www.chennaiuniversity.net www.chennaiuniversity.netChennaiUniversity.net1 EE2401 POWER SYSTEM OPERATION AND CONTROLUNIT I: INTRODUCTION Systemloadvariationloadcharacteristicsloadcurvesandload-durationcurve(daily, weeklyandannual)loadfactordiversityfactor.Importanceofloadforecastingandsimple techniquesofforecasting.Anoverviewofpowersystemoperationandcontrolandtheroleof computers in the implementation.(Qualitative treatment with block diagram). PART A 1. What is load curve? Thecurvedrawnbetweenthevariationsofloadonthepowerstationwithreferencetotimeisknown as load curve. Thereare three types,Dailyload curve, Monthly load curve, Yearly load curve. 2. What is daily load curve? The curve drawn between the variations of loadwith reference to various time period of dayis known as daily load curve. 3. What is monthly load curve? It is obtained from daily load curve. Average value of the power at a month for a different time periods are calculated and plotted in the graph which is known as monthly load curve. 4. What is yearly load curve? It is obtained from monthly load curve which is used to find annual load factor.5. What is connected load? It is the sum of continuous ratings of all the equipments connected to supply systems. 6. What is Maximum demand? It is the greatest demand of load on the power station during a given period. 7.What is Demand factor?It is the ratio of maximum demand to connected load.Demand factor= (max demand)/ (connected load)8. What is Load factor?The ratio of average load to the maximum demand during a given period is known as load factor. Load factor = (average load)/ (maximum demand)9. What is Average demand?The average of loads occurring on the power station in a given period (day or month or year) is known as average demand. www.chennaiuniversity.net www.chennaiuniversity.netChennaiUniversity.net2 Daily average demand= (no of units generated per day)/ (24 hours)Monthly average demand = (no of units generated in month)/ (no of hours in a month)Yearly average demand= (no of units generated in a year)/ (no of hours in a year)10. What is Diversity factor?Theratioofthesumofindividualmaximumdemandonpowerstationisknownasdiversity factor.Diversity factor = (sum of individual maximum demand)/(maximum demand). 11. What is Capacity factor? This is the ratio of actual energy produced to the maximum possible energy that could have been produced during a given period. Capacity factor= (actual energy produced)/ (maximum energy that have been produced) 12. What is Plant use factor?Itistheratioofunitsgeneratedtotheproductofplantcapacityandthenumberofhoursfor which the plant was in operation. Units generated per annum= average load * hours in a year13. What is Load duration curve?When the load elements of a load curve are arranged in the order of descending magnitudes the curve then obtained is called load duration curve.14. What is the necessity of voltage regulation? Allequipmentsinpowersystemaredesignedtooperatesatisfactorilyonlywhenthevoltage level on the system correspond to their rated values. So voltage regulation is very important. 15. What are the disadvantages of voltage regulation? The disadvantages of voltage regulation are, i) If voltage variation is more then a pre specified value, the performance of equipments is poor. ii) Life in most of the equipments is also sacrificed. 16. Give some category of load device The categories of load devices are, a. Motor devices 70% b. Heating and lighting 25% c. Electronic devices-5% 17. Why the frequency regulation is very important? Sincethespeedofinductionmotorsdependuponthesystemfrequency,regulationof power system frequency is very important. www.chennaiuniversity.net www.chennaiuniversity.netChennaiUniversity.net3 18. How the multitudes of devices are characterized? The multitude of devices are characterized by,a. Size b. Symmetry (single or three- phase) c. Load constancy (in respect to time, frequency and voltage) d. Use cycle (regular or random use). 19. What are the effects of load dependency on voltage and frequency? The effects of load dependency on voltage and frequency are, a. Serious problems are created in HP motors. b. If efficiency and power factor increases, current decreases. c. Generators are lead to shut down. 20. What is load management? As generator capacity has increased in price (to as much as $1000 per kilowatt) and as the fuelshortages put an extra squeeze on them, many electric utilities are finding it worthwhile to try to slave the load peaks. This is referred to as load management. 