Howard Medium Power Substation - Manual (Page 5)

Medium Power Substation Transformers
Document 2.4.06, Revision 4
June 2013
Tank Pressure
The tank vacuum/pressure gauge may indicate positive
or negative when the transformer is received, depending
on the relative fluid and ambient temperatures. This
indicates that the transformer is sealed effectively. If the
vacuum/pressure gauge reads zero, this may indicate
the possibility of a tank leak, although a zero reading can
occur occasionally with changing internal and external
temperatures. In the case of a zero reading the tank
should be pressure tested according to the instructions
on Page 14 (Pre-energization Tests and Checklist).
Report any suspected tank leaks to the Howard
Substation Transformer Division.
Moisture Test (for units shipped dry)
The transformer core and coil assembly was thoroughly
dried before shipping from the factory. Precautions
were taken to ensure that dryness is maintained during
shipment. Since it is possible that moisture could
enter the transformer tank due to mishandling or other
causes, perform a dew point test as described below to
ensure the core and coils are sufficiently dry.
A dew point measurement of the gas inside the
transformer tank can be used to estimate dryness of the
insulation system. Using this technique, it is assumed
that moisture content of the gas is in equilibrium with
the moisture content at the surface of the insulation
material.
The dew point test should be performed as described
below. The easiest way to ensure reasonable accuracy
is to take the dew point and temperature measurements
during the early morning hours, when the ambient
temperature and the insulation temperature are in
equilibrium. The gas in the transformer tank should be
at a slight positive pressure (5 PSI max.) when the dew
point measurement is made.
1. Use a dew point tester to measure the dew point
of the gas inside the transformer tank.
2. Using Chart A (Page 6), convert the dew point
measurement to vapor pressure.
3. Measure the temperature of the insulating
material.
4. Using Chart B (Page 6), estimate the moisture
content of the insulating material.
5. Moisture content above the curve is in the
acceptable range. Moisture content below the
curve is excessive and should be brought to the
attention of the Howard Industries Substation
Transformer Division before proceeding.
Internal Inspection
An internal inspection of the transformer tank is
rarely necessary and is required only when there are
indications that the transformer has received severe
impact damage during transit. If an internal inspection
is necessary, follow the instructions on Page 10 for
opening the transformer tank. Contact the Howard
Substation Transformer Division before proceeding with
any internal inspection.
It is important to follow a careful receiving inspection
program to record the condition of the transformer when
received and to identify the source of any damage that
may have occurred during shipping and handling. A
complete inspection of the transformer and detached
parts should be made as soon as possible to identify any
such damage.
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