Magnetic saturation bridge arm coupling reactor and flexible DC transmission system

Abstract

The invention discloses a magnetic saturation bridge arm coupling reactor and a flexible direct current transmission system, the magnetic saturation bridge arm coupling reactor comprises: two magnetic cores, six groups of coils, two permanent magnets and a direct current power supply; the first, third and fifth coils are wound on the first magnetic core, the second, fourth and sixth coils are wound on the second magnetic core, the first and second coils are connected end to end, the third and fourth coils are connected end to end, the first and third coils are connected at opposite ends, and the center tap is connected to the AC end of the converter valve, the taps of the fourth and second coils are respectively connected to the upper bridge arm and the lower bridge arm of the converter valve, the winding direction of the six groups of coils is the same, and all are right-hand spiral winding directions; two permanent magnets are respectively arranged on the first magnetic core and the second magnetic core; the fifth and sixth coils are connected end to end, and form a direct current excitation circuit with the direct current power supply; the two permanent magnets and the direct current excitation circuit form a hybrid excitation structure. The invention is applied to the converter valve of the flexible direct current transmission system, can realize power transmission, and can automatically realize circulating current and fault current limitation.

Description

Technical Field

[0001] The invention relates to the technical field of direct current transmission systems, and in particular to a magnetic saturation bridge arm coupling reactor and a flexible direct current transmission system. Background Art

[0002] As a new type of direct current transmission technology, flexible direct current transmission can achieve cross-regional optimization and efficient utilization of power resources by fully controlling the opening and closing of semiconductor devices to establish an AC voltage with controllable amplitude and phase, solve the grid connection and consumption of large-scale renewable energy, and has been widely used. As part of the flexible direct current engineering converter valve, the bridge arm reactor is located on the bridge arm unit of the modular multilevel converter. It has the function of coordinating power transmission. In addition, it can also suppress the circulation between the bridge arms and the bridge arm fault current that rises too fast during short circuit. The circulation between the bridge arms is caused by the time-sharing switching of different sub-modules, the imbalance of capacitor charging and discharging, and the voltage fluctuation of the sub-module capacitor, which makes the voltage between the various phase units of the converter valve not completely consistent. The bridge arm fault current is when a short circuit fault occurs on the DC side of the modular multilevel converter, before the converter valve is locked, the capacitor in the sub-module will generate a large short-circuit current on the bridge arm through the discharge circuit.

[0003] However, the current bridge arm reactors are traditional air-core reactors or iron-core reactors with constant inductance values, which cannot effectively suppress circulating currents. Long-term operation will cause damage to the converter valves. In addition, with the increase in voltage levels and the continuous access to new energy sources, the short-circuit current level of the power system is gradually increasing. When a fault occurs, the bridge arm reactor has a low inductance value and cannot effectively suppress the fault current. Therefore, it is necessary to propose a new bridge arm reactor suitable for flexible DC transmission systems based on the characteristics of the bridge arm current of modular multilevel converters, which has a variable inductance value to better limit circulating currents and fault currents without affecting the normal operation of the system. Summary of the invention

[0004] The present invention aims to solve the technical problems in the related art at least to a certain extent. To this end, the first object of the present invention is to provide a magnetic saturation bridge arm coupling reactor, whose inductance value is variable, conforms to the electrical characteristics of the flexible DC converter valve bridge arm, has three working states, and is automatically adjusted. It can realize power transmission in steady state, and can also show two larger inductance values ​​for circulating current and fault respectively, so as to better limit circulating current and fault current.

[0005] A second object of the present invention is to provide a direct current power transmission system.

[0006] To achieve the above object, the present invention is implemented through the following technical solutions:

[0007] A magnetic saturation bridge arm coupling reactor comprises: a first magnetic core, a second magnetic core, a first coil to a sixth coil, a first permanent magnet, a second permanent magnet and a direct current power supply; wherein the first coil, the third coil and the fifth coil are wound on the first magnetic core, the second coil, the fourth coil and the sixth coil are wound on the second magnetic core, the first coil and the second coil are connected end to end, the third coil and the fourth coil are connected end to end, the first coil and the third coil are connected at opposite ends, and the center tap is connected to the AC end of a converter valve, the taps of the fourth coil and the second coil are respectively connected to the upper bridge arm and the lower bridge arm of the converter valve, the first coil to the fourth coil have the same winding direction, and are all right-hand spiral winding directions; the first permanent magnet and the second permanent magnet are respectively arranged on the first magnetic core and the second magnetic core; the fifth coil and the sixth coil are connected end to end, and are all right-hand spiral winding directions, and form a direct current excitation circuit with the direct current power supply; the first permanent magnet, the second permanent magnet and the direct current excitation circuit form a hybrid excitation structure.

