Method for evaluating long-term stability of zinc oxide varistor based on polarity reversal test

Abstract

The application belongs to the field of electrical insulation detection of power equipment, and particularly discloses a long-term stability evaluation method of zinc oxide varistor based on polarity reversal test, which compares the voltage-current characteristics after polarity reversal at different time points in the direct current aging experiment, utilizes the characteristics that the deterioration of the reverse voltage-current characteristics of the zinc oxide varistor is more significant in the direct current aging test, and the change trend is highly consistent among different sample types, compares the change of leakage current after aging and before aging, effectively avoids many limitations of directly evaluating the long-term stability of zinc oxide varistor ceramic by aging power consumption, and more accurately and conveniently obtains the evaluation result of the long-term stability of the zinc oxide varistor product.

Description

Technical Field

[0001] This invention belongs to the field of electrical insulation testing of power equipment, specifically relating to a method for evaluating the long-term stability of zinc oxide varistors based on polarity reversal testing. Background Technology

[0002] Zinc oxide varistor ceramics (ZnO varistor ceramics) possess excellent nonlinear voltage-current characteristics and are commonly used for overvoltage protection in power systems and electrical equipment. They are a core material in metal oxide arresters (MOAs). The nonlinearity of ZnO varistors originates from the double Schottky barrier at their grain boundaries. Generally, when the double Schottky barrier is subjected to voltage for a prolonged period, the barrier height degrades, leading to a deterioration in nonlinearity and a decrease in overvoltage protection capability, posing a significant threat to the stable operation of power systems. Therefore, the long-term stability of ZnO varistors has always been one of their most important properties.

[0003] When conducting long-term stability tests on zinc oxide varistors, the aging power dissipation (i.e., leakage current under DC aging conditions) is the most direct evaluation standard for the aging state. When the aging power dissipation continuously increases during the aging test, the product is considered unstable; when the aging power dissipation continuously decreases during the aging test, the product is considered stable. Additionally, there are test results where the aging power dissipation initially decreases for a period during the early stages of aging and then continuously increases; this is generally considered an intermediate state, referred to as metastable.

[0004] However, aging power consumption is a very superficial characteristic, greatly affected by aging temperature and aging voltage. Aging tests on some stable zinc oxide varistors revealed an increase in power consumption at lower DC aging voltages; conversely, aging tests on some unstable zinc oxide varistors showed a decrease in power consumption under complex waveform aging conditions. These contradictory aging phenomena indicate that using aging power consumption alone as a criterion for the stability of zinc oxide varistors is insufficient, and a more accurate and rapid evaluation method is urgently needed. Summary of the Invention

[0005] In order to overcome the shortcomings of the prior art, the purpose of this invention is to provide a long-term stability evaluation method for zinc oxide varistors based on polarity reversal testing, so as to solve the problem that the current method of using only aging power consumption as the criterion for the stability of zinc oxide varistors is inaccurate.

[0006] To achieve the above objectives, the present invention employs the following technical solution:

[0007] This invention provides a method for evaluating the long-term stability of zinc oxide varistors based on a polarity reversal test, characterized by comprising:

[0008] Step 1: Measure the sample thickness and electrode area.

[0009] Step 2: Heat the sample to the aging temperature; keep the temperature constant, set one end of the sample as the high voltage end, and set the step voltage and threshold current according to the sample thickness and sample electrode area measured in Step 1. Test the forward volt-ampere characteristic curve and the reverse volt-ampere characteristic curve of the sample respectively to obtain the volt-ampere characteristic curve of the sample in the unaged state.

[0010] Step 3: Apply a continuous 0.1 to 0.9 times the varistor voltage as the aging voltage from the high-voltage end of the sample to perform the aging test;

[0011] Step 4: At different time points during the aging test, stop applying the aging voltage, apply the step voltage set in Step 2 from the high voltage end within 1 minute, and test the forward current-voltage characteristic curve of the sample; then reverse the polarity, apply the step voltage from the original electrode, and test the reverse current-voltage characteristic curve after the polarity reversal.

