CN-121983911-A - DC interruption/follow current device and temporary fault discrimination method

CN121983911ACN 121983911 ACN121983911 ACN 121983911ACN-121983911-A

Abstract

The invention relates to the technical field of flexible direct current transmission and relay protection, and discloses a direct current break and follow current device and a temporary fault judging method. The device is connected in parallel between a direct current breaker and a current limiting reactor and comprises an isolating switch, a capacitor bank and a charging and discharging branch. The capacitor is precharged in normal operation, the capacitor is discharged in fault to limit the fault current of the converter, and after the breaker is opened, the energy of the reactor is transferred to the capacitor for storage, so that the MOV energy consumption is greatly reduced. The judging method utilizes the RLC oscillation circuit naturally formed by the circuit and the capacitor after fault removal, acquires voltage signals and carries out spectrum analysis, and the permanent faults and the temporary faults are distinguished through the ratio of main frequency amplitude values, so that no additional signal is needed, and a criterion is provided for reliable reclosing. The invention effectively reduces the breaking cost and time and improves the reliability and efficiency of system fault recovery.

Inventors

CAO QIUYU
YU XIUYONG
LI ZHIYAN
ZHANG XINSONG
WANG CHAOYUE
SUN XIAOYING

Assignees

南通大学

Dates

Publication Date: 20260505
Application Date: 20251230

Claims (10)

1. A dc open-and-close current device connected in parallel between a line side of a hybrid dc breaker and a line side of a current limiting reactor, the device comprising: a first disconnecting switch connected in series on a connection line between the hybrid direct current breaker and the device; the capacitor parallel branch is connected between a node and the ground, and the node is positioned on the device side of the first isolating switch; the capacitor parallel branch circuit comprises a second isolating switch, a capacitor bank, a charging branch circuit and a discharging branch circuit; The first end of the second isolating switch is connected to the node, and the second end of the second isolating switch is connected to a first common node; the capacitor bank is connected between the first common node and ground; the charging branch comprises a charging diode and a charging resistor which are connected in series, the charging branch is connected in parallel between the second end of the second isolating switch and the high-voltage end of the capacitor bank, and the anode of the charging diode faces to the second end of the second isolating switch; the discharging branch circuit comprises a discharging diode and a controllable discharging switch which are connected in series, the discharging branch circuit is connected in parallel between the second end of the second isolating switch and the high-voltage end of the capacitor bank, and the cathode of the discharging diode faces to the second end of the second isolating switch.
2. A dc discontinuous current arrangement according to claim 1, further comprising a protection branch comprising a series connection of a protection switch and a damping resistor, the protection branch being connected in parallel across the capacitor bank.
3. The direct current interrupt flow device of claim 1, wherein the device is configured to: When the system operates normally, the second isolating switch and the discharging switch are controlled to be conducted, and the system direct-current voltage charges the capacitor bank through the charging branch circuit; when the charging current is detected to be lower than a preset threshold value, the conduction state of the discharging switch is maintained, so that the device is in an on-state.
4. The dc interrupt and freewheeling device of claim 1, wherein the capacitor bank provides current through the discharging switch and the discharging diode that are turned on to limit fault current from the inverter after a dc line fails and the hybrid dc breaker receives a break-gate signal.
5. The dc interrupt and continuous current device according to claim 1, wherein after the hybrid dc breaker breaks a fault current, magnetic field energy stored in the current limiting reactor is transferred to the capacitor bank through the capacitor parallel branch for storage.
6. A method for discriminating a temporary fault based on the dc discontinuous flow device according to any of claims 1 to 5, characterized in that the method does not require external active signal injection, comprising the steps of: the signal acquisition, namely acquiring a voltage oscillation signal at the side of a fault line after the fault line is disconnected by the hybrid direct current breaker; performing spectrum analysis, namely performing fast Fourier transform on the acquired voltage oscillation signals to obtain a spectrogram of the voltage oscillation signals; the characteristic frequency extraction, namely identifying a first main frequency with highest amplitude and a second main frequency with next highest amplitude from the spectrogram; calculating the ratio K=A1/A2 of the amplitude A1 of the first main frequency to the amplitude A2 of the second main frequency; Comparing the ratio K with a preset judging threshold K set ; If K is more than or equal to K set , judging that the permanent fault exists, and locking and reclosing; If K < K set , judging that the temporary fault exists, and executing automatic reclosing operation.
7. The method according to claim 6, wherein the discrimination threshold K set is determined by simulation or experiment to have a value ranging from 5 to 20.
8. The temporary fault discrimination method according to claim 6, wherein in the signal acquisition step, the preset duration is 20 milliseconds to 50 milliseconds.
9. The method according to claim 6, wherein the voltage oscillation signal is generated by an RLC second-order oscillation circuit naturally composed of a capacitor bank in the parallel capacitor branch, an equivalent inductance and an equivalent resistance of a fault line, and a fault point resistance after the fault is removed.
10. The temporary fault discrimination method according to claim 9, characterized in that: When the fault is a permanent fault, the transition resistance of the fault point is kept low impedance, and the oscillation main frequency of the RLC circuit is kept basically unchanged; When the fault is a temporary fault, after the hybrid direct current breaker is opened, the arc of the fault point is extinguished to cause the impedance of the hybrid direct current breaker to be sharply increased, so that the oscillation main frequency of the RLC circuit is changed, and a plurality of remarkable frequency components appear in the frequency spectrum of the voltage oscillation signal.

