CHAPTER- 9 HYDRO GENERATOR, CHARACTERISTICS

9.3.2Transient and Emergency Duty RequirementsA generator confirming to these guidelines will be suitable for withstanding exposure to transient event andemergency duty imposed on a generator because of power system faults.Sudden short circuit at the generator terminals: A generator should be capable of withstanding, withoutinjury, a 30 second, 3 phase short circuit at its terminals when operating at rated MVA and power factorand at 5% over voltage, with fixed excitation. The machine shall also be capable of withstanding, withoutinjury, any other short circuit at its terminals of 30s duration or less in accordance with IEEE C 50. 122005. Generator circuit breaker needs to be selected accordingly.Synchronizinga.Generators be designed to be fit for service without inspection or repair after synchronizing that iswithin the limits given below:i)ii)iii)Breaker closing angleGenerator voltage relative to systemFrequency difference 10%0% to 5% 0.067 HzAdditional information on synchronizing practices can be found in IEEE std. C37. 102TM- 1995.b. Faulty synchronizing is that which is outside the limits given above. Under some system conditions,faulty synchronizing can cause intense, short duration currents and torques that exceed thoseexperienced during sudden short circuits. These currents and torques may cause damage to thegenerator.c.Generators should be designed so that they are capable of coasting down from synchronous speed to astop after being immediately tripped off-line following a faulty synchronization. Any generator thathas been subject to a faulty synchronization should be inspected for damage and repaired as necessarybefore being judged fit for service after the incident. Any loosening for stator winding bracing andblocking and any deformation of coupling bolts, couplings, and rotor shafts should be corrected beforereturning the generator to service. Even if repairs are made after a severe out-of-phase synchronization,it should also be expected that repetition of less severe faulty synchronizations might lead to furtherdeterioration of the components.d.It should be noted that the most severe faulty synchronizations, such as 1800 or 1200 out-of-phasesynchronizing to a system with low system reactance to the infinite bus, might require partial or totalrewind of the stator, or extensive or replacement of the rotor, or both.Check synchronizing relay and auto synchronizing equipment need to be set accordingly.Normally synchronizing closing angle is kept 7%.Short-time volts/hertz variations: The manufacturer should provide a curve of safe short-time volts/hertzcapability. Identify the level of over flux above which the machine should never be operated, to avoidpossible machine failure. Unless otherwise specified, the curve apply for time intervals of less than 10 min.9.3.3Rotor Surface HeatingContinuous phase current unbalance: Generator above 5 MVA should be capable of withstanding, withoutinjury, the effects of a continuous phase current unbalance corresponding to a negative current of the valuesin table 9.1, provided the rated MVA is not exceeded the maximum expressed as a percentage of ratedstator current.Table –9.1 Continuous negative sequence current capabilityType of generator or generator/motorNon-connected amortisseur windingConnected amortisseur windingPermissible I2 (%)510213

