Island locking control method and system of power transmission system

Abstract

The invention provides an island locking control method and control system for a power transmission system, and the method comprises the steps: obtaining an island signal according to the operation state of a converter station connected with an AC power grid in an island state under the condition of receiving the island signal; determining a priority locking pole and a rear locking pole in the double poles of the converter station connected with the alternating current power grid in the island state; and controlling the locking moment of the rear locking pole not to be earlier than the locking moment of the priority locking pole. According to the scheme, the alternating current overvoltage level after island locking in the ultra-high voltage direct current transmission system can be greatly reduced, the direct current maximum transmission power limit value is improved, and the availability rate of the direct current system is improved.

Description

Technical Field

[0001] The present application relates to the technical field of direct current transmission, and in particular to an islanding blocking method and system for a power transmission system. Background Art

[0002] Currently, UHVDC transmission systems have achieved widespread engineering application. In UHVDC transmission systems, the high-end and low-end converters at the sending end can be located in the same converter station, connected to the same AC grid, or in different converter stations, connected to different AC grids, using a split-site construction method to achieve multi-source power supply.

[0003] Sending AC grids are often connected to the main grid via a small number of AC interconnection lines. When these lines are disconnected or a large number of generators are lost, the AC system weakens and experiences imbalances in active power generation and transmission. In these situations, the stabilization control system executes generator shedding according to a pre-set strategy and simultaneously sends an islanding signal to the DC system, requiring it to coordinate with the generator shedding sequence and block the DC power according to a specific timing, ensuring stable operation of the AC system.

[0004] After receiving the islanding signal, the DC system simultaneously blocks both poles with the same designed delay. Afterward, the bipolar blocking and fast-tripping AC filtering strategy is implemented. Even after this strategy is implemented, due to excess reactive power in the AC filter, the AC overvoltage may still not meet the equipment's overvoltage tolerance requirements, thus limiting the maximum DC transmission power. Furthermore, for UHVDC projects constructed at the sending end, the simultaneous blocking of both poles at the islanded station can lead to even higher AC overvoltages due to the continued power transmission from non-islanded stations. Summary of the Invention

[0005] The embodiments of the present application provide a method and system for controlling islanding of a power transmission system to solve the problem in the related art that AC overvoltage may affect system stability when an ultra-high voltage direct current (UHVDC) transmission system performs islanding.

[0006] To solve the above problems, the technical solutions provided by this application are as follows:

[0007] In a first aspect, the present application provides an islanding blocking control method for a power transmission system, comprising:

[0008] In the case of receiving an islanding signal, determining a priority blocking pole and a rear blocking pole in the bipolar portion of the converter station connected to the AC grid in the islanding state according to the operating state of the converter station connected to the AC grid in the islanding state;

[0009] The locking time of the rear locking pole is controlled to be no earlier than the locking time of the priority locking pole.

[0010] In one embodiment, the control method further includes:

[0011] The operating state of the sending-end AC power grid is monitored in real time, and when it is detected that the sending-end AC power grid is in an island state, the island signal is generated.

[0012] In one embodiment, when different converters are arranged in one converter station, the islanding signal is used to instruct all different converters of the same positive pole or the same negative pole in the converter station to be locked.

[0013] In one embodiment, the step of determining a priority blocking pole and a post blocking pole in a bipolar portion of a converter station connected to an AC grid in an islanded state according to an operating state of the converter station connected to the AC grid in an islanded state includes:

[0014] In the case of bipolar symmetrical operation of the converter station connected to the AC grid in an islanded state, determining one pole as the priority blocking pole and the other pole as the post-blocking pole;

[0015] In the case where one of the two poles of the converter station connected to the AC grid in an islanded state is in reduced voltage operation and the other pole is in full voltage operation with a dual valve group, determining that the pole in reduced voltage operation is the priority blocking pole and the pole in full voltage operation is the last blocking pole;

[0016] When one of the two poles of the converter station connected to the AC grid in an islanded state operates with a single valve group and the other operates with a double valve group, it is determined that the pole operating with a single valve group is the priority blocking pole and the pole operating with a double valve group is the last blocking pole.

[0017] In one embodiment, when different converters of the same pole are respectively arranged in converter stations at different locations, the islanding signal is used to instruct the converters in the converter station connected to the AC grid in the islanded state to be locked.

