A novel line protection method for flexible power transmission system based on parameter identification

Abstract

This invention discloses a novel line protection method for flexible power transmission systems based on parameter identification. The line protection method includes the following steps: acquiring fault transient voltage and current sampling data at the protection installation points on both sides of the flexible DC transmission line; performing phase-mode transformation on the sampling data to extract line-mode components; and calculating the measured impedance at the protection installation points in the forward and reverse directions; constructing general voltage and current parameter identification equations based on the measured impedances at the protection installation points in the forward and reverse directions; solving the constructed voltage and current parameter identification equations to obtain the resistance identification values ​​and inductance identification values ​​corresponding to the protection installation points on the rectifier side and inverter side of the line, respectively; and determining whether the resistance identification values ​​and inductance identification values ​​on both sides meet the forward fault identification criterion. If they do, the line is determined to be in-zone fault; otherwise, it is determined to be out-of-zone fault.

Description

Technical Field

[0001] This invention relates to the field of line protection technology, and in particular to a novel line protection method for flexible power transmission systems based on parameter identification. Background Technology

[0002] Flexible direct current (DC) transmission is a representative key technology in the development of new power systems. It features independent control of active and reactive power, and the ability to provide voltage support to the receiving-end grid. Compared to traditional DC transmission, it is more suitable for scenarios such as renewable energy generation grid connection and asynchronous grid interconnection. However, flexible DC transmission lines are long and span large spatial areas, making the reliability of their relay protection crucial to the safe and stable operation of the entire system.

[0003] Traditional DC transmission line main protection is mainly divided into single-ended quantity protection and double-ended quantity protection. Among them, single-ended quantity protection mainly uses the step characteristics of fault voltage to form protection criteria, which is easily affected by transition resistance and noise.

[0004] Dual-terminal protection is mainly divided into current differential protection and directional longitudinal protection. Due to the long length of DC transmission lines, differential protection often requires a long operating time. Existing directional longitudinal protection is significantly affected by transition resistance and fault distance, and its calculations are quite complex. Therefore, it is necessary to research a novel line protection method for flexible power transmission systems based on parameter identification. Summary of the Invention

[0005] In view of the above-mentioned prior art, the present invention provides a novel line protection method for flexible power transmission systems based on parameter identification, which mainly solves the technical problems existing in the background art.

[0006] To achieve the above objectives, the technical solution of this invention is implemented as follows: This invention discloses a novel line protection method for flexible power transmission systems based on parameter identification. The line protection method includes the following steps: Acquire fault transient voltage and current sampling data at the protection installation points on both sides of the flexible DC transmission line, perform phase mode transformation on the sampling data to extract line mode components, and calculate the measured impedance at the protection installation points in the positive and negative directions; A general voltage and current parameter identification equation is constructed based on the measured impedance at the installation location in both the forward and reverse directions. Solve the constructed voltage and current parameter identification equations to obtain the resistance identification values ​​and inductance identification values ​​corresponding to the protection installation locations on the rectifier side and inverter side of the line, respectively. Determine whether the dual-sided resistance and inductance values ​​meet the positive fault identification criteria. If they do, the line is determined to have an in-zone fault; otherwise, it is determined to have an out-of-zone fault.

[0007] Optionally, acquire fault transient voltage and current sampling data at the protection installation points on both sides of the flexible DC transmission line, and perform phase-mode transformation on the sampling data to extract line-mode components, specifically including:

[0008] The synchronous sampling devices installed at the rectifier and inverter sides are used to collect the instantaneous values ​​of the transient three-phase voltage and three-phase current during the protection time window after the initial traveling wave of the fault arrives at the protection installation location at a preset sampling frequency. The collected instantaneous values ​​of three-phase voltage and three-phase current are processed by Clarke phase-mode transformation to eliminate the interference of electromagnetic coupling between three-phase transmission lines on fault electrical quantities, and decouple to obtain mutually independent line-mode components and zero-mode components.

[0009] Optionally, the protection impedance in both forward and reverse directions can be calculated based on the line mode components, specifically including: The measured impedance under positive short-circuit fault conditions at the rectifier-side protection installation location is calculated using the following formula:

[0010] in, The measured impedance under positive-direction short-circuit fault conditions. The incident traveling wave of the line-mode voltage propagating along the line towards the rectifier-side protection installation point. For a line-mode current incident traveling wave that propagates synchronously with the voltage incident traveling wave, This refers to the line-mode voltage reflection traveling wave generated at the converter station boundary after the incident traveling wave reaches the rectifier-side protection installation location. For the line-mode current reflection traveling wave synchronized with the voltage reflection traveling wave, N is the total number of valid sampling points within the preset protection time window. The voltage traveling wave reflection coefficient at the rectifier-side protection mounting location. The line-mode wave impedance of a flexible DC transmission line. is the instantaneous value of the line-mode voltage measured at the rectifier-side protection mounting point, and i is the instantaneous value of the line-mode current measured at the rectifier-side protection mounting point. The equivalent impedance of the station side of the rectifier-side modular multilevel converter; The measured impedance in the opposite direction is calculated using the following formula:

[0011] in, The measured impedance under reverse short-circuit fault conditions. The measured line-mode voltage reverse traveling wave at the rectifier-side protection installation location. To and Synchronous corresponding line mode current reverse traveling wave For line mode wave impedance.

