CN112952776B - Current transfer circuit and method suitable for medium-voltage direct-current circuit breaker - Google Patents

Abstract

The invention relates to a current transfer circuit and a method suitable for a medium-voltage direct-current circuit breaker. The circuit includes: a main through-current branch, a fault current transfer branch and an energy consumption branch; the main through-current branch, the fault current transfer branch and the energy consumption branch are connected in parallel; the fault current transfer branch comprises a solid-state switch and an auxiliary commutation unit; the auxiliary current conversion unit comprises an energy storage capacitor, a first full-control power electronic device and a second full-control power electronic device; the positive electrode of the first full-control power electronic device is connected with the negative electrode of the second full-control power electronic device to form a first series string; the cathode of the first full-control type power electronic device is connected with the anode of the second full-control type power electronic device to form a second series string; the first series string is connected with the second series string in parallel, and the energy storage capacitor is connected between the first series string and the second series string in parallel, so that an H-bridge type circuit through-current topology is formed. The invention can realize the transfer of fault current and meet the requirement of quickly removing faults in the medium-voltage direct-current power distribution network.

Description

A current transfer circuit and method suitable for medium voltage DC circuit breaker

technical field

The invention relates to the field of medium-voltage direct current distribution network, in particular to a current transfer circuit and method suitable for medium-voltage direct current circuit breakers.

Background technique

In recent years, the medium-voltage DC distribution network has a good development prospect. In the future, the DC distribution network will have many advantages such as good economy, low energy consumption, high power supply reliability, convenient access to distributed power and energy storage devices, less land occupation and environmental protection. Advantages, the application range of DC power distribution is becoming wider and wider. Even though most of the existing forms of power transmission are AC-based, in recent years, with the rapid development of power electronics technology, the demand for electricity in major occasions such as rail transportation, aerospace transportation, and high-power medical equipment has provided a huge supply for the application of DC distribution networks. development space.

DC circuit breaker is the key equipment to protect low-damping DC system. Hybrid DC circuit breaker is a relatively mature technical solution currently applied, and the rapid development of power electronics technology also provides important technical support for it. The working principle of the hybrid DC circuit breaker is mainly to use the reliable cooperation of the main flow branch, the transfer branch and the energy consumption branch to realize the rapid breaking of the fault current. For the research on the transfer branch of the hybrid DC circuit breaker, the solutions proposed in many published documents and works at home and abroad are not the same. The auxiliary commutation measures can be configured in both the main flow branch and the transfer branch. road. Power semiconductor devices are usually used to form high-power semiconductor switch components, which are used as the core component of the failover branch to realize the auxiliary commutation function.

There are relatively mature current transfer methods for DC circuit breakers, which can be divided into two categories: impedance type and power type. The impedance type commutation method mainly refers to creating a high impedance state when the fault current is generated, and the power type commutation method mainly forces the fault current to transfer by introducing an independent voltage source or current source. At present, for the technical scheme of power supply current transfer, energy storage capacitors are mainly used to form active commutation conditions to force the main branch current to transfer. The published patent CN108390362A uses the primary side inductance coil and a certain number of fast conduction switches, and its main branch is still equipped with auxiliary power electronic switch modules. For DC circuit breakers used in medium voltage environments, additional equipment is required The water-cooling device has a certain improvement in economy, but the operation reliability is relatively poor; although the patent CN110401174A mentions that the auxiliary inductance La and the auxiliary capacitor Ca are introduced to form a resonant circuit, but its focus is to assist in completing the transfer of current. There are no practical application conditions for the inductance during the breaking process, and the circuit also has a certain impact on the process of circuit breaker current shutdown due to the introduction of auxiliary inductance; the patent CN105024369B mainly uses capacitors to store energy in advance to assist current transfer. Due to the use of inductive coupling coils, in The turn-off link current will oscillate, but its simple implementation and low cost are the main advantages. Patent CN109861183B has also proposed the use of capacitors to pre-store electric energy to assist current transfer, but due to the use of semi-controlled devices, the reliability of the device is relatively weak; on the other hand, because the internal auxiliary commutation structure involves more devices, compared with medium voltage , the reason why it is not suitable for application mainly lies in the fact that there are many devices, the control and structure are complex, and the floor space is large.

