Intenational Jounal on Electical Engineeing and Infomatics - Volume 10, Numbe 3, Septembe 2018 Contol of SCIG Based Constant Voltage Geneation Scheme fo Distibuted Powe Supply Rupa Misha and Tapas Kuma Saha Depatment of Electical Engineeing, National Institute of Technology Dugapu, Dugapu 713209, India Abstact: This pape addesses the supply of powe to the stand-alone load at constant voltage and constant fequency (CVCF), fom a vaiable souce distibuted geneation system. The inheent poblem of having a fixed output fom a vaiable input is pominent in all of the distibuted geneation systems, un by uncetain enewable souces like wind. This pape poposes a topology to maintain output load voltages and fequency duing changes in both input toque and load petubation. The contol is designed fo both the geneato side and load side convetes, connected as a back-to-back convete with one dc link capacito. The voltage acoss dc link capacito is maintained at a desied level by powe flow contol though the geneato side voltage souce convete (VSC). The poposed thee-stage contolle technique fo the geneato side convete is analysed and tested in all opeating conditions. The oute most dc link voltage eo is passing though the contolle is geneating the speed efeence of the speed contolle. Then the inne loop cuent contolle follows the speed contolle. The load side VSC is contolled to poduce a constant voltage at the output of the load side filte. The stand-alone system has successfully been tested in the simulation envionment. It is found to be capable of maintaining input powe level to the demand, even with 50% incement in load and 50% incement in input toque. These dynamic esponses of the vaiables ae found to be satisfactoy thoughout the opeation. Keywods: Induction Machine (IM), Sinusoidal Pulse wih modulation (SPWM), Squiel cage induction geneato (SCIG), Vecto contol (VC), Cuent contol. 1. Intoduction The impotance of enewable enegy has been appeciated as altenative esouces fo powe geneation in the last few decades. Wind enegy achieves a consideable fast gowth amongst all the enewable enegy esouces [1]. The synchonous geneato is not usually pefeed fo wind enegy convesion systems (WECS), as it opeates only at a constant speed while the wind speed is vaying. Geneally, the induction geneato (IG) is chosen fo the WECS because of the ability to opeate at vaiable speed constant fequency (VSCF) opeation [2]. Selection of the suitable geneato fo WECS is the challenging facto. The caggy constuction and low continuation cost of the squiel cage induction geneatos (SCIG) ae pefeed fo AC dive appliances. The equiement of eactive powe by the diectly gid connected SCIG leads to lowe efficiency and ealie was suppoted by shunt capacitos [3]. The voltage build-up of SCIG equies esidual magnetism, which seems to be unavailable with inaccessibility of its oto cicuit. Some methods like connection of shunt capacito at the stato teminal [4] o a battey connected excitation capacito is adjoined to the system. These conventional techniques ae howeve inceasing the complicacy of the topology, used fo distibuted powe geneation. Intefacing distibuted geneating system to the gid is achieved though back-to-back convete, uncontolled bidge ectifie and voltage souce invete, and matix convete in [5]. The back-to-back convete is gaining attention amongst all the available convetes. The vital eason fo choosing back-to-back convete is the ability to extact maximal powe fom the Received: August 14 th, 2017. Accepted: Septembe 20 th, 2018 DOI: 10.15676/ijeei.2018.10.3.7 513
Rupa Misha, et al. pime move [6]. The Double fed induction geneato (DFIG) fed to stand-alone load [7] o gid [8] is also available with this convete. The DFIG is the suitable option fo sites even with lowe wind speed [9]. By integating battey enegy stoage system (BESS) in DFIG, powe supply emains invaiant w..t. the wind speed. The SCIG diven stand-alone load though a VSC in [10], and the vecto contol (VC) technique is used fo geneating gate pulse. In [11], SCIG is feeding the load though a back-to-back convete. The geneato side convete is contolled though a VC technique. The basic citeion of load side convete is to contol the dc-link voltage [12]. The inne and oute loop bandwihs of the contolles ae sepaated by a decade in geneal cases [13]. A novel contol stategy of the DFIG fed to the gid-connected system has been analyzed in [14-16]. In [17], voltage and fequency contol in the stato of stand-alone doubly opeated induction geneato (DOIG) has been discussed thooughly. The design of a contolle fo DOIG based standalone VSCF application, by consideing filte, has