Modelling and Simulation of STATCOM for Reactive Power and Voltage Control.

Transcription

1 Journal of Multdscplnary Engneerng Scence and Technology (JMEST) ol. Issue, February Modellng and Smulaton of STTOM for Reactve Power and oltage ontrol.. U. Musa Dept. of Electrcal and Electronc Engneerng Unversty of Madugur Madugur, gera bstract The contnuous demand n electrc power system network has caused the system to be heavly loaded leadng to voltage nstablty. However, Flexble lternatng urrent Transmsson System (FTS) devces provde the necessary features to avod techncal problems n the power systems. They also ncrease the transmsson lne effcency. mong the FTS famly s Statc Synchronous ompensator (STTOM). It njects the compensatng current n phase quadrature wth lne voltage, t can emulate as nductve or capactve reactance so as to produce capactve power for the grd or draws nductve power from the ac grd to control power flow n the lne. Ths paper descrbes the approach of a unt connected FTS devce (STTOM), n whch the devce s modelled and used to provde controllable bus voltage and reactve power compensaton. Smulaton results obtaned usng MTL/SIMULK for selected 3-bus- 3-machne 500 k nterconnected power system. The fndngs reveal that the performance and behavour of the STTOM n controllng bus voltage and reactve power on 500 k transmsson network was effectve. The smulated results ow the advantages of usng STTOM. Keywords Flexble Transmsson System (FTS); Statc Synchronous ompensator (STTOM); Reactve Power ompensator; us oltage I. ITRODUTIO In nterconnected power systems, whch today are very complex, there s great need to mprove utlzaton whle stll mantanng relablty and securty. Whle some transmsson lnes are charged up to the lmt load, the others may have been overloaded, whch have an effect on the values of voltage and reduces system stablty and securty. Transmsson networks of present power systems are becomng progressvely more stressed because of ncreasng demand and lmtatons on buldng new lnes. One of the consequences of such a stressed system s the rsk of losng stablty followng a dsturbance. For ths reason, t s very mportant to control the power flows along transmssons lnes to meet transfer of power needs. Summarzng the current development, t must be Musa Mustapha Dept. of Electrcal and Electronc Engneerng Unversty of Madugur Madugur, gera musamustyy@gmal.com notced that both plannng and operaton of electrc networks are undergong fundamental and radcal changes n order to cope wth the ncreased complexty of fundng economc and relable network solutons. The necessty to desgn power system networks that provde the mal transmsson capacty and at the same tme resultng n mnmal cost s a great engneerng challenge. Despte that the reactve power problem have been successfully employed n some sample power system [1] and []. Stll much need to be done to ensure effectve and effcent dstrbuton of reactve power and voltage control n an electrc network. In an attempt to crcumvent the defcences of the conventonal methods, FTS technology whch was developed by [3] offers consderable advantage over the conventonal one's n terms of space reducton and performance [4]. FTS are a result of the development n the power electronc area and am to rapdly control electrcal sgnal [5]. Ths paper deals wth modelng and smulaton of Statc Synchronous ompensators, (STTOM), a unt compensaton devce from the famly of Flexble lternatng urrent Transmsson System, FTS. STTOM or Statc Synchronous ompensator s used for voltage compensaton at the recever end of a transmsson lnes, thus replacng banks of unt capactors. When used for ths purpose, STTOMs offer a number of advantages over banks of unt capactors, such as much tghter control of the voltage compensaton at the recever end of the ac transmsson lne and ncreased lne stablty durng load varatons [6]. It s buld based on power electroncs oltage Source onverter and can act as ether a source or snk of reactve power whch s ted to a transmsson lne. The STTOM regulates the voltage magntude at ts termnals by controllng the amount of reactve power njected nto or absorbed from the power system. When system voltage s low, the STTOM generates reactve power (STTOM capactve). When system voltage s hgh, t absorbs reactve power (STTOM nductve). So the unt controller s therefore a good way to control voltage n and around the pont of connecton through njecton of reactve current (leadng or laggng) alone or a combnaton of actve and reactve current for a more effectve voltage control and dampng of voltage dynamcs [7]. The real power (P) and reactve power (Q) are gven by:. sn (1) JMEST

