Maintenance of hydro-electric plantB.W. McMillan

Indexing terms: Generators, Natural resources, Engineering administration and management

Abstract: The paper discusses the maintenance of both pumped storage and conventional hydro-electric plantin the United Kingdom, dealing specifically with the experiences of one Group of the North of Scotland Hydro-Electric Board. The types of hydro-electric plant considered are geographically well spread and are subject to anumber of different hydraulic regimes. A computer program developed for the best utilisation of maintenanceresources entitled 'generation equipment maintenance' (GEM) is introduced and discussed.

1 Introduction

This paper attempts to cover the spectrum of maintenanceon hydro-electric plant in the United Kingdom, but wherespecific times and frequencies are quoted, they representthe experience in one Group of the North of ScotlandHydro-Electric Board.

The maintenance requirements of conventional hydroplant and pumped storage plant are markedly different,due to their widely differing operating regimes.

2 Operating regimes

Conventional hydro plant tends to be in the range of10-40 MW units, and normally operates either as peakingplant or mid-load factor plant. Exceptions to this are a fewsets situated in rivers where some compensation running isrequired at all times to utilise water discharge.

Peaking plant features 4, or possibly 5, starts and stopsper day, with only 2 or 3 running hours per day. Mid-loadfactor plant will have 2 to 3 starts and stops per day, and 8to 10 running hours. These are average figures, and it isnot unusual for mid-load factor plant, which is the maintype found, to do extended periods of 24 hour running inwinter, during periods of high run-off, and to be little usedfor long periods in the summer.

The wear and tear on main equipment following theseregimes is not high, and high frequency routine main-tenance is limited to circuit breakers and auxiliaries inorder to give good starting reliability. There will be manyoccasions in summer when maintenance can be arrangedwithout loss of water, or interference with the systemrunning programme.

Pump storage plant, on the other hand, is highly flexiblein its operation, and not particularly subject to run-offconstraints. At times of high system demand, generation isrequired both for energy supply and system security, andin periods of low system demand, pumps are needed tomaintain efficient loads on high-merit base load plant. Thismeans that there are seldom times when it is easy for thesystem to release pumped storage plant for maintenance,even for one day's duration. The machines range from90-300 MW in size, and may be called upon to perform upto 30 mode changes per day. About one-third of these willinvolve stopping and starting. The consequences of this arehigh acceleration/deceleration stresses, thermal cycling ofthe generator core and windings and repeated circuitbreaker operation. Typical running hours are up to 17 perday in one mode or other.

In the main, pump storage plant is of the reversibletype, whereby the generator also operates as a motor and

Paper 4331C, (P10), received 1st October 1984The author is with the North of Scotland Hydro-Electric Board, South CaledoniaArea, Blackfriars, Perth PHI 5LT, United Kingdom

the impeller operates either as a pump or turbine. This hasthe advantage of high utilisation of the plant, but a dis-advantage is that shaft reversals are necessary for changesbetween pump and generate modes, with consequentialwear and tear on main plant, circuit breakers, controlsystem components and auxiliaries. Pump storage plant inaddition is designed to operate in spin modes of bothpump and generate, where the turbine tailwater isdepressed by compressed air, thus reducing the frictionlosses, with the machine synchronised to the system. Itsloading time is thus reduced to approximately 40 seconds.This, however, means that the generator can rapidly swingbetween full load and virtually no load. These duty cyclescause severe thermal cycling of the core and windings, andthis has an adverse effect on their long-term life. Theretends also to be periods of high vibration during modetransfer. The pump storage duty is much more severe onthe plant.

3 Maintenance

Maintenance can be divided conveniently into two maincategories:

(i) Routine planned preventative maintenance(ii) Major overhauls.

The principal difference between these two items is that inthe first case, the main machine parts are not significantlydismantled, whereas in the second case the dismantling iscarried out completely for major part refurbishment.

3.1 Routine maintenanceIn the first case there are, in the plan, items which experi-ence has determined require servicing at intervals fromweeks up to 3 years. If they do not receive this servicingthey are likely to give poor performance or reduce theplant availability. All running main plant requires weeklyattention of a greasing, instrument monitoring, generalstation cleaning, and security checks. Beyond this, on con-ventional hydro plant, the most common frequency formaintenance for auxiliaries and brushgear, etc. is quarterly,although obviously this is influenced by any abnormalrunning hours. Circuit breakers being basically upratingsof distribution types, require maintenance on a threemonthly frequency on their auxiliary mechanisms, due tothe higher number of operations than is normal on dis-tribution systems. Auxiliaries and control sequence partsrequire checking. Annually, preventative checks are carriedout on the turbine and MIV. This work needs an outage of3-4 weeks. Vibration monitoring and pipe thicknesschecks provide useful guides to future maintenancerequirements, and fibre optics aids inspection in difficultareas.

