LCC-HVDC power transmission line differential protection method based on double-end station domain ratio braking fault direction identification

Abstract

The invention relates to an LCC-HVDC power transmission line differential protection method based on double-end station domain ratio braking fault direction identification, and belongs to the technical field of ultrahigh voltage direct current power transmission line protection. Firstly, a station domain measurement point and a current collection area are defined; a ratio braking fault direction identification criterion is constructed by using a rectification station domain measurement point current, and noise interference is avoided by prolonging a criterion time window; grounding electrode current is used as a basis for protecting and selecting a fault electrode, a ratio braking type fault direction identification element is installed, a restart strategy is adopted for protection, when a forward fault is judged, a rectification side trigger angle is forcibly subjected to phase shift restart according to 18 degrees to 120 degrees to 160 degrees to 18 degrees, and if an opposite-end locking signal is received, a rectification side trigger angle is not subjected to phase shift restart. And if so, the control system forcibly shifts the phase to 160-90 degrees immediately to lock the rectifier station. And if the fault outside the reverse region is judged through protection, the rectifier station is locked according to the trigger angle of the rectifier side of 18-160 degrees to 90 degrees. According to the method, the fault direction can be reliably identified, the anti-interference capability is relatively high, and the fault removal speed is greatly improved compared with longitudinal differential protection.

Description

Technical Field

[0001] The invention relates to a high voltage direct current (LCC-HVDC) transmission line differential protection method of a grid commutation converter based on double-end station area ratio braking fault direction identification, belonging to the technical field of ultra-high voltage direct current transmission line protection. Background Art

[0002] As the proportion of renewable energy such as wind power generation and photovoltaic power generation continues to increase, these power generation centers are distributed in remote areas far away from the load center, so a transmission technology with large transmission capacity and long transmission distance is needed to transmit electricity. Compared with AC transmission, ultra-high voltage direct current transmission has many advantages such as large transmission capacity, long transmission distance, convenient grid interconnection, and easy power regulation, so it has received widespread attention and development.

[0003] Considering that the UHV DC transmission system has a more complex structure and more diverse operating modes, the requirements for control and protection are also higher. UHV transmission lines are long distances, cross complex terrain and surrounding environments, and are more prone to failure than other components. According to statistics, transmission line failures account for about 50% of UHV DC system failures, so the protection of UHV transmission lines is particularly important.

[0004] At present, the traveling wave protection has a fast action, but is easily interfered, has high requirements for sampling frequency, is not sensitive to high-resistance grounding faults, and lacks mature setting principles; the reliability of differential undervoltage protection is higher than that of traveling wave protection, but its shortcomings are similar to those of traveling wave protection; longitudinal differential protection can detect high-resistance grounding faults, but has high requirements for communication and is greatly affected by the distributed capacitance current of the line, and has a slow response speed. Therefore, it is of great significance to propose a protection that is resistant to high resistance, has strong fast action ability, and has strong resistance to high resistance, in order to improve the protection principle and protection scheme of DC lines and ensure the safe and reliable operation of the system. Summary of the invention

[0005] The purpose of the present invention is to provide a LCC-HVDC transmission line differential protection method based on double-terminal station ratio braking fault direction identification, aiming to solve the technical problem that single-terminal DC line protection cannot infer the fault location through waveform characteristics, resulting in difficulty in ensuring reliable identification of fault direction, anti-interference ability and fault removal speed.

[0006] To achieve the above-mentioned object, the present invention provides a LCC-HVDC transmission line differential protection method based on dual-end station area ratio braking fault direction identification, which can select the corresponding protection and control system coordination strategy according to the actual judgment results of the startup criterion, the ratio braking fault direction identification of the rectifier side and the inverter side, and the fault pole selection criterion, and distinguish the instantaneous fault or permanent fault of the line, and improve the reliability and quickness of the protection under the premise of ensuring the protection selectivity. The method comprises:

[0007] When a DC transmission system fails, the fault area is determined based on the grounding electrode current and fault voltage gradient start-up criteria, the double-terminal station area ratio braking fault direction identification element criteria and the grounding electrode current flow direction selection fault electrode criteria;

[0008] Based on the fault area determined by the protection, determine the corresponding protection control coordination strategy;

[0009] The invention is applied to the protection of direct current transmission lines of large-capacity ultra-high voltage direct current transmission systems, wherein the system is a true bipolar transmission structure based on a grid-commutated converter, smoothing reactors and direct current filters are installed at both ends of the line, and the grid-commutated converter is a double 12-pulsation series valve group.

