Voltage-sharing electrode impedance measurement and scaling degree judgment method

Abstract

The invention discloses a voltage-sharing electrode impedance measuring and scaling degree judging method, and belongs to the field of voltage-sharing electrode state monitoring of a converter valve cooling system of a high-voltage direct-current transmission system. According to the method, a four-probe difference method is adopted to separate current injection and voltage detection channels, and contact resistance interference is eliminated; the capacitance of the double electric layers is obtained through fitting of a chronoamperometer method and an alternating current impedance spectrum; introducing a temperature and conductivity double-compensation model to correct environmental fluctuation; and establishing a multi-parameter joint discrimination rule of standardized impedance, double-layer capacitance and conduction current. Microohm-level impedance change identification is realized, environmental interference and scaling are effectively distinguished, submillimeter-level initial scaling is stably identified, the state identification accuracy rate reaches 96% or above, online monitoring can be carried out without stopping a converter station, the detection reliability is remarkably improved, and support is provided for safe operation of the converter valve.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of cooling of equipment in high-voltage direct-current transmission systems, and particularly relates to a method for measuring impedance of a grading electrode and determining a degree of fouling. BACKGROUND

[0002] The converter valve is the core equipment of the high-voltage direct-current transmission system, and the valve cooling system takes away the operating heat through circulating cooling water, which is the key to ensuring the safe and stable operation of the converter valve. The grading electrode, as an important component of the valve cooling system, can balance the potential difference between metal components, suppress leakage current, and effectively prevent electrochemical corrosion of the components. However, during operation, the grading electrode is prone to form an insulating fouling layer on the surface due to the electrode reaction (anode generates Al2O3 fouling layer) of [Al(OH)4]- ions in the cooling water medium. The fouling layer not only increases the electrode impedance and reduces the grading effect, but also blocks the cooling water pipe due to the shedding of the fouling layer, causing overheating failure of the converter valve. According to statistics, more than 25% of the converter valve failures are caused by valve cooling system problems, and the fouling of the grading electrode is one of the main causes.

[0003] The existing solution relies on regular maintenance and cleaning, but requires the shutdown of the converter station, causing significant economic losses, and cannot provide early warning of initial fouling, still posing a risk of fouling layer shedding and blocking. In terms of impedance measurement technology, the traditional single probe / two probe method has significant defects: first, the current injection and voltage detection paths are not separated, and the contact resistance and lead resistance directly interfere with the measurement results, making it impossible to accurately reflect the intrinsic impedance of the electrode; second, there is a lack of temperature and conductivity compensation mechanism, and the impedance value drifts due to fluctuations in environmental parameters, making it difficult to distinguish between "environmental influence" and "fouling itself"; third, it relies on a single impedance threshold to determine the state, which is easily disturbed by high-frequency noise and non-sinusoidal voltage waveforms, resulting in a high misjudgment rate and making it impossible to identify sub-millimeter initial fouling; fourth, it does not combine double-layer capacitance and conduction current parameters for multi-dimensional state verification.

[0004] In summary, there is an urgent need for an impedance measurement and verification method that can eliminate environmental interference, verify multiple parameters, and accurately determine the state of the electrode. SUMMARY

[0005] The present application provides a method for measuring impedance of a grading electrode and determining a degree of fouling, which eliminates the influence of contact resistance through a four-probe differential structure, realizes impedance value standardization through a temperature and conductivity double-compensation algorithm, and synchronously extracts double-layer capacitance and conduction current parameters. The three-parameter joint determination criterion effectively distinguishes between environmental disturbance and real fouling, improves the accuracy of initial fault identification, and provides reliable protection for the continuous and stable operation of the converter valve.

[0006] To achieve the above-mentioned purpose, the present application provides a method for measuring impedance of a grading electrode and determining a degree of fouling, comprising the following steps:

[0007] S1. Surface impedance of the equalizing electrode is measured by four-probe differential method, the excitation signal is injected by two current probes on the outside, and the potential difference is independently collected by two voltage probes on the inside;

[0008] S2. Four-probe impedance value is calculated based on the collected potential difference and injected current, and the influence of the contact resistance between the probe and the electrode on the measurement result is eliminated;

[0009] S3. Double-layer capacitance value C_dl of the electrode is obtained, and correlation analysis is performed with the four-probe impedance value, and the rationality of the measurement result is verified by using the negative correlation between the two in the fouling state;

[0010] S4. A temperature and conductivity double-compensation model is introduced to standardize and correct the four-probe impedance value data;

[0011] S5. Multi-parameter collaborative determination rules are established, and the equalizing electrode state is determined by combining the standard impedance Z_standard, the double-layer capacitance value C_dl and the conduction current I_conduct characteristic quantity.

