FIELD OF INVENTION
The invention relates to the field of stator winding of electrical machines in
general and to the field of Turbo generator in particular.
PRIOR ART
The individual bars of a regular large capacity synchronous turbo generator
stator winding is made with several numbers of all insulated copper strips with
glass / paper / fibre insulation. The bars are made first with shaping several
conductor strips into rectangular stack and bending them in to diamond shaped
coils. The insulation of the stack formation consist of inter strip, half and filler
material, which provide the required themoset resin to bind the strips together.

Hence, the bars are first thermally cured at 155 to 165° C for stipulated hours to
get thermally cured to the required shape.
OBJECTS OF THE INVENTION
The following are the main drawbacks of the Prior Art:
1. Winding losses could not be minimized.
2. Control over skin effect loss was less.
3. Lower efficiency with bigger size of the machine.
4. Chances of inter strip short are more.
To overcome these above drawbacks the following inventive measures were
undertaken.
1. Introduction of higher numbers of copper strips in Roebel bar design of
stator winding.
2. Introduction of transposition insulation with very high tearing strength.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention can now be described in detail with the help of the figures of the
accompanying drawings in which,
Figure 1 shows the cross section of stator bar (Roebel bars)
with multilayers insulation.
Figure 2 shows the transposition of conductors (Stator bars)
Figure 3 shows the schematic diagram of stator
winding (27 MW Turbo Generator)
Figure 4 shows the transposition details of Roebel bar
in 27 MW Turbo Generator.
Transposition of conductors (stator bar) as shown in Figure 2,of Roebel bar with
bending of plaited copper strips from one half to other and then travel from top
to bottom layer vertically. 360° transposition in Roebel bar means all the strips
travel from top to bottom and then up to the original top level in the given active

straight part of the generator. Figure 4, gives details of transposition i.e. the
factors guiding transposition like transposition pitch (p), conductor width (w),
transposition angle (a) and transposition length (LT). The value of the ratio
transposition pitch to conductor width and transposition angle, play important
role in determining the frictional force in inter strip friction. To avoid the peeling
and strip insulation, transposition insulation is provided as shown in Figure 1.
The multilayer insulation as shown in Figure 1, consisting of polyamide - polyster
- polyamide as reinforcement to this insulation has higher tearing strength due
to presence of polyester, which is sandwiched between two layers of polyamide
paper and this being the inventive step, provided the same in stator winding for
improved insulation. Figure 3, shows the schematic diagram of such stator
winding as described above.
During process of winding assembly, the coils are placed / lowered into the
stator slots as per the winding scheme (Figure 3). Subsequently, the bars are
electrically connected by soldering / brazing process to form phase graphs.

DESCRIPTION OF THE INVENTION
In the proposed method, a Roebel bar was designed with necessary
transposition insulation with very high tearing strength for effective inter strip
insulation. The individual bar has several insulated copper strips which are
Roebel transposed for 360 degrees. The transposition pitch to copper strip ratio
and angle of transposition play vital role in deciding the stresses on the
transposition insulation. Lesser is ratio and higher is the angle, higher will be the
stress which result in more inter strip / half short. With the introduction of the
new transposition insulation, the designer has an option to choose lower ratio by
introducing more conductors in given cross section of the bar. This also ensures
lower losses in the bars of the winding.
The new design of insulation scheme with introduction of multilayer (Nomex)
Polyamide-Polyster-Polyamide (Nomex), polyamide (commonly known as Nomex
- trade name) as transposition insulation makes the bars free from all shorts and
maintains the integrity of insulation against bar bouncing forces and resulting
vibration during machine operation. The insulation reinforcement can
accommodate lesser transposition pitch, in turn more number of copper strips in

a given cross section of the bar and lesser core length of the machine. This
results in lowering the skin effect losses of the machine and improved efficiency.
The proposed insulation finally results in reducing machine length and makes the
generator optimal in size.
The invention relates to the following topics: -
A. Basic principle (General Requirement)
B. Technical Data, Scheme of Generator Stator winding
C. Main components of Stator Bars
D. Roebel bars transposition, transposition pitch, pitch to strip width
ratio
E. Function
F. Advantages
The above topics are described in details individually as under: -

A. General Requirements of Stator Winding Bars: (Basic principle)
The Roebel bar is a single conductor of generator stator winding consisting of
several numbers of insulated copper strips stacked one over other with
suitable transposition. The transposition of insulated strips can be in multiple
of 180 degree, which means traverse of individual strip from top of the bar to
the bottom. This arrangement is similar to bending / twisting of bunch of
plaits of hair strands. Transposition of 360 degree means traverse of each
strip of copper from top to bottom and back to top in the given active /
straight length of the machine.
The Roebel Bar design of stator winding fulfills the following:
- Lesser winding losses due to transposition of strips with
respect to the flux generating surface of the generator
rotor.
- Skin effect (tendency of current to flow in the outer layers
of conductor) losses of the conductors are less.
- Higher the number of transposed conductors in a given
cross section of bar, lower is the skin effect losses.
- Thus the efficiency of the machine is higher.

