A method and system for flexible direct current single-ended protection based on transverse differential current

Abstract

The application relates to a kind of flexible direct current single-end protection method and system based on transverse differential current.The method first establishes the equivalent network model under the fault state of flexible direct current system, and based on the equivalent network model, transverse differential current is modeled, and the modeling result of transverse differential current is output;Single-end fault recognition mechanism containing single-end protection factor is constructed based on the modeling result of transverse differential current;In each sampling period, the current signals on both sides of the bridge arm are collected in real time, and the instantaneous value of the transverse differential current at the current time is calculated from the current signals on both sides of the bridge arm;Single-end protection factor is calculated from the instantaneous value of the transverse differential current;Based on single-end protection factor, the single-end fault recognition mechanism is used to determine whether there is internal fault at present, if there is, protection logic is triggered, and corresponding protection control signal is output.Compared with prior art, the application has the advantages of strong independence, strong real-time and high robustness.

Description

Technical Field

[0001] This invention relates to the field of relay protection technology, and in particular to a flexible DC single-ended protection method and system based on transverse differential current. Background Technology

[0002] Against the backdrop of large-scale grid connection of new energy sources and rapid development of power electronic equipment, flexible DC transmission technology, due to its advantages such as flexible control and controllable power flow, has gradually become a key supporting technology for long-distance power transmission, offshore wind power grid connection, and multi-terminal power supply systems. Flexible DC systems widely adopt voltage source converter (VSC) structures, connecting various ports through DC cables to form a complex topology network. In this process, the fault response mechanism, protection configuration, and control strategy of the transmission system exhibit characteristics drastically different from traditional AC systems. Especially during the transient process after a fault occurs, voltage and current changes extremely rapidly, making traditional protection methods relying on steady-state quantities and fault directionality inadequate.

[0003] In flexible DC transmission systems, due to the converter control strategy shielding the current direction and the low electrical inertia of the system, conventional techniques such as directional elements, current amplitude jumps, and power flow judgment are insufficient to accurately identify fault types and fault sections. Furthermore, the multi-terminal structure further exacerbates the complexity of protection system configuration, making protection methods relying solely on inter-station communication vulnerable to reliability risks in the event of communication failures, delays, or link failures. Therefore, in engineering practice, there is an urgent need to construct relay protection methods that are communication-independent, have rapid response, and can adapt to changes in multi-terminal system topology and operating states. In existing relay protection research, a relatively effective approach is to construct differential information criteria based on locally measured quantities of the converter. By comparing the current or voltage differences between different arms of the same converter, the asymmetric characteristics between phases or modules at the time of a fault are identified. These criteria are simple to implement, rely on few information, and are insensitive to changes in system parameters, and have been initially applied in modular multilevel converter (MMC) systems. However, traditional differential criteria mostly rely on empirical thresholds or fixed judgment logic. This approach is clearly insufficient in adaptability when faced with complex fault characteristics under various topologies and diverse operating modes.

[0004] Existing relay protection methods for flexible DC transmission systems generally rely on communication networks for fault information transmission and coordination. When communication is delayed, links fail, or nodes malfunction, protection actions are easily affected, making it difficult to meet the engineering requirements for independent and rapid protection of the system.

[0005] Current differential protection schemes mostly use a single criterion of converter arm current or module voltage, and rely on fixed thresholds or static rules, which are difficult to adapt to changes in operating conditions and are prone to failure to operate or false operation.

[0006] Most of them are based only on steady-state or transient current measurement data, lacking comprehensive utilization of the module's dynamic energy behavior, and cannot fully explore internal information to enhance the robustness of fault diagnosis.

[0007] In multi-terminal flexible DC systems, existing protection configurations lack universality and scalability. The division of protection areas and the deployment of strategies rely heavily on engineering experience and manual settings, which is not conducive to the unified promotion and application in complex access scenarios.

[0008] In summary, current relay protection schemes for flexible DC transmission systems generally suffer from problems such as reliance on fixed thresholds or static rules, low real-time performance, and low robustness. Summary of the Invention

[0009] The purpose of this invention is to overcome the shortcomings of the prior art by providing a flexible DC single-ended protection method and system based on transverse differential current.

[0010] The objective of this invention can be achieved through the following technical solutions:

[0011] According to one aspect of the present invention, a flexible DC single-ended protection method based on transverse differential current is provided, the method comprising the following steps:

[0012] S1. Establish an equivalent network model of the flexible DC system under fault conditions, and model the transverse differential current based on the equivalent network model, and output the transverse differential current modeling results.

