PV cell & STATCOM control technique for grid connected wind energy system to improve power quality

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1 P cell & STATCOM cotrol techique for grid coected wid eergy system to improve power quality Y.kameswara rao 1, Akula.Prada rao, Kadukuri.Sudheer 3 1PG Scholar, Dept. of EEE, ITAM College of Egieerig, ikhapatam, AP, Idia Associate Professor, Dept. of EEE, ITAM College of Egieerig, ikhapatam, AP, Idia 3Assistat Professor, Dept. of EEE, ITAM College of Egieerig, ikhapatam, AP, Idia Abstract Ijectig wid power ito a electric grid affects the power quality of the grid. The wid turbie i the grid system will ifluece the power quality of the system. The power quality measuremets deped o the performace of the wid turbie. Active power, reactive power, variatio of voltage, flicker, harmoics are the measuremets which are iflueced, ad these are measured accordig to atioal/iteratioal guidelies. The paper study demostrates the power quality problem due to istallatio of wid turbie with the grid. I this proposed scheme a P (Photo oltaic) cell alog with STATCOM (Static Competor) is coected at a poit of commo couplig with a BESS (battery eergy storage system) to mitigate the power quality issues. The battery eergy storage is itegrated to sustai the real power source uder fluctuatig wid power. The proposed cotrol scheme for the grid coected wid eergy geeratio system for power quality improvemet is simulated usig MATLAB/SIMULINK i power system block set. The effectiveess of the proposed scheme relives the mai supply source from the reactive power demad of the load ad the iductio geerator ad provides uity power factor at the source side. Idex Terms Iteratioal electro-techical commissio (IEC), power quality, wid geeratig system (WGS), photo voltaic cell(p). I. INTRODUCTION To have sustaiable growth ad social progress, it is ecesry to meet the eergy eed by utilizig the reewable eergy resources like wid, biomass, hydro, co-geeratio, etc I sustaiable eergy system, eergy coservatio ad the use of reewable source are the key paradigm. The eed to itegrate the reewable eergy like wid eergy ito power system is to make it possible to miimize the evirometal impact o covetioal plat [1]. The itegratio of wid eergy ito existig power system presets techical challeges that require cosideratio of voltage regulatio, stability, power quality problems. The power quality is a essetial customer- focused measure ad is greatly affected by the operatio of a distributio ad trasmissio etwork. The issue of power quality is of great importace to the wid turbie. I the fixed-speed wid turbie operatio, all the fluctuatio i the wid speed are trasmitted as fluctuatios i the mechaical torque, electrical power o the grid ad leads to large voltage fluctuatios. Durig the ormal operatio, wid turbie produces a cotiuous variable output power. These power variatios are maily caused by the effect of turbulece, wid shear, ad tower-shadow ad of cotrol system i the power system. Thus, the etwork eeds to maage for such fluctuatios. The power quality issues ca be viewed with respect to the wid geeratio, trasmissio ad distributio etwork, such as voltage g, swells, flickers, harmoics etc. However the wid geerator itroduces disturbaces ito the distributio etwork. Oe of the simple methods of ruig a wid geeratig system is to use the iductio geerator coected directly to the grid system. The iductio geerator has iheret advatages of cost effectiveess ad robustess.however; iductio geerators require reactive power for magetizatio. Whe the geerated active power of a iductio geerator is varied due to wid, absorbed reactive power ad termial voltage of a iductio geerator ca be sigificatly affected. A proper cotrol scheme i wid eergy geeratio system is required uder ormal operatig coditio to allow the proper cotrol over the active power productio. I the evet of icreasig grid disturbace, a battery eergy storage system for wid eergy geeratig system is geerally required to compete the fluctuatio geerated by wid turbie. A P cell STATCOM based cotrol techology has bee proposed for improvig the power quality which ca techically maage the power level with the commercial wid turbies. This proposed P cell STATCOM cotrol scheme for grid coected wid eergy geeratio for power quality improvemet has followig objectives. Uity power factor at the source side. Reactive power support oly from STATCOM to wid Geerator ad Load. Simple bag-bag cotroller for STATCOM to achieve fast dyamic respose. The paper is orgaized as fallows. The Sectio II itroduces the power quality issues ad its cosequeces of wid turbie. The Sectio III itroduces the grid coordiatio rule for grid quality limits. The Sectio I describes the topology for power quality improvemet. The Sectios, I, II describes the cotrol scheme, simulik discussio ad coclusio respectively. 3785