Pump storage plant, on the other hand, requires 5weekly checks and maintenance of brushgear and air blast

142 IEE PROCEEDINGS, Vol. 133, Pt. C, No. 3, APRIL 1986

circuit breaker mechanisms in order to prevent failure.During this outage, mechanical checks are carried out inthe guide vane and turbine areas, principally for slacknessor metal pick-up. This work takes approximately one shiftto complete, although in winter, where the return toservice for early tea-time peaks is mandatory, this willsometimes be carried out on two half-shifts. Annually,outages of up to 6 weeks are necessary to check main unitand auxiliaries. It is clear that, due to the much higher andharder duty cycle of pumped storage, a higher frequency ofpreventative maintenance routines is required than that fora conventional plant.

3.2 Major overhaulThe second category of maintenance is that where majorstripdown is required. The majority of conventional andpump storage plant is of vertical configuration, and inmost cases it is not practicable to remove the turbine com-ponents from below, thus necessitating rotor removal. Inany case, the maintenance engineer can only carry outlimited inspections of areas such as air ducts in the statorcore, the rotor damper bars or turbine runner seals, whenthe set is fully assembled. Experience has shown that aftera period of somewhere around 10 years operation it isprudent to have a major overhaul. There are not statutoryconstraints similar to boiler inspections on thermal plantwhich enforce such outages, and the period for inspectionis based upon a combination of experience of the plant andthe requirement, where possible, to carry out the workwith 'in house' resources. The wear and tear on turbinerunners and their seals tends to be caused by cavitation orsand erosion, and, to date, experience with pump storageand conventional hydro would not indicate a marked dif-ference in deterioration due to this cause between the twooperating regimes. However, the higher running levels ofvibration and the stresses associated with handling an air/water mix during mode transitions on pump storage,causes considerably greater wear on guide vane regulatinggear equipment and turbine bearings than is experiencedon conventional hydro. Thrust bearings have deteriorateddue to thermal ratcheting effects, requiring pad replace-ment as often as every two years with tin-based pads. Leadbased pads have performed better.

In so far as the main generators are concerned, themajor stripdown maintenance required for conventionalhydro is restricted to cleaning of air ducts, removing ofsurface contamination and occasionally tightening upwedging or bracings. This type of attention would be smallon conventional generators. As far as pump storage is con-cerned, the varying load cycle causing thermal cycling hasbeen responsible for deteriorations in the tightness of coresand degradation of winding insulation, necessitating amuch higher level of generator maintenance. In an attemptto schedule more accurately the deterioration of electricalwindings, a programme of dielectric loss analysis and tan-delta measurements has been undertaken, and this hasbeen reasonably successful in predicting the approach offailure on a number of windings. The intention is to sched-ule these generators for rewind before significant failure islikely to occur, in order that, in the case of conventionalplant, no major water loss takes place (typically £4000 +per day) because of an unplanned winter outage, or, in thecase of pump storage plant, no unplanned loss of avail-ability.

The maintenance of hydro-electric plant in the North ofScotland Hydro-Electric Board (NSHEB) tends to becarried out by a relatively small number of manual staff,typically 6 mechanical and 4 electrical craftsmen, with a

number of general duties or unskilled staff in support.Building and civil maintenance is carried out by a HeadOffice resource in the NSHEB and only routine work,such as screen cleaning, aqueduct running maintenanceand minor civil repairs, are carried out by local staff. Themaintenance workforce is usually located centrally withstation attendants at the locality, so transport must beorganised. It is common for the maintenance organisationto be responsible for at least 15 conventional generatorsand up to 4 pump storage units. The plant requiring main-tenance is spread across a large geographical area, withdistances of 50 km between a central working point andthe set requiring attention being not untypical. (Fig. 1)

N

Nant • / f • Sron Mor( / j Clachan. •Allt-na-Lairige

• Kilmelfort • s? !.s i°y

^^__^^_ Inverawe( * • Dalmally

/ * Cruachan

N.S.H.E.B/ S.sI.B

Fig. 1 Sloy-Awe generation gap

4 Organisation of work

It is necessary to arrange the maintenance in such a waythat the workforce is presented with a reasonably evenworkload throughout the year, and also that the plant beoverhauled in a reasonable time scale compatible withsystem demands, and any hydraulic constraints that mayapply.

