Dielectrometry measurements of moisture diffusion and temperature dynamics in oil impregnated paper insulated electric power cables

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2007.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Vita.Includes bibliographical references (p. 439-449).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.(cont.) Dielectrometry sensors for use in and z periodic geometries are designed and manufactured on a 4 mil PTFE substrate. An experimental setup is designed and built to provide a temperature and humidity controlled environment for measurements. Special clamping mechanisms are used to secure the sensor to the sample. Custom built hardware is used to excite the sensor at frequencies ranging from 0.005 Hz to 10,000 Hz (over six decades), measure and record the response, and deliver it to a computer for storage and analysis. Steady state measurements are performed at temperatures ranging from room temperature to 100 C. Plastics, woods, and the PILC cable samples are measured. Using the theoretical model, the effective permittivity is estimated from the measurements. An Arrhenius temperature dependence is observed for several materials including the PILC cables. We characterize the temperature dependence of these materials by a master curve and activation energy. Together they give a complete description of the effective permittivity's frequency and temperature dependence. Transient measurements of moisture diffusion are made at constant temperatures for several materials. For some experiments the boundary conditions limit the diffusion of moisture to one direction. When the z periodic sensor is used, six independent measurements are taken along the direction of diffusion. From the initial and final conditions and the theoretical model, a mapping between the electrical properties and the insulation moisture content is formed. Moisture concentration along the direction of diffusion is estimated and in some cases characterized by a simple diffusion coefficient.(cont.) The diffusion experiments provide evidence that moisture can move rapidly through the stranded conductors of the cables, delivering moisture to locations far removed from an initial breach. The experimental results show the electrical properties of the cable insulation is highly sensitive to the presence of moisture and changes in temperature. While using these sensors for field measurements may not be practical because they require the cable sheath to be removed, the technology provides a valuable, low cost tool for assessing cable health in an off-line setting.by Zachary M. Thomas.Ph.D