Replacement Generators to Modernize and Uprate Power Trains Limited
by the Generator
Dr. Thorsten Krol (presenter, thorsten.krol@siemens.com ), Kai-Uwe Paeselt (author, kaiuwe.paeselt@siemens.com ), Fabian Bremer (author, fabian.bremer@siemens.com ), Maren Wiese (author,
maren.wiese@siemens.com)
1
Abstract.............................................................................................................................................1
2
2.1
2.2
Initial Situation and Solution .............................................................................................................2
Aging of Generators .........................................................................................................................2
Modernization Portfolio.....................................................................................................................4
3
3.1
3.2
3.3
Modernization of Main Generator Components as Solution ............................................................4
Rotor Rewind....................................................................................................................................4
Stator Rewind and Restack ..............................................................................................................5
Replacement of Stator Midsection ...................................................................................................7
4
4.1
4.2
Generator Replacement as Solution ................................................................................................7
Replacement of Generators up to 300 MVA ..................................................................................10
Replacement of Generators larger 300 MVA .................................................................................11
5
Conclusion ......................................................................................................................................12
6
References .....................................................................................................................................12
1
Abstract
Siemens generators are well known for their long lifetime. They can be used in a power plant for several
decades, but aging increases the technical risk of their operation. To be able to reduce the risk and to operate the plant according to the valid requirements, a modernization of the generators might become necessary.
The modernization can be done by different approaches. Major solutions that can be performed are the modernization of the limiting components of the generator or the replacement of the entire generator. The best
approach for the power plant can be evaluated together with Siemens experts. Based on the fleet experiences Siemens provides the full scope of service, ranging from maintenance to modernizations and upgrades.
Rather than a modernization for very old gas- or water-cooled generators with low ratings, it might be more
economical for an exchange with a state-of-the-art air-cooled generator, matching or even exceeding the
rating of the old unit. Siemens supports the decision making with in depth analysis of the specific situation of
the power plant.
The exchange is enabled through a modular design concept, which adapts the generator to the existing
foundation, as well as to the axis level of the existing power train, so modification of the foundation or turbine
interface is unnecessary.
In addition, Siemens offers replacement of large generators to increase the efficiency, reliability and availability as well as the output of the power train. While developing such projects, Siemens takes care that the new
generator fits in the existing power plant. On customer’s request Siemens has proven that it is possible to
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Page 1 of 12
customize the generators in such a way that the new auxiliary systems can be utilized for both generators,
the existing and the new one. This enables the possibility to keep the existing generator as a spare.
The technology, the possibilities of uprating and the benefits will be explained by examples in theory and
reference projects.
2
Initial Situation and Solution
As power plants become older and reach their equipment lifetime, or if the operation regime has changed
drastically, plant owners are confronted with the question whether to modernize the plant or to replace major
components of the plant. Usually, there are many different lifetime extension programs offered by different
manufacturers including turbine update, refurbishment or even replacement.
2.1
Aging of Generators
Aging is a well expected and normal effect during the life cycle of a generator, but the severity and kind of
aging always depends on the generator type and the operating conditions.
With the increasing demand for higher efficiency and output, increased requirements are being put on base
load power plants. Some power plants originally intended for base load operation are now being used for
intermediate and peak load operation. With intermediate and peak load operation, plants have more
start/stop cycles than they do in base load. Increased start/stop cycles and operation on turning gear can put
additional stresses on generator components, potentially reducing their service life.
The major components which can experience problems due to thermal, mechanical and electrical stresses,
leading to accelerated life expenditure, are typically the stator and rotor windings, stator core, rotor body and
retaining rings.
The number and kind of starts, shutdowns and fault conditions of the generator can cause material fatigue in
the various rotor components, resulting in severe damage in the long run. In the rotor end-winding areas,
thermal expansion and the influence of centrifugal forces may cause displacement of winding parts or insulation damage, which can cause earth faults, interturn faults or excessive temperature rises [1]. The longer a
generator has been in service, the higher the probability that the soundness of the insulation has been compromised by mechanical stress or heat related damage.
