Multi-port direct current change-over switch based on controllable negative voltage source

Abstract

The invention discloses a multi-port direct current change-over switch based on a controllable negative voltage source, and belongs to the technical field of power equipment. Comprising X (X = 5 or 3) main branches, a commutation branch and X groups of controllable conduction switches, each main branch is composed of a through-flow vacuum switch, and the commutation branch is composed of a controllable negative voltage source and a square wave resonance direct current circuit breaker which are connected in series. The five main branches of the sending end are connected with a sending end positive pole rectifier and the like, and the three main branches of the receiving end are connected with a receiving end positive pole rectifier and the like and are all connected with the commutation branch through a controllable conduction switch. The controllable negative voltage source is formed by connecting a double-thyristor branch circuit and a negative voltage source in parallel, and the square wave resonance direct current circuit breaker comprises a temporary through-flow branch circuit and the like. The vacuum switch is used for replacing an SF6 switch, so that the switch is good in environmental protection property; the converter branch and the lightning arrester are shared, so that the use of high-price equipment is reduced, and the cost is reduced; a controllable square wave voltage source is utilized to excite high-frequency oscillation of an LC branch, and the element size is reduced; and rapid transfer of current in different directions can be realized, and the system response speed is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of power equipment, and particularly to a multi-port DC conversion switch based on a controllable negative pressure source. Background Art

[0002] In a conventional DC power transmission system, in the cases of converter valve or DC line failures, DC line maintenance, and changes in system power demand, etc., in order to ensure the stability, safety, and efficiency of the system, it is necessary to complete the switching of the system operation mode through the coordinated cooperation of a DC conversion switch and a disconnector.

[0003] As Figure 1 shown, the DC conversion switch is a two-port switch, which is composed of a current-carrying branch, a commutation branch, and an energy-absorbing branch. The current-carrying branch is composed of mechanical switches; the commutation branch is composed of an LC oscillation circuit; the energy-absorbing branch is composed of a lightning arrester group. The working process of the DC conversion switch is as follows: in response to a trip signal, the contacts of the mechanical switch separate, an arc is generated between the contacts, and the arc resistance shows a non-linear change; due to the negative impedance characteristic of the arc, the oscillating current in the LC circuit diverges and oscillates. When the sum of the load current and the oscillating current superimposed on the current-carrying branch passes through zero, the arc of the mechanical switch extinguishes, and the load current transfers to the commutation branch; the load current charges the capacitor C. When the voltage across the lightning arrester rises to the operating voltage of the lightning arrester, the lightning arrester operates to absorb energy and establish a transient voltage, and the load current transfers to the energy-absorbing branch; the lightning arrester absorbs energy, the load current decreases to zero, and the current conversion is completed.

[0004] Since the DC conversion switch is a two-port switch, and there are various necessary operation mode switching requirements in a conventional DC power transmission system, it is necessary to install multiple DC conversion switches in the conventional DC power transmission system to meet different operation mode switching requirements. As Figure 2 shown, according to the different lines connected to the ports, the DC conversion switches in the UHV conventional DC power transmission project are subdivided into a metallic return transfer breaker MRTB, an earth return transfer breaker ERTB, a neutral bus switch NBS, and a neutral bus grounding switch NBGS.

[0005] The ports of the MRTB are respectively connected to the sending - end neutral bus and the sending - end ground electrode line. Its main functions are: to transfer the load current from the ground return line with lower impedance to the positive / negative metal return line with higher impedance, and to complete the switching from the monopole - ground return operation mode to the monopole - metal return operation mode. The ports of the ERTB are respectively connected to the sending - end neutral bus and the positive and negative metal return lines. Its main functions are: to convert the load current from the positive / negative metal return line to the ground return line, in order to complete the switching from the monopole - metal return operation mode to the monopole - ground return operation mode. The ports of NBS1 are respectively connected to the sending - end neutral bus and the sending - end positive rectifier, the ports of NBS2 are respectively connected to the sending - end neutral bus and the sending - end negative inverter, the ports of NBS3 are respectively connected to the receiving - end neutral bus and the receiving - end positive rectifier, and the ports of NBS4 are respectively connected to the receiving - end neutral bus and the receiving - end negative inverter. The main functions of NBS1, NBS2, NBS3, and NBS4 are: after the converter is blocked, the NBS switches off to achieve the switching from the bipolar operation mode to the monopole - ground return operation mode. The ports of NBGS1 are respectively connected to the sending - end neutral bus and the sending - end grounding grid, and the ports of NBGS2 are respectively connected to the receiving - end neutral bus and the receiving - end grounding grid. The main function of NBGS is: when the ground electrode line fails, NBGS switches on to provide a grounding point for the DC system, enabling the unbalanced current between the positive and negative poles to flow through the grounding grid.