B. The invention will be explained with an exemplary embodiment
of the turbo generator, which has following specifications:
The stator winding bar with new insulation has been designed with the
winding scheme for a 27 MW, 6.6 kV, 2 pole, 3000 rpm, star connected, 3
phase single layer lap mode.
Rating: 27 MW, 6.6 kV, 3000 rpm TG
No. of slots: 42
No. of poles: 2
No. of phases: 3
No. of slots / poles / phase: 7
Pitch : 18 /21
No. of parallel paths: 1
Connection: Y
Active length of machine: 2040 mm

The winding is manufactured / assembled with individually manufactured bars
laid in stator core as per the winding scheme.
The Roebel bars details are: (Ref.: Figure 1) for details no. of strips: 2 x 29
all insulated 2.35 x 10.25 (txw), Bar pitch: 18/21, No. of turns/ bar: 1
Roebel Transposition: 360°
The stator bars are with single turn, individual turns are with insulated strips,
which are transposed for 360°. Strips are individually insulated before being
processed for stack formation.
C. Main components of a bar (Ref.: Figure 1):
A typically Roebel transposed bar consists of the following material:
(i) Double glass covered insulated copper strips in two halves (2 x
29)
(ii) Half insulation with thermoset epoxy glass fleece sheet

(iii) Half insulation extension with polyster mica tape
(iv) Transposition insulation, nomex (polyamide, generally known as
nomex) fleece sheet
(v) Transposition surface filler with FOM micanite putty.
(vi) Main ground wall insulation with resin poor mica tape.
(vii) Outer corona protection tape (conducting graphite paper tape)
Stack consolidation of the bars are done with components detailed from (i) to (v)
above and thermally curing the bars in a mould at 155 to 165° C for stipulated
period of time.
Normally the Roebel bar design of the stator bars are used for medium and large
sized high voltage generator stator winding. In such case, either single or
double layer star connected lap winding design is adopted. In case of the Roebel
bars, several insulated copper strips are transposed in single / two planes to
minimize the copper losses due to any possible difference in the induced electro
motive force (EMF) among the copper strips.

D. Roebel bar transposition: (Ref.: Figures 2 and 4)
The name Roebel is derived form the inventor Mr. Robert Roebel, the British
Scientist and Engineer, who invented the concept of transposition of strips in a
plaited way to equalize the flux linkage among all the strips irrespective of their
differential radial distance from the flux generating plane i.e. the rotor in case of
a synchronous generator. This results in equal induced voltage generation
among all the strips of the bar and avoid inter strip circulating short circuiting
current. In a Roebel bar, all the strips travel from top to bottom and then up to
the original top level (As done in plaiting of the hair strands of women) in the
given active straight part of the generator. This is called 360° transposition.
Depending upon the requirement, the transposition may be repeated in the
multiples of 180 ° or may be continuous in the active length of the machine.
Figure 2 gives an isometric view of transposed Roebel bar showing details of
bending of copper strips from one half to other and then travel from top to
bottom layer vertically.

Figure 4 gives details of transposition. The transposition pitch (p), conductor
width (w), transposition angle (α) and transposition length (LT).
Normally transposition length LT > / = Machine active length.
Here machine length = 2040 mm
No. of strips = 2 x 29
Conductor size = 2.35 x 10.25 mm, hence w = 10.25 mm
Transposition pitch = 2040 / 2 x 29 = 35.17 ~ 36 mm
Transposition pitch / conductor width = 36 / 10.25 = 3.51 (Tr)
Transposition angle = a = 45° C.
In normal circumstances, the transposition pitch to strip width ratio is taken > /
= 5 otherwise, the conductor strips at transposition bend will exert tearing /
frictional forces on each other resulting in failure of strip insulation. In the
present case the Tr is 3.15, which is very less from the view point of inter strip
friction. The transposition angle also plays important role in this frictional force
too.

