IOSR Journal of Electrical and Electronic Engineering (IOSRJEEE) ISSN : 78-676 Volume, Iue (May-June 0), PP 0-09 Review of D-STATCOM for Stability Analyi Pradeep Kumar, Niranjan Kumar & A.K.Akella 3 3 Deptt. of Electrical Engineering, National Intitute of Technology, Jamhedpur Jharkhand (INDIA) Abtract: A Static Compenator (STATCOM) i a flexible ac tranmiion ytem (FACTS) controller, which can either aborb or deliver reactive power to a power ytem. Ditribution STATic COMpenator (D- STATCOM) i propoed for compenation of reactive power and unbalance caued by variou load in ditribution ytem. Ditribution tatic compenator i baed on the VSC principle. A D-STATCOM inject a current into the ytem to correct the voltage ag, well and power factor. Ditribution Static Synchronou Compenator (D-STATCOM) i an effective meaure to maintain voltage tability and improve power quality of ditribution grid. Thi paper deal with the modeling and control cheme of D-STATCOM. A tability analyi of D-STATCOM i obtained by bode plot approach. The theoretical analyi and deign are verified by the reult. Keyword: Ditribution Sytem, Power Quality, Cutom Power Device, Shunt Compenation Device, Ditribution Static Compenator (D-STATCOM). I. INTRODUCTION The high power quality i requeted by manufacturing factorie and commercial building dealing with information. Thi circumtance i caued by the fact that poor power quality give the bad effect to the quality of product and therefore, reult in financial loe. According to EPRI report (995), the revenue loe due to poor power quality to U.S. buine alone were $400 billion per year. Power quality problem are caued by dynamic or non-linear load and interaction between the load and network. Outage, voltage ag and well, voltage flicker, harmonic interference, and unbalance are ome of the mot common problem encountered. Today, new technologie known a Cutom Power [], uing power electronic-baed concept, have been developed to provide protection from power quality problem. Generally, Cutom Power equipment are divided by erie-connected compenator like DVR (Dynamic Voltage Retorer), hunt-connected compenator like D-STATCOM (Ditribution Static Compenator), and erie and hunt compenator like UPQC (Unified Power Quality Compenator). In many intance, the ue of Ditribution Static Synchronou Compenator (D-STATCOM) can be ome of the mot cot-effective olution for thee type of power quality problem. When a fault happen in a ditribution network, udden voltage ag will appear on adjacent load. D-STATCOM intalled on a enitive load, retore the line voltage to it nominal value within the repone time of a few milliecond thu avoiding any power diruption to the load. Currently, mot of the STATCOM deign tudie are baed on the aumption of the balanced three-phae ytem. And almot all reearche are baed on the three-phae three-wire ytem Moreover thi paper preent the D-STATCOM uing IGBT voltage ource inverter with 0 khz witching frequency PWM operation for reactive power compenation in ditribution ytem. AC voltage direct control ha the advantage of improved harmonic performance, and ag voltage. II. STATIC SYNCHRONOUS COMPENSATOR Static Synchronou Compenator (STATCOM) i a voltage ource converter baed FACTS controller. It i a hunt controller mainly ued to regulate voltage by generating/aborbing reactive power. The chematic diagram of STATCOM i hown in Fig.. STATCOM ha no long term energy upport in the DC Side and cannot exchange real power with the ac ytem ; however it can exchange reactive power. The reactive power i varied by varying the magnitude of the converter output voltage. A mall phae difference exit between the converter output voltage and STATCOM bu voltage o that real power i drawn from the line to compenate for the loe. STATCOM are employed at ditribution and tranmiion level though for different purpoe. Page
III. VOLTAGE SOURCE CONVERTERS (VSC) In recent year, voltage ource converter technology ha made a great progre through the development of high power elf-turnoff type emiconductor device. The rating for converter of thi type in practical application ha already reached a high. Becaue of it advantage over the line commutated type in performance characteritic and compactne, variou application of the voltage ource converter have been developed and reearched. Three phae Voltage Source Converter (VSC) i heart of mot new FACTS and cutom power equipment. It may be employed a a erie or hunt element or combination of both, a in cae of Unified Power Flow Controller (UPFC). Multilevel Voltage Source Converter topology i uperior alternative to multipule arrangement for high power application like STATCOM. Voltage ource converter (VSC) are commonly ued to tranfer power between a dc ytem and an ac ytem or back to back connection for ac ytem with different frequencie, uch a variable peed wind turbine ytem[]. A baic VSC tructure i hown in Fig. where R and L repreent the reitance and inductance between the converter ac voltage (V C ) and the ac ytem voltage (V) and I i the current injected into the grid. A dc capacitor i connected on the dc ide to produce a mooth dc voltage. The witche in the circuit repreent controllable emiconductor, uch a IGBT or power tranitor. 6-pule D-STATCOM configuration with the IGBT ued a power device. The IGBT are connected anti parallel with diode for commutation purpoe and charging of the DC capacitor.for converter the mot important part i the equence of operation of the IGBT. The IGBT ignal are referred to the Pule Width Modulation (PWM) that will generate the pule for the firing of the IGBT. IGBT are ued in thi imulation becaue it i eay to control the witch on and off of their gate and uitable for the D-STATCOM. Fig.. Voltage ource converter (VSC) Page
