A method and system for electromagnetic transient modeling of a low frequency power transmission system

Abstract

The application discloses a kind of low-frequency power transmission system electromagnetic transient modeling method and system.The method of the application includes the following steps:1) determining the structure and parameters of low-frequency power transmission primary system;2) building low-frequency power transmission control and protection secondary system;3) complete model building and debugging: under the parameters of each electrical element and device of main circuit determined in step 1), according to the actual low-frequency power transmission engineering circuit structure, each electrical element and device of main circuit is connected to establish a primary system, and the low-frequency power transmission control and protection secondary system completed in step 2), build a complete M3C-based low-frequency power transmission system electromagnetic transient model;4) construct short-circuit fault and open-phase operation simulation module;5) construct model data export and electrical quantity visualization module.The low-frequency power transmission system electromagnetic transient model obtained by the application can simulate the actual field engineering primary system and secondary control protection system, and realize the steady-state and electromagnetic transient simulation close to the actual low-frequency power transmission project.

Description

Technical Field

[0001] This invention belongs to the technical field of low-frequency power transmission systems, specifically a method and system for electromagnetic transient modeling of low-frequency power transmission systems based on a modular multilevel matrix converter. Background Technology

[0002] Traditional high-voltage AC transmission capacity is limited by the capacity rise effect of long-distance transmission lines, while low-frequency transmission, by reducing the transmission frequency to decrease line reactance, has a greater advantage in increasing transmission capacity compared to high-voltage AC transmission. Low-frequency transmission is also suitable for transmitting medium- and long-distance offshore wind power and other renewable energy sources. Compared to flexible high-voltage direct current (HVDC) transmission for offshore wind power, low-frequency transmission eliminates the need for costly and complex offshore converter stations, making it a highly competitive solution for medium- and long-distance offshore wind power transmission. Furthermore, low-frequency transmission is more advantageous in addressing issues such as the difficulty in interrupting DC-side short-circuit current and the degradation of cable insulation caused by space charge accumulation in high-voltage cables, which are present in flexible HVDC transmission. Using a modular multilevel matrix converter (M3C) based on a full-bridge structure for low-frequency transmission can directly achieve AC-AC frequency conversion, and offers advantages such as modular design, controllable power factors on both the power frequency and low-frequency sides, fast dynamic response, and flexible power flow control.

[0003] Currently, there are no actual low-frequency power transmission projects based on M3C that have been put into operation, either domestically or internationally. In order to simulate the actual low-frequency power transmission projects based on modular multilevel matrix converters, accurately study the dynamic behavior of low-frequency power transmission and analyze fault characteristics, and provide a basis for the protection principle and configuration of AC low-frequency power transmission systems, it is necessary to establish a detailed, accurate, and realistic electromagnetic transient simulation model of the low-frequency power transmission system. Summary of the Invention

[0004] In response to the aforementioned existing low-frequency power transmission technologies, this invention provides a method and system for electromagnetic transient modeling of low-frequency power transmission systems based on a modular multilevel matrix converter. By establishing an electromagnetic transient model of the low-frequency power transmission system, it achieves simulation of steady-state and electromagnetic transient states that closely approximate actual low-frequency power transmission projects.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: an electromagnetic transient modeling method for low-frequency transmission systems based on M3C, comprising the following steps:

[0006] 1) Determine the structure and parameters of the low-frequency power transmission primary system;

[0007] 2) Establish a secondary system for low-frequency power transmission control and protection;

[0008] 3) Complete model building and debugging: Under the parameters of each electrical component and equipment of the main circuit determined in step 1), according to the actual low-frequency power transmission engineering circuit structure, connect each electrical component and equipment of the main circuit to establish a primary system and the low-frequency power transmission control and protection secondary system completed in step 2), and build a complete electromagnetic transient model of the low-frequency power transmission system based on M3C.

[0009] After completing the construction of the electromagnetic transient model of the low-frequency power transmission system, continuous simulation tests are conducted, and the actual waveforms are compared. The control parameters are continuously modified and the electromagnetic transient model is debugged to ensure the accuracy of the electromagnetic transient model, so that the low-frequency power transmission system can operate stably for a long time and the operating results are consistent with the actual engineering.

