud server, and the like. The terminal device can include, but is not limited to, a processor and a memory. Those skilled in the art can understand that the terminal device is not limited, and can include more or less components, or combine certain components, or different components, for example, the terminal device can also include an input / output device, a network access device, a bus, and the like.

[0138] The processor can be a central processing unit (CPU), and can also be other general-purpose processors, digital signal processors (DSP), application specific integrated circuits (ASIC), field-programmable gate arrays (FPGA) or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The general-purpose processor can be a microprocessor or the processor can also be any conventional processor.

[0139] The memory can be an internal storage unit of the terminal device, such as a hard disk or a memory of the terminal device. The memory can also be an external storage device of the terminal device, such as a plug-in hard disk, a smart media card (SMC), a secure digital (SD) card, a flash card, and the like. Further, the memory can include both the internal storage unit and the external storage device of the terminal device. The memory is used to store the computer program and other programs and data required by the terminal device. The memory can also be used to temporarily store data that has been output or will be output.

[0140] Those skilled in the art can clearly understand that, for the convenience and brevity of description, the specific working processes of the above-described system, device and unit can refer to the corresponding processes in the foregoing method embodiments, which will not be repeated here.

[0141] In several embodiments provided in the present application, it should be understood that the disclosed system, device and method can be implemented in other manners. For example, the described device embodiments are merely schematic, and the division of the units is merely a logical function division, and there can be another division manner in actual implementation. For example, a plurality of units or components can be combined or integrated into another system, or some features can be ignored or not executed. In addition, the displayed or discussed mutual couplings or direct couplings or communication connections can be indirect couplings or communication connections through some interfaces, devices or units, and can be electrical, mechanical or other forms.

[0142] The units described as separated components can or can not be physically separated, and the components displayed as units can or can not be physical units, i.e., can be located in one place, or can be distributed on a plurality of network units. In actual implementation, some or all of the units can be selected according to actual needs to achieve the purposes of the embodiments.

[0143] In addition, each functional unit in the embodiments of the present application can be integrated in a processing unit, or each unit can exist physically, or two or more units can be integrated in one unit. The integrated unit can be implemented in the form of hardware, or in the form of software function units.

[0144] When the integrated unit is implemented in the form of software function units and sold or used as an independent product, it can be stored in a computer readable storage medium. Based on such an understanding, the technical solutions of the present application essentially, or the part that contributes to the prior art, or all or a part of the technical solutions can be embodied in the form of a software product. The computer software product is stored in a storage medium, and includes several instructions for causing a computer device (which can be a personal computer, a server, or a network device, etc.) to execute all or part of the steps of the methods described in the various embodiments of the present application. The foregoing storage medium includes: U disk, mobile hard disk, read-only memory (ROM, Read-Only Memory), random access memory (RAM, Random Access Memory), magnetic disk or optical disk, and various other media that can store program codes.

[0145] The above-described embodiments are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacements for part of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for calculating the DC bus short-circuit current level of a multi-terminal flexible DC back-to-back system, applied to the multi-terminal flexible DC back-to-back system, wherein the multi-terminal flexible DC back-to-back system includes multi-terminal flexible transmission modules, each of the flexible transmission modules including an MMC, a transformer connected to the MMC, and an AC system connected to the transformer, characterized in that, The method for calculating the DC bus short-circuit current level includes the following steps: Acquire system data for each of the AC systems, electrical quantity data for each of the MMCs, and parameter data for each of the transformers; Based on the system data, parameter data, and electrical quantity data of the flexible transmission module at each end, the valve-side system short-circuit impedance, the first valve-side short-circuit impedance, and the first impedance data of the bridge arm reactor in the MMC at that end are calculated. Obtain a first manufacturing deviation for each transformer and a second manufacturing deviation for each arm reactor; update the first valve-side short-circuit impedance of each transformer based on the first manufacturing deviation to obtain the second valve-side short-circuit impedance of the transformer; update the first impedance data of each arm reactor based on the second manufacturing deviation to obtain the second impedance data of the arm reactor. The AC short-circuit current component input to the MMC at each end is calculated based on the system data, parameter data, valve-side system short-circuit impedance, second valve-side short-circuit impedance, and second impedance data of the flexible transmission module at each end. Obtain the withstand capability type of the MMC in each end of the flexible power transmission module, and calculate the discharge current of the DC capacitor in the MMC based on the withstand capability type and the electrical quantity data of the MMC at that end. The short-circuit current of the MMC when a DC bus inter-pole short-circuit fault occurs is calculated based on the discharge current and the AC short-circuit current component of the MMC. The short-circuit current level flowing through the DC bus in the multi-terminal flexible DC back-to-back system is obtained by calculating the short-circuit current of the MMC at each terminal.