21. Give the two major control loops of large generators? The two major control loops of large generators are, a. Automatic Voltage Regulator (AVR) b. Automatic Load Frequency Control (ALFC) 22. Write about AVR loop. Theautomaticvoltageregulator(AVR)loopcontrolsthemagnitudeoftheterminalvoltage (Vt). The latter voltage is continuously sensed, rectified and smoothed. This DCsignal,theresultingerrorvoltage,afteramplificationandsignalshapingservesastheinputtothe exciter which finally delivers the voltage (Vf) to the generator field winding. 23. How the ALFC loop is affected by AVR loop? AVR affect the magnitude of generated EMF (Eg). This generated EMF affects the generated real power. Therefore changes in AVR loop affect ALFC loop. 24. Write about load frequency mechanism. The frequency is closely related to the real power balance in the overall network. Under normaloperating conditions the system generators run synchronously and generate together the power at each moment is being drawn by all loads plus the real. 25. Give an important reason for voltage control. Thereallinelossesdependasmuchuponthereactiveasuponthereallinepowerflow. Itispossibletominimizetheselosses byselectinganoptimumpowerflow,intermsof realand reactive powers. So the voltage control is very important. www.chennaiuniversity.net www.chennaiuniversity.netChennaiUniversity.net4 26. What is the one distinct difference between P-f and Q-V control? ThesurplusofMEGAVARStendstoincrease thefrequencyofasystem. Thechangesarenotuniform but will be greatest at the buses where the Q surplus is the greatest. This is one distinct difference between P-f and Q-lVl control. PART B 1. Discuss about the recent trends in real time control of power system? Realtimecontrolofpowersystemmeansthesysteminworkingstate.Whilethepower system is in process control the machine at that time in the working field. The size of turbo generator units has increased. So it is difficult to operate them near critical parameters. Hence, it is necessary to acquire a large volume of data pertaining to plant and to processthemimmediately.Itisessentialtotakenecessarycontrolactionswithincritical time. Higher cost of fuels, environmental protection and rising inflationary spiral have placed a strain on the utility return on investment. New plant equipment and construction costs rise at an alarming rate. This means, more peak load demand must be met by older plants. Thus, monitoring and control systems must be fast enough to protect generation equipments against overloads, false and dangerous operating conditions, conventional electrical instruments can handlethesituation,ifinformationreceivingisoneatatime.Butwhenafloodof information hit all at once, a fast data processing equipment or system is required. Thus the computer is essential for the efficient and economic operation and control. Some of the major benefits of computerized operation and control of plant are: Increased plant safety. More equipment life. Increased plant availability. Higher plant efficiency. Minimum operating errors. In 1982, the United States derived its electricity from the following sources. 72 % from coal, gas, oil, 15 % from hydro power and 13 % from nuclear. But no energy form is entirely risk free. Use of energy made possible our present life styles. Thus the major goals for the future aretodevelopnewprimaryresourceswhichpromotebettermeansofgenerationand transmission and emphasis less wasteful use of electricity. Computer Configuration Trend: Thecomputersystemusedatpowerplantstationhasbeenundergoingcontinuous developmentovertheyears.Formerly,allthefunctionssuchasdataacquisition,logging display,controlandperformancecalculationswereperformerbycomputerprocessingunit (CPU).Insuchsystemfailureofanyoftheelegantleadstothetotalsystembreakdown. Thus, the need for a dual computer configuration arose which is quite costly. www.chennaiuniversity.net www.chennaiuniversity.netChennaiUniversity.net5 Thefurtheradvancementincommunicationtechnologyandpowerfulmicroprocessorshas resultedinthecheapandreliablemicroprocessorbasedDistributedProcessingSystem (DPS). It is based on the principle of LAN. Today, in all process industries including power plant, this system is employed for data acquisition and control. DPS consists of a number of microprocessors connected through data highway, which is passive in nature. Each processor isassignedaspecifictaskindependently.So,thefailureofoneoftheprocessorsdoesnot disturb the function of the other processors. Advantages of DPS Better reliability. Greater processing power. More responsiveness. Longer