WIND POWER Utilization
Technology Gowth in Wind Tubine Geneatos Wind tubine geneatos (WTGs) stated as fixed-speed wind tubines with conventional induction geneatos and capacito banks as static eactive copensatos. Capacitos supplied eactive powe fo the ai gap agnetic flux, which the induction geneatos could not poduce. Denak initially standadized on this odel, teing it the Danish Concept. These tubines contibuted 71.6% of the total WTGs thee by 2006. ate, squiel-cage otos in induction geneatos wee eplaced with wound otos. Vaiable oto esistance, vaiable speed copatibility with geas, and capacito banks becae standad featues.
Doubly fed induction geneatos (DFIGs) followed with patially ated powe electonic convetos. The convete helped to povide independent contol of active and eactive powe outputs of the WTGs. The PE convete ating was geneally at 30% of the WTG ating. Finally, TWGs with added functions in the PE convetos aived. The PE potion inceased the costs but gave bette contol and helped in the fault-idethough facility. This categoy constitutes just 0.2% of the total WTG population.
Natue of Wind Wind ay blow steadily duing cetain peiods, vaying by day, season, location, and so on. et us say the velocities fall within soe zones. The wind ay die down, falling alost to nil. Then it ay ise fo a vey low speed. Thee ay be a wind lull, when the wind dies out and then ises in shot busts. A wind gust is the opposite phenoenon to a wind lull. A vey stong wind is a sto. This natue of wind akes it an uneliable souce of powe due to its vaiability and uncetainty.
Coponents of A Wind Tubine Geneato The oto blades, whose pitch is adjustable as pe wind velocity so as to catch axiu wind enegy. The gea box which adjusts the p of the oto of the geneato as closely as possible to the gid synchonous fequency. The geneato, which convets echanical input into an electical output. Wind Tubine
1. Fo a given wind condition it should poduce axiu possible powe. This is possible when λ stands at λ opt. 2. Thee is a iniu wind condition below which the WTG becoes unstable. λ is epesents the cossove point fo the stable condition liit. At λ stall, the WTG will stall. two opeating conditions fo a WTG: Opeating Conditions
Note that the atio of WTG blade tip speed to wind speed, λ, plays an ipotant pat. The WTG contol should pefo in such a way that it is at λ opt unde diffeent conditions of wind load. The ate of change in λ is given by anothe quotient x: A WTG is in an unstable condition when becoes positive. The past figue shows powe vesus speed cuves of a wind tubine with wind velocity as a paaete. The dashed line is a bounday between high- and low-speed egions.
WIND POWER The figue elates the powe coefficient to the tip speed atio, λ, defined as the elationship between the oto blade tip speed and the fee speed of the wind fo seveal wind powe tubines. As stated ealie, in quantitative tes, the tip speed atio is defined as λ=v/v =wr/v, whee w is the angula speed of the tubine shaft, v the blade tangent tip speed, and R the length of each blade. It ephasizes the ipotance of knowing the pupose fo which the enegy will be used, to allow deteination of the best selection fo wind powe extaction.
