MODELLING SIMULATION AND ANALYSIS OF GRID CONNECTED WIND GENERATOR

Transcription

1 KITE/NCISRDC/IJARIIT/08/EEE/0 IJARIIT( ISSN: 5-X) MODELLING SIMULATION AND ANALYSIS OF GRID CONNECTED WIND GENERATOR Sandhya Thakur, Ullas Kumar Agrawal, Tomeshvar Dhvar, Dr. Mthlesh Sngh Assstant Professor, EEE Dept (KITE) Assstant Professor, EEE Dept (KITE) Assstant Professor, EEE Dept (KITE) Assstant Professor, EEE Dept (KITE) E-mal:- ABSTRACT Wnd generators an have a sgnfant mpat on the power flow, voltage profle and the power qualty for ustomers and eletrty supplers. Ths requres a sutable tool to analyze the nfluene of wnd generators on the dstrbuton system. Ths paper presents a method to fnd the steady-state voltage stablty regon for eah bus of a dstrbuton power system, onsderng the presene of wnd power generaton. The maxmum permssble load of eah bus s alulated, so that t an operate wth the voltage wthn the lmts allowed by the power system utltes. Keywords- Wnd generator; Voltage stablty; power generaton; steady-state..introduton Reently wnd power generaton has been experenng a rapd development n a global sale. The sze of wnd turbnes and wnd farms are nreasng qukly; a large amount of wnd power s ntegrated nto the power system. As the wnd power penetraton nto the grd nreases qukly, the nfluene of wnd turbnes on the power qualty and voltage stablty s beomng more and more mportant. It s well known that a huge penetraton of wnd energy n a power system may ause mportant problems due to the random nature of the wnd and the haratersts of the wnd generators. In large wnd farms onneted to the transmsson network (0 kv 0 kv) the man tehnal onstrant to take nto aount s the power system transent stablty that ould be lost when, for example, a voltage dp auses the swth off of a large number of wnd generators.in the ase of smaller nstallatons onneted to weak eletr grds suh as medum voltage dstrbuton networks, power qualty problems may beame a serous onern beause of the proxmty of the generators to the loads. The exstene of voltage dps s one of the man dsturbanes related to power qualty n dstrbuton networks. In developed ountres, t s known that from 75% up to 95% of the ndustral setor lams to the eletr dstrbuton ompanes are related to problems orgnated by ths dsturbane type. These problems arse from the fat that many eletral loads are not desgned to mantan ther normal use behavor durng a voltage dp. The am of ths paper s to ondut a voltage stablty analyss usng an teratve power system smulaton pakage, to evaluate the mpat of strategally plaed wnd generators on dstrbuton systems wth respet to the rtal voltage varatons and ollapse margns. Ths paper onludes wth the dsusson of wnd generators exellent optons for voltage stablty..induton mahnes Induton mahnes are used extensvely n the power system as nduton motors but are not wdely used as generators. Despte ther smplty n onstruton, they are not preferred as muh as synhronous generators. Ths s manly due to the defned relatonshp between the export of P and absorpton of Q. However, nduton generators have the benefts of provdng large dampng torque n the prme mover, whh makes t sutable for the applaton n fxed speed wnd 08. IJARIIT ALL rghts reserved. Webste: Page 9

