Power transmission line corrosion and safety assessment method based on grounding electrode current

Abstract

The invention relates to the technical field of power transmission line maintenance, and discloses a grounding electrode current-based power transmission line corrosion and safety evaluation method, which comprises the steps of monitoring a step excitation signal of a high-voltage direct-current power transmission system, controlling the tail end impedance of an OPGW (Optical Fiber Composite Overhead Ground Wire) to perform time sequence switching in a transient attenuation stage, and collecting bimodal transient current data; based on a transmission line-earth screen distribution parameter model containing electrochemical polarization characteristics, performing frequency domain transformation on the data, constructing a normalized differential shunt spectrum, and decoupling common-mode interference of an earth potential rise excitation source; inverting the charge transfer resistance and the double-electric-layer capacitance of the line pole tower by using a regularization algorithm; and calculating the instantaneous corrosion current density according to the inversion parameters, evaluating the corrosion state of the tower grounding device, and judging that the tower corrosion state belongs to an active corrosion stage. Through an active modulation and frequency domain difference algorithm, dependence on absolute parameters of an excitation source is eliminated, and quantitative evaluation of the grounding corrosion rate of the power transmission line tower under the excavation-free condition is realized.

Description

Technical Field

[0001] This invention relates to the field of power transmission line maintenance technology, specifically to a method for assessing power transmission line corrosion and safety based on grounding electrode current. Background Technology

[0002] Grounding devices on transmission line towers are critical facilities for ensuring the safe operation of power systems and providing lightning protection. Because they are buried in the soil environment for extended periods, grounding devices are inevitably affected by soil electrochemical corrosion. This is especially true under the monopolar polarity operation conditions of high-voltage direct current transmission systems, where stray currents in the ground accelerate the electrolytic corrosion process of the grounding electrode. Corrosion of the grounding device can lead to a reduction in effective cross-section, increased grounding resistance, and even breakage, seriously threatening the operational safety of transmission lines.

[0003] Traditional corrosion detection of grounding systems mainly relies on manual excavation inspection or electromagnetic field measurement. While excavation inspection is intuitive, it is labor-intensive, costly, and disrupts the original compacted soil structure, making it difficult to conduct a comprehensive survey of all towers along the entire line. Existing trenchless electromagnetic diagnostic techniques typically simplify the tower grounding system into a pure resistive network model, inferring the connection status by measuring port resistance. However, metal corrosion is essentially an electrochemical kinetic process involving charge transfer and double-layer effects. A simple resistive model ignores the polarization impedance characteristics of the metal-soil interface and cannot obtain key electrochemical parameters such as charge transfer resistance, thus making it difficult to quantitatively calculate the corrosion rate or differentiate the degree of corrosion activity.

[0004] Furthermore, passive monitoring using the ground current of a high-voltage direct current (HVDC) transmission system as an excitation source is a current research direction. However, in practical applications, this passive monitoring method faces the challenge of uncertain excitation source parameters. The rise in ground potential caused by the injected current at the DC grounding electrode is affected by various factors such as operating power, soil structure, and climatic conditions, exhibiting time-varying and random characteristics. Without the ability to accurately obtain the real-time amplitude and phase information of the excitation source, directly using monitoring data for parameter inversion leads to ill-posed problems in the equations, making it difficult to accurately separate the impedance characteristics of the line itself from the fluctuation interference of the excitation source, thus limiting the accuracy and reliability of the evaluation results. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for assessing transmission line corrosion and safety based on grounding electrode current, solving the problem of uncertain excitation source parameters faced by passive monitoring methods. The rise in ground potential caused by DC grounding electrode injection current is affected by various factors such as operating power, soil structure, and climatic conditions, exhibiting time-varying and random characteristics.

[0006] To achieve the above objectives, the present invention provides the following technical solution: The first aspect of the present invention provides a method for corrosion and safety assessment of transmission lines based on grounding electrode current. This method utilizes the step signal generated during the adjustment of the operating conditions of a high-voltage direct current transmission system as an excitation source, and combines active impedance modulation and frequency domain differential algorithms to invert the electrochemical polarization parameters and assess the corrosion status of the grounding device of the transmission line tower. Specifically, the method includes the following steps: Preferably, a wide-area transient monitoring unit is used to monitor the transformer neutral point current or substation ground potential rise signal in real time. By establishing a wideband continuous monitoring channel, sliding differential calculation is performed on the acquired signal to obtain the instantaneous rate of change characteristic value, and a dynamic trigger threshold is calculated based on the background noise floor. If the monitored instantaneous rate of change characteristic value exceeds the dynamic trigger threshold, it is determined that the system has generated a step excitation, and the excitation trigger time is locked.

[0007] Preferably, during the transient decay phase after the excitation trigger moment, the OPGW active modulation unit deployed at the substation incoming line structure is controlled to execute a timing active modulation strategy. This strategy drives the OPGW terminal impedance to alternately switch between a low-resistance state and a high-resistance state at a preset frequency, synchronously acquiring dual-mode transient current data from the OPGW terminal. The preset frequency must be higher than a multiple of the reciprocal of the transient current decay time constant and lower than the ratio of the speed of light to four times the total length of the transmission line, to avoid traveling wave reflection interference.

