FIELD OF THE INVENTION
The present invention generally relates to a manufacturing process of Roebel
bars (conductor strips) for stator windings of turbo generators. More particularly,
the invention relates to an improved Roebel transposed bar for stator windings,
which eliminates tearing of transposition insulation and inter-strip short circuit.
BACKGROUND OF THE INVENTION
Medium and large capacity Turbo generators (TG) stator winding coils are
generally manufactured in half coil/bar throughout the world. The primary
considerations in the manufacturing process are the size and weight of the bars
as well as the ease of insertion of the coils/bars in the stator slots. The individual
bars of a regular large capacity synchronous turbo generators stator winding is
made with several numbers of all insulated copper strips with glass fiber
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 thermoset resin to bind the strips together. Hence, the bars are first
thermally cured at 155-165 Deg. C for stipulated hours to get thermally cured to
the required shape.
The TG stator coils are first transposed for 360/540 deg. i.e. bending of each
strip from one half to other and then vertically traversing from top to bottom and
back to the top of bar in the total active / straight length of the bar. This is
known as ROEBEL TRANSPOSITION known popularly after the name of the
inventor. Subsequently, the bars are given solid shape through stack
consolidation, which includes placing several semi-cured B-stage insulation
materials within the copper strips and thermally curing them. Later on, the
stacked bars are insulated for the ground wall insulation with either resin rich or
resin poor epoxy glass mica tape and placed in the TG stator core. During the
process of winding assembly, the coils are placed / lowered in to the stator slots
as per the selected winding pattern. Subsequently, the bars are electrically
connected by soldering/brazing process to form phase groups. The inherent
weakness of failure of inter strip insulation may result in excessive winding losses
and early failure of the Generator.
The Roebel bar constitutes a single conductor of generator stator winding
consisting of several numbers of insulated copper strips stacked one over other
with required transposition. The transposition of the insulated strips can be in
multiple of 180 deg, which interalia 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 Deg. 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.
- Thus the efficiency of the machine is higher.
The concept of transposition of strips in a plaited way is known, which allows 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 turbo-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 in the given active straight part of the generator. This is
called 360 degrees transposition. Depending upon the requirement, the
transposition may be repeated or may be continuous in the active length of the
machine.
Figures 4A and 4B respectively shows a typical construction of transposed
conductor (Isometric View), and bar stacks of a 2-pole turbo-generator. Figure 5
shows or view of the overhang of Turbo Generator (TG) stator winding. The
winding is manufactured/assembled with individually manufactured bars laid in
stator core as per the winding pattern.
As shown in figure 1, a typical Roebel transposed bar consists of:
(i) Double glass covered insulated copper strips in two halves (1,2);
(ii) Half insulation with thermost epoxy glass fleece sheet (3,4);
(iii) Half Insulation Extension with fine mica cloth (6,7);
(iv) Transposition insulation, nomex glass fleece sheet (5);
(v) Transposition surface filler with FOM micanite putty (13);
(vi) Main ground wall insulation with resin poor mica tape (14,17); and
(vii) Outer corona protection tape (18,21).
Stack consolidation of the bars are done with components detailed at (i) to (v)
above. The bars are thermally cured in a mould at 155-165 deg. C for a
stipulated period of time.
Normally a Roebel Bar design of the stator bars are used for stator winding of
medium and large sized high voltage. Turbo generator (TG). 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.
The function of the transposition insulation in the stator bar is to electrically
isolate the strips at bend/transposition points and prevent electrical short. The
Inter half Insulation avoids electrical short between two halves of the conductor
strips stacked one over the other.
The basis 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.
OBJECTS OF THE INVENTION
It is therefore an object of the invention is to propose an improved Roebel
transposed bar for stator windings, which avoids inter-strip/inter half short in
stacked Roebel Bar.
Another object of the invention is to propose an improved Roebel transposed bar
for stator windings, which allows a flexibility of adding more number of
conductors in the stack so as to minimize the skin effect (tendency current flow
to the outer layers of the conductor).
A still another object of the invention is to propose an improved Roebel
transposed bar for stator windings, which reduces copper/winding losses leading
to higher efficiency of the turbo generator.
