Journal of Power and Energy Engneerng, 014,, 13-19 Publshed Onlne Aprl 014 n cres. http://www.scrp.org/journal/jpee http://dx.do.org/10.436/jpee.014.4030 Power Dstrbuton trategy Consderng Actve Power Loss for DFIGs Wnd Farm Xao Du, Ygong Zhang, Zhqang Da, Huan Lu, mng We, Jn Lu Insttute of Electrcal and Electronc Engneerng, North Chna Electrc Power Unversty, Bejng, Chna Emal: duxao0607@163.com Receved January 014 Abstract Wth the development of concentrated wnd power areas, new energy s dspatchng problems are more promnent wth ts fast expanson. However, we can maxmze the utlzaton of wnd power under power curtalment condtons by optmal wnd power dspatchng. The paper studes on the basc theores of wnd power turbnes, and analyses the power s control and output characterstcs of wnd turbne, whch analyses the double-fed nductor generator s excellent decouplng control of power and ts excellent reactve power output capablty. By studyng the characterstcs of wnd power output, ths paper provdes a strategy for optmal dspatch n wnd power generaton. The calculaton results show that the total actve and reactve power output of the wnd farm match the request of dspatch center. And the total actve loss and total reactve loss s the least n the meantme. Keywords Wnd Farm; Power Dstrbuton; Actve Power Loss 1. Introducton hortage of conventonal energy sources and problems of envronmental polluton have become ncreasngly promnent, and the use of renewable energy, such as wnd power as the next most mportant, clean and alternatve energy sources, to allevate the energy shortage has been urgent sgnfcance. Wth the nstalled capacty of wnd power turbnes contnue to ncrease, wnd power has an ncreasngly mportant effect on the power system. However, the output of conventonal power plants can be hghly controllable, but the power output of wnd farm depends on the wnd speed, whch s changeable, unstable, and hard to control. o ths has brought dffcultes to the power system schedulng and lmted the development of wnd power. o far, people have done amounts of researches on how the wnd power mpacts on the power system [1,] and the optmal control of wnd turbnes [3,4] and have made great progress. However, wnd power and wnd power output control to optmze operaton of the nternal electrc feld has yet to be further studed. o ths paper studes the output control and optmzes operaton of wnd farm based on the good performance of AC-excted doubly-fed wnd turbnes. At present, the new wnd farm manly uses large capacty of AC-excted doubly-fed wnd turbne as the man models. Compared to normally wnd turbne, doubly-fed wnd turbne s rotor usng AC exctaton operaton mode, t has fan speed range, the largest wnd energy effectve trackng. And confguraton of the nverter unt n the rotor crcut, only to deal wth the two-way flow of power, makes the converter small sze, lght weght and How to cte ths paper: Du, X., Zhang, Y.G., Da, Z.Q., Lu, H., We,.. and Lu, J. (014) Power Dstrbuton trategy Consderng Actve Power Loss for DFIGs Wnd Farm. Journal of Power and Energy Engneerng,, 13-19. http://dx.do.org/10.436/jpee.014.4030
low cost characterstcs, and electro-mechancal systems to acheve a flexble connecton. Because of ther great advantages n comparson of conventonal unts, so ths type of wnd turbne s used wdely. Obvously, the effectve control of output power n large-scale wnd farm, together wth the grd schedulng system, should be more and more mportant for wnd power development. Therefore, actve power control has become an urgent problem for the wnd farm grd connecton. Ths artcle has manly studed on the power control theory of wnd farm, the control algorthm of wnd turbne and the stablty of actve power control, and the mplementaton of actve power control n wnd farms and wnd turbnes.. The Overall Control ystem of a Wnd Farm wth DFIG Turbnes At any tme and n any wnd condtons, the system operator gves orders to the wnd farm, namely ether a maxmum producton or a producton regulaton, accordng to dfferent control tasks, exactly as a conventonal power plant. ystem operator supervses