been designed in [17-18]. Howeve the design of filte is clealy discussed in [19-21]. This design has been adopted fo SCIG based stand-alone system in this wok. The poposed wok developed a contol stategy to supply fixed voltage and fequency to a stand-alone load fom SCIG based distibuted geneation system. The contol stategy is consideing both load side voltage souce convete (VSC) and the geneato side VSC. Moeove, one filte is designed fo load side VSC to compensate the voltage dop of the filte at diffeent load level, in this wok. None of the pevious, wok available in the liteatues has shown the pefomance fo both the load and geneato side vaiations and supply of powe with fixed voltage and fequency in SCIG diven stand-alone mode. Specifically, the pesent wok ensues: (i) Constant voltage constant fequency (CVCF) supply fom the stand-alone system in all opeating conditions. (ii) A novel thee-stage contol is poposed fo the geneato side VSC to supply egulated dc voltage at the input of the mentioned load side VSC. (iii) The dc link voltage contolle is keeping the input powe level same as the load demand by adjusting the speed of the geneato in pesence of vaiation in input toque also. The poposed system is handling both input toque and output load demand vaiations, while supplying quality powe to one stand-alone load. 2. Mathematical Modelling The scheme of the complete powe and contol topologies ae pesented in Figue 1. The contol scheme includes SCIG diven by a constant pime move, a back-to-back voltage souce convete and the stand-alone load. The contol stategy of the geneato side VSC implemented is a vecto contol in oto flux oiented efeence fame. Simultaneously, the load side contol algoithm is obtained in synchonously otating load voltage oiented efeence fame. The dynamic model of SCIG is analyzed in oto flux efeence fame is as follows: dds Vds = Rsids + (1) d qs Vqs = Rsiqs + (2) dd Vd = Rid + = 0 (3) d q Vq = Riq + = 0 (4) Whee stato and oto flux linkages in the d and q axis ae expessed as: = L i + L i (5) ds s ds m d = L i + L i (6) qs s qs m q 514
Contol of SCIG Based Constant Voltage Geneation Scheme = L i + L i (7) d d m ds = L i + L i (8) q q m qs The toque in this fame can be witten as 2 p Te = Lmiqs (9) 32 Whee, Rs, R and Ls, L ae stato and oto esistance, inductance; L m is magnetizing inductance. iqs, ids and iq, id Stato and oto cuent in d-q axis; p is pole pai. Figue 1. Poposed contol stategy of SCIG fed to stand-alone load 3. Contol Stategies A. Load side VSC This convete contol stategy is to poduce a constant voltage and fequency at the output of the load filte as shown in Figue 1. The contol methodology is based upon pojection of the filte cuent in the otating efeence fame. The d-axis load voltage of the synchonous fame is aligned with fixed voltage vecto and q-axis voltage is kept zeo in this scheme. The line-to-line load voltage is consideed 230V and d axis voltage is kept at 163V. The cuent and voltage state equations, fo load side invete including filte, ae expessed as: di ( v ( i * ) v af ) = ai af f ac (10) l f di ( v ( i * ) v bf ) = bi bf f bc (11) l f di ( v ( i * ) v cf ) = ci cf f cc (12) l f 515
Rupa Misha, et al. ( iaf ila ) dv ac = (13) c f dv ( ibf ilb ) bc = (14) c f dv ( icf ilc ) cc = (15) c f The equivalent cicuit diagam of the load VSC in synchonously otating d-q efeence fame is as shown in Figue 2(a),(b). The simplified contolle diagam fo the load contolle is potayed in Figue 3. To facilitate contolle design, the (10)-(15) ae designed by using the d-q epesentation in the synchonous efeence fame. didf vdi = ( idf f ) + l f + vdc el f iqf (16) diqf vqi = ( iqf f ) + l f + vqc + el f idf (17) dvdc idf = idl + c f ec f vqc (18) dvqc iqf = iql + c f + ec f vdc (19) Whee iaf, ibf, icf and vac, vbc, vcc ae Filte cuent and voltage acoss capacito. l, c Denotes filte inductance and capacitance. B. Thee-stage contol technique of Geneato-side VSC Anothe inteesting objective of this pape is to keep the dc-link voltage constant, egadless of the amount of the load demand and input powe vaiation. The VC is developed with efeence fame aligned with the oto flux, which enable contolling active and eactive powe independently. The d q axis voltages can be witten as: di ds Lm d Vds = Rsids + Ls + Lsiqs ( m + e ) (20) L di qs Lm Vqs Rsi = qs + Ls + ( m e ) Lsids ( m e ) L + + + (21) f f Figue 2. Equivalent cicuit of the invete (a) d-axis, (b) q-axis 516
