A New Approach for Assessing Moisture Content of SF6 Insulated Equipment

This month’s Featured Technical Paper is from the Proceedings of the 2017 International Conference of Doble Clients. The paper is titled: A New Approach for Assessing Moisture Content of SF6 Insulated Equipment.

 

Abstract

Since the early 80s, a significant number of SF6-insulated equipment has been installed in Hydro-Québec.  In such equipment, moisture content is a key factor for the equipment condition assessment during maintenance operations.  Practical issues revealed some difficulties to meet the maximal moisture acceptance criteria specified by the manufacturers or by the standards.  These criteria are specified in terms of concentration (ppmV) or dew point (referred to an ambient temperature of 20°C).  An accurate assessment of moisture content for outdoor equipment is difficult to attain by users considering the lack of conversion rules or criteria for measurement taken at ambient temperatures different than 20°C. SF6-insulated equipment temperatures influence the gas humidity concentration.  Indeed, as the system temperature increases, water molecules are driven from the surfaces and bulk materials into the gas phase.  Conversely, as the system temperature decreases, water molecules from the gas phase are adsorbed onto the solid surfaces and absorbed into the solid insulator materials.  Thus, an SF6 system appears more or less humid according to the temperature.  Because measurements are rarely done at the referenced temperature and due to ppm and dew point temperature dependence, it is not possible to rely solely on these parameters to get an accurate assessment of moisture content.  As a consequence, users do not have enough information to ensure equipment operate within established criteria. 

In order to prevent failures associated to high moisture content and to facilitate the outdoor equipment moisture content assessment, Hydro-Québec has developed a new SF6 moisture assessment approach using relative humidity.  Four relative humidity condition zones have been established to guide the decision making regarding equipment condition.  Case studies with different moisture content and types of equipment will be presented.

 

Doble Service Agreement Clients

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