A LCC-HVDC transmission line differential protection method based on double-terminal station area ratio braking fault direction identification

Abstract

The application relates to an LCC-HVDC power transmission line differential protection method based on a double-terminal station area ratio brake fault direction identification, and belongs to the technical field of extra-high voltage direct current power transmission line protection. Firstly, a station area measuring point and a current collection area are defined; a ratio brake fault direction identification criterion is constructed by using rectifier station area measuring point current, noise interference is avoided by prolonging the criterion time window; a grounding electrode current is used as a basis for selecting a fault pole, a ratio brake type fault direction identification element is installed, the protection adopts a restart strategy, when the rectifier side trigger angle is judged as a forward fault, the rectifier side trigger angle is forcibly phase shifted and restarted according to 18 DEG-120 DEG-160 DEG-18 DEG, if a blocking signal from the opposite end is received, the control system immediately forcibly phase shifts to 160 DEG-90 DEG to block the rectifier station. If the protection judges that the fault is a reverse external fault, the rectifier side trigger angle is forced to phase shift according to 18 DEG-160 DEG-90 DEG to block the rectifier station. The application can reliably identify the fault direction, has strong anti-interference capability, and greatly improves the fault removal speed compared with longitudinal differential protection.

Description

Technical Field

[0001] This invention relates to a differential protection method for high voltage direct current (LCC-HVDC) transmission lines based on dual-end station domain ratio braking fault direction identification, belonging to the field of ultra-high voltage direct current transmission line protection technology. Background Technology

[0002] With the increasing use of renewable energy sources such as wind and solar power, these power generation centers are located in relatively remote areas far from load centers. Therefore, a power transmission technology with large capacity and long transmission distance is needed to transmit electricity. Ultra-high voltage direct current (UHVDC) transmission has many advantages over AC transmission, such as large transmission capacity, long transmission distance, convenient grid interconnection, and easy power regulation, and has therefore received widespread attention and development.

[0003] Given the more complex structure and diverse operating modes of ultra-high voltage direct current (UHVDC) transmission systems, the requirements for control and protection are also higher. Furthermore, UHVDC transmission lines are long, traversing complex terrains and surrounding environments, making them more prone to failure compared to other components. Statistics show that transmission line failures account for approximately 50% of all UHVDC system faults, making the protection of UHVDC transmission lines particularly crucial.

[0004] Currently, traveling wave protection operates rapidly, but it is susceptible to interference, requires high sampling frequency, is insensitive to high-resistance grounding faults, and lacks mature setting principles. Differential undervoltage protection has higher reliability than traveling wave protection, but its shortcomings are similar. Longitudinal differential protection can detect high-resistance grounding faults, but it has high communication requirements and is greatly affected by the distributed capacitance current of the line, resulting in a slow response speed. Therefore, proposing a protection system with high resistance to high impedance, high speed capability, and high resistance to high impedance is of great significance for improving the protection principle and scheme of DC lines and ensuring the safe and reliable operation of the system. Summary of the Invention

[0005] The purpose of this invention is to provide a differential protection method for LCC-HVDC transmission lines based on fault direction identification using a dual-end station domain ratio braking system. This method aims to solve the technical problem that single-end DC line protection cannot infer the fault location from waveform characteristics, which makes it difficult to guarantee reliable fault direction identification, anti-interference capability, and fault clearing speed.

[0006] To achieve the above objectives, this invention provides a differential protection method for LCC-HVDC transmission lines based on ratio-controlled fault direction identification at both ends of the station. This method selects appropriate coordination strategies for the protection and control systems based on the actual judgment results of the starting criteria, ratio-controlled fault direction identification on the rectifier and inverter sides, and fault pole selection criteria. It distinguishes between transient and permanent line faults, improving the reliability and speed of protection while ensuring protection selectivity. The method includes:

[0007] When a fault occurs in a DC transmission system, the fault area is determined by the ground electrode current and fault voltage gradient start-up criteria, the fault direction identification element criteria of the dual-end station area ratio braking, and the fault electrode selection criteria based on the direction of ground electrode current flow.

[0008] Based on the fault area identified by the protection, a corresponding protection and control coordination strategy is determined.

