Design of humidity sensor for condition monitoring of power transformer

Abstract

The life of the transformer highly depends upon the insulation quality of the transformer oil. Moisture is one of the main causes for the degradation of the insulation quality of the oil. It decreases the dielectric strength and accelerates the aging of the insulation system of the transformer. Content of the moisture in insulation is measured in terms of parts per million (ppm). There are two main sources of water contamination of the oil in high voltage transformers such as (i) moisture ingress through cellulose paper used for transformer winding, (ii) moisture ingress by atmospheric humidity through transformer breather. In addition, a fresh oil sample always contains certain amount of moisture. The water contamination rate through the breather greatly depends upon the dehydrating capability of the silica gel present in the breather. The humidity level in the breather varies between 10% RH to 90% RH depending upon the condition of silica gel. At present, there is no online method for condition monitoring of the silica gel of the breather. To avoid the need of frequent replacement of silica gel and for condition-based maintenance of the breather, a humidity sensor based auto regenerative dehydrating breather may be an advanced solution. Further, for protecting the high voltage transformer from the dangerous effect of excess moisture, there is a need of a humidity sensor, which directly detects moisture in ppm level (3-100 ppm) in the transformer (T/F) oil. Moisture measurement in ppm level is challenging, and a costly affair. The environment of the transformer is chemically harsh and there is a change in ambient temperature. Metal oxide thin film sensor is stable robust material which may be suitable for moisture measurement in oil and in breather. There are various technologies used for moisture measurement from ppm to ppb level. Popular technologies are Karl fisher titration, chilled mirror hygrometer, cavity ring down spectroscopy (CRDS), electrolytic, and polymer/oxide thin film sensors.