[0006] In summary, the traditional DC transmission system relies on two - port DC conversion switches (such as the metal return transfer switch MRTB and the earth return transfer switch ERTB) to achieve operation mode switching. Such switches mostly use sulfur hexafluoride (SF6) gas as the arc - quenching medium, and there are the following problems: Poor environmental protection: SF6 is a strong greenhouse gas, and its leakage causes great harm to the environment; Bulky equipment: The frequency of the traditional LC resonance circuit is low (<5 kHz), requiring large - capacity capacitors and reactors, resulting in a large volume; High cost: Multiple independent two - port switches need to be configured to meet different switching requirements, and the lightning arresters need to absorb a large amount of energy, with high construction costs.

[0007] In the prior art, the DC conversion switches of manufacturers such as ABB rely on the 6 negative resistance characteristic of the arc to achieve current transfer, but its dielectric recovery speed is limited, making it difficult to meet the high - frequency requirement. In addition, the lack of multi - port switches leads to the need to configure multiple independent switches in the system, occupying a large area and having complex control. Summary of the Invention

[0008] The object of the present invention is to provide a multi - port DC conversion switch based on a controllable negative pressure source, which solves the problems of bulky equipment, high cost, and poor environmental protection in the prior art by sharing commutation branches and controllable conduction switches.

[0009] To achieve the above object, the present invention provides a multi-port DC conversion switch based on a controllable negative pressure source, including X main branches, a commutation branch, and X groups of controllable conduction switches, where X = 5 or 3. Each of the X main branches is composed of a current-carrying vacuum switch, and the commutation branch is composed of a series connection of a controllable negative pressure source and a square-wave resonant DC circuit breaker.

[0010] Preferably, the multi-port DC conversion switch at the sending end of a conventional DC transmission system has 5 main branches. The main branch 1, main branch 2, main branch 3, main branch 4, and main branch 5 among the 5 main branches respectively electrically connect the sending-end positive rectifier, the sending-end grounding electrode line, the sending-end grounding grid, the positive / negative metal return line, and the sending-end negative reverse current inverter to the sending-end neutral bus; the commutation branch is connected to the sending-end neutral bus; the far busbar ends of the 5 main branches are each connected to the far busbar end of the commutation branch through a group of controllable conduction switches.

[0011] Preferably, the multi-port DC conversion switch at the receiving end of a conventional DC transmission system has 3 main branches. The main branch A, main branch B, and main branch C among the 3 main branches respectively electrically connect the receiving-end positive rectifier, the receiving-end grounding grid, and the receiving-end negative reverse current inverter to the receiving-end neutral bus; the commutation branch is connected to the receiving-end neutral bus; the far busbar ends of the 3 main branches are each connected to the far busbar end of the commutation branch through a group of controllable conduction switches.

[0012] Preferably, the controllable negative pressure source includes 2 thyristor branches connected in parallel and 1 negative pressure source. When the current in the main branch needs to be transferred, the negative pressure source is cut into the commutation branch by triggering the thyristor to achieve the transfer of the load current. The first thyristor branch is composed of a common cathode series connection of thyristor VT 1 and thyristor VT 2 ; the second thyristor branch is composed of a common anode series connection of thyristor VT 3 and thyristor VT 4 ; the anode of thyristor VT 1 and the cathode of thyristor VT 3 are connected, and a terminal is led out from the connection point, and this terminal is the near busbar end of the controllable negative pressure source; the anode of thyristor VT 2 and the cathode of thyristor VT 4 are connected, and a terminal is led out from the connection point, and this terminal is the far busbar end of the controllable negative pressure source; the negative pressure source is composed of 1 pre-charging capacitor and 1 diode assembly connected in reverse parallel. The function of the diode assembly is to provide a path for the load current after the current is transferred to the commutation branch; the negative electrode of the pre-charging capacitor is connected to the common cathode point of the first thyristor branch, and the positive electrode of the pre-charging capacitor is connected to the common anode point of the second thyristor branch.