[0010] The startup criteria are specifically:

[0011] Rectifier side protection start-up criteria:

[0012]

[0013] Inverter side protection start criteria:

[0014]

[0015] Among them, ΔU set is the voltage gradient threshold for the start-up criterion, |ΔU dcR (N)| and |ΔU dcI (N)| are the voltage gradient values ​​of the rectifier side and inverter side at the sampling time N, respectively, U dcR (Nj) and U dcI (Nj) are the sampling values ​​of the rectifier side and inverter side pole voltage before the jth sampling cycle at time N, i GND.set is the grounding electrode current threshold for the start judgment, i DC.set is the setting value for bipolar fault determination, i DCR+ / - and i DCI+ / - It is the positive and negative DC current on the rectifier side and the inverter side.

[0016] The double-terminal station area ratio braking fault direction identification element judgment criteria are specifically:

[0017] Rectifier station fault direction identification:

[0018]

[0019] Identification of inverter station fault direction:

[0020]

[0021] Among them, i op is the station area differential current, i ris the station braking current, i opR.set and i opI.set The lower limit of the rectifier and inverter station action current setting, S r is the proportional braking coefficient, i dcR+ 、i dcR- 、i GNDR are respectively the positive and negative line currents and the grounding current in the current collection area of ​​the rectifier station, i dcI+ 、i dcI- 、i GNDI They are respectively the positive and negative line currents and the grounding current in the inverter station current collection area. Among them, the differential current i op ≈0; under reverse fault, the differential current i op ≈i r ,i op Much larger than S r i r andi op.set .

[0022] The specific criterion for selecting the fault pole for the flow direction of the grounding pole current is:

[0023]

[0024] Among them, I GND.set The pole setting value for the grounding current fault is selected according to the maximum unbalanced current detected at the grounding pole. max.unb │To adjust, I GND.set =1.2│i max.unb │.

[0025] The specific determination of the fault area is:

[0026] DC system forward area line fault: After the fault wave reaches the protection measuring point, the rectifier station differential protection is in |ΔU dcR (N)|>ΔU set and|i GNDR (t)|>i GND.set After starting, after 6ms, the fault direction is judged as a station-wide forward fault and the pole selection is completed, the protection outlet is actuated, and the inverter station-wide fault direction judgment result is a non-station-wide fault, and the blocking ESOF signal cannot be sent to the rectifier station, then the comprehensive judgment result is a forward line fault;

[0027] Fault outside the forward zone of the DC system: The differential protection of the rectifier station is in |ΔU dcR (N)|>ΔU set and|i GNDR (t)|>i GND.setAfter starting, after 6ms, the fault direction is judged as a station-wide forward fault and the pole selection is completed. The protection judges the fault as a forward fault. The inverter station-wide ratio braking fault direction discrimination element judges the fault as a station-wide fault. The comprehensive judgment result is a forward out-of-area fault.

[0028] DC system reverse fault outside the zone: rectifier station differential protection in |ΔU dcR (N)|>ΔU set and|i GNDR (t)|>i GND.set After starting, after 6ms, the fault direction is judged to be a forward fault in the station area and the pole selection is completed. The protection judges the fault to be a reverse fault. The inverter station area ratio braking fault direction discrimination element judges the result to be a non-station area fault. The comprehensive judgment result is a reverse out-of-zone fault.

[0029] The fault area determined based on the protection is specifically determined as follows:

[0030] In case of a DC system forward area line fault, the control system prioritizes the trigger angle of the rectifier side to be forced to shift the phase to restart according to 18°-120°-160°-18°, and implements the trigger restart logic; and continuously waits for the blocking ESOF signal sent by the inverter station area ratio braking fault direction discrimination element to correct the control strategy;

[0031] Fault outside the forward zone of the DC system: The control system will prioritize the trigger angle of the rectifier side to force phase shift and restart according to 18°-120°-160°-18°, and implement the trigger restart logic; and constantly wait for the lockout ESOF signal sent by the inverter station ratio braking fault direction discrimination element to correct the control strategy. If a lockout signal is received from the other end during the restart process, it is judged to be a fault outside the forward zone. The control system will force phase shift and lock the rectifier station according to 160°-90° for the trigger angle of the rectifier side;

[0032] Fault outside the reverse zone of the DC system: The rectifier station protection judges that it is a reverse station fault, and the protection sends an ESOF signal to the rectifier station to force the rectifier station to phase shift and lock.

