A resonant direct current circuit breaker reclosing method and energy dissipation capacity configuration method

Abstract

The application discloses a resonant DC circuit breaker reclosing method and energy dissipation capacity configuration method, and belongs to the technical field of power systems. The application firstly controls the square wave excitation source to work in a positive resistance state in the reclosing stage, shortens the internal circulating current decay time and recovers the residual energy of the resonance capacitor; secondly, a modular sequential reclosing strategy is adopted, the series connected circuit breaker modules are tried to be reclosed one by one, the fault type is dynamically identified based on the fault current amplitude, the permanent fault is immediately locked and the secondary breaking is triggered, and the energy absorption of the arrester is reduced by more than 68% compared with the traditional direct reclosing; finally, an analytical calculation method of the energy capacity of the arrester is provided, without relying on full-condition electromagnetic transient simulation, and the calculation error is less than 5%. The application solves the technical problems that the circulating current is large, the secondary energy demand is high and the engineering adaptability is poor in the reclosing process of the resonant DC circuit breaker, can greatly reduce the equipment cost and volume, and supports the large-scale construction of the DC power grid.

Description

Technical Field

[0001] This invention relates to the field of power system technology, and in particular to a reclosing method for a resonant DC circuit breaker and a method for configuring energy dissipation capacity. Background Technology

[0002] With the rapid development of flexible DC transmission technology, DC power grids have become an important direction for the future development of power systems due to their significant advantages in new energy grid integration and inter-regional power transmission. However, the characteristics of DC system fault current rising rapidly and lacking a natural zero-crossing point place extremely high demands on the breaking capacity of DC circuit breakers.

[0003] Currently, DC circuit breakers are mainly divided into two types: mechanical and hybrid. Both require hundreds of fully controlled power devices connected in series to achieve voltage withstand, resulting in high cost, large size, and complex control, which seriously restricts the large-scale application of DC power grids. Resonant DC circuit breakers achieve zero-crossing current interruption by exciting an LC resonant circuit with a square wave excitation source. They require only a small number of low-voltage fully controlled devices to complete the commutation process, offering significant cost and size advantages. They are widely recognized as an important development direction for next-generation DC interruption technology.

[0004] However, existing reclosing schemes cannot be directly adapted to the special topology of resonant DC circuit breakers, mainly due to the following three technical problems: First, the secondary energy requirement of traditional direct reclosing schemes is too high. If the circuit breaker recloses to a permanent fault, the surge arrester needs to absorb the fault energy twice within hundreds of milliseconds. Taking the Zhangbei ±500kV flexible DC project as an example, when using the traditional direct reclosing scheme, the surge arrester needs to be equipped with an energy capacity of more than 150MJ, which significantly increases the equipment cost and size, and reduces the economic advantage of resonant DC circuit breakers.

[0005] Second, existing adaptive reclosing schemes have poor engineering adaptability. Fault type identification schemes based on residual voltage signals, voltage differentials at both ends of the line, or converter injection signals either have logical conflicts, insufficient accuracy in identifying high-impedance transient faults, or require cross-system coordination control, making them difficult to implement in engineering and hard to apply directly to modular series resonant DC circuit breakers.

[0006] Third, there is a lack of effective means to suppress the internal circulating current during reclosing of resonant DC circuit breakers. After the initial interruption, the resonant capacitor in the resonant DC circuit breaker module retains nearly 1 times the rated voltage, which will generate a high-amplitude, slow-decaying internal circulating current in the low-impedance converter circuit, threatening the insulation safety of the equipment and the life of power devices. However, no targeted suppression measures have been proposed in the existing technology.

[0007] Furthermore, existing surge arrester energy capacity design methods heavily rely on full-condition simulation verification using electromagnetic transient simulation software such as PSCAD, resulting in design cycles lasting several weeks. The lack of analytical calculation methods that can be directly applied in engineering seriously affects the engineering design efficiency of resonant DC circuit breakers. Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of the prior art and, based on the structural characteristics of resonant DC circuit breakers, provide a reclosing method and an energy dissipation capacity configuration method for resonant DC circuit breakers. This solves three major technical problems in the reclosing process: internal circulating current suppression, accurate fault type identification, and significant reduction of secondary fault energy. At the same time, it provides a simplified configuration method for surge arrester energy capacity, supporting the engineering application of resonant DC circuit breakers.

