Transformers are one of the most vital links in industrial type electric power systems and are among the most
reliable components in the system. If they are nor overloaded or otherwise abused, they will provide long,
Established records of reliability coupled with the lack of movement, noise, or other signs of action, often results
in general disregard and neglect. However, because a transformer failure is usually of a very serious nature,
requiring extensive repair and long down time, regular maintenance procedures are the best assurance of
continued long life and high reliability. For maintenance purposes, transformers may be divided into two general
categories in accordance with their insulating medium and construction. The categories are the oil-filled type and
dry type. Each has several variations that will be listed and covered under the specific maintenance
recommendations. Each also requires slightly different maintenance techniques but, in general, measurement of
the dielectric value and/or resistance of the insulation are the major maintenance test for all transformers.
LIQUID FILLED TRANSFORMERS
In a liquid transformer, the core and coils are immersed in the liquid. This liquid serves two purposes: first, it is
an important part of the insulation medium; and second, it serves to transfer heat away from the winding out to
where it is dissipated to the air by the cooling fins, tank surface, or radiator. There are three types of insulating
liquids in common use. These are:
1. Mineral Insulating Oil
Askarel is a nonflammable synthetic insulating fluid which is
subject to strict environmental controls and manufacturers instructions should be carefully followed. Askarel or
PCB's ( the better known name for it), comes under severe rules published by the EPA. Any handling for PCB's
should be done by a competent company, as any problems such as law suits, fines or any type of legal action
must be handled by the original owner and not by the company handling the PCB's.
CONSTRUCTION OF OIL FILLED TRANSFORMERS
There are several types of transformers construction regarding the preservation of the oils. By preservation, we
mean the exposure of the insulating oil to the atmosphere. These types are:
1. Free Breathing(open to the air).
2. Restricted Breathing (open to the air through dehydrating compounds).
3. Conservator or expansion tank (exposure to air limited to the conservator tank).
4. Sealed Tanks (a gas space above the liquid serves as a cushion for internal pressure).
5. Gas-Oil seal (Exposure to air to the oil in the auxiliary tank).
6. Inert (gas space above the liquid maintained under positive pressure by nitrogen gas)
In addition to oil preservation, the various cooling methods are:
1. Self cooled (heat is dissipated by the tank surfaces and cooling fins or tubes).
2. Forced Air Cooled (fans are employed to force air over the cooling surfaces to supplement the
3. Forced Air Cooled/Forced Oil Cooled (an oil pump circulates the oil through a fan blown oil-to-air heat
4. Water cooled (heat exchange by means of water pumped through a pipe coil installed inside or outside
the transformer tank.
Before going into the testing, inspecting and maintaining of transformers, I will explain (in laymans language) the
types of oil filled transformers, why they are oil filled and what is expected of each item. As I go through each
phase of transformer work, many more items will be pointed out. As this is one of the most important pieces of
electrical gear in your plant and the most neglected, too much cannot be said about them.
Routine inspections of power transformers should include observations of the operating conditions on a periodic
basis. The period may be once a shift, once a day, once a week, or less frequent as may be judged necessary
by the importance of the transformer, the severity of the operating environment and/pr the severity of the load. It
is recommended that a form be adopted by your company and attached to each transformer so that the periodic
rating and routine inspection findings can be recorded, and a condition rate charted. Although many plants differ
in their routine inspection, the following should be established routine for a 24 hour operation.
Load Current--------Daily----------and Record
Oil Temperature-----Daily----------and Record
Oil Level------------Weekly---------and Record
These readings for current, voltage and temperature should be taken during heaviest load conditions and the oil
level reading taken during the lowest load tests.
CURRENT AND VOLTAGE READINGS
Voltage and load current checks are important in routine inspections. If the current is found to exceed the full
current rating of the transformer, steps should be taken to reduce this load.
Over-voltages can be detrimental to the transformer and it serves. Under-voltage can be detrimental to the
load served. If either low or high voltage readings are discovered, steps should be taken immediately to
determine the cause and it remedied. Either of the problems listed above can cause windings and oils to
deteriorate and eventually breakdown.
Transformers are rated to carry their name plate load in KVA with a given heat rise when the ambient
temperature is at a standard level. These exact values are stated on the same plate. An example: an oil filled
transformer with a Class A insulation may be rated to deliver name plate capacity with a 55 degrees centigrade
temperature rise above 30 degrees centigrade (86 degrees Fahrenheit) ambient temperature. Temperature
readings should be taken at set intervals and a permanent record kept. The readings and recordings provide an
indication of the transformers performance and load conditions. Excessive temperatures indicate an overload or
that the cooling system is not working properly. Continued operations at excessive temperatures will increase the
deterioration of the oil, and result in reduced life of the winding insulation. Either one will greatly increase the
chance of total transformer failure.
OIL LEVEL INDICATOR AND PRESSURE/VACUUM GAUGES
Oil levels should be checked as previously outlined, especially after a long period of low load at low ambient
temperature. It is at this time that the oil should be at its lowest level. Pressure/Vacuum gauges which are found
on sealed type transformers are valuable indicators of the conditions of the seals. If the pressure does not
change with the changes of the oil
temperature, leaks above the oil level are indicated or the pressure/vacuum gauge is bad. In any indication of the
above, both items should be checked.
