Electrical auxiliary supply systems for hydro-electric power plants

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<ul><li><p>Electrical auxiliary supply systems forhydro-electric power plants</p><p>J.A. Wade. B.Sc.(Eng.). C.Eng., M.I.E.E.Indexing terms: Natural resources, Power systems and plant</p><p>Abstract: The paper examines the auxiliary equipment of hydro-electric generating stations and associateddams, spillways and other works in respect of the essentiality of electrical auxiliary supply to these equipments.Various alternative sources of auxiliary electrical supply are considered, the advantages and disadvantages ofthese are compared and proposals are made for high voltage distribution systems around the hydro-electricplant and associated works. Arrangements for DC supply systems and uninterruptable power supply (UPS)systems are also discussed.</p><p>1 IntroductionThe safety and continuity of output of a generating plantlargely depend on the reliability of the electrical supplies tothe auxiliaries. Hence careful consideration should begiven to the design of the electrical auxiliary distributionsystem.</p><p>As a point of interest, the electrical load which isrequired for running the various items of auxiliary plantand services on a hydro-electric project, including those atthe associated dam, spillway and other works, mayamount to about j % of the maximum station capacity inthe case of a normal hydro-electric station above ground.This may be up to 2 to 2\% of the station capacity in thecase of a pumped storage generating station situated belowground.</p><p>2 General auxiliaries and their supplyrequirements</p><p>In arriving at the arrangements required for an auxiliaryelectrical supply system, it is firstly pertinent to considerthe auxiliaries for which electrical supplies have to be pro-vided and the circumstances surrounding their use. Fromthis one can then evaluate whether the auxiliary supplieswhich should be provided in each case need to be highlysecure, whether a standby supply needs to be immediatelyavailable or can be less immediately available, and whethera standby supply is necessary under all operating condi-tions.</p><p>2. 1 Main auxiliary plant itemsThe main items of auxiliary plant which have to be provid-ed for from the electrical auxiliary power supply system,whether AC or DC, and the considerations of securitywhich are applicable in each case are generally as follows:</p><p>2.1.1 Turbine-generators: For turbine-generators the fol-lowing unit auxiliary items will need to be considered:</p><p>(a) Main inlet valves: The control system is by hydraulicopening of the valve with gravity closing. Other than theDC control system, no electrical auxiliary power require-ments exist for this fairly important item of auxiliaryequipment. It is an important item because main inletvalve closure is usually a part of the turbine-generator setshut-down sequence. Its closure will obviate shaft rotationat low speeds which could be caused by guide vane</p><p>Paper 4377C (P10, PI, P9), first received 19th September 1984 and in revised form22nd November 1985The author is with Kennedy &amp; Donkin, Consulting Engineers, Westbrook Mills,Godalming, Surrey GU7 2AZ, United Kingdom</p><p>leakage and which could have an undesirable effect onbearing wear, a point which will be mentioned later.</p><p>(b) Governor oil pumps and associated compressed aircushion for the guide vane control system: This system mustbe brought up to and maintained at appropriate oil levelsand air pressure before a start-up procedure can be com-menced, and must be so maintained during operation. Inthe event of auxiliary supply failure, sufficient oil capacityfor a number of operations is available. The system istherefore adequate to perform a shut-down procedure, andthe shut-down process would not be adversely affected bythe loss of AC supply.</p><p>(c) Generator excitation: It is some years sinceamplidyne sets were common, these requiring main andstandby driving motors. The usual current practice is touse excitation transformers at the generator terminals,these being effectively unit transformers, energising thethyristor converter system. Start-up and run-down periodsare adequately covered by the control of this system, asalso is the effect of short-circuit on the generator terminals.A DC field flashing supply is normally the only alternativesupply required, this being necessary for a brief period atstart-up. In the case of generator-motor units in pumpedstorage schemes a DC excitation supply may be requiredto maintain dynamic braking.</p><p>(d) Bearing oil systems: Oil supply to thrust and guidebearings during running conditions has been made bymain AC and standby DC pumps. However, the use ofself-lubricating bearings, in which the pumping action isinherent in the bearing design, has now become prevalent.Under the running condition with such bearings there isno need for a continuous oil feed from an external system.Such bearings may be suitable for use without hydrostaticlift. However, where starting is frequent (for example inpumped storage plants) and/or the bearing pressure ishigh, then a high pressure lifting or jacking oil pump isused to force oil between the sliding surfaces of the pads.In the case of generator-motor units in pumped storageschemes, this also reduces the stiction torque which isimportant in starting such sets in the pumping mode. Thispump may also be used during the run-down period on ashut-down sequence where, if this is likely to be long, thereis perhaps a greater danger of bearing wipe than at start-up.