Simulation of Distributed Power-Flow Controller (Dpfc)

Transcription

1 RESEARCH INVENTY: Internatonal Journal of Engneerng and Scence ISBN: , ISSN: , Vol. 2, Issue 1 (January 2013), PP Smulaton of Dstrbuted Power-Flow Controller (Dpfc) Sarmalla Pedakotaah and Santosh A Abstract: Ths paper descrbes the steady-state response and control of power n transmsson lne equpped wth FACTS devces. Detaled smulatons are carred out on two-machne systems to llustrate the control features of these devces and ther nfluence to ncrease power transfer capablty and mprove system relablty. The DPFC s derved from the unfed power-flow controller (UPFC) and DPFC has the same control capablty as the UPFC. The DPFC can be consdered as a UPFC wth an elmnated common dc lnk. The actve power exchange between the shunt and seres converters, whch s through the common dc lnk n the UPFC, s now through the transmsson lnes at the thrd-harmonc frequency. The nteracton between the DPFC, the network and the machnes are analyzed. Keywords: FACTS, DPFC, modellng, power transmsson, AC-DC power converson, semconductor devces, power system control. I. Introducton The flexble ac trans msson system (FACTS) technology s the applcaton of power electroncs n transmsson systems. The man purpose of ths technology s to control and regulate the electrc varables n the power systems. Ths s acheved by usng converters as a controllable nterface between two power system termnals. The resultng converter representatons can be useful for a varety of confguratons. Bascally, the famly of FACTS devces based on voltage source converters (VSCs) conssts of a seres compensator, a shunt compensator, and a shunt/seres compensator. The statc Compensator (STATCOM) s a shunt connected devce that s able to provde reactve power support at a network locaton far away from the generators. Through ths reactve power njecton, the STATCOM can regulate the voltage at the connecton node. The statc synchronous seres compensator (SSSC) s a seres devce whch njects a voltage n seres wth the transmsson lne. Ideally, ths njected voltage s n quadrature wth the lne current, such that the SSSC behaves lke an nductor or a capactor for the purpose of ncreasng or decreasng the overall reactve voltage drop across the lne, and thereby, controllng the transmtted power. In ths operatng mode, the SSSC does not nterchange any real power wth the system n steady-state. The unfed power-flow controller (UPFC) s the most versatle devce of the famly of FACTS devces, snce t s able to control the actve and the reactve power, respectvely, as well as the voltage at the connecton node. The Unfed Power Flow Controller (UPFC) s comprsed of a STATCOM and a SSSC, coupled va a common DC lnk to allow b-drectonal flow of actve power between the seres output termnals of the SSSC and the shunt output termnals of the STATCOM. Each converter can ndependently generate (or) absorb reactve power at ts own AC termnal. The two converters are operated from a DC lnk provded by a DC storage capactor. The UPFC s not wdely appled n practce, due to ther hgh cost and the susceptblty to falures. Generally, the relablty can be mproved by reducng the number of components; however, ths s not possble due to the complex topology of the UPFC. To reduce the falure rate of the components, selectng components wth hgher ratngs than necessary or employng redundancy at the component or system levels. Un fortunately, these solutons ncrease the ntal nvestment necessary, negatng any cost related advantages. Accordngly, new approaches are needed n order to ncrease relablty and reduce cost of the UPFC. The same as the UPFC, the DPFC s able to control all system parameters lke lne mpedance, transmsson angle and bus voltage. The DPFC elmnates the common dc lnk between the shunt and seres converters. The actve power exchange between the shunt and the seres converter s through the transmsson lne at the thrd-harmon c frequency. The seres converter of the DPFC emp loys the dstrbuted FACTS (D- FACTS) concept. Comparng wth the UPFC, the DPFC have two major advantages: 1) Low cost because of the low voltage solaton and the low component ratng of the seres converter and 2) Hgh relablty because of the redundancy of the seres converters and hgh control capablty. DPFC can also be used to mprove the power qualty and system stablty such as power oscllaton dampng, Voltage sag restoraton or balancng asymmetry. 25

