Flexible DC power transmission system receiving end near area AC line protection method, device and system based on MMC injection

Abstract

The embodiment of the invention relates to the technical field of power systems, in particular to a flexible direct current power transmission system receiving end near area alternating current line protection method, device and system based on MMC injection. The method comprises the following steps: monitoring the operation state of a near-region AC line at the receiving end of the flexible DC power transmission system in real time, and when the operation state meets a preset fault disturbance condition, generating a detection signal with a preset characteristic; a receiving end MMC converter of the flexible DC power transmission system is controlled, and a detection signal is injected into an AC line during a fault period; acquiring voltage response data and current response data after injection of the detection signal through a preset measurement point of the alternating current line; impedance phase feature extraction is carried out on the voltage response data and the current response data, extracted impedance phase feature parameters are compared with a preset phase threshold value, and a fault area is determined according to a comparison result. According to the method, internal and external faults of a near-area alternating-current line at the receiving end of the flexible direct-current power transmission system can be accurately identified, and the reliability and sensitivity of fault identification are improved.

Description

Technical Field

[0001] This invention relates to the field of power system technology, and in particular to a method, device and system for protecting near-field AC lines in a flexible DC transmission system based on MMC injection. Background Technology

[0002] With the development of new energy technologies (wind power, photovoltaics, etc.), large-scale grid connection of new energy has become the core direction of my country's energy structure transformation. Flexible DC transmission systems, with their outstanding advantages in new energy reception, power quality control, and inter-regional power transmission, have become a key technological carrier for new energy grid connection. In a typical topology of a flexible DC system, new energy power plants such as wind farms are collected via AC lines, converted into DC power by converter stations, and then transmitted to onshore converter stations via high-voltage DC lines and connected to the AC power grid. The safe and stable operation of the AC lines between the main transformer of the wind farm and the converter transformer on the wind farm side of the flexible DC system is directly related to the reliable transmission of new energy power.

[0003] For fault identification and protection of AC lines between the main transformer of a wind farm and the converter transformer on the flexible DC wind farm side, traditional methods for AC grid protection schemes mainly include overcurrent protection and differential protection, which rely on the abnormal voltage and current characteristics under the AC fundamental frequency to determine the fault. However, the controlled current source characteristics of the flexible DC converter station break the fault signal triggering logic of traditional protection, causing the circuit to be unable to present the fault state detected by overcurrent protection, differential protection, etc., making it difficult to identify the fault. At the same time, the receiving end near the flexible DC wind farm exhibits a double-end weak power supply characteristic, which also weakens the signal identification basis of traditional protection.

[0004] Therefore, how to propose an effective solution for the fault characteristics of near-field AC lines at the receiving end of a flexible DC system is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0005] The purpose of this invention is to provide at least one method, device, and system for protecting near-zone AC lines in a flexible DC transmission system based on MMC injection, which can accurately identify faults inside and outside the receiving zone of the near-zone AC lines in a flexible DC transmission system, thereby improving the reliability and sensitivity of fault identification.

[0006] To address the aforementioned technical problems, at least one embodiment of this application provides a method for protecting near-field AC lines in a flexible DC transmission system based on MMC injection, comprising: The system monitors the operating status of AC lines near the receiving end of the flexible DC transmission system in real time. When the operating status meets the preset fault disturbance conditions, a detection signal with preset characteristics is generated. Control the receiving-end MMC converter of the flexible DC transmission system to inject the detection signal into the AC line during a fault. Voltage response data and current response data after the injection of detection signal are collected through the preset measurement points of the AC line; Impedance phase features are extracted from the voltage response data and current response data to obtain impedance phase feature parameters; The impedance phase characteristic parameters are compared with a preset phase threshold, and the fault area is determined based on the comparison result in order to execute the corresponding line protection action.

[0007] In one embodiment, controlling the receiving-end MMC converter of the flexible DC transmission system to inject the detection signal into the AC line during a fault includes: The detection signal is injected into the AC line during a fault by utilizing the differential mode voltage formed by the upper and lower arms of the MMC converter.

[0008] In one embodiment, before injecting the detection signal into the AC line during a fault using the differential-mode voltage formed by the upper and lower arms of the MMC converter, the method further includes: Configure the number of sub-modules in the upper and lower arms of the MMC converter based on the signal characteristics of the detected signal; The differential-mode voltage formed by the upper and lower bridge arms of the MMC converter is used to inject the detection signal into the AC line during a fault. Specifically, the differential-mode voltage formed by the voltage output from the upper and lower bridge arms through the submodule is used to inject the detection signal into the AC line during a fault.

[0009] In one embodiment, configuring the number of sub-modules in the upper and lower arms of the MMC converter based on the signal characteristics of the detected signal includes: In the lower control loop of the MMC converter, differential additional values ​​corresponding to the detection signals are superimposed on the three phase lines respectively; Based on the reference values ​​of the bridge arm voltage under normal operating conditions, and in conjunction with the differential additional value, the reference values ​​of the voltage of the upper and lower bridge arms of the MMC converter are calculated. Based on the voltage reference values ​​of the upper and lower bridge arms, calculate the number of sub-modules that need to be deployed in the upper and lower bridge arms to match the voltage reference values, and deploy the corresponding number of sub-modules in the upper and lower bridge arms respectively.

[0010] In one embodiment, generating the detection signal with preset features includes: Even-order harmonics are generated as a detection signal with an amplitude of 0.12 per-unit value based on the rated voltage of the AC line.

[0011] In one embodiment, impedance phase feature extraction is performed on the voltage response data and current response data, including: The voltage response data and current response data are preprocessed by filtering; The EWT algorithm is used to decompose the preprocessed data and extract the target signal component that matches the frequency of the detection signal. The target signal component is input into the Prony algorithm for mode parameter identification to obtain the amplitude and phase information of the target signal component. The phase difference between the voltage target signal component and the current target signal component at the same measurement point is calculated and used as the impedance phase characteristic parameter.

[0012] In one embodiment, the preset phase threshold includes: a first threshold range and a second threshold range; the first threshold range is 0°~180°, and the second threshold range is -180°~0°; The impedance phase characteristic parameters are then compared with a preset phase threshold, and the fault region is determined based on the comparison result, including: When the impedance phase characteristic parameter is within the first threshold range, it is determined to be an in-zone fault; When the impedance phase characteristic parameter is within the second threshold range, it is determined to be an external fault.

[0013] At least one embodiment of this application also provides a receiving-end near-zone AC line protection device for a flexible DC transmission system based on MMC injection, comprising: The detection signal generation unit is used to monitor the operating status of the AC lines in the near-field area of ​​the receiving end of the flexible DC transmission system in real time. When the operating status meets the preset fault disturbance conditions, it generates a detection signal with preset characteristics. The detection signal injection unit is used to control the receiving-end MMC converter of the flexible DC transmission system to inject the detection signal into the AC line during a fault. The response acquisition unit is used to acquire voltage response data and current response data after the detection signal is injected through preset measurement points on the AC line; The feature extraction unit is used to extract impedance phase features from the voltage response data and current response data to obtain impedance phase feature parameters. The fault location unit is used to compare the impedance phase characteristic parameters with a preset phase threshold, determine the fault area based on the comparison result, and execute the corresponding line protection action.

[0014] At least one embodiment of this application also provides a receiving-end near-zone AC line protection system for a flexible DC transmission system based on MMC injection, comprising: The aforementioned MMC injection-based flexible DC transmission system receiving-end near-zone AC line protection device; The receiving-end MMC converter of the flexible DC transmission system is communicatively connected to the detection signal injection unit of the protection device, and is used to receive the control command of the detection signal injection unit. During a fault, the detection signal is injected into the near-zone AC line of the receiving end through the differential mode voltage of the upper and lower bridge arms. The detection device, set at the preset measurement point of the AC line, is communicatively connected to the response acquisition unit of the protection device. It is used to collect AC line voltage response data and current response data after the injection of the detection signal, and transmit the response data to the response acquisition unit. The protected flexible DC transmission system receiving-end near-area AC line connects the wind farm main transformer and the flexible DC wind farm side converter transformer.

