A method for power frequency side fault ride-through of an m3c frequency converter

Abstract

The application discloses a kind of M3C frequency changer power frequency side fault crossing method, comprising the following steps:1), real-time acquisition power grid voltage, judge whether positive sequence component is less than 0.9p.u.;2), when power grid fault occurs, real-time acquisition M3C frequency changer frequency division side and power frequency side power, according to the difference between frequency division side power Pf and power frequency side power Pg, as the modulation signal of unloading resistance connected in M3C frequency changer frequency division side, control unloading resistance trigger pulse;3), when power grid voltage positive sequence component is less than 0.5p.u., by soft cut link, the vertical voltage-sharing control of M3C frequency changer, diagonal voltage-sharing control is switched from power frequency side to frequency division side;4), after fault recovery, cut off unloading resistance, and by soft cut link, the vertical voltage-sharing control of M3C frequency changer, diagonal voltage-sharing control is switched back from power frequency side to power frequency side.The application can realize the reliable crossing of M3C frequency changer when power frequency side fault, need not and frequency division side wind / light and other converter unit communication, also need not change power frequency side and frequency division side control structure, can guarantee M3C each sub-converter, sub-converter each bridge arm between sub-module voltage balance during fault crossing.

Description

Technical Field

[0001] This invention relates to the field of high-capacity power electronic devices and control strategies, specifically to a method for fault ride-through on the power frequency side of an M3C frequency converter. Background Technology

[0002] Frequency division transmission reduces the reactance of the transmission system, decreases the charging current caused by the distributed capacitance of long-distance transmission lines, and increases the transmission capacity and distance by lowering the transmission frequency, thereby reducing the number of circuits and the length of the transmission line corridor.

[0003] 2. The M3C frequency converter, also known as a modular multilevel matrix converter, is a new type of AC-AC power conversion device. It can convert different amplitudes, frequencies, and power factors at the input and output terminals, combining the advantages of both traditional matrix converters (MC) and modular multilevel converters (MMC). It is applied in scenarios where frequency-division transmission systems are connected to the mains power grid, possessing significant technical advantages and development potential. Typically, in a frequency-division transmission system, the frequency division side houses wind power, photovoltaic, and other new energy power plants. The M3C frequency converter operates as a voltage source on the frequency division side and is connected to the mains power grid on the mains side. Therefore, mains fault ride-through is a mandatory requirement of the power grid for the M3C frequency converter. Summary of the Invention

[0004] The purpose of this invention is to provide a method for fault ride-through on the power frequency side of an M3C frequency converter. This method aims to achieve reliable ride-through without requiring communication with wind / solar converter units on the frequency division side, or changing the control structure on the power frequency side and the frequency division side. At the same time, it can ensure the voltage balance of each sub-converter and each bridge arm of the M3C during fault ride-through.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a method for fault ride-through on the power frequency side of an M3C frequency converter, comprising the following steps:

[0006] 1) Real-time acquisition of power grid voltage, and determination of whether the positive sequence component is less than 0.9 pu;

[0007] 2) When a fault occurs in the power frequency grid, the power on the frequency divider side and the power frequency side of the M3C inverter is collected in real time. The difference between the power on the frequency divider side Pf and the power frequency side Pg is used as the modulation signal connected to the unloading resistor on the frequency divider side of the M3C inverter to control the trigger pulse of the unloading resistor.

[0008] 3) When the positive sequence component of the power frequency grid voltage is less than 0.5pu, the vertical voltage equalization control and diagonal voltage equalization control of the M3C frequency converter are switched from the power frequency side to the frequency division side through a soft-switching circuit;

[0009] 4) After the fault is recovered, the unloading resistor is cut off, and the vertical voltage-sharing control and the diagonal voltage-sharing control of the M3C converter are switched from the power frequency side back to the power frequency side through a soft switching link.

[0010] As preferred, the vertical voltage-sharing control of the M3C converter in the step three is switched from the positive sequence d-axis α component and the d-axis β component of the bridge arm current on the power frequency side to the negative sequence d-axis zero sequence component and the q-axis zero sequence component on the frequency division side.

