Stator Rewind Solves Corona Problems

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COMBINED CYCLE JOURNAL, First Quarter 2007 67

GENERATOR OVERHAUL

T

om Hundley, plant manager,Elk Hills 35R Cogen, locatednear Bakersfield, Calif, said

he knew the day was comingwhen a new generator, or a rewind,would be required on his facility’stwo LM2500-powered gas-turbinepackages. He listened over the yearsto experts who participated in theWestern Turbine User Group Inc’sconferences (visit www.wtui.com)and they all essentially had the samemessage: Be prepared for major workor replacement after your air-cooledgenerator passes its tenth birthday.

The two units Hundley is respon-sible for were installed in 1994 and,as summer 2006 approached, the

annual borescope inspection on Unit2 revealed some deposits of whitepowder on stator coils characteristicof those associated with corona (Side-bar 1). There was sufficient evidenceto warrant pulling the rotor for acloser look.

First a little background: The ElkHills cogen facility is operated byProduction Services Network, a rela-tively new firm based in Aberdeen,Scotland, that provides O&M servic-

es to the oil and gas industry. Hund-ley, who is employed by ProductionServices, has worked in the Elk Hills

O&G field for nearly 30 years andhas managed the plant since 2001.He was not affiliated with the plantwhen it was built in 1994.

National Mechanical Services(NMS), National City, Calif, whichspecializes in generator work, washired to pull the rotor and assessthe condition of the stator windings. An independent consultant also wasengaged. NMS President Bill daSilva, well known in the land-based

Stator rewind

solves coronaproblem

1. What is corona?

Bill Moore, PE, National ElectricCoil’s director of development,

is well known by owner/operatorsfor his presentations at user-groupmeetings while the company’s man-ager of engineering. Here’s howMoore describes corona, plus histhoughts on prevention:

Partial discharge, sometimescalled corona, is quite common inair-cooled generators. It is caused bya partial voltage breakdown withinthe generator coil insulation, in gapsbetween the coil and the stator core,and/or in the end turns when the coils

are in close proximity to each other.Because it is not a complete

breakdown of the insulation system,

it does not cause a full electricalground. Over time, however, thesedischarges can “eat” at the insulation,causing its deterioration until a fullground does occur and the unit tripsoffline.

Evidence of partial-discharge (PD)activity often is visible to the eye,appearing as a white powder on thesurface of the stator winding (left).Severe PD damage on the outsidesurface of a stator coil shown belowwas attributed to a lack of semi-con-ductive coating on the surface of thecoil in the slot portion.

Prevention. Special equipment isneeded to detect PD activity. Statorslot couplers can be inserted under

the stator wedge to monitor magni-tude and frequency of the discharges.It is important to trend PD activityover time because different machineshave different baseline values.

Doubling of PD levels over aperiod of six months is cause forconcern. The machine should beopened up and inspected visually.Special corona suppression treat-ments can be applied to coil surfacesto minimize some types of PD activity.Photo, right, shows special cell- andcorner-section corona suppressiontreatment for a stator coil. Other

types of shielding arrangements canbe used as well, especially for phase-to-phase discharges.



68 COMBINED CYCLE JOURNAL, First Quarter 2007

GENERATOR OVERHAUL

Profiling a generatoroverhaul/rewindproject in the fieldand in the shop

National Mechanical Services’

personnel eased the Elk Hillsrotor out of the generator sta-

tor (1), bracing and blocking it incritical locations to prevent damage.Next, crane straps were installed (2)and the rotor positioned so the lift-ing rig could be attached (3). Therotor then was moved to the flatbed,packed in protective covers, andsecured for the trip cross-countryto National Electric Coil’s shops inColumbus, Ohio.

A stator rewind of the unit hadbeen decided on months earlier whenan annual borescope inspection

revealed corona, later confirmed bya rotor-out inspection that showeddiscoloration of the ground-wall insu-lation (4).

First step after the rotor wasremoved for the November outage:Label and document bus connectionsand other critical information beforeremoving the old windings (5). Onthis project, bus rings were reused,

but surge rings and blocking werereplaced.

When the rotor arrived in Colum-bus, its physical condition wasassessed to see if any damage hadoccurred in transit. None in evidenceand photographs were taken for a

permanent record. Next, the rotor wasinstalled in a lathe and run-out checksmade at more than 30 locations (6).

Rings off. Retaining rings wereprepped for removal (7) using a high-frequency heater. Ring measurementsof OD, ID, length, etc, were recordedand a dye-penetrant examination con-ducted to detect cracks or f laws in theforging (8).

Damper or amortisseur windingis visible under rotor end wedges (9).Note the brass squares used as build-up in front of the wedge. They hadbeen soldered onto a copper ring (at

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GENERATOR OVERHAUL

far left in 9) that encircles the rotor endwinding; damper windings normallyare silver-plated aluminum. This designrequired especially careful disassemblyof the rotor to prevent damage.

