470 mva generator lifetime extension...

Authors: Waldemar Kowalski Dr. Peter Lauter (STEAG GmbH) Maren Wiese Udo Weber Siemens AG Energy Sector Service Division

470 MVA Generator Lifetime Extension in Walsum, Germany

POWER-GEN Europe 2012, Cologne, Germany June 12-14, 2012

www.siemens.com/energy

AL: N; ECCN: N © Siemens AG 2013. All rights reserved. of 19

Table of Contents

1 Abstract ...............................................................................................................3

2 Initial Situation and Generator Lifetime Extension Scope ....................................3

3 Aligning the Shaft Train by Preparation of Foundation Beams ............................5

3.1 Hydraulic Lifting Equipment ...............................................................................5

4 Stator Rewind ......................................................................................................6

4.1 Advanced Stator End-Winding Design ................................................................9

4.2 Electrical Testing at Elevated Frequency ........................................................... 10

5 Rotor Inspection and Modernization .................................................................. 11

5.1 Rotor Rewind .................................................................................................... 12

5.1.1 Replacement of J-Straps .................................................................................... 13

5.1.2 Retaining Ring Shrink Fit Modification ............................................................. 14

5.1.3 Fast Rotor Rewind ............................................................................................. 16

6 Conclusion ........................................................................................................ 17

7 References ......................................................................................................... 17

8 Disclaimer ......................................................................................................... 18


1 Abstract Siemens generators are well known for their long lifetime. They can be used in a power plant

for several decades, but aging can increase the technical risk of their operation. The degree of

aging and the way in which it manifests depend on the specific generator model, the

individual operating conditions, and the history of the unit. When power plant generators

approach their equipment design lifetime, and if deterioration - e.g. due to grid impacts -

occurs, or if the operation regime is significantly changed, a modernization of the generators

might become necessary.

Siemens provides the full scope of generator service, ranging from maintenance to

modernizations and upgrades. In this paper the successful modernization of a 470 MVA

hydrogen-cooled generator operated by STEAG since 1988 in unit 9 of the Walsum

cogeneration plant in Duisburg, Germany is presented. The measure was implemented in

2011 during a planned outage and included a modernization of the main generator

components. An on site stator rewind, a rotor inspection, and based on the findings, a rotor

rewind resulting in a generator lifetime extension were performed.

2 Initial Situation and Generator Lifetime Extension Scope

In the Walsum cogeneration plant generators have been in operation since 1928. The unit 9

generator started its operation in 1988. The plant generates both, electricity and heat and

provides several areas with district heating by means of two distribution tracks. The first one

ends in Moers, the second one in Voerde. RWE Power is supplied with 333 MW electricity.

The Fernwärme Niederrhein GmbH receives up to 150 MJ/s heat and 10 cbm water per hour.

The nearby paper mill is provided with up to 120 t/h steam and 800 cbm water per day. The

on site located company Specialty Minerals Inc (SMI) produces products for paper

manufacturing using CO2, extracted from the flue gas and other deliveries.

Due to grid impacts (short circuits) near the power plant, which overloaded the generator

transiently and thereby highly increased the technical risk of its operation, in 2009 the plant

owner STEAG GmbH requested modernization and repair of the 23 year old KWU generator

type THDD 108/44, operating since 1988 (with approx. 180.000 operating hours/450 starts).

Friction dust and cracks indicated that the stator winding had been loosened by the extreme


transient mechanical stresses in the end-winding area which could have lead to an unplanned

outage over the time.

Given the importance of unit 9 for the supply of heat and electricity to the above mentioned

consumers, an immediate generator repair was not possible. Therefore, the modernization was

scheduled during a planned outage in 2011, lasting from May 18th until September 14th.

Meanwhile a large increase in bearing vibration was detected several times during load

increases from minimum to full load in the mornings. It was assumed that the higher vibration

levels were caused by a possible restraint at the rotor.

As Siemens offers various condition related and preventive measures to keep generators in

reliable condition; to extend their lifetime and output; and to avoid potential system downtime

and costly repairs, Siemens together with STEAG evaluated which service would be the best

for the main generator components.

Finally, Siemens was contracted to perform a stator rewind with advanced design features on

site and a rotor inspection in the manufacturing plant in Mülheim with an optional rotor

rewind. Based on the rotor findings, a rotor rewind with refurbished copper was implemented

which included a retaining ring shrink fit modification. Furthermore, the coupled shafts

needed to be aligned by preparation of the generator foundation beams. A retrofitting of the

generator supply systems according to ATEX and an exciter inspection were also performed,

but these two measures are not subject of this paper.

