HYDRO TURBINES

HYDRO TURBINES

TYPES OF HYDRO-TURBINES :A) Reaction Turbines1.Francis

2. Kaplan3. Propeller4. BulbB) Impulse Turbines1. Pelton

Head Range2mto70 m Kaplan30m to450 m Francis300m to1700 m Pelton

MAJOR COMPONENTS OF TURBINE:1.Draft Tube/Draft Tube Cone

POWER HOUSE BUILDING CONSISTS OF THREE MAIN AREAS NAMELY

1. Machine Hall/Unit Bay

2. Erection/Service Bay

3. ControlRoom/Auxiliary Bay2. Spiral Case3. Stay Ring/Vanes4. Distributor• Guide Vanes/Nozzles(Deflectors)• Top Cover/Head Cover• Lower Ring/Pivot/Bottom Ring5.Runner and Labyrinths6. Turbine shaft7.Turbine pit liner (Upper & Lower) 8.Turbine guide bearing• Housing• TGB Pads9. Servomotors10. Regulating ring/Regulating Mechanism11. Shaft seal12. Governor & OPU systemSpecific speed of a turbine: The specific speed (m-KW system)of a turbine is thespeed of a geometrically similar turbine that would develop one kW power under ahead of one meter.Specific Speed in M-KW SystemFrancis60to 400Kaplan300

to 1100Pelton4to 60

VALVES:There are two types of valves:1.Spherical valve: It is used where the head is high, i.e. to sustain high pressure.(For Heads above 200m)1.2.Butterfly valve: It is used where the inlet pressure of water is comparativelylower.(For Heads above 200m)

PROCEDURE FOR DIMENSIONING OF POWER HOUSE• Head Calculation.• Selection of specific speed and synchronous speed of turbine.• Fixing the turbine setting• Calculation of discharge diameter.• Calculation of spiral case dimensions• Calculation of draft tube dimensions• Calculation of Generator dimensions.• Finalization of overalldimensions of the power house.

HEAD CALCULATION• Avg. Gross Head = MDDL + 2/3(FRL - MDDL) -TWL(4 UnitsRunning)= 203 + 2/3(208 - 203) -184.24= 22.09 m.• Rated/Net Head = Avg. Gross Head - Head Loss= 22.09 - 0.75= 21.34 m.•  Max. Gross Head = FRL - min TWL= 208.00 - 181.78= 26.22 m•  Max. Net Head= Max. Gross Head-Head Loss= 26.22-0.75= 25.47 m• Min. Gross Head = MDDL - TWL(4 Units Running)= 203.00 - 184.24= 18.76m•  Min. Net Head

= Min. Gross Head - Head Loss=18.76 - 0.75=18.01 m.SELECTION OF MACHINE SPEED• Fromeconomical point of view, the turbine and generator should have the highestpracticable speed to develop given hydropower for given design head.However,final speed may be selected considering the following parameters:• Variation of head,• Silt content,• Cavitation,• Vibrations,• Drop in peak efficiency etc.• From the available formulae, the specific speed for a specific head is calculated.Then for even number of poles of generator, rated speed is obtained. On the basisof this rated speed, corrected specific speed is calculated.CALCULATION OF SPEED:• Specific speed w.r.t. Head– Kaplan Turbine, Ns = 2570 * H-0.5….HARZA= 2334 * H-0.5….USBR– Francis Turbine, Ns = 3470 * H-0.625….HARZA• Rated Speed–N = Ns * H5/4 * P-1/2• Synchronous speed (N=120f/p) nearest to Rated speed obtained from aboveformulae is selected.•Corrected Specific speed, Ns = N * P1/2/H5/4HYDRO GENERATORS

Hydro Generators are low speed salient pole type machines.Rotor is characterized by large diameter and short axial length.