[0011] As preferred, the diagonal voltage-sharing control of the M3C converter in the step one is switched from the negative sequence d-axis α component, the d-axis β component, the q-axis α component, and the q-axis β component of the bridge arm current on the power frequency side to the negative sequence d-axis α component, the q-axis α component, the d-axis β component, and the q-axis β component on the frequency division side.

[0012] As preferred, the frequency division side controllable unloading device in the step three includes but is not limited to a structure of directly connecting a resistor through a three-phase modular multilevel converter, a structure of first passing through uncontrolled rectification and then connecting a resistor through a modular multilevel converter on the direct current side, and the like.

[0013] As preferred, the M3C converter on the power frequency side in the step one is integrated into an alternating current power grid, the phase of the positive sequence component of the power grid voltage is obtained through a phase-locked loop, three groups of bridge arm currents iua / iva / iwa, iub / ivb / iwb, and iuc / ivc / iwc are divided, the phase of the positive sequence component of the grid voltage is taken as a vector angle, and vector transformation is respectively performed on the three groups of currents, as shown in the following formula:

[0014]

[0015]

[0016]

[0017] As preferred, αβ transformation is respectively performed on iad / ibd / icd and iaq / ibq / icq in the vector transformation calculation process, as shown in the following formula:

[0018]

[0019]

[0020] Double αβ transformation is performed on the sub-module voltage of each bridge arm, as shown in the following formula:

[0021]

[0022] As preferred, the M3C frequency converter in step one incorporates the AC power grid on the frequency division side, and divides the nine bridge arm currents into iua / iub / iuc, iva / ivb / ivc, iwa / iwb / iwc three groups, and performs vector transformation with the frequency division side phase as the vector angle, as shown in the following formula:

[0023]

[0024]

[0025]

[0026] As preferred, the frequency division side phase continues to perform αβ transformation on iud / ivd / iwd and iuq / ivq / iwq respectively, as shown in the following formula:

[0027]

[0028]

[0029] In the above technical solution, the M3C frequency converter power frequency side fault ride-through method provided by the application has the following beneficial effects: reliable ride-through is achieved without the need for communication with the frequency division side wind / light and other converter units, and without the need to change the power frequency side and frequency division side control structure, and the balance of the sub-module voltage between the M3C sub-converters and the bridge arms of the sub-converters can be ensured during fault ride-through. BRIEF DESCRIPTION OF DRAWINGS

[0030] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed in the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments described in the present application, and other drawings can also be obtained by those skilled in the art based on these drawings.

[0031] Figure 1 It is a system structure schematic diagram;

[0032] Figure 2 It is a M3C frequency converter circuit schematic diagram;

[0033] Figure 3 It is two common schematic diagrams of unloading parts (including but not limited to);

[0034] Figure 4 It is an unloading part control structure schematic diagram;

[0035] Figure 5 It is a M3C frequency converter power frequency side fault ride-through flowchart;

[0036] Figure 6Control structure diagram for power frequency side;

[0037] Figure 7 Control structure diagram for sub-frequency side;

[0038] Fig. 8(a) is a vertical voltage-sharing control structure diagram implemented on the power frequency side;

[0039] Fig. 8(b) is a vertical voltage-sharing control structure diagram implemented on the sub-frequency side;

[0040] Figure 9 Control structure diagram for horizontal voltage-sharing;

[0041] Fig. 10(a) is a diagonal voltage-sharing control structure diagram implemented on the power frequency side;

[0042] Fig. 10(b) is a diagonal voltage-sharing control structure diagram implemented on the sub-frequency side;

[0043] Figure 11 Switching structure diagram for vertical voltage-sharing control during power frequency fault ride-through;

[0044] Figure 12 Switching structure diagram for diagonal voltage-sharing control during power frequency fault ride-through;

[0045] Figure 13 Simulation waveform of power frequency transformer inverter side voltage when the power frequency side transformer grid side voltage of M3C inverter drops to zero during 2-3s;

[0046] Figure 14 Simulation waveform of power frequency q-axis current given and feedback when the power frequency side transformer grid side voltage of M3C inverter drops to zero during 2-3s;

[0047] Figure 15 Simulation waveform of bridge arm sub-module voltage when the power frequency side transformer grid side voltage of M3C inverter drops to zero during 2-3s;