[0009] The objective of this invention is achieved through the following technical solution: a reclosing method for a resonant DC circuit breaker, wherein the resonant DC circuit breaker is composed of n circuit breaker modules with identical structures connected in series, comprising the following steps: S1, Initial Disconnection Phase: After a fault occurs, control all circuit breaker modules to perform the initial disconnection operation to reduce the fault current to zero; S2. Sequential Overlapping and Circulating Current Suppression Stage: After a preset deionization waiting time, the circuit breaker modules are closed one by one in a preset order. Simultaneously, the square wave excitation source of each circuit breaker module is controlled to operate in a positive resistance state to absorb the residual energy of the resonant capacitor within each module and suppress the circulating current within the module. The system fault current amplitude is monitored in real time after each circuit breaker module is closed. S3. Fault Identification and Secondary Interruption Stage: If the fault current detected at any time exceeds the preset threshold, it is determined to be a permanent fault. The reclosing operation of the remaining unclosed circuit breaker modules is immediately stopped, and the closed circuit breaker modules are triggered to perform secondary interruption. If the fault current still does not exceed the preset threshold after all circuit breaker modules are closed, it is determined to be a transient fault, and the reclosing process is completed.

[0010] Furthermore, in step S2, controlling the square wave excitation source to operate in a positive resistance state is specifically achieved by turning off all fully controllable power devices of the square wave excitation source: when the resonant current flows counterclockwise, the current flows through the current path formed by the first diode, the pre-charge capacitor of the square wave excitation source, and the fourth diode; when the resonant current flows clockwise, the current flows through the current path formed by the third diode, the pre-charge capacitor of the square wave excitation source, and the second diode, making the square wave excitation source equivalent to a positive resistance, thereby realizing the transfer of residual energy of the resonant capacitor to the pre-charge capacitor.

[0011] In a preferred embodiment of the present invention, the deionization waiting time in step S2 is 200ms to 300ms, which is adapted to the fault point arc dielectric strength restoration requirements of DC transmission systems with voltage levels of ±500kV and below.

[0012] In a preferred embodiment of the present invention, in step S3, the preset threshold is 1.5 to 2.5 times the rated current of the resonant DC circuit breaker, preferably 2 times the rated current.

[0013] In a preferred embodiment of the present invention, in step S3, after the secondary interruption is completed, the circuit breaker modules that have not been reclosed are kept in the open state, and the interruption operation is only performed on the closed circuit breaker modules to reduce the energy consumption of the metal oxide surge arrester.

[0014] In a preferred embodiment of the present invention, in step S3, when the circuit breaker module is closed, the pre-charge capacitor voltage of the square wave excitation source can also be restored to the rated pre-charge value at the same time to ensure the reliability of the secondary interruption.

[0015] In a preferred embodiment of the present invention, the square wave excitation source includes: The fault interruption control module is used to control the fully controlled power device during the fault interruption phase so that the square wave excitation source works in a negative resistance state, generating a square wave voltage to excite the LC resonant circuit to achieve current zero-crossing interruption. The circulating current suppression control module is used to control the fully controlled power device during the reclosing phase so that the square wave excitation source operates in a positive resistance state to absorb the residual energy of the resonant capacitor and suppress the circulating current inside the module. The pre-charge recovery module is used to restore the pre-charge capacitor voltage of the square wave excitation source to the rated pre-charge value before the second interruption, so as to ensure the reliability of the next interruption.

[0016] In a preferred embodiment of the present invention, the circulating current suppression control module turns off all fully controlled power devices, so that the four diodes of the square wave excitation source and the pre-charge capacitor form a rectifier bridge, thereby achieving the equivalent characteristics of positive resistance.

[0017] In a preferred embodiment of the present invention, each circuit breaker module includes a current-carrying branch, an LC resonant converter branch, a metal oxide surge arrester, and a square wave excitation source.