2 KITE/NCISRDC/IJARIIT/08/EEE/0 IJARIIT( ISSN: 5-X) turbnes. The fxed speed wnd turbne uses a squrrel age nduton generator that s oupled to the power system through a onnetng transformer as shown n Fgure. Due to dfferent operatng speeds of the wnd turbne rotor and generator, a gearbox s used to math these speeds. The generator slp slghtly vares wth the amount of generated power and s therefore not entrely onstant. Fgure. Modellng wnd turbne onneted grd. However, beause these speed varatons are n the order of per ent ths wnd turbne s normally referred to as onstant speed. Nowadays, ths type of wnd turbne s nearly always ombned wth stall ontrol of the aerodynam power, although pth-ontrolled onstant speed wnd turbne types have been bult n the past. Induton mahnes onsume reatve power and onsequently, t s present prate to provde power fator orreton apators at eah wnd turbne. These are typally rated at around 0 per ent of the wnd farm apaty. As the stator voltage of most wnd turbne eletral generators s 690V, the onnetng transformer of the wnd turbne s essental for onneton to the dstrbuton network and should be onsdered when modelng the eletral nteraton wth the power system []..Impats of WGs. Connetng a generaton sheme to a dstrbuton network wll affet the operaton and performane of the network dependng on the sheme and ratng of the generator tself. The mpats are () Power Flows, () Voltage stablty, () Fault Analyss, and (v) Impat of WTs on the Networks. Let us dsuss eah of them n detal. A. Voltage Stablty A system experenes a state of voltage nstablty when there s a progressve or unontrollable drop n voltage magntude after a dsturbane, nrease n load demand or hange n operatng ondton. The man fator, whh auses these unaeptable voltage profles, s the nablty of the dstrbuton system to meet the demand for reatve power. Under normal operatng ondtons, the bus voltage magntude (V) nreases as Q njeted at the same bus s nreased. However, when V of any one of the system s buses dereases wth the nrease n Q for that same bus, the system s sad to be unstable. Although the voltage nstablty s a loalzed problem, ts mpat on the system an be wde spread as t depends on the relatonshp between transmtted P, njeted Q and reevng end V. These relatonshps play an mportant role n the stablty analyss and an be dsplayed graphally []. 08. IJARIIT ALL rghts reserved. Webste: Page 9

3 KITE/NCISRDC/IJARIIT/08/EEE/0 IJARIIT( ISSN: 5-X) B. PV Curves When onsderng voltage stablty, the relatonshp between transmtted P and reevng end V s of nterest. The voltage stablty analyss proess nvolves the transfer of P from one regon of a system to another, and montorng the effets to the system voltages. Ths type of analyss s ommonly referred to as a PV study. Fgure. P-V Charatersts. The Fgure shows a typal PV urve. It represents the varaton n voltage at a partular bus as a funton of the total atve power suppled to loads or snkng areas. It an be seen that at the knee of the PV urve, the voltage drops rapdly when there s an nrease n the load demand. Load flow solutons do not onverge beyond ths pont, whh ndates that the system has beome unstable. Ths pont s alled the Crtal pont. Hene, the urve an be used to determne the system s rtal operatng voltage and ollapse margn. Generally, operatng ponts above the rtal pont sgnfes a stable system. If the operatng ponts are below the rtal pont, the system s dagnosed to be n an unstable ondton [5]. Fgure. Q-V Charatersts. 08. IJARIIT ALL rghts reserved. Webste: Page 9