[0013] Preferably, the square-wave resonant DC circuit breaker includes a temporary current-carrying branch, an LC branch, and a MOV branch connected in parallel; The temporary current-carrying branch is composed of a commutation vacuum switch and a controllable square-wave voltage source connected in series, and is used to transfer the load current of the main branch; the LC branch is composed of a reactor and a capacitor connected in series, and is used to generate an oscillating current to make the current on the temporary current-carrying branch zero-crossing; the MOV branch is composed of a metal oxide varistor, and is used to absorb energy and establish a transient voltage to transfer the load current to other main branches.

[0014] Preferably, the controllable square-wave voltage source is composed of a trigger branch, a voltage-clamping branch, and a buffer branch connected in parallel. The trigger branch is composed of n switch branches (n = 1, 2, 3...) connected in parallel to realize that the trigger branch continuously switches between the on state and the off state at a certain frequency; the buffer branch is composed of a resistor and a capacitor connected in series. When the trigger branch is turned off, the load current is transferred to the buffer branch to charge the capacitor, so that the voltage across the voltage-clamping branch rises rapidly to the operating voltage of the varistor; the voltage-clamping branch is composed of a metal oxide varistor. When the trigger branch is turned off, the voltage-clamping branch can maintain the output voltage of the controllable square-wave voltage source equal to the residual voltage of the varistor; the i-th switch branch (i = 1, 2, 3..., n) is composed of two IGBT components connected in series with common emitters. The IGBT at the near-bus end is IGBT fi , and the IGBT at the far-bus end is IGBT bi , and each IGBT component is reversely connected in parallel with a diode to realize the bidirectional current flow of the trigger branch; all the IGBTs at the near-bus ends of the n switch branches are IGBT f , and all the IGBTs at the far-bus ends are IGBT b .

[0015] Preferably, the controllable conduction switch can be composed of a thyristor component. When the thyristor component is used to connect the main branch 2, the main branch 3, the main branch 4 and the commutation branch, a bidirectional conduction structure is required; when the thyristor component is used to connect the main branch 1, the main branch 5 and the commutation branch, only a unidirectional conduction structure is required; when the thyristor component is used to connect the main branch B and the commutation branch, a bidirectional conduction structure is required; when the thyristor component is used to connect the main branch A, the main branch C and the commutation branch, only a unidirectional conduction structure is required. The bidirectional conduction structure is two thyristor components reversely connected in parallel. The thyristor component with the conduction direction consistent with the current direction flowing out of the sending / receiving end neutral bus through the commutation branch is the forward thyristor component, and the thyristor component with the conduction direction opposite to the current direction flowing out of the sending / receiving end neutral bus through the commutation branch is the reverse thyristor component; the unidirectional conduction structure is a thyristor component. The thyristor components connecting the main branch 1 and the main branch C are forward thyristor components, and the thyristor components connecting the main branch 5 and the main branch A are reverse thyristor components.

[0016] Preferably, the controllable conduction switch may be composed of a vacuum trigger gap. When the vacuum trigger gap is used to connect the main branch 2, main branch 3, main branch 4 and the commutation branch, a bidirectional conduction structure is required; when the vacuum trigger gap is used to connect the main branch 1, main branch 5 and the commutation branch, only a unidirectional conduction structure is required; when the vacuum trigger gap is used to connect the main branch B and the commutation branch, a bidirectional conduction structure is required; when the vacuum trigger gap is used to connect the main branch A, main branch C and the commutation branch, only a unidirectional conduction structure is required. The bidirectional conduction structure is two vacuum trigger gaps connected in reverse parallel. The vacuum trigger gap with the conduction direction consistent with the current direction flowing out of the sending / receiving end neutral bus through the commutation branch is the forward vacuum trigger gap, and the vacuum trigger gap with the conduction direction opposite to the current direction flowing out of the sending / receiving end neutral bus through the commutation branch is the reverse vacuum trigger gap; the unidirectional conduction structure is a vacuum trigger gap. The vacuum trigger gap connecting the main branch 1 and the main branch C is the forward vacuum trigger gap, and the vacuum trigger gap connecting the main branch 5 and the main branch A is the reverse vacuum trigger gap.

