FIELD OF THE INVENTION
The invention relates to a terminal arrangement for high current carrying coil of
large Generator transformer (GT), or Station transformer (ST) or Unit
Transformer (UT) or Unit Auxiliary Transformers (UAT), which carry large current
in their HV or LV coils and connectable to Isolated Bus duct or Segregated Bus
duct.
More particularly, the invention relates to a coil termination method for high
current power transformer for connecting to isolated phase as well as segregated
phase bus ducts in power generating stations.
BACKGROUND OF THE INVENTION
Electric power is the most common form of energy which can be generated,
transmitted and utilized for different applications in Industry and Society .
Electric power is generated in a power station by harnessing the kinetic energy
of Water, Steam or Gas. The kinetic energy of the water or Steam or Gas rotates
the Turbine. The Shaft of the turbine is coupled with the shaft of an electric
machine called Electric Generator. The rotation of the turbine rotates the
generator thus producing electric power.
This electric power is generated at low voltage in the range of 11 KV to 15.75 KV
or 21 Kv Transmission of electric power at this low voltage to distant load centre
(places where power is utilized) is highly undesirable due to technical and
economical reasons. Accordingly, the generated power at low-voltage 11 kV or
15.75 kV or 21 kV is converted to a higher voltage such as 132 kV or 220 kV or
400 kV (or still higher voltages), at which the power can be

transmitted to distant load centre economically. For this purpose a power
transformer known as Generator transformer (GT) is employed.
A generator transformer (GT) is a vital equipment for transformer of power from
the generator to a load centre. The GT comprises at least two different coils for
inflow and out flow of power through it. Electrical power is generated at 11 kV
(or 15.75 kV or 21 kV) and transmitted into the transformer. Connection from
the generator is made to the LV coils of the transformer. From the LV Coil power
is transferred to the high voltage of the transformer through electro-magnetic
induction. From the high voltage coil of the transformer, the power is further
transmitted to the load centre.
To transmit large volume of electric power from power stations of capacity 200
MW (or 250 MW or 500 MW or 600 MW or 660 Mw or 800 MW), large generator
transformer (GT) of capacity 150 MVA or 250 MVA or 315 MVA or 330 MVA are
normally installed. The typical voltage rating of the generator transformer on
High Voltage side (connected to transmission line) is 132 kV or 220 kV or 400 kV
(or higher voltage) and on the Low voltage side (which is connected to
Generator), the voltage is in the tune of 11 kV or 15.75 kV or 21 kV. The current
in the each phase of High Voltage side-connected to the transmission line (in a 3
phase or 1 phase system) is in the tune of 1000 Ampere and the current in each
phase of Low voltage side connected to the generator (in a 3 phase or 1-phase
system) is in the tune of 8,000 Ampere or 10,000 ampere.
The current flowing from the generator to Low voltage side of the Transformer in
each phase is in the tune of 8,000 ampere to 10,000 ampere which is
transmitted from generator to transformer through heavy copper plates/flat
conductor of cross section of 8,000 sq. mm or 10,000 sq. mm. This heavy

copper plate/flat conductors are known as Bus-bar. So, for a 3 phase power
generating system, at least 3 Bus-bars are required which are housed in a sealed
metallic box known as Bus-duct.
Besides the Generator transformer (GT), there are other types of transformers
viz. System Transformer (ST), Unit Transformer (UT), Unit Auxiliary Transformers
(UAT) in a power generating station which also carry very high current in one or
more coils. In a generating station, the System transformer is required for back
charging from the grid. So, the LV side of the System transformer is made so as
to match the voltage level of the generator and correspondingly the associated
current is very high Bus-bars ( housed in bus-ducts ) are employed to carry this
high current from System transformer.
In a power station, there are many other utilities for running auxiliary
equipments like pumps, compressors, cooling equipments etc. For running these
equipments power is sourced through a transformer known as Unit transformer
(UT) or Unit Auxiliary transformers (UAT). The voltage of both the side of these
transformers viz. Input and Output side (conventionally referred as HV side and
LV side) are very low and the correspondingly the current is large. On many
occasions, bus-bars (housed in bus-ducts) are employed to carry these high
current.
Depending upon the system requirements, there are two different arrangement
of running the bus-bar (conductor) in the bus duct, in other words, there are two
different types of Bus-ducts. One type is known as Segregated Phase Bus Duct

