Low-conduction-loss direct-current circuit breaker with fault type judgment function and judgment method

Abstract

The invention provides a low-conduction-loss direct-current circuit breaker with a fault type judgment function and a judgment method, and belongs to the field of circuit fault detection. In order to solve the problems that in an existing hybrid high-voltage direct-current circuit breaker, the conduction loss of a load commutation switch is high, and a circuit device does not have the fault type discrimination capability, the direct-current circuit breaker comprises a load branch, a multifunctional branch, a main breaking branch and an energy absorption branch. The discrimination method comprises the following steps: in a normal operation mode, a load branch is conducted; the capacitor pre-charging mode operates in parallel to charge the capacitor C; when the circuit is short-circuited, a thyristor turn-off mode is entered, and the T1 is turned off; and entering a load commutation mode, converting the fault current to a main breaking branch, introducing an energy absorption branch, and entering an energy absorption mode. And injecting a controlled RLC oscillation current, analyzing current waveform characteristics to judge a direct current fault type, and performing reclosing or maintenance of the circuit breaker according to the analyzed direct current fault type.

Description

TECHNICAL FIELD

[0001] The present application relates to a low conduction loss DC circuit breaker with fault type judgment and a judgment method, and belongs to the field of circuit fault detection. BACKGROUND

[0002] Regions with abundant natural energy such as wind and solar energy often have low load levels and limited peak regulation capabilities, and thus need to transmit electric energy over long distances to high load demand areas. To meet the demand for long-distance and large-capacity power transmission, high-voltage direct current transmission technology is widely used due to its low transmission loss and construction cost advantage. Modular multilevel converter high-voltage direct current transmission (MMC-HVDC) has gradually replaced traditional alternating current transmission and high-voltage direct current systems based on line commutated converters due to its active / reactive power decoupling control, ability to supply power to passive grids, and no risk of commutation failure. To minimize construction and operation costs, power transmission lines usually use overhead line solutions, but they are susceptible to environmental factors that can cause direct current short circuit faults. In addition, MMCs usually use half-bridge sub-module topology, which does not have direct current fault clearing capability, so they must rely on direct current circuit breakers (DCCB) to isolate and open faults.

[0003] In an alternating current system, there are two natural zero-crossing points per cycle of the current, at which the magnetic energy stored in the system is reduced to zero, creating favorable conditions for extinguishing arcs and safe interruption. In a direct current system, a short circuit fault on the direct current side can cause the current to rise several times the rated value within a few milliseconds, and the direct current has no natural zero-crossing point. Direct interruption of the fault current can produce an arc, which seriously affects the service life of the switchgear. Therefore, a high-voltage direct current circuit breaker is a key device for isolating faults on the direct current side.

[0004] Direct current circuit breakers are mainly divided into mechanical, all-solid-state, and hybrid types. Mechanical direct current circuit breakers usually use the "artificial zero-crossing" method: when interrupting under non-zero current conditions, the fast mechanical switch produces an arc, and the pre-charged reverse current branch generates a high-frequency reverse current to extinguish the arc, thereby achieving fast interruption of the mechanical switch.

[0005] The all-solid-state DC circuit breaker (DCCB) uses power electronic switches to achieve fast arcless interruption during fault conditions. However, it needs to have strong breaking capability in high-voltage scenarios, which requires a large number of power electronic devices, resulting in significant conduction loss and making it difficult to be widely applied in large-capacity high-voltage DC power transmission projects. The hybrid DC circuit breaker combines the current-carrying insulation capability of mechanical switches with the breaking capability of solid-state switches: after fault detection, the current is first transferred to the solid-state switch branch, allowing the fast mechanical switch to break quickly, and the fault energy is dissipated by the metal oxide surge arrester (MOSA). This technology has become the mainstream choice for modern high-voltage DC power transmission systems. The relatively representative hybrid DC circuit breaker topology was proposed by ABB in 2012, and the circuit breaker is composed of three branches: the through-flow branch, the current transfer branch, and the energy absorption branch.

[0006] On this basis, many scholars at home and abroad have carried out a lot of improvement research on the topology optimization, loss control, and reliability improvement of hybrid DC circuit breakers. The specific technical paths and core features are as follows: Document 1 proposes a voltage division topology scheme with SCR and IGBT in series. The core of the design is to use SCR to bear most of the rated voltage during normal system operation, thereby significantly reducing the voltage stress of the series IGBT and reducing the number of IGBTs required. Document 2 proposes a coupled negative voltage commutation circuit. The key innovation of this topology is the introduction of an LC oscillation branch composed of a commutation capacitor and a commutation inductor, which establishes energy interaction with the main circuit through electromagnetic coupling. When the one-way UFD breaks, the LC branch generates a negative voltage in the opposite direction of the main current through resonance, forcing the UFD contact arc current to zero quickly. Although this scheme achieves lower conduction loss, it still has technical limitations: its commutation process relies on arc voltage, and the micro-arc between the UFD contacts will continue to erode the contact surface, leading to increased contact roughness and increased contact resistance, which may shorten the mechanical life of the UFD in the long term.

[0007] Document 3 proposes a capacitor-based active commutation branch, aiming to replace the cascaded IGBT modules used for active turn-off in traditional HDCCB. The commutation branch stores energy through a pre-charged capacitor; when the current needs to be cut off, the capacitor discharges to generate a reverse commutation current, forcing the main branch current to transfer to the commutation branch, thereby achieving current-free interruption of the main switch. However, the load branch of this topology still needs to be configured with IGBT as the current transfer switch, and it needs to be kept on for a long time during normal system operation to maintain the load current path.

[0008] The direct current circuit breaker proposed in document 4 adopts a fault current self-charging commutation scheme, and the topological characteristics thereof are that an extra pre-charging power supply is omitted, and the fault current is used to rapidly charge the capacitor when a fault occurs. When the capacitor voltage reaches a threshold, the commutation circuit is triggered to be turned on, and a reverse current is released to force the main branch SCR to be turned off. This scheme simplifies the power supply structure of the system, but the main branch still needs to be connected with an IGBT as an auxiliary commutation device, so that the system still cannot completely avoid the conduction loss caused by the IGBT.

