NORME INTERNATIONALE CEI IEC INTERNATIONAL STANDARD 62068 1 Première édition First edition 2003 07 Systèmes d''''isolation électrique – Contraintes électriques produites par des impulsions de tension app[.]
Vue d'ensemble
This article outlines the general procedures for assessing the ability of a System of Electrical Installations (SIE) to withstand damage from periodically applied voltage impulses Two methods are proposed based on the desired outcome: a selective test can be conducted for a single test voltage to evaluate additional parameters.
MIEs, or various physical constructions, are compared to the previously evaluated SIE to identify which MIE enhances endurance A single SIE can be assessed through simple tension tests under varying conditions, such as different humidity levels and variable impulse repetition rates, to determine the impact of these factors Additionally, endurance testing can be conducted to evaluate the relationship between tension impulse and lifespan for each SIE The SIE is tested at multiple tension levels while keeping other conditions generally constant A potential relationship between tension endurance and voltage amplitude may be represented by an inverse power law.
L est le temps écoulé jusqu'à la défaillance ou le nombre d'impulsions avant défaillance de l'objet soumis à l'essai (pour une probabilité donnée);
U est l'impulsion de tension appliquée; n est le coefficient connu d'endurance en tension (VEC); k est une constante.
D'autres relations sont aussi possibles Par exemple, le modèle exponentiel est:
L = Ae – hU (2) ó A et h sont des constantes.
The outcomes of an electric pulse endurance test or selection test are influenced by numerous factors beyond the inherent capacity of a SIE It is essential to specify and control these factors in all aging tests related to pulses Appendix A reviews these factors.
The following paragraphs outline the general testing procedures for the selection and endurance testing under impulse conditions The design of the object and the characteristics of the voltage impulse are determined by the Equipment Under Test (EUT) Therefore, specific testing procedures that identify the design of the test object and the impulse characteristics for a particular EUT will be detailed in future sections of IEC 62068.
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VEC exponent of the inverse power model or exponential model, which together with the coefficient k, describes the relationship between life and voltage
3.15 life either time or number of impulses to failure
This clause outlines the procedures for assessing an EIS's resistance to deterioration from repetitive impulse voltages, utilizing two distinct methods The first method involves a screening test at a single voltage to compare various EIMs or physical constructions against previously evaluated EIS, aiming to identify the option with superior endurance Additionally, a single EIS can be tested under varying conditions, such as humidity and impulse repetition rates, to analyze the impact of these variables The second method is an endurance test, which estimates the correlation between impulse voltage and lifespan for each EIS This involves evaluating the EIS at multiple voltage levels while maintaining consistent conditions, with the relationship between voltage endurance and magnitude potentially represented by an inverse power law.
L is the time to failure or number of impulses to failure of the test object (at a given probability);
U is the applied impulse voltage; n is the voltage endurance coefficient (VEC); k is a constant.
Other relationships are also possible For example, the exponential model is:
L = Ae – hU (2) where A and h are constants.
The outcomes of an impulse electrical endurance or screening test are influenced by various factors beyond the intrinsic capabilities of an Electrical Insulation System (EIS) It is essential to identify and manage these factors during any impulse-ageing test, as detailed in Annex A.
The following describes the general test procedures for impulse screening and endurance testing The design of the test object and the impulse-voltage characteristics depend on the
The article will detail specific test procedures that define the design of the test object and the impulse characteristics for a particular Electrical Insulation System (EIS), as outlined in future sections of the IEC.
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Objet à l'essai
L'objet soumis à l'essai contient un conducteur séparé du conducteur à la terre par une isolation électrique Certains modèles de SIE seront décrits dans de futures parties de la CEI
For each testing procedure, it is recommended to use a sample consisting of at least 11 test objects per voltage level Utilizing a larger number of specimens can enhance statistical accuracy, particularly when detecting minor differences is essential.
Méthode d'essai de sélection
Procédure d’essai
A sample of test objects must undergo a specified voltage impulse in accordance with the endurance voltage procedures outlined in IEC 60727-1 The application of a triggering current can be tailored for detecting failures in the tested objects.
Dans certains types d'essai, d'autres moyens de détection de défaillance peuvent être requis.
The selected test conditions must consider the applicability factors outlined in Annex A The voltage impulse characteristics should align with those specified in Article 5 If a future part of IEC 62068 describes a relevant SIE, its voltage impulse characteristics must be consistent with those presented in Table 1.
La tension d'essai choisie doit être pertinente pour que le processus de défaillance soit modélisé.
Mesures de TADP et TEDP
TADP and TEDP should be measured under impulse voltage rather than at industrial frequency If it can be demonstrated that TADP (TEDP) is the same under impulse voltage and at industrial frequency, then measurements for the tested object can be conducted solely at industrial frequency.
NOTE Les mesures normalisées telles que décrites dans la CEI 60270 ne s’appliquent pas pour des décharges partielles (DP) causées par des tensions impulsionelles.
The values of TADP and TEDP can significantly vary depending on the measuring instrument used Therefore, it is essential to specify the measurement system and the criteria for determining TADP and TEDP.
Traitement des données
Failure times should be analyzed using the two-parameter Weibull distribution Complete or censored tests can be conducted, provided that at least \((n + 1)/2\) specimens fail if \(n\) is odd, or \((n/2 + 1)\) if \(n\) is even.
Based on the scale and shape parameter estimates, which indicate a failure time with a probability of 63.2%, it is possible to determine the average and median failure times, the number of impulses before failure, and the failure percentages The method with the highest probability can be utilized for these calculations.
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The test object consists of a conductor insulated from the earth conductor Future sections of IEC 62068 will detail various EIS models Each test procedure should utilize a sample of at least 11 test objects for each voltage level.
A greater number of specimens may be needed if greater statistical significance is required to detect small differences.