2 Journal of Multdscplnary Engneerng Scence and Technology (JMEST) ol. Issue, February Q cos () Where E s the lne voltage of transmsson lne Is the generated voltage of the oltage Source onverter. X Is the equvalent reactance of nterconnecton transformer Is the phase angle of E wth respect to. The basc element s the Source oltage onverter (S) whch converts an nput D voltage to an voltage at the fundamental frequency wth a gven magntude and a controllable phase. The output voltage s dynamcally controlled n order to provde the requred reactve power to the network [8].. Operatonal Prncple of STTOM The fg. 1 ows the STTOM te on to a transmsson lne. Grd us Step-down Transformer 3ϕ Lne ontroller STTOM us E Fg.1. STTOM connected to the power system va couplng transformer. The STTOM consst of a couplng transformer,converter/nverter crcut, and a D capactor, whch s own n fg.1 above. For such an arrangement, the deal steady state analyss assumed that the actve power exchange between the system and the STTOM can be neglected, and only the reactve power can be exchanged between them. When the ampltude of the output voltage s rased over the system voltage, then the current flows va reactance from the nverter to the system and the nverter produces capactve power for the grd. On the other hand, f the output voltage ampltude s decreased under that of the grd, then the reactve current flows from the system to the nverter and the nverter draws nductve power. In addton, when the output voltage ampltude s balanced to the grd voltage, the reactve power flows become zero. The output of reactve current strongly depends on the thyrstor frng angle α that s gven by the phase ft between the STTOM-voltage E and the bus voltage. Dependng on ths frng angle α, the chargng state of the dc- capactor s changed and therefore the ampltude of the STTOM bus voltage E changes. The ampltude dfference of the STTOM bus voltage and the grd voltage together wth the transformer leakage reactance X T determnes the ampltude of reactve current that s njected nto the power system gven n equaton (3). α dc X T 3 In the same way, there s exchange between the reactve power of the nverter and the system, whch can be controlled by varyng the magntude of the output voltage [7, 8].. Mult ontrol Functon of STTOM In the practcal applcaton of a STTOM, t may be used for controllng one of the followng Parameters. 1. oltage magntude of the local bus to whch the STTOM s connected.. Reactve power njecton to the local bus, to whch the STTOM s connected. 3. Impedance of the STTOM. 4. oltage Injecton. 5. oltage magntude at a remote bus. 6. Power flow. 7. pparent power or current control of a local or remote transmsson lne. mong these control optons, control of the voltage of the local bus whch the STTOM s connected to, s the most recognzed control functon. The other control possbltes have not fully been nvestgated n power flow analyss [9, 10, 11]. II. METHODOLOGY. STTOM Model ased on the operatng prncple of the STTOM, the equvalent crcut can be derved, whch s gven n fg.. In the dervaton, t s assumed that,1.harmonc generated by the STTOM s neglected. The system as well as the STTOM s assumed three phase balanced The STTOM can be equvalently represented by a controllable fundamental frequency postve sequence voltage source. In prncple, the STTOM output voltage can be regulated such that the reactve power of the STTOM can be changed. P (Pont of ommon onnecton) Fg.. STTOM Equvalent rcut. + - us jq ased on the equvalent crcu, t can be JMEST

3 L3= 500k 180Km establed, s the voltage at bus, I s the current through the STTOM unt converter. P and Q are the unt converter branch actve and reactve power flows respectvely. The power flow drecton of P and Q s leavng bus. Z s the equvalent STTOM unt couplng transformer mpedance. From the equvalent crcut,suppose [9] then the power and flow constrants of the STTOM are: Q g Where b ( g ( g cos( ) b g jb 1 Z sn( ) b sn( cos( The operatng constrant of the STTOM s the actve power exchange va the D lnk as descrbed by: ( I ) ( I e e ) g ( g cos( ) b sn( The prncple of operaton of S based STTOMdepends on the control strategy for regulatng the nterchange of power between the