[0013] S2. Construct a single-ended fault identification mechanism with a single-ended protection factor based on the results of transverse differential flow modeling;

[0014] S3. In each sampling period, the current signals on both sides of the bridge arm are collected in real time, and the instantaneous value of the transverse differential current at the current moment is calculated from the current signals on both sides of the bridge arm.

[0015] S4. The single-end protection factor is calculated from the instantaneous value of the transverse differential current;

[0016] S5. Based on the single-ended protection factor, use the single-ended fault identification mechanism to determine whether there is an internal fault. If there is, trigger the protection logic and output the corresponding protection control signal; otherwise, jump to step S3 and maintain the monitoring state.

[0017] As a preferred technical solution, the specific process of establishing the equivalent network model of the flexible DC system under fault conditions in S1 includes:

[0018] Construct the first model;

[0019] Based on the first model, a second model is further constructed;

[0020] Based on the first and second models, considering the fault points, the flexible DC system is divided into multiple electrical subnets. Variable information including converter modulation factors and total voltage of submodules is introduced, and combined with the relationship between branch impedance and node voltage, an equivalent network model of the flexible DC system under fault conditions is further constructed.

[0021] The first model is the dynamic response model of the bridge arm of the modular multilevel converter under fault disturbance; the second model is the current differential characteristic model of the flexible DC system under normal conditions.

[0022] As a preferred technical solution, the specific construction process of the bridge arm dynamic response model of the modular multilevel converter under fault disturbance includes:

[0023] Establish the current balance relationship between the upper arm current and the lower arm current;

[0024] Based on Kirchhoff's voltage law, the relationship between the upper and lower bridge arm currents and the bus voltage is established; and the relationship between the sum of the module voltages and the capacitor voltage is established.

[0025] Substituting the relationships between the upper and lower arm currents and the bus voltage, and the relationship between the sum of the module voltages and the capacitor voltage, into the current balance relationship and simplifying it, we obtain the relationship between the DC side voltage and the output current and the arm current, which is the dynamic response model of the arm of the modular multilevel converter under fault disturbance.

[0026] As a preferred technical solution, the specific formula for the dynamic response model of the bridge arm of the modular multilevel converter under fault disturbance is as follows:

[0027]

[0028]

[0029]

[0030] in, This is the DC bus voltage; For the bridge arm resistance; For bridge arm inductance, Output current for the lower bridge arm; This refers to the output voltage of the lower bridge arm. Output current for the upper bridge arm; Upper bridge arm output voltage; This is the current output to the AC bus. This refers to the capacitor voltage of a single submodule. Number of submodules in each bridge arm; This indicates the total module voltage of the bridge arm.

[0031] As a preferred technical solution, the specific formula for the lateral differential flow modeling result is as follows:

[0032]

[0033] in, and This indicates the voltage at nodes near the fault point; The equivalent impedance of the line; This indicates the transverse differential current that forms after a fault occurs; This represents the normalized relative position of the fault point within the entire line segment, with a value range of [0, 1]. These are the positive and negative equivalent currents on both sides of the bridge arm, respectively.

[0034] As a preferred technical solution, the single-end protection factor in S2 is calculated from the instantaneous value of the transverse differential current. First, a time sliding window is set. Within this time sliding window, the absolute value of all instantaneous values ​​of the transverse differential current is integrated, and the continuous fluctuation amplitude is extracted to form the single-end protection factor.

[0035] As a preferred technical solution, the specific formula for the single-end protection factor is:

[0036]

[0037] Where S represents the cumulative amplitude of differential current fluctuation within the set time window. The number of sampling points within the set time window; for i The instantaneous value of the transverse differential flow at time t; A specific moment within the set time window; The preset action threshold.

[0038] As a preferred technical solution, the single-end fault identification mechanism in S2 is specifically as follows:

[0039] Set the action threshold of the single-ended protection factor and compare the single-ended protection factor with the preset action threshold; if the single-ended protection factor is greater than the preset action threshold, it is determined that there is an internal fault; otherwise, it is determined that there is no internal fault.

[0040] As a preferred technical solution, the protection control signals in S5 include circuit breaking control signals and interlocking control signals.

[0041] According to another aspect of the present invention, a flexible DC single-ended protection system based on transverse differential current is provided. The system includes a fault network modeling and differential current modeling module, a single-ended fault identification mechanism construction module, a differential current data acquisition and calculation module, and a fault determination and protection control module.