2 A. oltage ariatio II. POWER QUALITY ISSUES The voltage variatio issue results from the wid velocity ad geerator torque. The voltage variatio is directly related to real ad reactive power variatios. The voltage variatio is commoly classified as uder: oltage Sag/oltage Dips. oltage Swells. Short Iterruptios. Log duratio voltage variatio. The voltage flicker issue describes dyamic variatios i the etwork caused by wid turbie or by varyig loads. Thus the power fluctuatio from wid turbie occurs durig cotiuous operatio. The amplitude of voltage fluctuatio depeds o grid stregth, etwork impedace, ad phase-agle ad power factor of the wid turbies. It is defied as a fluctuatio of voltage i a frequecy Hz. The IEC specifies a flicker meter that ca be used to measure flicker directly. B. Harmoics The harmoic results due to the operatio of power electroic coverters. The harmoic voltage ad curret should be limited to the acceptable level at the poit of wid turbie coectio to the etwork. To esure the harmoic voltage withi limit, each source of harmoic curret ca allow oly a limited cotributio, as per the IEC guidelie. The rapid switchig gives a large reductio i lower order harmoic curret compared to the lie commutated coverter, but the output curret will have high frequecy curret ad ca be easily filter-out. C. Wid Turbie Locatio i Power System The way of coectig the wid geeratig system ito the power system highly iflueces the power quality. Thus the operatio ad its ifluece o power system deped o the structure of the adjoiig power etwork. D. Self Excitatio of Wid Turbie Geeratig System The self excitatio of wid turbie geeratig system (WTGS) with a asychroous geerator takes place after discoectio of wid turbie geeratig system (WTGS) with local load. The risk of self excitatio arises especially whe WTGS is equipped with competig capacitor. The capacitor coected to iductio geerator provides reactive power competio. However the voltage ad frequecy are determied by the balacig of the system. The didvatages of self excitatio are the fety aspect ad balace betwee real ad reactive power [5]. E. Cosequeces of the Issues The voltage variatio, flicker, harmoics causes the malfuctio of equipmets amely microprocessor based cotrol system, programmable logic cotroller; adjustable speed drives, flickerig of light ad scree. It may leads to trippig of cotractors, trippig of protectio devices, stoppage of sesitive equipmets like persoal computer, programmable logic cotrol system ad may stop the process ad eve ca damage of sesitive equipmets. Thus it degrade the power quality i the grid. III. GRID COORDINATION RULE The America Wid Eergy Associatio (AWEA) led the effort i the uited state for adoptio of the grid code for the itercoectio of the wid plats to the utility system. The first grid code was focused o the distributio level, after the blackout i the Uited State i August 003. The Uited State wid eergy idustry took a stad i developig its ow grid code for cotributig to a stable grid operatio. The rules for realizatio of grid operatio of wid geeratig system at the distributio etwork are defied as-per IEC The grid quality characteristics ad limits are give for refereces that the customer ad the utility grid may expect. Accordig to Eergy-Ecoomic Law, the operator of trasmissio grid is resposible for the orgaizatio ad operatio of itercoected system [6]. 