[0009] A DC transmission line protection system is applied to a large-capacity ultra-high voltage DC transmission system. The system is a true bipolar transmission structure based on a grid-commutated converter. Smoothing reactors and DC filters are installed at both ends of the line. The grid-commutated converter is a dual 12-pulse series valve group.

[0010] The specific start criterion is as follows:

[0011] Criteria for activating rectifier-side protection:

[0012]

[0013] Inverter-side protection activation criteria:

[0014]

[0015] Where, ΔU set To initiate the voltage gradient threshold criterion, |ΔU dcR (N)| and |ΔU dcI (N)| represents the voltage gradient values ​​on the rectifier side and inverter side at sampling time N, respectively. dcR (Nj) and U dcI (Nj) represent the sampled values ​​of the rectifier-side and inverter-side electrode voltages before the j-th sampling period at time N, respectively. GND.set To trigger the ground electrode current threshold criterion, i DC.set i is the setting value for bipolar fault detection. DCR+ / - and i DCI+ / - These are the positive and negative DC currents on the rectifier and inverter sides.

[0016] The specific criteria for the dual-end station domain ratio braking fault direction identification element are as follows:

[0017] Rectifier station domain fault direction identification:

[0018]

[0019] Inverter station fault direction identification:

[0020]

[0021] Among them, i op For the station differential current, i rFor the station braking current, i opR.set and i opI.set S sets the lower limit value for the operating current setting of the rectifier and inverter substation. r i is the proportional braking coefficient. dcR+ i dcR- i GNDR These represent the positive and negative line currents and the grounding electrode current in the rectifier station current collection area, respectively. dcI+ i dcI- i GNDI These are the positive and negative line currents and the grounding electrode current in the inverter station current collection area, respectively. Among them, the differential current i under a positive fault... op ≈0; Differential current i under reverse fault op ≈i r i op Much larger than S r i r andi op.set .

[0022] The specific criterion for selecting the fault electrode based on the direction of the grounding electrode current is as follows:

[0023]

[0024] Among them, I GND.set The grounding electrode current fault selection setting value is based on the maximum unbalanced current |i detected at the grounding electrode. max.unb │To set it up, I GND.set =1.2|i max.unb │.

[0025] The specific steps for determining the fault area are:

[0026] DC system forward line fault: After the fault traveling wave reaches the protection measuring point, the rectifier station differential protection is at |ΔU dcR (N)|>ΔU set and|i GNDR (t)|>i GND.set After starting, the fault direction is determined to be a forward fault in the station area after 6ms and the pole selection ends. The protection output is activated. The fault direction determination result of the inverter station area is a non-station area fault, and it is unable to send the blocking ESOF signal to the rectifier station. Therefore, the comprehensive judgment result is a forward line fault.

[0027] DC system positive out-of-range fault: rectifier station differential protection at |ΔU dcR (N)|>ΔU set and|i GNDR (t)|>i GND.setAfter starting, the fault direction is determined to be a positive fault in the station area after 6ms and the pole selection ends. The protection determines the fault to be a positive fault. The inverter station area ratio braking fault direction discrimination element determines the fault to be a station area fault. Therefore, the comprehensive judgment result is a fault outside the positive zone.

[0028] DC system reverse zone external fault: rectifier station differential protection at |ΔU dcR (N)|>ΔU set and|i GNDR (t)|>i GND.set After startup, the fault direction is determined to be a positive fault in the station area after 6ms and the pole selection ends. The protection determines the fault to be a reverse fault. The inverter station area ratio braking fault direction discrimination element determines that the fault is not in the station area. Therefore, the comprehensive judgment result is a fault outside the reverse zone.

[0029] The specific steps for determining the corresponding protection and control coordination strategy based on the fault area identified by the protection are as follows:

[0030] DC system forward line fault: The control system will prioritize the rectifier side trigger angle to force phase shift restart according to 18°-120°-160°-18°, implement the trigger restart logic; and continuously wait for the blocking ESOF signal sent by the inverter station domain ratio braking fault direction discrimination element to correct the control strategy;

[0031] For DC system out-of-direction fault: The control system will prioritize restarting the rectifier side firing angle according to a forced phase shift of 18°-120°-160°-18°, and implement the trigger restart logic; and continuously wait for the blocking ESOF signal sent by the inverter station domain ratio braking fault direction discrimination element to correct the control strategy. If a blocking signal is received from the other end during the restart process, it is judged as an out-of-direction fault, and the control system will block the rectifier station by forcibly shifting the rectifier side firing angle according to a forced phase shift of 160°-90°.