Figure 1: Abrasion of the rotor insulation
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The rotor body is a sturdy component which is less susceptible to damage. Nevertheless, it can be damaged
by overheating from negative sequence currents or asynchronous operation. When the rotor is energized
from standstill, very high currents flow in both the rotor surface and in the slot wedges. These currents may
cause melting of the slot wedges or create cracks in the rotor body, and may circulate through the shrink-fit
areas of the retaining rings [1].
Furthermore, the high electromagnetic flux and currents created inside a generator can lead to end-turn basket vibration. High vibration levels may loosen the stator winding and slot wedges, weaken insulation and
ultimately crack conductors and cause forced outages. Loose and broken laminations at the stepped core
end can cause damage to the stator bar insulation and can lead to formation of friction dust and stator earth
faults.
Figure 2: Friction dust due to looseness of end-winding structure
Especially for old paper insulated stator cores, the aging process is enforced by operational stresses. Abrasion of the paper insulation increases the electrical conductivity between laminations which can cause formation of hot spots inside the stator core. In the worst case, those hot spots can result in internal core burning
and melting.
Figure 3: Thermograph of stator core and localization of hot spots (in red)
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2.2
Modernization Portfolio
Siemens offers various condition related and preventive measures to keep generators in reliable condition, to
extend their lifetime and output, to avoid potential system downtime and costly repairs, and to minimize outage duration.
These include inspection, repair, modernization and replacement of generators, stators, rotors, exciters and
auxiliary systems. Based on fast, reliable and cost effective on-site inspections during minor, intermediate
and major generator overhauls, Siemens helps you to decide which service would be the best for the affected generator components.
This could be a stator repair and uprate ranging from a stator rewind with advanced design features, a fast
stator rewind and core repair to a new stator midsection or core restacking, e.g. using the well-proven donut
concept.
For the rotor, Siemens offers inspections including electrical testing and non-destructive examination (NDE),
spare rotors, rotor rewinds with old or new copper, and fast rotor rewinds to support short outages. Other
rotor services include short and long ring modifications, new gas baffles, exchange of retaining rings, improved slot liners, new J-Leads or an improved rotor end-winding and pole cross-over design.
Siemens also offers modernization services for generator auxiliary systems and exciter systems. These include an improved seal oil skid equipped with the latest instrumentation, filtration and cooling, services for
gas supply and stator water cooling systems, the axial split seal ring holder or our COOLGEN evaporative
cooler for air-cooled generators.
Increased costs and extended delivery time for spare parts and services for legacy excitation systems of
generators give economic reason for their modernization. For example, the replacement of original analogue
excitation systems with digital state-of-the-art systems reduces operational costs and at the same time increases the availability. Besides exciter replacement, Siemens offers exciter inspections, exciter rewinds,
refurbishments and spare exciter rotors.
In each case it must be determined which solution would be the most economic and efficient measure. For
very old generators a modernization or replacement of single components can be more expensive than a
complete generator replacement. For this case Siemens has developed two generator replacement solutions
which are introduced in section 4.
3
Modernization of Main Generator Components as Solution
One option to modernize turbo generators is to modernize or replace the main components of the generator.
In this section three representative modernization solutions for main generator components are introduced.
3.1
Rotor Rewind
If a rotor winding has been destroyed as result of a failure, or the deterioration is widespread, then a full rewind including mechanical and electrical testing, non-destructive examination (NDE) of the rotor body and
retaining rings, and rewind with new or refurbished copper may be the best option, permitting the owner to
have the winding returned to "as new" condition.
A rotor rewind includes the replacement of turn-to-turn and ground wall insulation to "reset" the life clock of
the insulation system. Reliability is increased because new materials are used in the insulation system which
is more durable than the materials in the original design.
With a rewind of the rotor plant owners can benefit from several new design features such as the enhanced
retaining ring shrink-fit seal, nonmagnetic 18Mn-18Cr retaining rings, redesigned J-straps, the corner brazed,
rectangular end-winding and an improved slot side angle material.
