116 CHAPTER 4 TABILITY ANALYI OF INDUCTION GENERATOR UING TATCOM 4.1 RENEWABLE WIND POWER UPPORT IN THE POWER YTEM Wind enegy is gaining apid momentum in the wold enegy balance. The installation of wind tubine geneatos has inceased to a lage extent and it has been epoted that 10% of the wold s electicity is tageted fom wind powe by 2020 (alman and Anita 2003). Due to this incease in demand of wind enegy, the eseach and development in the contol of wind powe to suppot the powe system netwok with a constant voltage pofile poses challenges. New gid codes fom the diffeent Tansmission ystem Opeation (TO) epesent how wind fams should behave in diffeent gid distubances in ode to maintain stability in the gid. Accoding to C.C.6.3.2, C.C.6.3.6 and C.C.6.3.8 of the gid code, wind fams must be able to povide automatic voltage contol at the PCC by continuous changes to thei eactive powe output. In ecent yeas, seveal gids connecting Wind Enegy Convesion ystems (WECs) consists of Fixed peed Induction Geneatos (FIG) since they ae obust, small size, uggedness, bushless design, self potection against oveload and shot cicuits, cost effective, simple constuction, maintenance fee opeation, povides lage damping toque and equies no synchonization device. FIGs impose voltage
117 instability in the gid. At the instant when a lage wind tubine with FIG is connected to the gid, lage tansient inush eactive cuent 2 to 3 times lage than the geneato ated cuent is dawn fom the gid fo magnetizing the stato. High in ush cuents causes both distubances to the gid and high toque spikes in the dive tain of wind tubines. Unless special pecautions ae taken, the inush cuents can be up to 5 to 7 times the ated cuent of the geneato. This causes voltage dip in the netwok. Fo illustation voltage dip pofile is shown in Figue 4.1. If the voltage dip exceeds 2 to 3%, the wind fam is disconnected fom the gid to avoid futhe decease in voltage which esults in voltage collapse. A tansient like this distubs the gid and the inability of a powe system to meet eactive powe demand will cause voltage instability, with the isk of eventual voltage collapse. t (secs) Figue 4.1 Voltage dip pofile The island opeation may occu in some situations incase of a gid collapse and the ability of opeating in such a situation could be useful fo minimizing the possible aea of blackout. The IEEE 1547 and the IEC 61400-21 standads ae the bases fo evaluating the impact of such wind-tubine geneation systems on the electic powe system. Theefoe,
eactive powe contol is necessay fo Fixed peed Wind Enegy Convesion systems (FWEC). 118 4.2 WIND POWER INTEGRATION CHALLENGE One of the majo challenges faced by the electicity industy is how to effectively integate significant amount of wind powe into the electicity system. A majoity of the wind tubines installed ae FIGs that absob eactive powe fom the system even duing nomal opeating conditions. A fixed-speed wind-geneato is usually equipped with a quiel-cage Induction Geneato (CIG) whose speed vaiations ae vey limited. This configuation uses capacito bank fo eactive powe compensation and has a geabox to match the otational speed of blades with that of the geneato as shown in Figue 4.2, which illustates the statup of a soft-state-fed induction geneato. Figue 4.2 Fixed-speed system with stall o active-stall contol The induction geneato based wind tubines ae the consume of eactive powe. o they need a eactive powe compensato to educe the eactive powe demand fom the tubine geneatos to the gid. It is usually done by continuously switching capacito banks as shown in
119 Figue 4.2. The value of the capacito is so chosen that the powe facto of the wind powe station becomes unity when it is opeating in the ated condition. The eactive powe compensation can be adjusted linealy by contolling the fiing angle of the thyisto switch in othe technology, such as Fixed-Capacito Thyisto-Contolled-Reacto (FC-TCR). Howeve, this method will geneate a hamonic poblem, because the thyisto switch cannot conduct a full cycle. ince the eactive powe of CIG vaies significantly due to the vaiation in the oto speed, a eactive powe compensato, which can adjust the eactive powe along with vaiations in wind speed, is equied to obtain a high powe facto fo the CIG (Piee Bousseau et al. 2006, lootweg 2003). Theefoe, contollable eactive powe (VAR) suppotes, such as TATCOM ae in some cases necessay to povide dynamic voltage suppot with thei actively contollable VAR injection, especially unde voltage depession. This eseach wok pesents esults fom the investigation into the impact of installing a TATCOM at an existing wind fam. This wind fam consists of FIGs and is integated though a weakly connected utility system. A TATCOM is to be installed at the PCC, whee the wind fam is integated with the utility system. It utilizes a new powe electonic device, the GTO, fo enhanced high-powe switching pefomance, simplified tiggeing technology, and oveall educed device and system costs. The contolles of the TATCOM ae designed accoding to commonly known contol pinciples aleady discussed in Chapte 2. The outputs of the TATCOM contolle ae amplified and used as the contollable inputs of the thee-phase voltage souce.