These values also express the negative-sequence current capability at reduced generator MVA capabilities,as a percentage of the stator current corresponding to the reduced capability.Continuous performance with non connected amortisseur windings is not readily predictable. Therefore, ifunbalanced conditions are anticipated, machines with connected amortisseur windings should be specified.Negative sequence relays (phase unbalance) be set accordingly.9.3.4Types of Generators and Configuration (Vertical or Horizontal)Vertical shaft generators are generally used. There are two types of vertical shaft hydro generatorsdistinguished by bearing arrangements.Umbrella type generators: These generators have combined bottom thrust and guide bearings and confinedto low operating speeds (up to 200 rpm) are the least expensive generator design. In semi umbrella typegenerators a top guide bearing is added. Umbrella/Semi Umbrella design is being increasingly used forslow speed vertical generator.Conventional generators: Prior to introduction of umbrella and semi umbrella designs conventional designcomprised of top-mounted thrust and guide bearing supported on heavy brackets, capable of supportingtotal weight of generator. All thrust bearing supported brackets take care the weight of generator rotatingparts. Turbine rotation parts and axial component of water thrust acting on turbine runner. A bottom guidebearing combined with turbine shaft is usually provided. This conventional design is used for high speeds(up to 1000 rpm) generators. Some medium size low flow turbine and tube turbine generators are horizontalshaft. Direct driven bulb turbine generators are also horizontal shaft generators located in the bulb. Peltonturbine coupled generators are mostly horizontal shaft.9.3.5Capacity and RatingkW Rating: kW capacity is fixed by turbine rated output. In a variable head power plant the turbine outputmay vary depending upon available head. In general the generator is rated for turbine output at rated head.In peaking power plant higher generator kW rating could be specified to take care of possible higherturbine output. Economic analysis is required for this purpose as the cost will increase and generatorcapacity remains unutilized when heads are low.The kilowatt rating of the generator should be compatible with the kW rating of the turbine. The mostcommon turbine types are Francis, fixed blade propeller, and adjustable blade propeller (Kaplan). Eachturbine type has different operating characteristics and imposes a different set of generator design criteria tocorrectly match the generator to the turbine. For any turbine type, however, the generator should havesufficient continuous capacity to handle the maximum kW available from the turbine at 100-percent gatewithout the generator exceeding its rated nameplate temperature rise. In determining generator capacity,any possible future changes to the project, such as raising the forebay (draw down) level and increasingturbine output capability, should be considered. Typical hydro generator capability curve is shown in figure9.3.kVA Rating and power factor: kVA and power factor is fixed by consideration of interconnectedtransmission system and location of the power plant with respect to load centre. These requirements includea consideration of the anticipated load, the electrical location of the plant relative to the power system loadcenters, the transmission lines, substations, and distribution facilities involved. A load flow study fordifferent operating condition would indicate operating power factor, which could be specified.(Turbine output in MW) x (Generator efficiency)Generator MVA Generator power factor9.3.6Electrical CharacteristicsElectrical Characteristics e.g. voltage, short circuit ratio, reactance, line charging capacity etc. mustconform to the interconnected transmission system. Large water wheel generators are custom designed to214

match hydraulic turbine prime over. Deviation from normal generator design parameters to meet systemstability needs can have a significant effect on cost. The system stability and other needs can be met bymodern static excitation high response systems and it is a practice to specify normal characteristics forgenerators and achieve stability requirements if any by adjusting excitation system parameter (ceilingvoltage/exciter response).9.3.6.1 Generator Terminal VoltageGenerator terminal voltage should be as high as economically feasible. Standard voltage of 11 kV or higheris generally specified for hydro generator.9.3.6.2 Insulation and Temperature RiseModern hydro units are subjected to a wide variety of operating conditions but specifications are preparedwith the intention of achieving a winding life expectancy of 35 years or more under anticipated operatingconditions. Present practice is to specify class F insulation system for the stator and rotor winding withclass B temperature rise over the ambient. Ambient temperature rise should be determined carefully fromthe temperature of the cooling water etc.If may be noted that as per IS the temperature rise specified over an ambient of 400C. Accordinglymaximum temperature for the insulation class under site conditions should be specified.The class F system is known as thermo setting insulation system. The epoxy resins systems may be dividedinto the following two major classes.i)ii)Resin rich systemResin poor systemResin poor technology needs sophisticated resin storage, transfer and impregnation plants. Most of largemachines commissioned in India have class F insulation resin rich.It is generally believed that stator insulation degradation occurs due to slot discharge as well as due toionization. Slot discharge is a result of poor contact between the conducting surface on generator coil andstator iron and built up of high surface potential. Discharges of this nature can be very severe because ofthe charges current involved and cause serious damage to the insulation. At some stage slot dischargesappear to or begin to increase between the surface of the insulation of the winding and laminated slot walland provide one of the deteriorating factor. Phenomenon of slot discharges is known since long and effortshave been made to evolve suitable methods for monitoring the state of erosion of insulation in anassembled machine. With adoption of epoxy mica insulation and use of increased current densitiesresulting in higher electro-dynamic forces, there is an increase in the intensity of these slots discharges. Slotdischarges have been reported in water wheel generators.Thermosetting insulation systems materials are hard and do not readily conform to the stator slot surface, sospecial techniques and careful installation procedures must be used in applying these materials to avoidpossible slot discharges. Special coil fabrication techniques, installation, acceptance and maintenanceprocedure are required to ensure long, trouble-free winding life.All components of stator and rotor insulation should be of class F insulation with class B temperature rise.9.3.6.3 Short Circuit RatioThe short circuit ratio of a generator is the ratio of field current required to produce rated open circuitvoltage to the field current required to produce rated stator current when the generator terminals are shortcircuited and is the reciprocal of saturated synchronous reactance. Normal short circuit ratios are givenbelow. Higher than normal short circuit ratio will increase cost and decrease efficiency.Generator Power Factor0.80.90.95Normal Short Circuit Ratio1.01.101.17215

