On-site regeneration of transformer oil
Regeneration
of transformer oil at the transformer’s location is an important
preventive measure of transformer servicing.
Transformer
life time is, in essence, the life time of its insulation system. The most widely
used insulation is liquid insulation (transformer oils) and solid insulation
(paper, i.e. cellulose insulation). The oil provides for at least 80% of the
electric strength in a transformer. Almost 85% of transformer failures are
caused by damaged insulation.
Transformer
oil is a good insulator when insulation paper is will impregnated: the oil
increases the breakdown voltage of the insulation which it saturates. Low
viscosity of the oil allows it to permeate solid insulation and dissipate heat
by transferring it to the cooling system. Therefore, liquid insulation is also
a cooling liquid. Oxidation stability of the oil allows it to endure high
temperatures and prevent significant damage to the insulation system.
Aging
or degradation of transformer oil is usually related to oxidation. As oxygen
and water appear in the oil, the oil oxidizes even of other conditions are
perfect. Contaminants generated by solid insulations also affect the quality of
transformer oil. Reactions which occur in the oil between unstable
hydrocarbons, oxygen and other catalyst, such as moisture, with such
accelerators as heat, lead to oil decomposition (oxidation).
Heat
and moisture, along with oxidation, act as primary accelerators of this process
and are the largest threat to solid insulation. If the cooling and insulation
system is serviced right, insulation system’s life time can be extended from 40
to 60 years. Unfortunately, oil oxidation cannot be entirely eliminated,
however, it can be controlled and slowed by oil treatment. One of the most
important transformer maintenance procedures is oil analysis scheduled at least
annually. Oil analysis is indicative of the overall insulation condition.
Moisture
is a combination of free water, water solved in the products of oil degradation,
solved and chemically bound water (it is a part of glucose molecules and is
necessary for maintaining the mechanical strength of cellulose). It is
impossible to completely dehydrate cellulose insulation.
Transformer
oil solves more water at higher temperatures. If the mixture of oil and water
is cooled, water will settle out of the oil. The oil will permeate solid
insulation, or become bound to oil degradation products. Moisture will
distribute itself between the oil and the paper. However, this distribution
will be uneven: paper absorbs water from the oil and retains it, in the areas
of highest voltage.
Damage to inside of coil winding
stack of oil filled transformer
Contaminants
are formed in the process of transformer operation. Oil decay products are
acidic, and they attack cellulose and metals; the acids also create soaps,
aldehyde and alcohol, which settle on the insulation, tank walls, breathing and
cooling systems in the form of sludge. Sludge forms faster in a heavily loaded,
hot transformer operated incorrectly. Sludge increases oil viscosity thus
reducing its cooling ability, which has further negative effect on transformer
life time.
Contamination
also causes insulation to shrink, destroys varnish and cellulose material. It
is a conductor for discharges and currents; being hygroscopic, it absorbs
moisture and leads to insulation overheating. Sediment forms on the core, which
increases transformer temperature.
Cellulose
material is the weakest link in the insulation system. Since transformer life
time is in essence the life time of its cellulose insulation, and since
cellulose degradation is irreversible, contaminants must be removed
immediately, until they damage the cellulose. A good maintenance program
extends its life time significantly.
Normal
servicing of power transformer should attain a practical life time of 50 – 75
years. However, the actual condition of insulation defines the difference of
real time operation between 20 – 50 years plus the transformer life time.
Experience shows that the most common cause for transformer failure is
inadequate servicing and incorrect operation.
Transformer oil before and after
regeneration by UVR 450/16
Transformer
oil can be completely regenerated and made as good as new. Insulation oil can be used indefinitely, if it is
processed regularly. The prospect of regenerating a batch of very poor quality
oil should be balanced against the relatively high cost of acquiring new oil.
Removal
of water and keeping the insulation dry is of utmost importance. Moisture
accelerates aging. 1% of moisture in cellulose accelerates aging by one order
of magnitude in comparison to 0.1% moisture content.
So, what are the main guidelines for preventive maintenance?
Purification
of transformer oil, including regeneration, is a method of extending
transformer life time.
The
objective of this process is to remove aging product from solid insulation and
oil before they damage the insulation system (insulation damage may be
determined by furan compounds).
A
well planned maintenance strategy aims to avoid accumulation of moisture in
insulation and make sure that the transformer always operates in a clean
environment.
To
stop or slow the aging process of transformer insulation, the oil must be kept
in the best possible condition. The following measures will help:
- Constant control of oil condition;
- Silica gel in the breather must be in good condition (blue). Never allow more than one third of the silica gel volume to change color to pink;
- Repair oil leaks as soon as one is detected;
- Start using an oil purification system for dehydration of oil to 10 ppm at most;
- Do not add oil contaminated with moisture (if the oil was kept in an open vessel);
- Start drying the oil as soon as moisture content exceeds 20 ppm or the breakdown voltage drops to below 50 kV;
- Keep a close eye on the oil’s acidity and regenerate the oil when it reaches the critical level of 0.2 mg KOH/g. Best use a Fuller’s earth system with renewable sorbent (Globecore CMM-R is a good choice);
Sometimes
oil should be changed (filtered, rinsed, and refilled). This procedure is best
performed on site. Oil is drained form the transformer. The interior of
transformer tank is rinsed with hot naphthene or regenerated oil to remove
sediment concentrations; then the transformer is filled with regenerated oil.