At present, the large-capacity DC interrupting technology researched in the field of medium-voltage DC distribution network is relatively backward compared with HVDC transmission. Most of the existing commercialized large-capacity DC interrupting technologies are aimed at low-voltage systems, so they cannot meet the needs of rapid fault removal. , but in practical applications, volume, cost and other issues need to be considered.

Contents of the invention

The purpose of the present invention is to provide a current transfer circuit and method suitable for medium-voltage DC circuit breakers, so as to solve the problem that the existing large-capacity DC breaking technology is aimed at low-voltage systems and cannot meet the needs of quick removal of faults in the medium-voltage DC distribution network The question of demand.

To achieve the above object, the present invention provides the following scheme:

A current transfer circuit suitable for a medium-voltage DC circuit breaker, including: a main flow branch, a fault current transfer branch, and an energy consumption branch;

The main flow branch, the fault current transfer branch and the energy consumption branch are connected in parallel;

The fault current transfer branch includes a solid-state switch and an auxiliary commutation unit; the auxiliary commutation unit includes an energy storage capacitor, a first fully-controlled power electronic device, and a second fully-controlled power electronic device;

The positive pole of the first full-control power electronic device is connected to the negative pole of the second full-control power electronic device to form a first series string; the negative pole of the first full-control power electronic device is connected to the second full-control power electronic device The positive electrode of the controlled power electronic device forms a second series string; the first series string is connected in parallel with the second series string, and the energy storage capacitor is connected in parallel with the first series string and the second series string Between, combined into an H-bridge circuit flow topology;

Fault current transfer stage: both of the first fully-controlled power electronic devices are turned off, and both of the second fully-controlled power electronic devices are turned on, and the energy storage capacitor is discharged to form a negative pressure, so that all Transfer the fault current through the main flow branch;

Fault current shutdown stage: the energy storage capacitor is completely discharged, the two first full-control power electronic devices are still in the shutdown state, and the first series string and the second series string are connected in parallel; after After a preset period of time, the two second fully-controlled power electronic devices are triggered to shut down, and the terminal voltage of the auxiliary converter unit gradually rises to the operating voltage of the energy-consuming branch. The branch circuit drains the residual fault current.

Optionally, the first fully-controlled power electronic device is of the same type as the second fully-controlled power electronic device;

When the flow mode of the auxiliary converter unit is a unidirectional flow mode, the first fully controlled power electronic device is a diode; the second fully controlled power electronic device is an insulated gate bipolar transistor;

When the flow mode of the auxiliary converter unit is a bidirectional flow mode, both the first fully-controlled power electronic device and the second fully-controlled power electronic device are insulated gate bipolar transistors.

Optionally, only a fast mechanical switch is configured in the main flow branch; the fast mechanical switch is used to meet the millisecond-level segmentation requirements of the medium-voltage DC circuit breaker.

Optionally, the energy consumption branch includes one or more lightning arresters; the action voltage of the lightning arresters is set based on the transient voltage generated by the auxiliary converter unit during the shutdown phase of the fault current.

Optionally, the energy storage capacitor is a film capacitor.

Optionally, the solid-state switch includes a direct series solid-state switch, a diode bridge solid-state switch, and a capacitor-containing full-bridge solid-state switch.

Optionally, after the fault current shutdown phase, if the storage capacitor still stores residual voltage, the residual voltage is used in the reclosing phase of the next period.

A method of current diversion suitable for medium voltage direct current circuit breakers, comprising:

Fault current generation stage: fault current is generated and only flows through the main current branch;

Fault current transfer stage: the two first fully-controlled power electronic devices in the fault current transfer branch are both turned off, the two second fully-controlled power electronic devices are both turned on, and the energy storage capacitor is discharged to form a negative pressure, so that The main flow branch diverts the fault current; the first series string is a series string formed by connecting the positive pole of the first fully-controlled power electronic device to the negative pole of the second fully-controlled power electronic device, and the second series string Connecting the negative electrode of the first fully-controlled power electronic device to the positive electrode of the second fully-controlled power electronic device to form a series string;

Fault current shutdown stage: After the energy storage capacitor is discharged, the two first fully-controlled power electronic devices are still in the off state, and the first series string and the second series string are connected in parallel; Two fully-controlled power electronic devices are triggered to shut down, the terminal voltage of the auxiliary converter unit in the fault current transfer branch gradually rises to the operating voltage of the energy-consuming branch, and the residual power is exhausted through the energy-consuming branch fault current.