120 4.2.1 Low Voltage Ride Though (LVRT) Capability The LVRT equies that a Wind Tubine (WT) does not tip even if the voltage dops to 0.15 pe unit fo about 0.625 seconds. If due to a fault, the voltage dops below this value, the wind tubine can be tipped until the system estoes and the wind tubine can be esynchonized. As pe FERC ode No. 661, a WT can take a maximum of 2.375 seconds to estoe to about 0.9 pe unit voltage afte the fault has been cleaed (Chompoo-inwai et al. 2005). Figue 4.3 LVRT equiement fo wind geneation facilities These ules ae moe stingent fo some gids which ae deived based on gid eliability equiement (Ullah et al. 2007). Ode No. 661 issued by FERC (Fedeal Enegy Regulatoy Commission) on June 2, 2005, sets specific wind powe equiements as shown in Figue 4.3, namely, LVRT, powe facto design citeia (eactive powe) and upevisoy Contol and Data Acquisition (CADA) capability. The gid codes ae specific to a paticula powe zone and they vay with espect to the voltage pofile equiement duing system
121 distubances. To obtain LVRT capability and to withstand the effects of voltage distubances ae the new challenges with the integation of wind fams into the powe system. Unde any voltage distubances, the FWTs must emain connected to the netwok to maintain voltage pofile. 4.3 TABILITY ANALYI OF INDUCTION GENERATOR BAED WIND TURBINE In the WEC with fixed speed CIGs, eactive powe is equied to maintain the ai gap flux and this leads to high inush cuent dawn fom the powe system. This in tun causes a voltage dop on the powe system netwok. Due to voltage dop, the geneato speed inceases and the electomagnetic toque deceases. Unde nomal condition the electo-magnetic toque T e is equal to the mechanical toque T m. But when T e deceases the geneato acceleates and consumes moe eactive powe which deceases the voltage futhe. This instability pushes the machine beyond its pull out toque and the geneato keeps inceasing its speed. This esults in ove speeding of the geneato and esults in voltage collapse and disconnection of geneatos fom the gid i.e., islanding opeation. To e-establish the magnetic field in the ai gap the voltage must be impoved. Voltage impovement can be made possible by enhancing the eactive powe supply which may pave way fo the geneatos to ide though the low voltage (Aillaga et al. 2007). Fo stability analysis, conside a FWEC which consists of a conventional CIG (asynchonous) geneato diectly coupled to the gid as shown in Figue 4.4. At the instant when the FIG is connected to the gid, high eactive cuent is dawn by the stato fo magnetization.
122 Unde nomal opeating condition : T e (electomagnetic toque) = T m (mechanical toque). In tems of tip speed atio, the equivalent electical toque poduced in the geneato is given by (aad aoud et al. 1998, Aillaga et al. 2007), T e = ½ C P ( )R 3 V eq 2 (4.1) whee - Ai density (kg/m 3 ); R - Wind tubine oto adius (m); V eq - Equivalent wind speed (m/s); - Pitch angle of wind tubine oto (deg); C p - powe coefficient which is a function of both aeodynamic efficiency coefficient. Mechanical Tubine toque is T m =K( s- G) (4.2) angle. whee K = tiffness facto, s = haft angle, G = Geabox Figue 4.4 Fixed speed wind enegy convesion system
123 The Cp- cuves ae shown in Figue 4.5 fo diffeent values of which depicts that as inceases, Cp deceases. Figue 4.5 C P cuves fo diffeent pitch angles Wind powe is conveted into mechanical powe by a wind tubine oto. The wind speed and the aeodynamic toque can be elated using the equation 1 3 R v 2 eq 2 Cp, (4.3) whee T - Aeodynamic toque (Nm) The electomagnetic toque equation of induction machine is RV R X 2 Te 2 KV 2 2 2 (4.4) At stat, due to high inush stato cuents voltage deceases. Fom Equation (4.4), when voltage deceases the electomagnetic toque deceases which esults in voltage collapse and ove speed of the geneato.
