Hybrid DC circuit breaker with adaptive reclosing capability and fault clearing judgment method

Abstract

The present invention relates to a hybrid DC circuit breaker with adaptive autoreclosing ability and a fault clearing discrimination method. The circuit breaker is composed of three branches and three devices. The three branches are a current-carrying branch, a transfer branch, and a charge-discharge branch, and the three devices are a metal oxide arrester, a pre-charging capacitor C, and a current-limiting reactor L dc . During the fault isolation period, the present invention inputs the pre-charging capacitor, reduces the operation voltage of the arrester, and thus reduces the number of IGBTs connected in parallel at both ends of the arrester. In addition, by reasonably determining the on-off sequence of each branch and retaining and controlling the energy in the pre-charging capacitor, the adaptive autoreclosing is achieved without reconnecting the converter station and the fault point.

Description

Technical Field

[0001] The present invention belongs to the field of flexible DC power grid protection and control, and in particular relates to a hybrid DC circuit breaker with adaptive reclosing capability and a fault clearing judgment method and system. Background Art

[0002] In recent years, large-scale renewable energy has been integrated into the power grid. However, the traditional AC power grid has limited capacity to absorb large-scale renewable energy, and even "wind and solar power abandonment" may occur. Flexible DC transmission technology based on modular multilevel converters (MMC) can maximize the integration of various renewable energy sources and loads, and is one of the effective means to absorb and connect large-scale renewable energy to the grid.

[0003] Overhead lines with large transmission capacity and low investment cost are the mainstream form of building flexible DC grids. However, the construction of overhead lines requires crossing complex geographical environments, which are easily affected by weather and environment, and have a high risk of failure. The flexible DC grid based on MMC is a "low impedance, low inertia" system. Once a DC fault occurs, the fault current quickly rises to dozens of times the rated current and lacks a natural zero crossing point, causing the problem of difficulty in interrupting the DC fault current. Therefore, it is crucial to study reliable fault clearing technology to cut off the fault current in time.

[0004] The DC circuit breaker is the preferred solution for fault clearing technology. It can cut off the fault line without locking the converter station, ensuring the power transmission of the non-fault line during the fault crossing process. Existing research on DC circuit breakers mainly focuses on three types: mechanical, all-solid-state and hybrid. Among them, the hybrid DC circuit breaker (HCB) takes into account the advantages of low conduction loss of mechanical DC circuit breakers and short breaking time of all-solid-state DC circuit breakers, becoming a fault clearing solution with great development potential. However, the main breaking branch of the traditional hybrid DC circuit breaker requires a large number of insulated gate bipolar transistors (IGBT) to be connected in series, resulting in high economic costs, and most of the existing hybrid DC circuit breakers do not have the ability to distinguish the type of fault.

[0005] In order to further reduce the manufacturing cost of HCB and improve the operational reliability of flexible DC system, the present invention proposes an HCB with adaptive reclosing capability. The HCB uses the cooperation between pre-charge capacitor and transfer branch to provide a zero current and nearly zero voltage disconnection condition for ultra-fast mechanical switch (UFD). The HCB of the present invention uses pre-charge capacitor during fault isolation to reduce the action voltage of the arrester, thereby reducing the number of IGBTs connected in parallel at both ends of the arrester. In addition, the HCB can realize the pre-charging of capacitors and the determination of fault properties by only using existing devices, expanding the functions of DC circuit breakers without additional investment costs. Summary of the invention

[0006] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a hybrid DC circuit breaker with adaptive reclosing capability and a fault clearing judgment method, which can reduce the manufacturing cost of the hybrid DC circuit breaker and avoid the secondary overcurrent and secondary overvoltage problems caused by blind reclosing of the hybrid DC circuit breaker to a permanent fault after fault isolation. The hybrid DC circuit breaker of the present invention uses a pre-charge capacitor during fault isolation to reduce the operating voltage of the lightning arrester, thereby reducing the number of IGBTs connected in parallel at both ends of the lightning arrester. In addition, by reasonably determining the on-off sequence of each branch, retaining and controlling the energy in the pre-charge capacitor, adaptive reclosing is achieved without reconnecting the converter station and the fault point.

