Moisture Measurement in SF6 - A KAHN article

Published by   – July 7, 2015

Most electric utilities and independent power producers use Sulfur Hexafluoride quenching and insulating gas in high voltage circuit breakers and switchgear installed in their electrical transmission and distribution systems. These circuit breakers require periodic maintenance, including agas – typically in the range of 50 to 100 PPMv (-50 to -40°F dewpoint). Historically, customers have used portable hygrometers to measure the moisture content of by shutting down the circuit breaker, extracting a sample of the gas from the breaker, flowing the sample through the hygrometer and venting the gas to atmosphere during the measurement process.

SF6 has been used in high voltage circuit breakers and switchgear in the US for more than 40 years. Recently, however, the U.S. Environmental Protection Agency (EPA) has designated SF6 as a greenhouse gas and has imposed regulations to limit the amount of SF6 that is released into the atmosphere. This presents a problem for power companies that vent SF6 during moisture measurement tests.

In anticipation of these regulations and in an effort to reduce SF6 emissions, improve worker safety (less frequent breaker shutdowns), minimize circuit breaker outages and lower maintenance costs, a forward-thinking mid-Atlantic electric utility developed a method to measure the moisture content of SF6 circuit breakers without venting the gas to atmosphere. This power company permanently installed Kahn Easidew 2-Wire Transmitters into single-phase and 3-phase SF6 breakers to provide an on-line measurement of moisture content. The company’s maintenance technician uses a Fluke meter, which he normally carries for multiple measurement tasks, to periodically measure the current output of the Easidew 2-Wire Transmitter. From the measured milliamp output current, the technician determines the moisture content of the SF6 by using a milliamp to PPMv conversion chart.

This utility company recently purchased and is now installing the first 24 Easidew 2-Wire Transmitters. Their plan is to purchase and install a total of 200 Easidew 2-Wire Transmitters for moisture measurement in SF6 circuit breakers installed in their electrical transmission and distribution network. The current output from the Easidew 2-Wire Transmitters is being measured with a Fluke meter and Kahn provided a power supply kit with cables. As an alternative, Kahn can supply a display kit consisting of a display meter in a NEMA 4 enclosure. Electric utilities that utilize SF6 circuit breakers in their electric power systems, and most do, face a similar moisture measurement challenge. They will benefit from a more environmentally friendly measurement method that uses Easidew 2-Wire Transmitters, which are permanently installed in the SF6 circuit breakers.

~ Credit - KAHN Instruments

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