Before designing materials and Electrical Insulation Systems (EIS) into specific products, it is essential to evaluate them thoroughly At this stage, the final form of the impulse is often unknown A screening test establishes a standardized set of test conditions and impulse-voltage characteristics for all materials under evaluation This common framework allows for a consistent basis to assess and compare different materials effectively.
It is also necessary to establish a fixed set of parameters so that evaluation of the effect of change in parameters can be compared realistically.
Test objects must undergo specified impulse voltage testing as outlined in IEC 60727-1, with a trip-current device serving as an effective method for monitoring failures For specific test objects, alternative failure detection methods may be necessary It is essential to consider the relevant factors in Annex A when selecting test conditions Additionally, the impulse-voltage characteristics should align with those specified in Clause 5, and future sections of IEC 62068 may provide further relevant details.
EIS, then the impulse-voltage characteristics should be consistent with those described in
The test voltage selected shall be relevant for the failure process being modelled.
The PDIV and PDEV should be assessed using impulse voltage instead of power-frequency voltage If it is demonstrated that the PDIV (PDEV) remains consistent under both impulse and power-frequency voltage, then measurements can be conducted solely at power-frequency voltage for the tested object.
NOTE Standard measurement methods as described in IEC 60270 are not applicable for partial discharges (PD) caused by impulse voltages.
As the values of PDIV and PDEV may vary significantly depending on the instrument used to make measurements, the measuring system and the criterion used to establish PDIV and
Time-to-failures shall be processed using the two-parameter Weibull probability distribution.
Tests can be conducted as either complete or singly censored, provided that a minimum of (n + 1)/2 specimens fail for odd n, or (n/2) + 1 for even n This approach relies on estimates of the scale and shape parameters, with the scale parameter reflecting the time-to-failure at a specified probability.
63,2 %), the mean and median time-to-failure and number of impulses to failure, as well as
This document is licensed to MECON Limited for internal use in Ranchi and Bangalore, provided by the Book Supply Bureau It can be utilized to estimate scale and shape parameters, and confidence intervals for these parameters and percentages can also be calculated, with a recommended probability of 90%.
Les procédures d'analyse statistiques sont décrites dans la CEI 61649.
Evaluation
Repeat the selection test for each system being evaluated or to assess the change of a single parameter Relative evaluations can then be made by comparing the failure time or the number of pulses before failure for a given probability: the longer the failure time or the greater the number of pulses before failure, the better the performance of the SIE This procedure will assist in selecting suitable candidates for the design of the equipment's SIE.
Méthode d'essai d'endurance
SIE de référence
Pour effectuer l’essai, choisir au moins trois niveaux d'impulsion de tension différents supérieurs aux contraintes en services souhaitées (pour les besoins d’accélération de l’essai).
Il est recommandé que la différence entre les niveaux de tension consécutifs soit au moins de
If \( n \) is known to be greater than 15, the consecutive voltage levels can differ by less than 10% Voltage levels are selected to ensure that the failure mechanism remains consistent across the range of test voltages The failure mechanisms of the SIE under test should resemble those in operational mode Various failure mechanisms can be identified, for instance, through microscopic examination of failure sites or by observing changes in the slope of a double logarithmic plot of voltage versus the number of impulses until failure (or time to failure), which may be influenced by voltage levels that are partially above or below TADP.
Conduct endurance testing on each test object at the selected voltages to determine the number of impulses or the time until failure Analyze the number of impulses before failure or the minutes until failure (for complete or censored tests) using the two-parameter Weibull function (refer to section 4.3.3).
Estimate the scale parameter values (such as median, mean, or another specified percentile) obtained for each level of test tension and create a logarithmic or semi-logarithmic plot.
Essai comparatif
Après avoir établi une courbe d'endurance pour un SIE de référence, un autre SIE candidat peut être évalué en utilisant la même procédure d'essai et les mêmes tensions d'essai.
A comparison of the VEC for each candidate SIE against the reference system reveals the relative degradation caused by voltage impulses Additionally, the failure time or the number of impulses before failure, for a given probability, can be compared at the lowest test voltage A greater difference between the candidate and the reference system indicates better endurance of the candidate SIE under operating conditions, assuming that the candidate SIE requires more impulses before failure.
To analyze the lifespan of each examined SIE, a regression method is employed to plot the data on a log-log graph, as outlined in Equation (1) If the resulting graph does not yield a straight line, it indicates a low correlation coefficient.
In cases where the failure rate is less than 0.85, a semi-log graph can be employed to plot either the logarithm of the number of impulses or the duration in minutes until failure against the applied voltage amplitude If the resulting graph is linear, it indicates that the lifespan model follows an exponential law (as shown in Equation (2)) However, if a nonlinear characteristic is observed, it suggests that the failure mechanism may vary at different voltage levels Consequently, it may be necessary to repeat the testing sequence at various voltage levels, with particular attention to the Average Time to Failure (TADP) and the Time to Event Distribution (TEDP).
The maximum likelihood method is effective for estimating scale and shape parameters, as well as failure percentiles Additionally, confidence intervals for these parameters and percentiles can be calculated, with a recommended probability level of 90%.
Statistical analysis procedures are described in IEC 61649.
Conduct this screening test for each system under evaluation or when assessing a single parameter change By comparing time-to-failure or the number of impulses to failure at a specified probability, relative evaluations can be made; a longer time-to-failure or a higher number of impulses indicates superior EIS performance This approach will aid in identifying appropriate candidates for the design of EIS equipment.
Select at least three distinct impulse-voltage levels for testing, ensuring they exceed the anticipated service stress to accelerate the test The voltage levels should differ by a minimum of 10%, although if \( n \) exceeds 15, smaller differences are permissible It is crucial that the selected voltage levels maintain consistent failure processes within the test range, mirroring those experienced under actual operating conditions Variations in failure processes can be identified through microscopic analysis of failure sites and changes in the slope of the log voltage versus log number of impulses to failure plot, which may shift due to test voltage levels above or below the partial discharge inception voltage (PDIV).
