Storage – Howard Medium Power Substation User Manual

Page 8

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Document 2.4.06, Revision 4

June 2013

Medium Power Substation Transformers


Transformer Storage

Transformers can be stored if properly prepared. It is

recommended that stored transformers be completely

assembled. Prior to storage, transformers should be

pressure tested according to the instructions on Page

14 (Pre-energization Tests and Checklist). The gas

space above insulating fluid should be pressurized with

dry nitrogen to a pressure of between two to three PSIG.

This will prevent a negative internal pressure which

might draw moisture into the tank. Transformers not

designed for outdoor use must be stored indoors. If

the transformer is not completely assembled, separate

components and accessories should be stored in a

clean dry area in their original shipping containers.

Transformer fluid containers or totes should be covered

or stored in a protected area to prevent exposure to

rain water or other contaminants. Condenser bushings

should be stored in an upright position.

Extended Storage

If the transformer is to be stored for an extended period

of time before being placed into service, it should

be placed on a firm level surface. The following test

procedure should be used prior to storage (in addition

to following the Receiving Inspection instructions

on Pages 4 and 5 and Transformer Storage

recommendations on this page).

1. Measure and record ambient temperature and

barometric pressure (for correction of test data).

2. Pressure test the tank using the procedure on

Page 14 (Pre-energization Tests and Checklist)

and check for fluid leaks. Following pressure

test, reduce tank pressure to two (2) PSIG.

3. Test insulating fluid for dielectric strength and

moisture content.

4. Test insulation with a 1000-volt or 2500-volt

Megger test set. (Omit if bushings have not been


5. Measure insulation power factor using Doble or

similar test set. (Omit if bushings have not been


6. Measure coil ratios in each tap position to

ensure proper operation of the de-energized

tap changer. (Omit if bushings have not been


7. Observe fluid level gauge to verify proper fluid


Sidewall-mounted bushings, if exposed and not

contained in a terminal chamber or switch enclosure,

should be protected from moisture and ultraviolet

damage with an appropriate covering. This covering

should be checked periodically to ensure it is intact.

Cover-mounted bushings which are shipped separately

from the transformer should be stored in a clean,

dry location in their original shipping cartons. These

bushings should be carefully inspected for damage prior

to installation on the transformer.

All space heaters (if supplied) in terminal chambers,

switch cabinets, control cabinets or other enclosures,

must be connected and energized at all times to control

moisture. Desiccant packets may be used to control

moisture in control cabinets only, if it is not practical to

energize the control cabinet space heater. Desiccant

packets must be replaced periodically to maintain


It is recommended that the transformer be inspected

periodically as described below while it is in extended

storage. Results should be recorded and compared with

previous results to ensure that no damage or change in

condition has occurred.

Quarterly Inspection

The quarterly inspection described below should be

performed after the first month of storage and then

quarterly thereafter.

1. Inspect protective covering on sidewall-mounted

bushings. Replace covering if damaged.

2. Inspect space heaters to ensure they are

working properly. Inspect desiccant packets (if

any) in control cabinets; replace if necessary.

3. Record fluid-level and pressure/vacuum

gauge readings. Ensure that tank pressure is

maintained at approximately 2 PSIG.

4. Measure and record ambient temperature and

barometric pressure.

5. If vacuum/pressure gauge or fluid level gauge

readings indicate a possible tank leak, perform

a tank pressure test according to instructions

found on Page 14 (Pre-energization Tests and

Checklist). Any leaks found should be repaired

immediately and then rechecked during

subsequent inspections.

6. Transformer fluid containers or totes should be

visually inspected for damage and any indication

of water intrusion or other contamination.