[0008] Preferably, based on a transmission line-to-ground grid distributed parameter model incorporating electrochemical polarization characteristics, frequency domain transformation and differential operations are performed on the collected dual-mode transient current data to construct a normalized differential shunt spectrum. The transmission line-to-ground grid distributed parameter model equates the transmission line to cascaded two-port network units, and the tower grounding admittance branches within these network units are characterized using an improved Randle equivalent circuit that includes ohmic resistance, charge transfer resistance, and double-layer capacitance.

[0009] Preferably, the normalized differential split spectrum The construction formula is as follows: ; In the formula, Angular frequency, The measured current spectrum phasor under low resistance state. This represents the measured current spectrum phasor under high impedance conditions. Based on transmission line theory, the current flowing through the OPGW is the excitation source of the ground potential. With system transfer admittance The product of the terms. The above formula uses the ratio of the difference numerator term to the common-mode denominator term to simplify and eliminate the ground potential rise excitation source factor commonly included in both the numerator and denominator. This decouples the common-mode interference of excitation source amplitude fluctuations on the measurement and extracts frequency domain features that are only related to the inherent impedance parameters of the line and ground grid.

[0010] Preferably, an objective function based on the least squares method is constructed, and the charge transfer resistance and double-layer capacitance of the grounding devices of each tower along the line are solved using the normalized differential current shunting spectrum inversion. Regarding the charge transfer resistance R... ct With double-layer capacitance C dl Given the order-of-magnitude difference, a logarithmic space transformation is performed on the parameter vector to be inverted, which contains the above parameters. At the same time, a spatial continuity regularization term constructed using a second-order difference operator is introduced into the objective function to constrain the smoothness of adjacent tower parameters and suppress the ill-posedness of numerical calculations.

[0011] Preferably, the charge transfer resistance R obtained from the inversion is... ct and double-layer capacitance C dl The corrosion rate index was calculated and the corrosion status of towers along the transmission line was assessed using the Stern-Geary equation. Specifically, the instantaneous corrosion current density i was calculated. corr : ; In the formula, For Stern-Gilley constant, This represents the effective current-dissipating surface area of ​​the tower's grounding electrode. Based on the numerical distribution range of charge transfer resistance and instantaneous corrosion current density, the corrosion state of the tower is determined to be either active corrosion, localized damage, or passivation and stabilization.

[0012] A second aspect of the present invention provides a transmission line corrosion and safety assessment system based on grounding electrode current, the system comprising a wide-area transient monitoring unit, an OPGW active modulation unit, and a central processing unit.

[0013] Preferably, the wide-area transient monitoring unit is used to monitor the transformer neutral point current or ground potential rise signal in real time, identify HVDC step excitation, and determine the trigger time.

[0014] Preferably, the OPGW active modulation unit is deployed at the substation incoming line structure and includes a power electronic switch module and a non-inductive damping resistor connected in parallel therewith, used to switch the OPGW end impedance according to control commands during the transient attenuation phase.

[0015] Preferably, the central processing unit is connected to the wide-area transient monitoring unit and the OPGW active modulation unit to perform the method described in the first aspect above, including controlling the time-series active modulation, constructing a physical model containing electrochemical polarization characteristics, calculating the normalized differential shunt spectrum, inverting electrochemical parameters, and evaluating the corrosion state.

[0016] This invention provides a method for assessing the corrosion and safety of transmission lines based on grounding electrode current. It offers the following advantages: 1. This invention achieves physical decoupling between the measurement results and the amplitude of the excitation source by constructing a normalized differential shunt spectrum. It utilizes the active modulation of the OPGW end impedance to obtain the dual-mode response and mathematically eliminates the uncertainty factor of the ground potential rise excitation source by calculating the ratio of the differential numerator to the common-mode denominator. This process effectively overcomes the limitation of traditional measurement methods that heavily rely on precise synchronous data of the ground current entering the HVDC grounding electrode. Even when the absolute amplitude or waveform details of the excitation source cannot be obtained, it can still accurately extract the frequency domain fingerprint characterizing the impedance characteristics of the line and ground grid from a distance, significantly improving anti-interference capability.

[0017] 2. This invention improves the accuracy of corrosion state assessment by introducing a distributed parameter model that incorporates electrochemical polarization characteristics. Unlike traditional models that merely equate tower grounding conductors to pure resistance, this invention employs an improved Randles equivalent circuit to describe grounding admittance, enabling the simultaneous inversion of two key electrochemical parameters: charge transfer resistance and double-layer capacitance. This modeling approach recreates the true electrochemical kinetics of metals in soil electrolytes, allowing for the quantitative calculation of instantaneous corrosion current density using the Stern-Gierley equation. This represents a technological leap from simple "electrical connectivity detection" to "quantitative corrosion rate assessment."

