FIELD OF THE INVENTION
The present invention generally relates to a novel combination of shields and
shunts in tandem to overcome bell tank bottom flange overheating problem in
large power transformers, and a method thereof to minimize the eddy losses and
hot spot temperatures in the transformer tank.
BACKGROUND OF THE INVENTION
In higher rating power transformers, above 100MVA, Bell shape tanks are
commonly used, owing to large weight of core and windings which are supported
on the flat portion of bottom tank. They are also preferred, for ease of winding
terminal gear assembly and maintenance at site. The bottom tank is generally
raised to the bottom yoke level and top half of tank are bolted to it through
flanges, which are isolated by gasket while electrically connected through bolts.
The bell shaped tank models have lower clearances from the winding bottom
ends and are subjected to high leakage magnetic field, which reaches the bottom
tank directly and induces large eddy currents. The eddy current in the bottom
tank try to link with the current induced in top tank through the flange joint
leading to overheating of bolts. This phenomena is more pronounced by LV (Low
Voltage) TG (Terminal Gear) in higher rating transformers which employ four
limb core design to meet the short circuit test requirements. The current and
resulting short circuit forces are minimized by splitting the windings into two
parallel coils and placing on the two centre limbs. The LV coils on either limbs
are connected in parallel by the high current carrying LV TG, which is routed at
the bottom portion of the winding. The location of bottom tank flange joint is
near to the winding bottom end, which is a very critical zone as it receives very

high leakage flux from the windings and also LV TG. Very high eddy currents are
induced in the bell tank which tend to expand and loop across the top and
bottom tank flange joint. The electrical contact between the top and bottom tank
through bolts is unlikely to be uniform, leading to concentration of currents in
certain bolts which have better electrical contact, resulting in overheating due to
non-availability of oil for cooling. There are instances where the vulnerable bolts
got melted due to continuous overheating.
The prior art consisted of arrangements such as tank isolation or copper links. In
tank isolation method, insulating sleeve is arranged over the fastening bolts
which isolates the bottom and top tank flanges and thus prevents overheating of
bolts as the eddy currents are constrained from forming the loops across the
flange joint. However, it was observed that over a period of time the insulation
sleeves give away under bolting pressure leading to bolt overheating as the
whole loop current flows through the single point of contact. The copper links
are arranged across the flange bolts and they provide low resistance path to the
eddy currents, however they do not minimize the losses and overheating of tank
rim, which in the long run will wear away the gasket and causes failures due to
oil leakages. With the new stringent customer requirement of maximum
allowable hot spot temperatures below 100oC, there is an acute need for
development of adequate solution to this long pending problem.
The economics in manufacture of large power transformers demand optimal
design of transformers with minimum losses and hot spot temperatures. The
authors have adopted a novel incorporation of shield and shunt arrangement in
tandem, which minimizes the induction of magnetic field in the critical bottom
tank flange region, and subsequent minimization of eddy losses and hot spots
temperatures.

Large Power Transformers, K Karsai, D. Kerenyi, and L. Kiss, Chapter
4.4.7. Discusses about eddy current losses excited by the field of high current
carrying conductors. The application of magnetic shunts and copper/aluminium
shields under different combination is explained. However, the usage of copper
screens to compliment the arrangement of shunts to shield the tank from
leakage flux is not envisaged. The combined effect of leakage flux from windings
and LV TG has significant impact on tank overheating problems in large power
transformers, which may not have been envisioned at that time.
Magnetic shunt, magnetic shunt arrangement and power device
US 20110298575 A1, WO 2010094671 A1: Propose 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 flux collected by proposed shunt model
leaves the bottom flux collectors enters the tank and do not address the above
defined problem. The additional flux due to LV lead routing is not envisaged.
Reduction of Stray Losses in Flange–Bolt Regions of Large Power
Transformer Tanks, IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS,
VOL. 61, NO. 8, AUGUST 2014: The design of the copper links was carried out
using a criterion of current density. It is recommended to place copper links in
the identified hot spot zones to minimize flange overheating problems. The
results were verified by measurements and simulations. The proposed solution
bypasses the eddy currents flowing through the bolts, but do not minimize the