Mechanical Powe The tubine echanical powe can be given by The ai density ρ can be coected by the gas law (ρ = P/RT) fo evey pessue and tepeatue with the following expession:
Wind geneato technologies Double Fed Induction Geneato DFIG Gid Geabox Convete Electical Exc. Synchonous Geneato EESG Gid Convete Convete Advantages Disadvantages Advantages Disadvantages Sooth gid-connection Multi-stage gea box Flux contol fo iniizing loss Extenal field excitation Reactive powe copensation Need caful potection PMs is not equied (less cost) Heavy weight Roto enegy can be fed to gid Contol coplexity Self Exc. Induction Geneato Voltage is contollable Expensive solution Peanent Magnet Synchonous Geneato Geabox SEIG Convete Gid PMSG Convete Gid Advantages Disadvantages Advantages Disadvantages flexible contol Moe expensive conv. Diect dive opeation High cost of PMs Absence of bushes Highe conv. losses Highe efficiency and enegy PM deag. at high T ess cost and aintenance Multi-stage gea box Highe powe to weight atio Manuf. Difficulties No additional powe supply 10
SEIG Dynaic Modeling + _ + _ R s R ls l λ ds λ d - ω e λ qs (ω e ω ) λ q i ds i d V ds + _ + _ R s R ls l λ qs λ q i qs i q V qs - (ω e ω ) λ d ω e λ ds d q ds qs d q ds qs e e e e e s s s e e s e s s K K V V i i i i p R p p R p p p R p p R ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( 0 0 0 0 0 1 t cq qs qs V dt i C V 0 1 t cd ds ds V dt i C V ) ( 2 2 3 ds q qs d e i i P T 2 2 2 ) ( ) ( ) ( 1 e s e s s s s s s e s e s e e s d q ds qs R R R R R R i i i i dt d d s ds q s qs ds d qs q s s e e K V K V V K V K R R 1 ) ( 2 t T e J J T J dt d 1 1 11
DFIG A thee-phase wound-oto induction achine can be set up as a doublyfed induction oto. In this case, the achine opeates like a synchonous oto whose synchonous speed (i.e., the speed at which the oto shaft otates) can be vaied by adjusting the fequency f Roto of the ac cuents fed into the oto windings. The sae wound-oto induction achine setup can also seve as a doubly-fed induction geneato. In this case, echanical powe at the achine shaft is conveted into electical powe supplied to the ac powe netwok via both the stato and oto windings. Futheoe, the achine opeates like a synchonous geneato whose synchonous speed (i.e., the speed at which the geneato shaft ust otate to geneate powe at the ac powe netwok fequency f Netwok can be vaied by adjusting the fequency of the ac cuents fed into the oto windings.
DFIG The ac cuents poduced by the geneato ae conveted into dc cuent by an AC/DC convete, then conveted by anothe AC/DC convete back to ac cuents that ae synchonous with the ac powe netwok. It is theefoe necessay fo the powe electonics devices used in such a cicuit to have the size and capacity to pocess 100% of the geneato output powe. The powe electonics devices used in doubly-fed induction geneatos, on the othe hand, need only to pocess a faction of the geneato output powe, i.e., the powe that is supplied to o fo the geneato oto windings, which is typically about 30% of the geneato ated powe. Consequently, the powe electonics devices in vaiable-speed wind tubines using doubly-fed induction geneatos typically need only to be about 30% of the size of the powe electonics devices used fo copaatively sized thee-phase synchonous geneatos.
AC/DC powe Convesion topologies: Thee ae seveal ways fo conveting the AC altenate (fo wind) powe to DC, as a fist stage of intefacing with the gid. Advantages: Siple design Disadvantages: A B C Uncontolled output age haonics Advantages: Contolled output age powe (MW) A Disadvantages: B C age haonics age size & weight D1 D3 D5 D4 D6 D2 Diode Rectifie T1 T3 T5 T4 T6 T2 Phase-contolled Thyisto Convete C C + oad + oad V dc V dc Advantages: Contolled output A Disadvantages: B C Poo powe facto age haonics Advantages: Contolled output ess haonics Disadvantages: A B C age switching loss Coplex contol D1 D3 D5 D4 D6 D2 Diode with Boost Convete C Fully-contolled IGBT Convete S S1 S3 S5 S4 S6 S2 C + oad _ + oad _ V dc V dc 14
DC/AC Invete inteface topologies fo ac-loads and utility gid connectivity: Z-souce invete Voltage o Cuent souce + _ C C Convete o Invete DC o AC Sall powe applications such as: Fuel cell Vehicles UPSs ASDs Voltage souce invete + V dc S1 S3 S5 C S4 S6 S2 _ AC Gid Utility R V gid Sall and ediu powe applications such as: Mediu voltage industial appl. Wind fas VAR copensatos active filtes FACTS 15
Cuent souce invetes (CSIs) can be utilized fo new ideas: Advantages: Boost design age powe (MWs) Disadvantages: age inducto size Fo Exaple: CSIs can be used fo Cascaded ultilevel invetes age switching fequency levels with SiC technologies + V dc _ S1 S3 S5 S4 i dc S6 S2 V gid age powe applications such as: plug-in HEVs PM oto contol PVs gid tie invetes Populsion dive systes AC Gid Utility R 16
Convete Modeling Voltage souce PWM convete The dynaic equations in the synchonous fae diectly such as i dc i i i d q ( s) ( s) 1. e s R 1. e s R d q ( s) v ( s) v d q ( s).. i ( s).. i d q ( s) ( s) e R a e b a + b e C R c c _ V dc i c Fo the convete output side R v dc. idc 1 CRs 17
Voltage souce PWM convete The convete plant odel in s-doain epesentation is 18
APPROPRIATE OCATION To select the ideal site fo placeent of wind powe tubines, it is necessay to study and obseve the existence of enough wind to ake extaction of enegy possible at a desied ate. Although flat plains ay have steady stong winds, fo sall-scale wind powe the best choice is usually along dividing lines of wates (i.e., the cests of ountains and hills). In those geogaphical locations, thee is good wind flow pependicula to the cest diection. Soe basic chaacteistics to be obseved fo defining a site ae: Wind intensities in the aea Distance of tansission and distibution netwoks Topogaphy Pupose of the enegy geneated Means of access
Pefoance Ipoveent though Blade Pitch Contol At low speeds, the pitch angle is alost zeo. Maxiu possible enegy is scooped up (axiu powe stategy). At high speeds, the pitch angle inceases. Beyond a cetain wind speed, autoatic echanical bakes apply and electical duping esistances ae used as loads.
