Multi-circuit MMC type flexible direct current transient voltage collaborative supporting method and system

Abstract

The invention discloses a multi-circuit MMC type flexible direct current transient voltage cooperative support method and system, and the method comprises the steps: calculating the fault features of key nodes of a power grid in various N-2 fault scenes, and dividing each converter station into a responsibility area of the key nodes based on the sensitivity coefficient of each converter station for the voltage of the key nodes; determining the priority of each converter station in the same responsibility area according to the number of the flexible DC lines; after a fault occurs, calculating fault dynamic characteristics of the key nodes by using real-time measurement data of the power grid; based on a matching result of the dynamic fault features and the calculated fault features, determining converter stations cooperatively supporting the key node voltage under the fault and priorities of the converter stations; determining a reactive power reference value of each converter station by taking power grid transient voltage stabilization as an optimization target; based on the priorities of the converter stations, allocating an execution time sequence of each converter station; and based on the execution time sequence and the reactive power reference value, rapid, accurate and adaptive cooperative support of the multi-loop MMC type flexible direct transient voltage current is realized.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of flexible direct current power transmission, and in particular, relates to a multi-loop modular multilevel converter (MMC) type flexible direct current transient voltage cooperative support method and system based on offline pre-computation and online fast retrieval. BACKGROUND

[0002] With the rapid development of flexible direct current power transmission (MMC-HVDC) technology, multi-loop MMC flexible direct current systems are increasingly widely used in power grids. However, when serious transient events such as N-2 line faults and DC bipolar blocking occur in the power grid, the traditional voltage support method has defects such as poor adaptability, high calculation delay, etc. Specifically, the offline strategy has poor adaptability, the traditional offline pre-planned fault scenarios are limited, and it is difficult to cover all dynamic changes in the actual operation of the power grid, and the support strategy cannot be adjusted according to the real-time operating state; the online calculation has high delay, the online voltage support method based on real-time simulation needs to call the full system model for transient calculation, which has large calculation amount and time consumption, and cannot meet the hundred-millisecond-level fast decision-making requirement; the multi-loop direct current cooperation is insufficient, the existing method does not clearly define the responsibility partition of the multi-loop MMC type flexible direct current, leading to control timing conflicts, such as power oscillation caused by simultaneous adjustment of different converter stations, or unreasonable reactive power distribution, such as insufficient utilization of near-zone converter stations and over-adjustment of far-zone converter stations; the support capacity for serious faults is weak, for extreme scenarios such as N-2 faults, the reactive power support capacity of single-loop MMC type flexible direct current is limited, and there is a lack of dynamic cooperative support mechanism for multi-loop direct current, which is easy to cause voltage instability. Moreover, the existing technology focuses on parameter optimization or local control strategy improvement at the single device level, and lacks a systematic cooperative mechanism; moreover, the traditional method uses equalization processing for fault impact assessment of multi-loop MMC, ignores the electrical distance and topological correlation between devices, and does not consider the influence of fault propagation path on voltage support effect. Therefore, there is an urgent need for a multi-loop MMC transient voltage support method that takes into account rapidity, adaptability and cooperation. SUMMARY

[0003] To solve the problems in the prior art, the present application provides a multi-loop MMC type flexible direct current transient voltage cooperative support method and system, which is suitable for transient voltage stability control under serious transient events (such as N-2 line faults and DC bipolar blocking) in an AC / DC hybrid power grid containing multi-loop MMC type flexible direct current. Through offline pre-computation, online fast retrieval and hierarchical cooperative control, multi-loop MMC type flexible direct current transient voltage cooperative support is performed, wherein a typical fault library and responsibility partition template are constructed through offline pre-computation, real-time fault characteristics are matched quickly in the online stage to generate a cooperative strategy, and finally a rapid, accurate, adaptive and cooperative multi-loop MMC transient voltage support of multi-loop MMC type flexible direct current is realized through global-regional-local three-level coordinated control.

[0004] The application adopts the technical solutions as follows.

[0005] The application provides a multi-loop MMC type flexible DC transient voltage collaborative support method, which comprises the following steps:

[0006] The historical operation data and the historical operation mode of the power grid are acquired, and the key nodes of the power grid under various N-2 fault scenarios are determined; the fault characteristics of each key node are calculated, each converter station is divided into the responsibility area of the key node based on the sensitivity coefficient of the key node voltage of each converter station, and the priority of each converter station in the same responsibility area is determined according to the number of flexible DC lines;

[0007] After a fault occurs, the fault dynamic characteristics of the key nodes are calculated by using the real-time measurement data of the power grid; the dynamic fault characteristics are matched with the calculated fault characteristics, and based on the matching result, the converter stations for collaboratively supporting the voltage of the key nodes under the fault and the priority thereof are determined;

[0008] The reactive power reference value of each converter station is determined with the transient voltage stability of the power grid as the optimization target; the execution time sequence of each converter station is allocated based on the priority of the converter station; and each converter station is controlled to perform voltage collaborative support based on the execution time sequence and the reactive power reference value.