[0008] Preferably, the first magnetic core and the second magnetic core are both U-shaped structures formed by an upper transverse yoke, a lower transverse yoke, a left core column and a right core column, and the upper transverse yoke, the lower transverse yoke, the left core column and the right core column are all solid core columns.

[0009] Preferably, the first coil and the third coil are respectively wound on the upper cross yoke and the lower cross yoke of the first magnetic core, the fifth coil is wound on the right core column of the first magnetic core close to the second magnetic core, and the first permanent magnet is embedded in the left core column of the first magnetic core away from the second magnetic core; the fourth coil and the second coil are respectively wound on the upper cross yoke and the lower cross yoke of the second magnetic core, the sixth coil is wound on the left core column of the second magnetic core close to the first magnetic core, and the second permanent magnet is embedded in the right core column of the second magnetic core away from the first magnetic core.

[0010] Preferably, the first permanent magnet is set with an upper end as an N pole and a lower end as an S pole; the second permanent magnet is set with an upper end as an S pole and a lower end as an N pole; the first permanent magnet and the second permanent magnet have the same size and excitation direction, and the first permanent magnet and the second permanent magnet generate clockwise excitation flux on the first magnetic core and the second magnetic core respectively.

[0011] Preferably, the DC excitation circuit generates clockwise excitation flux on the first magnetic core and the second magnetic core respectively.

[0012] Preferably, the number of turns of the first coil is equal to that of the fourth coil, and the number of turns of the third coil is equal to that of the second coil, wherein the number of turns of the first coil is greater than that of the third coil.

[0013] Preferably, the cross-sectional areas of the upper transverse yoke and the lower transverse yoke of the two magnetic cores are the same, and the two magnetic cores are made of soft magnetic materials.

[0014] Preferably, the magnetic saturation bridge arm coupling reactor is a single-phase iron core reactor for realizing power transmission and limiting circulating current and fault current.

[0015] Preferably, the center tap is connected to the single-phase AC end of the converter valve, and the taps of the fourth coil and the second coil are respectively connected to the single-phase upper bridge arm and the single-phase lower bridge arm of the converter valve.

[0016] To achieve the above-mentioned object, the second aspect of the present invention provides a flexible direct current transmission system, comprising a converter valve and the above-mentioned magnetic saturation bridge arm coupled inductor, wherein the magnetic saturation bridge arm coupled inductor is connected to the converter valve, and the converter valve is a modular multilevel converter.

[0017] The present invention has at least the following technical effects:

[0018] 1. The magnetic saturation bridge arm coupling reactor of the present invention adapts to the current characteristics of the flexible DC system, can effectively limit the interphase circulating current and fault current, does not affect the steady-state power transmission, and has a better working effect than the traditional bridge arm reactor.

[0019] 2. The magnetic saturation bridge arm coupling reactor of the present invention has three working states, does not require manual control, and can naturally change the inductance value according to the working conditions, which is highly efficient and saves manpower.

[0020] 3. The present invention can reasonably configure the inductance value of the magnetic saturation bridge arm coupling reactor under fault, so that it can not only replace the bridge arm reactor, but also replace the smoothing reactor in the flexible DC system, thereby improving the overall economy and reducing the system volume.

[0021] Additional aspects and advantages of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic structural diagram of a magnetic saturation bridge arm coupling reactor according to an embodiment of the present invention.

[0023] Figure 2 Schematic diagram of the connection between the magnetic saturation bridge arm coupling reactor and the converter valve according to an embodiment of the present invention.

[0024] Figure 3 This is a working principle diagram of the magnetic saturation bridge arm coupling reactor under normal power transmission according to an embodiment of the present invention.

[0025] Figure 4This is a working principle diagram of a magnetic saturation bridge arm coupled inductor under interphase circulating current according to an embodiment of the present invention.

[0026] Figure 5 This is a working principle diagram of the magnetic saturation bridge arm coupling reactor under fault conditions according to an embodiment of the present invention. DETAILED DESCRIPTION

[0027] The present embodiment is described in detail below, and examples of the embodiment are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and should not be construed as limiting the present invention.

