Insulating liquid maintenance
Sampling insulating liquid
ote:N A sample of the liquid should be taken when the unit
is warmer than the surrounding air to avoid conden-
sation of moisture on the liquid. Liquid samples must
be drawn from the sampling valve located at the
bottom of the transformer tank.
Containers used for sampling liquid should be clean and dry
large mouth glass bottles.
Do not permit the fluid to splash into the receiving
container. Splashing can introduce air and moisture into
the fluid. Rinse the bottle three times with the liquid being
sampled. Make sure the liquid being sampled is representa-
tive of the liquid in the unit.
Test samples should be taken only after the liquid has
settled for some time – eight hours for a barrel, up to
several days for a large transformer. Cold insulating liquid
is much slower in settling. Liquid samples from the trans-
former should be taken from the sampling valve at the
bottom of the tank.
WARNING
Do not lower fluid in tank to a level below the top
of the core clamp. Insufficient fluid risks unit failure,
which could result in severe personal injury or death to
exposed personnel.
When sampling, a metal or non rubber hose must be used
because oil leaches the sulfur found in rubber. The presence
of sulfur can prove harmful to transformer components.
When drawing samples from the bottom of the transformer
or large tank, sufficient liquid must first be drawn off to
ensure that the sample will be from the bottom of the tank,
and not the liquid stored in the sampling pipe.
Testing insulating liquid
For testing the dielectric strength of insulating liquids,
follow the procedure specified by the American Society
for Testing Materials in ASTM D-877, "Standard Method for
Testing Electrical Insulating Oils."
If, at any time, the dielectric strength of the liquid drops
below 26 kV, it should be filtered until it tests at 26 kV or
better.
Filtering insulating liquid
Transformer oil, R-Temp fluid or Envirotemp™ FR3™ fluid
can be filtered by means of a filter press. The filter press is
effective for removing all types of foreign matter, including
finely divided carbon and small amounts of moisture. The
purifier equipment consists of a specifically proportioned
filter press, a positive volume gear pump, driving motor,
combined drip pan and mixing tank, necessary piping,
valves, strainer, gauges and a drying oven.
Filtration should be continued until the dielectric test of the
insulating liquid is 26 kV or better.
CAUTION
When filtering any insulating fluid, the filtering
equipment must be free of contaminants and other
liquids. The presence of other liquids may alter the
physical and electrical characteristics of the fluid.
Applicable standards
1. IEEE Std C57.91™-2011 standard, "Guide for Loading
Mineral Oil Immersed Transformers."
2. IEEE Std C57.93™-2007 standard "Guide for Installation
and Maintenance of Liquid Immersed Power
Transformers."
3. IEEE Std C57.106™-2006 standard, "Guide for
Acceptance and Maintenance of Insulating Oil in
Equipment."
4. ASTM Specification #D 877, "The Standard Method of
Testing Electrical Insulating Oils."
5. IEEE Std C57.121™-1998 standard, “Guide for
Acceptance and Maintenance of Less Flammable
Hydrocarbon Fluid in Transformers.”
R-Temp fluid-filled transformers
Periodic maintenance tests for R-Temp fluid-filled
transformers should be performed on essentially the same
schedule as would be used for conventional mineral oil-filled
transformers of similar application. The same type of
sampling techniques should be used. Basic recommended
tests for R-Temp fluid-filled transformers are dielectric
strength, moisture content, and flash and fire point.
Dielectric strength
Although the transformer should operate satisfactorily
with a fluid dielectric strength of 22 kV, an R-Temp fluid
dielectric strength below 25 kV is an indication of excessive
contamination. In this case, the R-Temp fluid should be
replaced or filtered to remove the moisture or particulate
contamination.
The dielectric strength of R-Temp fluid should be tested in
accordance with ASTM D 877.
14 SubStation tranSformer inStallation and partS replacement information MN202002EN JUNE 2016
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