1) oltage Rise (u): The voltage rise at the poit of commo couplig ca be approximated as a fuctio of maximum apparet power S of the turbie, the grid impedaces R max ad X at the poit of commo couplig ad the phase agle [7], give i (1) Rcos X si U u S (1) max / S max max. apparet power, Where u voltage rise, phase differece is the omial voltage of grid. The Limitig voltage rise value is %. ) oltage Dips (d): The voltage dips is due to start up of wid turbie ad it causes a sudde reductio of voltage. It is the relative% voltage chage due to switchig operatio of wid turbie. The decrease of omial voltage chage is (). S d K () u Sk Where d is relative voltage chage, rated apparet power, short circuit apparet power, ad sudde voltage reductio factor. The acceptable voltage dips limitig value is 3%. 3) Flicker: The measuremets are made for maximum umber of specified switchig operatio of wid turbie with 10-mi period ad -h period are specified, as give i (3) Where P S lt C k (3) Sk P log term flicker. lt C Flicker coefficiet calculated from Rayleigh distributio of the wid speed. The Limitig alue for flicker coefficiet is about, for average time of h [8]. 4) Harmoics: The harmoic distortio is assessed for variable speed turbie with a electroic power coverter at the poit of commo coectio [9]. The total harmoic voltage distortio of voltage is give as i (4): k 3786

3 40 THD 100 (4) 1 where is the th harmoic voltage ad is the fudametal frequecy (50) Hz. The THD limit for 13 K is < 3 %. THD of curret is give as i (5) I I 100 THD (5) I 1 Where I is the th harmoic curret ad I 1 is the fudametal frequecy (50) Hz. The THD of curret ad limit for 13 K is <.5 %. 5) Grid Frequecy: The grid frequecy i Idia is specified i the rage of Hz, for wid farm coectio. The wid farm shall able to withstad chage i frequecy up to 0.5 Hz/s [9]. I. TOPOLOGY FOR POWER QUALITY IMPROEMENT The P cell STATCOM based curret cotrol voltage source iverter ijects the curret ito the grid i such a way that the source curret are harmoic free ad their phase-agle with respect to source voltage has a desired value. The ijected curret will cacel out the reactive part ad harmoic part of the load ad iductio geerator curret, thus it improves the power factor ad the power quality. To accomplish these goals, the grid voltages are sesed ad are sychroized i geeratig the curret commad for the iverter. The proposed grid coected system is implemeted for power quality improvemet at poit of commo couplig (PCC), as show i Fig. 1. The grid coected system i Fig. 1, cosists of wid eergy geeratio system ad battery eergy storage system with pv cell ad statcom. protectio agaist short circuit. The available power of wid eergy system is preseted as uder i (6). 1 3 Pwid A wid (6) 3 Where ( kg / m ) is the air desity ad A ( m ) is the area swept out by turbie blade, is the wid speed i wid mtr/s. It is ot possible to extract all kietic eergy of wid, thus it extract a fractio of power i wid, called power coefficiet Cp of the wid turbie, ad is give i (7). P C P (7) mech p wid where Cp is the power coefficiet, depeds o type ad operatig coditio of wid turbie. This coefficiet ca be express as a fuctio of tip speed ratio ad pitch agle. The mechaical power produce by wid turbie is give i (8) 1 Pmech 3 R widc p (8) Where R is the radius of the blade (m). B. BESS-STATCOM ad P CELL The battery eergy storage system (BESS) is used as a eergy storage elemet for the purpose of voltage regulatio. The BESS will aturally maitai dc capacitor voltage costat ad is best suited i STATCOM sice it rapidly ijects or absorbed reactive power to stabilize the grid system. It also cotrol the distributio ad trasmissio system i a very fast rate. Whe power fluctuatio occurs i the system, the BESS ca be used to level the power fluctuatio by chargig ad dischargig operatio. The battery is coected i parallel to the dc capacitor of STATCOM [10] [14]. The STATCOM is a three-phase voltage source iverter. The active power capability of a STATCOM is less. It ca be icreased by usig a suitable storage. The P cell is coected across the STATCOM ad a capacitace is coected o its DC lik ad coected at the poit of commo couplig. The STATCOM ijects a competig curret of variable magitude ad frequecy compoet at the bus of commo couplig. The P cell STATCOM will icrease the power trasfer capability of a trasmissio. Fig. 1 Grid Coected system for Power Quality Improvemet. A. Wid Eergy Geeratig System I this cofiguratio, wid geeratios are based o costat speed topologies with pitch cotrol turbie. The iductio geerator is used i the proposed scheme because of its simplicity, it does ot require a separate field circuit, it ca accept costat ad variable loads, ad has atural 3787

