Investigation of Self-Excited Induction Generators for Wind Turbine Applications

Transcription

1 February NEL/CP Invetigation of Self-Excited Induction Generator for Wind Turbine Application E. Muljadi and C.P. Butterfield National enewable Energy Laboratory Golden, Colorado J. Sallan and M. Sanz Univerity of Zaragoza Zaragoza, Spain Preented at the 999 IEEE Indutry Application Society Annual Meeting Phoenix, Arizona October 3-7, 999 National enewable Energy Laboratory 67 Cole Boulevard Golden, Colorado NEL i a U.S. Department of Energy Laboratory Operated by Midwet eearch Intitute Battelle Bechtel Contract No. DE-AC36-99-GO337

2 NOTICE The ubmitted manucript ha been offered by an employee of the Midwet eearch Intitute (MI), a contractor of the US Government under Contract No. DE-AC36-99GO337. Accordingly, the US Government and MI retain a nonexcluive royalty-free licene to blih or reproduce the blihed form of thi contribution, or allow other to do o, for US Government rpoe. Thi report wa prepared a an account of work ponored by an agency of the United State government. Neither the United State government nor any agency thereof, nor any of their employee, make any warranty, expre or implied, or aume any legal liability or reponibility for the accuracy, completene, or uefulne of any information, apparatu, product, or proce dicloed, or repreent that it ue would not infringe privately owned right. eference herein to any pecific commercial product, proce, or ervice by trade name, trademark, manufacturer, or otherwie doe not necearily contitute or imply it endorement, recommendation, or favoring by the United State government or any agency thereof. The view and opinion of author expreed herein do not necearily tate or reflect thoe of the United State government or any agency thereof. Available electronically at Available for a proceing fee to U.S. Department of Energy and it contractor, in paper, from: U.S. Department of Energy Office of Scientific and Technical Information P.O. Box 6 Oak idge, TN phone: fax: report@adoni.oti.gov Available for ale to the blic, in paper, from: U.S. Department of Commerce National Technical Information Service 585 Port oyal oad Springfield, VA 6 phone: fax: order@nti.fedworld.gov online ordering: Printed on paper containing at leat 5% watepaper, including % potconumer wate

3 Invetigation of Self-Excited Induction Generator for Wind Turbine Application Eduard Muljadi # Jeu Sallan* Mariano Sanz* Charle P. Butterfield # # National enewable Energy Laboratory 67 Cole Boulevard Golden, CO 84 Tel. (33) , Fax (33) Eduard_muljadi@nrel.gov, * Univerity of Zaragoza Electrical Engineering Department Maria de Luna 3, 55 Zaragoza, Spain Phone/Fax: / jallan@pota.unizar.e Abtract The ue of quirrel-cage induction machine in wind generation i widely accepted a a generator of choice. The quirrel-cage induction machine i imple, reliable, cheap, lightweight, and require very little maintenance. Generally, the induction generator i connected to the utility at contant frequency. With a contant frequency operation, the induction generator operate at practically contant peed (mall range of lip). The wind turbine operate in optimum efficiency only within a mall range of wind peed variation. The variable-peed operation allow an increae in energy captured and reduce both the torque peak in the drive train and the power fluctuation ent to the utility. In variable-peed operation, an induction generator need an interface to convert the variable frequency outt of the generator to the fixed frequency at the utility. Thi interface can be implified by uing a elf-excited generator becaue a imple diode bridge i required to perform the ac/dc converion. The ubequent dc/ac converion can be performed uing different technique. The ue of a thyritor bridge i readily available for large power converion and ha a lower cot and higher reliability. The firing angle of the inverter bridge can be controlled to track the optimum power curve of the wind turbine. With only diode and thyritor ued in power converion, the ytem can be caled up to a very high voltage and high power application. Thi paper analyze the operation of uch a ytem applied to a /3-hp elf-excited induction generator. It include the imulation and tet performed for the different excitation configuration. I. INTODUCTION Many type of generator concept have been ued and propoed to convert wind power into electricity. The ize of the wind turbine ha increaed during the pat ten year, and the cot of energy generated by wind turbine ha decreaed. The challenge i to build larger wind turbine and to produce cheaper electricity. Thu, there i a need to find a way to convert wind energy into electrical energy from wind turbine that can be caled up in power without extremely high cot penaltie. In thi paper, a combination of erie [] and parallel [] capacitor are ued to excite the induction generator while operating at variable peed. In the elf-excited mode, the induction generator i excited with three-phae ac capacitor. The frequency, the lip, the air gap voltage and the operating range of the ytem are affected by the characteritic of the induction generator and the choice capacitor ize. The operating lip in a elf-excited mode i generally mall and the variation of the frequency depend on the operating peed range. The ytem we teted ha the following component: - a /3-hp dc machine to repreent the wind turbine - a three-phae, /3-hp, induction generator driven by the dc machine INDEX TEMS Wind energy, variable-peed generation ytem, renewable energy, elf excitation. dc machine Induction machine Capacitor bank Diode bridge Figure. Scheme of the ytem Thyritor bridge