survivability. Better modularity. More system expandability. Easier maintenance. Functions and Facilities The function of the data acquisition and control system is to provide the operator with current plant information through graphic displays,group displays, alarms annunciations and so on, forthesafeoperationofplant.TheDPSfurtherprovidesdetailedhistoricalinformationfor diagnostic/reviewpurposesincaseofoutageandplantperformance.Thedataacquisition and control system performs the following general functions. Data acquisition and validation. Real time variable computations. Alarm monitoring and display. Performance and deviation calculation. Trends, events, reports and logs. Sequential control. Modulating control. Other related functions. In order to carry out these functions, some facilities are required. They are: Data Processing System: Theprocessinput/outputrequirementsarehighresolutiondigitalinputs,lowresolution digitalinputs,analoginputs4-20mADC,thermo-coupleinputs,RTDinputs,pulseinputs, analog outputs and contact outputs. Computer System Design Requirement for Power Plant: Powerplantisacomplexsystemwhichincludestheboiler,turbo-generator,coolingtower and the reheat system. The operating parameters of the plant are largely governed by the grid connected to it. Thus, DPS at power plant must have the following features: www.chennaiuniversity.net www.chennaiuniversity.netChennaiUniversity.net6 High availability. Good expandability. High data rate. Geographic distance spanning. Ability to support several hundred independent devices. Stability under high control. Self-diagnostic. Ease of reconfiguration and maintenance and low cost. Non-Conventional Energy Resources: i. Solar power. ii. Wind, wave, geothermal and tidal power. iii. Fossil fuel. Future Generating Equipment: Todaysmaximumgeneratorunitsizeisexceeding1GWandislimitedbytheallowable currentdensitiesusedinrotorandstatorwindings.Majoreffortsareusedtodevelop superconducting machines, where the winding temperature is kept close to absolute zero. The enormouscurrentandfluxdensitiesachievedinsuchmachinescoiledpossiblyincrease1 GW size unit to(5 6) GW. This would mean better generating economy. Automation: Itisthetermusedtodescribeabroadarrayofprocesses,productsandsystemsworkingin harmonytoautomatemanufacturing.Itincludeselementsoffactoryandplantautomation suchasrobotics,drives,PLCandcomputerizednumericalcontrol,production,planning processoptimization,opencontrolsystems;humanmachineinterfacesvisionsystemsand industrial networks. It is only in the last fewyears the companies have started incorporating software systems at all levels of production. Modularity & Reconfigurability: Design of machinery demands, connectivity solutions, which are reliableand simple to use. At higher levels of plant automation, it will become possible to run overseas factories on the same web browser used to control domestic production. Field Bus Networks: Inmanyfactories,hard-wiredcontrolsystemsarestillthenorm.Theyareusedtocontrol level to allow communication with the operation level. Decentralized Control: As control is being transferred to the field devices, electronic motor control can take place at themotoritself.Electricalconnectionsareintegratedinconnectorsthatlargelyeliminate www.chennaiuniversity.net www.chennaiuniversity.netChennaiUniversity.net7 wiringerrorsandallowmotorstobeconnectedanddisconnectedwithoutanyuseoftools. Allthecomponentsofadecentralizedautomationsystemwillbefedbyacontinuous common connection system for data and power. 2. What are system level and plant level controls? Thefunctionofanelectricpowersystemistoconvertenergyfromoneofthenaturally available forms to electrical from and to transport it to points of consumption. Aproperlydesignedandoperatedpowersystemshouldmeetthefollowingfundamental requirement. 1.Adequatespinningreservemustbepresenttomeettheactiveandreactivepower demand. 2. Minimum cost with minimum ecological impact. 3. The power quality must have certain minimum standards within the tolerance or limit such as,Constancy of frequency. Constancy of voltage (Voltage magnitude and load angle). Level of reliability. Factor affecting power quality: Switching surges. Lightning. Flickering of voltage. Load shedding. Electromagnetic interference. Line capacitance and line inductance. Operation of heavy equipment. Welding machine operation. The three main controls involved in powers are: 1. Plant Level Control (or) Generating Unit Control. 2. System Generation Control. 3. Transmission Control. 