Stability Enhancement of Wind Farm Connected Power System Using Superconducting Magnetic Energy Storage Unit

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1 Stability Enhancement of Wind Fam Connected Powe System Using Supeconducting Magnetic Enegy Stoage Unit Aghya Mita Dept. of Electical Engineeing Indian Institute of Technology Khaagpu Khaagpu, India Dheeman Chtejee Dept. of Electical Engineeing Indian Institute of Technology Khaagpu Khaagpu, India Abstact This pape deals with the enhancement of the tansient stability of the powe system integed with Doubly Fed Induction Geneo () based wind fams connected with Supeconducting Magnetic Enegy Stoage () unit. The fluctuion in the wind powe injection to the system due to vaiability in wind speed can also be educed by the intoduction of the unit. The study is caied out in SMIB system and WSCC - machine 9-bus system in PSCAD/EMTDC plfom. Keywo Tansient Stability; Powe System; ; ; I. INTRODUCTION With the incease in the envionmental impacts on the society due to the use of fossil fuel based powe plants, the use of enewable powe as an altene souce of enegy aises apidly. Wind due to its easy availability is one of the majo souces of altene enegy. Because of cetain advantages the vaiable speed based wind fams ae gaining populaity ove fixed speed squiel cage induction geneo (SCIG) based wind fams. based wind fams can opee in both sub-synchonous and supesynchonous speed egimes extacting maximum powe fom wind []. Thee is a fluctuion of output powe of the wind fam because of the intemittent and vaiable nue of the wind. So, when the wind powe plant is integed with the gid the fluctuion in geneed powe affect the powe system stability. An enegy stoage unit can educe the net fluctuion in injected powe to the gid. Reseach is going on the diffeent kind of enegy stoage units th can be used to impove the powe quality and stability of the system [, ]. The enegy stoage units can be listed as Supe Capacito Enegy Stoage (SCES), Compessed Ai Enegy Stoage system (CAESS), Fly Wheel Enegy Stoage System (FESS), Pumped Hydo Enegy Stoage System (PHESS), Btey Enegy Stoage System (BESS) and Supeconducting Magnetic Enegy Stoage (). Among all unit is vey popula while consideed to be connected with wind fam because of its high efficiency, high capacity and a vey fast esponse with espect to the othe []. SCES has also high efficiency and capacity but has vey long life time in compaison to SCES. BESS is also used as an enegy stoage unit in connection with wind fam but it has a limited life cycle along with its voltage and cuent limitions []. is a lage supeconducting coil. It is capable of stoing electic enegy in the magnetic field geneed by dc cuent flowing though the coil []. An applicion of unit to impove the powe dispch and dynamic pefomance duing intemittent misfie and fie-though faults is seen in [6]. Again applicion of to enhance the dynamic pefomance of duing voltage sag and swell is investiged in [7]. Applicion of is also seen in wind fam to impove the voltage stability [8]. The tansient stability enhancement of SCIG based wind fam is seen in [9-]. The enegy capacito system is used in [] to impove the tansient stability of a multi-machine powe system which includes SCIG based wind fam. Hee the unit is used to enhance the tansient stability of the powe system integed with based wind fam. The unit can also be able to educe the fluctuion in powe output of the wind fam due to vaiion in wind speed. The study is caied out in PSCAD/EMTDC simulo and the systems used fo the study ae single machine infinite bus system (SMIB) and WSCC -machine 9-bus system. II. MODEING OF POWER SYSTEM, AND A. Modeling of synchonous machine and the netwok The flux decay model of synchonous machine is consideed along with a stic excite of one gain and one time constant [, ]. The tansmission lines ae //$. IEEE