Optionally, after the fault current shutdown phase, if the storage capacitor still stores residual voltage, the residual voltage is used in the reclosing phase of the next period.

Optionally, the reclosing stage specifically includes reclosing once at the first moment, and then opening at a second moment; the second moment is a moment next to the first moment.

According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects: The present invention provides a current transfer circuit and method suitable for medium-voltage DC circuit breakers. By configuring the auxiliary commutation unit, the energy storage capacitor is pre-charged Realize the transfer of fault current with an appropriate amount of voltage to meet the needs of the medium-voltage DC distribution network for rapid fault removal; the present invention proposes a current transfer circuit and method for a medium-voltage hybrid DC circuit breaker to enable fault current transfer and shutdown functions Integration, and taking into account the volume, cost and work reliability requirements of the medium-voltage DC distribution network for the circuit breaker equipment, has important engineering significance for expanding the application range of DC circuit breakers.

Description of drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the accompanying drawings required in the embodiments. Obviously, the accompanying drawings in the following description are only some of the present invention. Embodiments, for those of ordinary skill in the art, other drawings can also be obtained based on these drawings without any creative effort.

Fig. 1 is a schematic diagram of a current transfer circuit suitable for a medium-voltage DC circuit breaker provided by the present invention;

Fig. 2 is a unidirectional flow structure diagram of the auxiliary converter unit provided by the present invention;

Fig. 3 is a bidirectional flow structure diagram of the auxiliary converter unit provided by the present invention;

FIG. 4 is a schematic diagram of a straight-series solid-state switch unit provided by the present invention;

Fig. 5 is a schematic diagram of a solid-state switch unit with a diode bridge provided by the present invention;

6 is a schematic diagram of a full-bridge solid-state switch unit with capacitance provided by the present invention;

Fig. 7 is the overall working flow diagram of the fault current transfer and shutdown process provided by the present invention;

Fig. 8 is the main control sequence diagram of the fault current transfer and shutdown process provided by the present invention;

Fig. 9 is the fault current transfer process provided by the present invention and the waveform diagram of the first reclosing situation;

Fig. 10 is a waveform diagram of the fault current transfer process and the second reclosing situation provided by the present invention.

detailed description

The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some, not all, embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

The purpose of the present invention is to provide a current transfer circuit and method suitable for medium-voltage DC circuit breakers, which can realize the transfer of fault currents and meet the demand for rapid removal of faults in the medium-voltage DC distribution network.

In order to make the above objects, features and advantages of the present invention more comprehensible, the present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

Fig. 1 is a schematic diagram of a current transfer circuit suitable for a medium-voltage DC circuit breaker provided by the present invention. As shown in Fig. 1, a current transfer circuit suitable for a medium-voltage DC circuit breaker includes: a main flow branch 1, The fault current transfer branch 2 and the energy consumption branch 3; the main flow branch 1, the fault current transfer branch 2 and the energy consumption branch 3 are connected in parallel; the fault current transfer branch 2 includes a solid state A switch 4 and an auxiliary commutation unit 5; the auxiliary commutation unit 5 includes an energy storage capacitor, a first fully-controlled power electronic device and a second fully-controlled power electronic device; the first fully-controlled power electronic device The positive pole is connected to the negative pole of the second fully-controlled power electronic device to form a first series string; the negative pole of the first fully-controlled power electronic device is connected to the positive pole of the second fully-controlled power electronic device to form a second A series string; the first series string and the second series string are connected in parallel, and the energy storage capacitor is connected in parallel between the first series string and the second series string, and are combined to form an H-bridge circuit for current flow Topology; fault current transfer stage: both of the first fully-controlled power electronic devices are turned off, both of the second fully-controlled power electronic devices are turned on, and the energy storage capacitor is discharged to form a negative pressure, so that the main flow branch 1 transfers the fault current; the fault current shutdown stage: the energy storage capacitor is completely discharged, and the two first fully-controlled power electronic devices are still in the off state, and the first series connection The series and the second series are connected in parallel; after a preset period of time, the two second fully-controlled power electronic devices are triggered to shut down, and the terminal voltage of the auxiliary converter unit 5 gradually increases to the specified The operating voltage of the energy-consuming branch 3 is used to drain the residual fault current through the energy-consuming branch 3 .

