Grounding protection device and grounding protection method suitable for flexible direct current system

Abstract

The invention discloses a grounding protection device and a grounding protection method suitable for a flexible direct current system, and belongs to the technical field of grounding protection of the flexible direct current system. The grounding protection device is characterized in that a control unit and a plurality of grounding protection devices are configured in the flexible direct current system of a bipolar topological structure; one end of the grounding protection device is connected with a non-grounding electrode of the converter, and the other end is grounded through a grounding point; when the control unit detects that a single-phase earth fault occurs in a valve side loop, the current converter is locked, and when bridge arm current crosses zero, the grounding protection device is controlled to be communicated with a non-grounding electrode corresponding to the current converter, and the non-grounding electrode of the current converter forms a new low-resistance path with the ground through a grounding point, so that the fault current is prevented from being established again, and the reliability of the current converter is improved. Therefore, the AC circuit breaker can perform normal arc extinguishing by means of a current zero crossing point at the moment, and contact ablation and insulation breakdown are avoided. By implementing the invention, the problem that the alternating current circuit breaker in the prior art cannot normally extinguish arc can be solved.

Description

Technical Field

[0001] The present invention relates to the technical field of grounding protection for flexible direct current (DC) systems, and in particular to a grounding protection device and a grounding protection method applicable to a flexible DC system. Background Art

[0002] A flexible DC system with a bipolar topology typically consists of a positive, negative, and neutral DC busbar. The multi-purpose converter (MMC) is connected to the AC system via a converter transformer. When a single-phase ground fault occurs on the converter valve side—the connection between the converter and the converter transformer—the faulted phase forms a conductive loop with the earth through the grounding point. This creates an abnormal current path through the AC system power supply, converter transformer, MMC valve assembly, and fault point.

[0003] After a single-phase grounding fault occurs on the valve side, the capacitor within the MMC will continue to discharge toward the fault point. Because the capacitor discharge current has a fixed direction, a significant DC component will be superimposed on the fault current. This DC component causes the fault current to deviate toward one side of the time axis, preventing it from crossing zero over time. This means the fault current loses its natural zero-crossing point. Conventional technology uses the arc-extinguishing principle of AC circuit breakers: AC circuit breakers rely on the arc to extinguish naturally when the current crosses zero. After the current crosses zero, the arc gap insulation strength quickly recovers, thus shutting off the fault current. If the fault current contains a DC component but no zero-crossing point, the arc current will not naturally cross zero when the circuit breaker opens, and the arc gap insulation strength will be difficult to recover, causing the arc to continue burning. Forced opening can cause problems such as contact erosion and insulation breakdown, threatening system safety.

[0004] Therefore, since the AC circuit breaker is directly used for phase tripping after a single-phase grounding fault occurs on the valve side, the impact of the fault current having no zero crossing point on arc extinction is ignored. The existing technology has the problem that the AC circuit breaker cannot extinguish the arc normally, which may cause irreversible damage to the equipment. Summary of the Invention

[0005] An embodiment of the present invention provides a grounding protection device and a grounding protection method suitable for a flexible DC system. When single-phase grounding is detected on the valve side, the target converter is locked, and when the bridge arm current is detected to be zero, the grounding protection device is controlled to be connected to the non-grounded electrode corresponding to the converter, so that the non-grounded electrode of the converter forms a new low-resistance path with the earth through the grounding point, so that the AC circuit breaker can rely on the current zero point to extinguish the arc normally, avoid contact erosion and insulation breakdown, thereby ensuring the performance of the equipment, and can effectively solve the problem in the prior art that the AC circuit breaker cannot extinguish the arc normally, which may cause irreversible damage to the equipment.

[0006] An embodiment of the present invention provides a grounding protection device applicable to a flexible direct current (HVDC) system, comprising: a flexible direct current (HVDC) system having a bipolar topology, a control unit, and a plurality of grounding protection devices; wherein each converter in the flexible direct current (HVDC) system is connected to a grounding protection device;

[0007] One end of the grounding protection device is connected to the non-grounded electrode corresponding to the converter, and the other end of the grounding protection device is connected to the ground;

[0008] The control unit is used to lock the target converter when a single-phase grounding fault is detected in the valve-side circuit of a target converter, and then control the grounding protection device to connect with the non-grounding electrode corresponding to the target converter when it is determined that the bridge arm current corresponding to the non-grounding electrode of the target converter is zero.