[0010] 4) Construct a short-circuit fault and phase loss operation simulation module: Add a fault module to the electromagnetic transient model of the low-frequency transmission system to simulate short-circuit faults and phase loss operation;

[0011] 5) Construct a model data export and electrical quantity visualization module: Add a voltage and current monitoring module, a sequence component analysis module, a differential current calculation module, and a data export module to the electromagnetic transient model of the low-frequency power transmission system, so that the voltage, current, sequence components, and differential current of each measurement point can be displayed on an oscilloscope, or the data can be exported for analysis through the data export module.

[0012] Further, in step 1), the structure of the low-frequency power transmission primary system is as follows: At two voltage levels of V... N Each substation will have a frequency converter station built within it, with a capacity of P. N Low-frequency f-type transmission was carried out between the two substations. N Power transmission; within each frequency converter station, the three-phase power at the industrial frequency is first stepped down to V by an industrial frequency transformer. m Then it is converted to low frequency by M3C. N Then it is stepped up to V by a low-frequency step-up transformer. N Finally, it is transmitted through a low-frequency transmission line.

[0013] Furthermore, in step 1), the parameters of the low-frequency power transmission primary system include the parameters of the power frequency transformer, the low-frequency step-up transformer, the power frequency grounding transformer, the power frequency transmission line, the low-frequency transmission line, the system equivalent impedance, and M3C.

[0014] Further, in step 2), the construction process of the low-frequency power transmission control and protection secondary system is as follows: Based on the functional requirements of the low-frequency power transmission secondary system, the control parameters of the M3C control system are determined. By utilizing library modules and writing programs, a custom module is formed and added to the main control system to realize the control strategy of the M3C converter valve of the low-frequency power transmission system, thereby completing the construction of the low-frequency power transmission control and protection secondary system.

[0015] Furthermore, in step 2), the M3C control system adopts a hierarchical control structure, namely system-level control, converter station-level control, and converter valve-level control system. The system-level control system is located at the top level of the control system. From the perspective of the overall safe and stable operation and optimized operation of the system, it converts the set values ​​of active and reactive physical quantities into reference values ​​for lower-level control, enabling the system to switch smoothly under different operating commands. The converter station-level control system is the core of the entire system control and is the link between the system-level and converter valve-level control systems. It includes dual closed-loop vector control based on outer and inner loop control on the input and output sides, as well as capacitor voltage equalization control. The converter valve-level control system is the bottom level of the control system. It can not only complete M3C modulation, but also realize capacitor voltage equalization control between sub-modules within the M3C bridge arm.

[0016] Furthermore, in step 4), based on the electromagnetic transient model built above, short-circuit fault modules are added to the power frequency transmission line, low-frequency transmission line, power frequency transformer, and low-frequency transformer protection zone, respectively. Three-phase reclosing and single-phase loss modules are added to the low-frequency transmission line. For faults that occur in actual engineering sites, different types of short-circuit faults at different locations between turns of the power frequency or low-frequency transmission line and power frequency or low-frequency transformer are simulated. The two-phase operation or single-phase operation mode of the low-frequency transmission line is simulated, as well as the three-phase tripping and reclosing of the circuit breaker during short-circuit faults are simulated.

[0017] Another technical solution adopted in this invention is: an electromagnetic transient modeling system for low-frequency power transmission systems based on M3C, comprising:

[0018] Primary system determination unit: used to determine the structure and parameters of the low-frequency power transmission primary system;

[0019] Secondary system construction unit: used to build a low-frequency power transmission control and protection secondary system;

[0020] Model building and debugging unit: Under the parameters of each electrical component and equipment of the main circuit determined by the primary system determination unit, the primary system is established by connecting each electrical component and equipment of the main circuit according to the actual low-frequency power transmission engineering circuit structure, and the low-frequency power transmission control and protection secondary system is completed in the secondary system building unit, thus building a complete electromagnetic transient model of the low-frequency power transmission system based on M3C.