This comprehensive generator modernization was successfully executed within four months

and included:

Generator disassembly

Transport of the rotor to manufacturing plant Mülheim

Aligning the shaft train by preparation of foundation beams

Stator rewind on site

Rotor inspection

Rotor rewind with refurbished copper and short ring modification

Exciter inspection

Retrofitting of generator supply systems according to ATEX

Generator reassembly


3 Aligning the Shaft Train by Preparation of Foundation Beams After generator disassembly and transport of the rotor from Duisburg to the Siemens

manufacturing plant in Mülheim, the generator was lifted to mill the foundation beams.

During the years of operation the foundation on the exciter end (EE) descended in a manner

that the generator position was too high. In order to support future trouble-free operation the

generator had to be realigned relative to the shaft train. As there were no alignment elements

left to lower the generator it was necessary to mill off material from the foundation beam

prior to the stator rewind.

For this purpose the generator needed to be lifted off the foundation which would restrict the

availability of the crane by occupying it for several days during the milling works. Thus, other

works could be delayed. In Walsum, lifting, milling of the foundation beam and placing the

generator back into the foundation took a total of approximately 10 days. Furthermore, many

crane systems installed in power plants are not designed to lift a 260 ton generator stator.

Therefore, Siemens field service experts have developed a compact hydraulic lifting

equipment for generators with a maximum load of 260 tons which can be used world-wide

and which has been used for the first time in Walsum.

3.1 Hydraulic Lifting Equipment

The hydraulic lifting equipment can lift a generator with a maximum weight of 260 tons to a

height of 1.00 m within only a few hours. By virtue of the maximum cylinder stroke of

0.20 m the lifting operation is performed in steps. The hydraulic equipment comprises two

lifting beams, four hydraulic cylinders, several support blocks and a control unit. The lifting

beams are flanged onto the two sides of the generator; each beam is lifted by two cylinders.

The support blocks are used for temporarily setting down the generator after each lifting step

before moving the cylinders to a higher level in preparation for the next lifting step. When the

final position has been reached, the generator is set down on the support blocks, the cylinders

retracted and disconnected. The generator can remain in the final position until work on the

foundation beams is completed. Throughout this time period, the overhead traveling crane can

be used for other works, e.g. for simultaneously performed measures in the turbine area which

helps to reduce outage times.


Fig. 1: Hydraulic Lifting Equipment for Generators

4 Stator Rewind

A stator rewind with new bars and new insulation materials extends the availability and

reliability of the generator. In Walsum, the stator inspection and rewind were performed on

site within 11 weeks. The rewind included the implementation of an advanced end-winding

design (see Section 4.1). To ensure that the stator core insulation was not damaged during the

works, a 500-Hz flux test with thermal imaging was performed prior to the disassembly of the

old bars as well as after the rewind (see Section 4.2). For dismantling the old stator bars and

for installing the new bars a wrecking crane was provided by STEAG.

Fig. 2: Wrecking crane for installation of stator bars

In general, high electromagnetic flux and currents created inside a generator can lead to end-

turn basket vibration. High vibration levels acting for many years of operation may loosen the

stator winding and slot wedges, weaken insulation and ultimately can lead to cracking of

conductors and cause forced outages. Loose and broken laminations at the stepped core end or

Lifting beam

Cylinder

Support blocks


loose parts can cause damage to the stator bar insulation and can lead to formation of friction

dust and stator earth faults.

During disassembly of the old stator bars several findings were detected in Walsum, which

confirmed the urgent need for a stator rewind. This includes, for example black friction dust

caused by looseness of the end-winding structure. The typically white friction dust was

partially black-colored due to oil contamination.

Fig. 3: Black friction dust at stator end-winding

Furthermore, displaced slot-side ripple springs have been detected which have been moved

during operation and caused thereby some damages in the stator core. After repairing the

affected areas, another 500-Hz flux test was performed in order to evaluate whether the

damages in the stator core would lead to a possible heating due to local insulation damage.

The flux test did not detect any hot spots.

Some loose slot-side ripple springs also caused serious damage of the high voltage insulation

of the stator bars. Another unexpected finding were the loose flux shields which needed to be

fixed prior to installation of the new winding.

Fig. 4: Displaced slot-side ripple spring


Fig. 5: Slot-side ripple spring destroying the bar insulation

During and after the rewind several inspections of the dielectric insulation condition of the

new stator winding were performed as part of the Siemens quality control system, including

partial discharge, loss factor and insulation current measurements as well as high-voltage

testing. The data obtained are taken as a basis for comparison for aging trend analyses in

subsequent measurements (fingerprints).

