An adaptive reclosing method based on improved dc circuit breaker active injection

Abstract

The application provides a kind of self-adaptive reclosing method based on improved DC circuit breaker active injection, it is related to the technical field of self-adaptive reclosing of flexible DC transmission line, and the method comprises the following steps: first, the structural characteristics of the improved DC circuit breaker are analyzed.In the fault nature discrimination stage, by putting in the charge-discharge branch in the improved DC circuit breaker, to establish the equivalent equation of discharge circuit, it is obtained that the energy of energy storage capacitor greatly attenuates at the initial stage of charge-discharge branch under permanent fault, and then the attenuation amplitude is small and slow discharge.While under transient fault, the energy waveform of energy storage capacitor basically does not attenuate and remains at a high value.Using the different attenuation characteristics of energy waveform to construct energy integral criterion to identify the fault nature of flexible DC transmission line.The application realizes the function of identifying the fault nature of flexible DC transmission line, and effectively improves the reclosing success rate of the line.

Description

Technical Field

[0001] This invention belongs to the field of DC power transmission technology and relates to an adaptive reclosing method based on active injection of an improved DC circuit breaker. Background Technology

[0002] my country's large-scale clean energy power generation bases are mainly distributed in the Northwest, North China, and Northeast regions. However, these regions have low population density and low industrial energy consumption. In contrast, the Central and Eastern regions are densely populated with strong industrial electricity demand, yet face energy shortages. Due to the significant dynamic characteristics of clean energy power generation, this instability poses challenges to grid connection. Using traditional AC or DC transmission technologies for grid connection easily leads to problems such as degraded power quality and frequent fluctuations, failing to effectively address the uncertainties of clean energy. Therefore, flexible DC transmission technology based on MMC (Multi-Mode Control) is gradually becoming the optimal technical solution for solving the grid connection problems of new energy sources and cross-regional power transmission. Because flexible DC transmission systems contain a large number of fully controlled electronic components, and the natural frequency of DC lines approaches zero, the entire transmission system exhibits low inertia. When using overhead lines for cross-regional power transmission, the probability of failure is far higher than that of cable lines due to the long-term exposure to the atmosphere. When a DC line fault occurs, the capacitor modules in the MMC converter discharge rapidly. Due to their low inertia, the current at the fault point can reach tens of kiloamperes within milliseconds. To protect the fully controlled electronic components from damage, the DC circuit breaker quickly interrupts the fault current within those milliseconds. However, during the fault recovery phase, using a traditional automatic reclosing scheme can cause a secondary impact on the DC system if the reclosing occurs on a permanent fault, threatening the system's operational safety. Therefore, it is necessary to design an adaptive reclosing scheme that identifies the nature of the fault before reclosing. Summary of the Invention

[0003] To address the aforementioned problems, this invention proposes an adaptive reclosing method based on active injection from an improved DC circuit breaker, which effectively solves the problems in the prior art.

[0004] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0005] An adaptive reclosing method based on active injection from an improved DC circuit breaker includes the following steps:

[0006] Step S1: When a fault occurs in a DC transmission line, the DC circuit breakers on both sides of the line are controlled to disconnect the fault current.

[0007] Step S2: After fault isolation, inject preset energy into the faulty line through the energy injection unit of the DC circuit breaker;

[0008] Step S3: Monitor the electrical response characteristics of the line after energy injection;

[0009] Step S4: Determine the nature of the fault based on the difference in the electrical response characteristics, and decide whether to perform reclosing based on the determination result.

[0010] Preferably, step S1 specifically includes:

[0011] Using an improved hybrid DC circuit breaker topology in a symmetrical monopole DC transmission system:

[0012] The improved hybrid DC circuit breaker topology includes a current-carrying branch, a transfer branch, a charge-discharge branch, and a metal oxide surge arrester;

[0013] The current-carrying branch includes two components connected in series: an ultra-fast mechanical switch UFD1 and a switch LCS composed of fully controlled devices; wherein, the LCS is an IGBT module structure connected in reverse series.

[0014] The transfer branch consists of a fully controlled switch Q and an ultra-fast mechanical switch UFD2, with the ultra-fast mechanical switch UFD2 initially in a normally open state.

[0015] The charging / discharging branch includes a buffer inductor L. a Energy storage capacitor C b Auxiliary switch K1, charging / discharging capacitor C a And anti-parallel thyristors T1 / T2.

[0016] Preferably, step S2 specifically includes:

[0017] When a fault current causes a disconnection, the charging and discharging capacitors in the charging and discharging branch have already been charged under normal system operating conditions.