[0009] Preferably, the ground protection device comprises: a switch device and a resistor;

[0010] The first port of the switch device is connected to the non-grounded electrode corresponding to the converter, and the second port of the switch device is connected to the first port of the resistor;

[0011] The second terminal of the resistor is grounded.

[0012] Preferably, the resistance of the resistor is greater than a preset current limiting resistance threshold; wherein the preset current limiting resistance threshold is the product of a reference resistance of the flexible DC system and a preset multiple.

[0013] Preferably, the ground protection device comprises: a switch device, a fixed resistor and a variable resistor;

[0014] The first port of the switch device is connected to the non-grounded electrode corresponding to the converter, and the second port of the switch device is connected to the first port of the fixed resistor;

[0015] The second port of the fixed resistor is connected to the first port of the variable resistor, and the second port of the variable resistor is grounded;

[0016] The control unit is further configured to adjust the resistance of the variable resistor according to a fault current corresponding to the single-phase grounding fault when a single-phase grounding fault is detected in the valve-side circuit of a target converter.

[0017] Preferably, the control unit is further configured to, when detecting a single-phase grounding fault in a valve-side circuit of a target converter, send a single-phase grounding fault signal to an opposite-side converter arranged on an opposite side of the target converter;

[0018] The control unit is further configured to cut off the AC source corresponding to the opposite-side converter and lock the opposite-side converter when detecting that the opposite-side converter receives a single-phase grounding fault signal.

[0019] Preferably, the control unit is used to take the converter as the target converter and determine that a single-phase grounding fault occurs in the valve side circuit of the target converter when it detects that the differential current of any phase in the three-phase electrical circuit corresponding to the converter is greater than the differential current threshold.

[0020] Preferably, each bridge arm in the three-phase electrical circuit corresponding to the converter is provided with a relay protection device;

[0021] The relay protection device is used to collect the bridge arm current of the bridge arm where it is located, and send the collected bridge arm current to the control unit.

[0022] Preferably, the control unit is further configured to generate a warning signal indicating that an abnormality exists in the variable resistor when it is detected that the resistance of the variable resistor exceeds a resistance threshold.

[0023] Based on the above-mentioned device embodiment, the present invention provides corresponding method embodiments.

[0024] An embodiment of the present invention provides a grounding protection method applicable to a flexible DC system, which is applied to a control unit in a grounding protection device applicable to a flexible DC system;

[0025] The grounding protection method comprises:

[0026] When a single-phase grounding fault is detected in a valve-side circuit of a target converter, the target converter is locked;

[0027] When it is determined that the arm current corresponding to the non-grounded electrode of the target converter is zero, the grounding protection device is controlled to be connected to the non-grounded electrode corresponding to the converter.

[0028] Preferably, it also includes:

[0029] sending a single-phase ground fault signal to an opposite-side converter disposed on an opposite side of the target converter;

[0030] The AC source corresponding to the opposite-side converter is cut off, and the opposite-side converter is locked at the same time.

[0031] The following beneficial effects are achieved by implementing the present invention:

[0032] An embodiment of the present invention provides a grounding protection device and a grounding protection method suitable for a flexible DC system. The grounding protection device of the present invention configures multiple grounding protection devices and integrates a control unit in a flexible DC system with a bipolar topology structure, and makes each converter correspond to a grounding protection device. One end of the grounding protection device is connected to the non-grounded pole of the converter, and the other end is connected to the ground through a grounding point; the control unit can monitor the valve side circuit status of the converter in real time. When single-phase grounding on the valve side is detected, the control unit immediately locks the target converter, and the discharge power supply of the internal capacitor of the target converter can be cut off, preventing the fault current from continuously superimposing the DC component; after locking the converter, the current of the bridge arm connected to the non-grounded pole gradually decays due to the loss of power drive. When the control unit detects that the bridge arm current is zero, indicating that the AC component of the fault current has naturally crossed zero, and the arc extinguishing conditions are met, the grounding protection device can be controlled to connect with the non-grounded electrode corresponding to the converter at the moment the bridge arm current crosses zero, so that the non-grounded electrode of the converter forms a new low-resistance path with the earth through the grounding point, while preventing the fault current from re-establishing. This allows the AC circuit breaker to extinguish the arc normally by relying on the current crossing zero, avoiding contact erosion and insulation breakdown, thereby ensuring the performance of the equipment. Compared with the prior art, the present invention can implement a timing control strategy of locking, zero-crossing detection, and grounding connection, allowing the AC circuit breaker to extinguish the arc normally at the zero-crossing point. Moreover, through the auxiliary effect of the grounding protection device, the DC bias in the original fault current is offset, prompting the current to return to the zero-crossing point, further ensuring that the AC circuit breaker can extinguish the arc normally. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 The figure is a structural diagram of a grounding protection device applicable to a flexible DC system provided by one embodiment of the present invention.