Obvious hydraulic constraints are the desirability not tolose stored water during the outages, and in some cases tohave the sets available for the passing of compensationwater in rivers. Broadly speaking, hydro plant can bemade available for working between April and Septemberwithout significant risk of water loss, but during the othermonths the run-off in the British catchments is such thatvery extensive cost penalties could be incurred. Fortu-nately, with the exception of peak system load times, pumpstorage is at present most economically available for main-tenance during the late autumn and early spring months.This helps considerably to even out the workload for staff.

5 GEM program

It is clear, however, that there are a number of constraintswhich must be optimised in order to make the best returns

IEE PROCEEDINGS, Vol. 133, Pt. C, No. 3, APRIL 1986 143

for the Board in terms of plant availability versus main-tenance costs. In order to accommodate these, the Boardhas developed a computer program for routine plannedpreventative maintenance called Generation EquipmentMaintenance, GEM for short. GEM schedules the entireroutine planned preventative maintenance scheme for eachgroup, on the basis of the maintenance frequencies as setout by the relevant engineering staff for all electrical,mechanical, civil and operational work. The method ofcontrol is that the job priority follows a straight line curve,designed to give it a value of 100 on due date and pro-portionately higher numbers thereafter. This curve is resetto zero when the work is done. In this way the work due ispresented by the computer in a manner such that one canreadily identify work which is overdue through the workcurrently due, right down to whatever advance notice ofwork has been preset. The system also accommodatesstatutory work which is not allowed to progress above100, and it is capable of scheduling work which requires tobe done on a particular calendar date, such as the install-ation of fish screens. The system operates from a computerterminal in the main group headquarters, and once a weeka schedule of work due is produced for integrating withany defects which may have arisen during the previousweek.

This schedule also identifies the labour resources necess-ary to complete each task and summarises it, for readyassessment. The maintenance planning staff can comparethe workload with the staff available and the consequencesof any decisions they make are readily apparent, based onthe priority numbers of the items concerned. Having sel-ected the work to be carried out, the computer programproduces all the work documentation necessary for presen-tation to the manual staff, including all references to workspecifications and manufacturer's recommendations,together with appropriate safety precautions. All printingis carried out at group headquarters.

When completed cards are returned they are entered viathe computer terminal: this automatically causes theprogram for the particular item to be reset to zero, and sothe climb up the priority curve begins again. Should anycard fail to be returned, a periodic 'work outstanding' list

is produced from the computer, which will identify theseomissions. In this way all work which is due to be carriedout is presented for consideration and also any work itemwhich becomes lost is also identified for appropriateaction. The program has been set up in order to providevarious management reports which allow for continuousassessment of maintenance performance and a readykeeping of maintenance records.

6 PERT

The GEM system does not do any evaluation of workloadagainst resources, and therefore does not help in the actualjob control. When some extensive outages have to bearranged, such as the annual overhaul of both convention-al and pump storage plant, or during the total stripdownsof such plant, it is obviously necessary to provide moredetailed project control. For this the Board utilises aPERT program, which operates on the traditional basisof critical path analysis.

Having set the flow network for all the tasks to be com-pleted, the computer is capable of being interrogated fromthe terminal, with any changes which may have occurredand provide rapid reassessments of end dates. It alsoallows maintenance engineers to test the beneficial natureof costly overtime work, the deployment of unsocial shiftsor utilisation of contractors. This type of computerprogram does not produce work order cards, but by cross-coding the system with the GEM it has proved possible forall work order cards to be produced by the computer.

7 Conclusion

The use of the computer has released engineers from semi-clerical and minute-by-minute control of work to carry outtheir technical function or devote more time to forwardplanning and budgetary control.

In conclusion, numbers of hydro units, spread over aconsiderable geographical area, can be maintained to ahigh degree of efficiency and availability as long as thelimited resources of men, transport and outage times arecarefully co-ordinated.

144 IEE PROCEEDINGS, Vol. 133, Pt. C, No. 3, APRIL 1986