[0018] In one embodiment, the step of determining a priority blocking pole and a post blocking pole in a bipolar portion of a converter station connected to an AC grid in an islanded state according to an operating state of the converter station connected to the AC grid in an islanded state includes:

[0019] In the case of bipolar symmetrical operation of the converter station connected to the AC grid in an islanded state, determining one pole as the priority blocking pole and the other pole as the post-blocking pole;

[0020] In the case where one of the two poles of the converter station connected to the AC grid in an islanded state is in reduced voltage operation and the other pole is in full voltage operation with a dual valve group, determining that the pole in reduced voltage operation is the priority blocking pole and the pole in full voltage operation is the last blocking pole;

[0021] When one of the two poles of the converter station connected to the AC grid in an islanded state operates with a single valve group and the other operates with a double valve group, it is determined that the pole operating with the double valve group is the priority blocking pole and the pole operating with the single valve group is the last blocking pole.

[0022] In one embodiment, the control method further includes:

[0023] After the priority blocking pole is blocked, the AC filter is cut off according to the reactive balance principle.

[0024] In one embodiment, the control method further includes:

[0025] After the priority blocking pole is blocked, the AC filter is cut off according to the minimum filter principle.

[0026] In one embodiment, the control method further includes:

[0027] After the priority blocking pole is blocked, the AC filter is quickly switched off according to the absolute minimum filter principle.

[0028] In one embodiment, the control panel method further includes:

[0029] During the blocking process, the de-bipolar power control is first performed on the rear blocking pole, and then the priority blocking pole is blocked.

[0030] In a second aspect, the present application provides an island blocking control system for a power transmission system, comprising:

[0031] A stabilization control system is configured to monitor the operating status of the sending-end AC power grid in real time and generate an islanding signal when the sending-end AC power grid is in an islanding state;

[0032] The DC system is configured to receive the islanding signal and, based on the operating status of the converter station connected to the AC grid in the islanding state, determine a priority blocking pole and a rear blocking pole in the bipolar portion of the converter station connected to the AC grid in the islanding state, and control the blocking moment of the rear blocking pole to be no earlier than the blocking moment of the priority blocking pole.

[0033] In one embodiment, the DC system includes a converter station, in which different converters of the same pole are centrally arranged;

[0034] The stabilization control system is further configured to generate an islanding signal when detecting that the sending-end AC power grid is in an islanding state, so as to instruct the DC system to lock different converters of the same pole.

[0035] In one embodiment, the DC system is further configured as follows:

[0036] In the case of bipolar symmetrical operation of the converter station connected to the AC grid in an islanded state, determining one pole as the priority blocking pole and the other pole as the post-blocking pole;

[0037] In the case where one of the two poles of the converter station connected to the AC grid in an islanded state is in reduced voltage operation and the other pole is in full voltage operation with a dual valve group, determining that the pole in reduced voltage operation is the priority blocking pole and the pole in full voltage operation is the last blocking pole;

[0038] When one of the two poles of the converter station connected to the AC grid in an islanded state operates with a single valve group and the other operates with a double valve group, it is determined that the pole operating with a single valve group is the priority blocking pole and the pole operating with a double valve group is the last blocking pole.

[0039] In one embodiment, the DC system includes a plurality of converter stations, the plurality of converter stations are located at different locations, and each converter station is provided with different converters of the same pole;

[0040] The stabilization control system is further configured to generate an islanding signal when detecting that the sending-end AC grid is in an islanding state, to instruct the DC system to lock the converter in the converter station connected to the AC grid in the islanding state.

[0041] In one embodiment, the DC system is further configured as follows:

[0042] In the case of bipolar symmetrical operation of the converter station connected to the AC grid in an islanded state, determining one pole as the priority blocking pole and the other pole as the post-blocking pole;

[0043] In the case where one of the two poles of the converter station connected to the AC grid in an islanded state is in reduced voltage operation and the other pole is in full voltage operation with a dual valve group, determining that the pole in reduced voltage operation is the priority blocking pole and the pole in full voltage operation is the last blocking pole;

[0044] When one of the two poles of the converter station connected to the AC grid in an islanded state operates with a single valve group and the other operates with a double valve group, it is determined that the pole operating with the double valve group is the priority blocking pole and the pole operating with the single valve group is the last blocking pole.

[0045] The present invention provides an islanding block control method and control system for a power transmission system. Upon receiving an islanding signal, the method determines a priority blocking pole and a subsequent blocking pole in the bipolar portion of the converter station connected to the islanded AC grid based on the operating status of the converter station. The subsequent blocking pole is controlled to be locked no earlier than the priority blocking pole. This method significantly reduces AC overvoltage levels after islanding blockage in UHVDC transmission systems, increases the maximum DC transmission power limit, and improves DC system availability. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For those skilled in the art, other drawings can be obtained based on these drawings without creative work.