[0034] Figure 2 This is a logic diagram of ground protection of a control unit provided by an embodiment of the present invention.

[0035] Figure 3 It is a schematic diagram of the installation of a grounding protection device provided by one embodiment of the present invention.

[0036] Figure 4 The figure is a flow chart of a grounding protection method applicable to a flexible DC system provided by one embodiment of the present invention. DETAILED DESCRIPTION

[0037] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0038] like Figure 1 As shown, in order to solve the problem in the prior art that the AC circuit breaker cannot extinguish the arc normally due to the direct phase tripping of the AC circuit breaker after a single-phase grounding fault occurs on the valve side, ignoring the impact of the fault current's lack of zero crossing on arc extinction, an embodiment of the present invention provides a grounding protection device suitable for a flexible DC system, comprising: a flexible DC system with a bipolar topology, a control unit, and multiple grounding protection devices; wherein each converter in the flexible DC system is connected to a grounding protection device;

[0039] One end of the grounding protection device is connected to the non-grounded electrode corresponding to the converter, and the other end of the grounding protection device is connected to the ground;

[0040] The control unit is used to lock the target converter when a single-phase grounding fault is detected in the valve-side circuit of a target converter, and then control the grounding protection device to connect with the non-grounding electrode corresponding to the target converter when it is determined that the bridge arm current corresponding to the non-grounding electrode of the target converter is zero.

[0041] Schematically, a flexible DC system with a bipolar topology structure actually refers to a DC transmission system in which a power transmission channel is formed by two electrodes, positive and negative, and a zero potential reference point is drawn out through a DC neutral bus.

[0042] In an embodiment of the present invention, a flexible DC system with a bipolar topology includes a positive busbar, a negative busbar, and a DC neutral busbar, forming a symmetrical electrical architecture. The neutral busbar, serving as the common connection point for the positive and negative poles, can directly establish a zero-potential reference point, providing a potential benchmark for the system.

[0043] In the above-mentioned flexible DC system with a bipolar topology, there are multiple converters. Schematically, the converter is a modular multilevel converter (MMC). The MMC is connected to the AC system through a converter transformer. Its DC side is connected to the positive and negative busbars respectively, and the neutral busbar serves as the potential reference point.

[0044] When the flexible DC system is in normal operation, the voltages at both poles are symmetrical relative to the neutral busbar zero potential, and the system realizes AC / DC power conversion and power transmission by adjusting the switching of MMC submodules.

[0045] When a single-phase grounding fault occurs on the converter valve side (the side connected to the converter transformer), the fault phase current forms a loop through the grounding point, and the internal capacitor of the MMC continues to discharge, which will cause the fault current to contain a DC component and no zero crossing point, making it impossible for the traditional AC circuit breaker to extinguish the arc normally. However, the present invention controls the grounding protection device to be connected with the non-grounded electrode corresponding to the converter, so that the non-grounded electrode of the converter forms a new low-resistance path with the earth through the grounding point, so that the AC circuit breaker can now rely on the current zero crossing to extinguish the arc normally.

[0046] The present invention configures multiple grounding protection devices in a flexible DC system with a bipolar topology structure, and each converter in the flexible DC system corresponds to a grounding protection device. One end of the grounding protection device is connected to a non-grounded electrode (such as a positive electrode or a negative electrode), and the other end is connected to the ground through a grounding point.