In general, the requirement for other than nominal short-circuit ratios can be determined only from astability study of the system on which the generator is to operate. If the stability study shows thatgenerators at the electrical location of the plant in the power system are likely to experience instabilityproblems during system disturbances, then higher short-circuit ratio values may be determined from themodel studies and specified.Refer Para 9.4 for more detailed discussion.9.3.6.4 Line Charging and Synchronous Condensing CapacityThis is the capacity required to charge an unloaded line. Line charging capacity of a generator havingnormal characteristics can be assumed to equal 0.75 of its normal rating multiplied by its short circuit ratio.If the generator is to be designed to operate as synchronous condenser. The capacity when operating overexcited as condensers can be as follows:Power Factor0.800.900.95Condenser Capacity65%55%45%9.3.6.5 ReactanceThe eight different reactances of a salient-pole generator are of interest in machine design, machine testing,and in system stability model studies. Lower than normal reactances of the generator and step-uptransformer for system stability will increase cost and is not recommended.Both rated voltage values of transient and subtransient reactances should be used in computations fordetermining momentary rating and the interrupting ratings of circuit breakers.Typical values of transient reactances for water wheel generators up to 25 MA are given below. Guaranteedvalues of transient reactances will be approximately 10% higher.Rated Sub-transient Reactance - xd′′MVA Rating10 - 25Speed RPM1500.261000.273000.259.3.6.6 Damper WindingA short circuit grid copper conductor in face of each of the salient poles is required to prevent pulling outof step the generator interconnected to large grid. Two types of damper windings may be connected witheach other, except through contact with the rotor metal. In the second, the pole face windings are connectedat the top and bottom to the adjacent damper windings.The damper winding is of major importance to the stable operation of the generator. While the generator isoperating in exact synchronism with the power system, rotating field and rotor speed exactly matched, thereis no current in the damper winding and it essentially has no effect on the generator operation. If there is asmall disturbance in the power system, and the frequency tends to change slightly, the rotor speed and therotating field speed will be slightly different. This may result in oscillation, which can result in generatorpulling out of step with possible consequential damage.The damper winding is of importance in all power systems, but more important to systems that tend towardinstability, i. e. systems with large loads distant from generation resources, and large intertie loads.216

In all cases, connected damper windings are recommended. If the windings are not interconnected, thecurrent path between adjacent windings is through the field pole and the rotor rim. This tends to be a highimpedance path, and reduce the effectiveness of the winding, as well as resulting in heating in the currentpath. Lack of interconnection leads to uneven heating of the damper windings, their deterioration, andultimately damage to the damper bars.The damper winding also indirectly aids in reducing generator voltage swings under some faults conditions.It does this by contributing to the reduction of the ratio of the quadrature reactance and the direct axisreactance, X q′′ / X d′′ . This ratio can be as great as 2.5 for a salient pole generator with no damper winding,and can be as low as 1.1 if the salient pole generator has a fully interconnected winding practice is toprovide X q′′ / X d′′ L 1.3.9.3.6.7 EfficiencyAs high an efficiency as possible which can be guaranteed by manufacturer should be specified. Calculatedvalues should be obtained from the manufacturer.For a generator of any given speed and power factor rating, design efficiencies are reduced by thefollowing:i.ii.iii.Higher Short-Circuit RatioHigher WR2Above-Normal Thrust9.3.6.8 Total Harmonic Distortion (THD)Limits: When tested on open circuit and at rated speed and voltage, the total harmonic distortion (THD) ofthe line-to-line terminal voltage, as measured according to the methods laid down in IS should not exceed5%.9.3.6.9 Phase SequencePhase sequence defines the rotor in which the phase voltages reach their positive maximum at the terminalsof the machine, and shall be agreed upon the manufacturer and purchaser. Typically this is given as a threeletter sequence, R, C, L, (right, center, left) or L, C, R (left, center, right), as defined by an observer lookingat the terminals from outside the machine. In the case of terminals on the top or bottom of the machine, thesequence is defined looking from the end of the machine nearest the terminals toward the centerline of themachine.Care must be exercised to ensure that the defined phase sequence of the machine is consistent with that ofthe connected equipment, particularly in situations where the plant layout requires otherwise identicalmachines to have different phase sequence.9.3.7Mechanical Characteristics9.3.7.1 Direction of RotationThe direction of the rotation of the generator should suit the prime mover requirements.9.3.7.2 Large Generator StatorThe stator frame is designed for rigidly and strength to allow it to support the clamping forces needed toretain the stator punching in the correct core geometry. Strength is needed for the core to resist deformationunder fault conditions and system disturbances. Also, the core is subjected to magnetic forces that tend todeform it as the rotor field rotates. In some large size machines, this flexing has been known to cause thecore to contact the rotor during operation. In one instance, the core deformed and contacted the rotor, the217