The main flow branch 1 in the present invention only includes a fast mechanical switch and does not contain a solid-state power electronic switch, thus eliminating the loss and heating problems caused by the long-term conduction of the solid-state switch 4 .

The fully controlled power electronic devices used in the auxiliary converter unit 5 should choose the same type of IGBT. If the selection of devices is inconsistent, in the transition process of the two commutation stages, the two IGBT parallel branches of the auxiliary commutation unit 5 may have uneven current flow, and a single branch device may be subjected to transient electrical stress. There is a risk of device failure.

The auxiliary commutation unit 5 is mainly responsible for the fault current transfer and shutdown process of the circuit breaker, and is integrated with the solid-state switch 4 and placed in the fault current transfer branch 2 .

Figure 2 is a one-way flow structure diagram of the auxiliary converter unit provided by the present invention; Figure 3 is a two-way flow structure diagram of the auxiliary converter unit provided by the present invention; 503 and 504 in Figure 2 are two of the same type. A fully-controlled power electronic device, the first fully-controlled power electronic device is a diode, 501 and 502 are second fully-controlled power electronic devices of the same type, and the second fully-controlled power electronic device is an insulated gate bipolar type transistor (Insulated Gate Bipolar Transistor, IGBT); 505 is a pre-charged energy storage capacitor before work, usually a film capacitor can be used; 506 and 507 in Figure 3 are the first fully-controlled power electronic devices, the first fully-controlled power electronics Both the electronic device and the second fully-controlled power electronic device are IGBTs.

Fig. 4 is a schematic diagram of a matching straight-series solid-state switch unit provided by the present invention; Fig. 5 is a schematic diagram of a matching diode bridge solid-state switching unit provided by the present invention; Fig. 6 is a matching full-bridge solid-state switching unit with capacitance provided by the present invention Schematic diagram; among them, the straight-series solid-state switch 401 adopts the method of directly connecting IGBT devices in series; the diode-bridge solid-state switch 402 is composed of diodes, intermediate IGBT devices, and buffer capacitors; the full-bridge solid-state switch 403 with capacitors mainly uses IGBT devices to form a full bridge way, where the capacitor acts as a snubber capacitor during the turn-off phase. The solid-state switching units corresponding to each matching scheme can be connected in series to expand the voltage application level.

The energy-consuming branch 3 includes one or more arresters; the operating voltage of the arrester is set based on the transient voltage generated by the auxiliary commutation unit 5 in the shutdown phase of the fault current; the arrester is a metal oxide lightning arrester.

A method of current diversion suitable for medium voltage direct current circuit breakers, comprising:

Fault current generation stage: the fault current is generated and only flows through the main current branch 1.

Fault current transfer stage: the two first fully-controlled power electronic devices in the fault current transfer branch 2 are both turned off, the two second fully-controlled power electronic devices are both turned on, and the energy storage capacitor is discharged to form a negative pressure. Make the main flow branch 1 divert the fault current; the first series string is a series string formed by connecting the positive pole of the first fully-controlled power electronic device to the negative pole of the second fully-controlled power electronic device, and the second In the series string, the negative pole of the first full-control power electronic device is connected to the positive pole of the second full-control power electronic device to form a series string.

Fault current shutdown stage: After the energy storage capacitor is discharged, the two first fully-controlled power electronic devices are still in the off state, and the first series string and the second series string are connected in parallel; Two fully-controlled power electronic devices are triggered to shut down, the terminal voltage of the auxiliary commutation unit 5 in the fault current transfer branch 2 gradually rises to the operating voltage of the energy consumption branch 3, through the energy consumption branch 3 Drain the residual fault current.

