Bushing tests

MORE complete information on the dielectric characteristics of high-voltage bushings has been obtained from power-factor tests by extending the test voltage above the values generally used and by supplementing the ordinary power-factor tests with laboratory examinations of deteriorated bushings. Most power-factor tests on high-voltage bushings have been made at a relatively low voltage, about 10 kv, regardless of the operating voltage of the bushing. These tests have been valuable in determining the general condition of the insulation. But by extending the test voltage to higher values, even above the operating voltage, it has been possible to detect defects in new bushings which would shorten their useful life and to detect certain types of deterioration in older bushings, both of such a nature that they are not shown by a low-voltage test. Laboratory examinations of deteriorated bushings have been valuable in determining the nature, extent, and cause of bushing deterioration. Such information is needed to properly evaluate power-factor test results. Summary The value of power-factor tests on bushings at higher voltages has been demonstrated. Tests at low voltages, while valuable, are inadequate. The power-factor and capacitance vs. voltage characteristics at voltages up to or above the operating voltage are important in judging the condition of the insulation. The nature of bushing deterioration and the significance of high power factor have been shown by laboratory examinations. The findings help to establish a basis for power-factor limits, and for improvements in design and maintenance.