Conduct endurance tests on each test object at the specified voltages to ascertain the number of impulses or time-to-failure Analyze the failure data, whether in terms of impulses or minutes, utilizing a two-parameter model for both complete and censored tests.
The Weibull function is utilized to estimate scale parameter values, such as the median, mean, or other specified percentiles, at each test-voltage level These values can then be plotted in either a log-log or log-linear (semi-log) coordinate system.
After a reference EIS endurance curve has been established, another candidate EIS can be evaluated using the same test procedure and test voltages.
A comparison of the Voltage Endurance Characteristics (VEC) for each candidate against the reference Electrical Insulation System (EIS) reveals the extent of degradation due to impulse voltage Additionally, the time-to-failure or the number of impulses to failure at a specified probability, determined at the lowest test voltage, can be analyzed A larger disparity between the candidate and the reference system indicates a superior expected endurance of the candidate EIS under operational conditions, provided that the candidate EIS necessitates more impulses to reach failure.
2 Draw a lifeline (calculated by a regression technique) for each examined EIS using a log-log plot according to
If the correlation coefficient is less than 0.85, a semi-log coordinate system should be employed, plotting the logarithm of either the number of impulses or the minutes to failure against voltage A straight line indicates that the life model aligns with the exponential model However, if a non-linear characteristic persists, it suggests that the failure process may vary at different voltage levels, necessitating a repeat of the test sequence with alternative test voltages while closely examining the PDIV and PDEV values.
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The statistical methods outlined in IEC 61649 can be employed to assess significant differences During comparative testing, it is advisable to have a sufficient number of specimens to detect differences at a 10% significance level when true differences exist.
5 Caractéristiques d'impulsion de tension d'essai
Table 1 outlines the range of voltage impulse characteristics that may be utilized in anticipation of a future section of IEC 62068, which will define specifications for a specific SIE It is advisable that any particular test features testing characteristics suitable for the environment of the equipment type in use Additionally, the voltage impulse measurement system should have a bandwidth of at least 10 MHz to accurately capture an impulse with a rise time of 40 ns.
Tableau 1 – Caractéristiques d'impulsion de tension d’essai
Polarité Unipolaire ou bipolaire (de préférence)
3 Des différences significatives peuvent être détectées en comparant la superposition des niveaux de confiance de chaque SIE.
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The statistical methods outlined in IEC 61649 are effective for evaluating significant differences It is advisable to ensure that comparison tests include a sufficient number of specimens to identify differences at the 10% significance level, should such differences exist.
Table 1 illustrates a range of impulse-voltage characteristics applicable in the absence of specific guidelines from IEC 62068 for a given EIS It is essential that the test characteristics align with the environmental conditions relevant to the equipment type Additionally, the impulse-voltage measurement system must possess a minimum bandwidth of 10 MHz to accurately capture a 40 ns rise-time impulse.
Table 1 –Test impulse-voltage characteristics
Polarity Bipolar (preferred) or unipolar
3 Significant differences can be detected by observing if the confidence levels for each EIS overlap.
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Généralités
Equipment circuits can experience impulse voltages due to voltage switching or lightning strikes The growing reliance on electronic devices increasingly subjects various electrical insulation systems to these impulse voltages Currently, the repetition rate of these impulses ranges from 0.5 to 20 kHz, with rise times included.
(0,1–1) às et des tensions crờtes pouvant dộpasser 2 fois la valeur de la tension nominale.
Short-duration constraints with high repetition rates can adversely affect SIEs in ways that differ from industrial frequency stresses The degradation may result from one or more of the following physical mechanisms.
– l’injection et l’extraction d’une charge d’espace dans le MIE;
– la fatigue électromécanique due aux impulsions de courant associées à l’application de tensions impulsionnelles aux SIE de fortes capacités;
– un échauffement diélectrique dû aux composantes hautes fréquences de la tension.
Voir la CEI 62068-2 pour plus d’informations.
La détérioration due à l’application d’une tension impulsionelle répétitive provenant des alimentations de l’ộlectronique de puissance peut apparaợtre, par exemple, dans les diffộrents types d’équipements électriques suivants:
– les bobinages de stators de moteurs bobinés de manière aléatoire;
– les bobinages rangés des machines moyenne tension;
– les alimentations de puissance et les capacités de filtrage;
– les câbles de liaisons de puissance;
Effet de la température
Electrical degradation can significantly increase at high temperatures The deterioration rate may rise if the dielectric losses of the insulating materials are elevated, leading to a further local temperature increase in areas with strong electric fields An increase in temperature also raises the permittivity of the insulating materials, resulting in greater stress at the cavity terminals, which lowers the breakdown voltage and enhances discharge activity.
In confined SIEs, an increase in temperature can reduce the size of cavities, leading to decreased discharge intensity and, consequently, a lower deterioration rate However, higher temperatures may also raise gas pressure within closed cavities, potentially impacting partial discharge activity Similarly, the trapping and release times of trapped electric charges may be shorter at elevated temperatures Therefore, it is essential to clearly specify the temperature of the test object during endurance testing.
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Equipment circuits can experience impulse voltages due to lightning or switching events The growing reliance on electronic technology is leading to repetitive impulse voltages affecting various electric insulation systems.
The typical repetition rate of these impulses ranges from 0.5 to 20 kHz, with an impulse rise time between 0.1 and 1 microseconds, and a peak voltage that exceeds twice the nominal supply voltage.
Short-duration, high-repetition impulses can adversely affect insulation systems in ways that differ from the effects of conventional a.c power-frequency voltage This electrical deterioration may arise from various physical processes.
– injection and extraction of space charges in the EIMs;
– electromechanical fatigue due to the current impulses resulting from voltage impulses applied to high capacitance EIS;
– dielectric heating due to the high-frequency components in the voltage.
See IEC 62068-2 for more information.
