Circulating current coupling type inter-line direct current power flow controller circuit

Abstract

The invention discloses a circulating current coupling type inter-line direct current power flow controller circuit, which is characterized in that a first resonant tank module is arranged between a first side port and a bridge arm connection point, and a second resonant tank module is introduced into a line, so that the problem of introducing alternating current harmonic current into a direct current power transmission line is effectively solved, and the reliability of the direct current power flow controller circuit is improved. And meanwhile, the device cost and loss are reduced. By optimizing the design and layout of the resonant tank module, dependence on additional devices is reduced while alternating-current circulation management is realized, so that effective control of the overall cost and loss of the device is realized on the basis of ensuring the system performance, and the device has the advantages of avoiding introducing alternating-current harmonic current into a direct-current transmission line, suppressing voltage fluctuation and improving the reliability of the device. Internal reactive power is reduced, current stress is reduced, system operation efficiency and reliability are improved, the number of sub-modules is reduced, and overall cost and loss of the device are reduced.

Description

Technical Field

[0001] This application relates to the field of DC power flow control technology, and in particular to a circulating coupled line-to-line DC power flow controller circuit. Background Technology

[0002] With the continuous development of flexible DC transmission technology, DC power grids will be widely used. To achieve complete controllability of power flow distribution in DC power grids, the first technical requirement is to distribute power flow among multiple lines at the same voltage level. In scenarios such as high-voltage DC transmission and transoceanic power transmission, DC power grids transmit large amounts of electrical energy over long distances, requiring reasonable distribution of power flow across lines to improve transmission efficiency and grid economy. Among existing DC power flow controller types, the line-to-line DC power flow controller is the most suitable topology for achieving power flow distribution among multiple lines at the same voltage level.

[0003] In addition to inserting an equivalent DC voltage source, a circulating-coupled inter-line power flow controller also inserts an equivalent AC voltage source into the line, with both sharing energy storage components. The equivalent AC voltage source generates an AC circulating current, enabling power transfer between the equivalent AC voltage sources and thus achieving energy balance between the equivalent DC voltage sources. However, due to the insertion of the equivalent AC voltage source, to avoid the power flow controller introducing AC current harmonics into the DC transmission line, current methods for suppressing the AC current generated by the AC voltage mainly involve inserting bridge arms composed of series submodules into the line. These bridge arms generate AC voltages with the same amplitude but opposite phase, making the amplitude of the inserted AC voltage zero, which can completely eliminate the AC current generated by the power flow controller in the line. However, this requires adding more components, thus increasing the cost and losses of the power flow controller. Summary of the Invention

[0004] This application provides a circulating current coupled line-to-line DC power flow controller circuit to address the technical problems of high cost and high loss in existing circulating current coupled line-to-line power flow controller equipment.

[0005] To solve the above-mentioned technical problems, this application provides a circulating coupled inter-line DC power flow controller circuit, including: a first side port, a first resonant slot module, multiple second side ports, multiple second resonant slot modules, and multiple bridge arm modules;

[0006] The first side port is connected to each of the second side ports respectively, and a second resonant slot module is connected in series on each line between the first side port and the second side port;

[0007] Each second side port corresponds to a bridge arm module. One end of each bridge arm module is connected between the corresponding second side port and the second resonant slot module, and the other end is connected to the other bridge arm modules to form a bridge arm connection point.

[0008] The first resonant slot is disposed between the first side port and the bridge arm connection point.

[0009] Preferably, the first resonant slot module is configured to present high impedance at the fundamental frequency to provide voltage support for the bridge arm connection point, and to achieve energy balance between the bridge arm modules by adjusting the circulating current flowing through the first resonant slot module.

[0010] Preferably, the first resonant slot module is composed of an inductor and a capacitor connected in parallel.

[0011] Preferably, the second resonant slot module is configured to provide a path for the AC circulating current inside the DC power flow controller to prevent the AC circulating current from flowing into the DC line connected to it.

[0012] Preferably, the second resonant slot module is composed of an inductor and a capacitor connected in series.

[0013] Preferably, the bridge arm module includes: a bridge arm inductor connected in series with multiple sub-modules.

[0014] Preferably, the submodule is a full-bridge submodule.