A further object of the invention is to propose an improved Roebel transposed
bar for stator windings, which enables to reduce the size of the turbo generator
making the machine compact.
SUMMARY OF THE INVENTION
According to the invention, polyester film is hot fused onto the individual
conductor strips and provided with glass yarn/fiber as the final layer of strip
insulation (Called DAGLASS copper strip). This results in electrically short free
stator bars due to high abrasive resistance of the polyester film in the conductor.
Hence additional half insulation (between two halves of the conductor stacks)
and transposition insulation (between two conductors at Roebel cross over) are
avoided.
According to the invention, a reinforcement of the conductor insulation in the
stack of Roebel bars of Generators ensures total flexibility of the inter half and
inter strip shorts in the coils. With the proposed insulation arrangement, it is
possible to have a lower value of ratio between transposition pitch to copper
strip width, which allows addition of more numbers of copper strip in a bar cross
section resulting in lower winding losses.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - shows a cross-section of stator bar in slot area according to prior
art,
Figure 2 - shows a cross-section of stator bar in slot area according to the
invention.
Figure 3A - shows transposition of stator (Roebel) bars
Figure 3B - shows stator (Roebel) bar stacks
Figure 4 - shows a view of TG-stator end winding according to the invention.
DETAIL DESCRIPTION OF THE INVENTION
According to the invention, a primary insulation is provided to the conductor
strips which is made up of polyester film fused on to the copper, before laying
glass yarn/ (Called DAGLAS copper) fiber thus building a layer with abrasive
resistance to prevent tearing of the insulation. This insulation is a Class-H
(Temperature Index of 180 Deg. C) material as opposed to only double glass
insulation of Class-F (Temperature index of 155 Deg. C) of prior art.
As shown in figure 2, the Roebel transposed stator bar comprises:
(i) DAGLASS copper strips in two halves )22,23);
(ii) Transposition surface filler with FOM micanite putty (13);
(iii) Main ground wall insulation with resin poor mica tape (14,17); and
(iv) Outer corona protection tape (18,21) after application of component
(iii), wherein a stack consolidation of the bars are done with the
components recited at (i) and (ii) with a hot curing vanish and
thermally curing the bars in a mould at 155-165 deg. C for a
stipulated period of time.
Advantages
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 less due to use of higher number
copper strips in a give cross section of the bar. This adds to the
improvement of efficiency of machine,
(iii) Higher efficiency leads to optimal size of machine
(iv) No necessity of additional materials for transposition (5) and inter half
(3,4,6,7) insulation,
(v) Bar stack manufacturing is simpler and cheaper.
WE CLAIM
1. An improved Roebel transposed bar for stator windings of turbogenerator,
comprising:
(i) DAGLASS copper strips in two halves (22,23);
(ii) Transposition surface filler with FOM micanite putty (13);
(iii) Main ground wall insulation with resin poor mica tape (14,17); and
(iv) Outer corona protection tape (18,21) after application of component
(iii), wherein a stack consolidation of the bars is done with the
components recited at (i) and (ii) with a hot curing vanish and
thermally curing the bars in a mould at 155-165 deg. C for a
stipulated period of time.
2. The Roebel transposed bar as claimed in claim 1, wherein the DAGLASS
copper strips comprises a polyester film fused on the copper strips preceded
by application of an abrasive resistance layer of glass yarn/fiber.
3. The Roebel transposed bar as claimed in claim 1 or 2, wherein the
insulation on the conductor strips is of class-H with temperature index of
180°C.

ABSTRACT

The invention relates to an improved Roebel transposed bar for stator windings
of turbogenerator, comprising (i) DAGLASS copper strips in two halves(22,23);
(ii) Transposition surface filler with FOM micanite putty (13); (iii) Main ground
wall insulation with resin poor mica tape (14,17); and (iv) Outer corona
protection tape (18,21), wherein a stack consolidation of the bars are done with
the components recited at (i) and (ii) with a hot curing vanish and thermally
curing the bars in a mould at 155-165 deg. C for a stipulated period of time.