FIELD OF INVENTION
The present invention relates to a method of in-situ replacement of water cooled
stator winding bars of 500 MW or higher turbogenerator. The present invention
emanates from the field of repair technology involving replacement of large size
hydrogen/water cooled (THDF type) Turbo generator stator winding at site. The present
invention in particular relates to a method of in-situ replacement of water cooled stator
winding bars corresponding to hydrogen/water cooled (THDF type) Turbo generators
having in it directly water cooled stator winding. In such Generators stator winding bar's
overhang profile is completely covered with epoxy and insulating clamping plates. More
particularly, the invention propose a methodology for replacement of Stator winding bar
in THDF type Generators at site itself.
Hydrogen/water cooled (THDF type) Turbo generator used in coal or gas or
nuclear fuel based thermal power plants shall hereinafter referred as 'generator' and
also, stator windings used in such generators shall hereinafter referred as 'winding'
unless stated otherwise.
Stator winding bars used in such generators shall hereinafter referred as ‘bars’
unless stated otherwise.

BACKGROUND OF THE INVENTION
Large size electrical machine, Generators in particular, used in coal or nuclear (as
a fuel) based power plants are operational worldwide and in India, particularly, since
mid-eighties. Large rotating electrical machines such as generators for production of
electrical power have a rotor and a stator which concentrically surrounds the rotor. This
stator is normally in the form of a laminated core, formed from individual laminates. For
high powers, the stator winding that is accommodated in the stator is formed from
individual winding bars which are inserted into corresponding slots in the laminated
core of the stator and are secured in the slots by means of slot closure wedges.
Winding, an important part of generator, are made of large number of insulated
rectangular copper conductors of small section transposed in slot portion to reduce
eddy and circulating currents. The overhang part is clamped between support ring and
clamping plates. The bars are coated with semi-conducting varnish to avoid corona and
flash over. Rotating electric machines such as electric motors or generators generally
have an annular stator defining a stator bore that houses a rotor.
The stator includes a plurality of laminations insulated from one another and
defining axial slots. Within the axial slots bars are housed (usually one or two bars but
also more bars can be housed in each slot); the bars are connected at their ends to
define stator windings. The bars include a conductor and insulation around it. The

conductor is usually defined by a plurality of transposed copper strands, each insulated
from the others. The insulation is usually defined by a mica tape impregnated with a
resin. The bars are firmly fixed in the slots so as to reduce the vibrations, thus
preventing insulation damages and electric discharges within the slots. The dielectric
performances of the insulation deteriorate during operation, so that the electric
machine could have a reduction of the expected lifetime. This ageing depends on the
design and operating conditions, as well as on the environmental conditions. For
example, the high dynamic forces acting during the operation at locations of resonance
of the mechanical stator end-winding system of stator end-windings and the problems
resulting therefrom, specifically in the case of large electrical machines, specifically of
turbo-generators with a directly water-cooled stator winding, are generally known.
During operation, operational vibrations, specifically in the case of resonance, lead to
high dynamic stresses of the end-windings, specifically in case of turbo-generators,
which may lead to a loosening of the entire stator winding assembly. Abrasions of
insulation and damages at the stator and the results thereof lead in turn to long shut-
down times for a replacement of the stators windings. Such stator end-winding
systems, also if they have been improved regarding dynamic stresses, must be
subjected from time to time to maintenance work whereby the generally complicated
and, regarding the maintenance, difficult structural designs of stator end-winding
systems lead to corresponding shut-down times.