Deterioration due to repetitive voltage impulses from electronic power supplies may, for example, occur in the following types of electrical equipment:
– random-wound motor stator windings;
– medium-voltage, form-wound stator windings;
– power-supply and filter capacitors;
Elevated temperatures significantly impact electrical degradation, as increased dielectric loss in electrical insulation materials (EIMs) can lead to higher local heating under electric stress This rise in insulation temperature enhances the dielectric permittivity of EIMs, resulting in increased electric stress in nearby air gaps and a lower partial discharge inception voltage, which escalates partial discharge (PD) activity In confined electrical insulation systems (EIS), higher temperatures may reduce void sizes, thereby decreasing PD intensity and the overall deterioration rate Additionally, increased gas pressure within closed voids at elevated temperatures can influence PD activity, while electric charge trapping and detrapping times may also shorten Therefore, it is crucial to specify the temperature of the test object during endurance testing.
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Effet de la contrainte mécanique
Mechanical stress, both static and dynamic, can significantly enhance electrical degradation through a synergistic effect described in IEC 60505 In fact, mechanical stress can create and/or exacerbate insulation defects, as the electric field associated with repetitive impulses can more easily increase partial discharge (PD) Additionally, it can contribute to damage caused by the energy released with each impulse, thereby lowering the energy barrier of the degradation process.
Effet de l'humidité et de l'environnement
Humidity in the environment of an electrical insulation system (EIS) affects the dielectric strength of air and the performance of the dielectric properties (DP) Additionally, the surrounding air humidity and/or the conditioning of the EIS surface can influence the distribution of electrical stress and the conduction of electric charges on the insulation surface, thereby altering the deterioration rate Therefore, it is essential to define and control humidity and environmental conditions during endurance testing.
Effet des caractéristiques d'amplitude de tension et d'impulsion de tension
In certain equipment, the voltage distribution can vary significantly depending on whether the voltage is impulsive or at industrial frequency The amplitude and duration of the electrical stress between components of the electrical installation (SIE) caused by these voltage impulse phenomena depend on the physical position of the electrical stress relative to the power supply connection (between phases or to ground), as well as the characteristics of the electrical circuit, including series capacitances, ground capacitances, resistances, and inductances Therefore, a rigorous design of the objects subjected to SIE testing is essential to accurately simulate the impact of voltage impulse stresses.
The rise time of voltage impulses can significantly impact aging rates, making it essential to define this parameter in testing In certain electrical insulation systems, such as those with multi-turn windings, a shorter rise time results in a greater proportion of voltage affecting adjacent turns.
Shorter rise times can lead to reduced endurance if partial discharges are the cause of degradation Additionally, the physical processes of deterioration may be influenced by rise time Furthermore, the accumulation of charges can be time-dependent, thereby affecting the distribution of the electric field.
The voltage amplitude significantly influences the aging rate of materials Generally, a higher applied test voltage leads to an increased aging rate Often, an inverse power model or an exponential model can effectively describe the relationship between voltage endurance and voltage amplitude.
A variety of aging processes induced by voltage impulses can occur in any specific electrical insulation system (EIS) Deterioration in an EIS may result from space charge injection and partial discharge processes It is essential to select the appropriate test voltage to simulate the desired deterioration process.
In general, the expected occurrence during service should be considered For instance, if the degradation caused by the injection of a space charge is the only process being simulated, it is important that the test voltage remains below the breakdown voltage.
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Mechanical stress, whether static or dynamic, significantly exacerbates electrical degradation due to a synergistic effect outlined in IEC 60505 This stress can create or enlarge defects in insulation, making it easier for the electric field from repetitive impulses to induce partial discharge (PD) Additionally, it contributes to the damage from the energy released by each impulse, lowering the energy barrier for the degradation process.
A.4 Effect of humidity and the environment
Humidity in the environment around an Electrical Insulation System (EIS) can significantly impact the breakdown strength of air and influence partial discharge (PD) activity Additionally, the moisture levels and surface conditions of the EIS affect the distribution of electrical stress and the conduction of charges on the insulation surface, which can alter the rate of deterioration Consequently, it is essential to define and control humidity and environmental conditions during endurance testing.
A.5 Effect of voltage magnitude and impulse-voltage characteristics
Voltage distribution in equipment can vary greatly between impulse and power-frequency voltages The electric stress experienced in insulation systems is influenced by the physical positioning of the stress in relation to the supply voltage connection, as well as the characteristics of the electric circuit, including capacitances, resistances, and inductances Therefore, meticulous design is essential.
EIS test objects is required to simulate properly the impact of impulse-voltage stresses.
The rise time of impulse voltage significantly influences the ageing rate of electrical insulation systems (EIS), particularly in designs with multiturn windings Shorter rise times can lead to a higher voltage across adjacent turns, potentially resulting in reduced endurance due to partial discharge-related degradation Additionally, the physical deterioration processes are affected by rise time, and the time-dependent accumulation of charges can alter the electric field distribution.
The magnitude of voltage significantly influences the ageing rate of materials, with higher applied test voltages typically leading to increased ageing This relationship can often be described using an inverse power model or an exponential model, highlighting the connection between voltage endurance and voltage magnitude.
Multiple aging processes can affect an Electrical Insulation System (EIS) due to voltage impulses, such as space charge injection and partial discharge It is crucial to select the appropriate test voltage to accurately simulate the expected deterioration process that will occur during service For instance, if the focus is solely on simulating deterioration from space charge injection, the test voltage must remain below the Partial Discharge Endurance Voltage (PDEV).
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Effet du taux de répétition d'impulsion
The repetition rate of impulse voltage can significantly influence the number of impulses required to induce failure High impulse repetition rates may lead to increased dielectric heating, resulting in fewer impulses before failure occurs Conversely, for certain deterioration processes dependent on dielectric properties and space charge, higher repetition rates may hinder various charge dissipation processes, thereby affecting the number of impulses needed to reach failure Therefore, it is essential to establish the appropriate repetition rate for testing.