[0047] A control unit is also integrated in the flexible DC system with a bipolar topology, which can monitor the status of the converter valve-side circuit in real time and execute the logical processes of locking the converter, detecting the bridge arm current, and controlling the connection of the ground protection device.

[0048] Specifically, the control logic of the control unit is:

[0049] Step 1: When single-phase grounding is detected on the valve side, the control unit immediately blocks the target converter, cuts off the discharge power supply of the internal capacitor of the MMC, and prevents the fault current from continuously superimposing the DC component.

[0050] Step 2: After the converter is locked, the current in the bridge arm connected to the non-grounded electrode gradually decays due to the loss of power. When the control unit detects that the bridge arm current is zero, it indicates that the AC component of the fault current has naturally passed zero, and the conditions for arc extinction are met.

[0051] Step 3: At the moment when the bridge arm current passes through zero, the control unit turns on the grounding protection device, so that the non-grounded electrode forms a new low-resistance path with the earth through the grounding point, which can quickly discharge the remaining charge and prevent the fault current from being established again, ensuring that the arc gap insulation strength can be restored when the circuit breaker is opened.

[0052] Indicatively, when the control unit determines whether a single-phase grounding fault occurs, the specific determination logic is as follows:

[0053] When the control unit detects that the differential current of any phase in the three-phase electrical circuit corresponding to the converter is greater than the differential current threshold, the converter is used as the target converter and determines that a single-phase grounding fault occurs in the valve-side circuit of the target converter.

[0054] In a preferred embodiment, each bridge arm in the three-phase electrical circuit corresponding to the converter is provided with a relay protection device;

[0055] The relay protection device is used to collect the arm current of the corresponding arm and send the collected arm current to the control unit, so that the control unit can judge whether the collected arm current is zero.

[0056] Furthermore, in the present invention, by first blocking the converter to cut off the source of the DC component and then waiting for the arm current to naturally pass through zero, it is ensured that the fault current no longer contains the DC component that keeps the arc burning continuously. At this time, the AC circuit breaker can rely on the current zero-crossing point to extinguish the arc normally, avoiding contact erosion and insulation breakdown.

[0057] Moreover, the grounding protection device is connected after the current passes through zero, which can provide a grounding path for the non-grounded pole of the converter, shorten the fault duration, and reduce the impact energy borne by the equipment.

[0058] Therefore, the control unit of the present invention realizes the precise switching of the grounding protection device by real-time monitoring of the zero-crossing state of the arm current, avoiding the wrong arc extinguishing timing caused by direct tripping in the traditional technology, and fundamentally solving the problem of equipment damage caused by the absence of a current zero-crossing point.

[0059] In the present invention, by controlling the non-grounded pole of the converter to be grounded through the grounding protection device, the fault current can recover the zero-crossing point, so that the AC circuit breaker can smoothly cut off the fault current. The grounding protection device of the present invention is applicable to a multi-terminal flexible DC system based on MMC with a bipolar wiring mode (i.e., having a bipolar topology structure) where the DC side is grounded through a resistor, so that the AC circuit breaker can rely on the current zero-crossing point to extinguish the arc normally.

[0060] In a preferred embodiment, the grounding protection device includes: a switching device and a resistor;

[0061] The first port of the switching device is connected to the non-grounded pole corresponding to the converter, and the second port of the switching device is connected to the first port of the resistor;

[0062] The second port of the resistor is grounded.

[0063] Schematically, the resistance value of the resistor is greater than a preset current-limiting resistance threshold value; wherein, the preset current-limiting resistance threshold value is the product of the reference resistance of the flexible DC system and a preset multiple.

[0064] Specifically, the resistor is a current-limiting resistor Rg, and the value range of Rg is 0 < Rg < 10R0, where the preset multiple is 10 and R0 is the reference resistance of the flexible DC system.

[0065] Furthermore, in a preferred embodiment, the grounding protection device includes: a switching device, a fixed resistor, and a variable resistor;

[0066] The first port of the switch device is connected to the non-grounded electrode corresponding to the converter, and the second port of the switch device is connected to the first port of the fixed resistor;

[0067] The second port of the fixed resistor is connected to the first port of the variable resistor, and the second port of the variable resistor is grounded;

[0068] The control unit is further configured to adjust the resistance of the variable resistor according to a fault current corresponding to the single-phase grounding fault when a single-phase grounding fault is detected in the valve-side circuit of a target converter.