[0018] Preferably, step S3 specifically includes:

[0019] During the energy injection process to determine the nature of the fault, the voltage is sampled, and the fault circuit discharge equation is established based on the constructed injection circuit. Different energy values ​​are obtained under permanent faults and transient faults.

[0020] Preferably, step S4 specifically includes:

[0021] In the early stage of discharge, the energy of the energy storage capacitor is fitted by the definite integral method to calculate the integral value of the energy storage capacitor. By reasonably setting the setting value, the nature of the fault in the flexible DC transmission line can be distinguished.

[0022] Preferably, in step S2:

[0023] Step S21: When no fault occurs, the improved DC circuit breaker will not operate and the system will operate normally. Under normal system operation, the charging and discharging branches will continue to be charged. When a short circuit fault is confirmed to occur, all the improved DC circuit breakers configured on both sides of the symmetrical single-pole flexible DC transmission system will operate, disconnect the fault current, and wait for the surge arresters in the improved DC circuit breakers HCB1 and HCB2 to absorb energy and complete the energy discharge.

[0024] Step S22: For a single-pole grounding fault, close switch K1 and turn on conductor T2 in HCB1 to form a single-pole discharge circuit with the grounding point to provide a stable electrical quantity to the fault point. For an inter-pole short-circuit fault, add a grounding switch K on the output side of the DC circuit breaker HCB2 of the negative line. a During an inter-pole short-circuit fault, K is closed. a Construct a complete fault current path.

[0025] Preferably, step S3 specifically comprises:

[0026] By modeling different discharge circuits using Kirchhoff's voltage and current laws, the following discharge circuit equations are obtained:

[0027] For a permanent unipolar ground fault, the energy storage capacitor C b The equation is:

[0028]

[0029] Where A=C a C b L a L eq B=C a C b L a R eq C=(C a +C b )L eq +L a C a D=(C a +C b )R eq U a (0 - ) is the capacitor C a The initial energy storage voltage;

[0030]

[0031]

[0032]

[0033]

[0034] In the above formula, C a C b and L a R represents the capacitor and inductor in the charging and discharging branch. l L eq The equivalent resistance and inductance of the line under positive ground fault conditions, R eq From the line equivalent resistance R l and transition resistance R f Composition; U a For the capacitor C in the charging and discharging branch a The initial voltage value, the electrical quantity is determined only by C. a Provided; k1, k2, k3, and k4 are undetermined coefficients, which are solved using the reduction method; , These are the voltage attenuation factors; , These are the two voltage oscillation angular frequencies in the discharge circuit;

[0035] For transient faults, the energy storage capacitor C b The equations are all:

[0036]

[0037] in,

[0038] .

[0039] Preferably, step S4 specifically comprises:

[0040] During transient faults, the energy storage capacitor exhibits constant-amplitude oscillations and the energy consumption is at its lowest, resulting in the highest corresponding integral value.

[0041] During a permanent fault, the increase in line resistance and transition resistance accelerates the rate and magnitude of energy loss, resulting in a decrease in the corresponding integral value. The setting value is set higher than the calculated value when a metallic ground fault occurs at the end of a single pole of the line, but lower than the calculated value under fault-free conditions; the criterion is:

[0042]

[0043] In the formula: t1 represents the connection time of the charging and discharging branch in the improved DC circuit breaker; Δt represents the integration time window, the range of which should be selected as a time window in which the capacitor energy has decayed slowly under a single-pole ground fault; E h E g These represent the integral values ​​calculated when the fault disappears and when a metallic grounding fault occurs at the end of a single pole of the line, respectively; F represents the energy integral setting value, the magnitude of which can be set to E. h +E gHalf of the sum; F can be expressed as:

[0044]

[0045] By setting the setting value, the nature of faults in flexible DC transmission lines can be effectively distinguished.

[0046] Compared with the prior art, the present invention has the following beneficial effects:

[0047] 1. This invention improves the topology of a hybrid DC circuit breaker by connecting an ultra-fast mechanical switch in series and a charging / discharging branch consisting of a charging / discharging capacitor, an energy storage capacitor, and an inductor in parallel on the original DC circuit breaker's transfer branch. This enables active energy injection in DC transmission line scenarios and accurately identifies the nature of flexible DC line faults by constructing an energy integration criterion based on the initial energy of the energy storage capacitor.

[0048] 2. Compared to existing technologies that utilize converters for energy injection, the solution of this invention requires only a simple modification to the existing hybrid DC circuit breaker topology, resulting in extremely low modification costs. This invention effectively overcomes the questionable feasibility of energy injection using MMC converters.