[0028] The magnetic saturation bridge arm coupled inductor and the DC power transmission system of this embodiment will be described below with reference to the accompanying drawings.

[0029] Figure 1 FIG. 1 is a schematic diagram of the structure of a magnetic saturation bridge arm coupling reactor according to an embodiment of the present invention. Figure 1 As shown, the magnetic saturation bridge arm coupling reactor includes a working winding, a core structure, a permanent magnet excitation structure and a DC excitation circuit. The working winding includes a first coil 21, a second coil 22, a third coil 23 and a fourth coil 24. Among them, the first coil 21 and the second coil 22 are connected end to end, and the winding direction is a right spiral; the third coil 23 and the fourth coil 24 are connected end to end, and the winding direction is a right spiral; the first coil 21 and the third coil 23 are wound in the same direction, the opposite ends are connected, and the center tap is connected to the AC output end; the second coil 22 and the fourth coil 24 are wound in the same direction, and are respectively connected to the upper bridge arm and the lower bridge arm of the converter valve. The core structure includes a first magnetic core 11 and a second magnetic core 12, on which two groups of working windings and a group of excitation windings are wound, for example, the first magnetic core 11 is wound with a first coil 21, a third coil 23 and a group of excitation windings, namely a fifth coil 25. The permanent magnet excitation structure includes a first permanent magnet 31 and a second permanent magnet 32, which are respectively embedded in the two magnetic cores. The DC excitation circuit includes two sets of excitation windings, namely the fifth coil 25, the sixth coil 26 and a DC source 41, wherein the two sets of excitation windings are both wound in a right spiral direction, connected end to end and connected in series with the DC source 41. It should be noted that the permanent magnet excitation structure and the DC excitation circuit form a hybrid excitation structure, which jointly generate an excitation bias for the two magnetic cores.

[0030] Please continue to refer to Figure 1 In this embodiment, the first magnetic core 11 and the second magnetic core 12 are both U-shaped structures formed by an upper cross yoke, a lower cross yoke, a left core column and a right core column, wherein the upper cross yoke, the lower cross yoke, the left core column and the right core column of the two magnetic cores are all solid core columns.

[0031] Specifically, the first coil 21 and the third coil 23 are respectively wound on the upper cross yoke and the lower cross yoke of the first magnetic core 11, the fifth coil 25 is wound on the right core column of the first magnetic core 11 close to the second magnetic core 12, the first permanent magnet 31 is embedded in the left core column of the first magnetic core 11 away from the second magnetic core 12, and the first permanent magnet 31 is set with an upper end of the N pole and a lower end of the S pole; the fourth coil 24 and the second coil 22 are respectively wound on the upper cross yoke and the lower cross yoke of the second magnetic core 12, the sixth coil 26 is wound on the left core column of the second magnetic core 12 close to the first magnetic core 11, the second permanent magnet 32 ​​is embedded in the right core column of the second magnetic core 12 away from the first magnetic core 11, and the second permanent magnet is set with an upper end of the S pole and a lower end of the N pole.

[0032] It should be noted that the two magnetic cores have the same size, specifically the cross-sectional areas of the upper cross yoke and the lower cross yoke, ie, the coil winding part, are the same, and both magnetic cores are made of soft magnetic material.

[0033] Furthermore, the first permanent magnet 31 and the second permanent magnet 32 ​​have the same size and excitation direction, and the first permanent magnet 31 and the second permanent magnet 32 ​​respectively generate clockwise excitation flux on the first magnetic core 11 and the second magnetic core 12. In addition, the DC excitation circuit generates clockwise excitation flux on the first magnetic core 11 and the second magnetic core 12 respectively.

[0034] It can be understood that the hybrid excitation structure in this embodiment is used to provide excitation flux. If only a permanent magnet excitation structure is used, the economic efficiency is poor. If only a DC excitation circuit is used, a large DC current needs to be provided, which will cause a large copper loss in the excitation winding. Therefore, the hybrid excitation structure used in this embodiment can reduce the copper loss of the coil and improve the economic efficiency.

[0035] In addition, the hybrid excitation provided by the permanent magnet excitation structure and the DC excitation circuit in this embodiment can make the magnetic core initially in the magnetic saturation region, and then the magnetic saturation degree of the magnetic core will change according to different working conditions, thereby producing different inductance effects.

[0036] In one embodiment of the present invention, the first coil 21 and the fourth coil 24 have the same number of turns, the third coil 23 and the second coil 22 have the same number of turns, and the first coil 21 has a greater number of turns than the third coil 23 .