For this reasons, in some cases refurbishment is needed. Refurbishment can
include stator and/or rotor repair or a full rewind (e.g. replacement of the stator bars to
provide a new winding) and can lead to a general lifetime extension of the electric
machine. In order to carry out the refurbishment or rewind, it is often needed to
remove the bars. In order to remove the bars, the bar ends that protrude from the
stator core (called end windings) are removed and replaced. In case of complete failure
due to any of the probable causes, in situ replacement of such winding bars is not
possible. Such stator winding replacement job, to have a variety of qualitative and
reliability enhancements, is preferred to be performed at manufacturing facility as the
stator can be rotated to achieve any position for work but at site stator is fixed at
foundation and it cannot be rotated. However, transporting the stator to manufacturing
works not only leads to forced outage but also results in added lead time.
So it was decided to attempt for replacement of bar at site itself.
OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose a method of in-situ
replacement of water cooled stator winding bars of 500 MW or higher turbogenerator,
which is able to eliminate the disadvantages of Prior Art.
Another object of the invention is to propose a method of in-situ replacement of
water cooled stator winding bars of 500 MW or higher turbogenerator, which is capable

of providing a retrofitting process to disassemble and then to assemble stator winding
at site itself.
A further object of the invention is to propose a method of in-situ replacement of
water cooled stator winding bars of 500 MW or higher turbogenerator, which enables
the retrofitting process to complete the job in less than half the time required in prior
Art.
SUMMARY OF THE INVENTION
Accordingly there is provided an in-situ process for replacement of
turbogenerator stator winding bars (3), in particular stator overhang winding bars (2),
involving partial dismantling of the generator (5), comprising the steps of:
- Identifying, isolating and marking the top (6) and dismantling covering on the
bars to be removed;
- Removing deposits from bar overhang (2) and pulling out the bars (6) from slot
(4) of the stator core (13);
- Cleaning the slot (4) for bits of insulation etc.;
- Visually examining the rest of the stator (15) for any damage etc.;
- Carrying out test of the remaining winding to ascertain its healthiness;
- Visually inspecting spare bars to be used for ensuring healthiness of insulation,
corona-protection coatings and carrying out IR and HV (ac) test;

- Blue-matching the contact faces of fresh bars;
- Fixing resin rope pieces (37) from where the top bars (6) were removed;
- Manually inserting and locking the new bar and carrying out tests for the winding
bar end connections and tests for electrical parameters of the stator winding;
- Filling the space among the adjacent bars in overhang portion with epoxy and
performing curing;
- Re-assembling insulating stud, top clamping plate (43), coolant connections (21,
22, 23, 24), electrical end connection (38, 39) and locking insulating bolts (38) of
locking shoes (31);
- Carrying out final electrical parameter testing of each phase of the entire
winding;
- Carrying out leak tightness test using pressurized fluid.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 is a perspective end illustration of an electric machine, particularly a
turbogenerator;
Figure 2 is a partial exploded illustration of a portion of the electric machine stator
shown in Figure 1, according to an aspect of the present invention;

Figure 3 illustrates a cross-sectional view of a stator slot according to an aspect of the
invention.
Figure 4 Schematically illustrates a stator end winding, as is known for an electrical
generator;
Figure 5 Sketch illustrates Stator winding bar overhang profile covered with Epoxy &
Top Clamping plate as well as Electrical end connections, Support Ring, Insulation Box,
Inlet & outlet water supply hoses.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
Figure 1 is a perspective end illustration of an electric machine, particularly a
turbogenerator (5) that includes a core (15) having a plurality of stator slots (4) to
accommodate a winding to generate a magnetic flux, according to an aspect of the
present invention; a partial, exploded view of the stator is illustrated by the reference
numeral (13) that is described in detail with reference to Figure 2;
Figure 2 is a partial exploded illustration (13) of a portion of the electric machine stator
shown in Figure 1, according to an aspect of the present invention;
Figure 3 illustrates a cross-sectional view of a stator slot (4) according to an aspect of
the invention. The stator core (15) is part of a dynamoelectric machine or electric

