Howard Medium Power Substation User Manual

Page 11

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Medium Power Substation Transformers

Document 2.4.06, Revision 4

June 2013


When internal work is being done on the transformer,

dry air should be used to ventilate the inside of the tank.

Dry air can be obtained in high-pressure cylinders or

from desiccant-type air-drying equipment. When using

dry air, the following precautions should be observed.

1. The temperature of dry air entering the

transformer should equal to or greater than the

temperature of the transformer fluid and at least

10°F higher than the dew point of the ambient


2. The dry air flow should be directed so as to

continuously purge the transformer gas space.

3. The dew point in the tank gas space should

never exceed 20°F.

While the tank is open, the following tests should be


1. A ratio test should be made on all windings

and on each tap position. Contact the Howard

Industries Substation Transformer Division if any

measurement is off-ratio by more than 0.5%.

2. Insulation resistance of each winding to all

other windings and from each winding to ground

should be measured. (Coil windings must be

completely covered with fluid.) Record the

temperature of the fluid. These readings should

be comparable with factory measurements.

3. Disconnect the core ground connection, when

accessible, on core-form transformers and

measure the resistance from the core to the

tank or end frames, using a 1000-volt Megger

test set. The resistance should exceed 100

megohms if the core is not covered with fluid or

200 megohms if the core is covered with fluid.

When the internal inspection is complete, reseal the

tank and refill with fluid (if any was removed prior to

inspection). The gas space should be filled with dry

air or dry nitrogen. Contact the Howard Substation

Transformer Division if any internal damage is found

during the internal inspection.

Installation of Detachable Radiators

When radiators are shipped detached, they should be

installed according to the instructions below.

1. verify that all valves on the tank are in the

closed position.

2. Remove shipping plates from the valves on the

tank and from the radiator flanges. Take care

not to damage gaskets.

3. Inspect for moisture and contamination

inside the radiator headers. Contact the

Howard Substation Transformer Division if any

contamination is found.

4. Clean all mating surfaces on the tank valves

and radiator flanges and apply a small amount

of petroleum jelly to the gaskets on the tank

valve flanges to hold them in place during

installation of the radiators.

5. lift the radiators into position using the lifting

eyes on top of the radiators and align holes

in the radiator flanges with holes in the valve

flanges. If fans are provided, attach the

radiators having fans first, then attach the other


6. Insert bolts in the top and bottom flanges, with

bolt heads adjacent to the transformer tank.

7. Tighten bolts evenly, alternating across corners,

until metal-to-metal tightness is achieved at all

four corners of each flange.