MODELING AND SIMULATION OF PWM BASED STATOM FOR REATIVE POWER ONTROL 1 N.Rajkumar, 2.Sharmeela 1 Student, Institute for Energy Studies, Dept. of Mechanical Engg., ollege of Engineering Guindy, 2 Assistant Professor (Sr. Gr) in EEE, Department of hemical Engg. A..Tech., Anna Uniersity, hennai, India Email : 1 nrajkumarbe@gmail.com, 2 sharmeela20@yahoo.co.in Abstract This dissertation is dedicated to Modeling and Simulation of STATic synchronous OMpensator (STATOM) for Reactie Power ontrol. Among Flexible A Transmission System (FATS) controllers, the STATOM has shown feasibility in terms of cost effectieness in a wide range of problem soling abilities from transmission to distribution leels. The Modular Multileel ascaded onerters (MMs) with separated Direct urrent (D) capacitors is the most feasible topology for use as a power conerter in the STATOM applications. To be able to operate in a high oltage application, a large number of D capacitors are utilized in a MM based STATOM. All D capacitor oltages must be balanced in order to aoid oeroltage on any particular link. MM based on Single Delta Bridge ells (SDBs) to a STATOM, particularly for reactie-power control. SDB is categorized by cascade connection of multiple single phase H-bridge (or full bridge) conerter cells per leg, hence simplifying flexible circuit design and low oltage steps. Modeling and Simulation of a 100V 5kVA Pulse Width Modulated (PWM) STATOM based on the SDB using MATLAB/Simulink, with focus on the operating principle and performance. Simulation results show that it can control positie sequence reactie power, negatie sequence reactie power and also low frequency actie power. Index Terms STATOM, multileel conerters, MM, reactie power, positie sequence reactie power, negatie-sequence reactie power. I. INTRODUTION The Family of MM is expected as one of the nextgeneration power conerters suitable for high-oltage or medium-oltage applications without line-frequency transformers [1] [21]. From power-circuit and conerter-cell configurations, it can be classified into the following [2]: 1) Single Star Bridge ells (SSBs) 2) Single Delta Bridge ells (SDBs) ) Double Star hopper ells (DSs) 4) Double Star Bridge ells (DSBs) The term bridge cell is a single-phase H-bridge (or full bridge) conerter, and the chopper cell is a bidirectional chopper consisting of a dc capacitor and two insulated-gate bipolar transistors. SSB is a STAtic Synchronous OMpenstator (STATOM) for oltage regulation [11], [1] and battery energy storage systems [14], [15]. Howeer, without significantly increasing the conerter-cell count, the SSB-based STATOM cannot draw any negatie sequence reactie power because it has no circulating current. The SDB, DS and DSB are suitable for STATOM for negatie sequence reactie power control because they hae the circulating current(s) that flow inside. onerter-cell count required for the DS is four times of that for the SSB. The authors of this paper hae described the DS acting as a STATOM with focus on control and performance [20], [21]. Attention has been paid to STATOMs based on the SDB [8]-[10]. The SDB seems to be a better choice than the DS from a practical point of iew because the conerter-cell count 100
required for the SDB is only 1.7 (= ) times of that for the SSB [2]. The authors of this paper hae described the SDB acting as STATOM with experimental erification using down scaled model of 100, 5kVA. Experimental results erify that it can control not only positie-sequence reactie power but also negatie sequence reactie power and low-frequency actie power at the same time. Howeer, no simulation erification has been made in the literature [1]. The aim of this paper is to proide modeling and simulation erification of an SDB-based pulse widthmodulated (PWM) STATOM for reactie-power control. This paper proposes a control method that is characterized by forming a feedback loop of the circulating current among the delta-connected clusters, leading to stable dc-mean oltage control of all the dc capacitors. This paper models and followed by simulation erification using a downscaled model rated at 100 V and 5 kva. Simulation results erify that it can control positie-sequence reactie power, negatie sequence reactie power and low-frequency actie power at the same time. II. MM-SDB ONFIGURATIONS Fig. 1 shows two kinds of SDB-based STATOM. The SDB is characterized by easily increasing the oltage and current ratings without using line frequency conerter transformers. Fig. 1(a) shows a 22-kV/-kV system with a grid transformer. Each cluster of the SDB is connected in delta configuration ia a single coupled inductor. The leakage inductance of the transformer works as ac-link inductors between the grid and the SDB. Fig. 1(b) shows a.