Contol of SCIG Based Constant Voltage Geneation Scheme Figue 3. Poposed contol methodology of load side convete The eo of dc link voltage is passing though PI contolle geneate speed command as shown in (22). * kid * = kpd + ( Vdc Vdc ) s (22) The efeence toque can be geneated as: * ki 1 * Te = kp1 + ( ) s (23) * * Whee, V, V Actual and Refeence dc-link voltage;, ae actual and efeence dc dc speed of synchonous efeence fame. When the speed eo passed though speed contolle, efeence toque is geneated in (23) and fom this, by the use of (9) the efeence value of q-axis component of stato cuent is calculated. The flux eo is passing though the PI contolle geneate the d-axis component of the machine cuent as given in (24). At the final stage, the eo of the d-axis component of the machine cuent passes though the PI contolle to geneate the d -axis voltage as potayed in (20). Figue 4. Fequency esponse of (a) dc link voltage contolle (b) speed contolle and (c) cuent contolle 517
Rupa Misha, et al. * ki 1 * ids = kp1 + ( ) s (24) The cuent contolles ae designed to cause the geneation of d and q axis voltages using k, k ae taken as thee times of the (20) and (21). The bandwihs of cuent contolles ( ) p i the speed contolle ( p1, i1) k k. Again, speed contolle bandwih is thee times of the voltage contolle. Fom the Bode plot shown in Figue 4(a), it is clea that bandwih of the voltage contolle is 767.8ad/s. In the case of speed and cuent contolle as shown in Figue 4(b), Figue 4(c), the system is inheently stable as the gain magin is infinite. Phase magin fo both the system is set at 90º at the fequency of 2280 ad/s in case of speed contol and 7620 ad/s fo cuent contol. The 2-phase to thee phase convesion has been achieved with the VC appoach. The pulse wih signal is geneated afte compaing this modulating signal with a caie wave. 4. System Desciption The system is tested unde the following condition: Machine specification: 1 hp,750 pm, 220 V, 7 A, 50 Hz. DC link voltage is 400 V. Filte paamete: Inducto with 8 mh, esistance is 0.1 Ω and capacitance 8.0938 µf. 5. Results and Discussion The contol technique and its implementation fo the SCIG descibed in the pevious section is simulated in MATLAB/SIMULINK envionment. The topology is validated with load impedance petubation and step change in input toque. A. Load Petubation The system is stating with 40% of ated load and then subjected to an incement of 100% in the active powe, in a step. Duing step change in stand-alone load, the ms value of the output cuent inceases fom 0.65A to1.25a, as shown in Figue 5. The d-axis and q-axis component of the load voltages ae potayed in Figue 6(a) and 6(b). The d axis component of the load voltage is maintaining at the desied level of 163V duing the study. The voltage contolles (the block diagam as shown in Figue 3) settle the tansient vaiation in these voltages, duing the load incement and decement, successfully within 0.8 s. The q-axis component of the load voltage is kept at the level zeo thoughout opeation successfully. With fixed d axis and zeo q-axis load voltage, the active powe 3 3 2 d d vi. The d-axis 2 d q becomes ( ) vi, and the eactive powe geneation becomes ( ) component of the load cuent is found to be inceasing duing a step change in load, as shown in Figue 6(c). The eactive powe demand also inceases duing this step change. Theefoe, the q-axis component of the load cuent is following the natue as potayed in Figue 6(d). The lagging eactive demand had met by an incement in this negative cuent. 518
Contol of SCIG Based Constant Voltage Geneation Scheme Figue 5. Rms value of the output cuent with load vaiation Figue 6. The tansient behavio of vaiables duing step change in load demand: (a) d-axis load voltage, (b) q-axis load voltage, (c) d-axis component of load cuent, and (d) q-axis component of load cuent The total hamonic distotion (THD) of the output voltage and cuent is keeping within a small limit by the use of filte. The THD of the load voltage found to be 2.79%, at steady state, as displayed in Figue 7(b) and the THD of the load cuent, at the same time, is 1.88%, as shown in Figue 7(d). The load vaiation is suppoted by a change in powe flow fom the machine side convete, which is contolled by the thee-stage contolle, developed in section 3 B. The tansient behavio of the dc-link voltage is potaying in Figue 8(a). The dc link voltage is maintaining Figue 7. The quality of: (a) Load voltage (b) FFT of load voltage (c) Load cuent (d) FFT of load cuent, with highe load. 519