[0009] In the prior art scheme, the load branch of the hybrid high-voltage direct current circuit breaker is mostly composed of an IGBT for series operation of an LCS. Compared with an SCR, the conduction loss of the IGBT is obviously higher. Since the high-voltage direct current system needs to be continuously operated for a long time, the energy consumption of the LCS caused by the conduction loss will continuously accumulate with the operation time. For a system that needs to be stably operated for a long time, the continuously increasing energy loss will eventually cause a large amount of unnecessary waste of electric energy, which not only reduces the energy efficiency of the system, but also increases the long-term operation cost. SUMMARY

[0010] The present application is to solve the problem of high conduction loss of the load commutation switch in the existing hybrid high-voltage direct current circuit breaker, and the circuit breaker does not have the fault type discrimination capability, and further proposes a low-conduction-loss direct current circuit breaker with fault type judgment and a discrimination method.

[0011] The technical solution adopted by the present application to solve the above problem is that the low-conduction-loss direct current circuit breaker with fault type judgment proposed by the present application comprises: A load branch for carrying current when the direct current circuit breaker is operated, realizing low conduction loss, and transferring fault current after a fault occurs; A multi-functional branch for reserving energy when the direct current circuit breaker is normally operated, and releasing a reverse current to make the current of the thyristor in the load branch zero-crossing and turn off after a fault occurs; A main breaking branch for carrying the fault current transferred by the load branch after a fault occurs; An energy absorption branch for absorbing and dissipating the energy generated by the fault current.

[0012] Further, the load branch comprises an ultra-fast mechanical switch UFD and a thyristor-diode full-bridge valve group T1. One end of the ultra-fast mechanical switch UFD is connected to the positive pole of a direct current bus, the other end is connected to the input end of the thyristor-diode full-bridge valve group T1, and the output end of the thyristor-diode full-bridge valve group T1 is connected to the negative pole of the direct current bus.

[0013] Further, the multi-functional branch includes the multi-functional branch thyristor T2, thyristor T3, thyristor T4, thyristor T5, thyristor T6, capacitor C, current limiting resistor R1, current limiting resistor R2, IGBT device T10, inductor L, mechanical switch T7, mechanical switch T8 and mechanical switch T9; The input end of the thyristor T2 is connected to the positive pole of the DC bus and one end of the ultra-fast mechanical switch UFD, the output end of the thyristor T2 is connected to the input end of the thyristor T3, the input end of the thyristor T4 and one end of the current limiting resistor R2 respectively, the output end of the thyristor T3 is connected to one end of the capacitor C and the input end of the thyristor T5; the output end of the thyristor T5 is connected to one end of the mechanical switch T7, the output end of the thyristor T7 and one end of the mechanical switch T8, the other end of the capacitor C is connected to the output end of the thyristor T4 and the input end of the thyristor T6, the output end of the thyristor T4 is connected to the input end of the thyristor T6, the output end of the thyristor T6 is adjacent to one end of the mechanical switch T8, the other end of the mechanical switch T8 is connected to one end of the current limiting resistor R1 and one end of the mechanical switch T9 respectively, the other end of the current limiting resistor R1 is grounded, and the other end of the mechanical switch T9 is connected to one end of the inductor L, and the other end of the inductor L is connected to the negative pole of the DC bus; The other end of the current limiting resistor R2 is connected to the emitter of the IGBT device T10, and the collector of the IGBT device T10 is connected to the other end of the inductor L and the negative pole of the DC bus.

[0014] Further, the main breaking branch includes an IGBT-diode full-bridge valve group T11, the IGBT-diode full-bridge valve group T11 is connected in parallel with the thyristor-diode full-bridge valve group T1, the input end of the IGBT-diode full-bridge valve group T11 is connected to the positive pole of the DC bus, and the output end is connected to the output end of the load branch.

[0015] Further, the energy absorption branch includes a metal oxide voltage limiter MOSA, one end of the metal oxide voltage limiter MOSA is connected to the output end of the main breaking branch, and the other end is connected to the negative pole of the DC bus.

[0016] The fault type discrimination method based on the low conduction loss DC circuit breaker comprises: Step 1: when the low conduction loss DC circuit breaker is in normal operation, a normal operation mode is entered, the thyristor-diode full-bridge valve group T1 is triggered to conduct by a pulse signal, the gate trigger signal is cancelled after conduction, the ultra-fast mechanical switch UFD is turned on, and the current flows through the load branch; Step 2: during the running of the normal operation mode, a capacitor pre-charging mode runs in parallel, the low conduction loss DC circuit breaker intermittently triggers the thyristor valve T2, after the thyristor T2 is triggered to conduct, the voltage of the positive pole of the DC bus is limited by the current limiting resistor R1, and flows to the capacitor C through the thyristor T2 to charge the capacitor C; Step 3: When a circuit short circuit occurs in the low-conduction-loss DC circuit breaker, the thyristor turns off, and the thyristor-diode full-bridge valve group T1 turns off; Step 4: After the thyristor-diode full-bridge valve group T1 turns off, the load commutation mode is entered, the fault current is commutated to the main breaking branch, the fault current is introduced into the energy absorption branch through the main breaking branch, and the energy absorption mode is entered, and the fault current is completely absorbed through the metal oxide voltage limiter MOSA; Step 5: After the fault current is completely absorbed, the capacitor energy release mode is entered, the capacitor C energy is completely discharged through the cooperation of the thyristor and the mechanical switch in the multifunctional main circuit, and the low-conduction-loss DC circuit breaker enters a completely isolated state; Step 6: A controlled RLC oscillation current is injected into the low-conduction-loss DC circuit breaker, the DC fault type is judged by analyzing the current waveform characteristics, and the circuit breaker is reclosed or repaired according to the analyzed DC fault type.

[0017] Further, step 3 specifically includes: When a circuit short circuit occurs in the low-conduction-loss DC circuit breaker, the thyristor T2, the thyristor T6, the mechanical switch T7, and the IGBT device T10 in the multifunctional branch are triggered, the current stored in the capacitor C flows reversely into the thyristor-diode full-bridge valve group T1, and the thyristor-diode full-bridge valve group T1 turns off.

[0018] Further, step 4 specifically includes: Step 4.1: After the thyristor-diode full-bridge valve group T1 turns off, the low-conduction-loss DC circuit breaker enters a completely broken state, the trigger signals of the thyristor T2, the thyristor T6, the mechanical switch T7, and the IGBT device T10 are removed, the IGBT-diode full-bridge valve group T11 is triggered to commutate the fault current to the main breaking branch, at this time, the UFD current in the load branch is zero, and a breaking signal is applied to it; after the UFD establishes a medium insulation distance that meets the requirements, a turn-off signal is applied to the IGBT-diode full-bridge valve group T11, so that the resistance of the IGBT-diode full-bridge valve group T11 gradually increases; Step 4.2: The fault current is injected into the energy absorption branch, and the energy absorption mode is entered, and the energy of the fault current is dissipated through the metal oxide voltage limiter MOSA.