[0009] The typical operation modes are screened from the historical operation modes of the power grid, and various N-2 fault scenarios are determined according to the historical operation data corresponding to the typical operation modes;

[0010] The time-domain simulation is performed on the various N-2 fault scenarios, and the voltage drop depth and the fault recovery time length of each node in the power grid under various fault types are determined; the nodes with the voltage drop depth greater than a set voltage threshold and the fault recovery time length greater than a set recovery time length are taken as the key nodes;

[0011] The sensitivity coefficient of each converter station to the voltage of the key nodes is calculated; each converter station is divided into the responsibility area of each key node according to the sensitivity coefficient; and the priority of each converter station in the same responsibility area is determined according to the number of flexible DC lines in the power grid and in the order from large to small of the sensitivity coefficient.

[0012] The typical operation modes include the wet season, the dry season, the high load, the low load, the high output of new energy and the low output of new energy; and the N-2 fault scenarios include the line N-2 tripping and the DC bipolar blocking.

[0013] The converter stations with the sensitivity coefficient not less than a first coefficient threshold are divided into the first responsibility area of the key nodes, the converter stations with the sensitivity coefficient less than the first coefficient threshold and not less than a second coefficient threshold are divided into the second responsibility area of the key nodes, and the converter stations with the sensitivity coefficient less than the second coefficient threshold are divided into the non-responsibility area of the key nodes.

[0014] If there are p flexible DC lines in the power grid, then there are p priority levels. Within the same responsibility area, converter stations are ranked in descending order of sensitivity coefficient, with the highest-ranked station being prioritized. Each converter station corresponds to the first priority, followed by others in the order of priority. Each converter station corresponds to the second priority, and so on, with the last one listed first. Each converter station corresponds to the p-th priority level, where p is a positive integer.

[0015] An offline fault database is established to store the fault characteristics and responsibility area characteristics of each key node. The fault characteristics include: fault type, key node i, voltage drop depth ΔU%, and fault recovery time t. rec The characteristics of the responsibility area include: key node i, converter stations in the first responsibility area and their corresponding priorities, and converter stations in the second responsibility area and their corresponding priorities.

[0016] After a fault occurs, the number of flexible DC lines that have failed is determined and used as a priority number.

[0017] Determine the voltage measurement value before the fault based on real-time power grid measurement data; calculate the fault dynamic characteristics of key nodes using the voltage measurement value before the fault.

[0018] When the dynamic fault characteristics match the fault characteristics stored in the offline fault database, the responsibility area characteristics of the key nodes stored in the offline fault database are called to determine the converter stations that collaboratively support the voltage of the key nodes under the fault and their priorities.

[0019] When the dynamic fault characteristics do not match the fault characteristics stored in the offline fault database, each converter station switches to droop control and updates the sensitivity coefficient of each converter station to the critical node voltage. Based on the absolute value of the change in the sensitivity coefficient before and after the update, and using the determined priority number, the converter stations that collaboratively support the critical node voltage under the fault and their priorities are determined.

[0020] The responsibility area characteristics of critical nodes stored in the offline fault database are invoked to determine the converter stations that collaboratively support the voltage of critical nodes under fault conditions and their priorities, including:

[0021] Adjust the priority of each converter station within the same responsibility area based on the number of faulty flexible DC lines; or, call the responsibility area characteristics of key nodes stored in the offline fault database, and use the priority of converter stations within each responsibility area as the priority of converter stations that collaboratively support the voltage of key nodes under real-time faults.

[0022] Update the sensitivity coefficients of each converter station to the voltage at critical nodes. Sort the converter stations in descending order of the absolute value of the change in sensitivity coefficients before and after the update. Then, consider the number of faulty flexible DC lines, p′, and prioritize the stations ranked first. The first priority corresponds to one converter station, and the second priority corresponds to one converter station, and so on, and the last priority corresponds to one converter station. The first priority corresponds to one converter station, and the second priority corresponds to one converter station, and so on, and the last priority corresponds to one converter station. The first priority corresponds to one converter station, and the second priority corresponds to one converter station, and so on, and the last priority corresponds to one converter station.

[0023] The reactive power reference value of each converter station is determined by taking the transient voltage stability of the power grid as the optimization target, including:

[0024] A global objective function is established by taking the weighted sum of the voltage deviation variance of the key nodes, the reactive power deviation of the converter station, and the voltage deviation variance of the non-key nodes as the optimization target, wherein the non-key nodes include the non-key nodes of the 220kV and above voltage level in the power grid;

[0025] The constraint condition of the global objective function is established;

[0026] The global objective function is iteratively solved under the constraint condition to obtain the reactive power reference value of each converter station.

[0027] The application also provides a multi-loop MMC type flexible DC transient voltage cooperative support system, which comprises:

[0028] An offline module is configured to obtain historical operation data and historical operation mode of the power grid, determine key nodes of the power grid under various N-2 fault scenarios, calculate fault characteristics of each key node, divide each converter station into a responsibility area of the key nodes based on the sensitivity coefficient of each converter station to the voltage of the key nodes, and determine the priority of each converter station in the same responsibility area according to the number of flexible DC lines;

[0029] An online module is configured to calculate the dynamic fault characteristics of the key nodes by using real-time measurement data of the power grid after a fault occurs, match the dynamic fault characteristics with the calculated fault characteristics, and determine the converter station and the priority of the converter station for cooperatively supporting the voltage of the key nodes under the fault based on the matching result;

[0030] A cooperative control module is configured to determine the reactive power reference value of each converter station by taking the transient voltage stability of the power grid as the optimization target, allocate the execution time sequence of each converter station based on the priority of the converter station, and control each converter station to cooperatively support the voltage based on the execution time sequence and the reactive power reference value.