[0007] The present invention solves the technical problem by the following technical solutions:

[0008] A hybrid DC circuit breaker with adaptive reclosing capability, characterized in that: the topological structure of the hybrid DC circuit breaker is composed of three branches and three devices;

[0009] The three branches are:

[0010] The current-carrying branch includes an ultra-fast switch UFD connected in series and an LCS composed of an IGBT connected in anti-series;

[0011] The transfer branch includes an IGBT switch group Q, an ultra-fast switch UFD1 and a thyristor valve group T1, wherein the IGBT switch group Q and the ultra-fast switch UFD1 are connected to the same branch, and the thyristor valve group T1 is connected to another branch;

[0012] The charging and discharging branch includes a thyristor valve group T2, a buffer inductor L2 and a buffer resistor R2. The thyristor valve group T2, the buffer inductor L2 and the buffer resistor R2 are arranged in series, and the three are connected in series in parallel at both ends of the thyristor valve group T1.

[0013] The three devices are:

[0014] Current limiting reactor L dc , one end is connected to the node between the through-flow branch and the transfer branch, and the other end is directly connected to the DC line;

[0015] A metallic oxide arrester MOA is connected in parallel at both ends of the IGBT switch group Q in the transfer branch;

[0016] A pre-charge capacitor C, one end of which is connected to the node between the transfer branch and the charge and discharge branch, and the other end of which is directly grounded;

[0017] The adaptive reclosing capability is achieved by conducting the charging and discharging branches after the fault is isolated and the line is de-ionized, detecting the capacitor current and comparing it with the set current threshold, thereby determining the fault type and performing reclosing or power outage maintenance operations based on the determination result.

[0018] A method for determining fault clearing of a hybrid DC circuit breaker with adaptive reclosing capability, characterized in that the method comprises the following steps:

[0019] At time t0, a single-pole grounding short circuit fault occurs in the system. The fault current flows through the current-carrying branch and rises rapidly. After a delay, the fault current amplitude reaches the protection device start-up value.

[0020] At time t1, the protection device detects a fault and immediately turns on the IGBT switch group Q and thyristor valve group T1 of the transfer branch. After a period of time, the IGBT in the LCS completes the current interruption, providing zero current and zero voltage interruption conditions for the UFD. The fault current is commutated to the transfer branch, controlling the UFD to start the trip.

[0021] At t2, UFD completes the opening action and applies a shut-off signal to Q in the transfer branch. The voltage across the arrester MOA rises rapidly to the starting voltage, the fault current flows through the arrester MOA, and the arrester MOA starts to discharge energy.

[0022] At t3, the energy is discharged, the fault current drops to zero, and UFD1 is controlled to open, completing the opening action and achieving physical isolation between the converter station and the faulty line. Since the fault point has not disappeared, the pre-charged capacitor C continues to discharge to the fault point.

[0023] At time t4, the pre-charge capacitor C reaches the maximum reverse voltage, the current on the fault line drops to zero, and the thyristor valve group T1 is naturally turned off, realizing fault clearing;

[0024] At time t5, the fault is isolated and the fault line is de-ionized, the thyristor valve group T2 is turned on, the reclosing current is detected and compared with the set current threshold to determine the fault type.

[0025] The advantages and beneficial effects of the present invention are:

[0026] 1. The hybrid DC circuit breaker with adaptive reclosing capability and the fault clearing judgment method of the present invention puts into use a pre-charge capacitor during fault isolation to actively clamp the DC side voltage and limit the peak value of the fault current. The presence of the pre-charge capacitor can reduce the action voltage of the MOA and transfer energy with the current limiting reactor, greatly reducing the energy dissipation demand of the lightning arrester.

[0027] 2. The hybrid DC circuit breaker and fault clearing judgment method with adaptive reclosing capability of the present invention retains and controls the energy in the pre-charged capacitor by reasonably determining the on-off sequence of each branch, thereby realizing adaptive reclosing without reconnecting the converter station and the fault point. In addition, the pre-charged capacitor is self-charged by the DC power grid, and there is no need to configure a separate charging device, which will not increase the investment cost.