(SPBD) and the other is known as Isolated Phase Bus Duct (IPBD).
In Segregated phase bus-duct (SPBD), the conductors from the three individual
phase (viz. U , V and W) of the Generators are housed in a single metallic
enclosure i.e. single bus-duct as shown in figure 1(a). In Isolated phase bus-duct
(IPBD) the conductors from the three individual phase (viz. U , V and W) are
housed in three separate metallic enclosures or bus-ducts as shown in figure
1(b). Both types of bus ducts are terminated on the tank of the Generator
transformer (or System transformer or Unit Transformer or Unit Auxiliary
transformer) and the low voltage coil of the transformer is connected t o the
bus-bars located inside the bus-duct as shown in figure 2(a) and 2(b) through
bushing. But, both the types of bus-bars have different layout, construction and
termination methods.
Since the arrangement of the bus-bar are different in the bus-ducts, the method
of connection from the bus-bar from the coil is also different. This means a
transformer manufactured for any particular type of bus-bar arrangement can
not be used for the other type of bus-bar arrangement.
As a result, the design and manufacture of the transformer has to be linked with
the determination and finalization of the particular type of arrangement of bus-
bar (and bus-duct) . This dependency causes delay in design, manufacture and
supply of the transformer to the power station causing overall delay in availability
of power to the utility.
In addition, there is a serious limitation for the interchangeability of the
transformers at site. The transformer manufactured suitable to one type of bus-
duct arrangement cannot be fitted onto the other type of bus-bar arrangement.

In the event of failure of 1 transformer at site, another transformer with different
type of bus-bar termination cannot be installed for power transmission causing
disruption in service.
All the above mentioned constraints are evenly applicable for the Station
Transformer (ST) and Unit Transformer (UT) and Unit Auxiliary Transformers
(UAT) where Bus-ducts are used for carrying large current.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a coil termination method for
high current power transformer for connecting to isolated as well as segregated
bus ducts in power generating stations.
Another object of the invention is to propose a coil termination method for high
current power transformer for connecting to isolated as well as segregated bus
bars in power generating stations, which is applicable for connecting both of
IPBD and SPBD to a generator transformer.
A still another object of the invention is to propose a coil termination method for
high current power transformer for connecting to isolated as well as segregated
bus ducts in power generating stations, which allows interchangeability of a
generator transformer (or System transformer or Unit Transformer or Unit
Auxiliary transformer) irrespective of the type of bus ducts.
Yet another object of the invention is to propose a coil termination method for
high current power transformer for connecting to isolated as well as segregated

bus ducts in power generating stations, which maximizes the transmission of
power by diverting/replacing a high current power transformer (GT or ST or UT
or UAT) from one power station to another power station in the event of break-
down or enhancement of power transmission capacity.
A further object of the invention is to propose a coil termination method for high
current power transformer for connecting to isolated as well as segregated bus
ducts in power generating stations, which enables a high current power
transformer (GT or ST or UT or UAT) for use in a bus-duct system with dissimilar
pitch (non-uniform distance between the bus bars of U, V & W phase) as well as
a bus duct system with different dissimilar pitch bus-bars of 3 phase power
Supply.
A still further object of the invention is to propose a coil termination method for
high current power transformer for connecting to isolated as well as segregated
bus ducts in power generating stations, which standardize the Low voltage coil
termination of a high current power transformer (GT or ST or UT or UAT).
CIIMMAPY OF THE INVENTION
There is provided a coil termination method for high current power transformer
for connecting to isolated as well as segregated bus ducts in power generating
stations, the bus ducts comprising copper plates carrying low voltage current
from a generator to the low voltage (LV) coils of the generator transformer
which upon conversion to high voltage through electro magnetic induction is
transferred to high-voltage (HV) side of the transformer connected to the
transmission lines of the power station. The tank of the transformer having cut-

outs (S & I); allowing high current carrying bushings with bus ducts (1,2,3,4,5)
to be electrically connected to corresponding transformer coils of different
current carrying phases (W,V & U) . The Segregated Phase Bus Duct (SPBD)
having a single metallic enclosure housing all the conductors from the individual
there phases, wherein the Isolated Phase Bus Duct (IPBD) having three separate
and independent metallic enclosures each housing conductor from each of the
three phases (W,V & U). The method comprising the steps of extending the coil
end connections (1,4,3,5) inside from the transformer tank; providing means for
connecting the said extended coil-ends with the respective bus-bars ; connecting
the bushing (2) and the bushing termination at cut-out end(s) for SPBD type bus
duct, relocating the bushings of W-phase (1) and U-phase (3) on the tank cover;
and connecting the bushing terminations to an isolated end (I) of the three
phases.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1(a) Shows the arrangement of the conductor inside the Segregated
Phase Bus-Duct (SBPD).
Figure 1(b) Shows the arrangement of conductor inside the Isolated phase Bus-
Duct (IPBD).
Figure 2(a) Shows the arrangement of the transformer termination suitable for
segregated Phase Bus Duct (SPBD)
Figure 2(b) Shows the arrangement of the transformer termination suitable for
Isolated Phase Bus-Duct (PBPD)
Figure 3(a) Show the connection of the coil ends to the bushings mounted 0 the
transformer tank for bus-bars in a Segregated Phase Bus-Duct (SPBD)
Figure 3(b) Show the connection of the coil ends to the bushings mounted 0 the
transformer tank for bus-bars in a Isolated Phase Bus Duct (IPBD).