Accoding to wing equation, the dynamic behavio of single otating mass wind mill dive tain is given by, 124 dw J dt Tm Te (4.5) The ove speed of the geneato depends on the inetia and duation of the distubance. 4.3.1 Voltage tabilization and Fluctuation Mitigation The dynamic voltage vaiations fom the wind tubines duing opeation ae quantified by flicke and step change. Voltage fluctuations o flicke duing the induction geneation switching should be limited to comply with the flicke emission limits. It is ecommended in the pevious eseach woks that in 10 kv 20 kv netwoks an installation of WT with a flicke emission of P st = 0.35 as a weighted ten minute aveage can be accepted. Howeve a wind tubine installation may be assumed as a PQ node, which may use 10 min aveage data (P mc and Q mc ) o 60 s aveage data (P 60 and Q 60 ) o 0.2 s aveage data (P 0.2 and Q 0.2 ). A wind fam with multiple wind tubines may be epesented with its output powe at the PCC. Ten-minute aveage data (P mc and Q mc ) and 60 s aveage data (P 60 and Q 60 ) can be calculated by summation of the output fom each wind tubine, whee as 0.2 s aveage data (P 0.2 and Q 0.2 ) may be calculated accoding to Equations (4.6) and (4.7). N wt N wt 0.2 Pn, i P0.2, i P n,i i 1 i 1 P (4.6) 2 N wt N wt 0.2 Q n,i Q 0.2, i Q n,i i 1 i 1 Q (4.7) 2
125 whee P n, i and n, i Q ae the ated eal powe and eactive powe of the individual wind tubine; goup. Nwt is the numbe of wind tubines in the The flicke emission fom a single wind tubine duing continuous opeation may be estimated by: P f = n C f ( k,va ) (4.8) k whee: Cf ( k,va ) is the flicke coefficient of the wind tubine fo the given netwok impedance phase angle, given annual aveage wind speed k, at the PCC, and fo the V a, at hub-height of the wind tubine. The flicke emission due to switching opeations of a single wind tubine can be calculated as P st = 18x N 0.31 10 xk f ( k) n k (4.9) whee: P st is the shot tem flicke level, K f ( step facto of the wind tubine fo the given k at the PCC. k ) is the flicke The flicke emission fom a cluste of wind tubines connected to the PCC can be estimated fom Equation (4.10). P st 18 k N wt i 1 N 10,i K f,i k n,i 3.2 0.31 (4.10)
126 whee N 10, i and N 120, i ae the numbe of switching opeations of the individual wind tubine within 10 minutes and 2 hous peiod espectively; and K is the flicke step facto of the individual wind tubine f, i n, i is the ated appaent powe of the individual wind tubine; Duing continuous opeation, the flicke coefficient of the wind tubine fo the actual k and Va at the site can be calculated by applying linea intepolation. Voltage fluctuations o the flicke emissions of wind tubines may be estimated with the coefficient and flicke step factos, C f and K f by applying linea intepolation to the table of data obtained fom the measuements, which ae usually povided by wind tubine manufactues (Piee Bousseau et al. 2006). Fom the table of data poduced fom the measuements at a numbe of specified impedance angles and wind speeds povided by wind tubine manufactues, the flicke emission fom a goup of wind tubines connected to the PCC is calculated using equation 1 N wt 2 P st C f,i k, V a (4.11) n,i k i 1 wind tubine; whee C (,V ) is the flicke coefficient of the individual f k a N wt is the numbe of wind tubines connected to the PCC. If the limits of the flickes emissions ae known, the maximum allowable numbe of switching opeations in a specified peiod, the maximum pemission flicke emission facto o the equied shot cicuit
capacity at the PCC o the maximum allowable numbe of wind tubines fo connection may be detemined. 127 4.3.2 Electical Modeling Issues and Requiements of Fixed peed Induction Geneatos Wind fams with induction geneatos geneate eal powe and consume eactive powe. Figue 4.6 illustates the single line diagam of a wind powe geneation unit, connected to a powe system netwok. The conventional steady state model is the most widely used model fo the analysis of the eactive powe esponse of an induction machine. To investigate the vaiation in the powe output of the wind fams using induction geneatos conside the equivalent cicuit of an induction machine as shown in Figue 4.7. Figue 4.6 ingle line diagam of a WEC be witten as The equivalent impedance as seen acoss the stato teminal can Z eq = R eq + jx eq (4.12) The cuent associated with an applied voltage V is I=IR+ j IX (4.13)
128 whee IR = VR eq / (R eq 2 + X eq 2 ) (4.14) IX = VX eq / (R eq 2 + X eq 2 ) (4.15) Figue 4.7 Equivalent cicuit of an induction geneato The tansmission of eal powe ove a powe line with impedance Z eq esults in a voltage dop V. V = RP XQ V (4.16) Wind tubines affect the voltage level in the PCC due to thei powe poduction. The active powe poduced by the tubine inceases the voltage, wheeas the eactive powe can incease o decease the voltage level. As active powe inceases eactive powe consumption inceases. On the gid with a high X/R atio the voltage deceases and fom Equation (4.16), V is diectly popotional to the eactive powe Q tansfeed. Hence fo efficient voltage contol an effective eactive powe contol stategy is equied. In an induction machine, by unning the machine at ove-synchonous speed the slip is negative which acquies a geneato chaacte.
129 With the wind tubine acting as the pime move, the mathematical elation fo the mechanical powe extacted fom wind and the coesponding mechanical toque is given as P m -½ e A Vw 3 C p (, ) (4.17) T m = K ( - G ) (4.18) The electical counte toque T e set up in the stato connected to the gid at the voltage V is T e = K RV 2 / R 2 + X 2 2 (4.19) If any fault occus in the gid the voltage deceases and the slip inceases. As slip inceases eactive powe consumption inceases which futhe deceases voltage. Accoding to Equation (4.19), when voltage deceases tipping of the geneato occus (as discussed in section 4.2). Fo stable opeation of a wind fam, voltage pofile is the main issue. To maintain voltage and to avoid ove speed of the induction geneatos, dynamic stability impovements and voltage contol technologies ae taken into consideation duing the analysis. To pevent voltage collapse and to obtain fast eactive powe suppot the excellent contollability of FACT devices has paved the way to flexible and dynamic contolles that ae capable of egulating the flow of active and eactive powe components. tability can be enhanced by poviding a dynamic compensation with TATCOM (Alan Mullane et al. 2005).