[0047] Attachment Figure 1 This is a partial flow chart of an island blocking control method for a power transmission system according to an embodiment of the present application;

[0048] Attachment Figure 2 Schematic diagram of more flowcharts of the islanding blocking control method for the power transmission system in the embodiment of the present application;

[0049] Attachment Figure 3 This is a schematic diagram of an optional AC / DC connection at the sending end of an ultra-high voltage direct current transmission system in an embodiment of the present application;

[0050] Attachment Figure 4 This is another optional AC / DC connection diagram of an ultra-high voltage direct current transmission system in an embodiment of the present application;

[0051] Attachment Figure 5 Schematic diagram of an island locking system of an ultra-high voltage direct current transmission system in an embodiment of the present application;

[0052] Attachment Figure 6 Schematic diagram of the island blocking logic of the UHVDC transmission system in the embodiment of the present application.

[0053] Description of reference numerals in the figures:

[0054] 101, first pole; 102, second pole; 103, first AC filter; 104, AC grid; 105, first pole of high-end station; 106, second pole of high-end station; 107, second AC filter; 108, AC grid of high-end station; 109, first pole of low-end station; 110, second pole of low-end station; 111, third AC filter; AC grid of low-end station 112;

[0055] 30. Island blocking control system of transmission system; 301. Stabilization control system; 302. DC system. DETAILED DESCRIPTION

[0056] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the embodiments described are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without making creative efforts are within the scope of protection of this application.

[0057] It will be understood that, unless explicitly described to the contrary, the word “comprise” and variations like “comprises” or “comprising” will be understood to imply the inclusion of stated elements but not necessarily the exclusion of other elements.

[0058] Reference Figure 1 As shown, according to the first aspect of the present application, a method for controlling islanding of a power transmission system is provided, comprising:

[0059] Step S101: when an islanding signal is received, determining a priority blocking pole and a rear blocking pole in a bipolar portion of a converter station connected to the AC grid in an islanding state according to the operating state of the converter station connected to the AC grid in an islanding state;

[0060] Step S102: Control the locking time of the rear blocking pole to be no earlier than the locking time of the priority blocking pole.

[0061] It should be noted that the voltage levels of the transmission system are generally divided into high voltage, extra-high voltage, and ultra-high voltage. For AC transmission systems, the transmission voltage level of 35-220kV is usually referred to as high voltage, the transmission voltage level of 330-750kV is referred to as extra-high voltage, and the transmission voltage level of 1000kV and above is generally referred to as ultra-high voltage. For DC transmission systems, DC transmission systems with a voltage level of ±500kV are generally referred to as high-voltage DC transmission systems. In the embodiments of the present application, the transmission system refers to an ultra-high voltage DC transmission system, that is, a transmission system with a DC voltage level of ±800kV and above.

[0062] Furthermore, in a DC transmission system, only the transmission link uses DC power; the power generation and consumption systems still use AC power. At the sending end of the transmission line, the AC power from the AC system is sent to a rectifier via a converter transformer in a converter station. This converts the high-voltage AC power into high-voltage DC power, which is then fed into the DC transmission line. The DC power is then sent via the transmission line to an inverter in a receiving-end converter station, where it is converted back into high-voltage AC power. This power is then transferred to the AC system via a converter transformer. In the related art, all established high-voltage DC transmission systems are two-terminal DC transmission systems, primarily consisting of a rectifier station, an inverter station, and a transmission line. These two-terminal DC transmission systems can be categorized into two operating modes: bipolar and unipolar. In bipolar operation, positive and negative conductors are connected to the positive and negative poles of the converter stations at both ends, forming a closed DC loop. The earth return formed by the grounding electrodes at both ends serves as a backup conductor for the transmission system. During normal operation, the DC current flows through the positive and negative conductors. In the embodiments of this application, a two-terminal DC transmission system refers to a bipolar DC system.

[0063] Reference Figure 2 As shown, in some embodiments of the present application, the islanding blocking control method of the power transmission system further includes step S100: monitoring the operating status of the sending-end AC power grid in real time, and generating an islanding signal when it is detected that the sending-end AC power grid is in an islanding state.

[0064] On this basis, the transmission system of the present application includes a DC system and a stabilization and control system. The transmission system of the present application monitors the operating status of the sending-end AC power grid in real time through the stabilization and control system. When the sending-end AC power grid is disconnected from the main grid due to a fault or other reasons, that is, the AC interconnection line between the sending-end AC power grid and the main grid is disconnected to form an island operation, the stabilization and control system will send a special control command signal to the DC system, that is, an island signal.