[0015] In one embodiment, the detection device includes an electronic current transformer and an electronic voltage transformer.

[0016] The embodiment of this application provides a method for protecting AC lines near the receiving end of a flexible DC transmission system based on MMC injection. This method, with the active injection of preset characteristic detection signals as its core, effectively compensates for the lack of natural fault signals in the near-receiving area of ​​the flexible DC system. Signal injection is achieved through the receiving-end MMC converter, eliminating the need for additional dedicated equipment and adapting to existing system topologies and control frameworks. Voltage and current response data after signal injection are collected through preset measurement points. Impedance phase characteristics are extracted based on this data, and the obtained parameters directly correlate with the capacitive-inductive differences of the fault area. The impedance phase characteristic parameters are compared with preset phase thresholds. The fault area is determined based on the comparison logic that the line exhibits inductive behavior during faults within the fault area and capacitive behavior during faults outside the fault area. This eliminates the need for complex parameter conversions or additional judgments, fundamentally ensuring the accuracy of fault area determination. Furthermore, the determination process relies solely on the direct comparison of characteristic parameters and preset thresholds, without depending on inherent line parameters, downstream system operating modes, or other external conditions. This avoids the impact of external factor fluctuations on the determination results, adapting to the complex operating conditions of the near-receiving area of ​​the flexible DC system. It can prevent maloperation or failure to operate, maximizing the protection of the normal operation of the intact line. Attached Figure Description

[0017] One or more embodiments are illustrated by way of example with reference to the accompanying drawings, and these illustrative descriptions do not constitute a limitation on the embodiments.

[0018] Figure 1 This is a flowchart of a method for protecting near-field AC lines in a flexible DC transmission system based on MMC injection, provided in one embodiment of this application. Figure 2 This is a block diagram of an MMC active injection strategy control provided in one embodiment of this application; Figure 3This is a schematic diagram of a receiving-end near-area AC power grid for a flexible DC transmission system provided in one embodiment of this application; Figure 4 This is a schematic diagram of a system fault equivalent model at characteristic frequencies under an external fault, provided in one embodiment of this application; Figure 5 This is an embodiment of the present application providing a characteristic frequency voltage and current phasor relationship diagram for an external fault. Figure 6 This is a schematic diagram of a system fault equivalent model at characteristic frequencies under an intra-regional fault, provided in one embodiment of this application; Figure 7 This is a schematic diagram of a system fault equivalent model at characteristic frequencies under an external fault after star-delta transformation, provided in one embodiment of this application. Figure 8 This application provides an embodiment of a characteristic frequency voltage and current phasor relationship diagram for an intra-regional fault. Figure 9 This is a flowchart of a fault current calculation provided in one embodiment of this application; Figure 10 This is a schematic diagram of a receiving-end near-zone AC line protection device for a flexible DC transmission system based on MMC injection, provided in one embodiment of this application. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the various embodiments of this application will be described in detail below with reference to the accompanying drawings. However, those skilled in the art will understand that many technical details have been provided in the various embodiments of this application to help readers better understand this application. However, the technical solutions claimed in this application can be implemented even without these technical details and various changes and modifications based on the following embodiments. The division of the various embodiments below is for the convenience of description and should not constitute any limitation on the specific implementation of this application. The various embodiments can be combined with and referenced by each other without contradiction.

[0020] In a typical topology of a flexible DC system, new energy power plants such as wind farms are collected via AC lines, converted into DC power by converter stations, and then transmitted to onshore converter stations via high-voltage DC lines and connected to the AC power grid. The operating environment of the AC lines between the main transformer of the wind farm and the converter transformer on the flexible DC wind farm side is fundamentally different from that of the traditional AC power grid. When a short-circuit fault occurs in the near-field area of ​​the receiving end, the original AC power grid protection mechanism generally faces the severe challenge of adaptation failure.

[0021] Firstly, the controlled current source characteristic of the flexible DC converter station breaks the fault signal triggering logic of traditional protection. Traditional AC power grids are mainly powered by synchronous generators (thermal power, hydropower, etc.), whose output characteristics are rigid. When a short-circuit fault occurs, the power source naturally outputs a significantly increased fault current. Traditional power frequency protection (such as overcurrent protection and differential protection) is based on this characteristic, triggering protection actions by detecting signals such as sudden current changes at the power frequency and the current difference between the two ends of the line. However, the core equipment of the flexible DC system, the modular multilevel converter (MMC), is essentially a power electronic controller, not a traditional synchronous generator. When a fault occurs, the converter station immediately activates a current-limiting control strategy, actively limiting the output current within a safe threshold to prevent damage to power devices, thus exhibiting a controlled current source characteristic. This causes the fault current surge signal relied upon by traditional protection to disappear. Overcurrent protection cannot operate because the tripping threshold is not reached, and differential protection cannot identify the fault because the current difference between the two ends of the line does not meet the expected criteria.

[0022] Secondly, the weak power supply characteristics of the receiving end near the grid weaken the signal identification basis of traditional protection. The AC lines near the receiving end of the flexible DC system are typical of a weak power supply system with two ends: one end is a new energy power station such as a wind farm (with large output fluctuations and weak power intensity), and the other end is a flexible DC converter station (a controlled current source, not a traditional strong power source). The design premise of traditional power frequency protection is a strong power grid, where the power frequency signal (50Hz) has clear characteristics and sufficient amplitude during a fault, making it easy for protection devices to capture. However, in the scenario of weak power supply at both ends, the power frequency signal itself is extremely weak during a fault. Coupled with the current limiting control of the converter station, traditional protection devices can hardly obtain effective power frequency fault characteristics, resulting in a significant decrease in protection sensitivity and even failure to operate.

[0023] Third, the converter control strategy and line parameter fluctuations disrupt the adaptability of traditional model-based identification and protection. While traditional line model-based identification and protection does not rely on power supply characteristics but rather identifies faults through changes in inherent line parameters (resistance, inductance, and capacitance), and is widely used in renewable energy grid-connected systems, its core assumption is that line parameters are relatively fixed. However, in flexible DC systems, the converter's operating modes (such as inner and outer loop control, circulating current suppression, and submodule capacitor voltage balance control) dynamically change the line's current distribution and equivalent parameters. Simultaneously, harmonic interference during faults and the dynamic adjustment of control strategies further distort line parameter characteristics, leading to significant deviations between the line impedance calculated by traditional model-based identification and protection and the actual situation. This makes it impossible to accurately determine the fault location and may even result in false tripping.

[0024] In addition, although conventional transient protection focuses on transient signals at the moment of a fault (such as high-frequency noise and voltage and current surges), the design of this type of protection does not fully consider the impact of the converter control strategy. The fault transient signal is distorted and suppressed by the rapid control action of the converter, resulting in a fundamental difference between the transient characteristics and traditional power grid faults. The criteria of conventional transient protection cannot be adapted, and the identification accuracy is greatly reduced.

[0025] Therefore, there is an urgent need to propose a new protection method that adapts to the fault characteristics of AC lines near the receiving end of flexible DC systems, to solve the problem of traditional protection failure, and to ensure the safe and stable operation of new energy grid-connected systems.

[0026] This invention proposes a method for protecting near-field AC lines in a flexible DC transmission system based on MMC injection. The implementation details of this method are described below. The following details are provided for ease of understanding and are not essential for implementing this solution.