If
the transformer is rinsed only through inspection opening, only approximately
10% of the interior surface will be cleaned. In this case, a film of
contaminated oil remains on most of the winding surface and the tank’s
interior. Keep in mind that up to 10% of the oil in the transformer permeates
cellulose insulation. The oil remaining in the insulation and the transformer
contains polarized structures and can poison a large amount of new or
regenerated oil.
If
the top lid is removed for rinsing, approximately 60% of the interior can be
cleaned. Better results can be achieved by using a Fuller’s earth regeneration
system, such as GlobeCore’s CMM-R, on a live transformer.
A
simple replacement of oil does not remove all sediment, which accumulates in
the cooling system and between the windings. This sediment will dissolve in new
oil and cause oxidation.
How to regenerate and remove contamination on site
Oil
may be regenerated directly in working transformer (prior analysis indicates if
such possibility exists, especially the DGA test). The oil is pumped from the
tank’s lower valve to the regeneration system, where it is purified,
regenerated and degassed before going back to the transformer through the
transformer expansion tank. The process continues until the oil is restored and
complies with standards or other specifications. If CMM-R unit is used, hot oil
is regenerated by percolation through fuller’s earth, then filtration and
vacuum degassing and dehydration.
The
difference between regeneration and purification is that regular purification
cannot remove such things as acids, aldehydes, cetones etc, solved in the oil.
Therefore, simple purification cannot change the oil color from dark to light
clear yellow. Regeneration, however, incorporates filtration and dehydration.
When
regenerating oil, the following results may be expected:
- Moisture content less than 10 ppm;
- Acidity below 0.02 mg KOH/g oil;
- Breakdown voltage of at least 70 kV;
- IFT at least 40 dynes/cm;
- Tan delta less than 0.003;
- Contaminants solved or suspended in the oil are removed;
- Oxidation stability restored;
- Oil color changed to clear light yellow;
- Solid insulation breakdown voltage improved.
Despite
the removal of solved or suspended solid contaminants, regular regeneration
cannot remove sediment. It is necessary to remove sediment if acidity of the
oil is above 0.15 mg KOH/g and IFT is less than 24 dynes/cm. Sludge removal
involves circulating hot oil through the transformer. The oil is heated to the
point when it becomes a solvent for the sludge. If the transformer is
operating, vibrations of the windings enhance the process.
Next
to consider is shrinking of insulation and dehydration of transformer oil.
Solid
insulation may shrink as a result of motion of loaded coil, specifically, under
shock loads; shrinking may become a source of premature failure. Shrinking also
comes as a result of cellulose degradation. On-site regeneration of transformer
oil does not cause insulation shrinking.
Experience
shows that if the transformer insulation is super-dry (up to +2% of dry
weight), shrinking does not occur. The regeneration process does not aim to dry
transformer insulation. It is impossible to dry the insulation within the time
of regeneration. High level of dehydration requires significant time.
Moving
moisture from insulation by thermal diffusion is a natural process of restoring
the balance between the winding insulation and the oil. The process rate
depends on the level of water diffusion through solid insulation.
Removal of sediment from transformer core
Insulation
forms and accumulates in cellulose fibers. During purification, the oil is
heated to the point when sediment becomes soluble in oil. The process
guarantees that the solved contaminants will be removed by regeneration and oil
will become clean.
Obviously,
regeneration and purification is broader than simple oil restoration.
Loss of furan values
Restoration
(regeneration or purification) or replacement of transformer oil destroys furan
compounds, which are used to measure the degree of polymerization (insulation
condition and life time). Furan analysis should be done before the process.
If
transformer oil is allowed to degrade beyond salvaging without regeneration or
purification, transformer life time decreases significantly. After
purification, a new base line for furan compounds control is established.
Future furan test must be referenced against this new base line.
Removal of aromatic compounds
Some
types of aromatic compounds may have anti-oxidation properties. Most
specifications require that the content of polyaromatic hydrocarbonate be equal
or less than 3%. Too much aromatic compounds reduce dielectric or impulse
strength and imcrease the oil’s ability to solve most of the solid insulation
submerged in oil. Oxidation stability of regenerated oil (after 164 hours at
100 degrees C) was 0.006% by weight, which is lower than the highest allowable
level of 0.1% by weight.
Before
regeneration is started, the whole system, including the hoses, is filled with
oil. Old oil and suspended contaminants accumulated in the bottom of the
transformer tank are removed from the lower part of the tank. Regenerated,
filtered and pure oil enters the transformer via the expansion tank above. This
is done to ensure the level of oil in the transformer is unchanged. The oil
freely circulates and the contaminants do not reenter the tank. Only the clean,
dehydrated, degassed oil returns to the transformer.
No comments:
Post a Comment