2 epesented by equivalent π stuctue. The loa ae consideed to be of constant impedance type. B. Modelling of the and its contolle The wind tubine and the oting mass is epesented by a two mass model [, 6]. The mechanical toque of the wind tubine is a function of wind velocity [-7]. The sto and the oto cicuits of ae epesented by thei espective diffeential equions fo a balanced and unsued condition [, 6]. The connection diagam of along with to the load bus of a powe system is as shown in Fig.. The is connected though a double cicuit line and a tansfome [8] to a load bus of the existing powe system as shown in Fig.. The unit is also connected to the same load bus though a tansfome. Total active and eactive powes deliveed to the gid by ae P = P + P = V i + V i + V i + V i () Q dg dg : n s s s GSC P s, Q s qs Unit qs qs = Q + Q = V i V i () qs Hee, P s and Q s ae the sto active and eactive powes. P is oto active powe, V, V qs, V d, and V q ae the sto and the oto d-axis and q-axis voltages espectively, i and i qs ae the sto d-axis and q-axis cuents, i d, and i q ae the oto d-axis and q-axis d d P P dg + jq dg + jx q q To system P, Q P GSC, P + jq Q GSC RSC GSC P DG+ jq DG Fig.. Connection of and to the system load bus In Fig., n s is the gea box io, and x ae esistance and eactance of the line connecting the to the system. RSC stan fo oto side convete and GSC stan fo gid side convete. The load connected to the system bus in the oiginal system (without ) is (P + jq ). An additional load of (P DG + jq DG ) is assumed to be connected to the bus as shown in Fig. [9, ]. Unde ed wind speed, the would supply this additional load as well as the losses in the connecting tansfome and line, while the steady ste powe flow in the oiginal netwok emains same. cuents espectively. GSC is opeed unity powe facto making its eactive powe (Q GSC ) equal to zeo. In this wok, the d-axis of the synchonously oting efeence fame is consideed to be oiented along the sto voltage V and the q-axis is leading the d-axis, hence V = V and Vqs = () ψ and ψqs ψs () Hee, ψ and ψ qs ae the sto d-axis and q-axis flux, ψ s is the sto flux. The electomagnetic toque thus can be expessed as T m e = ψ qs i () d ss Similaly, the sto eactive powe can be expessed as a function of q-axis oto cuent i q as = mv V ψ qs Q s iq (6) ss ss Whee, m is the mutual inductance, ss and ae sto and oto inductances. It can be seen fom () and (6) th the T e and Q s can be sepaely contolled by i d and i q espectively. Using these elions, the nomal field oiented contol [] of the RSC of the becomes as shown in Figue (a) and (b). T eef Q sef In Figue (a) obtained by T = K ω eef opt Fig. (a). d-axis contol block Fig. (b). q-axis contol block Te ef is the efeence toque and is ed = Te if ω ed ω ed Whee K opt in pu is given by [] opt Te ed ss mv ss mψ qs ψ if ω < ω ( S ) (7) K =.ρπr C p ω max tb λ (8) opt B Hee stan fo the ed toque, ω is the pe unit ω qs m I def + + I qef + + _ v qef oto speed, ed is the pe unit ed speed, C pmax is the maximum coefficient of pefomance; λ opt is optimum tip speed io, R is the wind tubine blade length, ρ is the ai density, S B and ω tb ae the base powe and the base speed of the wind tubine espectively. The amount of eactive powe to be supplied by the is set as the sto eactive powe _ i d i q PI PI v def

3 efeence ( Q ). s ef Reaangement of () and (6) gives the expessions of the efeences of the d-axis and q-axis oto cuents I ss def = T (9) e ef mψ qs ψ qs I = ss qef Qs + () mv ef m I def is compaed with the actual oto cuent and the eo signal is passed though a P-I contolle to genee the oto d-axis voltage efeence ( v ). d ef Similaly using a P-I contolle q-axis oto voltage efeence ( v ) can be geneed fom I q qef. Now, d- ef q- to a-b-c tansfomion on and gives the vd ef vq ef phase voltage efeences. These ae then used as the moduling signals in PWM to genee the switching pulses fo RSC. Simila to RSC, the sto voltage oiention is consideed fo the contol stuctue of GSC also fo the decoupled contol of active and eactive powe flowing though the convete. By maintaining the DC bus voltage constant it is ensued th the GSC allows bi-diectional flow of active powe between the gid and the RSC. The eactive powe flow is contolled such th the GSC is opeed unity powe facto. The pitch contolle of the wind tubine inceases the tubine blade pitch angle to educe the mechanical powe wheneve the wind speed (thus ω ) excee the ed value, to keep active powe output constant its ed value. To povide fault ide though capability, the oto of the is shot cicuited duing fault though cowba (an extenal esistance), thus making the acts like a singly excited machine. Fo a ealistic simulion, the cowba is made