IEC 62068 Edition 1 0 2013 03 INTERNATIONAL STANDARD NORME INTERNATIONALE Electrical insulating materials and systems – General method of evaluation of electrical endurance under repetitive voltage im[.]
Overview
Clause 4 outlines the procedures for assessing an EIS's resistance to deterioration from repetitive impulse voltages Two evaluation methods are available: a screening test at a single voltage to compare various EIMs or constructions against previously assessed EIS, aiming to identify the most durable option Additionally, a single EIS can be tested under varying conditions, such as humidity and impulse repetition rates, to analyze the impact of these variables.
IEC/TS 60034-18-42 provides a screening test example for stator winding stress grading coating An endurance test can be performed to assess the correlation between impulse voltage and lifespan for each evaluated Electrical Insulation System (EIS) The EIS is tested at various voltage levels while maintaining consistent conditions The relationship between voltage endurance and voltage magnitude may be expressed through 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 inherent 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.
This article outlines the general procedures for impulse screening and endurance testing, emphasizing that the design and quantity of test objects, along with their impulse-voltage characteristics, are determined by the specific EIS being modeled.
Test object
The test object consists of a conductor insulated from the earth conductor To achieve greater statistical significance in detecting small differences, a larger number of test objects is necessary Ideally, each test procedure should utilize a sample of at least five test objects per voltage level, as outlined in section 12.3.
Overheating at stress grading of test objects may be taken into account during endurance test when repetition frequency of test voltage impulse increases.
Screening test method
General
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 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 procedure
Test objects must undergo the specified impulse voltage as outlined in the voltage endurance procedures of IEC 60727-1 Utilizing a trip-current device can effectively monitor specimen failures, although alternative detection methods may be necessary for certain test objects It is essential to consider the relevant factors detailed in Annex A when selecting test conditions Additionally, the impulse-voltage characteristics should align with the specifications in Clause 5.
The test voltage selected shall be relevant for the failure process being modelled.
RPDIV and RPDEV measurements
The RPDIV and RPDEV shall be measured under impulse voltage, rather than PDIV and
PDEV under power-frequency voltage
NOTE RPDIV and PPDEV are measured as described in IEC/TS 61934
As the values of RPDIV and RPDEV may vary significantly depending on the instrument used to make measurements, the measuring system and the criterion used to establish RPDIV and
Data processing
Time-to-failures shall be processed using the two-parameter Weibull probability distribution
Tests can be conducted as either complete or singly censored, requiring that at least \((n + 1)/2\) specimens fail for odd \(n\) or \((n/2) + 1\) for even \(n\) By estimating the scale and shape parameters, which correspond to a time-to-failure probability of 63.2%, one can derive the mean and median time-to-failure, the number of impulses to failure, and failure percentiles The maximum likelihood method is effective for estimating these parameters, and it is advisable to calculate confidence intervals for the parameters and percentiles, with a recommended probability level of 90%.
Statistical analysis procedures are described in IEC 62539.
Evaluation
To evaluate each system or assess changes in a single parameter, repeat the screening test 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 performance of the EIM or EIS This process aids in selecting appropriate candidates for the design of EIM or EIS equipment.
Endurance test method
Reference EIS
To conduct the test effectively, select at least three distinct impulse-voltage levels that exceed the anticipated service stress to accelerate the testing process Ensure that the difference between consecutive voltage levels is a minimum of 10% However, if the value of \( n \) exceeds 15, the voltage levels can 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 Distinct 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 RPDIV threshold.
Conduct endurance tests on each test object at the specified voltages to ascertain the number of impulses to failure or the time-to-failure Analyze the failure data, whether complete or censored, utilizing the two-parameter Weibull function for processing.
Estimate the scale parameter values, such as the median, mean, or a specified percentile, for each test-voltage level Subsequently, plot these values using a log-log or log-linear (semi-log) coordinate system.
Comparison test
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 caused by impulse voltage Additionally, the time-to-failure or the number of impulses leading to failure, at a specified probability, can be assessed using the lowest test voltage.