[0018] 3. This invention employs an inversion strategy combining logarithmic space transformation and spatial regularization to address the numerical instability problem in multi-parameter inversion. To address the significant order-of-magnitude difference between charge transfer resistance and double-layer capacitance, the parameter sensitivity is balanced through logarithmic domain transformation, and regularization constraints are applied using the spatial continuity characteristics of transmission line tower corrosion. This algorithm effectively suppresses non-physical spurious solutions caused by field measurement noise, ensuring the convergence and robustness of the inversion results under complex electromagnetic environments, making it suitable for practical engineering applications. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the overall process of the method of the present invention; Figure 2 This is a schematic diagram illustrating the application scenarios and system architecture of the present invention; Figure 3 This is a schematic diagram of the circuit principle of the OPGW active modulation unit of the present invention; Figure 4 This is the equivalent circuit diagram of the transmission line-ground grid distributed parameter model containing electrochemical polarization characteristics of the present invention. Figure 5 This is a flowchart of the normalized differential split spectrum construction and parameter inversion algorithm of the present invention; Figure 6 This is a schematic diagram illustrating the corrosion status assessment and grading of the present invention.

[0020] Among them, 101 is the central processing unit; 102 is the wide-area transient monitoring unit; and 103 is the OPGW active modulation unit. Detailed Implementation

[0021] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] Example: Please see the appendix Figure 1 - Appendix Figure 6 This invention provides a method for assessing the corrosion and safety of transmission lines based on grounding electrode current, including a transmission line corrosion assessment system. This system is applied to AC transmission lines that have electromagnetic coupling or ground potential coupling with a high-voltage direct current transmission system. The system mainly includes: a central processing unit 101, a wide-area transient monitoring unit 102, and an OPGW active modulation unit 103.

[0023] The central processing unit 101 establishes communication connections with the wide-area transient monitoring unit 102 and the OPGW active modulation unit 103, respectively, to receive monitoring data and send control commands. The OPGW active modulation unit 103 is connected in series in the end grounding lead loop of the fiber optic composite overhead ground wire OPGW on the substation side of the AC transmission line, and this loop is connected to the substation's main grounding grid. The wide-area transient monitoring unit 102 is installed in the transformer neutral point grounding loop on the AC transmission line side or at a key node of the substation grounding grid, and is equipped with a broadband current transformer and a high-frequency voltage sensor.

[0024] The OPGW active modulation unit 103 includes an impedance switching circuit composed of a power electronic switch array. This circuit has at least two switchable impedance states: a low-resistance state and a high-resistance state. The low-resistance state corresponds to the operating mode where the OPGW is directly grounded or grounded through a small resistor, while the high-resistance state corresponds to the operating mode where a preset damping impedance is connected in series in the OPGW grounding loop. The wide-area transient monitoring unit 102 has a high-frequency sampling function, and the sampling frequency covers the transient signal frequency band generated during the adjustment of the operating conditions of the high-voltage direct current transmission system.

[0025] It also includes a method for assessing transmission line corrosion and safety based on grounding electrode current. This method relies on the aforementioned system and specifically includes the following steps: S201 utilizes a wide-area transient monitoring unit to collect in real time the ground potential signal of the AC transmission line ground grid and the DC bias current signal of the neutral point. S202, calculate the time change rate of the ground potential signal or the neutral point DC bias current signal. When the time change rate exceeds the preset trigger threshold, it is determined that the high voltage DC transmission system has undergone an operating condition adjustment that generates a step excitation, triggering the active evaluation process. S203, during the transient process after the step excitation is generated, the OPGW active modulation unit is controlled to perform impedance switching operation, and alternately switches between low impedance state and high impedance state according to a preset timing sequence; S204, synchronously collect transient response data under the low resistance state and high resistance state, the transient response data includes the transformer neutral point current waveform and the OPGW grounding lead current waveform; S205, perform frequency domain transformation on the transient response data, and calculate the normalized differential shunt function by utilizing the difference between the low-impedance state frequency domain data and the high-impedance state frequency domain data; S206. Based on the transmission line distributed parameter model with electrochemical polarization characteristics, the normalized differential shunt function is used as the target to perform inversion solution to obtain the charge transfer resistance and double-layer capacitance parameters of the grounding electrode of the tower along the line, and the tower corrosion status is evaluated accordingly.

[0026] In this embodiment, the specific process of non-intrusive identification and triggering of step excitation caused by the adjustment of the operating conditions of the HVDC system mainly relies on the wide-area transient monitoring unit for execution.

[0027] Specifically, a wideband continuous monitoring channel is established. The wide-area transient monitoring unit uses a Hall current sensor or a wideband shunt installed in the transformer neutral point grounding circuit to collect the current signal flowing through the neutral point. Using voltage transformers or RC dividers connected to the substation's main grounding grid, the ground potential rise signal V of the substation grounding grid relative to the distant ground is collected. g (t). During the acquisition process, a first-in-first-out (FIFO) circular storage area is configured. This storage area is continuously refreshed and retains historical data up to a preset duration (e.g., 1 to 5 seconds) before the current moment. This data is used to backtrack and acquire complete signal rising edges and background reference data after a triggered event occurs. Sampling frequency f s The frequency band is set to no less than 10kHz to cover the transient signal band during power regulation and switching between single-pole and bipolar operation modes in high-voltage direct current transmission systems.