[0015] Preferably, the full-bridge submodule includes: four switching devices and one energy storage capacitor, the four switching devices forming a full-bridge circuit, and the energy storage capacitor connected in parallel to the DC side of the full-bridge circuit.

[0016] Preferably, the switching device is an insulated gate bipolar transistor or a silicon carbide metal-oxide-semiconductor field-effect transistor.

[0017] Preferably, the number of the second side ports is at least two.

[0018] As can be seen from the above technical solutions, this application has the following advantages:

[0019] The solution provided in this application effectively solves the problem of avoiding the introduction of AC harmonic current into DC transmission lines by setting a first resonant slot module between the first side port and the bridge arm connection point, and introducing a second resonant slot module into the line, while reducing device cost and losses. This application optimizes the design and layout of the resonant slot modules, reducing reliance on additional components while achieving AC circulating current management. This results in effective control of overall device cost and losses while ensuring system performance. It offers advantages such as avoiding the introduction of AC harmonic current into DC transmission lines, suppressing voltage fluctuations, reducing internal reactive power, reducing current stress, improving system operating efficiency and reliability, reducing the number of sub-modules, and lowering overall device cost and losses. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a topology diagram of an embodiment of a circulating coupled line-to-line DC power flow controller circuit provided in this application.

[0022] Figure 2 The voltage and current waveforms of the three ports of a circulating coupled line-to-line DC power flow controller circuit provided in this application are shown.

[0023] Figure 3 A topology diagram of a full-bridge submodule in a circulating coupled line-to-line DC power flow controller circuit provided in this application.

[0024] Figure 4 This is a topology diagram of another embodiment of a circulating coupled line-to-line DC power flow controller circuit provided in this application. Detailed Implementation

[0025] This application provides a circulating current coupled line-to-line DC power flow controller circuit to address the problems of large size, high cost, and high loss in existing circulating current coupled line-to-line power flow controller equipment.

[0026] In the description of this application, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0027] Unless otherwise expressly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to the internal connection between two components. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0028] To make the inventive objectives, features, and advantages of this application more apparent and understandable, the technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described below are only some embodiments of this application, and not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0029] First, a detailed description of an embodiment of a circulating coupled inter-line DC power flow controller circuit provided in this application is as follows:

[0030] Please see Figure 1 The present application provides an embodiment of a circulating coupled line-to-line DC power flow controller circuit, which mainly comprises: a first side port, a first resonant slot module, multiple second side ports, multiple second resonant slot modules, and multiple bridge arm modules;

[0031] The first side port is connected to each of the second side ports respectively, and a second resonant slot module is connected in series on each line between the first side port and the second side port;

[0032] Each second side port corresponds to a bridge arm module. One end of each bridge arm module is connected between the corresponding second side port and the second resonant slot module, and the other end is connected to the other bridge arm modules to form a bridge arm connection point O.

[0033] The first resonant slot is disposed between the first side port and the bridge arm connection point O.

[0034] It should be noted that in practical applications, the first resonant slot module can be understood as a functional module used to adjust circuit characteristics. Its main function is to provide voltage support to the circuit through specific impedance characteristics. Specifically, the first resonant slot module can be implemented through a circuit composed of inductors and capacitors, such as a multi-stage LC parallel resonant circuit or a distributed parametric resonant circuit. Its main purpose is to present high impedance characteristics at the fundamental frequency, thereby providing stable voltage support for the bridge arm connection point O. Furthermore, the second resonant slot module can be designed as a path module that can guide the flow of current at a specific frequency. It can be implemented in various ways, such as using a multi-stage series resonant circuit or a resonant structure based on magnetic coupling. Its main purpose is to provide a low-impedance path for the internal AC circulating current while preventing the AC circulating current from flowing into the external DC line. In addition, the bridge arm module can be configured to contain different types of inductors and sub-module combinations, such as a combination of discrete inductors and half-bridge sub-modules, or a combination of integrated inductors and hybrid sub-modules. Its main function is to provide a path for DC current and participate in energy distribution.