[0065] Through the above scheme, after the DC system of the present application receives the islanding signal sent by the stabilization control system, it executes the priority blocking pole selection logic according to the DC operating status, and the converter station bipolar executes the time-sharing blocking logic, with the priority blocking pole blocking first and the other pole blocking later. This can greatly reduce the AC overvoltage level after blocking, increase the DC maximum transmission power limit and improve the DC system availability.

[0066] It should be noted that the converter station pole refers to the core unit that constitutes a complete DC transmission circuit in the high-voltage DC transmission system. Each pole represents an independent DC channel, which is responsible for transmitting DC power of the positive or negative pole and forming a loop through the grounding pole or another pole DC line.

[0067] In some embodiments of the present application, when different converters of the same polarity are arranged in a converter station, the islanding signal is used to instruct all different converters of the same positive polarity or the same negative polarity in the converter station to be locked. In these embodiments, the different converters of the same polarity (positive or negative) include a high-end converter and a low-end converter, both of which are centrally arranged in the same converter station. When the stabilization control system determines that the sending-end AC power grid is in an islanding state, it will send a locking instruction to require full-pole locking, that is, the high-end converter and the low-end converter of the same polarity will be shut down at the same time, and the DC power will be completely interrupted.

[0068] It should be noted that in the converter station, the "high end" and "low end" referred to in the embodiments of the present application refer to the different voltage levels of the converter in the DC transmission circuit. One pole of the UHVDC transmission system is generally composed of two converters connected in series, and each converter bears half of the DC voltage; for the ±800kV UHVDC transmission system, the high-end converter refers to a converter that bears a DC voltage of 400kV to 800kV or -400kV to -800kV, and the low-end converter refers to a converter that bears a DC voltage of 0kV to 400kV or 0kV to -400kV. In a conventional UHVDC transmission system, the high-end converter and the low-end converter are arranged in the same converter station; in a split-site UHVDC transmission system, the high-end converter and the low-end converter are arranged in converter stations at different geographical locations, and the high-end converter station and the low-end converter station are connected in series through a DC interconnection line.

[0069] Therefore, according to the different configurations of the DC transmission system, the pole locking methods in the embodiments of this application are also different. Figure 3 In the configuration shown, pole blocking means that both the high-side converter and the low-side converter of the same pole are blocked; if the DC transmission system adopts Figure 4 In the arrangement shown, the pole blocking refers to one pole blocking in the high-end converter station or one pole blocking in the low-end converter station.

[0070] More specifically, see Figure 3 FIG. 1 is a schematic diagram of an optional AC / DC connection at the sending end of a DC transmission system in an embodiment of the present application, wherein the first pole 101, the second pole 102, and the first AC filter 103 are connected to the same AC grid 104. When the AC grid 104 forms an island, the stabilization control system sends an island signal to the DC system. The DC system then blocks the first pole 101 and the second pole 102 according to the island signal and removes the first AC filter 103. Based on this, step S101 further includes:

[0071] In the case of bipolar symmetrical operation of the converter station connected to the AC grid in an island state, one pole is determined to be the priority blocking pole and the other pole is determined to be the last blocking pole. Figure 3In this case, either pole 101 or pole 102 is selected as the priority blocking pole, and the other pole is selected as the last blocking pole. For example, if pole 101 is selected as the priority blocking pole, pole 102 is selected as the last blocking pole. In this case, there is no need to determine the difference in the state between the poles, which helps shorten the protection operation time.

[0072] In the case where one of the two poles of the converter station connected to the AC grid in an islanded state is in step-down operation and the other pole is in full-pressure operation with a dual valve group, the pole in step-down operation is determined to be the first to be blocked, and the pole in full-pressure operation is the last to be blocked. Figure 3 In this scenario, if first pole 101 is operating at reduced voltage and second pole 102 is operating at full voltage with a dual-valve group, first pole 101 is selected as the first pole to be blocked, and second pole 102 is selected as the last pole to be blocked. In this case, the pole operating at reduced voltage is blocked first. Because the transmission efficiency of the pole operating at reduced voltage is lower, its impact on the total system power after blocking is minimal. Retaining the pole operating at full voltage allows for a higher DC power level, facilitating AC / DC power balance and minimizing grid voltage fluctuations.

[0073] In the case where one of the two poles of the converter station connected to the AC grid in an islanded state operates with a single valve group and the other operates with a double valve group, the pole operating with a single valve group is determined to be the first to be blocked and the pole operating with a double valve group is determined to be the last to be blocked. Figure 3 In this scenario, if the first pole 101 is operating with a single valve group and the second pole 102 is operating with a dual valve group, the first pole 101 is selected as the first pole to be blocked, and the second pole 102 is selected as the last pole to be blocked. In this case, the pole operating with a single valve group is blocked first. Because the pole operating with a single valve group has lower transmission efficiency, its blocking has a smaller impact on the total system power. Retaining the pole operating with a dual valve group can maintain a higher DC power level, which helps maintain AC and DC power balance and reduces grid voltage fluctuations.