machine, such as a motor or a generator. The stator core (15) includes a plurality of
radially extending stator slots (4) for housing windings or stator bars (6 and 7). As will
be appreciated, the stator core (15) extends around a central axis and the stator slots
(4), as well as the stator bars (6 and 7), extend longitudinally parallel to that axis and in
a generally radially inward direction. In the illustrated form, side ripple springs (16)
maintain the stator bars (6, 7) firmly against the opposite sides of the stator slot (4).
Radial space in the stator slots (4) may be taken up by intermediate radial fillers (8, 9).
A top retention assembly (30) includes stator wedges (12) that extend longitudinally
along a radially inner portion of the stator slots (4) with their lateral edges residing in
shaped grooves or dovetails (19) formed in the stator slots (4), and a top ripple spring
(10) positioned at least partially within stator slot (4) such that the ripple spring (10) is
adjacent to at least one slot filler (18). The top ripple spring (10) is then secured in
stator slot (4) using a plurality of stator wedge slides (11) and stator wedges (12);
Figure 4 Schematically illustrates a stator end winding, as is known for an electrical
generator, comprising a multiplicity of winding bars (3), which are electrically connected
to one another. Each of the winding bars (3) in this case has respective bar ends (2);
Figure 5 Sketch illustrates Stator winding bar (6, 7) overhang profile covered with
Epoxy & Top Clamping plate (33) as well as Electrical end connections (38 & 39),

Support Ring (25), Insulation Box (35), Inlet & outlet water supply hoses (21, 22, 23,
24).
The method of replacement of water cooled stator winding bars at site itself in
THDF type of turbogenerator (≥ 500 MW sets, voltage rating ≥ 21 kV) is as per below:
1. Isolating the bars 6 by first removing the water supply hoses viz. inlet and outlet
hoses (21, 22 & 23, 24) and then the end clamp connections viz. (38, 39). Marking the
bar numbers in the corresponding disassembled connecting parts (21, 22, 23, 24, 38 &
39) to facilitate of re-assembly. (Refer Figure 5)
2. Removing top retention assembly (30) consisting of slot wedges (12) that extend
longitudinally along a radially inner portion of the stator slots (4) with their lateral edges
residing in shaped grooves or dovetails (19) formed in the stator slots (4), packers (18),
ripple springs under wedge or top ripple spring (10) secured in slot (4) using a plurality
of stator wedge slides (11) and stator wedges (12), slot filler (8, 9) and side ripple
springs (16). Marking the slot numbers in the corresponding disassembled connecting
parts (8, 9, 11 & 12) for the purpose of re-assembly. (Refer Figure. 3)
3. Dismantling the locking shoes (31) and nuts (32) of the top clamping plates (33)
used for covering the bars (6) to be removed.

4. Dismantling and lifting the top clamping plates (33) of region from where the
bars are to be removed and removing corresponding spacer segments (34). Also,
removing the insulation boxes (35) corresponding to phase change over bar.
5. Chipping off the epoxy along the sides of the bar overhang (3) on both side
using hammer and chisel while taking care not to damage the insulation of the
neighboring bars.
6. Vacuum cleaning intermittently to remove the chipped off pieces of the
epoxy/insulation.
7. Pulling out the bars (6) from slot (4) and then taken out from the stator (5).
8. Cleaning the slot (4) for bits of insulation etc.
9. Visually examining the rest of the stator (5), core (15) for any damage etc. and
then cleaning it thoroughly of bits of insulation, carbon, grease, dirt using vacuum
cleaner etc.
10. Carrying out Insulation Resistance (IR) and High voltage (HV) (ac) test of the
remaining winding to ascertain its healthiness.
11. Visually inspecting spare winding bars to be used for ensuring healthiness of
insulation, corona-protection coatings. Then carrying out IR and HV (ac) test.

