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Resin for the end-windings contains
a flexibiliser which provides a small
degree of flexibility within the resin,
making it less susceptible to the
end-winding cracking often seen in
larger VPI coils
circulating current losses (a process
called Roebel transposition), or,
by changing winding covers to a
non-magnetic material. Improved
design software and modern winding
equipment also contribute to delivering
stator coils with a more efficient and
robust construction.
Stator core laminations
It is not only the stator windings that
can influence the efficiency of the
generator, the design and construction
of the stator core itself also has an
influence. As part of a generator
refurbishment, it is essential that
inspection and tests be carried out on
the stator core and rotor. These can
highlight any issues with the integrity
of the stator core which may have to
be replaced before any coils can be
installed.
Rebuilding the core using new
lamination segments which are
manufactured from a specific lowerloss grade of magnetic steel can result
in a 10% reduction in losses when
compared to the material used in the
original construction. In addition, the
rotor field coils can be stripped and
re-insulated with improved, Class F,
insulating materials.
of the coil dimensions within the
slot, which, combined with good
winding practice, minimises any voids
between the coil and the slot; resulting
in good control of partial discharge
(PD) activity within the slot. PD is
one of the main failure modes of the
insulation in rotating machines, making
it a good indicator of normal service life
expectancy.
The resin rich coil uses a thermosetting
epoxy resin infused mica tape for
the slot while the resin for the endwindings contains a flexibiliser
which provides a small degree of
flexibility within the resin, making it
less susceptible to the end-winding
cracking often seen in larger VPI coils.
The flexible end-winding tapes allow
the coil to be adjusted slightly during
installation in the stator, which ensures
a more consistent gap between the coil
sides in the end-winding.
Epoxy resin is more susceptible to PD
activity when compared to mica and
so the resin rich coils, which contain
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PECM Issue 14
less resin than the VPI equivalent,
provide better resistance to partial
discharge activity. In most cases the
use of modern insulating materials will
improve both the dielectric and thermal
performance when compared to those
used by the OEM during original
manufacture.
As part of a generator
refurbishment, it
is essential that
inspection and tests
be carried out on the
stator core and rotor.
Improvement in the control of
stray losses can also be achieved
by changing the method of stator
coil transposition in order to reduce
Sulzer can also redesign, manufacture
and fit new AC excitation systems
to replace DC systems. The main
benefit of this change is to remove
the possibility of deposits from carbon
brushes contaminating the stator
and rotor components; an added
benefit is the reduction in brush gear
maintenance time giving additional
generator uptime.
The use of modern materials for both
the stator core and the insulation
thereby helps to ensure the continued
reliable operation of a facility after the
generator has been refurbished, but it
can also increase the maximum output
of the newly repaired generator by over
10%.
High voltage testing for Hydro
generators
Stator coils can be independently
tested before being installed, giving
the repair centre and the client peace
of mind that every coil has passed the
numerous tests available.
The electrical testing of the completed
coil includes the Tan δ, which is a
measure of the integrity of the slot
wall insulation, where a lower figure
indicates a better quality coil. At
Sulzer, the figure achieved is always
less than half the international
standard, with an aim to realise less
than one third of the standard.