130 4.4 WIND TURBINE MODEL Modeling of wind tubine oto, blade and shaft needs complicated lengthy computations and needs infomation about oto geomety. Consideing only the electical behavio of the system, a simplified method of modeling of the wind tubine blade and shaft is nomally used. 4.4.1 Mechanical Model The mechanical model is selected with emphasis to include only the pats of the dynamic stuctue of the wind tubine, which ae impotant to the inteaction with the gid, i.e. which influences significantly on the fluctuations of the powe. Thus, only the dive tain is consideed in the fist place because this pat of the wind tubine has the most significant influence on the powe fluctuations. The mechanical model is illustated in Figue 4.8. Figue 4.8 Mechanical model fo the wind tubine The aeodynamic toque T ae is povided by the aeodynamic model and the wind tubine oto angle WTR povides the oto speed.
131 On the othe side the mechanical model intefaces to the geneato model with the ai gap toque T ag and the geneato speed gen which is deived fom the geneato angle position. The dive tain model is essentially a two mass model. To conside the electomechanical inteactions between the shaft system and the gid, the otating pat of the wind tubine is given by the two mass model. The masses used in the model coespond to a lage tubine oto inetia I WTR epesenting the blades and hub, and a small inetia I gen epesenting the induction geneato. Among all models, the two mass model is the most accuate model fo WEC. The wind tubine and the geneato oto ae modeled as two masses and the shaft as sping element. If w is the tubine s otational speed (ad/s); g is the geneato s speed; K s is the shaft stiffness (Nm/ad).; wg is the angula displacement between the shaft ends, then the two mass system with low stiffness shaft can be descibed as follows : d J dt T Tgen Ds ( g ) (4.20) d H g T K s Ds ( ) (4.21) g dt 2 g d g H g Te K s D s ( ) (4.22) g dt 2 g whee H g and H ae the inetia constants of the geneato oto espectively; w and g ae the wind tubine and geneato speed espectively; K s and D s ae the dive tain shaft stiffness and damping constants espectively; and g is the shaft tensional twist angle. In the
132 above equation T is the toque, is the angula displacement between the two ends of the shaft, is the angula speed, H is the inetia constant and K s is shaft stiffness. and g hee stands fo wind tubine and geneato paametes. Tip speed atio, Blade tip speed [R w (m/s)] Wind speed [v (m/s)] (4.23) 4.4.2 Dynamic dq Model Dynamic epesentation of FIG is based on 5 th ode model as shown in Figue 4.9 whee all the diffeential equations ae witten in dq abitay efeence fame. Fo shot tem voltage stability study, the 5 th ode model povides the most accuate esult. The 5 th ode model involves both the stato and the oto tansients. The ode of the model indicates the numbe of states involved in the electical equations togethe with one state in the oto speed equation. This model pedicts the eactive powe esponse also. Figue 4.9 dq model of wind tubine
133 To study the tansient and dynamic stability of lage powe systems and fo incopoating the dynamic chaacteistics of an induction machine into a digital compute pogam the synchonously otating efeence fame is moe convenient (Paul C. Kause 1986). Accoding to this model, the modeling equations in flux linkage fom ae as follows. V = i s R s + j + d s s s (4.24) dt V = 0 = i R + j( - ) + d s dt (4.25) s = i L s + i L m (4.26) = i L + i s L m (4.27) oto inductances. whee L m is the magnetizing eactance, L s, L is the stato and T e = s i s (4.28) Accoding to Mechanical dynamics d dt J Te Tm (4.29) I = L i L m (4.30) 2 = Lm Lm s i L is (4.31) L L
134 = m 2 m s L L L L L i (4.32) Tansient eactance = x 1 = s (L s 2 m L L ) (4.33) s = m 1 L L X i (4.34) Intoducing voltage components, m e L L j V (4.35) e 1 V j X i (4.36) dt d V _ jx i i R V e s 1 (4.37) Eliminating the oto cuent and expessing the oto flux in tems of V e, the oto equation is m 1 e 1 1 e o 1 e V X X j Jv i X X j V T 1 dt dv (4.38) x 1 = s 2 m L L Ls (4.39)
