Influence of Water Content on the Dielectric Characteristics of LN2-Impregnated PPLP for HTS Power Cables

High-temperature superconducting (HTS) power cables operate at low-, medium-, and high-voltage levels for electrical power transmission. Polypropylene-laminated paper (PPLP) is wrapped helically around the HTS layer and it is impregnated with liquid nitrogen (
$\text {LN}_{{2}}$
) to form a composite dielectric layer. During the installation and maintenance process of HTS cables, any leak of moisture in the cryostat will lead to immediate ice formation. Once the cable warms up, the ice will melt into water, which will be absorbed by the PPLP Kraft paper layer and is similar to the PPLP paper soaked with water. When the moist PPLP is cooled with
$\text {LN}_{{2}}$
, it will form ice and forms a composite material during operation. The objective of this article is to investigate the influence of water content on the breakdown strength, relative permittivity (
$\epsilon _{r} $
), and dielectric losses of
$\text {LN}_{{2}} $
-impregnated PPLP samples for different numbers of layers. Considering the worst-case scenario of moist samples, wet PPLP samples were tested for breakdown strength and dielectric properties at room temperature (RT) and
$\text {LN}_{{2}}$
temperature and were compared with dry PPLP samples. Two different experimental setups were developed to measure the breakdown strength and dielectric properties, respectively, for stacked PPLP layers. The wet and dry samples were tested for the ac
${V}$
–
${I}$
characteristics, the effect of PPLP stack thickness on dielectric strength, statistical analysis using two-parameter Weibull distribution, and computation of relative permittivity and loss factor from capacitance measurement.