Fig. 6: New stator end-winding with installed vibration detectors

With a stator rewind including the advanced RIGI-Flex® stator winding power plant owners

can benefit from:

higher reliability

improvement of the vibration behavior of the end-windings reduction of thermo-mechanical stresses

increasing of the robustness of the end-winding structure reduction of maintenance efforts for the stator winding

extension of unit lifetime


4.1 Advanced Stator End-Winding Design

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 original stator end-winding design for direct hydrogen-cooled stators was developed

around 30 years ago. The design of those generators is thus based on the know-how available

at that time. In contrast to the prior rigid type of end-winding support, which under certain

circumstances can impose a thermo-mechanical load on the stator bars, the newly engineered

flexible type of end-winding reinforcement stands out in particular for its optimized vibration

behavior and for imposing less of the aforementioned thermo-mechanical loads. Duly tested,

state-of-the-art insulating materials are used.

The new end-winding design is flexible but with high mechanical strength and requires little

maintenance. Through the axially free and tangentially as well as radially flexible end-

windings the natural frequency of the whole end-winding basket was lowered, which helps to

avoid operation of the generator end-winding close to resonance. This is referred to as "low

tuning design" which ensures permanent separation between two-lobe mode vibrations and

the double line frequency.

The overall end-winding structure exhibits homogeneous stiffness, which results from the use

of an additional inner ring and the utilization of perfectly matched materials reducing thermo -

mechanical stresses. The new design allows better resistance to transient effects without

plastic deformation through the use of damping components, specially designed radial gaps

and the use of Teflon®-coated surfaces.

The complete advanced stator end-winding design features consist of:

axially free and tangentially flexible end-winding structure homogeneous structure with matched materials

tighter parallel rings decoupled from coil basket low-tuned end-winding structure

protected sliding surfaces at support plate matched insulation materials


Fig. 7: Advanced end-winding design, crossover section of connection end

Fig. 8: Stator core with end-winding support structure

4.2 Electrical Testing at Elevated Frequency

The 500-Hz flux test with thermal imaging patented by Siemens already has a proven track

record in the field of generator stator core testing. This examination method is used to

measure the condition of the stator core insulation and thus to detect possible local insulation

damage. Insulation failure can result in local hot spots between several sheets of the core

during magnetization. These hot spots can be detected and localized with thermal imaging.

The measurement is performed at a frequency of 500 Hz, as significantly lower power is

required at high frequency than at grid frequency (50 or 60 Hz) to generate equivalent

magnetization losses in the stator core. However, since the stress on the insulation between

the laminations is comparable, the integrity of the core can be evaluated just as effectively.


The requisite instrumentation is also considerably smaller and lighter in comparison with that

needed for 50 Hz testing. This helps shorten test duration and assembly of the test setup. The

system is more transportable and simpler to implement worldwide.

Fig. 9: Example for thermal imaging of a stator with hot spots

5 Rotor Inspection and Modernization

In parallel to the stator rewind the generator rotor was transported to the Mülheim

manufacturing plant for inspection. A rotor inspection includes mechanical and electrical

measurements, disassembly of the retaining rings and parts of the end-winding spacer

arrangement, non-destructive examination (NDE) of various rotor components including the

tooth tops underneath the retaining rings, and inspection of the slot side insulation.

In the rotor end-winding areas, thermal expansions and the influence of centrifugal forces

may cause displacements of winding parts or insulating material which can entail earth faults,

intern faults and excessive temperature rises [1].

During inspection of the rotor winding it was discovered that the coil insulation has been

exposed to high thermal stresses during the years and that in the rotor body area displaced and

loose coil insulation parts had sealed some cooling ducts and thereby prevented optimal

cooling of the rotor. The restricted cooling of these areas lead to overheating and further

thermal damage of the coil insulation.


Fig. 10: Indication of high thermal stresses

In addition, several crack indications in the tooth flank area (see Fig. 11) were detected by

means of the magnetic particle inspection (NDE).

Fig. 11: Rotor tooth with indication in tooth flank area

The inspection results were discussed with STEAG and it was decided to perform a rotor

rewind with refurbished copper which had already been planned in advance by ordering the

necessary strategic spare parts. Due to the findings at the rotor tooths an unplanned short ring

modification had to be implemented. After rewind the rotor was balanced. In total, the rotor

inspection and modernization including the short ring modification were performed within 13

weeks.

