Fault current monitoring method for multi-terminal flexible direct current power transmission system considering active current limiting control of hybrid converter station

Abstract

The application discloses a kind of multi-terminal flexible DC transmission system fault current monitoring methods considering hybrid converter station active current limiting control.First, the equivalent model of DC line and the equivalent model of hybrid converter station in different response stages are established;Then, according to the topological structure of multi-terminal flexible DC transmission system, the spatial state equation set of the whole system is established;Solve the state space equation set, obtain the time domain response of fault current.The application realizes the accurate calculation of the fault current of multi-terminal flexible DC transmission system by fully considering the dynamic response characteristics of converter station active current limiting control, so as to realize the accurate monitoring of the fault current of multi-terminal flexible DC transmission system.

Description

Technical Field

[0001] This invention relates to the field of flexible DC transmission technology, and specifically to a fault current monitoring method for multi-terminal flexible DC transmission systems that takes into account active current limiting control of hybrid converter stations. Background Technology

[0002] With the accelerated global energy transition, especially the large-scale development and utilization of renewable energy sources such as offshore wind power, flexible DC transmission technology based on Modular Multilevel Converters (MMCs) has become a key carrier for constructing new power systems and multi-terminal DC grids due to its advantages such as high power quality and flexible control. However, compared with AC grids, DC grids have lower damping and inertia. Once a fault such as an inter-pole short circuit occurs, the fault current can rise sharply to several times the rated value within milliseconds, seriously threatening the safety of converter stations and transmission lines. Therefore, accurately grasping the transient development law of fault current is a prerequisite for optimizing protection configuration and improving the system's fault ride-through capability. Among various topologies, hybrid MMCs, with their DC fault self-clearing and ride-through capabilities, are gradually becoming a popular solution for solving DC-side fault problems.

[0003] Currently, fault current calculations for multi-terminal hybrid converter stations neglect the dynamic regulation of DC voltage by the proportional-integral controller within the active current limiting control system. In reality, as the fault current decreases, the controller output exits the saturation region, causing a change in the output voltage. Ignoring this dynamic process will result in significant monitoring errors during the fault current decline phase, failing to accurately reflect the true transient characteristics.

[0004] To address the aforementioned issues, a monitoring method that fully considers the dynamic response characteristics of active current limiting control at converter stations is needed to achieve accurate monitoring of fault currents in multi-terminal flexible DC transmission systems. Summary of the Invention

[0005] This application provides a fault current monitoring method for multi-terminal flexible DC transmission systems that takes into account the active current limiting control of hybrid converter stations. It can fully consider the dynamic response characteristics of the active current limiting control of converter stations and realize the accurate calculation of fault current of multi-terminal flexible DC transmission systems.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] In a first aspect, the present invention provides a fault current monitoring method for a multi-terminal flexible DC transmission system that considers active current limiting control of a hybrid converter station, comprising:

[0008] S1. Based on the main parameters of the hybrid converter station and DC line, establish equivalent circuit models of the multi-terminal flexible DC transmission system at different fault monitoring stages.

[0009] The hybrid converter station, denoted as MMC, is set up in a multi-terminal flexible DC transmission system, and multiple MMCs are set up. The positive terminal of each MMC is connected through the positive terminal of a DC line, and the negative terminal is connected through the negative terminal of a DC line, forming a ring network topology.

[0010] Each MMC is pre-installed with an active current limiting control system; when a fault occurs in the multi-terminal flexible DC transmission system and active current limiting control needs to be activated, the active current limiting control systems of all MMCs are activated simultaneously.

[0011] The different fault monitoring stages, including the first stage and the second stage, are defined as follows:

[0012] The first stage is defined as the moment when the active current limiting control system of MMC is activated; the second stage is defined as the time from the occurrence of a fault in the multi-terminal flexible DC transmission system to the activation of the active current limiting control system; the third stage is defined as the time after the active current limiting control is activated and the fault current is controlled to 0.

[0013] S11, Establish the equivalent circuit model of the hybrid converter station;

[0014] The equivalent circuit model of the hybrid converter station includes the first-stage equivalent circuit model and the second-stage equivalent circuit model of the converter station.

[0015] The first-stage equivalent circuit model of the converter station is an equivalent circuit model established in the first stage based on the current-limiting inductor, bridge arm inductor, sub-module capacitor, number of sub-modules, and sub-module resistance of MMC, through the principle of equivalence, consisting of equivalent capacitor, equivalent inductor, and equivalent resistance connected in series.

[0016] The second-stage equivalent circuit model of the converter station is as follows: In the second stage, the equivalent capacitor in the first-stage equivalent circuit model of the converter station is replaced with an equivalent voltage source, while the rest of the structure and parameters remain unchanged. The equivalent circuit model is established by connecting the equivalent resistance, equivalent inductance and equivalent voltage source in series. The voltage value of the equivalent voltage source is determined by the output of the active current limiting control system.

[0017] S12, Establish the equivalent circuit model of the DC line;

[0018] The DC line equivalent model is an equivalent circuit model established based on the unit resistance, unit inductance, and line length of the DC line, consisting of an equivalent inductance and an equivalent resistance connected in series.

[0019] S13, Establish the equivalent circuit model of the multi-terminal flexible DC transmission system;

[0020] The equivalent circuit model of the multi-terminal flexible DC transmission system includes the first-stage equivalent circuit model of the multi-terminal flexible DC transmission system and the second-stage equivalent circuit model of the multi-terminal flexible DC transmission system.

[0021] The first-stage equivalent circuit model of the multi-terminal flexible DC transmission system is established based on the first-stage equivalent circuit model of the converter station and the equivalent model of the DC line in the first stage.

[0022] The second-stage equivalent circuit model of the multi-terminal flexible DC transmission system is established in the second stage based on the second-stage equivalent circuit model of the converter station and the equivalent model of the DC line.

[0023] S2, Establish the state-space equation system;

[0024] The state-space equation set includes the first-stage state-space equation set for fault current monitoring and the second-stage state-space equation set for fault current monitoring.

[0025] The state-space equation set for the first stage of fault current monitoring is established based on the equivalent circuit model of the first stage of the multi-terminal flexible DC transmission system.

[0026] The state-space equation set for the second stage of fault current monitoring is established based on the equivalent circuit model of the second stage of the multi-terminal flexible DC transmission system.

[0027] S3, solve for the time-domain response of the fault current to monitor the fault current;

[0028] Solving the state-space equations of the first stage of the fault current monitoring system yields the time-domain response of the fault current in the multi-terminal flexible DC transmission system from the occurrence of a bipolar fault to the moment when the active current limiting control system is activated.

[0029] Solve the state-space equations of the second stage of the fault current monitoring to obtain the time-domain response of the multi-terminal flexible DC transmission system from the moment the active current limiting control system is activated to the moment the fault current is controlled to 0.

[0030] This invention achieves fault current monitoring in a multi-terminal flexible DC transmission system through the following steps: Fault monitoring is divided into two stages (with the active current limiting control activation time as the boundary); equivalent circuit models are established for the hybrid converter station, DC line, and multi-terminal system, respectively; a state-space equation set is established; and the time-domain response of the fault current is obtained by solving the equations.

[0031] Phased modeling: The first phase (fault occurrence → active current limiting control activation) uses an RLC series equivalent circuit; the second phase (active current limiting control activation → current drops to 0) replaces the equivalent capacitor with an equivalent voltage source, the value of which is determined by the output of the active current limiting control system.

[0032] Key innovative approach: In the second stage, the converter station is no longer regarded as a constant voltage source. Instead, the dynamic output of the control system is introduced, which overcomes the defect of the traditional method that ignores the voltage change after the PI controller leaves the saturation region.

[0033] State-space solution: By matrixing the state-space equations, the time-domain evolution of current and capacitor voltage in each loop of the multi-terminal system is accurately described;

[0034] This invention achieves accurate calculation of fault current through phased, dynamic control response modeling and state-space solving.

[0035] Based on the above, the specific method for establishing the equivalent circuit model of the multi-terminal flexible DC transmission system is as follows:

[0036] Define faulty loops and non-faulty loops;

[0037] If a certain MMC goes to the fault point only through the DC line and does not pass through other MMCs, then the MMC, the DC line connecting the fault point and the MMC, and the fault point together constitute a fault loop.

[0038] If there is no fault between two adjacent MMCs and they are only connected by a DC line, then the two adjacent MMCs and the DC line connecting the two adjacent MMCs constitute a non-faulty circuit.

[0039] In the first stage:

[0040] For the fault loop, a first-stage equivalent fault loop model is established based on the first-stage equivalent circuit model of the converter station MMC based on the fault loop, the DC line equivalent circuit model of the fault loop, and the fault point.

[0041] For non-faulty loops, a first-stage equivalent non-faulty loop model is established based on the first-stage equivalent circuit model of the converter station for each of the two adjacent MMCs of the non-faulty loop, and the equivalent circuit model of the DC line connecting the two adjacent MMCs.

[0042] The first-stage equivalent fault loop model and the first-stage equivalent non-fault loop model together constitute the first-stage equivalent circuit model of the multi-terminal flexible DC transmission system.

[0043] In the second phase:

[0044] For the fault loop, a second-stage equivalent fault loop model is established based on the second-stage equivalent circuit model of the converter station based on the MMC of the fault loop, the DC line equivalent circuit model of the fault loop, and the fault point.

[0045] For non-faulty loops, a second-stage equivalent non-faulty loop model is established based on the second-stage equivalent circuit model of the converter station for each of the two adjacent MMCs of the non-faulty loop, and the equivalent circuit model of the DC line connecting the two adjacent MMCs.

[0046] The second-stage equivalent fault loop model and the second-stage equivalent non-fault loop model together constitute the second-stage equivalent circuit model of the multi-terminal flexible DC transmission system.

[0047] This invention achieves accurate calculations by distinguishing between faulty and non-faulty loops and establishing equivalent faulty loop models and non-faulty loop models respectively.

[0048] Circuit differentiation: The fault circuit includes the fault point resistor R. f Non-faulty circuits do not contain fault points.