[0012] Optionally, a general voltage and current parameter identification equation can be constructed based on the measured impedance at the protection mounting points in both the forward and reverse directions, specifically including: Based on the inductive impedance characteristics under forward fault protection, the voltage and current equations for the forward protection measuring points are derived as follows:

[0013] in, For MMC bridge arm inductors, The inductance value of the current-limiting reactor connected in series at the installation point of the flexible DC transmission line protection; Based on the impedance characteristics of resistance-type measurements under reverse-direction faults, and considering the property that the line wave impedance is a constant, the voltage and current equations for the reverse-direction protection measuring points are derived as follows:

[0014] Based on the voltage and current equations under both forward and reverse fault conditions, a general voltage and current parameter identification equation applicable to both fault conditions is obtained:

[0015] Where R is the resistance value to be identified, L is the inductance value to be identified, and a is a constant term reserved to account for data errors.

[0016] Optionally, the general voltage and current parameter identification equations can be integrated on both sides and solved using the least squares method to obtain the resistance identification value and the inductance identification value.

[0017] Optionally, the fault identification criterion in the positive direction is:

[0018] in, To set the threshold for the resistor, For inductor setting threshold, , These are the tuning coefficients.

[0019] Optional, , The value ranges between 0.3 and 0.7.

[0020] Optionally, when the resistance identification value and inductance identification value obtained by the single-sided protection device meet the positive direction fault identification criterion, the protection device on that side has a positive direction fault.

[0021] Optionally, when both the protection devices on the rectifier side and the inverter side of the line determine that the fault is in the forward direction, the fault is determined to be within the fault zone of the flexible DC transmission line; when the protection device on either side of the line determines that the fault is in the reverse direction, the fault is determined to be outside the fault zone of the flexible DC transmission line.

[0022] The beneficial effects of this invention are as follows: Based on the essential difference between the back-side impedance models of the protection installation location under positive and negative direction faults in flexible DC transmission line protection, a general voltage and current parameter identification equation is constructed and the parameter solution and fault identification are completed by using the integral combined with the least squares method. The fault characteristics used are only affected by the fault direction and are not related to the fault distance, transition resistance, or fault type. It has a very strong ability to withstand transition resistance and can achieve reliable operation of faults within the zone and reliable blocking of faults outside the zone under various fault conditions such as different fault locations, high transition resistance, single-pole grounding, and double-pole short circuit. This solution achieves universal analysis of all fault types by leveraging the stability characteristics of line mode components. It solves the problems of traditional single-ended quantity protection being susceptible to interference from transition resistance and noise, traditional double-ended current differential protection having long operating times, and existing directional longitudinal protection having complex calculations and being greatly affected by fault conditions. It can quickly and accurately identify faults inside and outside the flexible DC transmission line area, achieve rapid fault clearing and isolation, and effectively ensure the safe and stable operation of the flexible DC transmission system. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only preferred embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 Diagram of a double-ended flexible DC transmission system; Figure 2 This represents the voltage and current data and the fitted plane for a positive-direction fault. Figure 3 This is the voltage and current data and the fitted plane for a fault in the opposite direction; Figure 4 This is a plane containing voltage and current data and data set when a positive metallic grounding fault occurs at the midpoint of the zone. Figure 5 The voltage and current data and the plane of the set when a positive metallic ground fault occurs at the midpoint of the zone from another angle; Figure 6 This is a flowchart illustrating a novel line protection method for a flexible power transmission system based on parameter identification, as described in this application. Detailed Implementation

[0025] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. In the following description, the expression "some embodiments" refers to a subset of all possible embodiments; however, it should be understood that "some embodiments" can be the same subset or different subsets of all possible embodiments and can be combined with each other without conflict.

[0026] In the following description, numerous specific details are set forth in order to provide a more thorough understanding of the invention. However, it will be apparent to those skilled in the art that the invention can be practiced without one or more of these details. In other instances, certain technical features well-known in the art have not been described in order to avoid obscuring the invention.