[0037] In this embodiment, the number of turns of the first coil 21 is greater than the number of turns of the third coil 23, which can facilitate the magnetic saturation bridge arm coupling reactor to achieve power transmission.

[0038] In one embodiment of the present invention, the magnetic saturation bridge arm coupling reactor is a single-phase iron core reactor, and the converter valve is a modular multilevel converter (MMC). The first working coil 21 and the second working coil 22 are connected in series and serve as the single-phase lower bridge arm inductor of the converter valve; the third working coil 23 and the fourth working coil 24 are connected in series and serve as the single-phase upper bridge arm inductor of the converter valve.

[0039] like Figure 2 As shown, the working winding of the magnetic saturation bridge arm coupling reactor is connected in the modular multilevel converter as a single-phase upper and lower bridge arm inductor. Specifically, the center tap of the first working coil 21 and the third coil 23 is connected to the converter valve, i.e., the single-phase AC terminal u of the modular multilevel converter. sa , the taps of the fourth coil 24 and the second coil 22 are connected to the upper bridge arm of the a phase and the lower bridge arm of the a phase of the modular multilevel converter respectively. In this embodiment, the modular multilevel converter has a three-phase AC side on one side and a DC side on the other side, and contains six upper and lower bridge arms of three phases in the middle, each bridge arm has N submodules, and the submodules are composed of capacitors, IGBTs (insulated gate bipolar transistors) and diodes. By controlling the number of submodules switched, the AC side can output a given value. When working stably, the current flowing through the bridge arm mainly includes: the grid current component, in steady state, the upper and lower bridge arms of each phase will evenly distribute this current, which is the power frequency AC current; the bridge arm circulating current component, this current component flows between the three-phase bridge arms, neither flows into the AC grid nor flows into the DC line, and is mainly a double frequency negative sequence current component. When the DC side has a bipolar fault, before the converter valve is not locked, it is mainly the capacitor in the submodule that discharges to the fault point, so the fault current flows between the upper and lower bridge arms and the DC side fault point, which is an attenuated oscillating current. The third coil 23 and the fourth coil 24 are connected in series and connected to the Nth submodule of the upper bridge arm of phase a, serving as the inductor of the upper bridge arm of phase a; the first coil 21 and the second coil 22 are connected in series and connected to the N+1th submodule of the lower bridge arm of phase a, serving as the inductor of the lower bridge arm of phase a.

[0040] In this embodiment, the inductance of the upper / lower bridge arm of the novel reactor is the sum of the equivalent inductance of the two corresponding coils. For example, the inductance of the upper bridge arm of phase a is the sum of the self-inductance and mutual inductance of the third coil 23 plus the sum of the self-inductance and mutual inductance of the fourth coil 24.

[0041] It should be noted that in Figure 1 Based on the structure of the magnetically saturated bridge arm coupled inductor, under different working conditions (such as normal power transmission, circulating current and fault), the self-inductance and coupling of the four working windings will automatically change, thereby automatically changing the total inductance value to achieve the effect of automatically adapting to the corresponding working conditions, such as achieving power transmission, limiting circulating current and fault current.

[0042] Figure 3-5 The steady-state AC component, interphase circulating current and the corresponding current flow direction and magnetic field change of the magnetic saturation bridge arm coupling reactor under fault are given respectively. By analyzing the working process, the specific working principle of the magnetic saturation bridge arm coupling reactor can be obtained.

[0043] First, the working winding in this embodiment is connected to the bridge arm of the converter valve, generating inductance to the outside, and the coupling characteristics between the coils and the magnetic characteristics of the magnetic core around which the coils are wound together affect the equivalent inductance value of the magnetic saturation bridge arm coupling reactor. In this embodiment, both magnetic cores are made of soft magnetic materials and can work between the magnetic saturation zone and the desaturation zone, and the magnetic permeability of the magnetic saturation zone is much smaller than that of the desaturation zone. The permanent magnet and the DC excitation circuit generate excitation flux in the same direction on the two magnetic cores, and jointly excite them. Under normal circumstances, the magnetic cores can enter a magnetic saturation state, and the magnetic permeability is small, so that the self-inductance of the windings wound on the magnetic cores can be small.