[0049] Model completeness: All MMCs and lines are included in the loop, forming a complete equivalent ring network;

[0050] Two-stage unified structure: The two stages differ only in the converter station model (equivalent capacitance vs. equivalent voltage source), which facilitates the unified construction of state-space equations;

[0051] This approach ensures the topological integrity and two-phase consistency of the system model, laying the foundation for state-space modeling.

[0052] Based on the above, the specific method for establishing the state-space equation system is as follows:

[0053] The loop topology is defined as including faulty loops and non-faulty loops;

[0054] Based on the loop topology of the first-stage equivalent circuit model of the multi-terminal flexible DC transmission system, the n×1 order first-stage capacitor-voltage matrix U is obtained. C1 The first-stage loop current matrix I1 is of order (n+1)×1, and the first-stage capacitor current matrix I is of order n×1. C1 ; where, the n×1 order first-stage capacitor-voltage matrix U C1 In the (n+1)×1 order first-stage loop current matrix I1, the matrix elements are the voltages of the equivalent capacitors in the loop topology; in the n×1 order first-stage capacitor current matrix I... C1 In the matrix, the elements represent the current flowing through the equivalent capacitance in the loop topology; n is the number of ports in the multi-terminal flexible DC transmission system.

[0055] Based on the loop topology of the first-stage equivalent circuit model of the multi-terminal flexible DC transmission system, the first-stage (n+1)×n order capacitor-voltage relationship matrix A is obtained; where...

[0056] In the first stage (n+1)×n order capacitor voltage relationship matrix A, the number of rows is the same as the number of loops in the loop topology, and the number of columns is the same as the number of equivalent capacitors in the loop topology;

[0057] In the first stage (n+1)×n order capacitor-voltage relationship matrix A, if the i-th loop in the loop topology of the first stage equivalent circuit model of the multi-terminal flexible DC transmission system has an equivalent capacitance C of MMC... j If the element in the i-th row and j-th column of the (n+1)×n order capacitor voltage relationship matrix A in the first stage is non-zero, then the element in the i-th row and j-th column is 0; when the element in the i-th row and j-th column is non-zero, if the capacitor voltage and the current reference direction of the circuit are the same, then the element in the i-th row and j-th column is 1; if the capacitor voltage and the current reference direction of the circuit are different, then the element in the i-th row and j-th column is -1.

[0058] Based on the loop topology of the first-stage equivalent circuit model of the multi-terminal flexible DC transmission system, the first-stage equivalent resistance matrix R of the first-stage equivalent circuit model of the multi-terminal flexible DC transmission system is obtained. m1 The first stage equivalent inductance matrix L m1 and the first-stage equivalent capacitance matrix C m1 ;

[0059] Wherein, matrix R m1 Matrix L m1 The number of rows and columns is the same as the number of loops in the loop topology, matrix C m1 The number of rows and columns is the same as the number of MMCs;

[0060] Matrix R m1 diagonal element R ii Represents the equivalent total resistance of loop i, and the remaining elements R ij This indicates that loop i and loop j share the equivalent resistance on the line; when the current directions of the two loops are the same, it is R. ij When the loop currents are in opposite directions, it is -R. ij When there is no shared line, R ij =0; R f The fault resistance value at the fault point;

[0061] Matrix L m1 diagonal element L ii Let L represent the equivalent total inductance of loop i, and the remaining elements L ij This indicates that loop i and loop j share the equivalent inductance on the line; when the current directions of the two loops are the same, it is L. ij When the loop currents are in opposite directions, it is -L. ij When there is no shared line, L ij =0;

[0062] Capacitor matrix C m1 The diagonal elements are the various MMCs. j The reciprocal of the equivalent capacitance, with all other elements being 0;

[0063] Based on the first stage (n+1)×n order capacitor voltage relationship matrix A and the first stage equivalent resistance matrix R m1 The first stage equivalent inductance matrix L m1 and the equivalent capacitance matrix C of the first stage m1 The following state-space equation set for the first stage of fault current monitoring is established:

[0064]

[0065] In the formula, t represents time;

[0066] Based on the loop topology of the second-stage equivalent circuit model of the multi-terminal flexible DC transmission system, the n×1 order second-stage equivalent voltage source voltage matrix U is obtained. C2 The second-stage loop current matrix I2 is of order (n+1)×1, and the active current limiting control output matrix P is of order n×1; where the second-stage equivalent voltage source voltage matrix U is of order n×1. C2 The matrix elements are the voltages of the equivalent voltage sources in the loop topology; the (n+1)×1 second-stage loop current matrix I2 has the elements being the currents of each loop in the loop topology; the n×1 active current limiting control output matrix P has the elements being the outputs of the active current limiting control system of each MMC in the loop topology.

[0067] Based on the loop topology of the second-stage equivalent circuit model of the multi-terminal flexible DC transmission system, the second-stage (n+1)×n order equivalent voltage source relationship matrix B is obtained.

[0068] In the second stage (n+1)×n order equivalent voltage source relation matrix B, if the k-th loop in the loop topology of the second stage equivalent circuit model of the multi-terminal flexible DC transmission system contains an equivalent voltage source U of MMC. h If the element in the k-th row and h-th column of the (n+1)×n equivalent voltage source relation matrix B in the second stage is non-zero, then the element in the k-th row and h-th column is 0; when the element in the k-th row and h-th column is non-zero, if the voltage direction of the equivalent voltage source is the same as the reference direction of the current in the loop, then the element in the k-th row and h-th column is 1; if the voltage direction of the equivalent voltage source is different from the reference direction of the current in the loop, then the element in the k-th row and h-th column is -1.

[0069] In the second stage (n+1)×n equivalent voltage source relation matrix B, the number of rows is the same as the number of loops in the loop topology, and the number of columns is the same as the number of equivalent voltage sources in the loop topology;

[0070] Based on the loop topology of the second-stage equivalent circuit model of the multi-terminal flexible DC transmission system, the second-stage equivalent resistance matrix R of the second-stage equivalent circuit model of the multi-terminal flexible DC transmission system is obtained. m2 The second-stage equivalent inductance matrix L m2 ;

[0071] Wherein, matrix R m2 Matrix L m2 The number of rows and columns is the same as the number of loops in the loop topology;

[0072] Matrix R m2 diagonal element R ii Represents the equivalent total resistance of loop i, and the remaining elements R ij This indicates that loop i and loop j share the equivalent resistance on the line; when the current directions of the two loops are the same, it is R. ij When the loop currents are in opposite directions, it is -R. ij When there is no shared line, R ij =0; R f The fault resistance value at the fault point;

[0073] Matrix L m2 diagonal element L ii Let L represent the equivalent total inductance of loop i, and the remaining elements L ij This indicates that loop i and loop j share the equivalent inductance on the line; when the current directions of the two loops are the same, it is L. ij When the loop currents are in opposite directions, it is -L. ij When there is no shared line, L ij =0;

[0074] Based on the second-stage (n+1)×n equivalent voltage source relation matrix B and the second-stage equivalent resistance matrix R m2 The second stage equivalent inductance matrix L m2 Based on the n×1 order active current limiting control output matrix P, the following state-space equation set for the second stage of fault current monitoring is established:

[0075]

[0076] In the formula, t is time, and U dcref This refers to the rated voltage on the DC side of the DC transmission system.

[0077] Based on the above, the equivalence principle adopted when establishing the first-stage equivalent circuit model of the converter station is as follows:

[0078]

[0079] In the formula, C, L, and R are the equivalent capacitance, equivalent inductance, and equivalent resistance values ​​after applying the equivalence principle; C eq L arm R on The values ​​of capacitance, arm inductance, and resistance of a single submodule in the hybrid converter station; L x N is the DC-side current-limiting inductance value of the hybrid converter station; N is the number of individual bridge arm submodules in the hybrid converter station.

[0080] Based on the above, the method for establishing the equivalent circuit model of the DC line is as follows:

[0081]

[0082] In the formula l d r d x represents the unit inductance and unit resistance values ​​of the DC line; d R is the length of the DC line; d L d These are the equivalent resistance and equivalent inductance values ​​of the DC line.

[0083] This invention employs matrix modeling, expressing the relationships between all loops, equivalent capacitances, and equivalent voltage sources using matrices A and B. This makes the method applicable to any multi-terminal system. Furthermore, in the second stage, through U... C2 =U dcref P directly links the equivalent voltage source matrix with the control output matrix P, realizing the coupling between control dynamics and circuit response, so that the system response can be solved accurately.

[0084] Based on the above, the control method of the active current limiting control system is as follows:

[0085] The difference between the actual value of the positive current and the reference value of the hybrid converter station is input, and the output is obtained after calculation by the proportional-integral circuit and the anti-integral saturation circuit.

[0086] The specific calculation formulas for the proportional-integral element and the anti-integral saturation element are as follows:

[0087]

[0088] In the formula, P u P is the output of the proportional-integral (PI) stage of the active current limiting control system. o ΔP is the output of the active current limiting control system; ΔP is the input of the active current limiting control system; I dcp This represents the actual value of the positive current of the hybrid converter station; I dcref This is the reference value for the positive electrode current of a hybrid converter station; K p For the proportionality coefficient and K iP is the integral coefficient; t1 is the control switching time, t is time; c For the output of the anti-integral saturation stage of the active current limiting control system; P h The upper limit of the output of the active current limiting control system and P l This is the lower limit of the output of the active current limiting control system.

[0089] The above scheme provides a quantifiable equivalent method to ensure that the model is consistent with the actual MMC behavior. Specifically, it performs equivalence based on the actual physical parameters of the MMC (submodule capacitance, bridge arm inductance, current-limiting inductance, submodule resistance, and number of submodules). The discharge path of the submodule capacitors is preserved, making it suitable for the rapid current rise phase in the initial stage of a fault. The formula is clearly derived (bridge arm series / parallel equivalence), possessing repeatability and engineering applicability.

[0090] Secondly, the present invention provides a fault current monitoring system for a multi-terminal flexible DC transmission system that considers active current limiting control of hybrid converter stations, comprising:

[0091] The equivalent circuit model establishment module is used to establish equivalent circuit models of multi-terminal flexible DC transmission systems at different fault monitoring stages based on the main parameters of hybrid converter stations and DC lines. It includes equivalent circuit model establishment modules for hybrid converter stations, DC lines, and multi-terminal flexible DC transmission systems.