[0048] Figure 16 Simulation waveform of M3C power frequency side current when the power frequency side transformer grid side voltage of M3C inverter drops to zero during 2-3s;

[0049] Figure 17 Simulation waveform of M3C sub-frequency side voltage when the power frequency side transformer grid side voltage of M3C inverter drops to zero during 2-3s;

[0050] Figure 18 Simulation waveform of M3C sub-frequency side current when the power frequency side transformer grid side voltage of M3C inverter drops to zero during 2-3s;

[0051] Figure 19 Simulation waveform of M3C bridge arm current when the power frequency side transformer grid side voltage of M3C inverter drops to zero during 2-3s. DETAILED DESCRIPTION

[0052] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the present application.

[0053] As shown in the figure, a method for power frequency side fault ride-through of an M3C frequency converter includes the following steps: Figures 1-19

[0054] 1) Real-time acquisition of power frequency grid voltage, judgment of whether the positive sequence component is less than 0.9 p.u.

[0055] 2) Real-time acquisition of M3C frequency converter frequency division side and power frequency side power when the power frequency grid fails, difference between the frequency division side power Pf and the power frequency side power Pg as the modulation signal of the unloading resistor connected to the frequency division side of the M3C frequency converter, control of the trigger pulse of the unloading resistor;

[0056] 3) When the power frequency grid voltage positive sequence component is less than 0.5 p.u., switching of the vertical voltage equalization control and the diagonal voltage equalization control of the M3C frequency converter from the power frequency side to the frequency division side through a soft switching link;

[0057] 4) After the fault is recovered, the unloading resistor is removed, and the vertical voltage equalization control and the diagonal voltage equalization control of the M3C frequency converter are switched back from the frequency division side to the power frequency side through a soft switching link.

[0058] Specifically, when the power frequency grid fails, the power frequency side of the M3C frequency converter maintains the average voltage stability control of the sub-module, the power frequency side reactive power outer ring is removed, the output grid-connected standard reactive current is required, the frequency division side maintains the AC voltage control; the unloading device pulse is enabled, the modulation signal of the unloading device is controlled according to the difference between the frequency division side and the power frequency side power; when the power frequency grid voltage positive sequence component is lower than 0.5 p.u., the vertical voltage equalization and the diagonal voltage equalization links of the M3C frequency converter are switched from the power frequency side to the frequency division side through a soft switching link, while realizing the power frequency fault ride-through and ensuring that the sub-module voltages of each sub-converter and each bridge arm of the sub-converter of the M3C are balanced; after the power frequency grid fault is recovered, the unloading device pulse is blocked, the vertical voltage equalization and the diagonal voltage equalization links of the M3C frequency converter are switched from the frequency division side back to the power frequency side through a soft switching link, and the power frequency reactive power ring is restored.

[0059] ​The technical scheme has the advantages that reliable crossing is realized without communication with the frequency side and the frequency division side wind / light converter unit, without changing the control structure of the frequency side and the frequency division side, and the balance of the sub-module voltage between the M3C sub-converters and the bridge arms of the sub-converters is ensured during fault crossing.

[0060] As a further provided embodiment of the application, the vertical voltage balance control of the M3C converter in step three is switched from the bridge arm current frequency side positive sequence d-axis alpha component and d-axis beta component to the bridge arm current frequency division side negative sequence d-axis zero sequence component and q-axis zero sequence component.

[0061] Further, the diagonal voltage balance control of the M3C converter in step three in the above embodiment is switched from the bridge arm current frequency side negative sequence d-axis alpha component, d-axis beta component, q-axis alpha component and q-axis beta component to the bridge arm current frequency division side negative sequence d-axis alpha component, q-axis alpha component, d-axis beta component and q-axis beta component.

[0062] Still further, the frequency division side controllable unloading device in step three in the above embodiment includes but is not limited to a structure of directly connecting a resistance through the three-phase modular multilevel converter and a structure of connecting a resistance through the modular multilevel converter on the DC side after uncontrolled rectification.