[0012] In an embodiment, in step S1, in the four-probe differential method, the excitation signal injected by the current probe has a frequency of 1 kHz and an amplitude of 10 mA alternating current, and the input impedance of the voltage probe is not less than 10 GΩ.

[0013] In an embodiment, in S2, the four-probe impedance value is obtained by Ohm's law, and the formula is as follows:

[0014] ; ①

[0015] In formula ①, Z diff is the four-probe differential impedance, V inner is the potential difference measured by the inner voltage probe, I outer is the current of the outer current probe, d is the distance between the two inner electrodes, and S is the effective area of the electrode.

[0016] In an embodiment, in step S3, the double-layer capacitance value C_dl is obtained by chronoamperometry method and AC impedance spectrum fitting, and the measurement frequency range is 10 mHz-100 kHz.

[0017] In an embodiment, in step S4, the temperature compensation and conductivity compensation double-compensation model is realized by establishing a multiple linear regression equation, and the temperature compensation model is as follows:

[0018] ; ②

[0019] In formula ②, Z is the standard impedance value; a is the temperature coefficient of conductivity, and is 0.02 / ℃; T is the measured temperature, in ℃;

[0020] The standard impedance value is substituted into the conductivity compensation model for secondary correction, and the formula calculation model is as follows:

[0021] ; ③

[0022] In formula 3, σ ref is the reference conductivity, and is 0.2 μS / cm; σ measured is the measured conductivity; and Z standard is the standardized impedance.

[0023] In an embodiment, four consecutive equalizing electrodes in the same valve section are selected as the measurement object, and a conductivity sensor is arranged at the adjacent internal cooling water sampling port to collect the measured conductivity σ measured ;

[0024] A temperature sensor is arranged on the surface of the equalizing electrode to adhere to the wall of the distribution pipe to collect the measured temperature T of the cooling water.

[0025] In an embodiment, four consecutive equalizing electrodes in the same valve section are selected as the measurement object, and a conductivity sensor is arranged at the adjacent internal cooling water sampling port to collect the measured conductivity σ measured ;

[0026] A temperature sensor is arranged on the surface of the equalizing electrode to adhere to the wall of the distribution pipe to collect the measured temperature T of the cooling water.

[0027] Compared with the prior art, the application has the following beneficial effects:

[0028] The key problems in equalizing electrode impedance measurement and state discrimination are solved, and the application has significant technical advantages and application value:

[0029] First, the four-probe differential method is used to separate the current injection and voltage detection paths, combined with a 1 kHz, 10 mA precise excitation signal and a high input impedance voltage probe, to eliminate the interference of contact resistance and lead resistance, realize the identification of micro-ohm level impedance change, solve the defects of insufficient measurement accuracy of traditional single / two-probe method, and provide a reliable data basis for subsequent analysis.

[0030] Second, the temperature and conductivity double compensation model is introduced, the impedance drift caused by environmental parameter fluctuation is corrected through the multivariate linear regression equation, the comparability of the standardized impedance under different working conditions is ensured, the misjudgment risk caused by environmental interference is overcome, and the measurement stability is improved.

[0031] ​Furthermore, a multi-parameter joint discrimination rule of standardizing impedance, double-layer capacitance and conduction current is established, the result rationality is verified by the negative correlation between impedance rise and capacitance reduction during fouling, single parameter misjudgment is avoided, sub-millimeter initial fouling can be stably identified, the state identification accuracy is more than 96%, and the pain point that the traditional method cannot early warn initial fouling is solved. BRIEF DESCRIPTION OF DRAWINGS

[0032] In order to more clearly illustrate the technical solutions of the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or prior art description will be briefly introduced. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0033] Figure 1 A flowchart of a voltage equalizing electrode impedance measurement and fouling degree determination method provided by the present application is shown in the figure.

[0034] Figure 2 A four-probe differential impedance and double-layer capacitance value scatter plot of a voltage equalizing electrode impedance measurement and fouling degree determination method provided by the present application is shown in the figure.