the behavor of the wnd farm through a complex control system. Dependng on the actual network status, the system operator ssues specfc demands to the wnd farm central control level, whch prepares and sends further reference sgnals to each wnd turbne local control level. The double control system of a wnd farm should nclude two parts: the wnd farm control level and the wnd turbne control level, as shown n Fgure 1. The wnd farm control level behaves as a sngle central unt. It controls the power producton of the wnd farm by sendng out actve and reactve power references to the wnd turbne control level. These power references are provded from the wnd farm control level based on several measurements n the pont of common couplng (PCC) and on the avalable power of each ndvdual wnd turbne. The wnd turbne control level addresses the local control system of each sngle wnd turbne and ensures that the references power whch s send from the wnd farm control level are reached. Each local wnd turbne control system s also bult-up wth a herarchcal structure. It contans a slow dynamc control level (control of speed and power) and a fast dynamc control level (electrcal control of the generator currents). The slow dynamc control level provdes reference sgnals both to the ptch system of the wnd turbne and to the fast dynamc control level [5]. The latter addresses the electrcal control of the converter. 3. The Optmzaton Problem of Power Dspatch n Wnd Farm 3.1. DFIG P-Q Characterstcs Generally, the reference value of the actve power that a DFIG should generate s establshed through optmum generaton curves, whch provde the actve power as a functon of the generator rotatonal speed. uch curves defne the maxmum-mechancal power the DFIG can extract from the wnd at any angular speed. The followng mathematcal expresson referred to as P-Q load curve relatng the stator-sde actve and reactve powers to the peak value of the rotor current, can be derved: g + Qg + 3 = 3 U IR s X X P U X. (1) 1, I R, X, X, s are the stator voltage, rotor current, magnetzng reactance, stator reactance and slp of DFIG respectvely; P g g are the output actve power and reactve power of DFIGs. Fgure shows the composed curve for the DFIG turbne P-Q output. It reveals that the P-Q load curves correspond approxmately to eccentrc crcumferences n the P-Q plane, whose eccentrcty along the Q-axs and Fgure 1. Framework of wnd farm double control system. 14
radus are U 3 X X and 3 U I R, respectvely. X Fgure. DFIG actve power capablty lmts. The P-Q characterstc curve for a specfed condton s completed by addng the maxmum and mnmum actve avalable powers. From ths fgure, t can be observed that when the avalable actve power at the wnd turbne s close to ts nomnal values, the avalable reactve power decreases. On the other hand, when the avalable actve power at the wnd turbne s close to ts mnmum techncal lmt, the largest amount of reactve power s avalable. 3.. Objectve Functon of Optmal Control Ths paper takes a DFIG wnd farm as an example, proposng a power allocaton strategy based on nonlnear optmzaton control algorthm, whch ams at mnmzng power loss n wnd farm and power devaton between dspatchng command and actual output. pecfc objectve functon and constrant condtons are also gven n ths paper, and then smulaton s conducted n a wnd farm wth 10 DFIGs as an example compared wth tradtonal control strategy results [6]. The goals of optmzaton control n wnd farms nclude two aspects. One s to realze the change of the wnd farm output follow schedulng requrements, whch means to reduce devaton between the actual power output of wnd farms and schedulng requrements. The other s to satsfy the actve loss mnmum n the power output of each unt. Therefore, the objectve functon can be expressed as nf = λ1 P + λq + λ3ploss. () where P = Pout Pref s devaton between actual actve power output and schedulng actve request; Q = Qout Qref s devaton between actual reactve power output and schedulng reactve request; P loss s actve power loss wthn wnd farm; λ 1, λ, λ 3 are weght coeffcents. 