[0012] Step 5: Read the leakage current I at 0.1 to 0.9 times the varistor voltage from the current-voltage characteristic curves before and after polarity reversal at different time points obtained in Step 4. L Compare the leakage current I after aging with that before aging. Lend with I L0 To assess the long-term stability of the samples.

[0013] Furthermore, the sample mentioned in step 1 is a zinc oxide varistor.

[0014] Furthermore, the aging temperature mentioned in step 2 is in the range of 80 to 150°C.

[0015] Furthermore, based on the sample thickness measured in step 1, the step voltage in step 2 is set to 0.5V / mm, and based on the sample electrode area measured in step 1, the current density is set to reach 1mA / cm². 2 Set the threshold current in step 2.

[0016] Furthermore, the varistor voltage mentioned in step 3 is a current density of 1 mA / cm² in the unaged state. 2 The voltage at that time.

[0017] Furthermore, the different time points for aging mentioned in step 4 are selected for testing every 24 hours to 72 hours.

[0018] Furthermore, in step 4, the positive direction refers to the test voltage having the same polarity as the aging voltage, while the negative direction refers to the test voltage having the opposite polarity to the aging voltage.

[0019] Furthermore, the aging process described in step 5 refers to aging for 350 to 500 hours.

[0020] Furthermore, the I mentioned in step 5 L0 This refers to the leakage current at 0.1 to 0.9 times the varistor voltage, obtained from the tested current-voltage characteristic curve under unaged conditions; I Lend This refers to the leakage current obtained from the volt-ampere characteristic curve obtained after aging, at 0.1 to 0.9 times the varistor voltage.

[0021] Furthermore, the method for evaluating the long-term stability of the sample described in step 5 is as follows: if the positive I Lend Greater than 2 times I L0 The above indicates that the sample is unstable; otherwise, the reverse leakage current should be used for judgment. There are two conditions for using the reverse leakage current for evaluation:

[0022] First type: If the reverse I Lend Greater than 10 times I L0 The sample was considered to have very poor long-term stability; Lend Greater than 5 times I L0 But less than 10 times I L0 The sample is considered to have good long-term stability; Lend Less than 5 times I L0 The sample is considered to have good long-term stability.

[0023] The second method: based on the reverse I obtained from different aging times t. L and Linear relationship, do For linear fitting, if the slope of the fitted line is greater than 1, the long-term stability of the sample is considered to be very poor; if the slope of the fitted line is greater than 0.1 but less than 1, the long-term stability of the sample is considered to be relatively good; if the slope of the fitted line is less than 0.1, the long-term stability of the sample is considered to be very good.

[0024] The present invention has at least the following beneficial effects:

[0025] 1. This invention compares the current-voltage characteristics after polarity reversal at different time points in a DC aging test. It utilizes the fact that the deterioration of the reverse current-voltage characteristics of zinc oxide varistors is more significant and the trend of change is highly consistent across different sample types during DC aging tests. By comparing the changes in leakage current after aging with that before aging, this invention effectively avoids many limitations of directly assessing the long-term stability of zinc oxide varistors through aging power consumption. It provides a more accurate and convenient way to obtain the long-term stability assessment results of zinc oxide varistors.

