Abstract
High dielectric strength of solid electrical insulation materials for high voltage applications is essential for high reliability and long-term performance.The IEC 60243 and ASTM D149 both describe very similar test methods to determine the short-term electric strength of solid insulation materials. A test sample, usually thin plaque, is placed in an electrode system with surrounding insulating oil. The voltage is then steadily increased until an electric breakdown occurs. Despite the relatively simple test setup, testing materials with high electric strength can be difficult. The breakdown channel is often located outside the active testing area of the electrodes and found at the electrode edge at the triple point between the electrode, test object, and surrounding oil.In this study, we have investigated different possibilities to control the electrical field enhancement in the vicinity of the electrode edges using silicone rubber, field grading silicone rubber, and a high permittivity oil. The testing was performed with semi-spherical electrodes and electrodes as recommended in IEC 60243 on polyethylene films. Electrodes covered with the field grading rubber increased the short-term breakdown strength compared to standard testing without modification. The high permittivity oil and silicone rubber seemed to have limited effect on the breakdown strength.
Highlights
Electric breakdown strength is an important property of insulation material for high voltage applications and test procedures to determine the short-term electric strength, as described in ASTM D149 and IEC 60243 [1, 2]
It was expected that oil 2 would have a positive effect on the breakdown strength value due to better electrical field grading by the higher permittivity
Still the majority of the breakdowns in both series A and B occurred close to the electrode edges. This could indicate that the high permittivity oil 2 both reduced the electrical field in the triple point close to the electrode edges as well as increased the inception voltage of discharges prior to breakdown
Summary
Electric breakdown strength is an important property of insulation material for high voltage applications and test procedures to determine the short-term electric strength, as described in ASTM D149 and IEC 60243 [1, 2]. Often the breakdowns are triggered by surface discharges at this triple point giving breakdown channels located close to the electrode edges or outside the active electrode area This implies that the results are dependent on how the electrical field distribution at the triple point is controlled by the rounded electrode edges, and by the matching of permittivity between test object and the immersion fluid. For XLPE cables it is common to use terminations circulated with deionized water having a permittivity close to 80 or other field grading materials to control the electrical field at the triple point Another approach to minimize the impact from the triple point is to use recessed test objects or electrodes molded into the test objects [3]. It is still desirable to use the procedure described in the IEC and ASTM standards with flat test objects since they are easy to produce in large quantities and in a repeatable way that allows testing on multiple test objects for statistical evaluation
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