[0035] The working principle of this application embodiment is as follows:

[0036] The circulating current coupled inter-line DC power flow controller circuit provided in this embodiment achieves its function through a specific structural design. A first-side port is connected to multiple second-side ports, with a second resonant slot module connected in series in each connection line. The function of the second resonant slot module is to provide a low-impedance path for the internal AC circulating current, thereby confining the AC circulating current within the controller and preventing it from flowing into the external DC line, thus solving the problem of introducing AC harmonic current into the DC transmission line. Furthermore, each second-side port corresponds to a bridge arm module. One end of the bridge arm module is connected between the corresponding second-side port and the second resonant slot module, while the other end forms a bridge arm connection point O with other bridge arm modules. This connection method ensures a normal path for the DC current and provides a clear flow path for the circulating current, reducing unnecessary energy loss.

[0037] Furthermore, the first resonant slot module is positioned between the first side port and the bridge arm connection point O. It exhibits extremely high impedance characteristics at the fundamental frequency, providing stable voltage support for the bridge arm connection point O and suppressing voltage fluctuations. Specifically, by adjusting the circulating current flowing through the first resonant slot module, energy balance between the bridge arm modules is achieved, thereby significantly reducing the internal reactive power circulating between the upper and lower bridge arms. The reduction in internal reactive power circulating current directly reduces the current stress on the power devices, improving system operating efficiency and reliability. Simultaneously, due to the optimization of the bridge arm electrical stress, the number of sub-modules (SMs) required to be connected in series to meet the same power level is reduced, ultimately achieving an effective reduction in overall device cost and losses.

[0038] More specifically, with Figure 1 Taking the illustrated topology as an example, this topology regulates the current in the two lines by controlling the DC component of the bridge arm voltage to change the voltage of the equivalent DC voltage source inserted in the two lines. However, since there is no external energy source, it relies solely on the power flow controller itself to achieve energy balance, and cannot achieve independent control of the two DC voltages. The DC component of one bridge arm voltage is used to regulate the current in one line, while the DC component of the other bridge arm voltage is used to regulate the total power absorbed by the power flow controller to achieve energy balance, resulting in the current in the other line being passively regulated.

[0039] The stable operation of the power flow controller depends on the capacitor voltage of each submodule SM in each bridge arm being maintained at the rated value. The capacitor voltage of the submodule SM mainly depends on the energy stored in the submodule SM. The energy stored in each part of the power flow controller is basically stored in the capacitor of the submodule SM. Therefore, by controlling the energy balance of each part of the power flow controller, the capacitor voltage of the submodule SM can be stabilized at the rated value. This mainly includes two aspects: (1) the total energy balance of the power flow controller; (2) the energy balance between the bridge arms.

[0040] The overall energy balance of the power flow controller ensures the stability of the sum of energy stored in all submodules (SM), i.e., maintaining the sum of the capacitor voltages of all submodules (SM) at their rated values. This is achieved by adjusting the DC component of the bridge arm voltage. Since the power flow controller needs to insert DC voltages into the line when regulating line power flow, and since DC current exists in the line, each inserted equivalent voltage source will release or absorb power. To ensure overall energy balance, the sum of the power released by all inserted equivalent voltage sources and the power flow controller's own losses should be zero. Because the inter-line power flow controller does not require an external power source, the proposed topology needs to change the value of the DC component of each bridge arm voltage to change the power released by each equivalent voltage source, ultimately achieving overall energy balance for the power flow controller. Simultaneously, line power flow control is also achieved by adjusting the DC component of the bridge arm voltage; therefore, the roles of the DC components of each bridge arm voltage in the proposed power flow controller will differ. One bridge arm voltage's DC component is used to control line power flow, while the other bridge arm voltage's DC component is used to adjust the power released by the equivalent voltage sources, ensuring that the sum of the power released by all inserted DC voltages is zero, thus achieving overall energy balance for the power flow controller.

[0041] After achieving overall energy balance in the power flow controller, the energy released by each bridge arm may differ, leading to an unreasonable distribution of energy stored in each bridge arm. This makes it impossible to guarantee that the voltage of the SM capacitors in all submodules remains stable at their rated values. Therefore, an energy balance control strategy between bridge arms is still needed to rationally distribute the energy stored in each bridge arm to ensure that the energy stored in each bridge arm remains stable at its rated value. The proposed energy balance between bridge arms is achieved by adjusting the AC component of the bridge arm voltage.

