FIELD OF INVENTION
The invention relates to a method of improving the quality of insulation provided
on the coils of a fully finished transformer in the works of transformer
manufacturer. More particularly, the invention relates to the improvement in the
dielectric tan delta value of a power transformer.
BACKGROUND OF THE INVENTION
In a large transformer, bulk volume of power is transmitted for which large size
High voltage and low voltage windings are provided around the magnetic core.
The core is kept at ground potential whereas the windings are kept at the
required level of voltage.
The core is made of electromagnetic steel laminations ( stampings ) cut and
assembled to required dimensions . The coil is made of multiple parallel copper
wires. The wires are wound around the core known as turns. These turns are
responsible for handling the voltage of the transformer. Higher is the voltage,
more is the number of turns. The multi parallel wires are responsible for the flow
of current in the transformer. Higher is the current, higher is the cross sectional
area of the conductor of the turns .To handle high volume of power the current /
voltage of the windings are very high . So, a common basic feature of large power
transformer necessarily includes coils with large number of turns of many
parallel conductors with large cross section area of conductor. Since the voltage
is dependent upon number of turn in the coils, the coils of the large power
transformers are also very large comprising large number of turns.
The HV (high voltage) and LV (low voltage) windings are isolated from the core,
which is at ground potential. HV and LV coils are also isolated from each other

to enact the transformer action. The isolation is provided by wood pulp based
boards and sheets of varying properties and insulating mineral oil. Press board,
pre-compressed press board, wooden blocks, paper sheets, paper strips, fiber
glass blocks are the typical insulating items in a power transformer.
The transformer is a labour intensive product. The insulating items are cut and
assembled manually. The coils are wound and assembled manually. Owing to
the manual involvement in the manufacture of transformer, copious amount of
dusts are generated and clogged to the core and coil assemblies. These dusts are
both insulating and non-insulating type. Even though the dusts are removed at
many different stages of manufacturing, some dust always stay stuck to the
core/coil assembly even after manufacture of the transformer. In addition, the
cellulose insulation, being hygroscopic in nature, absorbs moisture from the
atmosphere. It is pertinently desirable that the transformer stays clear of all
types of dust and moisture.
After the manufacture of the transformer, the core/coil assembly are subjected
to heating and vacuum so as to remove the moisture and dust. After processing
by the most advanced method VAPOUR PHASE PROCESING (involving heating
and vacuum) the core and coil are placed inside a steel tank and filled with oil.
The oil is filtered many times to remove foreign particles viz. dust particles from
it. The removal of the particles are determined by measuring the Tan delta value
of the transformer. The limiting value of the Tan delta is normally considered
0.005 to 0.007. So the processing and dust removal is considered to be
satisfactory only when the measured Tan Delta value is less than 0.005 to 0.007.
The oil after filling into the tank is thoroughly filtered employing sophisticated
filters which can filter particles up to 2 micron. Even after filtering the oil, some
foreign and suspended particles and dust particles of size less than 2 microns
are not removed. This is very difficult to remove from the oil filled tank housing

core coil assembly. These suspended and dust particles of size less than 2
micron are responsible for the deterioration of the Tan Delta value. Hence, it is
important to remove these suspended and dust particles from the transformer in
order to improve the Tan Delta values to the accepted value of less than 0.005.
The dust moisture may not immediately effect the performance of the
transformer. But these dusts are liable to get charged under the influence of
voltage and current in due course by virtue of power flow and transformer
operation after few years. So for a reliable fault free operation, it is essential to
remove the dust. Even though it is impossible to remove all the dust and
moisture content from the transformer, it is desirable to remove the dust to the
extent possible.
The presence of these particles inside the transformer is reflected by the Tan
Delta value measured on the transformer. In the event of the Tan delta value
exceeding the limit of 0.005, the transformer is not considered to be suitable for
operation. Hence, activities involving vacuum and heating are required to be
done on the transformer sometimes at site to achieve the desired Tan Delta value
less than 0.005.
Due to the stringent requirement of incremental rise in the Tan Delta values by
0.001 even after operation of the transformer, the collection and removal of the
suspended and foreign particles is carried out to a sufficiently large extent
during the manufacture of the transformer.
To alleviate the drawbacks of the prior art, the present method of invention
disclosed herein teaches about a reliable and proven method by virtue of which
the removal of particle is done even after complete processing of the transformer
by vapor phase process.