</p><p>However, the availability of hydrostatic lift or jackingoil supply may not be essential to enable 'black start' of agenerator to be undertaken. Equally, during the stoppingsequence the same would apply when the stopping timedoes not exceed about 15 minutes.</p><p>(e) Mechanical brakes: Hydro-electric units have highinertia and can therefore have long run-down periods,</p><p>148 IEE PROCEEDINGS, Vol. 133, Pt. C, No. 3, APRIL 1986</p></li><li><p>sometimes more than three or four hours, depending uponthe type of set and the operating mode (e.g. blown down orchurning water). Many smaller hydro-electric units, andeven some larger ones, do not have brakes, particularlywhen they are fitted with hydrostatic lifting equipment.When mechanical brakes are fitted, they are intended todeal with the following problems associated with the run-down period of the set:</p><p>Under a normal shut-down operation, when brakesmay be applied at about 25% speed for perhaps about oneminute, it is the intention</p><p>(i) to reduce the time at low speed where there is nohydrostatic lifting for the thrust bearing and thereby toreduce the risk of bearing wipe</p><p>(it) to hold the rotor at standstill until the main inletvalve has closed in cases where guide vane tolerances,erosion or sticking allow a leakage torque to continue.</p><p>Under emergency shut-down conditions, brakes wouldprobably be applied at about 50% speed and, dependingon the type of set, may be on from 5 to 15 minutes. Thevirtues of the use of mechanical brakes under these condi-tions are arguable but, if used, the emergency operationwould be initiated by:</p><p>(i) vibration(ii) bearing temperature(Hi) failure of oil circulation (if non self-lubricated</p><p>bearings)(iv) fire</p><p>Thus it follows that where mechanical brakes are used, acompressed air system is required to operate them whichmust be available under the shut-down procedure. Thiscompressed air system would be maintained fully chargedand have adequate capacity at the commencement of theshut-down procedure. Hence the loss of compressor ACsupplies would not affect the conditions of shut-downadversely.</p><p>(/) Generator cooling: Internal cooling air is normallyby rotor fan circulation, except in the case of reversiblegenerator-motor units when motor-driven ventilating fansare required. Cooling water for the heat exchangers mayrequire main and standby pump sets to establish watercirculation before start-up and to maintain it duringrunning. Failure of this circulation during a run-downwould not be of consequence as load would then havebeen removed.</p><p>(g) Generator-transformer cooling: The generator-transformers would probably have forced oil pumps and,depending on circumstances, either water circulation or aircooling. As in the case of the main generator cooling, thesupply to these auxiliaries needs to be established at start-up and maintained during running. Again, it would not beessential to maintain the electrical supply during the shut-down sequence because the load would first have beenremoved.</p><p>2.1.2 Station general services and head works services:For the station general services and headworks servicesassociated with the hydro-electric project, the followingprincipal items require consideration:</p><p>(a) Compressed air systems: These have been referred toabove in so far as they are required for certain items ofturbine-generator auxiliary equipment. In general, com-pressed air plant may be installed to fulfil any of the fol-lowing functions:</p><p>(i) turbine low load injection(ii) blow-down for condenser operation</p><p>(Hi) blow-down for spinning reserve mode for pumpedstorage plant.</p><p>The last function requires a reasonable level of electricalsupply security to the compressed air system in questionto ensure availability of the generator-motor for thevarious modes of operation which are important to its rolein the supply system.</p><p>(b) Drainage and dewatering pumps: Drainage pumpsmust be maintained in operation, particularly in under-ground power plants, although a short period of inter-ruption may be of no consequence. Dewatering pumpsmay present a larger load but as they are associated withan inspection and maintenance service their use can beprogrammed appropriately so as not to coincide with elec-trical supply loss conditions.</p><p>(c) Fire fighting systems of the water spray or delugetypes: These may use a compressed air service which mustbe maintained charged at all times in order to ensure therequired water pressure at the spray nozzles. The storagecapacity will be sufficient to tolerate loss of auxiliarysupply for a reasonable interval.</p><p>If DC fire pumps are alternatively employed very con-siderable addition to battery capacity may be needed.