12. Blue-matching the contact faces of new bars to be laid.
13. Fixing the resin rope pieces (37) (cut as per requirement) on the intermediate
plates (36) using tapes where the top bars (6) were removed.
14. Manually bringing the fresh bars (6) inside the core (15) and inserting it into the
respective slots (4) and locking them in slots (4) using clamps and at ends via end
connections (38, 39), using new tension discs and fasteners. Also fixed spacer
segments (34) and insulating selves at both the ends.
15. Carrying out Millivolt drop test of winding bar end connections.
16. Carrying out H.V. (ac) test of the affected phase prior to epoxy filling.
17. Assemble the packers (9 & 11), ripple springs (10) and slot wedges (12).
Carrying out wedge tightness test before locking the end wedge.
18. Filling the space between adjacent bars in overhang with epoxy. For increasing
the flow ability of the epoxy, the same can be heated for few minutes in electric heater.
19. After curing of the epoxy, insulating paint is to be done as well as insulating
studs to be assembled in the plurality of threaded holes of support ring (25).
20. Assembling the removed top clamping plate (33) using new resin ropes (37).

21. Tightening the top clamping plates (33) through nut (32) and further locking the
nut using locking shoe (31) arrangement. Further, locking insulating bolts (38) of these
locking shoes (31).
22. Then assembled the removed water supply hoses (21, 22, 23 & 24) using new
‘O’ rings.
23. Then final IR and HV (ac) of each phase of the entire winding was successfully
carried out.
24. Carrying out leak tightness test using primary water pressured through nitrogen.
25. Handing over the stator after cleaning and painting.
Through the method as described hereinabove, it is apparent that the proposed
method of in situ replacement of stator bars corresponding to large size
turbogenerators will provide an extremely critical method to replace stator bars in
significantly less time and completely eliminate the logistics cost as per prior art.
Since, the foregoing is considered as illustrative only of the principles of the
invention. Further, since numerous modifications and changes will readily occur by
those skilled in the art, it is not desired to limit the invention to the exact construction

and operation shown and described, and accordingly all suitable modifications and
equivalents may be resorted to, falling within the scope of the invention as claimed.

WE CLAIM
1. A method of in-situ replacement of water cooled stator winding bars of 500 MW
or higher turbogenerator comprising the steps of;
isolating the bars (6) by removing inlet and outlet water supply hoses (21, 22,
23, 24), end clamp connection (38, 39), stator wedges (12), stator wedge slides (11),
slot filler (8, 9);
marking the bar numbers and slot numbers on the corresponding disassembled
connecting parts to facilitate re-assembly;
removing top clamping plates (33) covering the bar (6);
chipping off epoxy along the sides of the bar overhang (3);
pulling out faulty bars (6) from slot (4) and then from stator (5);
cleaning the slot (4) for bits of insulation;
visually examining rest of stator (5), core (15) for any damage;
carrying out Insulation Resistance (IR) and High Voltage (HV) test of the
remaining winding to ascertain healthiness;
visually inspecting spare winding bars to ensure healthiness of insulation, corona
protection coatings and carrying out IR and HV test;
blue matching the contact faces of replaced bars;
fixing resin rope pieces (37) on intermediate plates (36) with tapes;

manually bringing fresh bars (6) inside the core (15) and inserting into respective
slots and locking them;
carrying out Millivolt drop test of winding bar end connections;
carrying out H.V. test of affected phase prior to epoxy filling;
filling the space between adjacent bars in overhang with epoxy;
curing the epoxy and carrying insulating paint;
wherein, the dismantled top clamping plate (33) with new resin ropes (37),
water supply hoses (21, 22, 23 & 24) with new ‘O’ rings, locking nut (32) along with
locking shoe (31) are assembled, when final electrical parameter testing of each phase
of the entire winding followed by leak tightness test using pressurized fluid are carried
out, when the stator is handed over after cleaning and painting.
2. The method as claimed in claim 1, wherein final electrical parameter testing
comprising IR and HV (ac) test of each phase of the entire winding.
3. The method as claimed in claim 1, wherein before filling the space between
adjacent bars in overhang, epoxy is heated for few minutes in electric heater to
enhance its flow ability.