active afte a ms delay of the occuence of the fault and is made inactive (by emoving the cowba) afte a simila time delay when the fault is cleaed. The convete is out of the cicuit when the cowba is connected. C. Modelling of the and its contolle The unit used in this wok consists of a supeconducting coil (), a voltage souce convete (VSC), a DC link capacito (C DC ) and a two quadant DC-DC choppe as shown in Fig. [-6]. The unit is then connected though a tansfome to the system gid. The VSC povides a powe electonic inteface between AC powe system and the supeconducting coil. The DC link voltage acoss C DC is maintained constant by the VSC. The contol stuctue of it is same as th of the To Gid Fig.. VSC based with a DC-DC choppe GSC of the. The supeconducting coil is chaged o dischaged by a two quadant DC-DC choppe. The DC-DC choppe is contolled the voltage acoss the coil (positive o negive) and the stoed enegy can then be chaged o dischaged. The duty cycle of PWM opeion to educe the wind geneo output powe fluctuions as an effect of the wind speed vaiability is decided by a PI contolle. The contol stuctue of the choppe is as shown in the Fig.. The duty cycle (D) detemines the diection and magnitude of powe exchange between the coil and the AC system as pesented in Table I. +. Fig.. Contol stuctue DC-DC choppe TABE I DUTY CYCE RUES D coil action D =. Standby. < D Chaging D <. Dischaging If D is equal to., no action will be taken by the supeconducting coil. In this condition, a bypass switch acoss the coil (shown in Fig. ) will be closed to avoid the dischaging of. The bypass switch is contolled in such a way th it will be closed if D is equal to. othewise it will be opened. When the gid powe deceases, D will educe accodingly to be in the ange of to.. The stoed enegy in the coil will be tansfeed to the AC system duing this time. Chaging of the coil takes place when D is in the ange of. to.. The elion between V and V DC, can be witten as []: V Tansfome P + _ P ef VSC PI Caie wave = ( D) () V DC _ DC-DC Choppe C DC + D Bypass Switch evel Compao Whee, V is the aveage voltage acoss the coil D is duty cycle V DC _ is the aveage voltage acoss the DC link capacito To DC-DC Choppe

4 Duing fault the is delibeely made to opee in chaging mode iespective of the value of the duty cycle so th the oveall system stability inceases due to incease in the demand of the system. III. SIMUATION RESUT The study is caied out on SMIB system [] and WSCC -machine 9-bus system [] in PSCAD/EMTDC plfom. The citical cleaing is compaed fo the cases with and without connected to the powe system integed with a wind fam. The esults ae taken fo constant as well as vaiable wind speed. A. Result fo SMIB system The SMIB system connected with a wind fam is shown in the Fig.. The SMIB system is modified by intoducing two load buses (Bus and bus ). Wind fam, of a capacity of MW ( units of MW each [6]), is connected to one of the load buses (Bus ). An aggeged model of the wind fam is consideed. The ing of unit is 6 MW, 7 MJ. Synchonous Geneo Additional Fig.. Schemic diagam of the SMIB system with wind fam and The wind speed (V w ) is consideed to be vaiable following a pofile [] as shown in Fig. 6 (a s window). Study is caied out to obseve the effectiveness of the unit to maintain the wind powe injected to the gid (P g ) nealy constant value unde this vaiable wind speed condition. Fig. 7 shows the vaiion of P g with and without in the system. It is obseved th in pesence of the the vaiion in the powe injected to the gid deceases even in the case of a vey high wind speed vaiion. The stability enhancement is studied fist with constant wind speed (V w ).7 m/s (close to its ed speed of m/s). At this wind speed, the wind fam will genee 9 MW. The -phase shot cicuit selfcleaing fault is consideed two diffeent buses viz. Oiginal Unit Infinite Bus V w (m/s) P g (pu) 8 6. with without Fig. 6. Wind pofile Fig. 7. Vaiion in P g with and without the pesence of synchonous geneo teminal bus (bus ) and one of the load buses (Bus ). The citical cleaing time (t c ) is calculed fo these two cases fist without in the system and then t c is calculed fo the same cases now with in the system. Fig. 8 shows the vaiion of elive delta (δ) of the alteno a fault cleaing time of 7 ms. The plot of δ (dotted line) becomes unbounded in absence of indicing an unstable system while the system becomes stable (bounded δ) when is intoduced in the system (continuous line). δ (ad) P g (pu) Fig. 8. Alteno elive oto angle vaiion fo a fault bus Fig. 9. Powe injected to gid fo a fault bus A B D C Refeence Powe with without - 6 with without 6

