OL. 3, NO. 5, OCTOE 008 ISSN 89-6608 AN Journa o Engineering and Appied Sciences 006-008 Asian esearch ubishing Networ (AN. A rights reserved. www.arpnournas.com EFOMANCE COMAISON OF OEN AND CLOSED LOO OEATION OF UFC Sarat Kumar Sahu, S. Suresh eddy and S.. Jayaram Kumar Department o Eectrica and Eectronics Engineering, NKIST, idyanagar, Neore, Andhraparadesh, India Department o Eectrica Engineering, JNTU, Hyderabad, Andhraparadesh, India E-Mai: sarata@redimai.com ASTACT Controing power ow in modern power systems can be made more exibe by the use o recent deveopments in power eectronic and computing contro technoogy. The Uniied ower Fow Controer is a FACTS device that can contro a the three system variabes namey, ine reactance, magnitude and phase ange dierence o the votages across the ine. The Uniied ower ow controer provides a promising means to contro power ow in modern power systems. Essentiay, the perormance depends on proper contro setting achievabe through a power ow anaysis program. This paper addresses comparison o the two steady-state modeing o U..F.C within the context o Load ow study o a power system. This mode is incorporated into an existing Newton-aphson Load ow agorithm. Keywords: FACTS, Load ows, UFC.. INTODUCTION With the deveopment o power systems, especiay the opening o eectric energy marets, it becomes more and more important to contro the power ow aong the transmission ine, thus to meet the needs o power transer. On the other hand, the ast deveopment o power eectronic technoogy has made FACTS (Fexibe A.C. Transmission Systems a promising path or uture power system. With the FACTS technoogy such as STATCON (Static Condenser, TCSC (Thyristor Controed Series capacitor, TC (Thyristor controed phase ange reguator, UFC [] (Uniied ower Fow Controer etc, the bus votages, ine impedances, and phase anges in the power system can be reguated rapidy and exiby. FACTS do not indicate a particuar controer, but a host o controers whom the system panner can choose based on cost beneit anaysis. The UFC is an advanced power systems device capabe o providing simutaneous contro o votage magnitude and active and reactive power ows, a this in adaptive ashion. Owing to its aost instantaneous speed o response and unrivaed unctionaity, it is we paced to sove most issues reating to power ow contro whie enhancing consideraby transient and dynamic stabiity. There are two aspects in handing the UFC in steady state anaysis [7]. When the UFC parameters are given, a power ow program is used to evauate the impact o the given UFC on the system under various conditions. In this case UFC is operated in open oop orm. As UFC can be used to contro the ine ow and bus votage, contro techniques are needed to derive the UFC contro parameters to achieve the required obective. In this case UFC is operated in cosed oop orm. In this paper UFC is treated to operate in both orms and the both the modeing methods are compared with each other.. CLOSED LOO OEATION.. Circuit description Considering the we-estabished modeing principe [] and the steady state UFC modes suggested in [] the one shown in Figure- was adopted or the study. The three controabe variabes namey votage magnitude ( T inected by the booster transormer, votage phase ange dierence (Φ T and the exciting transormer reactive current (I q can be reguated independenty with in the region deined by Γ = { T, Φ T, I q } T Є (0, Tmax Φ T Є (0,П I q Є (-I qmax, I qmax. The mathematica reations o UFC contro variabes are s = p t ( Ιs = Ιp Ιq Ι t ( arg(ι t = arg(p (3 Ι t = e( t Ι * s /p (4 arg(ι q = arg(p ± π/ (5.. ower equations o the UFC connected branch Consider a UFC with its boost transormer connected in series with a transmission ine. Assume that the exciting transormer is connected to the bus and the 33
OL. 3, NO. 5, OCTOE 008 ISSN 89-6608 AN Journa o Engineering and Appied Sciences 006-008 Asian esearch ubishing Networ (AN. A rights reserved. two terminas o the transmission ine are denoted as bus s and m respectivey. y using the UFC mode [3] iustrated in Figure- and pi equivaent circuit o the transmission ine, the branch with the UFC connected between bus and m can be modeed as shown in Figure-. Z = X and denote the parameters o the transmission ine. Y and Y m represent the system shunt admittance at bus and m, respectivey. www.arpnournas.com S m = c c = Se (6 * m = m m X (7 * t Se = e e = m X (8 S = (9 c c = m c pc (0 m ( E m F m sinδ = Iq ( E F cosδ ( m m E = C x c C yc E = C yc C x c (3 F = C y (4 F = ( C x (5 C x = X (6 C y = (7 δ = δ δ m, δ is the phase ange dierence between bus and m.3. Load ow equations Assume that or a given contro strategy the power S m on the UFC controed transmission ine -m is set to constant c c. y means o substitution theorem, this