A method for evaluating the state of a metallized film capacitor based on a frequency characteristic curve

Abstract

This invention discloses a method for evaluating the condition of a metallized film capacitor based on its frequency response curve, comprising: taking the capacitor to be tested and applying an alternating current across its terminals; and measuring the amplitude of the current across the capacitor. I M Calculate the phase difference between voltage and current; increase the current frequency by 50Hz, and the current magnitude reaches... I M Record the voltage magnitude at this point and calculate the phase difference between the voltage and current. Repeat the previous step until the measured current frequency reaches the required level for plotting the frequency response curve of the capacitance value. Calculate the capacitance value of the capacitor at different frequencies. Plot the frequency response curve of the capacitance value on the ordinate and the frequency on the abscissa. Extract characteristic parameters from the curve. Analyze whether the capacitor has failed and the cause of failure based on the characteristic parameters. This invention can determine the state of the capacitor dielectric while judging the capacitance value, thereby analyzing the cause of degradation.

Description

Technical Field

[0001] This invention belongs to the field of capacitor technology, specifically relating to a method for evaluating the condition of metallized film capacitors based on frequency response curves. Background Technology

[0002] Metallized film capacitors (MCCs) are capacitors made by winding metallized thin films. They possess advantages such as high energy density and high reliability, and are widely used in modular multilevel converters in flexible DC transmission systems. However, MCCs operate under high field strength conditions for extended periods, accompanied by temperature rise, leading to gradual performance degradation and threatening the safe and stable operation of flexible DC transmission systems. Therefore, monitoring the degradation status of MCCs is crucial for ensuring the reliability of flexible DC transmission systems. However, current methods for evaluating capacitor health only measure capacitance at a single frequency, which is insufficient for a comprehensive assessment of capacitor condition. Summary of the Invention

[0003] To address the aforementioned issues, this invention provides a method for evaluating the health of metallized film capacitors based on frequency response curves. By measuring capacitance values ​​at multiple frequencies, the health status of the capacitor can be evaluated from multiple dimensions.

[0004] The technical solution adopted by this invention to solve its technical problem is:

[0005] A method for evaluating the condition of metallized film capacitors based on frequency response curves, comprising the following steps:

[0006] S100: Take the capacitor to be tested and apply an alternating current across the capacitor.

[0007] S200: The phase of the voltage across a capacitor at 50Hz is recorded as... The amplitude and phase of the capacitor current at 50Hz were measured, and the magnitude of the amplitude was denoted as I. M Phase is denoted as Calculate the phase difference between voltage and current:

[0008] ;

[0009] S300: Based on the previous frequency, increase the current frequency by 50Hz and apply current until the current magnitude reaches I. M Record the magnitude U of the voltage at this time, and calculate the phase difference between the voltage and the current;

[0010] S400: Repeat step S300 until the measured current frequency reaches the required level for plotting the frequency response curve of the capacitance value; furthermore, generally, the current frequency should not be lower than 250Hz when the measurement is terminated in order to plot a meaningful frequency response curve.

[0011] S500: Based on the voltage, current, and phase difference values ​​at different frequencies, calculate the capacitive reactance X and capacitance C of the capacitor at the corresponding frequency. M ;

[0012] S600: Based on capacitance value C M Plot the frequency response curve of the capacitance value with the vertical axis as the ordinate and the frequency f as the horizontal axis.

[0013] S700: Extract characteristic parameters from the frequency response curve;

[0014] S800: Analyze whether the capacitor has failed based on the aforementioned characteristic parameters.

[0015] Preferably, the capacitor mentioned in step S100 is a metallized film capacitor.

[0016] Preferably, the frequency of the alternating current in step S100 is 50Hz.

[0017] Preferably, the current amplitude in step S200 is obtained as follows: The voltage across the capacitor is increased using a step-up voltage boosting method. When the voltage across the capacitor approaches its AC rated voltage, the voltage and current amplitudes at this point are recorded as the voltage amplitude and current amplitude, respectively. The magnitude of the current amplitude is denoted as I. M After recording the data, the voltage is reduced, and the phase difference between the voltage and current is calculated. .