To avoid the peeling of strip insulation, transposition insulation is provided as
mentioned in Section - C above.
E. Function (Transposition Insulation)
1. The function of transposition insulation in stator bar is to electrically isolate
the strips at bend points and prevent electrical short.
2. The proposed multilayer insulation consisting of Polyamide (commonly known
as Nomex)-Polyster -Polyamide (Nomex) as a reinforcement to this insulation
has higher tearing strength due to presence of polyester, which is sandwiched
between two layers of the polyamide paper.
(Ref.: Figure 1 where this multiplayer insulation position of the bar is shown.)
(i) Objective:

The basic objective of the proposed insulation scheme is to avoid inter strip /
inter half short in the stacked Roebel Bar.
If more conductors can be placed in a given cross section of the bar, the skin
effect (tendency of current to flow in the outer layers of the conductor) will be
less. Hence copper / winding losses will be less. This will result is higher
efficiency of the machine.
In such a case the same cross section of the machine can carry more current
and thus for a given output, the machine size will be less.
(ii) Principle: (innovation in insulation)
The basic principle of effective electrical insulation in copper strips in a Roebel
bar is (i) Electrical / Dielectric strength (ii) Mechanical strength and (iii)
Temperature Index / Thermal class of the material.

The primary insulation of polyamide (Nomex) known in general as used for
transposition insulation is a Class-H material with breaking strength of 230 N/cm
and dielectric strength of 35 kV / mm. The proposed NPN layer as insulation
reinforcement, as proposed method has identical thermal and electrical
properties but a breaking strength of 370 N / cm. The higher breaking strength
of this multilayer insulation is due to presence of polyester in between two layers
of polyamide paper which is processed with hot pressing.
F. Advantages:
The proposed method of reinforcement of transposition insulation in the stack of
Roebel bars of Generators ensured total freedom of inter half and inter strip
shorts in the coils. With the proposed insulation scheme, it is possible to have a
lower value of ratio between transposition pitch to copper strip width (Tr). This
gives the freedom to have more numbers of copper strip in a bar cross section
resulting in lower winding losses.

The advantage of this new insulation scheme can be summarized as:
(i) No inter strip / inter half short resulting in lesser losses in machine. Hence,
higher efficiency.
(ii) Skin effect losses in stator winding is less due to use of higher number
copper strips in a given cross section of the bar. This adds to the improvement
of efficiency of machine.
(iii) Higher efficiency leads to optimal size of machine.

WE CLAIM
1. An improved insulation for Roebel bar conductors in stator winding of Turbo
generators comprising:
Roebel bar conductors having plaited and thermally cured at 155 to 165° C
copper strips (10), such strips being doubly covered with glass insulation in two
halves, thermoset epoxy glass fleece sheet (11) as half insulation, polyester mica
tape (12) as half insulation extension, polyamide fleece sheet (15) as
transposition insulation, FOM mica tape putty (13) as transposition surface filler,
resin poor mica tape (16) as main ground wall insulation, conductive graphite
paper tape (17) as protection for outer corona;
characterized in that multilayer insulation is provided consisting of polyamide -
polyester - polyamide (14) as transposition reinforcement to this insulation
having higher tearing strength due to presence of sandwitched polyester layer.
2. An insulation for Roebel bar conductors as claimed in claim 1, wherein the
individual conductor strip is covered doubly with glass insulation.

3. An insulation as claimed in claim 1, wherein the half insulation is formed
with thermoset epoxy glass fleece sheet.
4. An insulation as claimed in claim 1, wherein the half insulation extension
is provided with polyester mica tape.
5. An insulation as claimed in claim 1, wherein the transposition insulation is
made of polyamide fleece sheet.
6. An insulation as claimed in claim 1, wherein the FOM micanite putty is
applied as transposition surface filler.
7. An insulation as claimed in claim 1, wherein the resin poor mica tape
forms the main ground wall insulation.
8. An insulation as claimed in claim 1, wherein the conducting graphite
paper tape is provided as protection for outer corona.

9. An insulation as claimed in claim 1, wherein layers of polyamide -
polyester - polyamide is provided as transposition reinforcement to this
insulation.
10. An insulation as claimed in claim 9, wherein the said polyester is
sandwiched between two layers of said polyamide paper.

An improved insulation for Roebel bar conductors in stator winding of Turbo generators comprising:
Roebel bar conductors having plaited and thermally cured at 155 to 165°C copper strips (10), such strips being doubly covered with glass insulation in two halves, thermoset epoxy glass fleece sheet (11) as half insulation, polyester mica tape (12) as half insulation extension, polyamide fleece sheet (15) as
transposition insulation, FOM mica tape putty (13) as transposition surface filler, resin poor mica tape (16) as main ground wall insulation, conductive graphite paper tape (17) as protection for outer corona; characterized in that multilayer insulation is provided consisting of polyamide - polyester - polyamide (14) as transposition reinforcement to this insulation having higher tearing strength due to presence of sandwitched polyester layer.