A direct current line single-end protection method based on modulus transformation and current curvature

Abstract

The application discloses a DC line single-end protection method based on modulus transformation and current curvature, and relates to the field of power grid protection. The method comprises the following steps: based on a flexible DC power transmission system, a fault equivalent model under a DC side fault condition is constructed by adopting a source-grid collaborative current limiting strategy, and a fault current limiting and line protection coordination scheme is proposed; time domain expressions of fault current modulus under different fault types are derived; modulus current waveform curves of each fault type are drawn, a plurality of sampling points are uniformly set on the modulus current waveform curves, and discrete current waveform curvatures are calculated; a fault polar selection criterion is generated, and a fault starting criterion is constructed; according to the discrete current waveform curvatures, external faults and internal faults are identified, and according to the determination result of the fault polar selection, a single-end protection scheme of the flexible DC power transmission system is executed. The application can enhance the coverage ability of the protection criterion to different fault types, and improve the reliability and adaptability of the protection action.

Description

Technical Field

[0001] This invention relates to the field of power grid protection, specifically to a single-ended protection method for DC lines based on modulus transformation and current curvature. Background Technology

[0002] Modular multilevel converter-based high-voltage direct current transmission technology (MMC-HVDC) has been widely used in large-scale new energy access scenarios such as photovoltaic, onshore and offshore wind power due to its advantages such as flexible and controllable operation, long transmission distance, low line loss, no commutation failure, and ability to support large-scale renewable energy grid connection and multi-terminal networking.

[0003] In flexible DC power grids, the rapid and reliable clearing of DC line faults is crucial for ensuring the safe and stable operation of the system. Compared to AC systems, DC systems have weaker damping, and fault currents lack a natural zero-crossing point. After a fault occurs, the current rises rapidly and its amplitude increases quickly. If faults cannot be identified and isolated in a timely manner, they can cause severe electrical stress on converter valves, DC lines, DC circuit breakers, and related primary equipment. This can also lead to converter lockout, resulting in power interruption and an expansion of the fault area. Therefore, DC line protection typically needs to complete fault initiation, fault zone / external identification, and fault pole selection within a very short time to meet the requirements of rapid fault clearing in flexible DC power grids.

[0004] Currently, the main protection of flexible DC lines mostly adopts protection methods based on single-ended electrical quantities. These methods utilize the characteristics of local voltage, current, and their transient changes to achieve fault identification, offering advantages such as no communication dependency, fast response speed, and relatively simple engineering implementation. For high-voltage flexible DC transmission lines, the main protection typically requires reliable operation within approximately 3 modulus fault current milliseconds. However, for metallic return bipolar DC transmission systems, due to their more complex line structure, grounding methods, and inter-pole coupling relationships, in addition to conventional pole-to-ground faults and inter-pole faults, special fault types such as pole-to-neutral faults may occur, placing higher demands on fault differentiation, fault pole selection, and protection criterion construction. Therefore, there is an urgent need to construct a fast and reliable single-ended protection method suitable for metallic return bipolar DC systems to improve the accuracy and adaptability of protection operations under complex fault scenarios.

[0005] Furthermore, to suppress the rapid rise of DC fault current and reduce the electrical stress on equipment such as converters and DC circuit breakers, flexible DC systems are typically equipped with source-side current limiting control, fault current limiters, or a combination of both. While fault current limiting measures can reduce peak fault current and improve equipment safety margins to some extent, their intervention can also alter the changing patterns of transient fault voltage and current, weakening the initial characteristics and differences of the fault. This can lead to reduced sensitivity of traditional protection criteria, difficulty in fault identification, and even the risk of protection failure or maloperation. Therefore, considering the impact of fault current limiting, constructing a fast protection method suitable for metallic return bipolar flexible DC transmission lines has become a critical technical problem that urgently needs to be solved. Summary of the Invention

[0006] To address the aforementioned shortcomings of existing technologies, this invention provides a single-ended protection method for DC lines based on modulus transformation and current curvature, which solves the problem of weakening protection characteristics by fault current limiting in flexible DC grids, as well as the inter-pole coupling problem in metal return bipolar DC lines.

[0007] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows: A method for single-ended protection of DC lines based on modulus transformation and current curvature is provided, comprising the following steps: S1: Based on the flexible DC transmission system, a fault equivalent model under DC-side fault conditions is constructed using a source-grid coordinated current limiting strategy. The fault characteristics of the flexible DC transmission system under fault current limiting and its impact on line protection are analyzed, and a coordinated scheme for fault current limiting and line protection is proposed.