Capacity of such generator varies from 500 KW to 500 MW.Power factor are usually 0.90 to 0.95 lagging.Available head is a limitation in the choice of speed of hydro generator.Standard generation voltage in our country is 3.3KV, 6.6KV, 11 KV ,13.8 KV, &

16KV at 50 Hz.Short Circuit Ratio varies from 1 to 1.4.

CLASSIFICATIONSClassification of Hydro Generators can be done with respectto the position of rotor( i) Horizontal(ii) Vertical (two types)a) Suspension Typeb) Umbrella TypeGENERATOR BARRELDi (Air gap diameter, select from fig. 8 on page no. 25 of BHEL curve)Da (outer core diameter)Df (Stator frame diameter)Db ( Inner diameter of generator barrel)UMBRELLA TYPE GENERATORCOMBINED LOWER THRUST & GUIDE BEARING

SELECTION OF NO. OF POLES Nsyn (Sync. Speed) = 120 FP

Synchronous Speed Of The Generator Depends Upon The Specific Speed Of TheTurbine Nsyn = Ns X Hn1.25 / Pt0.5

EXCITATION SYSTEM – COMPONENTSEXCITATION SYSTEM – COMPONENTS1.EXCITATION TRANSFORMER (DRY TYPE )2. RECTIFIER SYSTEM3.AUTOMATIC VOLTAGE REGULATOR4.FIELD FLASHING UNITS5.FIELD CIRCUIT BREAKER6. DISCHARGE RESISTORAIR COOLING SYSTEM

Generator is provided with a closed, recirculating air cooling system

The cooling pressure is created by fanning action of rotor spider

The air circulates through radial ducts provided in the rotor rim which allows acooling air flow to be distributed radially and uniformly all along the machine axis The air circulation path isspider-> rim -> inter-pole areas-> stator winding-> stator core radial duct-> aircoolers-> lower and upper floors-> lower and upper air baffles-> spiderBRAKING AND JACKING SYSTEM The hydro generators are provided with mechanical friction braking system whichhelps to stop the generator’s rotation after unit is stopped / tripped off-line The brakes are normally applied when the unit speed is slowed down to less than

25% of the rated speed to avoid wearing of thrust bearing pads Brake shoes situated on the lower bracket are pressed against the brake tracks on

the rotor to bring the machine to the rest Brake shoes are also used as jacks for lifting of the rotor for which the oil under

pressure (about 100 kg/cm2) is fed from high pressure pump unit.After jacking therotor can be maintained in lifted position by turning the locking nut and releasing oilpressure. In modern hydro electric generators specially Pelton wheels, electrical dynamicbraking is used in addition to mechanical braking system which will reduce wear onthe mechanical brakes The dynamic braking is initiated at around 50% of rated speed and maintaineduntil mechanical friction brakes are applied which are normally applied at 10 -15% inconjunction with dynamic braking

Fig 22. T.G. Set SectionDESIGN STUDYOUTPUT COEFFICIENT(derived from output equation of AC machines) (Pg-456,AK Sawhney)Output Equation:Q = C0 * D2 * L * Ns

Where, output coefficient, C0 = 11 * Bav * ac * Kw * 10^(-3)

Q = kVA rating of machine

Bav = specific magnetic loading

ac = specific electrical loadingKw = winding factor

From these equations we can infer that the volume of active parts is inversely

proportional to the value of output coefficient C0. Thus an increase in value of

Results in reduction in size and cost of machine and so looking from the economics

