GLOBAL POWER SYSTEM CONTROL USING A UNIFIED POWER FLOW CONTROLLER

Transcription

1 GLOBAL POWER SYSTEM CONTROL USING A UNIFIED POWER FLOW CONTROLLER Joseh Leung*, Davd J. Hll*, Yxn N and Ron Hu* * Deartment of Electronc Engneerng, Cty Unversty of Hong ong Deartment of Electrcal and Electronc Engneerng, The Unversty of Hong ong joe.leung@student.ctyu.edu.hk Abstract Ths aer exlores the use of a Unfed Power Flow Controller () for global ower system control. Coordnated control schemes are assgned to the, Power System Stablzer (PSS) and generator exctaton to mantan transent stablty and voltage stablty followng a dsturbance. The extenson to use of n damng nterarea oscllaton s also consdered. Under the roosed coordnated control schemes, the results show the combned wth other control devces can restore lne voltage, mrove transent stablty and enhance the damenng of nterarea oscllaton. eywords: FACTS,, transent stablty, voltage stablty, voltage regulaton, nterarea oscllaton INTRODUCTION Power systems are becomng ncreasngly more comlex due to the nterconnecton of regonal systems and deregulaton of the overall electrcty markets. Tradtonal ower system controls rmarly focus on local ssues. There needs to be develoed a global coordnated scheme wth great degree of flexblty to allow for ths comlexty, artcularly unforeseen crcumstances. Ths aer follows earler work by the authors [][], [3]. In [][], a novel global control strategy s ntroduced for coordnated transent stablty and voltage regulaton by exctaton control. In [3], the use of a Unfed Power Flow Controller () for ower flow control and mroved damng erformance s studed. In ths aer, we use the to mlement coordnated control over all three major dynamc ssues for ower systems, namely transent stablty, voltage regulaton and nterarea oscllaton followng a dsturbance. Transent stablty consders the roblem of loss of synchronsm among the synchronous machnes. oltage regulaton s the mantenance of reured voltage levels durng normal oeraton or after a dsturbance. When machnes from dfferent areas are nterconnected by weak tes, the roblem of nstablty wll be worsen due to the resence of nterarea mode oscllatons. These roblems are not mutually exclusve. For examle, varous controls can be used to mrove each of them, such as exctaton control. In the ast, the elmnaton of one roblem has sometmes ntroduced roblems n the other areas, e.g. hghseed exctaton for transent stablty led to selfexcted oscllatons [4]. Dfferent system behavour reures dfferent control actons at dfferent tmes and oeratng condtons. Global control rovdes a framework to coordnate dfferent knds of control across the whole system, geograhcally and for all oeratng stuatons,.e. for all states and nthelarge n the sense of stablty theory [4]. It s used to combne the ualtatve and uanttatve nformaton avalable n the system and to formulate the otmal control strategy at a artcular oeratng condton. It s used n ths aer to mantan the transent stablty and acheve voltage regulaton n dfferent scenaros. The s caable of controllng all the arameters affectng ower flow n the transmsson lne, smultaneously and selectvely [5]. Its flexblty makes t ossble to aly multle control laws wthn a sngle unt. Its effectveness n damng the system oscllaton s also demonstrated from the system freuency resonse. In ths aer, the above ssues are demonstrated n the modfed WSCC (Western Systems Coordnatng Councl) 9bus test system, whch has a smle form of a multmode meshed network. The control strateges are tested by usng the MATLAB Power System Toolbox (PST) []. The aer s structured as follows: The and the test system models are ntroduced n Secton. The concet of global control law s resented n Secton 3. Smulaton and results at dfferent scenaros are rovded n Secton 4. Concluson s resented n Secton 5. SYSTEM MODEL. Model and Its Drect Controls The model used n ths aer s adoted drectly from [3]. However, some of ts arameters are modfed to sut the examle system reurements. The schematc dagram s shown n Fgure. The man euatons are lsted below: dd Cd ( P P ) S B dt = where * n s!! P = Re! jx t s!!! P = Re! jx t R *