[0021] After completing the construction of the electromagnetic transient model of the low-frequency power transmission system, continuous simulation tests are conducted, and the actual waveforms are compared. The control parameters are continuously modified and the electromagnetic transient model is debugged to ensure the accuracy of the electromagnetic transient model, so that the low-frequency power transmission system can operate stably for a long time and the operating results are consistent with the actual engineering.

[0022] Short-circuit fault simulation building block: A short-circuit fault module is added to the electromagnetic transient model of the low-frequency transmission system to simulate short-circuit faults and phase loss operation;

[0023] Model data export and electrical quantity visualization building unit: Add voltage and current monitoring module, sequence component analysis module, differential current calculation module and data export module to the electromagnetic transient model of low frequency power transmission system, so that the voltage, current, sequence components and differential current of each measurement point can be displayed on an oscilloscope, or the data can be exported for analysis through the data export module.

[0024] The beneficial effects of this invention are as follows: The electromagnetic transient model established by this invention can be run directly in a computer. It can simulate different types of short-circuit faults at different locations, such as power frequency or low frequency transmission lines and power frequency or low frequency transformer turns, for faults that occur in actual engineering sites. It can also simulate two-phase operation or single-phase operation mode of low frequency transmission lines, and can also simulate the three-phase tripping and reclosing of circuit breakers during short-circuit faults.

[0025] The electromagnetic transient model established by this invention can realize the monitoring of three-phase voltage and current, sequence component analysis, differential current calculation and data export at each measurement point; the voltage, current, sequence component and differential current at each measurement point can be displayed by an oscilloscope, or the data can be exported by the module for analysis, which is convenient for analyzing fault characteristics and can be used for the protection and control principle test and algorithm implementation of low frequency power transmission system. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the low-frequency power transmission system of the present invention;

[0027] The diagram marks the measurement points and fault points; black squares represent measurement points, and diagonal arrows indicate short-circuit fault points. The diagram also shows protection zones for different parts of the low-frequency transmission system: power frequency transmission line protection, power frequency transformer protection, valve-controlled protection, low-frequency transformer protection, and low-frequency transmission line protection. Because the sending and receiving ends are symmetrical, only the sending end is marked in the diagram. Detailed Implementation

[0028] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0029] Figure 1 The diagram shown is a structural schematic of the low-frequency power transmission system included in this invention. It is the main circuit topology of a low-frequency power transmission system with two power supplies based on a modular multilevel matrix inverter (M3C). Power supplies A and B are both industrial frequency power supplies with a rated voltage V. N Power source A is the sending power source, and power source B is the receiving power source; the power frequency electricity passes through a power frequency Y / Δ transformer from V N The voltage drops to V m After passing through the #1 M3C converter, it is converted to a frequency of f. N The low-frequency alternating current maintains a constant voltage level; then, after passing through a low-frequency Δ / Y transformer, it flows from V... m Boost to VN The receiving-end and sending-end power supply circuits are symmetrical. The low-frequency side connects the sending-end and receiving-end power supplies via a low-frequency transmission line.

[0030] Example 1

[0031] This embodiment presents an electromagnetic transient modeling method for low-frequency power transmission systems based on modular multilevel matrix converters. The steps are as follows:

[0032] 1) Determine the structure and parameters of the low-frequency power transmission primary system.

[0033] The structure of a low-frequency power transmission primary system is as follows: In actual low-frequency power transmission projects, at two voltage levels of V... N Each substation will have a frequency converter station built within it, with a capacity of P. N Low-frequency f-type transmission was carried out between the two substations. N Power transmission; within each frequency converter station, the three-phase power at the industrial frequency is first stepped down to V by an industrial frequency transformer. m Then it is converted to low frequency by M3C. N Then it is stepped up to V by a low-frequency step-up transformer. N Finally, it is transmitted through a low-frequency transmission line.

[0034] The parameters of the low-frequency power transmission primary system are the actual electrical parameters provided by the actual low-frequency power transmission project, including the parameters of the power frequency transformer, low-frequency step-up transformer, power frequency grounding transformer, power frequency transmission line, low-frequency transmission line, system equivalent impedance, and converter M3C.

