Acceptable bubble size in potting materials

Air gaps or bubbles within the solid insulation used in high-voltage power supplies are discussed. These bubbles can be a cause of failure if the AC voltage across the gap or bubble is sufficient to cause ionization (corona). If the bubbles are small enough, they will not be a problem because there will not be enough voltage across the gas to cause ionization to start. The purpose of this work is to show how small is small enough. The results should apply equally well to naturally occurring bubbles in other kinds of insulation such as paper, Teflon, Kapton, and other plastics. The problem was approached by running an electrostatic field stress program for bubbles occurring in potting (encapsulation) materials, with various values of dielectric constant. A typical case is shown. Cases were run not only for bubbles but also for transverse cylinders. It was found that the voltage stress factor for bubbles and cylinders is surprisingly low, and it stays low even for large values of relative dielectric constant. It is concluded that since the maximum voltage stress in the potting material does not exceed 200 V/mil. even locally, bubbles up to 0.75 mm should be acceptable.<>