[0033] To achieve the above object, the present invention also provides a LCC-HVDC transmission line differential protection device based on double-end station area ratio braking fault direction identification, the device comprising:

[0034] A fault data acquisition module, used to acquire line fault characteristics;

[0035] Protection start module, used to determine whether the protection is started under fault conditions;

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

[0037] A fault pole selection module, used for selecting a fault pole according to the flow direction of the grounding pole current;

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

[0039] To achieve the above-mentioned object, the present invention also provides a storage medium, which is a computer-readable storage medium, and stores a computer program on the storage medium. When the computer program is executed by a processor, the steps of the LCC-HVDC transmission line differential protection method based on double-terminal station ratio braking fault direction identification as described above are implemented.

[0040] The present invention adopts the above technical solution and has the following beneficial effects: The present invention is aimed at ultra-high voltage direct current transmission lines, and can reliably identify the fault direction according to the fault characteristics and the station-area ratio differential protection, and has a strong ability to withstand transition resistance. The current value at the grounding electrode measuring end is used to realize the reliable identification of ground faults and AC faults within the station, and effectively avoids the control system from shifting phase in advance under AC system faults. The protection strategy of the present invention can truly solve the problem that single-ended DC line protection cannot reverse the fault location through waveform characteristics, and can quickly suppress the fault current for inverter station-area faults, which is conducive to the rapid clearing of faults. In summary, the present invention can be widely used in large-capacity ultra-high voltage direct current transmission lines. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The following will clearly and completely describe the technical solutions in the embodiments of the present application according to the drawings in the embodiments of the present application. It is obvious that the described embodiments are only some embodiments of the present application, not all. Based on the embodiments mentioned in the present application, all other embodiments obtained by those skilled in the art without performing creative work belong to the scope of protection of the present application.

[0042] Figure 1 is a strategy flow chart of the present invention;

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

[0044] Figure 3 This is the protection waveform diagram when the forward area line is grounded;

[0045] Figure 4 This is the protection waveform diagram when the converter valve in the area outside the forward zone is grounded;

[0046] Figure 5 This is the protection waveform diagram when the converter valve in the area outside the reverse zone is grounded;

[0047] Figure 6This is a protection simulation timing diagram of a true bipolar ultra-high voltage direct current system under faults in different regions according to an embodiment of the present invention. DETAILED DESCRIPTION

[0048] The following is a clear and detailed description of the technical solutions of the embodiments of the present application in conjunction with the accompanying drawings and specific implementation methods. For the convenience of description, it is obvious that the described embodiments are part of the embodiments of the present application, rather than all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present application.

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

[0050] In response to the frequent occurrence of transient faults in large-capacity ultra-high voltage direct current transmission lines, the present invention proposes a differential protection method, device and medium for LCC-HVDC transmission lines based on double-end station ratio braking fault direction identification, including: the protection can reliably identify the fault direction and effectively cut off the permanent fault of the line according to the fault characteristics and the actual judgment results of the double-end station ratio braking direction discrimination element; it can ensure the reliable restart of the rectifier converter station under transient faults, greatly shorten the power outage time, and has a strong ability to withstand transition resistance. Reliable identification of ground faults and AC faults within the station is achieved based on the current value at the grounding electrode measurement end, effectively avoiding the control system from forcing phase shifting in advance under AC system faults. The present invention can truly solve the problem that single-end DC line protection cannot reverse the fault location through waveform characteristics, and can quickly suppress the fault current for inverter station faults, which is conducive to the rapid clearing of faults.