[0035] Figure 3 A fouling thickness-multi-parameter curve diagram of a voltage equalizing electrode impedance measurement and fouling degree determination method provided by the present application is shown in the figure.

[0036] Figure 4 A measurement diagram of a voltage equalizing electrode impedance measurement and fouling degree determination method provided by the present application is shown in the figure. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solutions and advantages of the embodiments of the present application more clear, the technical solutions in the embodiments of the present application will be described clearly and completely. Obviously, the described embodiments are some embodiments of the present application, not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor also belong to the scope of protection of the present application.

[0038] Reference Figures 1 to 4 As shown in the figure, the voltage equalizing electrode impedance measurement and fouling degree determination method provided by the present application includes the following steps:

[0039] S1. Surface impedance of the equalizing electrode is measured by four-probe differential method. The excitation signal is injected by two current probes on the outside, and the potential difference is collected by two voltage probes on the inside. In the four-probe differential method, the excitation signal injected by the current probe is an alternating current with a frequency of 1 kHz and an amplitude of 10 mA, and the input impedance of the voltage probe is not less than 10 GΩ. The potential difference between the inside electrodes is captured by the high input impedance voltage probe to eliminate the influence of contact resistance and lead resistance on the measurement.

[0040] It should be noted that the four-probe differential method effectively avoids the measurement deviation caused by contact resistance by separating the current injection and voltage detection paths, ensuring that the potential difference obtained only reflects the intrinsic impedance characteristics of the electrode surface; combined with precisely controlled excitation signal parameters, it can realize the identification of micro-ohm level impedance changes and provide reliable data basis for subsequent multi-parameter joint analysis.

[0041] S2. Four-probe impedance value is calculated based on the collected potential difference and injected current to eliminate the influence of probe and electrode contact resistance on the measurement results.

[0042] The four-probe impedance value is obtained by Ohm's law, as shown in the following formula:

[0043] ; ①

[0044] In formula ①, Z diff is the four-probe differential impedance, V inner is the potential difference measured by the inside voltage probe, I outer is the current of the outside current probe, d is the distance between the two inside electrodes, and S is the effective area of the electrode. This impedance value reflects the AC response characteristics of the electrode surface at a specific frequency. When the electrode surface is fouled or corroded, the effective conduction area decreases, resulting in a significant increase in the four-probe differential impedance Z diff .

[0045] S3. The double-layer capacitance value C_dl of the electrode is obtained and correlated with the four-probe impedance value for analysis. The negative correlation between the two in the fouling state is used to verify the rationality of the measurement results. The double-layer capacitance value C_dl is obtained by chronoamperometry method and AC impedance spectrum fitting, and the measurement frequency range is 10 mHz-100 kHz.

[0046] chronoamperometry method applies a step voltage to the electrode and records the current response, and the charge quantity is obtained by integrating the current-time curve, and the double-layer capacitance value C_dl is calculated by combining the potential step amplitude. The alternating current impedance spectrum is applied to the small amplitude sinusoidal perturbation under the open circuit potential, and the capacitance value at the intersection point of the high frequency semicircle arc diameter and the low frequency diffusion line in the Nyquist diagram is fitted. When the fouling covers the electrode surface, the double-layer capacitance value C_dl decreases with the decrease of the effective specific surface area, and the four-probe differential impedance Z diff The positive correlation trend is negatively correlated, and the combined analysis can exclude the false judgment caused by the environmental noise interference of a single parameter, and improve the state recognition accuracy.

[0047] Referring to Figure 2 , the scatter plot of four-probe differential impedance Z diff and double-layer capacitance value C_dl, if the data points show a significant negative correlation distribution, and meet the power law relationship trend, it is determined that the measurement result is reliable; otherwise, signal interference or probe contact abnormality needs to be checked. Further, combined with the temperature and conductivity compensation model, the double-layer capacitance value Z diff is corrected in real time to eliminate the influence of environmental parameter fluctuation, and finally realize high-precision online monitoring of fouling degree. The standardized impedance value Z_standard after compensation and the double-layer capacitance C_dl constitute a two-dimensional criterion, which is combined with the historical calibration curve to map the state; through the trend analysis of multiple parameters in the sliding time window, the initial fouling characteristics (double-layer capacitance value Z diff slowly rises, and the double-layer capacitance C_dl drops sharply) are identified, and the sub-millimeter scale scale layer is accurately warned.