[0042] The fault network modeling and differential current modeling modules are used to establish an equivalent network model of the flexible DC system under fault conditions, and to model the transverse differential current based on the equivalent network model, and output the transverse differential current modeling results.

[0043] The single-ended fault identification mechanism construction module is used to construct a single-ended fault identification mechanism containing a single-ended protection factor based on the results of transverse differential current modeling.

[0044] The differential current data acquisition and calculation module is used to acquire the current signals on both sides of the bridge arm in real time during each sampling period, calculate the instantaneous value of the transverse differential current at the current moment from the current signals on both sides of the bridge arm, and calculate the single-end protection factor from the instantaneous value of the transverse differential current.

[0045] The fault determination and protection control module is based on a single-ended protection factor and uses a single-ended fault identification mechanism to determine whether there is an internal fault. If there is, the protection logic is triggered and the corresponding protection control signal is output; otherwise, the monitoring state is maintained and the differential current data acquisition and calculation stage is executed.

[0046] Compared with the prior art, the present invention has the following beneficial effects:

[0047] 1. In this invention, a single-ended fault identification mechanism is constructed based on the lateral differential current of the current signals on both sides of the bridge arm. The weak asymmetric components between the upper and lower bridge arms are extracted during the initial stage of a fault, enabling local identification of the fault polarity. This method relies entirely on single-ended data combined with theoretical modeling to achieve fault identification, eliminating the need for cross-end communication networks to transmit fault information. This solves the problem in existing methods where communication delays and link failures affect protection actions. A joint criterion system of differential current signals and module energy information is constructed, enabling local fault identification in flexible DC transmission systems that does not depend on communication or current direction information, thus satisfying the independence and real-time requirements of fault identification in flexible DC systems.

[0048] 2. In this invention, a modular multilevel converter arm dynamic response model and a system normal state current differential characteristic model are first constructed. Then, by combining the fault point subnetting and introducing variables such as the converter modulation factor and the total voltage of the submodule, a fault equivalent network model is constructed. This makes fault modeling no longer dependent on empirical assumptions, but rooted in the system topology and electrical parameters. It solves the problem that traditional differential criteria rely on empirical thresholds and are not adaptable to various topologies. This achieves the beneficial effect of making the model adaptable to complex fault characteristics under diverse operating modes.

[0049] Based on the current balance relationship and Kirchhoff's voltage law, and combined with the relationship between module voltage and capacitor voltage, the dynamic response model of the MMC bridge arm is derived. This model integrates multi-dimensional information such as bridge arm current, bus voltage, and sub-module capacitor voltage, rather than relying solely on a single current quantity. This solves the problem that current differential protection schemes use a single criterion and are difficult to adapt to changes in operating conditions. It fully explores the internal electrical and energy dynamic information of the converter and enhances the robustness of fault diagnosis.

[0050] A dynamic response model for the MMC bridge arm is constructed by quantifying the relationships between DC bus voltage, bridge arm resistance / inductance, and bridge arm current. The trend of module capacitor voltage change is introduced as an auxiliary quantity, and the dynamic energy change characteristics of submodules are utilized to enhance the ability to identify current flow direction and fault sections. Simultaneously, fault modeling is transformed from qualitative description to quantitative calculation, avoiding the subjective bias of traditional empirical thresholds. This solves the problem that traditional differential criteria cannot adapt to complex fault characteristics due to fixed logic. It achieves accurate characterization of bridge arm dynamic response under different topologies or operating loads, providing a unified theoretical benchmark for differential current calculation. This method possesses good topology adaptability and robustness, and is suitable for relay protection applications in flexible DC systems under complex conditions such as multi-terminal, long-distance, and renewable energy integration.

[0051] 3. In this invention, based on parameters such as the node voltage near the fault point, the equivalent impedance of the line, and the normalized location of the fault point, a specific formula for the transverse differential current is derived. This makes the differential current calculation no longer dependent on static rules, but dynamically reflects the influence of the fault location and line parameters on the differential current. This solves the problem that the current differential protection scheme relies on a fixed threshold and is difficult to adapt to diverse operating modes. It enables accurate quantification of the differential current magnitude under different fault types such as single-pole grounding and bridge arm short circuit, thereby improving the accuracy of fault identification.

[0052] By setting a time sliding window, the absolute value of the instantaneous value of the transverse differential current is integrated to extract the continuous fluctuation amplitude, forming a single-ended protection factor. This makes the protection factor no longer a static current quantity at a single moment, but reflects the dynamic accumulation characteristics of the differential current over a period of time. This solves the problem of current protection schemes relying on a single steady-state / transient current quantity and being prone to false tripping due to instantaneous interference. By capturing the continuous fluctuation characteristics of the differential current, the adaptability to complex fault identification is enhanced.