1. Plant Level Control (or) Generating Unit Control The plant level control consists of: i. Governor control or Prime mover control. ii. Automatic Voltage Regulator (AVR) or Excitation control. i. Governor control or Prime mover control GovernorcontrolorPrimemovercontrolsareconcernedwithspeedregulationofthe governorandthecontrolofenergysupplysystemvariablessuchasboilerpressure, temperatureandflows.Speedregulationisconcernedwithsteaminputtoturbine.With www.chennaiuniversity.net www.chennaiuniversity.netChennaiUniversity.net8 variation in load, speed of governor varies as the load is inversely proportional to speed. Thespeedofthegeneratorvariesandthegovernorsensesthespeedandgivesa commandsignal,sothat,thesteaminputoftheturbineischangedrelativetotheload requirement. ii.Automatic Voltage Regulator (AVR) or Excitation control The function of Automatic Voltage Regulator (AVR) or Excitation control is to regulate generator voltage and relative power output. As the terminal voltage varies the excitation control, it maintains the terminal voltage to the required standard and the demand of the reactive power is also met by the excitation control unit. These controls are depicted in given figure 1. Figure 1: Plant and System Level Controls 2. System Generation Control Thepurposeofsystemgenerationcontrolistobalancethetotalsystemgenerationagainst systemloadandlosses,sothat,thedesiredfrequencyandpowerinterchangewith neighboring systems are maintained. This comprises of: i.Load Frequency Control (LFC). ii.Economic Dispatch Control (EDC). www.chennaiuniversity.net www.chennaiuniversity.netChennaiUniversity.net9 iii.System Voltage Control. iv.Security control. i. Load Frequency Control (LFC). Thisinvolvesthesensingofthebusbarfrequencyandcompareswiththetielinepower frequency.Thedifferenceofthesignalisfedtotheintegratoranditisgiventospeed changerwhichgeneratesthereferencespeedforthegovernor.Thus,thefrequencyofthe tie line is maintained as constant. ii. Economic Dispatch Control (EDC). When the economical load distribution between a number of generator units is considered, it is found that the optimum generating schedule is affected when an incremental increased atoneoftheunitsreplacesacompensatingdecreaseateveryotherunit,intermofsome incrementalcost.Optimumoperationofgeneratorsateachgeneratingstationatvarious station load levels is known as unit commitment. iii. System Voltage Control. Thisinvolvestheprocessofcontrollingthesystemvoltagewithintolerablelimits.This includesthedevicessuchasstaticVARcompensators,synchronouscondenser,tap changing transformer, switches, capacitor and reactor. Thecontrolsdescribedabovecontributetothesatisfactoryoperationofthepowersystem bymaintainingsystemvoltages,frequencyandothersystemvariableswithintheir acceptable limits. They also have a profound effect on the dynamic performance of power system and on its ability to cope with disturbances. iv. Security control Themainobjectiveofrealtimepowersystemoperationrequiresaprocessguidedby controlanddecisionsbasedonconstantmonitoringofthesystemcondition.Thepower system operation is split into two levels. LEVEL 1: Monitoring and Decision TheconditionofthesystemiscontinuouslyobservedIthecontrolcentresbyprotective relaysforfaultsorcontingenciescausedbyequipmenttroubleandfailure.Ifanyofthese monitoringdevicesidentifiesasufficientlysevereproblematthesampletime,thenthe system is in an abnormal condition. If no such abnormality is observed, then the system is in a normal condition. LEVEL 2: Control Ateachsample,thepropercommandsaregeneratedforcorrectingtheabnormalityon protecting the system from its consequences. If on abnormality is observed, then the normal operation proceeds for the next sample interval. Central controls also playan important role in modern power systems. Today systems are composedofinterconnectedareas,whereeachareahasitsowncontrolcentre.Thereare www.chennaiuniversity.net www.chennaiuniversity.netChennaiUniversity.net10 manyadvantagestointerconnections.Theinterconnectedareascansharetheirreserve power to handle anticipated load peaks and unanticipated generator outages. Interconnected areascanalsotoleratedlargeloadchangeswithsmallerfrequencydeviationsatspinning reserve and standby provides a reserve margin. Thecentralcontrolcentreinformationincludingareafrequency,generatingunitoutputs and tie line power floes to interconnected areas. This information is used by automatic load frequency control in order to maintain area frequency at its scheduled values. 3. Explain (i) Load Forecasting, (ii) Unit Commitment and (iii) Load Scheduling. 