Download Photo 7: Stator coils can be independently tested

ADVANCED REPAIRS MEAN IMPROVED EFFICIENCY AND LONGEVITY FOR
HYDRO POWER SYSTEMS
An increased focus on controlling greenhouse gas emissions, coupled with escalating
demand for electricity and slow progress on nuclear projects is leading to an
expansion of alternative renewable energy sources. Leading the pack at the moment
is hydro electricity, the basic principles of which were commercialised in the late
1880's and now accounts for around 16% of global electricity generation capacity.
One of the advantages of Hydro-power is actually the age of its base level technology;
as a mature generation method, the equipment is tried and tested and although a large
proportion of it is due for refurbishment, it means that refinements in materials and
manufacturing techniques can be applied to existing installations.
Improved generation efficiency and longevity can now be built-in during routine
maintenance, delivering 10% increases in power output from the same source, without
re-investing in replacement hardware, surely a massive win-win for the industry. It
should be noted that not all repairers offer the same level of service and expertise, so
hydro generator operators need to choose carefully.
Installed base
The variety of plants is huge, from the smallest Pelton wheel to the largest
hydroelectric power station in the world, the 22,500MW Three Gorges Dam in China.
However, the principle remains the same, using flowing water to turn a turbine and
generate electricity. The key to a successful installation is efficiency and reliability,
both of which are benefitting from the latest technological advancements in the sector.
In recent years, many hydro projects have begun operating in ways not envisioned
during their original design and new installations have been made possible through
improvements in technology. Of course, work is still needed to determine how
equipment can be improved through new materials technology as well as identifying
actions and tools for predicting and preventing failures.
Turbine technology
As hydropower increases in popularity so the demand for new installations grows.
Thanks to inherent shortcomings with traditional technology, it may not be suitable to
meet these demands in some locations, such as those with large fish populations.
Here, development programmes, such as the Alden turbine project, have produced
new designs that significantly improve the mortality rate of fish travelling downstream
while also improving generating efficiency.
Hydro turbine research has also produced a new fish-friendly design criteria for Kaplan
and Francis turbines that can be incorporated either into units during rebuild projects
or in new hydroelectric facilities. In the USA, the Department of Energy (DOE) has
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implemented a new hydro programme called "The Advanced Hydropower Turbine
System Program" (AHTS) which aims to develop technology to maximise the use of
hydropower while minimising the environmental effects.
Refurbishment of existing turbine assemblies can now include the latest materials
technology such as the High Velocity Oxygen Fuel (HVOF) coating system, which
uses tungsten carbide materials. This and other similar coating and wear protection
systems can be used to protect water turbines and draft tubes against erosion from
water-born sediment and reduce cavitation effects for Pelton, Kaplan, and Francis
turbines.
Improving generator efficiency
Advances have been made in the design and construction of the high voltage stator
coils used within the generator, which can provide improved efficiency and reliability.
The manufacture of high voltage coils is a very precise science; design and
construction methods will vary between suppliers, as will the testing procedures, which
form the bulk of the proof the client has of the quality of the finished product.
Obviously, when repairs are unplanned, speed is crucial in getting the equipment back
up and running, but this cannot be achieved to the detriment of quality, which has
much longer lasting implications. With regard to the testing protocols for these high
voltage coils the hydro electric industry works to higher standards than some industrial
sectors.
For example, Sulzer's Birmingham Service Centre, where replacement coils are
manufactured for use worldwide, subjects its coils to a series of demanding tests both
in-house and by external testing experts to ensure that these standards are met and in
most cases surpassed in order to deliver a guaranteed product.
Maintaining high standards of design and construction has a number of key benefits,
most notably reliability and operational efficiency. Employing a process of continual
measurement against specification throughout the manufacturing process will ensure
correct initial fit as well as continued reliable service.
Advances in coil structure & insulation improve longevity
In addition to the myriad of small differences in design, construction and materials from
one manufacturer to another, there are two distinct insulation methods of coils in
generator construction. It is worth focussing on this aspect of generator repair because
it can make a profound difference to the efficiency and performance of a unit, often
without having to replace the other major components.
Replacement coils or bars will be manufactured either using the heated and
individually pressed Resin Rich insulation system or a vacuum pressure impregnated
(VPI) resin bath method. While smaller applications might prefer the VPI method, this
can become unmanageable for larger installations such as large generators.
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The design and construction of the resin rich coil provides accurate control 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 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. 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 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.