After the fault current shutdown phase, if the storage capacitor still stores residual voltage, the residual voltage is used for the reclosing phase of the next period; the reclosing phase specifically includes reclosing once at the first moment , opening at the second moment; the second moment is the next moment of the first moment.

In practical applications, the specific process of the current transfer method suitable for medium-voltage DC circuit breakers provided by the present invention is as follows:

As shown in Figure 7, it is divided into the fault current generation stage, the second full-control power electronic device 501 and the second full-control power electronic device 502 conduction and the energy storage capacitor 505 discharge stage, the second full-control power electronic device 501 and the second fully-controlled power electronic device 502 are in the two parallel branch flow-through phase, the second fully-controlled power electronic device 501 and the second fully-controlled power electronic device 502 trigger the shutdown phase, and the lightning arrester energy consumption phase. Specifically, FIG. 8 has shown the control sequence of the pre-charging of the energy storage capacitor 505, the fast mechanical switch and the devices 501 and 502 of the present invention.

It needs to be clarified that the auxiliary converter unit 5 in the present invention has a bidirectional current flow function, so only taking the current flowing from left to right during the normal operation of the system as an example, the following five stages are specifically described in chronological order:

A) Fault current generation stage (before time t ₁ ): the fault current is generated and only flows through the main current branch 1.

B) Fault current transfer phase (time period t ₁ -t ₂ ): IGBT devices 501 and 502 are controlled to be turned on, while devices 506 and 507 are strictly controlled to be turned off. At this time, the energy storage capacitor 505 starts to discharge, and the devices 501 , 502 and the capacitor 505 are connected in series.

C) Fault current transfer branch 2 flow stage (t ₂ ~ t ₃ time period): the energy storage capacitor 505 is completely discharged at time t ₂ , and the devices 506 and 507 are still strictly controlled to be turned off. At this time, the device 501 and the device 507 The freewheeling diodes of the device 502 are connected in series with the freewheeling diodes of the device 506, and the two series branches are connected in parallel to flow.

D) Fault current shutdown phase ( _{t3 -} t4 time period): devices 501 and 502 are triggered to shut down, capacitor 505 is connected in series with the freewheeling diodes of devices 506 and 507, the residual current forms a path, and the auxiliary commutation unit 5 The terminal voltage gradually builds up and gradually rises to the operating voltage of the arrester in the energy-consuming branch 3. After the arrester reaches the operating voltage, the residual fault current only flows through the energy-consuming branch 3 and is exhausted. At this time, the capacitor 505 of the auxiliary converter unit 5 still has a residual voltage, which is used for reclosing in the next stage.

E) Reclosing stage (t ₄ ~ t ₄ time period): The specific steps are to reclose once at t ₄ and then open again at t _5. This situation is divided into two ways.

One is that the transfer branch recloses first, as shown in Figure 9, that is, the auxiliary converter unit 5 is controlled to pass the current, and if a fault occurs, the fault current is immediately broken, that is, the above-mentioned stages B) to D) are repeated. If the fault current is exhausted, Then there is no need to break the fault current again, and the energy storage capacitor 505 can still maintain a part of the voltage.

The second is to directly control the closing of the fast mechanical switch of the main current branch 1, as shown in Fig. 10, considering that the energy storage capacitor 505 in the auxiliary commutation unit 5 still has residual voltage, so it has the conditions for auxiliary commutation again. When the whole machine is switched off again, the operation process of the circuit breaker can still be realized by repeating the above steps A) to D). After the whole machine is broken, the capacitor energy storage 505 in the auxiliary converter unit 5 still has a partial voltage.

In the present invention, after the pre-charged energy storage capacitor is discharged during the fault current transfer process, it can also be used as a buffer capacitor during the fault current shutdown process, that is, the auxiliary commutation unit 5 establishes a voltage during the shutdown phase to cooperate with the energy consumption branch 3 The operating voltage required for the operation of the arrester; at the same time, the residual voltage of the capacitor can also be conveniently used for the fault current transfer process when the circuit breaker breaks the next fault, saving the cost of recharging the capacitor; in addition, the residual voltage of the capacitor Usually when the circuit breaker works next time, it can be flexibly controlled to complete the secondary charging.