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Siemens uses class F material for the rotor turn-to-turn insulation based on its excellent wear characteristics
and high friction factor. The excellent wear characteristics lead to prolonged rotor life; the high friction factor
minimizes relative slippage of the coil turns, allowing the entire coil stack to act as a unit rather than as individual turns. The combination of Teflon backed by glass epoxy insulation provides dimensional stability under high loads and at maximum rotor temperature.
As availability and reliability are very important indicators for product quality, it is essential that repair times
are reduced to avoid long outages. The Siemens generator manufacturing plant Mülheim has demonstrated
its proficiency with reduced repair times with the successful implementation of a fast rotor rewind in record
time. Within only 20 days, beginning from the receipt of the rotor in the factory and ending with the delivery, a
rotor rewind with new copper was implemented for a 153 MVA hydrogen-cooled generator. This greatly reduced factory processing time could only be achieved with comprehensive planning including an efficient
spare parts strategy, and by providing sufficient manufacturing resources. Furthermore, there had to be a
special focus on reducing any risks concerning the time schedule and the expected findings during project
handling. The manufacturing plant Mülheim with its many decades of generator manufacturing, engineering
and project experience offers the best conditions for these challenging tasks.
With the fast rotor rewind Siemens Energy offers plant owners a solution which helps to avoid unnecessary
power plant outage and production downtimes. However, for each customer it must be individually determined which exact scope of work can be implemented in the available time slot depending on the generator
type.
In general, a rotor rewind offers the following potential advantages:
ƒ
Improvement of operational flexibility (not only base load operation)
ƒ
Higher availability and reliability due to new design features and materials
ƒ
Extended lifetime
3.2
Stator Rewind and Restack
Siemens offers a broad selection of stator repair services which can be performed conveniently on site, ranging from small winding and core repairs to a complete or partial core restack and rewind.
With a stator rewind with an advanced RIGI-Flex® stator winding the power plant owner can benefit from:
ƒ
Higher Reliability
ƒ
Reduction of maintenance efforts for the stator winding
ƒ
Improvement of the vibration behavior of the end-windings
ƒ
Reduction of thermo-mechanical stresses
ƒ
Increasing of the robustness of the end-winding structure
ƒ
Reduction of maintenance efforts for the stator winding
ƒ
Extension of unit lifetime
The advanced stator end-winding design can be applied to most of the Siemens stators with direct cooling,
and to some of those manufactured by other OEMs. Installation of this advanced design technology requires
a rewind which can be performed on site or in one of the Siemens facilities around the globe, generally during a major overhaul.
The complete advanced stator end-winding design features consist of:
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ƒ
Axially free and tangentially flexible end-winding structure
ƒ
Homogeneous structure with matched materials
ƒ
Tighter parallel rings decoupled from coil basket
ƒ
Low-tuned end-winding structure
ƒ
Protected sliding surfaces at support plate
ƒ
Matched insulation materials
ƒ
Withstand short circuit conditions
For core refurbishment, e.g. due to hot spots inside the stator core, Siemens has developed an on-site core
restack with bonded lamination packs which reduces the core stacking time. These so-called donuts are
compressed stacks of laminations which are vacuum pressure impregnated and thermally cured. The donuts
are electrically tested prior to installation in the unit, which reduces the possibility of lamination shorts during
core assembly.
A core restack with donuts can either be performed with the stator frame in vertical or in horizontal position
on the foundation. In Figure 4, the restack concept using donuts in horizontal position is shown.
This new method for stator core replacement on-site with the stator frame in horizontal position has several
benefits during the outage including:
ƒ
Shorter outage duration than restack with individual laminations where the frame is typically lifted
into the vertical position
ƒ
No need to disconnect the stator auxiliaries like iso-phase bus
ƒ
No need to lift the stator frame from the foundation
ƒ
No need to realign stator frame
ƒ
No cost associated with heavy lifting
ƒ
Less internal labor costs
ƒ
No safety concerns associated with lifting the stator frame
At American Electric Power’s, 493 MVA, Conesville Unit 6, Siemens replaced the stator core using donuts for
the first time at a power plant site. The outage was in April and May 2005; the total time required to replace
the core iron was 16 days, which is 38 days less compared to a former restack of Conesville 5, a sister unit
of Conesville 6, where the entire stator core was replaced in 2003 by hand stacking individual laminations
which took 54 days. Instead of the 110,000 individual lamination stacking interfaces, Conesville 6 has only
35 bonded core pack stacking interfaces [2].