[0069] Specifically, the switching device of the present invention can be a large-capacity high-speed switch FSR (Fast Switching Rel ay), which is installed between the non-grounded pole and the resistor (i.e., the resistor after the fixed resistor and the variable resistor are connected in series) of the flexible DC system converter, and is used to quickly connect or disconnect the grounding protection device in the event of a fault. It is a power electronic switching device.

[0070] Schematically, when the flexible DC system is operating normally, the FSR is in the disconnected state and the non-grounded electrode is isolated from the ground; when the control unit determines that ground protection needs to be activated (for example, the bridge arm current drops to zero), the FSR quickly closes, so that the non-grounded electrode is grounded through the resistor, forming a complete path from non-grounded electrode → FSR → resistor Rg → ground.

[0071] The closing timing of the FSR is triggered by the control unit based on the bridge arm current monitoring results, ensuring that it is connected to the ground loop after the MMC is locked and the bridge arm capacitor is discharged, avoiding the impact of the capacitor charging current on the resistor.

[0072] The collaborative mechanism of resistance current limiting and zero-crossing recovery proposed in the present invention is as follows: the resistors in the ground protection device (which can be divided into fixed resistors and variable resistors) act through voltage division, so that the AC voltage, after being divided by the converter transformer, bridge arm reactance and resistors, generates a voltage opposite to the DC component of the fault current, offsetting the DC bias and forcing the fault current to return to zero-crossing, thus solving the problem of arc extinguishing difficulty in AC circuit breakers.

[0073] Schematically, when the fault current is large, the control unit adjusts the variable resistor to reduce the resistance, enhance the voltage division effect, and quickly restore the zero point; when the fault current is small, increasing the resistance of the variable resistor can reduce the grounding loss and improve the system economy.

[0074] In a multi-terminal flexible DC system, the variable resistor can be dynamically adjusted according to the severity of the fault at each end, balancing the potential at each end, suppressing cross-end circulating current, and avoiding system abnormalities caused by potential imbalance.

[0075] Compared to fixed resistors, the present invention uses variable resistors to adapt to fault characteristics under different operating conditions (such as varying ground resistances and system operating modes), expanding the applicability of the protection device. After the fault is cleared, the variable resistor can also return to a higher resistance value, reducing ground loop losses during normal operation and extending the life of the device.

[0076] Therefore, the grounding protection device of the present invention solves the problems of arc extinguishing difficulty of AC circuit breakers and high cost of DC circuit breakers in traditional technologies through precise control of the switching device, current limiting and voltage dividing effects of the resistor, and dynamic adjustment of the variable resistor, and improves the reliability of the flexible DC system under ground faults.

[0077] In a preferred embodiment, the control unit is further configured to generate a warning signal indicating that an abnormality exists in the variable resistor when it is detected that the resistance of the variable resistor exceeds a resistance threshold.

[0078] In schematic form, if the resistance of the variable resistor exceeds the resistance threshold, it may lead to insufficient current limiting capability, unable to effectively offset the DC bias of the fault current, and thus unable to restore the current zero point, making it difficult for the AC circuit breaker to extinguish the arc and lose the protection function. The present invention can generate an early warning signal to promptly prompt the operation and maintenance personnel to handle it, thereby avoiding the expansion of the fault due to abnormal resistance.

[0079] In a preferred embodiment, the control unit is further configured to, when detecting a single-phase grounding fault in a valve-side circuit of a target converter, send a single-phase grounding fault signal to an opposite-side converter located on an opposite side of the target converter;

[0080] The control unit is further configured to cut off the AC source corresponding to the opposite-side converter and lock the opposite-side converter when detecting that the opposite-side converter receives a single-phase grounding fault signal.

[0081] It's understandable that when a single-phase ground fault occurs on the valve side of the target converter, the fault current could be transmitted through the DC line to the opposite converter, causing voltage fluctuations in the opposite AC system or malfunctioning of protection. Sending a fault signal and blocking the opposite converter cuts off the cross-terminal path of the fault current, preventing damage to the opposite converter due to overcurrent or overvoltage, and maintaining system stability on the non-faulty side.