135 is Tansient open - cicuit time constant of the induction geneato T o = L R (4.40) T e = 1 i e s V (4.41) The d-q tansfomation equations (Paul C. Kause 2002) in the synchonous efeence fame ae V V V V c qs c ds c q c d ( R - s L c m L p L p s )L m ( R s - s L L m s L p p )L m -( - R s L m s - p L m L p )L ( s R c L - L m m p )L L p i c qs i c ds i c q i c d (4.42) This model epesents the numbe of systems involved in the electical equations togethe with one state in the oto speed equation. In this wok, using the squiel-cage induction geneato detailed model, c V q and c V d ae set to zeo. Tansfoming fom abc to dqo vaiables, the toque is given as cos c 2 2 2 Tabc sin sin sin s (4.43) 3 3 3 1 2 cos 1 2 2 3 cos 1 2 2 3
136 The electomagnetic toque is given as T e = 3 P c c c c Lm iqs id - ids iq Nm (4.44) 2 2 4.5 FACT BAED OLUTION New netwok technologies that facilitate inceased powe tansfes on the gid including voltage egulation, system damping and powe flow contol have been though by the use of FACT. FACT contolles contol the voltage fom the high side of the netwok duing steady state and tansient conditions incopoating powe electonic devices. FACT devices can be used in wind powe systems to impove the tansient and dynamic stability of the oveall powe system (Gacia Gonzales et al. 2000, iisukpaset et al. 2002). A fast dynamic va compensato is needed to addess these issues moe effectively, as has been pointed out in many liteatue papes (Haizea Gaztanaga et al. 2007, Dong et al. 2001). Compaed to all conventional FACT contolles TATCOM has many advantages with its natues (Rao et al. 2000). TATCOM is a powe electonic based FACT device that can typically povide much faste contol and with lowe losses than the taditional compensatos such as synchonous condenses. The main motivation fo choosing TATCOM in wind fams is its ability to povide bus ba system voltage suppot eithe by supplying and/o absobing eactive powe into the system. The TATCOM is the best option available fo poviding efficient voltage quality in the powe system. One of the most impotant advantages of using TATCOM ove a thyisto based VC is because of its compensating cuent is not loweed as the voltage dops. The majo applications ae: voltage stability enhancement, damping
137 tosional oscillations, powe system voltage contol and powe system stability impovement. These applications can be implemented with a suitable contol (voltage magnitude and phase angle contol) as discussed in Chapte 2. Hence in this chapte a CMC based TATCOM contolle is poposed fo impoved powe flow contol. econdly, a model of the WEC and TATCOM fo steady-state and dynamic impact study is developed in the MATLAB simulation envionment. Moeove the system voltage contol and stability issues ae analyzed, and finally, a TATCOM contol stategy fo voltage fluctuation suppession is pesented, and the dynamic simulations ae used to veify the pefomance of the poposed CMC based TATCOM and its contol stategy. 4.5.1 Poposed CMC based TATCOM The advanced high powe electonics pomise evolutionay incease in thei pefomance, flexibility and cost effectiveness of the electicity tansmission and distibution. These electonics enable full ealization of FACT technologies with potential fo optimum tuning and pecise contol of all powe cicuits. With ecent powe semiconducto technologies, a taditional 2-level is not competent fo FACT contolle applications because of` the needing fo bulky zigzag tansfomes and seies/paallel switches to match hamonics, voltage and powe specifications (Wells et al. 2007, Jiang et al. 2005). To eliminate the coupling tansfome a high voltage convete has to be tied to the PCC of the gid. To achieve this, identical H-bidge convetes with same seies output teminals ae connected in seies and lage voltage blocking capacity is achieved. In this wok, a high powe modula VC using H-Bidge Building Block (HBBB) is poposed fo stability analysis of
138 FWTs. To implement TATCOM at MVA level, a CMC as shown in Figue 4.10 is used due to its high voltage output without tansfome. In eactive powe compensation, CMC based TATCOM with sepaated dc souces ae pefeed due to many easons (Tolbet et al. 2005) as aleady discussed in detail in Chapte 3. The cascaded convete equies fewe main components and has the same stuctue fo each level; the desiable powe ating of the system can theefoe be simply adjusted by connecting a diffeent numbe of the identical modules (Lee et al. 2003). A geat combination of the TATCOM concept and the CMC topology connected to a wind geneato is shown in Figue 4.10. Figue 4.10 CMC based topology The main advantage of the multi- level invete cicuits is the desiability to poduce quasi-hamonic neutalized output voltage wavefoms without magnetic wavefom summation cicuits.