[0027] It should be understood that the present invention can be embodied in various forms and should not be construed as being limited to the embodiments set forth herein. Rather, providing these embodiments will make the disclosure thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Furthermore, the terminology used herein is intended only to describe particular embodiments and is not intended to limit the invention. When used herein, the singular forms “a,” “an,” and “the” are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the terms “compose” and / or “comprising,” when used in this specification, identify the presence of the stated features, integers, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups. When used herein, the term “and / or” includes any and all combinations of the associated listed items.

[0028] It should also be noted that when an element is referred to as being "fixed to" another element, it can be directly attached to the other element or there may be an intervening element. When an element is referred to as being "connected to" another element, it can be directly connected to the other element or there may be an intervening element. The terms "vertical," "horizontal," "inner," "outer," "left," "right," and similar expressions used herein are for illustrative purposes only and do not represent the only possible implementation.

[0029] To fully understand this invention, a detailed structure will be presented in the following description to illustrate the technical solution proposed by this invention. Optional embodiments of the invention are described in detail below; however, in addition to these detailed descriptions, the invention may have other embodiments.

[0030] Please refer to the attached document. Figure 6 This application discloses a novel line protection method for flexible power transmission systems based on parameter identification. The line protection method includes the following steps: S1. Obtain fault transient voltage and current sampling data at the protection installation points on both sides of the flexible DC transmission line, perform phase mode transformation on the sampling data to extract line mode components, and calculate the measured impedance at the protection installation points in the positive and negative directions. In some embodiments of step S1, fault transient voltage and current sampling data at the protection installation points on both sides of the flexible DC transmission line are acquired, and the sampling data are subjected to phase-mode transformation to extract line-mode components, specifically including: After a line fault occurs and triggers the protection activation element, the synchronous sampling device with high-precision time synchronization capability, which is pre-configured at the protection installation points on the rectifier and inverter sides, collects the instantaneous values ​​of the transient three-phase voltage and the instantaneous values ​​of the three-phase current within a preset protection time window after the initial traveling wave of the fault arrives at the protection installation point at a preset sampling frequency.

[0031] The preset sampling frequency can preferably be 50kHz, and the preset protection time window can preferably be a 2ms period after the initial traveling wave of the fault arrives at the protection installation point. This protection time window covers the fault transient period without secondary reflection interference after the initial traveling wave arrives and before the second traveling wave head arrives, which can effectively avoid the interference of subsequent traveling wave reflection process on the fault electrical quantity characteristics. At the same time, the synchronous sampling devices on the rectifier side and the inverter side achieve precise alignment of sampling time based on a globally unified time synchronization reference, ensuring the synchronization and consistency of the fault electrical quantity analysis at both ends. The collected instantaneous values ​​of three-phase voltage and three-phase current are processed by Clarke phase mode transformation to eliminate the interference of electromagnetic coupling between three-phase transmission lines on the fault electrical quantity, and decouple to obtain mutually independent line mode components and zero mode components.

[0032] Because zero-mode components attenuate rapidly and have poor amplitude stability during transmission in long-distance flexible DC transmission lines, and are easily affected by line-to-ground distribution parameters and fault types, they cannot provide stable and effective electrical quantity support for fault analysis under all fault scenarios. In contrast, line-mode components can exist stably in various common fault types of flexible DC transmission lines, such as single-pole grounding faults and double-pole short-circuit faults, and have low attenuation and stable traveling wave propagation characteristics during transmission. Therefore, the line-mode components obtained after phase-mode transformation are selected as the basic electrical quantities for the subsequent measurement impedance calculation at the forward and reverse installation locations of the protection system. This provides effective electrical quantity data with no coupling interference and strong universality for the subsequent fault equivalent model analysis and parameter identification process.

[0033] The line mode component serves as the basic electrical quantity for fault transient analysis. A preset protection time window is selected as the analysis period, which is between the arrival of the initial traveling wave of the fault at the protection installation location and the arrival of the second traveling wave head. During this period, the fault traveling wave does not undergo secondary reflection and refraction, which can effectively avoid the interference of multiple wave head superposition on the fault electrical quantity characteristics and ensure the accuracy of fault equivalent model analysis.

[0034] In some embodiments of this application, the core analysis object is the rectifier-side protection installation location, and the fault condition analysis is carried out using a positive electrode fault as an example.

