Modular series high voltage direct current circuit breaker grading reclosing method based on fault type identification

Abstract

The application relates to a modular series type high-voltage direct-current circuit breaker grading reclosing method based on fault type identification. After the first break of the modular series type high-voltage direct-current circuit breaker and the preset deionization time, the reclosing is performed in a step-by-step closing module mode. Meanwhile, the line current is monitored in real time after each stage of closing, and the line current is compared with a threshold value. If the line current is lower than the threshold value, the closing is continuously performed until all the modules are closed, and the fault is determined as transient. If the line current reaches or exceeds the threshold value, the closing is immediately stopped, and the second break of the closed modules is performed, and the fault is determined as permanent. The application integrates the fault identification into the grading reclosing process, significantly reduces the reclosing impulse current and the energy absorption requirement of a metal oxide arrester (MOA) under the permanent fault, effectively protects the power electronic equipment, and reduces the cost of the circuit breaker.

Description

Technical Field

[0001] This invention belongs to the field of DC transmission system protection and control technology, specifically relating to a modular series high-voltage DC circuit breaker graded reclosing method based on fault type identification. Background Technology

[0002] In DC transmission systems, DC circuit breakers (DCBs) are crucial equipment for ensuring the safe and reliable operation of the system. While traditional mechanical DC circuit breakers (MCBs) and hybrid DC circuit breakers (HCBs) are technologically mature, their high cost and large size, requiring numerous power electronic components, limit their large-scale application in multi-terminal power grids. High-voltage DC circuit breakers (M-DCBs), composed of multiple resonant circuit breaker modules connected in series, have become a research hotspot due to their low cost and compact structure. Each circuit breaker internally generates a high-frequency oscillating current through LC resonance to create an artificial zero-crossing point, thereby achieving fault current interruption.

[0003] However, existing research mainly focuses on the fault-breaking process of modular series DC circuit breakers, with insufficient attention paid to their reclosing characteristics. Traditional direct reclosing methods are insensitive to fault types. Under permanent faults, reclosing can trigger severe secondary inrush currents, which not only damage power electronic equipment such as converter valves but also force the circuit breaker to interrupt the fault current again in a very short time. This necessitates the configuration of extremely high energy absorption capacity in the internal metal oxide surge arresters (MOAs), such as the MOAs in the Zhangbei project, which require a capacity of approximately 150 MJ. Existing adaptive reclosing methods, such as those based on residual current, residual voltage, small current injection, or parallel resistance, either suffer from logical flaws, excessively long delays, or are insensitive to high-resistance faults and have difficult parameter design, making them unsuitable.

[0004] Therefore, there is an urgent need for a modular series-type high-voltage DC circuit breaker reclosing method that can effectively suppress reclosing impact and reduce MOA capacity requirements. Summary of the Invention

[0005] The purpose of this application is to overcome the shortcomings of the prior art and provide a modular series high voltage DC circuit breaker graded reclosing method based on fault type identification. It aims to solve the technical problem that the existing method has a large inrush current under permanent faults, which leads to excessive MOA capacity configuration requirements. At the same time, it can effectively identify and handle transient and permanent faults.