Rupa Misha, et al. Figue 8. The tansient behavio of vaiables duing step change in load demand: (a) dc link voltage, (b) geneato speed Figue 9. The tansient behavio of vaiables duing step change in load demand: (a) q-axis component of machine cuent, and (b) d-axis component of machine cuent, (c) q-axis component of machine voltage, and (d) d-axis component of machine voltage at the desied 400V level, with less than 4.5% tansient vaiation duing both incement and decement in the load at 6s and 15s espectively. The speed command of the geneato is inceasing though the dc link voltage contolle duing the load demand incement. The input toque duing this tansient is consideed as constant, and so is able to geneate moe powe to meet the load demand with inceased speed. The actual and efeence speed of the geneato, duing this load vaiation, ae as shown in Figue 8(b). The actual speed is closely following the efeence one, in this figue. The speed contolle woks successfully duing the decement of the load also. The q-axis component of the machine cuent emains at the constant level of 3.3 A, with tansient changes duing the load vaiation. The tansient behavio of this cuent is displaying in Figue 9(a). The diect axis component of machine cuent emains invaiant duing the load tansient. The natue of this cuent is potayed in Figue 9(b). Howeve, the d and q axis components of machine voltage ae changing to accommodate the incement in the load powe demand, as potayed in Figue 9(c) and Figue 9(d).The total powe supplied by the machine side convete inceases by 314W due to the change in the consideed load demand. B. Vaiation in Input toque Consideing the load is constant, a step incement in the input toque is poviding to the system to veify the opeation of the contol with vaiable input. The input toque is inceasing 520
Contol of SCIG Based Constant Voltage Geneation Scheme by 50% at 15 s as pesented in Figue 10. The changes on the stato side and load side vaiables, because of this input vaiation, ae showing in Figue 11 and Figue 13. The d-axis component of the load voltage is going though a small tansient distubance at 15 s and settled back to the efeence level, 163 V, as shown in Figue 11(a). Howeve, the q-axis component of the load voltage is maintaining constant at zeo though the contolle, as shown in Figue 11(b). Figue 10. Input toque vaiation Figue 11. The tansient behavio of vaiables duing step change in input toque: (a) d-axis load voltage, (b) q-axis load voltage, (c) d-axis component of load cuent, and (d) q-axis component of load cuent Figue 12. Quality of: (a) Load voltage (b) Load cuent (c) FFT of load voltage (d)fft of load cuent, with highe input toque 521
Rupa Misha, et al. The constant voltage at the output of filte allows the load cuent to be same, even with the change in the input toque. The d-axis and q-axis components of the load cuents ae so constant thoughout the opeation and shown in Figue 11(c) and 11(d). The total hamonic distotion (THD) of the output voltage and cuent is keeping within a small limit by use of the filte in this case also. The THD of the load voltage is finding to be 2.66% at steady state, as shown in Figue 12(c) and the THD of the load cuent, at the same time, is of 2.02%, as shown in Figue 12(d). Both Figue 7 and Figue 12 ae potaying the efficacy of the filte output contol fo this stand-alone system. The developed voltage contol is impoving the powe quality even with 100%vaiation in load and 50% vaiation in input toque. The dc link voltage is successfully contolling to maintain at 400 V level, with 2% vaiation in tansient condition as shown in Figue 13(a). The opeating speed is educing with highe input toque to supply constant load powe. The speed contolle is successfully following the efeence command as shown in Figue 13(b). It is also obseving fom Figue 14 (a), that the q-axis component of the machine cuent, inceases to -4.5A fom -3.2A in the esult of the incement in inceased input toque at 15 s. Howeve, the d and q axis components of machine voltage ae changing to meet the same load powe demand, as potayed in Figue 14(c), Figue 14 (d). The total powe supplied by the machine side convete is keeping same fo the change in the input toque. The cuent contolle is successfully contolling the actual cuent to follow the efeence, thoughout the opeation. The d-axis component of machine cuent is constant, as potayed in Figue 14 (b). As a esult, thee is no such vaiation in d-axis voltage, as shown in Figue 14(d). Figue 13. The tansient behavio of vaiables duing step change in input powe: (a) dc link voltage, (b) geneato speed Figue 14. The tansient behavio of vaiables duing step change in input powe: (a) q-axis component of machine cuent, and (b) d-axis component of machine cuent (c) q-axis component of machine voltage, and (d) d-axis component of machine voltage 522