[0031] The application also provides a terminal, which comprises a processor and a storage medium.

[0032] The application also provides a computer readable storage medium, which stores a computer program.

[0033] The beneficial effects of the present application at least include that, compared with the prior art, in the multi-loop flexible DC transient voltage control strategy proposed by the present application, the transient voltage support responsibility is dynamically allocated to each MMC converter station based on the power grid topology characteristics and the fault propagation path, and the three-level linkage collaborative control is performed from the global power grid, the key node responsibility area and the local converter, which is the multi-time scale coordination between multiple converter stations, realizes the real cross-device collaborative control, improves the transient voltage stability margin of the AC / DC hybrid power system, shortens the voltage recovery time after the fault, and provides key technical support for the safety of the power grid under the background of high proportion of new energy access.

[0034] The offline pre-computation and online fast retrieval are coordinated, the offline pre-computation converts the complex online simulation into template matching, so that the online decision-making time is significantly shortened, and the rapidity of voltage support is significantly improved; the responsibility partition and priority are dynamically adjusted through real-time fault feature matching, the problem that the traditional offline strategy cannot cover the dynamic changes of the power grid is solved, and the adaptability of voltage support is significantly improved; the global power grid transient voltage optimization and partition execution timing coordination strategy avoids the power oscillation caused by the simultaneous adjustment of multiple MMCs, improves the efficiency of reactive power distribution, and optimizes the cooperativity of voltage support; the multi-loop MMC collaborative support is realized for extreme scenarios such as N-2 fault, so that the key node voltage recovery time is shortened, the voltage drop depth is reduced, voltage instability is effectively prevented, and the support capacity is significantly enhanced. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1 is a flowchart of a multi-loop MMC type flexible DC transient voltage collaborative support method provided by the present application. DETAILED DESCRIPTION

[0036] In order to make the purpose, technical scheme and advantages of the present application clearer, the technical scheme of the present application will be described clearly and completely below in combination with the drawings in the embodiments of the present application. The embodiments described in the present application are only a part of the embodiments of the present application, not all the embodiments. Based on the spirit of the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the present application.

[0037] The multi-loop MMC type flexible DC includes multiple converter stations, each converter station adopts a modular multilevel converter, and the present application provides a multi-loop MMC type flexible DC transient voltage collaborative support method, as shown in Figure 1 , which comprises:

[0038] Step 1, obtain historical operation data and historical operation mode of the power grid, determine the key nodes of the power grid under various N-2 fault scenarios; calculate the fault characteristics of each key node, divide each converter station into the responsibility area of the key node based on the sensitivity coefficient of each converter station to the voltage of the key node, and determine the priority of each converter station in the same responsibility area according to the number of flexible DC lines.

[0039] Specifically, step 1 is an offline pre-computation stage, including:

[0040] Step 1.1, select typical operation modes from the historical operation modes of the power grid, and determine various N-2 fault scenarios according to the historical operation data corresponding to the typical operation modes;

[0041] Typical operation modes include but are not limited to: wet season, dry season, high load, low load, high output of new energy, and low output of new energy; N-2 fault scenarios include but are not limited to: line N-2 tripping, DC bipolar blocking and other serious fault combinations;

[0042] Step 1.2, time-domain simulation is performed on various N-2 fault scenarios to determine the voltage drop depth and fault recovery time of each node in the power grid under various fault types; nodes with voltage drop depth greater than a set voltage threshold and fault recovery time greater than a set recovery time are regarded as key nodes;

[0043] Various fault types include but are not limited to: single-phase or three-phase ground fault of lines or buses under different voltage levels (220 kV to 1000 kV), and single-pole or bipolar blocking fault of EHVDC;

[0044] In the embodiment, each node in the flexible DC includes but is not limited to: a load center and an EHVDC converter station;

[0045] The voltage drop depth satisfies the following relationship:

[0046]

[0047] In the formula, ΔU% is the voltage drop depth, U0 is the voltage before the fault, U min is the minimum voltage in the set time period after the fault; in the embodiment, the set time period is 200 ms;

[0048] The fault recovery time t rec is the time period from the fault time to the time when the voltage recovers to 90% U0;

[0049] In the embodiment, the set voltage threshold is 20%, and the set recovery time is 1 s.

[0050] Step 1.3: Calculate the sensitivity coefficient of each converter station to the voltage of the critical node; based on the sensitivity coefficient, divide each converter station into the responsibility area of ​​each critical node; based on the number of flexible DC lines in the power grid, determine the priority of each converter station in the same responsibility area in descending order of sensitivity coefficient;

[0051] Among them, converter stations with sensitivity coefficients not less than the first coefficient threshold are assigned to the first responsibility area of ​​the key node, converter stations with sensitivity coefficients less than the first coefficient threshold but not less than the second coefficient threshold are assigned to the second responsibility area of ​​the key node, and converter stations with sensitivity coefficients less than the second coefficient threshold are assigned to the non-responsibility area of ​​the key node.

