Presented at EPRI Utility Generator Predictive Maintenance and Refurbishment ConferencePhoenix, Arizona, December 3, 1998.

Preventive Maintenance and Overhaul Experience for Rotating Brushless Exciters

and other Excitation Systems

by

Tom Figiel Jim Bothwell William Moore, P.E. Commonwealth Edison Consultant National Electric Coil

ABSTRACT

Rotating brushless exciters were originally developed to overcome the problems associated withgeared exciters, brushes and brush dust, collector rings, fires, the potential danger to personnelchanging brushes on line, and the routine maintenance required for brush type exciters.i Althoughthese units were originally designed to improve reliability, over the years new problems havesurfaced, reducing their reliability and pointing to the need to do a certain measure ofpreventative maintenance. The brushless exciter did eliminate the maintenance associated withreplacing carbon brushes. It also eliminated the generation of carbon dust, the source of fires onunits that were not maintained well. What was not anticipated, however, was that these rotatingexciters, developed their own unique operating problems. The number of possible failure modesactually increased!

Age, contamination, and mechanical stress are a few of the principal contributors. Catastrophicbrushless exciter failures have prompted some owners to replace their brushless exciters withstatic excitation systems. The cost to purchase and install a replacement static excitation systemis very high. A less expensive but equally reliable solution is to carry out important preventivemaintenance activities and keep failures from occurring. This paper presents importantpreventive maintenance activities, necessary to ensure high reliability. Common failure modesare reviewed as well as important overhaul items that should be addressed. For example,contamination, including the entrance of foreign objects is, possibly, the number one cause ofbrushless exciter failures. This paper presents options to prevent or at least minimizecontamination and, also, steps to take if contamination occurs, in order to inhibit a potentialfailure. Other component failures are addressed, including fuses and diodes, phase leads, glassand wire banding, and coils. Testing methods and guidelines are included to provide the readerwith a reference for future work. Finally, a case history is provided, along with photodocumentation.

INTRODUCTION

In 1950, Westinghouse received a patent covering the basic features of a rotating brushlessexciter.ii In 1960, the world’s first 180 kW, 250 volt, 720 amp, 3600 RPM brushless exciter wasplaced into service.iii Although brushless exciters were originally designed to improve reliability,over the years new problems have surfaced, reducing their reliability and pointing to the need to

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do a certain measure of preventative maintenance. Significant, catastrophic failures, could force aunit out of service for a long, long time. During a 20 year time span, from 1965 to 1985, lostMwh for exciters (6,156) exceeded that for rotor windings (5,981) according to data supplied byNERC/GADS (North American Electric Reliability Council/Generator Availability DataSystems). iv

In addition to overcoming various reliability and maintenance problems germane to brush-typeexciters, brushless exciters were also developed because of the amperage limitations of thebrush-type exciter and the low efficiency. The amperage limitation on brushes at the time ofbrushless exciter development was about 6,000 amperes. An 800 MW generator, for instance,would utilize about 10,000 amperes in the generator rotor at full load. The current carryingability of the "screw-in" diodes available at the time limited the amount of direct current (DC)that could be produced. When more current was needed, a second or tandem exciter was added.This arrangement put two brushless exciters in parallel supplying current to the generator rotor.When “pancake” or “hockey puck” diodes were developed, which could handle much highercurrents, the size of brushless exciters was greatly reduced. These higher currents, however,resulted in higher risks of failure, due to the close proximity of these higher current carrying non-insulated components to each other.

GENERAL OVERVIEW

A typical rotating brushless exciter is made up of the following major components: a pilotexciter, an AC generator, and the rectifier or diode wheels. A photograph of the majorcomponents is shown in Figure 1. In general, the theory of operation is as follows: Alternatingcurrent (AC) produced by the permanent magnet generator (PMG) is rectified to direct current(DC) inside the voltage regulator cabinet. A controlled direct current is fed to the stationary fieldof the AC exciter. This induces a voltage in the AC exciter armature. This alternating current isthen rectified by the rotating diode wheels back to direct current and fed by leads directlythrough to the generator bore copper main leads and finally to the main rotor winding. Thecomplicated nature of the brushless exciter design, along with its many rotating, high currentcarrying components, has led to many different types of failures. Outlined below are thepredominant brushless exciter failure modes as experienced by the authors. Recommendationsfor prevention of these failures are discussed.