induction of eddy current densities. This arrangement does not minimize the hot
spot temperatures and over a period of time the gasket gets worn out. It was
also reported that over a period of time overheating of copper links was
observed.
Transformer Engineering Design and Practice, 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 bell tank from
assembly point of view.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved arrangement of
shield and shunts in tandem to minimize the overheating of bell tank bottom
flange joint in large power transformers. The invention is more pertinent to
single phase generator transformers with four limb core design and LV leads
taken out from the winding bottom end.
Another object of the invention is to propose an arrangement of copper shields,
of appropriate dimensions, in between the top tank wall and magnetic shunts, in
isolation from both. The proposed wall shunt arrangement shifts the epicentre of
flux which enters the tank from the wall shunt bottom end, while the copper
shield generates the required anti flux which neutralizes the incident normal flux
on to the tank.
Yet another object of the invention is to propose a system for shunting and
shielding of stray flux from reaching the tank, especially the additional flux

generated by high current carrying LV bottom leads. This improvement shall
minimize the hot spot temperatures across the bottom tank flange joint region,
thereby substantially improving the life of gasket.
Another object of the invention is to propose a system for minimization of eddy
losses in transformer tanks by reducing the concentration of eddy currents at the
critical bottom tank flange joint region.
Yet another object of the invention is to propose a system which enables further
reduction of tank clearances, especially with reference to LV lead routing, while
minimizing the tank losses and hot spot temperatures.
A further object of the invention to propose a system which minimizes the heat
generation in transformer tank, rather than using external copper links, tank
isolation using isolated bolts or heat sink arrangement for dissipation of
generated heat.
A still further object of the invention is to propose a system for prevention of
power transformer failures due to overheating of tank flange 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
Failure of generator transformers (GT) above 200 MVA due to overheating and
melting of bolts across the bottom flange joint in bell type tanks are widely
reported. Detailed investigations were carried out and it was observed that the

above problem is pronounced in case of GT’s with four limb core designs, which are
commonly adopted for short circuit test requirements to minimize the forces,
wherein additional flux due to high current carrying LV TG are induced along the
critical bottom tank flange joint region. The conventional design methods to
constrain the stray flux by arrangement of magnetic shunts on tank wall is
observed to be inadequate, while other solutions such as tank isolation and
arrangement of external copper links were observed to be ineffective in long run.
The leakage flux from the windings and LV TG is collected by the wall shunts
arranged on the top tank wall. The collected flux tends to enter the tank wall
region below the shunt, which happens to be the low permeable path, leading to
high concentration of flux at the wall shunt ends. This flux in turn sets up high
eddy currents in this area, which try to expand and link with the currents in
bottom tank via flange joint, causing localized losses and hot spots. Modelling of
LV leads and comprehending the effect of superimposed leakage flux from
winding and LV TG, on the transformer tank, is quite a challenge. A detailed
analysis of induced eddy current distribution on the transformer bottom and top
tank was carried out with reference to location of wall shunts, yoke shunts,
flange joint and LV lead routing. Different shunting and shielding techniques
were incorporated into the 3D transformer model and parametric analysis was
carried out to evaluate the losses and hot spot temperatures.
An effective solution is developed, with a specific combination of shunts and
shields, which shifts the epicenter of eddy current loops and also shields the
critical tank regions from winding leakage flux and LV TG flux. The model is
being incorporated in the manufacture of 265 MVA GT. With the incorporation of
proposed development, it is envisaged that there would be reduction of load
losses to the tune of 10%-15% and the maximum hot spot temperature on tank

will be limited to 90oC. The invention reduces the capitalization cost and also
improves the service life of equipment.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Figure 1 - Thermography scan of 400kV 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 large power rating single phase generator transformers with short circuit
test requirements are designed as shown in Fig.1 with four limb core
construction (1) with windings split into two halves (2) and placed on central
limbs. This design is generally adopted to minimize short circuit forces as the
current is split between the parallel winding sections. The bell tank (3) model
offers better withstand strength to the dynamic short circuit forces, as they
provide additional mechanical support to the winding structure. However,
overheating of bottom flange joint (4) and fastening bolts (5) for the above
models has been an overdue problem.
Complete 3D coupled electromagnetic and thermal analysis was carried out on
the above 265MVA GT model and it was observed that the leakage flux collected