[0052] In this embodiment, each converter station in the flexible DC transmission system is denoted as MMC. j , j = 1, 2, ..., n, where n is the number of converter stations; the sensitivity coefficient of each converter station to the voltage of the critical node satisfies the following relationship:

[0053]

[0054] In the formula, S ij MMC for converter station j The sensitivity coefficient U to the voltage of critical node i i For the critical node i voltage, Q j MMC for converter station j Output reactive power;

[0055] In the embodiment, the first coefficient threshold S th1 The value is 0.3, and the second coefficient threshold S th2 The value is 0.1; S ij Not less than S th1 The converter station is assigned to the first responsibility area of ​​key node i, S ij Less than S th1 And not less than S th2 The converter station is assigned to the second responsibility area of ​​key node i, S ij Less than S th2 The converter stations are assigned to the non-responsibility area of ​​key node i; if there are p flexible DC lines in the power grid, then there are p priorities. Within the same responsibility area, the converter stations are sorted in descending order of sensitivity coefficient, with the first-ranked station being prioritized. Each converter station corresponds to the first priority, followed by others in the order of priority. Each converter station corresponds to the second priority, and so on, with the last one listed first. The pth priority corresponds to the pth converter station, and p is a positive integer; in the embodiment, the number of the flexible DC lines to be controlled is 3, and there are 5 converter stations in the first responsibility area, then according to the order from large to small of the sensitivity coefficients, the first two converter stations in the front correspond to the first priority, the two converter stations after the sorting correspond to the second priority, and the last one converter station corresponds to the third priority;

[0056] S ij Not only the dynamic responsibility zoning of the converter station is realized, but also S ij According to the physical meaning of S ij The greater S j The stronger the voltage support ability of the MMC ij of the converter station to the key node i, so S ij characterizes the response order of each converter station in supporting the voltage of the key node under various N-2 fault scenarios.

[0057] Step 1.4, an offline fault library is established to store the fault characteristics of each key node and the responsibility area characteristics; wherein the fault characteristics include: fault type, key node i, voltage drop depth ΔU%, and fault recovery time t rec ; the responsibility area characteristics include: key node i, converter stations in the first responsibility area and the corresponding priority of each converter station, converter stations in the second responsibility area and the corresponding priority of each converter station.

[0058] The offline precalculation carried out by the present application does not involve full calculation or full time scale calculation of the flexible DC, but for the screened N-2 fault scenarios, the fault characteristics and responsibility area characteristics of each converter station under different fault types are calculated, and the time scale used for calculation is often minute level or even second level, which is a kind of local quantity calculation, avoiding the problems of data delay and information lag of traditional offline calculation, and at the same time, through offline precalculation, the fault characteristics and responsibility area characteristics of each key node are provided for online retrieval matching.

[0059] In the offline precalculation stage, the response speed and support ability of the converter station to the power grid voltage support demand are improved through responsibility zoning, avoiding the problems of insufficient utilization of near-area converter stations and over-regulation of far-area converter stations, then the priority of the converter station in each responsibility area is determined according to the number of flexible DC lines existing in the power grid, realizing the basic collaborative control strategy, solving the problem of control time sequence conflict caused by the response order of the converter station cluster performing voltage support in the prior art, and avoiding power oscillation caused by simultaneous regulation of multiple converter stations.

[0060] Step 2, after the fault occurs, the fault dynamic characteristics of the key node are calculated by using the real-time measurement data of the power grid; the dynamic fault characteristics are matched with the calculated fault characteristics, and based on the matching result, the converter station and its priority for cooperatively supporting the voltage of the key node under the fault are determined.

[0061] Specifically, step 2 is an online fast decision-making stage, including:

[0062] Step 2.1, determining the number p' of faulty flexible DC lines after the fault occurs as the priority number; p'≤p, p' is a positive integer;

[0063] Specifically, the faulty flexible DC line is located by combining the switch state measurement with the graph theory algorithm.

[0064] Step 2.2, determining the voltage measurement value before the fault according to the real-time measurement data of the power grid;

[0065] Specifically, the real-time measurement data of the power grid is obtained by the wide-area measurement system (WAMS), including but not limited to: the voltage phasor V i ∠θ i of the key node i, the current phasor I l ∠φ l of the line l, the frequency f, the active power P, and the reactive power Q, with a sampling period ≤10 ms; the measurement error is corrected based on the state estimation (SE) to obtain the real-time topology and operating point (V0, P0, Q0) of the power grid, wherein V0, P0, and Q0 are the voltage measurement value, the active power measurement value, and the reactive power measurement value before the fault, respectively.

[0066] Step 2.2, calculating the fault dynamic characteristics of the key node using the voltage measurement value before the fault;

[0067] The fault dynamic characteristics of the key node include: voltage drop dynamic characteristics and fault recovery duration;

[0068] Wherein, the voltage drop dynamic characteristics satisfy the following relationship:

[0069]

[0070] In the formula, is the voltage drop dynamic characteristics, and V0 is the voltage measurement value before the fault;

[0071] In the online fast decision-making stage, the voltage drop depth calculated using the voltage measurement value before the fault has dynamic characteristics, which is different from the voltage drop depth obtained by offline precalculation.