[0035] For protection against forward short-circuit faults, based on the superposition theorem, a DC power supply of equal magnitude and opposite direction to the line voltage before the fault is superimposed at the fault point. The fault voltage incident traveling wave generated by this DC power supply Fault current incident traveling wave The incident traveling wave propagates along the line towards the rectifier-side protection installation point. Upon reaching the rectifier-side protection installation point, it is reflected at the converter station boundary, generating a corresponding line-mode voltage reflected traveling wave. Traveling wave with line mode current reflection Where i is the traveling wave sequence number arriving at the protection installation point within the protection time window, and based on the boundary conditions of traveling wave propagation and the voltage and current traveling wave constraint relationship of the line mode components, the measured impedance under the positive direction short-circuit fault condition at the rectifier-side protection installation point is derived. The measured impedance The complete expression is:

[0036] in, The measured impedance under positive-direction short-circuit fault conditions. The incident traveling wave of the line-mode voltage propagating along the line towards the rectifier-side protection installation point. For a line-mode current incident traveling wave that propagates synchronously with the voltage incident traveling wave, This refers to the line-mode voltage reflection traveling wave generated at the converter station boundary after the incident traveling wave reaches the rectifier-side protection installation location. For the line-mode current reflection traveling wave synchronized with the voltage reflection traveling wave, N is the total number of valid sampling points within the preset protection time window. The voltage traveling wave reflection coefficient at the rectifier-side protection mounting location. The line-mode wave impedance of a flexible DC transmission line. is the instantaneous value of the line-mode voltage measured at the rectifier-side protection mounting point, and i is the instantaneous value of the line-mode current measured at the rectifier-side protection mounting point. The equivalent impedance of the station side for the rectifier-side modular multilevel converter.

[0037] During the initial traveling wave phase of the fault, the power electronic devices on the converter station side did not switch on. The bridge arm inductance of the converter station and the current-limiting reactor connected in series on the line side constituted the core equivalent impedance on the back side of the protection installation. The equivalent impedance of the rectifier-side modular multilevel converter on the station side is... The expression is:

[0038] Therefore, the measured impedance at the protection installation point under a positive direction fault can be determined. It is a purely inductive impedance, and its value is consistent with the inductive reactance of the total equivalent inductance on the converter station side. It is not affected by the fault distance or transition resistance, but is determined only by the fault direction and the inherent equipment parameters of the converter station and line. The measured impedance under the forward short-circuit fault condition at the inverter side protection installation point adopts the same derivation logic and calculation method as the rectifier side. It is only necessary to replace the equipment parameters in the formula with the corresponding current-limiting reactor inductance value and converter bridge arm inductance value on the inverter side.

[0039] For the reverse short-circuit fault condition, where the fault point is located on the back side of the converter station at the protection installation location, within the preset protection time window after the initial traveling wave of the fault reaches the protection installation location and before the second traveling wave head arrives, only a single fault reverse traveling wave propagates on the line towards the fault point, without superposition interference from the incident traveling wave and the secondary reflected wave. Based on the inherent property that the line wave impedance corresponding to the line mode component is a constant determined solely by the line's own structural parameters and exhibits pure resistive characteristics, the measurable impedance under the reverse short-circuit fault condition at the rectifier-side protection installation location is derived. The measured impedance The complete expression is:

[0040] in, The measured impedance under reverse short-circuit fault conditions. The measured line-mode voltage reverse traveling wave at the rectifier-side protection installation location. To and Synchronous corresponding line mode current reverse traveling wave The impedance of the line mode wave is given. Therefore, the measured impedance at the protection installation point under reverse-direction fault conditions can be determined. As a purely resistive impedance, its value is completely consistent with the line-mode impedance of the line. This forms a quantifiable and identifiable essential difference from the inductive impedance characteristics under forward faults, providing a core model basis for the construction of subsequent general parameter identification equations and fault direction discrimination. The measured impedance under reverse short-circuit fault conditions at the inverter-side protection installation point adopts the same derivation logic and calculation method as the rectifier side. Only the line-mode impedance in the formula needs to be replaced with the line-mode impedance parameter of the corresponding line on the inverter side.

[0041] S2. Construct a general voltage and current parameter identification equation based on the measured impedance at the protection installation location in the positive and negative directions; Specifically, based on the station-side equivalent impedance of the rectifier-side modular multilevel converter... Based on the volt-ampere characteristics of the inductor and the boundary conditions for fault traveling wave propagation, the impedance expression in the Laplace domain is converted into a time-domain expression, and the voltage-current equations for the protection measuring point under positive-direction faults are derived as follows:

[0042] in, For MMC bridge arm inductors, The inductance value of the current-limiting reactor connected in series at the installation point of the flexible DC transmission line protection; This formula clarifies that under positive direction fault conditions, the rate of change of line mode voltage and line mode current at the protection measuring point is linearly proportional, corresponding to the inductive impedance characteristic under positive direction faults. This characteristic is determined only by the fault direction and the inherent equipment parameters of the converter station and line, and is not affected by the fault distance or transition resistance, providing a stable characteristic basis for fault direction identification.