Effet de la polarité de l'impulsion
The oscillatory nature of the impulse can influence the deterioration rate Unipolar impulses between the conductor and ground typically cause less deterioration than bipolar impulses of the same amplitude Additionally, for test objects exposed to non-uniform electric fields, the polarity of the applied voltage can impact endurance The specific shape of the impulse, aside from the rise time, does not appear to significantly affect endurance For instance, a test object subjected to a square impulse or a triangular impulse (with the same peak amplitude, rise time, and repetition rate) may exhibit approximately the same endurance.
Document de référence
Overview
This clause outlines the procedures for assessing an EIS's resistance to deterioration from repetitive impulse voltages, utilizing two distinct methods The first method involves a screening test at a single voltage to compare various EIMs or physical constructions against previously evaluated EIS, aiming to identify the option with superior endurance Additionally, a single EIS can be tested under varying conditions, such as humidity and impulse repetition rates, to analyze the impact of these variables The second method is an endurance test, which estimates the correlation between impulse voltage and lifespan for each EIS This involves evaluating the EIS at multiple voltage levels while maintaining consistent conditions, with the relationship between voltage endurance and magnitude potentially represented by an inverse power law.
L is the time to failure or number of impulses to failure of the test object (at a given probability);
U is the applied impulse voltage; n is the voltage endurance coefficient (VEC); k is a constant.
Other relationships are also possible For example, the exponential model is:
L = Ae – hU (2) where A and h are constants.
The outcomes of an impulse electrical endurance or screening test are influenced by various factors beyond the intrinsic capabilities of an EIS It is essential to identify and regulate these factors during any impulse-ageing test, as detailed in Annex A.
The following describes the general test procedures for impulse screening and endurance testing The design of the test object and the impulse-voltage characteristics depend on the
The article will detail specific test procedures for modeling Electrical Impedance Spectroscopy (EIS), including the design of the test object and the impulse characteristics relevant to each EIS These descriptions will be provided in future sections of the IEC documentation.
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L'objet soumis à l'essai contient un conducteur séparé du conducteur à la terre par une isolation électrique Certains modèles de SIE seront décrits dans de futures parties de la CEI
For each testing procedure, it is recommended to use a sample consisting of at least 11 test objects per voltage level Utilizing a larger number of specimens can enhance statistical accuracy, particularly when detecting minor differences is essential.
It is essential to assess materials and the SIE before their integration into a specific product At this stage, the final form of the impulse is often unknown Selection tests establish a single set of testing conditions and voltage impulse characteristics to be applied to all evaluated materials A common set of parameters is necessary to ensure that different materials can be compared on the same basis.
Il est aussi nécessaire d'établir un ensemble de paramètres figés de sorte que l'évaluation de l'effet d'un changement de ces paramốtres puisse ờtre comparộe de faỗon rộaliste.
A sample of test objects must undergo a specified voltage impulse in accordance with the endurance testing procedures outlined in IEC 60727-1 The application of a triggering current can be tailored for detecting failures in the tested objects.
Dans certains types d'essai, d'autres moyens de détection de défaillance peuvent être requis.
The selected test conditions must consider the applicability factors outlined in Annex A The voltage impulse characteristics should align with those specified in Article 5 If a future part of IEC 62068 describes a relevant SIE, its voltage impulse characteristics must be consistent with those detailed in Table 1.
La tension d'essai choisie doit être pertinente pour que le processus de défaillance soit modélisé.
4.3.2 Mesures de TADP et TEDP
TADP and TEDP should be measured under impulse voltage rather than at industrial frequency If it can be demonstrated that TADP (TEDP) is the same under impulse voltage and industrial frequency, then measurements for the tested object can be conducted solely at industrial frequency.
NOTE Les mesures normalisées telles que décrites dans la CEI 60270 ne s’appliquent pas pour des décharges partielles (DP) causées par des tensions impulsionelles.
The values of TADP and TEDP can significantly vary depending on the measuring instrument used Therefore, it is essential to specify the measurement system and the criteria for determining TADP and TEDP.
Failure times should be analyzed using the two-parameter Weibull distribution Complete or censored tests can be conducted, provided that at least \((n + 1)/2\) specimens fail if \(n\) is odd, or \((n/2 + 1)\) if \(n\) is even.
Based on the scale and shape parameter estimates, which indicate a failure time with a probability of 63.2%, it is possible to determine the average and median failure times, the number of impulses before failure, and the failure percentages The method with the highest probability can be utilized for these calculations.
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Test object
The test object consists of a conductor insulated from the earth conductor Future sections of IEC 62068 will detail various EIS models Each test procedure should utilize a sample of at least 11 test objects for each voltage level.
A greater number of specimens may be needed if greater statistical significance is required to detect small differences.
Screening test method
Before designing materials and Electrical Insulation Systems (EIS) into specific products, it is essential to evaluate them thoroughly At this stage, the final form of the impulse is often unknown A screening test establishes a standardized set of test conditions and impulse-voltage characteristics for all materials under evaluation Having a common set of parameters is crucial for ensuring that different materials can be assessed on an equal basis.
It is also necessary to establish a fixed set of parameters so that evaluation of the effect of change in parameters can be compared realistically.
Test objects must undergo the specified impulse voltage as outlined in IEC 60727-1, with a trip-current device serving as an effective method for monitoring failures For specific test objects, alternative failure detection methods may be necessary It is essential to consider the relevant factors detailed in Annex A when selecting test conditions Additionally, the impulse-voltage characteristics should align with those specified in Clause 5, and future sections of IEC 62068 may provide further relevant information.
EIS, then the impulse-voltage characteristics should be consistent with those described in
The test voltage selected shall be relevant for the failure process being modelled.
