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Transformer Maintenance

Specialists in Medium Voltage Maintenance & Repairs
Medium Voltage Equipment
1. Transformers Insulating Oil Analysis:
The analysis of insulating oils provides information about the oil, but also enables the detection of
other possible problems, including contact arcing, aging insulating paper and other latent faults and
is an indispensable part of a cost-efficient electrical maintenance program. The tests monitor various
aspects of the transformer such as:
Oil condition - Moisture, Acid Content, Dielectric Strength (kV), IFT & Tan Delta
Internal working - Dissolved Gas Analysis (DGA)
Paper condition - Furan analysis
Contaminants - PCB and Corrosive Sulphur
If a proactive approach is adopted based on the condition of the transformer oil, the life of the
transformer can be extended.
1.1. Physically taking a sample from a transformer:
There are several environmental variables, such as temperature, precipitation, etc., to consider
before collecting a sample. The ideal situation for collecting a sample from an electrical apparatus is
35°C or higher, zero percent humidity and no wind. Cold conditions, or conditions when relative
humidity is in excess of 70 percent, should be avoided, as this will increase moisture in the sample.
Step 1: Check for positive pressure by first observing the pressure vacuum gage, if one is present,
then check for positive pressure at a sample outlet by placing a slug of insulating fluid in a piece of
clear plastic tubing and attaching it to the sample outlet. While observing the slug of insulating fluid,
slowly crack the sample outlet valve open. If the slug moves towards the electrical apparatus, a
negative pressure exists. Sampling should be discontinued. If the slug moves away from the
electrical apparatus, a positive pressure exists. Samples can be obtained safely.
Step 2: Place a flush-oil container under the main drain valve and remove the security pipe plug from
the drain valve. Wipe the inside of the valve and threads with a clean lint-free cloth. Drain at least
1.9 L of fluid into the flush oil container to flush the drain valve and drain valve extension. One of
two procedures may then be used to prepare the sample outlet.
Procedure A - Install the sample adapter on the drain valve with a piece of oil resistant tubing
attached. Flush the valve at installed sample adapter, flushing at least one more quart of fluid into
the flush oil container before collecting sample.
Procedure B - Install the drain valve security plug. Attach oil-resistant tubing to the sample port on
the drain valve and flush at least one more quart of fluid into the flush-oil container before collecting
the fluid in the sample container.
1.2. Transformer Insulating Oil Tests:
1.2.1 Moisture Analysis
Why is it important to determine the moisture content in transformer oil?
• To determine if there are any leaks in the transformer
• Increase in moisture accelerates insulating paper degradation
• To determine if the decrease in insulating strength is due to high water
• Paper and oil degradation
1.2.2 Dielectric Strength
Dielectric strength is the voltage at which the electric breakdown of the
oil occurs. By analyzing the voltage and the amount of contaminants the
Insulating quality can be predicted.
1.2.3 Acidity
High acidity will cause oil degradation and sludge formation which will cause paper degradation,
cooling problems and internal corrosion.
1.2.3 Dissolved Gas Analysis (DGA)
Dissolved gases in transformer oils are inherent gases and gases that form due to the breakdown of
the paper, oil under stress or degradation, or mechanical faults.
With regular DGA testing the following problems can be detected:
• Overheating • Loose connections
• Breakdown in insulation • Arcing
• Partial discharge • Corona
• Overloading • Sparking
1.2.4 Furan
Furan are degradation by-products of the insulation paper found in transformers.
Analysing furan is important in predicting the degradation of the insulating paper.
The following will destroy insulation paper:
• Water (moisture) • Oxygen
• Heat • Oil oxidation
1.2.5 Corrosive Sulphur
Corrosive sulphur forms acidic conditions in transformers.
This is a vital test as DGA, MAD and furan tests may indicate normal operation even when failure is
imminent. If Corrosive Sulphur oil is found, oil will need to be replaced.
1.2.6 Interfacial Tension
At the interface formed between Oil and water, the molecular cohesive and adhesive forces cause a
“film” to form which is analogous to an elastic sheet with an associated tension force. This property
is called the interfacial tension (IFT) and it is very sensitive to impurities in either liquid. As
transformer oil degrades over time or as contaminants are introduced, the IFT between it and pure
water will drop, thus indicating deteriorating health of the transformer.