For the integration of internal equipment of the device, the current transfer system proposed by the present invention can reduce the functional redundancy of the circuit breaker device; at the same time, the current transfer method proposed by the present invention mainly uses diodes and full-control solid-state switches 4 and avoids Using passive components such as auxiliary inductors can improve the reliability of the circuit breaker's current transfer and turn-off transient characteristics. To sum up, the present invention aims to improve the cost and use space of the device, and provides certain guiding significance for the application optimization of the medium-voltage DC circuit breaker.

Each embodiment in this specification is described in a progressive manner, each embodiment focuses on the difference from other embodiments, and the same and similar parts of each embodiment can be referred to each other. As for the system disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and for the related information, please refer to the description of the method part.

In this paper, specific examples have been used to illustrate the principle and implementation of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core idea; meanwhile, for those of ordinary skill in the art, according to the present invention Thoughts, there will be changes in specific implementation methods and application ranges. In summary, the contents of this specification should not be construed as limiting the present invention.

Claims (7)

1. A current transfer circuit suitable for a medium-voltage DC circuit breaker, characterized in that it includes: a main flow branch, a fault current transfer branch, and an energy consumption branch; The main flow branch, the fault current transfer branch and the energy consumption branch are connected in parallel; The fault current transfer branch includes a solid-state switch and an auxiliary commutation unit; the auxiliary commutation unit includes an energy storage capacitor, a first fully-controlled power electronic device, and a second fully-controlled power electronic device; The positive pole of the first full-control power electronic device is connected to the negative pole of the second full-control power electronic device to form a first series string; the negative pole of the first full-control power electronic device is connected to the second full-control power electronic device The positive electrode of the controlled power electronic device forms a second series string; the first series string is connected in parallel with the second series string, and the energy storage capacitor is connected in parallel with the first series string and the second series string Between, combined into an H-bridge circuit flow topology; The first fully-controlled power electronic device is of the same type as the second fully-controlled power electronic device; When the flow mode of the auxiliary converter unit is a unidirectional flow mode, the first fully controlled power electronic device is a diode; the second fully controlled power electronic device is an insulated gate bipolar transistor; When the flow mode of the auxiliary converter unit is a bidirectional flow mode, both the first fully-controlled power electronic device and the second fully-controlled power electronic device are insulated gate bipolar transistors; Fault current generation stage: fault current is generated and only flows through the main current branch; Fault current transfer stage: both of the first fully-controlled power electronic devices are turned off, both of the second fully-controlled power electronic devices are turned on, the energy storage capacitor is discharged, and the two second fully-controlled power electronic devices are turned on. The type power electronic device and the energy storage capacitor are connected in series to form a negative pressure, so that the main flow branch diverts the fault current; The current stage of the fault current transfer branch: the energy storage capacitor is completely discharged at the initial moment of the fault current transfer branch flow stage, and the two first fully-controlled power electronic devices are still strictly controlled to be turned off. At this time, the first series connection The second fully-controlled power electronic device in the string is connected in series with the freewheeling diode of the first fully-controlled power electronic device, and the second fully-controlled power electronic device in the second series string is connected to the first fully-controlled power electronic device. The freewheeling diodes are connected in series, and the two series branches are connected in parallel to flow; Fault current shutdown stage: after a preset time period, the two second fully-controlled power electronic devices are triggered to shut down, and the terminal voltage of the auxiliary converter unit gradually increases to the action of the energy-consuming branch voltage, draining the residual fault current through said dissipating branch; After the fault current shutdown phase, if the energy storage capacitor still stores a residual voltage, the residual voltage is used for the reclosing phase of the next period; Reclosing stage: After reclosing once at the beginning of the reclosing stage, it will be opened at the end of the reclosing stage again. This situation can be divided into two ways: One is to reclose the transfer branch first, that is, to control the flow of the auxiliary converter unit, and immediately break the fault current if a fault occurs, that is, to repeat the fault current transfer stage to the fault current shutdown stage. If the fault current is exhausted, it does not need to be broken again Fault current, and the energy storage capacitor can still maintain a part of the voltage; The second is to directly control the fast mechanical switch closing of the main flow branch. Considering that the energy storage capacitor in the auxiliary commutation unit still has residual voltage, it has the conditions for auxiliary commutation again. When the whole machine completes the opening again, The action process of the circuit breaker can still be realized by repeating the fault current generation stage to the fault current shutdown stage. After the whole machine is broken, the capacitor energy storage in the auxiliary converter unit still has a part of the voltage. 