[0035] 2) Establish a low-frequency power transmission control and protection secondary system

[0036] Current simulation tools contain many built-in modules in their basic module libraries, which can meet many basic requirements and functions, facilitating the creation of basic simulation models. However, in actual use, some functions still cannot be implemented, especially the control strategy of M3C, whose internal calculation logic is quite complex, and existing modules are difficult to meet simulation requirements.

[0037] The M3C control system adopts a hierarchical control structure, namely system-level control, converter station-level control, and converter valve-level control system. The system-level control system is located at the top level of the control system. It focuses on the overall safe, stable, and optimized operation of the system, converting the set values ​​of active and reactive physical quantities into reference values ​​for lower-level control, enabling the system to switch smoothly under different operating commands. The converter station-level control system is the core of the entire system control and serves as the link between the system-level and converter valve-level control systems. It includes dual closed-loop vector control based on outer and inner loop control on the input and output sides, as well as capacitor voltage equalization control. The converter valve-level control system is the lowest level of the control system. By employing appropriate modulation strategies, it can not only complete M3C modulation but also achieve capacitor voltage equalization control among submodules within the M3C bridge arm.

[0038] To implement the above control strategy and meet its special functions, a custom module is formed by using library modules and writing programs based on control principles and requirements. This module is then added to the main control system to implement basic or special functions, thereby realizing the control strategy of the M3C converter valve in the low-frequency power transmission system and completing the construction of the low-frequency power transmission control and protection secondary system.

[0039] 3) Complete model building and debugging

[0040] Under the parameters of each electrical component and device in the main circuit determined in step 1), and according to the actual low-frequency power transmission engineering circuit structure, connect each electrical component and device in the main circuit to establish a primary system and the low-frequency power transmission control and protection secondary system completed in step 2), and build a complete electromagnetic transient model of the low-frequency power transmission system based on M3C.

[0041] After completing the construction of the electromagnetic transient model of the low-frequency power transmission system, continuous simulation tests are conducted, and the actual waveforms are compared. The control parameters are continuously modified and the electromagnetic transient model is debugged to ensure the accuracy of the electromagnetic transient model, so that the low-frequency power transmission system can operate stably for a long time and the operating results are consistent with the actual engineering.

[0042] When operating in steady state, the power transmitted by the low-frequency power transmission system is P. N The power frequency and low frequency grid-side voltages are V. N The power frequency and low frequency valve-side voltages are V. m .

[0043] 4) Construct a simulation module for short-circuit faults and phase loss operation.

[0044] To provide a theoretical basis for the protection principle and configuration of low-frequency power transmission projects, based on the electromagnetic transient model established earlier, short-circuit fault modules are added to the protection zones of power frequency transmission lines, low-frequency transmission lines, power frequency transformers, and low-frequency transformers, respectively. Three-phase reclosing and single-phase loss modules are added to the low-frequency transmission lines.

[0045] With the addition of the above modules, electromagnetic transient simulation of low-frequency power transmission systems can be easily realized. It can simulate different types of short-circuit faults at different locations such as power frequency or low-frequency power transmission lines, power frequency or low-frequency transformer grounding or inter-turn faults, and can also simulate two-phase operation or single-phase operation mode of low-frequency power transmission lines. It can also simulate the three-phase tripping and reclosing of circuit breakers during short-circuit faults.

[0046] 5) Construct a module for model data export and electrical quantity visualization.

[0047] To facilitate the observation and analysis of electrical quantities such as voltage and current at important measurement points, a voltage and current monitoring module, a sequence component analysis module, a differential current calculation module, and a data export module have been added to the electromagnetic transient model of the low-frequency power transmission system.

[0048] The aforementioned modules can display and export waveforms of three-phase voltage and current, differential current, positive sequence, negative sequence, and zero sequence components of transmission lines and transformers at various measurement points. By observing waveforms and analyzing data, the voltage and current characteristics, sequence component characteristics, and differential current characteristics under short-circuit faults can be analyzed, facilitating the analysis of fault characteristics and the implementation of protection principles and algorithms.

