FILTERING OF TRANSFORMER OIL
The filtering of transformers is one of the lest expensive, best known ways of preserving the life of the unit. There are many articles,
notes, ways and means written on how to filter a transformer. We at Southern Substation, Inc., use only the methods and equipment
recommended by the transformer manufacturer, and not those recommend by the transformer manufacturer, and not those
recommended by the manufacturing of filtering equipment. All transformer recommend the use of a composite type filter and not
Fullers Earth. Fullers Earth has the effect of over-refining the oil by removing some of the oils natural inhibitors. However, at the
owners discretion, this method may be used and 0.2% inhibitor be added afterwards. Sometimes, transformers are allowed to let the
oil deteriorate to a point where this type of filtering is necessary. However, this condition should never be allowed to happen and with
the proper testing and maintenance, this will not happen.
Southern Substation, Inc., uses the Cuno Division of AMF type Zeta-Plus filtering unit. We have two filtering units, one capable of 30
gallons per minute and one capable of 50 gallons per minute. Both are sealed-type units and can be done in any type of weather. It is
not recommended by any manufacturer that their transformers be filtered while energized. Southern Substation, Inc., does not filter any
energized transformers. The type of filter cartridge use by Southern Substation, Inc., is manufactured also by Cuno and is specially
designed for their Zeta-Plus unit and is the most desirable on the market today. All of our technicians here attended the Allis Chalmers
college of Knowledge on the proper maintenance and filtering of transformers
INSULATION TEST, INSULATION RESISTANCE TEST,
DIELECTRIC ABSORPTION TEST, POLARIZATION INDEX
All of the above tests are outlined in Hi-potting section of this site. I would like to point out an old rule of thumb for insulation
resistance test. The minimum acceptance value of insulation resistance is simply "one megohm per thousand volts", with one
megohm minimum. This test should be made from each windings.
CONCLUSION AND SUMMARY OF TEST
Because of the complex nature of oil and the ways in which it can change in service, probably no single test will ever replace all others
for determining oil condition. In determining whether an oil is suitable for service in a transformer, as complete data as possible is
desirable. Particularly, the results of the maintenance test which indicate the condition of the oil with reference to its desirable
properties for service. Rapid changes in any of these properties are particularly significant in indicating oil deterioration. If the most
significant of the tests indicate rapid deterioration, the oil should be replaced of filtered.
DRY TYPE TRANSFORMERS
Dry Type transformers operate in air or gas rather than oil. The two general types of construction are open (or ventilated), dry
transformers and the sealed (or closed), tank type. The sealed transformer is cooled and insulated by an inert-high dielectric gas
such as nitrogen, sulphur, hexaflouride, or perflouropropane. The air serves as an insulating medium and also to dissipate heat from
the windings. Two standard insulation classes are Class B (80 degrees centigrade rise), and Class H (150 degrees centigrade rise).
ROUTINE INSPECTIONS, CURRENT AND VOLTAGE READINGS AND TEMPERATURE READINGS.
The same applies to these as was outlined in Oil filled transformer section. Of course, omitting the parts pertaining to oil. These
items should be added to the routine inspection. Louvers are ventilating openings in the enclosure of ventilated dry type transformers.
They should be checked to see that they are not clogged with dirt or any other type of obstruction that would hamper proper ventilation.
If these louvers are covered by air filters, these should be checked and replaced periodically. The operation of intended ventilating fans
should be checked for proper operation. Most dry type transformers are usually installed in a vault or a particular room. The
temperature of the vault or room should be checked, measured and recorded at regular intervals. Proper and adequate ventilation is
essential to the operation of the transformer at its full rated value. Any item that could prevent the free-circulation air around the
transformer should be removed.
If forced air fans are used to ventilate the vault or room, their proper operation should be checked periodically. It is recommended that
vault or room fans be operated by a temperature controlled fan relay. Corrosion of the transformer enclosure, the
intrusion of dirt, as well as evidence of water leaks into the room or vault, should also be carefully checked and corrective measures
taken as required.
SPECIAL INSTRUCTIONS AND MAINTENANCE
First the transformer under test and maintenance should be de-energized in accordance with Section 10-4. After following the steps
outlined in that section, covers the openings of the enclosure of ventilated dry type transformers should be removed. A check be made
for the accumulation of dirt or other contaminants (manufacturers products) on the windings, insulators and other insulating surfaces
or wire cooling air flow may be impaired. A close check should be made for loose or cracked insulators or coil spaces and to assure
that coil clamps are properly torqued. The turn insulation as well as the barriers separating the primary and secondary windings and
the windings for the iron core, should be checked for corrosion, overheating, tightness and then torqued to ASA standards. The
windings may be cleaned of dirt, dust and manufacturers contaminants with a vacuum cleaner and blower. A small portable air
compressor may be used if the air is clean and dry and not used at a pressure of over 30PSI. In particular, ventilating ducts and the top
and bottom of the windings should be cleaned. The use of liquid cleaners should be used only when it is known for a fact that they will
not damage, nor have a deteriorating effect on the windings insulations. As long as ventilated dry type transformers are energized,
humidity conditions are unimportant. However, if the transformer is to be down for an extended period of time, proper measures must
be taken to assure the temperature of the windings remain high enough so as not to absorb any moisture. All insulation tests should
be performed the same as with oil type transformers.
Grounding systems on both oil and dry type transformers are a must to be checked. This includes both grounding electrodes and
ground wire connections. An adequate grounding system is necessary to:
1. Provide a discharge path for lighting.
2. Prevent an induced voltage caused by surges or power lines.
3. Maintain a reference point of potential for instrumentation safety.
Periodic testing is necessary because variations in soil resistivity are caused by changes of soil temperature, soil moisture,
conductive salts in the soil and corrosion of the ground connectors. The test equipment used for this test should be a ground resistant
test set designed for this purpose. This instrument is direct reading with a scale calibrated in ohms of ground resistance.
FAULT GAS ANALYSIS
Fault gas analysis test is one of the most neglected tests on sealed transformers. The analysis of the percentage of combustible
gases present in the nitrogen cap of sealed, pressurized oil filled transformers can provide information as to the likelihood of incipient
faults in the transformer. When arcing or excessive heating occurs below the top surface of the oil, some oil decomposes. Some of
the products of the decomposition are combustible gases which rise to the top of the oil and mix with the nitrogen blanket above the
oil. The test set used for this test is designed for that purpose only. A small amount of nitrogen is removed from the transformer and
analyzed. The set has a direct reading scale calibrated in percentiles of combustible gas. Ordinarily, the nitrogen blanket in a
transformer will have less than 1/2 % combustible content. As a problem develops over a period of time the combustible content can
rise 10 to 15%. The combustible gas created by this condition is acetylene gas. If oxygen is present in the air space above the oil,
then the condition for an explosion is prominent.