[0017] Therefore, the multi-port DC conversion switch based on a controllable negative pressure source with the above structure according to the present invention has the following beneficial effects: By sharing the commutation branch and the controllable conduction switch, the present invention can transfer the load current of multiple main branches and use the square-wave resonant DC circuit breaker to quickly transfer the load current to other main branches to complete the operation mode switching, reducing the use of high-cost equipment such as high-voltage capacitors and metal lightning arresters, and greatly reducing the cost of the DC conversion switch; the controllable negative pressure source is cut into the commutation branch, greatly increasing the speed of the load current transferring to the commutation branch, reducing the performance requirements for the main branch current-carrying vacuum switch, and reducing the cost of the DC conversion switch; using a vacuum switch instead of a sulfur hexafluoride switch as the main switch of the square-wave resonant DC circuit breaker has the advantage of environmental protection; using a controllable square-wave voltage source to make the oscillating current of the LC branch diverge and oscillate further reduces the cost of high-voltage capacitors and high-voltage reactors.

[0018] The technical solution of the present invention will be further described in detail below with reference to the drawings and embodiments. Description of the Drawings

[0019] Figure 1 It is a schematic diagram of the topological structure of the existing DC conversion switch; Figure 2 It is a schematic diagram of the DC conversion switch configuration scheme in the existing conventional DC power transmission system; Figure 3 It is a schematic diagram of the topological structure of the multi-port DC conversion switch based on a controllable negative pressure source according to the embodiment of the present invention and its connection to the sending / receiving end lines of the conventional DC power transmission system; Figure 4 Schematic diagram of the line connection of the commutation branch for the embodiment of the present invention; Figure 5 Schematic diagram of the line connection of the controllable square-wave voltage source SWO for the embodiment of the present invention. Detailed implementation manners

[0020] The technical solutions of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0021] Unless otherwise defined, the technical terms or scientific terms used in the present invention shall have the ordinary meanings understood by those of ordinary skill in the field to which the present invention belongs. The "first", "second" and similar terms used in the present invention do not denote any order, quantity or importance, but are only used to distinguish different components. The terms such as "comprising" or "including" mean that the elements or objects appearing before this term cover the elements or objects listed after this term and their equivalents, without excluding other elements or objects. The terms such as "connected" or "coupled" are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. The terms such as "upper", "lower", "left" and "right" are only used to represent relative positional relationships, and when the absolute position of the object being described changes, the relative positional relationship may also change accordingly.

[0022] Embodiment 1 As Figure 3 shown, a topology of a multi-port DC conversion switch based on a controllable negative voltage source and its connection method in a conventional DC power transmission system.

[0023] The multi-port DC conversion switch at the sending end includes 5 main branches, a commutation branch and a controllable conduction switch. The main branch 1, main branch 2, main branch 3, main branch 4, and main branch 5 are sequentially connected to the positive rectifier, the grounding electrode, the grounding grid, the positive / negative metal return line, the negative current reverser and the sending-end neutral bus. Each of the 5 main branches consists of a current-carrying vacuum switch. The commutation branch is connected to one end of the main branch via a bus, and the other end is connected via a controllable conduction switch (in the embodiment, the controllable conduction switch is taken as an example of a thyristor component). Since the current directions of the grounding electrode, the grounding grid, and the positive / negative metal return line are not unique, the controllable conduction switches connecting the main branch 2, main branch 3, and main branch 4 are composed of two thyristor components connected in reverse parallel; since the current of the positive rectifier is always a forward current (flowing out of the bus), the controllable conduction switch connected to the main branch 1 is a forward conduction switch; since the current of the negative current reverser is always a reverse current (flowing towards the bus), the controllable conduction switch connected to the main branch 5 is a reverse conduction switch.

[0024] As Figure 4As shown in the figure, the commutation branch consists of a controllable negative pressure source and a square-wave resonant DC circuit breaker. The controllable negative pressure source consists of a pre-charging capacitor C 2 , a current-carrying diode V t and four thyristor assemblies. Among them, thyristor VT 1 and thyristor VT 3 are the thyristor assemblies at the near-bus end, and thyristor VT 2 and thyristor VT 4 are the thyristor assemblies at the far-bus end. If the commutation branch needs to conduct forward current, then trigger thyristor VT 1 and thyristor VT 4 to cut the negative pressure source into the commutation branch; if the commutation branch needs to conduct reverse current, then trigger thyristor VT 2 and thyristor VT 3 to cut the negative pressure source into the commutation branch.

