Howard Medium Power Substation User Manual

Page 5

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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


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


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.