[0018] This invention also provides a method for configuring the energy dissipation capacity of a resonant DC circuit breaker surge arrester, applied to the above-mentioned reclosing method, using the following analytical formula to calculate the total energy absorption capacity required by the metal oxide surge arrester:

[0019] Where L is the total equivalent inductance of the fault circuit, including the smoothing reactor inductance, the converter arm inductance, and the line equivalent inductance; Iint is the breaking current; Udc is the rated voltage of the DC system; TO is the rise time of the fault current to the breaking value; and Irat is the rated line current.

[0020] In a preferred embodiment of the present invention, the total energy absorption capacity is calculated for the most severe fault scenario at the line outlet, and the calculation result is used as the final selected capacity of the metal oxide surge arrester.

[0021] Compared with the prior art, the present invention has the following advantages and beneficial effects: (1) This invention effectively shortens the decay time of the internal circulating current during the reclosing stage by using the positive resistance control strategy of the square wave excitation source. The decay time of the circulating current is greatly reduced from 3.8ms in the traditional mode to 0.475ms, and the decay speed is increased by 87.5%. This avoids the problem of device overheating and insulation aging caused by long-term circulating current, fundamentally solves the internal energy circulation problem unique to resonant DC circuit breakers, and significantly improves the reliability and service life of the equipment.

[0022] (2) The modular sequential reclosing strategy proposed in this invention utilizes the characteristic difference that the fault current under permanent faults is much higher than the charging current under transient faults. The fault nature can be determined before the fault current develops to a dangerous amplitude, which significantly limits the rise rate of the secondary fault current. This reduces the energy absorption of the surge arrester during the secondary interruption process by more than 95% compared with the traditional direct reclosing. It fundamentally avoids the redundancy requirement of the surge arrester to be designed for two full-capacity energy absorptions in the traditional scheme, greatly reduces the size and cost of the surge arrester, and enhances the economic advantages of the resonant DC circuit breaker.

[0023] (3) The surge arrester energy analysis calculation method provided by the present invention breaks the limitation of existing design relying on a large number of electromagnetic transient simulations. It simplifies the original design process that required several weeks of iteration into an algebraic calculation based on the system short-circuit inductance, rated voltage and fault response time. The error between the calculation results and the PSCAD simulation results is less than 5%. While ensuring the configuration accuracy, it significantly improves the engineering design efficiency of the DC circuit breaker energy absorption branch.

[0024] (4) The square wave excitation source of the present invention has multiple working modes and realizes multiple functions such as fault interruption, circulating current suppression and energy recovery through the same set of hardware topology. No additional hardware equipment is required, which improves the integration and utilization of the equipment and reduces the overall system cost. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the overall structure of the modular series resonant DC circuit breaker in this embodiment; Figure 2 This is a simulation waveform diagram of reclosing under a permanent fault in this embodiment; Figure 3 This is a simulation waveform diagram of reclosing under transient fault conditions in this embodiment; Figure 4 This is a comparison diagram of the resonant current waveforms under different operating modes of the square wave excitation source in this embodiment; Figure 5 This is a waveform diagram showing the effect of circulating current suppression state on the internal circulating current of the resonant DC circuit breaker in this embodiment; Figure 6 This is a schematic diagram of the equivalent circuit and waveforms of the surge arrester's energy dissipation process in this embodiment. Detailed Implementation

[0026] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0027] like Figure 1 As shown, this embodiment provides a modular series resonant DC circuit breaker, composed of n identical circuit breaker modules connected in series, where n is a positive integer greater than or equal to 2. Each circuit breaker module includes a current-carrying branch, an LC resonant converter branch, a metal oxide surge arrester, and a square wave excitation source. This modular structure is suitable for high-voltage DC transmission scenarios, enabling flexible expansion of voltage levels through module series connection, and the coordinated functions of each branch ensure the reliability of the breaking and reclosing processes.

[0028] The current-carrying branch consists of a high-speed mechanical switch and a small number of fully controlled power devices connected in series, used to carry the rated current during normal operation; the LC resonant commutation branch consists of a resonant inductor and a resonant capacitor connected in series, used to realize current commutation and zero-crossing interruption; the metal oxide surge arrester is connected in parallel across the circuit breaker module, used to absorb fault energy during the interruption process and limit overvoltage; the square wave excitation source is connected in parallel across the resonant capacitor, used to generate a square wave voltage to excite the LC resonant circuit.