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Figure 1: Major exciter components

CONTAMINATION INCLUDING FOREIGN OBJECTS

Contamination from dirt, fly ash, or other foreign objects in the exciter, especially in the diodewheel component area, can lead to significant failures. Most of these exciter components are air-cooled, with once-through ventilation of filtered, cooled air. Dirt and fly ash can enter excitersthrough filters, open doors, window seal leaks, door seal leaks, housing seal leaks, and so on.These contaminants can build up on the heat sinks and eventually provide an electrical path fromthe diode heat sinks to the diode wheel forging, bypassing the insulation between the heat sinkand the diode wheel. Exciters, such as at the Kincaid station, (Mark II) have a double layer of G-11 around the inside diameter of the diode wheel. These models allow dirt and fly ash to build upsufficiently to bridge the space between adjacent heat sinks. Exciters like ComEd’s Collins(Mark III) utilize a “tray” insulator under each heat sink module, so the build up of dirt and flyash could not progress as far as it does in the Mark II exciters before resulting in a short. Thesingle-wheel Mark III exciters also have a small gap between the end of the heat sinks and thecollector ring where dirt or a foreign object can lead to a short.

Dirt and fly ash can build up around the diodes separating the AC and DC components, providingan electrical path. “Tracking” from one component to another can occur, shorting thecomponents together or “tracking” to ground surfaces. Once a tracking path is burned in byvoltage, the very high current in the exciter does the damage. The flash and arcing is similar to anair arc, which is used to cut steel. The ensuing flow or arc of alternating current does damagesignificant enough to the diode wheel components to render the exciter inoperable and causes aforced outage.

Foreign objects causing failures have included loose shim stock, which worked its way out frombetween the exciter frame and the floor plate; loose objects left in the exciter house; tools left inthe diode wheel; and wire strands from the vent filters on top of the exciter house.v

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Figure 2 shows what remains of some of the diode wheel insulation, after one exciter was forcedout of service. The ground and ensuing arcing and burning was so severe, the diode wheelforging was unusable.

Figure 2: Burned remains of diode wheel insulation

The diode wheels cannot be readily seen on most units, hence the build up dirt and fly ash is notoften noticed. Some utilities have gone to periodic cleaning of the diode wheels by removing thestatic air baffles and cleaning the diode wheels with brushes and vacuum cleaners.

ComEd, because of the inevitable build up of dirt and fly ash in its coal plants, overhauls itsbrushless exciters every eight to ten years. A complete disassembly of the unit is done, includingall components of the diode wheels. The AC winding along with the fuses and diodes are testedas described later in this paper. Insulation is cleaned or replaced. Damaged wiring on the exciterskid is replaced. Glass banding is inspected and repaired, if necessary. Electrical connections areimproved. The unit is properly aligned and leveled. It is believed that the minor cost of thispreventive maintenance is especially necessary for the brushless exciters and well worth themoney spent. Figure 3 shows one of the exciters from Kincaid station, being leveled andprepped for shipment, after being overhauled at the National Electric Coil service facility.

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Figure 3: Leveled exciter prior to preparation for shipping

Of course, it is best to prevent contaminants from entering the exciter in the first place. It ispossible to retrofit air-cooled brushless exciters with a positive pressure ventilation system.vi Thissystem inhibits accumulation of dust and dirt on the diode wheels by maintaining greater thanatmospheric pressure inside the exciter house.

POOR ELECTRICAL CONNECTIONS

It is conceivable that a large number of brushless exciter failures have been attributed to foreignobjects, even though the object was never found. We suspect that the cause of these failures, inmany instances, is poor electrical connections between the heat sink leads and the heat sinks.Poor electrical connections at these locations cause the high currents to flow through themounting bolts instead of across the contact surfaces, heating the bolts, which, in turn, vaporizethe resin molecules in the insulation and ultimately provide a path between the AC and DCcomponents.

Outlined below, in sequential steps, is one scenario involving the failure of the insulationbetween heat sinks and the diode wheels: Alternating current passes from the outboard heat sink through the heat sink lead connection

point, the heat sink lead, and the inboard diode wheel heat sink lead connection point to theinboard diode wheel heat sink. One heat sink lead connects two heat sinks, one heat sink ineach diode wheel.

The inadequate electrical connection between the heat sink leads and the heat sinks causesthe alternating current to pass through the steel bolt attaching each heat sink lead to itsrespective heat sink. By design, no current should pass through the bolting of an electricalconnection. All current should pass across the interface between conductors.