[0027] Example 1: The specific process of the receiving-end near-area AC line protection method for flexible DC transmission systems based on MMC injection in this embodiment can be described as follows: Figure 1 As shown, it includes: Step 101: Monitor the operating status of the AC lines in the near-field area of ​​the receiving end of the flexible DC transmission system in real time. When the operating status meets the preset fault disturbance conditions, generate a detection signal with preset characteristics.

[0028] The operational status of the AC lines near the receiving end of the flexible DC transmission system is continuously monitored in real time. These AC lines are located between the main transformer at the wind farm and the converter transformer on the wind farm side. The focus is on tracking real-time changes in key electrical quantities such as voltage and current. Specific monitoring methods are not limited in this embodiment; for example, voltage transformers can be used to collect voltage signal lights on the lines, which will not be elaborated further here.

[0029] The preset fault disturbance condition is a pre-defined criterion used to determine whether a line fault may occur. When the monitored line operating state (such as sudden changes in voltage or current) meets this preset criterion, the detection signal generation mechanism is triggered. This mechanism generates a sinusoidal detection signal with specific technical characteristics according to pre-determined core parameters. All characteristics are determined based on the principles of adapting to MMC operating constraints, ensuring detection sensitivity, and reducing the impact on the power grid. This embodiment does not limit the specific generation mechanism and core parameters of the detection signal; they can be set according to actual detection needs and will not be elaborated further here.

[0030] Additionally, it should be noted that the preset fault disturbance conditions need to be set in accordance with the monitoring method. Without limiting the monitoring method, the preset fault disturbance conditions are also not limited here. One preset fault disturbance condition is that the sudden change value of any one phase voltage in the three-phase voltage of the line exceeds 10% of the rated voltage, and the sudden change state lasts for at least three consecutive sampling points, but it is not limited to this.

[0031] Step 102: Control the receiving-end MMC converter of the flexible DC transmission system to inject a detection signal into the AC line during a fault.

[0032] The controlled current source characteristics of the flexible DC converter suppress natural overcurrent and voltage surges during faults. Furthermore, the weak power supply characteristics at the receiving end near the MMC converter weaken the signal identification basis of traditional protection systems. To address this, during a fault, the protection device sends a detection signal injection control command to the receiving-end MMC converter of the flexible DC transmission system, activating a preset signal injection strategy. By actively injecting detection signals, a clear and stable fault identification signal source can be artificially constructed, compensating for the inadequacy of natural fault signals.

[0033] Step 103: Collect voltage response data and current response data after the detection signal is injected through the preset measurement points on the AC line.

[0034] After the pre-defined detection signal is stably injected into the near-field AC line of the flexible DC transmission system via the receiving-end MMC converter, the data acquisition process at the pre-defined measurement points on the AC line is immediately initiated. The acquisition process focuses on the changes in the electrical quantities of the line after the injection of the detection signal, and captures the three-phase voltage data and three-phase current data at each pre-defined measurement point in real time as voltage response data and current response data.

[0035] The acquisition process must ensure the integrity and validity of the data, fully retain the characteristic frequency, amplitude and phase correlation information corresponding to the detected signal, and strictly synchronize the acquisition timing of voltage and current response data to avoid distortion of subsequent feature analysis due to timing deviations.

[0036] Step 104: Extract impedance phase features from voltage response data and current response data to obtain impedance phase feature parameters.

[0037] After acquiring voltage and current response data at preset measurement points on the AC line, a preset feature extraction process is initiated. Through targeted data processing, irrelevant signal components such as grid fundamental wave, harmonics, and measurement noise are eliminated, and the effective response components that match the frequency of the injected detection signal are focused on. Subsequently, based on the effective voltage and current response components at the same measurement point, the impedance phase characteristic parameters at the characteristic frequency are extracted by calculating the phase correlation between the two. These parameters directly reflect the impedance phase attributes of the line at the detection signal frequency.

[0038] Step 105: Compare the impedance phase characteristic parameters with the preset phase threshold, determine the fault area based on the comparison result, and execute the corresponding line protection action.

[0039] After obtaining the impedance phase characteristic parameters at the characteristic frequency, these parameters are compared with a preset phase threshold. The preset phase threshold is a pre-defined criterion based on the difference in impedance phase characteristics between faults inside and outside the line area, and includes at least the phase range where the line is inductive and the phase range where the line is capacitive.

[0040] Based on the above judgment results, the protection device immediately executes the corresponding protection action. For example, for faults within the zone, it quickly activates the preset fault isolation measures (such as triggering the circuit breaker to trip) to prevent the fault from expanding and affecting the transmission of new energy power. For faults outside the zone, it does not activate unnecessary protection actions to ensure the normal operation of the line. Here, the protection actions of the line corresponding to different fault areas are not limited, and can be set according to actual usage needs. Here, only the above protection method is used as an example.

[0041] Traditional protection relies on the overcurrent and voltage surge characteristics of natural fault signals. However, in flexible DC systems, this approach fails to effectively identify faults in the near-end region due to the controlled current source characteristics of the converter and the weak power supply characteristics at both ends. This step, based on the phase characteristic comparison of actively injected signals, leverages the impedance phase characteristic parameters to clearly reflect the line's capacitive-inductive characteristics. These characteristics are directly related to the fault location (inductive for faults within the zone, capacitive for faults outside the zone). By comparing with preset thresholds, abstract phase parameters can be transformed into clear fault area determination results, constructing a determination logic independent of natural fault signals and achieving effective fault identification. Furthermore, the comparison rules are directly based on the core physical characteristic of the difference in capacitive-inductive properties between faults inside and outside the zone. There are no complex parameter conversions or logical deductions, and it does not rely on external factors such as inherent line parameters, grid operating modes, or converter control strategies. This avoids the impact of external condition changes on the determination results, reduces error sources in the determination process, ensures the accuracy of fault area determination, and prevents false trips or failures to trip.

[0042] Based on the above introduction, the AC line protection method for the receiving end near-zone of a flexible DC transmission system based on MMC injection provided in this embodiment takes the active injection of preset characteristic detection signals as its core, effectively compensating for the lack of natural fault signals in the receiving end near-zone of the flexible DC system. Signal injection is achieved by relying on the receiving end MMC converter, eliminating the need for additional dedicated equipment and adapting to existing system topologies and control frameworks. Voltage and current response data after signal injection are collected through preset measurement points. Impedance phase characteristics are extracted based on the voltage and current response data. The obtained parameters are directly related to the capacitive-inductive differences of the fault area. The impedance phase characteristic parameters are compared with preset phase thresholds. The fault area is determined based on the comparison logic that the line is inductive during faults within the area and capacitive during faults outside the area. No complex parameter conversions or additional judgments are required, ensuring the accuracy of fault area determination from the root. Furthermore, the determination process relies only on the direct comparison of characteristic parameters and preset thresholds, without depending on inherent line parameters, downstream system operating modes, or other external conditions, avoiding the influence of external factor fluctuations on the determination results. This adapts to the complex operating conditions of the receiving end near-zone of the flexible DC system, preventing protection maloperation or failure to operate, and maximizing the protection of the normal operation of the intact line.

[0043] Example 2: In traditional solutions, there are no strict technical restrictions on the frequency and amplitude of the detection signal. However, when a fault occurs, the AC line will be accompanied by a large number of odd harmonic transient components, which are prone to frequency aliasing with the fault transient signal. This makes it difficult to distinguish between the effective signal and the interference signal in the subsequent acquired response data. At the same time, there is no unified benchmark for amplitude setting. If it is too large, it may aggravate the system voltage distortion during the fault and affect the stable operation of the converter. If it is too small, the signal will be easily submerged by noise after the signal is attenuated during transmission through the line, and the accuracy of impedance phase feature extraction cannot be guaranteed.