[0018] Preferably, the capacitive reactance X in step S500 is calculated in the following manner:

[0019] ,

[0020] In the formula, U represents the measured voltage, and I M Indicates the applied current. This represents the phase difference between voltage and current.

[0021] Preferably, the capacitance value C in step S500 is... M Calculated as follows:

[0022] ,

[0023] In the formula, f is the frequency at which the capacitance value is calculated, and X is the capacitive reactance of the capacitor at that frequency.

[0024] Preferably, the characteristic parameters in step S700 include: the capacitance value of the capacitor at a frequency of 50 Hz, and the frequency corresponding to the inflection point of the frequency response curve, denoted as f. M .

[0025] Preferably, S800 further includes the following steps:

[0026] S801: Calculate the ratio of the tested capacitor to its capacitance at 50 Hz before aging. ;

[0027] S802: When the capacitance value of a capacitor at 50 Hz drops below 95% of its unaged capacitance value at 50 Hz, that is... This is considered the end of the capacitor's lifespan;

[0028] S803: Determine the threshold for the cutoff frequency according to application requirements. If the cutoff frequency exceeds this threshold, the capacitor is also considered to be in failure.

[0029] S804: According to Determine the cause of capacitor failure;

[0030] S805: Determine the cause of capacitor failure based on the corner frequency.

[0031] Preferably, the cause of failure described in S804 is determined in the following way: when the capacitance value of the capacitor at 50 Hz drops to less than 95% of the capacitance value at 50 Hz before aging, the cause of failure includes cumulative self-healing discharge or electrochemical corrosion.

[0032] Preferably, the failure cause described in S805 is determined as follows: Since actual capacitors are not ideal capacitors, the structure of the capacitor is represented by a series-parallel equivalent model. Based on this model, the capacitance value C is calculated. M The expression is:

[0033] ,

[0034] Among them, R P It is the equivalent parallel capacitance, characterizing the state of the dielectric. C represents the capacitance value of the ideal capacitor, and L... S It is the equivalent series inductance formed by the winding structure of the capacitor, and f represents the frequency of the voltage and current during measurement.

[0035] Preferably, the transition frequency varies with R P R increases as the medium deteriorates. P As the frequency decreases, the corner frequency shifts towards higher frequencies. Therefore, the state of the medium can be determined based on the corner frequency.

[0036] The technical advantages of this invention are as follows:

[0037] 1) By measuring the capacitance value at multiple frequencies, this invention can determine the capacitance value of a capacitor and the state of the capacitor dielectric at the same time, thus providing a more comprehensive assessment of the capacitor's health status.

[0038] 2) This invention can also analyze the causes of degradation and avoid system failures caused by the degradation of the medium. Attached Figure Description

[0039] Figure 1 This is a flowchart of a metallized film capacitor state assessment method based on frequency response curves in one embodiment of the present invention.

[0040] Figure 2 This is a flowchart illustrating the capacitor failure mechanism determination in one embodiment of the present invention;

[0041] Figure 3 This is an equivalent model diagram of a series-parallel metallized film capacitor in one embodiment of the present invention;

[0042] Figure 4 This is a frequency response curve of the capacitance value of a normal capacitor when it is not aged, as shown in one embodiment of the present invention.

[0043] Figure 5 This is a frequency response curve of the capacitance value of a normal capacitor after aging, as shown in one embodiment of the present invention.

[0044] Figure 6 This is a frequency response curve of the capacitance value of a capacitor without armor and potting compound in one embodiment of the present invention when it has not aged.

[0045] Figure 7 This is a frequency response curve of the capacitance value of an unarmored and unencapsulated capacitor after aging, according to one embodiment of the present invention. Detailed Implementation

[0046] The following will refer to the appendix. Figures 1 to 7 Specific embodiments of the invention are described in detail below. While specific embodiments of the invention are shown in the accompanying drawings, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the invention and to fully convey the scope of the invention to those skilled in the art.

[0047] It should be noted that certain terms are used in the specification and claims to refer to specific components. Those skilled in the art will understand that different terms may be used to refer to the same component. This specification and claims do not distinguish components based on differences in terminology, but rather on differences in function. The terms "comprising" or "including" used throughout the specification and claims are open-ended and should be interpreted as "comprising but not limited to." The following descriptions are preferred embodiments for carrying out the invention; however, these descriptions are for the purpose of understanding the general principles of the specification and are not intended to limit the scope of the invention. The scope of protection of this invention is determined by the appended claims.