[0006] The technical problem solved by this application is achieved through the following technical solution: A graded reclosing method for a modular series-connected high-voltage DC circuit breaker based on fault type identification, wherein the modular series-connected high-voltage DC circuit breaker is composed of multiple circuit breaker modules connected in series, and each circuit breaker module includes a main current-carrying branch, a current transfer branch, and an energy absorption branch; the steps of the method are as follows: S1. Initial fault interruption: When a fault is detected in the DC line, the first interruption command is sent to all the circuit breaker modules connected in series. Each circuit breaker module operates simultaneously and generates a resonant current through its internal current transfer branch, forcing the current in the main current branch to cross zero and transferring the fault current to the energy absorption branch for dissipation, thus completing the first fault interruption. S2, Fault Deionization Waiting: After the first interruption is completed, the DC transmission system enters the fault deionization waiting stage and presets the fault deionization time. During this period, all circuit breaker modules remain in the open state and share the system voltage, so that the voltage at the fault point remains zero, providing time for the dielectric insulation strength of the fault point to recover. S3. Staged reclosing and fault type identification: After waiting for a preset fault de-ionization time, staged reclosing begins, specifically as follows: S3.1. Close each series-connected circuit breaker module in a preset order, and detect the fault current in the line in real time after closing each module or group of modules. S3.2 Compare the detected fault current with the preset second interruption current threshold; If the fault current is less than the second breaking current threshold, the current fault is determined to be a transient fault and has been cleared by itself; the subsequent circuit breaker module closing instructions continue to be executed until all modules are completely closed and the system returns to normal operation. If the fault current reaches or exceeds the second interruption current threshold, the current fault is determined to be a permanent fault. Immediately stop sending closing commands to subsequent circuit breaker modules that have not yet closed, and keep them in the open state. Send a second interruption command to one or more circuit breaker modules that have already closed, so that they can perform the fault interruption operation again. S4. Secondary interruption of permanent faults: After receiving the second interruption command, the closed circuit breaker module generates resonant current again through its internal current transfer branch to complete the second interruption of the persistent fault current, and the system eventually remains in the open state.

[0007] Moreover, the preset fault deionization time of S2 is determined according to the voltage level of the DC transmission system. The preset fault deionization time of the 500kV multi-terminal flexible DC system is 200~300ms.

[0008] Furthermore, the S3 graded reclosing starts from the circuit breaker module closest to the beginning of the line and closes sequentially towards the far end, with the time interval between the closing of each two circuit breaker modules fixed at 10ms.

[0009] Moreover, the second interrupting current threshold of S3 is greater than the maximum load current of the DC line under rated operating conditions, and less than the minimum fault current value that the circuit breaker can safely interrupt.

[0010] Compared with the prior art, the present invention has the following beneficial effects: 1. The energy capacity required by the MOA of the present invention can be reduced by 38%, which significantly reduces the cost and size of the circuit breaker; 2. This invention integrates fault type identification into the dynamic process of graded reclosing. It does not require additional voltage or current injection devices or line communication. It can quickly and reliably distinguish between transient faults and permanent faults simply by monitoring whether the current after each reclosing exceeds the threshold. 3. For permanent faults, this invention can stop subsequent reclosing and trigger secondary interruption before the fault current rises to a dangerous level (when it reaches a preset threshold), thus avoiding the huge impact and repeated actions caused by the complete reclosing of permanent faults in the traditional method. Attached Figure Description

[0011] Figure 1 This is a circuit breaker module topology diagram of the modular series-connected high-voltage DC circuit breaker of the present invention; Figure 2 A schematic diagram of a high-voltage DC circuit breaker consisting of multiple circuit breaker modules connected in series. Figure 3 The following are the timing and current / voltage waveforms of the method of the present invention under permanent fault conditions; Figure 4 The diagram shows the timing and current / voltage waveforms of the method of the present invention under transient faults. Detailed Implementation

[0012] The present application will be further described in detail below through specific embodiments. The following embodiments are merely descriptive and not limiting, and should not be used to limit the scope of protection of the present application.

