FIELD OF THE INVENTION
The present invention generally relates to a method of reducing the stray loss
and associated over-heating of the clamp plate of a power and system
transformer. In particular, the present invention restricts, increase in the
temperature of transformer oil by preventing over-heating of the clamp plate.
More particularly, the present invention relates to a method for enhancement of
efficiency and reliability of large rating power and system transformer by
harnessing shielded clamp plate for reduction of local heating and stray loss.
BACKGROUND OF THE INVENTION
A power transformer is a vital equipment for transformer of power from one
circuit (system) to another circuit. The power transformer comprises at least two
different electrical circuits (windings) for inflow and out flow of power through it.
The windings are made of high grade electrolytic paper covered conductor. The
windings are isolated from each other and also from the other structural parts of
the transformer using insulating boards and paper sheets isolated by oil ducts.
The winding are conventionally mounted concentrically over the core of the
transformers. The core is made of laminated high-grade silicon steel (CRGO-Cold
Rolled Grain Oriented Steel) sheets of small thickness. The thin steel sheets are
piled together to make the core of the transformer, as shown in figure 1. The
pallet of thin sheets are kept together with the help of fabricated Steel plates
known as Clamp plate. The clamp plates are joined to other fabricated parts
which is used for holding the core and to form an assembled core assembly, as
shown in figure 2.
The coils are kept concentrically around the core-clamp plate assembly as shown
in figure 3 and there exists a mechanical space axially and radially between the

coils and the core. The clamp plate is positioned between the core and the coil
i.e. one surface of the clamp plate is faced towards the core and the other
surface of the clamp plate is faced towards the coils. The clamp plate are also
used for lifting the assembled core comprising the core along with the coils.
The whole core-coil assembly is housed in a metallic container known as Tank.
The tank is full of di-electric fluid known as oil and the core - coil assembly is
fully immersed in oil; which serves the purpose of insulation and cooling. Barring
the winding, all the metallic structures inside the transformer namely, core,
clamp plate, End frame, Tank are kept at ground potential. The coils are
mounted concentrically around the core limb as shown in fig - 3. Even though
the arrangement of the placement of coils and their combination is decided by
different electrical parameters of the transformer; conventionally the high current
carrying L V coil is placed closest to the Core limb and other coils are kept
radially away from the L V coil. The transfer of electric power from HV to LV
circuit (or vice versa) takes place on the principle of electro - magnetic induction.
The core carries necessary MMF (Magneto Motive force) for setting up of electro
- magnetic induction corresponding to the voltage in the coil. During the transfer
of electrical energy from one circuit ( winding ) to another, electromagnetic field
is set up between the coils - which links the power input and output windings .
However a part of the magnetic field (flux) does not link the coil and it is
diverted towards the core, clamp plate and other earthed potential structures
rather than linking the coils as shown in fig-3. This phenomena is known as
fringing of the flux and this flux is known as Stray flux.
The fringing of the flux as shown in the fig-4a and fig-4b are maximum at the
ends of the coils adjacent to the core limb .In conventional power transformers,
the fringing of the flux takes place at the top and bottom of the coil. In System
transformers with Split type coil arrangements, the LV coils (normally 2 coils J

are arranged vertically one over the other spaced apart by maintaining an axial
distance between them. The fringing of the flux takes place at the axial distance
between the two L V coils and also at the ends of the coils. This fringed stray
flux interfaces the clamp plate owing to proximity.
The clamp plate being a load bearing member is manufactured from Structural
steel viz. Mild Steel or High tensile steel. Both the above material being magnetic
material in nature cause additional energy loss when the stray flux impinges on
the surface of the clamp plate facing the coils. This energy loss also causes local
over heating zones on the clamp plate and surrounding oil inside the transformer.
Since the power transformer is completely immersed in the insulating oil, the
local over heating of the Clamp plate due to Stray loss causes over heating of the
oil. The over heating triggers deterioration of oil and generates hydro carbon
gases. The generation of the gases also reduces the di-electric strength of the oil,
which leads to break down of the transformer. To eliminate the gases from the
oil, the transformer oil has to be processed which calls for shut down of the
transformer and loss of power supply to utility effecting the reliability of the
power supply.
This problem apparently could be resolved to some extent by provision of stain
less steel clamp plate. But, the stain less steel material is costly, less effective for
load bearing and difficult for fabrication due to it's intrinsic properties in
comparison to the conventionally used Mild steel and High tensile steel material
used for Clamp plate. Also the Stain less steel clamp plate looses a large part of
it's non-magnetic property after fabrication. As a result, provision of stainless
steel clamp plate has a limitation for the practical solution to the problem.