[0042] Because of the phase difference between the AC components of the bridge arm voltages, a circulating current exists in the circuit that flows only within the power flow controller. This circulating current also has a phase difference with the AC components of the individual bridge arm voltages, causing the equivalent AC voltage sources inserted into each bridge arm to absorb or release active power, thus achieving power transfer between the bridge arms. By controlling the amplitude and phase of the AC voltage to control the amplitude and phase of the circulating current, the amount of power transferred between different bridge arms is adjusted to achieve energy balance among the bridge arms, ensuring that the sum of the SM capacitor voltages of each bridge arm's submodules remains stable at its rated value.

[0043] For example, such as Figure 2As shown, when the output current at port B is increased, the voltage of the parallel resonant tank becomes an AC voltage with a phase angle of 0, and the DC component of the current is equal to the output current at port A. The upper arm voltage corresponding to port B is an AC voltage with a forward DC bias, and the lower arm voltage corresponding to port C is an AC voltage with a reverse DC bias. The DC components of the upper and lower arms are equal to the corresponding line currents. The AC components of the currents in the upper and lower arms are equal to the AC circulating current generated by the corresponding arm voltages. The upper arm absorbs DC power and releases AC power, while the lower arm releases DC power and absorbs AC power, thus achieving power balance.

[0044] In summary, this technical solution, through the coordinated operation of various modules, not only avoids the introduction of AC harmonic currents but also optimizes the system's operating efficiency, achieving the goal of reducing costs and losses.

[0045] Building upon this, this application further proposes that the first resonant tank module is composed of an inductor and a capacitor connected in parallel. The second resonant tank module is composed of an inductor and a capacitor connected in series.

[0046] Specifically, the first resonant slot module refers to a circuit structure capable of precise tuning to the fundamental frequency, which can be implemented using an inductor and capacitor connected in parallel. This parallel structure, by adjusting the parameters of the inductor and capacitor, can generate extremely high impedance at a specific frequency, thereby providing stable voltage support for the bridge arm connection point O. Its purpose is to optimize system operating efficiency while reducing the circulation of internal reactive power.

[0047] The second resonant slot module refers to a structure that provides a path for the AC circulating current inside a DC power flow controller and prevents the AC circulating current from flowing into the connected DC line. In practical applications, this module achieves its function through a series combination of an inductor and a capacitor. It can form a low-impedance path at the AC circulating current frequency, allowing the internal AC circulating current to pass smoothly, while presenting a high impedance at the DC line frequency, effectively isolating AC harmonics and preventing them from flowing into the DC line. The purpose of this design is to simplify the device structure and avoid the increased cost and complexity caused by relying on large inductors or additional bridge arm submodules (SM) in existing technologies.

[0048] In detail, the first resonant slot module enhances the voltage stability of the bridge arm connection point O and reduces the electrical stress of the system through its high impedance characteristics at the fundamental frequency. Simultaneously, this module achieves dynamic energy distribution among the bridge arm modules through precise control of the circulating current. This design not only solves the potential energy unevenness problem between bridge arm modules but also significantly improves the system's operating efficiency and reliability. Based on this, the first resonant slot module, together with the first side port, the second side port, and the bridge arm modules, forms a complete energy management and voltage support system. By introducing the first resonant slot module, the system can maintain efficient energy transmission while reducing the circulating flow of reactive power, thereby optimizing overall performance.

[0049] Meanwhile, the introduction of the second resonant slot module allows the AC circulating current inside the DC power flow controller to flow in a dedicated path without affecting the connected DC lines. This design cleverly combines the characteristics of resonant circuits, precisely tuning the inductor and capacitor parameters to achieve extremely low impedance at the target frequency, thereby effectively attracting and accommodating the internal AC circulating current. Furthermore, due to the presence of the second resonant slot module, the controller can achieve efficient AC current isolation without relying on large-size filter inductors or additional bridge arms. This not only reduces the number of components but also significantly lowers the system's complexity and cost. Based on this, this module, together with the first resonant slot module and the bridge arm module, further optimizes the overall circuit performance, improves operating efficiency and reliability, and effectively reduces device cost and losses.

[0050] Based on the above embodiments, this application further proposes a bridge arm module comprising: a bridge arm inductor connected in series with multiple sub-modules SM.

