Discharge characteristics of air gaps stressed by AC, DC and impulse voltages obtained at proficiency tests in Japan

The Japanese Accreditation Board (JAB) has carried out proficiency tests accorded with ISO/IEC Guide 43-1:1997 to confirm the skill of high voltage testing laboratories in Japan every year since 2002. Contents of these tests are to measure gap length vs. discharge voltage characteristic of "rod-rod" and "rod-plane" gaps stressed by AC, DC, lightning impulse or switching impulse voltages. An average of seven laboratories participated in the proficiency test every year; each z-score is compared and the confirmed ability is promulgated. Each combination of the voltage type and the gap type was selected every four to six years. These tests were carried out at gap lengths from 10 to 100 cm, and the humidity range was from 2 g/m3 to 13 g/m3. The huge data obtained through those proficiency tests are re-analyzed, and the relationships among the gap length, the discharge voltage and the atmospheric humidity are evaluated. As a result, linear relationships and small (less than 3 %) dispersion are obtained between the gap length and the discharge voltage at each voltage type. At the rod-plane electrode stressed by AC voltage, the discharge voltage Ed [kV] can be estimated by the formula Ed = 0.351d + 6.3 where d is in mm. In this case, the maximum deviation of the measurement points from the regression line is 1.9 %. The maximum deviation is 3 % at other types of voltages except DC. The maximum standard deviation of 11 % is found at 10 cm gap lengths of rod-rod gap stressed by DC voltage. At rod-plane gap stressed by +DC voltage, the maximum deviation is about 7.8 %. IEC60052 defines "Ed = 2 + 0.534 d" for this relationship, and the measured values have less than 2.5 % deviation with this formula when the gap length is longer than 30 cm; however, large deviation of more than 20% is found when the gap length is short. As regards dependence on the humidity, it is found that dispersion is small. Atmospheric correction produces quite satisfactory results. These results demonstrate that the high voltage test laboratories in Japan possess excellent measuring techniques and that obtained data are worth to be added to the worldwide database.