[0032] DC system reverse zone external fault: The rectifier station domain protection judges the fault as a reverse station domain fault, and the protection sends an ESOF signal to the rectifier station to force phase shift blocking.

[0033] To achieve the above objectives, the present invention also provides a differential protection device for LCC-HVDC transmission lines based on dual-end station domain ratio braking fault direction identification, the device comprising:

[0034] The fault data acquisition module is used to acquire line fault characteristics;

[0035] The protection activation module is used to determine whether the protection should be activated under fault conditions;

[0036] A station-area ratio-controlled fault direction identification module is used to determine the fault direction;

[0037] The fault electrode selection module is used to select the fault electrode based on the direction of the grounding electrode current.

[0038] The fiber optic communication channel is used to obtain the ESOF signal sent by the station ratio braking fault direction identification module.

[0039] To achieve the above objectives, the present invention also provides a storage medium, which is a computer-readable storage medium, and stores a computer program thereon. When the computer program is executed by a processor, it implements the steps of the LCC-HVDC transmission line differential protection method based on dual-end station domain ratio braking fault direction identification as described above.

[0040] This invention, employing the above technical solutions, possesses the following beneficial effects: For ultra-high voltage direct current (UHVDC) transmission lines, this invention can reliably identify the fault direction based on fault characteristics and station-area ratio differential protection, and exhibits strong resistance to transition resistance. It utilizes the current value at the grounding electrode measuring terminal to reliably identify grounding faults and AC faults within the station, effectively avoiding premature phase shifting of the control system under AC system faults. This invention's protection strategy truly solves the problem that single-ended DC line protection cannot infer the fault location from waveform characteristics, and can quickly suppress fault current for inverter station-area faults, facilitating rapid fault clearance. In summary, this invention can be widely applied to large-capacity UHVDC transmission lines. Attached Figure Description

[0041] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments mentioned in this application without creative work are within the scope of protection of this application.

[0042] Figure 1 This is a flowchart illustrating the strategy of the present invention;

[0043] Figure 2 This is a simulation circuit diagram of a true bipolar ultra-high voltage DC system according to an embodiment of the present invention;

[0044] Figure 3 The waveform diagram for protection when the line is grounded in the positive region;

[0045] Figure 4 The protection waveform diagram is shown when the converter valve in the area outside the positive zone is grounded.

[0046] Figure 5 The waveform diagram for protection when the converter valve in the reverse zone is grounded is shown.

[0047] Figure 6This is a timing diagram of the protection simulation under different fault conditions in the true bipolar ultra-high voltage DC system according to an embodiment of the present invention. Detailed Implementation

[0048] The technical solutions of the embodiments of this application will be clearly and thoroughly described below with reference to the accompanying drawings and specific embodiments. For ease of description, it is obvious that the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0049] It should be understood that spatial relative terms, such as "above," "below," "left," and "right," may be used in the text. These spatial relative terms are used to indicate the different positions of related devices or signals in the diagram.

[0050] To address the frequent transient faults in high-capacity ultra-high voltage direct current (UHVDC) transmission lines, this invention proposes a differential protection method, device, and medium for LCC-HVDC transmission lines based on dual-end station-area ratio-controlled fault direction identification. The method includes: reliable fault direction identification based on fault characteristics and the actual judgment results of dual-end station-area ratio-controlled direction discrimination elements, effectively clearing permanent line faults; reliable restart of the rectifier-converter station under transient faults, significantly shortening outage time, and strong resistance to transition resistance. Reliable identification of in-station grounding faults and AC faults is achieved based on the current value at the grounding electrode measurement terminal, effectively avoiding premature forced phase shifting of the control system under AC system faults. This invention truly solves the problem that single-end DC line protection cannot infer fault location from waveform characteristics, and can quickly suppress fault current for inverter station-area faults, facilitating rapid fault clearing.

[0051] Specific embodiments of the present invention will now be described in more detail with reference to the accompanying drawings.