In the present invention, a method is suggested to reduce this effect of Stray
flux on the clamp plate, reduce the energy loss, reduce the temperature of the
clamp plate and surrounding oil, reduce the deterioration of oil, reduces all other
negative effects of heating on oil and increase the reliability of the power supply.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a method for controlling an
increase in local temperature of Clamp plate in a split-winding power transformer.
Another object of the invention is to propose a method for controlling an
increase in local temperature of Clamp plate in a split-winding power transformer,
in which the Stray loss occurring in the clamp plate of the transformer is reduced
to enhance the efficiency of the transformer.
A still another object of the invention is to propose a method for controlling an
increase in local temperature of Clamp plate in a split-winding power transformer,
which restricts rise in temperature of the transformer oil in the vicinity of clamp
plate due to reduction of the temperature of the clamp plate.
Yet another object of the invention is to propose a method for controlling an
increase in local temperature of Clamp plate in a split-winding power transformer,
which decreases the maintenance cost of the transformer and increase reliability
of the power supply.
A further object of the invention is to propose a method for controlling an
increase in local temperature of Clamp plate in a split-winding power transformer,
which extends the operational life of the transformer.

SUMMARY OF THE INVENTION
According to the invention, there is provided a method for enhancement of
efficiency and reliability of large rating power and system transformer by
harnessing shielded clamp plate for reduction of local heating and stray loss. The
transformer comprising at least two different electrical windings formed of
current conductors covered with electrolytic paper, the windings being mounted
concentrically over a core of the transformer with insulating boards and papers
isolating said windings from the other structural components of the transformer
remaining at ground potential, wherein the core of the transformer is formed by
piling a plurality of thin steel sheets on fabricated steel plates constituting clamp
plates, the clamp plates when fabricated to corresponding structural components
allows the transformer core to be held thereon; a plurality of coils mounted
around the core assembly maintaining an axial and redial distance; and a
metallic container acting as a tank and filled with di-electric fuel housing the
complete core assembly for transferring power between the low voltage circuit
and the high voltage circuit of the transformer, providing an additional sheet of
non-magnetic material over the existing clamp plate such that the sheet facing
the coils; selecting the dimension and location of the additional sheet such that
the generated stray electro-magnetic flux terminates on this sheet and disabled
to extend to the clamp plate.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRA WINGS
Fig. 1 - Shows a stacking arrangement of core laminated sheets of a power
transformer.
Fig. 2 - Shows the conventional arrangement of core-clamp plate assembly
in a power transformer.

Fig. 3a - Shows the conventional core-coil assembly in a Power transformer.
Fig. 3b - Shows the conventional core-coil assembly in a Split winding Power
transformer with 2 LV coils
Fig. 4a - Shows the path of Stray flux impinging on the clamp plate in a
conventional Power transformer according to prior art.
Fig. 4b - Shows the path of Stray flux impinging on the clamp plate in a Split
winding Power transformer according to the invention.
Fig. 5 - Arrangement of shield over the Clamp plate according to the
invention.
DETAILED DESCRIPTION OF THE INVENTION
A transformer is used for transferring electrical power from one circuit to another
circuit under the principle of Electro-magnetic induction. In the event of flow of
electrical energy, an electromagnetic field is established which inter alia the coils
of input and output side of the two circuits respectively, and energy is
transferred. However, a small part of the electro- magnetic field (flux) known as
Stray flux finds a different path owing to the physical location of the coils with
respect to the other metallic structures available in the proximity. This can be
seen from the mapping at Fig- 4a and Fig - 4b from the plotting of the Electro-
magnetic field.
The clamp plate of the transformer is a fabricated metallic structure present in
the in the vicinity of Stray flux causing the loss due to the Stray flux.