[0008] S2: Construct a bipolar DC transmission system with metal return based on a flexible DC transmission system, and construct a pole mode transformation matrix for the bipolar DC transmission system based on independent operation of positive and negative poles to decouple the lines and derive the time-domain expression of the fault current modulus under different fault types. S3: Based on the time-domain expression of the fault current modulus for different fault types, plot the modulus current waveform curves for each fault type, and uniformly set several sampling points on the modulus current waveform curves. Normalize the current values ​​and times corresponding to the sampling points and calculate the curvature of the discrete current waveform. S4: Obtain the modulus fault current components of the positive and negative lines based on the time-domain expression of the fault current modulus under different fault types, and generate fault pole selection criteria. S5: Based on the voltage regulation coefficients of the MMCs at both ends of the line in the fault equivalent model. K M1 , K M2 The expression is used to obtain the differential of the modulus fault voltage, and a fault initiation criterion is constructed. S6: Determine whether the protection system should start the protection program based on the fault start criteria. When the protection program is started, execute the fault current limiting and line protection coordination scheme to perform fault current limiting. At the same time, identify external and internal faults based on the curvature of discrete current waveforms, and execute the single-ended quantity protection scheme of the flexible DC transmission system based on the fault pole selection determination result.

[0009] Furthermore, the fault characteristics of the flexible DC transmission system under fault current limiting and its impact on line protection are analyzed, specifically including: S11: Constructing an equivalent fault model based on Kirchhoff's Current Law (KCL) and Voltage Law (KVL) with voltage regulation coefficients at the converter stations at both ends of the line. K M1 , K M2 expression; S12: Analyze the fault current as a function of the voltage regulation coefficient when a fault occurs in a flexible DC transmission system and a current limiting strategy is adopted by building a simulation model of a double-ended flexible DC transmission system. K M1 , K M2 The relationship between the short-circuit resistance and the short-circuit resistance is shown in the graph. S13: The influence relationship of fault current when different current limiting strategies are adopted is obtained based on the simulation model of the double-ended flexible DC transmission system; S14: Based on the voltage regulation coefficient K M1 , K M2 The effects of fault characteristics on line protection are analyzed using expressions, change relationship diagrams, and influence relationship diagrams.

[0010] Furthermore, the coordination scheme between fault current limiting and line protection includes: The timing sequence of actions based on the coordinated scheme of fault current limiting and line protection includes, in sequence, the time of fault occurrence. t 0. When the protection sends an action signal t 动作 Fault current limiting action time t 限流 DC circuit breaker operating time t DCCB ; When the fault occurs, at the time of the fault occurrence t The protection device first uses the transient information at the initial stage of the fault to complete the fault identification; After fault identification and fulfillment of protection criteria, the protection will issue an action signal at the specified time. t 动作Simultaneously, start commands are sent to the fault current limiter (FCL) and the DC circuit breaker (DCCB), and the fault current limiter (FCL) begins operation; the DC circuit breaker (DCCB) starts operation at the moment of DC circuit breaker tripping. t DCCB Complete the interruption and finally remove the fault.

[0011] Furthermore, the fault types of flexible DC transmission systems include: positive-to-ground fault PG, negative-to-ground fault NG, positive-to-return fault PM, negative-to-return fault NM, and positive-to-negative fault PN.

[0012] Furthermore, the bipolar DC transmission system is based on a pole-mode transformation matrix that operates independently of the positive and negative poles. for: ; in, x For the electrical quantities of a bipolar DC transmission system line, x + , x m , x These are the electrical quantities for the positive terminal, the metallic return line, and the negative terminal, respectively. x 1. x 2. x 0 represents the 1-mode, 2-mode, and 0-mode components, respectively; The 1-mode component represents the case where the positive electrode operates independently, the 2-mode component represents the case where the negative electrode operates independently, and the 0-mode component represents the grounding component.

[0013] Furthermore, the method for calculating the curvature of discrete current waveforms is as follows: ; ; ; in, This represents the tangent angle corresponding to two sampling points on the modulus current waveform curve. i Number the sampling points. The first i +1、 i The normalized value of the current value corresponding to each sampling point l This is the normalized value of the discrete time interval between adjacent sampling points on the time coordinate. The arc lengths corresponding to the two sampling points. The curvature of the discrete current waveform at the sampling point.

[0014] Furthermore, the fault start criterion is: ; in, These are the voltage protection start thresholds for mode 1 and mode 2, respectively. u 1. u 2 represents the positive and negative operating voltages, respectively. t For time, This is the reliability coefficient.