[0029] In this embodiment, the square wave excitation source adopts an H-bridge topology, including four fully controlled power devices, four anti-parallel diodes, and a pre-charge capacitor. By controlling the on and off states of the fully controlled power devices, the square wave excitation source can operate in different states: when operating in a negative resistance state, energy is injected into the resonant circuit, increasing the amplitude of the excitation resonant current; when operating in a positive resistance state, energy is absorbed from the resonant circuit, accelerating the decay of the resonant current.

[0030] Based on the above-mentioned resonant DC circuit breaker, this embodiment provides a reclosing method for a resonant DC circuit breaker, including the following steps: Step S1, Initial Interruption Phase: When a fault occurs in the DC line, the control system detects the fault current and simultaneously sends a trip command to all circuit breaker modules, controlling all circuit breaker modules to perform the initial interruption operation. The specific interruption process is as follows: First, the fully controlled power devices in the current-carrying branch are turned off, forcing the fault current to transfer to the LC resonant commutator branch; at the same time, the square wave excitation source is controlled to operate in a negative resistance state, generating a square wave voltage to excite the LC resonant circuit, causing the resonant current amplitude to gradually increase until the current in the current-carrying branch crosses zero, and the high-speed mechanical switch completes the tripping; subsequently, the fault current is completely transferred to the surge arrester branch, where the surge arrester absorbs the fault energy and limits the overvoltage until the fault current decays to zero, completing the initial interruption.

[0031] After the fault current drops to zero, the control system controls the square wave excitation source of each circuit breaker module to switch its operating mode from negative resistance state to bypass state.

[0032] Specifically, controlling the square wave excitation source to operate in a positive resistance state is achieved by turning off all fully controllable power devices of the square wave excitation source. In this state, the four anti-parallel diodes of the square wave excitation source and the pre-charge capacitor form an uncontrolled rectifier bridge: when the resonant current is greater than zero, the current flows through the current path formed by the first diode D1, the pre-charge capacitor of the square wave excitation source, and the fourth diode D4; when the resonant current is less than zero, the current flows through the current path formed by the third diode D3, the pre-charge capacitor of the square wave excitation source, and the second diode D2. Regardless of the direction of the resonant current, the pre-charge capacitor is always in a charging state, making the square wave excitation source equivalent to a positive resistance characteristic, thus transferring the residual energy of the resonant capacitor to the pre-charge capacitor and accelerating the decay of the internal circulating current.

[0033] like Figure 4 As shown, when the square wave excitation source operates in a negative resistance state, it releases energy to the resonant capacitor, and the amplitude of the resonant current gradually increases; when it operates in a positive resistance state, it absorbs energy from the resonant capacitor, and the amplitude of the resonant current gradually decreases.

[0034] Step S2, Sequential Overlap and Circulation Suppression Stage: After a preset deionization waiting time, the control system closes the circuit breaker modules one by one in a preset sequence. Simultaneously, the square wave excitation source of each circuit breaker module is controlled to operate in a positive resistance state to absorb the residual energy of the resonant capacitor within each module and suppress circulating current within the module. The deionization waiting time is set to 200ms to 300ms, with the specific value determined based on the voltage level and fault characteristics of the DC system. In this embodiment, a value of 300ms is used to ensure sufficient recovery of the dielectric strength of the fault point arc and to prevent transient faults from re-breaking down during reclosing.

[0035] When circuit breaker modules are closed one by one in a preset order, the closing time interval between two adjacent circuit breaker modules is 5ms to 10ms, and in this embodiment, it is set to 7ms. After each circuit breaker module is closed, the control system monitors the system fault current amplitude in real time. Due to the sequential reclosing strategy, only one circuit breaker module is activated at a time, and the system voltage is gradually applied to the fault circuit, significantly limiting the rate of increase of the fault current.

[0036] like Figure 5 As shown, during the reclosing process, actively adjusting the square wave excitation source to a circulating current suppression state can significantly reduce the decay time of the internal circulating current from 3.8ms in the traditional natural decay mode to 0.475ms, increasing the decay rate by 87.5%. This effectively eliminates the risk of long-term high-current impact caused by the residual voltage of the resonant capacitor after the first interruption, and significantly reduces the insulation and thermal stability pressure of the mechanical switch.