[0044] To address this issue, this embodiment further clarifies the specific implementation method for generating a detection signal with preset characteristics: that is, generating an even-order harmonic detection signal with an amplitude based on the rated voltage of the AC line and set to a per-unit value of 0.12.

[0045] The selection of the detection signal frequency should consider both meeting the performance constraints of the injection device and ensuring that the line at the other end of the non-injection source can deflect the injected characteristic frequency signal, thereby improving the detection sensitivity. The response speed of the MMC is affected by factors such as the submodule capacitance, arm inductance, and controller parameters. The open-loop time constant expression of the MMC is shown in Equation (1).

[0046] (1) Where ωAC represents the fundamental AC frequency, and Larm represents the inductance of the MMC bridge arm. CSM, N, and RL represent the submodule capacitance, the number of submodules, and the equivalent loss resistance, respectively.

[0047] For the characteristic signal, the non-injection source can be equivalent to an LC parallel resonant circuit. The cutoff frequency of the LC filter is its resonant frequency, as shown in equation (2).

[0048] (2) During normal modulation, the harmonics output by the MMC and the grid harmonics are mainly odd-order components. Therefore, to highlight fault characteristics, the harmonics injected into the MMC should theoretically be high-frequency even-order harmonic current signals.

[0049] The basic principle for selecting the amplitude of the detection signal is to minimize the impact on the power grid while ensuring the detection accuracy of the measuring equipment and the tolerance of the power electronic components of the converter.

[0050] Currently, electronic current transformers are widely used in flexible DC transmission projects. Compared to electromagnetic current transformers, electronic current transformers do not have saturation issues, offer a wider measurement bandwidth, and achieve measurement accuracy within 1%. Therefore, the amplitude of the detection signal should not be lower than the error range of the measuring equipment, i.e., not lower than 0.01 per-unit value of the rated voltage. When considering the injected signal, the safety margin of the submodule switching process and the converter valve arm current must be taken into account. Excessive injected signal amplitude may affect the safe operation of the injected equipment; a comprehensive assessment of the impact of the injected signal on the equipment is necessary. Considering the cost of capacitors and power devices, a reasonable range is approximately 10% to 15%, meaning the voltage fluctuation component of the submodule capacitor should be lower than 0.15 per-unit value. In summary, this method selects 0.12 per-unit value as the signal amplitude to optimize current protection performance.

[0051] The basic principle for selecting the signal injection duration (Δt) is to minimize the signal injection time while ensuring effective signal information extraction, so as to avoid affecting the normal operation of the circuit. Taking into account factors such as system inertia, controller response delay, and transformer measurement delay, the injection duration can ultimately be defined as Δt = 100ms.

[0052] Example 3: In the above embodiments, the specific extraction method and process for the impedance phase feature extraction of voltage and current response data in step 104 are not limited. Traditional feature extraction often uses conventional algorithms such as Fourier transform. However, AC line response data during a fault is significantly complex: on the one hand, the data contains fault transient components, line noise, and harmonic interference, which are typical non-stationary signals; on the other hand, the Fourier transform has limited ability to decompose non-stationary signals and is prone to problems such as frequency leakage and feature confusion, resulting in large deviations in the extracted impedance phase parameters, or even failing to accurately reflect the true impedance characteristics of the fault area, thereby affecting the reliability of fault determination and the timeliness of protection actions.

[0053] To address this issue, this embodiment proposes a specific implementation process for impedance phase feature extraction: First, the voltage response data and current response data are preprocessed by filtering; then, the EWT (Empirical Wavelet Transform) algorithm is used to decompose the preprocessed data and extract the target signal component that matches the frequency of the detected signal; subsequently, the target signal component is input into the Prony algorithm for mode parameter identification to obtain the amplitude and phase information of the target signal component; finally, the phase difference between the voltage target signal component and the current target signal component at the same measurement point is calculated as the impedance phase feature parameter.

[0054] Methods for extracting amplitude and phase information of characteristic signals include wavelet transform, fast Fourier transform (FFT), and the Prony algorithm. While wavelet transform can effectively capture the time-varying characteristics of signals, it cannot provide accurate phase information; FFT reduces computational complexity by reducing multiplication and addition operations, but its accuracy is limited by factors such as the selection of the data window and the DC attenuation component; the Prony algorithm can describe the transient characteristics of signals, but it is relatively sensitive to noise.

[0055] This method proposes an EWT-Prony harmonic detection and identification algorithm. Its basic principle is as follows: the EWT algorithm is used to decompose the noisy harmonic signal, obtaining a series of signal components. After filtering out the noise components, soft-threshold wavelet denoising is applied to the remaining signal components, thereby enhancing the harmonic signal. By determining the precise number of signal components and defining the dimensional range of the Prony algorithm based on this number, the denoised harmonic signal will undergo mode parameter identification. This method uses the Prony algorithm to extract the amplitude and phase of the characteristic signal based on the measured values ​​at both ends. Its mathematical model is shown in equation (3).

[0056] (3) In the formula, where, Indicates the measurement signal. For amplitude, As the attenuation factor, For frequency, For phase, This represents the model order. As the model order increases, the computational complexity of the Prony algorithm grows exponentially.

[0057] In this method, filtering preprocessing can effectively remove noise and irrelevant interference components, laying a clean data foundation for subsequent feature extraction; the EWT algorithm can adaptively match the frequency characteristics of non-stationary signals, accurately separate the target component with the same frequency as the detection signal, and avoid feature confusion; the Prony algorithm can efficiently extract the amplitude and phase information of the target signal, and the identification error is greatly reduced compared with traditional algorithms; the impedance phase characteristic parameters obtained based on the accurate phase difference can truly reflect the impedance phase characteristics of the fault area, making the comparison results with the preset threshold more valuable, thereby greatly improving the accuracy of fault area determination and the speed and reliability of line protection action.

[0058] Example 4: To avoid the problems of high hardware cost, large injection delay, and rapid signal attenuation caused by relying on external dedicated injection devices or simply using the converter port voltage superposition signal for detection signal injection in this method, this method controls the receiving-end MMC converter of the flexible DC transmission system to inject the detection signal into the AC line. The injection method of the receiving-end MMC converter to inject the detection signal into the AC line is not limited in the above embodiments. For example, it can be achieved by superimposing a weak excitation signal on the converter modulation wave or by using the short-time conduction timing difference of the bridge arm switching devices. In order to further optimize the injection accuracy and coupling effect of the detection signal, reduce signal interference and attenuation under fault transient environment, and improve the fidelity of subsequent response data, this embodiment proposes a method of signal injection through differential mode voltage. That is, using the differential mode voltage formed by the upper and lower bridge arms of the MMC converter, the detection signal is injected into the AC line during the fault.

[0059] This injection method achieves precise internal injection of the detection signal through differential mode voltage of the bridge arm. It ensures strong coupling between the injected signal and the faulty line. The differential mode voltage injection method has strong signal directivity and small attenuation, which can effectively avoid coupling interference and signal attenuation of fault transient signals. This makes the collected response data more realistically reflect the impedance phase characteristics of the fault area and provides high-fidelity data support for fault diagnosis.

[0060] The detection signal itself possesses specific amplitude and frequency characteristics. If the differential-mode voltage is directly generated by relying on the existing sub-module configuration of the bridge arm, the difference in output voltage between the upper and lower bridge arms may not match the characteristics of the detection signal, leading to amplitude deviation and frequency distortion in the injected signal. This, in turn, affects the accuracy of subsequent voltage and current response data and weakens the accuracy of fault diagnosis. To further achieve accurate matching between the detection signal and the differential-mode voltage, a preprocessing step can be added before generating the differential-mode voltage from the upper and lower bridge arms of the MMC converter to inject the detection signal: that is, the number of sub-modules of the upper and lower bridge arms of the MMC converter is specifically configured according to the signal characteristics of the detection signal. Correspondingly, the specific implementation of the subsequent differential-mode voltage injection is also optimized as follows: the differential-mode voltage is generated by connecting the voltages of the upper and lower bridge arms through the configured sub-modules, and the detection signal is injected into the AC line during a fault.