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Figure 4: Stator core restack with donuts in horizontal position on foundation
3.3
Replacement of Stator Midsection
Another option instead of stator restack and rewind is the replacement of the entire stator midsection. The
new stator midsection includes a new stator frame, stator core, winding and terminal box. The main advantage of a new modernized stator midsection is that it is manufactured in the manufacturing plant. Thus, after
transportation to the plant it allows a fast implementation and a shorter plant downtime during replacement.
New generator midsections are manufactured using state of the art technology that is currently being incorporated in the latest line of new Siemens generators, including new stator windings and support system using the latest RIGI-Flex® design. The stator core incorporates new class F insulation materials as well as
new axial bolting for improved long term core tightness.
The replacement of the stator midsection has been successfully realized for a power plant which was looking
for a repair due to findings in the stator end-winding region. The referenced power plant consists of 6 power
trains in service for 15 to 20 years with hydrogen-cooled generators. All power trains in the plant were of
similar design. The scope was to deliver a spare stator midsection as a risk mitigation measure for future
operation and to rewind the stators on the foundation. The customer required that the generator should be
back on the grid after the normal major outage of maximum 3 months duration. The refurbishment of all midsections should be executed within 51 months and the major work be performed by local personnel.
The units were rewound by Siemens using a swap concept where a modernized stator midsection was delivered from the manufacturing plant Mülheim. The modernized midsection was exchanged during the first
regular major outage with the midsection from the first unit. The unit returned to commercial operation while
the used stator midsection was rewound. Due to limited time between the following major outages a new
design to connect the top and bottom layer bars was developed. Using the new connection technology the
time required for a stator rewind was reduced to only 2 months. The entire stator rewind of all 6 units was
executed successfully in 40 months, which was about 12 months earlier than the customers required date.
4
Generator Replacement as Solution
Modernization or replacement of the stator or rotor is one option but there are also cases where it is more
efficient to replace the entire generator. A generator replacement might be unavoidable, when:
ƒ
the generator is damaged and a repair is impossible
ƒ
modifying main generator components does not provide the necessary power output after turbine refurbishment
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ƒ
the generator maintenance and operating costs for a new generator justify the replacement
ƒ
the generator lifetime shall be extended to 20 years and more
ƒ
the generator maintenance requires a long downtime of the plant
ƒ
the original supplier is out of business
ƒ
spare parts are no longer available
In these cases plant owner are confronted with the challenge to replace the generator without modifying the
foundation, as modifications of the foundation carry the highest risk and also the highest costs in a generator
replacement job.
As investigations have shown that in the above mentioned cases, a modernization or exchange of individual
components and a reproduction of the original machine is economically and technically inappropriate, Siemens has developed a generator replacement or so-called “footprint” concept.
The replacement generators are taken from the Siemens type series of generators of proven and reliable
design and will be fitted into an enclosure and base frame, which will fit into the existing foundation. The idea
may sound simple, but there are many conditions that have to be checked and calculated to ensure that the
new machine will fit into the power train and will run smoothly for the next 25 years.
To find a matching generator from the Siemens type series fulfilling the electrical requirements such as reactance, voltage, stator current or short circuit parameters of the old generator, the plant owner needs to provide all technical data, foundation drawings, grid requirements and interface connections of the existing generator. With these data Siemens is able to choose the matching generator from the type series which will fit
into the electrical system (see Figure 5).
The parameters from the reference generator will be handed over to the plant owner to agree on the matching replacement concept. In the unlikely case no reference generator exists to meet the necessary electrical
parameters, they can be reached by modifying a standard generator in close cooperation with the plant
owner.
