Document 2.4.126, Revision 4
October 2017
29
Three-Phase Padmounted Distribution Transformers
voltage level and sinusoidal purity, load conditions
and acoustic conditions at the installation site.
Unusual sounds should be investigated, as this
might indicate a potential problem.
Refer to NEMA Standards Publication TR-1,
Transformers, Regulators and Reactors, and IEEE
Standard C57.12.90, IEEE Standard Test Code
for Liquid-Immersed Distribution, Power, and
Regulating Transformers, for more information
about design sound levels and factory sound
testing.
Molded-Case Circuit Breakers
Molded-case circuit breakers are sealed devices
requiring minimal maintenance. Refer to inspection
and maintenance instructions provided by the
breaker manufacturer. Test sets are available from
breaker manufacturers that can be used to test and
calibrate breakers.
Other Accessory Devices
Other accessory devices, such as a gauges and
valves typically require no maintenance except for
replacement in the event of malfunction or damage.
Sampling and Testing the Fluid
Before sampling the insulating fluid, de-energize
the transformer from a remote location and make
sure all bushings and terminals are effectively
grounded. Samples should be drawn from the
bottom of the tank. Refer to ASTM D923 Standard
Practices for Sampling Electrical Insulating Liquids
for recommended sampling procedures. Also, refer
to any sampling recommendations supplied by the
fluid test equipment manufacturer.
It is recommended that a fluid sample be drawn
annually and tested for dielectric strength.
Dielectric strength should measure at least 26 kV.
Filtering the Fluid
Insulating fluids such as mineral oil, R-Temp or
seed-based fluids can be filtered using a filter
press system. A filter press can remove particle
contaminates as well as small amounts of moisture.
Follow the operating instructions provided by the
filter press system manufacturer.
Continue to filter the fluid until the dielectric test
result is 26kV or greater.
When filtering any particular type of insulating
fluid, make sure the filter press system is not
contaminated with any other type of fluid.
Contamination of the fluid may alter its chemical
or physical characteristics and could reduce its fire
point.
Removing or Lowering the Fluid
Should it be necessary to remove or lower the
insulating fluid, the following procedure should be
used.
1. De-energize the transformer and make
sure the tank, bushings, and terminals are
effectively grounded.
2. If cover removal is required, remove the
cover as outlined in the “Opening the
Transformer Tank” section.
3. Use a clean, dry temporary storage
container to contain the fluid.
4. Use clean pumps and hoses that have not
been contaminated by different types of
fluids. Hoses must be designed for handling
the particular fluid in the transformer (As an
example, rubber hoses should not be used
with mineral oil.).
5. Connect the pump intake line to the drain
valve at the bottom of the transformer tank.
6. Place the pump discharge line nozzle at the
bottom of storage container.
7. Pump slowly, and do not allow the fluid
to splash into the container, as this will
introduce air and moisture into the fluid.
8. Do not lower the insulating fluid below the
top of the core/coil clamp pan. Exposing
coils may allow moisture to contaminate coil
insulation.
Filling with Fluid
When refilling the transformer with insulating
fluid, fill with the same type of fluid. Do not mix
different types of fluids. Care should be taken to
avoid introduction of air bubbles during the filling
process. After refilling is complete, allow 24 hours
for dissipation of air bubbles before energizing the
transformer. Trapped air bubbles can reduce the
insulation value of the fluid and cause a flash-over.