[0092] The hybrid converter station, denoted as MMC, is set up in a multi-terminal flexible DC transmission system, and multiple MMCs are set up. The positive terminal of each MMC is connected through the positive terminal of a DC line, and the negative terminal is connected through the negative terminal of a DC line, forming a ring network topology.

[0093] Each MMC is pre-installed with an active current limiting control system; when a fault occurs in the multi-terminal flexible DC transmission system and active current limiting control needs to be activated, the active current limiting control systems of all MMCs are activated simultaneously.

[0094] The different fault monitoring stages, including the first stage and the second stage, are defined as follows:

[0095] The first stage is defined as the moment when the active current limiting control system of MMC is activated; the second stage is defined as the time from the occurrence of a fault in the multi-terminal flexible DC transmission system to the activation of the active current limiting control system; the third stage is defined as the time after the active current limiting control is activated and the fault current is controlled to 0.

[0096] The equivalent circuit model establishment module for the hybrid converter station includes a first-stage equivalent circuit model establishment module and a second-stage equivalent circuit model establishment module for the converter station.

[0097] The module for establishing the first-stage equivalent circuit model of the converter station is configured as follows:

[0098] In the first stage, based on the current-limiting inductor, bridge arm inductor, submodule capacitor, number of submodules, and submodule resistance of MMC, an equivalent circuit model of the converter station in the first stage is established by means of equivalent capacitor, equivalent inductor and equivalent resistance connected in series.

[0099] The module for establishing the second-stage equivalent circuit model of the converter station is configured as follows:

[0100] In the second stage, the equivalent capacitor in the first stage equivalent circuit model of the converter station is replaced with an equivalent voltage source, while the rest of the structure and parameters remain unchanged. A second stage equivalent circuit model of the converter station is established, consisting of an equivalent resistor, an equivalent inductor, and an equivalent voltage source connected in series. The voltage value of the equivalent voltage source is determined by the output of the active current limiting control system.

[0101] The DC line equivalent circuit model establishment module is configured as follows:

[0102] Based on the unit resistance, unit inductance, and line length of the DC line, an equivalent circuit model of the DC line consisting of an equivalent inductance and an equivalent resistance connected in series is established.

[0103] The equivalent circuit model establishment module for multi-terminal flexible DC transmission system includes a first-stage equivalent circuit model establishment module for multi-terminal flexible DC transmission system and a second-stage equivalent circuit model establishment module for multi-terminal flexible DC transmission system;

[0104] The first-stage equivalent circuit model establishment module of the multi-terminal flexible DC transmission system is used to establish the first-stage equivalent circuit model of the multi-terminal flexible DC transmission system based on the first-stage equivalent circuit model of the converter station and the equivalent model of the DC line in the first stage.

[0105] The second-stage equivalent circuit model establishment module of the multi-terminal flexible DC transmission system is used to establish the second-stage equivalent circuit model of the multi-terminal flexible DC transmission system based on the second-stage equivalent circuit model of the converter station and the equivalent model of the DC line in the second stage.

[0106] The state-space equation set establishment module is used to establish the state-space equation set; the state-space equation set includes the first-stage state-space equation set for fault current monitoring and the second-stage state-space equation set for fault current monitoring;

[0107] The state-space equation set for the first stage of fault current monitoring is established based on the equivalent circuit model of the first stage of the multi-terminal flexible DC transmission system.

[0108] The state-space equation set for the second stage of fault current monitoring is established based on the equivalent circuit model of the second stage of the multi-terminal flexible DC transmission system.

[0109] The fault current monitoring module is used to monitor the fault current based on the time-domain response of the fault current obtained from the solution.

[0110] Solving the state-space equations of the first stage of the fault current monitoring system yields the time-domain response of the fault current in the multi-terminal flexible DC transmission system from the occurrence of a bipolar fault to the moment when the active current limiting control system is activated.

[0111] Solve the state-space equations of the second stage of the fault current monitoring to obtain the time-domain response of the multi-terminal flexible DC transmission system from the moment the active current limiting control system is activated to the moment the fault current is controlled to 0.

[0112] Thirdly, the present invention provides a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the steps of the multi-terminal flexible DC transmission system fault current monitoring method considering active current limiting control of hybrid converter stations.

[0113] Fourthly, the present invention provides a controller comprising: the aforementioned computer-readable storage medium; and one or more processors for executing a program in the computer-readable storage medium.

[0114] This invention has outstanding substantive effects and significant progress compared to the prior art, specifically:

[0115] Currently, fault current calculations for multi-terminal hybrid converter stations neglect the dynamic regulation of DC voltage by the proportional-integral controller (PI controller) within the active current limiting control system, assuming the converter station maintains a constant voltage after active current limiting control is activated. In reality, as the fault current decreases, the controller output exits the saturation region, causing a change in the converter station's output voltage. Ignoring this dynamic process will result in significant calculation errors during the fault current decrease phase, failing to accurately reflect the true transient characteristics of the fault current. This invention fully considers the dynamic response characteristics of the converter station's active current limiting control. Instead of treating the converter station after active current limiting control as a constant voltage source, it treats its output voltage as a variable dynamically adjusted by the PI controller. This dynamic is introduced into the fault current calculation through state-space equations, thereby achieving accurate fault current calculation in multi-terminal flexible DC transmission systems. Attached Figure Description

[0116] Figure 1 This is a schematic diagram of a multi-terminal flexible DC transmission system provided in an embodiment of this application.

[0117] Figure 2 This is a flowchart illustrating the steps of a short-circuit fault calculation method for a multi-terminal flexible DC transmission system provided in this application embodiment.

[0118] Figure 3This is a schematic diagram of the first-stage equivalent circuit model of a multi-terminal flexible DC transmission system after a fault, provided in an embodiment of this application.

[0119] Figure 4 This is a schematic diagram of the second-stage equivalent circuit model of a multi-terminal flexible DC transmission system after a fault, provided in an embodiment of this application.

[0120] Figure 5 This is an active current limiting control block diagram of a multi-terminal flexible DC transmission system provided in the embodiments of this application.

[0121] Figure 6 The fault provided in this application embodiment is located on the line. 12 A schematic diagram comparing the calculated and simulated values ​​of the loop current during the intermediate period.

[0122] Figure 7 This is a schematic diagram comparing the calculated and simulated values ​​of the loop current at the DC side outlet of MMC1, provided in an embodiment of this application.

[0123] Figure 8 This is a schematic diagram comparing the calculated and simulated values ​​of the loop current at the DC side outlet of MMC2, provided in an embodiment of this application. Detailed Implementation

[0124] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0125] Example 1

[0126] This embodiment provides a fault current monitoring method for a multi-terminal flexible DC transmission system that takes into account active current limiting control of hybrid converter stations.

[0127] This embodiment divides the fault monitoring of the entire multi-terminal flexible DC transmission system into two fault monitoring stages (the first stage and the second stage) based on the activation time of the active current limiting control system. It establishes equivalent circuit models and state-space equations for the multi-terminal flexible DC transmission system in different fault monitoring stages, performs fault calculations in the time domain, and fully considers the dynamic response characteristics of the active current limiting control of the converter station, thus enabling accurate calculation of the fault current of the multi-terminal flexible DC transmission system.

[0128] Specifically, refer to Figure 1The diagram shows a schematic of a multi-terminal flexible DC transmission system provided in this embodiment, specifically a multi-terminal flexible DC transmission system based on MMC, that is, a system with MMC as the core unit, and the system topology can adopt a ring network architecture.

[0129] Specifically, the active current limiting control system in the hybrid converter station can be activated within 2ms after a fault occurs in the multi-terminal flexible DC transmission system. When a fault occurs in the multi-terminal flexible DC transmission system and active current limiting control needs to be activated, the active current limiting control systems of all MMCs are activated simultaneously.

[0130] Specifically, refer to Figure 2 The diagram illustrates a flowchart of a short-circuit fault calculation method for a multi-terminal flexible DC transmission system according to an embodiment of this application, which may specifically include the following steps:

[0131] Step S201: Based on the main parameters of the hybrid converter station and DC line, establish the equivalent circuit model of the multi-terminal flexible DC transmission system at different fault calculation stages.

[0132] Specifically, the fault calculation of the entire multi-terminal flexible DC transmission system is divided into two fault monitoring stages, with the moment when the active current limiting control system is put into operation as the boundary. The first stage is from the time the fault occurs until the active current limiting control system is put into operation, and the second stage is from the time the active current limiting control system is put into operation until the fault current is controlled to 0.

[0133] Specifically, if a certain MMC is connected to the fault point only via a DC line and does not pass through other MMCs, then the MMC, the DC line connecting the fault point and the MMC, and the fault point constitute a fault loop; if there is no fault between two adjacent MMCs and they are connected only via a DC line, then the two adjacent MMCs and the DC line connecting the two adjacent MMCs constitute a non-fault loop.

[0134] First, the equivalent circuit model of the converter station in the first stage is constructed. The equivalent circuit model of the converter station in the second stage is obtained by modifying the equivalent circuit model of the first stage.

[0135] Specifically, in the first stage, the hybrid converter station is equivalent to a circuit model consisting of equivalent capacitance, equivalent inductance, and equivalent resistance connected in series. The specific equivalence principle is as follows:

[0136] (1)

[0137] In the formula, C, L, and R are the equivalent capacitance, equivalent inductance, and equivalent resistance values ​​of the equivalent circuit model of the hybrid converter station; C eq L arm R on For the capacitance, arm inductance, and resistance values ​​of individual submodules in a hybrid converter station; Lx N represents the DC-side current-limiting inductance value of the hybrid converter station; N represents the number of individual bridge arm submodules in the hybrid converter station.

[0138] Based on the first-stage equivalent circuit model of the converter station, the internal equivalent capacitor is replaced with an equivalent voltage source, and the equivalent voltage source is connected in series with the equivalent inductance and equivalent resistance of the first-stage equivalent circuit model of the converter station to obtain the second-stage equivalent circuit model of the converter station.