MVA
0
100
200
300
400
500
600
700
800
900
1000 MVA
Air-cooled
Industrial
SGen-100A
Air-cooled
SGen-100A
SGen-1000A
Hydrogencooled
SGen-2000H
2-pole SGen-3000W → 1400 MVA
4-pole SGen-4000W → 2200 MVA
Hydrogen/
Water-cooled
SGen-3000W
SGen-4000W
Figure 5: Portfolio of existing generator designs from Siemens
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As the next step, the geometrical boundaries need to be checked to fit the reference generator to the existing
foundation and power train and, if possible, with the piping. Reference drawings of the geometry of the opening in the foundation as well and maximum foundation loads at various locations need to be submitted if
available. Otherwise a measurement program needs to be set up during a regular outage where the power
train, the foundation etc. can be measured utilizing modern laser technology equipment.
In order to fit into the existing foundation, the following geometrical interfaces of the reference generator are
customized:
ƒ
Re-calculation of rotor coupling and shaft to fit the turbine sets. The axis height of the power train
needs to be aligned to the power train and the coupling needs to be fit to the turbine side.
ƒ
Customization of base frame to fit existing foundation. The base frame to foundation connection
needs to be adapted to fit to the existing foundation.
ƒ
Pedestal or plug-in type bearings
ƒ
Placement of the bushings
ƒ
Brushless or static excitation
ƒ
Position of cooler
The adopted base frame is calculated with modern CAE methods to ensure the same mechanical behavior
as there would be in a non-customized generator from the type series. In order to perform the calculation, the
design and calculation teams are working together with 3D state-of-the-art drawing and structural analysis
software. The designed and modified parts are directly imported into the calculation software, where a dynamic mesh is created of the part. Natural frequencies are carefully calculated to ensure that customer requirements and/or international standards and best industry practice are met during operation, start up and
run down (see Figure 6).
Figure 6: Calculation of natural frequencies of stator midsection
Once the necessary modifications are designed, the generator is released for manufacturing and will be
manufactured in the same generator plant as the Siemens new application generators. The same quality
procedures, measurements and tests are performed and the replacement generator is of identical quality and
performance as a complete “off the shelf” product. Special tests, such as a complete performance test can
be carried out in order to verify the calculated values. Bump tests will verify the natural frequencies of the
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casing and ensure that no resonant frequency is excited during the operation. Customers can arrange witness points to be informed about the status of their new generator.
Figure 7: Customized reference generator which fits into existing foundation
4.1
Replacement of Generators up to 300 MVA
There are several possible replacement possibilities, but one major market is the replacement of Siemens
OEM and other OEM generators using a standard air-cooled generator instead of a hydrogen-cooled generator. Due to recent developments with respect to higher efficiencies and using modern technology, today’s
air-cooled generators reach power outputs of about 300MW - the same power output and efficiency as hydrogen-cooled generators of 25 – 30 years ago. Compared to a hydrogen-cooled generator, the new aircooled generator has much less operating equipment and auxiliaries. This results in lower operating costs
and decreased risk.
Especially in the eastern European countries (former RGW) there are still a number of 200 MW class hydrogen-cooled generators, which are more than 25 years in operation. These generators have reached or exceeded their expected design life. For these generators (e.g. TWW200) Siemens has the right solution, experience and reference for an air-cooled replacement generator.
One of these 200 MW class replacement jobs that Siemens has already successfully delivered was a replacement of four 200 MW hydrogen-cooled generators of non Siemens design in the Czech Republic. The
customer did not want to perform modifications on the existing foundations but was interested in the replacement of his generators to gain the advantages of the modern state-of-the-art air-cooled Siemens technology.
In this case, the original generator manufacturer went out of business, so the customer could not get a 1:1
rebuilding. Siemens was therefore contracted to replace the hydrogen-cooled generator with an air-cooled
generator from the Siemens type series. The first of the four replacement generators was delivered within 16
month after the purchase order was placed. A quicker delivery would have been possible, but there was no
need due to the long lead time for the turbine replacement. Today, all 4 generators are delivered. One is
already in operation.