[0049] 3. The present invention is simple to operate, requires no external power supply, and uses the system itself to charge and discharge the branch circuit. It is easy to design and can repeatedly determine the nature of the fault. Attached Figure Description

[0050] Figure 1 A diagram of the existing hybrid DC circuit breaker topology;

[0051] Figure 2 This is a topology diagram of an improved hybrid DC circuit breaker;

[0052] Figure 3 The current flow path in the improved DC circuit breaker during normal system operation;

[0053] Figure 4 A diagram showing the operating status of the current-carrying branch in an improved DC circuit breaker during fault isolation.

[0054] Figure 5 A diagram showing the operating status of the transfer branch in an improved DC circuit breaker during fault isolation.

[0055] Figure 6 The discharge current flow diagram for the charging and discharging branch;

[0056] Figure 7 This is a structural diagram of a half-bridge MMC symmetrical single-pole power transmission system;

[0057] Figure 8A schematic diagram of the adaptive reclosing process for flexible DC transmission lines;

[0058] Figure 9 The current flow diagram and equivalent circuit diagram for a permanent unipolar ground fault discharge circuit;

[0059] Figure 10 For complex frequency domain circuits under permanent unipolar ground faults;

[0060] Figure 11 The current flow diagram and equivalent circuit diagram of the discharge circuit under a permanent indirect ground fault;

[0061] Figure 12 This is the equivalent circuit diagram of the discharge circuit under transient conditions.

[0062] Figure 13 This is a comparison of the energy waveforms of the energy storage capacitor under permanent and transient faults. Detailed Implementation

[0063] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0064] The following is in conjunction with the appendix Figures 1 to 13 The specific embodiments of the present invention will be described in further detail below.

[0065] Improved hybrid DC circuit breaker topology, such as Figure 1 The diagram shows a hybrid DC circuit breaker topology in the prior art, with three core components: a current-carrying branch, a transfer branch, and an energy-dissipating branch. The current-carrying branch consists of a UFD1 connected in series with an LCS: the former is an ultra-fast mechanical switch, and the latter is a fully controlled device switch composed of reverse-connected IGBT modules. The transfer branch is composed of a fully controlled electronic device Q. The energy-dissipating branch consists of a metal oxide surge arrester. During a fault, the short-circuit current is attenuated by gradually engaging the surge arrester. To achieve adaptive reclosing using circuit breakers in a symmetrical unipolar DC transmission system, this invention improves the existing hybrid DC circuit breaker topology as follows: [Example of improvements would be inserted here]. Figure 2 As shown:

[0066] This improved hybrid DC circuit breaker topology consists of a current path (current-carrying branch), a converter unit (transfer branch), an energy absorption and release circuit (charge-discharge branch), and a discharge device (metal oxide surge arrester). The current-carrying branch includes two components connected in series: an ultra-fast mechanical switch UFD1 and a switch LCS composed of fully controlled devices. The LCS is a reverse-connected IGBT module structure. The transfer branch consists of a fully controlled switch Q and an ultra-fast mechanical switch UFD2, the latter initially being normally open. The charge-discharge branch includes a buffer inductor L. a Energy storage capacitor C b Auxiliary switch K1, charging / discharging capacitor C a And anti-parallel thyristors T1 / T2.

[0067] Existing hybrid DC circuit breaker topologies can only perform fault current interruption. This invention, by modifying them, enables active injection-type adaptive reclosing functionality through the addition of charging and discharging branches.

[0068] Working principle and configuration principles of improved DC circuit breakers:

[0069] When a DC transmission system is operating normally, the current flow path in the improved DC circuit breaker is as follows: Figure 3 As shown by the dashed line. During normal operation, only the current-carrying branch is active, with both LCS and UFD1 in the conducting state. Triggering thyristor T1 is sufficient to activate the DC system for C. a Charging. The charging process takes place at C. a The charging process ends when the voltage reaches the system's rated voltage. At this point, the charging current is zero, T1 automatically disconnects, and charging is complete. When charging is finished, UFD2 is closed to prepare for fault isolation.

[0070] When the system detects a fault, it immediately activates the fully controlled switch Q in the transfer branch, establishing a zero-voltage shutdown environment for the fully controlled device LCS in the current-carrying branch. After the LCS is fully disconnected, the fault current will be diverted from the current-carrying branch to the transfer branch. At this time, a tripping command is sent to the ultra-fast mechanical switch UFD1, causing UFD1 to achieve arc-free disconnection under zero current and near-zero potential conditions. Ultimately, UFD1 fully trips and reaches its rated opening distance. The operating state of its improved DC circuit breaker is as follows: Figure 4 As shown.