[0048] To facilitate understanding of the embodiments of the present invention, further explanations and descriptions will be provided below with reference to the accompanying drawings and specific embodiments. The accompanying drawings do not constitute a limitation on the embodiments of the present invention.

[0049] This invention provides a method for evaluating the condition of metallized film capacitors based on frequency response curves, such as... Figure 1 As shown, the evaluation method includes the following steps:

[0050] S100: Take the capacitor to be tested and apply a 50Hz AC current across the capacitor.

[0051] S200: Measure the voltage and phase across a capacitor at 50Hz. The voltage is denoted as U, and the phase as... The amplitude and phase of the capacitor current at 50Hz were measured, and the magnitude of the amplitude was denoted as I. M Phase is denoted as To calculate the phase difference between voltage and current, the formula for calculating the phase difference at 50Hz is:

[0052] ;

[0053] S300: Based on the previous frequency, increase the current frequency by 50Hz and apply current until the current magnitude reaches I. M Record the magnitude of the voltage at this time, as well as the phase difference between the voltage and the current;

[0054] S400: Repeat step S300 until the measured current frequency reaches the level required to plot the frequency response curve of the capacitance value; generally, the current frequency should not be lower than 250 Hz when the measurement is terminated, so as to plot a meaningful frequency response curve.

[0055] S500: The capacitive reactance of the capacitor at different frequencies is calculated using the formula:

[0056] ,

[0057] In the formula, U represents the measured voltage, and I M Indicates the applied current. This represents the phase difference between voltage and current.

[0058] The capacitance value at each frequency is calculated using the following formula:

[0059] ,

[0060] In the formula, f is the frequency at which the capacitance value is calculated, and X is the capacitive reactance of the capacitor at that frequency.

[0061] S600: Based on capacitance value C M Plot the frequency response curve of the capacitance value with the vertical axis as the ordinate and the frequency f as the horizontal axis.

[0062] S700: Calculate the capacitance value of the capacitor at a frequency of 50 Hz. Find the frequency corresponding to the inflection point of the frequency response curve and denote it as f. M ;

[0063] S800: Analyze whether the capacitor has failed and the cause of failure based on the two characteristic quantities obtained in S700.

[0064] like Figure 2 As shown, S800 includes the following steps:

[0065] S801: Calculate the ratio of the tested capacitor to its capacitance at 50 Hz before aging. ;

[0066] If the capacitance of the tested capacitor at 50Hz is recorded as C, and the capacitance without aging is recorded as C0, then The calculation formula is:

[0067]

[0068] S802: When the capacitance value of a capacitor at 50 Hz drops below 95% of its unaged capacitance value at 50 Hz, that is... This is considered the end of the capacitor's lifespan;

[0069] S803: Determine the threshold for the cutoff frequency based on application requirements. If the cutoff frequency exceeds this threshold, the capacitor is also considered to have failed.

[0070] Furthermore, based on the dielectric condition requirements of capacitor applications, the change in capacitance value during normal aging of a capacitor can determine the range of the cutoff frequency. The cutoff frequency is the frequency corresponding to the lowest capacitance value. For example, in Example 1, when the capacitor ages to its failure state, the cutoff frequency increases from 50Hz to 150Hz. Therefore, for capacitors of the same model, 150Hz can be used as the cutoff frequency threshold. If the application scenario has high requirements for dielectric condition, 100Hz can also be used as the threshold.

[0071] S804: Determine whether a capacitor has failed due to electrode area loss by comparing its capacitance value to its capacitance value at 50Hz before aging. The failure occurs when the capacitance value at 50Hz drops below 95% of its capacitance value at 50Hz before aging. The causes of failure include cumulative self-healing discharge or electrochemical corrosion.

[0072] S805: Determine whether a capacitor has failed due to dielectric degradation based on its cutoff frequency: The capacitor's structure employs the following... Figure 3 The series-parallel equivalent model is shown. Based on this model, the capacitance value C is calculated. M The expression is:

[0073] ,

[0074] Among them, R P It is the equivalent parallel capacitance, characterizing the state of the dielectric. C represents the capacitance value of the ideal capacitor, and L... S It is the equivalent series inductance formed by the winding structure of the capacitor, and f represents the frequency of the voltage and current during measurement.