[0052] Example 1: As Figure 1 The diagram shown is a strategy flowchart of a differential protection method for LCC-HVDC transmission lines based on fault direction identification using a ratio braking system in a dual-terminal station area, according to this application. The method in this embodiment can effectively select the appropriate control strategy to handle DC system faults by utilizing the discrimination results of the protection initiation element, fault direction discrimination element, and fault pole selection element in the dual-terminal station area, while ensuring correct data sampling. The method includes:

[0053] When a DC transmission system fails, the fault area is determined by the ground electrode current and fault voltage gradient start-up criteria, the fault direction identification element criteria of the double-end station area ratio braking, and the fault electrode selection criteria based on the direction of ground electrode current flow.

[0054] Based on the fault area identified by the protection, a corresponding protection and control coordination strategy is determined.

[0055] A DC transmission line protection system is applied to a large-capacity ultra-high voltage DC transmission system. The system is a true bipolar transmission structure based on a grid-commutated converter. Smoothing reactors and DC filters are installed at both ends of the line. The grid-commutated converter is a dual 12-pulse series valve group.

[0056] Both DC faults and AC fault ride-throughs cause DC voltage fluctuations. An improved voltage gradient algorithm and grounding current are used as the protection activation criteria.

[0057] Criteria for activating rectifier-side protection:

[0058]

[0059] Inverter-side protection activation criteria:

[0060]

[0061] Where, ΔU set The activation criterion voltage gradient threshold is set according to the minimum DC voltage gradient U caused by AC fault ride-through within 0.6 ms. dc.T To adjust ΔU set =K U.rel U dc.T K U.rel K is the voltage gradient start-up reliability coefficient. U.rel =0.9; |ΔU dcR (N)| and |ΔU dcI (N)| represents the voltage gradient values ​​on the rectifier side and inverter side at sampling time N, respectively. dcR (Nj) and U dcI (Nj) represent the sampled values ​​of the rectifier-side and inverter-side electrode voltages before the j-th sampling period at time N, respectively. GND.set To trigger the grounding electrode current threshold, the maximum unbalanced current i caused by the imbalance of the two electrode parameters is used. max.unb Absolute value tuning i GND.set =1.3i max.unb i DC.set The setting value for bipolar fault diagnosis is based on the maximum DC i caused by an AC fault. DC.max To adjust i DC.set =1.1i DC.max i DC.set This is the setting value for bipolar fault detection. DCR+ / - and i DCI+ / - These are the positive and negative DC currents on the rectifier and inverter sides, respectively. U dc.T =0.05pu, ΔU set =0.045pu, iGND.set =150A, i DC.max =8.1kA, i DC.set =8.9kA.

[0062] Faults in the DC area outside each station area are defined as station-area forward faults, while faults within the station area are defined as reverse station-area faults. To facilitate the differentiation of fault direction, the positive and negative line current measurement points and the grounding electrode current measurement point belonging to the same end are defined as station area measurement points, and the area between the station area measurement points and the converter transformer valve side is defined as the current collection area. Based on Kirchhoff's Current Law (KCL), a ratio braking is introduced into the fault direction criterion, and the station area ratio braking fault direction identification criterion is constructed based on the difference in current characteristics flowing through the current collection area.

[0063] Rectifier station domain fault direction identification:

[0064]

[0065] Inverter station fault direction identification:

[0066]

[0067] Among them, i op For the station differential current, i r For the station braking current, i opR.set and i opI.set S sets the lower limit value for the operating current setting of the rectifier and inverter substation. r i is the proportional braking coefficient. dcR+ i dcR- i GNDR These represent the positive and negative line currents and the grounding electrode current in the rectifier station current collection area, respectively. dcI+ i dcI- i GNDI These are the positive and negative line currents and the grounding electrode current in the inverter station current collection area, respectively. Among them, the differential current i under a positive fault... op ≈0; Differential current i under reverse fault op ≈i r i op Much larger than S r i r andi op.set。

[0068] i opR.set and i opRI.setf The maximum differential current i caused by forward faults in the rectifier and inverter substations are calculated separately. op.max To adjust, take i op.set =K rel.un ×i op.max Reliability coefficient K rel.un =1.3, where: iopR.max =│i GNDR +i dcR+ +i dcR- │ max i opI.max =│i GNDI +i dcI+ +i dcI- │ max。

[0069] Based on the principle that the current flows in opposite directions under positive and negative grounding faults, the criterion for selecting the fault electrode based on the direction of the grounding current is as follows:

[0070]

[0071] Among them, I GND.set The grounding electrode current fault selection setting value is based on the maximum unbalanced current |i detected at the grounding electrode. max.unb │To set it up, I GND.set =1.2|i max.unb |=138A. Based on the actual engineering protection and fault identification time window of approximately 5ms, the fault direction identification and pole selection time is set to 6ms.