[0071] After the fully controlled switch Q in the transfer branch receives the turn-off command, the fault current will be transferred from the transfer branch to the surge arrester MOA. The voltage on the surge arrester will rise rapidly. When it reaches the surge arrester's operating threshold, the MOA is connected to the fault circuit to begin releasing the fault current. After a very short energy release phase, the MOA current drops to zero, and the system immediately triggers the ultra-fast mechanical switch UFD2 to perform the opening operation. Finally, the MMC converter station and the faulty line are completely physically isolated. The operating state of the improved DC circuit breaker in this stage is as follows: Figure 5 As shown.

[0072] After fault isolation and deionization, K1 is synchronously closed, trigger T2, and the charging / discharging branch is activated. Through capacitor C... a For the faulty circuit and zero-energy storage capacitor C b Power supply. Capacitor C b The capacitance value is capacitor C. a It is more than ten times that of the previous value, and the specific value will be given in the simulation analysis. Finally, the C in the charging and discharging branch will be used. b The provided energy storage is used to determine the nature of the fault; capacitor C a The discharge current flow path is as follows Figure 6 As shown.

[0073] When a single-pole grounding or double-pole short-circuit fault occurs, the traditional hybrid DC circuit breakers on both sides of the transmission line will quickly trip to disconnect the faulty line, causing a complete system shutdown. In this situation, there is no electromagnetic coupling between the positive and negative lines, making it impossible to establish electrical mutual coupling between them. At the same time, the remaining electrical quantity on the faulty line decays rapidly, resulting in a lack of stable electrical quantities for fault determination.

[0074] Therefore, the above-mentioned improved DC circuit breaker is applied to, for example... Figure 7 The equivalent model of the half-bridge MMC symmetrical unipolar transmission system shown is presented.

[0075] To ensure the symmetry of the symmetrical monopolar flexible DC transmission system, improved DC circuit breakers are installed on both sides of the DC line.

[0076] Line fault nature identification:

[0077] Based on the above improvements, this invention proposes an adaptive reclosing method for flexible DC transmission lines based on active injection from an improved DC circuit breaker for single-pole grounding faults or inter-pole short-circuit faults. The processing flow is as follows: Figure 8 As shown. Based on the different nature of the fault (permanent or transient), the following two handling methods can be used:

[0078] First scenario: Under permanent fault conditions, specifically for single-pole grounding faults:

[0079] 1. When a fault current causes a disconnection, the charging and discharging capacitors in the charging and discharging branches have already been charged under normal system operation. Thyristor conduction is performed for different short-circuit fault types to construct different discharge circuits between the charging and discharging branches and the fault point, thereby achieving energy injection.

[0080] (1) If no fault occurs, the improved DC circuit breaker will not operate and the system will operate normally;

[0081] (2) If a short circuit fault is confirmed, the fault current is interrupted and the surge arrester in the improved DC circuit breaker HCB1 is allowed to absorb and dissipate the energy completely; then the auxiliary switch K1 in HCB1 and the conducting thyristor T2 are closed to form a single-pole discharge circuit to provide a stable electrical quantity to the fault point.

[0082] 2. During the energy injection process to determine the nature of the fault, the voltage is sampled, and the discharge equation of the fault circuit is established based on the constructed injection circuit: In the early stage of discharge, the energy of the energy storage capacitor is fitted by the definite integral method, and the integral value of the energy storage capacitor is calculated.

[0083] (1) Under permanent fault conditions, the current path and equivalent circuit of the single-pole ground fault discharge circuit are as follows: Figure 9 As shown. Among them, Figure 9 (a) in the diagram is the discharge circuit current flow diagram. Figure 9 (b) in the diagram is the equivalent current diagram of the discharge loop. According to circuit principles, this zero-input response discharge process can be analyzed by constructing differential equations based on a higher-order equivalent dynamic circuit. Therefore, after performing a Laplace transform, as shown... Figure 10 As shown. Modeling the loop using Kirchhoff's voltage and current laws, the corresponding equations for the mesh current relationships in the complex frequency domain can be obtained:

[0084] (1)

[0085] In the formula: I1(s) is the left mesh current; I2(s) is the right mesh current; all mesh currents are taken in the clockwise direction. C a C b and L a R represents the capacitor and inductor in the charging and discharging branch. l L eq The equivalent resistance and inductance of the line under positive ground fault conditions, R eq From the line equivalent resistance R l and transition resistance R f Composition, R f For transition resistance; U a For the capacitor C in the charging and discharging branch a The initial voltage value, the electrical quantities in the diagram are determined only by C. a supply.