2 = m d B θ = θ S ϕ () where, m, m are the coeffcents reresentng the PWM control; s and R are the sendng and recevng end voltages; S B, B are system MA and oltage base; and ϕ and ϕ are the frng angle of the two converters. S T P, I = θ S S X t n : Conv I d I d Conv = θ d = θ = θ m C I d ϕ m Fgure : schematc dagram Two basc local control systems are used for the. The shunt element uses automatc voltage control as the rmary control wth dc caactor voltage beng ket constant see Fgure 3. Constant real and reactve ower flow control s used for the seres element see Fgure. There are two sulementary controllers for contngency oeraton. Reactve Power Control (AR Control) of the shunt element can be swtched n to rovde extra reactve ower reurements for mantanng voltage stablty see Fgure 3. Addtonal hase angle control of the seres element s avalable for further mrove transent stablty see Fgure. n ϕ : P X t T R = R θ R sh AR Control sh d sh dref ref 3 T s ϕ sh AR Control Sref T s S and are the and comonents of the shunt converter current sh Fgure 3: Shunt element control block dagrams. Test System Fgure 4 shows the WSCC 9bus test system. It s a multmode meshed network wth 3 generators and 3 major loads. Transent models are used for the machnes, whch are eued wth IEEE DC exctaton systems and Power System Stablzers (PSS). Governors and turbnes are not modelled n ths smulaton; ther slow resonse renders them nsgnfcant n transent studes. The s nserted n as an addtonal lne between bus 5 and 7. Orgnal lne medances from [] are used whle the machnes and exctaton system arameters are modfed accordng to [] sh 6 4 ref 4 3 m Phase Angle Control θ θ R S θ ref 7 T s 7 w P L Q L P Lref Q Lref Power Control 5 T s 6 5 T s 6 w Note: = ϕ = tan ( w P and Q L w ) L = n B m d are the real and reactve ower on the lne and are the comonents of comensat on Fgure : Seres element control block dagrams and voltage ϕ, the seres Fgure 4: Modfed WSCC 9bus test system 3 GLOBAL CONTROL LAW Global control s a framework wthn whch varous control methods can be coordnated to handle comlex systems. It offers the ossblty to coordnate control actons across the whole system geograhcally and for all oeratng stuatons n an adatve way [][]. The global control law wthout tunng used n [] has the followng general form = v µ v () where, µ s the weghtng functon of the th local controllers; v s the nut for the th local controller. The global control law v thus takes the average of the ndvdual control laws, weghted by the oeratng re

3 gon membersh functons []. Any tye of local controller from tradtonal PID controller to the modern nonlnear controller such as rulebased, fuzzy or modelbased desgns such as feedback lnearzaton could be used under the global control law. Transent Controller oltage Controller Global Control weghtng functon µ δ ( F, F ) µ ( F, F ) PSS Exctaton Others Coordnated Control Laws Fgure 5: Global controller block dagram Dsturbance Power System Outut Fgure 5 shows the block dagram of the roosed global control law adoted from []. It frstly determnes the network oeratng condtons and then actvates the corresondng controller (transent or voltage controller) for that secfc stuaton. The nuts to the controller are the change n rotor angle (ω) and the change n voltage ( ) at the adjacent buses of the. The corresondng controller (transent or voltage controller) wll be actvated when a contngency arses accordng to the followng scheme: F F ( ω, ) = α ω α ( ) ( ω, ) αω = α ( ) F > F µ δ ( F, F ) = f F F > F µ ( F, F ) = f ( 3) F < where µ δ and µ v are the weghtng membersh functons for the transent controller and voltage controller resectvely. α and α are ostve scalng constants whch can be chosen accordng to the senstvty reurement. F s the redetermned senstvty level above whch corresondng control acton wll be actvated. Also, µ δ µ v = when the controller s swtched on. The global controller n [] uses traezodshaed lke membersh functons and offers a smooth swtchng from one control acton to another. In ths aer the global controller, however, s desgned to be dscrete to nvestgate the caablty of the under dfferent fault condtons. Once the network condton after the dsturbance s determned and the global controller acton s chosen (ether transent or voltage control), the control tasks are delegated to the ndvdual local controllers and ther control actons wll be coordnated accordng to the redetermned control schemes. These schemes are demonstrated n the next secton. 4 SIMULATIONS AND RESULTS 4. Transent Stablty Transent stablty focuses on the mantenance of stablty of the machne ower angles followng a major dsturbance. In ths examle, a 3hase ground fault s aled to bus 7. Escalaton of rotor angles s detected and transent controller s actvated. The fault s cleared by dsconnectng one of the two arallel lnes between bus 5 and bus Control Scheme The control scheme adoted s as follows: ) The s byassed durng a major dsturbance for rotecton of the unt; ) When the fault s cleared, the wth all the basc controls s automatcally renserted and an addtonal hase angle controller s smultaneously swtched on; 3) At the end of the frst swng of the ower angle, gans of the local machnes Power System Stablzers (PSS s) are ncreased from ther base levels to uckly damen the subseuent local oscllatons. (Note that ths acton of varyng the gans of PSS s s uncommon n the ndustral. However, a tunng caablty s antcated n general global control schemes []). Angle (degree) Machne Angles Machne rotor angle when clearng tme > 5 cycles Machne 3 rotor angle when clearng tme > 5 cycles Machne 3 rotor angle when clearng tme = 5 cycles Machne rotor angle when clearng tme = 5 cycles Fgure 6: Loss of synchronsm n the frst swng. 4.. Results The crtcal clearng tme of the orgnal system wthout the global control scheme and the s 5 cycles see Fgure 6. The control scheme extends the stablty lmt. Fgure 7 shows an ncrease of swng angle when the crtcal clearng tme s extended to 6 cycles. As shown n Fgure 7a, the maxmum rotor angle of machne ncreases to 5. The shunt element of the