3.3. Optmal Control Constrants 1) The node power equaton are ( cosθ + snθ ) ( snθ cosθ ) P = U U j j Gj j B j j (3) Q = U U j j Gj j B j j, U j are voltage ampltude of nodes and j respectvely; θj = θ θ j s voltage phase angle dfference of node and j; G j, B j are mutual conductance and susceptance of admttance matrx respectvely; P, Q are njected actve power and reactve power of node. ) Upper and lower bounds the unt actve power s P P. (4) 0 g g _ max where, P g_max s the maxmum output power of DFIGs, whch can be obtaned from wnd speed forecastng curve and wnd turbnes power curve. 3) Upper and lower bounds of reactve power unt Reactve power of doubly-fed wnd power generator s made up of the reactve power emtted or absorbed from both the stator sde and the network sde. Whle n operaton, typcally the power factor of grd sde con- 15
verter controller s set to 1.0, therefore, the reactve power of the njecton system s smlar to the stator sde reactve power. When stator voltage s constant, operatng ranges of actve and reactve power are lmted by the rotor sde converter maxmum current. If the actve power s a constant value, reactve power output range s P X g U Qg 3 U IR 3 X 1 s X (5) X P g U Qg 3 U IR 3 X 1 s X, I, R X, X, s are the stator voltage, rotor current, magnetzng reactance, stator reactance and slp of DFIG respectvely; P g g are the output actve power and reactve power of DFIGs. 4) The node voltage constrants s U U U. (6) mn max mn max s Per-unt value of the node voltage n wnd farm; U, U are the mnmum and maxmum lmt for node voltage. The voltage constrant range s taken as [0.9, 1.1] n ths paper. 4. Calculaton odel Verfcaton 4.1. odel Data and Control trateges A wnd farm consstng of 10*1.5 W DFIGs s used as an example. As t s shown n Fgure 3, ten wnd turbnes are dvded nto two groups, through 10 kv double-crcut lne access to 10 kv bus bar of 110 kv step up substaton, after step up to 110 kv by a transformer, whch s connected wth nfnte power grd [7]. trategy 1: Dstrbute power n the tradtonal way, accordng to the proporton of the wnd power capacty output of each unt, are shown as follows Pg,max Pref _ = P n P = 1 g,max (7) Qg,max Qref _ = Q n Q = 1 g,max where, P g,max g,max are predcton maxmum powers of DFIG; P are dspatchng commands for wnd farm; P ref _ ref _ are power nstructons assgned to each turbne. Fgure 3. chematc of a wnd farm wth 10 turbnes. 16
trategy : Usng optmal control strategy, set λ1 = 1/, λ = 1/, λ3 = 0, whch means t only takes mnmzng actve and reactve power devaton n power output and schedulng requrements as objectve functon, regardless of actve power loss. trategy 3: Usng optmal control strategy, set λ1 = 1/3, λ = 1/3, λ3 = 1/3, whch means t not only takes mnmzng actve and reactve power devaton n power output and schedulng requrements as objectve functon, but also takes actve power loss nto consderaton. If wnd speed forecastng curve s known, the maxmum power of each wnd turbne can be captured accordng to the wnd turbnes power curve. Assumng that the maxmum power capture for 10 wnd turbnes of ths perod are as shown n Table 1. 4.. Calculaton Results The output dstrbuton unts of each DFIG under dfferent control strateges are gven n Tables and 3 when schedulng request s large as 5 W/ Var and small as 3 W/1 Var respectvely. As t can be seen from Tables and 3, tradtonal strategy 1 s only dstrbutes power due to predcton power capacty, but n fact the output power can t meet the dspatchng requrement. In Table, accordng to strategy 1, the power command s 5 W, whle the total output s 4.71 W because there s 0.9 W power loss. o strategy 1 can t satsfy schedulng demand and power loss s large. Both strategy and strategy 3 can satsfy the actve and reactve power of schedulng demand. Compared strategy wth strategy 3, when the actve power loss s to be consdered, the actve power loss wthn the wnd farm s less and the operaton of the wnd farm s more effcent. From dstrbuton sets of strategy 3, t can be seen that actve and reactve power output of unts close to connecton node are larger, such as node unt, 4, 1, 14 n Table whose actve power output have reached ther maxmum output, however, power output of unts far from connecton node are smaller, such as node unt 10, Table 1. axmum power avalable n each turbne. Turbne number 4 6 8 10 axmum power 0.65 0.57 0.71 0.68 0.64 Turbne number 1 14 16 18 0 axmum power 0.55 0.55 0.71 0.59 0.71 Total 6.36 Table. Power dspatch of each turbne (5 W/ Var). Turbne number trategy 1 trategy trategy 3 P 1 Q 1 P Q P 3 Q 3 0.50 0.0 0.56 0.8 0.68 0.35 4 0.49 0. 