[0072] Step 2.3, when the dynamic fault characteristics match the fault characteristics stored in the offline fault library, the responsibility area characteristics of the key node stored in the offline fault library are called to determine the converter station and its priority for supporting the voltage of the key node under real-time fault;

[0073] Specifically, the dynamic fault feature of the key node is matched with the fault features stored in the offline fault library, including: the cosine similarity algorithm is used to calculate the similarity between the dynamic fault feature and the fault features stored in the offline fault library, and the similarity greater than the set threshold indicates that the dynamic fault feature matches the fault features stored in the offline fault library, which is a non-limiting and preferred selection;

[0074] When the dynamic fault feature of the key node i matches the fault feature of the key node i stored in the offline fault library, the converter stations supporting the voltage of the key node i in real time are determined according to the responsibility area features of the key node i stored in the offline fault library, and the priorities of the converter stations corresponding to the priorities; wherein, in the face of real-time fault of the flexible DC line, when the voltage is cooperatively supported, the priority of each converter station in the same responsibility area can be adjusted in real time according to the number of flexible DC lines that have failed, so as to effectively cover the dynamic changes of line faults in the actual operation of the power grid, adjust the support strategy according to the real-time operation state of the power grid, or directly call the responsibility area features of the key node stored in the offline fault library in order to improve the operability of the engineering implementation, and the priority of each converter station in the responsibility area determined in the offline stage is used as the priority of the converter station cooperatively supporting the voltage of the key node in real time.

[0075] Step 2.4, when the dynamic fault feature does not match the fault feature stored in the offline fault library, each converter station switches to droop control and updates the sensitivity coefficient of each converter station to the voltage of the key node; according to the absolute value of the change amount of the sensitivity coefficient before and after the update, the number of priorities determined is used to determine the converter stations supporting the voltage of the key node in real time and the priorities of the converter stations;

[0076] When the dynamic fault feature does not match the fault feature stored in the offline fault library, it means that a new fault scenario has occurred, and there is a possibility of failure in online retrieval, each converter station switches to droop control, and the output of the reactive power is adjusted adaptively to support the voltage of the power grid, and a warning of failure of cooperative control is sent, so as to ensure the rapidity of voltage support;

[0077] The sensitivity coefficients of each converter station to the voltage of the key node are updated, the converter stations are sorted in the order from large to small according to the absolute value of the change amount of the sensitivity coefficient before and after the update, the number of flexible DC lines that have failed p', the converter stations sorted in the front correspond to the first priority, the converter stations sorted after correspond to the second priority, and so on, and the converter stations sorted at the end correspond to the p' priority; when the dynamic fault feature of the key node i does not match the fault feature of the key node i stored in the offline fault library, the absolute value of the change amount of the sensitivity coefficient before and after the update is ij| actually represents the location of the converter station on the fault propagation path, the closer the converter station to the fault line on the fault propagation path, the larger |ΔS ij Therefore, the converter stations in the near area are divided into the first priority, and the converter stations in the far area are divided into the remaining priorities, so as to realize an active responsibility partition, which is dynamically adjusted in real time according to the change of the fault line, and takes into account the adaptability and synergy of voltage support.

[0078] In the embodiment, step 2.4 is implemented in the near-line service, and after offline pre-computation, the sensitivity coefficient calculation model and the calculation results are loaded into the near-line service. When a mismatch is found in the online retrieval stage, each converter station switches to droop control while sending a calculation request to the near-line service, and the re-determination of the converter stations and their priorities for real-time fault voltage support is performed, so as to avoid the risk of transient voltage instability of the power grid caused by retrieval failure due to scene mismatch in the dispatching process. At the same time of realizing the timely support of the converter station to the power grid voltage at the fault moment, the near-line service carries out calculation in an asynchronous non-blocking mode to quickly determine the converter stations and their priorities for responding to voltage support in the new scene; and the dynamic fault characteristics and the calculation results of the near-line service are used to update the offline fault library to store new fault characteristics and responsibility area characteristics.

[0079] Step 3: determining the reactive power reference value of each converter station with the power grid transient voltage stability as the optimization target; based on the priority of the converter station, allocating the execution time sequence of each converter station; based on the execution time sequence and the reactive power reference value, controlling each converter station to perform voltage collaborative support.

[0080] Specifically, step 3 is a hierarchical collaborative control stage, including:

[0081] Step 3.1: establishing a global objective function with the weighted sum of the key node voltage deviation variance, the converter station reactive power deviation, and the non-key node voltage deviation variance as the optimization target, as follows:

[0082]

[0083] In the formula, minF is the global objective function, w1, w2, and w3 are weight coefficients, and w1>w2>w3, w1+w2+w3=1, ΔU i is the voltage deviation of the key node i, m is the number of key nodes, Q j_ref , Q j_oper is the reactive power reference value and actual value of the MMC j of the converter station, ΔV i′ is the voltage deviation of the non-key node i', m' is the number of non-key nodes;

[0084] In the embodiment, the non-critical nodes include non-critical nodes of 220 kV and above voltage levels in the power grid.

[0085] Step 3.2, constraints of the global objective function are established, including:

[0086] 1) reactive power constraints of the converter station MMC j , as follows:

[0087] Q j_min ≤ Q j_ref ≤ Q j_max

[0088] In the formula, Q j_min and Q j_max are the lower limit and the upper limit of the reactive power of the converter station MMC j , respectively.

[0089] 2) safety constraints of the voltage of the critical nodes, as follows:

[0090] U i_min ≤ U i ≤ U i_max

[0091] In the formula, U i_min and U i_max are the lower limit and the upper limit of the voltage of the critical node i, respectively.

[0092] 3) response time constraints of the converter station MMC j , as follows:

[0093] t j ≤ t delay

[0094] In the formula, t j is the response time of the converter station MMC j , t delay is the response time of the reactive power of the converter station, and in the embodiment, t delay ≤ 20 ms.

[0095] Step 3.3, the global objective function is iteratively solved under the constraints to obtain the reactive power reference value of each converter station MMC j .