[0028] Specifically, the process involves performing sliding differential operations and noise suppression on the signal. The central processing unit 101 reads the discrete sampled sequence from the memory area. Corresponding to I neu or V gThe sampled values ​​are used to calculate the instantaneous rate of change characteristic value of the signal using the sliding window algorithm. To effectively eliminate the interference of 50Hz or 60Hz power frequency components on DC step detection, a differential filter operator is constructed, and its calculation formula is as follows: ; In the formula: For the first The characteristic value of the rate of change of each sampling point; This is the current signal sample value; The differential step size is set to an integer or half-integer multiple of the number of sampling points corresponding to the power frequency cycle (e.g., ...). This utilizes the periodic subtraction characteristic to cancel out steady-state power frequency signals; The length of the smoothing window is used to smooth random impulse noise.

[0029] Specifically, the determination of excitation events is based on an adaptive threshold. The calculated feature values... With dynamic trigger threshold A comparison is made. Among them, the dynamic trigger threshold... The calculations depend on a real-time updated background noise basis. The specific calculation logic is as follows: ; ; In the formula: This is the signal-to-noise ratio gain coefficient, with a typical value range of 3 to 10; A fixed safety margin is provided to prevent false triggering in extremely low noise environments; L is the length of the background noise statistics window; To lag the data points, ensure that the data segment used to calculate background noise is located before the current detection window, thus avoiding the rising edge of the signal contaminating the calculation of the noise floor.

[0030] When satisfied And the duration exceeds the judgment time limit T dur When the system determines that the high-voltage direct current transmission system has undergone an operational condition adjustment (such as power increase / decrease, blocking, or single-polarity operation switching), a step-type stray current excitation is generated in the ground circuit. The system records the current time as the start time t0 of the transient event.

[0031] Specifically, an active modulation trigger command is generated. After confirming time t0, the central processing unit 101 determines the trigger command based on preset delay parameters. The control signal is generated and sent to the OPGW active modulation unit 103. This delay parameter... The setting value is 10ms to 50ms after the first peak of the transient signal. This is intended to avoid the electromagnetic transient oscillation process at the beginning of the step and to start modulation when the signal enters the stable stage of monotonic decay. This ensures that the extracted impedance characteristics mainly reflect the electrochemical polarization characteristics rather than the high-frequency inductance effect. The specific selection and installation wiring of Hall current sensors, broadband shunts and RC voltage dividers are well-known technologies in this field and will not be elaborated here.

[0032] In this embodiment, the timing active modulation strategy of the OPGW terminal impedance is executed by the OPGW active modulation unit 103 deployed at the substation incoming line structure, specifically covering the hardware loop construction, impedance state definition and dynamic timing control process.

[0033] Specifically, a dual-mode impedance switching loop is constructed. The OPGW active modulation unit 103 is connected in series between the grounding down conductor of the optical fiber composite overhead ground wire (OPGW) and the substation's main grounding grid. This unit consists of a high-power power electronic switching module K. SW With non-inductive damping resistor R damp They are connected in parallel and equipped with overvoltage protection branches. Power electronic switch module K SW Insulated-gate bipolar transistors (IGBTs) or gate turn-off thyristors (GTOs) are selected, with an integrated anti-parallel diode to provide a reverse freewheeling path. The non-inductive damping resistor R... damp Select a power-type metal film resistor or ceramic resistor array. The resistance value R of this resistor is... val The resistance value is set to be 0.8 to 1.5 times the sum of the DC resistance of the OPGW metal sheath of the transmission line under test and the equivalent grounding impedance of the towers along the line. The purpose of selecting this resistance range is to ensure that, while maintaining the electrical continuity of the OPGW circuit, the change in the amplitude of the shunt current flowing through the OPGW before and after the switch operation reaches more than 10% of the total current, thereby ensuring the signal-to-noise ratio of the subsequent differential signal. The overvoltage protection branch uses a metal oxide varistor (MOV) connected in parallel across the switch module, with the operating voltage set to 80% of the rated withstand voltage of the switch device.

[0034] Specifically, the impedance state switching logic is defined. The central processing unit 101 sends the gate control signal S to the power electronic switching module through the fiber optic isolated drive interface. gate (t), controlling the equivalent impedance Z of the OPGW terminal to ground L (t) Switching between low-resistance and high-resistance states. When the control signal is high, the system is in a low-resistance state (state A), the power electronic switch module is saturated and conducting, short-circuiting the OPGW terminal to the ground grid, maintaining the conventional grounding method of the transmission line. At this time, the equivalent impedance is... Approximately zero; when the control signal is low, the system is in a high-impedance state (state B), the power electronic switch module is turned off, and the OPGW ground current is forced to flow through the non-inductive damping resistor. At this time, the equivalent impedance is approximately zero. Equal to R v al.

[0035] Specifically, transient synchronization modulation timing is executed. Upon receiving the active modulation trigger command, the central processing unit 101 generates a square wave modulation sequence to drive the power electronic switches. This sequence begins at time t. start Duration T win During modulation, the impedance Z at the end of the OPGW... L The time-domain variation law of (t) satisfies: ; In the formula: t start The modulation start time is the value of the HVDC excitation trigger time. Superimposed delay amount ; T win The modulation window length is set to cover the DC transient response until it decays to the background noise floor. f mod The modulation frequency, whose value range simultaneously satisfies the following two constraints: T decay The time constant of the transient current is used to ensure that at least 5 complete modulation cycles can be collected during the transient decay process; ;,in For the speed of light, L line To ensure the modulation period is much longer than the round-trip time of the traveling wave along the transmission line, and to eliminate the influence of traveling wave reflections caused by distributed capacitance on impedance measurements, a typical value is set between 20Hz and 200Hz, considering the above constraints.