[0015] Fault start-up logic: As long as any fault voltage differential If the calculated value meets the fault start criterion, then the protection program is started.

[0016] Furthermore, the method for identifying external and internal faults based on the curvature of discrete current waveforms is as follows: By discrete current waveform curvature Obtain the curvature of the mode 1 and mode 2 current waveforms K 1. K 2. If the calculated value of the curvature of any one of the current waveforms satisfies the fault identification criterion, it is considered that a fault has occurred within the zone; otherwise, it is considered that a fault has occurred outside the zone. ; in, These represent the curvature of the current waveforms in modes 1 and 2 during the most severe fault outside the zone. K set1 , K set2 These are the set threshold values ​​for the curvature tuning of the 1st and 2nd mode current waveforms, respectively.

[0017] The beneficial effects of this invention are as follows: The single-ended quantity protection designed in this invention serves as the main line protection. Compared to backup protection, the main protection can identify and operate on faults more quickly, thereby minimizing the impact of the fault. Based on its role as the main protection, a coordinated approach is adopted, where current limiting after fault identification and synchronous operation of the circuit breaker are implemented. Because the main protection operates rapidly and the fault current rise is limited, the required duration of the current limiting component is relatively low. Therefore, the coordinated approach of fault current limiting and circuit breaker activation satisfies the fault current suppression requirements under the rapid operation of the main protection while minimizing the impact of current limiting on the protection, thus meeting the requirements of both fault current limiting and line protection.

[0018] The earliest electrical quantity abrupt change in a line fault usually manifests as a sharp change in the shape of its local current waveform. Therefore, the curvature of the current waveform curve can enable rapid fault identification. This invention can enhance the coverage of protection criteria for different fault types and improve the reliability and adaptability of protection actions. Attached Figure Description

[0019] Figure 1 Schematic diagram of a flexible DC transmission system; Figure 2Schematic diagram of the DC fault current limiting equivalent model; Figure 3 Graph showing the relationship between fault current variation under different voltage regulation coefficients and short-circuit resistances; Figure 4 The influence of different current limiting strategies on fault current of modulus fault current is shown in the figure. Figure 5 Timing diagram of fault current limiting and protection coordination for modulus fault current; Figure 6 Parameter model diagram of a metal-return bipolar DC system with modulus fault current; Figure 7 Schematic diagram of a bipolar DC transmission line fault with metal return current modulus fault current. Figure 8 Equivalent circuit diagram of each modulus component network of the modulus fault current in the complex frequency domain. Figure 9 Schematic diagram of fault-selective phase plane; Figure 10 A flowchart of a single-ended quantity protection scheme for a flexible DC transmission system. Detailed Implementation

[0020] The specific embodiments of the present invention are described below to enable those skilled in the art to understand the present invention. However, it should be understood that the present invention is not limited to the scope of the specific embodiments. For those skilled in the art, various changes are obvious as long as they are within the spirit and scope of the present invention as defined and determined by the appended claims. All inventions utilizing the concept of the present invention are protected.

[0021] A single-ended protection method for DC lines based on modulus transformation and current curvature includes the following steps: S1: Based on the flexible DC transmission system, a fault equivalent model under DC-side fault conditions is constructed using a source-grid coordinated current limiting strategy. The fault characteristics of the flexible DC transmission system under fault current limiting and its impact on line protection are analyzed, and a coordinated scheme for fault current limiting and line protection is proposed.

[0022] The flexible DC transmission system in this embodiment is as follows: Figure 1 As shown, it consists of a positive line, a negative line, a metallic return line, a converter station (MMC), an AC test system (AC), a current limiting reactor (Current Fault Current Limiting Reactor, CLR), a fault current limiter (FCL), and a DC circuit breaker (DCCB).

[0023] When a fault occurs in a flexible DC transmission system, the converter station's MMC (Multi-Mode Controller) controls the output voltage to step down, working in conjunction with the grid-side fault current limiting device (FCL) to suppress the rise of the fault current. The fault equivalent model under DC-side fault conditions using the source-grid coordinated current limiting strategy is as follows: Figure 2 As shown.

[0024] Figure 2 middle, i 1. i 2 represents the current at both ends of the line; R M1 , L M1 , C M1 , R M2 , L M2 , C M2 These are the equivalent resistance, inductance, and capacitance parameters of the MMC at both ends of the line; L 1. R 1. L 2. R 2 represents the inductance and resistance at both ends of the line; L C1 , L C2 These are the CLR parameters of the current-limiting reactors at both ends of the line; R f Grounding resistance; m For measuring elements; u C1 , u C2 This is the equivalent capacitance voltage across the two ends of the line; u FCL1 , u FCL2 These are the fault current limiters (FCLs) connected at both ends of the line, respectively.