[0026] 2. This invention utilizes the characteristic that the degradation of the current-voltage characteristics of zinc oxide varistors after polarity reversal is more significant than the forward change during DC aging testing. It was found that for samples with different stability, the reverse leakage current continuously increases during aging, but the increase is much greater for unstable samples than for stable samples. This method innovatively uses the leakage current after polarity reversal as a parameter to evaluate the long-term stability of zinc oxide varistors, avoiding many limitations of aging power consumption assessment methods and enabling accurate evaluation of the long-term stability of zinc oxide varistors. Attached Figure Description

[0027] The accompanying drawings, which form part of this specification, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0028] Figure 1 The power consumption variation curve of a stable zinc oxide varistor under different voltages during aging;

[0029] Figure 2 The power consumption variation curve of an unstable zinc oxide varistor under a 12-pulse voltage waveform during aging;

[0030] Figure 3 The JE variation curve of an unstable zinc oxide varistor under a 12-pulse voltage waveform;

[0031] Figure 4 The curves show the changes in reverse leakage current of stable and unstable zinc oxide varistors during aging tests in embodiments of the present invention. Detailed Implementation

[0032] The present invention will now be described in detail with reference to the accompanying drawings and embodiments. It should be noted that, unless otherwise specified, the embodiments and features described herein can be combined with each other.

[0033] The following detailed description is exemplary and intended to provide further detailed explanation of the invention. Unless otherwise specified, all technical terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art. The terminology used in this invention is for describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention.

[0034] This invention provides a method for evaluating the long-term stability of zinc oxide varistors based on a polarity reversal test, comprising:

[0035] Step 1: Measure the sample thickness and electrode area of ​​the zinc oxide varistor sample;

[0036] Step 2: Heat the zinc oxide varistor to the aging temperature range of 80–150°C; keep the temperature constant, set one end of the zinc oxide varistor as the high-voltage end, and set a stepped voltage of 0.5V / mm based on the sample thickness measured in Step 1. Based on the sample electrode area measured in Step 1, set a current density of 1mA / cm². 2 Set a threshold current and test the forward and reverse current-voltage characteristic curves of the sample to obtain the current-voltage characteristic curve of the sample in the unaged state.

[0037] Step 3: Apply a continuous 0.1 to 0.9 times the varistor voltage as the aging voltage from the high-voltage end of the sample to conduct the aging test. The varistor voltage is the current density of 1 mA / cm² in the unaged state. 2 The applied voltage must remain stable during the aging test.

[0038] Step 4: At different time points during the aging test, stop applying the aging voltage, apply a test step voltage from the high-voltage end within 1 minute, and test the forward current-voltage characteristic curve of the sample; then reverse the polarity, apply a step voltage from the original electrode, and test the reverse current-voltage characteristic curve after the polarity reversal; the different time points of aging are selected by testing once every 24h to 72h; forward means that the polarity of the test voltage is the same as the aging voltage, and reverse means that the polarity of the test voltage is opposite to the aging voltage;

[0039] Step 5: Read the leakage current I at 0.1 to 0.9 times the varistor voltage from the current-voltage characteristic curves before and after polarity reversal at different time points obtained in Step 4. L Compare the leakage current I after aging with that before aging. Lend with I L0 To evaluate the long-term stability of zinc oxide varistors.

[0040] "After aging is complete" refers to aging for 350 to 500 hours.

[0041] I L0 This refers to the leakage current at 0.1 to 0.9 times the varistor voltage, obtained from the tested current-voltage characteristic curve under unaged conditions; I Lend This refers to the leakage current obtained from the volt-ampere characteristic curve obtained after aging, at 0.1 to 0.9 times the varistor voltage.

[0042] The method for evaluating the long-term stability of zinc oxide varistors is as follows: if the forward I... Lend Greater than 2 times I L0 The above indicates that the zinc oxide varistor is unstable; otherwise, the reverse leakage current should be used for judgment. There are two conditions for using reverse leakage current for evaluation, which can be determined depending on the situation: First: If the reverse leakage current I... Lend Greater than 10 times I L0It is believed that the long-term stability of this zinc oxide varistor is very poor; Lend Greater than 5 times I L0 But less than 10 times I L0 It is believed that the zinc oxide varistor has good long-term stability; I Lend Less than 5 times I L0 The first method considers the zinc oxide varistor to have excellent long-term stability; the second method uses the reverse I obtained from different aging times t. L and Linear relationship, do Linear fitting is used to determine the long-term stability of the zinc oxide varistor. If the slope of the fitted line is greater than 1, the long-term stability is considered to be very poor. If the slope of the fitted line is greater than 0.1 but less than 1, the long-term stability is considered to be relatively good. If the slope of the fitted line is less than 0.1, the long-term stability is considered to be very good.