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

[0052] Example 1: Figure 1 As shown, it is a strategy flow chart of a LCC-HVDC transmission line differential protection method based on double-terminal station ratio braking fault direction identification of the present application. The method in this embodiment can effectively select a corresponding control strategy to handle DC system faults by using the judgment results of the protection start element, fault direction judgment element, and fault pole selection element of the double-terminal station domain under the premise of ensuring correct data sampling. The method includes:

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

[0054] Based on the fault area determined by the protection, determine the corresponding protection control coordination strategy;

[0055] The invention is applied to the protection of direct current transmission lines of large-capacity ultra-high voltage direct current transmission systems, wherein the system is a true bipolar transmission structure based on a grid-commutated converter, smoothing reactors and direct current filters are installed at both ends of the line, and the grid-commutated converter is a double 12-pulsation series valve group.

[0056] According to the DC voltage fluctuation caused by DC fault and AC fault ride-through, the improved voltage gradient algorithm and grounding electrode current are used as the protection start-up criterion;

[0057] Rectifier side protection start-up criteria:

[0058]

[0059] Inverter side protection start criteria:

[0060]

[0061] Among them, ΔU set The voltage gradient threshold is the starting criterion. The minimum DC voltage gradient U caused by AC fault ride-through within 0.6 ms is dc.T To adjust ΔU set =K U.rel U dc.T , K U.rel is the voltage gradient starting reliability factor, K U.rel =0.9; |ΔU dcR (N)| and |ΔU dcI (N)| are the voltage gradient values ​​of the rectifier side and inverter side at the sampling time N, respectively, U dcR (Nj) and U dcI (Nj) are the sampling values ​​of the rectifier side and inverter side pole voltage before the jth sampling cycle at time N, i GND.set The maximum unbalanced current i caused by the unbalanced parameters of the two poles is the starting criterion of the grounding electrode current threshold. max.unb Absolute value setting GND.set =1.3i max.unb .i DC.set It is the setting value for bipolar fault determination, according to the maximum DC i caused by AC fault DC.max To adjust i DC.set =1.1i DC.max ,i DC.set It is the setting value for bipolar fault determination. DCR+ / - and i DCI+ / - It is the positive and negative DC current of the rectifier side and the inverter side. dc.T =0.05pu, ΔU set =0.045pu,iGND.set =150A,i DC.max =8.1kA,i DC.set =8.9kA.

[0062] It is stipulated that the faults in the DC area outside each station area are station area forward faults, and the faults in the station area are reverse station area faults. In order to facilitate the distinction of the fault direction, the positive and negative line current measurement points and the grounding current measurement points belonging to the same end are defined as station area measurement points, and the area between the station area measurement points and the converter valve side is defined as the current collection area; according to Kirchhoff's current law (KCL), ratio braking is introduced in the fault direction judgment criterion, and the station area ratio braking fault direction identification criterion is constructed according to the difference in current characteristics flowing through the current collection area;

[0063] Rectifier station fault direction identification:

[0064]

[0065] Identification of inverter station fault direction:

[0066]

[0067] Among them, i op is the station area differential current, i r is the station braking current, i opR.set and i opI.set The lower limit of the rectifier and inverter station action current setting, S r is the proportional braking coefficient, i dcR+ 、i dcR- 、i GNDR are respectively the positive and negative line currents and the grounding current in the current collection area of ​​the rectifier station, i dcI+ 、i dcI- 、i GNDI They are respectively the positive and negative line currents and the grounding current in the inverter station current collection area. Among them, the differential current i op ≈0; under reverse fault, the differential current i 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 the forward fault in the rectifier and inverter stations is 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] According to the principle that the grounding electrode current flows in opposite directions under positive and negative grounding faults, the grounding electrode current flow direction selection fault electrode criterion is specifically:

[0070]

[0071] Among them, I GND.set The pole setting value for the grounding current fault is selected according to the maximum unbalanced current detected at the grounding pole. max.unb │To adjust, I GND.set =1.2│i max.unb │=138A. According to the actual protection time window for identifying lightning faults in the project, which is about 5ms, the fault direction identification and pole selection time is set to 6ms.