by the wall shunts (6) leaves at the bottom end and enters the tank wall region
below the shunt, which happens to be the low permeable path, leading to high
concentration of flux at the wall shunt ends, which gets further enhanced by the
additional leakage flux from the LV winding bottom TG (7, 8), which are routed
along the tank wall and terminated in the turrets (9). This flux in turn sets up
high eddy currents in the limited tank region below the wall shunt as shown in
Fig.2a. The induced currents try to expand and link with the eddy currents in
bottom tank via flange joint, causing localized losses and hot spots as shown in
Fig.2b. The maximum hot spot temperature was estimated as 150, though the
hot spot is limited to a small region near the flange joint it effects the gasket life
and dielectric strength of oil in the long run. Hence, from service life point of
view it is very important to limit the hot spot temperatures rather than develop
mechanisms to dissipate them. The existing solutions are inadequate in limiting
the hot spot temperatures, while the widely adopted arrangement of copper links
to shunt the current looping through bolts, as shown in Fig.3a, were found to be
ineffective as the high temperature in the flange region, as shown in Fig.3b,
wears away the gasket in the long run.
The modelling of LV leads and comprehending the effect of superimposed
leakage flux from winding and LV TG, on the transformer tank, is quite a
challenge. The present inventors carried-out extensive analysis on transformer
tank models, to study the effect of different configurations of copper shields and
magnetic shunts around the bell tank bottom flange joint, on eddy current
distribution and hot spot temperatures. From the investigations it was observed
that lead routing has a significant effect on the eddy current density distribution
in the critical tank region below the wall shunts. Therefore, it is very important to
minimize the induction of magnetic field in this region, accordingly a novel
arrangement of copper shields behind the lower portion of modified top tank wall
shunts is developed as shown in Fig.4. Unlike the conventional practise of

arranging wall shunts with minimum distance from tank flange joint, the
proposed invention requires placement of wall shunts away from the flange joint
by at appropriate distance (6) and arrangement of copper shield of proportionate
dimensions with its central axis in line with the shunt bottom edge, and isolated
from both tank wall and shunts (10). The leads routed along the tank wall height
induce eddy currents in the tank wall, which add up to the currents in the critical
zone near the bottom flange, accordingly wall shunt pairs were arranged behind
the lead covering an area of at least 800mm with minimum gap between them
(6). The eddy currents in bottom tank are minimized by replacing yoke shunts by
copper shields, which are welded on bottom tank covering the active flux zone
(11). The developed arrangement minimizes flux jumping from wall shunt
bottom end to top tank wall and also the eddy currents on bottom tank.
Coupled electromagnetic and thermal analysis was carried out on the
transformer model with copper shields on top and bottom tanks with modified
wall shunt arrangement. The developed arrangement resulted in reduction of
stray losses by about 10%-15% and the maximum hot spot temperatures on the
tank surface were limited to 90oC and the comparison between initial design and
proposed design are presented in Table.1. The simulation results are matching
the experimental results. It is very important to analyse the induced current
loops in both bottom and top tanks and also the effect of lead routing before
finalizing the design. The proposed solution with a specific combination of shunts
and shields, shifts the epicentre of induced current loops away from the flange
joint, and provides larger area for eddy current to generate opposing flux and
effectively shield the critical tank regions from winding leakage flux and LV TG
flux. The development offers an efficient solution to the overheating problem in
bell tank lower flange joint in large power transformers.

WE CLAIM:
1) An arrangement of copper shields (10, 11) and magnetic shunts (6) above
the bottom flange joint of high voltage Bell type transformer tank (3),
characterized by minimization of flux jumping from wall shunt bottom end
to tank wall, resulting minimization of eddy current loss of large power
transformer.
2) The arrangement as claimed in claim 1, wherein, the wall shunts (6)
unlike the conventional practice, are placed away from the flange joint
and should also cover an area of minimum 800mm behind the lead with a
minimum gap between the shunt pairs, resulting minimization of magnetic
field effect and eddy current loss.
3) The arrangement as claimed in claim 1, is further supplemented by
replacing, yoke shunts by copper shields (10, 11), welded on bottom tank
covering active flux zone. This arrangement further reduces the induction
of eddy currents in bottom tank.