4 C. System Operatio Fig. Operatio of system The shut coected STATCOM with battery eergy storage is coected with the iterface of the iductio geerator ad o-liear load at the PCC i the grid system. The STATCOM competor output is varied accordig to the cotrolled strategy, so as to maitai the power quality orms i the grid system. The curret cotrol strategy is icluded i the cotrol scheme that defies the fuctioal operatio of the STATCOM competor i the power system. A sigle STATCOM usig isulated gate bipolar trasistor is proposed to have a reactive power support, to the iductio geerator ad to the oliear load i the grid system. The mai block diagram of the system operatioal scheme is show i Fig.. CONTROL SCHEME The cotrol scheme approach is based o ijectig the currets ito the grid usig bag-bag cotroller. The cotroller uses a hysteresis curret cotrolled techique. Usig such techique, the cotroller keeps the cotrol system variable betwee boudaries of hysteresis area ad gives correct switchig sigals for STATCOM operatio. The cotrol system scheme for geeratig the switchig sigals to the STATCOM is show i Fig. 3. The cotrol algorithm eeds the measuremets of several variables such as three-phase source curret i Sabc, DC voltage dc, iverter curret i iabc with the help of sesor. The curret cotrol block, receives a iput of referece * curret i Sabc ad actual curret i Sabc are subtracted so as to activate the operatio of STATCOM i curret cotrol mode [16] [18]. A. Grid Sychroizatio I three-phase balace system, the RMS voltage source amplitude is calculated at the mplig frequecy from the source phase voltage, sb, sc ad is expressed, as mple template sm, mpled peak voltage, as i (9). sm sb sc (9) 3 The i-phase uit vectors are obtaied from AC source phase voltage ad the RMS value of uit vector u, u, u as show i (10). u sb sc, sb sc, usb usc (10) sm sm sm The i-phase geerated referece currets are derived usig i-phase uit voltage template as, i (11) * * * i I. u, i I. u, i I. u (11) sb sb where is proportioal to magitude of filtered source voltage for respective phases. This esures that the source curret is cotrolled to be siusoidal. The uit vectors implemet the importat fuctio i the grid coectio for the sychroizatio for STATCOM. This method is simple, robust ad favourable as compared with other methods [18]. B. Bag-Bag Curret Cotroller Bag-Bag curret cotroller is implemeted i the curret cotrol scheme. The referece curret is geerated as i (10) ad actual curret are detected by curret sesors ad are subtracted for obtaiig a curret error for a hysteresis based bag-bag cotroller. Thus the ON/OFF switchig sigals for IGBT of STATCOM are derived from hysteresis cotroller [19]. The switchig fuctio S for phase a is expressed as (1). i ( i * HB) S 0 i ( i * HB) S 1 (1) A A A where HB is a hysteresis curret-bad, similarly the switchig fuctio c. sc 1 sc S, S ca be derived for phases b ad C. oltage Source Curret Cotrol B C The three phase ijected curret ito the grid from STATCOM will cacel out the distortio caused by the oliear load ad wid geerator. The IGBT based three-phase iverter is coected to grid through the trasformer. The geeratio of switchig sigals from referece curret is simulated withi hysteresis bad of The choice of arrow hysteresis bad switchig i the system improves the curret quality. The cotrol sigal of switchig frequecy withi its operatig bad, as show i Fig. 4. The choice of the curret bad depeds o the operatig voltage ad the iterfacig trasformer impedace. The competed curret for the oliear load ad demaded reactive power is provided by the iverter. Fig.3 Cotrol scheme 3788

