Moisture and temperature effects on the dielectric spectrum of transformer insualtion materials

Abstract

The presence of moisture in a transformer deteriorates the transformer insulation by decreasing its electrical, mechanical, and thermal strength. Therefore it is of great importance to monitor the moisture condition in both liquid and solid insulation to assure transformer performance. There are commercially available sensors to measure the moisture in oil. When the transformer system is in equilibrium, existing moisture partitioning curves for oil-paper system can be used to find the moisture in paper from the oil measurement. A comprehensive study of the moisture equilibrium curves are given for an overview of the classic curves and their history, and providing useful information on the relationships among them and their validity. Solubility tests for differently conditioned oil are performed. Results show the solubility of moisture in oil increases significantly only in severely aged transformer oil. When the system is not in equilibrium, the moisture curves are not applicable and the three-wavelength interdigital dielectrometry sensor developed at the MIT Laboratory for Electromagnetic and Electronic Systems is a unique way to measure the spatial profile of the moisture distribution in transformer pressboard. Preliminary measurements of oil and oil-impregnated pressboard are given in this thesis. A joint research group effort led to development of an improved design of the three-wavelength sensor which reduces the problems associated with the previous design and simplifies the inversion algorithm to convert the electrical signal to dielectric properties. The thesis focuses on the design and implementation of experimental studies based on interdigital dielectrometry using the new three-wavelength sensor. As a first step to relate measurable dielectric properties to absorbed moisture, the moisture and temperature effects on the dielectric spectrum of oil-free pressboard are measured using a parallel-plate geometry sensor for nine moisture levels and five temperature levels. A dielectric model for biological tissue is adopted here for cellulose structured pressboard. A universal curve is found relating dielectric properties to moisture concentration and temperature by fitting the data to the model. Preliminary measurements of oil-impregnated pressboard show similar characteristics. The moisture diffusion process of oil-free transformer pressboard is monitored using the new interdigital dielectrometry three-wavelength sensor in a specially constructed bench-top apparatus. Experiments are performed for five different temperatures at various moisture levels. The time evolution of the moisture spatial profile in transformer pressboard is estimated non-destructively. Experimental results have a good agreement with theoretical analysis on the effects of pressboard thickness and temperature on the diffusion process. The diffusion coefficient for oil-free pressboard as a function of temperature and moisture concentration is estimated and compared with literature values. Numerical methods of solving the diffusion equation are developed.