4 acts lke a mdont comensator from the generaton end to the load and rovde voltage suort. In Fgure 7b, when the hase angle control of the seres element of the s ncororated n the ost fault erod, t shfts the ower angle and ncreases the transent stablty lmt. Fgure 7c shows the subseuent machne rotor angle oscllatons after the frst swng are further damened when the PSS s of the local machnes gradually ncrease ther controller gans. The smulatons showed that t s the most effectve way to damen the local oscllaton. Fgure 8 shows the dc caactor voltage durng the transent and ost fault erod. The control scheme attemts to mantan constant dc voltage n the transent scenaro. Angle (degrees) Angle (degrees) Angle (degrees) (a) Machne Angles wth (b) Machne Angles wth and Phase Angle Control (c) Machne Angles wth and subseuent PSS Damng Fgure 7: Stablty margn enhancement under varous control strateges at crtcal clearng tme = 6 cycles oltage (u) dc Caactor oltage Tme (second) Fgure 8: dc caactor voltage n the ost fault erod 4. oltage Stablty In ths scenaro, bus 5 s assumed to reure ts voltage to be mantaned above.85 u. An addtonal AR controller s nstalled to rovde extra reactve ower and to mantan the voltage of the selected bus see Fgure. Fgure 9 llustrates ths dea when reactve ower s suddenly demanded and voltage of bus 5 decreases as a result. oltage (u) Bus 5 voltage wth AR control Bus 5 oltages Bus 5 voltage wthout AR control Fgure 9: Bus 5 voltage durng a dsturbance The transmsson lne between bus 5 and bus 7 s modeled as a weak te, whch s also a doublecrcut. Followed by the loss of one of the two lnes, the system exerenced voltage collase. 4.. Control Scheme The control scheme s scheduled as follows: ) The basc controls of the contnue to oerate durng transent stablty mode. If the voltage at the bus beng montored (Bus 5 n ths case) dros below the lmt, sulementary AR control wll be swtched n to regulate the selected bus voltage; ) The amount of extra reactve ower outut s lmted by the change n dc caactor voltage. It s set to ±% n our case; 3) Other longterm controls are later on brought nto servce, accordng to ther own resonse tme. These controls nclude the swtchn on caactor banks, generator exctaton control or n the worst case, load sheddng. 4.. Results Fgure shows that when the s nstalled, t successfully defers the voltage from collasng. Exctaton gans of the generators are readjusted at t = sec and the system s gradually restored. It shows that the s effectve n rovdng mmedate voltage suort to the system before other devces wth slower resonse tme (e.g. mechancal swtchng of caactor banks) are brought nto servce. Fgure shows the dc caactor voltage for voltage nstablty scenaro. By varyng the dc caactor voltage

5 wthn ts allowable lmt, the sendng end voltage s mantaned. Fgure shows that voltage suort by the s more effectve for the buses near the, as exected. In the long term, the ower system should be desgned and revewed such that adeuate voltage stablty margn s ensured. oltage (u) Bus 5 voltage wthout Bus 5 oltages Bus 5 voltage wth Fgure : Bus 5 voltages wth and wthout Caactor dc oltage 4.3 Interarea Oscllaton Oscllatory nstablty, n general, corresonds to one grou of generators swngng aganst another grou of generators. A Power System Stablzer (PSS) s wdely used by ndustry nowadays as a means to damen local oscllaton modes. The basc rncle of a PSS s to ntroduce a comonent of electrcal torue n hrase wth the rotor angle n order to damen the oscllatory oscllatng rotor angles [4]. Whle ts sgnfcance for damng local modes of oscllaton s well recognzed, ts caablty to further damen nterarea oscllaton s relatvely weaker [6]. The network detals of the WSCC 9bus system are further modfed accordng to [] n order to roduce a strong resonance system caused by nterarea oscllaton. The major nterarea oscllatng freuency s.49 Hz Results When a small dsturbance s aled to bus 8 n the system wth no for. second, the machnes oscllate aganst each other and the voltage at bus 8 escalates see Fgure 3. The system wth, however, remans stable. Freuency resonses of the machne rotor angles reveal that the has sgnfcantly fltered the.49 Hz freuency. In general t lowers the magntude of rotor angle oscllaton along the low freuency sectrum see Fgure 4 and 5..7 Bus 8 oltages olt (u) System wthout olt (u) Fgure : Dc caactor voltage durng the dsturbance. System wth.5 Bus 4, 7, oltages Bus Fgure 3: Instablty due to InterArea Oscllatons olt (u) Bus 7 Bus Fgure : oltages of other buses durng the dsturbance. 5 CONCLUSION In ths aer, a global control scheme s mlemented whch determnes the crtera for the selecton of the transent or voltage control algorthms. Two sets of local controls, whch use varous controls of the and other system control devces, are suggested for mrovng transent stablty and voltage stablty. Fnally, the freuency resonse of the system wth the ncluson of shows sgnfcant mrovement n damng of system oscllaton.