Efficiency of a WTG Aveage efficiency of a WTG is defined as the atio of enegy deliveed to gid to the enegy at the tubine oto shaft. As the enegy is tansitted fo one ebe to the next in the tansission syste of a WTG, losses ae incued.
osses in a WTG Aveage and ated efficiencies fo the thee diffeent types of WTGs ae 82 86% at low wind speeds and 89.7 89.9% at high wind speeds. Thus, weathe foecast and past statistical data fo ipotant equieents fo efficiency and eliability when integating wind fa enegies into today s ega gids.
Flickes in the Output of a WTG Thee ae two ain causes of flicke in the supply fo a WTG: Mechanically Related Causes Moto tubine ibalance Roto blades passing in font of the wind stuctue Stuctual odes due to echanical eigenfequencies (fequencies at which thee is echanical esonance) Rotational sapling The flickes caused by these echanical causes have a egula patten, low aplitude, and a low-fequency ange of 0.65 to 0.71 Hz. Wind Velocity Related Causes. Wind flow has egula busts that can cause flicke. This flicke has a high aplitude and a ange of 0.01 Hz 10 Hz. This flicke is objectionable and has been investigated deeply.
CONNECTION OF WIND ENERGY PANTS TO THE GRID THE GRID CODE In the ealy days of wind electicity geneation, the plant sizes wee sall. With an induction geneato, thee was no poble of synchonizing with gid fequency. Extenal capacitos took cae of voltages; when thee was a distubance in the gid leading to low voltages at the point of connection, the wind plants wee disconnected and stayed disconnected until the gid distubance was cleaed. Today, wind plant sizes have inceased. Should a wind plant get disconnected due to a gid distubance, it could aggavate the situation. A gid code fo inteconnection has evolved.
Gid Code The ain featues of the gid codes ae: A low-voltage ide-though (VRT) is essential fo getting into a gid. Accuate powe contol at a PF of ±0.95 has to be aintained at the point of connection. Accuate plant odels ust be subitted. SCADA data ust be supplied as ageed with the syste opeato.
ow-voltage Ride-though Wind enegy fas ae now a sizeable constituent of the powe supply. If a fault develops in the gid, with voltages at the point of coon coupling between the gid and the wind fa, falling to low levels duing the fault cleaance tie, the wind fa should not disconnect. In othe wods, it ust have a low-voltage ide-though capacity. If a fault occus in the gid syste to which a wind enegy fa is connected, the voltage at PCC dips to a low pecentage, depending on the seveity and location of the fault. The contol syste cleas the fault within the tie fae specified unde gid discipline.
A RESISTIVE BRAKING OF A WTG The figue below shows powe-vesus-speed chaacteistics of a WTG. The top cuve epesents conditions with a seies daping beak esisto (SDBR). The botto cuve shows the sae without SDBR. A gid fault occus at point 1 and the SBDR is switched in. The chaacteistics change ove to the botto one at point 2. It tavels along 2 3 within the duation of the fault. At point 3, the SDBR is shoted and the chaacteistics jup to point 4. Howeve, actual WTG at this changeove is at a low speed, coesponding to point 3, and also has a low powe output. So the chaacteistics tavel pack to point 5, which coincides with the oiginal point 1. The SDBR has pevented a unaway incease in WTG speed.