2. The method for calculating the DC bus short-circuit current level of a multi-terminal flexible DC back-to-back system according to claim 1, characterized in that, The system data includes the system short-circuit impedance and the system maximum operating voltage. The electrical quantity data includes the inductance of the bridge arm reactor, the DC capacitor value, the DC voltage, the number of power modules, and the nominal rated current of the power devices. The parameter data includes the grid-side rated voltage, the valve-side rated voltage, the transformer capacity, and the short-circuit impedance percentage. Based on the system data, the parameter data, and the electrical quantity data of each flexible transmission module, the valve-side system short-circuit impedance, the first valve-side short-circuit impedance, and the first impedance data of the bridge arm reactor in the MMC at that end are calculated, including: The valve-side system short-circuit impedance of the transformer is calculated based on the system short-circuit impedance, the grid-side rated voltage, and the valve-side rated voltage. The first valve-side short-circuit impedance of the transformer is calculated based on the valve-side rated voltage, the transformer capacity, and the short-circuit impedance percentage. The first impedance data of the bridge arm reactor is obtained by calculating the inductance of the bridge arm reactor.

3. The method for calculating the DC bus short-circuit current level of a multi-terminal flexible DC back-to-back system according to claim 2, characterized in that, Based on the withstand capability type and the electrical quantity data of the MMC at this end, the discharge current of the DC capacitor in the MMC is calculated as follows: If the withstand capability type is one that does not consider the current withstand capability of power devices, the discharge current of the DC capacitor in the MMC is calculated based on the inductance of the bridge arm reactor, the capacitance value of the DC capacitor, the DC voltage and the number of power modules in each MMC. If the withstand capability type considers the current withstand capability of the power device, the discharge current of the DC capacitor in the MMC is calculated based on the nominal rated current of the power device in each MMC.

4. The method for calculating the DC bus short-circuit current level of a multi-terminal flexible DC back-to-back system according to any one of claims 1-3, characterized in that, Based on the short-circuit current calculations of the MMC at each terminal, the short-circuit current level flowing through the DC bus in the multi-terminal flexible DC back-to-back system is obtained as follows: If a DC inter-electrode short circuit occurs at the DC side outlet of the MMC at any end, the initial short-circuit current level corresponding to the DC bus flowing through the MMC at that end is obtained by calculating the short-circuit current of the MMC at each end. The initial short-circuit current level with the largest value is selected from the initial short-circuit current levels of the multiple terminals as the short-circuit current level flowing through the DC bus in the multi-terminal flexible DC back-to-back system.

5. The method for calculating the DC bus short-circuit current level of a multi-terminal flexible DC back-to-back system according to claim 4, characterized in that, Also includes: The initial short-circuit current level for a DC inter-electrode short circuit at the DC side outlet of the MMC is calculated using the short-circuit level formula based on the short-circuit current at each terminal of the MMC. The short-circuit level formula is as follows: In the formula, Ishort_i is the short-circuit current when a DC inter-electrode short circuit occurs at the DC side outlet of the i-th MMC, N is the number of terminals of the flexible transmission module, Ishort_j is the short-circuit current of the j-th MMC, and Idc_short_i is the initial short-circuit current level when a DC inter-electrode short circuit occurs at the DC side outlet of the i-th MMC.