point of view the value of output coefficientshould be as high as possible.Now we see that output coefficient is proportional to specific magnetic and electricloading.Therefore the size and cost of the machine decreases if we use increasedvalues of specific magnetic and electric loading. Hence economically these valuesshould be as high as possible. their limit is decided by analyzing the effect ofincreased loadings on performance characteristics of machine. Too high values mayhave adverse effects on temperature rise,efficiency,power factor(in case of inductionmotors) and commutation conditions (in case of dc machines).Therefore optimumvalues are selected.We can calculate the output coefficient from a graph (Large AC Machines, JH Walker, Figure1-1 page 4.) if we know the number of poles of the machine. The graph is obtained byanalyzing the published data of 40 generators in manufacture in USA, Canada, UK,Japan a EuropeMACHINE PARAMETERSBore Diameter : It is the inner diameter of the stator core.Flywheel Effect:(or Mechanical Inertia is defined in terms of the start up timeof the unit)(Standard Handbook of Powerplant Engineeringby Thomas C. Elliott, Kao Chen, Robert Swanekamp)Tm = (WR2 * n2) / [(1.6 * 10^6)P]Where n = rotational speed of unit in rounds/minP = full gate turbine capacity in H.P.

WR2 = Product of revolving parts of unit and square of radius of gyration

(turbine runner, shaft and generator rotor), lb-ft2

For preliminary design studies in which the unit WR2 is not known, its value maybeestimated from the following U.S. Bureau of Reclamation formulas:Turbine WR2 = 23,800 [P / n^(3/2)]^(5/4)Generator WR2 = 356,000 [kVA / n^(3/2)]^(5/4)The heavy pole pieces produce a flywheel effect on a slow speed rotor. This helps tokeep the angular speed constant and reduce variations in voltage and frequency ofthe generator output.In our design we have used the formula:Flywheel effect (GD2) is computed as follows:

Generator WR2= 15000 x (KVA/ N3/2)5/4Where KVA = Unit rating in KVAN= Unit speed in RPMPage no. 1.51. Power Engineer’s Handbook by TNEB Engineer’sAssociation, ChennaiGD2 = 4 x WR2Number of poles :Can be calculated as P=120f/NNumber of poles :Can be calculated as P=120f/NWhere f =frequency of outputN=speed of the rotorAir gap Diameter calculation (same as bore diameter)a.Di Obtained from BHEL graph (Air Gap diameter)b.Di= (60 * Vr) / (pi * N)pi=22/7Where Vr = Max. Peripheral velocity.

It can be obtained from Fig. 1-2 Page 5, Large AC Machines by J.H.

Walker

The bigger of the above two diameters is selected.Stator Core and frame length calculation:Stator core length is the gross length of the stator. It can be calculated usingthe formula for output coefficient.The output coefficient can be obtained fromgraph and air gap diameter calculated above. Once these two are knownstator length can be calculated using the formula:Stator core length,Lt = W/ (Ko* Di2 * N)Where W = Rated KVA of machineKo =  Output coefficient obtained from curve(Fig 1-1, Page 4, LargeAC Machines by J.H. Walker.)N= Rated RPM of the machine

Where f =frequency of outputN=speed of the rotorAir gap Diameter calculation (same as bore diameter)a.Di Obtained from BHEL graph (Air Gap diameter)b.Di= (60 * Vr) / (pi * N)pi=22/7Where Vr = Max. Peripheral velocity.

It can be obtained from Fig. 1-2 Page 5, Large AC Machines by J.H.

Walker

The bigger of the above two diameters is selected.Stator Core and frame length calculation:

Stator core length is the gross length of the stator. It can be calculated usingthe formula for output coefficient.The output coefficient can be obtained fromgraph and air gap diameter calculated above. Once these two are knownstator length can be calculated using the formula:Stator core length,Lt = W/ (Ko* Di2 * N)Where W = Rated KVA of machineKo =  Output coefficient obtained from curve(Fig 1-1, Page 4, LargeAC Machines by J.H. Walker.)N= Rated RPM of the machineRadial and Axial VentilationThe ventilating systems can be classified into three types depending upon how theair passes over the heated machine parts ,as :-(a)Radial,(b)Axial.Radial Ventilating System :This system is most commonly employed because themovement of rotor induces a natural centrifugal movement of air, which may beaugmented by provisions of fans if required .The advantages of radial system are :(1)minimum energy losses for ventilation(2)sufficiently uniform temperature rise of machine in the axial directionThe disadvantages are :(1) It makes the machine lengths larger as space for ducts has to be providedalong the core length .(2) The ventilating system sometimes becomes unstable in respect to quantity ofcooling air flowing.STATOR DESIGNINGPole pitch is defined as the peripheral distance between two consecutive poles. Itmay be expressed as number of slots, degrees .(electrical or mechanical)Calculated as :ψ= pi x Di/PWhere Pi (constant) =22/7Di =Air gap diameter in metersP=No. of polesPole Arc= Pole pitch * 0.7Gross area of air gap/pole = Stator core length x pole pitchn a typical hydro generator wound for 11-16kV experience shows that toobtain flux densities in the stator and rotor which are satisfactory both as tomagnetizing ampere-turns and core loss and to obtain acceptable values of thetransient reactances , a mean flux density (Bm) of 0.6-0.7 Wb/m^2 should beassumed.Flux per pole (φ) =Mean flux density * Pole pitch (ψ)* Length of core * 0.01Assuming a suitable value of Bm, the flux per pole can be calculated.In the preliminary stage, tentative value of number of turns per phase can becalculated asTph= (k1k2 Vph)/4.44fφWe can assume the value ofk1k2 as 1.1Calculation of number of parallel paths .Total current in a slot should not exceed 5000 A. (Current in Slot should lie between3000 to 5000A as per CEA)

If I be the rated current per phase and there be p parallel paths then current perconductor is I/p , and current per slot is 2*I/pThis should not exceed the limit of 5000 A.5000 > 2 * I / pthis gives a minimum value of p , the value of p greater than or equal to this valuewhichsatisfies other designing constraints are chosen as the appropriate number ofparallel paths.After the calculation of turns per phase we can calculate the approximate no. ofstator slots.No. of slots is given by,Ns = (no. of phases) *Tph * (no. of parallel paths) / (turns percoil)Note: Turns per coil = 1 for bar windingNumber of conductor per slots = 2 ( for bar winding)Number of conductors in series per phase = Nc= Z x S/ (Parallel path x 3)WhereZ = No. of conductors per slot andS = Total no of slotsStator slot pitch = Pi x Di/ total no of slotsSlot angle (Mechanical) = 2*Pi / S(P = no of poles S = Total no of slots) inradiansSlot angle (Electrical) =P * (Mechanical Slot Angle) /2MODIFIED CALCULATIONTurns per phase as calculated from slot selection = No of slots / (3 x No ofparallel paths)New Flux per pole=k1 x k2 x rated generator voltage/(4.44 x Turns per phasex f)f = 50 Hzk1 x k2 = 1.1Modified Flux density = Flux per pole / (Stator core length x pole pitch)Where, Stator core length and pole pitch are expressed in metersMaximum Flux density (Bg) = Modified flux density / Form FactorRADIAL LENGTH OF AIR GAPIn the absence of specified values of Xd (direct axis synchronous reactance in p.u.)and Xl( leakage reactance in p.u.) on a 0.9 pf machine a value of unity may beassumed for the former and 0.15 p.u. for the latter.The value of armature reaction (Ma) may be calculated asMa = (2.12* Iph *Tph* ka)/(Np *k1*k2)(ampere-turn /pole)WhereI ph =current per phaseTph = turns per phaseKa=Amplitude factor obtained from the graph(given on page 79 ,fig 5-1,Large AC machines byJ.H.Walker)k1*k2=1.1