[0043] For the reverse fault protection condition, the derived results of the measured impedance show that the measured impedance at the protection installation point is equal to the line-mode wave impedance of the flexible DC transmission line. The line-mode wave impedance is a constant determined solely by the structural parameters of the line's inductance and capacitance per unit length. During the initial traveling wave phase of a fault, it exhibits purely resistive characteristics and is unaffected by fault location, transition resistance, or other fault conditions. Based on Ohm's law for resistive elements and the boundary conditions for fault traveling wave propagation, the voltage and current equations for the protection measuring points under reverse-direction faults are derived as follows:

[0044] This formula clarifies that under reverse fault conditions, the line-mode voltage and line-mode current at the protection measuring point are linearly proportional. This corresponds to the resistive impedance characteristic under reverse faults, which forms a quantifiable and identifiable essential difference from the inductive impedance characteristic under forward faults. This provides a core model foundation for the construction of subsequent general parameter identification equations. The forward and reverse fault voltage and current equations at the inverter-side protection installation point adopt the same derivation logic as the rectifier side, requiring only the replacement of the corresponding equipment parameters with the inherent equipment parameters of the inverter side.

[0045] By combining the voltage and current equations of protection measurement points under both forward and reverse fault conditions, a unified identification of impedance model differences under these two fault conditions is achieved without switching the analysis model based on the fault direction. This improves the computational efficiency and engineering practicality of the protection method. Simultaneously, considering model errors and sampling measurement data errors present in actual engineering scenarios, a universal voltage and current parameter identification equation applicable to all fault conditions is constructed. The expression of this universal voltage and current parameter identification equation is as follows:

[0046] Where R is the resistance value to be identified, L is the inductance value to be identified, and a is a constant term reserved to account for data errors.

[0047] This general equation enables the synchronous fitting and identification of resistance and inductance parameters under fault conditions. In the forward direction fault, the theoretical identification value of the resistance parameter to be identified approaches 0, and the theoretical identification value of the inductance parameter to be identified approaches the total equivalent inductance value on the converter station side. In the reverse direction fault, the theoretical identification value of the inductance parameter to be identified approaches 0, and the theoretical identification value of the resistance parameter to be identified approaches the line mode wave impedance. This allows for accurate determination of the fault direction.

[0048] S3. Solve the constructed voltage and current parameter identification equations to obtain the resistance identification value and inductance identification value corresponding to the protection installation points on the rectifier side and inverter side of the line, respectively. Specifically, to suppress the amplification effect of random noise generated during the sampling process by the numerical difference algorithm and improve the anti-interference capability and calculation accuracy of the parameter identification results, this embodiment uses time-domain integration combined with least squares linear regression to solve for the resistor parameters, inductor parameters, and constant compensation terms in the general equation. First, time-domain integration is performed on both sides of the general voltage and current parameter identification equation within a preset protection time window to obtain the parameter identification equation in integral form, the expression of which is:

[0049] For data within a certain time period in the time domain, and for discrete sampled data within a preset protection time window, the parameter identification equation in integral form described above is discretized and converted into a matrix expression suitable for calculation by digital protection devices. Based on the discrete sampled data, the integral term is numerically calculated using the trapezoidal rule. The discretized integral equation can then be organized into the following standard linear regression matrix form:

[0050] In the formula, U is the observation vector constructed based on measured line-mode voltage sampling data, I is the coefficient matrix constructed based on measured line-mode current sampling data, and β is the parameter vector containing all parameters to be identified. The specific expressions for the observation vector U, the coefficient matrix I, and the parameter vector β are as follows:

[0051] In the above formula T S The sampling interval is... T S arrive nT S To protect the time window, T represents matrix transpose. Through the above matrix transformation, the parameter fitting problem is transformed into a standardized linear regression problem, which is adapted to the discrete calculation logic of digital protection devices. At the same time, it provides a standardized calculation model for the global optimal solution of the least squares method.

[0052] Based on the standardized linear regression matrix equation described above, the optimal unbiased estimate of the parameter vector β is obtained using the least squares method. With the objective of minimizing the sum of squared residuals between the observed and fitted values, the optimal solution expression for the parameter vector is derived as follows:

[0053] By performing matrix operations on the optimal solution, the identification values ​​of the resistor parameter to be identified, the identification values ​​of the inductor parameter to be identified, and the optimal estimate of the constant compensation term can be directly extracted. The first element of the parameter vector β is the resistor identification value at the corresponding protection installation location, and the second element is the inductor identification value at the corresponding protection installation location. Following the integral discretization processing and least squares solution logic that is completely consistent with the rectifier-side protection installation location, the synchronous sampling data of line-mode voltage and line-mode current collected at the inverter-side protection installation location are calculated in parallel to obtain the corresponding resistor identification value and inductor identification value at the inverter-side protection installation location.

