A control strategy for grid-side converter of DFIG under unbalanced condition based on Dig SILENT/Power Factory

Transcription

1 IOP Conference Seres: Earth and Envronmental Scence PAPER OPEN ACCESS A control strategy for grdsde converter of DFIG under unbalanced condton based on Dg SILENT/Power Factory To cte ths artcle: Pngpng Han et al 018 IOP Conf. Ser.: Earth Envron. Sc Vew the artcle onlne for updates and enhancements. Ths content was downloaded from IP address on 19/10/018 at 3:03

2 IOP Publshng IOP Conf. Seres: Earth and Envronmental Scence (018) 0505 do : / /108/5/0505 A control strategy for grdsde converter of DFIG under unbalanced condton based on Dg SILENT/Power Factory Pngpng Han 1, Hatan Zhang 1,*, Lngq Chen 1 and Xaoan Zhang 1 New Energy Utlzaton and Energy Savng Laboratory, Hefe Unversty of Technology, Hefe 30009, Chna Intellgent Manufacturng Insttute, Hefe Unversty of Technology, Hefe 30009, Chna Correspondng author emal: @16.com Abstract. The models of doubly fed nducton generator (DFIG) and ts grdsde converter (GSC) are establshed under unbalanced grd condton based on DIgSILENT/PowerFactory. Accordng to the mathematcal model, the vector equatons of postve and negatve sequence voltage and current are deduced n the postve sequence synchronous rotatng reference frame dq0 when the characterstcs of the smulaton software are consdered adequately. Moreover, the reference value of current component of GSC n the postve sequence frame dq0 under unbalanced condton can be obtaned to mprove the tradtonal control of GSC when the natonal ssue of unbalanced current lmts s combned. The smulated results ndcate that the control strategy can restran negatve sequence current and the two tmes frequency power wave of GSC s ac sde effectvely. The voltage of DC bus can be mantaned a constant to ensure the unnterrupted operaton of DFIG under unbalanced grd condton eventually. 1. Introducton Wth the rapd development of DFIG n recent years, some theores and control strateges for low voltage rde through (LVRT) under symmetrcal fault has been ncreasng mature[1], and on the bass of ths, the unnterrupted operaton and control of DFIG under unbalance fault and condton has become one of the mportant trends of wnd power technology development. The State Grd Corporaton of Chna has promulgated Techncal rule for connectng wnd farm to power network [4]. It shows that durng the normal operaton of power system, the negatve sequence voltage unbalance of power system common connecton pont should not exceed %, short term may not exceed 4%. Many researches have been done n [57], such as dynamc modelng and control, enhanced control of backtoback PWM voltagesource converter and torque rpple elmnaton. However, the characterstcs of the smulaton software are not consdered adequately, and t s dffcult to realze them. In ths paper, the models of DFIG and ts GSC are establshed under unbalanced grd condton based on Dg SILENT/Power Factory. The vector equatons of postve and negatve sequence voltage and current are deduced n the postve sequence synchronous rotatng reference frame dq0 accordng to the mathematcal model. Moreover, the reference value of current component of GSC n the postve sequence frame dq0 under unbalanced condton can be obtaned when the natonal ssue of unbalanced Content from ths work may be used under the terms of the Creatve Commons Attrbuton 3.0 lcence. Any further dstrbuton of ths work must mantan attrbuton to the author(s) and the ttle of the work, journal ctaton and DOI. Publshed under lcence by IOP Publshng Ltd 1

3 IOP Publshng IOP Conf. Seres: Earth and Envronmental Scence (018) 0505 do : / /108/5/0505 current lmts s combned. At last, an mprove control strategy of GSC s put forward to restran negatve sequence current.. Model of GSC under unbalanced condton The man crcut of GSC s shown n Fg. 1. When the grd works under unbalanced condton, the vector equatons of postve and negatve sequence voltage and current n the postve and negatve sequence synchronous rotatng reference frame dq0 are expressed as (1)[8]. di Utdq RgIgdq jlgigdq Ugdq Lg dt di Utdq RgIgdq jlgigdq Ugdq Lg dt gdq gdq (1) In (1), Ugdq, Igdq, and Utdq are the space vectors of the GSC s AC voltage, GSC s AC current, and grd voltage. The superscrpts such as, represent the postve and negatve sequence components. Fgure 1. Man crcut of DFIG s grd sde converter The space vector dagram of GSC s ac voltage under unbalanced condton s shown n Fg. accordng to (1), where δ s the angular dfference of postve and negatve sequence synchronous rotatng reference frame dq0. ε s the ntal value of δ, and δ=ωtε. q u q q u dq q u q q dq δ d dq u d u dq d ω d u d d ω Fgure. The space vector dagram of GSC s ac voltage