[0082] In a two-terminal flexible DC system, when a ground fault occurs on the valve side of phase A at one end, if the converter on the opposite side is not locked, the phase A bridge arm of the converter on the opposite side may discharge to the fault point through the DC line, causing the AC circuit breaker on the opposite side to bear abnormal current. After locking, the discharge circuit can be cut off.

[0083] In the bipolar topology system of the present invention, after the opposite-side converter is locked, the potential of its non-grounded pole is no longer affected by the fault end, thereby avoiding cross-end circulating current caused by the large potential difference between the two ends, and ensuring the stability of the DC neutral bus zero potential reference point.

[0084] The control unit of the present invention can trigger the protection logic of the opposite converter (non-faulty end) by sending a fault signal after the target converter (faulty end) is locked, causing it to disconnect the AC power source and lock out, forming a coordinated mechanism that eliminates the fault on the faulty end and isolates the non-faulty end for self-protection. If only the faulty end is locked out while the opposite end remains inactive, the opposite converter may continue to deliver power to the faulty end, causing the fault current to persist and preventing the zero-crossing point from being effectively restored.

[0085] Furthermore, after the opposite converter disconnects the AC source, its valve-side current rapidly decays, reducing the interruption burden on the AC circuit breaker at the fault end. Furthermore, locking both converters prevents the failure of zero-crossing recovery of the fault current due to power backflow from the opposite side. This embodiment of the present invention further ensures that the AC circuit breaker can successfully extinguish the arc.

[0086] Furthermore, after the opposite-side converter is locked, the fault-end ground fault protection device is activated, preventing interference from opposite-side power injection on the fault current, ensuring a more accurate voltage divider effect from the resistor, thereby quickly restoring the zero-crossing point of the fault current. If the opposite-side converter is not locked, the capacitor discharge of the opposite-side converter may offset the current-limiting effect of the resistor, resulting in a failure of zero-crossing recovery.

[0087] In a preferred embodiment, a ground protection logic diagram of a control unit using a non-grounding electrode grounded via a resistor is shown as follows: Figure 2 The specific process is as follows:

[0088] During operation of the flexible DC system, the ground fault protection device's control unit monitors the converter valve-side circuit in real time. If a sudden zero-sequence current or zero-sequence voltage shift is detected, it considers a single-phase ground fault on the AC side and triggers the following process:

[0089] Step 1: First, if the phase differential protection is activated, proceed to step 2 (local fault determination); if the phase differential protection is not activated, proceed to step 4 (opposite end fault determination);

[0090] Step 2: Determine if a single-phase grounding fault has occurred on the valve side of the local converter, mark the local converter as the target converter, and send a single-phase grounding fault signal (which may include the fault type, location, and occurrence time) to the opposite converter, initiating the opposite-side coordinated protection logic.

[0091] Step 3: Preparation for locking and grounding the local converter:

[0092] Send a blocking command to the local converter to block the local converter, monitor the upper arm capacitor discharge current, and continuously determine whether the upper arm capacitor discharge current is zero. If it is zero, proceed to step 5 (grounding the non-grounded electrode through a resistor).

[0093] Step 4: Check whether the fault information sent by the opposite side is received. If so, enter the opposite side coordinated protection process; if not, operate normally;

[0094] If the fault information is received from the opposite-side converter, the AC source corresponding to the local converter will be cut off (blocking the opposite-side power from being fed back to the fault point), the command converter will be locked, and power output will be stopped.

[0095] Step 5: Control the fast switching resistor (FSR) to close, and the non-grounded electrode at this end is grounded through the variable resistor and the fixed resistor. The fault current is limited and divided by the resistor, which can force the fault current waveform to return to the zero point (creating conditions for arc extinguishing of the AC circuit breaker).

[0096] Therefore, in this embodiment of the present invention, the signal sent by the faulty end triggers the coordinated shutdown of all converters on the opposite side, preventing the fault from spreading across a multi-terminal network. Compared to the traditional mode of single-end shutdown while the opposite side continues to operate, this invention can reduce the magnitude of the fault current, significantly alleviating the interruption pressure on the AC circuit breaker at the faulty end.