1. Load forecasting: The load on their systems should be estimated in advance. This estimation in advance is known as load forecasting. Load forecasting based on the previous experience without any historical data. Classification of load forecasting: ForecastingLead TimeApplication Very short timeFew minutes to half an hour Realtimecontrol,realtime security evaluation. Short termHalf an hour to a few hours Allocationofspinning reserve,unitcommitment, maintenance scheduling.Medium termFew days to a few weeks Planningorseasonalpeak-winter, summer. Long termFew months to a few years Toplanthegrowthofthe generation capacity. Need for load forecasting: To meet out the future demand. Longtermforecastingisrequiredforpreparingmaintenancescheduleofthe generating units, planning future expansion of the system. For day-to-day operation, short term load forecasting demand and for maintaining the required spinning reserve. Veryshorttermloadforecastingisusedforgenerationanddistribution.Thatis, economic generation scheduling and load dispatching. Mediumtermloadforecastingisneededforpredictedmonsoonactingandhydro availability and allocating. www.chennaiuniversity.net www.chennaiuniversity.netChennaiUniversity.net11 2. Unit Commitment: Theunitcommitmentproblemistominimizesystemtotaloperatingcostswhile simultaneouslyprovidingsufficientspinningreservecapacitytosatisfyagivensecurity level. In unit commitment problems, we consider the following terms. A short term load forecast. System reserve requirements. System security. Startup costs for all units. Minimum level fuel costs for all units. Incremental fuel costs of units. Maintenance costs. 3. Load Scheduling (Load Dispatching): Loading of units are allocated to serve the objective of minimum fuel cost is known as load scheduling. Load scheduling problem can be divided into: i.Thermal scheduling. ii.Hydrothermal scheduling. i. Thermal scheduling. Theloadingofsteamunitsareallocatedtoservetheobjectiveofminimumfuelcost. Thermalschedulingwillbeassumedthatthesupplyundertakinghasgotonlyform thermal or from steam stations. ii. Hydrothermal scheduling. Loadingofhydroandthermalunitsareallocatedtoservetheobjectiveofminimumfuel cost is known as hydrothermal scheduling. SchedulingofhydrounitsarecomplexbecauseofnaturaldifferencesIthewatersheds, manmade storage and release elements used to control the flow of water are difficult. Duringrainyseason,wecanutilizehydrogenerationtoamaximumandtheremaining period,hydrogenerationdependsonstoredwateravailability.Ifavailabilityofwateris notenoughtogeneratepower,wemustutilizeonlythermalpowergeneration.Mostly hydroelectricgenerationisusedtomeetoutpeakloads.Therearetwotypesof hydrothermal scheduling. a) Long range hydro schedulingb) Short range hydro scheduling. a) Long range hydro schedulingLong range hydro scheduling problem involves the long range forecasting of water availabilityandtheschedulingofreservoirwaterreleasesforanintervaloftime thatdependsonthereservoircapacities.Longrangehydroschedulinginvolves www.chennaiuniversity.net www.chennaiuniversity.netChennaiUniversity.net12 fromIweektoIyearorseveralyears.Longrangehydroschedulinginvolves optimizationofstatisticalvariablessuchasload,hydraulicinflowsandunit availabilities. b) Short range hydro scheduling. Shortrangehydroschedulinginvolvesfromonedaytooneweekorhour-by-hour scheduling of all generation on a system to achieve minimum production cost foe a given period. Assumingload,hydraulicinflowsandunitavailabilitiesareknown,foragiven reservoir level, we can allocated generation of power using hydro plants to meet out the demand, to minimize the production cost. Thelargestcategoryofhydrothermalsystemincludesabalancebetween hydroelectricandthermalgenerationresources.Hydrothermalschedulingis developed to minimize thermal generation production cost. 4. Explain the need for voltage and frequency regulation in power system. A power system is said to be well designed if it gives a good quality of reliable supply, which means that the voltage levels must be within reasonable limits. Practically, all the equipments on the power system are designed to operate satisfactorily within voltage variations of around 5%.Ifthevoltagevariationismorethanapre-specifiedvalue,theperformanceofthe equipments is also sacrificed. ThevoltageatthegeneratingstationsandthefrequencydecidestheKWloadingofthe generating stations and the loading through the interconnectors. Need for Voltage Regulation in Power System: Knowledge of voltage regulation helps in maintaining the voltage at the load terminals within