[0025] A fault equivalent model is constructed based on Kirchhoff's Current Law (KCL) and Voltage Law (KVL), and the voltage regulation coefficients of the MMCs at both ends of the line converter station are used. K M1 , K M2 expression; ; in, t For time, u 1. u 2 represents the positive and negative operating voltages, respectively.

[0026] Voltage regulation coefficient K M1 ,K M2 It is used to characterize the degree of reduction in the equivalent output voltage of the DC side of the converter station MMC during a fault, and its value is generally taken as 0.7~1.0.

[0027] This embodiment analyzes the fault current as a function of the voltage regulation coefficient when a fault occurs in a flexible DC transmission system and a current limiting strategy is adopted. This is achieved by building a simulation model of a double-ended flexible DC transmission system. K M1 , K M2 The relationship between the short-circuit resistance and the simulation results is shown in the graph below. Figure 3 As shown; and the influence relationship of fault current when different current limiting strategies are adopted is obtained, such as Figure 4 As shown, based on the voltage regulation coefficient K M1 , K M2 expression, Figure 3 and Figure 4 It is known that current limiting strategies on both the source and grid sides will cause changes in voltage and current on the transmission line, reduce the rate of change and amplitude of fault quantities, and weaken fault characteristics. This may hinder fault identification, affect the reliability and sensitivity of protection, and may even cause protection to fail to operate.

[0028] Considering the impact of fault current limiting on protection performance, a coordinated scheme is proposed to synchronize the action of the fault current limiter's modulo fault current (FCL) and the DC circuit breaker's modulo fault current (DCCB) after fault identification. Fault current limiting is an active control process; the current limiting device can only be put into operation and participate in fault current suppression after the fault is detected. To minimize the impact of the current limiting circuit on protection criterion extraction and operational performance, the fault current limiter's modulo fault current (FCL) and the DC circuit breaker's modulo fault current (DCCB) are configured to share the same set of start signals. This avoids weakening the fault characteristics due to fault current limiting, thus preventing a decrease in the sensitivity and reliability of the protection. However, the current limiting time is relatively short in this case. For systems using hybrid DC circuit breakers (DCCB), there is a mechanical switch action delay (typically about 3ms) during the effective action time of the fault current limiting. Modulo fault current... The timing sequence of the fault current limiting and line protection coordination scheme is as follows: Figure 5 As shown, where, I 0 represents the steady-state operating current of the transmission line before the fault occurred; t 0、 t 动作 , t 限流 , t DCCBThese are the times of fault occurrence, protection activation signal issuance, fault current limiting action, and DC circuit breaker operation. When a fault occurs, the protection device first uses initial transient information to identify the fault. Once the protection criteria meet the operating conditions, it activates the protection at the specified time. t 动作 Simultaneously, start commands are sent to the fault current limiter (FCL) and the DC circuit breaker (DCCB). The fault current limiter (FCL) begins operation, while the DC circuit breaker (DCCB), affected by the mechanical switch operation delay, stops operation at a specific time. t DCCB The fault is interrupted and ultimately cleared. Since the fault current limiting device is activated only after protection identification is completed, its operating period mainly corresponds to the mechanical switching delay process of the DCCB. Therefore, the impact of the current limiting component on the protection criterion extraction and identification results is minimal. Thus, protection identification, fault current limiting, and DCCB interruption form a clear sequential relationship in time, ensuring both the effective use of the original fault transient information by the main protection and the coordinated operation between fault current limiting and fault clearing.

[0029] S2: Construct a bipolar DC transmission system with metal return based on a flexible DC transmission system, and construct a pole mode transformation matrix for the bipolar DC transmission system based on independent operation of positive and negative poles to decouple the fault current, and derive the time-domain expression of the fault current modulus under different fault types.

[0030] In this embodiment, the lumped parameter model of the metal-return bipolar DC transmission system is as follows: Figure 6 As shown, Figure 6 middle, R , L These are the line resistance and inductance, respectively. C , G These are the line-to-ground capacitance and conductance, respectively. M For inter-pole mutual inductance, the subscripts 1 and 2 represent the electrical quantities at the left and right ends of the line, respectively.

[0031] The fault types of bipolar DC transmission systems include the following five types: The diagrams for different fault types are as follows: positive-to-ground fault (PG), negative-to-ground fault (NG), positive-to-return fault (PM), negative-to-return fault (NM), and positive-to-negative fault (PN). Figure 7 As shown. Figure 7 In the diagram, +, m, and - represent the positive terminal, metallic return line, and negative terminal of the circuit, respectively. R f For transition resistance; u f , i f These are the fault point voltage and fault current, respectively.