[0053] 4. In this invention, the fault is determined by comparing the single-ended protection factor with the preset action threshold. This solves the problem in the current differential protection scheme that the static rules are difficult to adapt to changes in operating status and are prone to failure to operate / false operation. It achieves the beneficial effect of accurately distinguishing internal / external faults even under operating conditions such as low system load and fluctuations in new energy output.

[0054] 5. In this invention, the protection control signal includes a circuit breaker control signal and a blocking control signal, forming a protection action based on a fault severity matching and hierarchical control strategy determined by a precise differential current criterion. This solves the problem of false tripping caused by a single action logic in the current protection scheme, and achieves the beneficial effect of reducing unnecessary power outage losses and improving the rationality of protection actions while ensuring system safety. Attached Figure Description

[0055] Figure 1 This is a schematic diagram of the steps of a flexible DC single-ended protection method based on transverse differential current in this invention;

[0056] Figure 2 This is the time-domain equivalent circuit diagram of the modular multilevel converter submodule under fault conditions in the embodiment;

[0057] Figure 3 This is a simplified equivalent circuit model diagram of the system under non-fault conditions in the embodiment;

[0058] Figure 4 This is a flowchart illustrating the implementation of a flexible DC single-ended protection strategy based on transverse differential current in this embodiment. Detailed Implementation

[0059] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0060] To address the protection challenges in flexible DC transmission systems, such as unclear fault direction, indistinct current abrupt changes, and strong communication dependence, this paper proposes a protection method for flexible DC transmission systems based on joint discrimination of converter arm lateral differential current measurement and module energy information. This method establishes a differential current calculation mechanism between the upper and lower arms of the same converter, combined with the voltage change direction of the submodules, to achieve highly reliable identification of fault polarity and fault region. It can effectively distinguish between internal and external fault types and possesses advantages such as no communication dependence, fast response, and adaptability to various topologies. It is particularly suitable for the relay protection needs of flexible DC systems in practical engineering scenarios including wind power integration, long-distance transmission, and multi-terminal distribution.

[0061] Example 1

[0062] In this real-time example, a flexible DC single-ended protection method based on transverse differential current is adopted. The method steps are as follows: Figure 1 As shown, it specifically includes:

[0063] S1. Establish an equivalent network model of the flexible DC system under fault conditions, and model the transverse differential current based on the equivalent network model, and output the transverse differential current modeling results.

[0064] S2. Construct a single-ended fault identification mechanism with a single-ended protection factor based on the results of transverse differential flow modeling;

[0065] S3. In each sampling period, the current signals on both sides of the bridge arm are collected in real time, and the instantaneous value of the transverse differential current at the current moment is calculated from the current signals on both sides of the bridge arm.

[0066] S4. The single-end protection factor is calculated from the instantaneous value of the transverse differential current;

[0067] S5. Based on the single-ended protection factor, use the single-ended fault identification mechanism to determine whether there is an internal fault. If there is, trigger the protection logic and output the corresponding protection control signal; otherwise, jump to step S3 and maintain the monitoring state.

[0068] The specific implementation process of this method includes:

[0069] Step 01. Establish a modular multilevel converter fault modeling structure:

[0070] First, to characterize the transient behavior of the flexible DC transmission system under fault conditions, a time-domain dynamic model of the Modular Multilevel Converter (MMC) needs to be constructed. This model serves as the core support for the differential current criterion of this invention and can reflect the instantaneous response characteristics of the arm current, voltage, and submodule energy within the converter.

[0071] In this scheme, the three-phase MMC submodule structure is considered during modeling. The time-domain equivalent circuit of the modular multilevel converter submodule under fault conditions is as follows: Figure 2 As shown, it includes several sub-modules (SMs) of the upper and lower bridge arms, and bridge arm resistors. Bridge arm inductor and DC bus voltage Taking phase A as an example, the output voltage and current of the upper bridge arm are: , The lower bridge arm is , The current output to the AC bus is The current in the upper arm and the current in the lower arm satisfy the current balance relationship:

[0072]

[0073] Based on Kirchhoff's voltage law, the relationship between the voltage of the upper and lower bridge arms and the bus can be written as:

[0074]

[0075] Simultaneously, based on the relationship between the sum of module voltages and capacitor voltage:

[0076]