[0097] In a preferred embodiment, Figure 3 The installation diagram of the ground protection device shown is a ground protection architecture diagram of a two-terminal flexible DC transmission system (bipolar topology). Its core purpose is to quickly isolate the fault and limit the fault current when a single-phase ground fault occurs on the converter valve side, ensuring the normal operation of the rest of the system and thus ensuring that the AC circuit breaker can extinguish the arc normally.

[0098] Specifically, in Figure 3 In the system topology (two-terminal bipolar), the AC side is as follows: there is an AC source (such as a new energy power station or AC grid) on each side, which is connected to the converter through a transformer (transformer 1-4).

[0099] DC side: adopts bipolar topology, with positive and negative DC lines connecting the converters at both ends (MMC1-MMC4), forming an energy transmission channel from positive pole to DC line to negative pole;

[0100] Ground protection device: The positive and negative poles of each converter are equipped with a ground protection device consisting of "FSR (fast switch) + current limiting resistor Rg" to quickly limit current when a fault occurs.

[0101] Specifically, taking a single-phase grounding fault occurring on the valve side of transformer 1 as an example, at this time, the phase differential protection at end 1 operates quickly, and MMC1 at end 1 is identified as the fault end. The control unit can send a grounding fault message to the opposite end (i.e., MMC3 at end 3). At the same time, MMC1 at end 1 enters a locked state. When it is detected that the charging current of the upper arm capacitor of MMC1 is zero, the FSR switch in the protection device 1 is controlled to close, and the non-grounded pole of MMC1 is grounded through resistor Rg. At this time, the AC circuit breaker at end 1 can be disconnected normally.

[0102] Indicatively, at this time, the phase differential protection at terminal 3 is not activated, and terminal 3 is identified as a non-fault terminal. However, terminal 3 receives the ground fault information sent by terminal 1, so terminal 3 cuts off the AC source (i.e., disconnects the grid-side AC circuit breaker), and at the same time, the MMC3 at terminal 3 enters a locked state.

[0103] At this time, the phase differential protection at terminals 2 and 4 does not operate, terminals 2 and 4 are identified as non-fault terminals, and terminals 2 and 4 do not receive ground fault information, so terminals 2 and 4 remain in operation without change.

[0104] Therefore, the embodiment of the present invention can quickly locate the fault end through phase differential protection, and can ensure reliable disconnection of the fault end circuit breaker by locking the converter, monitoring the capacitor discharge current, and controlling the connectivity of the grounding protection device.

[0105] Moreover, the embodiment of the present invention can only trigger the locking and AC source disconnection of the fault-associated end (opposite end), while other non-associated ends remain in operation, thereby blocking the cross-end transmission of the fault and minimizing the scope of power outage, thereby improving the continuity of system power supply.

[0106] In a preferred embodiment, the present invention installs a ground fault protection device comprising a high-speed switch in series with a current-limiting resistor at the non-grounded DC-side electrode of each converter at each end of the flexible DC network. This allows the AC voltage to be divided through the converter transformer, converter reactor, and current-limiting resistor. When a single-phase ground fault occurs on the valve side of the flexible DC system, the ground fault protection device can rapidly restore the zero-crossing point of the fault current, allowing the AC circuit breaker to extinguish arcs smoothly, thus ensuring the safe and reliable operation of the flexible DC grid.

[0107] like Figure 4 As shown, based on the above-mentioned various embodiments of the grounding protection device applicable to the flexible DC system, the present invention provides a corresponding method embodiment;

[0108] An embodiment of the present invention provides a grounding protection method applicable to a flexible DC system, which is applied to a control unit in a grounding protection device applicable to a flexible DC system;

[0109] The grounding protection method comprises:

[0110] When a single-phase grounding fault is detected in a valve-side circuit of a target converter, the target converter is locked;

[0111] When it is determined that the arm current corresponding to the non-grounded electrode of the target converter is zero, the grounding protection device is controlled to be connected to the non-grounded electrode corresponding to the converter.

[0112] In a preferred embodiment, the grounding protection method applicable to a flexible DC system further includes:

[0113] sending a single-phase ground fault signal to an opposite-side converter disposed on an opposite side of the target converter;

[0114] The AC source corresponding to the opposite-side converter is cut off, and the opposite-side converter is locked at the same time.