Improvement in the control of stray losses can also be achieved by changing the
method of stator coil transposition in order to reduce circulating current losses (a
process called Roebel transposition), or, by changing winding covers to a nonmagnetic 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 lower-loss 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.
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
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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.
Further testing can be carried out on sample hydro-generation coils by independent
laboratories, including the thermal endurance test, which is carried out at 30kV for 500
hours, a simple enough task for a high quality coil. However, a more arduous test is
done at 35kV for 250 hours, however all the coils provided by Sulzer have passed this
standard as well.
In the majority of cases, the client will specify the test parameters for the coils, which
are normally specified to operate at 11kV. As a baseline assessment, coils are
subjected to 23.9kV for 400 hours while being heated to normal operating
temperature, around 120°C. This test is in accordance with the demanding IEEE
standard 1553, which relates specifically to hydro generator coils.
Additional coils can be initially subjected to the withstand test, which evaluates the
ability of the coils to operate in overvoltage conditions which may be expected during
their life. Coils are expected to pass this test which is conducted at 28kV, as well as a
further test at 60kV and an end-winding test at 22kV, with no detectable issues with
the insulation. While the IEEE standards set out the framework for these tests, the
exact test voltages and durations are specified by the client to ensure that the coils
meet the requirements of the individual application.
High Speed Rotor Balancing
As the stator is completed, so attention falls to the rotor, which can also benefit from
the improved insulation materials technology. Rotor field coils will usually be
reinsulated as part of a refurbishment project along with the replacement of any
damaged components. Clearly, the rotor will need to be re-balanced before it is
returned to service.
Dynamic balancing of rotating elements is an important aspect of the manufacture and
the repair of any turbo machinery. A rotating element that is out of balance can cause
major operational difficulties, which may prevent the timely start-up of a facility if it has
to be re-balanced. Furthermore, the unbalanced element can cause internal damage
that will rob a machine of its design efficiency, reduce machine reliability, and increase
the costs of operation and maintenance.
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For any applications that require high speed balancing, this process requires a
specialised balancing bunker, but very few of these are available to the independent
repair market. Globally, Sulzer has invested in a number of overspeed balancing pits
which can be used as part of each repair project for its clients. Though, due to their
rarity, Sulzer has also made these available to OEMs and other customers. Each pit is
equipped with advanced electronics and diagnostics to provide state-of-the-art troubleshooting capabilities.
Pumped storage systems
Typically used as fill-in generation at times of peak demand, although green in terms
of generation, they obviously consume power when pumping water back up to the high
level storage. Efficiency in the motors, pump sets and the generators is crucial to an
installation's viability as they are normally run on very low percentage margins. An
improvement in generating, or pumping efficiency, especially during routine
maintenance work could therefore make the difference between a site being viable or
not.
Some pumped storage installations use separate turbines and pumps while others
employ reversible turbine/generators which can act as both pump and turbine. One
cost effective alternative approach is to use a reverse-running centrifugal pump, which
can improve the efficiency of the pumping cycle and thus reduce operating costs.
Maintaining these pumps and the associated generators requires expertise and
experience as well as access to the latest in pump design innovations, which Sulzer,
as one of the world's leading pump manufacturers, can provide.
Preventative maintenance
A power station needs to optimise output in order to meet demand as efficiently as
possible, but, inevitably there comes a time when maintenance repairs, either
electrical or mechanical, will be required. Therefore, a programme of periodic,
preventative maintenance can be combined with condition monitoring techniques to
improve the reliability and longevity of the equipment.
Employing a range of condition monitoring equipment, combined with suitable analysis
techniques, can provide an accurate assessment of the status of turbines and
generators, allowing operators to ensure continued production. One of the most useful
tools is vibration analysis, which can indicate potential problems with bearings and
large rotating parts; combined with thermal imaging, it can produce an accurate
indication of the overall status of the plant.
Additional testing of the electrical windings, especially partial discharge (PD) analysis,
can also provide very useful information on the overall condition of the generator. PD
can cause insulation degradation, which if not remedied, can lead to reduced output of
the generator and premature failure of the insulation system causing stator failures.
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The appropriate use of condition monitoring provides an excellent maintenance tool
and can ensure efficient generator output.
Thermographic surveys
The use of infra-red cameras for predictive and preventative maintenance is becoming
more common, especially when assessing high value equipment or where an
unexpected failure could cause considerable disruption. These cameras can be used
to analyse the running temperatures of bearings, motors, electrical connections and
other equipment and, as such, can indicate potential areas for concern without having
to stop any of the machinery.