[0036] Specifically, fault-priority blocking protection is implemented. During the execution of the modulation strategy, the wide-area transient monitoring unit continuously compares the power frequency current component flowing through the OPGW with the preset safety threshold I. safe Once the monitored value exceeds I safe The central processing unit 101 immediately executes an interrupt operation, sending the control signal S... gete (t) Forced lockout to a constant high level keeps the power electronic switching module always on. This protection mechanism ensures that the automatic bypass damping resistor R is activated in the event of a short-circuit fault in the transmission line. dampIt provides a low-impedance fault current discharge path. For the specific isolation design of the IGBT drive circuit and the thermal capacity calculation of the heat dissipation system, those skilled in the art can design according to the power electronics engineering handbook and relevant electrical standards; these are well-known technologies in the field and will not be elaborated upon here.

[0037] In this embodiment, the central processing unit 101 constructs a distributed parameter network model incorporating electrochemical polarization characteristics based on the principles of electrochemical kinetics and the theory of multi-conductor transmission lines. The specific modeling process is as follows: Specifically, a chain-like network topology for the transmission line grounding grid is constructed. The AC transmission lines affected by HVDC stray currents are divided into N cascaded two-port network units. Each network unit adopts an "inverted L-shaped" equivalent circuit structure, that is, along the signal transmission direction (from the substation to the end of the line), it consists of an OPGW longitudinal impedance branch (series) and a tower grounding admittance branch (parallel). k is defined as the tower index number (k=1, 2, ..., N), where k=1 corresponds to the first tower closest to the substation and its preceding span.

[0038] An electrochemical equivalent circuit model (EECM) for the tower grounding electrode is established. Unlike traditional models that treat the grounding electrode as a constant resistance, this embodiment treats the corroded grounding electrode as an electrochemical corrosion cell system with nonlinear dynamic response characteristics. For the k-th tower, its complex frequency domain grounding impedance is... The improved Randle equivalent circuit is used for description: ; In the formula, ω is the angular frequency. It is an ohmic resistance, which is composed of the current dissipation resistance of the soil around the grounding electrode and the conduction resistance of the metal grounding electrode itself. The polarization impedance at the corrosion interface characterizes the charge transfer and accumulation process at the metal-soil electrolyte interface, and its mathematical expression is as follows: ; In the formula: The imaginary unit; R ct,k The charge transfer resistance is a parameter that directly reflects the ease of corrosion reaction; its value decreases significantly during the active corrosion stage. C dl,k It is the double-layer capacitance, a parameter generated by the double-layer effect at the corrosion interface, and its capacitance value is proportional to the microscopic surface area of ​​the corrosion.

[0039] Specifically, determine the longitudinal transmission parameters of the OPGW. For the k-th unit, the longitudinal series impedance Z of the k-th span is... s,k The calculation is as follows: ; In the formula: l k Let be the physical length of the k-th span. z int The internal impedance per unit length of the OPGW conductor is determined by the conductor's DC resistance and skin effect. z earth The mutual impedance per unit length of the ground loop is calculated using Carson's infinite ground loop formula. This parameter is the angular frequency. With local soil resistivity The function.

[0040] Specifically, the transmission line matrix equations under non-uniform excitation are derived. Considering the non-uniform distribution of ground potential rise (GPR) centered on the DC grounding electrode when the HVDC system's operating conditions are adjusted, a ground potential distribution model is constructed. (For example, the 1 / r potential decay model of a hemispherical grounding electrode), combined with the normalized calibration of the measured ground potential amplitude of the substation, thereby estimating the ground potential excitation phasor at the bottom of the kth tower. .

[0041] definition The voltage to ground at the input port (OPGW side) of the k-th network unit. Let be the OPGW current flowing into the k-th network element. Based on the above "inverted L" topology, Kirchhoff's Voltage Law (KVL) and Current Law (KCL) are applied to establish the input state variables of the k-th element. With output state variables transitive relationships: ; Among them, the transmission matrix With the stimulus injection vector The specific derivation of each element is as follows: ; Specifically, establish the system boundary conditions. At the end of the transmission line (node ​​N+1), set the boundary equations based on the actual grounding condition of the OPGW (direct grounding or insulation). At the beginning of the transmission line (node ​​1, substation side), the boundary conditions are controlled by the OPGW active modulation unit. The reference direction of current I1 is defined as flowing from the substation to the transmission line. When the active modulation unit is in impedance state Z... L (i.e. Z) low or Z high When ), the boundary equations at the beginning and end satisfy: ; This boundary condition indicates that by changing ZL The value of this value can forcibly change the voltage V across the entire line. k With current I k The distribution state is thus used to provide an independent set of observation equations for subsequent difference inversion.

[0042] For the specific numerical calculation method of the Carson formula in the above model, and the specific selection of the geopotential distribution model, such as the uniform soil model or the horizontally stratified soil model, those skilled in the art can choose the appropriate well-known algorithm based on the actual geological survey data, which will not be elaborated here.