[0019] Further, step 5 specifically includes After the fault current is completely absorbed, the capacitor energy release mode is entered, and the thyristor T3, the thyristor T6, the IGBT device T10, the mechanical switch T8 and the mechanical switch T9 are triggered to form a capacitor C discharge loop through the current limiting resistor R2, and when the current of the capacitor C is reduced to 0, the thyristor T3, the thyristor T6, the IGBT device T10, the mechanical switch T8 and the mechanical switch T9 are turned off, and the low-conduction-loss DC circuit breaker enters a complete isolation state.

[0020] Further, step 6 specifically comprises: Step 6.1: injecting a controlled RLC oscillation current into the low-conduction-loss DC circuit breaker, if the low-conduction-loss DC circuit breaker is out of fault, triggering the super-fast mechanical switch UFD to be closed and the thyristor-diode full-bridge valve group T1 to be turned on, and restoring the normal operation of the low-conduction-loss DC circuit breaker, if the low-conduction-loss DC circuit breaker is still in a fault state, step 6.2 is performed. Step 6.2: the resistance R f The inductor L, the capacitor C and the current resistor form an RLC oscillation circuit, the RLC oscillation circuit generates a single-peak oscillation current, by analyzing the current amplitude of the single-peak oscillation current, if the current amplitude is greater than a first preset value, it is a permanent fault, if the current amplitude is less than a second preset value, it is a transient fault, if it is a transient fault, the super-fast mechanical switch UFD is triggered to be closed and the thyristor-diode full-bridge valve group T1 is turned on, and the normal operation of the low-conduction-loss DC circuit breaker is restored, if it is a permanent fault, the isolation state of the low-conduction-loss DC circuit breaker is maintained, and the line maintenance of the circuit breaker is waited, after the maintenance is completed, the super-fast mechanical switch UFD is triggered to be closed and the thyristor-diode full-bridge valve group T1 is turned on, and the normal operation of the low-conduction-loss DC circuit breaker is restored.

[0021] The beneficial effects of the present application are: 1. The present application adopts thyristors as load commutation switches, which can realize extremely low conduction loss in normal operation, thereby reducing operation cost. The circuit breaker design of the present application can realize the breaking of bidirectional current, and is suitable for the case that the current polarity may change in the MMC-HVDC system.

[0022] 2. The DC circuit breaker designed in the present application integrates fault type detection function, which can distinguish transient fault from permanent fault, provides basis for reclosing decision, and improves power supply reliability. The full-bridge module composed of bidirectional thyristors and diode bridges is integrated in the forced commutation switch, which reduces the number of power electronic switching devices and simplifies the topology structure.

[0023] 3. The present application avoids misreclosing under permanent fault condition through fault type discrimination and intelligent reclosing function, and enhances the reliability of the system. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 Structure diagram of low conduction loss DC circuit breaker with fault type judgment; Figure 2 Structure diagram of bidirectional current flow based on diode bridge; Figure 3 Circuit diagram of circuit breaker in normal operation mode; Figure 4 Circuit diagram of circuit breaker in capacitor pre-charging mode; Figure 5 Circuit diagram of circuit breaker in thyristor turn-off mode; Figure 6 Circuit diagram of circuit breaker in load commutation mode; Figure 7 Circuit diagram of circuit breaker in energy absorption mode; Figure 8 Circuit diagram of circuit breaker in complete disconnection state; Figure 9 Circuit diagram of circuit breaker in capacitor energy release mode; Figure 10 Circuit diagram of circuit breaker after injecting controlled RLC oscillation current.

[0025] Figure 11 Flow diagram of fault type discrimination method based on low conduction loss DC circuit breaker; Figure 12 Current response diagram of RLC oscillation loop under different fault currents, Figure 12 wherein (a) is a current response diagram of RLC oscillation loop when fault resistance Rf=50Ω, and (b) is a current response diagram of RLC oscillation loop when fault resistance Rf=2Ω; Figure 13 Current-resistance relationship curve, Figure 13 wherein (a) is a current response diagram of RLC oscillation loop when current is 400kV and resistance is [45, 60]Ω, and (b) is a current response diagram of RLC oscillation loop when current is 500kV and resistance is [45, 60]Ω; Figure 14 Comparison diagram of energy dissipated by current-limiting resistor R2 and energy dissipated by metal oxide voltage limiter MOSA; Figure 15 Model diagram of pseudo bipolar MMC-HVDC system built in simulation experiment; Figure 16 Existing hybrid DC circuit breaker topology diagram; Figure 17 A simulation diagram of the capacitor charging process; Figure 18 The waveform diagram shows the fault current interruption process. Figure 19 This is a schematic diagram of the fault current disconnection trigger sequence; Figure 20 Voltage waveform diagram for key components; Figure 21 A schematic diagram of DC voltage under the intervention of a low-conduction-loss DC circuit breaker; Figure 22 Schematic diagram of current characteristics under permanent and transient faults; Figure 23 This is a schematic diagram of the experimental results of the segmentation process. Figure 23 In the diagram, (a) is a schematic diagram of the experimental results of the disconnection process, and (b) is a schematic diagram of the experimental results of the fault identification process. Detailed Implementation

[0026] Specific implementation method one: Combining Figure 1 and Figure 2 This embodiment describes a bidirectional current-carrying structure based on a diode bridge. The MMC-HVDC system inherently possesses the ability to flexibly adjust power flow direction, and the current polarity is not fixed, potentially changing with the system's operating state. This necessitates that the DC circuit breaker installed at the converter outlet be capable of reliably interrupting bidirectional current. Traditional topology designs typically employ reverse-series IGBTs or anti-parallel thyristors to achieve bidirectional current carrying capacity. However, both of these schemes require two power electronic switching devices, resulting in one set of devices remaining idle in unidirectional current carrying scenarios where frequent current reversal is not required, leading to low device utilization efficiency. To reduce costs and improve the economics of the DC circuit breaker, this embodiment proposes a bidirectional current-carrying structure based on a diode bridge, as follows: Figure 2 As shown, this structure achieves bidirectional current carrying capacity through a single switching device in conjunction with a diode, significantly improving the utilization rate of the switching device while reducing equipment costs at the same current level.

[0027] like Figure 1 As shown, the structure of the low conduction loss DC circuit breaker with fault type judgment described in this embodiment includes: a load branch, a multi-functional branch, a main breaking branch, and an energy absorption branch.