[0061] By precisely configuring the number of sub-modules, the difference in output voltage between the upper and lower bridge arms (i.e., differential mode voltage) is made to closely match the characteristics of the detection signal in terms of key parameters, thereby avoiding signal distortion from the source, ensuring the integrity and accuracy of the injected signal, and providing high-fidelity data support for subsequent impedance phase feature extraction.

[0062] To accurately respond to the dynamic characteristics of the detection signal, such as amplitude and frequency, while also considering the arm voltage balance during normal operation of the MMC converter, this embodiment further proposes a specific configuration method for the number of submodules. First, in the lower control loop of the MMC converter, differential additional values ​​corresponding to the detection signal are superimposed on the three-phase lines respectively. Then, based on the arm voltage reference value under normal operating conditions, combined with the differential additional value, the voltage reference values ​​of the upper and lower arms of the MMC converter are calculated. Finally, based on the voltage reference values ​​of the upper and lower arms, the number of submodules to be deployed to match the voltage output requirement is accurately calculated, and the corresponding number of submodules are deployed in the upper and lower arms respectively.

[0063] The receiving-end MMC converter station of the flexible DC transmission system consists of sub-modules composed of semiconductor switching devices with low rated voltage, which are cascaded to form a high-voltage inverter. The basic control strategies required for the MMC include inner and outer loop control strategies, circulating current suppression strategies, and sub-module capacitor voltage balancing strategies. The frequency domain form of the basic dynamic equations of the MMC in the dq coordinate system is shown in Equation (4).

[0064] (4) Where imd(s) and imq(s) represent the d-axis and q-axis components of the three-phase AC current at the MMC valve terminals, respectively; usd(s) and usq(s) represent the d-axis and q-axis components of the three-phase AC bus voltage at the MMC converter station, respectively; and ufd(s) and ufq(s) represent the d-axis and q-axis components of the three-phase differential mode modulation voltage of the MMC, respectively. It can be seen from formula (4) that the output current of the MMC depends on the system voltage and the bridge arm differential mode voltage.

[0065] Based on formula (4), the transfer function relationship between the three-phase differential modulation voltage of the MMC and the three-phase AC current at the MMC valve terminal can be obtained, providing a theoretical basis for the harmonic signal injection method. Based on this, this method considers using differential injection to inject a detection signal with a definite frequency and amplitude from the converter end into the near-field AC grid terminal at the receiving end. The injection strategy control block diagram is as follows: Figure 2 As shown.

[0066] Parameter UN1 represents the q-axis voltage command value of the positive-sequence voltage outer loop. Based on the above analysis, this value is Vref under steady-state conditions; when the negative-sequence current increases to the limit and the MMC side initiates buck fault ride-through control, UN1 is calculated by the corresponding control algorithm. In summary, UN1 can be summarized as follows: (5) Wherein, UN2 and IN2 represent the magnitudes of the negative sequence voltage and current, respectively.

[0067] Figure 2 In the middle, when the injection strategy activation signal uctrl is triggered, an additional value u*detj (j=a, b, c) will be added in the lower control loop of MMC in differential mode. This value corresponds to the reference values ​​up and un of the upper and lower bridge arm voltages, as shown in formula (6).

[0068] (6) Where u*difj (j=a, b, c) and U*dc represent adaptive correction functions for the differential mode voltage reference values ​​of the upper and lower bridge arms and the DC side voltage reference values.

[0069] The number of submodules Np and Nn participating in the upper and lower arms at any given time are as follows: (7) The expression for the detected signal is: (8) Where ωdet, U*det, and φctrl represent the frequency, amplitude, and initial phase angle of the detection signal generated by the injection strategy, respectively.

[0070] This configuration achieves precise coupling between the dynamic characteristics of the detection signal and the arm voltage output by independently superimposing differential values ​​on the three phases of the lower control loop. It relies on iterative calculations based on the arm voltage reference value under normal operating conditions, avoiding disruption to the original control logic of the converter and effectively ensuring arm voltage balance and system stability. Furthermore, it ensures precise matching of the arm output voltage to the detection signal requirements through quantitative calculation of the number of submodules, significantly improving the fidelity of the injected signal and the operability of the configuration process. Of course, this embodiment is not limited to this method of configuring the number of submodules. For example, a difference coefficient between the number of upper and lower arm submodules can be preset based on the amplitude and frequency characteristics of the detection signal. By dynamically adjusting the number of submodules by real-time acquisition of the arm output voltage feedback value, the differential mode voltage of the upper and lower arms can be made to match the detection signal parameters. Other implementation methods can be referred to the description in this embodiment and will not be elaborated further here.

[0071] Example 5: In the fault zone determination stage of the near-zone AC line protection at the receiving end of the flexible DC transmission system, the above embodiments do not define the specific range of the threshold and the corresponding determination logic. In traditional schemes, the preset phase threshold often suffers from problems such as vague division and lack of specificity: either only a single threshold boundary is set without scientifically dividing it based on the essential differences in impedance phase characteristics when faults occur inside or outside the zone; or the threshold range does not match the operating characteristics of the near-zone lines at the receiving end of the flexible DC transmission system, which easily leads to situations where faults inside the zone are misjudged as outside the zone and vice versa, thereby causing protection to fail to operate or to maloperate, threatening the safe and stable operation of the system.

[0072] This embodiment proposes a specific configuration and judgment rule for a preset phase threshold: the preset phase threshold includes a first threshold range and a second threshold range, wherein the first threshold range is 0°~180° and the second threshold range is -180°~0°; correspondingly, the specific implementation of step 105, which compares the impedance phase characteristic parameter with the preset phase threshold to determine the fault area, is as follows: when the impedance phase characteristic parameter is within the first threshold range, it is determined to be an in-area fault; when the impedance phase characteristic parameter is within the second threshold range, it is determined to be an out-of-area fault.

[0073] Figure 3 The diagram shows a schematic of the AC power grid near the receiving end of a flexible DC transmission system. The "receiving-end MMC converter station" on the left is the flexible DC converter station; the DC line is connected to the left side of the converter station, and the AC line to be protected is connected to the right side. "M" and "N" on the line are two measurement points (used to detect injected signals); "f1-f5" on the line indicate potential short-circuit fault locations.

[0074] Taking external fault f5 single-phase grounding as an example, Figure 4This is the equivalent model of the system fault at the characteristic frequency under external fault conditions. In the figure, uM(t) and iM(t) represent the characteristic frequency voltage and characteristic frequency current at measurement point M, respectively; uN and iN(t) represent the characteristic frequency voltage and characteristic frequency current at measurement point N, respectively; iMC(t) and iNC(t) are the characteristic frequency currents analyzed by the grounding capacitance on the M and N sides, respectively; LL, RL, and CL represent the lumped parameters of inductance, resistance, and capacitance of the Π-type equivalent circuit of the line at the characteristic frequency, respectively; and Rg is the transition resistance value during the fault.

[0075] From Kirchhoff's laws, we can obtain equation (9): (9) Analysis reveals that, at the characteristic frequency, the relationship between the characteristic frequency voltage and the characteristic frequency current of the ground capacitance branches on both sides of the line is as follows: (10) Define the characteristic frequency differential current icd(t) and the characteristic frequency differential voltage ucd(t) as follows: (11) (12) Combining equations (9), (10), (11), and (12), the relationship between the characteristic frequency differential voltage ucd(t) and the characteristic frequency differential current icd(t) at both ends of the faulty line within the area satisfies: (13) By utilizing the capacitance and inductance characteristics of electrical components and combining them with Kirchhoff's laws, it can be seen that the differential voltage phase lags behind the differential current, indicating that the line model exhibits capacitive characteristics at this characteristic frequency. The phasor relationship between voltage and current at the characteristic frequency during an external fault is as follows: Figure 5 As shown.