[0077] in, This refers to the capacitor voltage of a single submodule. The number of submodules in each bridge arm, This represents the total module voltage of the bridge arm. Further substituting into the current balance relationship and simplifying, we obtain the expression for the relationship between the DC side voltage, output current, and bridge arm current:

[0078]

[0079] This expression characterizes the dynamic response of the MMC arm under fault disturbances, possessing dual modeling capabilities for both current disturbances and module voltage responses. Based on the above formula derivation, the MMC converter can be equivalent to a first-order dynamic circuit model. (The text then abruptly shifts to a seemingly unrelated topic: "consisting of series resistors...") ,inductance Module voltage source With voltage source This equivalent model provides a modular foundation for subsequent fault modeling of the entire flexible DC system, and can effectively reflect the time-domain evolution characteristics of key variables during a fault without unfolding complex sub-module-level simulations.

[0080] Step 02. Construct a current differential characteristic model of the flexible DC system under normal conditions:

[0081] Based on the local modeling of the converter, an equivalent circuit model of the entire flexible DC transmission system is further established to analyze the current distribution and symmetry characteristics under normal operating conditions. The flexible DC system consists of two modular multilevel converter stations (MMC1 and MMC2) connected by DC transmission lines. Its simplified equivalent circuit model under non-fault conditions is as follows: Figure 3 As shown; the main electrical parameters are: , These represent the resistance of the transmission line segments respectively; , For line inductance; , This is the grounding capacitor on the converter side; , These are the coupling resistor and inductor in the middle of the line, respectively; , The sum of the voltages of the MMC module submodules at both ends; the central point is the connection point for the DC transmission line.

[0082] Under fault-free conditions, the system has a symmetrical structure, and the voltage and current remain balanced. The currents on the left and right sides of any cross-section should satisfy the following relationship:

[0083]

[0084] This means that the currents at both ends of the system are always equal, and there is no current differential at any sampling time. This relationship can serve as a normal operating condition reference baseline for subsequent differential current judgment. If an internal fault or module failure occurs, the system will disrupt the current symmetry, leading to current differential. This significantly increases the accuracy, thereby enabling fault identification and section determination. In the formula, For forward current transmission, This represents the reverse transmission current. This relationship reflects that, under conditions of balanced system structure and stable operation, a DC line should not exhibit transverse differential current at any point in time. The current symmetry characteristic model constructed in this step provides a theoretical basis for establishing fault criteria based on "transverse differential current," and also reveals the dynamic balance mechanism of a flexible DC system under normal conditions, serving as a core benchmark for differential protection.

[0085] Step 03. Establish the equivalent network model and differential current criterion expression for the flexible DC system under fault conditions:

[0086] Based on the normal operating condition modeling of the flexible DC transmission system, this scheme further constructs an equivalent network model of the system under fault scenarios to characterize the current response during a single-pole grounding fault. Considering typical fault points, the system is divided into multiple electrical subnets, introducing variables such as converter modulation factors and total voltage of submodules. Combining the relationship between branch impedance and node voltage, Kirchhoff's current law and voltage law equations under fault conditions are established. The equivalent model is composed of a branch impedance matrix coupled with node voltage vectors, and its output is the steady-state voltage response of each key node. Based on this modeling result, the equivalent current difference between the left and right ends of the fault point is defined as a criterion. The lateral differential current is expressed as the difference between the node voltage difference and the ratio of the equivalent impedance.

[0087]

[0088] in , , and This indicates the voltage at nodes near the fault point; The equivalent impedance of the line; This indicates the transverse differential current that forms after a fault occurs; This represents the normalized relative position of the fault point within the entire line segment, with a value ranging from 0 to 1. These are the positive and negative equivalent currents on both sides of the bridge arm, respectively. The current difference forms the physical basis for judging internal faults. Under normal or external fault conditions, this differential current should be approximately zero; when an internal fault occurs, this difference deviates significantly from zero, providing a basis for subsequent protection mechanism construction.

[0089] Step 04. Extract differential current response features and construct a single-ended fault identification mechanism:

[0090] To achieve rapid fault detection and identification in flexible DC systems under communication-free conditions, this step constructs a protection action criterion based on the aforementioned transverse differential current modeling results. The response characteristics of the transverse differential current are as follows: when an internal fault is triggered, the value increases rapidly; while under external fault or normal conditions, its fluctuation amplitude is limited. Therefore, a single-ended protection factor can be constructed by accumulating its amplitude within a certain sampling time window. The number of sampling points within the time window is set to N, and the instantaneous value of the transverse differential current at each moment is recorded. Then, a one-end protection factor S can be constructed, and its expression is:

[0091]

[0092] Where S represents the cumulative amplitude of differential current fluctuation within the set time window. S is then compared with a preset action threshold. The system compares the values; if S exceeds the threshold, the fault is determined to be located in the section under the jurisdiction of this terminal; otherwise, the monitoring status is maintained. This method does not rely on current direction determination and requires no communication coordination, making it particularly suitable for practical operating scenarios with weak current or limited communication.