Then ,Air gap Ampere Turn (open circuit)Mg =Ma/(Xd-Xl)(source :Page 79 equation 5-2 ,Large AC machines J.H.Walker)where,Xl = leakage reactance in p.u.Xd = direct axis synchronous reactance p.u.The value of φ we used earlier was based on an assumed value ofBm=0.675 Wb/m^2 and this corresponds approximately to Bg =0.85Wb/m^2.ThenMa=0.796*ge*Bg * 10^4gap = 1.26 x Air gap Ampere –turn / (Max flux density)(source page 79,81 Large AC Machines by J.H.WalkerSHORT CIRCUIT RATIOThe short ratio (SCR) of a synchronous machine is defined as the ratio of fieldcurrent required to produce rated voltage under open circuit conditions to the fieldcurrent required to circulate rated current at short circuit.Short circuit ratio is the reciprocal of synchronous reactance Xd ,if Xd isdefined in per unit value for rated voltage and rated current. The value of Xd for agiven load is affected by saturation conditions then exist, while SCR is specific andunivalued for a given machine as it is defined at the rated voltage.For salient pole hydro electric generators SCR varies from 1.0 to 1.1.EFFECT OF SCR ON MACHINE PERFORMANCE(a) Voltage RegulationA low value of SCR means large synchronous reactance .Thus the machine hasgreater changes in fluctuations of load. The inherent voltage regulation of themachine is poor.(b) Stability.A low value of SCR has a lower stability limit as the maximum power output of themachine is inversely proportional to Xd.(c) Parallel OperationMachines with a low value of SCR are also difficult operate in parallel because ahigh value of Xd gives a small value of synchronizing power. This power isresponsible for keeping the machines in synchronism. Also the transmission lineimpedance adds up to the machine impedances thus it further reduces thesynchronizing power as the machines are weakly held in synchronism. They becomemore sensitive to torque and voltage disturbances.

(d) Short circuit currentA small SCR indicates a small value of short circuit current as Xd is high. But this isnot a problem as short circuit currents can be limited and the machines need not bedesigns with low values of SCR.(e) Self ExcitationMachines feeding long transmission lines should not be designed with a small SCR(high Xd) as this would lead to large voltages on open circuit produced by selfexcitation owing to large capacitive currents drawn by the transmission lines.We have seen that a machine with low value of SCR has a lower stability limitand a low value of inherent voltage regulation. On the other hand a higher value ofSCR means a high value of short circuit current. Also the machine designed with ahigher value of SCR has a long air gap which means that the mmf required by thefield is large. Hence a machine with higher SCR is costlier to build. Present trend isto design the machine with a low value of SCR . This is due to the recentadvancement in the fast acting control and excitation systems.CALCULATION OF MEAN LENGTH OF A TURN.The MLT is assumed to be made up of the following portions: The length of coil inthe slot (Lc) ,the length of the straight portion extending from the core to the angledportion of the end winding (Ac), the angled portion (Y) and the portion at the endconsisting either of the evolutes (multi-turn-coil) or clips (single turn bar) . The MLTis then given byMLT =2*Lc +4(Ac+Bd)+4Y,(Lc is in cms)Where

Ac + Bd is obtained from fig 3-9 ,Large AC Machines J.H.Walker.

And Y =Pdsecθ3/2

Pd = [pi *(100Dg + 2ds)/Np]*[percentage coil pitch/100]Ds=depth of slotAnd sin θ3 =Xc/λs1λs1 = pi*(Dg100 +2ds)/NsXc = coil pitch at end winding= width of insulated coil + clearance (w)NUMBER OF RADIAL VENTILATING DUCTS.nd = 0.26(Lc100 -12.5)for duct width = 6.6mm(page 68, 69 Large AC Machines J.H.Walker)Le (effective length of core) = (Lc – nd*wr*.01)Wherend=number of radial ventilating ductswr=width of duct beam in cm(Page 70,Large AC Machines by J.H. Walker.)Active length of stator core = Stacking factor x Length of core ductWhere, stacking factor = 0.93(Page 89,Large AC Machines by J.H. Walker.)ARMATURE WINDINGS, COILS AND THEIR INSULATIONSThere are two types of coils :1.Single turn bar

2.Multi turn barSingle turn coil :A single turn bar winding is used in machines when thearmature current per circuit exceeds 1500 A.As the current is quite large so the cross-section of the conductors used is verylarge and so bars used are subdivided into many parts to reduce the eddycurrent losses in them.Basic structure of the conductors used:There are two conductors in a slot if the bar winding is used. EEach conductorconsists of two vertical stacks of copper laminations insulated by either asbestos

or glass rovings.