4 IOP Publshng IOP Conf. Seres: Earth and Envronmental Scence (018) 0505 do : / /108/5/0505 Because the control module of DFIG n Dg SILENT/Power Factory software only contans postve sequence d and q axs components, the followng transformaton of negatve sequence d and q axs components should be done to project them to postve sequence frame dq0: ugd ugd cos ( d axs) ( ugd sn )( q axs) ugq ugq sn ( d axs) ugq cos ( q axs) gd gd cos ( d axs) ( gd sn )( q axs) gq gq sn ( d axs) gq cos ( q axs) () The projectons of negatve sequence d and q components on postve sequence frame dq0 are two tmes frequency components. Thus, the d and q axs components for GSC of DFIG (all the d and q axs components wthout superscrpts are postve sequence components whch postve and negatve components are calculated to, such as (3)) can be expressed as (3). ugd ugd ugd cos ugq sn ugq ugq ugd sn ugq cos gd gd gd cos gq sn gq gq gd sn gq cos (3) The whole d and q axs components are of two tmes frequency. Then, the actve and reactve power can be deduced as (4). 3 g gd gd gq gq gd gd gq gq gd gd gq gq gd gd gq gqcos gd gq gq gd gd gq gq gd 3 cos ugdgd ugqgq ugdgd ugqgq P u u u u u u u u u u u u Qg ugdgq ugqgd ugdgq ugqgd ugdgq ugqgd ugdgd ugqgq As the voltage and current of GSC, the power also contans second harmonc. The steady of DC bus voltage of DFIG may be threatened f the second harmonc s not restraned. The second harmonc n actve and reactve power can be restraned effectvely f the negatve sequence components of GSC are elmnated. Accordng to Rotatng electrcal machnes Ratng and performance GB , the negatve sequence components of three phase alternators cannot exceed 5% of postve sequence components, as shown n (5),where ε can be looked upon as unbalance factor of negatve sequence current and ts value s sn sn (4) 1 (5) Accordng to symmetrcal component method, the equaton (6) s as follows, where a=e j10. I I I a(1) 1 a a Ia 1 1 a a a() Ib a(0) Ic (6) Ia(1), Ia() and Ia(0) are the three sequence current components of a phase. The subscrpt a wll be omtted n ths paper followng. 3

5 IOP Publshng IOP Conf. Seres: Earth and Envronmental Scence (018) 0505 do : / /108/5/0505 Because the connecton way of DFIG s star connecton, there s no zero sequence current n the system no matter the system works n what knd of operaton mode. The relatonshp should be satsfed. Thus, (6) can be expressed as (7). I1 1 1 I 3 I 1 a aa a a a a Ib (7) The nstantaneous current of a and b phase can be expressed as (8).And accordng to ths, the rotatng phasors are shown n (9). a Iamcos( t a) b Ibmcos( t b) Ia Iam cos( ta ) jiam sn( ta ) a a j Ibm cos( tb ) jibm sn( tb ) b I b b (8) (9) The nstantaneous postve and negatve sequence current can be obtaned by substtutng Eq. (9) nto Eq. (7), such as the followng equaton (10) [910]. 1 1 a a a 1 1 a a a a 1 I a a Re Re j 3 1 a a a 3 1 a a a b Ib b (10) Wthout zero sequence current n the system, Park transformaton can be smplfed as (11), where θ s the angle between frame abc and postve sequence frame dq0. a 4 cos sn d b 3 cos( 3) sn( 3) q (11) Equaton (1) can be obtaned by substtutng Eq. (11) nto Eq. (10). 1 1 a a a a 1 a Re j 3 1 a a a b b Re 1 a aa cos sn cos sn d cos( 3) sn( 3) cos( 3) sn( 3) q 1 4 d 4 j 3 1 a a a 3 q 3 (1) The values of postve sequence d and q axs current should be adjust to ensure the postve and negatve sequence current satsfy Eq. (5). Eq. (1) can also be used to obtan negatve sequence current and determne whether t exceeds the lmts. 3. GSC s control system desgn The mproved control dagram to restran negatve sequence current s shown as Fg. 3. 4