[0048] S4. Introduce a temperature and conductivity double compensation model to standardize and correct the four-probe impedance value data.

[0049] When the temperature of the internal cooling water changes the ion migration rate, and then changes the double-layer structure of the electrode interface, it causes the impedance measurement value to drift. By real-time acquisition of water temperature and combining the equation for temperature compensation, the measurement deviation caused by thermal effect is eliminated; at the same time, according to the measured conductivity σmeasured obtained in S2, substitute into the standard temperature compensation model, the calculation model is as follows:

[0050] ; ②

[0051] In formula ②, is the standard impedance value; ɑ is the conductivity temperature coefficient, which is 0.02 / ℃; T is the measured temperature, unit ℃.

[0052] It should be noted that temperature compensation only eliminates the thermal-induced impedance drift, but cannot correct the impedance essence change caused by fouling. Therefore, a conductivity compensation mechanism needs to be introduced simultaneously to distinguish the medium conductivity change from the surface state evolution. The ratio of the measured conductivity σmeasured to the reference conductivity σ_ref (0.2 μS / cm) is used as a correction factor to participate in the normalized impedance calculation, ensuring that Z_standard is still comparable under different water quality conditions.

[0053] wherein the standard impedance value is substituted into the conductivity compensation model for secondary correction, and the formula calculation model is as follows:

[0054] ; ③

[0055] In formula ③, σ ref is the reference conductivity, taking a value of 0.2 μS / cm; σ measured is the measured conductivity; and Z standard is the normalized impedance.

[0056] It should be noted that the introduction of the temperature and conductivity double compensation model significantly improves the stability and comparability of impedance measurement, so that the normalized impedance Z_standard can truly reflect the fouling state of the electrode surface. By simultaneously correcting the environmental interference and medium fluctuation, the influence of non-surface factors on the measurement value is effectively removed, ensuring that the early fouling characteristics can still be accurately identified under complex working conditions.

[0057] S5. Establishing a multi-parameter collaborative judgment rule, combining the normalized impedance Z_standard, the double-layer capacitance value C_dl and the current probe current I outer characteristics to realize the discrimination of the equalizing electrode state.

[0058] The three-parameter joint judgment criterion is used for state recognition, and when any two of the following conditions are met, it is determined that the equalizing electrode state is invalid: the normalized impedance Z_standard ≥ 1 MΩ, the double-layer capacitance value C_dl ≤ 80% of the non-fouling state, and the conductive current I_conduct ≤ 70% of the non-fouling state.

[0059] It should be noted that the three-parameter joint judgment criterion effectively avoids the risk of single-index misjudgment, especially when the internal cooling water quality fluctuates or the sensor is temporarily disturbed, the discrimination stability can still be maintained. Combined with the field measurement data verification, the accuracy rate of this method for identifying abnormal states such as early fouling, surface pollution and electrode aging is more than 96%, which is significantly better than the traditional single-parameter threshold method.

[0060] Therefore, the standardized impedance Z_standard is taken as a core index, and the double-layer capacitance value C_dl and the Iouter are used for cooperative verification, so that the performance degradation of the grading electrode can be graded early warned and accurately positioned, and strong support can be provided for reliable operation of the converter valve cooling system. Further, the multi-parameter cooperative determination mechanism can train and optimize the threshold boundary through historical data, and adaptively adjust the criterion sensitivity in long-term operation.

[0061] Optionally, four continuous grading electrodes in the same valve section are selected as the measurement objects, and conductivity sensors are arranged at adjacent internal cooling water sampling ports to collect the measured conductivity σ measured A temperature sensor is arranged on the surface of the grading electrode and attached to the wall of the distribution pipe to collect the measured temperature T of the cooling water.

[0062] In this embodiment, the outer two electrodes E3 and E6 are connected to a 1 kHz alternating current constant current source as a current injection end, the inner electrodes E4 and E5 are connected to a voltage difference acquisition module to obtain V_inner; the double-layer capacitance C_dl between E4 and E5, the temperature sensor output T and the conductivity sensor output σmeasured are synchronously read by an LCR meter. After the collected data is temperature-compensated to 25℃, the conductivity is normalized by substituting the formula ③, and the standardized impedance Z_standard is obtained. Then, the double-layer capacitance value C_dl and the Iouter (optional direct current conduction current) are combined for three-parameter joint discrimination: when any two of the standardized impedance Z_standard≥1MΩ, the double-layer capacitance value C_dl≤80% of the original value or the conduction current I_conduct≤70% of the original value are met, the electrode is determined to be invalid. The verification results show that this scheme can stably identify the initial scaling under various working conditions, and significantly improve the detection reliability.

