Atmospheric correction of insulation withstand voltage test values for North American switchgear

The insulation test of the North American medium voltage air insulated armored switchgear standard IEEE C37.20.2 follows the IEEE STD4-2013 standard, which has clear provisions for test voltage, atmospheric correction, etc., slightly different from the IEC standard. The standard specifies that the dielectric rating is applicable under standard conditions (temperature=20 ℃, pressure=101.3 kPa [14.7 psi], absolute humidity 11 g/m3). Of course, if it occurs during testing, it is under standard conditions. Therefore, the insulation withstand voltage test must correct the voltage required during the test period to match the actual test conditions and make it the same as the performance under standard conditions.

The standard requirements aim to generate reproducible testing conditions and parameters that are unlikely to be affected by specific testing laboratory choices. The consistency of the experiment will not vary due to differences in the laboratory, location, and time of the experiment. During dielectric testing, equipment, conductors, and insulation components must be clean and non-contact.

The result is that the test showed performance under "ideal" (and reproducible) conditions, and the performance at the installation site will not match the values obtained during design testing. Various standards and application guidelines recognize this and stipulate the use of "protective margins" in insulation coordination research. This attempts to coordinate the rated dielectric value with the actual capacity of the equipment in actual installation, where insulation may degrade due to age, unfavorable atmospheric conditions, unclean equipment and insulation surfaces, and other factors. However, our domestic laboratories often do not make corrections and conduct tests based on a coefficient of 1, which is called raising standards. As a testing station or certification agency, they do not conduct atmospheric correction tests. Testing agencies comply with standards, rather than setting their own standards. This is unscientific and violates standards. Members of the International Short Circuit Alliance STL strictly adhere to the standards, and the insulation withstand voltage values in their test reports are corrected by atmospheric conditions. Unlike the atmospheric correction of IEC standards, North American standards have different correction factors for positive and negative polarity test values of power frequency withstand voltage and lightning impulse withstand voltage, and must be calculated according to IEEE STD4-2001 standard.

The following is an explanation of AI's calculation values for the IEEE Std4-2013 standard atmospheric coefficient. According to the IEEE Std4-2013 standard, the Atmospheric Correction Factor is used to correct the effect of differences in atmospheric conditions (temperature, pressure, humidity) from standard atmospheric conditions on insulation strength during high-voltage testing. The calculation formula for this coefficient is: k=d × fk=d \ timesfk=d × f. The range of values for the correction coefficient is: the maximum value of the atmospheric correction coefficient is 1, corresponding to standard atmospheric conditions (without correction). When the actual environment is worse than the standard conditions (such as a decrease in high-altitude air pressure, an increase in temperature, or a decrease in humidity), the correction factor will be less than 1, thereby reducing the test voltage to avoid over assessment of the equipment. When the actual environment is better than the standard conditions, the correction factor is still calculated as 1 and will not be greater than 1. If the calculated correction factor is greater than 1, possible reasons and solutions: Calculation error: Check whether the formula application and parameter input are correct, such as whether the units of temperature, air pressure, and humidity are consistent, and whether standard atmospheric condition parameters have been misused. Data anomaly: Actual measured atmospheric parameters may have errors or outliers, requiring re measurement or verification of the data. Scope of application: The IEEE Std4-2013 standard is mainly aimed at atmospheric correction for high-voltage testing. If it involves other fields (such as power correction, remote sensing atmospheric correction, etc.), reference should be made to the corresponding standards and methods in the field. Summary: The atmospheric correction factor of IEEE Std4-2013 will not exceed 1. If the calculation result is abnormal, priority should be given to checking the calculation process and data accuracy to ensure compliance with standard requirements. Currently, more and more foreign customers are choosing domestic testing stations for testing. Therefore, domestic testing stations must strictly adhere to standards and cannot follow domestic practices, which will affect foreign customers' doubts about the capabilities of domestic testing stations. Engineers from various certification agencies also need to study the standards more, follow the standards, and not be stubborn, do not make progress, and do not learn.

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