OBJECTS OF THE INVENTION:-
An object of the invention is to propose a method for improving the quality of the
insulation of transformer by removing the dust and suspended particles from it.
Another object of the invention is to propose a method to reduce the
commissioning time at site during installation of the transformer thereby
eliminating the delay in the commissioning schedule of the project involving
transformer.
Yet another object of the invention is to propose a method of protecting the
transformer from the hazardous effect of the dust and suspended particles
during the working of the transformer.
Yet another object of the invention is to propose a method to provide long life to
the transformer ensuring better reliability and more service to the utility.
A still further object of the invention is to propose a method to provide trouble
free service of the transformer and reduce the maintenance activity of the
transformer thereby reducing the time and cost to the utility.
DETAILED DESCRIPTION OF THE INVENTION
A power transformer is an oil filled outdoor installed electrical machine to
selectively alter the voltage of the generation or transmission line located in the
switch yard of a sub-station or generating station .Large power transformers are
meant for handling large bulk of power .It is either connected at the generating
station to boost the generating voltage level or to inter-connect two different

transmission lines of different voltages so as to facilitate flow of power from
source to utility.
The transformer is a machine as well as labor intensive product. There is a
innumerable interaction between the machine and manual skill. The core is
made of CRGO steel sheets cut from CRGO roll. These sheets are piled up
assembled manually to form a magnetic core - that is used for passage of
magnetic flux caused by the voltage of the energizing winding.
The core is surrounded by the high voltage and low voltage coils made of
insulated copper conductors. These coils are prepared manually over rqtating
winding machine with adjustments earlier to pressing and adjustments done
manually.
The coils are mounted on the core with the help of over head crane under
manual supervision. After the assembly, the terminals of the windings are
connected to the respective bushings and insulated manually to be suitable for
the required current and voltage. The completed core-coil assembly etc are put
inside a metallic steel container known as tank.
So, during the manufacture of the transformer, the core and coil are exposed to
external ambient condition leading to deposition of many types of dust on the
core and coil. The coils are insulated with cellulose based insulation (for power
transformer) which has a tendency to absorb moisture from the ambient air.
Since the core and coil are manufactured and assembled with human
intervention, some amount of dust invariably gets into the transformer.

A large amount of these dusts are removed by the process of the transformer
which involves heating , application of vacuum and rinsing by hot oil before the
transformer is put to test .
However, some amount of dust of very small size in the order of 1 micron and
less remains inside the transformer even after thorough cleaning and processing
. Even with the finest mesh available, these dusts of size less than 2 micron are
present .These dusts are primarily suspended particles on the surface of coil and
float in the oil. Whenever, oil is filled or removed these dusts stick on to the
surface of coils. The presence of these dusts are reflected in the testing of the
transformer for tests like capacitance and Tan Delta values.
In the proposed method removal of very finer dusts is done. In the process, the
transformer is kept filed with oil for a long period and kept idle. As a result, the
dust floating in the oil settle down slowly and slowly on the base of the tank. The
transformer when tested for the capacitance and Tan Delta values under this
condition, reveals significant improvement. The settled dusts are finally removed
from the bottom of the tank by slow removal of the oil from the bottom.
So, in this process, dust particles of size less than 2 microns (which pass
through the finest wire mesh filters) are removed from the transformer thereby
improving the insulation quality of the insulation of the transformer .

We claim:-
1. A method of removal of fine particles in a power transformer, the said method
comprising the steps of
- keeping the power transformer static for a predetermined period of time;
- allowing fine particles to settle on the base of the power transformer tank;
- removal of the settled fine particle from the base of the tank by means of slow
removal of oil.
2. The method of removal of fine particles in a power transformer as claimed in
claim 1, wherein the power transformer is necessarily an oil immersed power
transformer.
3. The method of removal of fine particles in a power transformer as claimed in
claim 1, as illustrated in the accompanying drawings.