[0096] Step 3.2, based on the priority of the converter station, the execution timing of each converter station is allocated.

[0097] The execution timing offset Δt k, k is the priority order number, in the embodiment, the execution time sequence offset of the first priority is Δt1=0 ms, the execution time sequence offset of the second priority is Δt2=50 ms, the execution time sequence offset of the third priority is Δt3=100 ms, and the different execution time sequences ensure that the high-priority converter station acts first to support voltage, and the low-priority converter station acts after the voltage support of the high-priority converter station plays a stabilizing role, effectively avoiding power oscillation.

[0098] When the converter station receives the execution time sequence and the reactive power reference value, the switching is performed to the cooperative control mode, the switching quantity or the modulation ratio is adjusted through the fixed reactive power control adjustment submodule, the fast reactive power output is realized, and through the three-level coordination of the global, regional and local, the cooperative reactive power control of the multi-loop MMC type flexible DC is realized.

[0099] The application further provides a multi-loop MMC type flexible DC transient voltage cooperative support system, which comprises:

[0100] The offline module is used for acquiring historical operation data and historical operation modes of the power grid, determining key nodes of the power grid under various N-2 fault scenarios, calculating fault characteristics of each key node, dividing each converter station into a responsibility area of the key node based on a sensitivity coefficient of each converter station to voltage of the key node, and determining the priority of each converter station in the same responsibility area according to the number of flexible DC lines.

[0101] The online module is used for calculating the dynamic fault characteristics of the key node by using real-time measurement data of the power grid after a fault occurs, matching the dynamic fault characteristics with the calculated fault characteristics, and determining the converter station and the priority of the converter station for cooperatively supporting voltage of the key node under the fault based on the matching result.

[0102] The cooperative control module is used for determining the reactive power reference value of each converter station with the power grid transient voltage stability as an optimization target, allocating the execution time sequence of each converter station based on the priority of the converter station, and controlling each converter station to cooperatively support voltage based on the execution time sequence and the reactive power reference value.

[0103] In the embodiment, the typical operation mode is summer peak load, the receiving end load is 50 million kilowatts, the new energy output is 15 million kilowatts, there are three converter stations and two flexible DC lines, the active power of the converter station MMC1 is 6 million kilowatts, the active power of the converter station MMC2 is 6 million kilowatts, the N-1 and N-2 fault scenarios are constructed, in the offline stage, the simulation calculation obtains that the key node is the receiving end load center node N1, the ΔU% of the key node is 25%, the t rec= 300 ms; the sensitivity coefficient of the converter station MMC1 to N1 is 0.4 p.u. / p.u., the sensitivity coefficient of the converter station MMC2 to N1 is 0.2 p.u. / p.u., and the sensitivity coefficient of the converter station MMC3 to N1 is 0.05 p.u. / p.u.; offline calculation determines that the converter station MMC1 is the first responsibility area of N1, the converter station MMC2 is the second responsibility area of N1, and the converter station MMC3 is the non-responsibility area of N1;

[0104] The template stored in the offline fault library is {fault type: three-phase permanent ground fault of line L1-L2, key node: N1, responsibility partition: [MMC1, MMC2], priority: [1, 2]};

[0105] The WAMS detects that the line L1-L2 trips, the voltage of the node N1 drops to 0.72 p.u., and ΔU% = 37.3%; the cosine similarity between the fault dynamic characteristics and the pre-stored fault characteristics is 0.92, which is greater than the set threshold value 0.8, and is a high match; the responsibility area characteristics of the node N1 are directly called to check the operating state of each converter station, wherein the available reactive power capacity of the converter station MMC1 is 1500 MVar, and the available reactive power capacity of the converter station MMC2 is 1000 MVar;

[0106] When the global optimization of the grid voltage is performed, the weight coefficients are 0.6, 0.3 and 0.1 respectively, the rolling optimization obtains that the reactive power reference value of the converter station MMC1 is 1200 MVar, the reactive power reference value of the converter station MMC2 is 800 MVar, and the execution time sequence offset of the converter station MMC1 (first priority) is 0 ms, and the execution time sequence offset of the converter station MMC2 (second priority) is 50 ms;

[0107] After the converter station MMC1 receives the instruction, the reactive power output is increased from 500 MVar to 1200 MVar at 0 ms, and the response time is 5 ms; after the converter station MMC2 receives the instruction, the reactive power output is increased from 300 MVar to 800 MVar at 50 ms, and the response time is 5 ms;

[0108] 100 ms after the fault, the voltage of the node N1 recovers to 0.85 p.u., and ΔU% = 15%, t rec = 150 ms.

[0109] In the present application, offline pre-computation converts complex online simulation into template matching, so that the online decision time is less than or equal to 100 ms, compared with the decision time of more than 1 s in the traditional method, which meets the golden 100 ms demand of transient voltage control, and significantly improves the rapidity of voltage support; through real-time fault feature matching, the responsibility partition and priority are dynamically adjusted, the problem that the traditional offline strategy cannot cover the dynamic change of the power grid is solved, and the adaptability of voltage support is significantly improved; the global power grid transient voltage optimization and partition execution timing coordination strategy avoids the power oscillation caused by the simultaneous adjustment of multiple MMCs, compared with single MMC control, the reactive power distribution efficiency is improved by more than 30%, and the synergy of voltage support is optimized; the method proposed in the present application realizes the collaborative support of multiple MMCs for N-2 fault and other extreme scenarios, so that the voltage recovery time of the key node is shortened by 50%, in the embodiment, the voltage recovery time is reduced from more than 200 ms to less than 100 ms, the voltage drop depth is reduced by 20%, the voltage instability is effectively prevented, and the support capacity is significantly enhanced.