[0049] Example 2

[0050] This embodiment provides an electromagnetic transient modeling system for low-frequency power transmission systems based on M3C, which consists of a primary system determination unit, a secondary system construction unit, a complete model construction and debugging unit, a short-circuit fault simulation construction unit, and a model data export and electrical quantity visualization construction unit.

[0051] 1. Primary system determination unit

[0052] The structure of a low-frequency power transmission primary system is as follows: At two voltage levels of V... N Each substation will have a frequency converter station built within it, with a capacity of P. N Low-frequency f-type transmission was carried out between the two substations. N Power transmission; within each frequency converter station, the three-phase power at the industrial frequency is first stepped down to V by an industrial frequency transformer. m Then it is converted to low frequency by M3C. N Then it is stepped up to V by a low-frequency step-up transformer. N Finally, it is transmitted through a low-frequency transmission line.

[0053] The parameters of the low-frequency power transmission primary system are the actual electrical parameters provided by the actual low-frequency power transmission project, including the parameters of the power frequency transformer, low-frequency step-up transformer, power frequency grounding transformer, power frequency transmission line, low-frequency transmission line, system equivalent impedance, and M3C.

[0054] 2. Secondary System Construction Unit

[0055] Based on the functional requirements of the low-frequency power transmission secondary system, the control parameters of the M3C control system are determined. By utilizing library modules and writing programs to form a custom module, it is added to the main control system to realize the control strategy of the M3C converter valve in the low-frequency power transmission system, thereby completing the construction of the low-frequency power transmission control and protection secondary system.

[0056] The M3C control system adopts a hierarchical control structure, namely system-level control, converter station-level control, and converter valve-level control system. The system-level control system is located at the top level of the control system. It focuses on the overall safe, stable, and optimized operation of the system, converting the set values ​​of active and reactive physical quantities into reference values ​​for lower-level control, enabling the system to switch smoothly under different operating commands. The converter station-level control system is the core of the entire system control and serves as the link between the system-level and converter valve-level control systems. It includes dual closed-loop vector control based on outer and inner loop control on the input and output sides, as well as capacitor voltage equalization control. The converter valve-level control system is the lowest level of the control system. By employing appropriate modulation strategies, it can not only complete M3C modulation but also achieve capacitor voltage equalization control among submodules within the M3C bridge arm.

[0057] 3. Complete model building and debugging unit

[0058] Under the parameters of each electrical component and equipment in the main circuit determined by the primary system determination unit, and according to the actual low-frequency power transmission engineering circuit structure, the electrical components and equipment in the main circuit are connected to establish the primary system, and the low-frequency power transmission control and protection secondary system is completed in the secondary system construction unit, thus building a complete electromagnetic transient model of the low-frequency power transmission system based on M3C.

[0059] After completing the construction of the electromagnetic transient model of the low-frequency power transmission system, continuous simulation tests are conducted, and the actual waveforms are compared. The control parameters are continuously modified and the electromagnetic transient model is debugged to ensure the accuracy of the electromagnetic transient model, so that the low-frequency power transmission system can operate stably for a long time and the operating results are consistent with the actual engineering.

[0060] When operating in steady state, the power transmitted by the low-frequency power transmission system is P. N The power frequency and low frequency grid-side voltages are V. N The power frequency and low frequency valve-side voltages are V. m .

[0061] 4. Short-circuit fault simulation building unit

[0062] Based on the electromagnetic transient model established above, short-circuit fault modules are added to the power frequency transmission line, low frequency transmission line, power frequency transformer, and low frequency transformer protection zone, respectively. Three-phase reclosing and single-phase loss modules are added to the low frequency transmission line.

[0063] With the addition of the above modules, electromagnetic transient simulation of low-frequency power transmission systems can be easily realized. It can simulate different types of short-circuit faults at different locations such as power frequency or low-frequency power transmission lines, power frequency or low-frequency transformer grounding or inter-turn faults, and can also simulate two-phase operation or single-phase operation mode of low-frequency power transmission lines. It can also simulate the three-phase tripping and reclosing of circuit breakers during short-circuit faults.