[0044] For steady-state power transmission of the converter valve, the bridge arm current flows between the DC side and the AC side, and there is symmetrical AC current in the upper and lower bridge arms. Figure 3 As shown, since the first coil 21 and the third coil 23 are connected to the opposite ends and connected to the AC end, the magnetic flux generated by the first coil 21 and the third coil 23 on the first magnetic core 11 is in opposite directions, and the steady-state AC component is reversely coupled, and the equivalent inductances are mutually reduced. For example, the equivalent inductance of the first coil 21 is self-inductance-mutual inductance. However, since the number of turns of the first coil 21 is greater than that of the third coil 23, the working magnetic fluxes of the two coils are not equal and are not completely offset. There is still a certain equivalent inductance to the outside, which is not zero. The winding method of the fourth coil 24 and the second coil 22 is symmetrical to that of the first and third coils. Similarly, their equivalent inductances are mutually reduced and are not zero. Since the system current is small in steady state and due to the effect of the hybrid excitation structure, the two cores work in the magnetic saturation state and remain unchanged, and the magnetic permeability is small, so the self-inductance of the working windings wound on the two cores is small. Due to reverse coupling, the equivalent inductance of each working winding is self-inductance-mutual inductance, so the total equivalent inductance of the magnetic saturation bridge arm coupling reactor is small, which does not affect the normal operation of the system and can achieve steady-state power transmission of the system.

[0045] For the interphase circulation of the converter valve, Figure 4As shown, the bridge arm current flows between the upper and lower bridge arms, which is mainly the double frequency negative sequence AC component. The first coil 21 and the third coil 23 are coupled in the same direction, that is, they both generate clockwise magnetic flux, and the equivalent inductance is enhanced. It can be understood that at this time, the equivalent inductance of the first coil 21 and the third coil 23 are both self-inductance + mutual inductance. The fourth coil 24 and the second coil 22 are similar, and the equivalent inductance is also enhanced. In this embodiment, the amplitude of the interphase circulating current is small, and because the excitation magnetic flux generated by the hybrid excitation structure is large, the two magnetic cores are in a magnetic saturation state and remain unchanged, and the magnetic permeability is small, so the self-inductance of the working winding wound on the two magnetic cores is small. However, compared with the previous working condition, due to the in-phase coupling, that is, the equivalent inductance is self-inductance + mutual inductance, the total equivalent inductance is greatly increased, which plays a role in limiting the interphase circulating current.

[0046] For DC side faults, such as Figure 5 As shown, the bridge arm current increases rapidly, and the capacitor in the bridge arm submodule discharges to the fault point. The generated fault current flows between the upper and lower bridge arms and the DC side fault point, which is an attenuated oscillating current, that is, half of the working cycle is a positive direction current, and half of the working cycle is a reverse direction current. Assuming that the positive direction current is from DC-in and DC+out, the working windings of the first magnetic core 11, that is, the first coil 21 and the third coil 23, are coupled in the same direction, and the equivalent inductance of the two working windings increases. In addition, since the fault current is very large, the magnetic flux generated by the two working windings is very large, and the large counterclockwise magnetic flux generated by the hybrid excitation structure offsets each other, so that the first magnetic core 11 is in a desaturated state. When the first magnetic core 11 is in a desaturated state, the magnetic permeability increases, and the self-inductance of the two working windings of the first magnetic core 11 increases, so the equivalent inductance of the two working windings of the first magnetic core 11 will increase significantly. At this time, since the working magnetic flux and the excitation magnetic flux in the second magnetic core 12 are both clockwise and cannot be offset, the second magnetic core 12 remains in a magnetic saturation state. Therefore, the self-inductance of the two working windings in the second magnetic core 12 is small, but due to the unidirectional coupling, the equivalent inductance will also be greater than that of the first working condition.

[0047] When the current is in the reverse direction, the working state is reversed, the second magnetic core 12 is in the desaturated state, and the first magnetic core 11 is in the magnetic saturation state. Therefore, when an oscillating current is generated during a fault, the first magnetic core 11 and the second magnetic core 12 will be alternately desaturated. At any time, there are always two groups of working windings entering the desaturation zone, with large magnetic permeability and large self-inductance. Therefore, compared with the previous working condition, the total equivalent inductance is further increased, which plays a role in limiting the fault current.

[0048] Therefore, the magnetic saturation bridge arm coupling reactor of this embodiment can achieve corresponding effects under different working conditions through the above structure. The magnetic saturation bridge arm coupling reactor of this embodiment has three working states for the electrical characteristics of the bridge arm current of the converter valve of the flexible DC transmission system, can produce different inductance values, and automatically adjust, which can cooperate to realize power transmission in steady state, and can also show two larger inductance values ​​for interphase circulating current and fault respectively, so as to better limit circulating current and fault current.

