Dielectrometry Measurements of Moisture Diffusion and Temperature Dynamics in Oil Impregnated Paper Insulated Electric Power Cables

摘要

Paper insulated lead covered (PILC) cables have played an important role in underground power distribution for a hundred years. Replacing aged PILC before failure is critical to managing power distribution. Three prominent failure mechanisms accelerate cable aging: temperature stresses, moisture ingress, and partial discharge. The research focuses on the effect of temperature and moisture on the effective (complex) permittivity of the cable insulation. Measurements are performed using cylindrical dielectrometry sensors designed to be wrapped around the cable. The lead sheath of the cable is removed so that the sensors can be placed directly in contact with the insulation. From the measurements, the electrical properties of the material as a function of temperature and transient moisture diffusion are found. A theoretical treatment of the interdigital dielectrometry sensors with a cylindrical geometry is presented. Two classes of the geometry are studied. The periodic sensor geometry has electrodes aligned with the cylindrical axis and periodic around the circumference. The z periodic sensor geometry has the electrodes forming rings around the cylinder that are periodic along the cylindrical axis. The material is modeled by concentric rings of homogeneous materials. The electric field solution consists of an infinite summation of Fourier series terms. In finding the field solution and as a consequence of it, the potential, electric field lines, and the impedance between the driving and sensing electrodes are found. A generalized solution to the planar dielectrometry sensor topology is also presented. This solution allows for an arbitrary placement and excitation of electrodes, providing the analytical tools for a new generation of sensors.