[0032] like Figure 6 As shown, the addition of a metallic return line in a bipolar DC transmission system makes the inter-pole coupling more complex, causing electrical quantities between different lines to influence each other, which may lead to maloperation of protection systems on non-faulty lines. Therefore, to reduce the coupling effect and simplify the modeling and calculation of subsequent protection algorithms, a new pole-mode transformation matrix is ​​needed to analyze the bipolar DC transmission system with metallic return line.

[0033] The polar mode transformation is a linear transformation method. Its mathematical essence is to transform originally coupled extrema into independent moduli by selecting different basis vectors in a vector space. The transformation matrix from the original basis vectors to the new basis vectors is called the polar mode transformation matrix. Different transformation matrices will yield different results, and different moduli will represent different physical meanings.

[0034] Pole-mode transformation can convert coupled electrical quantities into independent modulus components, thereby achieving line decoupling and simplifying fault analysis. Combined with fault boundary conditions, the modulus domain expression of the fault quantity can be further derived to reveal its evolution and mechanism of action. Modulus characteristics can also provide support for the construction of protection criteria and the design of fault pole selection methods, contributing to further optimization of protection schemes.

[0035] Bipolar DC transmission systems are based on pole mode transformation matrices that operate independently of the positive and negative poles. for: ; in, x For the electrical quantities of a bipolar DC transmission system line, x + , x m , x These are the electrical quantities for the positive terminal, the metallic return line, and the negative terminal, respectively. x 1. x 2. x 0 represents the 1-mode, 2-mode, and 0-mode components, respectively; The 1-mode component represents the case where the positive electrode operates independently, the 2-mode component represents the case where the negative electrode operates independently, and the 0-mode component represents the grounding component.

[0036] Assuming the bipolar DC transmission system remains perfectly symmetrical except at the fault location and can be equivalently represented as a linear network, it satisfies the superposition theorem. Taking a positive-pole ground fault on a single-ended transmission line as an example, after the fault occurs, the converter station MMC can be equivalently represented as a constant voltage source; further, through the pole-mode transformation matrix... The transformation yields the system's modulus component network, and applying Thevenin's theorem to each modulus component network provides a complex frequency domain equivalent circuit oriented towards the fault point, such as... Figure 8 As shown, where: U dc＋ This represents the complex frequency domain quantity of the rated voltage at the fault point before the positive electrode fault. U f , I f These represent the voltage and current complex frequency domain quantities at the fault location, respectively. U f1 , U f2 , U f0 These represent the voltage complex frequency domain components of mode 1, mode 2, and mode 0 after pole-mode transformation, respectively. I f1 , I f2 , I f0 These represent the complex frequency domain components of the current in mode 1, mode 2, and mode 0 after pole-mode transformation, respectively. Z 1∑ , Z 2∑ , Z 0∑ These are the equivalent impedances for mode 1, mode 2, and mode 0, respectively. R f This indicates the fault grounding resistance.

[0037] Based on Kirchhoff's voltage law (KVL), the relationship between the complex frequency domain components of voltage and the complex frequency domain components of current is constructed as follows: ; in, U dc＋ This is the complex frequency domain quantity of the rated voltage at the fault point before the positive electrode fault. U f1 , U f2 , U f0 These are the voltage complex frequency domain components of mode 1, mode 2, and mode 0 after pole-mode transformation, respectively. I f1 , I f2 , I f0 These are the complex frequency domain components of the current in mode 1, mode 2, and mode 0 after pole mode transformation, respectively. Z 1∑ , Z 2∑ , Z 0∑ These are the equivalent impedances for 1-mode, 2-mode, and 0-mode, respectively.

[0038] This embodiment takes a positive-to-ground fault PG as an example to illustrate the derivation process of the modulus fault current expression when a positive-to-ground fault PG occurs. The derivation process of the modulus fault current expression for other fault types is similar. The boundary conditions for a positive-to-ground fault PG in a bipolar DC transmission system are as follows: ; in, U f+ This is the voltage complex frequency domain quantity at the positive terminal fault point. I f+ This is the complex frequency domain quantity of the current at the positive terminal fault point. Let be the complex frequency domain quantity of the current in the metal loop. I f- This is the complex frequency domain quantity of the current at the negative terminal fault point. R f For fault grounding resistance; Establish the boundary conditions, the relationship between the voltage complex frequency domain components and the current complex frequency domain components, and the pole mode transformation matrix for a positive-to-ground fault PG. ,get: ; ; in, These represent the complex frequency domain components of the fault voltage after pole-mode transformation: mode 1, mode 2, and mode 0. These represent the complex frequency domain components of the fault current after pole-mode transformation: mode 1, mode 2, and mode 0. s It is a complex frequency variable.