4.6 TEADY TATE VOLTAGE CONTROL WITH TATCOM 139 A TATCOM can continuously povide the eactive powe demand of a wind fam unde vaious conditions with apid contol (Qingguang et al. 2004). The TATCOM continuously maintains the voltage at the wind fam connection by injecting o absobing eactive powe. PI contol scheme is used fo maintaining the efeence voltage of the wind fam. PI contolle is applied fo its simplicity and obustness. All voltage contol blocks and thei inteconnection is shown in Figue 4.11. Figue 4.11 Block diagam fo voltage contol using TATCOM The function of PI is to egulate the multilevel invete so that it stays close to the nominal opeating point in the pesence of distubances and noise. The input to the PI contolle is the voltage eo signal fom the
140 voltage compaato. The output of the PI contolle is the modulating signal which is used to geneate the gating pulses. A step esponse of 10% change in the contolle voltage efeence is used fo tuning the PI contolle of the TATCOM. Unde the weakest system condition, the maximum pecentage oveshoot is 10.6% of the step change fo the TATCOM contolles. The settling time at which the eo educed to a value within 1% of the steady state value is 2.8 cycles fo TATCOM. Two PI contolles can be implemented to egulate the dc link voltage and the eactive cuent espectively. TATCOM impoves the steady state stability limit when the eal powe poduced by an induction geneato exceeds and lage amount of eactive powe is consumed. The TATCOM contol scheme is shown in Figue 4.12. Figue 4.12 TATCOM contol scheme In steady state, the fiing pulses fo the TATCOM switches have to be synchonized to the bus voltage such that the fundamental component of the voltage injected by the convete leads the supply voltage
141 by the contol angle ( ). This synchonization can be achieved by a PLL as shown in Figue 4.12 which poduces the phase angle of the bus voltage as an output ( ). t t = + t (4.45) whee is the elative phase of the bus voltage with espect to a synchonously otating efeence fame, is the opeating fequency in ad/sec, t anges fom 0 to 2 and is poduced by a PI contolle (saw tooth geneato). An oscillato poduces the output, sin t and cos t, which ae used to compute the quantities Vp and V as V p = V sin t + V cos t (4.46) V = V cos - V sin t t (4.47) whee V = 2 3 v a 1 2 v b 1 2 v c (4.48) V = 1 2 v c vb (4.49) In steady state V = V s in t (4.50) V = V s cos t (4.51)
142 If V p (ef) = Vs V (ef) = 0 (4.52) whee V s is the line to line voltage at the convete bus. PLL tacks the phase of the voltage by feedback contol of and diving it to zeo. K c V and T c ae the contolle paametes. Unde voltage unbalance, the PLL opeates satisfactoily (Peng et al. 2001). The feed fowad of fequency vaiation ( ) compensates fo the change in fequency. If the change in fequency is not pedictable, an additional integal tem may be used in the contolle to achieve the same esult. Figue 4.13 implified model of the CMC based TATCOM in both abc and dqo coodinates The effective mitigation of sag and hamonics depends on the effectiveness of the contolle design. The contol system employed in the TATCOM system maintains the magnitude of the bus voltage constant by
143 contolling the magnitude and/o phase shift of the voltage souce convete s output voltage. Reactive powe exchange is achieved by popely contolling i q, (Rodiguez et al. 2007). The DC capacito voltage is maintained at a constant value and this voltage eo is used to detemine the efeence fo the active powe to be exchanged by the invete. The total instantaneous powe in abc quantities can be tansfomed into q-d-o quantities as follows P abc = V a I a +V b I b +V c I c (4.53) = 3/2 (V d I d +V q I q ) +1/3V 0 I 0 (4.54) d Id 0 - L Id Vsd - Vcd L (4.55) dt I L 0 I V - V q q sq cq d dt (4.56) V V cdef cqef V V sd sq LI LI qef def d L I dt d L I dt def qef (4.57) Cef 2 cdef 2 cqef V V V (4.58) V 1 cqef tan (4.59) V cdef t 1 V dc ( t) i dc (t ) dt (4.60) c
144 To egulate the capacito voltage, a small phase shift is intoduced between the convete voltage and the powe system voltage. A small lag of the convete voltage with espect to the voltage at the PCC causes eal powe to flow fom the powe system to the TATCOM, while the eal powe is tansfeed fom the TATCOM to the powe system by contolling the convete voltage so that it leads the voltage at the PCC. The phase angle of the utility voltage is of vital impotance fo the opeation of most of the advanced powe electonic devices connected to the electic utility, since it has a diect effect on thei contol algoithms. In ode to lock the phase angle of the utility voltage in a obust way, a PLL is used. The outputs of the contolle ae i def and i qef which ae the efeence cuents in the dq coodinates which ae needed to calculate the powe injections by the TATCOM as in Equations (4.61) and (4.62). P = V i (i d cos i +i q sin i ) = v d i d +v q i q (4.61) Q = V i (i d sin i -i q cos i ) = -v d i d +v q i q (4.62) whee i d and i q ae the efeence d and q axis cuents of the ac system. The fundamental magnitude and the hamonic spectum ae contolled vaying the switching angles,. 4.7 PERFORMANCE EVALUATION OF VC BAED TATCOM To evaluate the pefomance of the conventional TATCOM fo a CIG, a thee-phase pototype with a utility line voltage of 25 kv and a utility fequency of 50 Hz is developed. Both the eal powe and the eactive powe of induction geneato ae popotional to the oto speed and the powe facto of the induction geneato is vey poo. The DC bus
voltage of the TATCOM depends on the maximum and minimal values of compensation eactive powe. 145 (a) without TATCOM (b) with TATCOM Figue 4.14 Reactive powe absobed by the induction geneato The induction geneatos ae connected to the system at the PCC whee the conventional TATCOM is connected to compensate the eactive powe absobed by the induction geneatos duing its stating. The utility system at the PCC is 25 kv. The TATCOM povides eactive powe to the induction geneato duing its opeation. o the powe facto of the cicuit gets impoved afte applying the TATCOM. Figues 4.14 (a) and (b) shows the simulation esults of the eactive powe absobed by the induction geneato with and without TATCOM. With TATCOM cicuit the eactive powe is compensated and becomes positive.
146 (a) without TATCOM (b) with TATCOM Figue 4.15 Active powe (a) without TATCOM (b) with TATCOM Figue 4.16 Powe facto Figues 4.15 (a) and (b) shows the active powe in the cicuit with and without TATCOM in the cicuit. The powe facto of the system gets impoved when the TATCOM is applied to the system. Figues 4.16 (a) and (b) shows the powe facto of the cicuit which is 0.78 when thee is no compensation cicuit in the system. With TATCOM the powe facto is impoved to 0.95.
