SMALL POWER GENERATOR FOR WIND POWER PLANTS

Transcription

1 Zeszyty Problemowe Maszyny Elektryczne Nr 8/29 9 Karel hmelík, Stanslav Mšák Techncal Unversty of Ostrava, Ostrava SMALL POWER GENERATOR FOR WND POWER PLANTS GENERATORY MAŁEJ MOY DO ELEKTROWN WATROWYH Abstract: the most mportant Renewable Energy Sources n the zech Republc are: water plants, bomass, bogas, wnd power plant, lqud bofuel and photovoltac systems. Most progressve RES producton growth we can observe n past ten years at bomass, bogas and water plants. But most dynamcally grows Wnd Power Plants electrc energy producton. Systems of Wnd Power Plant wth generators wth nomnal power more than 2 MW grows mostly. Systems of Wnd Power Plant wth nomnal power more than 5 kw are developed at present too. These systems are used for producton of electrc energy n autonomous electrc systems for sngle-famly house, remote areas and so on.. ntroducton The power of wnd s knd of renewable sources on the world. For utlzaton these sources are used wnd power-plants wth electrcal generators. The generator transforms mechancal power of wnd to electrcal power. The nducton generator s used mostly for transformaton of mechancal energy. The nducton generator has these characterstcs mportant for use n wnd-power plants: easy control and operatng of generator, cheaper technology of producton, less weght, hgh relablty, possblty of use standard nducton motor. Dsadvantage of usng nducton generator s necessty supply and control of reactve energy. The control system wth synchronous generator wth permanent magnets s extended n the present tme. The reason of expanson of usage s dmnshng prce of generator and hgher effcency of system n comparson to system wth nducton generator. The possblty of use small nducton generator manufactured as an nducton motor and synchronous generator wth permanent magnets workng n autonomous power network and workng n parallel run wth dstrbuton network wll be descrbed n ths paper. 2. nducton motor as a generator Small wnd power plants can work wth nducton generator manufactured as a motor, the recomputaton some nomnal parameters of machne for generator mode s necessary. The modfcaton energy flow of machne occurs n the generator mode. The characterstcs parameters of nducton motor are classfed by these rules [2]: The output power s mechancal power on the shaft of machne and t s solved by formula (). P SH π n M = ω M = () 3 where M (N m) s moment on the shaft and n (rpm) are revoluton per mnute of the shaft. The nput power of motor s electrcal power n termnals of machne accordng to equaton (2) P = 3 U cosϕ (2) EL Where U s rated voltage, s rated current and cosφ (-) s rated power factor. Losses are defned as a dfference between nput and output power, see equaton (3). P LOS = P P (3) NP OUT The nput power s mechancal power on shaft accordng to formula () and output power s electrcal power accordng to formula (2) for generator mode of nducton machne. The lose flow s changed and losses are refunded by mechancal energy. Some values of electrcal and magnetc quanttes are changed for ths replacement n the machne. Ths fact s ntroduced by equvalent crcut n the Fg..

2 2 Zeszyty Problemowe Maszyny Elektryczne Nr 8/29 Z Z2 Z2 Z R2 (-s)/s U U U Z Z R2 (-s)/s Fg.. The equvalent crcut of nducton machne Equvalent crcuts of a) nducton motor and b) nducton generator are shown n the Fg.. The nner generated voltage s gven by formula (4) for motor mode of nducton machne. U = U Z (4) Further, the nner generated voltage s gven by formula (5) for generator mode of nducton machne. U = U+ Z. (5) Where U s nner generated voltage, U s termnal voltage and Z s voltage drop of stator. For case, when the termnal voltage s constant n motor and generator mode of nducton machne, the nner generated voltage s hgher about double stator voltage drop of. The sze of nner generated voltage s nfluenced by magnetc flow n machne accordng to formula (6), U φ = (6),44 N f k 4 v The mountng magnetc flow grows up magnetc nductance, magnetc losses and magnetc current. The magnetc current s dependent on power factor of the machne and sze of reactve energy. n case, when the nducton machne works wth decreased termnal voltage about double value of stator voltage drop, the machne retans ts constant power factor. The decreased termnal voltage and losses replacement lmtatve of rated power of machne. The rated power of the nducton machne for generator mode s only 5 to 7 percent of rated power for the motor mode. Therefore, the usng machne wth double sze of rated power and machne runnng wth decreases termnal voltage s necessary for usng nducton motor as a generator n practce. U 3. nducton generator workng n autonomous network Most of small wnd-power plants are nstalled n remote countres for supply n autonomous networks. The supply of reactve energy to the termnal of nducton generator s needful for creaton of electromagnetc feld n the nducton generator. The reactve energy s obtaned from the capactors. t s assumed that sze of the generator nductance s same as a capactor capactance accordng to the formula (7). X = X L (7) The formula (7) respects smplfcaton, when the no-load current s reflected as magnetzaton current. The value of requred capactve reactance wll be obtaned by formula: U and then value of the capacty: f X = (8) = 2 π f X (9) Every electrc source s characterzed by nner resstance wth value R. Ths resstance determnes so called hardness source. n case, when the nducton machne s used as source of electrc energy, the nner resstance s not only one of mportant parameter of electrc source. The leakage reactance affects runnng state of nducton machne too. The leakage reactance s dependent on load, voltage and frequency. Therefore, the constant termnal voltage s requred. The termnal voltage s proportonal to capacty of capactor bank. Thus, the controllable capactor bank s requred too. The ncreasng capacty of condenser battery s attended by ncrease of machne losses. When the electrc source s loaded, the termnal voltage s decreased. Ths voltage drop s gven by nner resstance of the source R and determnes hardness of the source. And next, the value of voltage drop nfluences reactve current (see formula ()). = 2 π f U f () When the voltage drop s ncreased by vrtue of ncrease n load value, the slp s decreased. The slp s zero at synchronous speed and ts value s decreased to negatve value wth speed ncreasng. At complyng wth constant rotaton