[0086] Energy storage capacitor C b voltage U m1 (s) is:

[0087] (2)

[0088] In the formula: A=C a C b L a L eq B=C a C b L a R eq C=(C a +C b )L eq +L a C a D=(C a +C b )R eq U a (0 - ) is the capacitor C a The initial energy storage voltage.

[0089] For U m1 (s) can be obtained by performing the inverse Laplace transform:

[0090] (3)

[0091] Energy storage capacitor C b Energy W m1 (t) and voltage U m1 (t) has the following relationship:

[0092] (4)

[0093] Substituting equation (3) into equation (4) yields the energy storage capacitor C. b Energy W m1 (t) is:

[0094] (5)

[0095] Due to resistive losses, the entire formula is... Multiplied by, capacitance C bThe energy waveform exhibits a significant energy decay in the initial stage of discharge, followed by a decrease in amplitude decay and a slow approach to zero. Simultaneously, the formula contains two terms multiplied by different angular frequencies and a term multiplied by the same angular frequency, indicating coupling and orthogonal oscillations between frequency components. Initially, the exponent factor is 1, at which point the energy is primarily determined by the various sine / cosine terms, resulting in large oscillation amplitudes and a strong dual-frequency mixed oscillation in the energy waveform, with significant fluctuations in energy levels. Because... , The attenuation factors are different; as the discharge time increases, The oscillations under the attenuation factor component become less noticeable, and ultimately mainly consist of... The decay factor component dominates, resulting in significant energy fluctuations over certain time intervals, followed by a decrease in amplitude, creating an energy "envelope" effect. Due to the influence of the exponential decay term, the energy of the energy storage capacitor gradually approaches zero over time, eventually completing its discharge.

[0096] Under permanent fault conditions, for extremely indirect fault conditions

[0097] 1. When a fault current causes a disconnection, the charging and discharging capacitors in the charging and discharging branches have already been charged under normal system operation. Thyristor conduction is performed for different short-circuit fault types to construct different discharge circuits between the charging and discharging branches and the fault point, thereby achieving energy injection.

[0098] (1) If no fault occurs, the improved DC circuit breaker will not operate and the system will operate normally;

[0099] (2) If a short-circuit fault is confirmed, the fault current is interrupted, and the energy absorbed by the surge arrester in the improved DC circuit breaker HCB1 is completely dissipated; then, the auxiliary switch K1 in HCB1 and the conducting thyristor T2 are closed. At this time, since both improved DC circuit breakers on both sides of the transmission line are open, the energy generated by the charging and discharging branch in HCB1 cannot flow from the positive pole to the negative pole, and the inter-pole path is interrupted. To solve this problem, a grounding switch K is installed on the output side of the DC circuit breaker HCB2 on the negative pole line. a By closing K a Construct an inter-electrode discharge circuit to provide a stable electrical quantity to the fault point.

[0100] 2. During the energy injection process to determine the nature of the fault, the voltage is sampled, and the discharge equation of the fault circuit is established based on the constructed injection circuit: In the early stage of discharge, the energy of the energy storage capacitor is fitted by the definite integral method, and the integral value of the energy storage capacitor is calculated.

[0101] (1) Under permanent faults, the discharge circuit current flow diagram and equivalent circuit diagram under indirect faults are as follows: Figure 11 As shown, where, Figure 11(a) in the diagram is the discharge circuit current flow diagram. Figure 11 (b) in the diagram is the equivalent diagram of the discharge circuit current.

[0102] Transition resistance R during inter-electrode short circuit f It is negligible; its discharge characteristics are similar to those of a single-pole grounding fault, except that the line impedance increases to twice its normal value. For detailed analysis, please refer to the analysis of single-pole grounding faults. Under inter-pole short-circuit fault conditions, the discharge circuit still exhibits underdamped characteristics, and its capacitance C... b The voltage waveform shows an oscillating discharge until zero, with its energy dropping sharply in the early stages of discharge.

[0103] The second type is under transient faults, specifically single-pole short-circuit faults or inter-pole short-circuit faults:

[0104] 1. When a fault current causes a disconnection, the charging and discharging capacitors in the charging and discharging branches have already been charged under normal system operation. Thyristor conduction is performed for different short-circuit fault types to construct different discharge circuits between the charging and discharging branches and the fault point, thereby achieving energy injection.