[0139] Secondly, the equivalent circuit model of the DC line is constructed. The DC line is equivalent to an equivalent circuit model consisting of an equivalent inductor and an equivalent resistance connected in series. The specific equivalence principles are as follows:

[0140] (2)

[0141] In the formula l d r d For the unit inductance and unit resistance values ​​of a DC line; x d R is the length of the DC line; d L d These are the equivalent resistance and equivalent inductance values ​​of a DC line.

[0142] Finally, an equivalent circuit model of the multi-terminal flexible DC transmission system is constructed; the equivalent circuit model of the multi-terminal flexible DC transmission system includes a first-stage equivalent circuit model and a second-stage equivalent circuit model.

[0143] Specifically, in the first phase:

[0144] For the fault loop, a first-stage equivalent fault loop model is established based on the first-stage equivalent circuit model of the converter station MMC based on the fault loop, the DC line equivalent circuit model of the fault loop, and the fault point.

[0145] For non-faulty loops, a first-stage equivalent non-faulty loop model is established based on the first-stage equivalent circuit model of the converter station for each of the two adjacent MMCs of the non-faulty loop, and the equivalent circuit model of the DC line connecting the two adjacent MMCs.

[0146] The first-stage equivalent fault loop model and the first-stage equivalent non-fault loop model together constitute the first-stage equivalent circuit model of the multi-terminal flexible DC transmission system.

[0147] In the second phase:

[0148] For the fault loop, a second-stage equivalent fault loop model is established based on the second-stage equivalent circuit model of the converter station based on the MMC of the fault loop, the DC line equivalent circuit model of the fault loop, and the fault point.

[0149] For non-faulty loops, a second-stage equivalent non-faulty loop model is established based on the second-stage equivalent circuit model of the converter station for each of the two adjacent MMCs of the non-faulty loop, and the equivalent circuit model of the DC line connecting the two adjacent MMCs.

[0150] The second-stage equivalent fault loop model and the second-stage equivalent non-fault loop model together constitute the second-stage equivalent circuit model of the multi-terminal flexible DC transmission system.

[0151] Step S202: Establish the state-space equation set corresponding to each equivalent circuit model, including the first stage state-space equation set for fault current monitoring and the second stage state-space equation set for fault current monitoring.

[0152] The loop topology is defined to include faulty loops and non-faulty loops.

[0153] Based on the loop topology of the first-stage equivalent circuit model of the multi-terminal flexible DC transmission system, the n×1 order first-stage capacitor-voltage matrix U is obtained. C1 The first-stage loop current matrix I1 is of order (n+1)×1, and the first-stage capacitor current matrix I is of order n×1. C1 ; where, the n×1 order first-stage capacitor-voltage matrix U C1 In the (n+1)×1 order first-stage loop current matrix I1, the matrix elements are the voltages of the equivalent capacitors in the loop topology; in the n×1 order first-stage capacitor current matrix I... C1 In the matrix, the elements represent the current flowing through the equivalent capacitance in the loop topology; n is the number of ports in the multi-terminal flexible DC transmission system.

[0154] Based on the loop topology of the first-stage equivalent circuit model of the multi-terminal flexible DC transmission system, the first-stage (n+1)×n order capacitor-voltage relationship matrix A is obtained; where...

[0155] In the first stage (n+1)×n order capacitor voltage relationship matrix A, the number of rows is the same as the number of loops in the loop topology, and the number of columns is the same as the number of equivalent capacitors in the loop topology;

[0156] In the first stage (n+1)×n order capacitor-voltage relationship matrix A, if the i-th loop in the loop topology of the first stage equivalent circuit model of the multi-terminal flexible DC transmission system has an equivalent capacitance C of MMC... jIf the element in the i-th row and j-th column of the (n+1)×n order capacitor voltage relationship matrix A in the first stage is non-zero, then the element in the i-th row and j-th column is 0; when the element in the i-th row and j-th column is non-zero, if the capacitor voltage and the current reference direction of the circuit are the same, then the element in the i-th row and j-th column is 1; if the capacitor voltage and the current reference direction of the circuit are different, then the element in the i-th row and j-th column is -1.

[0157] Based on the loop topology of the first-stage equivalent circuit model of the multi-terminal flexible DC transmission system, the first-stage equivalent resistance matrix R of the first-stage equivalent circuit model of the multi-terminal flexible DC transmission system is obtained. m1 The first stage equivalent inductance matrix L m1 and the first-stage equivalent capacitance matrix C m1 ;

[0158] Wherein, matrix R m1 Matrix L m1 The number of rows and columns is the same as the number of loops in the loop topology, matrix C m1 The number of rows and columns is the same as the number of MMCs;

[0159] Matrix R m1 diagonal element R ii Represents the equivalent total resistance of loop i, and the remaining elements R ij This indicates that loop i and loop j share the equivalent resistance on the line; when the current directions of the two loops are the same, it is R. ij When the loop currents are in opposite directions, it is -R. ij When there is no shared line, R ij =0; R f The fault resistance value at the fault point;

[0160] Matrix L m1 diagonal element L ii Let L represent the equivalent total inductance of loop i, and the remaining elements L ij This indicates that loop i and loop j share the equivalent inductance on the line; when the current directions of the two loops are the same, it is L. ij When the loop currents are in opposite directions, it is -L. ij When there is no shared line, L ij =0;

[0161] Capacitor matrix C m1 The diagonal elements are the various MMCs. j The reciprocal of the equivalent capacitance, with all other elements being 0;

[0162] Based on the first stage (n+1)×n order capacitor voltage relationship matrix A and the first stage equivalent resistance matrix R m1 The first stage equivalent inductance matrix L m1and the equivalent capacitance matrix C of the first stage m1 The following state-space equation set for the first stage of fault current monitoring is established:

[0163]

[0164] In the formula, t represents time.

[0165] Based on the loop topology of the second-stage equivalent circuit model of the multi-terminal flexible DC transmission system, the n×1 order second-stage equivalent voltage source voltage matrix U is obtained. C2 The second-stage loop current matrix I2 is of order (n+1)×1, and the active current limiting control output matrix P is of order n×1; where the second-stage equivalent voltage source voltage matrix U is of order n×1. C2 The matrix elements are the voltages of the equivalent voltage sources in the loop topology; the (n+1)×1 second-stage loop current matrix I2 has the elements being the currents of each loop in the loop topology; the n×1 active current limiting control output matrix P has the elements being the outputs of the active current limiting control system of each MMC in the loop topology.

[0166] Based on the loop topology of the second-stage equivalent circuit model of the multi-terminal flexible DC transmission system, the second-stage (n+1)×n order equivalent voltage source relationship matrix B is obtained.

[0167] In the second stage (n+1)×n order equivalent voltage source relation matrix B, if the k-th loop in the loop topology of the second stage equivalent circuit model of the multi-terminal flexible DC transmission system contains an equivalent voltage source U of MMC. h If the element in the k-th row and h-th column of the (n+1)×n equivalent voltage source relation matrix B in the second stage is non-zero, then the element in the k-th row and h-th column is 0; when the element in the k-th row and h-th column is non-zero, if the voltage direction of the equivalent voltage source is the same as the reference direction of the current in the loop, then the element in the k-th row and h-th column is 1; if the voltage direction of the equivalent voltage source is different from the reference direction of the current in the loop, then the element in the k-th row and h-th column is -1.

[0168] In the second stage (n+1)×n equivalent voltage source relation matrix B, the number of rows is the same as the number of loops in the loop topology, and the number of columns is the same as the number of equivalent voltage sources in the loop topology;

[0169] Based on the loop topology of the second-stage equivalent circuit model of the multi-terminal flexible DC transmission system, the second-stage equivalent resistance matrix R of the second-stage equivalent circuit model of the multi-terminal flexible DC transmission system is obtained. m2 The second-stage equivalent inductance matrix L m2 ;

[0170] Wherein, matrix R m2 Matrix L m2The number of rows and columns is the same as the number of loops in the loop topology;

[0171] Matrix R m2 diagonal element R ii Represents the equivalent total resistance of loop i, and the remaining elements R ij This indicates that loop i and loop j share the equivalent resistance on the line; when the current directions of the two loops are the same, it is R. ij When the loop currents are in opposite directions, it is -R. ij When there is no shared line, R ij =0; R f The fault resistance value at the fault point;

[0172] Matrix L m2 diagonal element L ii Let L represent the equivalent total inductance of loop i, and the remaining elements L ij This indicates that loop i and loop j share the equivalent inductance on the line; when the current directions of the two loops are the same, it is L. ij When the loop currents are in opposite directions, it is -L. ij When there is no shared line, L ij =0;

[0173] Based on the second-stage (n+1)×n equivalent voltage source relation matrix B and the second-stage equivalent resistance matrix R m2 The second stage equivalent inductance matrix L m2 Based on the n×1 order active current limiting control output matrix P, the following state-space equation set for the second stage of fault current monitoring is established:

[0174]

[0175] In the formula, t is time, and U dcref This refers to the rated voltage on the DC side of the DC transmission system.

[0176] Step S203: Solve the state-space equations corresponding to different stages to obtain the time-domain response of the fault current for each fault monitoring stage.

[0177] Solving the state-space equations of the first stage of the fault current monitoring system yields the time-domain response of the fault current in the multi-terminal flexible DC transmission system from the occurrence of a bipolar fault to the moment when the active current limiting control system is activated.

[0178] Solve the state-space equations of the second stage of the fault current monitoring to obtain the time-domain response of the multi-terminal flexible DC transmission system from the moment the active current limiting control system is activated to the moment the fault current is controlled to 0.

[0179] Example 2

[0180] This embodiment takes a four-terminal flexible multi-terminal flexible DC transmission system as an example to illustrate the specific implementation process of the fault current monitoring method of the multi-terminal flexible DC transmission system that takes into account the active current limiting control of the hybrid converter station.

[0181] like Figure 1 As shown, there are four hybrid converter stations (MMC1, MMC2, MMC3, and MMC4). Each arm of the hybrid converter station is connected in series with an MMC submodule and an arm inductor. The hybrid converter station uses 75% full-bridge submodules and 25% half-bridge submodules. The DC side of the MMC is connected to the DC line via a current-limiting inductor. Each hybrid converter station is interconnected through positive and negative DC lines to form a multi-terminal flexible DC transmission system topology with a ring network.