147 (a) without TATCOM (b) with TATCOM Figue 4.17 THD measuement Figues 4.17 (a) and (b) shows that the THD measued without TATCOM in the cicuit is 5.28% and with TATCOM it is educed to 2.38%. 4.8 TET YTEM WITH CMC BAED TATCOM Figue 4.18 shows the single-line diagam of the powe system used fo this study. A 220 kw wind fam consisting of CIG diven by FWT is connected to a powe gid though a step-up tansfome T 1 and a powe line. The Wind Tubine Geneato (WTG) with a ated powe capacity of 220 kw is consideed hee. A TATCOM is shunt connected at the 22 kv bus (the high voltage teminal of the tansfome T 1 ) to povide dynamic eactive compensation. To educe the size of the TATCOM, a fixed capacito bank is used to supply about 10 Mva eactive powe at the nominal voltage condition. The paametes of the system components ae as follows. CIG: ated powe = 220 kw, ated stato voltage = 440 V, stato esistance = 0.0079 pu, oto esistance = 0.025 pu, stato leakage inductance = 0.07939 pu, oto leakage inductance = 0.4 pu, magnetizing
inductance = 4.4 pu; Tansfome T 1 : tuns atio = 440 V/22 kv, equivalent leakage eactance = 0.06 pu; Tansfome T 2 : tuns atio = 22 kv/13 kv. 148 Figue 4.18 ingle line diagam of the test system In the poposed wok, a test system with a load equiing a voltage 22 kv is connected to the gid as shown in Figue 4.19 and simulated with CMC in Matlab/imulink softwae package. Figue 4.19 Active powe without TATCOM
149 Figue 4.20 Reactive powe without TATCOM Figue 4.21 Gid voltage without TATCOM Figue 4.22 Load voltage without TATCOM
150 Figue 4.23 Active powe with 3-level invete TATCOM Figue 4.24 Reactive powe with 3-level invete TATCOM Figue 4.25 Gid voltage with 3-level invete TATCOM
151 Figue 4.26 Load voltage with 3-level invete TATCOM The simulations ae caied out with the incopoation of CMC based TATCOM in the system and the esults obtained ae compaed with those obtained without the pesence of TATCOM in the system. The paametes measued at thee locations such as Gid side, Load side and Wind fam side ae shown fom Figue 4.19 to Figue 4.26. Table 4.1 gives the compaison of the simulation esults obtained with and without CMC based TATCOM. Table 4.1 Compaison of simulation esults Paametes Measued Without TATCOM (Pe Unit values) With 3-level VI TATCOM (Pe Unit values) Gid active powe 0.2 0.28 Gid eactive powe 0.21 0.2 Gid voltage 0.99 0.99 Load active powe 0.29 0.38 Load eactive powe 0.12 0.2 Load voltage 0.97 0.99
4.8.1 Reactive Powe Compensation using 5-level CMC based TATCOM 152 The 5-level CMC TATCOM simulation cicuit featues two conventional full-bidges seially connected togethe with thei powe ails connected to sepaate isolated DC voltage supplies. This section pesents the simulation esults of the test system with CMC based TATCOM. Figue 4.27 Active powe with cascaded 5-level invete TATCOM Figue 4.28 Reactive powe with cascaded 5-level invete TATCOM
153 Figue 4.29 Load voltage with cascaded 5-level invete TATCOM Figue 4.30 Gid voltage with cascaded 5-level invete TATCOM 4.9 COMPARION OF REULT The following simulation esults fom Figue 4.31 to Figue 4.36 shows the implementation of diffeent TATCOM modules (multipulse, 3-level, 5-level) in the test system and its compaison without TATCOM.
154 Figue 4.31 Compaison of gid active powe Figue 4.32 Compaison of gid eactive powe Figue 4.33 Compaison of load active powe
155 Figue 4.34 Compaison of load eactive powe Figue 4.35 Compaison of wind fam active powe Figue 4.36 Compaison of wind fam eactive powe
thee cases. 156 Table 4.2 shows the compaison of esults obtained fo all the Table 4.2 Compaison of esults with 3-level and 5-level TATCOM Measued Paametes Without TATCOM (p.u) With 3-level VITATCOM (p.u) With 5-level CMC TATCOM (p.u) Gid active powe 0.24 0.28 0.54 Gid eactive powe 0.21 0.2 0.017 Gid voltage 0.99 0.99 1 Load active powe 0.29 0.38 0.68 Load eactive powe 0.12 0.2 0.18 Load voltage 0.97 0.98 1.0 4.10 PERFORMANCE EVALUATION OF ACMC TATCOM The test system compises of two 275 kva wind fam being connected to a 440 V distibution system and expots powe to a 22 kv gid though a step up tansfome, with a 10 kw load has been connected to the PCC and in addition a TATCOM has been intefaced at the PCC. Wind tubines use FIG consisting of CIG and a pitch angle contol system. In FIG, the stato is connected diectly to the 50 Hz gid and the oto is being coupled to the pime move. The FIG technology allows extacting maximum enegy fom the wind fo low wind speed by optimizing the tubine speed. The optimum tubine speed poducing
157 maximum mechanical enegy fo a given wind speed is popotional to the wind speed. Fo wind speeds lowe than 10 m/s the oto is unning at sub synchonous speed. At high wind speed it is unning at hype synchonous speed. Figue 4.37 Geneation of caie signals Phase Disposition Technique is used fo Caie Geneation of Caie fequency - 10 khz. The Modulation index, m a is selected as 0.9. In multi caie based sine PWM, Phase Disposition (PD) is chosen because of educed switching losses at highe modulation index. Only fou caies ae used as shown in Figue 4.37 to develop 9-level output instead of (M-1) caies fo the same M level output.