4 KITE/NCISRDC/IJARIIT/08/EEE/0 IJARIIT( ISSN: 5-X) C. QV Curves Voltage stablty depends on how the varatons n Q and P affet the voltages at the load buses. The nfluene of reatve power haratersts of deves at the reevng end s more apparent n a QV relatonshp. It shows the senstvty and varaton of bus voltages wth respet to reatve power njetons or absorptons. Fgure shows a typal QV urve, whh s usually generated by a seres of load-flow solutons. Fgure shows a voltage stablty lmt at the pont where the dervatve dq/dv s zero. Ths pont also defnes the mnmum reatve power requrement for a stable operaton. An nrease n Q wll result an nrease n voltage durng normal operatng ondtons. Hene, f the operatng pont s on the rght sde of the urve, the system s sad to be stable. Conversely, operatng ponts n the left sde of the graph are deemed to be unstable [,5]. D. PQ urves. The maxmum permssble loadng of a system, wthn the voltage stablty lmt, s usually determned from the wellknown P V urve or Q V urve. The P V urve s plotted for a onstant power fator and the Q Vurve s plotted for a onstant power. A seres of omputer smulatons s requred to generate a famly of these urves [6]. There are other methods to fnd the voltage stablty lmt of a system, suh as multple load flow solutons, sngularty rteron of the Jaoban matrx [7]. It s an establshed fat that the voltage ollapse ours when the system load nreases beyond a ertan lmt. If the lmtng values of P and Q are known, the voltage stablty margn for a gven operatng pont an dretly be determned. Ths requres the plottng of voltage stablty boundary of the system n P Q plane, however, usng the lmtng values of P and Q. To the best knowledge of the author, no suh work has been reported so far that an determne the voltage stablty margn usng the P Q urve. The lmtng or rtal values of P and Q at the voltage ollapse pont are frst determned and then used to plot the voltage stablty boundary n P Q plane. Unlke the onventonal P V or Q V urves, no fxed value of power or power fator s used n generatng the stablty boundary [8]. Usng the above urve, the voltage stablty margn n terms of P, Q, or S (for a gven power fator) an easly be determned when the ntal operatng pont s known. Consder the equvalent π model of a dstrbuton lne onneted between buses and j as shown n Fg.. Fgure. The π model dstrbuton lne onneted between bus and bus j. We an formulate the relatonshp between njeted urrent and voltage at any buses based on generalzed ABCD parameters as follows: V AV BI () j where A = + ZY and B = Z. The omplex form of A and B an be expressed as shown n eq. (). A = a + ja and B = b + jb () 08. IJARIIT ALL rghts reserved. Webste: Page 95

5 KITE/NCISRDC/IJARIIT/08/EEE/0 IJARIIT( ISSN: 5-X) The reevng end urrent, I, an be expressed: I ( P jq ) V () Substtute A and B from () and I from () nto () resultng n (). Or Where V ( a av ( b jb )( P jq ja) V j () V j j av j 0 j (5) a a b ( P a ( a b ( a b b P a a b ; Q ; a b ); ; Q ) a b The soluton of eq. (5) s the square of the reevng end voltage. Thus the reevng end voltage an be alulated from eq. (6). V ); ; ) b b a V j (6) a There are two solutons for eq. (6), the lower soluton les on the lower part of the P-V urve and s unstable []. Thus, the avalable soluton s a stable one on the upper half, whh an be expressed as n eq. (7). b b a V j (7) a The pont where the two trajetores,.e. stable and unstable lnes, are joned s the nose or bfuraton pont. In addton, ths s the pont where the maxmum power an be transferred, whh s the ondton b a. Substtute oeffents of the quadrat equaton from eq. () nto eq. (7) and rearrange, we obtan (8). ( V P ) P ( V Q ) Q Q V (8) 08. IJARIIT ALL rghts reserved. Webste: Page 96

6 KITE/NCISRDC/IJARIIT/08/EEE/0 IJARIIT( ISSN: 5-X) The relatonshp between P and Q of eq. (8) s a lous of the ollapsng pont on the P-Q plane whh separates the operatng ponts nto feasble and nfeasble regons. The alulaton method to obtan the predted ollapsng pont an be llustrated n Fg.5. Fgure 5. The predted ollapsng pont alulaton. E. Test System Fg. 6 shows the smplfed dagram of test system. It has 0 buses operatng at the voltage of.7 kv, 9 ruts/transformers, and the hypothetal wnd power s onneted at bus 0 at 0.69 kv. The detaled of the system s not showed but t has only one rut of 8 kv []. The power wnd farm s a varable speed dret drve synhronous generator onneted to the Pont of Common Couplng, (PCC) by a full-load onverter. It an be represented by a P-Q bus or a P-V bus, sne t has the apaty to ontrol the reatve power. The loads are dstrbuted onsderng the thermal lmts of the ondutors. The lne parameters, loads and Generator data are shown n Tables I. The load s nreased wth steps of 0.5% n all loads, startng from a normal operatng pont. The lmts of the safe voltage n dstrbuton buses are 0.95 to.05 per unt. The system was smulated onsderng two dfferent senaros. TABLE I. LINE PARAMETERS OF DISTRIBUTION SYSTEM IN PER UNIT ON 0 MVA BASE. From To Resstane Reatane IJARIIT ALL rghts reserved. Webste: Page 97