2 II. Dpfc Topology: By ntroducng the two approaches outlned n the prevous secton (elmnaton of the common DC lnk and dstrbuton of the seres converter) nto the UPFC, the DPFC s acheved. Smlar as the UPFC, the DPFC conssts of shunt and seres connected converters. The shunt converter s smlar as a STATCOM, whle the seres converter employs the DSSC concept, whch s to use multple sngle -phase converters nstead of one three-phase converter. Each converter wthn the DPFC s ndependent and has ts own DC capactor to provde the requred DC voltage. The confguraton of the DPFC s shown n Fgure 1. Fgure 1: DPFC confguraton As shown, besdes the key components - shunt and seres converters, a DPFC also requres a hgh pass flter that s shunt connected to the other sde of the transmsson lne and a transformer on each sde of the lne. The reason for these extra components wll be explaned later. The unque control capablty of the UPFC s gven by the back-to-back connecton between the shunt and seres converters, whch allows the actve power to freely exchange. To ensure the DPFC has the same control capablty as the UPFC, a method that allows actve power exchange between converters wth an elmnated DC lnk s requred. III. Dpfc Operatng Prncple Actve Power Exchange Wth Elmnated Dc Lnk Wthn the DPFC, the transmsson lne presents a common connecton between the AC ports of the shunt and the seres converters. Therefore, t s possble to exchange actve power through the AC ports. The method s based on power theory of non-snusodal co mponents. Accordng to the Fourer analyss, nonsnusodal voltage and current can be expressed as the sum of snusodal functons n dfferent frequences wth dfferent ampltudes. The actve power resultng from ths non-snusodal voltage and current s defned as the mean value of the product of voltage and current. Snce the ntegrals of all the cross product of terms wth dfferent frequences are zero, the actve power can be expressed by: P V I cos (1) 1 Where VI and I are the voltage and current at the th harmonc frequency respectvely, and s the correspondng angle between the voltage and current. Equaton (1) shows that the actve powers at dfferent frequences are ndependent from each other and the voltage or current at one frequency has no nfluence on the actve power at other frequences. The ndependence of the actve power at dfferent frequences gves the possblty that a converter wthout a power source can generate actve power at one frequency and absorb ths power from other frequences. By applyng ths method to the DPFC, the shunt converter can absorb actve power from the grd at the fundamental frequency and nject the power back at a harmonc frequency. Ths harmonc actve power flows through a transmsson lne equpped wth seres converters. Accordng to the amount of requred actve powe r at the fundamental frequency, the DPFC seres converters generate a voltage at the harmonc frequency, there by absorbng the actve power from harmonc components. Neglectng losses, the actve power generated at the fundamental frequency s equal to the power absorbed at the harmonc frequency. For a better understandng, Fgure 2 ndcates how the actve power s exchanged between the shunt and the seres converters n the DPFC system. The hgh-pass flter wthn the DPFC blocks the fundamental frequency components and allows the harmonc components to pass, thereby provdng a return path for the harmonc components. The shunt and seres converters, the hgh pass flter and the ground form a closed loop for the harmonc current. 26

3 IV. Usng Thrd Harmonc Components Fgure 2: Actve power exchange between DPFC converters Due to the unque features of 3rd harmonc frequency components n a three-phase system, the 3rd harmonc s selected for actve power exchange n the DPFC. In a three-phase system, the 3rd harmonc n each phase s dentcal, whch means they are zero -sequence components. Because the zero-sequence harmonc can be naturally blocked by trans- formers and these are wdely ncorporated n power systems (as a means of changng voltage), there s no ext ra flter requred to prevent harmonc leakage. As ntroduced above, a hghpass flter s requred to make a closed loop for the harmonc current and the cut off frequency of ths flter s approxmately the fundamental frequency. Because the voltage solaton s hgh and the harmonc frequency s close to the cut off frequency, the flter wll be costly. By usng the zero-sequence harmonc, the costly flter can be replaced by a cable that connects the neutral pont of the transformer on the rght sde n Fgure 2 wth the ground. Because the -wndng appears open-crcut to the 3rd harmonc current, all harmonc current wll flow through the Y- wndng and concentrate to the groundng cable as shown n Fgure 3. Therefore, the large hgh-pass flter s elmnated. Fgure 3: Utlze grounded transformer to flter zero-sequence harmonc Another advantage of usng the 3rd harmonc to exchange actve power s that the groundng of the transformers can be used to route the harmonc current n a meshed network. If the network requres the harmonc current to flow through a specfc branch, the neutral pont of the the sde opposte to the shunt converter, wll be grounded and vce versa. transformer n that branch, at Fgure 4 shows a smple example of routng the harmonc current by usng the groundng of the transformer. Because the floatng neutral pont s located on the transformer of the lne wthout the seres converter, t s an open-crcut for 3rd harmonc components and therefore no 3rd harmonc current wll flow through ths lne. Fgure 4: Route the harmonc current by usng the groundng of the transformer 27