[0054] Based on the resistance and inductance identification values ​​obtained from the above solution process, their values ​​have a clear and unique correspondence with the fault direction, unaffected by fault distance, transition resistance, or fault type. This provides a core quantitative basis for subsequent fault direction determination and fault identification within and outside the protection zone. When a positive fault occurs at the protection installation location, the theoretical value of the parameter to be identified is:

[0055] Under positive fault conditions, the resistance identification value theoretically approaches 0, and the inductance identification value theoretically approaches the total equivalent inductance value on the converter station side, exhibiting significant inductive impedance characteristics.

[0056] When a reverse fault occurs at the protection installation location, the theoretical value of the parameter to be identified is:

[0057] That is, under the reverse fault condition, the inductance identification value theoretically approaches 0, and the resistance identification value theoretically approaches the constant line-mode wave impedance of the line, exhibiting significant resistive impedance characteristics.

[0058] S4. Determine whether the identification values ​​of the resistance and inductance on both sides meet the positive direction fault identification criteria. If they do, determine that the line has an internal fault; otherwise, determine that it is an external fault.

[0059] Specifically, based on the inherent structural parameters of flexible DC transmission lines and converter station equipment, a resistance setting threshold is preset. With inductor setting threshold The resistance setting threshold With inductor setting threshold The tuning expressions are as follows:

[0060] in, To set the threshold for the resistor, For inductor setting threshold, , The reliability setting factor is less than 1, in order to balance the reliability and sensitivity of the protection action. , The value range is set between 0.3 and 0.7. This setting method allows the protection threshold to be adapted to the line and equipment parameters under different engineering scenarios, while avoiding the interference of model errors and measurement errors on the protection criteria in actual engineering.

[0061] For the protection devices on the rectifier and inverter sides of the line, the resistance and inductance identification values ​​obtained by the least squares method on the corresponding sides are compared with the preset resistance setting threshold. Inductance setting threshold By comparing the results, a unilateral positive direction fault identification criterion is constructed. The expression of the unilateral positive direction fault identification criterion is as follows:

[0062] When the resistance and inductance identification values ​​corresponding to a single-sided protection device simultaneously meet the aforementioned positive-direction fault identification criteria, it is determined that the protection device on that side has detected a positive-direction fault. When the resistance and inductance identification values ​​corresponding to a single-sided protection device cannot simultaneously meet the aforementioned positive-direction fault identification criteria, it is determined that the protection device on that side has detected a reverse-direction fault. The core logic of this criterion is based on the essential difference between the parameters to be identified under positive and reverse-direction faults. Under positive-direction faults, the resistance identification value theoretically approaches 0, and the inductance identification value theoretically approaches the total equivalent inductance on the converter station side. Under reverse-direction faults, the inductance identification value theoretically approaches 0, and the resistance identification value theoretically approaches the line-mode wave impedance. This enables accurate and reliable fault direction identification. Furthermore, based on the fault direction determination results of the protection devices on the rectifier side and inverter side of the line, a fault determination criterion for the longitudinal protection zone is constructed. The expression for the fault determination criterion for the zone is as follows:

[0063] in and These represent the resistance and inductance values ​​on the rectifier side, respectively. and These represent the resistance and inductance identification values ​​on the inverter side, respectively.

[0064] When both the protection devices on the rectifier side and the inverter side of the line determine that the fault is in the forward direction, that is, when the above-mentioned fault determination criteria for inside and outside the zone are met simultaneously, it is determined that the flexible DC transmission line has an inside-zone fault, and the protection action is immediately triggered to complete the rapid disconnection and isolation of the faulty line. When the protection device on either side of the line determines that the fault is in the reverse direction, that is, when the above-mentioned fault determination criteria for inside and outside the zone cannot be met, it is determined that the flexible DC transmission line has an outside-zone fault, and the protection device is reliably blocked and does not operate. The fault characteristics used in this longitudinal joint criterion are independent of the fault distance and transition resistance, and are only affected by the fault direction. Combined with the noise immunity characteristics of the integral and least squares methods, it can achieve rapid and accurate identification of faults inside and outside the zone under complex operating conditions with high transition resistance and strong noise interference.