5 Durig the operatio the circuit breaker is set to tur ON automatically at time t=1sec.after tur ON of circuit breaker the differece i source curret ad load curret ca be see i the figure 7 ad figure 8 below. Fig.4 Hysteresis bad cotrol I. SIMULATION STUDIES AND RESULTS Fig.7 Source curret Fig.8 Load curret Fig.5 Proposed scheme i simulik The wid eergy geeratig system is coected with grid havig the oliear load. The performace of the system is measured by switchig the circuit breaker at time t=1s i the system ad how the P cell i coordiatio with the STATCOM respods after the ON time of circuit breaker is show usig simulatio. Poit of commo couplig (PCC) The Iductio geerator, the load ad the P cell STATCOM cotroller is coected at the poit of commo couplig alog with battery eergy storage system (Bess).The plat model of PCC voltage cotroller of the P system is derived cosiderig both reactace ad resistace of the etwork to which the P system is coected. At PCC the voltage regulatio is attaiable with iverter iterfaced sources by dyamically cotrollig the amout of reactive power ijected ito grid by idividual systems. The supply voltage is costat at the PCC.The curret at the PCC differs after the tur ON time of the circuit breaker. The supply voltage ad the curret at PCC ca be viewed i the figure 9. Fig.6 Curret waveforms (a) Source curret (b) Load curret (c) Ijected iverter curret (d) Wid geerator curret(iductio geerator) Fig.9 oltage ad curret at PCC 3789

6 Power Quality Improvemet It is observed that the source curret o the grid is affected due to the effects of oliear load ad wid geerator, thus purity of waveform may be lost o both sides i the system. The iverter output voltage uder P cell STATCOM cotroller operatio with load variatio is obtaied. The dyamic load does affect the iverter output voltage. The source curret with ad without P cell STATCOM cotroller operatio is show i Fig. 7.Before tur ON of the cotroller the source has udergoe some disturbaces. The power factor is maitaied costat throughout the process. This shows that the uity power factor is maitaied for the source power whe the P cell STATCOM cotroller is i operatio. The power factor is show i the figure 10. curret waveform is show i Fig.1 with its FFT.The THD of source curret is recorded as.89%. It is show that the THD has bee improved cosiderably ad withi the orms of the stadard. Fig.1a Source curret Fig.10 Uity power factor The curret waveform before ad after the cotroller operatio is aalyzed. The FFT of this waveform is expressed ad the THD of this source curret at t=0.05s durig the OFF coditio of the cotroller is 7%, as show i figure 11. Fig.1b FFT of Source curret The load curret at time t=0.05 is give ad the FFT of this load curret at the time t=0.05 is aalyzed ad preseted i the figure 13. Fig.11a Source curret Fig.13a Load curret Fig.11b FFT of source curet The power quality improvemet is observed at poit of commo couplig, whe the cotroller is i ON coditio. The cotroller is placed i the operatio at 0.13 s ad source Fig.13b FFT of Load curret 3790

7 Fig.14a Load curret Fig.16a Trasfer fuctio Fig.14b FFT of load curret The load curret at t=0.05 has a THD value of 11.9 whe the cotroller is i OFF coditio. As soo as the cotroller is tured ON at 0.1sec the load curret observes a chage i its THD by recordig a value 11.63%. Fig.16b FFT of trasfer fuctio The FFT aalysis for the trasfer fuctio is show i figure 16.The THD of the trasfer fuctio observed as 7% durig OFF coditio of cotroller ad.89% after the cotroller is made ON. The power quality improvemet is observed at poit of commo couplig, whe the cotroller is i ON coditio. The above tests with proposed scheme has ot oly power quality improvemet feature but it also has sustai capability to support the load with the eergy storage through the batteries. II. CONCLUSION Fig.15a Trasfer fuctio The paper presets the P CELL STATCOM-based cotrol scheme for power quality improvemet i grid coected wid geeratig system ad with o liear load. The power quality issues ad its cosequeces o the cosumer ad electric utility are preseted. The operatio of the cotrol system developed for the P CELL STATCOM-BESS i MATLAB for maitaiig the power quality is simulated. It has a capability to cacel out the harmoic parts of the load curret. It maitais the source voltage ad curret i-phase ad support the reactive power demad for the wid geerator ad load at PCC i the grid system, thus it gives a opportuity to ehace the utilizatio factor of trasmissio lie. The itegrated wid geeratio ad P CELL STATCOM cotroller with BESS have show the outstadig performace. Thus the proposed scheme i the grid coected system fulfils the power quality orms as per the IEC stadard Fig.15b FFT of trasfer fuctio 3791

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