[0025] As Figure 4 shown, the square-wave resonant DC circuit breaker consists of a temporary current-carrying branch, an LC branch, and a MOV branch. The temporary current-carrying branch includes a commutation vacuum switch VCB and a controllable square-wave voltage source SWO; the LC branch includes a high-voltage capacitor C 1 and a high-voltage reactor L; the MOV branch includes a lightning arrester MOV 1 .

[0026] The working process of the square-wave resonant DC circuit breaker is as follows: the load current flows through the temporary current-carrying branch, the VCB opens, the contacts start to separate, and then the controllable square-wave voltage source is selectively triggered according to the direction of the load current so that the square-wave voltage and the LC resonance frequency satisfy the same-frequency / multiple-frequency relationship. An oscillating current starts to be generated in the LC branch and gradually diverges. The oscillating current is superimposed on the temporary current-carrying branch to extinguish the arc between the VCB contacts, and the load current is transferred to the LC branch to charge the capacitor C 1 . The voltage across the MOV branch gradually rises. When the voltage across both ends rises to the operating voltage of MOV 1 , MOV 1 operates to establish a transient voltage and absorb energy, and the load current is transferred to other main branches.

[0027] As Figure 5 shown, the controllable square-wave voltage source SWO consists of a trigger branch, a voltage-clamping branch, and a buffer branch. The trigger branch consists of n switch branches connected in parallel. Each switch branch consists of an IGBT fi and an IGBT bi (i = 1, 2, 3, …, n), and the IGBT fi is reversely paralleled with a diode V fi , and the IGBT bi is reversely paralleled with a diode V bi ; the voltage-clamping branch consists of a lightning arrester MOV 1Composition; The buffer branch is composed of a resistor R and a capacitor C 3 and is formed.

[0028] The working principle of the controllable square-wave voltage source SWO is as follows: By controlling the on and off of the IGBT through the same-frequency / frequency-division control method, the output voltage of the square-wave voltage source U SWO has a multiple-frequency relationship with the resonance frequency of the LC branch. When the IGBT is turned on, the load current flows through the trigger branch, U SWO and is 0; when the IGBT is turned off, the load current transfers to the buffer branch to charge the capacitor C 3 and the voltage across the clamping branch rises. When it rises to the operating voltage of MOV 1 , MOV 1 operates, and the output voltage of the square-wave voltage source U SWO rises to the residual voltage of MOV 1 . U E .

[0029] When the trigger branch is composed of 1 switch branch, the same-frequency / frequency-division control method is achieved by reducing the switching frequency of the IGBT . The specific implementation method is as follows: First, turn off the IGBT 1 and keep it off for 2N - 1 half-waves of the capacitor voltage. Wait until the capacitor voltage reaches the Nth positive peak, then turn on the IGBT 1 and keep it on for 1 half-wave. Wait until the capacitor voltage drops from this positive voltage peak to the negative voltage peak, then turn off the IGBT again 1 , and cycle into the process of turning off the IGBT 1 for 2N - 1 half-waves and turning on for 1 half-wave.

[0030] Embodiment 2 As Figure 3As shown in the figure, the multi-port DC conversion switch at the receiving end includes three main branches, a commutation branch, and a controllable conduction switch. The main branch A, the main branch B, and the main branch C are connected to the positive inverter, the grounding grid, the negative inverter, and the receiving end neutral bus in sequence. Each of the three main branches consists of a current-carrying vacuum switch. The commutation branch is connected to one end of the main branch via a bus, and the other end is connected via a controllable conduction switch (in the embodiment, the controllable conduction switch is taken as an example of a thyristor component). Since the current direction of the grounding grid is not unique, the controllable conduction switch connected to the main branch B consists of two thyristor components connected in reverse parallel; since the current of the positive inverter is always a reverse current (flowing towards the bus), the controllable conduction switch connected to the main branch A is a reverse conduction switch; since the current of the negative inverter is always a forward current (flowing out from the bus), the controllable conduction switch connected to the main branch C is a forward conduction switch. Its working principle is similar to that of the sending end, and the efficient transfer of current is achieved through a controllable negative voltage source and a square wave resonant DC circuit breaker.