[0028] 3. The hybrid DC circuit breaker of the present invention has adaptive reclosing capability and a fault clearing judgment method. The hybrid DC circuit breaker of the present invention greatly reduces the number of fully controlled IGBT devices used and has clear control logic and simple principles. It has certain advantages in economy and breaking performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a diagram of a four-terminal flexible DC transmission system;

[0030] Figure 2 It is a topological structure diagram of a DC circuit breaker of the present invention;

[0031] Figure 3 It is an equivalent circuit diagram of the DC circuit breaker in the normal operation stage of the present invention;

[0032] Figure 4 It is an equivalent circuit diagram of the DC circuit breaker fault detection stage of the present invention;

[0033] Figure 5 It is an equivalent circuit diagram of the DC circuit breaker fault isolation stage of the present invention;

[0034] Figure 6 It is an equivalent circuit diagram of the fault nature determination stage of the DC circuit breaker under permanent fault of the present invention;

[0035] Figure 7 This is a value analysis diagram of the buffer resistor of the DC circuit breaker of the present invention. DETAILED DESCRIPTION

[0036] The present invention is further described in detail below through specific examples. The following examples are only illustrative and not restrictive, and the protection scope of the present invention cannot be limited thereto.

[0037] Figure 1 It is a ±500kV four-terminal flexible DC transmission system, in which the hybrid DC circuit breaker of the present invention is installed on each DC line in the system. The topological structure of the hybrid DC circuit breaker with adaptive reclosing capability and the fault clearing judgment method of the present invention is first given below, and then the working principle of the hybrid DC circuit breaker of the present invention in the whole process from fault occurrence to fault clearing in the four-terminal flexible DC transmission system is analyzed, and finally the parameter design and required quantity of the circuit breaker components are analyzed.

[0038] 1. Hybrid DC circuit breaker topology

[0039] The topological structure of the hybrid DC circuit breaker of the present invention is as follows: Figure 2 As shown in the figure, the DC circuit breaker is mainly composed of three branches and three devices. The three branches are the flow branch, the transfer branch and the charge and discharge branch, and the three devices are the metal oxide surge arrester (MOA), the pre-charge capacitor C and the current limiting reactor L. dc The flow branch is composed of ultra-fast mechanical switch UFD1 and LCS in series. Among them, LCS is composed of IGBT modules in reverse series. The transfer branch is composed of main disconnect switch Q, ultra-fast mechanical switch UFD2 and thyristor T1. Among them, the main disconnect switch Q is composed of IGBT modules in series. The charge and discharge branch is composed of thyristor T2, buffer inductor L c and snubber resistor R c constitute.

[0040] 2. Working principle of hybrid DC circuit breaker

[0041] The working principle of the hybrid DC circuit breaker during normal system operation and fault period is introduced respectively. During normal system operation, LCS and UFD1 in the current branch are in the on state.

[0042] In addition, the HCB of the present invention can use the DC system to charge the pre-charge capacitor C, without the need to equip the capacitor with a separate charging device. The working principle of the charging stage is: during the normal operation of the system, the thyristor T2 is triggered, and the DC system is connected to the buffer inductor L c and snubber resistor R c Charge the pre-charge capacitor C. When the capacitor voltage rises to the rated voltage of the system, the charging current drops to zero, the thyristor T2 is naturally turned off, and the charging process ends. After charging is completed, close the ultra-fast mechanical switch UFD2 to prepare for fault isolation. The equivalent circuit during normal operation is as follows Figure 3 As shown. Among them, U dc is the system DC voltage, R L is the line resistance, L L and L dcare line reactance and current limiting reactance respectively. c Perform analytical calculation and assume that the voltage of the pre-charge capacitor C is u c , according to the voltage and current constraint relationship between KVL and the capacitor element:

[0043]

[0044] The solution is:

[0045] i c =D1e αt cos(βt)+D2e αt sin(βt) (2)

[0046] in:

[0047]

[0048] From equations (2) and (3), we can see that the charging current i c The magnitude and charging speed of the buffer inductor L c and snubber resistor R c The influence of c and R c The parameters can limit the amplitude of the charging current and avoid a large impact on the DC system.

[0049] Taking a single-pole ground short circuit fault as an example, the working principle of the hybrid DC circuit breaker during a fault is introduced. During a fault, the working process of the HCB of the present invention is mainly divided into a fault detection stage, a fault isolation stage and a fault nature determination stage.

[0050] The following will introduce the working principle of the HCB of the present invention during the fault period in combination with these three stages:

[0051] 1) Fault detection stage

[0052] At time t0, a single-pole grounding short circuit fault occurs in the system, and the fault current flows through the current-carrying branch and continues to rise.