[0075] Because C will continue until the capacitor ages and fails. M The capacitance value changes only to 95% of the unaged value at 50Hz, a very small change. S Basically unchanged, R P However, it can vary by 3 to 4 orders of magnitude, so C M Only with R P It is related to f. The corner frequency varies with R. P R increases as the medium deteriorates. P As the frequency decreases, the corner frequency shifts towards higher frequencies. Therefore, the state of the medium can be determined based on the corner frequency.

[0076] Example 1:

[0077] Take a normal capacitor and accelerate its aging under DC voltage. The main cause of failure at this point is cumulative self-healing. Measure and calculate the frequency response curves of the healthy capacitor and the capacitor that failed under normal degradation in the range of 50Hz to 1000Hz. The frequency response curve of the capacitance value under healthy conditions is shown below. Figure 4 As shown, when the capacitor has good dielectric insulation, R P Usually greater than 10 6 Ω, calculated based on the basic parameters of a capacitor, indicates a cutoff frequency of 50Hz or 100Hz. From Figure 4 As can be seen, the cutoff frequency of the capacitance frequency response curve is 100 Hz, indicating that the capacitor dielectric is in good condition.

[0078] When the capacitance of a capacitor drops to 95% of its unaged capacitance at 50Hz, the frequency response curve of the capacitor is as follows: Figure 5 As shown. From Figure 5 As can be seen, the corner frequency is 150Hz. In the equivalent series-parallel model, R P The value decreases to approximately 50% of the capacitance value at 50Hz before aging, indicating that the dielectric has deteriorated to some extent.

[0079] Example 2:

[0080] An unarmored and unencapsulated capacitor was subjected to accelerated aging under DC voltage. Under the influence of water vapor and oxygen, the dielectric within the capacitor deteriorated rapidly. The frequency response curves of the healthy capacitor and the deteriorated capacitor were measured and calculated within the range of 50 Hz to 1000 Hz. The frequency response curve of the capacitance value under healthy conditions is shown below. Figure 6 As shown. From Figure 6 As can be seen, the cutoff frequency of the capacitance frequency response curve is 50 Hz, indicating that the capacitor dielectric is in good condition.

[0081] When the capacitance of a capacitor drops to 95% of its unaged capacitance at 50Hz, the frequency response curve of the capacitor is as follows: Figure 7 As shown. From Figure 7 As can be seen, the corner frequency is 1000Hz. In the equivalent series-parallel model, R P The value has decreased to at least 2.25% of the capacitance value at 50Hz when it was not aged. This indicates that in addition to the decrease in capacitance value caused by normal self-healing discharge, the dielectric and conductive parts of the capacitor have also undergone severe deterioration.

[0082] Compared with the prior art, the technical advantages of the present invention are as follows:

[0083] 1. Existing methods for assessing capacitor condition commonly involve monitoring the capacitance value at a single frequency. However, this method only reflects the capacitance value and does not address the causes of capacitor degradation. This new method, by measuring capacitance values ​​at multiple frequencies, not only determines the capacitance value but also the condition of the capacitor dielectric, providing a more comprehensive assessment of the capacitor's health and allowing for analysis of the causes of degradation. In Examples 1 and 2, the capacitor capacitance value both decreased to 95% of its unaged value at 50Hz, but the causes of degradation differed. This method can be used to analyze the different causes of degradation.

[0084] 2. In Example 2, severe degradation of the capacitor's dielectric material leads to a decrease in the film breakdown field strength. The system's safety and reliability are threatened until the capacitance value drops to 95% of its unaged value at 50Hz. Therefore, detecting the capacitor's dielectric condition is crucial. Compared to traditional methods, this method enables dielectric condition detection, preventing system failures caused by dielectric degradation.

[0085] 3. When the capacitance value measured at 50Hz is inaccurate before aging, the capacitor may not be considered faulty even when its capacitance drops below 95% of its unaged capacitance at 50Hz. This method can determine the dielectric degradation of the capacitor while measuring the capacitance value. Since a decrease in capacitance is accompanied by dielectric degradation, by simultaneously measuring both parameters and assessing the capacitor's health status, severe degradation can be avoided.