branch -m can be detached as shown in Figure-3 in which S m represents power rom bus m and S represents power rom bus. For each other additiona UFC, its corresponding branch can be deat with simiary. Ater modiying a o the UFC embedded branches the oad ow equations can be written as oows: Gi Li = i G cosδ sinδ i i i i (8 Gi Li = i G cosδ sinδ i i i i (9 i=,,3 n; but i,m G L = ( G cosδ sinδ (0 = m G cosδ sinδ Gm Lm c ( Gm Lm = m G cosδ sinδ c ( G L = ( G cosδ sinδ (3.4. Load ow computation Since S m = c c is set as constant or the given contro requirement, S = can be treated as a specia oad varying with respect to the votages and m. As a resut, the UFC s have aready been decouped rom the system and the oad ow equations (8-3 can be soved by standard Newton aphson oad ow program..5. Computation or the UFC contro setting Ater the oad ow computation converges, the contro setting o the UFC can be computed as oows. First and are computed rom (7. Note that c and c are given and (6 gives Se = e e = c c (4 t φ t = ( e e ( X /( m δ m (5 [( ( X ] t = c c /m (6 φ t = γ β δ m (7 X γ = arctg[ e /( e ], β = arctg (8 From (5 and (6 T and Φ T can be determined readiy once the oad ow cacuation converges or the given ( c, c. 34
OL. 3, NO. 5, OCTOE 008 ISSN 89-6608 AN Journa o Engineering and Appied Sciences 006-008 Asian esearch ubishing Networ (AN. A rights reserved. www.arpnournas.com 3. OEN LOO OEATION 3.. Circuit description The steady state mode is based [4] on two idea votage source converters. One in series with the ine and one are in shunt with the ine. The output votage o the series converter is added to the AC termina votage o via the series connected couping transormer. The inected votage cr acts as an AC series system votage source changing the eective sending end votage as seen rom node m. The product o transmission ine current Im and series votage source cr, determines the active and reactive power exchanged between the series converter and AC system. The rea power demanded by the series converter is suppied orm the AC power system by the shunt converter via the common DC in. 3.. Equivaent circuit and power equations The UFC equivaent circuit shown in Figure-4 is used to derive steady state mode. The equivaent circuit consists o two idea votage sources representing the undamenta Fourier series component o the switched votage waveorms at the AC converter terminas. The idea votage sources are: vr = vr (cos θ vr sin θ vr (9 cr = cr (cos θ cr sin θ cr (30 The rea and reactive powers inected at the nodes, m and aso at series converter and shunt converter. At node, = G cr (G m vr (G vr m (G m m θ cr m θ vr θ m θ m θ cr vr θ vr (3 = m cr (G m vr (G vr m (G m θ cr m θ vr θ m θ m θ cr vr θ vr At node 'm', m = m G mm m (G m m θ m m θ m cr (G mm m θ cr m = m mm mm m θ cr m (G m m θ m m θ m cr (G mm m θ cr mm m θ cr At series converter, cr = cr G mm cr (G m cr θ m cr θ cr m (G mm cr θ m cr = cr mm mm cr θ m cr (G m cr θ m cr θ cr m (G mm cr θ m mm cr θ m At shunt converter, vr = vr G vr vr (G vr vr θ vr vr θ vr = vr vr vr (G vr vr θ vr vr θ Where Y = G = Z Z cr vr Y Z mm = G mm mm = cr (3 (33 (34 (35 (36 (37 (38 (39 (40 35
OL. 3, NO. 5, OCTOE 008 ISSN 89-6608 AN Journa o Engineering and Appied Sciences 006-008 Asian esearch ubishing Networ (AN. A rights reserved. www.arpnournas.com Y m = G m m = Z cr (4 Y vr = G vr vr = Z vr (4 Assuming a ree oss converter operation, the UFC neither absorbs nor inects active power with respect to the A.C system. The dc ine votage dc, remains constant. The active power associated with series converter becomes the DC power dc *I. The shunt converter must suppy an equivaent amount o DC power to maintain dc constant. Hence the active power suppied to the shunt converter vr, must satisy the active power demanded by series converter, cr i.e., cr vr = 0 (43 The UFC inearised power equations are combined with the inearised system o equations corresponding to the rest o networ as [ (X ] = [ J][ X] (44 where T ( X [ m m cr cr ] (45 = bb is the power mismatch and superscript T indicates transposition. X is the soution vector and J is the acobian matrix, where [ X ] = [ ] (46 bb T θ θ m vr m θ cr cr θ vr This steady state mode is suitabe or interpoation into an existing Newton-aphson oad ow agorithm. In common with a other controabe pant component modes avaiabe in agorithm, the UFC state variabes and interpoation inside the Jacobian matrix and mismatch equations, eading to a best iterative soution. 