[0037] Step S3, Fault Identification and Secondary Interruption Stage: The control system compares the real-time monitored fault current amplitude with a preset threshold. If the fault current detected at any time exceeds the preset threshold, it is determined to be a permanent fault. The reclosing operation of the remaining unclosed circuit breaker modules is immediately stopped, and the closed circuit breaker modules are triggered to perform secondary interruption. If the fault current still does not exceed the preset threshold after all circuit breaker modules are closed, it is determined to be a transient fault. The reclosing process is completed, and the circuit breaker returns to normal operation.

[0038] The preset threshold is 1.5 to 2.5 times the rated current of the resonant DC circuit breaker, and in this embodiment, it is preferably 2 times the rated current. This threshold is selected based on the following principle: Under transient faults, the current during the sequential reclosing process is mainly the charging current of the line capacitor, with an amplitude typically in the hundreds of amperes, far lower than the rated current; while under permanent faults, the fault current rises rapidly with the increase in the number of reclosing modules, reaching thousands of amperes in a short time. By setting a reasonable current threshold, the fault type can be accurately identified before the fault current develops to a dangerous amplitude.

[0039] During the secondary interruption process, unclosed circuit breaker modules remain open, and interruption is only performed on closed circuit breaker modules. Since the number of closed modules is less than the total number of modules, and the fault current has not yet reached its maximum value, the energy that the surge arrester needs to absorb is significantly reduced. Furthermore, before triggering the closed circuit breaker modules to perform secondary interruption, the control system can also control the square wave excitation source to restore the voltage of the pre-charge capacitor to its rated pre-charge value, ensuring the reliability of the secondary interruption.

[0040] The square wave excitation source described in this embodiment includes a fault interruption control module and a circulating current suppression control module.

[0041] The fault interruption control module is used to control the fully controlled power devices during the fault interruption phase so that the square wave excitation source operates in a negative resistance state, generating a square wave voltage to excite the LC resonant circuit to achieve current zero-crossing interruption. Specifically, the fault interruption control module controls the conduction timing of the four fully controlled power devices in the H-bridge, causing the square wave excitation source to output a square wave voltage in phase with the resonant current, continuously injecting energy into the resonant circuit, causing the amplitude of the resonant current to continuously increase until current zero-crossing interruption is achieved.

[0042] The circulating current suppression control module is used to control the fully controlled power devices during the reclosing phase so that the square wave excitation source operates in a positive resistance state to absorb the residual energy of the resonant capacitor and suppress the circulating current inside the module. Specifically, the circulating current suppression control module turns off all fully controlled power devices, causing the four anti-parallel diodes of the square wave excitation source and the pre-charge capacitor to form an uncontrolled rectifier bridge, achieving the equivalent positive resistance characteristic. This transfers the residual energy of the resonant capacitor to the pre-charge capacitor, accelerating the decay of the circulating current.

[0043] Furthermore, the square wave excitation source also includes a pre-charge recovery module, used to restore the pre-charge capacitor voltage of the square wave excitation source to the rated pre-charge value during reclosing, ensuring the reliability of the next interruption. This is achieved by absorbing residual energy in the resonant capacitor to charge the pre-charge capacitor, restoring its voltage to the rated value.

[0044] The square wave excitation source in this embodiment achieves multiple working modes through the same hardware topology, without the need for additional hardware devices, thereby improving the integration and utilization of the device and reducing the overall system cost.

[0045] like Figure 2 The figure shows the reclosing simulation waveform under a permanent fault. Figure 2 In Figure (a), the fault current waveform is shown. The circuit breaker module receives the first trip command at t=1.003s. Once the fault current in the main line is completely transferred from the LC resonant converter branch to the energy-absorbing branch, the metal oxide surge arrester begins to absorb energy, causing the fault current to gradually decay to zero.

[0046] After the line fault deionization process is completed, module 1 performs reclosing first at t=1.306s, and induces a high-frequency, rapidly decaying transient circulating current inside the module; then, module 2 follows up with reclosing with a 7ms delay step; as subsequent modules of each level reclose in a step-by-step manner, the fault current gradually increases; when it reaches the 3kA setting threshold at t=1.332s, the system successfully identifies the current fault as a permanent fault.