4 - variou et of capacitor to provide reactive power to the induction generator - a three-phae diode bridge to rectify the current provided by the generator - a dc reactor to mooth the dc current and to limit the current peak on the dc bu - a three-phae thyritor bridge to convert the power from the dc bu to the utility. The organization of thi paper will be preented a follow: Section II of thi paper i devoted to wind turbine operation and the propoed ytem. Section III i devoted to the concept of elf excitation of an induction generator. In Section IV, we preent erie compenation and in Section V, the combination of erie and parallel compenation i preented. The lat ection ummarize the reult. II. WIND TUBINE In wind park, many wind turbine are equipped with fixed frequency induction generator. Thu the power generated i not optimized for all wind condition. To operate a wind turbine at it optimum at different wind peed, the wind turbine hould be operated at it maximum power coefficient (C p-optimum =.3-.5). To operate around it maximum power coefficient, the wind turbine hould be operated at a contant tip-peed ratio, which i proportional to ratio of the rotor peed to the wind peed. A the wind peed increae, the rotor peed hould follow the variation of the wind peed. In general, the load to the wind turbine i regulated a a cube function of the rotor rpm to operate the wind turbine at the optimum efficiency. The aerodynamic power generated by wind turbine can be written a: 3 P =.5ρAC p V [] Where the aerodynamic power i expreed a a function of the pecific denity (ρ) of the air, the wept area of the blade (A) and the wind peed (V). To operate the wind turbine at it optimum efficiency (C p-optimum ), the rotor peed mut be varied in the ame proportion a the wind-peed variation. If we can track the wind peed preciely, the power can alo be expreed in term of the rotor peed. 3 P = K p rpm [] The power decribed by equation [] will be called P ideal. Thi i the power to be generated by the generator at different rotor rpm. One way to convert a wind turbine from fixedpeed operation to variable-peed operation i to modify the ytem from a utility-connected induction generator to a elfexcited operation. Ideally, if the inertia of the wind turbine rotor i negligible, the rotor peed can follow the variation of the wind peed if the outt power of the generator i controlled to produce the power-peed characteritic decribed in equation. Thu the wind turbine will alway operate at C p-optimum. In reality, the wind turbine rotor ha a ignificantly large inertia due to the blade inertia and other component. The wind turbine operation can only in the vicinity of C p-optimum. However, compared to fixed-peed operation, the energy captured in variable-peed operation i ignificantly higher. With variable-peed operation and ufficiently large rotor inertia, there i a buffer between the energy ource (wind) and energy ink (utility). Allowing the rotor peed to vary ha the advantage of uing the kinetic energy to be tranferred in and out of the rotor inertia. Thu, the aerodynamic power, that fluctuate with the wind int, i filtered by the inertia before it i tranmitted to the utility grid. Thi concept i very imilar to the ue of dc filter capacitor at the dc bu of a dc-dc converter. The dc capacitor filter the voltage ripple o that the voltage outt preented to the load will be a mooth outt voltage. It i expected that the turbulent content in wind int will not be tranmitted directly to the mechanical drive (gearbox) of the wind turbine thu the mechanical tre and fatigue on mechanical component can be relieved. Thu, the lifetime of the mechanical drive and other component of the wind turbine can be extended by variable-peed operation. III. SELF-EXCITED INDUCTION GENEATO The induction machine i modeled uing the teady-tate equivalent circuit hown in Figure. Detail derivation of equation for elf-excited induction generator can be found in many paper [3-4]. The frequency dependent reactance i caled by per unit frequency. Per unit frequency (f ) i the ratio of operating frequency to the rated frequency (6Hz). The table operation of the ytem can be utained at any moment when the balance of real power and reactive power can be maintained. The balance of real power i etablihed mainly between the power produced in the rotor and the power conumed from the tator winding through the power converter. The balanced of reactive power i etablihed between the ac capacitor and the air-gap flux condition at any operating condition. From the equivalent circuit hown in Figure a, the equivalent circuit can be repreented in term of it admittance. The admittance of the tator branch conit of the tator reitance, tator leakage inductance, the tator load reitance repreenting the power drawn by the diode bridge at the tator, and the excitation capacitor. Given the load reitance, the capacitor ize, the tator leakage inductance and tator reitance, the tator admittance can be found. The tator admittance can be repreented a:

5 I X f r X lr f The balance of real power can be found by equating the real part of the tator branch and the real part of the rotor branch: Z L V t E X m f r (-)/ e ( Y ) = r + ( X lrf ) r + [6] Y = e(y ) j Im(Y ) [3] + The rotor branch can be repreented a: Y r Y E Y m Y r r + r r ( X f ) + ( X f ) lr j = [4] The magnetizing branch can be repreented a: Ym j X = [5].93 Figure. Equivalent circuit of the induction machine m X lr f lr The lip can be olved from Equation 6. The next thing to olve i the airgap voltage at thi operating point. The relationhip of the imaginary part of the rotor branch, the tator branch, and the magnetizing branch i given in Equation 7. It can be ued to olve X m. From X m and the operating frequency, the airgap voltage (E 6HZ f ), the magnetizing curve, and the operating frequency, the airgap flux and airgap voltage can be found. Im(Y ) X f + X f lr = r m + ( X lrf ) + [7] The relationhip between the magnetizing inductance L m a a function of flux linkage (E/ω) i given in Figure 3. It i hown how the value of L m decreae a the induction generator aturate. From the L m, the airgap voltage can be found. The terminal voltage, the tator and rotor current, the airgap voltage, and int power can be comted. IV. PAALLEL COMPENSATION In thi ection, the parallel compenation i invetigated. The three-phae ac capacitor are connected in parallel with the load at the terminal of the induction generator. The load i a power converter coniting of the diode bridge and the thyritor bridge. To the generator, the diode bridge preent a unity power factor load. Although the current waveform at the int to the diode bridge i ditorted, the preence of the Lm (Henry) L m. Induction machine L FLUX m.398 Flux (Volt/(rad/)) Figure 3. Magnetizing curve of the induction machine C Thyritor bridge Figure 4. Parallel compenation

6 power (watt) Pideal L i varied to track Pideal, C=7uF L =ohm, C=7uF L and C are varied to track Pideal and to keep I < Irated frequency (Hz) Figure 5. Power veru frequency of a parallel excited induction generator tator current (A) L = ohm, C=7uF L i varied to track Pideal, C=7uF L, C are varied to track P ideal and to keep I < I rated frequency (Hz) Figure 6. Stator current veru frequency of a parallel excited induction generator ac capacitor help to mooth out the current hape. The power outt of the generator can be adjuted by controlling the firing angle of the phae-controlled inverter at the utility ide. Thu, the utility i een by the generator a an adjutable energy ink. For teady-tate analyi and conidering only the fundamental component of the current waveform, the load can be replaced a an adjutable reitive load L. Figure 4 how the phyical diagram of parallel compenation. The load and the capacitor compenation can be repreented a Z Load. The admittance of the tator and the load can be expreed a: Y (ZLoad + Z ) = [8] where Z load and Z are given a: Z Load L = [9] + jωc Z = + jx f [] l Uing the equation above and the aturation curve of X m, the operating condition of the induction generator i examined. The adjutable converter load i repreented by reitive load. Three cae are examined. The firt one i a contant reitive load ( L = ohm) in parallel with a contant capacitive load (C=7µF). The frequency i varied and the operating point are comted. The econd one i the ame a the firt one except the reitance of the load i varied o that the outt power track the ideal power to optimize the wind turbine operation. The third one i to change the capacitor ize in three different value while changing the reitive load to optimize the wind turbine operation. The capacitor ize i tarted at 7 µf and the reitive load i changed a the frequency i increaed. When the tator current reache it rated value, another capacitor value i ued and the reitance i varied again and o on. We how in Figure 5 that to track the P ideal, the load mut be repreented a a variable reitance. Uing a contant reitance load, the generated power increae above the rated power of the generator a the frequency i increaed. With a variable-reitance load, it i poible to follow the ideal power for a wide range of frequency variation a hown by the dah-dot line. Similarly, with three different capacitor, it i poible to follow ideal power for a wide range of operating frequencie. In Figure 6, the tator current for three different cae are hown. For contant reitance load, the power cannot follow the Pideal, and the tator current goe above the rated current. By changing the value of the capacitor for different range of operating frequency, it i poible to keep the tator current below it rated value. It i hown from Figure 6 that a the tator current goe above it rated, a new capacitor value i applied to the ytem. Note that there are dicontinuitie of the operating point when changing the capacitance value. In Figure 7, the value of magnetizing inductance L m a the frequency change are hown for three different cae. The Lm (H) L = ohm, C=7uF L i varied to track Pideal, C=7uF L and C are varied to track P ideal and to keep I < Irated frequency (Hz) Figure 7. Magnetizing inductance L m for a parallel excited induction generator