[0028] The load branch consists of an ultra-fast mechanical switch (UFD) and a thyristor-diode full-bridge valve group (T1), which together achieve low conduction losses during normal operation. One end of the ultra-fast mechanical switch (UFD) is connected to the positive terminal of the DC bus, and the other end is connected to the input terminal of the thyristor-diode full-bridge valve group (T1). The output terminal of the thyristor-diode full-bridge valve group (T1) is connected to the negative terminal of the DC bus.

[0029] The multi-functional branch circuit consists of thyristors T2-T6, capacitor C, current-limiting resistors R1 and R2, IGBT device T10, inductor L, and mechanical switches T7-T9. By controlling the power electronic switching device and the mechanical switches, capacitor C can be injected in reverse into thyristor T1 to force its turn-off. The input terminal of thyristor T2 is connected to the positive terminal of the DC bus and one end of the ultra-fast mechanical switch UFD. The output terminal of thyristor T2 is connected to the input terminals of thyristor T3 and T4, and one end of the current-limiting resistor R2. The output terminal of thyristor T3 is connected to one end of capacitor C and the input terminal of thyristor T5. The output terminal of thyristor T5 is connected to one end of mechanical switch T7, the output terminal of thyristor T7, and one end of mechanical switch T8. The other end of capacitor C is connected to the output terminal of thyristor T4 and the input terminal of thyristor T6. The output terminal of T4 is connected to the input terminal of thyristor T6. The output terminal of thyristor T6 is adjacent to one end of mechanical switch T8. The other end of mechanical switch T8 is connected to one end of current-limiting resistor R1 and one end of mechanical switch T9. The other end of current-limiting resistor R1 is grounded. The other end of mechanical switch T9 is connected to one end of inductor L. The other end of inductor L is connected to the negative terminal of DC bus. The other end of current-limiting resistor R2 is connected to the emitter of IGBT device T10. The collector of IGBT device T10 is connected to the other end of inductor L and the negative terminal of DC bus.

[0030] The main interrupting branch uses an IGBT-diode full-bridge valve group T11 to improve the utilization rate of switching devices and serve as a short-term path for fault current flow. The IGBT-diode full-bridge valve group T11 is connected in parallel with the thyristor-diode full-bridge valve group T1. The input terminal of the IGBT-diode full-bridge valve group T11 is connected to the positive terminal of the DC bus, and the output terminal is connected to the output terminal of the load branch.

[0031] The energy absorption branch dissipates residual energy from the fault current through a metal oxide voltage limiter (MOSA), ensuring safe system disconnection. One end of the MOSA is connected to the output of the main disconnection branch, and the other end is connected to the negative terminal of the DC bus.

[0032] Specific Implementation Method Two: Combining Figures 3-14 This implementation method is described as follows: Figures 3-10 As shown, the circuit of a low-conduction-loss DC circuit breaker with fault type detection sequentially undergoes eight operating modes during fault breaking: normal operation mode, capacitor pre-charging mode, thyristor turn-off mode, load commutation mode, breaking mode, energy absorption mode, capacitor energy release mode, and fault type detection mode. The capacitor energy release mode is used to release the stored energy in the capacitor after the fault current is broken, creating conditions for fault type detection. This embodiment uses a forward current as an example to illustrate the fault current breaking process of the DC circuit breaker.

[0033] likeFigure 11 As shown, the steps of the fault type discrimination method based on the low-conduction-loss DC circuit breaker include: S1: In the normal operation mode, the thyristor-diode full-bridge valve group T1 is turned on, and the ultra-fast mechanical switch UFD is turned on, and the current flows through the load branch; As shown in the figure, Figure 3 At this stage, the thyristor T1 is triggered by a pulse signal. After the thyristor is turned on, the gate trigger signal is removed, and the ultra-fast mechanical switch is in the on state. At this time, the low-conduction-loss DC circuit breaker has no fault current, and the current flows through the load branch, and the circuit breaker is in the normal operation state.

[0034] S2: In the parallel operation capacitor pre-charging mode, the thyristor valve T2 is intermittently triggered, the voltage at the positive pole of the DC bus is limited by the current-limiting resistor R1, and is charged to the capacitor C through the thyristor T2; As shown in the figure, Figure 4 This mode coexists with the normal operation mode, and the circuit breaker intermittently triggers the thyristor T2, so that the capacitor C is charged through the pre-charging branch. The current-limiting resistor R1 is used to suppress the impact current when the capacitor is connected to the DC bus. When the capacitor voltage rises to equal the DC bus voltage, the pre-charging branch current decreases to zero, and the thyristor naturally turns off. The capacitor C voltage is maintained by intermittent trigger pulses to ensure that the capacitor stores enough energy to achieve reliable breaking when a fault occurs.

[0035] S3: Enter the thyristor turn-off mode to turn off the thyristor-diode full-bridge valve group T1; As shown in the figure, Figure 5 When the circuit breaker has a short-circuit fault, the current rises sharply, and the control system continuously triggers the thyristor T2, the thyristor T6, the mechanical switch T7, and the IGBT device T10 in the multi-functional branch after detecting the circuit breaker fault, so that the current stored in the capacitor C flows reversely into the thyristor-diode full-bridge valve group T1, forcing the thyristor-diode full-bridge valve group T1 to turn off.

[0036] S4: Enter the load commutation mode to commutate the fault current to the main breaking branch; After the thyristor-diode full-bridge valve group T1 is turned off, the low-conduction-loss DC circuit breaker enters the complete breaking state, and trigger signals are applied to the thyristor T2, the thyristor T3, the thyristor T6, the mechanical switch T8, and the mechanical switch T9; The short-circuit fault of the low conduction loss DC circuit breaker is mostly transient fault. After the DC circuit breaker successfully breaks the fault current, the system needs to be restored to normal operation through reclosing operation. The feasibility of reclosing depends on the judgment of the fault nature, that is, confirming whether there is still a permanent short-circuit fault in the system, which is a prerequisite for performing reclosing. The proposed topology has a fault type detection function, which can distinguish between transient fault and permanent fault, providing a key basis for reclosing decision, avoiding misreclosing under permanent fault condition, and ensuring system reliability.

[0037] As shown in Figure 8 , the circuit breaker is in a completely broken state, the system energy storage element has no residual energy, and all switch devices are in an off state.