[0043] In this embodiment, the central processing unit 101 processes the acquired dual-mode transient data, eliminates common-mode interference through frequency domain differential operations, and extracts independent components characterizing corrosion polarization features. The specific execution process is as follows: Specifically, the transient data undergoes time-domain truncation and windowing segmentation. The central processing unit 101 reads the raw current waveform data uploaded by the wide-area transient monitoring unit. raw (t). Simultaneously, the switching action timing signal or synchronous trigger pulse fed back from the OPGW active modulation unit is read. Using the rising and falling edges of the switching action signal as a time reference, i... raw (t) is precisely divided into two data subsets: time series segments corresponding to the low-resistance state (state A, switch on). And the time series segment corresponding to the high-resistivity state (state B, switch off). To suppress spectral leakage caused by aperiodic truncation, a smoothing window function (e.g., Hanning window) is applied to each data segment, with a window length T. win Set to an integer multiple of the modulation period.

[0044] Specifically, the frequency domain transformation of the dual-modal response. This involves the windowed time-domain sequence. and Perform Fast Fourier Transform (FFT) to map the time-domain signal to the complex frequency domain. From the frequency-domain data, extract only the modulation frequency f. mod and its odd harmonic components (3f) mod， 5f mod，... The amplitude and phase at point () constitute a discrete current spectrum phasor sequence. and Odd harmonics are selected to avoid even-symmetric interference that may exist at even harmonics, and to cover the frequency band of the electrochemical impedance spectroscopy using multi-frequency data.

[0045] Specifically, a normalized differential split-current function is constructed. The central processing unit 101 uses the spectral data from the two states described above to calculate the normalized differential split-current function. The function is defined as follows: ; In the formula: The measured current phasor when the OPGW terminal is directly grounded; The measured current phasor is when a damping resistor is connected in series at the end of the OPGW.

[0046] Specifically, it performs automatic decoupling of common-mode background interference. Based on the aforementioned transmission line network model, the current phasor flowing through the OPGW... With the earth potential excitation source They satisfy a linear transitive relationship, that is... , This is the system transfer admittance, which includes line parameters and ground grid impedance. Therefore, the normalized differential shunt function can be expanded as follows: ; As can be seen from the above derivation, the excitation source terms in the numerator and denominator... They cancel each other out. This process achieves complete decoupling of physical quantities: The value depends only on the inherent structural properties of the grounding impedance parameters of the towers along the transmission line and the longitudinal impedance parameters of the OPGW, and is independent of the absolute amplitude and waveform characteristics of the HVDC ground potential rise. If corrosion is present, due to the electrochemical double-layer capacitance C... dl The existence of system transfer admittance Y sys It exhibits nonlinearity in the complex frequency domain, making It exhibits specific impedance trajectory characteristics on the complex plane. These trajectory characteristics directly reflect the electrochemical polarization state of the tower grounding conductor.

[0047] Specifically, spectral validity masking and preprocessing are involved. To avoid instability in numerical calculations, the central processing unit sets a validity mask threshold. Only when the denominator modulus is At that time, the corresponding frequency point data was retained. Subsequently, the calculated... The curve is smoothed in the frequency domain using a Savitzky-Golay filter to remove measurement noise and generate the final observation vector Y for subsequent inversion. obs .

[0048] The specific algorithm implementation of the Fast Fourier Transform (FFT) and the definition of the coefficients of the window function can be implemented by those skilled in the art using existing digital signal processing toolkits, which are well-known technologies in the field and will not be elaborated here.

[0049] In this embodiment, the central processing unit 101 uses a numerical optimization algorithm to solve for the electrochemical parameters of the towers along the line based on the physical model and the measured normalized differential current split spectrum, and evaluates the corrosion state accordingly. The specific process is as follows: Specifically, the parameter vector to be solved and the solution space are constructed. This involves constructing the parameter vector along the transmission line. The electrochemical parameters of the base tower grounding device are defined as unknown variables to be inverted. A parameter vector is constructed. This vector contains the charge transfer resistance R of each base tower. ct,k and double-layer capacitance C dl,k .

[0050] Considering the resistance parameter R ct With capacitance parameter C dl Numerically, there may be differences of several orders of magnitude (e.g., R). ct It could be 10 2 Level, while C dl It could be 10 -3 To avoid ill-conditioned problems during numerical optimization, a logarithmic space transformation is performed on the parameter vector, and a new vector to be solved is defined. .

[0051] Simultaneously, based on the geometric dimensions of the tower grounding electrode and the local soil physicochemical properties, upper and lower limits of physical constraints for each parameter in the logarithmic domain are set. For example, based on the double-layer theory, the limiting range of the double-layer capacitance per unit area is set as follows: to C is calculated based on the surface area of ​​the grounding electrode. dl The constraint boundary.

[0052] Specifically, the forward computation operator and objective function are defined. The central processing unit 101 calls a preset transmission line distributed parameter model containing electrochemical polarization characteristics. For any given parameter vector... First, its exponent is reduced to the physical parameter X, and then substituted into the model to calculate the theoretical current response in the low-resistance and high-resistance states, thereby obtaining the theoretical normalized differential shunt function. Construct a regularized least squares objective function. Its expression is as follows: ; In the formula: M is the number of effective frequency points selected; W m These are frequency-weighted coefficients; This is a regularization parameter used to balance the data fitting residuals with the smoothness of the solution; L is the second-order difference operator matrix and the Laplace matrix. The term is used to apply spatial continuity constraints.