[0053] After a delay of Δt0 (fault detection time, generally 3ms), the protection device detects the fault at time t1.

[0054] The equivalent circuit of the fault detection stage is as follows: Figure 4 In order to simplify the calculation process, the switching characteristics of the MMC submodule capacitor during fault isolation are ignored, and the MMC is equivalent to a capacitor with an amplitude of U dc DC voltage source, equivalent inductance L s And the equivalent resistance R s According to KVL:

[0055]

[0056] The solution is:

[0057]

[0058] In the formula, R eq =R s +R L , L eq =L s +L dc +L L ,i dc (t 0_ ) is the steady-state current during the normal operation of the system. From formula (5), we can see that during the fault period, the current limiting reactor L dc The existence of limits the rising speed of the fault current.

[0059] 2) Fault isolation stage

[0060] At t1, the main disconnect switch Q and thyristor T1 of the transfer branch are immediately turned on to provide zero voltage shutdown conditions for the LCS in the current-carrying branch. At t1", the LCS completes current interruption, and the fault current is commutated to the transfer branch. At this time, a trip signal is applied to UFD1. During the UFD1 tripping period, since the conduction voltage drop of the main disconnect switch Q and thyristor T1 is very small, and the two ends of UFD1 only bear part of the conduction voltage drop, the voltage across UFD1 is almost zero, thereby ensuring that the ultra-fast mechanical switch UFD1 is arc-free under zero current and nearly zero voltage conditions. Since the triggering and shutdown speed of the IGBT is very fast, only tens of us, in order to simplify the calculation process, it is assumed in the theoretical analysis that t1=t1'=t1". After a delay of Δt1 (the ultra-fast mechanical switch tripping time, generally 2ms), UFD1 completes the tripping action at t2 and reaches its rated opening distance. At this time, a shutdown signal is applied to the main disconnect switch Q in the transfer branch, and the voltage across the MOA rises rapidly. After reaching the operating voltage, the MOA is forced to connect to the fault circuit and start to discharge the remaining energy. After the energy discharge delay of Δt2, the MOA current is reduced to zero, and UFD2 is controlled to open. At t3, UFD2 completes the opening operation and reaches its rated opening distance, realizing the physical isolation of the converter station and the fault line. During the entire fault isolation period, the pre-charged capacitor C forms a discharge circuit with the fault point through the current limiting reactor.

[0061] The equivalent circuit of the fault isolation stage is as follows: Figure 5 As shown, switch S1 is equivalent to the breaking operation of the main disconnect switch Q, and switch S2 is equivalent to the breaking operation of the ultra-fast mechanical switch UFD2.

[0062] Since the fault current flow paths are different during t1-t2 and t2-t3, the solutions are performed for these two stages separately.

[0063] During t1-t2, the fault current flow path is as follows Figure 5 As shown by the red dotted line in the middle. According to the voltage and current constraints of KVL, KCL and capacitor elements, we get:

[0064]

[0065] During t2-t3, MOA is forced to connect to the fault circuit, and MOA is equivalent to a DC voltage source U MOA , the fault current flow path is as follows Figure 5 As shown by the green line in the middle. According to the voltage and current constraints of KVL, KCL and capacitor elements, we get:

[0066]

[0067] It is difficult to directly obtain analytical solutions for equations (6) and (7). The commonly used method for solving nonlinear ordinary differential equations is the fourth-order Runge-Kutta method, which has high accuracy and stability. Therefore, this paper uses the fourth-order Runge-Kutta method to solve u c 、i dc and i c The numerical solution of . From the numerical solution results, we can know that: u c and U MOA Influence of fault current i dc The amplitude and decay rate, L dc and C affect the pre-charge capacitor voltage u c The decay rate.

