Measurements and modeling of moisture diffusion processes in transformer insulation using interdigital dielectrometry sensors

Abstract

The presence of moisture in a transformer deteriorates the transformer insulation by decreasing its electrical, mechanical, and thermal strength. Therefore, it is important 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, moisture partitioning curves for the oil-paper system can be used to find the moisture in paper from the oil measurement. A comprehensive study of the classic moisture equilibrium curves is given, historical mistakes are corrected, and all relevant concepts are clarified. This research serves as a useful tool to utilities and manufacturers. A new set of moisture equilibrium curves is constructed for moisture in oil up to the saturation moisture content. A measurement technique exploiting the linearity between the relative humidity of the oil and the moisture content of the oil to indirectly measure the oil solubility is developed. Solubility tests are performed for differently conditioned oils: Fresh Shell Diala AX oil, lab-aged Shell Diala A oil, used oil from Ramapo Substation, and used oil from Texas Utilities. Results show that aging under normal operation at service temperature is not very likely to significantly change the water solubility. When the system is not in equilibrium, the equilibrium moisture curves are not applicable and the three-wavelength interdigital dielectrometry sensor developed at the MIT Laboratory for Electromagnetic and Electronic Systems provides a unique way to measure the spatial profile of the moisture distribution in transformer pressboard.

(cont.) reduces the problems associated with the previous design and simplifies the inversion algorithm to convert the sensor 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 electrode 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 in 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 non-destructively estimated.

Experimental results show good agreement with theoretical analysis of the moisture diffusion equation on the effects of moisture level, 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. Numerical algorithms for solving the non-linear diffusion equation arederived. Literature results of the diffusion coefficient for cellulose insulation are compared and analyzed. The interdigital sensor setup is also combined in a transformer oil Couette Facility that simulates the transformer environment for studying temperatureand moisture transients in pressboard. The understanding of moisture-related processes in power transformers can be enhancedwith this methodology. Finally, a newly promoted in-situ time-domain measurement technique is implemented using the Tettex Recovery Voltage Meter. Time domain measurements are performed for oil-impregnated pressboard in both parallel-plate andcoaxial cylinder electrode geometries (the Couette Facility). The results for temperature and aging effects show good agreementwith theory.