[0038] S5: The fault current is introduced into the energy absorption branch through the main breaking branch, and enters the energy absorption mode, and the fault current is completely absorbed through the metal oxide voltage limiter MOSA; As shown in Figure 6 , the trigger signals of the thyristors T2, T6, the mechanical switch T7 and the IGBT device T10 are removed, and the IGBT-diode full-bridge valve group T11 is triggered at the same time, so as to commutate the fault current to the main breaking branch; At this time, the ultra-fast mechanical switch UFD current in the load branch is reduced to zero, and a breaking signal is applied to it; After the ultra-fast mechanical switch UFD establishes a medium insulation distance meeting the requirements, a turn-off signal is applied to the IGBT-diode full-bridge valve group T11, so that the resistance of the IGBT-diode full-bridge valve group T11 gradually increases.

[0039] S6: Enter the capacitor energy release mode, and completely discharge the energy storage of the capacitor C through the cooperation of the thyristors and mechanical switches in the multifunction main circuit, so that the low conduction loss DC circuit breaker enters a completely isolated state; As shown in Figure 7 , the fault current is injected into the energy absorption branch to enter the energy absorption mode, and this process is similar to the energy absorption mechanism of the traditional hybrid DC circuit breaker. The voltage across the MOSA gradually reaches the clamping level, and the energy of the fault current is dissipated through the metal oxide voltage limiter MOSA. Thus, the DC circuit breaker completes the whole process of breaking the fault current, and realizes the rapid and safe isolation of the fault current.

[0040] S7: Inject a controlled RLC oscillation current, judge the DC fault type by analyzing the current waveform characteristics, and perform reclosing or maintenance of the circuit breaker according to the analyzed DC fault type; As shown in Figure 9As shown, the trigger thyristor T3, the trigger thyristor T6, the IGBT device T10, the mechanical switch T8 and the mechanical switch T9 form a capacitor C discharge loop through the current-limiting resistor R2, and when the current of the capacitor C is reduced to 0, the trigger thyristor T3, the trigger thyristor T6, the IGBT device T10, the mechanical switch T8 and the mechanical switch T9 are turned off, and the low-conduction-loss DC circuit breaker enters a completely isolated state. At this time, the system energy storage element has no residual energy, and all the switching devices are in the off state.

[0041] A controlled RLC oscillation current is injected into the low-conduction-loss DC circuit breaker, and if the low-conduction-loss DC circuit breaker is still in a fault state, trigger signals are applied to the trigger thyristor T2, the trigger thyristor T3, the trigger thyristor T6, the mechanical switch T8 and the mechanical switch T9, as shown in FIG. 4. Figure 10 As shown, the type of DC fault can be determined by analyzing the current waveform characteristics.

[0042] If the low-conduction-loss DC circuit breaker is out of fault, the super-fast mechanical switch UFD is triggered to be closed and the trigger thyristor-diode full-bridge valve group T1 is turned on, so as to restore the normal operation of the low-conduction-loss DC circuit breaker; if the circuit breaker is still in a fault state, the resistance R f The inductance L, the capacitor C and the resistance R form an RLC oscillation loop. At this time, the damping resistor is the fault resistance itself, and the resistance value is very small, which leads to a single-peak oscillation current in the loop. The current amplitude is used as the basis for fault type determination, and provides a basis for the reclosing decision. The inductance L in the RLC loop can naturally limit the current amplitude, and through reasonable parameter design, the oscillation current can be restricted in a safe operation range The RLC oscillation loop generates a single-peak oscillation current, and by analyzing the current amplitude of the single-peak oscillation current, if the current amplitude is greater than a first preset value, it is a permanent fault, and if the current amplitude is less than a second preset value, it is a transient fault. Then, according to the fault type determination result obtained by the above analysis, if it is a transient fault, the super-fast mechanical switch UFD is triggered to be closed and the trigger thyristor-diode full-bridge valve group T1 is turned on, so as to restore the normal operation of the low-conduction-loss DC circuit breaker; if it is a permanent fault, the isolation state of the low-conduction-loss DC circuit breaker is maintained, and the line maintenance of the circuit breaker is waited for, and after the maintenance is completed, the super-fast mechanical switch UFD is triggered to be closed and the trigger thyristor-diode full-bridge valve group T1 is turned on, so as to restore the normal operation of the low-conduction-loss DC circuit breaker.

[0043] The mechanism realizes safe fault type determination, energy discharge and seamless conversion between the breaking and operating states, and effectively improves the system reliability and operation efficiency.

[0044] After the fault type determination is completed, the parameter design of the elements in the circuit is described in the embodiment, and specifically as follows. Safety current: According to the aforementioned reclosing mechanism analysis, the parameters of the capacitor C and the inductor L need to be reasonably designed during the reclosing process to achieve rapid fault type discrimination while avoiding excessive oscillation current that endangers system safety.

[0045] Let the DC bus voltage Un be 500 kV and the rated current Ir be 2 kA. The total sum of the smoothing reactor at the outlet side of the converter station and the line inductance is 100 mH. The fault resistance R f is set to 2 Ω-50 Ω, and the safety current threshold during fault type discrimination is set to 10 kA. The resistance of the thyristor valve group in the circuit is ignored, so we have: (1); In formula (1), a is the system damping coefficient, ω0 is the natural oscillation frequency of the circuit, L L is the inductance value, C and Ic is the current value of the capacitor. To make the current exhibit decaying oscillation characteristics, the system needs to operate in an underdamped state, as the reverse current generated by oscillation is needed to achieve the commutation of the thyristor valve groups T3 and T6 when the line is still in a fault state. The underdamping constraint condition is: (2); When the circuit breaker is in underdamped oscillation, its current response is: (3); In formula (3), ω d ωu is the underdamped oscillation frequency, i and i(t) is the circuit current response.

[0046] Reliable turn-off of thyristors: According to the working principle of the circuit breaker, the DC circuit breaker needs to always be ready for fault interruption, requiring the capacitor to store sufficient energy during normal operation, which is reflected in the capacitor voltage. To ensure readiness, the capacitor is charged from the DC bus every 10 seconds. This study stipulates that the capacitor voltage should not be lower than 0.8 times the rated voltage (Un). Assuming that there is no natural leakage without a discharge circuit, the charging circuit resistance is 5 MΩ. After the capacitor is charged and disconnected, its natural voltage decay follows the RC circuit model, and the capacitor voltage expression and required capacitor value can be obtained from the following formula (where Roff represents the resistance of the valve group in the off state): (4); In formula (4), t is the time, C is the capacitance value, and Vc is the capacitor voltage.

[0047] To achieve reliable turn-off of the thyristors, the capacitor needs to force the thyristor circuit current to zero and maintain the thyristor turn-off time T qThis also imposes a constraint on the capacitance value. Let the rated current be I n , and the DC fault detection threshold be I f . When the circuit breaker is working in the thyristor blocking state, the process is equivalent to the discharge process of the RC circuit composed of the resistance R2 and the capacitance C. The RC loop discharge current is opposite to the system rated current Ir, and the following relationship needs to be met: (5).