[0074] In other embodiments of the present application, the DC transmission system of the present application utilizes a sending-end split-address construction, meaning that different converters on the same pole are located at different converter stations. In this case, the islanding signal is used to instruct the converters in the converter station connected to the islanded AC grid to block. In these embodiments, the high-end converter and low-end converter of the same pole are located at converter stations in different geographical locations. Depending on the type of islanding signal, the stabilization control system can choose to block only the pole containing a converter station, while maintaining partial power transmission at the other converter station.

[0075] More specifically, see Figure 4As shown, it is a schematic diagram of the AC and DC connection of the sending end of the UHVDC transmission system with sending-end split-address construction, wherein the first pole 105 of the high-end station, the second pole 106 of the high-end station and the second AC filter 107 are connected to the high-end station AC grid 108, and the first pole 109 of the low-end station, the second pole 110 of the low-end station and the third AC filter 111 are connected to the low-end station AC grid 112.

[0076] On this basis, in some embodiments of the present application, the high-end station AC power grid 108 forms an island, and the stabilization control system will send a high-end station island signal to the DC system. The DC system then needs to lock the high-end station first pole 105 and the high-end station second pole 106, and cut off the second filter 107. After the high-end station is locked, the low-end station first pole 109 and the low-end station second pole 110 form a current loop through the bypass switch of the high-end station to the station, and continue to transmit power to the receiving end.

[0077] In some other embodiments of the present application, the low-end station AC power grid 112 forms an island, and the stabilization control system will send a low-end station island signal to the DC system. The DC system needs to lock the low-end station first pole 109 and the low-end station second pole 110, and cut off the third AC filter 111. After the low-end station is locked, the high-end station first pole 105 and the high-end station second pole form a current loop through the bypass switch knife of the low-end station, and continue to transmit power to the receiving end.

[0078] In the embodiments where the high-end station AC grid 108 or the low-end station AC grid 112 of the split-site construction forms an island, step S101 specifically includes:

[0079] In the case of bipolar symmetrical operation of the converter station connected to the AC grid in an island state, one pole is determined to be the priority blocking pole and the other pole is determined to be the last blocking pole. Figure 4 If the high-end AC grid is in an islanded state, either high-end station first pole 105 or high-end station second pole 106 can be selected as the priority blocking pole, with the other pole being the last blocking pole. If the low-end AC grid is in an islanded state, either low-end station first pole 109 or low-end station second pole 110 can be selected as the priority blocking pole, with the other pole being the last blocking pole. In this case, there is no need to additionally determine the difference in inter-pole states, which helps shorten protection operation time.

[0080] In the case where one of the two poles of the converter station connected to the AC grid in an islanded state is in step-down operation and the other pole is in full-pressure operation with a dual valve group, the pole in step-down operation is determined to be the first to be blocked, and the pole in full-pressure operation is the last to be blocked. Figure 4In this case, if the high-end AC grid is in an islanded state, and the high-end station first pole 105 and the low-end station first pole 109 are operating at reduced voltage, while the high-end station second pole 106 and the low-end station second pole 110 dual valve groups are at full pressure, the high-end station first pole 105 is selected as the priority blocking pole, and the high-end station second pole 106 is selected as the last blocking pole. If the low-end AC grid is in an islanded state, and the high-end station first pole 105 and the low-end station first pole 109 are operating at reduced voltage, while the high-end station second pole 106 and the low-end station second pole 110 dual valve groups are at full pressure, the low-end station first pole 109 is selected as the priority blocking pole, and the low-end station second pole 110 is selected as the last blocking pole. In this case, the priority blocking of the pole operating at reduced voltage results in less current flowing through the pole operating at full pressure during the blocking process, taking into account the transfer between power valve groups, thus helping to reduce AC overvoltage levels.