[0072] The specific fault area is as follows:

[0073] DC system forward line fault: After the fault traveling wave reaches the protection measuring point, the control system will increase the phase shift of the rectifier side firing angle; the rectifier station differential protection will operate at |ΔU dcR (N)|>ΔU set and|i GNDR (t)|>i GND.set After restarting, the fault direction is determined to be a positive fault in the station area after 6ms, and the pole selection ends. After confirming that the lightning interference has been avoided, the protection output is activated. The control system will prioritize the rectifier side trigger angle to force a phase shift restart according to 18°-120°-160°-18°, and implement the trigger restart logic. It will continuously wait for the blocking ESOF signal sent by the inverter station area ratio braking fault direction discrimination element to correct the control strategy. If the rectifier station does not receive the emergency shutdown signal (ESOF) sent by the inverter station area ratio braking fault direction discrimination element during the restart process, it can be determined that there is a grounding fault in the line area. If the system returns to normal after the restart, it will continue to operate. If the system encounters a fault again, it will be directly blocked and shut down.

[0074] DC system positive-side fault: After the fault traveling wave reaches the protection measuring point, the control system will increase the phase shift of the rectifier side firing angle; the rectifier station differential protection will be in the range of |ΔU dcR (N)|>ΔU set and|i GNDR (t)|>i GND.setAfter startup, the fault direction is determined to be a positive fault in the station area after 6ms, and pole selection ends. After confirming that lightning interference has been avoided, the protection output activates. The control system prioritizes a forced phase shift restart of the rectifier side firing angle according to 18°-120°-160°-18°, implementing the trigger restart logic. It continuously waits for the ESOF signal sent by the inverter station area ratio braking fault direction discrimination element to correct the control strategy. During the restart process, a blocking signal is received from the other end, indicating a fault outside the positive zone. The control system then forces a phase shift to block the rectifier station by a forced phase shift of the rectifier side firing angle to 160°-90°.

[0075] DC system reverse zone external fault: rectifier station differential protection at |ΔU dcR (N)|>ΔU set and|i GNDR (t)|>i GND.set After startup, and following a 6ms fault direction determination and pole selection process, the fault is determined to be a reverse station-domain fault. The protection then sends an ESOF signal to the rectifier station to force phase shift blocking. Before the forced phase shift, the rectifier-side firing angle will follow the reference current variation.

[0076] Based on the fault area identified by the protection system, the corresponding protection and control coordination strategy is determined as follows:

[0077] DC system forward line fault: The control system will prioritize the rectifier side trigger angle to force phase shift restart according to 18°-120°-160°-18°, implement the trigger restart logic; and continuously wait for the blocking ESOF signal sent by the inverter station domain ratio braking fault direction discrimination element to correct the control strategy;

[0078] For DC system out-of-direction fault: The control system will prioritize restarting the rectifier side firing angle according to a forced phase shift of 18°-120°-160°-18°, and implement the trigger restart logic; and continuously wait for the blocking ESOF signal sent by the inverter station domain ratio braking fault direction discrimination element to correct the control strategy. If a blocking signal is received from the other end during the restart process, it is judged as an out-of-direction fault, and the control system will block the rectifier station by forcibly shifting the rectifier side firing angle according to a forced phase shift of 160°-90°.

[0079] DC system reverse zone external fault: The rectifier station domain protection judges the fault as a reverse station domain fault, and the protection sends an ESOF signal to the rectifier station to force phase shift blocking.

[0080] A differential protection device for LCC-HVDC transmission lines based on dual-end station ratio braking fault direction identification, the device comprising:

[0081] The fault data acquisition module is used to acquire line fault characteristics;

[0082] The protection activation module is used to determine whether the protection should be activated under fault conditions;

[0083] A station-area ratio-controlled fault direction identification module is used to determine the fault direction;

[0084] The fault electrode selection module is used to select the fault electrode based on the direction of the grounding electrode current.

[0085] The fiber optic communication channel is used to obtain the ESOF signal sent by the station ratio braking fault direction identification module.