[0048] In formula (5), is the thyristor blocking time, is the circuit current response.

[0049] Parameter determination: Considering the actual engineering limit of the inductance size, the inductance L L of 20 mH is selected. This value takes into account the current limiting performance and physical compactness requirements in actual applications. When the fault resistance Rf = 50 Ω, according to formula (2), the capacitance value needs to be less than 192 μF to meet the underdamped condition, and its current response is shown in Figure 12 (a). At the same time, when the fault resistance Rf = 2 Ω, according to formula (5), the capacitance value needs to be greater than 8.96 μF, and its current response is shown in Figure 12 (b). To ensure that the capacitance voltage is stably maintained at 400 kV or more within a 10-second charging period.

[0050] To reduce the capacitor loss and meet the actual engineering design requirements, while ensuring the reliable commutation of the thyristor, the capacitance value of 37 μF is finally selected in this embodiment. The current-resistance relationship curve is shown in Figure 13 based on the numerical solution of formula (5). Under the condition that the fault current threshold is set to 8 kA, the maximum allowed resistance value of R2 is calculated to be about 47 Ω. Figure 13 It is shown that the resistance value R = 45 Ω can not only ensure that the current is within the safe limit, but also provide sufficient reverse current amplitude to suppress the influence of line stray parameters on the commutation capability.

[0051] The energy dissipation task is mainly borne by the MOSA. To verify the heat dissipation feasibility of R2, this embodiment compares the energy that R2 and the MOSA need to dissipate through simulation. For the resistance R2, the process of bearing the current includes two stages: one is the "thyristor blocking stage", and the other is the "capacitor energy release stage". In the "thyristor blocking stage", R2 bears the reverse commutation current generated by the capacitor, which promotes the current of the thyristor to zero; in the "capacitor energy release stage", R2 is responsible for releasing the residual energy in the commutation capacitor after fault breaking. Figure 14 The comparison results of the energy that R2 and the MOSA need to dissipate in the two stages are shown.

[0052] The low conduction loss DC circuit breaker (TC-HCB) with fault type judgment provided in the application has performance comparison with the prior art as shown in Table 1.

[0053] Table 1

[0054] In the circuit breaker topology provided in the embodiment, the heat dissipation problem of R2 can be effectively solved by sharing the heat dissipation device with MOSA. This scheme does not need additional cost investment and can guarantee the safety of energy dissipation in the fault breaking process.

[0055] Specific embodiment three: to verify the fault breaking and fault discrimination ability of the low conduction loss DC circuit breaker (TC-HCB) with fault type judgment provided in the application in the high voltage DC power transmission system, Simulation platform building: based on PSCAD / EMTDC, a pseudo bipolar MMC-HVDC system model as shown in Figure 15 is built.

[0056] The TC-HCB is installed downstream of the smoothing reactor on the outgoing line side of the converter station to evaluate its performance under fault conditions. The system parameters and circuit breaker configuration are shown in Table 2.

[0057] Table 2

[0058] On the basis of these parameters, the existing DC circuit breaker is compared and analyzed in the present embodiment, and the DC circuit breaker used in Zhangbei project is used for comparison and analysis. Three types of DC circuit breaker structures used in Zhangbei four-terminal flexible DC project are selected, i.e. mechanical DC circuit breaker, coupled negative pressure type DC circuit breaker and hybrid DC circuit breaker, and the topology of the hybrid DC circuit breaker used in Zhangbei project is as shown in Figure 16 .

[0059] The topology structure provided in the application is compared. The comparison conditions are based on rated power 500kV / 2kA, breaking capacity 500kV / 12kA, and MOSA rated voltage is set as 1.5 times of the system rated voltage (750kV), and the results are shown in Table 3.

[0060] Table 3

[0061] As for the selection of thyristor devices, the application adopts TDK4453002DH. According to its official data manual, the maximum forward voltage drop of the device under the condition of rated current 2kA is about 1.34V, and the calculated no-load loss power is 2.68kW. In addition, the current borne by the device during the off process is always lower than the Itsm threshold value, and no additional parallel devices are needed to ensure the stability of operation. In contrast, the single tube power consumption of IGBT device 25SNA3000K452300 is as high as 31.2kW, and at least two IGBTs need to be connected in parallel to meet the current sharing requirement when transferring current to the commutation branch, which will increase the total loss power to 62.40kW. The use of TC-HCB structure can reduce the conduction loss of the DC circuit breaker by 95.71%.

[0062] Capacitor pre-charging simulation: First, the capacitor pre-charging process is verified. Under forward current conditions, the capacitor is charged forward by a single pulse trigger of valve group T2, and the capacitor voltage rises to the same value as the DC rated voltage, after which T2 valve group is naturally turned off; under reverse current conditions, trigger thyristors T4 and T5 once to charge the capacitor to zero current, and then the valve group is naturally turned off. The charging process and capacitor voltage waveform are shown in Figure 17 After 1.5s trigger, the capacitor rises to the DC rated voltage in about 350ms, which fully meets the functional requirements of the system 10s charging cycle.

[0063] Fault current breaking simulation: This step verifies the breaking capacity of the DC circuit breaker. At t=1.998s, a double-pole short-circuit fault with a resistance of 2Ω is set on the DC side, and considering the communication and detection delay of the control system, the DC circuit breaker starts breaking 1ms after the fault occurs. The breaking process waveform is shown in Figure 18 , where IM is the main breaking branch current. The action timing of each element is shown in Figure 19 .

[0064] Before t1=1.998s, the system is in a normal load current operating state, and the DC current is 2kA. The trigger signal of valve group T1 has been removed, and the thyristor is maintained in conduction by the system current.

[0065] At t1=1.998s, a short-circuit fault occurs in the DC cable, and due to the restriction of smoothing reactors and stray inductance, the fault current rises at a limited rate.

[0066] At t2=2.001s, the DC circuit breaker starts to act. The capacitor C voltage is equal to the system rated DC voltage, and valve groups T3, T6, T7 and T10 are triggered to generate a reverse current to force the thyristor current to zero and turn off. After the thyristor is turned off, the DC current flows through the path containing resistor R2 as shown in Figure 12 , which further limits the amplitude of the fault current, and at the same time, the main breaking branch valve group T11 is activated to shunt part of the fault current for subsequent commutation.