The design of the copper damperwinding used on the Elk Hills rotor is

considered unusual when comparedto generators of a similar age madeby other OEMs. The copper was laidin layers and joints were soldered.Note the buckling of the second layerof copper under the first layer’s failed joint (10).

A temporary set of slip rings wasused to induce a current into the rotorto conduct flux-probe and other tests(11). In 12, the rotor is fully assembledwith its exciter and ready for high-speed balancing.

Stator rewind. NEC field person-nel constructed an enclosure on the

front end of the generator housing toprovide shelter for technicians and

storage for tools and materials (13).Removal of the old windings is shownin 14. Cramped workspace and thehard epoxy-impregnated slot fillersmade coil removal difficult and time-consuming (15).

Some of the coils are shown readyfor the scrap heap in 16. Dark sec-tions visible on a few coils illustratethe effects of corona activity.

Every inch of the core was thor-oughly cleaned, then checked andrechecked to be sure all resin anddebris had been removed (17). Theoriginal surge rings of insulated steel

were replaced with fiberglass rings,which are stronger (18).

Last of the bottom bars is installedin 19. Wedges are driven into placeto maintain the proper bar-to-slotcontact and tightness (20). At the con-nection end of the machine, original

bus rings were tested; no repairs wererequired. Permanent ties and block-ing were installed when photo 21 wastaken; note that leads are prepared forthe brazing of clips.

After turn-to-turn testing for shorts,clips were brazed to connect the topand bottom coils—two at the turbineend and one on the connection end ofthe machine (22). Brazing complete,clips await installation of protectivecaps (23). El CID test confirmed thatcore iron had not been damaged dur-ing coil installation (24). Protective capswere installed and unit painted (25).

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70 COMBINED CYCLE JOURNAL, First Quarter 2007

GENERATOR OVERHAUL

aero community, recalled that thedamage suggested a rewind—ora new generator—as soon as theplant’s operating schedule couldaccommodate the work.

Hundley said preliminary num-bers suggested that a replacementBrush generator would not have

cost much more than a rewind; how-ever, the long lead time associatedwith a new unit was unacceptable.It would have put the plant at riskgiven the condition of the existinggenerator.

With the rotor out the followingsteps were taken to assure, to theextent possible, reliable operationuntil the stator could be rewound:

1. NMS cleaned up affected areasof the stator and made temporaryrepairs using an insulating epoxy.

2. The consultant recommendedthe installation of partial-discharge

analysis (PDA) instrumentation totrend PD activity. The instrumen-tation was installed by HamptonTedder Technical Services, Mont-clair, Calif. The thought here was

that if the condition of the statordid not deteriorate, the unit wouldcontinue to operate until the Novem-ber planned outage. If PD activityincreased to the point that failurewas considered imminent, the plantwould take a forced outage to rewind.To learn more about partial dis-

charge and predictive diagnostics,visit www.partial-discharge.com.

3. National Electric Coil, whichcommitted to rewinding the statorand conducting a full shop inspec-tion of the rotor within the plant’sdemanding outage window, wasbrought in to survey the generatorand take the samples necessary toplan the project and make replace-ment coils (Sidebar 2).

Assessment and repairs complete,NMS reinstalled the rotor and theplant successfully restarted the unit.Hundley had only good things to say

about the abilities of NMS and itsProject Manager Rudy Gaitan: Therotor was pulled in about a day anda half and replaced within three;no problems encountered. NMS’ da

Silva attributes that performance toan experienced in-house staff.

NEC’s Gary Barton, project man-ager—operations, said he had twoimmediate tasks: (1) Get the datagathered during the Elk Hills sitevisit to engineering so work couldbegin on the design of new coils,

and (2) develop a plan and tooling toremove the resin-saturated Dacron®felt filler used to secure the originalcoils in the slots in the iron core.

Barton said the 28-day windowfor the entire project was mademore challenging by the resin-satu-rated felt, which is very difficult toremove. A hiccup at this stage couldadversely affect schedule. NormallyBarton would add a few days to theoutage “just in case,” but this wasnot possible for the Elk Hills proj-ect. Also, NEC had to be preparedto deal with a forced outage, should

corona activity escalate to a criticallevel. This demanded an acceler-ated schedule for coil design andmanufacture.

In addition to a tight sched-

2. Inside an air-cooled generator

Key to the long-term health of anypiece of equipment is following

to the letter the inspection and main-tenance instructions provided by themanufacturer. Regarding the mainstator windings (1 in the diagram),which convert rotor power into voltsand amps, your primary goal is tokeep them clean, says Derek King,who manages Brush Turbogenera-tors Inc, Houston.