[0063] As another further provided embodiment of the application, the M3C converter frequency side is connected to the AC power grid in step one, the phase of the frequency grid voltage positive sequence component is obtained through a phase-locked loop, nine bridge arm currents are divided into iua / iva / iwa, iub / ivb / iwb and iuc / ivc / iwc three groups, the grid voltage positive sequence component phase is used as a vector angle, and vector transformation is performed on the three groups of currents respectively, as shown in the following formula:

[0064]

[0065]

[0066]

[0067] Then, iad / ibd / icd and iaq / ibq / icq are subjected to αβ transformation respectively, as shown in the following formula:

[0068]

[0069] Double αβ transformation is performed on the sub-module voltage of each bridge arm, as shown in the following formula:

[0070]

[0071] The M3C converter frequency side control structure is as follows: Figure 6As shown in the figure, all sub-module capacitor voltage average value and reactive power decoupling control are realized.

[0072] As a further provided embodiment of the application, in step one, the M3C frequency converter incorporates the AC power grid on the frequency division side, and divides the nine bridge arm currents into three groups of iua / iub / iuc, iva / ivb / ivc, and iwa / iwb / iwc, and performs vector transformation with the phase on the frequency division side as the vector angle, as shown in the following formula:

[0073]

[0074]

[0075]

[0076] Then, iud / ivd / iwd and iuq / ivq / iwq are respectively subjected to αβ transformation, as shown in the following formula:

[0077]

[0078]

[0079] The control structure of the M3C frequency converter on the frequency division side is as shown in the figure Figure 7 , and the outlet voltage on the frequency division side is stabilized.

[0080] During normal operation, the vertical voltage equalization control of the M3C frequency converter is realized on the power frequency side, and the control structure is as shown in Fig. 8(a); the average voltage control is realized on the frequency division side, and the control structure is as shown in Fig. 9(a); and the diagonal voltage equalization control is realized on the power frequency side, and the control structure is as shown in Fig. 10(a). Figure 9

[0081] When the power frequency grid fails, it is determined whether the positive sequence component is less than 0.9 p.u.; the difference between the power Pf on the frequency division side and the power Pg on the power frequency side of the M3C frequency converter is calculated as a modulation signal of the unloading resistor, and the trigger pulse of the unloading resistor is controlled.

[0082] When the positive sequence component of the power frequency grid voltage is less than 0.5 p.u., in order to ensure the effect of vertical and diagonal voltage equalization control, the vertical voltage equalization control and the diagonal voltage equalization control of the M3C frequency converter are switched from the power frequency side to the frequency division side through a soft start link, the control structure of the vertical voltage equalization control realized on the frequency division side is as shown in Fig. 8(b); and the control structure of the diagonal voltage equalization control realized on the frequency division side is as shown in Fig. 10(b).

[0083] After the fault is recovered, the unloading resistor is removed, and the vertical voltage equalization control and the diagonal voltage equalization control of the M3C frequency converter are switched from the frequency division side back to the power frequency side through a soft cut link. The power frequency fault ride-through process of the M3C frequency converter is as shown in the figure Figure 5 . ​

[0084] According to the power frequency fault ride-through enabling signal, the output of the vertical and diagonal voltage sharing control implemented at the power frequency side is multiplied by a ramp signal between 1 and 0, and the output of the vertical and diagonal voltage sharing control implemented at the split frequency side is multiplied by a ramp signal between 0 and 1. The soft switching structure between the power frequency side and the split frequency side of the vertical and diagonal voltage sharing control is shown as Figure 11 .

[0085] Those skilled in the art will appreciate that embodiments of the application can be supplied as a method, a system, or a computer program product. Accordingly, the application can take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the application can take the form of a computer program product on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROMs, optical storage devices, etc.) embodying computer-readable program code.

[0086] The application is described in reference to the flowchart and / or block diagrams of the method, apparatus (system) and computer program product according to embodiments of the application. It will be understood that each block of the flowchart and / or block diagrams, and combinations of blocks in the flowchart and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general purpose computer, special purpose computer, embedded processing device, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in the flowchart and / or block diagram block or blocks. Figure 1 one or more flowcharts and / or blocks Figure 1 means for functionally implementing the

[0087] These computer program instructions can also be stored in a computer- readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instructions which implement the function specified in the flowchart and / or block diagram block or blocks. Figure 1 one or more flowcharts and / or blocks Figure 1 means for functionally implementing the

[0088] The computer program instructions can also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart and / or block diagram block or blocks. Figure 1 one or more flowcharts and / or blocks Figure 1 means for functionally implementing the

[0089] The principles and implementation manners of the present application are described by using specific examples, and the above examples are only used to help understand the method and core idea of the present application; meanwhile, for those skilled in the art, according to the idea of the present application, the specific implementation manners and application ranges will be changed, and the above description should not be understood as a limitation of the present application.