Contol of SCIG Based Constant Voltage Geneation Scheme 6. Conclusion Two contolles ae designed fo geneato side VSC and fo load side VSC to achieve fixed voltage and fequency output with both input toque and load impedance vaiation, in this wok. The thee -stage contolle is designed fo the geneato side VSC to maintain the dc link voltage constant thoughout the opeation. The load side VSC is contolling simultaneously to povide a constant voltage at the output. Moeove, one filte is incopoated to educe the hamonics at the load end, and the aleady designed load side contolle compensates the dop of the load voltage. The contol is implemented with both load petubation and input toque vaiation successfully. It is found that: The machine side convete contol is managing the efeence speed to extact the powe fom the input. As a esult, dc link voltage emains constant thoughout the opeation. It is also obseved that dc link voltage is having only 2 % vaiation duing tansient condition fo input toque and output load petubation, and is settled within 0.8 sec. The total powe supplied by the geneato side VSC inceases due to the incement in the load demand, while the total powe supplied by the machine side convete is kept at same level duing the change in the input toque. The speed inceases to accommodate the inceased load powe, while deceases with the incement in the input toque. The contol of load side VSC is successfully keeping the load voltage constant thoughout the study. The tansient vaiations duing changes ae settling to povide constant d-axis and q-axis components of load voltage. The designed filte is capable of keeping the THD fo load cuent less than 2%. The system is so capable of futue extension towads gid-connected opeation. 7. Refeences [1]. M. Kalanta and S. M. Mousavi G., Dynamic behavio of a stand-alone hybid powe geneation system of a wind tubine, micotubine, sola aay and battey stoage, Applied Enegy, vol. 87, no. 10, pp. 3051 3064, 2010. [2]. C. N. Bhende, S. Misha, and S. G. Malla, "Pemanent Magnet Synchonous Geneato- Based Standalone Wind Enegy Supply System," IEEE Tansactions Sustainable Enegy, vol. 2, no. 4, pp. 361-373, 2011. [3]. R. Cadenas and R. Pena, "Sensoless vecto contol of induction machines fo vaiablespeed wind enegy applications," IEEE Tansactions Enegy Convesion, vol. 19, no. 1, pp. 196-205, 2004. [4]. N. H. Malik and A. H. Al-Bahani, "Influence of the teminal capacito on the pefomance chaacteistics of a self-excited induction geneato," IEE Poceedings C - Geneation, Tansmission and Distibution, vol. 137, no. 2, pp. 168-173, 1990. [5]. J. Lee and Y. S. Kim, "Sensoless fuzzy-logic-based maximum powe point tacking contol fo a small-scale wind powe geneation systems with a switched-mode ectifie," IET Renewable Powe Geneation, vol. 10, no. 2, pp. 194-202, 2016. [6]. R. Teodoescu and F. Blaabjeg, "Flexible contol of small wind tubines with gid failue detection opeating in stand-alone and gid-connected mode," IEEE Tansactions Powe Electonics, vol. 19, no. 5, pp. 1323-1332, 2004. [7]. R. Pena, J. C. Clae, and G. M. Ashe, "Doubly fed induction geneato using back-toback PWM convetes and its application to vaiable-speed wind-enegy geneation," IEE Poceedings - Electic Powe Applications, vol. 143, no. 3, pp. 231-241,1996. [8]. G. Tapia, G. SantamaÍa, M. Telleia and A. Suspeegui, "Methodology fo Smooth Connection of Doubly Fed Induction Geneatos to the Gid, IEEE Tansactions Enegy Convesion, vol. 24, no. 4, pp. 959-971, 2009. [9]. Manaullah, A. K. Shama, H. Ahuja, and A. Singh, "Pefomance compaison of DFIG and SCIG based wind enegy convesion systems," Computational Intelligence on Powe, Enegy and Contols with thei impact on Humanity (CIPECH), 2014 Innovative Applications of, Ghaziabad, pp. 285-290,2014. 523
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Contol of SCIG Based Constant Voltage Geneation Scheme Rupa Misha is cuently woking towads the Ph.D. degee in the Depatment of Electical Engineeing, National Institute of Technology, Dugapu, India. He eseach inteests ae Machine dives, Powe-electonic and its applications and enewable enegy. Tapas Kuma Saha eceived the eceived the B.E. degee in electical engineeing fom the Jalpaigui Govenment Engineeing College, Univesity of Noth Bengal, Jalpaigui, India, in 1997, the M.E in Electical Machines fom the Jadavpu Univesity, Kolkata, India, in 1999, and the Ph.D. degee fom the Electical Engineeing Depatment, Indian Institute of Technology, Khaagpu, India, in 2009.He is cuently an Associate Pofesso in the Depatment of Electical Engineeing, National Institute of Technology Dugapu, India. His cuent eseach inteests ae machine dives, powe electonics, enewable enegy and the gid integated distibuted geneations. 525