The PDIV and PDEV should be assessed using impulse voltage instead of power-frequency voltage If it is demonstrated that the PDIV (PDEV) values are comparable under both impulse and power-frequency voltage, then testing can be conducted solely with power-frequency voltage for the object being evaluated.
NOTE Standard measurement methods as described in IEC 60270 are not applicable for partial discharges (PD) caused by impulse voltages.
As the values of PDIV and PDEV may vary significantly depending on the instrument used to make measurements, the measuring system and the criterion used to establish PDIV and
Time-to-failures shall be processed using the two-parameter Weibull probability distribution.
Tests can be conducted as either complete or singly censored, provided that a minimum of \((n + 1)/2\) specimens fail for odd \(n\) or \((n/2) + 1\) for even \(n\) This approach relies on estimates of the scale and shape parameters, with the scale parameter reflecting the time-to-failure at a specified probability.
63,2 %), the mean and median time-to-failure and number of impulses to failure, as well as
This document is licensed to MECON Limited for internal use in Ranchi and Bangalore, as supplied by the Book Supply Bureau It can be utilized to estimate scale and shape parameters, and confidence intervals for these parameters and percentages can also be calculated, with a recommended probability of 90%.
Les procédures d'analyse statistiques sont décrites dans la CEI 61649.
Repeat the selection test for each system being evaluated or to assess the change of a single parameter Relative evaluations can then be made by comparing the time to failure or the number of pulses before failure for a given probability: the longer the time to failure or the greater the number of pulses before failure, the better the performance of the SIE This procedure will assist in selecting suitable candidates for the design of the equipment's SIE.
Pour effectuer l’essai, choisir au moins trois niveaux d'impulsion de tension différents supérieurs aux contraintes en services souhaitées (pour les besoins d’accélération de l’essai).
Il est recommandé que la différence entre les niveaux de tension consécutifs soit au moins de
If \( n \) is greater than 15, the consecutive voltage levels can differ by less than 10%, as referenced in Equation (1) The voltage levels are selected to ensure that the failure mechanism remains consistent across the entire range of test voltages The failure mechanisms of the SIE under test should resemble those in operational mode Various failure mechanisms can be identified through microscopic examination of failure sites or by observing changes in the slope of a double logarithmic plot of voltage versus the number of impulses until failure (or time to failure), which may be influenced by voltage levels that are partially above or below TADP.
Conduct endurance testing on each test object at the selected voltages to determine the number of impulses or the time until failure Analyze the number of impulses before failure or the minutes until failure (for complete or censored tests) using the two-parameter Weibull function (refer to section 4.3.3).
Estimate the scale parameter values (such as median, mean, or another specified percentile) obtained for each level of test tension and create a logarithmic or semi-logarithmic plot.
Après avoir établi une courbe d'endurance pour un SIE de référence, un autre SIE candidat peut être évalué en utilisant la même procédure d'essai et les mêmes tensions d'essai.
A comparison of the Voltage Endurance Characteristic (VEC) for each candidate in relation to the reference SIE reveals the relative degradation caused by voltage impulses Additionally, the failure time or the number of impulses before failure, for a given probability, can be compared at the lowest test voltage The greater the difference between the candidate and the reference system, the better the desired endurance of the candidate SIE under operating conditions, assuming that the candidate SIE requires more impulses before failure.
To analyze the lifespan of each examined SIE, we will plot the data using a log-log graph based on a regression method, as outlined in Equation (1) If the resulting graph does not yield a straight line, we will assess the correlation coefficient to evaluate the fit of the model.
In cases where the failure rate is less than 0.85, a semi-log graph can be employed to plot either the logarithm of the number of impulses or the duration in minutes until failure against the applied voltage amplitude If the resulting graph is linear, it indicates that the lifespan model follows an exponential law (Equation (2)) However, if a nonlinear characteristic is observed, it suggests that the failure mechanism may vary at different voltage levels Consequently, it may be necessary to repeat the testing sequence at various voltage levels, with particular attention to the Time to Average Degradation Point (TADP) and Time to End of Degradation Point (TEDP).
The maximum likelihood method is effective for estimating scale and shape parameters, allowing for the calculation of failure percentiles Additionally, confidence intervals for these parameters and percentiles can be determined, with a recommended probability level of 90%.
Statistical analysis procedures are described in IEC 61649.
Conduct this screening test for each system under evaluation or when assessing a single parameter change By comparing time-to-failure or the number of impulses to failure at a specified probability, relative evaluations can be made; a longer time-to-failure or a higher number of impulses indicates superior EIS performance This approach will aid in identifying appropriate candidates for the design of EIS equipment.
Endurance test method
Select at least three distinct impulse-voltage levels for testing, ensuring they exceed the anticipated service stress to accelerate the test The voltage levels should differ by a minimum of 10% However, if the value of \( n \) exceeds 15, consecutive voltage levels may vary by less than 10% It is crucial that the selected voltage levels maintain consistent failure processes within the test range, mirroring those experienced under actual operating conditions Different failure processes can be identified through microscopic analysis of failure sites and by observing changes in the slope of the log voltage versus log number of impulses to failure (or log time-to-failure) plots, particularly when test voltage levels are above or below the partial discharge inception voltage (PDIV).
Conduct endurance tests on each test object at the specified voltages to ascertain the number of impulses or time-to-failure Analyze the failure data, whether in terms of impulses or minutes, utilizing a two-parameter model for both complete and censored tests.
The Weibull function is utilized to estimate scale parameter values, such as the median, mean, or other specified percentiles, at each test-voltage level These values can then be plotted in either a log-log or log-linear (semi-log) coordinate system.
After a reference EIS endurance curve has been established, another candidate EIS can be evaluated using the same test procedure and test voltages.
A comparison of the Voltage Endurance Characteristics (VEC) for each candidate against the reference Electrical Insulation System (EIS) reveals the extent of degradation due to impulse voltage Additionally, the time-to-failure or the number of impulses to failure at a specified probability, determined at the lowest test voltage, can be analyzed A larger disparity between the candidate and the reference system indicates a superior expected endurance of the candidate EIS under operational conditions, provided that the candidate EIS necessitates more impulses to reach failure.