Fig.3.Simulink Model of Voltage Source Converter Fig.4. Waveform of Inverter and Load voltage IV. BASIC CONFIGURATION AND OPERATION OF DSTATCOM When a STATCOM i employed at the ditribution level or at the load end for power factor improvement and voltage regulation alone it i called D-STATCOM. Fig.5 how a baic configuration diagram of the D-STATCOM. The D-STATCOM mainly conit of DC voltage ource behind elf-commutated inverter uing IGBT and coupling tranformer. The IGBT inverter with a DC voltage ource can be modeled a a variable voltage ource. The ditribution power ytem can alo be modeled a a voltage ource. 3 Page
Two voltage ource are connected by a reactor repreenting the leakage reactance of the tranformer. The principle operation mode of the D-STATCOM output current, I which varie depending upon V o I ( V Vo ) / X () Fig.5.Single-line diagram of D-STATCOM Where V, V o, X are the ytem voltage, output voltage of the IGBT-baed inverter, the total ckt reactance repectively. If V o i equal to V, then no reactive power i delivered to the ytem. If V o i greater than V, the phae angle of I i leading with repect to the phae angle of V by 90 degree. Thu, a leading reactive power flow in the Capacitive Mode of the D-STATCOM. If V O i lower than V, the phae angle of I i lagging with repect to the phae angle of V by 90 degree. Thu, a lagging reactive power flow in the Inductive Mode of the D-STATCOM. The quantity of the reactive power flow i proportional to the difference between V and V O. 4 Page
Fig.6. Operation mode of D-STATCOM V. MODELING AND CONTROL SCHEME OF D-STATCOM A. Principle Of propoed control cheme-: A implified block diagram of the internal control for a converter with internal voltage control capability i hown in Fig.7. The input ignal are the bu voltage V, the converter output current io and the reactive current reference, I Q Re 5 Page
The deired terminal voltage veru output current characteritic of the compenator can be etablihed by minor control loop a hown chematically in Fig.8. Fig.8. Control model conidering implementation of the V-I lope. A ignal proportional to the amplitude of the compenating current ki Q with an ordered polarity i derived and ummed to the reference V Ref. The effective reference V Ref controlling the terminal voltage thu become V Ref = V Ref +ki Q () B. Tranfer Function and Dynamic Performance: Fig. 9.Baic tranfer function block diagram The baic tranfer function block diagram hown in Fig. 9 can characterize the dynamic behavior of the compenator in the normal compenating range. Thi block diagram i derived directly from the baic control cheme hown in Fig.8. In the linear operating range of the compenator, the terminal voltage V T can be expreed from Fig.8 in term of the internal voltage V and the reference voltage V Ref a follow: V T GG X V VRe f (3) G G HX G G HX Since the objective i to etablih how well terminal voltage i regulated againt the ytem voltage, So V Ref = 0 V (4) G G HX V T 6 Page
Conider mall variation only. Then amplitude variation of terminal voltage and ytem voltage can be expreed in following form: V T (5) V G G HX Where G / k T G e T d H T Table: Parameter of D-STATCOM Parameter Main time contant of PI controller Amplitude meauring circuit time contant Tranport lag of D-STATCOM Value T = 0 (m) T = 8 (m) T d = 0. (m) Reactive part of ytem impedence X = 9.5 Regulation lope K = 0.03 G 33.33 0 G 0. e S H= 8 = 0. = 0. e (neglecting higher order term) G G HX 6 3 83.6 37.37 8. (6) From relation (5) and (6) we get V V T 3 6 83.6 8. 3 6 83.6 8. 38.37 7 Page
Phae (deg) Magnitude (db) >> num=[6 83.6 8. ]; >> den=[6 83.6 8. 38.37 ]; >> bode(num,den) >> y=tf(num,den) Tranfer function: 6 ^3 + 83.6 ^ + 8. + ---------------------------------- 6 ^3 + 83.6 ^ + 8. + 38.4 >> margin(y) Review of D-STATCOM for Stability Analyi C. Bode-plot command when D-STATCOM regulate voltage VI. RESULT AND DISCUSSION The Bode plot i an important tool for tability analyi of cloed-loop ytem. Gain and Phae margin can be determined from the bode diagram. Relative tability can be determined from the bode diagram uing margin function. The margin function i invoked in conjunction with bode function to compute the gain and phae margin. Here treating MATLAB a one of the tool to deign and analyze control ytem. From Fig. 0 it i clear that both Phae margin and Gain margin have poitive value o the D-STATCOM ytem i table. 0 Bode Diagram Gm = 7.5 db (at.07 rad/ec), Pm =.3 deg (at.38 rad/ec) 0-0 -40-60 0-90 -80-70 -360 0-3 0-0 - 0 0 0 Frequency (rad/ec) Fig.0. Bode-plot to demontrate the tability of D-STATCOM VII. CONCLUSION Cutom power device like DVR, D-STATCOM, and UPQC can enhance power quality in the ditribution ytem. Baed on the power quality problem at the load or at the ditribution ytem, there i a choice to chooe particular cutom power device with pecific compenation. The working principle of D- STATCOM i explained in two mode (capacitive and inductive) of operation. Modeling and control cheme of D-STATCOM i done with the help of tranfer function block diagram and tability analyi i invetigated uing bode plot with oberving gain margin and phae margin of a ytem tranfer function. Acknowledgment The author are thankful to Government of India. One of the author Mr. Pradeep Kumar i thankful to All India council of Technical Education (AICTE), Minitry of Human Reource Development (MHRD), Govt. of India for providing financial aitant to do the reearch work under Technical Quality Improvement Programme (TEQIP). 8 Page
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