[0053] This constraint is based on the physical fact that the soil environment and corrosion level of adjacent towers of transmission lines typically exhibit spatially gradual variations, without abrupt changes. Introducing this term can effectively suppress anomalies in the parameter inversion of individual towers caused by measurement noise.

[0054] Specifically, nonlinear iterative optimization is performed. The trust region reflection algorithm or the Levenberg-Marquardt algorithm is used to optimize the objective function. The algorithm is iteratively minimized to find the solution. During the iteration process, the algorithm is continuously corrected within the constraints. until the rate of change of the objective function value is less than the preset convergence threshold. After the iteration is complete, the optimal estimate is output. That is, to obtain the R of each tower along the line. ct,k and C dl,k .

[0055] Specifically, the corrosion rate index is calculated. This is based on the charge transfer resistance R obtained through inversion. ct,k The instantaneous corrosion current density i of the grounding electrode of each tower was calculated using the Stern-Geary equation. corr,k : ; In the formula: B is the Stern-Gilly constant, which is set to a value of 26mV to 52mV depending on the grounding electrode material such as galvanized steel and the soil type; S k The effective current-dissipating surface area of ​​the grounding electrode of the kth tower is a parameter pre-stored in the transmission line ledger database or determined based on the measured power frequency grounding resistance value R of the tower. grid Using formula Make an estimate.

[0056] Specifically, multi-dimensional corrosion status assessment and classification are performed. The central processing unit 101 combines inversion parameters and calculation indicators to automatically assess the grounding corrosion status of each tower: 1. Determination of active corrosion: If R ct,k Below the activity threshold R th1 And corrosion current density i corr,k Greater than the preset rate threshold i th The tower was determined to be in the "active corrosion stage," indicating that there was a violent electrochemical dissolution reaction at the metal interface. 2. Local damage assessment: If R ct,k Located in the interval It was determined to be in the "local damage stage"; 3. Passivation stability determination: If R ct,k Above the passivation threshold R th2This is determined to be the "passivation and stabilization stage". The aforementioned threshold R... th1 and R th2 It was calibrated based on the polarization curve test data of standard corrosion test pieces under similar soil conditions.

[0057] Finally, the system outputs assessment data containing the corrosion levels of each tower and generates alarm signals for towers determined to be in the "active corrosion stage".

[0058] For the specific iteration step size control and matrix operation implementation of the numerical optimization algorithm involved in the above steps, those skilled in the art can call existing scientific computing libraries (such as LAPACK or MINPACK) to execute them, which are well known technologies in the field and will not be described in detail here.

Claims

1. A method for ground electrode current based transmission line corrosion and safety assessment, the method comprising: The method comprises the following steps: Step S1: Real-time monitoring of transformer neutral point current or substation ground potential rise signal by wide-area transient monitoring unit, identification of step excitation caused by HVDC system operating condition adjustment, and determination of excitation trigger time through dynamic trigger threshold; Step S2: In the transient decay stage after the excitation trigger time, the OPGW active modulation unit deployed at the substation incoming line framework is controlled to execute the timing active modulation strategy, so that the OPGW terminal impedance is switched between low resistance state and high resistance state at a preset frequency, and the double-mode transient current data at the OPGW terminal is synchronously collected; Step S3: Based on the transmission line-ground network distributed parameter model containing electrochemical polarization characteristics, frequency domain transformation and difference operation are performed on the double-mode transient current data to construct a normalized differential current spectrum to decouple the common-mode interference of ground potential rise; Step S4: A target function based on the least square method is constructed, and the charge transfer resistance and double-layer capacitance of the grounding device along the line are solved by inversion using the normalized differential current spectrum; Step S5: According to the charge transfer resistance and double-layer capacitance obtained by inversion, the corrosion rate index is calculated and the corrosion state of the tower along the transmission line is evaluated.

2. The method of claim 1, wherein, The process of identifying step excitation and determining dynamic trigger threshold in step S1 includes: A wideband continuous monitoring channel is established, and a sliding difference operation is performed on the collected signal to obtain an instantaneous change rate characteristic value; The background noise base within a preset time window before the current time is calculated, the background noise base is multiplied by a signal-to-noise ratio gain coefficient, and then a fixed safety margin is added to obtain a dynamic trigger threshold; When the instantaneous change rate characteristic value exceeds the dynamic trigger threshold and the duration exceeds the determination time limit, it is determined that a step excitation occurs, and the time is locked as the excitation trigger time.

3. The method of claim 1, wherein the method is based on ground electrode current. The timing active modulation strategy in step S2 specifically includes: Set a delay parameter to make the modulation start time lag 10ms-50ms behind the excitation trigger time to avoid the transient signal first wave oscillation area; A square wave modulation sequence is generated to drive the power electronic switch module in the OPGW active modulation unit; When the control signal is high, the power electronic switch module is turned on, and the OPGW terminal impedance is in a low resistance state with an approximate value of zero; When the control signal is low, the power electronic switch module is turned off, and the OPGW terminal impedance is in a high resistance state, at which time the OPGW ground current is forced to flow through the non-inductive damping resistor in parallel with the power electronic switch module.