[0051] Specifically, a bridge arm inductor is an electrical component with inductive characteristics, which can be implemented using air-core inductors, iron-core inductors, or magnetic ring inductors. In practical applications, bridge arm inductors act as buffers and filters by suppressing rapid changes in current, thereby reducing ripple current generation. Their purpose is to reduce the current stress flowing through the submodule SM, improving system stability and efficiency. A submodule SM refers to a unit module that provides voltage support and power handling capabilities. It can be implemented through various circuit topologies, such as half-bridge or full-bridge submodule SMs. Its purpose is to meet the required voltage level and power demands of the system by connecting multiple submodule SMs in series.

[0052] Specifically, the bridge arm inductor and multiple sub-modules (SMs) are connected in series to form a complete bridge arm module. The bridge arm inductor is located at the rear end of the bridge arm module, and the multiple sub-modules (SMs) are connected in series before the bridge arm inductor, passing through the inductor and finally connecting to the bridge arm connection point O. This structural design allows the bridge arm inductor to effectively suppress current ripple and optimize the current path, thereby reducing the electrical burden on the sub-modules (SMs). Simultaneously, the series connection of multiple sub-modules (SMs) provides flexible voltage regulation and power handling capabilities, ensuring stable operation of the bridge arm module under various operating conditions.

[0053] Through the above technical solutions, the bridge arm module effectively solves the problems of current ripple and electrical stress while achieving circulating current control and voltage support, thereby improving the system's operating efficiency and reliability. This design cleverly combines the filtering characteristics of the inductor with the power handling capability of the submodule SM, laying the foundation for the overall performance optimization of the DC power flow controller.

[0054] More specifically, the aforementioned submodule SM is a full-bridge submodule, such as... Figure 3 As shown, a full-bridge submodule refers to a full-bridge circuit structure consisting of four switching devices and one energy storage capacitor. It can be implemented using insulated-gate bipolar transistors (IGBTs) or silicon carbide metal-oxide-semiconductor field-effect transistors (MOSFETs) as switching devices. In practical applications, the energy storage capacitor is connected in parallel to the DC side of the full-bridge circuit. This aims to reduce the need for additional components through a compact and efficient layout, while optimizing the generation and control of the AC voltage. Through precise control of the four switching devices, the required AC voltage can be effectively generated to eliminate the influence of AC circulating current. Furthermore, the introduction of the energy storage capacitor not only maintains circuit stability but also avoids the redundancy problem of multiple devices commonly found in traditional submodule SM implementations. This design choice significantly reduces device complexity and energy loss, thereby improving the overall operating efficiency of the device. In addition, since the full-bridge submodule and the bridge arm inductor are connected in series to form a bridge arm module, this combination reduces the number of submodules required to be connected in series while meeting the same power level, ultimately achieving an effective reduction in overall device cost and losses. Through the above technical solution, the full-bridge submodule effectively optimizes the device structure design while maintaining the ability to eliminate AC circulating current, thereby improving the cost-effectiveness of the overall device.

[0055] Furthermore, the aforementioned switching device is an insulated gate bipolar transistor or a silicon carbide metal-oxide-semiconductor field-effect transistor.

[0056] In practical applications, switching devices are key components used to control the on / off state of circuits. They can be implemented using insulated-gate bipolar transistors (IGBTs) with high-voltage switching characteristics, or using silicon carbide metal-oxide-semiconductor field-effect transistors (MOSFETs) with low conduction losses and high switching frequency characteristics. The purpose of introducing these two types of switching devices is to optimize switching performance, thereby effectively reducing overall system losses and costs, while improving efficiency and reliability.

[0057] It should be noted that, under normal circumstances, IGBTs (Insulated Gate Bipolar Transistors) are preferred as switching devices. However, in some embodiments, silicon carbide (SiC) switching devices can be used instead of IGBTs. This can increase the rated voltage of the submodule SM capacitor, thereby increasing the range of the DC component of the bridge arm voltage and further widening the adjustment range of the proposed power flow controller.

