Characterization of polyurethane foam dielectric strength

Internal flashovers in hollow insulation systems can produce serious damage. Frequently, SF₆ or N₂ is used to eliminate this problem, but possible gas leakage may endanger the insulation. This paper proposes the use of polyurethane foams to fill the hollow spaces in insulation systems. Thus far, few publications deal with the dielectric properties of foams. This paper demonstrates the dielectric strength of three different foams which are investigated using ac and lightning impulse voltages under different humidity and temperature conditions. The results show that polyurethane foams have 2-3 times better dielectric strength than air. The breakdown strength decreases with the thickness of the foam; temperature and humidity have negligible effects on the breakdown voltage. The major parameter is the size of the voids in the foam. Reducing the size of the voids increases the breakdown strength. This can be achieved by improving the manufacturing technology.