[0076] The range of the phase difference between voltage and current at the characteristic frequency is shown in the equation: (14) In the formula, Represents the characteristic frequency differential voltage phasor form. It represents the phasor form of the differential current at the characteristic frequency.

[0077] Regarding the calculation method for characteristic frequency fault quantities during faults within the zone, taking single-phase ground fault F3 within the zone as an example... Figure 6 This is the equivalent model of system faults at characteristic frequencies under internal faults. After the star-delta transformation, the equivalent model of system faults at characteristic frequencies under internal faults can be equivalently represented as follows: Figure 7 ,exist Figure 7In the equations, the equivalent impedances Z13, Z12 and Z23 of the line at the characteristic frequency are equivalently processed by star-delta transformation, and the equivalent impedances of the line can be expressed as equations (15), (16) and (17).

[0078] (15) (16) (17) Where ωdet is the characteristic angular frequency, i12 is the equivalent branch current shunt at the characteristic frequency of the fault on the M side, i23 is the equivalent branch current shunt at the characteristic frequency of the fault on the N side, R13 and L13 are the resistance and inductance of the equivalent impedance Z13 at the characteristic frequency of equivalent branch 13, R23 and L23 are the resistance and inductance of the equivalent impedance Z23 at the characteristic frequency of equivalent branch 23, and R12 and L12 are the resistance and inductance of the equivalent impedance Z12 at the characteristic frequency of equivalent branch 12.

[0079] Similarly, internal faults also occur: (18) According to the current splitting principle, the current distribution coefficient of the outgoing line is set as shown in equation (19).

[0080] (19) Therefore, the relationship between the currents i13 and i23 flowing through each branch and the differential current icd(t) is shown in equation (20).

[0081] (20) Through analysis Figure 6 The equivalent model of the fault characteristic frequency in the area shown, and by applying Kirchhoff's voltage law, can derive equations (21) and (22).

[0082] (twenty one) (twenty two) Combining the above equations, the relationship between the characteristic frequency differential voltage ucd(t) and the characteristic frequency differential current icd(t) at both ends of the faulty line within the area satisfies: (twenty three) Based on Kirchhoff's laws, the differential voltage phase leads the differential current, indicating that the line model exhibits inductive characteristics at this characteristic frequency. The phasor relationship between voltage and current at the characteristic frequency during an in-zone fault is as follows: Figure 8 As shown in the figure. When there is a fault in the region, the range of the phase difference between voltage and current at the characteristic frequency is shown in equation (24).

[0083] (twenty four) The characteristic frequency line model exhibits inductive characteristics under internal faults and capacitive characteristics under external faults. The impedance ZH of the characteristic frequency line model is defined and calculated by equation (25). The fault location is determined based on the capacitive or inductive characteristics of the characteristic frequency line impedance ZH.

[0084] (25) Therefore, the protection criterion is (26) In the formula, arg(ZH) represents the phase angle of the line model impedance ZH at the characteristic frequency. When the equivalent impedance exhibits capacitive characteristics, the phase angle is negative; when it exhibits inductive characteristics, the phase angle is positive. When arg(ZH) > +Dset, it is determined to be an in-zone fault; when arg(ZH) < -Dset, it is determined to be an out-of-zone fault. The safety margin Dset = 5° is set considering the following factors: typical measurement error of electronic transformers (phase error < 1°), system frequency drift effect (< 0.5°), phase estimation deviation of the signal extraction algorithm (EWT-Prony) (< 2°), and the reserved additional safety margin (1.5°). The first threshold range of 0° to 180° corresponds to the inherent phase difference characteristics of the voltage and current target signal components when there is a fault within the zone. Faults within the zone will cause the line impedance to exhibit an inductively dominated electrical response, and the phase difference between voltage and current naturally falls within this range. The numerical setting is directly anchored to this physical law to ensure the accuracy of fault identification within the zone. The second threshold range of -180° to 0° precisely matches the phase response characteristics when there is a fault outside the zone. Under faults outside the zone, the equivalent impedance characteristics of the line are reversed, and the phase difference between voltage and current exhibits an inverse distribution. This numerical range precisely covers this inherent difference, enabling accurate differentiation of faults outside the zone.

[0085] This numerical setting based on the physical nature of the fault fundamentally avoids the problem of threshold division being disconnected from the actual electrical response, making the fault zone determination based on objective laws. It simplifies the determination logic and significantly reduces the probability of misjudging faults inside or outside the zone, effectively eliminating the risk of protection failure or maloperation, and providing a scientific and practical basis for the accurate and reliable execution of line protection actions.

[0086] Example 6: To enhance understanding, this embodiment describes an implementation process; see [link to implementation details]. Figure 9 .

[0087] First, a refined modeling and fault equivalent network construction were carried out for the two-end DC transmission system. The modeling needs to cover the core components of the system, including the flexible DC receiving-end MMC converter, converter transformer, smoothing reactor, DC line, and grounding electrode system. For the MMC converter, equivalent parameters are calculated based on the bridge arm equivalent resistance, inductance, and submodule balancing capacitance. For the converter transformer, the effects of short-circuit impedance and connection group are considered. For the DC line, a frequency-dependent model is used to characterize the distributed parameter characteristics. Then, based on actual fault scenarios (such as in-zone / out-zone faults, single-phase grounding of phase A / two-phase short circuit of phases A and B), targeted fault equivalent networks are drawn: the location and type of the fault point are clearly defined, the fault path is equivalent to a lumped parameter impedance (e.g., the grounding resistance of the fault point needs to be considered for single-phase grounding faults), the parameters of non-faulty branches are simplified, and a closed-loop circuit of power source-equivalent impedance-fault point is formed, providing a clear circuit model for current calculation. Based on this, the fault current without considering current limiting control is obtained: Based on Kirchhoff's laws and combined with the resistance, inductance, and capacitance parameters of the equivalent network, the time-domain differential equation or complex frequency domain equation of the fault current is established. All current limiting control links (such as low-voltage current limiting and active current limiting) are ignored, and only the discharge characteristics of the submodule capacitor and the inherent impedance of the system are considered. The natural current change curve in the initial stage of the fault is obtained by solving. This current reflects the severity of the fault when the system has no control intervention.

[0088] The process then proceeds to the active current limiting control activation decision stage. The core decision indicators are the DC voltage threshold (typically 0.9 times the rated DC voltage) and the fault current change rate: when the detected DC voltage is below the set threshold, or the fault current rise rate exceeds a preset limit (e.g., greater than 2kA / ms), active current limiting control is activated; otherwise, the original unlimited fault current calculation result is maintained. If active current limiting control is activated, virtual resistors (Rvirt) and virtual reactance (Lvirt) need to be precisely added to the DC transmission system models at both ends. The virtual resistor is simulated through a control algorithm, and its core function is to dampen circulating current oscillations during faults and optimize power distribution accuracy. Its value needs to balance the current limiting effect with the risk of voltage drop (typically 0.1-0.5Ω). The virtual reactance is used to make the converter output impedance inductive, adapting to the power decoupling requirements of droop control and avoiding reactive power cross-interference. Parameter settings need to match the system switching frequency and inherent impedance characteristics. After adding virtual components, the equivalent network is reconstructed. The short-circuit current, which takes into account the active current limiting control strategy, is calculated using the corrected loop impedance equation. This current can effectively suppress the peak value and rise rate, reducing the equipment's withstand pressure.