[0072] Determine the fault area specifically:

[0073] Fault in the forward area of ​​the DC system: After the fault wave reaches the protection measuring point, the control system increases the phase shift of the trigger angle on the rectifier side; the differential protection in the rectifier station area is |ΔU dcR (N)|>ΔU set and|i GNDR (t)|>i GND.set After restarting, after 6ms, the fault direction is judged as a forward fault in the station area and the pole selection is completed. After confirming that the lightning interference has been avoided, the protection outlet is activated. The control system prioritizes the trigger angle of the rectifier side according to 18°-120°-160°-18° forced phase shift restart, and implements the trigger restart logic; and constantly waits for the lockout ESOF signal sent by the inverter station area ratio braking fault direction discrimination element to correct the control strategy; during the restart process, the rectifier station does not receive the emergency shutdown signal (ESOF) signal sent by the inverter station area ratio braking fault direction discrimination element, which can be judged as a grounding fault in the line area. If the system returns to normal after the restart, it will keep running; if the system encounters a fault again, it will be directly locked and stopped;

[0074] Fault outside the forward zone of the DC system: After the fault wave reaches the protection measuring point, the control system increases the phase shift of the trigger angle on the rectifier side; the differential protection of the rectifier station area is at |ΔU dcR (N)|>ΔU set and|i GNDR (t)|>i GND.setAfter starting, after 6ms, the fault direction is judged as a forward fault in the station area and the pole selection is completed. After confirming that the lightning interference has been avoided, the protection outlet is activated. The control system prioritizes the trigger angle of the rectifier side to restart according to the forced phase shift of 18°-120°-160°-18°, and implements the trigger restart logic; and constantly 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, the locking signal from the other end is received and it is judged as a fault outside the forward area. The control system forces the trigger angle of the rectifier side to shift the phase by 160°-90° to lock the rectifier station;

[0075] DC system reverse fault outside the zone: rectifier station differential protection in |ΔU dcR (N)|>ΔU set and|i GNDR (t)|>i GND.set After starting, after 6ms fault direction judgment and pole selection, the judgment result is a reverse station fault, and the protection sends an ESOF signal to the rectifier station to force the rectifier station to shift the phase. Before forced phase shift, the trigger angle of the rectifier side will follow the reference current change.

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

[0077] In case of a DC system forward area line fault, the control system prioritizes the trigger angle of the rectifier side to be forced to shift the phase to restart according to 18°-120°-160°-18°, and implements the trigger restart logic; and continuously waits for the blocking ESOF signal sent by the inverter station area ratio braking fault direction discrimination element to correct the control strategy;

[0078] Fault outside the forward zone of the DC system: The control system will prioritize the trigger angle of the rectifier side to force phase shift and restart according to 18°-120°-160°-18°, and implement the trigger restart logic; and constantly wait for the lockout ESOF signal sent by the inverter station ratio braking fault direction discrimination element to correct the control strategy. If a lockout signal is received from the other end during the restart process, it is judged to be a fault outside the forward zone. The control system will force phase shift and lock the rectifier station according to 160°-90° for the trigger angle of the rectifier side;

[0079] Fault outside the reverse zone of the DC system: The rectifier station protection judges that it is a reverse station fault, and the protection sends an ESOF signal to the rectifier station to force the rectifier station to phase shift and lock.

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

[0081] A fault data acquisition module, used to acquire line fault characteristics;

[0082] Protection start module, used to determine whether the protection is started under fault conditions;

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

[0084] A fault pole selection module, used for selecting a fault pole according to the flow direction of the grounding pole current;

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

[0086] A 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, the steps of the LCC-HVDC transmission line differential protection method based on double-terminal station ratio braking fault direction identification as described above are implemented.

[0087] In this embodiment, if Figure 2 The simulation circuit model of the true bipolar ultra-high voltage direct current system shown in the figure is divided into three areas, protecting the area outside the reverse area on the left, the forward line area on the right, and the area outside the forward area. The area outside the reverse area includes a rectifier converter (double 12-pulse series valve group) with a transmission capacity of 5000MW, a voltage level of ±800kV, a total length of the forward line on the right is 1373kM, and the inverter station outside the forward area on the right is also a double 12-pulse series valve group structure.

[0088] In order to verify the effectiveness and correctness of the method of the present invention, this embodiment sets up line end grounding, forward zone external grounding, and reverse zone external pole grounding for analysis, simulates the above-mentioned true bipolar ultra-high voltage DC system simulation circuit model, and obtains the protection simulation waveform diagram, wherein, Figure 3 This is the protection waveform diagram of the line end grounding in the forward area. Figure 4 This is the protection waveform diagram of the grounding area outside the forward zone. Figure 5 This is the protection waveform diagram of the grounding area outside the reverse zone. Figure 6 This is the protection simulation timing diagram of the true bipolar UHV DC system under faults in different areas.