2 Draw a lifeline (calculated by a regression technique) for each examined EIS using a log-log plot according to
If the correlation coefficient is less than 0.85, indicating that a straight line is not achieved, a semi-log coordinate system can be employed to plot the logarithm of either the number of impulses or the time to failure against voltage A straight line in this representation suggests that the life model aligns with the exponential model However, if a non-linear characteristic persists, it may indicate a change in the failure process at varying voltage levels, necessitating a repeat of the test sequence with different voltages while closely examining the PDIV and PDEV values.
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The statistical methods outlined in IEC 61649 can be employed to assess significant differences During comparative testing, it is advisable to have a sufficient number of specimens to detect differences at a 10% significance level when true differences exist.
5 Caractéristiques d'impulsion de tension d'essai
Table 1 outlines the range of voltage impulse characteristics that may be utilized in anticipation of a future section of IEC 62068, which will define specifications for a specific SIE It is advisable that any particular test features testing characteristics suitable for the equipment type's environment Additionally, the voltage impulse measurement system should have a bandwidth of at least 10 MHz to accurately capture an impulse with a rise time of 40 ns.
Tableau 1 – Caractéristiques d'impulsion de tension d’essai
Polarité Unipolaire ou bipolaire (de préférence)
3 Des différences significatives peuvent être détectées en comparant la superposition des niveaux de confiance de chaque SIE.
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The statistical methods outlined in IEC 61649 are effective for evaluating significant differences It is advisable to ensure that comparison tests include a sufficient number of specimens to identify differences at the 10% significance level, should such differences exist.
Table 1 illustrates a range of impulse-voltage characteristics applicable in the absence of a future IEC 62068 standard, which will define specific characteristics for a given EIS Each test must possess characteristics suitable for the equipment's environment Additionally, the impulse-voltage measurement system should feature a minimum bandwidth of 10 MHz to accurately capture a 40 ns rise-time impulse.
Table 1 –Test impulse-voltage characteristics
Polarity Bipolar (preferred) or unipolar
3 Significant differences can be detected by observing if the confidence levels for each EIS overlap.
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Electrical equipment circuits can experience impulse voltages due to voltage switching or lightning strikes The growing reliance on electronic devices increasingly subjects various electrical insulation systems to these impulse voltages Currently, the repetition rate of these impulses ranges from 0.5 to 20 kHz, with rise times included.
(0,1–1) às et des tensions crờtes pouvant dộpasser 2 fois la valeur de la tension nominale.
Short-duration constraints with high repetition rates can adversely affect SIEs in ways that differ from industrial frequency stresses The degradation may result from one or more of the following physical mechanisms.
– l’injection et l’extraction d’une charge d’espace dans le MIE;
– la fatigue électromécanique due aux impulsions de courant associées à l’application de tensions impulsionnelles aux SIE de fortes capacités;
– un échauffement diélectrique dû aux composantes hautes fréquences de la tension.
Voir la CEI 62068-2 pour plus d’informations.
La détérioration due à l’application d’une tension impulsionelle répétitive provenant des alimentations de l’ộlectronique de puissance peut apparaợtre, par exemple, dans les diffộrents types d’équipements électriques suivants:
– les bobinages de stators de moteurs bobinés de manière aléatoire;
– les bobinages rangés des machines moyenne tension;
– les alimentations de puissance et les capacités de filtrage;
– les câbles de liaisons de puissance;
Electrical degradation can significantly increase at high temperatures The deterioration rate may rise if the dielectric losses of the insulating materials are elevated, leading to a localized temperature increase in areas with strong electric fields An increase in temperature also raises the permittivity of the insulating materials, resulting in greater stress at the cavity boundaries, which lowers the breakdown voltage and enhances discharge activity.
In confined SIEs, an increase in temperature can reduce the size of cavities, leading to decreased discharge intensity and, consequently, a lower deterioration rate However, higher temperatures may also raise gas pressure within closed cavities, potentially impacting partial discharge activity Similarly, the trapping and detrapping times of trapped electric charges may be shorter at elevated temperatures Therefore, it is essential to clearly specify the temperature of the test object during endurance testing.
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General
Equipment circuits can experience impulse voltages due to lightning or switching events The growing reliance on electronic technology is leading to repetitive impulse voltages affecting various electric insulation systems.
The typical repetition rate of these impulses ranges from 0.5 to 20 kHz, with an impulse rise time between 0.1 and 1 microsecond, and a peak voltage that exceeds twice the nominal supply voltage.
Short-duration, high-repetition impulses can adversely affect insulation systems in ways that differ from the effects of conventional a.c power-frequency voltage This electrical deterioration may arise from various physical processes.
– injection and extraction of space charges in the EIMs;
– electromechanical fatigue due to the current impulses resulting from voltage impulses applied to high capacitance EIS;
– dielectric heating due to the high-frequency components in the voltage.
See IEC 62068-2 for more information.
Deterioration due to repetitive voltage impulses from electronic power supplies may, for example, occur in the following types of electrical equipment:
– random-wound motor stator windings;
– medium-voltage, form-wound stator windings;
– power-supply and filter capacitors;
Effect of temperature
Elevated temperatures significantly impact electrical degradation, as increased dielectric loss in electrical insulation materials (EIMs) can lead to higher local heating under electric stress This rise in insulation temperature enhances the dielectric permittivity of EIMs, resulting in increased electric stress in nearby air gaps and a lower partial discharge inception voltage, which escalates partial discharge (PD) activity In confined electrical insulation systems (EIS), higher temperatures may reduce void sizes, thereby decreasing PD intensity and the overall deterioration rate Additionally, increased gas pressure within closed voids at elevated temperatures can influence PD activity, while electric charge trapping and detrapping times may also shorten Therefore, it is crucial to specify the temperature of the test object during endurance testing.
