installation, and from moving contacts. To detect free conducting particles
inside a gas-insulated apparatus or equipment in the fi eld is by performing g
a 60 Hz high potential test at the manufacturers’ recommended test voltage.
Another method of locating conducting particles in the fi eld is to use an
ultrasonic translator detector which includes a microphone, an amplifi er,
and an ultrasonic signal generator.
4.6.1.2 Moisture
The SF6 gas shipped from the factory has very low moisture content, less
than 40 ppm by volume. Moisture is usually introduced into the gas during
installation by inadequate evacuation of the equipment before fi lling. Water
molecules adhering to the solid surfaces inside the equipment will diffuse
into the gas after fi lling. Normally the gas-insulated equipment is evacuated
to about 200 μm (0.2 mm Hg) before fi lling, and then checked for moisture
content within a few days. It should be recognized that the relative humidity
will change with variations in temperature and pressure. The moisture content
of the gas is higher during summer months when the temperature is
high and lower in winter when more moisture adheres to solid surfaces than
the gas. It is not a simple process to determine the moisture content in SF6
gas, therefore several factors should be considered. They are sensitivity and
accuracy of the measuring equipment, operating pressure of the instrument
and the system being tested, temperature, moisture absorption by the solid
insulating components, sampling method being used, and manufacturers’
operating requirements. There are several instruments and methods for
detecting moisture in the gas. The most common techniques and instruments
are (1) dew point method, (2) electrolytic cell method, and (3) capacitance
method using aluminum oxide hygrometer or silicone hygrometer. After the
moisture content of the SF6 has been determined, the next step is to determine
the adequacy of the gas dryness. Therefore, this has to be compared
against the manufacturer of the equipment maximum allowable moisture
level for safe operation of the equipment. In general the SF6 gas is considered
to be acceptably dry when the probability of moisture condensation in form
of water at all foreseeable operating temperatures and pressures is very low.
When taking samples of the gas for moisture determination, certain precautions
should be followed. These are (1) all electrical safety rules must be followed,
(2) ensure that the system is not subject to wide variations in
temperature, (3) keep the whole system temperature well above the highest
temperature at which water can condense from the gas, and (4) sampling
lines should be kept as short and simple as possible.
4.6.1.3 Oil Contamination
Oil and oil vapor containing free carbon molecules can cause fl ashover of the
SF6 gas. Operating experience has shown that clean oil and oil vapor free of
carbon does not degrade the performance of the gas-insulated equipment in
any way.