The 5/16” steel bolts, not being adequate electrical conductors, heat up when the alternatingcurrent passes through the bolts, instead of across the conductor interface.

Heat produced in the bolts, vaporizes the resin in the thin layer of insulation, which is underthe heat sinks and between the heat sinks and the diode wheels.

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When the epoxy is vaporized, the insulating material, between the heat sinks and the diodewheels, disintegrates.

The elimination of insulation between the heat sinks and the diode wheels provides anelectrical path that bypasses the diodes and fuses on the diode wheels.

Once a short circuit electrical path is established between a heat sink and the diode wheel, anair arc condition erupts. This condition can engulf multiple bare heat sink leads until thevoltage regulator, Loss of Field protective relays, or the operator, responding to the erraticgenerator and/or VAR swinging, recognizes the condition and trips the unit.

A combination of factors can cause inadequate electrical connections between heat sink leadsand heat sinks: Copper heat sink leads are relatively soft as compared to steel bolts and washers. Copper deforms over time with mechanical stress, due to the tightening of the heat sink lead

connection bolt. Heat sink leads have “slots” instead of “holes” at junction points, in order to provide a

measure of flexibility when installing the heat sinks in the diode wheels. The original washer distorts and can “pull” into the slot in the heat sink lead, expanding the

width of the slot, and hence, the heat sink lead. When a washer “pulls” or “cups” into the slot in the heat sink lead, the copper leaves bow

away from the connection surface. The original washer used is a standard 5/16” ID washer with an 11/16” OD, 1/16” thick.

Insulating varnish routinely found on the connection points between the heat sink leads and heatsinks is a poor connection, but it is a symptom of poor mechanical joints, not the cause of a poorconnection. If the connection point were adequate, varnish painted on after assembly would notbe able to penetrate and coat the connection surface.

Solution and Improvement

Correction of inadequate electrical connections, at the heat sink lead to the heat sink interface,results in restoring a good electrical path for all current to pass through the electrical junction inpreference to the bolts. The inadequate connections are corrected by thoroughly cleaning theconnection points and reassembling the connections using a 5/16” stainless steel washer that is1/8” thick and has a 7/8” outside dimension (OD). The improved washer, as compared to the oldwasher, increases the compression area on the heat sink lead, provides uniform compression ofthe heat sink lead against the heat sink connection point, and will not deform.

MOISTURE

The presence of contaminated water, condensation, or any type of moisture can also cause failureof the diode wheel components. Electrical “tracking,” as described earlier, can occur withmoisture in the same way as it does with dirt or fly ash. Moisture can also lower the insulationresistance of the diode wheel components and the winding.

Outdoor generating units in high humidity areas are prone to having moisture form in the exciterhouse through condensation on the cooling coils. Condensation was so much of a problem atFlorida Power & Light’s (FPL) Martin, Manatee and Sanford stations that the cooling waterwould often be shut off when the units cycled off at night.

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Moisture can also be a problem on outdoor units, if the seal between the exciter house and thesole plate is not adequate. One of the units in southern Florida was found to be sucking water offthe turbine deck and into the exciter house in the area of the PMG. The problem was found whenthe rotor ground detector indicated a problem. The unit was shut down and the exciter houseswabbed and then vacuumed. The base was temporarily sealed with a bead of RTV. A morepermanent fix was enacted during the next refueling outage. Better seals and their correctinstallation solve this problem.

Of course, cooler leaks, inside the exciter housing, can also be a source of moisture. Cooler leaksshould be repaired immediately.

VIBRATION

Other than design deficiencies, failed components, or system disturbances, many vibrationproblems can be caused by improper installation. It can be prudent to high speed balance exciterrotors, especially if the AC exciter rotor is rewound, or if a substantial number of components arereplaced. Instead of utilizing correct alignment and installation procedures, some installers havetried to overload the brushless exciter bearing in order to control vibration. They “load” thebearing by installing the exciter with a larger gap at the bottom of the coupling prior to actuallycoupling the exciter and main generator rotor. When the coupling is bolted, the shaft pushesdown on the exciter bearing. In one case, this solution to vibration led to the fatiguing andbreaking of the exciter shaft just behind the coupling. In other cases, it removed the load on thegenerator bearing causing the generator rotor to go into “oil whip.” The generator shaft just“wobbled” in the generator bearing.