Figure 4 Modified terminal arrangement of the coil termination of the generator
transformer for connecting to bus-bars of Segregated as well as Isolated phase
bus-duct.
Figure 5 Tank cover showing WS" and "1" cut-outs to suit the mounting bushing
and terminations of bus-bars of Segregated as well as Isolated phase bus-duct.
DETAILED DESCRIPTION HF THE INVENTION
Electrical current produced in a Generator is normally in the magnitude of 8,000
to 10,000 ampere. Large copper plates/Flats are required for carrying this
current from the generator up-to the Generator transformer. This large copper
plates/Flats are called bus-bar, which is housed in a sealed metallic box known
as Bus-duct. There are 2 types of Bus duct viz. Segregated Phase Bus-Duct
(SBPD) and Isolated Phase bus duct (IPBD) . In a Segregated Phase Bus-Duct
(SBPD) the conductors from the 3 individual phase are housed in a single
metallic enclosure as shown in figure 1(a). In an isolated phase bus duct (IPBD),
the conductors from 3 individual phases from Generators are housed in 3
separate metallic enclosures as shown in figure 1(b). Both type of the bus-duct
has different construction, layout connection and termination on the transformer
tank; as shown in figures 2(a) and 2(b). The bus-bar inside the bus duct is
joined electrically to a bushing termination coming out from the transformer.
Terminating lead of the Low voltage (LV) coil of the generator transformer is
connected to the LV bushings on the tank cover. Figure 3(a) and 3(b) show the
connection of the coil ends to the bushings mounted on the transformer tank.

Since the bus-bar are required to be connected to the "coil ends" of the
transformer, the "coil ends" of the transformer should be made matching with
the termination of the bus-bar-running between the generator and the
Transformer . As a result each type of bus duct/bus-bar will a different type of
arrangement of the "coil ends" of the transformer, which causes serious
limitation on interchangeability and replacement of the units .
To resolve the problem, the termination arrangement of the "coil end" is
modified as shown in figure 4.
The suggested method as shown in figure 4. In figure 4, the coil-end connection
inside the tank is extended on both the sides. Provision for connection from the
bus-bar is made at both the ends of the (4). For connecting to the bus-bar in a
segregated phase bus-duct (SPBD), the bushing located on the tank cover and
the bushing termination is connected to a segregated end (S).
Whenever the requirement arises for connecting to an IPBD, the bushing of "W"
phase (1) and that of "U" phase (3) are re-located on the tank cover and the
bushing terminations can be connected to an isolated end (I)
With this arrangement, the coil of the high current power transformer (GT or ST
or UT or UAT) can be connected conveniently to suit the bus-bars of Segregated
Phase Bus-Duct (SPBD) or Isolated Phase Bus-Duct (IPBD) as per requirement.
Suitable cut-outs are made on the tank cover to make provision for locating the
bushings for Segregated bus-duct or Isolated bus-duct as shown in figure 5. In
the event of use of SPBD, the particular "S" cut-out on the tank cover is used
and the "I" cut-outs are closed. In the event of use of IPBD, the particular "I"
cut-outs on the tank cover are used and the "S" cut-out is closed.

Thus by employing the suggested method, a high current power transformer (GT
or ST or UT or UAT) can be made suitable for connecting to Segregated phase
bus-duct (SPBD) or Isolated phase bus-duct (IPBD) for transmission of electric
power. Hence the arrangement is a Universal arrangement.
This invention can also be extended for use with Segregated or Isolated bus-duct
with unequal distance between the bus-duct of different phases (dis-similar
pitch). A transformer working with 1 set of bus-duct with a specified distance
between the bus-duct of the 3 phases (U-1,V-1,W-1) can be made suitable for
another power station with different set of bus duct with different distance
between the bus bars of the 3 phases (U-2,V-2,W-2).

WE CLAIM :
1. A coil termination method for high current power transformer for
connecting to isolated as well as segregated bus ducts in power
generating stations, the bus ducts comprising copper plates or cables
carrying low voltage current from a generator to the low voltage (LV) coils
of the generator transformer which upon conversion to high voltage
through electro magnetic induction is transferred to high-voltage (HV) side
of the transformer connected to the transmission lines of the power
station, the tank of the transformer having cut-outs (S-I) allowing high
current carrying bushings with bus ducts (1,2,3,4,5) to be electrically
connected to corresponding transformer coils of different current phases
(W,V,U), the segregated phase bus duct (SPBD) having a single metallic
enclosure housing all the conductors from the individual there phases,
wherein the isolated phase bus conductor (IPBD) having three metallic
enclosures each housing conductor from each of the three phases
(W,V,U), the method comprising the steps of :-
extending the coil end connections (1,4,3,5) inside from the transformer
tank;
providing means for connecting the said extended coil-ends with the
respective bus-bars;
connecting the bushing (2) and the bushing termination at cut-out end(s)
for SPBD type bus duct,

- relocating the bushings of W-phase (1) and U-phase (3) on the tank
cover; and
- connecting the bushing terminations to an isolated end (I) of the three
phases.