5.1 Rotor Rewind

A rotor rewind includes the replacement of turn-to-turn and ground wall insulation to "reset"

the life clock of the insulation system and can be done by using either new or the refurbished


copper. Reliability is increased in both cases because new materials are used in the insulation

system which is more durable than the materials in the original design.

In general, a rotor rewind offers the following potential advantages:

improvement of operational flexibility (not only base load operation)

higher availability and reliability due to new design features and materials extended lifetime

With a rewind of the rotor, plant owners can also benefit from several new design features

depending on the requirements and the history of the rotor such as the improved retaining ring

shrink-fit, nonmagnetic 18Mn-18Cr retaining rings and improved J-straps.

Fig. 12: Rotor body ready for non destructive examination

5.1.1 Replacement of J-Straps The rotor field current is carried to the rotor end-winding through the axial and radial leads.

The J-straps connect the radial leads to the rotor coils. During operation, the end-windings

expand radially due to the centrifugal forces. In addition, thermal expansion causes the end-

windings to expand axially. Even though the J-straps reflected the state of the art, the forces

acting on them during operation cause the J-straps to deflect which, in turn, can lead to fatigue

cracking. Damage of one of the two J-straps may result in a forced outage and cracking could

results in worst case in extensive damage to the rotor forging, winding, and exciter end (EE)

retaining ring. In addition, insulation and copper contamination of the rest of the components

in the ventilation circuit could occur.

Therefore, Siemens has developed an improved J-strap assembly as part of its ongoing

product development process containing several design enhancements, resulting in an

improved speed cycle calculated design life. The revised J-strap design is intended to

incorporate flexibility features, which allow better for axial and radial expansion of the


windings, while keeping stresses low. Support of the J-strap during operation is also

improved, resulting in decreased deformations and improved fatigue life. The replacement

requires rotor removal, removal of the retaining rings as well as removal and re-installation of

the A and B coil. New radial bolts may be required depending on the design.

For Walsum the 23 years old J-straps have been replaced with new original “in-kind” design

J-straps as there where no cracks discovered in the original J-strap design.

Fig. 13: Improved J-strap design

5.1.2 Retaining Ring Shrink Fit Modification Due to the findings in the rotor tooth flank area an unplanned short ring fit modification had

to be implemented for the Walsum rotor before installing the new winding.

The generator retaining ring is attached to the rotor body by a radial shrink fit and an axial

key in a keyway machined into the retaining ring and rotor body. The keyway is turned at an

offset from the body end face. The portions of the rotor teeth located under the retaining rings

are referred to as the rotor tooth tops.

When the generator is at standstill, the shrink fit of the retaining ring on the rotor compresses

the tooth tops. This creates compressive stresses in the tooth tops. During operation, the rotor

coils are forced outwards until they lay against the slot wedges (in the rotor body area). The

centrifugal forces of the slot contents load the tooth tops radially outwards, causing the tooth

tops to go in tension. The stresses in the tooth tops alternate between high compressive stress

at standstill, and high tensile stress at speed. These stresses occur each time the unit started

and stopped, and can cause lowcycle fatigue cracks. These can be found underneath the tooth

tops and at the axial cooling holes in the rotor body.

Curved form allows greater

elongation in axial and radial

directions

Curved support plate

reduces notch effect


Cracks in the tooth tops can damage the rotor teeth to such an extent that the retaining rings

can no longer be shrunk off or on. Furthermore, the retaining rings can be shifted tangentially

as a function of crack propagation.

Fig. 15: Shrink fit connection between rotor body and retaining ring

The modification of the retaining ring shrink fit to prevent potential tooth top cracking

includes four measures which can also be used to repair rotors that already exhibit light tooth

top cracking as for instance in Walsum (see Fig. 11). The decision on which modifications are

to be implemented depends on the NDE results.

The four measures are summarized as “short ring modification” and comprise geometrical

changes to decrease the alternating stresses on the tooth tops, and the removal of cracked

and/or fatigue damaged material in the tooth tops:

Non destructive examination (NDE)

Tooth top removal outboard of keyway: Removal of the tooth-top sides to remove

cracked and/or fatigue damaged material (see Fig. 16). The depth of removal is

determined by Siemens based on the NDE results.

Compressive Loads at Standstill From Shrink Fit

Tensile Loads at Running Speed From Slot Content Centrifugal Force

Axial Loads From Locking Key

Stress Concentration at Fillet Radii and Holes

Fig. 14: Loads and crack initiation areas in rotor tooth top

End plate Max. compressive

load at standstill Key

Retaining ring

Coils Rotor

body


Shrink fit modification: The compressive load peak on the rotor body corner at standstill

(see Fig. 15) is reduced by tapering the shrink fit area in the retaining ring nose.