158 Figue 4.38 witching signals using multicaie based PWM Figue 4.38 shows the switching angles obtained using caie based PWM contol stategy. Figue 4.39 Load voltage pofile
159 Fo the test system which includes a gid, step down tansfome and a non linea load the nominal voltage, cuent, eal powe and eactive powe appeas acoss the load that is obseved as shown fom Figue 4.39 to Figue 4.41. Without wind fam the voltage at PCC is aound 380 V and when wind fam is integated into the powe system, the voltage pofile educes to 220 V. This dop in voltage pofile is impoved with TATCOM and the voltage impoves again to 380 V. Figue 4.40 Load cuent wavefoms Without wind fam the cuent at PCC is aound 18 A. With wind fam the cuent at PCC inceases and then deceases to about 30 A. With TATCOM the cuent at PCC is about 15 A.
160 Figue 4.41 Real powe wavefoms Without wind fam the eal powe at PCC is aound 10 kw. With wind fam the eal powe at PCC inceases and it is about 10 kw. With TATCOM the eal powe at PCC is about 5 kw. Figue 4.42 Reactive powe wavefoms
161 Without wind fam the eactive powe at PCC is about 800 va. With wind fam the eactive powe is consumed fom the gid and with TATCOM the eactive powe injected at PCC is aound 5000 va. When wind fam is connected to PCC fo the same test system without wind fam thee is a voltage dop and thee is also a eduction in the eactive powe due to the eactive powe being consumed by the induction geneato. Finally when a CMC based TATCOM is incopoated to the test system, voltage pofile gets impoved as the eactive powe has been injected fom the TATCOM to the PCC. Figue 4.43 Pe phase voltage output of CMC Figue 4.44 7-level voltage output of CMC
162 Figue 4.45 THD fo 7-level CMC based TATCOM Figue 4.46 THD fo 9-level CMC based TATCOM Figue 4.47 THD fo 9-level ACMC based TATCOM
163 When compaed to 9-level CMC TATCOM, 9-level ACMC TATCOM gives eduction in THD value as obseved in Figue 4.46 and Figue 4.47. 4.11 HARDWARE IMPLEMENTATION In hadwae implementation, a 3-level, 5-level CMLIs based convete have been constucted using the MOFET as the switching device. The hadwae model has been built as shown in Figue 4.48 and tested to veify the concept. Fo a 3-phase system, the numbe of pulses, p, can be fomulated by P= (M-1) x 6 (4.63) whee M is the numbe of levels; DRIVER CIRCUIT1 DRIVER CIRCUIT 2 PIC CACADED H-BRIDGE 1 CACADED H-BRIDGE 2 Figue 4.48 Hadwae cicuit implementation
164 Figue 4.49 ingle H 1 bidge output wavefom in CRO Figue 4.50 ingle H 2 bidge output wavefom in CRO
165 Figue 4.51 Cascaded 5-level invete output wavefom in CRO Figues 4.49 and 4.50 shows the pe phase voltage output wavefoms of bidges H I and H 2 espectively. Figue 4.51 shows the 3 phase cascaded 5-level invete output wavefom measued in CRO. 4.12 CONCLUION Fo the wind fam voltage fluctuation suppession using a CMC based TATCOM, the methodology to conduct an impact study of a TATCOM on the integation of a lage wind fam into a weak loop powe system is descibed. The specific issues and solutions of the studied wind fam system ae illustated. Fo the system study, the models fo the system, wind fam and TATCOM ae developed. This chapte discusses the application of conventional, CMC and ACMC TATCOMs to impove the voltage quality of gid connected fixed speed Induction geneato wind tubine systems. The TATCOM when connected in shunt in the system can povide fast and smooth eactive powe contol and effectively contol the system voltage level duing the intoduction of induction geneatos and also duing the continuous opeation. The poposed CMC and ACMC
166 TATCOM topologies offes seveal advantages ove the conventional VC TATCOM such as educed powe loss, modula layout, less hamonic contents, the output changing linealy with input and the absence of costly, bulky coupling tansfome. The simulations made in Matlab shows the impovement made in the magnitudes of the eactive powe and voltages when compaed with the 3-level invete based TATCOM. In the poposed wok, a complete tansient stability and steady state model fo the TATCOM has been implemented and poved that by contolling eactive powe the voltage egulation and maximum active powe flow can be achieved.