[0065] To verify the effectiveness of this invention, a bipolar flexible DC transmission system was constructed in PSCAD / EMTDC, as shown below. Figure 1 As shown. The line length is 500km, the line-mode impedance is 252Ω, and the current-limiting reactor inductance value is... l line 100mH; MMC bridge arm inductance l arm The value is 50mH. The protection sampling frequency is 50kHz, and the protection time window is 2ms. Taking the protection at point R as an example, simulation analysis is performed. The fault line mode traveling wave arrives at point R at 0ms. Δ set The setting value is 161kV; R set Set to 126Ω L set Set to 0.067H, such as Figure 1 When a positive metallic ground fault occurs at the midpoint (250km from the rectifier side) of the ±500kV true bipolar flexible DC transmission system shown, the voltage and current data measured at the protection point R, and the parametric equations obtained by least squares fitting are as follows: Figure 2 As shown. When a positive metallic ground fault occurs at the rectifier-side reactor outlet, the voltage and current data measured at protection point R, and the parametric equations obtained by least squares fitting, are as follows: Figure 3As shown in the figure, the black curve represents the voltage and current data, and the blue plane represents the plane of the fitted parametric equations. It can be seen that the voltage and current data largely overlap with the fitted plane, and the fitting results all satisfy the goodness-of-fit criteria of Rsquared > 0.99 and SSE < 0.01. Figure 2 Identified L =0.1328H, R =0.024Ω, and ( l line +2 / 3 l arm The value of H is 0.133, which is approximately equal to the fitted L, verifying the analysis of the positive direction fault in step 1, and it can be determined to be a positive direction fault. Figure 3 Identified L =-0.001H, R =252.85Ω, while the line-mode wave impedance is 252Ω, which is approximately equal to the fitted R, verifying the analysis of the reverse fault in step 1, and it can be determined to be a reverse fault. The simulation results verify the correctness of the forward and reverse fault analysis of the protection system and the effectiveness of the parameter identification method in this paper.

[0066] Tables 1 and 2 show the protection operation results for single-pole grounding faults and double-pole short-circuit faults, respectively. It can be seen from the tables that this protection is almost unaffected by fault distance and transition resistance. During faults within the protection zone... L All were identified as approximately 0.13H (two decimal places are retained in the table, the same below). R The identification value does not exceed 1Ω, providing sufficient margin for reliable operation compared to the threshold; in case of external faults... R The values ​​are all around 252Ω. L The identified value is approximately 0H, far below the action threshold, indicating reliable no action. This is because the measured impedance characteristics used in this method are independent of transition resistance and fault distance, and are only affected by the fault direction. Transition resistance and fault distance primarily affect the amplitude of fault electrical quantities. The least squares parameter identification method does not directly utilize the absolute values ​​of fault electrical quantities, but instead fits the parametric equations of fault voltage and current. Therefore, the influence of transition resistance and fault distance is avoided in the fault feature extraction method.

[0067] Table 1. Protection Action Results for Single-Pole Ground Faults

[0068] Table 2. Protection Action Results for Bipolar Short Circuit Faults

[0069] Noise with a signal-to-noise ratio of 20 dB was added to the sampled data to protect the measured voltage and current data, as well as the parametric equations obtained by least squares fitting, as shown below. Figure 4As shown in the figure. The black curve represents voltage and current data, and the blue plane represents the plane containing the fitted parametric equations. The parameter identification results are as follows: L =0.12H, R =-4.59Ω.

[0070] from Figure 4 As can be seen, some black data points have deviated from the blue parametric equation plane. This is due to noise causing the data to deviate from the true value. However, if we change the observation angle of the three-dimensional plane and look at it from the direction perpendicular to the Z-axis of the blue plane, that is... Figure 5 As can be seen, the black data points are evenly distributed above and below the blue parametric equation plane. This is because noise has the characteristic of zero expectation in the time domain, which translates to the uniform distribution of black data points above and below the blue parametric equation plane in three dimensions. The least squares method aims to find a fitting result that minimizes the error with the known data. The fitting result tends to pass through the central area of ​​the data, exhibiting good anti-interference capabilities. Furthermore, to avoid the impact of differential amplification noise on the recognition effect, this method transforms the parametric recognition equation into an integral algorithm. Therefore, the recognized value of L does not change significantly and can still reliably identify a fault in the positive direction. Multiple sets of simulation data verify that this protection can reliably identify faults inside and outside the protection zone under noise levels of 20 dB.

[0071] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. The scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A novel line protection method for flexible power transmission systems based on parameter identification, characterized by, Line protection methods include the following steps: Acquire fault transient voltage and current sampling data at the protection installation points on both sides of the flexible DC transmission line, perform phase mode transformation on the sampling data to extract line mode components, and calculate the measured impedance at the protection installation points in the positive and negative directions; A general voltage and current parameter identification equation is constructed based on the measured impedance at the installation location in both the forward and reverse directions. Solve the constructed voltage and current parameter identification equations to obtain the resistance identification values ​​and inductance identification values ​​corresponding to the protection installation locations on the rectifier side and inverter side of the line, respectively. Determine whether the dual-sided resistance and inductance values ​​meet the positive fault identification criteria. If they do, the line is determined to have an in-zone fault; otherwise, it is determined to have an out-of-zone fault.