1.2.7 Tan Delta
Tan delta measures the insulation quality of transformer oil and will indicate if there is any
1.2.8 Polychlorinated Biphenyl (PCB) Analysis
PCBs were released in the 1970's during the oil crisis in order to bulk up transformer oil volume. Due
to the high toxicity and environmental impact of PCBs it is now been legislated to know the PCB
content of your transformer oil.
2. Life span of Transformers
To determine the life expectancy, you also need to consider the transformer type, environmental
conditions, operating conditions, and the oil and paper degradation processes.
The degradation of the insulation system largely determines the extent of a transformer's life.
2.1 Solid Insulation:
The solid insulation (cellulose based products) in transformers degrades with time at rates which
depends on the temperature, moisture content, oxygen and acids in the insulation system. Heat and
moisture are the main enemies of the solid paper insulation with oxidation as the primary
Moisture consists of free water, suspended water (trapped in oil decay products), dissolved water
and chemically bound water (part of the chemical structure of the glucose molecule and necessary
to maintain the mechanical strength of the cellulose).
The complete removal of moisture from cellulose insulation is therefore impossible.
2.2 Insulation System of the Transformer:
When electrical energy is transformed from one voltage to another, heat is developed. This
temperature must be within limits so that it will not adversely affect the dimensional stability of the
materials used in the construction of the transformer.
In power transformers, the high electrical stresses and amount of heat developed require both solid
and liquid insulation, with the fluid conducting the generated heat away to cooling surfaces, where it
gets dissipated.
The life of the transformer is the life of the internal insulation system.
The most widely used systems are liquid insulation (transformer oil) and solid insulation (Kraft paper,
pressboard, wood i.e. cellulose products). The insulating oil provides approximately 80% of the
dielectric strength of a transformer. Transformer oil is a good insulating medium and when
impregnated in paper, board and cloth, increases the dielectric strength of it further. The low
viscosity of oil also allows it to penetrate the solid insulation setting up convection currents for
conveying the heat from the core materials to the radiators. The liquid insulation therefore also
serves as a coolant and its oxidation stability allows it to operate at high temperatures for long
Understanding the design of the transformer as well as the operational history is essential for
making reliable diagnosis. The design of the components must enhance the life expectancy of the
assembly as a whole due to operations under extreme conditions, rapid aging and wear and tear will
occur, thereby shortening the life of the transformer.
3. Transformer Insulating Oil Purification:
Transformer oil purification is a process to remove sludge, dissolved moisture and dissolved gases
from the oil and by doing so, improving the Dielectric Strength (kV) of the oil.
Purification is done by using a heat vacuum system to achieve excellent results.
Step 1: The first step in this serial process is to raise the oil temperature to a desired level, generally
up to 65 C. This aids to give the oil latent heat which later aids to dissociate the moisture and gases
from oil in the degassing chamber. Additionally, the viscosity of the oil drops which aids in better
filtration to some extent. Recommended heater design should consider an area of heating of about 1
to 2 W/sq.cm. In the transformer oil purification plant, the heating system is protected against over
pressure built up and excess temperature rise.
Step 2: The second step of transformer oil purification plant is removal of sludge and dirt from the
transformer oil.
Step 3: The third step in the oil filter machine is dehydration of transformer oil and degasification of
the transformer oil.
These processes of dehumidification of transformer oil and removal of gasses is executed in the
degassing chamber. The dissolved water oil separation or dissolved gas oil separation is possible at
reduced pressure, i.e. vacuum, due to difference of boiling point of water, gas and transformer oil.
In the process of separation of gases from the oil it becomes important to retain the aromatic
hydrocarbons so that the original properties of the oil are retained. When the water level in the oil is
above saturation level of the transformer oil, oil is observed in free water. Removal of free water can
be done by power driven centrifuge or by coalescing principle, where the latter is more effective and
economical in practice.