[0039] Ignoring capacitance to ground and conductance, the equivalent impedance expressions for mode 1, mode 2 and mode 0 are obtained by pole-mode transformation; ; in, R ＋ , L ＋ For the positive terminal circuit resistance and inductance, M +m This is the coupling inductance between the positive terminal line and the metal return line; Combining the above equations and performing an inverse Laplace transform, we obtain the expression for the modulus fault current when a positive-to-ground fault (PG) occurs: ; In the formula: u dc＋ This is the time-domain quantity of the rated voltage at the fault point before the positive electrode fault. These are the time-domain fault currents for mode 1, mode 2, and mode 0, respectively.

[0040] Similarly, based on the boundary conditions and polar mode transformation matrices of each fault type... Using the same method described above, expressions for the fault components of each modulus current under different fault types were derived, and the results are summarized below: Negative electrode to ground fault (NG): ; Positive electrode return line fault PM: ; Negative pole to return line fault NM: ; Positive electrode to negative electrode fault PN: .

[0041] S3: Based on the time-domain expression of the fault current modulus for different fault types, plot the modulus current waveform curves for each fault type, and uniformly set several sampling points on the modulus current waveform curves. Normalize the current values ​​and times corresponding to the sampling points and calculate the curvature of the discrete current waveform.

[0042] The method for calculating the curvature of discrete current waveforms is as follows: ; ; ; in, The tangent angles corresponding to the two sampling points. i Number the sampling points. The first i +1、 i The normalized value of the current value corresponding to each sampling point l This is the normalized value of the discrete time interval between adjacent sampling points on the time coordinate. The arc lengths corresponding to the two sampling points. The curvature of the discrete current waveform at the sampling point.

[0043] The faster the modulus current waveform curve turns within a unit arc length, the more severe its local curvature, and the greater the corresponding curvature; conversely, when the curve approaches a straight line, the tangent direction hardly changes with the arc length, and the curvature approaches zero.

[0044] When a fault occurs within the protection zone of a DC transmission line, the modulus current rises sharply, and the modulus current waveform curve shows significant changes, exhibiting a large curvature. Conversely, when a fault occurs outside the protection zone, the modulus current waveform curve is relatively flat with a smaller curvature. Introducing curvature into the protection of flexible DC transmission lines, the curvature of the modulus current waveform is calculated to reflect the difference in current waveform characteristics when faults occur within or outside the protection zone, thereby identifying faults within and outside the protection zone and designing a single-ended quantitative protection scheme for the transmission line.

[0045] S4: Based on the time-domain expression of the fault current modulus under different fault types, obtain the modulus fault current components of the positive and negative lines, and generate fault pole selection criteria.

[0046] In this embodiment, based on the response differences in the 1-mode, 2-mode, and 0-mode currents under different fault types, the modulus fault current components obtained from the time-domain expressions of the fault current modulus under different fault types are shown in Table 1 below: Table 1 Modular Fault Current Components As shown in Table 1, when different faults occur in the flexible DC transmission system, the modulus fault current exhibits different characteristics in terms of magnitude, polarity, and variation. Therefore, it can be used as a basis for fault type identification and fault pole selection. Table 1 shows that the 0-mode fault current is mainly used to characterize pole-to-ground faults: under a positive pole-to-ground fault (PG), its modulus fault current component increases in the positive direction; under a negative pole-to-ground fault (NG), it increases in the negative direction. For pole-to-metal loop faults and inter-pole faults, the 0-mode fault current does not produce a significant response. Further analysis of the 1-mode and 2-mode fault currents reveals that the 2-mode fault current is zero for a positive pole-to-loop fault (PM), and the 1-mode fault current is zero for a negative pole-to-loop fault (NM). However, both the 1-mode and 2-mode fault currents show effective responses under a positive pole-to-negative fault (PN).

[0047] This demonstrates that different fault types exhibit distinguishable behavior in the modulus fault current component, providing a basis for constructing fault pole selection criteria. Therefore, the pole selection criteria are constructed as shown in Table 2, and the fault pole selection phase plane is as follows: Figure 9 As shown.