6 Freuency Plot of Rotor Angle () Magntude B = 3k, S B = MA, n =.5, n =.5 X t =.5, X t =.5 (all n u of 3k base) u magntude System wth 3 Fre/Hz System wthout Fgure 4: Freuency resonse of machne rotor angle magntudes u magntude Freuency Plot of Rotor Angle (3) Magntude System wth 7 3 Fre/Hz System wthout Fgure 5: Freuency resonse of machne 3 rotor angle magntudes Ths aer focuses rmarly on the alcatons of the for mlementng Global Control n varous system scenaros. From the smulaton and results, t has been shown that the, wth ts varous control modes, s caable of remedyng transent stablty and voltage stablty roblems, and suressng nterarea oscllaton. It s also a sutable tool for mlementng global control strateges. These results also demonstrate the caablty of Global Control for comlex system ssues. Further work s needed on the followng areas: Evaluate the range of controllablty of each control n each artcular system stuaton; Coordnate all roblems wth otmzed weghng functons and smooth swtchng schemes; and Further exlore the oortunty of cooeratng between the PSS and to damen dfferent modes of oscllaton. APPENDIX The s secfed as follows: ACNOWLEDGEMENT The research s sonsored by Hong ong Research Grant Councl under Central Allocaton Grant Number CtyU /C. REFERENCES [] D. J. Hll, Y. Guo, N. Larsson and Y. Wang, Global Hybrd Control of Power Systems, Prernts of Bulk Power System Dynamcs and Control, August 63,, Onomch, Jaan [] Y. Guo, D. J. Hll and Y. Wang, Global Transent Stablty and oltage Regulaton for Power Systems, IEEE Trans. on Power Systems, vol. 6, no. 4, , November [3] Z. Huang, Y. N, C. M. Shen, F. F. Wu, S. Chen and B. Zhang, Alcaton of Unfed Power Flow Controller n Interconnected Power Systems Modelng, Interface, Control Strategy, and Case Study, IEEE Trans. on Power Systems, vol. 5, no.,. 8784, May [4] P. undur, Power System Stablty and Control, McGrawHll Inc., 994 [5] N. G. Hngoran and L. Gyugy, Understandng FACTS, IEEE Press, [6] M. J. Gbbard, D. J owles and P. Pourbek, Interactons Between, and Effectveness of, Power System Stablzers and FACTS Devce Stablzers n Multmachne Systems, IEEE Trans. on Power Systems, vol. 5, no., , May [7] X. Le, E. N. Lerch and D. Povh, Otmzaton and Coordnaton of Damng Controls for Imrovng System Dynamc Performance, IEEE Trans. on Power Systems, vol. 6, no. 3, , August [8] M. len, G. J. Rogers, S. Moorty and P. undur, Analytcal Investgaton of Factors Influencng Power System Stablzers Performance, IEEE Trans. on Energy Converson, vol. 7, no. 3, , Setember 99 [9] P. undur, M. len, G. J. Rogers and M. S. Zywno, Alcaton of Power System Stablzers for Enhancement of Overall System Stablty, IEEE Trans. PWRS4, 6466, May 989 [] P. M. Anderson, A. A. Fouad, Power System Control and Stablty, IEEE Press, 994 [] I. Dobson, J. Zhang, S. Greene, H. Engdahl and P. W. Sauer, Is Strong Modal Resonance a Precursor to Power System Oscllatons, IEEE Trans. On Crcuts and Systems, vol. 48, no. 3, , March [] J. H. Chow, Power System Toolbox, A Set of Coordnated Mfles for Use wth MATLAB, Cherry Tree Scentfc Software, 996