[0093] Step 05. Build and deploy a robust protection mechanism for the application:

[0094] Building upon the completion of lateral differential current modeling and the establishment of a single-ended fault identification mechanism, to ensure the practicality and reliability of this solution in flexible DC transmission systems, it is necessary to further construct a protection application mechanism with real-time judgment and disturbance rejection capabilities, and achieve system-level deployment and coordinated response. Specifically, the protection system operation logic is designed as follows:

[0095] 1) During each sampling period, the system collects the current signals on both sides of the bridge arm in real time and calculates the corresponding node voltage based on the known structural parameters;

[0096] 2) Calculate the transverse differential current index at the current moment based on the relationship between the node voltage difference and the equivalent impedance;

[0097] 3) Set a time sliding window, perform absolute value integration on the transverse differential flow, extract the continuous fluctuation amplitude, and form a single-end protection factor;

[0098] 4) Compare the single-ended protection factor with the preset threshold. If the limit is exceeded, the protection logic is triggered, and corresponding control signals such as circuit breaking and blocking are output.

[0099] 5) The system has local independent decision-making capabilities, does not rely on communication coordination, and can adapt to extreme engineering conditions such as non-full selection communication and weak current channels, ensuring the stable and reliable execution of protection strategies.

[0100] Through a five-level progressive mechanism of "modeling, criterion construction, time-domain feature extraction, single-end identification, and local protection," this solution realizes an intelligent protection method for flexible DC transmission systems that requires no communication coordination, has rapid response capabilities, and is highly sensitive to typical internal faults. It has good engineering deployability and cross-scenario adaptability, and can be widely applied in complex DC transmission projects such as offshore wind power transmission and high-voltage DC interconnection, providing key guarantees for system operation safety.

[0101] In this scheme, a differential current measurement criterion is proposed based on the lateral differential current of the upper and lower bridge arms of the converter. The weak asymmetric component between the upper and lower bridge arms is extracted in the early stage of the fault, so as to realize the local identification of the fault polarity.

[0102] By introducing the trend of module capacitor voltage change as an auxiliary quantity, and utilizing the dynamic energy change characteristics of submodules, the ability to identify current flow direction and fault sections is enhanced.

[0103] A joint criterion system of differential current signal and module energy information is constructed to realize local fault identification of flexible DC transmission system that does not rely on communication and current direction information.

[0104] The method has good topology adaptability and robustness, and is suitable for relay protection applications of flexible DC systems under complex operating conditions such as multi-terminal, long-distance, and new energy access.

[0105] Example 2

[0106] In this embodiment, a flexible DC single-ended protection system based on transverse differential current is adopted. The system includes a fault network modeling and differential current modeling module, a single-ended fault identification mechanism construction module, a differential current data acquisition and calculation module, and a fault judgment and protection control module.

[0107] The fault network modeling and differential current modeling modules are used to establish an equivalent network model of the flexible DC system under fault conditions, and to model the transverse differential current based on the equivalent network model, and output the transverse differential current modeling results.

[0108] The single-ended fault identification mechanism construction module is used to construct a single-ended fault identification mechanism containing a single-ended protection factor based on the results of transverse differential current modeling.

[0109] The differential current data acquisition and calculation module is used to acquire the current signals on both sides of the bridge arm in real time during each sampling period, calculate the instantaneous value of the transverse differential current at the current moment from the current signals on both sides of the bridge arm, and calculate the single-end protection factor from the instantaneous value of the transverse differential current.

[0110] The fault determination and protection control module is based on a single-ended protection factor and uses a single-ended fault identification mechanism to determine whether there is an internal fault. If there is, the protection logic is triggered and the corresponding protection control signal is output; otherwise, the monitoring state is maintained and the differential current data acquisition and calculation stage is executed.

[0111] The specific implementation of this system is the same as in Example 1.

[0112] The specific process for applying this system to single-ended protection of flexible DC systems is as follows: Figure 4 As shown, Figure 4 The system comprises a basic modeling layer, a fault criterion construction layer, a data acquisition and processing layer, a decision-making and execution layer, and a final deployment layer.