7 value of L m indicate the level of aturation at any operating condition. The firt two cae that ue a ingle value of capacitor how very high aturation level at higher operating frequencie, while uing three different capacitor ize enable the ytem to operate at an average or almot contant level of aturation. V. SEIES-PAALLEL COMPENSATION The ytem configuration for erie-parallel compenation i hown in Figure 8. In thi configuration, both the erie capacitor C and the parallel capacitor C affect the level of excitation. The erie capacitance C help to increae the level of excitation and counteract the voltage drop acro the tator reitance and tator leakage inductance. Induction Machine Z In thi cae, the load impedance i: Load C C = +. (5) jω C + jω C L Utility In Equation 5, the terminal of the induction generator i connected to the erie capacitor and the parallel capacitor i connected in parallel with the diode bridge. The parallel capacitor compenate the main reactive power needed by the L Thyritor bridge Figure 8. Serie-parallel excitation of the induction machine 3 Power and Pideal veru rotor peed Stator current (A) rotor peed (rpm) Figure. Stator current veru rpm for a erieparallel excitation induction machine induction generator to counteract the main magnetizing winding L m, which ize varie with the level of aturation. A teady-tate analyi i performed to undertand the behavior of the ytem. The compenation choen i the parallel capacitor C=4 µf and the erie compenation capacitor C =5 µf). A in the previou ection, the power converter i imulated by an adjutable reitive load L. Apparently, a normal operating condition can be achieved by uing thi pair of capacitance value. The ideal power can be achieved and the tator current tay below the rated value in the entire range of operation. In Figure 9, the outt power of the generator (olid line) and P ideal (dahed line) are hown. The ytem can track the ideal power for a wide range of operating frequencie. In Figure, the tator current a a function of the frequency i hown. Note that the tator current tay within the rated value in the entire range of operation. The level of excitation can be hown by the variation of inductance L m a the frequency i varied. Thi variation i hown in Figure. Comparing Figure and Figure 7, it i clear that the erie compenation can atifactorily maintain the level of aturation. It i hown that in the lower frequency, the aturation level i very low and the tator current required to track the ideal power i alo low. Thu both the iron loe and the copper loe are low in the low frequency, which indicate that the operation of the ytem i efficient even in the lower frequency region. Power (watt) Lm (Henry) rotor peed (rpm) frequency (Hz) Figure 9. Power (in watt) veru rotor peed for erie-parallel excited generator Figure. Magnetizing inductance (in H) veru frequency for erie-parallel excited generator