[0182] Reference Figure 1 This diagram illustrates a bipolar fault in a four-terminal flexible multi-terminal flexible DC transmission system provided in this embodiment, assuming a fault occurs on line... 12 A bipolar fault f0 occurs. The method in this embodiment can accurately calculate the fault current in a multi-terminal flexible DC transmission system experiencing a bipolar fault.

[0183] When a bipolar fault f0 occurs in a four-terminal or multi-terminal flexible DC transmission system, a total of five circuits appear as follows:

[0184] (1) Fault circuit 1: MMC1, line 12 The DC line to the left of f0 and the fault point f0 constitute the fault circuit;

[0185] (2) Fault circuit 2: MMC2, line 12 The DC line to the right of f0 and the fault point f0 constitute the fault circuit;

[0186] (3) Non-faulty circuit 3: MMC2, line 23 And MMC3 forms a non-faulty loop;

[0187] (4) Non-faulty circuit 4: MMC3, line 34 And MMC4 forms a non-faulty loop;

[0188] (5) Non-faulty circuit 5: MMC1, line 41 And MMC4 constitutes a non-faulty loop.

[0189] Based on such Figure 1 The bipolar fault shown is that on the line 12 A bipolar fault occurs at point f0. When calculating the fault current, based on the converter station equivalence principle and MMC... j (Where j=1,2,3,4) Number of submodules Nj Submodule capacitor C eqj Bridge arm inductor L armj Current-limiting inductor L xj With submodule resistor R onj MMC is equivalent to RLC model.

[0190] For a flexible multi-terminal flexible DC transmission system with n (n=4) terminals, MMC can be used. j Equivalent to the equivalent resistance R j Equivalent inductance L j Equivalent capacitance C j Series circuit:

[0191] (3)

[0192] MMC j The line to the fault point f0 is equivalent to R. dj0 L dj0 For example, there exists R. d10 L d10 This indicates the line from MMC1. 12 The equivalent resistance and equivalent inductance of the fault point f0 on the MMC. i (where i = 1, 2, 3…n) to MMC j DC lines between ij Equivalent to R dij L dij For example, there exists R. d23 L d23 , representing the equivalent resistance and equivalent inductance of the DC line from MMC2 to MMC3.

[0193] (4)

[0194] In the formula l dij r dij DC line ij The unit inductance and unit resistance values; x dij DC line ij Length; R dij L dij DC line ij The equivalent resistance and equivalent inductance values.

[0195] It should be noted that, in the embodiments of this application, for a given DC system, the parameters of the hybrid converter station and DC line in the four-terminal flexible DC transmission system are known.

[0196] In the first stage, from the occurrence of the fault to the activation of the active current limiting control system, the equivalent circuit model of the multi-terminal flexible DC transmission system in the first stage is as follows: Figure 3 As shown. Where p j Represents the positive pole, n j Represents the negative electrode; MMC j Equivalent to R j L j and C j A branch circuit consisting of multiple lines; a line ij Both the positive and negative electrodes are equivalent to R. dij and L dij Branches formed by series connection.

[0197] Based on the state-space method, when a bipolar fault occurs in a four-terminal system, a total of 5 loops are generated. Among them, the current reference direction of fault loop 1 and fault loop 2 is the direction from MMC to the fault point; the current reference direction of non-fault loops 3, 4 and 5 is counterclockwise.

[0198] The 4×1 order capacitor voltage matrix U can be listed separately. C A 5×1 order loop current matrix I, and a 4×1 order capacitor current matrix I flowing through the capacitor branch. C .

[0199] (5)

[0200] Based on the circuit topology, the current in the capacitor branch is equal to the sum of the currents in all loops flowing through that branch.

[0201] (6)

[0202] Based on the circuit topology, the relationships between the voltages of each capacitor are obtained, thus establishing a 5×4 order capacitor voltage relationship matrix A for the first stage. Among these,

[0203] In fault circuit 1, there is an MMC equivalent capacitance C1, and the voltage of capacitor C1 is in the same direction as the current reference direction of fault circuit 1. Therefore, the element in the first row and first column of the 5×4 order capacitor voltage relationship matrix A in the first stage is 1, and the remaining elements in the first row are 0.

[0204] In fault circuit 2, there is an MMC equivalent capacitance C2, and the voltage of capacitor C2 is in the same direction as the current reference direction of fault circuit 2. Therefore, the element in the second row and second column of the 5×4 order capacitor voltage relationship matrix A in the first stage is 1, and the remaining elements in the second row are 0.

[0205] In the non-faulty loop 3, there are MMC equivalent capacitors C2 and C3. The voltage of capacitor C2 is in the same direction as the current reference direction of the non-faulty loop 3, and the voltage of capacitor C3 is in the opposite direction to the current reference direction of the non-faulty loop 3. Therefore, in the first stage 5×4 order capacitor voltage relationship matrix A, the element in the 3rd row and 2nd column is 1, the element in the 3rd row and 3rd column is -1, and the remaining elements in the 3rd row are 0.

[0206] In the non-faulty loop 4, there are MMC equivalent capacitors C3 and C4. The voltage of capacitor C3 is in the same direction as the current reference direction of the non-faulty loop 4, and the voltage of capacitor C4 is in the opposite direction to the current reference direction of the non-faulty loop 4. Therefore, in the first stage 5×4 order capacitor voltage relationship matrix A, the element in the 4th row and 3rd column is 1, the element in the 4th row and 4th column is -1, and the remaining elements in the 4th row are 0.

[0207] In the non-faulty loop 5, there are MMC equivalent capacitors C1 and C4. The voltage of capacitor C1 is opposite to the current reference direction of the non-faulty loop 4, while the voltage of capacitor C4 is in the same direction as the current reference direction of the non-faulty loop 5. Therefore, in the first stage 5×4 order capacitor voltage relationship matrix A, the element in the 5th row and 1st column is -1, the element in the 5th row and 4th column is 1, and the remaining elements in the 5th row are 0.

[0208] Finally, the first-stage 5×4 order capacitor-voltage relationship matrix was obtained. (7)

[0209] based on Figure 3 The loop topology of the first-stage equivalent circuit model of the multi-terminal flexible DC transmission system shown is used to obtain the first-stage equivalent resistance matrix R. m The first stage equivalent inductance matrix L m and the first-stage equivalent capacitance matrix C m As shown in the following formulas respectively.

[0210] (8)

[0211] With the fault occurrence time t0 as t0, the capacitor voltage U C The initial value in the first stage is the voltage U on the DC side of the MMC at the moment the fault occurs. Ct0 The initial value of the loop current I in the first stage is the DC line current I at the moment the fault occurs. t0 .

[0212] The state-space equations for the first stage of fault current monitoring are established as follows:

[0213] (9)

[0214] Substitute into equations (3) to (8), and substitute in the initial value U of the capacitor voltage at the moment the fault occurs. Ct0 and the initial value of the loop current It0 Finally, the time-domain response of the fault current in the multi-terminal flexible DC transmission system from the occurrence of a bipolar fault to the activation of the active current limiting control system at time t1 can be calculated. That is, the fault current from the occurrence of a fault in the DC system to the activation of the active current limiting control at time t1, and the loop current value I at time t1. t1 and capacitor current value I Ct1 .

[0215] The second-stage equivalent circuit model of the multi-terminal flexible DC transmission system is as follows: Figure 4 As shown, the equivalent capacitance inside the first-stage equivalent circuit model of the converter station is replaced with the equivalent circuit model of the second stage of the converter station.

[0216] Based on the second-stage equivalent circuit model of the hybrid converter station, a 5×4 order equivalent voltage source relationship matrix B is derived. Specifically:

[0217] In fault circuit 1, there is an MMC equivalent voltage source U1, and the voltage of the equivalent voltage source U1 is in the same direction as the current reference direction of fault circuit 1. Therefore, the element in the first row and first column of the 5×4 order equivalent voltage source relation matrix B in the second stage is 1, and the remaining elements in the first row are 0.

[0218] In fault circuit 2, there is an MMC equivalent voltage source U2, and the voltage of the equivalent voltage source U2 is in the same direction as the current reference direction of fault circuit 2. Therefore, the element in the second row and second column of the 5×4 order equivalent voltage source relation matrix B in the second stage is 1, and the remaining elements in the second row are 0.

[0219] In the non-faulty loop 3, there are MMC equivalent voltage sources U2 and U3. The voltage of the equivalent voltage source U2 is in the same direction as the current reference direction of the non-faulty loop 3, and the voltage of the equivalent voltage source U3 is in the opposite direction to the current reference direction of the non-faulty loop 3. Therefore, in the second stage 5×4 order equivalent voltage source relation matrix B, the element in the 3rd row and 2nd column is 1, the element in the 3rd row and 3rd column is -1, and the remaining elements in the 3rd row are 0.

[0220] In the non-faulty loop 4, there are MMC equivalent voltage sources U3 and U4. The voltage of the equivalent voltage source U3 is in the same direction as the current reference direction of the non-faulty loop 4, while the voltage of the equivalent voltage source U4 is in the opposite direction to the current reference direction of the non-faulty loop 4. Therefore, in the second stage 5×4 order equivalent voltage source relation matrix B, the element in the 4th row and 3rd column is 1, the element in the 4th row and 4th column is -1, and the remaining elements in the 4th row are 0.

[0221] In the non-faulty loop 5, there are MMC equivalent voltage sources U1 and U4. The voltage of the equivalent voltage source U1 is opposite to the current reference direction of the non-faulty loop 4, and the voltage of the equivalent voltage source U4 is in the same direction as the current reference direction of the non-faulty loop 5. Therefore, in the second stage 5×4 order equivalent voltage source relation matrix B, the element in the 5th row and 1st column is -1, the element in the 5th row and 4th column is 1, and the remaining elements in the 5th row are 0.

[0222] Finally, the second-stage 5×4 order equivalent voltage source relation matrix was obtained. (10).