3 Zeszyty Problemowe Maszyny Elektryczne Nr 8/29 2 speed of the shaft, synchronous speed n s wll be changed accordng to formula (): n= n.( ). s s = konst () The frequency of electrc source s gven by formula (2): n p f = (2) 6 ( s) Voltage and frequency drop wll have consderable feld suppresson effect; therefore nducton generator wll be comparable to soft source wth dervatonal characterstcs. Overloadng wll be attended by decreasng of reactve energy suppled by capactor and machne wll be complete feld suppressed. Voltage drop wthn knee of dervatonal loadng characterstcs s bonded to unload drve system. Snce certan value of voltage exsts, condenser batteres are not able to excte machne and voltage on termnals drops to zero. Power factor of load has nfluence to hardness of source too. n the case, that nducton generator s loaded by nductve load, the loadng characterstc s softer, and for possblty of next load s requred hgher value capacty of capactor bank. However, when the capacty n the crcut s ncreased, runnng effcency of generator s decreased. Generator startng must proceed wth dsconnected loadng. nduced remanent voltage develops current flowng through capactors, whch wll excte the generator. n case of connected load, the current by wndngs of machne wll be dvded to load current and current flowng through capactors, accordng to mpedances ratos of load and capactor. Last mentoned facts specfy condtons for optmal load of generator at autonomous network runnng []: a) the moment of load connecton s determned by value of termnal voltage of nducton generator, b) the value of load mpedance s selected accordng to load characterstc, c) the maxmal effcency of nducton generator s at area knee of load characterstc; n ths area s generated loaded optmally, d) nducton generator may work all types of the load. However, all of no-resstance loads must be fully compensated. Realzaton of control optmal run of the generator can be realzed by voltage guards or multfuncton programmable automat wth some analog nputs for example. 4. nducton generator workng n parallel run wth dstrbutve network Problems of small nducton generators runnng n parallel cooperaton wth dstrbutve network are n elmnate of swtchng transent phenomena. The reactve energy s supply by the dstrbutve network. The generator s connected to the network when hs rpm (revoluton per mnute) are n over-synchronous area. The connecton of nducton generator to dstrbutve network s followed by over-voltages and over-currents. Typcal transent phenomena of swtchng nducton generator are drawn n the Fg ,36-7,34-7,32-7,3-7,28-7,26-7,24 (s) -7,22 (f le asg3.adf x-v ar t) Us 3 ; Ug 2 Fg. 2. Transent phenomena of swtchng nducton generator to the dstrbutve network The over-current s around multple of rated current n ths example. Relevant oscllaton of current wth swtchng frequency may cause voltage fluctuaton n power network. There are some methods of elmnate of these transent phenomena. Usng frequency converter or softstart s possble to use for example, but these methods are too expensve for small generators. The connecton of nducton generator wth small power to the dstrbutve network s possble to realze n few steps. The frst step s swtch generator to power grd over resstors by contactor. After a few seconds pause s swtch man contactor and dsconnect contactor of resstors. Ths method elmnates the transent phenomena of swtchng [3]. Overcurrent rases only fragment of rated current and ts value s dependent on value of resstor R. The relaton between maxmum value of current and sze of resstor s ntroduced n Fg. 3. Ths relaton was obtaned for nducton gen