[0081] In the case where one of the two poles of the converter station connected to the AC grid in an islanded state operates with a single valve group and the other operates with a double valve group, the pole operating with the double valve group is determined to be the first to be blocked and the pole operating with the single valve group is determined to be the last to be blocked. Figure 4 In this case, if the high-end AC grid is in an islanded state, with the high-end station first pole 105 operating with a single valve group and the high-end station second pole 106 operating with two valve groups, the high-end station first pole 105 is selected as the priority blocking pole, and the high-end station second pole 106 is selected as the last blocking pole. If the low-end AC grid is in an islanded state, with the low-end station first pole 109 operating with a single valve group and the low-end station second pole 110 operating with two valve groups at full pressure, the low-end station second pole 110 is selected as the priority blocking pole, and the low-end station first pole 109 is selected as the last blocking pole. In this case, prioritizing the blocking of the pole operating with two valve groups results in less current flowing through the pole operating at full pressure during the blocking process compared to prioritizing the blocking of the pole operating with a single valve group, taking into account the power transfer between valve groups. This helps reduce AC overvoltage levels.

[0082] In step S102, the priority blocking pole and the subsequent blocking pole adopt a time-sharing blocking control strategy. Specifically, the priority blocking pole triggers the blocking operation with a first delay t1, and the subsequent blocking pole triggers the blocking operation with a second delay t2. Where t1 ≤ t2, and the values ​​of t1 and t2 range from 0 ms to 200 ms, the subsequent blocking pole delays blocking by a preset time t2-t1 relative to the priority blocking pole. It should be noted that the first delay t1 and the second delay t2 refer to the time intervals from the detection of a relevant fault signal or the fulfillment of a blocking condition by the system to the actual execution of the blocking operation. By ensuring that the priority blocking pole is locked earlier than or simultaneously with the subsequent blocking pole, the AC overvoltage level after islanding in the UHVDC transmission system can be significantly reduced, the DC maximum transmission power limit can be increased, and the DC system availability can be improved.

[0083] Optionally, in some embodiments of the present application, a fast-cut AC filter operation is executed when the priority blocking pole is locked. It should be noted that the fast-cut AC filter operation can also be omitted. That is, after preferentially blocking one pole, no AC filter is removed. After both poles are locked, the corresponding filter removal logic is executed. After preferentially blocking one pole, executing the fast-cut filter operation first and then blocking the other pole can help reduce reactive power surplus after bipolar blocking and lower AC overvoltage levels.

[0084] In the operation of fast-cutting the AC filter, specifically, the AC filter can be quickly cut off according to the reactive power balance principle. Based on the DC power value of the pole where the converter station is currently in normal operation and continuously transmitting power, the minimum reactive power compensation required is calculated, and the excess filter groups that exceed the demand are cut off to maintain reactive power balance and avoid excessive AC voltage due to excess reactive power.

[0085] In some embodiments, the AC filter can also be quickly switched according to the minimum filter principle, retaining the minimum number of filter groups that meet the current operating conditions, such as the minimum reactive compensation required to maintain the operation of the converter valve, and all other filters are cut off.

[0086] In some other embodiments, the AC filter may be quickly switched according to the absolute minimum filter principle, retaining the absolute minimum filter group that meets the current operating conditions, to prevent the risk of system resonance after the AC filter is completely removed.

[0087] Optionally, in some embodiments of the present application, when performing pole blocking, bipolar power control is first performed on the last blocked pole, and then the priority blocked pole is blocked. De-bipolar power control refers to switching the pole originally in bipolar coordinated control mode to an independent control mode, such as independent pole power control or pole current control, thereby releasing its power coordination relationship with the other pole to adapt to system blocking or fault handling requirements.

[0088] Reference Figure 5 As shown, according to the second aspect of the present application, an island blocking control system 30 of a power transmission system is provided, which is used to implement the control method of the aforementioned embodiment. The control system can achieve all the beneficial effects of the aforementioned control method, which will not be described in detail in this application.

[0089] Specifically, the control system includes a stabilization control system 301 and a DC system 302. The stabilization control system 301 is configured to monitor the operating status of the sending-end AC grid in real time and generate an islanding signal when the sending-end AC grid is in an islanded state. The DC system 302 is configured to receive the islanding signal and, based on the operating status of the converter station connected to the islanded AC grid, determine a priority blocking pole and a subsequent blocking pole in the bipolar state of the converter station connected to the islanded AC grid, and control the blocking time of the subsequent blocking pole to be no earlier than the blocking time of the priority blocking pole.

[0090] Reference Figure 6 As shown, after the stabilization control system 301 detects that an island has formed in the AC system, it sends an island signal to the DC system 302. After receiving the island signal from the stabilization control system 301, the DC system 302 executes the priority blocking pole selection logic according to the DC operating state. The DC system bipolar performs the time-sharing blocking logic, with the priority blocking pole blocking first and the other pole blocking later. The delay of the priority blocking pole is t1, and the delay of the later blocking pole is t2, where t1≤t2.