[0223] The input to the active current limiting control system is the difference between the actual and reference values ​​of the positive current of the hybrid converter station. Its output is calculated by a proportional-integral (PI) circuit and an anti-integral saturation circuit. Its control block diagram is shown below. Figure 5 As shown, the specific calculation formula is as follows:

[0224] (11)

[0225] P ui For MMC i The output of the proportional-integral (PI) stage in an active current limiting control system, P oi For MMC i The output of the active current limiting control system; the input ΔP of the active current limiting control system. i For MMC i Actual value of positive current I dcpi With DC current reference value I dcrefi The difference; K pi and K ii MMC i The proportional and integral coefficients of the active current limiting control system; t1 is the control switching moment, t is time; P ci For MMC i Output of the anti-integral saturation stage in an active current limiting control system; P hi and P li MMC i The active current limiting control system outputs upper and lower limits.

[0226] It should be noted that for a given multi-terminal flexible DC transmission system, the proportional coefficient, integral coefficient, upper output limit, lower output limit, and DC current reference value are parameters of the active current limiting control system, and are all known quantities.

[0227] In this embodiment, based on the output of the active current limiting control system of each hybrid converter station, a 4×1 order active current limiting control output matrix P is obtained:

[0228] (12)

[0229] 4×1 order second-stage equivalent voltage source voltage matrix U C2 The output matrix P of the 4×1 order active current limiting control is related to the rated voltage U of the DC side of the multi-terminal flexible DC transmission system. dcref The product of these. It should be noted that for a given multi-terminal flexible DC transmission system, the rated voltage of the DC side of the converter station is the same.

[0230] Second-stage equivalent resistance matrix R m2 Second-stage loop current matrix I2, second-stage equivalent inductance matrix L m2 With the first stage equivalent resistance matrix R m1 First-stage loop current matrix I1, first-stage equivalent inductance matrix L m1 Maintain consistency. Specifically, the initial value of the loop current in the second stage can be derived from the loop current value I at the end of the first stage. t1 Sure.

[0231] The system state-space equations for the second stage are established as follows:

[0232] (13)

[0233] Substitute into equations (10) to (12), and use the loop current value I at the end of the first stage. t1 By substituting the value of the active current limiting control output matrix P into the solution, the fault current time-domain response of the multi-terminal flexible DC transmission system from the moment the active current limiting control system is activated (t1) to the moment the fault current is controlled to 0 can be obtained, which is the fault current in the second stage (t1 to t2).

[0234] Simulation verification

[0235] Built on the PSCAD / EMTDC platform, such as Figure 1 The four-terminal flexible DC transmission system shown adopts a unipolar symmetrical connection method. The active current limiting control system is activated at a time of 2ms. Simulation verification shows the occurrence of a metallic fault (R... f The line fault current (=0Ω).

[0236] The parameters of the active current limiting control system, line parameters, and converter station parameters are shown in Tables 1 to 3 below:

[0237] Table 1 Parameters of Active Current Limiting Control System

[0238] parameter <![CDATA[MMC1]]> <![CDATA[MMC2]]> <![CDATA[MMC3]]> <![CDATA[MMC4]]> proportionality coefficient 4 4 4 5 Integral coefficient 10000 10000 10000 10000 Output limit 1 1 1 1 Output lower limit -0.5 -0.5 -0.5 -0.5

[0239] Table 2 Line Parameters

[0240] parameter <![CDATA[Line 12 > <![CDATA[Line 23 > <![CDATA[Line 34 > <![CDATA[Line 41 > Resistance (mΩ / km) 5 5 5 5 Inductance (mH / km) 0.41 0.41 0.41 0.41 Length (km) 100 100 100 100

[0241] Table 3 Converter station parameters

[0242] System parameters <![CDATA[MMC1]]> <![CDATA[MMC2]]> <![CDATA[MMC3]]> <![CDATA[MMC4]]> AC system frequency / Hz 50 50 50 50 Rated capacity / MVA 800 800 800 800 Rated DC voltage / kV 400 400 400 400 Number of full-bridge submodules / 150 150 150 150 Number of half-bridge sub-modules / each 50 50 50 50 <![CDATA[Sub-module capacitor C eq / mF]]> 10 10 10 10 <![CDATA[Arm inductance L arm / mH]]> 29 29 29 29 <![CDATA[Current-limiting inductor L x / mH]]> 20 20 20 20 <![CDATA[Sub-module resistance R on / mΩ]]> 1.361 1.361 1.361 1.361

[0243] like Figure 6 As shown, before 0ms, all hybrid converter stations were in normal operating condition. At 0ms, the line... 12 A short-circuit fault f0 occurred. Before the active current limiting control system was activated, the fault current mainly originated from the capacitor discharge of the sub-modules inside the hybrid converter station. If the fault current was too large at this time, it would cause irreversible damage to the system components. At 2ms, the active current limiting control system of the hybrid converter station was activated, suppressing the fault current by dynamically adjusting the DC voltage, and successfully controlling the fault current to 0kA within 8ms after the fault occurred. In the entire transient process from the occurrence of the fault to its final clearance, the calculated value of the fault current based on the method of this application is in good agreement with the simulation value of PSCAD / EMTDC software.

[0244] To further verify the accuracy of the calculation method proposed in this application, short-circuit faults were set at the DC-side outlets of MMC1 and MMC2, respectively. The comparison between the calculation results and simulation results is as follows: Figure 7 and Figure 8 As shown in the figure. The results show that the calculation results of the proposed method are highly similar to the simulation curves of the mainstream international simulation software PSCAD / EMTDC. This fully demonstrates that the proposed method can fully consider the dynamic response characteristics of the active current limiting control of the hybrid converter station, thereby achieving accurate calculation of the fault current of the multi-terminal flexible DC transmission system, and thus achieving accurate monitoring of the fault current of the multi-terminal flexible DC transmission system.

[0245] Example 3

[0246] This embodiment provides a fault current monitoring system for a multi-terminal flexible DC transmission system that considers active current limiting control in hybrid converter stations, including:

[0247] The equivalent circuit model establishment module is used to establish equivalent circuit models of multi-terminal flexible DC transmission systems at different fault monitoring stages based on the main parameters of hybrid converter stations and DC lines. It includes equivalent circuit model establishment modules for hybrid converter stations, DC lines, and multi-terminal flexible DC transmission systems.

[0248] The hybrid converter station, denoted as MMC, is set up in a multi-terminal flexible DC transmission system, and multiple MMCs are set up. The positive terminal of each MMC is connected through the positive terminal of a DC line, and the negative terminal is connected through the negative terminal of a DC line, forming a ring network topology.

[0249] Each MMC is pre-installed with an active current limiting control system; when a fault occurs in the multi-terminal flexible DC transmission system and active current limiting control needs to be activated, the active current limiting control systems of all MMCs are activated simultaneously.

[0250] The different fault monitoring stages, including the first stage and the second stage, are defined as follows:

[0251] The first stage is defined as the moment when the active current limiting control system of MMC is activated; the second stage is defined as the time from the occurrence of a fault in the multi-terminal flexible DC transmission system to the activation of the active current limiting control system; the third stage is defined as the time after the active current limiting control is activated and the fault current is controlled to 0.

[0252] The equivalent circuit model establishment module for the hybrid converter station includes a first-stage equivalent circuit model establishment module and a second-stage equivalent circuit model establishment module for the converter station.

[0253] The module for establishing the first-stage equivalent circuit model of the converter station is configured as follows:

[0254] In the first stage, based on the current-limiting inductor, bridge arm inductor, submodule capacitor, number of submodules, and submodule resistance of MMC, an equivalent circuit model of the converter station in the first stage is established by means of equivalent capacitor, equivalent inductor and equivalent resistance connected in series.

[0255] The module for establishing the second-stage equivalent circuit model of the converter station is configured as follows:

[0256] In the second stage, the equivalent capacitor in the first stage equivalent circuit model of the converter station is replaced with an equivalent voltage source, while the rest of the structure and parameters remain unchanged. A second stage equivalent circuit model of the converter station is established, consisting of an equivalent resistor, an equivalent inductor, and an equivalent voltage source connected in series. The voltage value of the equivalent voltage source is determined by the output of the active current limiting control system.

[0257] The DC line equivalent circuit model establishment module is configured as follows:

[0258] Based on the unit resistance, unit inductance, and line length of the DC line, an equivalent circuit model of the DC line consisting of an equivalent inductance and an equivalent resistance connected in series is established.

[0259] The equivalent circuit model establishment module for multi-terminal flexible DC transmission system includes a first-stage equivalent circuit model establishment module for multi-terminal flexible DC transmission system and a second-stage equivalent circuit model establishment module for multi-terminal flexible DC transmission system;

[0260] The first-stage equivalent circuit model establishment module of the multi-terminal flexible DC transmission system is used to establish the first-stage equivalent circuit model of the multi-terminal flexible DC transmission system based on the first-stage equivalent circuit model of the converter station and the equivalent model of the DC line in the first stage.

[0261] The second-stage equivalent circuit model establishment module of the multi-terminal flexible DC transmission system is used to establish the second-stage equivalent circuit model of the multi-terminal flexible DC transmission system based on the second-stage equivalent circuit model of the converter station and the equivalent model of the DC line in the second stage.

[0262] The state-space equation set establishment module is used to establish the state-space equation set; the state-space equation set includes the first-stage state-space equation set for fault current monitoring and the second-stage state-space equation set for fault current monitoring;

[0263] The state-space equation set for the first stage of fault current monitoring is established based on the equivalent circuit model of the first stage of the multi-terminal flexible DC transmission system.

[0264] The state-space equation set for the second stage of fault current monitoring is established based on the equivalent circuit model of the second stage of the multi-terminal flexible DC transmission system.

[0265] The fault current monitoring module is used to monitor the fault current based on the time-domain response of the fault current obtained from the solution.

[0266] Solving the state-space equations of the first stage of the fault current monitoring system yields the time-domain response of the fault current in the multi-terminal flexible DC transmission system from the occurrence of a bipolar fault to the moment when the active current limiting control system is activated.

[0267] Solve the state-space equations of the second stage of the fault current monitoring to obtain the time-domain response of the multi-terminal flexible DC transmission system from the moment the active current limiting control system is activated to the moment the fault current is controlled to 0.