[0048] Table 2. Criteria for Fault Polarity Selection Based on Modulus Fault Current in, ; For reliability coefficient, These are the set current thresholds for mode 1, mode 2, and mode 0, used to distinguish between normal fluctuations and fault responses. In this embodiment, the current thresholds... Δ 1. Δ 2. The rated current values ​​for both Mode 1 and Mode 2 are taken as 10% of the current values ​​during normal system operation. Since the rated current value for Mode 0 is 0 during normal system operation, the current threshold is... Δ A value of 0 can be obtained, and the current threshold is selected based on the simulation model and typical operating conditions. Δ 0 is 0.05kA.

[0049] S5: Based on the voltage regulation coefficients of the MMCs at both ends of the line in the fault equivalent model. K M1 , K M2 The expression is used to obtain the differential of the modulus fault voltage, and a fault initiation criterion is constructed. ; in, These are the voltage protection start thresholds for Mode 1 and Mode 2, respectively.

[0050] Fault start-up logic: As long as any fault voltage differential If the calculated value satisfies the above fault initiation criteria, then the protection program will be activated.

[0051] This invention selects the differential of the modulus fault voltage as the fault initiation criterion and adopts a maximum value type setting method, selecting the minimum value in the maximum value set of each simulation data as the setting basis to set the fault initiation criterion.

[0052] S6: Determine whether the protection system should start the protection program based on the fault start criteria. When the protection program is started, execute the fault current limiting and line protection coordination scheme to perform fault current limiting. At the same time, identify external and internal faults based on the curvature of discrete current waveforms, and execute the single-ended quantity protection scheme of the flexible DC transmission system based on the fault pole selection determination result.

[0053] In principle, the earliest electrical quantity change in a line fault usually manifests as a sharp change in its local waveform shape. Therefore, the curvature of the current waveform can enable rapid fault identification. Simultaneously, as a protection criterion, it must also meet reliability requirements. Therefore, selecting a larger reliability coefficient ensures the protection's anti-interference capability and reliability. The discrete current waveform curvature calculated in step S3... Obtain the curvature of the mode 1 and mode 2 current waveforms K 1. K 2. The protection criteria are designed as follows: when the calculated value of the curvature of any current waveform satisfies the fault identification criterion, it is considered that an in-zone fault has occurred; otherwise, it is considered an out-of-zone fault.

[0054] ; in, These represent the curvature of the current waveforms in modes 1 and 2 during the most severe fault outside the zone. K set1 , K set2 These are the set threshold values ​​for the curvature tuning of the 1st and 2nd mode current waveforms, respectively.

[0055] Based on the fault initiation, fault current limiting, fault identification, and fault polarity selection results described above, the process for designing a single-ended quantity protection scheme for a flexible DC transmission system is as follows: Figure 10 As shown, when the protection system detects a fault, it starts the protection program. Based on the fault identification and fault polarity selection results, it acts simultaneously to improve the protection speed. If and only if both identify the fault type and fault polarity, the fault current limiting program and the DC circuit breaker (DCCB) are started to achieve fault current limiting and fault isolation.

Claims

1. A single-ended protection method for DC lines based on modulus transformation and current curvature, characterized in that, Includes the following steps: S1: Based on the flexible DC transmission system, a fault equivalent model under DC-side fault conditions is constructed using a source-grid coordinated current limiting strategy. The fault characteristics of the flexible DC transmission system under fault current limiting and its impact on line protection are analyzed, and a coordinated scheme for fault current limiting and line protection is proposed. S2: Construct a bipolar DC transmission system with metal return based on a flexible DC transmission system, and construct a pole mode transformation matrix for the bipolar DC transmission system based on independent operation of positive and negative poles to decouple the lines and derive the time-domain expression of the fault current modulus under different fault types. S3: Based on the time-domain expression of the fault current modulus for different fault types, plot the modulus current waveform curves for each fault type, and uniformly set several sampling points on the modulus current waveform curves. Normalize the current values ​​and times corresponding to the sampling points and calculate the curvature of the discrete current waveform. S4: Obtain the modulus fault current components of the positive and negative lines based on the time-domain expression of the fault current modulus under different fault types, and generate fault pole selection criteria. S5: Based on the voltage regulation coefficients of the MMCs at both ends of the line in the fault equivalent model. K M1 , K M2 The expression is used to obtain the differential of the modulus fault voltage, and a fault initiation criterion is constructed. S6: Determine whether the protection system should start the protection program based on the fault start criteria. When the protection program is started, execute the fault current limiting and line protection coordination scheme to perform fault current limiting. Meanwhile, based on the curvature of the discrete current waveform, faults outside and inside the zone are identified, and based on the fault polarity determination results, a single-ended quantity protection scheme for the flexible DC transmission system is implemented.