[0110] The present disclosure can be a system, a method, and / or a computer program product. The computer program product can include a computer readable storage medium (or media) having computer readable program instructions thereon for causing a processor to carry out aspects of the present disclosure.

[0111] The computer readable storage medium can be a tangible device that can retain and store instructions for use by an instruction execution device. The computer readable storage medium can be, for example, but is not limited to, an electronic storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of the computer readable storage medium include the following: a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), a static random access memory (SRAM), a portable compact disc read-only memory (CD-ROM), a digital versatile disk (DVD), a memory stick, a floppy disk, a mechanically encoded device such as punch-cards or punched tape, a holographic storage medium, and any suitable combination of the foregoing. A computer readable storage medium, as used herein, is not to be construed as being transitory signals per se, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through a waveguide or other transmission media (e.g., light pulses passing through a fiber-optic cable), or electrical signals transmitted through a wire.

[0112] Computer readable program instructions described herein can be downloaded to respective computing / processing devices from a computer readable storage medium or to an external computer or external storage device via a network, for example, the Internet, a local area network, a wide area network and / or a wireless network. The network can comprise copper transmission cables, optical transmission fibers, wireless transmission, routers, firewalls, switches, gateway computers and / or edge servers. A network adapter card or network interface in each computing / processing device receives computer readable program instructions from the network and forwards the computer readable program instructions for storage in a computer readable storage medium within the respective computing / processing device.

[0113] Computer readable program instructions for carrying out operations of the present disclosure can be assembly instructions, instruction-set-architecture (ISA) instructions, machine instructions, machine dependent instructions, microcode, firmware instructions, state-setting data, or either source code or object code written in any combination of one or more programming languages, including an object oriented programming language such as Smalltalk, C++ or the like, and conventional procedural programming languages, such as the "C" programming language or similar programming languages. The computer readable program instructions can execute entirely on the user's computing device, partly on the user's computing device, as a stand-alone software package, partly on the user's computing device and partly on a remote computing device or entirely on the remote computing device or server. In the latter scenario, the remote computing device can be connected to the user's computing device through any kind of network, including a local area network (LAN) or a wide area network (WAN), or the connection can be made to an external computing device, for example, through the Internet using an Internet Service Provider. In some embodiments, electronic circuitry including, for example, programmable logic circuitry, field-programmable gate arrays (FPGA), or programmable logic arrays (PLA) can execute the computer readable program instructions by utilizing state information of the computer readable program instructions to personalize the electronic circuitry, in order to perform aspects of the present disclosure.

[0114] Finally, it should be noted that the above-mentioned embodiments are merely intended for describing and illustrating, but not limiting the technical solutions of the present application. Although the present application has been described in detail with reference to the above embodiments, those skilled in the art should understand that the specific embodiments of the present application can be modified or replaced by equivalents without departing from the spirit and scope of the present application. Any modification or equivalent replacement without departing from the spirit and scope of the present application should be covered within the protection scope of the claims of the present application.

Claims

1. A method for transient voltage support of a multi-loop MMC-type flexible HVDC, characterized in that, The method comprises the following steps: obtaining historical operation data and historical operation mode of the power grid, and determining key nodes of the power grid under various N-2 fault scenarios; calculating fault characteristics of each key node, dividing each converter station into a responsibility area of the key node based on the sensitivity coefficient of each converter station to the voltage of the key node, and determining the priority of each converter station in the same responsibility area according to the number of flexible DC lines; after the fault occurs, calculating the dynamic fault characteristics of the key node by using real-time measurement data of the power grid; matching the dynamic fault characteristics with the calculated fault characteristics, and determining the converter station and its priority for supporting the voltage of the key node under the fault based on the matching result; determining the reactive power reference value of each converter station with the power grid transient voltage stability as the optimization target; distributing the execution time sequence of each converter station based on the priority of the converter station; and controlling each converter station to perform voltage cooperative support based on the execution time sequence and the reactive power reference value.

2. The multi-loop MMC type flexible DC transient voltage cooperative support method according to claim 1, wherein: selecting a typical operation mode from the historical operation mode of the power grid, and determining various N-2 fault scenarios according to the historical operation data corresponding to the typical operation mode; performing time-domain simulation on various N-2 fault scenarios to determine the voltage drop depth and fault recovery time length of each node in the power grid under various fault types; and taking the node with a voltage drop depth greater than a set voltage threshold and a fault recovery time length greater than a set recovery time length as a key node; calculating the sensitivity coefficient of each converter station to the voltage of the key node; dividing each converter station into a responsibility area of each key node according to the sensitivity coefficient; determining the priority of each converter station in the same responsibility area according to the number of flexible DC lines in the power grid and in the order from large to small of the sensitivity coefficient.

3. The multi-loop MMC type flexible DC transient voltage cooperative support method according to claim 2, wherein: the typical operation mode includes: wet season, dry season, high load, low load, high output of new energy, and low output of new energy; and the N-2 fault scenario includes: line N-2 trip, DC bipolar blocking.