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A.3 Effet de la contrainte mécanique
Mechanical stress, both static and dynamic, can significantly enhance electrical degradation through a synergistic effect described in IEC 60505 In fact, mechanical stress can create and/or expand insulation defects, as the electric field associated with repetitive impulses can more easily increase partial discharge (PD) Additionally, it can contribute to damage caused by the energy released with each impulse, thereby lowering the energy barrier of the degradation process.
A.4 Effet de l'humidité et de l'environnement
Humidity in the environment of an electrical insulation system (EIS) affects the dielectric strength of air and the performance of the dielectric properties (DP) Additionally, the surrounding air humidity and/or the conditioning of the EIS surface can influence the distribution of electrical stress and the conduction of electric charges on the insulation surface, thereby altering the deterioration rate Therefore, it is essential to define and control humidity and environmental conditions during endurance testing.
A.5 Effet des caractéristiques d'amplitude de tension et d'impulsion de tension
In certain equipment, the voltage distribution can vary significantly depending on whether the voltage is impulsive or at industrial frequency The amplitude and duration of the electrical stress between components of the electrical installation (SIE) caused by these voltage impulse phenomena depend on the physical position of the electrical stress relative to the power supply connection (between phases or to ground), as well as the characteristics of the electrical circuit, including series capacitances, ground capacitances, resistances, and inductances Therefore, a rigorous design of objects subjected to SIE testing is essential to accurately simulate the impact of voltage impulse stresses.
The rise time of voltage impulses can significantly impact aging rates, making it essential to define this parameter in testing In certain electrical systems, such as those with multi-turn windings, a shorter rise time results in a greater proportion of voltage affecting adjacent turns.
Shorter rise times can lead to reduced endurance if partial discharges are responsible for degradation Additionally, the physical processes of deterioration may be influenced by rise time Furthermore, the accumulation of charges can be time-dependent, thereby affecting the distribution of the electric field.
The voltage amplitude significantly influences the aging rate of materials Generally, higher applied test voltages lead to an increased aging rate Often, an inverse power model or an exponential model can effectively represent the relationship between voltage endurance and voltage amplitude.
A variety of aging processes induced by voltage impulses can occur in any specific electrical insulation system (EIS) Deterioration in an EIS may result from space charge injection and partial discharge processes It is essential to select the appropriate test voltage to simulate the desired deterioration process.
In general, the expected occurrence during service should be considered For instance, if the degradation caused by the injection of a space charge is the only process being simulated, it is important that the test voltage remains below the breakdown voltage.
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Effect of mechanical stress
Mechanical stress, whether static or dynamic, significantly exacerbates electrical degradation due to a synergistic effect outlined in IEC 60505 This stress can create or enlarge defects in insulation, making it easier for the electric field from repetitive impulses to induce partial discharge (PD) Additionally, it contributes to the damage from the energy released by each impulse, lowering the energy barrier for the degradation process.
Effect of humidity and the environment
Humidity in the environment around an Electrical Insulation System (EIS) can significantly impact the breakdown strength of air and influence partial discharge (PD) activity Additionally, the moisture levels and surface conditions of the EIS affect the distribution of electrical stress and the conduction of charges on the insulation surface, which can alter the rate of deterioration Consequently, it is essential to define and control humidity and environmental conditions during endurance testing.
Effect of voltage magnitude and impulse-voltage characteristics
Voltage distribution in equipment can vary greatly between impulse and power-frequency voltages The electric stress experienced in insulation systems is influenced by the physical positioning of the stress in relation to the supply voltage connection, as well as the characteristics of the electric circuit, including capacitances, resistances, and inductances Therefore, meticulous design is essential.
EIS test objects is required to simulate properly the impact of impulse-voltage stresses.
The rise time of impulse voltage significantly influences the ageing rate of electrical insulation systems (EIS), particularly in multiturn windings Shorter rise times can lead to a higher voltage across adjacent turns, potentially resulting in reduced endurance due to partial discharge-related degradation Additionally, the physical deterioration processes are affected by rise time, and the time-dependent accumulation of charges can alter the electric field distribution.
The magnitude of voltage significantly influences the ageing rate of materials, with higher applied test voltages typically leading to increased ageing This relationship can often be described using an inverse power model or an exponential model, highlighting the connection between voltage endurance and voltage magnitude.
Multiple aging processes can affect an Electrical Insulation System (EIS) due to voltage impulses, including space charge injection and partial discharge It is crucial to select the appropriate test voltage to accurately simulate the expected deterioration process that will occur during service For instance, if the focus is solely on simulating deterioration from space charge injection, the test voltage must remain below the Partial Discharge Endurance Voltage (PDEV).
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A.6 Effet du taux de répétition d'impulsion
The repetition rate of impulse voltage can significantly influence the number of impulses required to induce failure High impulse repetition rates may lead to increased dielectric heating, resulting in fewer impulses before failure occurs Conversely, for certain deterioration processes dependent on discharge phenomena and space charge, higher repetition rates may hinder various charge dissipation processes, thereby affecting the number of impulses needed to reach failure Therefore, it is essential to establish the appropriate repetition rate for testing.
A.7 Effet de la polarité de l'impulsion
The oscillatory nature of the impulse can influence the deterioration rate Unipolar impulses between the conductor and ground typically result in less deterioration compared to bipolar impulses of the same amplitude Additionally, for test objects exposed to non-uniform electric fields, the polarity of the applied voltage can impact endurance The specific shape of the impulse, aside from the rise time, appears to have minimal effect on endurance For instance, a test object subjected to either a square or triangular impulse (with identical peak amplitudes, rise times, and repetition rates) may exhibit approximately the same endurance.
CEI 62068-2: Système d’isolation électrique – Contraintes électriques produites par des impulsions de tension appliquées périodiquement – Partie 2: Etat de l’art
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