[0043] For stable and unstable zinc oxide varistors, the changes in current-voltage characteristics after polarity reversal are extremely different. The unstable varistor shows a continuous and rapid increase in reverse leakage current during the aging process, while the reverse leakage current of the stable sample does not change much.

[0044] Example 1

[0045] DC aging experiments were conducted on stable zinc oxide varistors under different aging voltages. Three samples were placed in a 120℃ environment, and their initial volt-ampere characteristics were tested and the varistor voltage was read. Constant aging voltages of 0.3 times, 0.8 times, and 0.9 times the varistor voltage were applied, and the aging power dissipation was continuously recorded. The results are as follows: Figure 1 As shown, the power consumption at 0.8 times and 0.9 times the varistor voltage steadily decreased over several hundred hours of aging, while the power consumption at 0.3 times the varistor voltage steadily increased after 1100 hours of aging. Clearly, lower aging voltages do not compromise the stability of the zinc oxide varistor, indicating that directly using power consumption as a criterion might misjudge a stable zinc oxide varistor as unstable.

[0046] Example 2

[0047] An aging test was conducted on an unstable zinc oxide varistor under a 12-pulse waveform. The sample was placed in an environment of 135℃, and its initial voltage-current characteristics were tested and the varistor voltage was read. A 12-pulse voltage waveform with a continuous DC component of 0.82 times the varistor voltage was applied as the aging voltage. During the aging process, the change in power dissipation was continuously monitored, and the results are as follows. Figure 2 As shown. Aging was stopped after 311 hours, and the volt-ampere characteristics before and after polarity reversal were tested. The sample was then kept at the aging temperature for high-temperature recovery. After 309 hours of recovery, the volt-ampere characteristics before and after polarity reversal were tested again, and the results are as follows. Figure 3 As shown.

[0048] Depend on Figure 2 It can be seen that during the 12-pulse waveform aging of the zinc oxide varistor, the aging power consumption steadily decreases, but through... Figure 3 It can be observed that the nonlinear characteristics of this sample have deteriorated severely and irreversibly, indicating that it is not a stable zinc oxide varistor. Therefore, directly using aging power consumption as a criterion may lead to unstable zinc oxide varistors being mistakenly judged as stable, posing a safety hazard to the stable operation of power systems and equipment.

[0049] Example 3

[0050] DC aging experiments were conducted on both stable and unstable zinc oxide varistors. The samples were placed in a 120°C environment, and their initial volt-ampere characteristics were tested, with the varistor voltage recorded. An aging voltage of 0.8 times the varistor voltage was applied continuously as the aging voltage for accelerated aging tests. During the aging process, the aging voltage was paused at regular intervals, and the volt-ampere characteristics before and after polarity reversal were tested. Aging was terminated after a certain time. The change in reverse leakage current at 0.8 times the varistor voltage was recorded from the volt-ampere characteristics after polarity reversal, and its change with aging time was compared.

[0051] from Figure 4 As can be seen, the difference in leakage current changes after polarity reversal is significant between stable and unstable zinc oxide varistors. For unstable zinc oxide varistors, the reverse leakage current continuously increases throughout the entire test voltage range, significantly exceeding the initial value. In contrast, even after polarity reversal, the change in the volt-ampere characteristic of stable zinc oxide varistors remains small; although the leakage current increases somewhat, it does not exceed twice the initial value before aging. Therefore, this method avoids the various limitations of directly assessing the long-term stability of zinc oxide varistors through aging power dissipation, and provides a more accurate and convenient way to obtain the long-term stability assessment results of zinc oxide varistors.