Visually inspect for dust, oil, for-eign-object damage (FOD), etc, atleast annually. Continuous monitor-ing of winding temperature allowstracking of trends that would indicatethe presence of crud and the needfor more frequent attention. If youfind crud, use a vacuum, brushes,

and/or lint-free cloth to clean thewindings. For stubborn deposits,wipe sparingly with orange oil orequivalent. Take care to avoid forc-ing contaminants into the insulation’screvices.

Suggested annual electrical testsinclude measuring (a) the polariza-tion index to determine the extentof moisture and dirt contamination,and (b) winding resistance to detectthe presence of broken conduc-tors. Also, conduct a reduced hi-pot(short for high-potential) test toinsure insulation integrity.

Stator core is the magneticheart of the stator (2). Air-inlet and–outlet temperatures are monitoredto ensure against damage from anover-temperature condition causedby a blockage of cooling-air flow.Check the instruction book for yourmachine’s temperature limits.

Annually, or more frequently,visually inspect for contaminationor signs of distress—such as dis-coloration, FOD, etc. Every 100,000hours Brush recommends an El CID(electromagnetic core imperfectiondetection) test to ensure that thereare no shorted laminations.

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1. Main stator windings2. Stator core3. Exciter permanent

magnet generator4. Exciter field5. Exciter armature

6. Exciter fan7. Diode wheel8. Rotor and body wedges9. Retaining end caps

13. End frame14. Winding supports15. Fan shroud16. Mounting feet/

hold-down bolts17. Fans18. Stator casing and frame

10. Exciter end of machine11. Oil seals12. Main bearing (exciter end)

19. Drive end/coupling20. Air inlet21. Air outlet22. Access doors23. Access to heater and

back of stator24. Sole plates




GENERATOR OVERHAUL

ule, NEC had to remain flexible onthe start date, as did subcontrac-tor NMS. In fact the project startedabout a week later than originallyplanned because of delays in com-pleting annual preventive mainte-nance on Unit 1.

On the second day of the planned

outage, NMS had the generator rotorsecured on the flatbed truck andready for the four-day trip to Colum-bus. The NEC repair shop wouldhave two weeks to complete its rings-off inspection and address any issuesidentified before shipping it back tothe site.

Once the rotor was out of the31,176-kVA, 13.8-kV machine, NECtechnicians began removing the oldwindings from the 42 stator slots.They were aided by a new tool,designed by the NEC engineeringdepartment, to facilitate removal of

the difficult slot filler material in acramped 3-ft-diam bore. To be surecritical field operations proceededsmoothly and without interruption,Barton acted as a proactive liaisonbetween the technicians and thecorporate engineering group whenquestions arose. Conference calls andrelay of job-progress photos by e-mailwere frequent.

Onsite personnel were chal-lenged by having to do a perfect job

in resin removal, this to prevent corehot spots and ensure proper fit-up of the new coils. Material removed fromthe slots, including the old coils, wasscrapped.

The new windings were manufac-tured well in advance of the outage,to ensure their availability had there

been a forced outage requiring ear-lier overhaul of the generator. Theywere at the plant before the rewindspecialists arrived. NEC engineersoptimized the design of the new coilsso more copper could be installedin the slots. Benefits include cooleroperation and a slight increase inpower production.

Also, ripple spring fillers wereused, instead of the conventionalpacking originally installed, to holdthe coils in place. The expectation:They will last for the remainder of the generator’s intended service life.

Barton said that felt side packingoften degrades over time, allowingthe coils to move. When this occurs itcan facilitate the onset of corona.

Other generator-related fieldwork conducted in parallel with therewind was cleanup of the generatorbearings by a specialty contractor toeliminate coking in evidence. Serviceincluded sump cleaning and a newcharge of bearing oil. Such activity isnot typical on this type of project.

While field work proceeded, therotor was being serviced in NEC’sColumbus shops. The procedure wasstandard, one with a proven successrate. After the rotor was off-loadedfrom the flatbed, runout and jour-nals were checked, and followinga thorough visual inspection (with

plenty of photos to record machinecondition), the retaining rings wereremoved.

Tests included the standard “meg-ger,” plus a polarization index or PI,to measure insulation resistance andverify coil cleanliness. After testing,the rotor was cleaned, reassembled,and balanced at high speed.

Back in the oil patch. The sta-tor rewind was straight-forwardafter the old coil was removed; test-ing verified the electrical integ-rity of the new field. Hundley saidthe machine has been running well

since restart, which went off withouta hitch. He added that the machine’soutput actually has increased byapproximately 400 kW with the newstator. Hundley doesn’t seem likehe could be happier with the perfor-mance of NEC and NMS—true pro-fessionals, he said. The accompany-ing outage photo album presents avisual record of the project. A rewindof Unit 1 at the plant is scheduledfor the fall. ccj

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Stator Rewind Solves Corona Problems

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