ONE YEAR INSPECTIONS AND REPAIRS
The following work and inspection should without fail, be done at least once a year. The first precaution should
always be the de-energizing should always be handled by an approved positive lock out provision. This is the
assure against unplanned re-energizing and the resulting hazard to both equipment and personnel. Positive tests
should be performed to assure that the lock-out procedure has been followed and that no voltage exists on the
equipment. When the transformers has been safely de-energized and locked out, ground jumpers should be
placed on all phases on both high and low side to ground. After all the above is done and secure, the following
work should be done.
Both primary and secondary connections should be checked for signs of overheating and corrosion. The
insulator bushings should be checked for cracks or chipped skirts and the gasketed bases for leaks. The
insulators should be cleaned of any contaminations and any leaks should be repaired. Pressure relief valves
should be checked to assure that there are no leaks or corrosion and that the unit operates properly. Any
damaged valves or diagrams should be replaced.
The tap changer compartment, tank, cooling fins, tubes, radiators and all gaskets or any other openings should
be checked for leaks, dirt or corrosion. Leak repair, cleaning or painting should be done as required.
The conservator tank, inert gas and dehydrating breather equipment should be checked and tested in
accordance with the units manufacturers recommendations. Contamination or impairment of the insulating oils
should be carefully avoided during any type of test.
If any insulating oil is to be added to the transformer the oil should be at least as warm as the surrounding air.
To prevent areation (air bubbles from being in the oil), it should be applied at the bottom of the transformer.
The proper means for this is by a filter press pump. If in an emergency and a filter press pump is not available,
the oil can be added through the top. It is recommended that at least two thickness of unsized muslin be used to
strain the oil through. If large amounts of oil must be added in this manner, the transformer should remain
de-energized long enough to allow the air bubbles to dissipate. Prior to adding the oil, a dielectric test should be
performed on the oil even if the oil is new. A minimum test value for mineral oil is 26KV and for Askarel oil 30KV
OIL MAINTENANCE AND ANALYSIS
The dielectric strength of the insulating oil is of the most concern. Thus, a major part of the maintenance effort
is directed at detecting any variance from established levels or any deterioration that would result in a reduction
of the dielectric strength.
The oxygen in the air, combined with the heat developed during normal operation, will result in the formation of
acids and sludge in the oil.
This sludge will settle on the horizontal parts of the windings and at the bottom of the tank and interferes with
the normal circulation of the oil and its ability to dissipate heat. The sludge can also reduce the flash-over value
of the insulating surface.
Moisture is the most dangerous contaminant of insulating oils. As small as ten parts per million by volume can
lower the dielectric strength of the oil below its acceptable value.
Of the many test that have been developed to determine the condition of insulating oils, the following three are
considered sufficient for average maintenance requirements.
1. The dielectric breakdown voltage test.
2. The acidity test
3. The color test
All three of these tests, plus many more can be and are performed in the field by Southern Substation, Inc.,
technicians. I will now outline each of the preceding tests, showing how they are performed and the results to look
The dielectric breakdown test measures the ability of an insulating oil to withstand electrical stress up to a point
of failure. This test indicates at what voltage the oil will allow a flash-over or breakdown. Water, dirt or conducting
particles will cause the oil to breakdown below its acceptable levels. The proper testing procedure is as follows:
A sample of the oil to be tested is placed in a clean "standard" cup so that it covers the two disc electrodes that
are one inch in diameter and have their faces apart by one-tenth of an inch. Voltage is applied across the
electrodes and increased gradually until a flash-over occurs. It is an accepted practice to run a minimum of three
tests and the average of the three being considered as the actual dielectric strength of the oil. The minimum
acceptable value is 22KV for mineral oil, and 25KV for Askarel. Some manufacturers recommend that any
reading below this that the transformer be de-energized immediately
The acidity of oil is a measure of how much it has oxidized and thus deteriorated and how great is the
propensity to form sludge. Acidity is measured by a neutralization number. This number ranges from 0.03 to
1.504 as covered in ASTM specification number D877 and/or D1816. This number represents the milligrams of
potassium hydroxide (KOH) required to neutralize the acid in one gram of oil. NOTE: Acidity in Askarel does not
indicate deterioration but does indicate a possible chemical reaction with organic insulation.
The color test is performed by visually comparing the color of the oil to a color chart provided by most oil
manufacturers. The color scale on these charts range from 0.5 to 8.0 with new oil having a color number of 1.0 or
less. New oil will appear to be clear to a light straw color, while a dark red to black oil indicates sludge or other
If the results of any of the three recommend field tests (dielectric test, acid test, color test) are close to or below
the acceptable limits more complicated tests may need to be performed. Among these tests are the power point,
flash point, viscosity, specific gravity, interfacial tension, free ions, power factor, water content, nitrogen or inert
gas, oxygen (Fyrite) or any of the other tests listed by ASTM D117. Southern Substation, Inc., technicians have
the equipment in our mobile laboratory unit to perform any of the above tests.
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MORE INFORMATION ON OIL TESTING.