[0086] The foregoing general description of the invention and its specific embodiments should not be construed as a limitation on the technical solution of the invention. Those skilled in the art, based on the disclosure of this application, can add, reduce, or combine the disclosed technical features in the foregoing general description and / or specific embodiments (including examples) without departing from the constituent elements of the invention, to form other technical solutions within the scope of protection of this application.

Claims

1. A method for evaluating the state of a metallized film capacitor based on its frequency response curve, characterized in that, The method includes the following steps: S100: Take the capacitor to be tested and apply an alternating current across the capacitor. S200: Measures the phase of the voltage across the capacitor at 50Hz, and measures the amplitude and phase of the capacitor current at 50Hz. The magnitude of the amplitude is denoted as I. M Calculate the phase difference between voltage and current; S300: Based on the previous frequency, increase the current frequency by 50Hz and apply current until the current magnitude reaches I. M Record the magnitude U of the voltage at this moment, and calculate the phase difference between the voltage and the current. ; S400: Repeat step S300 until the measured current frequency reaches the required level for plotting the frequency response curve of the capacitance value. S500: Based on the voltage, current, and phase difference values ​​at different frequencies, calculate the capacitive reactance X of the capacitor at the corresponding frequency to obtain the capacitance value C at different frequencies. M ; S600: Capacitance value C at different frequencies M Plot the frequency response curve of the capacitance value with the vertical axis as the ordinate and the frequency f as the horizontal axis. S700: Extract characteristic parameters from the frequency response curve of the capacitance value; S800: Analyze whether the capacitor has failed based on the aforementioned characteristic parameters; in, The S800 includes the following steps: S801: Calculate the ratio of the tested capacitor to its capacitance at 50 Hz before aging. ; S802: When the capacitance value of a capacitor at 50 Hz drops to less than 95% of its capacitance value at 50 Hz before aging, i.e. η<95%, the capacitor is considered to have reached the end of its lifespan. S803: Determine the threshold for the cutoff frequency according to application requirements. If the cutoff frequency exceeds this threshold, the capacitor is also considered to be in failure. S804: Determine the cause of capacitor failure based on η; S805: Determine the cause of capacitor failure based on the cutoff frequency; The cause of failure described in S805 is determined in the following manner: The structure of a capacitor is represented by an equivalent series-parallel model. Based on this model, the capacitance value C is calculated. M The expression is: ， Among them, R P It is the equivalent parallel resistance, C represents the capacitance of the ideal capacitor, and L S It is the equivalent series inductance formed by the winding structure of the capacitor, and f represents the frequency of the voltage and current during measurement.

2. The evaluation method according to claim 1, characterized in that, The frequency of the alternating current in step S100 is 50Hz.

3. The evaluation method according to claim 1, characterized in that, The magnitude of the current in step S200 is obtained in the following way: Use the step-up voltage increase method to raise the voltage across the capacitor. When the voltage across the capacitor approaches its AC rated voltage, record the current at this point.

4. The evaluation method according to claim 1, characterized in that, The capacitive reactance X mentioned in step S500 is calculated as follows: ， In the formula, U represents the measured voltage, and I M Indicates the applied current. This represents the phase difference between voltage and current.

5. The evaluation method according to claim 4, characterized in that, The capacitance value C mentioned in step S500 M Calculated as follows: ， In the formula, f is the frequency at which the capacitance value is calculated, and X is the capacitive reactance of the capacitor at that frequency.

6. The evaluation method according to claim 1, characterized in that, The characteristic parameters mentioned in step S700 include: the capacitance value of the capacitor at a frequency of 50 Hz, and the frequency corresponding to the inflection point of the frequency response curve, denoted as f. M .

7. The evaluation method according to claim 1, characterized in that, The cause of failure described in S804 is determined in the following manner: When the capacitance of a capacitor drops to less than 95% of its capacitance at 50 Hz before aging, the cause of failure may include cumulative self-healing discharge or electrochemical corrosion.

8. The evaluation method according to claim 1, characterized in that, The method further determines the state of the medium based on the transition frequency.

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