4 The harmonc at the frequences lke 3rd, 6th, 9th... are all zero-sequence and all can be used to exchange actve power n the DPFC. However, the 3rd harmonc s selected, because t s the lowest frequency among all zero-sequence harmoncs. The relatonshp between the exchanged actve power at the th harmonc frequency P and the voltages generated by the converters s expressed by the well known the power flow equaton and gven as: V V sh, se, P sn( sh, se, X ) (2) Where X s the lne mpedance at th frequency, and s the voltage magntudes of the harmonc of the shunt and seres converters, and s the angle dfference between the two voltages. As shown, the mpedance of the lne lmts the actve power exchange capacty. To exchange the same amount of actve power, the lne wth hgh mpedance requres hgher voltages. Because the transmsson lne mpedance s mostly nductve and proportonal to frequency, hgh transmsson frequences wll cause hgh mpedance and result n hgh voltage wthn converters. Consequently, the zero -sequence harmonc wth the lowest frequency - the 3rd harmonc - has been selected. V. Dpfc Control To control multple converters, a DPFC conssts of three types of controllers: central control, shunt control and seres control, as shown n Fgure 5. Fgure 5: DPFC control block dagram The shunt and seres control are localzed controllers and are responsble for mantanng ther own converters parameters. The central control takes care of the DPFC functons at the power system level. The functon of each controller s lsted: Central control: The central control generates the reference sgnals for both the shunt and seres converters of the DPFC. Its control functon depends on the specfcs of the DPFC applcaton at the power system level, such as power flow control, low frequency power oscllaton dampng and balancng of asymmetrcal components. Accordng to the system requrements, the central control gves correspondng voltage reference sgnals for the seres converters and reactve current sgnal for the shunt converter. All the reference sgnals ge nerated by the central control concern the fundamental frequency components. Seres control: Each seres converter has ts own seres control. The controller s used to mantan the capactor DC voltage of ts own converter, by usng 3 rd harmonc frequency components, n addton to generatng seres voltage at the fundamental frequency as requred by the central control. Shunt control: The objectve of the shunt control s to nject a constant 3 rd harmonc current nto the lne to supply actve power for the seres converters. At the same tme, t mantans the capactor DC voltage of the shunt converter at a constant value by absorbng actve power from the grd at the fundamental frequency and njectng the requred reactve current at the fundamental frequency nto the grd. VI. Laboratory Results Smulaton Model of the seres converter control 28

5 RESEARCH INVENTY: Internatonal Journal of Engneerng and Scence ISBN: , ISSN: , Vol. 2, Issue 1 (January 2013), PP An expermental setup has been bult to verfy the prncple and control of the DPFC. One shunt converter and sx sngle phase seres converters are bult and tested n a scaled network, as shown n Fg. 7. Two solated buses wth phase dfference are connected by the lne. Wthn the expermental setup, t he shunt converter s a sngle-phase nverter that s connected between the neutral pont of the Y Δ transformer and the ground. The nverter s powered by a constant dc-voltage source. The specfcatons of the DPFC expermental setup are lsted n the Table I. Fg. 7. DPFC expermental setup crcut Table I: Specfcaton Of The Dpfc Expermental Setup Wthn the setup, multple seres converters are controlled by a central controller. The central controller gves the reference voltage sgnals for all seres converters. The voltages and currents wthn the setup are measured by an osclloscope and processed n computer by usng the MATLAB To verfy the DPFC prncple, two stuatons are demonstrated: the DPFC behavour n steady state and the step response. In steady state, the seres converter s controlled to nsert a voltage vector wth both d- and q- component, whch s Vse,d,ref = 0.3 V and Vse,q,ref = 0.1 V. Fgs show one operaton pont of the DPFC setup. For clarty, only the waveforms n one phase are shown. The voltage njected by the seres converter, the current through the lne, and the voltage and current at the Δ sde of the transformer are llustrated. Fg. 8. DPFC operaton n steady state: lne current. Fg. 9. DPFC operaton n steady state: seres converter voltage. 29