prescribedlimitsunderfluctuatingloadconditions,byemployingsuitablevoltagecontrol equipment. The following points are to be considered. The transmission lines and the distribution lines need voltage control at various stages to maintain the voltage at the last consumers premises within permissible limits. Variations in supply voltage are detrimental in various aspects. Below normal voltage substantially reduces the light output from incandescent lamps. Above normal voltage reduces the life of the lamps. Motorsoperatedatbelownormalvoltagedrawabnormallyhighcurrentsandmay overheat, even when carrying no more than the rated horse power load. If the voltage of the system deviates from the nominal value, the performance of the devices suffers and its life expectancy drops. Thereallinelossesdependasmuchuponthereactivelineasupontherealtime power flow. The reactive line flow depends upon line end voltages. www.chennaiuniversity.net www.chennaiuniversity.netChennaiUniversity.net13 Byadjustingtheexcitationofthegeneratoratthesendingendbelowacertainlimit may result in instability of the system and excitation above certain level will result in overheating of the rotor. Service voltages are usually specified by a nominal value and the voltage maintained is 5 % of the nominal value. Need for Frequency Regulation in Power System: Knowledge offrequency regulation helps in maintaining the systemfrequency that is speed ofthealternatorwithinprescribedlimitsunderfluctuatingloadconditions,byusingspeed governor and integral controller. In a network, considerable drop in frequency occurs due to high magnetizing currents in induction motors and transformers. The following points to be considered. In any power system, if the frequency changes there wont be required receiving end voltage. If we connected two systems in parallel, it will spoil the system. Thegeneratorturbines,particularlysteamdrivenonesaredesignedtooperateata very precise speed. Most of AC motors rub at speeds that are directly related to the frequency. Theoveralloperationofapowersystemcanbemuchbettercontrolledifthe frequency error is kept within strict limits. Alargenumberofelectricallyoperatedclocksareused.Theyarealldrivenbe synchronousmotorsandtheaccuracyoftheseclocksisafunctionnotonlyofa frequency error, but actually of the integral of this error. Constantturbinespeedisanimportantrequirement.Thevelocityoftheexpanding steam is beyond our control and the turbine efficiency requires perfect speed match. Unusual deviations in the frequency can be detected earlier. Whentwosystemsworkingatdifferentfrequenciesaretobetiedtogethertomake same frequency, frequency converting stations or links are required. 5.Draw and explain the basic P-f and Q-v control loops. BASIC PF AND QV CONTROL LOOPS StaticchangesPiintherealbuspoweraffectthebusphaseangleandnotthebus voltagemagnitudes.Thischangeaffectsthereallineflowsandnotthereactiveline flows. StaticchangesQiinthereactivepoweraffectthebusvoltagemagnitudesandthe phase angle. This change affects the reactive line flows and not the real line flows. A static change in the reactive bus power affects the bus voltage at the particular bus and has little effect on the magnitude of voltage. QV Control Loop: The automatic voltage regulator circuit or QV control loop as shown in given figure 2. This loop is used for voltage control. This bus bar voltage is stepped down using a potential transformer to a small value of voltage. This is sent to the rectifier circuit which converts the www.chennaiuniversity.net www.chennaiuniversity.netChennaiUniversity.net14 ACvoltageintoDCvoltageandafiltercircuitusedinthisremovestheharmonics.The voltageVi,thusrectifiediscomparedwithareferencevoltageVrefinthecomparatoranda voltage error signal is generated. The amplified form of this voltage gives a condition for the excitertoincreasethefieldcurrentbasedonitspolarity.Theoutputofthegeneratoris steppedupusingatransformerandfedtothen=busbar.Thusthevoltageisregulatedand controlled in this control circuit. PF Control Loop: ThiscontrolloopcircuitisdividedintoprimaryandsecondaryAutomaticLoadFrequency Control (ALFC) loop structures as shown in given figure 2. Figure 2: Automatic load frequency and voltage regulator control loops Primary ALFC: Thecircuitprimarilycontrolsthesteamvalveleadingtotheturbine.Aspeedsensorsenses thespeedoftheturbine.Thisiscomparedwithareferencespeed,governorwhosemain activity is to control the speed of the steam by closing and opening of the control valve. That is, if the differential speed is low, then the control valve is opened to let out the steam at high