[0031] Therefore, the present invention adopts a multi-port DC conversion switch based on a controllable negative voltage source with the above structure, replaces the SF6 switch with a vacuum switch, eliminates greenhouse gas emissions, and conforms to the trend of green energy. The commutation branch and the lightning arrester are shared, reducing the number of core components such as high-voltage capacitors and reactors, and lowering the equipment cost. The controllable square wave voltage source excites the high-frequency oscillation of the LC branch, significantly reducing the volume of the capacitor and inductor; through the controllable negative voltage source and the same-frequency / double-frequency control method, the rapid transfer of current in different directions is achieved, improving the system response speed.

[0032] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that they can still modify or equivalently replace the technical solutions of the present invention, and these modifications or equivalent replacements cannot make the modified technical solutions deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A multi-port DC conversion switch based on a controllable negative pressure source, characterized in that: It comprises X main branches, a commutation branch and X groups of controllable conduction switches, wherein X=5 or 3, each of the X main branches is composed of a flow-through vacuum switch, and the commutation branch is composed of a controllable negative pressure source and a square wave resonant DC circuit breaker connected in series.

2. A multi-port DC conversion switch based on a controllable negative pressure source according to claim 1, characterized in that: The multi-port DC conversion switch at the sending end of a conventional DC transmission system comprises five main branches, which are respectively connected to a sending end positive rectifier, a sending end grounding electrode line, a sending end grounding grid, a positive / negative metal return line, a sending end negative inverter and a sending end neutral bus, and the commutation branch is connected to the sending end neutral bus; the remote bus ends of the five main branches are each connected to the remote bus end of the commutation branch through a group of controllable conduction switches.

3. The multi-port DC conversion switch based on a controllable negative pressure source according to claim 1, characterized in that: The multi-port DC conversion switch at the receiving end of a conventional DC transmission system includes three main branches, which are respectively connected to the receiving end positive rectifier, the receiving end grounding grid, the receiving end negative inverter and the receiving end neutral bus; the commutation branch is connected to the receiving end neutral bus; the remote bus ends of the three main branches are connected to the remote bus ends of the commutation branches through controllable conduction switches.

4. The multi-port DC conversion switch based on a controllable negative pressure source according to claim 1, characterized in that: The controllable negative pressure source comprises two dual thyristor branches connected in parallel and a negative pressure source; One of the dual thyristor branches is a thyristor , The other double thyristor branch is composed of a thyristor , Common anode series composition; The negative pressure source is composed of a pre-charge container and a diode assembly connected in reverse parallel, the negative electrode of the pre-charge container is connected to the common cathode point of the first double thyristor branch, and the positive electrode is connected to the common anode point of the second double thyristor branch.

5. The multi-port DC conversion switch based on a controllable negative pressure source according to claim 1, characterized in that: The square wave resonant DC circuit breaker comprises a temporary current-carrying branch, an LC branch and an MOV branch connected in parallel; The temporary current-passing branch is connected in series with a commutation vacuum switch and a controllable square wave voltage source; The LC branch is a series connection of a reactor and a capacitor; The MOV branch is composed of a metal zinc oxide arrester.

6. The multi-port DC conversion switch based on a controllable negative pressure source according to claim 1, characterized in that: The controllable conduction switch is a thyristor assembly or a vacuum trigger gap; When the current direction of the main branch is not unique, a bidirectional conduction structure is adopted, that is, two thyristor components or vacuum trigger gaps are connected in reverse parallel; When the current direction of the main branch is unique, a unidirectional conduction structure is adopted, that is, a single thyristor component or a vacuum trigger gap.

7. The multi-port DC conversion switch based on a controllable negative pressure source according to claim 5, characterized in that: The controllable square wave voltage source comprises a trigger branch, a clamping branch and a buffer branch connected in parallel; The trigger branch is composed of n switch branches connected in parallel, where n is a positive real number; The buffer branch is composed of a resistor and a capacitor in series; The clamping branch is composed of a metal zinc oxide arrester.

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