[0013] An innovative method for graded reclosing of modular series-connected high-voltage DC circuit breakers based on fault type identification is as follows: Figure 1 , 2 As shown, the modular series-connected high-voltage DC circuit breaker is composed of multiple circuit breaker modules connected in series. Each circuit breaker module includes a main current-carrying branch, a current transfer branch, and an energy absorption branch. The steps of the method are as follows: S1. Initial fault interruption: When a fault is detected in the DC line, the first interruption command is sent to all the circuit breaker modules connected in series. Each circuit breaker module operates simultaneously and generates a resonant current through its internal current transfer branch, forcing the current in the main current branch to cross zero and transferring the fault current to the energy absorption branch for dissipation, thus completing the first fault interruption. S2. Fault Deionization Waiting: After the first interruption is completed, the DC transmission system enters the fault deionization waiting stage and presets the fault deionization time. During this period, all circuit breaker modules remain open and share the system voltage, keeping the voltage at the fault point at zero, providing time for the dielectric insulation strength at the fault point to recover. The preset fault deionization time is determined according to the voltage level of the DC transmission system. The preset fault deionization time for a 500kV multi-terminal flexible DC system is 200~300ms. S3. Staged reclosing and fault type identification: After waiting for a preset fault de-ionization time, staged reclosing begins, specifically as follows: S3.1. Close each series-connected circuit breaker module in a preset order, and detect the fault current in the line in real time after closing each module or group of modules. S3.2 Compare the detected fault current with the preset second interruption current threshold; If the fault current is less than the second breaking current threshold, the current fault is determined to be a transient fault and has been cleared by itself; the subsequent circuit breaker module closing instructions continue to be executed until all modules are completely closed and the system returns to normal operation. If the fault current reaches or exceeds the second interruption current threshold, the current fault is determined to be a permanent fault. Immediately stop sending closing commands to subsequent circuit breaker modules that have not yet closed, and keep them in the open state. Send a second interruption command to one or more circuit breaker modules that have already closed, so that they can perform the fault interruption operation again. The graded reclosing starts from the circuit breaker module closest to the beginning of the line and closes sequentially towards the far end. The time interval between closing two circuit breaker modules is fixed at 10ms. The second breaking current threshold is greater than the maximum load current of the DC line under rated operating conditions and less than the minimum fault current value that the circuit breaker can safely break. S4. Secondary interruption of permanent faults: After receiving the second interruption command, the closed circuit breaker module generates resonant current again through its internal current transfer branch to complete the second interruption of the persistent fault current, and the system eventually remains in the open state.

[0014] This embodiment uses a typical scenario from a four-terminal flexible DC transmission project as an example to illustrate the invention in detail.

[0015] The system parameters are as follows: rated voltage ±500kV, fault line length 100km, rated current 1.5kA. The high-voltage DC circuit breaker consists of five circuit breaker modules (DCB1~DCB5) with a rated voltage of 100kV connected in series. The internal parameters of each module are: resonant capacitor 4.7746µF, resonant inductance 53.052µH, excitation source capacitor 5mF, and its pre-charge voltage 3.3kV. The preset fault deionization time is 300ms, the time interval for graded reclosing is 10ms, and the second breaking current threshold used to determine the fault type is 2kA.

[0016] Scenario 1: Permanent Failure 1. Initial Fault Interruption: At t=1.0s, a permanent double-pole short-circuit fault occurs on the line. The protection device operates, and at t=1.006s (3ms protection delay + 3ms mechanism action delay), all five circuit breaker modules simultaneously receive the first interruption command. Each module generates a gradually increasing resonant current through the current transfer branch, causing the current of the ultra-fast mechanical switch MS in the main current-carrying branch to cross zero. The fault current is transferred to the MOA for absorption, completing the first interruption. At this time, all MSs are in the open state.

[0017] 2. Fault de-de ...

[0018] 3. Graded reclosing and fault type identification: After t=1.306s, the staged reclosing begins. First, a closing command is sent to the DCB1 module, which closes the MS, and fault current begins to appear in the line.

[0019] After 10ms, a closing command is sent to the DCB2 module. As DCB2 closes, the equivalent impedance of the line decreases, and the fault current... I f Further increase; fault current detected. I~f~ The preset 2kA threshold has been reached. The system immediately determines that the fault is permanent. Subsequently, the system stops sending closing commands to the subsequent DCB3, DCB4, and DCB5 modules, keeping them in the open state; at the same time, it immediately sends a second opening command to the already closed DCB1 and DCB2 modules.

[0020] 4. Secondary interruption of permanent faults: After receiving the instruction, DCB1 and DCB2 modules restart their current transfer branches to complete the second interruption of the continuous fault current; finally, all modules remain in the open state. The energy absorbed by the MOA during the second interruption is much less than that during the first interruption, which verifies that the method of the present invention can effectively reduce the MOA capacity requirement.

[0021] Scenario 2: Transient Failure The fault occurred at t=1.0s, but disappeared on its own after only 10ms.