speed,therebyincreasingturbinesspeedandviceversa.Thecontrolofspeedinturn controls the frequency. www.chennaiuniversity.net www.chennaiuniversity.netChennaiUniversity.net15 Secondary ALFC: Thiscircuitinvolvesafrequencysensorthatsensesthefrequencyofthebusbarand compares it with Tie line power frequencies in the signal mixer. The output of this is an Area ControlError(ACE)whichissenttothespeedchangerthroughintegrator.Thespeed changergivesthereferencespeedtothegovernor.Integralcontrollerisusedtoreducethe steady state frequency change to zero. After this part of the circuit, is the introduction of the Primary ALFC loop whose function has already been described.Thus,thetwoloopstogetherhelpincontrollingthespeedwhichinturncontrolsthe frequency, since N f. Using the relation, Speed 120 fNP=Where, f is frequency in Hz and P is number of poles. 6. Draw the load curve and load duration curve. Explain the importance of these curves in connection with economic operation of power system? Thegeneratingstationsmaybesteam,hydro,nuclear,dieseloranyothertype.Thisfactor mainly depends upon the natural sources available in the areas. The power station should be asnearaspossibletothecentreoftheloadsothatthetransmissioncostandlossesare minimum. The scheme employed should be such that extension could be madeto meet with the increase in demand in future, without incurring heavy expenditure. LOAD CURVES: Load Curves: Load on the power system is seldom constant. It varies from time to time. The curve showing the variation of load on the power station with respect to time is known as a load curve. It can be plotted on a graph taking load on Y-axis and time on X-axis. Daily Load Curve: The curve showing the variation of load on a whole day i.e., 24 hours with respect to time is known as daily load curve. The load variations are recorded half-hourly or hourly on a whole day (24 hours). Typical daily load curve is as shown in figure 3. Monthly Load Curve: Thecurveshowingthevariationofloadofthemonthwithrespecttotimeisknownas monthly load curve. It can be obtained from the daily load curves of that month. It can be plotted bycalculating the average values of power over a month at different times of the day. It is used to fix rate of energy tariff. www.chennaiuniversity.net www.chennaiuniversity.netChennaiUniversity.net16 Yearly or Annual Load Curve: The curve showing the variation of load of the year with respect to time is known as yearly or annual load curve. Itcanbeobtainedfromthemonthlyloadcurvesofthatyear.Itisusedtodeterminethe annual load factor. Figure 3: Daily load curve The load curves supply the following informations: The variation of the load during different hours of the day. The area under the curve represents the total number of units generated in a day. Thepeakofthecurverepresentsthemaximumdemandonthestationonthe particular day. Theareaundertheloadcurverepresentedisdividedbythenumberofhours,gives the average load on the power station. The ratio of the area under the load curve to the total area of the rectangle in which it contained, gives load factor. LOAD DURATION CURVE: This is another type of curve which indicated the variation of load, but with the loads arranged indescendingorderofmagnitude,i.e.,thegreatestloadontheleftandlesserloadstowards right.Fromthiscurve,theloadfactorofthestationcanalsobedetermined.Typicalload duration curve is as shown in figure 4. www.chennaiuniversity.net www.chennaiuniversity.netChennaiUniversity.net17 Figure 4: Load duration curve Important Terms for Deciding the Type and Rating of Generating Plant: Connected Load: Fordecidingthetypeandratingofgeneratingplant,itisnecessarythatengineeringmaybe familiar with the following important terms. Thesumofthecontinuousratingsofalltheelectricalequipmentconnectedtothesupply system us known as connected load. Maximum Demand: The greatest of all short time interval averaged during a given period on the power station is calledthemaximumdemand.Itisthemaximumdemandwhichdeterminesthesixeandthe cost of the installation. Demand Factor: Theratioofactualmaximumdemandonthesystemtothetotalratedloadconnectedtothe system is called the demand factor. It is always less than the unity. Maximum DemandDemand Factor = Connected LoadAverage Load or Demand: The average load or demand on the power station is the average of loads occurring at various events. KWhr Supplied in a dayDaily Average Load = 24KWhr Supplied in a month Monthly Average Load = 24 30KWhr Supplied in a year Annual Average Load = 24 365Load Factor: Loadfactorisdefinedastheratioofaverageloadtothemaximumdemandduringacertain