[0113] 1. Basic Modeling Layer:

[0114] Converter and system structural parameter modeling: As the starting point of the process, quantitative modeling of the structural and electrical parameters of the converter and flexible DC system is completed, providing basic support for subsequent analysis.

[0115] This module is divided into two parallel sub-processes:

[0116] Establish a modular converter arm electrical model: focus on the electrical characteristics of the core unit of the converter, and characterize the dynamic response of components such as sub-modules and arm impedance.

[0117] Construct the equivalent circuit of the DC system under normal operating conditions: describe the current and voltage balance relationship when the system is fault-free, and use it as a benchmark for fault judgment.

[0118] 2. Fault Criterion Construction Layer:

[0119] DC network modeling and transverse differential current criterion construction under fault conditions: Based on the previous normal operating condition model, the equivalent model of DC network under fault scenarios is derived, and then the transverse differential current (the difference in current between the two sides of the bridge arm or line) is defined as the core criterion for fault identification, clarifying the quantitative correlation between differential current and fault.

[0120] 3. Data Acquisition and Processing Layer:

[0121] Real-time acquisition of bridge arm current and conversion of key node voltage: The converter bridge arm current is acquired in real time by sensors, and the voltage of key nodes is calculated by combining system parameters, providing raw data for differential current calculation.

[0122] The mechanism for judging the integral amplitude of the transverse differential current within the sliding time window: For real-time differential current data, the sliding time window technique is used to calculate the integral amplitude of the differential current, thereby quantifying the severity and persistence characteristics of the fault.

[0123] 4. Decision-making and execution level:

[0124] This layer contains three parallel decision branches, covering all operational scenarios:

[0125] If the differential current amplitude exceeds the threshold, a blocking or short-circuit command will be issued. If the differential current integral amplitude exceeds the preset fault threshold, it is determined to be an internal fault, and the protection action will be triggered immediately.

[0126] If the differential current amplitude does not exceed the threshold, continue monitoring the differential current in the sliding window: if the differential current is within the normal range, continue to collect and analyze the differential current data of subsequent windows to maintain real-time monitoring of the system.

[0127] Abnormal signal identification and channel rejection mechanism: In response to electromagnetic interference, data anomalies, and other situations, identification logic is designed to reject invalid or interfering signals and avoid misjudgment.

[0128] 5. Final Deployment Layer:

[0129] Build local closed-loop protection logic and achieve independent operation and deployment: Integrate all preceding links to form local closed-loop protection logic. The system does not rely on external communication and can independently complete fault identification and protection execution, adapting to engineering scenarios with limited or weak communication.

[0130] In summary, by setting up fault network modeling and differential current modeling modules, single-ended fault identification mechanism construction modules, differential current data acquisition and calculation modules, and fault judgment and protection control modules, this system can flexibly adapt to multi-terminal flexible DC systems—without needing to redesign core modules due to topology changes, only the modeling parameters need to be adjusted. This solves the problems of existing protection configurations relying on engineering experience and lacking universality and scalability, achieving the beneficial effect of unified promotion and deployment in complex access scenarios such as offshore wind power transmission and multi-terminal DC interconnection.

[0131] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and these modifications or substitutions should all be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A flexible DC single-ended protection method based on transverse differential current, characterized in that, The method includes the following steps: S1, establishing an equivalent network model of the flexible DC system under fault conditions, and modeling the transverse differential current based on the equivalent network model, outputting the transverse differential current modeling results; S2, constructing a single-ended fault identification mechanism containing a single-ended protection factor based on the transverse differential current modeling results; S3, in each sampling period, acquiring the current signals on both sides of the bridge arm in real time, and calculating the instantaneous value of the transverse differential current at the current moment from the current signals on both sides of the bridge arm; S4, calculating the single-ended protection factor from the instantaneous value of the transverse differential current; S5, based on the single-ended protection factor, using the single-ended fault identification mechanism to determine whether there is an internal fault. If there is, the protection logic is triggered, and the corresponding protection control signal is output; otherwise, the process jumps to step S3 and maintains the monitoring state. The specific formula for the lateral differential flow modeling result is as follows: in, and This indicates the voltage at nodes near the fault point; The equivalent impedance of the line; This indicates the transverse differential current that forms after a fault occurs; This represents the normalized relative position of the fault point in the entire line segment, with a value range of [0,1]. These are the positive and negative equivalent currents on both sides of the bridge arm, respectively.