0.55 0.31 0.65 0.31 6 0.5 0.19 0.55 0.6 0.55 0.5 8 0.4 0.18 0.53 0.35 0.46 0.3 10 0.46 0.19 0.5 0.1 0.40 0.18 1 0.47 0. 0.55 0.31 0.65 0.37 14 0.43 0.3 0.53 0.36 0.6 0.35 16 0.51 0.19 0.55 0.6 0.5 0.0 18 0.54 0.0 0.54 0.1 0.40 0.19 0 0.66 0.18 0.54 0.1 0.40 0.15 um 5.00.00 5.4.76 5.33.58 Total power output of wnd farm 4.50 1.39 5.00.00 5.00.00 Power loss 0.50 0.61 0.4 0.76 0.33 0.58 17
Table 3. Power dspatch of each turbne (3 W/1 Var). trategy 1 trategy trategy 3 Turbne number P 1 Q 1 P Q P 3 Q 3 0.30 0.10 0.31 0.15 0.34 0.0 4 0.9 0.11 0.31 0.1 0.3 0.16 6 0.31 0.09 0.31 0.10 0.15 0.16 8 0.5 0.09 0.30 0.1 0.15 0.09 10 0.8 0.10 0.30 0.09 0.15 0.01 1 0.8 0.11 0.31 0.14 0.34 0.1 14 0.6 0.11 0.30 0.1 0.3 0.17 16 0.31 0.09 0.31 0.10 0.15 0.10 18 0.3 0.11 0.30 0.09 0.15 0.00 0 0.40 0.09 0.30 0.09 0.15 0.00 um 3.00 1.00 3.05 1.1 3.04 1.10 Total power output of wnd farm.80 0.89 3.00 1.00 3.00 1.00 Power loss 0.0 0.11 0.05 0.1 0.04 0.10 0. The reason s that electrcal dstance of unts close to connecton node are shorter and ther power loss are lower whle electrcal dstance of those far away are longer and power loss are larger. As a result, the unt whch s closer should provde more power output to grd n order to meet the objectve functon that mnmzes the power loss wthn wnd farm. 5. Conclusons In ths paper, a global optmzaton strategy s developed for actve and reactve power dspatch n a wnd farm. Ths strategy allows followng wnd farm dspatch center requests, regardng actve/reactve power to be generated. The approach was tested wth a small wnd farm havng 10 generators and wth dfferent wnd power avalablty scenaros and generaton requests. The results proved the effectveness of the developed approach, demonstratng ts practcal value n cases where wnd farm are requested to follow specfc tme sequence generaton profles. The proposed approach s flexble enough to be used for dfferent operatonal strateges. In the dspatch problem, the capablty curves of DFIGs of the wnd park and crcut lne power loss are taken nto account. The re-desgn analyss llustrates that ncrease n ncome of 5-10% s possble f the operatonal strategy s optmzed wth respect to both wnd condtons and electrcty market spot prce. Acknowledgements Ths work was supported by the Fundamental Research Funds for the Central Unverstes of Chna. References [1] Le, H.T. and antoso,. (007) Analyss of Voltage tablty and Optmal Wnd Power Penetraton Lmts for a Non- Radal Network wth an Energy torage ystem. IEEE Power Engneerng ocety General eetng, Tampa, 4-8 June 007, 1-8. [] Jauch, C., ørensen, P. and Bak-Jensen, B. (004) Internatonal Revew of Grd Connecton Requrements for Wnd Turbnes. Nordc Wnd Power Conference, Chalmers Unversty of Technology. [3] Pena, R., Clare, J.C. and Asher, G.. (1996) Doubly Fed Inducton Generator Usng Back-to-Back PW Converters and Its Applcaton to Varable-peed Wnd Energy Generaton. IEE Proceedngs of Electrc Power Applcatons, 143, 31-41. [4] Ekanayake, J. Holdsworth, L. and Jenkns, N. (003) Control of DFIG wnd turbnes. Power Engneer Journal, 17, 8-18
3. http://dx.do.org/10.1049/pe:0030107 [5] Hansen, A.D., ørensen, P., Iov, F. and Blaabjerg, F. (006) Centralzed Power Control of Wnd Farm wth Doubly Fed Inducton Generators. Renewable Energy, 3, 936-951. [6] Zhao, B., Wang,.-Y., L, H. and Yang, C. (01) Optmal Power Dspatch and Control of Wnd Farm Based on Nonlnear Interor Pont Algorthm. Power ystem Protecton and Control, 40, 4-30. [7] oyano, C.F. and Lopes, J.A.P. (007) An Optmzaton Approach for Wnd Turbne Commtment and Dspatch n a Wnd Park. Electrc Power ystems Research, 79, 71-79. 19