4 22 Zeszyty Problemowe Maszyny Elektryczne Nr 8/29 erator (, kw, 6poles,,4kV). The M s ampltude of over-current. M 2,5 M / (-) Fg. 3. The relaton between over-current and sze of resstor R The optmal swtchng of small nducton generator to power network s shown n Fg u ,5,5 m=f(r) m/=f(r) R (Ω) t (s) [s] -2.. Fg. 4. The optmal swtchng of small nducton generator to the dstrbutve network Transent phenomena are segmented to two transent phenomena wth mnmal over-currents and wthout over-voltages. The vew of optmal swtchng s realzed for connecton of nducton generator, kw, 6poles by resstor 5 Ohm and 22 rpm n Fg Synchronous generator as a source of electrc energy at wnd power plant The regulaton of voltage at termnal of machne s not possble for case usng of synchronous machne wth permanent magnets. The value of termnal voltage s defned by characterstc parameters of machne (materal parameters of permanent magnet) and by revolutons per mnute. Ths dependence s called as a exctng characterstc (see Fg. 5) U n (mn - ) U=f(n) Y connecton U=f(n) D connecton Fg. 5. The exctng characterstc of machne The drecton of curve s called as a voltage constant and ths s characterstc parameter of synchronous machne wth permanent magnet. The synchronous machne s possble to use as a source of electrc energy for wnd power plant as well as nducton generator. Advantage of usng synchronous machne wth permanent magnet s unnecessary source of reactve energy. Smlarly, as n the case of usng nducton machne, s possble to use synchronous generator wth permanent magnet as a source of electrc energy for autonomous network. The load n the form of resstor heatng spral s mostly used for water heatng n the famly house. Ths type of load s not dependent on the frequency of output voltage. The synchronous generator wth permanent magnet s possble to operate parallel wth dstrbutve network too. The system on the Fg. 6 s applcable to operatng of wnd power plant to 5 kva. T u G (t) SG S S u S (t) DS 4V/5 Hz Fg. 6. The block scheme of control system of synchronous generator wth permanent magnet. The synchronous generator must be operated so that termnal voltage has equvalent value as a voltage of dstrbutve network for synchronous revolutons per mnute. Synchronzng cond-

5 Zeszyty Problemowe Maszyny Elektryczne Nr 8/29 23 tons have to be realzed for swtchng synchronous generator to dstrbutve network. For reducton of current surges have to be realzed these condtons: - dentcal phase sequences for generator and dstrbutve network, - dentcal frequency of generator and dstrbutve network, - dentcal ampltude generator voltage and voltage of dstrbutve network, - dentcal phase generator voltage and voltage of dstrbutve network n the moment of swtchng. The swtchng by control system of wnd power plant s realzed subsequently: nstantaneous values of generator voltage u G (t) and dstrbutve network voltage u S (t) s traced by synchronzng devce S. The synchronous generator s connected to dstrbutve network at the moment completon of swtchng condtons. The example of swtchng synchronous generator 5 kva to dstrbutve network s shown n the Fg. 8. Ths paper was created under research project MSM Bblography []. Nevřala J.: The nducton Machnes for Wnd- Power Plants, Dploma Thess, VŠB-TU Ostrava 25. [2]. hmelík, K.: Asynchronní a synchronní elektrcké stroje, VŠB TU Ostrava, 22, SBN [3]. Nevřala, J.; hmelík, K.; Mšák, S.: Větrné elektrárny, Časops pre elektrotechnku a energetku EE, Bratslava, /27, page 75-78, SSN Authors Doc. ng. Karel hmelík, ng. Stanslav Mšák, Ph.D. Techncal Unversty of Ostrava, Faculty of Electrcal Engneerng and omputer Scence, Department of Electrc Machnes and Apparatus, 7. lstopadu 5, Ostrava-Poruba, zech Republc. e-mal: karel.chmelk@vsb.cz, stanslav.msak@vsb.cz ,6,8,,2,4 (s),6 (f le smpm.adf t) U2g U3g U23s ; x-v ar U23g U2s Fg. 8. The course of generator voltages (U 2g, U 23g, U 3g ), voltages of dstrbutve network (U 2s, U 23s ) and course of current at the moment of generator connecton to dstrbutve network 6. oncluson Smple systems of wnd power plants wth nomnal power to 5 kw are ntroduced wthn ths paper. nducton generator and synchronous generator wth permanent magnets were used as a source of electrc energy at wnd power plant. Results from measurng characterstc parameters of mentoned types of generators for autonomous network and parallel cooperaton wth dstrbutve network are shown n the paper too. Future research wll be drected to compare yeld of these systems on measure base.