2. The flexible DC single-ended protection method based on transverse differential current according to claim 1, characterized in that, The specific process of establishing the equivalent network model of the flexible DC system under fault conditions in S1 includes: Construct the first model; Based on the first model, a second model is further constructed; Based on the first and second models, considering the fault points, the flexible DC system is divided into multiple electrical subnets. Variable information including converter modulation factors and total voltage of submodules is introduced, and combined with the relationship between branch impedance and node voltage, an equivalent network model of the flexible DC system under fault conditions is further constructed. The first model is the dynamic response model of the bridge arm of the modular multilevel converter under fault disturbance; the second model is the current differential characteristic model of the flexible DC system under normal conditions.

3. The flexible DC single-ended protection method based on transverse differential current according to claim 2, characterized in that, The specific construction process of the bridge arm dynamic response model of the modular multilevel converter under fault disturbance includes: Establish the current balance relationship between the upper arm current and the lower arm current; Based on Kirchhoff's voltage law, the relationship between the upper and lower bridge arm currents and the bus voltage is established; and the relationship between the sum of the module voltages and the capacitor voltage is established. Substituting the relationships between the upper and lower arm currents and the bus voltage, and the relationship between the sum of the module voltages and the capacitor voltage, into the current balance relationship and simplifying it, we obtain the relationship between the DC side voltage and the output current and the arm current, which is the dynamic response model of the arm of the modular multilevel converter under fault disturbance.

4. The flexible DC single-ended protection method based on transverse differential current according to claim 3, characterized in that, The specific formula for the dynamic response model of the bridge arm of the modular multilevel converter under fault disturbance is as follows: in, This is the DC bus voltage; For the bridge arm resistance; For bridge arm inductance, Output current for the lower bridge arm; This refers to the output voltage of the lower bridge arm. Output current for the upper bridge arm; Upper bridge arm output voltage; This is the current output to the AC bus. This refers to the capacitor voltage of a single submodule. Number of submodules in each bridge arm; This indicates the total module voltage of the bridge arm.

5. The flexible DC single-ended protection method based on transverse differential current according to claim 1, characterized in that, The single-end protection factor in S2 is calculated from the instantaneous value of the transverse differential flow. First, a time sliding window is set. Within this time sliding window, the absolute value of all instantaneous values ​​of the transverse differential flow is integrated, and the continuous fluctuation amplitude is extracted to form the single-end protection factor.

6. The flexible DC single-ended protection method based on transverse differential current according to claim 5, characterized in that, The specific formula for the single-ended protection factor is as follows: Where S represents the cumulative amplitude of differential current fluctuation within the set time window. The number of sampling points within the set time window; for i The instantaneous value of the transverse differential flow at time t; A specific moment within the set time window; The preset action threshold.

7. A flexible DC single-ended protection method based on transverse differential current according to claim 5, characterized in that, The single-ended fault identification mechanism in S2 is specifically as follows: Set the action threshold of the single-ended protection factor and compare the single-ended protection factor with the preset action threshold; if the single-ended protection factor is greater than the preset action threshold, it is determined that there is an internal fault. Conversely, if the condition is not met, it is determined that there is no internal fault.

8. A flexible DC single-ended protection method based on transverse differential current according to claim 1, characterized in that, The protection control signals in S5 include circuit breaking control signals and interlocking control signals.

9. A flexible DC single-ended protection system based on transverse differential current, characterized in that, The system operates using a flexible DC single-ended protection method based on transverse differential current as described in any one of claims 1-8. The system includes a fault network modeling and differential current modeling module, a single-ended fault identification mechanism construction module, a differential current data acquisition and calculation module, and a fault determination and protection control module. The fault network modeling and differential current modeling modules are used to establish an equivalent network model of the flexible DC system under fault conditions, and to model the transverse differential current based on the equivalent network model, and output the transverse differential current modeling results. The single-ended fault identification mechanism construction module is used to construct a single-ended fault identification mechanism containing a single-ended protection factor based on the results of transverse differential flow modeling. The differential current data acquisition and calculation module is used to acquire the current signals on both sides of the bridge arm in real time during each sampling period, calculate the instantaneous value of the transverse differential current at the current moment from the current signals on both sides of the bridge arm, and calculate the single-end protection factor from the instantaneous value of the transverse differential current. The fault determination and protection control module is based on a single-ended protection factor and uses a single-ended fault identification mechanism to determine whether there is an internal fault. If there is, the protection logic is triggered and the corresponding protection control signal is output. Conversely, if the condition is not met, the monitoring state will be maintained, and the process will jump to the differential data acquisition and calculation stage.

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