[0022] 1. The initial fault interruption and fault de-idling waiting process are the same as those for permanent faults.

[0023] 2. Graded reclosing and fault type identification: After 1.306 seconds, the staged reclosing process began. Modules DCB1, DCB2, DCB3, DCB4, and DCB5 were closed sequentially. Since the fault point had disappeared during the deionization process, there was no sustained fault current in the line after DCB1 closed. Even with the subsequent modules closing, the line current was merely a rapidly decaying oscillating current caused by the line's distributed capacitance, with an amplitude far below the 2kA threshold. Because the detected current never reached 2kA throughout the entire staged reclosing process, the system determined it to be a transient fault; therefore, the system did not trigger a second interruption command.

[0024] 3. System Recovery: The tiered reclosing command continues to execute until all five DCB modules are closed. The line voltage gradually recovers, and after a brief period of oscillation, the current recovers to the rated value of 1.5kA, and the system resumes normal operation.

[0025] Although embodiments and drawings of the present invention have been disclosed for illustrative purposes, those skilled in the art will understand that various substitutions, variations 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 method for graded reclosing of a modular series-connected high-voltage DC circuit breaker based on fault type identification, characterized in that: The modular series-connected high-voltage DC circuit breaker is composed of multiple circuit breaker modules connected in series. Each circuit breaker module includes a main current-carrying branch, a current transfer branch, and an energy absorption branch. The steps of the method are as follows: S1. Initial fault interruption: When a fault is detected in the DC line, the first interruption command is sent to all the circuit breaker modules connected in series. Each circuit breaker module operates simultaneously and generates a resonant current through its internal current transfer branch, forcing the current in the main current branch to cross zero and transferring the fault current to the energy absorption branch for dissipation, thus completing the first fault interruption. S2, Fault Deionization Waiting: After the first interruption is completed, the DC transmission system enters the fault deionization waiting stage and presets the fault deionization time. During this period, all circuit breaker modules remain in the open state and share the system voltage, so that the voltage at the fault point remains zero, providing time for the dielectric insulation strength of the fault point to recover. S3. Staged reclosing and fault type identification: After waiting for a preset fault de-ionization time, staged reclosing begins, specifically as follows: S3.

1. Close each series-connected circuit breaker module in a preset order, and detect the fault current in the line in real time after closing each module or group of modules. S3.2 Compare the detected fault current with the preset second interruption current threshold; If the fault current is less than the second interrupting current threshold, the current fault is determined to be a transient fault and has been cleared by itself. Continue executing the step-by-step closing instructions of subsequent circuit breaker modules until all modules are fully closed and the system returns to normal operation; If the fault current reaches or exceeds the second interruption current threshold, the current fault is determined to be a permanent fault. Immediately stop sending closing commands to subsequent circuit breaker modules that have not yet closed, and keep them in the open state. Send a second interruption command to one or more circuit breaker modules that have already closed, so that they can perform the fault interruption operation again. S4. Secondary interruption of permanent faults: After receiving the second interruption command, the closed circuit breaker module generates resonant current again through its internal current transfer branch to complete the second interruption of the persistent fault current, and the system eventually remains in the open state.

2. The modular series-type high-voltage DC circuit breaker graded reclosing method based on fault type identification according to claim 1, characterized in that: The preset fault deionization time of S2 is determined according to the voltage level of the DC transmission system. The preset fault deionization time of the 500kV multi-terminal flexible DC system is 200~300ms.

3. The modular series-type high-voltage DC circuit breaker graded reclosing method based on fault type identification according to claim 1, characterized in that: The S3 graded reclosing starts from the circuit breaker module closest to the beginning of the line and closes sequentially towards the far end, with the time interval between the closing of each two circuit breaker modules fixed at 10ms.

4. The method for graded reclosing of a modular series-type high-voltage DC circuit breaker based on fault type identification according to claim 1, characterized in that: The second interrupting current threshold of S3 is greater than the maximum load current of the DC line under rated operating conditions, and less than the minimum fault current value that the circuit breaker can safely interrupt.

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