[0047] Figure 2(b) shows the timing of the circuit breaker action signal. The pre-closed modules 1-3 immediately perform a second emergency opening action, while the remaining modules 4-5 that have not yet closed immediately stop the action and remain in a normally open isolation state.

[0048] Figure 2 (c) shows the energy absorption waveform of the surge arrester. The simulation results further confirm that the energy absorbed by the surge arrester during the secondary fault interruption is much lower than that during the initial interruption stage, which is more than 68% lower than the traditional direct reclosing scheme. This strongly proves that this method can significantly reduce the configuration heat capacity and energy dissipation requirements of the surge arrester.

[0049] like Figure 3 The figure shows the simulation waveform of reclosing under transient fault. Figure 3 In Figure (a), the fault current waveform is shown. Once the deionization process of the medium at the fault point is completed and the arc is completely extinguished, there will be no residual fault current in the main line. As the circuit breaker modules at each level close in succession, each module only needs to withstand the steady-state system voltage. During the sequential adaptive reclosing process, the equivalent capacitance of the fault line is charged in a stepwise manner by the modular multilevel converter. At the same time, the main circuit will continue to exhibit a damped oscillation with an amplitude within several hundred amperes until the DC circuit breaker at the opposite end is successfully reclosed.

[0050] Figure 3 (b) shows the timing sequence of the circuit breaker action signal. All modules completed the reclosing in sequence and no secondary interruption action was triggered.

[0051] This embodiment also provides a method for configuring the energy dissipation capacity of a resonant DC circuit breaker surge arrester, applied to the reclosing method described above. For example... Figure 6 As shown, the equivalent circuit and waveform diagram of the energy dissipation process of the surge arrester proposed in this invention are illustrated. Based on the energy dissipation process of the surge arrester, an analytical formula for the energy capacity configuration of the surge arrester is derived. This embodiment analyzes the most severe fault scenario at the line outlet, where the system rated DC voltage U... dc The voltage is 500kV, the current-limiting inductance L is 120mH, and the circuit breaker tripping time T is... o =6ms, rated operating current I rat The current is 1.5 kA. Subsequently, the above system parameters are substituted into the theoretical calculation formula for surge arrester energy for quantitative solution. The specific calculation process is as follows:

[0052] Calculations show that the theoretical energy absorption requirement of the surge arrester under the most severe fault condition is approximately 126.5 MJ. To ensure the thermal safety operating boundary and electrical reliability of the circuit breaker during long-term service under actual complex conditions, this embodiment rounds up to determine the energy capacity configuration parameter of the selected surge arrester as 127 MJ. Comparison of the configuration scheme calculated based on the formula of this invention with the accurate multi-condition iterative simulation results from PSCAD electromagnetic transient simulation software shows that the error is less than 5%. This effectively avoids the cumbersome multi-condition iterative simulation process while ensuring configuration accuracy, significantly improving the engineering design efficiency of the DC circuit breaker's energy absorption branch.

[0053] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A reclosing method for a resonant DC circuit breaker, wherein the resonant DC circuit breaker is composed of n identical circuit breaker modules connected in series, characterized in that... Includes the following steps: S1, Initial Disconnection Phase: After a fault occurs, control all circuit breaker modules to perform the initial disconnection operation to reduce the fault current to zero; S2, Sequential Overlap and Circulating Current Suppression Stage: After a preset deionization waiting time, the circuit breaker modules are closed one by one in a preset order. At the same time, the square wave excitation source of each circuit breaker module is controlled to work in a positive resistance state to absorb the residual energy of the resonant capacitor in each module and suppress the circulating current inside the module. The system fault current amplitude is monitored in real time after each circuit breaker module is closed. S3. Fault Identification and Secondary Interruption Stage: If the fault current detected at any time exceeds the preset threshold, it is determined to be a permanent fault. The reclosing operation of the remaining unclosed circuit breaker modules is immediately stopped, and the closed circuit breaker modules are triggered to perform secondary interruption. If the fault current still does not exceed the preset threshold after all circuit breaker modules are closed, it is determined to be a transient fault, and the reclosing process is completed.

