Document 2.4.126, Revision 4
October 2017
28
Three-Phase Padmounted Distribution Transformers
Electrical Tests (cont.)
1. Insulation Resistance Test. Refer to
“Insulation Resistance Test” for instructions.
2. Turns Ratio Test. Refer to “Ratio Test” for
instructions
3. Insulation Power Factor Test
4. Fluid quality tests, such as moisture
content, power factor, dielectric strength
and dissolved gas analysis.
Exterior Paint Finish
Any damage to the exterior paint finish that exposes
the primer coat or bare metal should be repaired
immediately in order to prevent corrosion. Areas
to be repaired should be thoroughly clean and dry.
The surface should be sanded to remove rust, loose
paint flakes and other debris. The surface should
then be cleaned with a suitable solvent to remove
any oil, grease or other contaminates. At lease two
coats of a high-quality touchup paint should be
applied to the damaged area. Bare metal should
receive a primer coat before applying the final
finish. Touch-up paint is available in aerosol cans
from the Howard Industries Transformer Division.
Fluid Leaks
Check the tank for indication of fluid leaks, looking
carefully at weld seams and at tank fittings, such
as bushings, gauges, plugs and valves. Fluid leaks
should be repaired as soon as possible to prevent
moisture contamination of the insulating fluid and
to prevent internal flashover due to low fluid level.
If a fluid leak is suspected, investigate thoroughly
to determine if an actual leak does exist on the
transformer. False indications of a leak can occur
as a result of residual fluid that was not sufficiently
cleaned after the transformer was filled with fluid.
In some cases silicone lubricant used to install
high-voltage bushing inserts can flow onto the front
panel, giving a false indication of a fluid leak.
In addition to the presence of fluid residue, a low
reading on the fluid level gauge and a constant
zero reading on the pressure/vacuum gauge (which
does not vary over time as a function of transformer
loading and ambient temperature) are also
indications of a possible fluid leak.
To verify that a fluid leak exists, clean the
suspected leak area with an appropriate solvent
to completely remove the fluid and observe the
area for reappearance of fluid. Pressurize the tank
with dry air or dry nitrogen through the pressure
test fitting to a pressure of 3-4 PSIG. Let the tank
stand under pressure for one to two hours, then
examine the tank and fittings for leaks. Leaks above
the fluid level can be detected by applying soap
solution to all welds, joints, pipe fittings, and cable
connections.
In many cases a small pin-hole tank leak or leak
from a bushing, gauge, valve or other fitting can be
repaired on site. Pin-hole and weld seam leaks can
usually be repaired by welding on a de-energized
transformer. Welding on radiator panels is not
recommended due to the thinner gauge material.
Very small pin-hole leaks can sometimes be
repaired using an epoxy patch kit designed to repair
oil leaks.
Bushing leaks can sometimes be corrected by
tightening the bushing clamp bolts. Do not exceed
the recommended torque values listed in Table
2 to prevent the possibility of bushing or gasket
damage.
Audible Sound Level
It is normal for transformers to emit an audible
humming sound, which is primarily caused by
alternating magnetic flux in the transformer core.
Amplitude and harmonic content of the sound is
influenced by transformer size, the energizing
WARNING
FAILURE TO FOLLOW THE INSTRUCTIONS BELOW
MAY RESULT IN DEATH OR SEVERE PERSONAL
INJURY, AND /OR DAMAGE TO THE EQUIPMENT.
• De-energize transformer from a remote
location before opening cabinet to perform
inspection or maintenance.
• Make sure all transformer terminals and
bushings have zero voltage.
• Make sure that the transformer is properly
grounded.
• Fluid leaks should be repaired as soon as
they are discovered.