6 IOP Publshng IOP Conf. Seres: Earth and Envronmental Scence (018) 0505 do : / /108/5/0505 gd cosδ gq snδ u dc_ref dg_ref PI PI u dc dg u dg a b c Sgnal processng d q d q PLL δ qg_ref qg u qg PI gd snδ gq cosδ Fgure 3. Improved control dagram of GSC Accordng to Eq. (3), the feedback quanttes of negatve sequence current are added to the reference value of postve sequence d and q axs current of GSC to elmnate negatve sequence current. The negatve sequence current decson module s also added to the protecton module of GSC to verfy the strategy n Fg. 3. The flow chart s shown as Fg. 4. d q d q Equaton (3) Equaton (14) 1 Satsfy equaton(6) Yes Normal operaton No GSC block Fgure 4. Overcurrent judgment flowchart for negatve sequence current 4. Analyss of smulaton and examples To valdate the avalablty of the control strategy under unbalanced condton, models of DFIG and ts GSC are establshed n DgSILENT/PowerFactory. The specfc parameters of GSC are shown n the Table 1. Table 1. Smulaton parameters of sngle GSC Rated AC voltage rated voltage of DC lnk rated power ncomng lne nductance 690 [V] 1.15 [kv] [MVA] 1 [mh] 5

7 IOP Publshng IOP Conf. Seres: Earth and Envronmental Scence (018) 0505 do : / /108/5/0505 As shown n Fg. 5, the system begns to operate under unbalanced condton at.5s and get rght at.7s. Smulaton results of tradtonal control are shown n ths fgure. Obvously, the rato between negatve sequence current and postve sequence current s too large, and the negatve sequence current exceeds the lmt / p.u / p.u / t/s Fgure 5. Smulaton results of tradtonal control strategy / p.u. 1 / p.u / t / s Fgure 6. Smulaton results of mproved control strategy 6

8 IOP Publshng IOP Conf. Seres: Earth and Envronmental Scence (018) 0505 do : / /108/5/0505 Smulaton results are shown n Fg. 6 when mproved control strategy s used under the same smulated condton. The rato between negatve sequence current and postve sequence current s small and t satsfy Eq. (5). A defcency les n the fgure s that a two tmes frequency wave whose ampltude s ±4% may occur n GSC s postve sequence current. But the wave s controllable. Concluson Detaled theoretcal analyss of GSC s runnng mechansms under unbalanced grd voltage condton s done on the bass of the mathematcal models of DFIG and ts GSC and models of them n DIgSILENT/PowerFactory. The vector equatons of postve and negatve sequence voltage and current are deduced n the postve sequence synchronous rotatng reference frame dq0 when the characterstcs of the smulaton software are consdered adequately. And the negatve sequence current s projected to postve sequence frame dq0. Then an mproved control strategy s put forward to restran negatve sequence current. At last, compared wth the tradtonal control strategy, the negatve sequence current can be restran more effectvely when the mproved control strategy s used. The power wave of GSC can be depressed as well to mantan DC bus stablty. Acknowledgments Ths work was fnancally supported by Open Fund of State Key Laboratory of Operaton and Control of Renewable Energy & Storage Systems. References [1] Ahmed M.A. Hadar, Kashem M. Muttaq, Mehrdad Tarafdar Hagh, A coordnated control approach for DC lnk and rotor crowbars to mprove fault rdethrough of DFIG based wnd turbnes, IEEE Transactons on Industry Applcatons, PP(017) pp.11 [] Techncal rule for connectng wnd farm to power network, Bejng: The State Grd Corporaton of Chna, 009 [3] Xu L, Wang Y, Dynamc modelng and control of DFIGbased wnd turbnes under unbalanced network condtons, IEEE Transactons on Power Systems, 007, (1), pp [4] Hu Jabng, He Ykang, Nan Heng, Enhanced control of DFIG used backtoback PWM voltagesource converter under unbalanced grd voltage condtons, Journal of Zhejang Unversty SCIENCE A, 007, 8(8), pp [5] Park H G, AboKhal A G, Lee D C, et al, Torque rpple elmnaton for doublyfed nducton motors under unbalance source voltage, 7th Internatonal Conference on Power Electroncs and Drve Systems (PEDS), Thaland, 007 [6] Hu Jabng, He Ykang, Modelng and control of grdconnected voltage sourced converters under generalzed unbalanced operaton condtons, IEEE Transacton on Energy Converson, 008, 3(3), pp [7] Yuan Xufeng, Cheng Shje, Wen Jngyu, An mproved method of nstantaneous symmetrcal components and ts detecton for postve and negatve sequence current, Proceedngs of the CSEE, 8(008), pp.558 (n Chnese) [8] Iravan M R, KarmGharteman M., Onlne estmaton of steady state and nstantaneous symmetrcal components, IEE ProcGener Transm, Dstrb., 003, 150(5), pp