[0086] A storage medium, which is a computer-readable storage medium, stores a computer program thereon, which, when executed by a processor, implements the steps of the LCC-HVDC transmission line differential protection method based on dual-end station domain ratio braking fault direction identification as described above.

[0087] In this embodiment, as Figure 2 The simulation circuit model of the true bipolar ultra-high voltage direct current system shown is divided into three regions: the left reverse protection zone, the right forward line zone, and the right forward line zone. The reverse protection zone includes a rectifier-converter with a transmission capacity of 5000MW (dual 12-pulse series valve group) and a voltage level of ±800kV. The total length of the right forward line is 1373km. The inverter station in the right forward line zone also has a dual 12-pulse series valve group structure.

[0088] To verify the effectiveness and correctness of the method of this invention, this embodiment sets up line end grounding, forward zone external grounding, and reverse zone external electrode grounding for analysis. The simulation circuit model of the above-mentioned true bipolar ultra-high voltage DC system is simulated to obtain protection simulation waveform diagrams. Figure 3 This is a waveform diagram of the protection system under grounding at the end of the line in the positive direction region. Figure 4 This is a waveform diagram of the protection system under positive external grounding. Figure 5 This is a waveform diagram of the protection system under grounding outside the reverse zone. Figure 6 This is a simulation timing diagram of protection under different fault conditions in a true bipolar ultra-high voltage DC system.

[0089] from Figure 3 It can be seen that when a fault occurs in the forward region of the DC system, after the fault traveling wave reaches the protection measuring point, the differential protection in the rectifier station domain is at |ΔU dcR (N)|>ΔU set and|i GNDR (t)|>i GND.set After startup, the fault direction was determined to be a positive fault in the station area after 6ms, and pole selection ended. After confirming that lightning interference had been avoided, the protection output activated. The control system prioritized a forced phase-shift restart of the rectifier side trigger angle according to 18°-120°-160°-18°, implementing the trigger restart logic. The inverter station area ratio braking fault direction discrimination element was in |ΔUdcI (N)|>ΔU set and|i GNDI (t)|>i GND.set After startup, and after 6ms, the fault direction is determined and the fault pole selection is completed. The fault is determined to be a non-inverter station domain fault, and no blocking signal is sent to the rectifier station. Therefore, the rectifier station continues the restart process. If the system returns to normal after the restart, it will continue operating; if the system encounters another fault, it will be directly blocked and shut down.

[0090] from Figure 4 It can be seen that when a fault occurs outside the forward zone of the DC system, after the fault traveling wave reaches the protection measuring point, the differential protection in the rectifier station domain is at |ΔU dcR (N)|>ΔU set and|i GNDR (t)|>i GND.set The system started subsequently; however, after 6.6 ms, the inverter substation ratio braking fault direction discrimination element determined the fault to be in the inverter substation area, and therefore sent a blocking signal to the rectifier substation. The rectifier substation received the ESOF signal at 11.2 ms, and the control system forcibly phase-shifted the rectifier-side firing angle by 160°-90° to block the rectifier substation. Because the ESOF signal was received before the rectifier substation protection activated, the protection action was ineffective.

[0091] from Figure 5 It can be seen that when a fault occurs in the area outside the positive zone of the DC system, the differential protection of the rectifier station area is in |ΔU dcR (N)|>ΔU set and|i GNDR (t)|>i GND.set After starting, the fault direction is determined and the pole selection is completed in 6ms. The determination result is a reverse station domain fault. The protection sends an ESOF signal to the rectifier station to force phase shift blocking of the rectifier station.

[0092] from Figure 6 As can be seen, the protection operation timing diagrams are shown for different fault zones. For forward faults, if the rectifier station protection does not receive an ESOF signal from the inverter station before operation, the control first initiates a restart procedure, and then blocks the rectifier converter after receiving the ESOF signal. For reverse faults, if the ratio braking direction discrimination element in the rectifier station determines that it is a reverse fault, it directly sends an ESOF signal to the rectifier station to block the rectifier converter.