[0049] Furthermore, the present invention also provides a flexible direct current transmission system, comprising a converter valve and the above-mentioned magnetic saturation bridge arm coupling reactor, wherein the magnetic saturation bridge arm coupling reactor is connected to the converter valve, and the converter valve is a modular multi-level converter.

[0050] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "comprise a ..." do not exclude the existence of other identical elements in the process, method, article or device including the elements.

[0051] Although the content of the present invention has been described in detail through the above preferred embodiments, it should be appreciated that the above description should not be considered as a limitation of the present invention. After reading the above content, it will be apparent to those skilled in the art that various modifications and substitutions of the present invention will occur. Therefore, the protection scope of the present invention should be limited by the appended claims.

Claims

1. A magnetic saturation bridge arm coupling reactor, characterized in that: include: A first magnetic core, a second magnetic core, a first coil to a sixth coil, a first permanent magnet, a second permanent magnet and a DC power supply; wherein the first coil, the third coil and the fifth coil are wound on the first magnetic core, the second coil, the fourth coil and the sixth coil are wound on the second magnetic core, the first coil and the second coil are connected end to end, the third coil and the fourth coil are connected end to end, the first coil and the third coil are connected at opposite ends, and the center tap is connected to the AC end of the converter valve, the taps of the fourth coil and the second coil are respectively connected to the upper bridge arm and the lower bridge arm of the converter valve, the first coil to the fourth coil have the same winding direction, and are all right-hand spiral winding directions; the first permanent magnet and the second permanent magnet are respectively arranged on the first magnetic core and the second magnetic core; the fifth coil and the sixth coil are connected end to end, and the winding direction is right-hand spiral winding direction, and form a DC excitation circuit with the DC power supply; the first permanent magnet, the second permanent magnet and the DC excitation circuit form a hybrid excitation structure; The first magnetic core and the second magnetic core are both U-shaped structures formed by an upper transverse yoke, a lower transverse yoke, a left core column and a right core column, and the upper transverse yoke, the lower transverse yoke, the left core column and the right core column are all solid core columns; The first coil and the third coil are respectively wound on the upper transverse yoke and the lower transverse yoke of the first magnetic core, the fifth coil is wound on the right core column of the first magnetic core close to the second magnetic core, and the first permanent magnet is embedded in the left core column of the first magnetic core away from the second magnetic core; the fourth coil and the second coil are respectively wound on the upper transverse yoke and the lower transverse yoke of the second magnetic core, the sixth coil is wound on the left core column of the second magnetic core close to the first magnetic core, and the second permanent magnet is embedded in the right core column of the second magnetic core away from the first magnetic core; The first permanent magnet is provided with an N pole at the upper end and an S pole at the lower end; the second permanent magnet is provided with an S pole at the upper end and an N pole at the lower end; the first permanent magnet and the second permanent magnet have the same size and excitation direction, and the first permanent magnet and the second permanent magnet generate clockwise excitation magnetic flux on the first magnetic core and the second magnetic core respectively; The DC excitation circuit generates clockwise excitation flux on the first magnetic core and the second magnetic core respectively; The first coil and the fourth coil have the same number of turns, and the third coil and the second coil have the same number of turns, wherein the number of turns of the first coil is greater than the number of turns of the third coil.

2. The magnetic saturation bridge arm coupling reactor according to claim 1, characterized in that: The cross-sectional areas of the upper transverse yoke and the lower transverse yoke of the two magnetic cores are the same, and the two magnetic cores are made of soft magnetic materials.

3. The magnetic saturation bridge arm coupling reactor according to claim 1 or 2, characterized in that: The magnetic saturation bridge arm coupling reactor is a single-phase iron core reactor used for realizing power transmission and limiting circulating current and fault current.

4. The magnetic saturation bridge arm coupling reactor according to claim 3, characterized in that: The center tap is connected to the single-phase AC end of the converter valve, and the taps of the fourth coil and the second coil are respectively connected to the single-phase upper bridge arm and the single-phase lower bridge arm of the converter valve.

5. A flexible direct current transmission system, characterized in that: It comprises a converter valve and a magnetic saturation bridge arm coupling reactor as described in any one of claims 1 to 4, wherein the magnetic saturation bridge arm coupling reactor is connected to the converter valve, and the converter valve is a modular multi-level converter.

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