Another installation technique also has caused some problems. In some cases, long shims havebeen used between the brushless exciter base and the sole plate. These long shims could not bereadily sized for the gap between the exciter and the sole plate, because the PMG end of theexciter was higher than the coupling end. In these cases, a general size shim was selected, whichmeant that the exciter frame was bowed when it was tightened down.

We suggest that the correct shims should be Quick Shims or some other commercially availableshim material that is installed around each of the eight hold down bolts. The installation shouldbe just like any motor or pump. The exciter base is a rigid body with eight hold down points. It isthe opinion of the authors that if manufacturers had designed and welded small pads to thebottom of the exciter frame at the hold down points, installers would have been less inclined toimprovise with long shims.

The first brushless exciters had six or eight hold down bolts. The potential for failures, orpossible subsequent damage from failures, brought about the use of large foundation studs closerto the centerline of the exciter. The function of these studs was to prevent a severe unbalancefrom tearing the exciter base off its foundation or sole plate. It is likely that this preventivemeasure had its roots in a real occurrence in which the hold down bolts had been snapped. Theauthors are aware of at least one brushless exciter, located in the Western US, that was run withenough unbalance to destroy the hold down bolts of the bearing pedestal.

However, in a proper installation, the nuts on the foundation studs should be only finger tight.They can be double nutted or have a bead of RTV applied to prevent the nuts from backing off,but they should not be tightened. The studs are in an area of the exciter base where shims cannot

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be installed. Consequently, tightening the nuts on the studs will distort the frame and put it in astressed condition, in the same way that a snare drum that will amplify vibration.

Again, the brushless exciter should be installed like any motor or pump. Shims are installed ateach of the eight hold-down bolt locations and soft foot should be checked at each bolt.

Of course, during an overhaul, if all the diode wheel components are removed, it is veryimportant to return them to the same location, so as not to upset the original balance. Unequalweighting of these components, due to their location on the large diameter diode wheel, can havesignificant impact on rotor unbalance.vii When an original component is replaced, thereplacement part weight must not vary significantly from the original part’s weight, according tothe manufacturer’s recommendations.

GLASS BANDING

Glass banding, although an improvement over wire banding, has failed on occasion and causedits share of lost generation time. If the banding is not initially installed correctly, either due to alack of tension, or a poor resin content, the banding will become a chronic source of maintenanceproblems for years to come. At both FPL and ComEd, exciters have shed sections of glass bandover the years. This can raise a great deal of concern for station personnel who believe the bandis about to give way. In general, though, the glass banding sections are small, not significantlyreducing the “hold strength” of the band. Repairing the glass band is often possible by sandingthe frayed fibers, and recoating them with a good epoxy resin. This provides an adequatetemporary solution, until another section peels off. Rebanding is the only permanent solution.

Cracks in the glass bands that hold the exciter rotor end windings in place can be quitedisturbing, when observed for the first time. Circumferential cracks, as shown in Figure 4, arenot of much concern, unless the band is moving axially or it continues to fray and unravel. Thistype of crack should be repaired, and can be sanded and sealed with a good epoxy resin.However, a crack running in the axial direction, is cause for great concern, since the fibers aretearing in a direction that may allow the band to burst. This type of glass band separation must beaddressed immediately.

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Figure 4: Circumferential cracks in glass bands

On some exciter designs, risers or phase leads between the exciter windings and the conductorsfastened to the shaft were made out of straight pieces of copper with a “U” shape at the top abovethe solder point. Banding was installed at the factory in the “U” area to maintain the position ofthe risers or phase leads. In operation, the straight pieces of copper moved outward and cut intothe banding. On occasion, the banding failed and all of the risers or phase leads spun out. Theunits looked like flowers when the dust settled. As a solution, the risers or phase leads werechanged to curved, multilayer copper pieces with soldered/solid ends that attached to thegenerator leads and connection points at near the shaft. This, along with improved glass bandingtechniques, minimized the number of glass band failures in this area. Fatigue cracking of thesecopper pieces, as described in the next section, still continued to occur.

PHASE LEAD CRACKING

Design of the phase leads has progressed from rigid type connections to more flexible loops orrisers. Fatigue cracking and failures still occur, however. These connections, generally made withbrazed or soft soldered joints, are highly stressed, since the copper becomes annealed from theheating during operation. With a low yield point, changes in boundary conditions, and repeatedspeed cycles, the copper can fatigue and eventually can fail.