[0091] Reference Figure 3 As shown, in some embodiments of the present application, the DC system includes a converter station, in which different converters of the same pole are centrally arranged; the stabilization control system is also configured to generate an islanding signal when it detects that the sending-end AC power grid is in an islanding state, to instruct the DC system to lock all different converters of the same pole.

[0092] On this basis, the DC system is further configured as follows: in the case of symmetrical bipolar operation of a converter station connected to an AC grid in an islanded state, any one pole is determined to be the priority blocking pole, and the other pole is determined to be the last blocking pole; in the case of one pole of the bipolar operation of a converter station connected to an AC grid in an islanded state operating at reduced voltage and the other pole operating at full voltage with a dual valve group, the pole operating at reduced voltage is determined to be the priority blocking pole, and the pole operating at full voltage is determined to be the last blocking pole; in the case of one pole of the bipolar operation of a converter station connected to an AC grid in an islanded state operating at a single valve group and the other pole operating at a dual valve group, the pole operating at the single valve group is determined to be the priority blocking pole, and the pole operating at the dual valve group is determined to be the last blocking pole.

[0093] Reference Figure 4 As shown, in some embodiments of the present application, the DC system includes multiple converter stations, the multiple converter stations are located in different positions, and each converter station is provided with different converters of the same pole; the stabilization control system is further configured to generate an islanding signal when it detects that the sending-end AC power grid is in an islanding state, to instruct the DC system to lock the converters in the converter stations connected to the AC power grid in the islanding state.

[0094] On this basis, the DC system is further configured as follows: in the case of symmetrical bipolar operation of a converter station connected to an AC grid in an islanded state, any one pole is determined to be the priority blocking pole, and the other pole is determined to be the last blocking pole; in the case of one pole of the bipolar operation of a converter station connected to an AC grid in an islanded state operating at reduced voltage and the other pole operating at full voltage with a dual valve group, the pole operating at reduced voltage is determined to be the priority blocking pole, and the pole operating at full voltage is determined to be the last blocking pole; in the case of one pole of the bipolar operation of a converter station connected to an AC grid in an islanded state operating at a single valve group and the other pole operating at a dual valve group, the pole operating at the dual valve group is determined to be the priority blocking pole, and the pole operating at the single valve group is determined to be the last blocking pole.

[0095] In combination with the above, the islanding blocking control method and system of the power transmission system provided in the present application receives the islanding signal sent by the stabilization control system through the DC system, and then executes the priority blocking pole selection logic according to the DC operating status. The DC system bipolar executes the time-sharing blocking logic, and the priority blocking pole is blocked first, and the other pole is blocked later. This can greatly reduce the AC overvoltage level after blocking, increase the DC maximum transmission power limit and improve the DC system availability.

[0096] In summary, although the present application has been disclosed as above with preferred embodiments, the above preferred embodiments are not intended to limit the present application. Ordinary technicians in this field can make various changes and modifications without departing from the spirit and scope of the present application. Therefore, the scope of protection of the present application shall be based on the scope defined by the claims.

Claims

1. A method for controlling island blocking of a power transmission system, characterized in that: include: In the case of receiving an islanding signal, determining a priority blocking pole and a rear blocking pole in the bipolar portion of the converter station connected to the AC grid in the islanding state according to the operating state of the converter station connected to the AC grid in the islanding state; The locking time of the rear locking pole is controlled to be no earlier than the locking time of the priority locking pole.

2. The control method according to claim 1, characterized in that: The control method further includes: The operating state of the sending-end AC power grid is monitored in real time, and when it is detected that the sending-end AC power grid is in an island state, the island signal is generated.

3. The control method according to claim 1, wherein: In the case that different converters are arranged in one converter station, the island signal is used to instruct all the different converters with the same positive pole or the same negative pole in the converter station to be locked.

4. The control method according to claim 3, characterized in that: The step of determining a priority blocking pole and a rear blocking pole in a bipolar portion of a converter station connected to an AC grid in an islanded state according to an operating state of the converter station connected to the AC grid in an islanded state comprises: In the case of bipolar symmetrical operation of the converter station connected to the AC grid in an islanded state, determining one pole as the priority blocking pole and the other pole as the post-blocking pole; In the case where one of the two poles of the converter station connected to the AC grid in an islanded state is in reduced voltage operation and the other pole is in full voltage operation with a dual valve group, determining that the pole in reduced voltage operation is the priority blocking pole and the pole in full voltage operation is the last blocking pole; When one of the two poles of the converter station connected to the AC grid in an islanded state operates with a single valve group and the other operates with a double valve group, it is determined that the pole operating with a single valve group is the priority blocking pole and the pole operating with a double valve group is the last blocking pole.