[0089] from Figure 3 It can be seen that when the DC system has a positive fault, after the fault wave reaches the protection measuring point, the rectifier station differential protection is dcR (N)|>ΔU set and|i GNDR (t)|>i GND.set After starting, after 6ms, the fault direction is judged as a forward fault in the station area and the pole selection is completed. After confirming that the lightning interference has been avoided, the protection output is activated. The control system prioritizes the trigger angle of the rectifier side according to 18°-120°-160°-18° forced phase shift restart, and implements the trigger restart logic; the inverter station area ratio braking fault direction discrimination element is in |ΔUdcI (N)|>ΔU set and|i GNDI (t)|>i GND.set After restarting, after 6ms, the fault direction is judged and the fault pole selection is completed. It is judged that the fault is not a fault in the inverter station domain, 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 keep running; if the system encounters a fault again, it will be directly blocked and stopped.

[0090] from Figure 4 It can be seen that when a fault occurs outside the forward zone of the DC system, after the fault wave reaches the protection measuring point, the differential protection of the rectifier station area is dcR (N)|>ΔU set and|i GNDR (t)|>i GND.set After that, it started; but after 6.6ms, the inverter station ratio braking fault direction discrimination element judged that the fault was an inverter station fault, so it sent a blocking signal to the rectifier station. The rectifier station received the ESOF signal at 11.2ms, and the control system forced the rectifier side trigger angle to shift by 160°-90° to block the rectifier station. Because the ESOF signal was received before the rectifier station protection action, the protection action was invalid.

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

[0092] from Figure 6 It can be seen that the action sequence diagram of protection under different regional faults. For a positive direction fault, when the ESOF signal sent by the inverter domain is not received before the rectifier domain protection is activated, the control first adopts the restart procedure, and then locks the rectifier converter after receiving the ESOF signal. For a reverse fault, if the ratio braking direction discrimination element of the rectifier domain judges that the result is a reverse fault, the ESOF signal is directly sent to the rectifier station to lock the rectifier converter station.

[0093] The above embodiments are only for illustrating the technical idea of ​​the present application, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the present application. It should be pointed out that, for a person of ordinary skill in the art, any changes made on the basis of the technical solution without departing from the concept of the present application, all belong to the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the attached claims.

Claims

1. A LCC-HVDC transmission line differential protection method based on double-terminal station ratio braking fault direction identification, characterized in that: The method comprises: When a DC transmission system fails, the fault area is determined based on the grounding electrode current and fault voltage gradient start-up criteria, the double-terminal station area ratio braking fault direction identification element criteria and the grounding electrode current flow direction selection fault electrode criteria; Based on the fault area determined by the protection, determine the corresponding protection control coordination strategy; The invention is applied to the protection of direct current transmission lines of large-capacity ultra-high voltage direct current transmission systems, wherein the system is a true bipolar transmission structure based on a grid-commutated converter, smoothing reactors and direct current filters are installed at both ends of the line, and the grid-commutated converter is a double 12-pulsation series valve group.

2. A LCC-HVDC transmission line differential protection method based on double-terminal station ratio braking fault direction identification according to claim 1, characterized in that: The startup criteria are specifically: Rectifier side protection start-up criteria: Inverter side protection start criteria: Among them, ΔU set is the voltage gradient threshold for the start-up criterion, |ΔU dcR (N)| and |ΔU dcI (N)| are the voltage gradient values ​​of the rectifier side and inverter side at the sampling time N, respectively, U dcR (Nj) and U dcI (Nj) are the sampling values ​​of the rectifier side and inverter side pole voltage before the jth sampling cycle at time N, i GND.set is the grounding electrode current threshold for the start judgment, i DC.set is the setting value for bipolar fault determination, i DCR+ / - and i DCI+ / - It is the positive and negative DC current on the rectifier side and the inverter side.