[0105] (1) If no fault occurs, the improved DC circuit breaker will not operate and the system will operate normally;

[0106] (2) If a short circuit fault is confirmed, the fault current is interrupted and the surge arrester in the improved DC circuit breaker HCB1 is allowed to absorb and dissipate the energy completely; then the auxiliary switch K1 in HCB1 and the conducting thyristor T2 are closed to form a discharge circuit to provide a stable electrical quantity to the fault point.

[0107] 2. During the energy injection process to determine the nature of the fault, the voltage is sampled, and the discharge equation of the fault circuit is established based on the constructed injection circuit: In the early stage of discharge, the energy of the energy storage capacitor is fitted by the definite integral method, and the integral value of the energy storage capacitor is calculated.

[0108] In the event of a transient fault, when the charging / discharging branch of the improved DC circuit breaker is connected to the fault circuit, the original discharge path is interrupted. Considering the effect of the line-to-ground capacitance, the charging / discharging capacitance C in the discharge circuit at this time... a Only the energy storage capacitor C is given b In addition to the discharge to ground capacitance, the branches all exhibit capacitive characteristics. Therefore, to analyze the energy storage capacitor C... b The energy change characteristics can be simplified by ignoring the line resistance. The equivalent circuit diagram of the discharge circuit under fault-free conditions is as follows: Figure 12 As shown. To obtain the energy storage capacitor C b The relationship between energy and time is expressed by formulating the nodal voltage relationship equation in the complex frequency domain, as shown in equation (6), where Energy storage capacitor C b The voltage at both ends.

[0109] (6)

[0110] in For capacitance to ground, the energy storage capacitor C b Energy W m2 (s) is:

[0111] (7)

[0112] For W m2 (s) can be obtained by performing the inverse Laplace transform:

[0113] (8)

[0114] In the formula:

[0115] (9)

[0116] Equation (8) clearly demonstrates the energy W m2 (t) consists of a DC component and two cosine terms of different frequencies. The presence of the DC component shifts the waveform upwards, preventing it from oscillating around zero and instead causing it to oscillate around 3A² / 2. Because the fundamental component is much larger than the second harmonic component, the peak value of the energy waveform is slightly sharper and the trough value is slightly flatter than that of a standard cosine wave. Considering the line resistance in the fault circuit, which is very small, the amplitude of this energy waveform only slightly decreases in the initial stage of discharge, and it continues to oscillate at high frequency around 3A² / 2 as the axis of symmetry.

[0117] Furthermore, the energy definite integral of the energy storage capacitor differs between transient and permanent faults, as follows: Figure 13 .

[0118] Under permanent fault conditions, the capacitor continuously dissipates energy through the grounding point. In the initial stage of discharge, it exhibits a large-scale oscillation and decay, followed by a smaller decay and slow discharge.

[0119] In the event of a transient fault, the energy discharge channel of the capacitor disappears, and it can only discharge through the line-to-ground capacitor. The energy waveform of the energy storage capacitor will show high-frequency oscillation, but its amplitude is relatively stable and remains at a high value for a short period of time, and basically does not decay.

[0120] By setting appropriate settings, the nature of faults in flexible DC transmission lines can be effectively distinguished.

[0121] The overall process of this invention is as follows:

[0122] First, the tripping strategy is determined based on the different fault types occurring on the target DC transmission line. The tripping strategy is as follows: regardless of the fault occurring on the DC transmission line, all improved DC circuit breakers on both sides of the line will trip.

[0123] In addition, an improved hybrid DC circuit breaker topology is used in DC transmission line systems:

[0124] This improved hybrid DC circuit breaker topology consists of a current path (current-carrying branch), a converter unit (transfer branch), an energy absorption and release circuit (charge-discharge branch), and a discharge device (metal oxide surge arrester). The current-carrying branch includes two components connected in series: an ultra-fast mechanical switch UFD1 and a switch LCS composed of fully controlled devices. The LCS is a reverse-connected IGBT module structure. The transfer branch consists of a fully controlled switch Q and an ultra-fast mechanical switch UFD2, the latter initially being normally open. The charge-discharge branch includes a buffer inductor L. a Energy storage capacitor C b Auxiliary switch K1, charging / discharging capacitor C a And anti-parallel thyristors T1 / T2.