6. A device for calculating the DC bus short-circuit current level of a multi-terminal flexible DC back-to-back system, applied to the multi-terminal flexible DC back-to-back system, wherein the multi-terminal flexible DC back-to-back system includes multi-terminal flexible transmission modules, each of the flexible transmission modules including an MMC, a transformer connected to the MMC, and an AC system connected to the transformer, characterized in that, The DC bus short-circuit current level calculation device includes: a data acquisition module, a data calculation module, a data update module, a current component calculation module, a discharge current calculation module, a short-circuit current calculation module, and a short-circuit current level calculation module; The data acquisition module is used to acquire system data of each AC system, electrical quantity data of each MMC, and parameter data of each transformer; The data calculation module is used to calculate, based on the system data, parameter data and electrical quantity data of the flexible transmission module at each end, the valve-side system short-circuit impedance, the first valve-side short-circuit impedance and the first impedance data of the bridge arm reactor in the MMC at that end; The data update module is used to obtain a first manufacturing deviation of each transformer and a second manufacturing deviation of each bridge arm reactor, update the first valve side short-circuit impedance of each transformer according to the first manufacturing deviation to obtain the second valve side short-circuit impedance of the transformer; and update the first impedance data of each bridge arm reactor according to the second manufacturing deviation to obtain the second impedance data of the bridge arm reactor. The current component calculation module is used to calculate, based on the system data, parameter data, valve-side system short-circuit impedance, second valve-side short-circuit impedance, and second impedance data of each flexible transmission module, to obtain the AC short-circuit current component input to the MMC at that end. The discharge current calculation module is used to obtain the withstand capability type of the MMC in each end of the flexible transmission module, and calculate the discharge current of the DC capacitor in the MMC based on the withstand capability type and the electrical quantity data of the MMC at that end. The short-circuit current calculation module is used to calculate the short-circuit current of the MMC when a DC bus inter-pole short-circuit fault occurs based on the discharge current and the AC short-circuit current component of the MMC. The short-circuit current level calculation module is used to calculate the short-circuit current level flowing through the DC bus in the multi-terminal flexible DC back-to-back system based on the short-circuit current of each MMC.

7. The DC bus short-circuit current level calculation device for a multi-terminal flexible DC back-to-back system according to claim 6, characterized in that, The system data includes the system short-circuit impedance and the system maximum operating voltage. The electrical quantity data includes the inductance of the bridge arm reactor, the capacitance value of the DC capacitor, the DC voltage, the number of power modules, and the nominal rated current of the power devices. The parameter data includes the grid-side rated voltage, the valve-side rated voltage, the transformer capacity, and the percentage of short-circuit impedance. The data calculation module includes a first short-circuit impedance calculation submodule, a second short-circuit impedance calculation submodule, and an impedance calculation submodule. The first short-circuit impedance calculation submodule is used to calculate the valve-side system short-circuit impedance of the transformer based on the system short-circuit impedance, the grid-side rated voltage, and the valve-side rated voltage. The second short-circuit impedance calculation submodule is used to calculate the first valve-side short-circuit impedance of the transformer based on the valve-side rated voltage, the transformer capacity and the short-circuit impedance percentage. The impedance calculation submodule is used to calculate the first impedance data of the bridge arm reactor based on the inductance of the bridge arm reactor.

8. The DC bus short-circuit current level calculation device for a multi-terminal flexible DC back-to-back system according to claim 7, characterized in that, The discharge current calculation module includes a first calculation submodule and a second calculation submodule; The first calculation submodule is used to calculate the discharge current of the DC capacitor in each MMC based on the inductance of the bridge arm reactor, the capacitance value of the DC capacitor, the DC voltage, and the number of power modules, according to the tolerance type being "not considering the current tolerance of power devices". The second calculation submodule is used to calculate the discharge current of the DC capacitor in each MMC based on the nominal rated current of the power device, considering the current-carrying capacity of the power device in the tolerance type.

9. The DC bus short-circuit current level calculation device for a multi-terminal flexible DC back-to-back system according to claim 6, characterized in that, The short-circuit current level calculation module is also used to calculate the initial short-circuit current level corresponding to the DC bus when a DC inter-electrode short circuit occurs at the DC side outlet of any MMC at one end, based on the short-circuit current of each MMC at each end; and to select the initial short-circuit current level with the largest value from the initial short-circuit current levels of multiple ends as the short-circuit current level flowing through the DC bus in the multi-end flexible DC back-to-back system.

10. A terminal device, characterized in that, Including the processor and memory; The memory is used to store program code and transmit the program code to the processor; The processor is configured to execute, according to the instructions in the program code, the method for calculating the DC bus short-circuit current level of the multi-terminal flexible DC back-to-back system as described in any one of claims 1-5.

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