converter/nverter crcut and the grd and t depends also on the output voltage of the converter/nverter crcut. If the magntude of the voltage of the converter s equal to the voltage of the grd, the nterchange of reactve power between the STTOM and the grd s equal to zero. In contrast, f the voltage of the converter s less than the grd voltage at pont of common connecton (P), the STTOM absorbs reactve power (draws laggng current). However, f the STTOM controlled happens to be n such a way that the output voltage of the converter s hgher than the grd voltage at P,, reactve power s njected nto the grd. lso, note that the capacty for njectng reactve power nto the grd s lmted by the mumvoltage and the mum current allowed by the semconductors [9]. In practce, t s also necessary to control the actve power exchange of the STTOM by regulatng the phase angle between the voltage at the S ( ) and the voltage at the Ps so that the S absorbs actve power (4) (5) (6) Journal of Multdscplnary Engneerng Scence and Technology (JMEST) ol. Issue, February The S output voltage must fall wthn the allowed lmts of operaton: mn, Where s the voltage ratng of the STTOM, Whle mn s the mnmal voltage lmt of the STTOM..The current flowng through a STTOM I, must be less than the current ratng: that s Where s the current ratng of the STTOM converter whle I s the magntude of current through the STTOM and gven by In contrast, t s necessary to nclude external restrcton of the grd voltage at the P. ccordng to the specfc regulaton of the grd operator, the grd voltage at the P must be wthn certan mn allowed lmts. [9].. Modellng and Smulaton of 500 k System ase Study. To study a system, t s sometmes possble to experment wth the system tself. The goal of the system smulaton s to predct how a system performs when t s buld. So, t s not feasble to experment wth a system when t s already put nto use. It s very costly, dangerous and often mpossble to make experment wth real systems. Provded that models are adequate descrptons of realty (they are vald), expermentng wththem can save money, sufferng and tme.the block buldng prncple employed n modelng help organze system descrpton by solatng subsystem and dentfyng ther nput and output.fg. 3 ows the sngle lne dagram of system case study model whch s used to analyse the mpact of STTOM on the 500 k power network. 8500M Staton 300MW us 1 L= 500k 75km STTOM 6500M Staton us 00MWW from the grd to mantan a constant voltage for the D lnk [9, 11].. Restrcton of Operaton 1. In a STTOM, the mum reactve power that can be suppled to the grd depends on the mum voltage and current permtted by the power sem-conductor, so t s necessary to nclude the followng restrcton. L1= 500k 40km 9000M Staton us 3 Fg. 3. Sngle lne dagram of the proposed nterconnected power system wth STTOM nstalled as system study. JMEST

4 The STTOM s used to control the power flow along planned 500 k nterconnected lne. The STTOM s nstalled at the us 1of the 40km lne 1 planned transmsson lne between 500 k us 1 (Secton ) and us 3 (Secton ). The STTOM s employed to regulate 1 us voltage at (Secton ), Journal of Multdscplnary Engneerng Scence and Technology (JMEST) ol. Issue, February and reactve power flow through 1. It conssts of 100- M, three-level, 48-pulse GTO-based converter; t's connected n unt at bus 1. The MTL / SIMULIK model of the case study system s own n Fg. 4. L1= 500 k _40 km Staton (8500 M) Power System Equvalent a a b b 300 MW L= 500 k _75km c L3= 500 k_180 km 00 MW Staton (6500M) Power System Equvalent c 3 Staton (9000M) Power System Equvalent p dcp Pulses Q a b m dcm c a,aia asec, a IaPrm (pu) STTOM 500 k, 100M Q (Mvar) Q (Mvar) mes ref (pu) meas ref (pu) [abc_1] abc dc dc [Iabc_1] Iabc Pulses Sgnals and Scopes STTOM dcp Multmeter STTOM ontroller +100 Mvar/-100 Mvar 48-pulse GTO STTOM The Typcal Overhead Transmsson Lne parameters of the 500 k Transmsson lne system was taken from [1]. III. RESULTS D DISUSSIO The smulaton was prepared usng MTL/SIMULIK package avalable n MTL 9.1. The STTOM characterstc s smulated for bus voltage and reactve power flow control. From fg. 5, STTOM s n the voltage control mode and ts reference voltage s set to ref = 1.0 P.U. The voltage drop of the regulator s 0.03pu/100. Therefore when the STTOM operatng pont changes from fully capactve (+100 Mvar) to fully nductve (-100 Mvar) the STTOM voltage vares between = 0.97pu and = 1.03pu. Fg. 4. MTL/SIMULIK Study