Replacing an old hydrogen-cooled generator with a standard air-cooled generator offers the following potential advantages:
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ƒ
Generator substitution takes place using a modular design that is adapted to the respective installation (shaft height, anchoring, flange connection, ventilation etc.) resulting in short downtime
ƒ
If required the nominal output of the generator can be increased, especially when considering a future power enhancement of the entire machine unit
ƒ
Increased operational safety and availability as the lifetime off all components is reset
ƒ
No risk of unplanned findings or damage during modernization
ƒ
Reduced life cycle cost and increased safety due to less auxiliaries
ƒ
Longer service life
4.2
Replacement of Generators larger 300 MVA
Larger water- or hydrogen-cooled generators with an output of more than 300 MVA cannot be replaced with
standard air-cooled generators. These generators have often been customized and cannot be replaced with
a standard generator. An individual engineering study will be implemented by Siemens experts in order to
identify the matching replacement generator.
Due to the latest developments and increasing efficiency in the last years it is possible to replace some water-cooled generators by hydrogen-cooled generators, and to replace smaller hydrogen-cooled generators by
air-cooled generators for the same power range.
In the following an example is given for a customized replacement of the entire generator including auxiliaries. The requested service was to uprate an existing generator operating since 1974 in a nuclear power
plant in Sweden.
Siemens was contracted to replace the non-Siemens 600 MW water-cooled generator including a new excitation system. One of the main reasons for generator replacement was a capacity enhancement prior to a
steam turbine modernization. The new generator should allow an uprate of 24% as well as the modernization
of the auxiliaries in order to fulfill the international requirements including ATEX.
After clarifying the technical boundary conditions, four possible replacement generators were identified that
would best fit into the foundation and fulfill the technical requirements. Siemens determined together with the
customer which of the options would best fit the boundary conditions and which option should be implemented.
The upgrade work included:
ƒ
850 MVA generator with advanced stator end-winding design
ƒ
New excitation system
ƒ
Auxiliary systems (seal oil system, hydrogen system, stator cooling water system)
ƒ
Generator instrumentation and control system
The generator was completely developed, manufactured and shop tested in about 25 months after order
entry. It was shipped to site a few weeks before the outage started and arrived on time. The new generator
and the auxiliaries were installed during the planned outage. The customer took over the generator 5 weeks
after the outage started and was very satisfied with the improved generator efficiency as well as the preparation and performance of site personnel. Since commercial operation the customer is still very satisfied with
the higher rating and the unit is operating without major issues.
Since the upgrade was completed and the unit returned to operation, the modernization provides the customer with the following benefits:
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5
ƒ
Increased generator efficiency and availability
ƒ
Higher capacity to withstand transients
ƒ
Higher performance and output
ƒ
No foundation works required since the generator was custom-tailored
ƒ
Operable old generator, which can be used as a spare
ƒ
Custom-tailored auxiliary systems which can be utilized for both generators, old and new
Conclusion
In conclusion, it is worth thinking about a replacement generator instead of repairing the old one. A comprehensive modernization of very old generators can be more expensive than the replacement with a new stateof the-art generator, as the modernization of single generator components may include a cost-intensive redesign.
If plant owners decide to replace the generator they will benefit from state-of-the-art technology, an ensured
spare parts supply, a lower insurance premium and comprehensive and customized service contracts including most recently information on product updates.
However, a generator replacement always requires an experienced partner with a proven design that is flexible enough to be adapted to the existing foundation and interfaces.
The concept of replacement using footprint generators is limited to those generators which can be seen as
standard generators. Generators with higher output in the range of fossil applications have been customized
in most cases, so an individual study to identify the matching generator and boundaries to the auxiliaries is
necessary.
Whether you are considering a generator modernization or replacement - Siemens provides any service in
any location and supports plant owners all over the world in finding the right solution.
6
References
[1] Turbine Generator Life Extension and Upgrading, International conference on residual life of power plant
equipment - prediction and extension, Dieter Lambrecht, Wolfgang Schier, Rainer Gern, Siemens AG, 1989
[2] Conesville 6 Generator Stator Core Replacement using Bonded Core Packs, CIGRE Study Committee A1
and EPFL Joint Colloquium On Large Electrical Machines, James A. Cook, James R. Michalec, 2005
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