8 I. Xc Xl i i i Xc i f i Figure. Voltage drop acro reactance and the reactance (Xc-Xl) veru frequency It hould be pointed out that the erie-parallel compenation operate moothly becaue there i no need to change the capacitor ize. The capacitor pair choen can be ued for the entire range of operation. The erie capacitor hould be able to carry the rated current a the induction generator, while the parallel capacitor hould be rated to withtand the outt voltage at the terminal of the power converter. Both the erie and parallel capacitor are ac capacitor. It i intereting to oberve the erie capacitor compenate for the voltage-drop acro the tator leakage inductance. Figure how the total reactance (Xc-Xl) and the voltage drop acro it. In thi circuit, the erie capacitive-reactance i more dominant than the tator leakage reactance, the total reactance drop a the frequency increae. However, a the frequency increae, the demand for the tator current alo increae. Thu the voltage drop acro the total reactance (Xc-Xl) i almot contant for the entire range of operation. In Figure 3, the actual data from experimental reult collected in the laboratory i drawn on the ame figure a the ideal power P ideal. The experimental data fit the ideal curve down to a peed of 9 rpm, one-half of the rated peed. The tet reult hown verify that for the teady-tate calculation preented in thi ection. Note that in optimum operation of a wind turbine, the power i a cube function of the rotor peed. (W) Xl i voltage drop acro reactance reactance 7 7 Speed (rpm) Figure 3. Power veru rotor peed from experimental data Thu, if the upper limit i et to P rated, the lower limit that i 5% of the rated peed will generate about /8 (.5%) of the rated power. A. Generator voltage and current waveform Figure 4a how the phae voltage and line current in the generator when working in rated condition. The voltage waveform i affected by the current ditortion. Figure 4b, how the frequency pectrum of the current waveform. The current waveform contain minimal harmonic component. The parallel capacitor at the outt terminal acro the diode bridge filter the harmonic current entering the generator. It i expected that the harmonic loe due to current harmonic i reduced ignificantly. The torque lation which i normally aociated with current harmonic i expected to be very mall a well. The effect of torque lation on the peed fluctuation i further reduced by the fact that the blade inertia i very large compare to normal load. Thu the rotor peed fluctuation due to harmonic i barely be noticeable t (m) Figure 4. Voltage and current waveform at the generator terminal outt B. Utility voltage and current waveform Vgen (V) 5*I(A).5 Power (ideal) Power (tet) Igen t() (a) Figure 5 how the utility voltage and current. The latter ha the typical hape of a thyritor inverter current, with high ripple. It frequency pectrum how the high value of the fifth and eventh harmonic. With proper filtering at the (b)

9 5 VI. CONCLUSIONS Iline t (m) Vline (V) 5*Iline(A) (a) (b) We invetigated the propoed generator for application in wind power generation. In the firt tage of implementation, a proof of concept of the generator wa invetigated. Thi paper demontrate the technical viability of uing a elfexcited induction generator in variable-peed wind generation. Thi greatly implifie the power tage needed to connect to the utility compared with inverter-fed induction generator. The erie-parallel compenation ytem track the maximum power curve of the wind turbine over a wide range of peed, with a pair of capacitor value. The power tage of the power converter conit of diode and thyritor, thu, thi ytem can be caled up to a higher voltage and higher power to proce very high power in wind power generation. It main drawback i the non-inuoidal outt current generated at the grid. However, many cheme have been propoed to improve the power quality at the outt of the phae-controlled inverter uch a paive filter, active filter, or other topologie. VII. ACKNOWLEDMENTS f (H z) Figure 5. Utility phae voltage and line current. a) Current and voltage waveform b) Frequency pectrum of the utility line current The author wih to thank Jerry Bianchi for hi aitance during the tet et up of thi generator. We wih to acknowledge our management at National enewable Energy Laboratory and the U.S. Department of Energy for encouraging u and approving the time and tool we needed for thi project. DOE upported thi work under contract number DE-AC36-98-GO337. EFEENCES point of common coupling the current harmonic can be reduced to an acceptable level. C. Parameter Determination Tet A no-load circuit tet and lock rotor tet are performed. The parameter of the induction generator are comted from the tet data, and the reult are lited in Table. Table. Induction Machine Data from the Tet Induction Machine Data eitance eactance at 6Hz = 5.7 Ω X = X r = 5.76 Ω r = 4.35 Ω X m,nom = 88. Ω 4 pole ated power = ¾ HP Line Voltage = 3 volt Frequency = 6 Hz. C.F. Wagner, Self-Excitation of Induction Motor with Serie Capacitor. AIEE Tranaction, February 94, Vol. 6, pp E. Muljadi, B. Gregory, and D. Broad Self-excited Induction Generator for Variable-Speed Wind Turbine Generation. Powerytem World 96 Conference, La Vega, Nevada. Alternative Energy Seion, September 7-3, 996, pp T.F. Chan Self-excited Induction Generator Driven by egulated and Unregulated Turbine. IEEE Tranaction on Energy Converion, June 996,Vol., No.. 4. K.E. Halleniu, P Va, J.E. Brown. The Analyi of a Saturated Self-Excited Aynchronou Generator. IEEE Tranaction on Energy Converion, June 99, Vol. 6, No..