[0089] Finally, the converter station's lockout status is determined. The determination is based on the peak arm current and the fault duration: when the MMC converter arm current exceeds twice the rated current (IGBT safe operating limit), or when the fault causes the system voltage and current to exceed the stable range for an extended period, the converter station triggers a lockout command; otherwise, the current short-circuit current result considering current limiting control is output. If the converter station is locked out, the system model needs to be updated to construct a new fault equivalent network: after lockout, the MMC converter stops switching submodules, equivalent to an arm series resistive load (or open circuit state). The original converter's equivalent power supply and control loops fail, and the fault circuit consists only of the DC line, smoothing reactor, and remaining energy storage elements. The short-circuit current is recalculated based on the new model. At this point, the current mainly originates from the decaying discharge of the system's remaining energy. Its amplitude and decay rate need to be solved in conjunction with the equivalent impedance after lockout and the initial energy storage parameters, ultimately providing accurate current data support for line protection actions.

[0090] A flexible DC transmission system model was built using PSCAD / EMTDC to verify the proposed fault current calculation method. The arg(ZH) values ​​for the three phases A, B, and C were obtained for different conditions inside and outside the transmission zone, as shown in Tables 1 and 2 below.

[0091] Table 1 Protection characteristics of different types of out-of-area faults

[0092] Table 2 Protection characteristics of different types of faults within the zone

[0093] Table 1 (Characteristics of Protection Against External Faults) focuses on the fault location f5 (outside the zone) and lists the arg(ZH) values ​​of phases A, B, and C under four types of external faults: single-phase grounding of phase a, two-phase short circuit of phases a and b, etc. These values ​​are all in the range of -180° to 0° (e.g., -85.51°, -83.23°), which is consistent with the phase threshold range corresponding to external faults, confirming the parameter's ability to identify external faults.

[0094] Table 2 (Characteristics of Fault Protection within the Zone) covers four types of faults within the zone, including single-phase grounding of phase a and two-phase short circuit of phases a and b. The corresponding arg(ZH) values ​​of the three phases A, B, and C (e.g., 67.93° and 50.28°) are all in the range of 0° to 180°, matching the phase threshold range of faults within the zone. At the same time, the unmarked phases (no valid data) during single-phase and two-phase faults also conform to the electrical response characteristics of the faulty phase, demonstrating the effectiveness of this parameter in distinguishing faults within the zone.

[0095] The two tables, using arg(ZH) data from actual fault scenarios, support the judgment rule that arg(ZH) between -180° and 0° indicates an external fault and between 0° and 180° indicates an internal fault. This shows that the proposed method can correctly identify faults inside and outside the fault zone, and has high sensitivity, without relying on complex setting calculations.

[0096] Example 7: This embodiment relates to a receiving-end near-area AC line protection device for a flexible DC transmission system based on MMC injection. A schematic diagram of the receiving-end near-area AC line protection device for this embodiment based on MMC injection is shown below. Figure 10 As shown, it includes: a detection signal generation unit 201, a detection signal injection unit 202, a response acquisition unit 203, a feature extraction unit 204, and a fault location unit 205.

[0097] Among them, the detection signal generation unit 201 is used to monitor the operating status of the AC line in the near-field area of ​​the receiving end of the flexible DC transmission system in real time. When the operating status meets the preset fault disturbance conditions, it generates a detection signal with preset characteristics. The detection signal injection unit 202 is used to control the receiving-end MMC converter of the flexible DC transmission system to inject detection signals into the AC line during a fault. The response acquisition unit 203 is used to acquire voltage response data and current response data after the injection of a detection signal through a preset measurement point on the AC line; Feature extraction unit 204 is used to extract impedance phase features from voltage response data and current response data to obtain impedance phase feature parameters; The fault location unit 205 is used to compare the impedance phase characteristic parameters with the preset phase threshold, determine the fault area based on the comparison result, and execute the corresponding line protection action.

[0098] The near-zone AC line protection device for the receiving end of a flexible DC transmission system based on MMC injection provided in this embodiment has the following features: a detection signal generation unit that monitors the line's operating status in real time and accurately generates preset characteristic signals when a fault occurs; a detection signal injection unit that relies on the MMC converter at the receiving end of the flexible DC system to achieve endogenous injection, eliminating the need for external dedicated devices, simplifying the architecture while reducing hardware costs and injection delays; a response acquisition unit that accurately captures voltage and current response data after the injection signal through preset measurement points; a feature extraction unit that specifically processes non-stationary response data under fault scenarios, obtaining accurate impedance phase characteristic parameters through a scientific extraction process, effectively resisting interference and avoiding feature confusion; and a fault location unit that quickly and accurately determines faults within and outside the zone based on dual-phase threshold division that closely matches the physical nature of the fault, avoiding protection failure or false tripping.

[0099] The device's various units work together in a closed loop, from signal generation, injection, and acquisition to feature extraction and fault location. It is not only adapted to the electrical characteristics of near-field lines at the receiving end of the flexible DC transmission system, but also significantly improves the timeliness, accuracy, and reliability of protection actions, providing a solid guarantee for the safe and stable operation of the system.

[0100] It should be noted that the contents of the MMC injection-based flexible DC transmission system receiving-end near-area AC line protection device provided in this embodiment can be referred to in conjunction with the MMC injection-based flexible DC transmission system receiving-end near-area AC line protection method provided in the above embodiments, and the repeated parts will not be described again in this embodiment.

[0101] Furthermore, it is worth mentioning that all units involved in this embodiment are logic modules. In practical applications, a logic unit can be a physical unit, a part of a physical unit, or a combination of multiple physical units. In addition, to highlight the innovative aspects of this application, this embodiment does not introduce units that are not closely related to solving the technical problems proposed in this application; however, this does not mean that other units are absent from this embodiment.

[0102] Example 8: Another embodiment of this application relates to a receiving-end near-zone AC line protection system for a flexible DC transmission system based on MMC injection, comprising: a receiving-end near-zone AC line protection device for the flexible DC transmission system, a receiving-end MMC converter for the flexible DC transmission system, a detection device, and a receiving-end near-zone AC line for the flexible DC transmission system.

[0103] The core of the system is the near-zone AC line protection device for the receiving end of the flexible DC transmission system based on MMC injection. It integrates five functional units: detection signal generation, detection signal injection, response acquisition, feature extraction, and fault location, undertaking the decision-making and control tasks of the entire process: The detection signal generation unit monitors the operating status of the protected line in real time and generates a detection signal with preset characteristics when the preset fault disturbance conditions are met; the detection signal injection unit is responsible for issuing control commands and coordinating the execution of signal injection; the response acquisition unit receives externally acquired data and transmits it to the feature extraction unit; the feature extraction unit extracts impedance phase characteristics from the response data and outputs feature parameters; the fault location unit determines the fault area and triggers the corresponding protection action by comparing the feature parameters with the preset phase threshold.

[0104] Specifically, the receiving-end MMC converter of the flexible DC transmission system establishes a stable communication connection with the detection signal injection unit of the protection device, specifically receiving control commands issued by this unit. Its core function is to accurately inject the detection signal into the near-field AC line at the receiving end during a line fault, relying on its own topology characteristics, by forming a differential mode voltage through the series output voltage of the upper and lower bridge arms. No additional dedicated injection device is required, which not only adapts to the original system topology but also ensures the strong coupling and stability of the injected signal.

[0105] The detection device, installed at a preset measurement point on the AC line, serves as a data acquisition terminal and communicates with the response acquisition unit of the protection device. Its core function is to capture the AC line voltage and current response data in real time after the injection of the detection signal. During the acquisition process, it ensures the real-time performance, integrity, and accuracy of the data. The processed response data is then stably transmitted to the response acquisition unit of the protection device, providing a high-quality, high-fidelity data source for subsequent impedance phase characteristic extraction.