5. The control method according to claim 1, characterized in that: In the case where different converters are respectively arranged at different locations in a converter station, the islanding signal is used to instruct the converters in the converter station connected to the AC grid in an islanded state to be locked.

6. The control method according to claim 5, characterized in that: The step of determining a priority blocking pole and a rear blocking pole in a bipolar portion of a converter station connected to an AC grid in an islanded state according to an operating state of the converter station connected to the AC grid in an islanded state comprises: In the case of bipolar symmetrical operation of the converter station connected to the AC grid in an islanded state, determining one pole as the priority blocking pole and the other pole as the post-blocking pole; In the case where one of the two poles of the converter station connected to the AC grid in an islanded state is in reduced voltage operation and the other pole is in full voltage operation with a dual valve group, determining that the pole in reduced voltage operation is the priority blocking pole and the pole in full voltage operation is the last blocking pole; When one of the two poles of the converter station connected to the AC grid in an islanded state operates with a single valve group and the other operates with a double valve group, it is determined that the pole operating with the double valve group is the priority blocking pole and the pole operating with the single valve group is the last blocking pole.

7. The control method according to any one of claims 1 to 6, characterized in that: The control method further includes: After the priority blocking pole is blocked, the AC filter is cut off according to the reactive balance principle.

8. The control method according to any one of claims 1 to 6, characterized in that: The control method further includes: After the priority blocking pole is blocked, the AC filter is cut off according to the minimum filter principle.

9. The control method according to any one of claims 1 to 6, characterized in that: The control method further includes: After the priority blocking pole is blocked, the AC filter is quickly switched off according to the absolute minimum filter principle.

10. The control method according to any one of claims 1 to 4, characterized in that: The control method further includes: During the blocking process, the de-bipolar power control is first performed on the rear blocking pole, and then the priority blocking pole is blocked.

11. An island blocking control system for a power transmission system, characterized in that: include: A stabilization control system is configured to monitor the operating status of the sending-end AC power grid in real time and generate an islanding signal when the sending-end AC power grid is in an islanding state; The DC system is configured to receive the islanding signal and, based on the operating status of the converter station connected to the AC grid in the islanding state, determine a priority blocking pole and a rear blocking pole in the bipolar portion of the converter station connected to the AC grid in the islanding state, and control the blocking moment of the rear blocking pole to be no earlier than the blocking moment of the priority blocking pole.

12. The control system according to claim 11, characterized in that: The DC system includes a converter station, in which different converters are centrally arranged; The stabilization control system is further configured to generate an islanding signal when detecting that the sending-end AC power grid is in an islanding state, so as to instruct the DC system to lock different converters with the same positive or negative poles.

13. The control system according to claim 12, characterized in that: The DC system is further configured as follows: In the case of bipolar symmetrical operation of the converter station connected to the AC grid in an islanded state, determining one pole as the priority blocking pole and the other pole as the post-blocking pole; In the case where one of the two poles of the converter station connected to the AC grid in an islanded state is in reduced voltage operation and the other pole is in full voltage operation with a dual valve group, determining that the pole in reduced voltage operation is the priority blocking pole and the pole in full voltage operation is the last blocking pole; When one of the two poles of the converter station connected to the AC grid in an islanded state operates with a single valve group and the other operates with a double valve group, it is determined that the pole operating with a single valve group is the priority blocking pole and the pole operating with a double valve group is the last blocking pole.

14. The control system according to claim 11, characterized in that: The DC system includes a plurality of converter stations, the plurality of converter stations are located at different positions, and each converter station is provided with different converters with the same positive or negative pole; The stabilization control system is further configured to generate an islanding signal when detecting that the sending-end AC grid is in an islanding state, to instruct the DC system to lock the converter in the converter station connected to the AC grid in the islanding state.

15. The control system according to claim 14, characterized in that: The DC system is further configured as follows: In the case of bipolar symmetrical operation of the converter station connected to the AC grid in an islanded state, determining one pole as the priority blocking pole and the other pole as the post-blocking pole; In the case where one of the two poles of the converter station connected to the AC grid in an islanded state is in reduced voltage operation and the other pole is in full voltage operation with a dual valve group, determining that the pole in reduced voltage operation is the priority blocking pole and the pole in full voltage operation is the last blocking pole; When one of the two poles of the converter station connected to the AC grid in an islanded state operates with a single valve group and the other operates with a double valve group, it is determined that the pole operating with the double valve group is the priority blocking pole and the pole operating with the single valve group is the last blocking pole.

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