To resolve the problem of reduction in a Stray loss occurring in a Clamp plate of
a transformer a separate sheet of non-magnetic material, for example, copper or
Aluminum sheet is provided over the conventional clamp plate of the transformer.
This sheet acts as a Shield on the clamp plate. It is known that conventional
clamp plate is fabricated from HTS (High Tensile steel) or Mild Steel as per the
requirement of high mechanical strength required for rigidity to tightly hold the
core assembly and also to lift the core assembly of the transformer. A sheet of
Aluminum or copper is placed on the clamp plate, facing the coil (Fig-5). The
stray electro-magnetic flux terminates on this Sheet rather than on the Clamp
plate. The Shield which is made from Aluminum or Copper sheet even though
not suitable for load bearing activities but not affected by the Stray flux owing to
their non - magnetic property. This non-magnetic sheet behaves as a flux
deflector. It reflects back the stray flux impinging on it.
Since the stray flux does not interact with the main clamp plate (responsible for
core building and lifting of core-coil assembly), all the effect of Stray flux on the
clamp plate for example additional power loss or over heating or gas generation
or oil deterioration are eliminated. So all the activities relating to processing of
the transformer and the associated shut down time of the transformer reduces.
Thereby the reliability of the transformer increases. Also there is a substantial
reduction in the stray loss (energy loss) resulting increase in the over all
efficiency of the transformer.

WE CLAIM
1. A method for enhancement of efficiency and reliability of large rating
power and system transformer by harnessing shielded clamp plate for
reduction of local heating and stray loss, the transformer comprising:
at least two different electrical windings formed of current conductors
covered with electrolytic paper, the windings being mounted concentrically
over a core of the transformer with insulating boards and papers isolating
said windings from the other structural components of the transformer
remaining at ground potential, wherein the core of the transformer is
formed by piling a plurality of thin steel sheets on fabricated steel plates
constituting clamp plates, the clamp plates when fabricated to
corresponding structural components allows the transformer core to be
held thereon;
a plurality of coils mounted around the core assembly maintaining an axial
and redial distance; and
a metallic container acting as a tank and filled with di-electric fuel housing
the complete core assembly for transferring power between the low
voltage circuit and the high voltage circuit of the transformer, the method
is characterized in that:-
- providing an additional sheet of non-magnetic material over the
existing clamp plate such that the sheet facing the coils;
- selecting the dimension and location of the additional sheet such
that the generated stray electro-magnetic flux terminates on this
sheet and disabled to extend to the clamp plate.

2. The method as claimed in claim 1, wherein the additional sheet is selected
from aluminum and copper to act as a flux deflector.
3. The method as claimed in claim 1, wherein the clamp plate is formed of
high tensile steel or mild steel in conjunction with a non-magnetic material.

ABSTRACT

The invention relates to a method for controlling an increase in local temperature
of Clamp plate in a split-winding power transformer, the transformer comprising:
at least two different electrical windings formed of current conductors covered
with electrolytic paper, the windings being mounted concentrically over a core of
the transformer with insulating boards and papers isolating said windings from
the other structural components of the transformer remaining at ground
potential, wherein the core of the transformer is formed by piling a plurality of
thin steel sheets on fabricated steel plates constituting clamp plates, the clamp
plates when fabricated to corresponding structural components allows the
transformer core to be held thereon; a plurality of coils mounted around the core
assembly maintaining an axial and redial distance; and a metallic container
acting as a tank and filled with di-electric fuel housing the complete core
assembly for transferring power between the low voltage circuit and the high
voltage circuit of the transformer, providing an additional sheet of non-magnetic
material over the existing clamp plate such that the sheet facing the coils;
selecting the dimension and location of the additional sheet such that the
generated stray electro-magnetic flux terminates on this sheet and disabled to
extend up to the clamp plate, thereby eliminating all the effects of additional and
local heating inside the transformer.