2. The reclosing method for a resonant DC circuit breaker according to claim 1, characterized in that, In step S2, controlling the square wave excitation source to operate in a positive resistance state is specifically achieved by turning off all fully controllable power devices of the square wave excitation source: When the resonant current flows counterclockwise, the current flows through the current path formed by the first diode, the pre-charge capacitor of the square wave excitation source, and the fourth diode. When the resonant current flows clockwise, the current flows through the current path formed by the third diode, the pre-charge capacitor of the square wave excitation source, and the second diode, making the square wave excitation source equivalent to a positive resistor, thereby realizing the transfer of the residual energy of the resonant capacitor to the pre-charge capacitor.

3. The reclosing method for a resonant DC circuit breaker according to claim 1, characterized in that, In S2: the deionization waiting time is 200ms to 300ms, which is suitable for the fault point arc dielectric strength restoration requirements of DC transmission systems with voltage levels of ±500kV and below; In step S2, when the circuit breaker modules are closed one by one in a preset order, the closing time interval between two adjacent circuit breaker modules is 5ms to 10ms.

4. The reclosing method for a resonant DC circuit breaker according to claim 1, characterized in that, In S3: the preset threshold is 1.5 to 2.5 times the rated current of the resonant DC circuit breaker.

5. The reclosing method for a resonant DC circuit breaker according to claim 1, characterized in that, In S3: After the secondary interruption is completed, the circuit breaker modules that have not been reclosed are kept in the open state, and the interruption operation is only performed on the closed circuit breaker modules to reduce the energy consumption of the metal oxide surge arrester.

6. The reclosing method for a resonant DC circuit breaker according to claim 1, characterized in that, In step S3, before triggering the closed circuit breaker module to perform secondary interruption, the step of restoring the pre-charge capacitor voltage of the square wave excitation source to the rated pre-charge value is also included to ensure the reliability of secondary interruption.

7. The reclosing method for a resonant DC circuit breaker according to claim 1, characterized in that, The square wave excitation source includes: The fault interruption control mode is used to control the fully controlled power device during the fault interruption phase so that the square wave excitation source works in a negative resistance state, generating a square wave voltage to excite the LC resonant circuit to achieve current zero-crossing interruption. The circulating current suppression control mode is used to control the fully controlled power device during the reclosing phase so that the square wave excitation source operates in a positive resistance state to absorb the residual energy of the resonant capacitor and suppress the circulating current inside the module. The pre-charge recovery mode is used to restore the pre-charge capacitor voltage of the square wave excitation source to the rated pre-charge value before the second interruption, so as to ensure the reliability of the next interruption.

8. The reclosing method for a resonant DC circuit breaker according to claim 7, characterized in that, The circulating current suppression control module shuts down all fully controlled power devices, enabling the four diodes of the square wave excitation source and the pre-charge capacitor to form a rectifier bridge, thereby achieving positive resistance equivalent characteristics.

9. The reclosing method for a resonant DC circuit breaker according to claim 7, characterized in that, Each circuit breaker module includes a current-carrying branch, an LC resonant converter branch, a metal oxide surge arrester, and a square wave excitation source; the left end of the current-carrying branch is connected to the left end of the LC resonant converter branch and the left end of the metal oxide surge arrester, respectively; the right end of the current-carrying branch is connected to the right end of the square wave excitation source; the right end of the LC resonant converter branch is connected to the left end of the square wave excitation source; and the right end of the metal oxide surge arrester is connected to the right end of the resonant capacitor in the LC resonant converter branch.

10. A method for configuring the energy dissipation capacity of a resonant DC circuit breaker surge arrester, based on the method described in any one of claims 1 to 6, characterized in that, The total energy absorption capacity required for a metal oxide surge arrester is calculated using the following analytical formula: ， Where L is the total equivalent inductance of the fault circuit, including the smoothing reactor inductance, the converter arm inductance, and the line equivalent inductance; Iint is the breaking current; Udc is the rated voltage of the DC system; TO is the rise time of the fault current to the breaking value; Irat is the rated current of the line; the total energy absorption capacity is calculated for the fault scenario at the line outlet, and the calculation result is used as the final selection capacity of the metal oxide surge arrester.

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