machine was tripped by a ground fault, and intense heating caused local stator tooth iron melting, whichdamaged the stator winding ground insulation.In machines with split phase windings where the split phase currents are monitored for machine protection,the variation in the air gap causes a corresponding variation in the split phase currents. If the variations aresignificant, the machine will trip by differential relay action, or the differential relays will have to bedesensitized to prevent tripping. Desensitizing the relays will work, but it reduces their effectiveness inprotecting the machine from internal faults.9.3.7.3 Rotor Assembly Critical SpeedsA rotor dynamic analysis of the entire shaft system should be performed. This analysis should include theprime mover, generator, and any other rotating components. This analysis should include lateral andtorsional shaft system response to the various excitations that are possible within the operational dutiesallowed by the standards. When the turbine generator is purchased as a set, it would be typical that themanufacturer should perform this analysis. When shaft components are purchased from differentmanufacturers, the purchaser should arrange to have this analysis. Critical speeds of the generator rotorassembly should not cause unsatisfactory operation within the speed range corresponding to the frequencyrange agreed in accordance with 9.3.1.4. The generator rotor assembly shall also operate satisfactorily for areasonable period of time at speeds between standstill and rated speed agreed upon by the prime mover andgenerator designers. The turbine generator set shaft vibration at operating speed should be within limitsspecified by ISO: 7919-5 for machine sets in hydraulic power generating and pumping plants.9.3.7.4 BearingsThe allowable hydraulic thrust provided in standard generator design is satisfactory for use with a Francisrunner, but a Kaplan runner requires provision for higher-than-normal thrust loads. It is important that thegenerator manufacturer have full and accurate information regarding the turbine.Specifications for generators above 10 MW, and for generators in unmanned plants, should requireprovisions for automatically pumping oil under high pressure between the shoes and the runner plate of thethrust bearing just prior to and during machine startup, and when stopping the machine.9.3.7.5 Noise Level and VibrationUnder all operating conditions, the noise level of generator should be less than 85-95 dB (A) at a distanceof 1 meter radialy & 1.5 m from floor of operating. In order to prevent undue and harmful vibrations, allmotors should be statically and dynamically balanced in accordance with IEEE std. C50.12-2005. Testprocedure for verification should be based on ISO 3746. Acoustic treatment may be necessary to achievedecreasing sound pressure levels at 90 db.9.3.7.6 Over speed withstandIt is general practice in India to specify all hydro generators to be designed for full turbine runawayconditions (IS: 4722-2001, table 3 Clause 1). The stresses during design runaway speed should not exceedtwo-thirds of the yield point.American practice as per Army Corps Engineers Design Manual is as follows;Generators below 360 rpm and 50,000 kVA and smaller are normally designed for 100% over speed.9.3.7.7 Flywheel EffectThe flywheel effect (WR2) of a machine is expressed as the weight of the rotating parts multiplied by thesquare of the radius of gyration. The WR2 of the generator can be increased by adding weight in the rim ofthe rotor or by increasing the rotor diameter. Increasing the WR2 increases the generator cost, size andweight, and lowers the efficiency. The need for above-normal WR2 should be analyzed from twostandpoints, the effect on power system stability, and the effect on speed regulation of the unit. Speedregulation and governor calculation are discussed in the guidelines for selection of turbines

iv) Advanced manufacturing technology v) Formation of large grids requires special design consideration for operation and stability. Fig. 9.1: Bhakra Left Bank Power House (100 MVA, 90 MW, 11 kV,0.9 pf, 3 phase, 50 Hz, 166.4 RPM, 36 Poles Vertical Wheel Water Generator Commissioned in 1960)