4. The multi-loop MMC type flexible DC transient voltage cooperative support method according to claim 2, wherein: the converter station with a sensitivity coefficient not less than a first coefficient threshold is divided into a first responsibility area of the key node, the converter station with a sensitivity coefficient less than the first coefficient threshold and not less than a second coefficient threshold is divided into a second responsibility area of the key node, and the converter station with a sensitivity coefficient less than the second coefficient threshold is divided into a non-responsibility area of the key node.

5. The multi-loop MMC type flexible DC transient voltage cooperative support method according to claim 2, wherein: If there are p flexible DC lines in the power grid, then there are p priority levels. Within the same responsibility area, converter stations are ranked in descending order of sensitivity coefficient, with the highest-ranked station being prioritized. Each converter station corresponds to the first priority, followed by others in the order of priority. Each converter station corresponds to the second priority, and so on, with the last one listed first. Each converter station corresponds to the p-th priority level, where p is a positive integer.

6. The multi-loop MMC type flexible DC transient voltage cooperative support method according to claim 5, wherein: An offline fault library is established to store fault features and responsibility area features of each key node, wherein the fault features include: fault type, key node i, voltage drop depth ΔU%, and fault recovery time length t rec ; the responsibility area features include: key node i, converter stations in the first responsibility area and priorities corresponding to each converter station, converter stations in the second responsibility area and priorities corresponding to each converter station.

7. The multi-loop MMC type flexible DC transient voltage cooperative support method according to claim 6, wherein: after the fault occurs, the number of flexible DC lines where the fault occurs is determined as the priority number. determine a voltage measurement value before the fault according to real-time measurement data of the power grid; calculate a dynamic fault characteristic of the key node using the voltage measurement value before the fault; when the dynamic fault characteristic matches a fault characteristic stored in the offline fault library, call the responsibility zone characteristic of the key node stored in the offline fault library to determine the converter station and its priority for cooperatively supporting the voltage of the key node under fault; when the dynamic fault characteristic does not match the fault characteristic stored in the offline fault library, each converter station switches to droop control and updates the sensitivity coefficient of each converter station to the voltage of the key node; according to the absolute value of the change amount of the sensitivity coefficient before and after the update, using the determined number of priorities, determine the converter station and its priority for cooperatively supporting the voltage of the key node under fault.

8. The multi-circuit MMC-type flexible DC transient voltage cooperative support method according to claim 7, characterized in that, calling the responsibility zone characteristic of the key node stored in the offline fault library to determine the converter station and its priority for cooperatively supporting the voltage of the key node under fault comprises: adjusting the priority of each converter station in the same responsibility zone according to the number of flexible DC lines that have occurred faults; or calling the responsibility zone characteristic of the key node stored in the offline fault library, taking the priority of the converter station in each responsibility zone as the priority of the converter station for cooperatively supporting the voltage of the key node under real-time fault.

9. The multi-circuit MMC-type flexible DC transient voltage cooperative support method according to claim 7, characterized in that, Update the sensitivity coefficients of each converter station to the voltage at critical nodes. Sort the converter stations in descending order of the absolute value of the change in sensitivity coefficients before and after the update. Count the number of faulty flexible DC lines, p′, and then select the stations ranked first. Each converter station corresponds to the first priority, followed by others in the order of priority. Each converter station corresponds to the second priority, and so on, with the last one listed first. Each converter station corresponds to the p′-th priority, where p′≤p and p′ is a positive integer.

10. The multi-circuit MMC-type flexible DC transient voltage cooperative support method according to claim 1, characterized in that, determining the reactive power reference value of each converter station with the power grid transient voltage stability as the optimization objective comprises: establishing a global objective function with the weighted sum of the key node voltage deviation variance, the converter station reactive power deviation, and the non-key node voltage deviation variance as the optimization objective; wherein the non-key nodes include non-key nodes of 220kV and above voltage levels in the power grid; establishing the constraint condition of the global objective function; iteratively solving the global objective function under the constraint condition to obtain the reactive power reference value of each converter station.

11. A multi-circuit MMC-type flexible DC transient voltage coordinated support system, used to implement the multi-circuit MMC-type flexible DC transient voltage coordinated support method of any one of claims 1 to 10, characterized in that, comprise: an offline module for obtaining historical operation data and historical operation modes of the power grid, and determining key nodes of the power grid under various N-2 fault scenarios; calculating the fault characteristic of each key node, dividing each converter station into a responsibility zone of the key node based on the sensitivity coefficient of each converter station to the voltage of the key node, and determining the priority of each converter station in the same responsibility zone according to the number of flexible DC lines; an online module for calculating the dynamic fault characteristic of the key node using real-time measurement data of the power grid after the fault occurs; matching the dynamic fault characteristic with the calculated fault characteristic, and determining the converter station and its priority for cooperatively supporting the voltage of the key node under fault based on the matching result; a cooperative control module for determining the reactive power reference value of each converter station with the power grid transient voltage stability as the optimization objective; distributing the execution timing of each converter station based on the priority of the converter station; and controlling each converter station to cooperatively support the voltage based on the execution timing and the reactive power reference value.

12. A terminal comprising a processor and a storage medium; characterized in that: the storage medium is configured to store instructions; the processor is configured to operate in accordance with the instructions to perform the steps of the method of any of claims 1-10.

13. A computer readable storage medium having stored thereon a computer program, characterized in that, The program, which when executed by the processor, implements the steps of the method of any of claims 1-10.

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