period of time such as a day or a month or a year is called the load factor. www.chennaiuniversity.net www.chennaiuniversity.netChennaiUniversity.net18 Average DemandLoad Factor = Maximum DemandIf the plant is operated for T hours, Average Demand T Units Generated in T hoursLoad Factor = Maximum Demand T Maximum Demand T= T = 24 for daily load curve. T = 24 7 for weakly load curve. T = 24 365 for annual load curve. Significance of Load Factor: Loadfactorisalwaysgreaterthanunity,becauseaverageloadissmallerthan maximum demand. It is used to determine the overall cost per unit generated. If the load factor is high, cost per unit generated is low. Diversity Factor: The ratio of sum of the individual maximum demands of all the consumers supplied by it to the maximum demand of the power station is called the diversity factor. Sumof individual maximum demandsDiversity Factor = Maximum Demand of power stationIt is always greater than the unity. Because of maximum demand of different consumers do not occur at same time. Therefore, Maximum Sumof individual Demand on the < maximum power station demands ` ` ) )If diversity factor is more, the cost of generation of power is low. Factors to improve the diversity factor: Givingincentivestosomeconsumerstouseelectricityinthenightorlightload periods. Using day-light saving. Staggering the office timings. Havingtwo-parttariffinwhichconsumerhastopayanamountdependantonthe maximum demand of consumer uses. Coincidence Factor: The reciprocal of the diversity factor is called the coincidence factor. www.chennaiuniversity.net www.chennaiuniversity.netChennaiUniversity.net19 Maximum Demand of power stationCoincidence Factor = Sumof individual maximum demandsIt is always less than the unity. Capacity Factor or Plant Factor: Capacityfactorisdefinedastheratiooftheaverageloadtotheratedcapacityofthepower plant. Average DemandCapacity Factor = Rated Capacity of the power plantUnits or KWhrs generated= Plant Capacity Number of Hours Utilisation Factor: It is a measure of the utility of the power plant capacity and is the ratio of maximum demand to the rated capacity of the power plant. It is less than the unity. Maximum Demand on the power stationUtilisation Factor = Rated Capactiy of the power stationPlant Operating Factor (or) Plant Use Factor: It is defined as the ratio of the actual energy generated during a given period to the product of capacityofplantandthenumberofhourstheplanthasbeenactuallyoperatedduringthe period. Total KWhr GeneratedPlant Use Factor = Rated Capacity of the plant Number of operating hours Reserve Capacity: It is the difference between plant capacity and maximum demand. Reserve Capacity = Plant Capacity Maximum Demand. 7. Whatisspinningreserveanddoesthisreservehelpinoperatingapowersystem efficiently? How is clod reserve different from hot reserve? Inanyarea,thekindoffuelavailablecost,availabilityofsuitablesitesforahydrostation, thenatureofloadtobesupplied,areconsideredbychoosingthetypeofgeneration.The minimum capacity of the generating station must be such as to meet the maximum demand. Installed Reserves: www.chennaiuniversity.net www.chennaiuniversity.netChennaiUniversity.net20 Installedreserveisthatgeneratingcapacitywhichisthepowerintendedtobealways available. Installed reserve can be kept low by the achievement of good diversity factor. Spinning Reserves: Spinningreserveisthatgeneratingcapacitywhichisconnectedtothebusandisreadyto take load Cold Reserves: Coldreserveisthatreservegeneratingcapacitywhichisavailableforservicebutisnotin operation. Hot Reserves: Hot reserve is that reserve generating capacity which is in operation bus is not in service. ASSIGNMENT QUESTION 8. A generating station has the following daily load cycle :Time (Hours) 0-6 6-10 10-12 12-16 16-20 20-24Load (MW) 2025 30 253520Draw the load curve and calculate, (1) Maximum demand, (2) Units generated per day, (3) Average load, (4) Load factor.9.A power station has to meet following demand:Group A: 200 KW between 8 A.M and 6 P.M Group B: 100 KW between 6 A.M and 10 A.MGroup C: 50 KW between 6 A.M and 10 A.M Group D: 100 KW between 10 A.M and 6 P.M and then between 6 P.M and 6 A.MPlot the daily Load curve and determine diversity factor, units generated per day and load factor. 10. (i)Themaximumdemandonapowerstationis 100MW.Iftheannualloadfactoris40%, calculate the total energy generated in a year. (ii)Ifthemaximumdemandonthestationis2500KWandthenumberofKWhgenerated per year is 45 106, determine (1) Diversity factors, (2) Annual load factor.