2. The current transfer circuit suitable for medium-voltage DC circuit breakers according to claim 1, wherein only fast mechanical switches are arranged in the main flow branch; the fast mechanical switches are used to realize medium-voltage direct current Millisecond segmentation requirements for circuit breakers. 3. The current transfer circuit suitable for medium-voltage DC circuit breakers according to claim 1, wherein the energy-consuming branch circuit includes one or more lightning arresters; the action voltage of the lightning arresters is based on the fault current shutdown The transient voltage setting generated by the auxiliary converter unit during shutdown during the shutdown phase. 4. The current transfer circuit suitable for medium-voltage DC circuit breakers according to claim 1, wherein the energy storage capacitor is a film capacitor. 5 . The current transfer circuit suitable for medium-voltage direct current circuit breakers according to claim 1 , wherein the solid-state switches include straight-series solid-state switches, diode-bridge solid-state switches, and full-bridge solid-state switches with capacitors. 6. A current transfer method suitable for a medium-voltage DC circuit breaker, characterized in that the current transfer method suitable for a medium-voltage DC circuit breaker is applied to any one of claims 1-5. A current transfer circuit for a DC circuit breaker, the current transfer method applicable to a medium-voltage DC circuit breaker includes: Fault current generation stage: fault current is generated and only flows through the main current branch; Fault current transfer stage: both of the first fully-controlled power electronic devices are turned off, both of the second fully-controlled power electronic devices are turned on, the energy storage capacitor is discharged, and the two second fully-controlled power electronic devices are turned on. The type power electronic device and the energy storage capacitor are connected in series to form a negative pressure, so that the main flow branch diverts the fault current; The current stage of the fault current transfer branch: the energy storage capacitor is completely discharged at the initial moment of the fault current transfer branch flow stage, and the two first fully-controlled power electronic devices are still strictly controlled to be turned off. At this time, the first series connection The second fully-controlled power electronic device in the string is connected in series with the freewheeling diode of the first fully-controlled power electronic device, and the second fully-controlled power electronic device in the second series string is connected to the first fully-controlled power electronic device. The freewheeling diodes are connected in series, and the two series branches are connected in parallel to flow; Fault current shutdown stage: after a preset time period, the two second fully-controlled power electronic devices are triggered to shut down, and the terminal voltage of the auxiliary converter unit gradually increases to the action of the energy-consuming branch voltage, draining the residual fault current through said dissipating branch; After the fault current shutdown phase, if the energy storage capacitor still stores a residual voltage, the residual voltage is used for the reclosing phase of the next period; Reclosing stage: After reclosing once at the beginning of the reclosing stage, it will be opened at the end of the reclosing stage again. This situation can be divided into two ways: One is to reclose the transfer branch first, that is, to control the flow of the auxiliary converter unit, and immediately break the fault current if a fault occurs, that is, to repeat the fault current transfer stage to the fault current shutdown stage. If the fault current is exhausted, there is no need to break again Fault current, and the energy storage capacitor can still maintain a part of the voltage; The second is to directly control the fast mechanical switch closing of the main flow branch. Considering that the energy storage capacitor in the auxiliary commutation unit still has residual voltage, it has the conditions for auxiliary commutation again. When the whole machine completes the opening again, The action process of the circuit breaker can still be realized by repeating the fault current generation stage to the fault current shutdown stage. After the whole machine is broken, the capacitor energy storage in the auxiliary converter unit still has a part of the voltage. 7. The current transfer method suitable for medium-voltage DC circuit breakers according to claim 6, wherein the reclosing stage specifically includes reclosing once at the first moment, and opening at the second moment; The second moment is the next moment of the first moment.

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