5 Fig. 9 shows the vaiion of P g fo the same condition both with and without in the system. The compaisons of the t c with and without fo these two fault locions ae shown in Table II. With a vaiable wind speed, the instant of the occuence of the fault makes some diffeence in system condition as the wind speed and its gadient diffes, as shown by points A, B, C and D on the wind pofile of Figue 6. Study is caied out consideing these five points (one a time) as the fault occuence instant. The coesponding wind spee ae V wa, V wb, V wc, and V wd. At points A and C the coesponding wind spee have same value but one has a downwad diection ( A) and othe has an upwad diections. The wind spee V wb, V wc and V wd have upwa diections. Again V wb is coesponding to minimum wind speed along the pofile. At D wind speed is gee than the ed speed of m/s. The compaisons of the t c with and without these fou instants ae also shown in Table II. It is seen fom the Table II th with the intoduction of t c always inceases both with constant and vaiable wind speed. Fault TABE II COMPARISON OF t c Constant wind speed.7 m/s t c Fault without t c with Impovement Vaiable wind speed V w (m/s) t c without t c with Impovement V wa =. 6 9 V wb =. 6 V wc =. 6 8 V wd =. 6 7 V wa = V wb = V wc = V wd = B. Result fo -machine 9-bus system An aggeged wind fam model of capacity MW ( units of ing MW each) is integed with the WSCC -machine, 9-bus system as shown in Fig.. The distubance consideed is a -phase shotcicuit fault in one of the load buses. The ing of unit is MW, 7 MJ. The t c fo a fault bus point D on the wind pofile without is 9 ms, so the system becomes unstable with fault cleaing time of 9 ms as can be seen in Fig. (dotted line with unbounded δ ). Fo the same condition the system becomes stable with bounded δ when is connected to the system δ (ad) Fig.. WSCC -machine 9-bus system with and connected to bus 8. - with without - 6 Fig.. Alteno elive oto angle vaiion fo a fault bus (continuous line in Fig. ). The levels of δ fo the two cases befoe the fault ae slightly diffeent as due to the intoduction of the the steady ste powe level changes. The t c in this case inceases to ms (Table III). Fo the above condition the plots of electomagnetic toque (T e ) both with and without ae shown in Fig.. As the system becomes unstable with a fault cleaing time of 9 ms an instant D when is absent, T e shows an unstable nue along the time axis (dotted line), wheeas a steady nue of T e (continuous line) is visible because of a stable system condition when is pesent in the system. T e (pu) Gen Unit 7 with without 8 Gen ocal Gen 6 Fig.. electomagnetic toque vaiion fo a fault bus 9 6

6 Table III shows the compaison of t c fo 9-bus system both fo constant and vaiable wind speed conditions fo two diffeent fault locions of bus and bus 6 with wind fam and bus 8. The last column of Table III shows the impovement in stability of the powe system in tems of t c. As in SMIB system hee also it is seen th thee is an impovement of tansient stability when is connected and popely contolled. 8 8 Fault TABE III COMPARISON OF t c Constant wind speed.9 m/s t c without t c with Impovement.7.89 ms ms Vaiable wind speed Fault 6 V w (m/s) t c without t c with Impovement V wa =... ms V wb = ms V wc = ms V wd =..9. ms V wa =...6 ms V wb = ms V wc = ms V wd = ms IV. CONCUSION This pape deals with the enhancement of the tansient stability of powe system connected with a wind fam when unit is intoduced in the system. The study shows th thee is always an impovement in the stability in tems of the citical cleaing time in pesence of unit both fo constant and vaiable wind speed conditions. The systems consideed fo the study ae single machine infinite bus system and WSCC -machine 9-bus system and the study is caied out in PSCAD/EMTDC simulo. REFERENCES [] M. V. A. Nunes, J. A. P. opes, H. H. Zun, U. H. Bezea and R. G. Almeida, Influence of the vaiable-speed wind