[0115] It should be noted that those skilled in the art can clearly understand that, for the sake of convenience and brevity, the specific working process of the grounding protection method described above can refer to the corresponding process in the aforementioned device embodiment, and will not be repeated here.

[0116] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A grounding protection device suitable for a flexible DC system, characterized in that: include: A flexible direct current system with a bipolar topology, a control unit, and several grounding protection devices; wherein each converter in the flexible direct current system is connected to a grounding protection device; One end of the grounding protection device is connected to the non-grounded electrode corresponding to the converter, and the other end of the grounding protection device is connected to the ground; The control unit is used to lock the target converter when a single-phase grounding fault is detected in the valve-side circuit of a target converter, and then control the grounding protection device to connect with the non-grounding electrode corresponding to the target converter when it is determined that the bridge arm current corresponding to the non-grounding electrode of the target converter is zero.

2. A grounding protection device suitable for a flexible DC system according to claim 1, characterized in that: The grounding protection device includes: a switch device and a resistor; The first port of the switch device is connected to the non-grounded electrode corresponding to the converter, and the second port of the switch device is connected to the first port of the resistor; The second terminal of the resistor is grounded.

3. A grounding protection device suitable for a flexible DC system according to claim 2, characterized in that: The resistance of the resistor is greater than a preset current limiting resistance threshold; wherein the preset current limiting resistance threshold is the product of a reference resistance of the flexible DC system and a preset multiple.

4. The grounding protection device for a flexible DC system according to claim 1, characterized in that: The ground protection device includes: a switch device, a fixed resistor and a variable resistor; The first port of the switch device is connected to the non-grounded electrode corresponding to the converter, and the second port of the switch device is connected to the first port of the fixed resistor; The second port of the fixed resistor is connected to the first port of the variable resistor, and the second port of the variable resistor is grounded; The control unit is further configured to adjust the resistance of the variable resistor according to a fault current corresponding to the single-phase grounding fault when a single-phase grounding fault is detected in the valve-side circuit of a target converter.

5. A grounding protection device suitable for a flexible DC system according to claim 3 or 4, characterized in that: The control unit is further configured to send a single-phase grounding fault signal to an opposite-side converter arranged on an opposite side of the target converter when a single-phase grounding fault is detected in a valve-side circuit of a target converter; The control unit is further configured to cut off the AC source corresponding to the opposite-side converter and lock the opposite-side converter when detecting that the opposite-side converter receives a single-phase grounding fault signal.

6. A grounding protection device suitable for a flexible DC system according to claim 5, characterized in that: The control unit is used to take the converter as the target converter and determine that a single-phase grounding fault occurs in the valve-side circuit of the target converter when it detects that the differential current of any phase in the three-phase electrical circuit corresponding to the converter is greater than the differential current threshold.

7. A grounding protection device suitable for a flexible DC system according to claim 6, characterized in that: Each bridge arm of the three-phase electrical circuit corresponding to the converter is provided with a relay protection device; The relay protection device is used to collect the bridge arm current of the bridge arm where it is located, and send the collected bridge arm current to the control unit.

8. The grounding protection device applicable to a flexible DC system according to claim 7, characterized in that: The control unit is further configured to generate a warning signal indicating that an abnormality exists in the variable resistor when it is detected that the resistance of the variable resistor exceeds a resistance threshold.

9. A grounding protection method applicable to a flexible DC system, characterized in that: A control unit used in a grounding protection device for a flexible DC system as claimed in any one of claims 1 to 8; The grounding protection method comprises: When a single-phase grounding fault is detected in a valve-side circuit of a target converter, the target converter is locked; When it is determined that the arm current corresponding to the non-grounded electrode of the target converter is zero, the grounding protection device is controlled to be connected to the non-grounded electrode corresponding to the converter.

10. The grounding protection method applicable to a flexible DC system according to claim 9, characterized in that: Also includes: sending a single-phase ground fault signal to an opposite-side converter located on an opposite side of the target converter; The AC source corresponding to the opposite-side converter is cut off, and the opposite-side converter is locked at the same time.

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