[0106] The protected flexible DC transmission system's receiving-end near-area AC line is connected between the wind farm's main transformer and the flexible DC wind farm-side converter transformer. This line receives the detection signal injected by the MMC converter and exhibits corresponding electrical response characteristics when a fault occurs. By capturing relevant data through detection equipment, fault protection is ultimately achieved based on the decision output of the protection device, ensuring the safe and stable operation of itself and upstream and downstream equipment.

[0107] The specific type of detection equipment used to collect AC line response data in the system is not limited in this embodiment, and may include electronic current transformers (ECT) and electronic voltage transformers (EVT). The electronic current transformer is responsible for accurately capturing the current response data of the AC line after the injection of the detection signal, while the electronic voltage transformer is specifically responsible for collecting the corresponding voltage response data.

[0108] ECT (Optical Current Transformer) avoids the inherent defects of traditional electromagnetic transformers, such as magnetic saturation and ferroresonance. It can accurately capture current response data even in complex scenarios such as fault transients and harmonic interference, and is particularly adept at capturing weak current components corresponding to injected detection signals. EVT (Electronic Voltage Transformer), with its excellent insulation performance and fast dynamic response characteristics, can accurately track instantaneous voltage changes and fully collect voltage response information during faults. Together, they form a high-fidelity, multi-dimensional data source, laying a solid foundation for subsequent impedance phase feature extraction and accurate fault determination. It should be noted that this embodiment is not limited to this; other types of detection devices can be configured, such as optical fiber current transformers (OCT), optical fiber voltage transformers (OVT), and digital integrated transformers, all of which can be referred to the description in this embodiment and will not be elaborated further here.

[0109] Those skilled in the art will understand that the above embodiments are specific embodiments for implementing this application, and in practical applications, various changes can be made to them in form and detail without departing from the spirit and scope of this application.

Claims

1. A method for protecting near-field AC lines in a flexible DC transmission system based on MMC injection, characterized in that, include: The system monitors the operating status of AC lines near the receiving end of the flexible DC transmission system in real time. When the operating status meets the preset fault disturbance conditions, a detection signal with preset characteristics is generated. Control the receiving-end MMC converter of the flexible DC transmission system to inject the detection signal into the AC line during a fault. Voltage response data and current response data after the injection of detection signal are collected through the preset measurement points of the AC line; Impedance phase features are extracted from the voltage response data and current response data to obtain impedance phase feature parameters; The impedance phase characteristic parameters are compared with a preset phase threshold, and the fault area is determined based on the comparison result in order to execute the corresponding line protection action.

2. The method for protecting near-field AC lines in a flexible DC transmission system based on MMC injection according to claim 1, characterized in that, Controlling the receiving-end MMC converter of the flexible DC transmission system to inject the detection signal into the AC line during a fault includes: The detection signal is injected into the AC line during a fault by utilizing the differential mode voltage formed by the upper and lower arms of the MMC converter.

3. The method for protecting near-field AC lines in a flexible DC transmission system based on MMC injection according to claim 2, characterized in that, Before injecting the detection signal into the AC line during a fault using the differential-mode voltage formed by the upper and lower arms of the MMC converter, the method further includes: Configure the number of sub-modules in the upper and lower arms of the MMC converter based on the signal characteristics of the detected signal; The differential-mode voltage formed by the upper and lower bridge arms of the MMC converter is used to inject the detection signal into the AC line during a fault. Specifically, the differential-mode voltage formed by the voltage output from the upper and lower bridge arms through the submodule is used to inject the detection signal into the AC line during a fault.

4. The method for protecting near-field AC lines in a flexible DC transmission system based on MMC injection according to claim 3, characterized in that, Based on the signal characteristics of the detected signal, the number of sub-modules in the upper and lower arms of the MMC converter is configured, including: In the lower control loop of the MMC converter, differential additional values ​​corresponding to the detection signals are superimposed on the three phase lines respectively; Based on the reference values ​​of the bridge arm voltage under normal operating conditions, and in conjunction with the differential additional value, the reference values ​​of the voltage of the upper and lower bridge arms of the MMC converter are calculated. Based on the voltage reference values ​​of the upper and lower bridge arms, calculate the number of sub-modules that need to be deployed in the upper and lower bridge arms to match the voltage reference values, and deploy the corresponding number of sub-modules in the upper and lower bridge arms respectively.

5. The method for protecting near-field AC lines in a flexible DC transmission system based on MMC injection according to claim 1, characterized in that, The detection signal that generates preset features includes: Even-order harmonics are generated as a detection signal with an amplitude of 0.12 per-unit value based on the rated voltage of the AC line.

6. The method for protecting near-field AC lines in a flexible DC transmission system based on MMC injection according to claim 1, characterized in that, Impedance phase feature extraction is performed on the voltage response data and current response data, including: The voltage response data and current response data are preprocessed by filtering; The EWT algorithm is used to decompose the preprocessed data and extract the target signal component that matches the frequency of the detection signal. The target signal component is input into the Prony algorithm for mode parameter identification to obtain the amplitude and phase information of the target signal component. The phase difference between the voltage target signal component and the current target signal component at the same measurement point is calculated and used as the impedance phase characteristic parameter.

7. The method for protecting near-field AC lines in a flexible DC transmission system based on MMC injection according to claim 1, characterized in that, The preset phase threshold includes: a first threshold range and a second threshold range; the first threshold range is 0°~180°, and the second threshold range is -180°~0°; The impedance phase characteristic parameters are then compared with a preset phase threshold, and the fault region is determined based on the comparison result, including: When the impedance phase characteristic parameter is within the first threshold range, it is determined to be an in-zone fault; When the impedance phase characteristic parameter is within the second threshold range, it is determined to be an external fault.

8. A receiving-end near-zone AC line protection device for a flexible DC transmission system based on MMC injection, characterized in that, include: The detection signal generation unit is used to monitor the operating status of the AC lines in the near-field area of ​​the receiving end of the flexible DC transmission system in real time. When the operating status meets the preset fault disturbance conditions, it generates a detection signal with preset characteristics. The detection signal injection unit is used to control the receiving-end MMC converter of the flexible DC transmission system to inject the detection signal into the AC line during a fault. The response acquisition unit is used to acquire voltage response data and current response data after the detection signal is injected through preset measurement points on the AC line; The feature extraction unit is used to extract impedance phase features from the voltage response data and current response data to obtain impedance phase feature parameters. The fault location unit is used to compare the impedance phase characteristic parameters with a preset phase threshold, determine the fault area based on the comparison result, and execute the corresponding line protection action.

9. A receiving-end near-zone AC line protection system for a flexible DC transmission system based on MMC injection, characterized in that, include: The receiving-end near-zone AC line protection device for a flexible DC transmission system based on MMC injection as described in claim 8; The receiving-end MMC converter of the flexible DC transmission system is communicatively connected to the detection signal injection unit of the protection device, and is used to receive the control command of the detection signal injection unit. During a fault, the detection signal is injected into the near-zone AC line of the receiving end through the differential mode voltage of the upper and lower bridge arms. The detection device, set at a preset measurement point on the AC line, is communicatively connected to the response acquisition unit of the protection device. It is used to acquire AC line voltage response data and current response data after the injection of detection signal, and transmit the response data to the response acquisition unit. The protected flexible DC transmission system receiving-end near-area AC line is connected between the wind farm main transformer and the flexible DC wind farm side converter transformer.

10. The receiving-end near-zone AC line protection system for flexible DC transmission systems based on MMC injection according to claim 9, characterized in that, The testing equipment includes: electronic current transformers and electronic voltage transformers.

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