4. Transformer Insulating Oil Change:
Transformer oil is a mineral based oil that is commonly used in transformers for its chemical
properties and dielectric strength. This oil in your transformer acts as an insulator and a cooling
agent. Over time, the oil will degrade resulting in the potential for faults and costly repairs.
If some contaminant levels are significantly high, it may be more economically practical to replace
the oil.
The quality of your transformer oil will affect its insulation and cooling properties. Under normal
operating conditions, a minimal breakdown of oil quality will occur from oxidization and
contamination. These are summarized as follows:
4.1. Oxidization is acid that forms in the oil when it comes in contact with oxygen. The acid will form
sludge which settles on the windings of the transformer resulting in reduced heat dissipation. The
windings will run hotter thereby creating more sludge which in turn will create even more heat. The
high acid content and increased temperatures will accelerate the deterioration of the insulating
qualities of the oil and if left untreated will cause the transformer to fail.
4.2. Contamination commonly found in transformer oil includes water and particulate. The presence
of either of these contaminants will reduce the insulating qualities of your transformer oil.
4.3. If you have decided to replace the oil in the transformer, either Virgin (New) or Regenerated oil
can be used.
If the transformer tank is capable of vacuum, it should be filled under vacuum based on
manufacturer’s recommendations. If the tank cannot withstand a vacuum, the oil should be
degasified into the transformer and circulated through the degasifier three times the volume of the
transformer. This will help remove any moisture present in the insulation of the transformer.
New oil often requires further degasification to remove air and moisture added during the
transportation and handling process. This will increase the life expectancy of the oil in the
5. Transformer Oil Regeneration
Oil regeneration is extraction of contaminants from oil in order to restore its original properties to
be used equally with fresh oils.
5.1 Aging Transformer Oil Forms Sludge
As transformer oil ages, it oxidizes and begins to break down. The by-products of the degradation
process include acids, aldehydes, and peroxides, which bind together to form sludge. Sludge attacks
the cellulose insulation, inhibits oil flow, and traps heat inside the transformer. Eventually the
dielectric gap is bridged, resulting in failure of the transformer.
Aging is the result of physical and chemical processes that change oil. The main cause of aging oil is
exposure to high temperatures and contact with air that leads to oxidation, decomposition,
polymerization and condensation of hydrocarbons. Another cause of aging is contamination with
metal particles, water and dust. Their accumulation leads to build up of slurries, resinous and
asphaltic compounds, coke, soot, various salts and acids in the oils. The oil in which aging process
occurs, cannot fully perform its functions.
5.2 Oil reclamation extends unit life
The regeneration system restores transformer oil by removing acidity, sludge and soluble oil decay
products with Fullers Earth treatment technology. When the treatment is complete, the regenerated
oil meets or exceeds international standards for new oil with lower acidity and improved tan delta,
interfacial tension, and oxidation stability.
5.3 Regeneration analysis results
5.4 Physical methods of regeneration
Physical methods of regeneration do not change the chemical properties of oil. They remove only
mechanical impurities (metal particles, sand, dust, as well as tar, asphalt and coke-like substances,
5.4 Regeneration by physical methods include:
1. Sedimentation. This method is often used as the first stage in regeneration. The contamination
particles in oil settle down, due to gravity;
2. Filtration - separates suspensions into clean liquid and wet sediment with the help of filters;
Washing with water and dry washing to remove acidic products from oil (water-soluble lowmolecular acids, salts of organic acids).
5.5 Physicochemical method
Physicochemical methods are based on the use of coagulants and adsorbents. Coagulants promote
the coarsening and precipitation of fine-dispersed asphalt-resinous substances in oil. Adsorbents
selectively absorb organic and inorganic compounds. These methods remove asphalt and resinous
compounds, emulsified and dissolved water from oil. Adsorptive treatment with bleaching clays
neutralizes free acid in acid-treated oil, unstable oxidized and sulphurized products as well as traces
of sulphonic acid. In addition, clay treatment leads to higher resistance to oil oxidation at high
temperatures and increased colour stability. This process is used in clay polishing plants for waste oil
re-refining and transformer oil regeneration systems for the reclamation of old transformer oil to asnew condition.