Keyway radius enlargement: As part of the tooth top modifications, the outboard keyway

fillets are undercut and enlarged as necessary. Thus, the stresses in the keyway radii are less

concentrated and lead to lower peak levels.

Fig. 16: Tooth top removal outboard of keyway

For Walsum, only a reduced short rind modification scope was required. That is, fatigued

material was removed in the fillet radii and keyway radii area. Removal of the tooth-top sides

as shown in Fig. 16 on the right side was not necessary.

5.1.3 Fast Rotor Rewind To reduce outage times Siemens can also perform a so-called fast rotor rewind. As due to the

stator rewind enough time was available, a cost-reduced rotor rewind using refurbished

copper was implemented for Walsum. But if repair times must kept short, a fast rotor rewind

with new copper would be the best solution. Siemens has therefore optimized processes in

planning, manufacturing and spare parts procurement so that a rewind for a 2-pole rotor can

be performed in as little as 30 days in the manufacturing plant. Even better cycle times have

already been realized. For instance, a rotor rewind for a hydrogen-cooled generator with 153

MVA was implemented in only 20 days. The rewind starts with the receipt of the rotor in the

factory and ends with shipment.

With the fast rotor rewind Siemens Energy offers a solution which helps to extend lifetime,

reliability and availability of the generator rotor with less downtime. 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.

Keyway radii Fillet radii


6 Conclusion The stator rewind with the new end-winding design along with the rotor rewind including a

short ring modification contributes to increased operating reliability and availability; and to

durable power operation of the generator for the next years – not only for base load operation

as in the original design, but also for intermediate and peak load operation. Furthermore, the

usage of the newly developed hydraulic generator lifting equipment helped to reduce the

outage time by not occupying the crane for lifting the generator.

The close cooperation of STEAG and Siemens experts during planning and implementation of

this comprehensive generator modernization has lead to successful realization within four

months.

The unexpected findings in the stator and rotor caused a two week extension of the original

project schedule. Nevertheless, Siemens finished the works five days earlier than the

rescheduled date, and the generator started turning gear operation on September, 14th.

In October 2011 power plant director Rainer Borgmann and the head of electrical and control

engineering, Horst Ivartnik, both STEAG GmbH, stated „Your staff succeeded in realizing

the complex works for the stator rewind including all organizational challenges and in putting

the generator into operation on time. This happened also because the high level of personal

and effective commitment of your staff.”

7 References [1] Dieter Lambrecht, Wolfgang Schier, Rainer Gern, Siemens AG (1989): Turbine Generator

Life Extension and Upgrading, International conference on residual life of power plant

equipment - prediction and extension


8 Disclaimer These documents contain forward-looking statements and information – that is, statements

related to future, not past, events. These statements may be identified either orally or in

writing by words as “expects”, “anticipates”, “intends”, “plans”, “believes”, “seeks”,

“estimates”, “will” or words of similar meaning. Such statements are based on our current

expectations and certain assumptions, and are, therefore, subject to certain risks and

uncertainties. A variety of factors, many of which are beyond Siemens’ control, affect its

operations, performance, business strategy and results and could cause the actual results,

performance or achievements of Siemens worldwide to be materially different from any

future results, performance or achievements that may be expressed or implied by such

forward-looking statements. For us, particular uncertainties arise, among others, from changes

in general economic and business conditions, changes in currency exchange rates and interest

rates, introduction of competing products or technologies by other companies, lack of

acceptance of new products or services by customers targeted by Siemens worldwide,

changes in business strategy and various other factors. More detailed information about

certain of these factors is contained in Siemens’ filings with the SEC, which are available on

the Siemens website, www.siemens.com and on the SEC’s website, www.sec.gov. Should one

or more of these risks or uncertainties materialize, or should underlying assumptions prove

incorrect, actual results may vary materially from those described in the relevant forward-

looking statement as anticipated, believed, estimated, expected, intended, planned or

projected. Siemens does not intend or assume any obligation to update or revise these

forward-looking statements in light of developments which differ from those anticipated.

Trademarks mentioned in these documents are the property of Siemens AG, its affiliates or

their respective owners.


Published by and copyright © 2012: Siemens AG Energy Sector Freyeslebenstrasse 1 91058 Erlangen, Germany

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All rights reserved. Trademarks mentioned in this document are the property of Siemens AG, its affiliates, or their respective owners.

Subject to change without prior notice. The information in this document contains general descriptions of the technical options available, which may not apply in all cases. The required technical options should therefore be specified in the contract..

470 mva generator lifetime extension...

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