The ide-though peiods fo a WTG ust be natually uch longe than those allowed unde fault-cleaance codes. Danish gid codes specify testing each WTG as pe the specified voltage pofile
MODEING OF A WIND TURBINE GENERATOR It is desiable fist to undestand how a vastly spead electicity powe syste opeates physically. The following gives a bief sketch. A tansission syste opeato (TSO) looks afte load following and powe quality on a vey sall scale tie scale, say on a inute o 10 inute basis. Fo this, he has a schedule of powe offes fo vaious geneatos. The TSO has also a schedule of tie-bound equieents fo the custoes. He atches these and balances the load. The TSO also has an updated chat of tansission facilities with all thei chaacteistics in his copute. He selects an optiu oute fo a load dispatch. This oute has iniu opeating losses and costs. Moden fast-opeating coputes and accuate data ae essential fo his wok. Supplying of accuate odels of WTGs is copulsoy fo this eason.
Method Electical and echanical paaetes of a WTG ae conveted into algebaic quantities. These algebaic notations ae used to develop algoiths to aive at chaacteistic functions. Most of the gids in the wold equie a WTG dynaic odel to be subitted to the tansission syste opeato fo peission to join the gid. Typical Iish gid equieents ae listed below. Any WTG geate than 5 kw ust subit a odel incopoating the following featues: 1. Geneato geneal chaacteistics 2. Tubine geneato and dive tain echanical chaacteistics 3. Vaiation of powe coefficients and pitch angle to tip speed atio 4. Blade pitch contol 5. Convete contols 6. Reactive coponents 7. Potection elays Tie pe step fo siulation should not to exceed 5 icoseconds. Although odels fo siulations fo theal and hydo geneatos have long been used and standadized, those fo WTGs ae still evolving and thee ae no standads.
Pesent Poble Aeas in Modeling 1. Sececy by WTG anufactues who ae developing the newest achines. 2. Non-standadization, beginning with nubeing and naing coponents. 3. Nueical instability aising out of ounding off and tuncating pactices, aong othes. 4. A lage nube of diffeing odels.
Unit Coitent and Scheduling With powe aketing gowing in scale, load balancing becoes ipotant. Fo this, the syste opeato ust know accuately how uch unit coitent he can have fo the wind fa and how uch capacity he should hold in eseve. This is a deanding task, consideing the eatic natue of wind and the agnitude of loads to be handled. But the syste opeato has any tools in his hand, including dynaic scheduling and accuate houly weathe foecasts.
Dynaic Scheduling Dynaic Scheduling. With accuate odeling of the syste and coputeized softwae he can find out what can happen to the syste when an electic coponent is added o subtacted o contolled in powe output. All syste coponents ust be put in odels fo accuate siulation. Modeling ight to the last details becoes ipotant. Since the WTG technology is fast developing, standads fo odeling these ae not yet in place. Take the case of a 4.5 MW TWG weighing 450 t developed by Enecons in Geany. The vey size contibutes to VRT on sall duation faults. This lage piece of appaatus is not the last step in WTG developent.
Weathe Foecasts Accuate Houly Weathe Foecasts. Weathe is not all that eatic. Weathe behavio falls into a patten daily, seasonal, peiodical, and geogaphy specific. Excusions out of this patten ight be in a band that can be anticipated. This, along with daily weathe foecast by eteoology depatents, can help the syste opeato on unit coitents fo the wind fa on the day-ahead schedule as well as on the daily schedule, balancing the faily closely. The syste opeato need not coit too uch capacity to eseves. In fact, although wind enegy costs ae next to nil, thei opeational costs go lagely towad unit coitents.
CAPACITY FACTOR OF A WTG Effective load-caying capacity (ECC) is defined as the aount of additional load that can be eliably seved. In deteining this capacity, two factos ente into the pictue. A load seved duing peak hous has the highest going aket ate. If thee is a OP duing this peiod, cost of powe is taken into fo deteining the capacity facto of a WTG. This peiod will not necessaily always coincide with the peiod of delivey at the ated output of a WTG. Thus, WTG capacity duing the peiod of OP counts against its ated capacity fo planning puposes, unit coitents, and so on. Capacity value of a WTG has been shown to ange appoxiately fo 10% to 40% of the wind plant ated capacity.