System Model usng STTOM [13] bus voltage of fg. 5 at 0.979pu. 100-Mvar STTOM regulates voltage on the bus 3 of a 500 k nterconnected power system. Result n fg. 5 reveals that, when the bus voltage 3 s 1.0pu, the STTOM s out of servce. Thus, f the reference voltage ref s set to 1.0pu, the STTOM doesn t exhbt current nterchange (zero current). Fg. 5. STTOM oltage Magntude Measurement ( p.u) Result n fg. 5 ows the STTOM voltage source 1.0p.u at tme t= 0 second, whle fg. 6 reveals reactve power at 0-Mvar at tme t= 0 sec respectvely. t t = 0.1 second, the voltage source of fg. 5 suddenly decrease to 0.955pu. s a result, Fg. 6 dsplay STTOM reacton by generatng reactve power of Q=+70Mvar (capactve mode) n order to keep the Fg. 6. hange n Reactve Power (Mvar) t tme t= 0. second, fg. 5 reveals a rapd ncrease of source voltage from 0.979p.u to 1.03p.u. s a result, fg. 6 dsplays STTOM reacton by changng ts operatng pont from capactve mode (+70Mvar) to nductve mode (-75Mvar) to keep voltage at 1.01pu. t ths pont the STTOM absorbs -75Mvar as own n fg. 6. t tme t=0.3second, fg. 5 and 6 also dsplayed how the voltage source s set back to ts nomnal value, and that of STTOM operatng pont comesup to 0 Mvar respectvely. JMEST

5 Fg. 7.ehavour of the D bus voltage (k) Fnally, result n fg. 7 ows the behavour of D bus voltage. t tme t= 0.1 second of fg. 5 when a arp drop n source voltage s notced, the fg. 7 reveals D voltage response by rasng from 19.3 k to 0.4 k to keep the voltage wthn the operatng range. On the other hand, at t=0. sec of fg. 5, when a sudden ncrease n voltage s observed, the D voltage has lowered to 18. k. t ths pont, the STTOM absorbs reactve power. I. OLUSSIO The STTOM s a unt devce used n mprovng the bus voltage profle. It s commonly used to mantan a constant voltage across ac transmsson lnes and also serve as automatc reactve power control. The MTL/SIMULIK envronment was used to smulate a model of power system wth STTOM connected to an nterconnected power system. The control and performance of STTOM ntended for nstallaton on a transmsson lne for power qualty mprovement s presented. The STTOM dynamc response s very fast (n mllsecond) and able to pass from capactve mode of operaton to an nductve one n a few cycles. When the source voltage decreases, the STTOM reacts by generatng reactve power, so the D voltage ncreases; ths s the capactve mode. On the other hand, when the voltage ncreases, the STTOM reacts by absorbng the reactve power, so the D voltage decreased. Ths s the nductve mode. Smulaton results ow the effectveness of STTOM for regulatng bus voltage and control reactve power flow through the lne. REFEREES [1] G.. akare, G. K. enayagamoorthy and U. O. lyu "Reactve power and voltage control of the geran grd system usng Mcro-Genetc lgorthm" Proceedngs of IEEE Power Engneerng Socety Journal of Multdscplnary Engneerng Scence and Technology (JMEST) ol. Issue, February General Meetng, 005. [] M.. H. El-Sayeed, "Ruled based approach for real tme reactve power control n nterconnected power system", Expert System wth pplcatons, 14, 1998, pp [3].G. Hngoran G. Gyugy Lazlo "Understandng FTS: oncepts & Technology of Flexble Transmsson Systems "IS ( ). [4] Rcardo Da`valos Mar`n, Detaled analyss of a mult-pulse STTOM, Pre-doctoral thess, May 003. [5] Senzo Mkhze, Prof.. S. Rgby, DSP-ased control of STTOM: fnal report, Unversty of Kwazulu-natal, Faculty of Engneerng, ov [6] S. Hadjer, Fatha Ghezal and S.. Zd: Smulaton of a three level 48 pulses STTOM. [7] Mchael Merkle and mr M. Mr: Detaled dynamc modellng of a 50Mr STTOM va network smulaton n tme doman : Proc. of IEEE anadan onference of Electrcal and omputer Engneerng, 001. [8] Md. azrul Islam, M.d rfur Kabr and Yaro Kazuge: Desgn and smulaton of STTOM to mprove power qualty. Internatonal Journal of Innovaton and ppled Studes. ISS ol.3 o. 3 July 013, pp [9] Xao-Png Zhang, hrstan Rehtanz, ka Pal. Flexble Transmsson Systems: Modellng and ontrol (006). Sprnger-erlag erln, Hedelberg 006. [10] Statc Synchronous ompensator (STTOM) ourseware Sample by Lab-olt Ltd. [11] depoju G.. Komolafe O.. nalyss and Modellng of STTOM: omparson of Power Injecton and urrent Injecton Models n Power Flow Study Internatonal Journal of dvanced Technology ol. 36 ov [1] Prabha Shankar Kundur, Power System Stablty and control, Power System Engneerng Seres R.R. Donnelly and Sons ompany. IS X [13] MTL software- matlabcentral. JMEST