[0058] Specifically, in the aforementioned circulating-coupled inter-line DC power flow controller circuit, the bridge arm module includes a bridge arm inductor connected in series and multiple sub-modules SM, with each sub-module SM being a full-bridge sub-module. The full-bridge sub-module includes four switching devices and one energy storage capacitor. These four switching devices constitute the full-bridge circuit, and the energy storage capacitor is connected in parallel to the DC side of the full-bridge circuit. By selecting insulated-gate bipolar transistors (IGBTs) or silicon carbide metal-oxide-semiconductor field-effect transistors (MOSFETs) as switching devices, stable operation under high-voltage conditions can be ensured, and energy loss can be significantly reduced. This specific combination not only solves the loss and cost problems caused by improper selection of switching devices but also improves the overall system performance. For example, IGBTs utilize their high-voltage switching characteristics and reliability to ensure stability under high-voltage conditions; while silicon carbide MOSFETs, with their low conduction losses and high switching frequency characteristics, reduce energy loss. This design optimizes switching performance, reduces overall system losses and costs, and simultaneously improves efficiency and reliability.

[0059] More specifically, this application further proposes that the number of second-side ports is at least two.

[0060] It should be noted that the second-side port refers to the interface used to connect multiple DC lines and implement power flow control. It can be implemented using physical ports, electrical connection ports, or other adapted interface forms. The purpose is to ensure that the circuit can support the power flow distribution requirements among multiple DC lines, thereby improving the system's scalability and application flexibility. When there are two second-side ports, the topology is as follows: Figure 1 As shown, when the number of ports on the second side is greater than two, its topology can be found in [reference needed]. Figure 4 .

[0061] In detail, during actual operation, multiple second-side ports are connected to different DC lines, forming multiple independent power flow channels. Based on this, the lines between the first-side port and each of the second-side ports are connected in series via second resonant slot modules, ensuring that the AC circulating current is effectively confined internally and does not flow into the DC lines. Simultaneously, one end of the bridge arm module is connected between the corresponding second-side port and the second resonant slot module, while the other end converges to form bridge arm connection point O, thereby achieving balanced energy distribution and transmission. This design significantly improves the system's applicability and efficiency in multi-line power flow control scenarios. Furthermore, by rationally configuring the number of second-side ports, it can flexibly adapt to the needs of DC grids of different sizes, further optimizing the overall system's performance and economy.

[0062] The above-described embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A circulating-coupled inter-line DC power flow controller circuit, characterized in that, include: The system comprises a first side port, a first resonant slot module, multiple second side ports, multiple second resonant slot modules, and multiple bridge arm modules. The first side port is connected to each of the second side ports respectively, and a second resonant slot module is connected in series on each line between the first side port and the second side port; Each second side port corresponds to a bridge arm module. One end of each bridge arm module is connected between the corresponding second side port and the second resonant slot module, and the other end is connected to the other bridge arm modules to form a bridge arm connection point. The first resonant slot is disposed between the first side port and the bridge arm connection point.

2. The circulating current coupled inter-line DC power flow controller circuit according to claim 1, characterized in that, The first resonant slot module is configured to present high impedance at the fundamental frequency to provide voltage support for the bridge arm connection point, and to achieve energy balance between the bridge arm modules by adjusting the circulating current flowing through the first resonant slot module.

3. The circulating current coupled inter-line DC power flow controller circuit according to claim 2, characterized in that, The first resonant slot module consists of an inductor and a capacitor connected in parallel.

4. The circulating current coupled inter-line DC power flow controller circuit according to claim 1, characterized in that, The second resonant slot module is configured to provide a path for the AC circulating current inside the DC power flow controller to prevent the AC circulating current from flowing into the DC line connected to it.

5. The circulating current coupled inter-line DC power flow controller circuit according to claim 4, characterized in that, The second resonant slot module consists of an inductor and a capacitor connected in series.

6. The circulating current coupled inter-line DC power flow controller circuit according to claim 1, characterized in that, The bridge arm module includes: a bridge arm inductor connected in series and multiple sub-modules.

7. A circulating coupled inter-line DC power flow controller circuit according to claim 6, characterized in that, The submodule is specifically the full-bridge submodule.

8. The circulating current coupled inter-line DC power flow controller circuit according to claim 7, characterized in that, The full-bridge submodule includes four switching devices and one energy storage capacitor. The four switching devices form a full-bridge circuit, and the energy storage capacitor is connected in parallel to the DC side of the full-bridge circuit.

9. A circulating current coupled inter-line DC power flow controller circuit according to claim 8, characterized in that, The switching device is an insulated gate bipolar transistor or a silicon carbide metal-oxide-semiconductor field-effect transistor.

10. A circulating coupled inter-line DC power flow controller circuit according to claim 1, characterized in that, The number of the second-side ports is at least two.

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