[0125] During normal operation, only the current-carrying branch is active, with both LCS and UFD1 in the conducting state. Triggering thyristor T1 is sufficient to activate the DC system for C. a Charging. The charging process takes place at C. a The charging process ends when the voltage reaches the system's rated voltage. At this point, the charging current is zero, T1 automatically disconnects, and charging is complete. When charging is finished, UFD2 is closed to prepare for fault isolation.

[0126] When the system detects a fault, it immediately activates the fully controlled switch Q in the transfer branch, establishing a zero-voltage shutdown environment for the fully controlled device LCS in the current-carrying branch. After the LCS is completely disconnected, the fault current will be diverted from the current-carrying branch to the transfer branch. At this time, a tripping command is sent to the ultra-fast mechanical switch UFD1, causing UFD1 to achieve arc-free disconnection under zero current and near-zero potential conditions. Ultimately, UFD1 is fully tripped and reaches its rated opening distance. After receiving the shutdown command, the fully controlled switch Q in the transfer branch will be diverted from the transfer branch to the surge arrester MOA. The voltage on the surge arrester will rise rapidly. When it reaches the surge arrester's operating threshold, the MOA is connected to the fault circuit to begin releasing the fault current. After a very short energy release phase, the MOA current drops to zero, and the system then triggers the ultra-fast mechanical switch UFD2 to perform an opening operation, ultimately achieving complete physical isolation between the MMC converter station and the faulty line.

[0127] After fault isolation and deionization, K1 is synchronously closed, triggering T2 to engage the charging / discharging branch. Through capacitor C... a For the faulty circuit and zero-energy storage capacitor C b Power supply. Ultimately, the C in the charging / discharging branch is utilized. b The provided energy storage is used to determine the nature of the fault.

[0128] The main types of faults occurring on the line are single-pole grounding faults and inter-pole short-circuit faults. During the fault determination phase, the charging and discharging circuit of the improved DC circuit breaker is connected to the faulty line. Because the circuit structure differs between the presence and absence of a fault, the energy storage capacitor C in the charging and discharging branch under fault and non-fault conditions can be analyzed. b The energy waveform differences can be used to distinguish whether a fault exists.

[0129] When a positive ground fault occurs under a permanent fault, the energy storage capacitor C b The energy is continuously released through the grounding point, exhibiting a large oscillation and decay in the early stage of discharge, followed by a smaller decay and slow discharge.

[0130] When a short-circuit fault occurs between poles under a permanent fault condition, the improved DC circuit breakers on both the positive and negative poles simultaneously trip, disconnecting the circuit. At this time, the current generated in the charging and discharging branches of the improved DC circuit breaker cannot flow between the poles. To solve this problem, a grounding switch K is added to the output side of the DC circuit breaker HCB2 on the negative pole line. a By closing K during an inter-pole short-circuit fault. a Construct a complete fault current path. The circuit discharge situation and specific analysis are similar to those of a single-pole grounding fault, except that the line impedance is twice that of a single-pole grounding fault. Under inter-pole short-circuit fault conditions, the discharge circuit still exhibits underdamped characteristics, and its capacitance C... b The voltage waveform shows an oscillating discharge until zero, with its energy dropping sharply in the early stages of discharge.

[0131] In the case of transient faults, whether it is a positive ground fault or an inter-electrode short circuit fault, the energy discharge channel of the capacitor disappears and can only be discharged through the stray capacitance of the line. The energy waveform of the energy storage capacitor will show high-frequency oscillation, but its amplitude is relatively stable and remains at a high value for a short period of time, and basically does not decay.

[0132] In the initial stage of charging and discharging branch operation, the differentiated energy waveform attenuation characteristics can provide a clear physical criterion for fault identification. By performing definite integral calculations on the energy waveforms of the energy storage capacitor for permanent and transient faults respectively, and setting the setting value, the reclosing effect can be effectively achieved.

[0133] The beneficial effects of this invention are as follows:

[0134] This invention enables the identification of the nature of faults in flexible DC transmission lines and effectively improves the reclosing success rate of flexible DC transmission lines.