4. CASE STUDY 4.. Open oop contro A ive-bus networ has been used to show quantitativey, how the UFC perorms. The power system mode was drawn in simuin and it was simuated using SAT/SIMULINK. The origina networ is modiied to incude UFC as shown in Figure-5, which compensates the ine between buses 3 and 4 The UFC is used to reguate the active and reactive power owing in the ine at a pre speciied vaue. The oad ow soution or the modiied networ is obtained by using Newton-aphson method. Once the oad ow is converged the ina vaues o votages at 3 rd and 4 th node are taen and the UFC contro setting is determined. The maximum amount o active power exchanged between the UFC and the AC system wi depend on the robustness o UFC shunt bus i.e., bus 3.Since the UFC generates its own reactive power, the generator at the st bus reduces its reactive power as observed rom the simuation studies It must be noted that seected UFC initia conditions have a good impact on the convergence o oad ow probem in open oop system. I the initia vaues o UFC are not chosen in a proper way, the oad ow probem diverges, because the initia conditions are estimated by using the pre speciied power ows. 4.. Cosed oop contro The origina networ is modiied to incude UFC which compensates the ine between buses 3 and 4. The UFC is used to reguate the active and reactive powers eaving UFC towards node 4 at speciied vaues. Moreover the UFC shunt converter is set to reguate the node votage at bus 3 at p.u. (Or any other speciied vaue.the converged oad ow resuts can be used to determine the contro settings o UFC. In cosed oop contro, the contro setting o UFC can be determined directy without iteration and hence have no need or any assumption o initia vaues. Keeping the oad demand at the buses where UFC is connected, the pre speciied rea and reactive power ows are graduay increased to see how the contro setting o UFC is changed. As the pre speciied rea power is increased, the UFC series votage is increasing in quadrature with the system votage. The resuts o both the cose oop and open oop systems are given in Tabes and, respectivey. The ina converged vaues o UFC series and shunt votage sources are aso mentioned in Tabe-. In both the cases to compare the resuts the pre speciied power ow rom bus 3 to bus 4 is ept same and studies are carried out. The number o iterations or convergence in open oop system is more when compared with the 36
OL. 3, NO. 5, OCTOE 008 ISSN 89-6608 AN Journa o Engineering and Appied Sciences 006-008 Asian esearch ubishing Networ (AN. A rights reserved. www.arpnournas.com cosed oop system. It is because o the actor that the number o non inear equations in open oop system is more when compared with cosed oop system. Load demand (MW us 3 Tabe-. Cosed oop contro. us 4 re-speciied ine ow (p.u UFC votage Ange (Φ t c c ( t pu radians 45 40 0. 0.0 0.007 -.988 45 40 0.4 0.0 0.009 -.833 45 40 0.34 0.0 0.474 -.683 45 40 0.40 0.03 0.08 -.650 45 40-0.34 0.0 0.059 -.50 Tabe-. Open oop contro. Load demand UFC parameters (MW us 3 us 4 cr θ cr vr θ vr 45 40 0.0648 -.597.0493-0.0950 45 40 0.0590-0.87 0.9783-0.0853 ow cacuation. IEE roceedings on generation transmission distribution. 46(4: 365-369. [4] C.. Fuerte Esquive, Acha E. 997. Uniied power ow controer: a critica comparison o Newton- aphson UFC agorithms in power ow studies. IEE roceedings on generation, transmission distribution. 44(5: 437-444. [5] C. Fuerte Esquive, Acha E. 996. Newton-aphson agorithms or the reiabe soution o arge power networs with embedded FACTS devices. IEE roceedings on generation, transmission, distribution. 43(5: 447-454. [6] C.. Fuerte Esquive, Acha E. 998. Eicient obect oriented power systems sotware or the anaysis o arge scae networs containing FACTS controed branches. IEEE transactions on power systems. o. 3, No., May. [7] Yong Hua Song and Aan T. Johns. Fexibe AC transmission systems. [8] W. Stagg and A.H. E-abiad. Computer method in power system anaysis. 45 40 0.0488-0.579 0.99-0.0703 45 40 0.0403-0.4594 0.86-0.0583 45 40 0.0646 -.855.0566-0.7 5. CONCLUSIONS In this paper a steady state mode based on power ow between two buses is presented. It is observed that in cosed oop operation as the pre speciied rea power ow is increased the votage ange (Ф T is reaching near 90 o. In open oop operation prior to the incorporation o UFC the power transer between bus 3 and 4 is 8-5..Ater the UFC is incorporated the power ow has been changed to 34-5. This is done by appropriatey choosing the initia conditions o the UFC. The disadvantage in open oop operation is that we have to choose appropriate initia conditions other wise we may not get the soution or the oad ow. EFEENCES [] Gyugui L. 99. Uniied power ow contro concept or exibe AC transmission system. IEE roceedings- C. 39(4: 33-333. [] Nabavi-Naii A., Iravani M.. 996. Steady state and dynamic modes o Uniied power ow controer or power system studies. IEEE Transactions power systems. (4: 937-943. [3] W.L. Fang, H.W. Ngan. 999. Contro setting o Uniied power ow controers through a robust oad 37