ASTM D3300 Standard Test Method for Dielectric Breakdown Voltage of Insulating Oils of Petroleum Origin Under Impulse Conditions
8. Procedure
8.1 Set the electrode spacing to the desired value.
8.2 Rinse the test cell with a portion of the sample and discard this liquid. Slowly fill the cell with the test liquid, being careful to avoid entraining air bubbles. Allow it to set undisturbed for 2 min prior to testing.
8.2.1 For the test cell shown in Fig. 1, unscrew the upper electrode holding assembly to fill it with the sample oil while holding the cell at an angle to prevent splashing, which could create air bubbles. Screw the top portion down until the metal flange seats firmly.
8.3 Connect the fixed electrode to ground and the movable electrode to the impulse generator.
8.4 Apply the impulse wave of specified polarity starting approximately 40 kV below the expected breakdown level. Apply three impulse waves at each voltage level. Allow a minimum of 30 s between each test.
8.5 Increase the voltage level in steps of 10 kV or less until breakdown occurs, noting the crest voltage level at breakdown. It is necessary to have at least three withstand levels prior to breakdown.
8.5.1 Measure the breakdown voltage using techniques specified in IEEE Standard 4.
8.6 After each breakdown, change the point electrode and follow 8.1 and 8.2.
8.7 Make five breakdown tests on five specimens from the same sample. Maintain at least two significant digits in the results.
8.8 Criterion for Statistical Consistency:
8.8.1 Calculate the mean and standard deviation of the five breakdowns as follows:
8.8.2 Using the impulse crest voltage breakdown values determined in 8.7, calculate the mean value using the equation in 8.8.1. Determine that the range of the five breakdowns is no greater than 33.3 % of the mean value. If the range is acceptable, report this mean value as the impulse breakdown voltage. If the range exceeds 33.3 % of the mean value of the five breakdowns, then conduct five additional breakdowns and obtain a new mean breakdown value for the ten breakdowns. Determine the range of the ten breakdowns and if the range is less than 54.6 % of the mean of the ten breakdowns, report this mean value as the impulse breakdown voltage. If the allowable range is exceeded, the error is too large. Investigate the cause of the error and repeat the tests.
NOTE 1 - The criterion for statistical consistency specified apply only to negative polarity waves if point-to-sphere electrodes are used.
8.9 It may be necessary to partially immerse the test cell in oil to prevent external flashover. This is necessary with the cell shown in Fig. 1.
8.10 If a second insulating liquid is to be tested, thoroughly clean the test cell in accordance with 6.2.