[0067] At t3=2.0015s, the valve groups T10 and T7 are closed, and the t2-t3 period provides sufficient turn-off time for the thyristor. The UFD current gradually decays to zero, and a breaking signal is applied thereto, achieving zero-current arcless breaking.

[0068] At t4=2.004s (2.5ms after the UFD starts breaking), the UFD establishes sufficient medium insulation distance, and a turn-off signal is applied to the main breaking branch valve group T11, and the fault current is gradually dissipated by the MOSA.

[0069] At t5=2.007s, the DC fault is completely isolated.

[0070] Figure 20 The voltage changes of the key components in the breaking process are shown. The voltage curve clearly shows that, in each key stage of the breaking process, the peak voltage borne by each key component does not exceed its rated withstand voltage. In combination with the verification of Figure 18 , the voltage characteristics confirm that all key components work in a safe voltage range during the entire breaking process, effectively avoiding voltage breakdown failure, and ensuring that the TC-HCB current breaking task is completed smoothly and reliably.

[0071] Figure 21 The DC voltage changes before the fault occurs and after the fault breaking is completed are shown. It can be seen that the DC voltage is stably maintained at 500kV before the fault; after the breaking process is completed, the DC voltage drops to 0kV. This voltage characteristic shows that the DC circuit breaker has successfully completed the fault current breaking process.

[0072] Fault type discrimination simulation: The fault discrimination capability simulation is carried out for a 2Ω bipolar short-circuit fault. Based on the foregoing principle, the fault type discrimination function is simulated, and the valve groups T3, T6, T2, T8 and T9 are triggered at a specified time after the fault is completely cleared, taking the forward current as an example. The branch current characteristics under permanent fault and transient fault are as Figure 22 shown, and the two present obvious current peak differences, and the oscillation behaviors are significantly different.

[0073] By comparing the current differences, the reclosing safety of the converter station can be evaluated, thereby enhancing the functional integrity of the TC-HCB. In the simulation, the fault type discrimination is carried out at t=3.0s, and the conduction path currents under transient fault and permanent fault are as Figure 22 shown. The peak current is 2.42kA under transient fault, and 5.64kA under permanent fault. The valve group is closed after the RLC branch half-cycle oscillation, which realizes fault type discrimination while ensuring that the loop current is in a safe range.

[0074] Experimental verification: To verify the feasibility of the proposed topology, a low-voltage TC-HCB breaker functional verification platform is built. Although it cannot replace comprehensive verification, it can effectively test the effectiveness of the topology function and reduce the risk of subsequent development. The switch devices in the platform are controlled by the dSPACE MicroLabBox real-time simulator, and the current sensor is configured to measure the key current value, which is fed back to the simulator after being collected by the ADC. The Agilent N8944A power supply is used on the DC side, and the fault is simulated by connecting a small resistor in parallel with a standard resistor and setting the switch device. The experimental parameters are shown in Table 4.

[0075] Table 4

[0076] The experimental results are shown in Figure 23 , and the prototype successfully verifies the fault current breaking and fault type discrimination capabilities: Breaking process: Based on the aforementioned breaking principle, the TC-HCB switch devices are controlled by the dSPACE MicroLabBox. The experimental results of the breaking process are shown in Figure 23 (a). The system enters the normal current-carrying state at 0 ms, a short-circuit fault occurs at 2 ms, and the protection system starts to act at 3 ms. By 4 ms, the main branch thyristor is turned off, and the current commutation process is completed. The UFD establishes a safe insulation distance within 2.15 ms, and the main breaking branch completely isolates the fault at 6.15 ms. Due to the limitation of the current level of the low-voltage prototype, the MOSA energy dissipation stage is omitted.

[0077] Fault discrimination process: As shown in Figure 23 (b), both permanent and transient faults in the low-voltage system produce under-damped oscillations, but the peak current of the permanent fault is always higher than that of the transient fault, and the fault type can be distinguished by the theoretical criterion.

[0078] The above is only a preferred embodiment of the present application, and is not intended to limit the present application in any form. Although the present application has been disclosed as above with a preferred embodiment, it is not intended to limit the present application. Any skilled person in the art can make some changes or modifications to the disclosed technical content without departing from the scope of the technical solution of the present application, and any simple modification, equivalent replacement, and improvement of the above embodiments within the scope of the technical solution of the present application, in accordance with the technical essence of the present application, within the spirit and principles of the present application, are all within the protection scope of the present application.

Claims

1. A low conduction loss DC circuit breaker with fault type determination, characterized by, Comprise: Load branch, for carrying current when the DC circuit breaker is running, realizing low conduction loss, transferring fault current after fault occurs; Multifunctional branch, by controlling the cooperation of power electronic switching devices and mechanical switches, reserving energy when the DC circuit breaker is running normally, releasing reverse current to make the thyristor in the load branch current zero and turn off after fault occurs; Main breaking branch, for carrying the load branch transferred fault current after fault occurs; Energy absorption branch, for absorbing and dissipating the energy generated by the fault current.

2. The low conduction loss DC circuit breaker with fault type determination according to claim 1, characterized in that, The load branch comprises an ultra-fast mechanical switch UFD and a thyristor-diode full-bridge valve group T1, one end of the ultra-fast mechanical switch UFD is connected to the positive pole of the DC bus, the other end is connected to the input end of the thyristor-diode full-bridge valve group T1, and the output end of the thyristor-diode full-bridge valve group T1 is connected to the negative pole of the DC bus.

3. The low conduction loss DC circuit breaker with fault type determination according to claim 1, characterized in that, The multifunctional branch comprises a multifunctional branch composed of thyristors T2, T3, T4, T5, T6, a capacitor C, current limiting resistors R1 and R2, an IGBT device T10, an inductor L, mechanical switches T7, T8 and T9; The input end of the thyristor T2 is connected to the positive pole of the DC bus and one end of the ultra-fast mechanical switch UFD, the output end of the thyristor T2 is connected to the input end of the thyristor T3, the input end of the thyristor T4 and one end of the current limiting resistor R2 respectively, the output end of the thyristor T3 is connected to one end of the capacitor C and the input end of the thyristor T5; the output end of the thyristor T5 is connected to one end of the mechanical switch T7, the output end of the thyristor T7 and one end of the mechanical switch T8, the other end of the capacitor C is connected to the output end of the thyristor T4 and the input end of the thyristor T6, the output end of the thyristor T4 is connected to the input end of the thyristor T6, the output end of the thyristor T6 is adjacent to one end of the mechanical switch T8, the other end of the mechanical switch T8 is connected to one end of the current limiting resistor R1 and one end of the mechanical switch T9 respectively, the other end of the current limiting resistor R1 is grounded, and the other end of the mechanical switch T9 is connected to one end of the inductor L, and the other end of the inductor L is connected to the negative pole of the DC bus; The other end of the current limiting resistor R2 is connected to the emitter of the IGBT device T10, and the collector of the IGBT device T10 is connected to the other end of the inductor L and the negative pole of the DC bus.