By applying infra-red technology, engineers are only required to carry out maintenance
procedures when they are required, rather than on a fixed basis, which improves
efficiency and reduces maintenance costs. Sulzer believes that more clients can
benefit from this technology and is offering free thermographic surveys to both new
and existing customers.
Complete Hydropower solutions
Sulzer is a leading provider of engineering solutions for the power generation sector,
from on-going condition monitoring to complete turnkey projects. The company has
developed a large network of service centres capable of maintaining all rotating
equipment such as turbines, generators, motors and pumps as well as employing
experienced engineers, equipped to complete projects on site if necessary.
The high voltage coil manufacturing facility based at the Birmingham Service Centre,
employs an in-house copper rolling mill to enable round-the-clock coil production to
meet even the tightest deadline. Using the latest CAD systems, precise quality control
and extensive testing, modern coil production can significantly improve the
performance and efficiency of any generator which is due for refurbishment.
Employing the latest materials technology and manufacturing processes enables the
hydropower sector to maximise the efficiency and reliability of the existing
infrastructure as well as develop new opportunities. Sulzer has the expertise, tools and
equipment to provide engineering support to its clients for any scale of project, from
engineering supervision to completion of the largest repairs to turbines, generators
and pumps.
Author: Graeme Robertson, Head of Operations - UK for Sulzer
Graeme Robertson works as Head of Operations Electro-Mechanical UK for Sulzer
and has worked for the company since 1998 when he started as General Manager for
the Falkirk Service Centre.
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Graeme has since progressed to the Birmingham Head Office where he oversees 7
UK Electro-Mechanical Service Centres. His specialties include technical electromechanical knowledge and commercial management.
For additonal photos please contact Philip Howe: [email protected]
Photo Captions:
Photo 1: Improved generation efficiency and longevity can now be built-in during
routine maintenance, delivering 10% increases in power output from the same source
Photo 2: Advances have been made in the design and construction of the high voltage
stator coils used within the generator, which can provide improved efficiency and
reliability
Photo 3: The resin rich coil uses a thermosetting epoxy resin infused mica tape for the
slot
Photo 4: Some pumped storage installations employ reversible turbine/generators
which can act as both pump and turbine
Photo 5: As part of a generator refurbishment, it is essential that inspection and tests
be carried out on the stator core and rotor
Photo 6: 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
Photo 7: Stator coils can be independently tested before being installed, giving the
repair centre and the client peace of mind
Photo 8: For applications that require high speed balancing, this calls for a specialised
balancing bunker, but very few of these are available to the independent repair market
Photo 9: The balancing pit in Birmingham incorporates an HV motor and advanced
electronics and diagnostics to provide state-of-the-art trouble-shooting capabilities
Photo 10: Graeme Robertson, Head of Operations - UK for Sulzer
About Sulzer
Sulzer is the leading worldwide, independent service provider for large rotating
equipment. With technically advanced and innovative service and maintenance
support solutions, Sulzer provides a turnkey service that provides its customers with
the peace of mind to focus on their core operations.
Included in this package is a highly efficient and dependable high-voltage coil
manufacturing and supply service, delivered from a purpose built facility within the
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Birmingham Service Centre, UK. It is recognized for producing very high quality coils
for high voltage motors and generators; designed, manufactured and shipped by a
highly skilled and dedicated team to ensure fast and reliable service.
With an in-house copper rolling mill, Sulzer customers can benefit from round-theclock manufacturing to ensure that every coil is delivered on schedule. In conjunction
with constant quality control and full HV on-site testing facilities, Sulzer can deliver
quality, precision and speed, any time, day or night.
For further information click here or e-mail: [email protected], or phone
+44 (0)121 766 6161.
The image(s) distributed with this press release may only be used to accompany this
copy, and are subject to copyright. Please contact DMA Europa if you wish to license
the image for further use.
Editor Contact
DMA Europa Ltd. : Chris Holman
Tel: +44 (0)1562 751436
Fax: +44 (0)1562 748315
Web: www.dmaeuropa.com
Email: [email protected]
Reader Contact
Sulzer : Harpreet Suree, Marketing and Communications Officer – UK and North
Sea, Rotating Equipment Services, Sulzer
Tel: +44 (0)121 766 6161
Web: www.sulzer.com
Email: [email protected]
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