7 KITE/NCISRDC/IJARIIT/08/EEE/0 IJARIIT( ISSN: 5-X) Frst: the wnd power generaton s onneted and operates as a P-Q bus wth unt power fator. Hs partular operatng pont for atual operaton state s P = 7.5 MW and Q = 0.0 MVAR. The Wnd Power an also be represented by a P-V G G bus wth the reatve power lmts, so that when the lmts are reahed, t beomes a P-Q bus. In modern wnd turbne system t s possble to ontrol the power fator, provdes n ths way, an addtonal soure of reatve to help the power systems to ontrol de voltage. Seond: wthout Wnd Power Generaton. Fg. 7, 8 show the frst senaro and the seond for the load buses of the system. Fgure 6. Dstrbuton Power System Test. Fgure 7. Loadng n bus - Voltage stablty regon wth the ontrbuton of wnd power. 08. IJARIIT ALL rghts reserved. Webste: Page 98

8 KITE/NCISRDC/IJARIIT/08/EEE/0 IJARIIT( ISSN: 5-X) Fgure 8. Loadng n bus - Voltage stablty regon wthout wnd power. Fgure 9..Conluson A very effent method s presented to determne the maxmum permssble load that an be allowed n eah bus of a dstrbuton power system. To ensure the relablty of the results, a omparson between the proposed and a prevous analytal method s arred out n a smple two bus system onneted by a sngle transmsson lne. The results are very lose and the proposed method was appled on a dstrbuton power system. The presene of wnd power s onsdered n order to analyze the effet of ths generaton on voltage operaton and at the voltage stablty lmts. The voltage stablty lmt regon and a sub-regon n whh the voltages at the bus are n aeptable ranges are defned for dfferent power fators. For pratal purposes, the PQ urves an be useful to determne the apaty of ertan buses of the system to support an nreasng n the load demand. Of ourse, to meet de new demand, the lmts of seure voltages must be determned. P-Q urves an help the utltes n the hoe the best pont to onnet the wnd farms. 5.Referenes [] P.Kundur: Power: System Stablty and Control, New York: MGraw-Hll, 99. [] Ake Anderson; Power qualty of wnd turbnes; Thess Degree of Dotor of Phlosoph; Chalmers Unversty of Tehnology; Goteborg Swedent 000. [] Frtz W. Mohn, C. Zambron de Souza: Trang PV and QV Curves wth the Help of a CRIC Contnuaton Method, IEEE Trans. on Power Systems, Vol., Nº, pp.5-, 006. [] MGraw-Edson Company, Power System Dvson: Dstrbuton-System Proteton Manual. [5] M. Chnhlla, S. Arnalte, J.C. Burgos, and J.L. Rodríguez: Power lmts of grd-onneted modern wnd energy systems, Renewable Energy, September 005, pp [6] Rhard Pwko, Nholas Mller, Juan Sanhez Gasa, Xaomng Yuan, Renhang Da, James Lyons; Integratng Large Wnd Farms nto Weak Power Grds wth Long Transmsson Lnes; IEEE/PES Transmsson and Dstrbuton Conferene & Exhbton, Chna, P, MW U, kv [7] L. S. Barros, W. S. Mota, D. F. P. Moura; Matrx Method to Lnearzaton and State Spae Representaton of Power Systems Contanng Doubly Fed Induton Mahnes Operatng as Wnd Generators; 006 IEEE PES Transmsson and Dstrbuton Conferene and Exposton Latn Amera, Venezuela. [8] Poul Sørensen, Ana Hansen, Lorand Janos, John Beh, Brgtte Bak-Jensen; Smulaton of Interaton between Wnd Farm and Power System; Rsø Natonal Laboratory, Rosklde Deember IJARIIT ALL rghts reserved. Webste: Page 99