CAPACITY CREDIT CONSIDERATIONS How do the capacity consideations affect wind enegy installations? In any counties today, ajo utilities, ostly based on theal geneation, have to buy geen enegy such as wind enegy at pices stipulated by the enegy egulato. Typically, the basis fo these pices is fixed on the following factos: Capacity facto Opeations and aintenance cost eductions fo the utilities Electical loss eduction fo the utilities Envionental benefit Houly capacity factos ae deteined. The aveage of top 50% of the load hous (on both sides of the peak hous) is close to the estiation of the capacity facto fo wind enegy convetes (WECs). In the deteinistic ethod, histoical data on wind is collected and the capacity facto is calculated. In the pobabilistic ethod, this is calculated fo the wind data statistics and foecasts. This gives too high a capacity. Thus, the capacity facto is ipotant fo WEC s since it diectly affects thei eanings. The capacity facto is also ipotant fo the gid opeational point of view, fo daily planning as well as fo houly opeations. The capacity facto is ipotant fo futue plannes of the gid syste. Pobabilistic ethods ight be oe useful hee.
WIND PENETRATION IMIT The wind penetation is defined as the extent to which wind powe can be added to a powe syste without copoising its opeational eliability. Opeational eliability in tun is defined in tes of a syste s dynaic esponse, equiing the syste fequency to stay within the liits of potection fo a set of wind-elated events. This akes acceptable syste dynaics the sole citeia fo inceasing wind penetation. Wind penetation siply eans the extent to which wind powe can be added to a powe syste without affecting its opeational eliability. This eliability depends on the extent of eseves, extenal to the wind capacity, which ae available to the syste, intenally o extenally.
The gid ule fo Iish Electic Systes equies that the piay eseve in a syste should be 75% of the single lagest feed of the syste. This should be available within 5 to 15 seconds subsequent to the event of the feed going out. Again, if the load facto of a syste is 60% and the aveage wind plant capacity is 40%, then the penetation has been woked out by Sith as 30%. Aveage wind penetations has been between 20% to 30%.
WIND ENERGY FARMS With ising contibution by wind powe to electicity systes, individual WTGs o clustes theeof ae giving up thei place in the enegy scenaio to wind enegy fas. These fas have a few notewothy featues. They ae geneally located away fo load centes. Connecting the into the gid syste equies alost dedicated tansission as. Wheeas WTGs can be planned, installed, and put in opeation within a shot tie span, connecting tansission links equie long-tie planning and execution. Existing tansission lines ight not have the capacity to cay the wind fa powe, as happened in the case of an East Euopean custoe who contacted fo wind enegy fo a Dutch supplie. An iegula flow on the diect line could also affect load flows and voltage egulation on the neighboing lines. This necessitates cental contol fo the syste opeato ove the opeation and contibution by the wind enegy fa.
MAINTENANCE OF WTG Pobles applying to odeling also apply to standadizing aintenance. Manufactues teat failue data as popietay and ae not vey coopeative in disclosing this. oads ae high and of a fluctuating natue. So fa, aintenance is of a coective type. Downtie, loss of poduction, and epai costs ae high. Pobles ae aggavated in offshoe installations. Peventive aintenance is desiable. Geaed WTGs opeate though a weak shaft with tosion stesses. Quite often, thee is echanical esonance in the syste, siila to electical esonance. At this echanical esonance fequency (about 1 2 Hz), thee is shaft elaxation at shaft faults, leading to failues. Including echanical chaacteistic in odeling is essential.
Hoe Assignent Given the MATAB/SIMUINK odel of the wind enegy convesion syste exaple explained duing the lectue, apply the following changes to the odel, 1. Replace the siple step change wind input with a popely-scaled actual wind patten and coent on the effect of the wind speed vaiations on the tubine output powe and voltage. 2. Change the ating of the 500 kw load to any anothe ating value in the ange of (1-3 MW), and change the ating of the 2 MVA plant to any value in the ange of (4-6 MVA), then apply appopiate design changes to the ating of wind tubine in ode to successfully supply the local loads without violating the voltage o fequency liits on the gid connection point.