A larger disparity between the candidate and the reference system indicates improved expected endurance of the candidate EIM or EIS under operational conditions, provided that the candidate EIM or EIS necessitates more impulses to reach failure The statistical methods outlined in the study support this conclusion.
IEC 62539 can be used to assess significant differences It is recommended that the
3 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 In this system, the logarithm of either the number of impulses or the time to failure in minutes is plotted against voltage.
If a straight line is observed, the life model aligns with the exponential model as described in Formula (2) Conversely, if a non-linear characteristic is present, it suggests that the failure process may vary at different voltage levels.
The test sequence may have to be repeated with different test voltages, investigating carefully the RPDIV and
RPDEV values comparison tests should have enough specimens to detect differences at the 10 % significance level if indeed there are differences 4
Table 1 illustrates the range of impulse-voltage characteristics, emphasizing the need for test characteristics that align with the specific environment and equipment type To accurately capture a 40 ns rise-time impulse, the impulse-voltage measurement system must possess a minimum bandwidth of 10 MHz.
Table 1 – Test impulse-voltage characteristics
Repetition rate (Up to 10 000) Hz
Polarity Bipolar (preferred) or unipolar
4 Significant differences can be detected by observing if the confidence levels for each EIS overlap
Equipment circuits can experience impulse voltages due to lightning strikes or switching impulses 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 10 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 alternating current (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
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 lowering the partial discharge inception voltage, which in turn escalates partial discharge (PD) activity In confined electrical insulation systems (EIS), higher temperatures may reduce void sizes, thereby decreasing PD intensity and deterioration rates However, thermal cycling can create or enlarge voids, potentially increasing PD amplitude and frequency Additionally, elevated temperatures can raise gas pressure within closed voids, influencing PD behavior, while also shortening electric charge trapping and detrapping times Therefore, it is crucial to specify the temperature of the test object during endurance testing.
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 magnitude and duration in insulation systems are influenced by the physical positioning of the stress concerning 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 rates This relationship can often be described using an inverse power model or an exponential model, illustrating how voltage endurance correlates with voltage magnitude.
Multiple aging processes can affect an Electrical Insulation System (EIS) due to voltage impulses Deterioration may arise from both space charge injection and partial discharge processes It is crucial to select a test voltage that accurately simulates the expected deterioration process during service For instance, if the focus is solely on simulating deterioration from space charge injection, the test voltage must remain below the Relative Partial Discharge Electric Voltage (RPDEV).
A.6 Effect of impulse repetition rate
Vue d'ensemble
Article 4 outlines the general procedures for assessing a System of Electrical Insulation (SIE) against deterioration caused by periodically applied impulse stresses Two methods are proposed based on the desired outcome: a) A selection test can be conducted using a single test voltage to evaluate other Insulation Materials (MIE) or different physical constructions in comparison to the previously assessed SIE, aiming to identify the MIE or construction that enhances endurance Additionally, a single SIE can be evaluated at a single test voltage under varying test conditions, such as different humidity levels and variable impulse repetition rates, to determine the impact of these factors.
The CEI/TS 60034-18-42 standard provides an example of a selection test for a stator winding stress distribution coating Additionally, an endurance test can be conducted to assess the relationship between impulse voltage and lifespan for each insulation system to be evaluated The insulation system is tested at various voltage levels while keeping other conditions generally constant A potential relationship between voltage 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 la tension impulsionnelle appliquée; n est le coefficient 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 impulse endurance testing The design and quantity of the objects being tested, as well as the characteristics of the impulse voltage, are determined by the SIE undergoing the test.
Objet à l'essai
The tested object features a conductor that is electrically insulated from the ground conductor To achieve greater statistical accuracy in detecting minor differences, a larger number of tested objects is beneficial When feasible, it is recommended to use a sample of at least five test objects per voltage level for each testing procedure, as outlined in section 12.3 of IEC/TS 60034-18-42:2008.
