FIELD OF THE INVENTION
The present invention generally relates to an improved transformer tank and
a method thereof to minimize the eddy losses and hot spot temperatures.
More particularly, the invention relates to an improved tank for large
generator transformers to minimize the eddy losses and hot spot
temperatures.
BACKGROUND OF THE INVENTION
Overheating of structural components of power transformers is of huge
concern to the utilities as the failure of the power transformer, not only
causes power outage but also incurs enormous financial loss. The most
economical design of a power transformer is achieved by restricting the stray
losses and hot spot temperatures within the limited tank dimensions. The
stray losses in structural components of large rating transformers account for
about 20% of the total load losses, and the concentration of eddy currents at
localized regions in the tank may lead to overheating of the tank and
subsequent failure of the transformer. The low voltage (LV) turrets which
enclose the high current carrying leads are the most vulnerable components
for large power transformers, these turrets being invariably made of stainless
steel (SS) material to minimize the eddy losses and hot spot temperatures.
The induction of eddy currents in non-magnetic SS material is lower in
comparison to that in the mild steel (MS) material, due to lower relative
permeability and interalia results in lower losses due to the induction by line
lead conductors in the proximity of tank and turret.
However, in certain cases, it has been observed that though the flux densities
are lower on the tank and turret walls but very high hot spot temperatures
were measured near the interface of SS turret and MS tank wall.
Investigations were carried out by the present inventors and it was noticed

that such a phenomena can occur, even when the eddy currents of lower
density are estimated in the structural parts, due to the distribution of eddy
currents in the tank wall and turret, which resulting in hot spots at the
interface joint.
The material composition near the interface joint and location of magnetic
field induction governed by lead routing has a major role in determining the
distribution of eddy current density and it is very important to study this
effect before finalizing the design. The present inventors have adopted a
novel method of designing tank and turret configuration, based on material
configuration, to minimize the hot spots temperatures at the critical regions.
The prior art with reference to the general transformer design guidelines are
based on estimation of average magnetic flux density and eddy current
density induced in the tank. However, the prior art do not consider the effect
of eddy current concentration near the interface of the tank wall and turrets,
especially when the interface configuration is made of different materials, and
the hot spot temperatures on the tank are under estimated. Some
manufacturers employ heat sinks on the external region of the flange joint to
meet the maximum allowable temperature requirements as per standard
during heat run test. However, this solution do not minimize the
concentration of eddy currents, leading to deterioration of the insulation in
the long run.
The economics in manufacture of large power transformers demand optimal
design of the transformer tank and LV turret with minimum hot spot
temperatures, which concludes that a detailed study of the distribution of
eddy currents including an accurate estimation of the current densities
including the hot spot temperatures is very important to ensure the service
life of the power transformer.

US 20110298575 A1, WO 2010094671 A1: teaches magnetic shunt for
magnetic shielding of a power device. It includes magnetic flux collectors,
and a magnetically permeable bridge configured to magnetically connect the
magnetic flux collectors and form the magnetic shunt as a single structural
unit. To avoid the penetration of stray magnetic fields into ferromagnetic
conductive bodies of a power device, which causes losses due to eddy-
currents and overheating/hot-spots, the above-mentioned magnetic screens
in the form of conductive shields or magnetic shunts may be used. The
shunts are not applicable to the above defined problem as they cannot screen
the tangential flux generated by leads. The arrangement of screens in the
flange joint region from assembly point of view is also not feasible.
The non-patent literature entitled “IEEE TRANSACTIONS ON INDUSTRIAL
ELECTRONICS, VOL. 61, NO. 8, AUGUST 2014”, describes configuration of
copper links using a criterion of current density. It has been recommended in
the publication to place copper links in the identified hot spot zones to
minimize flange overheating problems. The results were verified by
measurements and simulations. This solution is not applicable for tank wall
and turret interface configuration with dissimilar materials, as the eddy
currents do not flow through the copper links across the interface.
The non-patent literature entitled “Transformer Engineering Design and
Practice, by S. V. Kulkarni & S. A. Khaparde, Chapter 5.6 Stray Loss in tank,”
proposed a complex arrangement of overlapping shunts covering the flange
joint to shield the curb joint. This arrangement is not possible for
transformers with top flange joint with very low space for fixing vertical
shunts. For flange bolt overheating the better option would be to connect the
two parts by metallic strips (links) made of high conductivity materials to
provide a low resistance alternative path to the induced currents in the tank
and cover. This arrangement did not minimize the hot spot temperatures.

OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved tank for
large generator transformer to minimize the eddy losses and hot spot
temperatures.
Another object of the invention is to propose an improved tank for large
generator transformer to minimize the eddy losses and hot spot
temperatures, in which the eddy losses in transformer tanks is minimized by
reducing the concentration of eddy currents at the tank wall turret interface.
A still another object of the invention is to propose an improved tank for large
generator transformer to minimize the eddy losses and hot spot
temperatures, which provides solution to tank flange overheating problems
for the existing transformer designs with dissimilar material configuration at
the tank wall and tank cover interface, without modifying the tank
dimensions.
Yet another object of the invention is to propose an improved tank for large
generator transformer to minimize the eddy losses and hot spot
temperatures, which enables further reduction of tank clearances while
minimizing the tank losses and hot spot temperatures.
A further object of the invention is to propose an improved tank for large
generator transformer to minimize the eddy losses and hot spot
temperatures, which minimizes the heat generation in transformer tank,
rather than using external copper links or heat sink arrangement for
dissipation of generated heat.

A still further object of the invention is to propose an improved tank for large
generator transformer to minimize the eddy losses and hot spot temperatures
which prevents power transformer failures due to overheating of tank and
turret interface region and improve the service life of the equipment by
preventing failure of gaskets due to high hot spot temperatures around them.
SUMMARY OF THE INVENTION
The turrets provide support for mounting of the electrical bushings including
covering of high current carrying leads. The tangential magnetic fields
generated by the LV leads, especially for high power transformers, induce
eddy currents in the turrets leading to higher losses and overheating. The
conventional practice of designing transformer tanks with mild steel material
with necessary clearances is being replaced by optimized designs
incorporating nonmagnetic stainless steel (SS) material. Though the material
cost of SS is manifold times higher than MS, but optimized designs with SS
turrets and tank covers are being developed which offer lower loss
capitalization and also further reduction in tank clearances. But, in certain
higher rating transformers with SS turret/tank cover hot spots of very high
temperature were observed. The arrangement of copper links, based on
criterion of current density, in the identified hot spot zones were
incorporated, but this solution was unproductive. Similarly, shields also
proved to be ineffective in minimizing the hot spot temperatures.
The present inventors carried-out extensive analysis on transformer tank
models above 200MVA rating, to co-relate the effect of incorporating different
materials across the tank wall and turret interface on eddy current
distribution and hot spot temperatures. It was observed that the average
eddy current density in the tank was below the design value, but distribution
of eddy currents in the tank wall and turret/tank cover were non-uniform with

concentration of currents at the interface joint. The widely adopted
arrangement of copper links from tank wall to cover were also ineffective.
Detailed electromagnetic and thermal coupled analysis was carried out and it
was ascertained that by replacing an earmarked portion of MS tank wall near
the interface joint with SS material will minimize the eddy current
concentration at the interface and there by substantially reducing the hot
spot temperatures. Studies were carried out to ascertain the weld ability of
MS and SS and also the strength of flange, and were found to be acceptable.
The lead routing and material properties have greater impact on the
concentration of eddy currents and the estimated hot spot temperatures
obtained from simulation results were found to be in good agreement with
the measured values. The present invention proposes an optimal
configuration for design of tank wall and turret interface joint, with dissimilar
materials, in large power transformers to prevent overheating problems.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Thermography scan of 765kV generator transformer during heat
run test
Figure 2 - Electromagnetic model of original transformer
Figure 3 - Eddy current and thermal distribution in original transformer tank
Model
Figure 4 - Electromagnetic model of transformer with improved tank
Figure 5 - Eddy current and thermal distribution in proposed transformer tank
model

DETAILED DESCRIPTION OF THE INVENTION
The most economical design of a power transformer, taking capitalization
costs into account, should offer a compact design with minimum stray losses.
The design clearances between tank (LV side) and active part are dependent
on induced flux density and eddy current density and resulting hot spot
temperatures. Design of tank with clearances below the required values will
lead to increase in tank eddy losses, while large tank clearances make the
equipment uneconomical. The LV turrets are critical structural components
which enclose the high current carrying leads. High eddy current densities are
induced in the turrets and tank walls due to the proximity of high current
carrying leads, resulting in higher losses and overheating. The transformer
tanks are designed with MS for economical and mechanical reasons.
However, SS turrets are invariably used in large power transformers, as MS
turrets have high magnetic permeability and result in higher eddy losses. The
choice of tank and turret material, thickness, clearances and employment of
screens and shields are the critical factors which govern the optimal design of
transformer with trade-off between the tank clearances and losses.
The stray losses in structural components of large rating transformers
account for 20-30% of the total load losses, which can result in substantial
increase in tank hot spot temperatures. While it is assumed that a minimum
clearance, based on estimated magnetic flux density and induced eddy
current density on the tank wall surface, is sufficient to control the induced
eddy losses and hot spot temperatures in tank. However, it was observed in
certain higher rating power transformers, like 275 MVA generator
transformer, very high hot spot temperatures were measured during type
tests as shown in Fig.1. A single phase generator transformer consisting of a
double layer LV winding [7] with incoming and outgoing leads [5] through a
plurality of independent cylindrical SS turrets [4] mounted on a SS