</p><p>(d) Spillway gates: On the hydraulic side an item whichmay require secure electrical auxiliary supplies is the spill-way gate system. These are hydraulically actuated. Theimportance of maintaining control of these gates underflood conditions is often sufficient to justify provision of alocal low voltage diesel generator set in addition to theprovision of a reliable AC auxiliary supply. The diesel gen-erator set, which may only be 100 or so kVA, can be amobile unit.</p><p>(e) Intake (penstock) gates: Gravity operation wouldusually be adopted if they are required to be shut underemergency conditions, although some types of gates maybe hydraulically actuated in which case a compressed airsystem situated locally would need to be maintainedcharged.</p><p>2.2 Assessment of auxiliary plant electrical supplyrequirements</p><p>The above review indicates that in hydro-electric plantsone does not generally experience the high degree of essen-tiality of auxiliary plant supply that is needed in a thermalpower plant, where, for instance, such items as boiler feedpumps and condensate extraction pumps are of criticalimportance.</p><p>If one wishes to have a 'black start' capability it will benecessary usually to have a diesel generator set or aux-iliary hydro set sufficient in rating to start oil pumps andcooling water circulation and to charge compressed airsystems and battery systems if the black out has been oflong duration.</p><p>In the running condition all auxiliaries are, of course,essential to the continuous operation of a turbine-generator set. However, a reasonable degree of tolerancecan exist in changeover time to a standby supply.</p><p>The total loss of electrical auxiliary supply is not likelyto cause damage, even in the case of the bearings system,provided that a shut-down sequence is initiated in goodtime and that mechanical brakes are applied when the cir-cumstances warrant them. If mechanical brakes are notused, a DC hydrostatic lift pump may be necessary tomaintain bearing oil films below about 20% speed.</p><p>It will, of course, be appreciated that control, alarms,recording and signalling systems must be available withoutfailure under all conditions to enable the necessary</p><p>IEE PROCEEDINGS, Vol. 133, Pt. C, No. 3, APRIL 1986 149</p></li><li><p>sequences to proceed. The power requirements for theseare not large and are met by DC batteries and/or uninter-ruptable AC power supply systems under auxiliary supplyfailure conditions.</p><p>Lighting systems, similarly essential to the maintenanceof control of the plant, must be supplied to the minimumlevels necessary under supply failure conditions.</p><p>3 Consideration of sources of AC auxiliarysupplies</p><p>The primary source of auxiliary supply is a high voltageAC system, preferably to ensure security at least two HVAC systems. However, apart from the system into whichthe hydro-electric plant is generating, alternative or sec-ondary AC systems are often not available or are unreli-able in the environments in which hydro-electric plantstend to be situated. Sometimes, however, a reasonable highvoltage (or extra high voltage) AC supply is installed at theconstruction stage and this may continue to be used aftercompletion of construction as an alternative auxiliary elec-trical supply source.</p><p>3.1 Main electrical system sourcesIf a hydro-electric plant has a fairly high merit position inthe system, such that maximum continuity of output fromit is desirable, or its size is such that its loss will be serious,then it becomes preferable to obtain the auxiliary electricalsupply source from as close to the generator terminals aspossible. The following alternative methods present them-selves, offering a descending order of security in the orderas given:</p><p>3.1.1 Connection to the generator terminals at gener-ation voltage: This conventional approach, being that of atrue unit transformer, provides a high degree of securityfor the auxiliary supply whilst the generating set is runningbut:</p><p>(a) it cannot be used for start-up which must thereforebe effected from an auxiliary supply on the other side ofthe generator circuit breaker or from another system</p><p>(b) if phase isolated bus ducting is used between the gen-erator terminals and the generator-transformer terminals,then the auxiliary transformer must be three single phaseunits to maintain the phase isolation. Fault levels maycause problems with the auxiliary transformer tee-connection or, alternatively, fault limiting reactors mayneed to be provided in the tee-off</p><p>(c) if the voltage regulation is based upon a generatorvoltage range of 10% (with no transformer on-load tapchanging) a greater range of voltage regulation will need tobe provided on the low voltage side of the auxiliary trans-former.</p><p>3.1.2 Connection to the generator-transformer terminalsin cases where a generator circuit breaker is used: Sucha supply will be available as a 'start-up' supply as the syn-chronising would be performed on the generator circuitbreaker. However, the disadvantages are similar to thos...</p></li></ul>