During endurance testing, overheating can occur due to the distribution of stresses in the tested objects, especially when the frequency of the test voltage impulse increases.
Méthode d'essai de sélection
Généralités
It is essential to assess materials and the impulse voltage before they are incorporated 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 impulse voltage 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 fixe de paramètres, de sorte que l'évaluation de l'effet d'un changement de ces paramốtres puisse ờtre comparộe de faỗon rộaliste.
Procédure d’essai
Test samples must be subjected to a specified impulse voltage according to the IEC 60727-1 voltage endurance procedures The application of a triggering current can be tailored for detecting failures in the tested objects.
In certain types of test objects, additional failure detection methods may be necessary The selected testing conditions should consider the applicability factors outlined in Annex A Furthermore, the characteristics of impulse voltage must align with those specified in Article 5.
La tension d'essai choisie doit être pertinente pour que le processus de défaillance soit modélisé.
Mesures de la tension d'apparition de décharges partielles répétitives (RPDIV) et de la tension d'extinction de décharges partielles répétitives (RPDEV)
et de la tension d'extinction de décharges partielles répétitives (RPDEV)
Les RPDIV et RPDEV doivent être mesurées sous tension impulsionnelle, et non à la fréquence industrielle, comme la PDIV et la PDEV
NOTE Les RPDIV et RPDEV sont mesurées tel que décrit dans la CEI/TS 61934
The values of RPDIV and RPDEV can vary significantly depending on the measuring instrument used Therefore, it is essential to specify the measurement system and the criteria for determining RPDIV and RPDEV.
Traitement des données
Failure times should be analyzed using the two-parameter Weibull probability distribution Complete or censored tests can be conducted, provided that at least \((n + 1)/2\) specimens are failed if \(n\) is odd, or \((n/2) + 1\) if \(n\) is even Based on the estimates of the scale and shape parameters—where the scale parameter corresponds to a 63.2% probability of failure—the mean and median times to failure, the number of impulses before failure, and failure percentages can be calculated The method with the highest likelihood can be employed to estimate the scale and shape parameters.
Les intervalles de confiance des paramètres et des pourcentages peuvent être aussi calculés; une probabilité de 90 % est recommandée
Les procédures d'analyse statistique sont décrites dans la CEI 62539
Repeat this selection test for each system to be 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 impulses before failure for a given probability: the longer the time to failure or the greater the number of impulses before failure, the better the performance of the MIE or SIE This procedure will assist in selecting suitable candidates for the design of the MIE or SIE of the equipment.
Méthode d'essai d'endurance
SIE de référence
To conduct the test, select at least three different impulse voltage levels that exceed the expected service stresses for the purpose of accelerating the test It is advisable that the difference between consecutive voltage levels be at least a specified minimum.
When n is greater than 15, the consecutive voltage levels can differ by less than 10%, as referenced in Formula (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 Distinct 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 to failure (or time to failure), which may result from test voltage levels that are partially above or below the RPDIV.
Conduct endurance testing on each test object at the selected voltage levels to determine the number of impulses before failure or the time until failure Analyze the number of impulses or time until failure (for complete or censored tests) using the two-parameter Weibull function Estimate the scale parameter values (median, mean, or another specified percentage) obtained for each voltage level 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 impulse voltage Additionally, the time to failure 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 MIE or SIE under operating conditions, assuming that the candidate MIE or SIE requires more impulses before failure The statistical methods outlined provide further insights into this analysis.
CEI 62539 can be utilized to assess significant differences In 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 de tension impulsionnelle d'essai
Table 1 illustrates a range of impulse voltage characteristics It is essential that any specific test features testing characteristics suitable for the equipment's environment Additionally, the impulse voltage 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 de tension impulsionnelle d’essai
Taux de répétition (Jusqu'à 10 000) Hz
Polarité Unipolaire ou bipolaire (de préférence)
To analyze the lifespan of each examined SIE, a regression method is employed to create a logarithmic plot, following Formula (1) If the resulting plot does not yield a straight line (with a correlation coefficient