[0064] 5. Model data export and electrical quantity visualization building unit

[0065] By adding a voltage and current monitoring module, a sequence component analysis module, a differential current calculation module, and a data export module to the electromagnetic transient model of a low-frequency power transmission system, the voltage, current, sequence components, and differential current at each measurement point can be displayed on an oscilloscope, or the data can be exported for analysis through the data export module. This enables the system to be used for testing the protection and control principles and implementing algorithms for low-frequency power transmission systems.

[0066] The above-described embodiments are used to specifically illustrate the present invention, but should not be construed as limiting the scope of protection of the present invention. Those skilled in the art can make changes or modifications to the present invention after understanding its spirit and principles to achieve equivalent purposes, and such equivalent changes and modifications should all be covered within the scope defined by the claims of the present invention.

Claims

1. A method for electromagnetic transient modeling of low-frequency transmission systems based on M3C, characterized in that, Including the following steps: 1) Determine the structure and parameters of the low-frequency power transmission primary system; 2) Establish a secondary system for low-frequency power transmission control and protection; 3) Complete model building and debugging: Under the parameters of each electrical component and equipment of the main circuit determined in step 1), according to the actual low-frequency power transmission engineering circuit structure, connect each electrical component and equipment of the main circuit to establish a primary system and the low-frequency power transmission control and protection secondary system completed in step 2), and build a complete electromagnetic transient model of the low-frequency power transmission system based on M3C. After completing the construction of the electromagnetic transient model of the low-frequency power transmission system, continuous simulation tests are conducted, and the actual waveforms are compared. The control parameters are continuously modified and the electromagnetic transient model is debugged to ensure the accuracy of the electromagnetic transient model, so that the low-frequency power transmission system can operate stably for a long time and the operating results are consistent with the actual engineering. 4) Construct a short-circuit fault and phase loss operation simulation module: Add a fault module to the electromagnetic transient model of the low-frequency transmission system to simulate short-circuit faults and phase loss operation; 5) Construct a model data export and electrical quantity visualization module: Add a voltage and current monitoring module, a sequence component analysis module, a differential current calculation module, and a data export module to the electromagnetic transient model of the low-frequency power transmission system, so that the voltage, current, sequence components, and differential current of each measurement point can be displayed on an oscilloscope, or the data can be exported for analysis through the data export module; In step 4), based on the electromagnetic transient model built earlier, short-circuit fault modules are added to the power frequency transmission line, low-frequency transmission line, power frequency transformer, and low-frequency transformer protection zone, respectively. Three-phase reclosing and single-phase loss modules are added to the low-frequency transmission line. For faults that occur in actual engineering sites, different types of short-circuit faults at different locations between turns of the power frequency or low-frequency transmission line and power frequency or low-frequency transformer are simulated. The two-phase operation or single-phase operation mode of the low-frequency transmission line is simulated, as well as the three-phase tripping and reclosing of the circuit breaker during short-circuit faults are simulated.

2. The electromagnetic transient modeling method for low-frequency transmission systems as described in claim 1, characterized in that, In step 1), the structure of the low-frequency power transmission primary system is as follows: At two voltage levels... V N Each substation will have a frequency converter station built within it, with a capacity of [missing information]. P N Low frequency was carried out between the two substations. f N Power transmission; within each frequency converter station, the three-phase power at the industrial frequency is first stepped down by an industrial frequency transformer. V m Then it is converted to low frequency by M3C. f N Then, it is stepped up by a low-frequency step-up transformer to... V N Finally, it is transmitted through a low-frequency transmission line.

3. The electromagnetic transient modeling method for low-frequency transmission systems as described in claim 1, characterized in that, In step 1), the parameters of the low-frequency power transmission primary system include the parameters of the power frequency transformer, the low-frequency step-up transformer, the power frequency grounding transformer, the power frequency transmission line, the low-frequency transmission line, the system equivalent impedance, and M3C.

4. The electromagnetic transient modeling method for low-frequency transmission systems as described in claim 1, characterized in that, In step 2), the construction process of the low-frequency power transmission control and protection secondary system is as follows: Based on the functional requirements of the low-frequency power transmission secondary system, the control parameters of the M3C control system are determined. By utilizing library modules and writing programs, a custom module is formed and added to the main control system to realize the control strategy of the M3C converter valve of the low-frequency power transmission system, thereby completing the construction of the low-frequency power transmission control and protection secondary system.