4. The method of claim 3, wherein the method is based on ground electrode current. The parameter configuration of the OPGW active modulation unit satisfies: The resistance value of the non-inductive damping resistor is set to 0.8 to 1.5 times the sum of the DC resistance value of the OPGW metal sheath of the measured transmission line and the parallel equivalent grounding impedance value along the line tower; The value of the preset frequency needs to meet two conditions: first, it is higher than the inverse of the preset multiple of the transient current decay time constant; second, it is lower than the ratio of four times the length of the transmission line to the speed of light to eliminate the traveling wave reflection interference caused by the distributed capacitance.

5. The method of claim 1, wherein the method is based on ground electrode current. The construction process of the transmission line-ground network distributed parameter model containing electrochemical polarization characteristics in step S3 includes: The power transmission line is divided into cascaded inverted L-shaped two-port network units, each of which is composed of an OPGW longitudinal impedance branch in series and a tower grounding admittance branch in parallel; The parameters of the OPGW longitudinal impedance branch are determined by the internal impedance of the conductor and the mutual impedance of the earth return circuit calculated based on the Carson formula; The parameters of the tower grounding admittance branch are described by an improved Randles equivalent circuit, which includes an ohmic resistance and a corrosion interface polarization impedance composed of a charge transfer resistance and a double-layer capacitance in parallel; Based on Kirchhoff's law, a transmission line matrix equation is established to describe the voltage and current transmission relationship of each network unit.

6. The method of claim 1, wherein, The specific process of constructing the normalized differential current spectrum in step S3 is as follows: Calculate the difference between the measured current spectral phasor in the low resistance state and the measured current spectral phasor in the high resistance state as the differential numerator; Calculate the sum of the measured current spectral phasor in the low resistance state and the measured current spectral phasor in the high resistance state as the common mode denominator; Divide the differential numerator by the common mode denominator to obtain the normalized differential current spectrum; The calculation process uses the linear ratio relationship of the earth potential rise excitation factor contained in the numerator and the denominator to eliminate the influence of the amplitude fluctuation of the earth potential rise on impedance inversion.

7. A ground current based transmission line corrosion and safety assessment method as claimed in claim 6, wherein, The specific steps of the frequency domain transformation include: Use the switching action timing signal of the active modulation unit as the time reference to accurately segment the double-mode transient current data into low resistance state data segments and high resistance state data segments; Apply a smoothing window function to the low resistance state data segments and high resistance state data segments respectively and perform fast Fourier transform; Extract the amplitude and phase at the preset frequency and its odd harmonic frequencies to form a discrete frequency spectrum sequence for calculating the normalized differential current spectrum.

8. The method of claim 1, wherein the method is based on ground current. The process of the inversion solution in step S4 includes: Perform a logarithmic space transformation on the to-be-inverted parameter vector containing the charge transfer resistance and the double-layer capacitance to balance the order of magnitude difference between different physical parameters; Construct a regularized least squares objective function, which includes a data fitting residual term and a spatial continuity regularization term; the data fitting residual term is the weighted sum of squares of the deviation between the theoretical calculation value and the measured observation value; the spatial continuity regularization term uses a second-order difference operator to constrain the smoothness of adjacent tower parameters; Use the trust region reflection algorithm or the Levenberg-Marquardt algorithm to iteratively optimize the objective function until the objective function converges, obtaining the optimal estimated values of the electrochemical parameters of each tower along the line.

9. The method of claim 1, wherein the method further comprises: The evaluation of the corrosion state of the towers along the power transmission line in step S5 includes: Calculate the corrosion rate index using the Stern-Geary equation, which is the product of the Stern-Geary constant and the ratio of the charge transfer resistance to the effective dispersion area of the tower grounding body, to obtain the instantaneous corrosion current density; If the charge transfer resistance is lower than the active threshold and the instantaneous corrosion current density is higher than the rate threshold, it is determined that the tower is in the active corrosion stage; If the charge transfer resistance is between the active threshold and the passivation threshold, it is determined that the tower is in the local damage stage; If the charge transfer resistance is higher than the passivation threshold, it is determined that the tower is in the passivation stable stage.

10. A ground electrode current based transmission line corrosion and safety assessment system according to the ground electrode current based transmission line corrosion and safety assessment method according to any one of claims 1-9, characterized by, The method comprises the following steps: A wide-area transient monitoring unit is used to monitor the transformer neutral current or ground potential rise signal in real time, identify the step excitation caused by the HVDC system operating condition adjustment and determine the triggering time; An OPGW active modulation unit is deployed at the substation incoming line framework, used to switch the OPGW terminal impedance according to the control instruction in the transient decay stage; A central processing unit is connected with the wide-area transient monitoring unit and the OPGW active modulation unit, used to execute the method as claimed in any one of claims 1 to 9, including the control timing active modulation, the construction of the physical model containing the electrochemical polarization characteristics, the calculation of the normalized differential current spectrum, the inversion of the electrochemical parameters and the evaluation of the corrosion state.

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