6 Fg. 10. DPFC operaton n steady state: bus voltage and current at the Δ sde of the transformer. The constant thrd-harmonc current njected by the shunt converter evenly dsperses to the three phases and s supermposed on the fundamental current, as shown n Fg. 8. The voltage njected by the seres converter also contans two frequency components n Fg. 9. The ampltude of the pulse wdth modulated (PWM) waveform represents the dc-capactor voltage, whch s well mantaned by the thrd-harmonc component n steady state. As shown, the dc voltage has a small oscllaton; however, t does not nfluence the DPFC control. Fg. 10 demonstrates the thrd-harmonc flterng by the Y Δ transformers. There s no thrdharmonc current or voltage leakng to the Δ sde of the transformer. The DPFC controls the power flow through transmsson lnes by varyng the voltage njected by the seres converter at the fundamental frequency. Fgs llustrate the step response of the expermental setup. A step change of the fundamental reference voltage of the seres converter s made, whch conssts of both actve and reactve varatons, as shown n Fg. 11. As shown, the dc voltage of the seres converter s stablzed before and after the step change. To verfy f the seres converter can nject or absorb actve and reactve power from the grd at the fundamental frequency, the power s calculated from the measured voltage and current n Fgs. 12 and 13. The measured data n one phase are processed n the computer by usng MATLAB. To analyze the voltage and current at the fundamental frequency, the measured data that contans harmonc dstorton are fltered by a low-pass dgtal flter wth the 50-Hz cut off frequency. Because of ths flter, the calculated voltage and current at the fundamental frequency have a 1.5 cycle delay to the actual values, thereby causng a delay of the measured actve and reactve power. Fg. 14 llustrated the actve and reactve power njected by the seres converter. A comparson s made between the measured power and the calculated power. We can see that the seres converters are able to absorb and nject both actve and reactve power to the grd at the fundamental frequency. Fg. 11. Reference voltage for the seres converters. Fg. 12. Step response of the DPFC: seres converter voltage. Fg. 13. Step response of the DPFC: lne current. 30

7 Fg. 14. Step response of the DPFC: actve and reactvse power njected by the seres converter at the fundamental frequency. Fg. 15. Step response of the DPFC: bus voltage and current at the Δ sde of the transformer. Concluson Ths paper has presented a new concept called DPFC. The DPFC emerges from the UPFC and nherts the control capablty of the UPFC, whch s the smultaneous adjustment of the lne mpedance, the transmsson angle, and the bus-voltage magntude. The common dc lnk between the shunt an d seres converters, whch s used for exchangng actve power n the UPFC, s elmnated. Ths power s now transmtted through the transmsson lne at the thrd-harmonc frequency. The seres converter of the DPFC employs the D-FACTS concept, whch uses multple s mall sngle-phase converters nstead of one large-sze converter. The relablty of the DPFC s greatly ncreased because of the redundancy of the seres converters. The total cost of the DPFC s also much lower than the UPFC, because no hgh -voltage solaton s requred at the seres-converter part and the ratng of the components of s low. The DPFC concept has been verfed by an expermental setup. It s proved that the shunt and seres converters n the DPFC can exchange actve power at the thrd-harmonc frequency, and the seres converters are able to nject controllable actve and reactve power at the fundamental frequency. References [1] Y.-H. Song and A. Johns, Flexble ac Transmsson Systems (FACTS) (IEE Power and Energy Seres), vol. 30. London, U.K.: Insttuton of Electrcal Engneers, [2] N. G. Hngoran and L. Gyugy, Understandng FACTS: Concepts and Technology of Flexble AC Transmsson Systems. New York: IEEE Press, [3] L.Gyugy, C.D. Schauder, S. L.Wllams, T. R. Retman,D. R. Torgerson, And A. Edrs, The unfed power flow controller: A new approach to power transmsson control, IEEE Trans. Power Del., vol. 10, no. 2, pp , Apr [4] A.-A. Edrs, Proposed terms and defntons for flexble ac transmsson system (facts), IEEE Trans. Power Del., vol. 12, no. 4, pp , Oct [5] K. K. Sen, Sssc-statc synchronous seres compensator: Theory, modelng, and applcaton, IEEE Trans. Power Del., vol. 13, no. 1, pp , Jan [6] M. D. Deepak, E. B. Wllam, S. S. Robert, K. Bll, W. G. Randal, T. B. Dale, R. I. Mchael, and S. G. Ian, A dstrbuted statc seres compensator system for realzng actve power flow control on exstng power lnes, IEEE Trans. Power Del., vol. 22, no. 1, pp , Jan [7] D. Dvan and H. Johal, Dstrbuted facts A new concept for realzng grd power flow control, n Proc. IEEE 36th Power Electron. Spec. Conf. (PESC), 2005, pp [8] Y. Zhhu, S.W. H. de Haan, and B. Ferrera, Utlzng dstrbuted power flow controller (DPFC) for power oscllaton dampng, n Proc. IEEE Power Energy Soc. Gen. Meet. (PES), 2009, pp [9] Y. Zhhu, S. W. H. de Haan, and B. Ferrera, Dpfc control durng shunt converter falure, n Proc. IEEE Energy Convers. Congr. Expo. (ECCE) 2009, pp [10] Y. Sozer and D. A. Torrey, Modelng and control of utlty nteractve nverters, IEEE Trans. Power Electron., vol. 24, no. 11, pp , Nov

8 BIOGRAPHY Sarmalla Pedakotaah, II-M.Tech P.E, Department of EEE, SV Engneerng College, Suryapet. Santosh A, M.Tech P.E, Department of EEE, SV Engneerng College, Suryapet. 32