[0268] The fault current monitoring system for a multi-terminal flexible DC transmission system in this embodiment, which considers active current limiting control of a hybrid converter station, corresponds to the fault current monitoring method for a multi-terminal flexible DC transmission system in Embodiment 1, and will not be described again here.

[0269] Example 4

[0270] This embodiment provides a computer-readable storage medium storing a computer program that, when executed by a processor, implements the steps of the fault current monitoring method for a multi-terminal flexible DC transmission system, which includes active current limiting control of a hybrid converter station, as described in Embodiment 1.

[0271] Example 5

[0272] This embodiment provides a controller, including: the computer-readable storage medium described in Embodiment 4; and one or more processors for executing a program in the computer-readable storage medium.

[0273] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product implemented on one or more computer-usable storage media containing computer-usable program code. The solutions in the embodiments of this application can be implemented using various computer languages.

[0274] Although preferred embodiments of this application have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this application.

[0275] Obviously, those skilled in the art can make various modifications and variations to this application without departing from the spirit and scope of this application. Therefore, if such modifications and variations fall within the scope of the claims of this application and their equivalents, this application also intends to include such modifications and variations.

Claims

1. A method for monitoring fault current in a multi-terminal flexible DC transmission system considering active current limiting control in a hybrid converter station, characterized in that, include: S1. Based on the main parameters of the hybrid converter station and DC line, establish equivalent circuit models of the multi-terminal flexible DC transmission system at different fault monitoring stages. The hybrid converter station, denoted as MMC, is set up in a multi-terminal flexible DC transmission system, and multiple MMCs are set up. The positive terminal of each MMC is connected through the positive terminal of a DC line, and the negative terminal is connected through the negative terminal of a DC line, forming a ring network topology. Each MMC is pre-installed with an active current limiting control system; when a fault occurs in the multi-terminal flexible DC transmission system and active current limiting control needs to be activated, the active current limiting control systems of all MMCs are activated simultaneously. The different fault monitoring stages, including the first stage and the second stage, are defined as follows: The first stage is defined as the moment when the active current limiting control system of MMC is activated; the second stage is defined as the time from the occurrence of a fault in the multi-terminal flexible DC transmission system to the activation of the active current limiting control system; the third stage is defined as the time after the active current limiting control is activated and the fault current is controlled to 0. S11, Establish the equivalent circuit model of the hybrid converter station; The equivalent circuit model of the hybrid converter station includes the first-stage equivalent circuit model and the second-stage equivalent circuit model of the converter station. The first-stage equivalent circuit model of the converter station is an equivalent circuit model established in the first stage based on the current-limiting inductor, bridge arm inductor, sub-module capacitor, number of sub-modules, and sub-module resistance of MMC, through the principle of equivalence, consisting of equivalent capacitor, equivalent inductor, and equivalent resistance connected in series. The second-stage equivalent circuit model of the converter station is as follows: In the second stage, the equivalent capacitor in the first-stage equivalent circuit model of the converter station is replaced with an equivalent voltage source, while the rest of the structure and parameters remain unchanged. The equivalent circuit model is established by connecting the equivalent resistance, equivalent inductance and equivalent voltage source in series. The voltage value of the equivalent voltage source is determined by the output of the active current limiting control system. S12, Establish the equivalent circuit model of the DC line; The DC line equivalent model is an equivalent circuit model established based on the unit resistance, unit inductance, and line length of the DC line, consisting of an equivalent inductance and an equivalent resistance connected in series. S13, Establish the equivalent circuit model of the multi-terminal flexible DC transmission system; The equivalent circuit model of the multi-terminal flexible DC transmission system includes the first-stage equivalent circuit model of the multi-terminal flexible DC transmission system and the second-stage equivalent circuit model of the multi-terminal flexible DC transmission system. The first-stage equivalent circuit model of the multi-terminal flexible DC transmission system is established based on the first-stage equivalent circuit model of the converter station and the equivalent model of the DC line in the first stage. The second-stage equivalent circuit model of the multi-terminal flexible DC transmission system is established in the second stage based on the second-stage equivalent circuit model of the converter station and the equivalent model of the DC line. S2, Establish the state-space equation system; The state-space equation set includes the first-stage state-space equation set for fault current monitoring and the second-stage state-space equation set for fault current monitoring. The state-space equation set for the first stage of fault current monitoring is established based on the equivalent circuit model of the first stage of the multi-terminal flexible DC transmission system. The state-space equation set for the second stage of fault current monitoring is established based on the equivalent circuit model of the second stage of the multi-terminal flexible DC transmission system. S3, solve for the time-domain response of the fault current to monitor the fault current; Solving the state-space equations of the first stage of the fault current monitoring system yields the time-domain response of the fault current in the multi-terminal flexible DC transmission system from the occurrence of a bipolar fault to the moment when the active current limiting control system is activated. Solve the state-space equations of the second stage of the fault current monitoring to obtain the time-domain response of the multi-terminal flexible DC transmission system from the moment the active current limiting control system is activated to the moment the fault current is controlled to 0.

2. The fault current monitoring method for a multi-terminal flexible DC transmission system considering active current limiting control of a hybrid converter station as described in claim 1, characterized in that, The specific method for establishing the equivalent circuit model of the multi-terminal flexible DC transmission system is as follows: Define faulty loops and non-faulty loops; If a certain MMC goes to the fault point only through the DC line and does not pass through other MMCs, then the MMC, the DC line connecting the fault point and the MMC, and the fault point together constitute a fault loop. If there is no fault between two adjacent MMCs and they are only connected by a DC line, then the two adjacent MMCs and the DC line connecting the two adjacent MMCs constitute a non-faulty circuit. In the first stage: For the fault loop, a first-stage equivalent fault loop model is established based on the first-stage equivalent circuit model of the converter station MMC based on the fault loop, the DC line equivalent circuit model of the fault loop, and the fault point. For non-faulty loops, a first-stage equivalent non-faulty loop model is established based on the first-stage equivalent circuit model of the converter station for each of the two adjacent MMCs of the non-faulty loop, and the equivalent circuit model of the DC line connecting the two adjacent MMCs. The first-stage equivalent fault loop model and the first-stage equivalent non-fault loop model together constitute the first-stage equivalent circuit model of the multi-terminal flexible DC transmission system. In the second phase: For the fault loop, a second-stage equivalent fault loop model is established based on the second-stage equivalent circuit model of the converter station based on the MMC of the fault loop, the DC line equivalent circuit model of the fault loop, and the fault point. For non-faulty loops, a second-stage equivalent non-faulty loop model is established based on the second-stage equivalent circuit model of the converter station for each of the two adjacent MMCs of the non-faulty loop, and the equivalent circuit model of the DC line connecting the two adjacent MMCs. The second-stage equivalent fault loop model and the second-stage equivalent non-fault loop model together constitute the second-stage equivalent circuit model of the multi-terminal flexible DC transmission system.