sheet [6] which is an integral part of a MS tank cover [3] and carried to the
tank wall [1] by a flange joint [2], as shown in Fig.2. The tank is optimally
constructed with clearances based on estimation of induced field and eddy
current density on the tank. The hot spot temperatures were above the
limiting values given by IEC 60076-7 standard, which can jeopardize the
dielectric strength of the equipment. Various arrangements based on common
practices such as external copper links, heat sinks and shields were carried
out, but were ineffective in limiting the hot spot temperatures.
Investigations were carried out on 3D electromagnetic transformer model to
evaluate the reason for high hot spot temperatures. It was observed that the
eddy current distribution [11] in the MS tank and SS turret being opposed to
each other, a high concentration of currents near the interface joint is
generated as shown in Fig. 3. It is the distribution of eddy currents apart
from their magnitudes which determine the hot spot temperatures on the
surface of structural components. Based on the detailed studies, the present
invention proposes a novel method based on material composition at tank
and turret interface, which minimizes the losses and hot spot temperatures in
large power transformers.
The distribution of eddy currents on the tank surfaces is quite complex,
especially at critical turret-tank wall interface region, and more so when
dissimilar materials are used and the analytical techniques may not yield the
required results. Manufacturers are following different configurations such as
single or double housings for go and return conductors, of different shapes
and sizes, certain portions of tank cover beyond the turret area are also being
converted from conventional mild steel composition to stainless steel. The
conventional design procedures based on estimation of induced flux and eddy
current densities, may not yield accurate results and the manufactured

transformer may be rejected if it does not pass the type tests and the
transformer may have to be redesigned. The proposed invention presents a
solution to overheating problems in transformers with tank wall and turret
interface configuration with different materials, where an earmarked a portion
of tank wall near the F turret interface joint is earmarked and be replaced
with SS material [10] as shown in Fig.4. The 3D electromagnetic field analysis
of transformer should be carried out to identify the critical regions of MS tank
wall near the interface joint, where high current concentration occurs, so as
to be earmark for replacement with SS material. The proposed modification
will increase the surface area for eddy current distribution, thereby reducing
the losses and substantially minimizing the hot spot temperatures as shown
in Fig.5. The comparison of hot spot temperatures for various configurations
of transformer tank are presented in Table.1. Studies were carried out to
ascertain the weld-ability of earmarked portion of MS tank with SS plate and
it was proposed to have Single “V” joint with metal arc welding for the tank
portion ensuring interpass temperature below 200oC. A reduction in yield
strength of flange [9] due to introduction of SS material in the critical region
was evaluated to be about 18%, and it was proposed to counter-balance this
reduction by increasing the thickness proportionately.


The proposed system offers economical design of transformer with minimum
clearances on LV side of tank. It also reduces the eddy current losses in tank
and substantially minimizes the hot spot temperatures. The proposed
invention can also be implemented on transformers in service, with have
overheating problems near the tank-turret interface configuration with
dissimilar materials.

WE CLAIM :
1. An improved tank for large generator transformer to minimize the eddy
losses and hot spot temperatures, the generator transformer
consisting of LV winding [7] with incoming and outgoing leads [5]
through a plurality of independent cylindrical SS turrets [4] mounted
on a SS sheet [6] which is an integral part of a MS tank cover [3] and
carried to the tank wall [1] by a flange joint [2], the tank is optimally
constructed with clearances based on estimation of induced field and
eddy current density on the tank, the eddy current distribution [11] in
the MS tank and SS turret being opposed to each other, a high
concentration of currents near the interface joint is generated, a
portion of tank wall near the turret interface joint is earmarked and be
replaced with SS material [10], 3D electromagnetic field analysis of
transformer should be carried out to identify the critical regions of MS
tank wall near the interface joint, where high current concentration
occurs, so as to be earmark for replacement with SS material.
2. The improved tank for large generator power transformer as claimed
in claim 1, wherein the concentration of eddy currents at the tank wall
turret interface is reduced to minimize the eddy losses in the
transformer tank s.
3. The improved tank as claimed in claim 1, wherein the tank clearances
are is reduced in addition to minimizing the tank losses and hot spot
temperatures.

4. The improved tank as claimed in claim 1, wherein the heat generation in
transformer tank, is reduced without employing external copper links or
heat sinks for dissipation of generated heat.
5. The improved tank as claimed in claim 1, wherein the overheating of tank
wall and turret interface region is reduced and wherein the wear of
gaskets due to high hot spot temperatures is prevented.