Description

DC interruption/follow current device and temporary fault discrimination method Technical Field The invention relates to the technical field of flexible direct current transmission and relay protection, in particular to a direct current break-make current device and a temporary fault judging method. Background The flexible direct current technology based on the voltage source converter (voltagesourceconverter, VSC) has the advantages of active power and reactive power decoupling control, convenience in constructing a direct current power grid and the like, and is an important technical approach for realizing high-efficiency grid connection of new energy. However, after the VSC fails on the dc side, the capacitor may be equivalently discharged through the inductor, so that the fault current rises rapidly, the dc fault current does not have a natural zero crossing point, and the insulated gate bipolar transistor device itself has weak capability of withstanding the over-current, so that the factors together make the breaking technique of the dc line fault an important bottleneck for further development of the flexible dc system. After the protection device identifies the direct current line fault, the most ideal protection scheme is to cut off the fault current rapidly and selectively by the direct current breakers (DCcircuitbreaker, DCCB) at the two ends of the line. Although various topologies such as hybrid, mechanical and solid-state dc circuit breakers and improvements thereof have been proposed at present, the cost of dc switching is still high, and the technical maturity of high-voltage high-capacity dc circuit breakers still needs to be improved. In order to suppress the rising rate and amplitude of the fault current, the prior art generally connects large current limiting reactors in series in the dc line. Although this reactor effectively limits the fault current, during the opening of the dc breaker, a large amount of the magnetic field energy stored in the reactor needs to be dissipated through the metal oxide arrester (MetalOxideSurgeArrester, MOA) energy absorbing branch inside the dc breaker. This results in: (1) MOV energy absorption requirements are enormous and cost prohibitive, requiring the deployment of large capacity, bulky MOV modules. (2) The heat dissipation requirement is high, huge energy dissipation generates a large amount of heat, a complex cooling system is needed, and the cost and the volume are further increased. (3) The on-off time is long, the MOV energy consumption process is relatively long, and the whole fault clearing time is delayed. In addition, existing schemes face a high risk in reclosing strategies that most faults in the dc line are temporary faults (e.g., lightning strikes, bird damage, etc.). To achieve fast power restoration, an automatic reclosing strategy needs to be introduced. However, most of the existing DCCB products do not fully consider the requirement of reclosing in a short time during design, and if the circuit breaker coincides with a permanent fault, severe secondary impact is caused to the equipment. To increase reclosing success rate, accurate discrimination between permanent and temporary faults must be achieved. The existing judging method mainly faces two major challenges, namely firstly, the transient voltage and current information after faults are simply utilized, the DCCB can cut off fault lines rapidly, so that the length of a data window for protection judgment is very short, and the accuracy is difficult to guarantee, secondly, the active signal injection method (such as the injection signal generated by utilizing a modularized multi-level converter or an internal capacitor of the DCCB) proposed by some researches is easy to interfere in a multi-terminal network, even the normal operation of a non-fault area is possibly influenced, and the accuracy requirement on fault detection is high. Therefore, a new dc switching-on/off and fault recovery integrated scheme is urgently needed in the art, which can effectively reduce the energy burden of the MOV, reduce the switching-off cost and time, and can rapidly and accurately judge the fault property without depending on external injection signals, thereby providing decision basis for reliable reclosing. Disclosure of Invention The invention provides a direct current break-make current device and a temporary fault judging method, which are used for solving the problems. In a first aspect, the present invention provides a dc cut-off and freewheeling device connected in parallel between a line side of a hybrid dc breaker and a line side of a current limiting reactor, the device comprising: a first disconnecting switch connected in series on a connection line between the hybrid direct current breaker and the device; the capacitor parallel branch is connected between a node and the ground, and the node is positioned on the device side of the first isolating switch; the capacitor parallel branch ci