4. The low conduction loss DC circuit breaker with fault type determination according to claim 1, characterized in that, The main breaking branch comprises an IGBT-diode full-bridge valve group T11, which is connected in parallel with the thyristor-diode full-bridge valve group T1, the input end of the IGBT-diode full-bridge valve group T11 is connected to the positive pole of the DC bus, and the output end is connected to the output end of the load branch.

5. The low conduction loss DC circuit breaker with fault type judgment according to claim 1, characterized in that, The energy absorption branch comprises a metal oxide voltage limiter MOSA, one end of the metal oxide voltage limiter MOSA is connected to the output end of the main breaking branch, and the other end is connected to the negative pole of the DC bus.

6. The fault type identification method based on the low conduction loss DC circuit breaker, applied to the low conduction loss DC circuit breaker with fault type judgment according to any one of claims 1-5, characterized in that, Comprise: Step 1: In normal operation mode, the thyristor-diode full-bridge valve group T1 is triggered to conduct by the pulse signal, and the gate trigger signal is removed after conduction, and the ultra-fast mechanical switch UFD is turned on, and the current flows through the load branch; Step 2: In the process of running in the normal operation mode, the capacitor pre-charging mode runs in parallel, and the low-conduction-loss DC circuit breaker intermittently triggers the thyristor valve T2, and after the thyristor T2 is triggered to conduct, the voltage at the positive pole of the DC bus is limited by the current-limiting resistor R1, and flows to the capacitor C through the thyristor T2 to charge the capacitor C; Step 3: In the event of a circuit short circuit of the low-conduction-loss DC circuit breaker, the thyristor turn-off mode is entered to turn off the thyristor-diode full-bridge valve group T1; Step 4: After the thyristor-diode full-bridge valve group T1 is turned off, the load commutation mode is entered to commutate the fault current to the main breaking branch, and the fault current is introduced into the energy absorption branch through the main breaking branch, and the energy absorption mode is entered, and the fault current is completely absorbed by the metal oxide voltage limiter MOSA; Step 5: After the fault current is completely absorbed, the capacitor energy release mode is entered, and the capacitor C energy is completely discharged through the cooperation of the thyristor and mechanical switch in the multifunctional main circuit, and the low-conduction-loss DC circuit breaker enters a completely isolated state; Step 6: A controlled RLC oscillation current is injected into the low-conduction-loss DC circuit breaker, the DC fault type is judged by analyzing the current waveform characteristics, and the circuit breaker is reclosed or repaired according to the analyzed DC fault type. 7.The fault type discrimination method based on low-conduction-loss DC circuit breaker according to claim 6, characterized in that, Step 3 specifically comprises: In the event of a circuit short circuit of the low-conduction-loss DC circuit breaker, the stored current in the capacitor C is reversed to flow into the thyristor-diode full-bridge valve group T1 by triggering the thyristor T2, the thyristor T6, the mechanical switch T7 and the IGBT device T10 in the multifunctional branch, so that the thyristor-diode full-bridge valve group T1 is turned off. 8.The fault type discrimination method based on the low-conduction-loss DC circuit breaker according to claim 6, characterized in that, Step 4 specifically comprises: Step 4.1: After the thyristor-diode full-bridge valve group T1 is turned off, the low-conduction-loss DC circuit breaker enters a completely broken state, the trigger signals of the thyristor T2, the thyristor T6, the mechanical switch T7 and the IGBT device T10 are removed, and the fault current is commutated to the main breaking branch by triggering the IGBT-diode full-bridge valve group T11; at this time, the current of the ultra-fast mechanical switch UFD in the load branch is reduced to zero, and a breaking signal is applied thereto; after the ultra-fast mechanical switch UFD establishes a medium insulation distance meeting the requirements, a turn-off signal is applied to the IGBT-diode full-bridge valve group T11, so that the resistance of the IGBT-diode full-bridge valve group T11 gradually increases; Step 4.2: The fault current is injected into the energy absorption branch, and the energy absorption mode is entered, and the energy of the fault current is dissipated by the metal oxide voltage limiter MOSA. 9.The fault type discrimination method based on low-conduction-loss DC circuit breaker according to claim 6, characterized in that, Step 5 specifically comprises After the fault current is completely absorbed, the capacitor energy release mode is entered, and the capacitor C is discharged through the current limiting resistor R2 by triggering the thyristor T3, the thyristor T6, the IGBT device T10, the mechanical switch T8 and the mechanical switch T9, and when the current of the capacitor C is reduced to 0, the thyristor T3, the thyristor T6, the IGBT device T10, the mechanical switch T8 and the mechanical switch T9 are turned off, and the low-conduction-loss DC circuit breaker enters a completely isolated state. 10.The fault type discrimination method based on the low-conduction-loss DC circuit breaker according to claim 6, characterized in that, Step 6 specifically comprises: Step 6.1: injecting a controlled RLC oscillation current into the low-conduction-loss DC circuit breaker, if the low-conduction-loss DC circuit breaker is in a fault-free state, triggering the super-fast mechanical switch UFD to be closed and the thyristor-diode full-bridge valve group T1 to be turned on, and restoring the normal operation of the low-conduction-loss DC circuit breaker, if the low-conduction-loss DC circuit breaker is still in a fault state, proceeding to step 6.2; Step 6.2: Resistance R at the fault occurrence f With the inductance L, the capacitance C constitutes an RLC oscillation circuit, and a current resistance with low resistance value is used as a fault resistance. The RLC oscillation circuit generates a single-peak oscillation current. By analyzing the current amplitude of the single-peak oscillation current, if the circuit amplitude is greater than a first preset value, it is a permanent fault, and if the current amplitude is less than a second preset value, it is a transient fault. If it is a transient fault, the ultra-fast mechanical switch UFD is closed and the thyristor-diode full-bridge valve group T1 is turned on to restore the normal operation of the low-conduction-loss DC circuit breaker. If it is a permanent fault, the isolation state of the low-conduction-loss DC circuit breaker is maintained, and the line maintenance of the circuit breaker is waited for. After the maintenance is completed, the ultra-fast mechanical switch UFD is closed and the thyristor-diode full-bridge valve group T1 is turned on to restore the normal operation of the low-conduction-loss DC circuit breaker.

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