5. The electromagnetic transient modeling method for low-frequency transmission systems as described in claim 4, characterized in that, In step 2), the M3C control system adopts a hierarchical control structure, namely system-level control, converter station-level control, and converter valve-level control system. The system-level control system is located at the top level of the control system. From the perspective of the overall safe, stable, and optimized operation of the system, it converts the set values ​​of active and reactive physical quantities into reference values ​​for lower-level control, enabling the system to switch smoothly under different operating commands. The converter station-level control system is the core of the entire system control and serves as the link between the system-level and converter valve-level control systems. It includes dual closed-loop vector control based on outer and inner loop control on the input and output sides, as well as capacitor voltage equalization control. The converter valve-level control system is the lowest level of the control system. It can perform M3C modulation and realize capacitor voltage equalization control between sub-modules within the M3C bridge arm.

6. An electromagnetic transient modeling system for low-frequency transmission systems based on M3C, used to implement the electromagnetic transient modeling method for low-frequency transmission systems according to any one of claims 1-5, characterized in that, include: Primary system determination unit: used to determine the structure and parameters of the low-frequency power transmission primary system; Secondary system construction unit: used to build a low-frequency power transmission control and protection secondary system; Complete model building and debugging unit: Under the parameters of each electrical component and equipment of the main circuit determined by the primary system determination unit, the primary system is established by connecting each electrical component and equipment of the main circuit according to the actual low-frequency power transmission engineering circuit structure, and the low-frequency power transmission control and protection secondary system is completed in the secondary system building unit, thus building a complete electromagnetic transient model of the low-frequency power transmission system based on M3C. After completing the construction of the electromagnetic transient model of the low-frequency power transmission system, continuous simulation tests are conducted, and the actual waveforms are compared. The control parameters are continuously modified and the electromagnetic transient model is debugged to ensure the accuracy of the electromagnetic transient model, so that the low-frequency power transmission system can operate stably for a long time and the operating results are consistent with the actual engineering. Short-circuit fault simulation building block: A fault module is added to the electromagnetic transient model of the low-frequency transmission system to simulate short-circuit faults and phase loss operation; Model data export and electrical quantity visualization building unit: Add voltage and current monitoring module, sequence component analysis module, differential current calculation module and data export module to the electromagnetic transient model of low frequency power transmission system, so that the voltage, current, sequence components and differential current of each measurement point can be displayed on an oscilloscope, or the data can be exported for analysis through the data export module.

7. The low-frequency power transmission system electromagnetic transient modeling system according to claim 6, characterized in that, In the aforementioned primary system determination unit, the structure of the low-frequency power transmission primary system is as follows: At two voltage levels... V N Each substation will have a frequency converter station built within it, with a capacity of [missing information]. P N Low frequency was carried out between the two substations. f N Power transmission; within each frequency converter station, the three-phase power at the industrial frequency is first stepped down by the industrial frequency transformer. V m Then it is converted to low frequency by M3C. f N Then, it is stepped up by a low-frequency step-up transformer to... V N Finally, it is transmitted through a low-frequency transmission line.

8. The low-frequency transmission system electromagnetic transient modeling system as described in claim 6, characterized in that, In the primary system determination unit, the parameters of the low-frequency power transmission primary system include the parameters of the power frequency transformer, the low-frequency step-up transformer, the power frequency grounding transformer, the power frequency transmission line, the low-frequency transmission line, the system equivalent impedance, and the M3C.

9. The low-frequency transmission system electromagnetic transient modeling system as described in claim 6, characterized in that, In the secondary system construction unit, the control parameters of the M3C control system are determined according to the functional requirements of the low-frequency power transmission secondary system. A custom module is formed by using library modules and writing programs, and added to the main control system to realize the control strategy of the M3C converter valve of the low-frequency power transmission system, thereby completing the construction of the low-frequency power transmission control and protection secondary system.

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