[0068] 3) Fault nature determination stage

[0069] At time t3, the capacitor energy continues to transfer to the current limiting reactor, and the fault line current continues to rise. According to the KVL and capacitor element voltage and current constraints, we get:

[0070]

[0071] Solving for the capacitor current i c for:

[0072]

[0073] in:

[0074]

[0075] After the energy transfer delay of Δt3, the current limiting reactor L dcAll the energy in the circuit is transferred to the pre-charge capacitor C. The pre-charge capacitor C reaches the maximum reverse voltage at time t4, the current in the discharge circuit drops to zero, and the thyristor T1 is naturally turned off, thus clearing the fault. In order to avoid blindly reclosing on a permanent fault during reclosing, the nature of the fault should be determined after the fault is isolated and the fault line is de-ionized. After a delay of Δt4 (the time for the fault line to be de-ionized, generally 300ms after the fault is isolated), the fault line is de-ionized. At time t5, turn on the thyristor T2, and use the difference in the current amplitude in the charging and discharging branch under permanent faults and transient faults to determine the nature of the fault. If the fault is a transient fault, the fault point disappears, and the reverse discharge circuit of the pre-charge capacitor C cannot be formed. The reclosing current i hz Almost zero; if the fault is a permanent fault, the fault point exists, and the pre-charge capacitor C discharges to the fault point. The equivalent circuit is as follows Figure 6 shown.

[0076] Under permanent fault, the pre-charge capacitor voltage u c and reclosing current i hz for:

[0077]

[0078] in:

[0079]

[0080] From equations (11) and (12), it can be seen that the peak value of the reclosing current and the forward voltage of the pre-charge capacitor depend on the buffer inductance L c , buffer resistor R c and the size of the pre-charge capacitor C. At time t6, the fault is repaired or the normal operation is restored according to the judgment result.

[0081] 3. Parameter design and quantity design of hybrid DC circuit breaker

[0082] 3.1 Parameter design of the circuit breaker of the present invention

[0083] 1) Current limiting reactor L dc

[0084] From formula (5), we can see that the fault current reaches its peak value at time t1, and its climbing speed is proportional to L dc In addition, it can be seen from formula (11) that in the fault nature judgment stage, the peak value and climbing speed of the reclosing current are also inversely proportional to L dc is inversely proportional to the size.

[0085] However, in actual engineering, high inductance reactors are not only difficult to manufacture but also reduce the dynamic performance of the DC system. The inductance value cannot be increased infinitely. Therefore, referring to the design of the ±500kV flexible DC project in Zhangbei, my country, Ldc Take it as 150mH.

[0086] 2) Pre-charge capacitor C

[0087] When the fault current is commutated to the transfer branch, the pre-charge capacitor C can clamp the line side voltage and reduce the voltage difference between the converter station and the line side, so that the fault current shows a downward trend. In order to achieve reliable soft shutdown of the fault current, during the period when the fault current flows through the MOA, the voltage u on the pre-charge capacitor C c The following formula must be satisfied:

[0088] u c >U dc -U MOA (13)

[0089] From equations (6) and (7), we can see that the voltage u on the pre-charge capacitor C is c It is related to the capacitance value. When the capacitance value is large, u c The falling speed is slow, and the arrester action voltage U MOA Smaller; when the capacitance value is small, u c The falling speed is fast, and the arrester action voltage U MOA Since the main disconnect switch Q of the transfer branch is connected in parallel with the arrester, both ends of the main disconnect switch Q need to withstand the arrester action voltage U MOA Therefore, a larger pre-charge capacitor value helps to reduce the withstand voltage requirement of the main disconnect switch Q.

[0090] In addition, after the fault line is isolated, the pre-charge capacitor C continues to discharge to the fault point, and the line-side fault current continues to increase. From formula (9), it can be seen that the peak value of the line-side fault current is closely related to the pre-charge capacitor value, and its peak value is not allowed to exceed the maximum line current I dc_max , must meet the following requirements:

[0091]

[0092] in:

[0093]

[0094] The upper limit of the pre-charge capacitance value can be determined by formula (12).

[0095] In summary, the selection of the pre-charge capacitor value needs to comprehensively consider the withstand voltage level of the switch group Q and the magnitude of the line-side fault current. The present invention recommends that the pre-charge capacitor C be 350 μF.