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Figure 5: Fatigue cracks in exciter phase leads

Figure 5 shows a looped phase lead design, consisting of two copper laminations, 0.060 inchesthick. The underside copper conductor had failed in three separate and different locations. For atwo-strap design, when the underside strap fails, it causes additional increased load to the topsideconductor. This increased load will soon cause the remaining strap to fail. A thorough redesignby NEC, including FEA (finite element analysis) as shown in Figure 6, resulted in a design offour conductors, of half the original thickness, formed to a pure catenary shape. Stresses werereduced by assuring the component was put “in tension only” and by carefully controlling theboundary conditions during assembly. The unit is back in service and has been runningsuccessfully for the past year.

Figure 6: Redesigned phase lead profile

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BEARING INSULATION

Maintaining the integrity of the insulated bearing during reassembly of the exciter is crucial. Abrushless exciter with a grounded bearing pedestal might run a week or so, depending on theseverity of the ground, before enough metal is removed from the exciter bearing to allow themagnets on the PMG to contact the stator windings of the PMG and cause failure. Some utilitieshave installed double insulated bearing pedestals. Those that have not, must assure bearinginsulation integrity. The final check should be performed while doing the swing check - when thebearing is rolled out and the oil drain line is connected.

To perform a swing check, the exciter shaft is put in a sling, usually nylon, and the exciterbearing is rolled out. Dial indicators are installed on both sides of the exciter shaft near the slingsupport and the turbine-generator is put on turning gear. If the coupling bolts have not beentightened uniformly or if there is something on the coupling face the shaft will "swing" awayfrom the high spot or the area that is not as tight. Total run out should be less than 0.0015 inchesbut most often it is zero or less than 0.001 inches.

ELECTRICAL COMPONENT TESTING

Electrical component testing of the exciter and its varied components should be done during theoverhaul. Fuse element resistances should be checked to assure the elements are not neckingdown (a process that precedes mechanical failure of the fuse element). Compare fuse resistancesand replace any that vary widely from the calculated average. Diodes almost always fail opencircuit, so the fuse does not fail and lift the indicating pin or flag. Checking the diodes during anoutage is the only time that an open diode will be found. Perform a diode check with a Flukemultimeter, and then with a 500 vdc megger. The windings on the PMG, the stator poles androtor winding are very durable, but they should have their insulation resistance checked, also.

OTHER IMPORTANT ISSUES FOR GOOD OVERHAULS

It is not recommended to do a steam clean and bake on any of the exciter components, since themechanical stress from this operation, may damage the insulation. The best alternative is to handclean with alcohol-dampened rags.

The integrity of the RTD’s and/or thermocouples should be verified. Replace any that aredefective. Repair or replace any frayed wiring found during inspection. Bearing and oil sealclearances should be checked and brought back to original sizes. These, especially, are critical tomaintaining proper vibration levels. Fan to shroud clearances should be measured upondisassembly. Special attention during reassembly is necessary to maintain the proper clearances.The use of shims may be required.

SUMMARY

The authors have extensive experience in the design, operation, maintenance and repair ofrotating brushless exciters. Based on this experience, predominant failure modes of brushlessexciters are identified and discussed. Preventive maintenance solutions to mitigate these failuremodes are offered. Important considerations when overhauling these units are also presented.

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By implementing regular preventive maintenance activities, such as described above, acceptablelevels brushless exciter reliability can be maintained.

For further information, the authors can be reached by email, at the following addresses:

Mr. Tom Figiel:thomas.g.figiel@ucm.com

Mr. Jim Bothwell: jsbothwell@aol.com

Mr. Bill Moore:WMOORENEC@aol.com

i “Brushless Excitation of 660 MW Generators,” W.W. Holburn, Journal of Science and Technology,Vol. 37, No. 2, 1970.ii Westinghouse Advanced School in Power Systems Engineering, “Update on Brushless Exciter,” D.I.Gorden, October 8, 1974.iii Westinghouse Advanced School in Power Systems Engineering, “Update on Brushless Exciter,” D.I.Gorden, October 8, 1974.iv NERC/GADS Summary Data on Generator Unavailability by Cause Code, 1965 to 1985.v “Suggested Solutions to Generator and Exciter Problems,” J. Bothwell, EPRI Generator ReliabilityWorkshop, 1985.vi “Positive Pressure Ventilation System,” A. Spisak, ASME - International Joint Power GenerationConference, Baltimore, MD, 1998.vii “Improvements for Generator Rotor Unbalance,” W. G. Moore, ASME - International Joint PowerGeneration Conference, Baltimore, MD, 1998.