[0090] The embodiment of the present application also provides a specific implementation manner of an electronic device capable of implementing all steps in the method in the above embodiment, and the electronic device specifically includes the following contents.

[0091] a processor, a memory, a communications interface and a bus;

[0092] The processor, the memory and the communications interface complete mutual communication through the bus.

[0093] The processor is used for calling a computer program in the memory, and the processor implements all steps in the method in the above embodiment when executing the computer program, for example, the processor implements the following steps when executing the computer program:

[0094] 1) Real-time acquisition of a power frequency power grid voltage, and judgment of whether a positive sequence component is less than 0.9 p.u.

[0095] 2) Real-time acquisition of power on a frequency division side and a power frequency side of an M3C frequency converter when a power frequency power grid fails, and taking a difference between the power on the frequency division side and the power on the power frequency side as a modulation signal of an unloading resistor connected to the frequency division side of the M3C frequency converter, to control a trigger pulse of the unloading resistor.

[0096] 3) When the positive sequence component of the power frequency power grid voltage is less than 0.5 p.u., switching, through a soft switching link, vertical voltage sharing control and diagonal voltage sharing control of the M3C frequency converter from the power frequency side to the frequency division side.

[0097] 4) After the failure is recovered, the unloading resistor is removed, and the vertical voltage sharing control and the diagonal voltage sharing control of the M3C frequency converter are switched back from the frequency division side to the power frequency side through the soft switching link.

[0098] The embodiment of the present application also provides a computer readable storage medium capable of implementing all steps in the method in the above embodiment, and the computer readable storage medium stores a computer program, and the computer program is executed by a processor to implement all steps in the method in the above embodiment, and the effects are as follows.

[0099] A power frequency transformer frequency converter side voltage simulation waveform interface Figure 13 );

[0100] Simulation waveform interface of the given and feedback of the power frequency q-axis current when falling to zero Figure 14

[0101] Simulation waveform interface of the bridge arm submodule voltage when falling to zero Figure 15

[0102] Simulation waveform interface of the power frequency side current of M3C when falling to zero Figure 16

[0103] Simulation waveform interface of the frequency division side voltage of M3C when falling to zero Figure 17

[0104] Simulation waveform interface of the frequency division side current of M3C when falling to zero Figure 18

[0105] Simulation waveform interface of the bridge arm current of M3C when falling to zero Figure 19

[0106] ​​​​​​The various embodiments in this specification are described in a progressive manner. Similar or identical parts between embodiments can be referred to mutually. Each embodiment focuses on its differences from other embodiments. In particular, for hardware + program embodiments, since they are basically similar to method embodiments, the description is relatively simple; relevant parts can be referred to the descriptions in the method embodiments. Although the embodiments in this specification provide the method operation steps as shown in the embodiments or flowcharts, more or fewer operation steps may be included based on conventional or non-inventive means. The order of steps listed in the embodiments is merely one possible execution order among many steps and does not represent the only execution order. In actual device or terminal product execution, the methods can be executed in the order shown in the embodiments or drawings or in parallel (e.g., in a parallel processor or multi-threaded processing environment, or even a distributed data processing environment). The terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, product, or apparatus that includes a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, product, or apparatus. Without further limitations, the presence of other identical or equivalent elements in the process, method, product, or apparatus that includes said elements is not excluded. For ease of description, the above devices are described in terms of function, divided into various modules. Of course, in implementing the embodiments of this specification, the functions of each module can be implemented in one or more software and / or hardware, or the module implementing the same function can be implemented by a combination of multiple sub-modules or sub-units, etc. The device embodiments described above are merely illustrative. For example, the division of units is only a logical functional division; in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be through some interfaces; the indirect coupling or communication connection between devices or units may be electrical, mechanical, or other forms. This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It should be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing device to produce a machine, such that the instructions, which are executable by the processor of the computer or other programmable data processing device, produce instructions for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0107] Those skilled in the art will appreciate that embodiments of the present specification can be devised for a method, a system, or a computer program product. Accordingly, embodiments of the present specification can be embodied in the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, embodiments of the present specification can be in the form of a computer program product on one or more computer-usable storage media (including, but not limited to, disk memory, CD-ROM, optical storage medium, etc.) embodying computer usable program code. Each of the various embodiments of the present specification is described in a progressive manner, and reference can be made to other embodiments for the same or similar parts. Each embodiment focuses on the differences from other embodiments. In particular, for system embodiments, since they are basically similar to method embodiments, the description is relatively simple, and reference can be made to the description of the method embodiments. In the description of the present specification, the description of the terms "one embodiment", "some embodiments", "an example", "a specific example", or "some examples" means that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the embodiments of the present specification.