[0093] The above embodiments are merely illustrative of the technical concept of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of this patent application. It should be noted that any modifications made by those skilled in the art to the technical solution without departing from the concept of this application fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A differential protection method for LCC-HVDC transmission lines based on dual-end station ratio braking fault direction identification, characterized in that, The method includes: When a fault occurs in a DC transmission system, the fault area is determined by the ground electrode current and fault voltage gradient start-up criteria, the fault direction identification element criteria of the dual-end station area ratio braking, and the fault electrode selection criteria based on the direction of ground electrode current flow. Based on the fault area identified by the protection, a corresponding protection and control coordination strategy is determined. A DC transmission line protection system is applied to a large-capacity ultra-high voltage DC transmission system. The system is a true bipolar transmission structure based on a grid-commutated converter. Smoothing reactors and DC filters are installed at both ends of the line. The grid-commutated converter is a dual 12-pulse series valve group. The specific start criterion is as follows: Criteria for activating rectifier-side protection: ； Inverter-side protection activation criteria: ； Where, Δ U set To trigger the voltage gradient threshold criterion, and Sampling time N The voltage gradient values ​​on the rectifier side and inverter side. U dcR ( Nj )and U dcI ( Nj (representing time) N No. j Sampled values ​​of rectifier-side and inverter-side electrode voltages before each sampling period. i GND.set To trigger the ground electrode current threshold criterion, i DC.set This is the setting value for bipolar fault detection. i DCR+ / - and i DCI+ / - These are the positive and negative DC currents on the rectifier and inverter sides, respectively. The specific criteria for the dual-end station domain ratio braking fault direction identification element are as follows: Rectifier station domain fault direction identification: ； Inverter station fault direction identification: ； Among them, i op For the station differential current, i r For the station braking current, i opR.set and i opI.set S sets the lower limit value for the operating current setting of the rectifier and inverter substation. r i is the proportional braking coefficient. dcR+ i dcR- i GNDR These represent the positive and negative line currents and the grounding electrode current in the rectifier station current collection area, respectively. dcI+ i dcI- i GNDI These are the positive and negative line currents and the grounding electrode current in the inverter station current collection area, respectively.

2. The differential protection method for LCC-HVDC transmission lines based on dual-end station domain ratio braking fault direction identification according to claim 1, characterized in that, The specific criterion for selecting the fault electrode based on the direction of the grounding electrode current is as follows: ； in, I GND.set The grounding electrode current fault selection setting value is based on the maximum unbalanced current detected at the grounding electrode. Come and fix it. .

3. The differential protection method for LCC-HVDC transmission lines based on dual-end station ratio braking fault direction identification according to claim 1, characterized in that, The specific steps for determining the corresponding protection and control coordination strategy based on the fault area identified by the protection are as follows: DC system forward line fault: The control system will prioritize a phase-shift restart of the rectifier side trigger angle according to 18°-120°-160°-18°, and implement the trigger restart logic. It continuously waits for the blocking ESOF signal sent by the inverter station domain ratio braking fault direction discrimination element to correct the control strategy; DC system positive zone external fault: The control system will prioritize the phase shift restart of the rectifier side trigger angle according to 18°-120°-160°-18°, and implement the trigger restart logic; It continuously waits for the blocking ESOF signal sent by the inverter station domain ratio braking fault direction discrimination element to correct the control strategy. If a blocking signal is received from the other end during the restart process, it is judged to be a fault outside the positive zone. The control system will force the rectifier side firing angle to phase shift and block the rectifier station according to 160°-90°. DC system reverse zone external fault: The rectifier station domain protection judges the fault as a reverse station domain fault, and the protection sends an ESOF signal to the rectifier station to force phase shift blocking.

4. An apparatus for implementing the differential protection method for LCC-HVDC transmission lines based on dual-end station ratio braking fault direction identification as described in claim 1, characterized in that, The device includes: The fault data acquisition module is used to acquire line fault characteristics; The protection activation module is used to determine whether the protection should be activated under fault conditions; A station-area ratio-controlled fault direction identification module is used to determine the fault direction; The fault electrode selection module is used to select the fault electrode based on the direction of the grounding electrode current. The fiber optic communication channel is used to obtain the ESOF signal sent by the station ratio braking fault direction identification module.

5. A storage medium, characterized in that, The storage medium is a computer-readable storage medium, and a computer program is stored on the storage medium. When the computer program is executed by a processor, it implements the steps of the differential protection method for LCC-HVDC transmission lines based on dual-end station domain ratio braking fault direction identification as described in any one of claims 1 to 3.

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