[0052] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A method for evaluating the long-term stability of zinc oxide varistors based on polarity reversal testing, characterized in that, include: Step 1: Measure the sample thickness and electrode area. Step 2: Heat the sample to the aging temperature; keep the temperature constant, set one end of the sample as the high-voltage end, and set a stepped voltage of 0.5V / mm based on the sample thickness measured in Step 1. Based on the sample electrode area measured in Step 1, set a current density of 1mA / cm². 2 Set a threshold current and test the forward and reverse current-voltage characteristic curves of the sample to obtain the current-voltage characteristic curve of the sample in the unaged state. Step 3: Apply a continuous 0.1 to 0.9 times the varistor voltage from the high-voltage end of the sample as the aging voltage to conduct an aging test; Step 4: At different time points during the aging test, stop applying the aging voltage, and apply the stepped voltage set in Step 2 from the high-voltage end within 1 minute to test the forward current-voltage characteristic curve of the sample; then reverse the polarity and apply the stepped voltage from the original electrode to test the reverse current-voltage characteristic curve after the polarity reversal; the forward direction refers to the test voltage polarity being the same as the aging voltage, and the reverse direction refers to the test voltage polarity being opposite to the aging voltage. Step 5: Read the leakage current I at 0.1~0.9 times the varistor voltage from the current-voltage characteristic curves before and after polarity reversal at different time points obtained in Step 4. L Compare the leakage current I after aging with that before aging. Lend with I L0 The long-term stability of the sample is evaluated; the method for evaluating the long-term stability of the sample is as follows: if the positive I Lend More than 2 times I L0 The above indicates that the sample is unstable; otherwise, the reverse leakage current should be used for judgment. There are two conditions for using the reverse leakage current for evaluation: First type: If the reverse I Lend Greater than 10 times I L0 They concluded that the sample had very poor long-term stability. I Lend Greater than 5 times I L0 But less than 10 times I L0 The sample is considered to have good long-term stability. I Lend Less than 5 times I L0 The sample is considered to have good long-term stability. The second method: based on the reverse I obtained from different aging times t. L and The linear relationship, do I L ~ For linear fitting, if the slope of the fitted line is greater than 1, the long-term stability of the sample is considered to be very poor; if the slope of the fitted line is greater than 0.1 but less than 1, the long-term stability of the sample is considered to be relatively good; if the slope of the fitted line is less than 0.1, the long-term stability of the sample is considered to be very good.

2. The method for evaluating the long-term stability of zinc oxide varistors based on polarity reversal testing according to claim 1, characterized in that, The sample mentioned in step 1 is a zinc oxide varistor.

3. The method for evaluating the long-term stability of zinc oxide varistors based on polarity reversal testing according to claim 1, characterized in that, The aging temperature mentioned in step 2 is in the range of 80~150℃.

4. The method for evaluating the long-term stability of a zinc oxide varistor based on a polarity reversal test according to claim 1, characterized in that, The varistor voltage mentioned in step 3 is based on a current density of 1 mA / cm² in the unaged state. 2 The voltage at that time.

5. The method for evaluating the long-term stability of a zinc oxide varistor based on a polarity reversal test according to claim 1, characterized in that, The aging test in step 4 is conducted every 24 hours to 72 hours.

6. The method for evaluating the long-term stability of a zinc oxide varistor based on a polarity reversal test according to claim 1, characterized in that, The aging process described in step 5 refers to aging for 350 to 500 hours.

7. The method for evaluating the long-term stability of a zinc oxide varistor based on a polarity reversal test according to claim 6, characterized in that, The I mentioned in step 5 L0 This refers to the leakage current at 0.1 to 0.9 times the varistor voltage, obtained from the tested current-voltage characteristic curve under unaged conditions; I Lend This refers to the leakage current obtained from the volt-ampere characteristic curve obtained after aging, at 0.1 to 0.9 times the varistor voltage.

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