3. The fault current monitoring method for multi-terminal flexible DC transmission systems considering active current limiting control of hybrid converter stations according to claim 2, characterized in that, The specific method for establishing the state-space equation set is as follows: The loop topology is defined as including faulty loops and non-faulty loops; Based on the loop topology of the first-stage equivalent circuit model of the multi-terminal flexible DC transmission system, the n×1 order first-stage capacitor-voltage matrix U is obtained. C1 The first-stage loop current matrix I1 is of order (n+1)×1, and the first-stage capacitor current matrix I is of order n×1. C1 ; Wherein, the first-stage capacitor-voltage matrix U of order n×1 C1 In the (n+1)×1 order first-stage loop current matrix I1, the matrix elements are the voltages of the equivalent capacitors in the loop topology; in the n×1 order first-stage capacitor current matrix I... C1 In the matrix, the elements represent the current flowing through the equivalent capacitance in the loop topology; n is the number of ports in the multi-terminal flexible DC transmission system. Based on the loop topology of the first-stage equivalent circuit model of the multi-terminal flexible DC transmission system, the first-stage (n+1)×n order capacitor-voltage relationship matrix A is obtained; where... In the first stage (n+1)×n order capacitor voltage relationship matrix A, the number of rows is the same as the number of loops in the loop topology, and the number of columns is the same as the number of equivalent capacitors in the loop topology; In the first stage (n+1)×n order capacitor-voltage relationship matrix A, if the i-th loop in the loop topology of the first stage equivalent circuit model of the multi-terminal flexible DC transmission system has an equivalent capacitance C of MMC... j If the element in the i-th row and j-th column of the (n+1)×n order capacitor voltage relationship matrix A in the first stage is non-zero, then the element in the i-th row and j-th column is 0; when the element in the i-th row and j-th column is non-zero, if the capacitor voltage and the current reference direction of the circuit are the same, then the element in the i-th row and j-th column is 1; if the capacitor voltage and the current reference direction of the circuit are different, then the element in the i-th row and j-th column is -1. Based on the loop topology of the first-stage equivalent circuit model of the multi-terminal flexible DC transmission system, the first-stage equivalent resistance matrix R of the first-stage equivalent circuit model of the multi-terminal flexible DC transmission system is obtained. m1 The first stage equivalent inductance matrix L m1 and the first-stage equivalent capacitance matrix C m1 ; Wherein, matrix R m1 Matrix L m1 The number of rows and columns is the same as the number of loops in the loop topology, matrix C m1 The number of rows and columns is the same as the number of MMCs; Matrix R m1 diagonal element R ii Represents the equivalent total resistance of loop i, and the remaining elements R ij This indicates that loop i and loop j share the equivalent resistance on the line; when the current directions of the two loops are the same, it is R. ij When the loop currents are in opposite directions, it is -R. ij When there is no shared line, R ij =0; R f The fault resistance value at the fault point; Matrix L m1 diagonal element L ii Let L represent the equivalent total inductance of loop i, and the remaining elements L ij This indicates that loop i and loop j share the equivalent inductance on the line; when the current directions of the two loops are the same, it is L. ij When the loop currents are in opposite directions, it is -L. ij When there is no shared line, L ij =0; Capacitor matrix C m1 The diagonal elements are the various MMCs. j The reciprocal of the equivalent capacitance, with all other elements being 0; Based on the first stage (n+1)×n order capacitor voltage relationship matrix A and the first stage equivalent resistance matrix R m1 The first stage equivalent inductance matrix L m1 and the equivalent capacitance matrix C of the first stage m1 The following state-space equation set for the first stage of fault current monitoring is established: In the formula, t represents time; Based on the loop topology of the second-stage equivalent circuit model of the multi-terminal flexible DC transmission system, the n×1 order second-stage equivalent voltage source voltage matrix U is obtained. C2 The second-stage loop current matrix I2 is of order (n+1)×1, and the active current limiting control output matrix P is of order n×1; where the second-stage equivalent voltage source voltage matrix U is of order n×1. C2 The matrix elements are the voltages of the equivalent voltage sources in the loop topology; the (n+1)×1 second-stage loop current matrix I2 has the elements being the currents of each loop in the loop topology; the n×1 active current limiting control output matrix P has the elements being the outputs of the active current limiting control system of each MMC in the loop topology. Based on the loop topology of the second-stage equivalent circuit model of the multi-terminal flexible DC transmission system, the second-stage (n+1)×n order equivalent voltage source relationship matrix B is obtained. In the second stage (n+1)×n order equivalent voltage source relation matrix B, if the k-th loop in the loop topology of the second stage equivalent circuit model of the multi-terminal flexible DC transmission system contains an equivalent voltage source U of MMC. h If the element in the k-th row and h-th column of the (n+1)×n equivalent voltage source relation matrix B in the second stage is non-zero, then the element in the k-th row and h-th column is 0; when the element in the k-th row and h-th column is non-zero, if the voltage direction of the equivalent voltage source is the same as the reference direction of the current in the loop, then the element in the k-th row and h-th column is 1; if the voltage direction of the equivalent voltage source is different from the reference direction of the current in the loop, then the element in the k-th row and h-th column is -1. In the second stage (n+1)×n equivalent voltage source relation matrix B, the number of rows is the same as the number of loops in the loop topology, and the number of columns is the same as the number of equivalent voltage sources in the loop topology; Based on the loop topology of the second-stage equivalent circuit model of the multi-terminal flexible DC transmission system, the second-stage equivalent resistance matrix R of the second-stage equivalent circuit model of the multi-terminal flexible DC transmission system is obtained. m2 The second-stage equivalent inductance matrix L m2 ; Wherein, matrix R m2 Matrix L m2 The number of rows and columns is the same as the number of loops in the loop topology; Matrix R m2 diagonal element R ii Represents the equivalent total resistance of loop i, and the remaining elements R ij This indicates that loop i and loop j share the equivalent resistance on the line; when the current directions of the two loops are the same, it is R. ij When the loop currents are in opposite directions, it is -R. ij When there is no shared line, R ij =0; R f The fault resistance value at the fault point; Matrix L m2 diagonal element L ii Let L represent the equivalent total inductance of loop i, and the remaining elements L ij This indicates that loop i and loop j share the equivalent inductance on the line; when the current directions of the two loops are the same, it is L. ij When the loop currents are in opposite directions, it is -L. ij When there is no shared line, L ij =0; Based on the second-stage (n+1)×n equivalent voltage source relation matrix B and the second-stage equivalent resistance matrix R m2 The second stage equivalent inductance matrix L m2 Based on the n×1 order active current limiting control output matrix P, the following state-space equation set for the second stage of fault current monitoring is established: In the formula, t is time, and U dcref This refers to the rated voltage on the DC side of the DC transmission system.

4. The fault current monitoring method for a multi-terminal flexible DC transmission system considering active current limiting control of a hybrid converter station according to any one of claims 1-3, characterized in that, The equivalence principle adopted when establishing the first-stage equivalent circuit model of the converter station is as follows: In the formula, C, L, and R are the equivalent capacitance, equivalent inductance, and equivalent resistance values ​​after applying the equivalence principle; C eq L arm R on The values ​​of capacitance, arm inductance, and resistance of a single submodule in the hybrid converter station; L x N is the DC-side current-limiting inductance value of the hybrid converter station; N is the number of individual bridge arm submodules in the hybrid converter station.

5. The fault current monitoring method for a multi-terminal flexible DC transmission system considering active current limiting control of a hybrid converter station according to any one of claims 1-3, characterized in that, The method for establishing the equivalent circuit model of the DC line is as follows: In the formula l d r d x represents the unit inductance and unit resistance values ​​of the DC line; d R is the length of the DC line; d L d These are the equivalent resistance and equivalent inductance values ​​of the DC line.

6. The fault current monitoring method for a multi-terminal flexible DC transmission system considering active current limiting control of a hybrid converter station according to any one of claims 1-3, characterized in that, The control method of the active current limiting control system is as follows: The difference between the actual value of the positive current and the reference value of the hybrid converter station is input, and the output is obtained after calculation by the proportional-integral circuit and the anti-integral saturation circuit. The specific calculation formulas for the proportional-integral element and the anti-integral saturation element are as follows: In the formula, P u P is the output of the proportional-integral (PI) stage of the active current limiting control system. o ΔP is the output of the active current limiting control system; ΔP is the input of the active current limiting control system; I dcp This represents the actual value of the positive current of the hybrid converter station; I dcref This is the reference value for the positive electrode current of a hybrid converter station; K p For the proportionality coefficient and K i P is the integral coefficient; t1 is the control switching time, t is time; c For the output of the anti-integral saturation stage of the active current limiting control system; P h The upper limit of the output of the active current limiting control system and P l This is the lower limit of the output of the active current limiting control system.

7. A fault current monitoring system for a multi-terminal flexible DC transmission system considering active current limiting control in a hybrid converter station, characterized in that, include: The equivalent circuit model establishment module is used to establish equivalent circuit models of multi-terminal flexible DC transmission systems at different fault monitoring stages based on the main parameters of hybrid converter stations and DC lines. It includes equivalent circuit model establishment modules for hybrid converter stations, DC lines, and multi-terminal flexible DC transmission systems. The hybrid converter station, denoted as MMC, is set up in a multi-terminal flexible DC transmission system, and multiple MMCs are set up. The positive terminal of each MMC is connected through the positive terminal of a DC line, and the negative terminal is connected through the negative terminal of a DC line, forming a ring network topology. Each MMC is pre-installed with an active current limiting control system; when a fault occurs in the multi-terminal flexible DC transmission system and active current limiting control needs to be activated, the active current limiting control systems of all MMCs are activated simultaneously. The different fault monitoring stages, including the first stage and the second stage, are defined as follows: The first stage is defined as the moment when the active current limiting control system of MMC is activated; the second stage is defined as the time from the occurrence of a fault in the multi-terminal flexible DC transmission system to the activation of the active current limiting control system; the third stage is defined as the time after the active current limiting control is activated and the fault current is controlled to 0. The equivalent circuit model establishment module for the hybrid converter station includes a first-stage equivalent circuit model establishment module and a second-stage equivalent circuit model establishment module for the converter station. The module for establishing the first-stage equivalent circuit model of the converter station is configured as follows: In the first stage, based on the current-limiting inductor, bridge arm inductor, submodule capacitor, number of submodules, and submodule resistance of MMC, an equivalent circuit model of the converter station in the first stage is established by means of equivalent capacitor, equivalent inductor and equivalent resistance connected in series. The module for establishing the second-stage equivalent circuit model of the converter station is configured as follows: In the second stage, the equivalent capacitor in the first stage equivalent circuit model of the converter station is replaced with an equivalent voltage source, while the rest of the structure and parameters remain unchanged. A second stage equivalent circuit model of the converter station is established, consisting of an equivalent resistor, an equivalent inductor, and an equivalent voltage source connected in series. The voltage value of the equivalent voltage source is determined by the output of the active current limiting control system. The DC line equivalent circuit model establishment module is configured as follows: Based on the unit resistance, unit inductance, and line length of the DC line, an equivalent circuit model of the DC line consisting of an equivalent inductance and an equivalent resistance connected in series is established. The equivalent circuit model establishment module for multi-terminal flexible DC transmission system includes a first-stage equivalent circuit model establishment module for multi-terminal flexible DC transmission system and a second-stage equivalent circuit model establishment module for multi-terminal flexible DC transmission system; The first-stage equivalent circuit model establishment module of the multi-terminal flexible DC transmission system is used to establish the first-stage equivalent circuit model of the multi-terminal flexible DC transmission system based on the first-stage equivalent circuit model of the converter station and the equivalent model of the DC line in the first stage. The second-stage equivalent circuit model establishment module of the multi-terminal flexible DC transmission system is used to establish the second-stage equivalent circuit model of the multi-terminal flexible DC transmission system based on the second-stage equivalent circuit model of the converter station and the equivalent model of the DC line in the second stage. The state-space equation set establishment module is used to establish the state-space equation set; the state-space equation set includes the first-stage state-space equation set for fault current monitoring and the second-stage state-space equation set for fault current monitoring; The state-space equation set for the first stage of fault current monitoring is established based on the equivalent circuit model of the first stage of the multi-terminal flexible DC transmission system. The state-space equation set for the second stage of fault current monitoring is established based on the equivalent circuit model of the second stage of the multi-terminal flexible DC transmission system. The fault current monitoring module is used to monitor the fault current based on the time-domain response of the fault current obtained from the solution. Solving the state-space equations of the first stage of the fault current monitoring system yields the time-domain response of the fault current in the multi-terminal flexible DC transmission system from the occurrence of a bipolar fault to the moment when the active current limiting control system is activated. Solve the state-space equations of the second stage of the fault current monitoring to obtain the time-domain response of the multi-terminal flexible DC transmission system from the moment the active current limiting control system is activated to the moment the fault current is controlled to 0.

8. A computer-readable storage medium having a computer program stored thereon, characterized in that, When executed by the processor, the program implements the steps of the fault current monitoring method for multi-terminal flexible DC transmission systems that includes active current limiting control of hybrid converter stations as described in any one of claims 1-6.

9. A controller, comprising: The computer-readable storage medium as described in claim 8; and one or more processors for executing the program in the computer-readable storage medium.

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