[0063] The conduction current I_conduct is obtained by applying a 5V direct current voltage between the electrodes and measuring the loop current, which is significantly affected by the degree of interface contamination; when an insulating deposit is formed on the surface of the electrode, the charge transfer resistance increases, resulting in a significant decrease in the conduction current I_conduct. Combined with the change trend of the standardized impedance Z_standard and the double-layer capacitance value C_dl, the light pollution and the serious failure state can be distinguished, and the diagnosis accuracy is improved.

[0064] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that; they can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for measuring the impedance of an equalizing electrode and determining the degree of scaling, characterized in that: Includes the following steps: S1. The surface impedance of the equalizing electrode is measured using the four-probe differential method. Excitation signals are injected through two current probes on the outer side, and the potential difference is collected independently by two voltage probes on the inner side. S2. Calculate the impedance values ​​of the four probes based on the acquired potential difference and injection current to eliminate the influence of the contact resistance between the probes and electrodes on the measurement results; S3. Obtain the double-layer capacitance value C_dl of the electrode and perform correlation analysis with the impedance value of the four probes. Utilize the negative correlation between the two under the scaling condition to verify the rationality of the measurement results. S4. Introduce a dual compensation model for temperature and conductivity to standardize and correct the impedance data of the four probes; S5. Establish a multi-parameter collaborative judgment rule, combining the standardized impedance Z_standard, the double-layer capacitance C_dl, and the conduction current I_conduct characteristic quantity to realize the judgment of the voltage equalization electrode state.

2. The method for measuring the impedance of an equalizing electrode and determining the degree of scaling according to claim 1, characterized in that: In step S1, in the four-probe differential method, the excitation signal injected by the current probe is an AC current with a frequency of 1 kHz and an amplitude of 10 mA, and the input impedance of the voltage probe is not less than 10 GΩ.

3. The method for measuring the impedance of an equalizing electrode and determining the degree of scaling according to claim 1, characterized in that: In step S2, the impedance value of the four probes is calculated using Ohm's law, as shown in the following formula: ； ① In formula ①, Z diff For four-probe differential impedance, V inner I is the potential difference measured by the inner voltage probe. outer d is the current of the outer current probe, d is the distance between the two inner electrodes, and S is the effective area of ​​the electrode.

4. The method for measuring the impedance of an equalizing electrode and determining the degree of scaling according to claim 1, characterized in that: In step S3, the double-layer capacitance value C_dl is obtained by fitting the AC impedance spectrum using the chronoamperometry method, with a measurement frequency range of 10 mHz to 100 kHz.

5. The method for measuring the impedance of an equalizing electrode and determining the degree of scaling according to claim 1, characterized in that: In step S4, the dual compensation model of temperature compensation and conductivity compensation is achieved by establishing a multiple linear regression equation. The temperature compensation model is shown below: ； ② In formula ②, α is the standard impedance value; α is the temperature coefficient of conductivity, with a value of 0.02 / ℃; T is the measured temperature, in ℃. The standard impedance value Substituting the values ​​into the conductivity compensation model for secondary correction, the calculation formula is shown below: ； ③ In formula ③, σ ref For reference conductivity, a value of 0.2 μS / cm is used; σ measured Z represents the measured conductivity. standard This is the standardized impedance.

6. The method for measuring the impedance of an equalizing electrode and determining the degree of scaling according to claim 1, characterized in that: Four consecutive pressure equalization electrodes within the same valve section were selected as the measurement objects, and conductivity sensors were placed at adjacent internal cooling water sampling ports to collect the measured conductivity σ. measured ; A temperature sensor is placed on the surface of the equalizing electrode and attached to the wall of the water distribution pipe to collect the measured temperature T of the cooling water.

7. The method for measuring the impedance of an equalizing electrode and determining the degree of scaling according to claim 1, characterized in that: In step S5, a three-parameter joint judgment criterion is used for state identification. The equalizing electrode is determined to be in failure when any two of the following conditions are met: normalized impedance Z_standard ≥ 1MΩ, double layer capacitance C_dl ≤ 80% of that in the scale-free state, and conduction current I_conduct ≤ 70% of that in the scale-free state.

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