[0108] In the present specification, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. In addition, those skilled in the art can combine and combine the different embodiments or examples described in the present specification and the features of the different embodiments or examples without contradiction. The above is only an embodiment of the embodiments of the present specification and is not intended to limit the embodiments of the present specification. The embodiments of the present specification can have various modifications and changes for those skilled in the art. Any modification, equivalent replacement, improvement, etc. within the spirit and principles of the embodiments of the present specification shall be included in the scope of the claims of the embodiments of the present specification.

Claims

1. A method for M3C frequency converter line side fault ride through, characterized in that, It comprises the following steps: 1) Real-time acquisition of power grid voltage, judge whether the positive sequence component is less than 0.9p.u.; 2) When the power grid fails, real-time acquisition of M3C frequency converter power on the frequency side and the frequency side, according to the difference between the frequency side power Pf and the frequency side power Pg, as the modulation signal of the unloading resistance connected to the frequency side of the M3C frequency converter, control the trigger pulse of the unloading resistance; 3) When the positive sequence component of the power grid voltage is less than 0.5p.u., the vertical voltage sharing control and the diagonal voltage sharing control of the M3C frequency converter are switched from the frequency side to the frequency side through the soft switching link; 4) After the fault is recovered, the unloading resistance is removed, and the vertical voltage sharing control and the diagonal voltage sharing control of the M3C frequency converter are switched back to the frequency side through the soft switching link.

2. The method for M3C converter line side fault ride through according to claim 1, characterized in that, The vertical voltage sharing control of the M3C frequency converter is switched to the negative sequence d-axis zero sequence component and the q-axis zero sequence component of the bridge arm current on the frequency side.

3. The method of claim 1, wherein, The diagonal voltage sharing control of the M3C frequency converter is switched to the negative sequence d-axis α component, q-axis α component, d-axis β component and q-axis β component of the bridge arm current on the frequency side.

4. The method of claim 1, wherein, It also includes a controllable unloading device on the frequency side, which includes a structure directly connected to the resistance through the three-phase modular multilevel converter, and a structure connected to the resistance through the modular multilevel converter on the DC side after uncontrolled rectification.

5. The method of claim 1, wherein, The M3C frequency converter is connected to the AC power grid on the frequency side, and the positive sequence component phase of the power grid voltage is obtained through the phase-locked loop, and the nine bridge arm currents are divided into iua / iva / iwa, iub / ivb / iwb, iuc / ivc / iwc three groups, and the three groups of currents are respectively vector transformed with the positive sequence component phase of the power grid voltage as the vector angle, as shown in the following formula:

6. The method of claim 5, wherein, During the vector transformation calculation, iad / ibd / icd and iaq / ibq / icq are respectively αβ transformed, as shown in the following formula: The sub-module voltage of each bridge arm is double αβ transformed, as shown in the following formula:

7. The method of claim 1, wherein, The M3C frequency converter is connected to the AC power grid on the frequency side, and the nine bridge arm currents are divided into iua / iub / iuc, iva / ivb / ivc, iwa / iwb / iwc three groups, and the three groups of currents are respectively vector transformed with the frequency side phase as the vector angle, as shown in the following formula:

8. The method of claim 7, wherein, The frequency side phase continues to respectively αβ transform iud / ivd / iwd and iuq / ivq / iwq, as shown in the following formula:

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