geneos in tansient stability magin of the conventional geneos integed in electical gi, IEEE Tans. on Enegy Convesion, vol. 9, no., Decembe, pp [] S. C. Smith, P. K. Sen and B. Koposki, Advancement of enegy stoage devices and applicion in electical powe system, Powe and Enegy Society Geneal Meeting - Convesion and Delivey of Electical Enegy in the st Centuy, 8 IEEE, - July 8, pp. -8. [] R. Gupta et al, Applicion of enegy stoage devices in powe system, Intenional Jounal of Engineeing, Science and Technology vol., no.,, pp [] Mohd. H. Ali, B. Wu, R. A., Dougal, "An oveview of applicions in powe and enegy systems," IEEE Tans. on Sustainable Enegy, vol., no., Apil, pp [] W. Buckles and W. Hassenzahl, Supeconducting magnetic enegy stoage, IEEE Powe Eng. Rev., vol., no., May, pp. 6. [6] A. M. S. Yunus,, A. Abu-Siada, and M. A. S., Masoum, "Applicion of unit to impove powe dispch and dynamic pefomance duing intemittent misfie and fiethough faults," IEEE Tans. on Applied Supeconductivity, vol., no., Aug., pp [7] A. M. S. Yunus,, M. A. S. Masoum and A. Abu-Siada, Applicion of to enhance the dynamic pefomance of duing voltage sag and swell, IEEE Tans. on Applied Supeconductivity, vol., no., Aug., pp [8] J. Shi, Y.J. Tang,. Ren, J.D. i and S.J. Chen, Applicion of in wind fam to impove voltage stability, Physica C: Supeconductivity, vol. 68, issues -, Septembe 8, pp.. [9] Mohd. H. Ali, M. Pak, In-Keun Yu, T. Mua and J. Tamua, Impovement of wind-geneo stability by fuzzy-logiccontolled, IEEE Tans. on Industy Applicions, vol., no., May/June 9, pp. -. [] S. S. Chen,. Wang, Z. Chen and Wei-Jen ee, "Powe-flow contol and tansient-stability enhancement of a lage-scale wind powe geneion system using a supeconducting magnetic enegy stoage () unit," Powe and Enegy Society Geneal Meeting, 8 IEEE, - July 8, pp.-6. [] M. R. I. Sheikh, S. M. Muyeen, R. Takahashi, T. Mua and J. Tamua, Tansient stability enhancement of wind geneo using supeconducting magnetic enegy stoage unit, 8 th Intenional Confeence on Electical Machines, 8 (ICEM), 6-9 Sept. 8, pp. -6. [] S. M. Muyeen, R. Takahashi, Mohd. H. Ali, T. Mua and J. Tamua, Tansient stability augmention of powe system including wind fam by using ECS, IEEE Tans. on Powe Systems, vol., no., August 8, pp [] P. W. Saue and M. A. Pai, Powe System Dynamics and Stability, Peason Educion Asia, Fist Indian Repint,. [] P. Kundu, Powe System Stability and Contol, McGaw- Hill, New Yok, 99. [] F. Mei and B. Pal, Modal analysis of gid-connected doubly fed induction geneos, IEEE Tans. on Enegy Convesion, Vol., No., Septembe 7, pp [6] F. Mei, Small signal modeling and analysis of doubly fed induction geneos in wind powe applicions, Ph.D. Thesis 7, Impeial College ondon, Univesity of ondon. [7] S. K. Salman and A.. J. Teo, Windmill modeling consideion and factos influencing the stability of a gidconnected wind powe-based embedded geneo, IEEE Tans. on Powe Systems, Vol. 8, No., May, pp [8] P. edesma and J. Usaola, Doubly fed induction geneo model fo tansient stability analysis, IEEE Tans. on Enegy Convesion, Vol., No., June, pp [9] M. K. Donnelly, J. E. Dagle, D. J. Tudnowski, and G. J. Roges, Impacts of the distibuted utility on tansmission system stability, IEEE Tans. on Powe Systems, Vol., No., May 996, pp [] A. Mita and D. Chtejee, A sensitivity based appoach to assess the impacts of integion of vaiable speed wind fams on the tansient stability of powe systems, Renew. Enegy, Vol. 6, Decembe, pp [] I. D. Hassan, R. M. Bucci, and K. T. Swe, " MW powe conditioning system development and simulion", IEEE Tans. on Powe Electonics, vol. 8, July 99, pp [] S. M., Bahma, Distance elay with out-of-step blocking function using wavelet tansfom, IEEE Tans. on Powe Delivey, vol., no., July 7, pp [] R. Dta, Roto side contol of gid-connected wound oto induction machine and its applicion to wind powe geneion, PhD Thesis, Indian Institute of Science, Bangaloe.

THE UNIVERSITY OF NEW SOUTH WALES. School of Electrical Engineering & Telecommunications

THE UNIVERSITY OF NEW SOUTH WALES. School of Electrical Engineering & Telecommunications THE UNIESITY OF NEW SOUTH WAES School of Electical Engineeing & Telecommunications EE97 POWE EETONIS FO ENEWABE AND DISTIBUTED GENEATION EXAMINATION Session (Supplementay Exam) TIME AOWED: 3 hous TOTA