[0135] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An adaptive reclosing method based on active injection from an improved DC circuit breaker, characterized in that, Includes the following steps: Step S1: When a fault occurs in a DC transmission line, the DC circuit breakers on both sides of the line are controlled to disconnect the fault current. Step S2: After fault isolation, inject preset energy into the faulty line through the energy injection unit of the DC circuit breaker; Step S3: Monitor the electrical response characteristics of the line after energy injection; Step S4: Determine the nature of the fault based on the difference in the electrical response characteristics, and decide whether to perform reclosing based on the determination result; Step S1 specifically includes: Using an improved hybrid DC circuit breaker topology in a symmetrical monopole DC transmission system: The improved hybrid DC circuit breaker topology includes a current-carrying branch, a transfer branch, a charge-discharge branch, and a metal oxide surge arrester; The current-carrying branch includes two components connected in series: an ultra-fast mechanical switch UFD1 and an IGBT module structure switch LCS composed of fully controlled devices connected in reverse series. The transfer branch consists of a fully controlled switch Q and an ultra-fast mechanical switch UFD2, with the ultra-fast mechanical switch UFD2 initially in a normally open state. The charging / discharging branch includes a buffer inductor L. a Energy storage capacitor C b Auxiliary switch K1, charging / discharging capacitor C a And anti-parallel thyristors T1 / T2; In step S2: Step S21: When no fault occurs, the improved DC circuit breaker will not operate and the system will operate normally. Under normal system operation, the charging and discharging branches will continue to be charged. When a short circuit fault is confirmed to occur, all the improved DC circuit breakers configured on both sides of the symmetrical single-pole flexible DC transmission system will operate, disconnect the fault current, and wait for the surge arresters in the improved DC circuit breakers HCB1 and HCB2 to absorb energy and complete the energy discharge. Step S22: For a single-pole grounding fault, the auxiliary switch K1 and thyristor T2 in the improved DC circuit breaker HCB1 of the positive pole line are closed to form a single-pole discharge circuit with the grounding point to provide a stable electrical quantity to the fault point. For an inter-pole short-circuit fault, a grounding switch K is added to the output side of the improved DC circuit breaker HCB2 of the negative pole line. a In the event of an inter-electrode short circuit fault, the grounding switch K is closed. a Construct a complete fault current path.

2. The adaptive reclosing method based on active injection of an improved DC circuit breaker according to claim 1, characterized in that, Step S3 specifically includes: During the energy injection process to determine the nature of the fault, the voltage is sampled, and the fault circuit discharge equation is established based on the constructed injection circuit. Different energy values ​​are obtained under permanent faults and transient faults.

3. The adaptive reclosing method based on active injection of an improved DC circuit breaker according to claim 2, characterized in that, Step S4 specifically includes: In the early stage of discharge, the energy of the energy storage capacitor is fitted by the definite integral method, and the integral value of the energy storage capacitor is calculated. By reasonably setting the setting value, the nature of the fault in the flexible DC transmission line can be distinguished.

4. The adaptive reclosing method based on active injection of an improved DC circuit breaker according to claim 3, characterized in that, Step S3 specifically involves: By modeling different discharge circuits using Kirchhoff's voltage and current laws, the following discharge circuit equations are obtained: For a permanent unipolar ground fault, the energy storage capacitor C b The energy equation is: ； ； ； ； ； ； In the above formula, C a C b and L a R represents the capacitor and inductor in the charging and discharging branch. l L eq R represents the equivalent resistance and inductance of the line under a positive ground fault condition. eq From the line equivalent resistance R l and transition resistance R f Composition; U a For the capacitor C in the charging and discharging branch a The initial voltage value, the electrical quantity is determined solely by the capacitor C. a Provided; k1, k2, k3, and k4 are undetermined coefficients, which are solved using the reduction method; , These are the voltage attenuation factors; , These are the two voltage oscillation angular frequencies in the discharge circuit; For transient faults, the energy storage capacitor C b The energy equation is: ； in, ， In the formula This is the capacitance to ground.

5. The adaptive reclosing method based on active injection of an improved DC circuit breaker according to claim 4, characterized in that, Step S4 specifically involves: During transient faults, the energy storage capacitor exhibits constant-amplitude oscillations and the energy consumption is at its lowest, resulting in the highest integral value. In the event of a permanent fault, the increase in line resistance and transition resistance will accelerate the rate and magnitude of energy loss, thereby reducing the corresponding integral value. The setting value is set higher than the calculated value when a metallic grounding occurs at the end of a single pole of the line, and lower than the calculated value when the fault has disappeared. The criterion is: ； In the formula: t1 represents the connection time of the charging and discharging branch in the improved DC circuit breaker; Δt represents the integration time window, the range of which should be selected as a time window in which the capacitor energy has decayed slowly under a single-pole ground fault; E h E g These represent the integral values ​​calculated when the fault disappears and when a metallic grounding fault occurs at the end of a single pole of the line, respectively; F represents the energy integral setting value, the magnitude of which can be set to E. h +E g Half of the sum; F can be expressed as: ； By setting the setting value, the nature of faults in flexible DC transmission lines can be effectively distinguished.

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