The advantage of using glassis that it gives a high space factor.

The two vertical stacks used are also insulated from each other.

The dimensions of individual strand is determined partly by electricalconsiderations so as to reduce the eddy current losses to less than 1/5 the ofI^2R losses and partly by the manufacturer considerations.

Further the eddy current loss in the top coil side is more than that in the lowerone so there is a difference in the rise of temp of the two.This temp rise difference is reduced by increasing the no of strands in the top coilsidethere by reducing the thickness of the strands in the top coil side.To reduce the circulating current losses it is essential to use some form oftransposition of conductor laminations in the slots.In the transposition each conductor lamination is arranged to move continuouslythrough all positions in depth of coil side so that the leakage reactanceof all theconductor laminations is equalized so that no circulating current flows. Roebeltransposition is widely used for this purpose

Bitumen mica folium applied to the slot portion of the bar while mica tape onthe overhang portion was most commonly used insulating materials earlier.The mica tape 0.13 mm thick and 20 mm wide is wrapped by hand up to 20half layers. So this process is both time consuming and expensive.2.Epoxy Novalak mica paper insulation system:The rows of conductor stacks are bound with epoxy based resins. This isdone by usingtwo highly loaded epoxy glass separators. The stack is thenpressed at 160 degree Celsius to form a rigid mass. This type of constructiondoes not require the filling of external voids.

The over hang insulation is in the form of a no of layers offlexible isopthalatevarnished polyester backed mica flake tapes. The insulation of the slot portionconsists of a no of half lap layers of epoxy novalak bonded glass backed micapaper tape.This system permits the machine to be operated at a higher temp rise due toits greater thermal conductivity.Multi turn coil:In this type of coils an additional insulation between betweenindividual turns has to be provided. The interturn insulation mustbe designed towithstand surges of magnitude 1.5 times of the line voltages.The inter turn insulation used is mica tape half overlap and asbestos. The thicknessof the mica tape is 0.13 mm and that of asbestos 0.38 mm.Multi turn coils epoxy novalak mica paper system : The epoxy novalak mica paperinsulation used is different for the slot portion of the conductor and the over hang.Novalak mica paper tapes are used for the slot portion while isopthalate varnishedmica flake tapes are used for the over hang.(source: Pg-744, A.K.Sawhney)WINDINGSTWO TYPES:1.Concentrated windings:these the mainly used in design of field windingsfor salient pole machine2.Distributed windings: are used in stator and rotor of all the ac machines

ARMATURE WINDINGS:1. Closed windings: are used for dc machines and ac commutatormachines .2.Open windings : are used only for ac machines like synchronousmachines and induction machines.Related Terms:1. Pole pitch:peripheral distance between adjacentpoles.2. Coil span:peripheral distance between two coil sides.3.Full pitch coil:coil span = pole pitch4. Chorded coil:coil span < pole pitchCLOSED WINDINGS :Two types:1. Lap windings: a = P2.Wave windings:a = 2Where,a = no of parallel pathsP = no of polesLap Windings :yb= 2C / P +/-Kyw= yb– yf= 2yc= 1Where,C = no of coils

P=no of polesyc= commutator pitchyb= back pitchyw= winding pitchK=Fraction or integer such thatyb is an odd integer.Wave windings :yc=(C + 1) / (P/2)yw= 2  ycy w=  y b  + y fWhere,C = no of coilsP=no of polesyc = commutator pitchyb = back pitchyw= winding pitchNote: Above relations are given only for progressive windingsasretrogressive windings are rarely being used.