[0096] 3) Buffer resistor R c With snubber inductor L c

[0097] During the charging phase, the pre-charge capacitor C and the buffer resistor R c , snubber inductor L c A second-order circuit is formed. During charging, only the thyristor T2 is turned on. In order to charge the pre-charge capacitor C to the rated voltage of the system, the second-order circuit needs to maintain an overdamped state, that is:

[0098]

[0099] In the fault identification stage, if the fault is a permanent fault, the pre-charge capacitor C is connected to the buffer resistor R c With snubber inductor L c Discharge to the fault point. From equations (11) and (12), it can be seen that the buffer resistor R c With snubber inductor L c The value of affects the peak value of the reclosing current and the amplitude of the forward voltage on the pre-charge capacitor C after the fault judgment stage. c and L c The influence of the peak value of the reclosing current and the forward voltage amplitude on the pre-charge capacitor C after the fault judgment stage is completed. Here, L is temporarily set c is 0.1H. Since the pre-charge capacitor C is 350μF, according to formula (16), the buffer resistor R c The value must be greater than 33Ω. Figure 7 Given the snubber resistor R c The reclosing current and pre-charge capacitor voltage waveforms are shown in Figure 3 when the resistance is 35Ω, 40Ω, and 45Ω. Figure 7 It can be seen that the buffer resistor R c The larger the value, the smaller the peak value of the reclosing current flowing through the thyristor T2, which helps to reduce the overcurrent tolerance of the thyristor T2. c The larger the value, the smaller the amplitude of the forward voltage on the pre-charge capacitor after the fault judgment stage. It takes longer time to charge the pre-charge capacitor to the rated voltage of the system when dealing with the next fault. Therefore, considering the limitations of overcurrent tolerance and recovery time, the buffer resistance value should not be too large. The present invention recommends that the buffer resistance R c The value is 40Ω.

[0100] Determine the snubber resistor R c After the parameters are determined, the snubber inductance L can be determined according to formula (16): c The selection of specific parameters can refer to the above method, and the analysis will not be repeated here.

[0101] 3.2 Design of the number of circuit breakers of the present invention

[0102] 1) Number of flow branch elements

[0103] The flow branch is composed of a load transfer switch LCS and an ultra-fast mechanical switch UFD1. When the system is operating normally, the IGBT in the LCS element is in the on state, and the voltage on the IGBT is zero. When the protection detects a fault, the main disconnect switch Q and thyristor T1 of the transfer branch are immediately turned on and the LCS is turned off. The voltage across the flow branch is almost zero, so there is no need to connect too many IGBTs in series in the LCS. According to the above analysis, assuming that the rated voltage and rated current of the IGBT in the LCS are U IGBT ,I IGBT The maximum voltage and current on LCS are U IGBT_max ,I IGBT_max Considering that LCS needs to have bidirectional current flow capability, the number of IGBTs required in LCS is:

[0104]

[0105] 2) Number of transfer branch components

[0106] The transfer branch consists of the main disconnect switch Q, the ultra-fast mechanical switch UFD2 and the thyristor T1. When the system is operating normally, the main disconnect switch Q is in the disconnected state, and both ends of Q are subjected to the difference between the single-pole rated voltage and the voltage on the pre-charge capacitor C. Since the ultra-fast mechanical switch UFD2 is turned on only after the pre-charge capacitor C is charged to the rated voltage of the system, the pressure at both ends of the main disconnect switch Q is almost zero during normal operation. When the protection detects a fault, the main disconnect switch Q is turned on and the pressure at both ends is zero; when UFD1 completes the opening action and reaches the rated opening distance, the main disconnect switch Q is turned off, and both ends of Q are subjected to the arrester action voltage U MOA According to the above analysis, assuming that the rated voltage and rated current of the IGBT in the main disconnect switch Q are U Q ,I Q , the maximum current flowing through the main disconnect switch Q is I Q_max Considering that the main disconnect switch Q does not need to have bidirectional current flow capability, the number of IGBTs required in the main disconnect switch Q is:

[0107]

[0108] When the system is operating normally, in order to charge the pre-charge capacitor C, it is necessary to turn on the thyristor T2. The two ends of the thyristor T1 are subjected to the voltage on the charge and discharge branch, that is, the buffer resistor R c and snubber inductor L cvoltage on the pre-charge capacitor; when the pre-charge capacitor is charged to the rated voltage of the system, the thyristor T2 is naturally turned off, and the difference between the unipolar rated voltage across the thyristor T1 and the voltage on the pre-charge capacitor C is zero; when the protection detects a fault, the thyristor T1 is turned on, and the pressure across T1 is zero; when the fault line is isolated, the pre-charge capacitor C continues to discharge to the fault point until the discharge loop current drops to zero, and the thyristor T1 is naturally turned off. After T1 is turned off, it needs to withstand the maximum reverse voltage U provided by the pre-charge capacitor after being reversely charged. cb_max According to the above analysis, assuming that the rated voltage and rated current of thyristor T1 are U T1 ,I T1 , the maximum current flowing through T1 is I T1_max , then the number of thyristors T1 required is:

[0109]

[0110] 3) Number of charge and discharge branch components

[0111] The charging and discharging branch consists of thyristor T2 and buffer inductor L c and snubber resistor R c Since the charging and discharging branch is connected in parallel with the thyristor T1, when the thyristor T2 is in the off state, T2 is subjected to the voltage across the thyristor T1, that is, the maximum reverse voltage U of the pre-charge capacitor C. cb_max ; When the thyristor T2 is in the on state, the voltage across T2 is zero. Based on the above analysis, assuming that the rated voltage and rated current of thyristor T2 are U T2 ,I T2 , the maximum current flowing through T2 is I T2_max , then the number of thyristors T2 required is:

[0112]

[0113] Although the embodiments and drawings of the present invention are disclosed for illustrative purposes, those skilled in the art will appreciate that various substitutions, changes and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the scope of the present invention is not limited to the contents disclosed in the embodiments and drawings.

Claims

1. A hybrid DC circuit breaker with adaptive reclosing capability, characterized in that: The topological structure of the hybrid DC circuit breaker consists of three branches and three devices; The three branches are: The current-carrying branch includes an ultra-fast switch UFD connected in series and an LCS composed of an IGBT connected in anti-series; The transfer branch includes an IGBT switch group Q, an ultra-fast switch UFD1 and a thyristor valve group T1, wherein the IGBT switch group Q and the ultra-fast switch UFD1 are connected to the same branch, and the thyristor valve group T1 is connected to another branch; The charging and discharging branch includes a thyristor valve group T2, a buffer inductor L2 and a buffer resistor R2. The thyristor valve group T2, the buffer inductor L2 and the buffer resistor R2 are arranged in series, and the three are connected in series in parallel at both ends of the thyristor valve group T1. The three devices are: Current limiting reactor L dc , one end is connected to the node between the through-flow branch and the transfer branch, and the other end is directly connected to the DC line; A metallic oxide arrester MOA is connected in parallel at both ends of the IGBT switch group Q in the transfer branch; A pre-charge capacitor C, one end of which is connected to the node between the transfer branch and the charge and discharge branch, and the other end of which is directly grounded; The adaptive reclosing capability is achieved by conducting the charging and discharging branches after the fault is isolated and the line is de-ionized, detecting the capacitor current and comparing it with the set current threshold, thereby determining the fault type and performing reclosing or power outage maintenance operations based on the determination result.

2. The fault clearing determination method for a hybrid DC circuit breaker with adaptive reclosing capability according to claim 1, characterized in that: The method comprises the following steps: At time t0, a single-pole grounding short circuit fault occurs in the system. The fault current flows through the current-carrying branch and rises rapidly. After a delay, the fault current amplitude reaches the protection device start-up value. At time t1, the protection device detects a fault and immediately turns on the IGBT switch group Q and thyristor valve group T1 of the transfer branch. After a period of time, the IGBT in the LCS completes the current interruption, providing zero current and zero voltage interruption conditions for the UFD. The fault current is commutated to the transfer branch, controlling the UFD to start the trip. At t2, UFD completes the opening action and applies a shut-off signal to Q in the transfer branch. The voltage across the arrester MOA rises rapidly to the starting voltage, the fault current flows through the arrester MOA, and the arrester MOA starts to discharge energy. At t3, the energy is discharged, the fault current drops to zero, and UFD1 is controlled to open, completing the opening action and achieving physical isolation between the converter station and the faulty line. Since the fault point has not disappeared, the pre-charged capacitor C continues to discharge to the fault point. At time t4, the pre-charge capacitor C reaches the maximum reverse voltage, the current on the fault line drops to zero, and the thyristor valve group T1 is naturally turned off, realizing fault clearing; At time t5, the fault is isolated and the fault line is de-ionized, the thyristor valve group T2 is turned on, the reclosing current is detected and compared with the set current threshold to determine the fault type.

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