2. The DC line single-ended protection method based on modulus transformation and current curvature according to claim 1, characterized in that, The analysis of the fault characteristics of the flexible DC transmission system under fault current limiting and its impact on line protection specifically includes: S11: Constructing an equivalent fault model based on Kirchhoff's Current Law (KCL) and Voltage Law (KVL) with voltage regulation coefficients at the converter stations at both ends of the line (MMC). K M1 , K M2 expression; S12: Analyze the fault current as a function of the voltage regulation coefficient when a fault occurs in a flexible DC transmission system and a current limiting strategy is adopted by building a simulation model of a double-ended flexible DC transmission system. K M1 , K M2 The relationship between the short-circuit resistance and the short-circuit resistance is shown in the graph. S13: The influence relationship of fault current when different current limiting strategies are adopted is obtained based on the simulation model of the double-ended flexible DC transmission system; S14: Based on the voltage regulation coefficient K M1 , K M2 The effects of fault characteristics on line protection are analyzed using expressions, change relationship diagrams, and influence relationship diagrams.

3. The DC line single-ended protection method based on modulus transformation and current curvature according to claim 1, characterized in that, The fault current limiting and line protection coordination scheme includes: The timing sequence of actions based on the coordinated scheme of fault current limiting and line protection includes, in sequence, the time of fault occurrence. t 0. When the protection sends an action signal t 动作 Fault current limiting action time t 限流 DC circuit breaker operating time t DCCB ; When the fault occurs, at the time of the fault occurrence t The protection device first uses the transient information at the initial stage of the fault to complete the fault identification; After fault identification and fulfillment of protection criteria, the protection will issue an action signal at the specified time. t 动作 Simultaneously, start commands are sent to the fault current limiter (FCL) and the DC circuit breaker (DCCB), and the fault current limiter (FCL) begins operation; the DC circuit breaker (DCCB) starts operation at the moment of DC circuit breaker tripping. t DCCB Complete the interruption and finally remove the fault.

4. The DC line single-ended protection method based on modulus transformation and current curvature according to claim 1, characterized in that, The fault types of the flexible DC transmission system include: positive to ground fault PG, negative to ground fault NG, positive to return line fault PM, negative to return line fault NM, and positive to negative fault PN.

5. The DC line single-ended protection method based on modulus transformation and current curvature according to claim 1, characterized in that, The bipolar DC transmission system is based on a pole mode transformation matrix that operates independently of the positive and negative poles. for: ； in, x For the electrical quantities of a bipolar DC transmission system line, x + , x m , x These are the electrical quantities for the positive terminal, the metallic return line, and the negative terminal, respectively. x 1. x 2. x 0 represents the 1-mode, 2-mode, and 0-mode components, respectively; The 1-mode component represents the case where the positive electrode operates independently, the 2-mode component represents the case where the negative electrode operates independently, and the 0-mode component represents the grounding component.

6. The DC line single-ended protection method based on modulus transformation and current curvature according to claim 1, characterized in that, The method for calculating the curvature of discrete current waveforms is as follows: ； ； ； in, This represents the tangent angle corresponding to two sampling points on the modulus current waveform curve. i Number the sampling points. The first i +1、 i The normalized value of the current value corresponding to each sampling point l This is the normalized value of the discrete time interval between adjacent sampling points on the time coordinate. The arc lengths corresponding to the two sampling points. The curvature of the discrete current waveform at the sampling point.

7. The DC line single-ended protection method based on modulus transformation and current curvature according to claim 5, characterized in that, The fault initiation criterion is as follows: ； in, These are the voltage protection start thresholds for mode 1 and mode 2, respectively. u 1. u 2 represents the positive and negative operating voltages, respectively. t For time, This is the reliability coefficient. Fault start-up logic: As long as any fault voltage differential If the calculated value meets the fault start criterion, then the protection program is started.

8. The DC line single-ended protection method based on modulus transformation and current curvature according to claim 6, characterized in that, The method for identifying external and internal faults based on the curvature of discrete current waveforms is as follows: By discrete current waveform curvature Obtain the curvature of the mode 1 and mode 2 current waveforms K 1. K 2. If the calculated value of the curvature of any one of the current waveforms satisfies the fault identification criterion, it is considered that a fault has occurred within the zone; otherwise, it is considered that a fault has occurred outside the zone. ； in, These represent the curvature of the current waveforms in modes 1 and 2 during the most severe fault outside the zone. K set1 , K set2 These are the set threshold values ​​for the curvature tuning of the 1st and 2nd mode current waveforms, respectively.

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