3. The LCC-HVDC transmission line differential protection method based on double-terminal station ratio braking fault direction identification according to claim 1 is characterized in that: The double-terminal station area ratio braking fault direction identification element judgment criteria are specifically: Rectifier station fault direction identification: Identification of inverter station fault direction: Among them, i op is the station area differential current, i r is the station braking current, i opR.set and i opI.set The lower limit of the rectifier and inverter station action current setting, S r is the proportional braking coefficient, i dcR+ 、i dcR- 、i GNDR are respectively the positive and negative line currents and the grounding current in the current collection area of ​​the rectifier station, i dcI+ 、i dcI- 、i GNDI They are respectively the positive and negative line currents and the grounding current in the inverter station current collection area. Among them, the differential current i op ≈0; under reverse fault, the differential current i op ≈i r ,i op Much larger than S r i r andi op.set。 4. A LCC-HVDC transmission line differential protection method based on double-terminal station ratio braking fault direction identification according to claim 1, characterized in that: The specific criterion for selecting the fault pole for the flow direction of the grounding pole current is: Among them, I GND.set The pole setting value for the grounding current fault is selected according to the maximum unbalanced current detected at the grounding pole. max.unb │To adjust, I GND.set =1.2│i max.unb │.

5. The LCC-HVDC transmission line differential protection method based on double-terminal station ratio braking fault direction identification according to claim 1 is characterized in that: The specific determination of the fault area is: DC system forward area line fault: After the fault wave reaches the protection measuring point, the rectifier station differential protection is in |ΔU dcR (N)|>ΔU set and|i GNDR (t)|>i GND.set After starting, after 6ms, the fault direction is judged as a station-wide forward fault and the pole selection is completed, the protection outlet is actuated, and the inverter station-wide fault direction judgment result is a non-station-wide fault, and the blocking ESOF signal cannot be sent to the rectifier station, then the comprehensive judgment result is a forward line fault; Fault outside the forward zone of the DC system: The differential protection of the rectifier station is in |ΔU dcR (N)|>ΔU set and|i GNDR (t)|>i GND.set After starting, after 6ms, the fault direction is judged as a station-wide forward fault and the pole selection is completed. The protection judges the fault as a forward fault. The inverter station-wide ratio braking fault direction discrimination element judges the fault as a station-wide fault. The comprehensive judgment result is a forward out-of-area fault. DC system reverse fault outside the zone: rectifier station differential protection in |ΔU dcR (N)|>ΔU set and|i GNDR (t)|>i GND.set After starting, after 6ms, the fault direction is judged to be a forward fault in the station area and the pole selection is completed. The protection judges the fault to be a reverse fault. The inverter station area ratio braking fault direction discrimination element judges the result to be a non-station area fault. The comprehensive judgment result is a reverse out-of-zone fault.

6. A LCC-HVDC transmission line differential protection method based on double-terminal station ratio braking fault direction identification according to claim 1, characterized in that: The fault area determined based on the protection is specifically determined as follows: In case of a line fault in the forward area of ​​the DC system: the control system prioritizes the trigger angle of the rectifier side according to 18°-120°-160°-18° for forced phase shift restart and implements the trigger restart logic; And keep waiting for the lockout ESOF signal sent by the inverter station area ratio braking fault direction discrimination element to correct the control strategy; Fault outside the forward zone of the DC system: The control system prioritizes the trigger angle of the rectifier side according to 18°-120°-160°-18° for forced phase shift restart and implements the trigger restart logic; And keep waiting for the lockout ESOF signal sent by the inverter station area ratio braking fault direction discrimination element to correct the control strategy. If a lockout signal is received from the other end during the restart process, it is judged as a fault outside the forward zone. The control system will force the rectifier side trigger angle to shift the phase by 160°-90° to lock the rectifier station. Fault outside the reverse zone of the DC system: The rectifier station protection judges that it is a reverse station fault, and the protection sends an ESOF signal to the rectifier station to force the rectifier station to phase shift and lock.

7. A LCC-HVDC transmission line differential protection device based on double-terminal station ratio braking fault direction identification, characterized in that: The device comprises: A fault data acquisition module, used to acquire line fault characteristics; Protection start module, used to determine whether the protection is started under fault conditions; The station-area ratio-braking fault direction identification module is used to determine the fault direction; A fault pole selection module, used for selecting a fault pole according to the flow direction of the grounding pole current; The optical fiber communication channel is used to obtain the ESOF signal sent by the station area ratio braking fault direction identification module.

8. 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 the processor, the steps of the LCC-HVDC transmission line differential protection method based on double-terminal station ratio braking fault direction identification according to any one of claims 1 to 6 are implemented.

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