2. A novel line protection method for a flexible power transmission system based on parameter identification according to claim 1, characterized in that, Acquire fault transient voltage and current sampling data at the protection installation points on both sides of the flexible DC transmission line, and perform phase-mode transformation on the sampling data to extract the line-mode component. Specifically, this includes: The synchronous sampling devices installed at the rectifier and inverter sides are used to collect the instantaneous values ​​of the transient three-phase voltage and three-phase current during the protection time window after the initial traveling wave of the fault arrives at the protection installation location at a preset sampling frequency. The collected instantaneous values ​​of three-phase voltage and three-phase current are processed by Clarke phase-mode transformation to eliminate the interference of electromagnetic coupling between three-phase transmission lines on fault electrical quantities, and decouple to obtain mutually independent line-mode components and zero-mode components.

3. A novel line protection method for a flexible power transmission system based on parameter identification according to claim 2, characterized in that, The protection impedance in both forward and reverse directions is calculated based on the line modulus components, specifically including: The measured impedance under positive short-circuit fault conditions at the rectifier-side protection installation location is calculated using the following formula: in, The measured impedance under positive-direction short-circuit fault conditions. The incident traveling wave of the line-mode voltage propagating along the line towards the rectifier-side protection installation point. For a line-mode current incident traveling wave that propagates synchronously with the voltage incident traveling wave, This refers to the line-mode voltage reflection traveling wave generated at the converter station boundary after the incident traveling wave reaches the rectifier-side protection installation location. For the line-mode current reflection traveling wave synchronized with the voltage reflection traveling wave, N is the total number of valid sampling points within the preset protection time window. The voltage traveling wave reflection coefficient at the rectifier-side protection mounting location. The line-mode wave impedance of a flexible DC transmission line. is the instantaneous value of the line-mode voltage measured at the rectifier-side protection mounting point, and i is the instantaneous value of the line-mode current measured at the rectifier-side protection mounting point. The equivalent impedance of the station side of the rectifier-side modular multilevel converter; The measured impedance in the opposite direction is calculated using the following formula: in, The measured impedance under reverse short-circuit fault conditions. The measured line-mode voltage reverse traveling wave at the rectifier-side protection installation location. To and Synchronous corresponding line mode current reverse traveling wave For line mode wave impedance.

4. A novel line protection method for a flexible power transmission system based on parameter identification according to claim 3, characterized in that, Based on the measured impedance at the protection mounting points in both the forward and reverse directions, a general voltage and current parameter identification equation is constructed, specifically including: Based on the inductive impedance characteristics under forward fault protection, the voltage and current equations for the forward protection measuring points are derived as follows: in, For MMC bridge arm inductors, The inductance value of the current-limiting reactor connected in series at the installation point of the flexible DC transmission line protection; Based on the impedance characteristics of resistance-type measurements under reverse-direction faults, and considering the property that the line wave impedance is a constant, the voltage and current equations for the reverse-direction protection measurement points are derived as follows: Based on the voltage and current equations under both forward and reverse fault conditions, a general voltage and current parameter identification equation applicable to both fault conditions is obtained: Where R is the resistance value to be identified, L is the inductance value to be identified, and a is a constant term reserved to account for data error.

5. A novel line protection method for a flexible power transmission system based on parameter identification according to claim 4, characterized in that, By integrating both sides of the general voltage and current parameter identification equation and solving it using the least squares method, the resistance identification value and the inductance identification value are obtained.

6. A novel line protection method for a flexible power transmission system based on parameter identification according to claim 5, characterized in that, The criteria for identifying faults in the positive direction are: in, To set the threshold for the resistor, For inductor setting threshold, , These are the tuning coefficients.

7. A novel line protection method for a flexible power transmission system based on parameter identification according to claim 6, characterized in that, , The value ranges between 0.3 and 0.

7.

8. A novel line protection method for a flexible power transmission system based on parameter identification according to claim 7, characterized in that, When the resistance and inductance values ​​obtained by a single-sided protection device meet the positive direction fault identification criteria, the protection device on that side has a positive direction fault.

9. A novel line protection method for a flexible power transmission system based on parameter identification according to claim 8, characterized in that, When both the protection devices on the rectifier side and the inverter side of the line determine that the fault is in the forward direction, the flexible DC transmission line is determined to be in an area where the fault occurs; when the protection device on either side of the line determines that the fault is in the reverse direction, the flexible DC transmission line is determined to be in an area where the fault occurs.

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