-kv system with no grid transformer. Each cluster of the SDB is connected ia three noncoupled inductors. The SDB can be connected directly to the.-kv grid because the noncoupled inductors work as ac-link inductors. III. IRUIT ONFIGURATION OF THE SDB A. ircuit onfiguration Fig. 2 shows the detailed circuit configuration of the 100V 5kVA STATOM used in MATLAB/Simulink. Each cluster of the SDB consists of cascade connection of three bridge cells (i.e., single-phase fullbridge PWM conerters), and the three clusters are connected in delta configuration ia a single coupled inductor L. The SDB is connected to three-phase ac mains of 100V (line to line in rms) ia a three-phase ac-link inductor Ls that corresponds to the leakage inductance of the grid transformer in Fig. 1(a). Here, u, w, and wu are the cluster oltages, iu, iw, and iwu are the cluster currents, and p and q are the instantaneous actie and reactie powers at the P. The following relations exist between the compensating currents and the cluster currents. The compensating currents and the supply currents are the same in Fig. because no load (arc furnace) is connected. iu=iu iwu i=iw iu iw=iwu iw. (1) Fig. 2 ircuit configuration of the 100V 5kVA MM SDB PWM STATOM Fig. 1 SDB-based STATOM. (a) 22-kV/-kV system with a grid transformer. (b).-kv system with no grid transformer 101
D apacitor of Bridge ell 14 mf/ 0.9 F D apacitor Voltage Reference V 0 V Unit apacitance onstant H 5 ms/2.9 s arrier frequency fc 2 khz Fig. oupled inductor used Let the circulating current flowing inside the deltaconnected clusters be i Z. It is defined as i Z (2) 1 ( i u B. Simulation Parameters Used i u i Table I summarizes the simulation parameters of the SDB which will be used in simulation later on. TABLE I Simulation Parameters Description Rated apacity Rated Line to Line rms oltage Vs wu ) Rating 5kVA 100V Rated Line Frequency ω/2 50 Hz Rated Line urrent I 29 A Rated luster urrent 2I/ 4 A Equialent Switching frequency fc 12 khz A Link inductor oupled inductor Ls L 0.5 mh (8%) 27 mh (7%) % inductance is on a three phase, 100V 5kVA and 50Hz base Each carrier frequency of phase-shifted PWM for bridge cells was set as f = 2 khz. The command of each dccapacitor oltage was set as V = 0 V. The dc capacitor was set as either = 1.4 mf (H = 5 ms) or = 0.9 F (H = 2.9 s). The ac-link inductor and the coupled inductor were set as L = 0.5 mh (8%) and L = 2. mh (7%), respectiely.. MATLAB/SIMULINK Model Deelopment As shown in Fig 4(a) to 4(d), MATLAB/SIMULINK R2010a used to Model a ircuit configuration of 100V 5kVA MM SDB PWM STATOM and MATLAB/SIMULINK deeloped models shown in Fig 4(a) to 4(d). S 102
Fig. 4(a) MATLAB/SIMULINK Modeling of ircuit configuration of 100V 5kVA MM SDB PWM STATOM Fig. 4(b) MATLAB/SIMULINK Modeling of u-phase H-Bridges circuit Fig. 4(c) MATLAB/SIMULINK Modeling of -phase H-Bridges circuit 10
Here,, u, w,and are instantaneous alues containing both ac and dc components. It is desirable to extract only the dc components (i.e., ( u) dc,( ) dc,( w) dc and ( ) dc because the existence of the ac components deteriorates the controllability. Fig. 4(d) MATLAB/SIMULINK Modeling of w-phase H-Bridges circuit IV. ONTROL METHODS OF MM- SDB BASED PWM STATOM Fig. 5 shows the MATLAB/SIMULINK Model -block diagram of the dc-capacitor oltage control. Voltage control of the nine floating dc capacitors in Fig. 2 can be diided into the following: 1) cluster-balancing control; 2) circulating-current control; ) indiidual-balancing control. A. luster-balancing ontrol Fig. 5(a) shows the MATLAB/SIMULINK Modelblock diagram of the cluster-balancing control. The oltage major loop forces the aerage oltage of each cluster, namely,, u and w oltage of the three clusters as u c w, to follow the aerage where they are defined 1 1 ju; j () 1 w w j; c c c c Fig. 5 MATLAB/SIMULINK Model-Block Diagram of dc-capacitor oltage control. (a) luster-balancing control. (b) irculating-current control. (c) Indiidualbalancing control. The following methods can be utilized to extract the dc components: 1) the method using a low-pass filter [11] 2) the method using a feed forward control; ) the method using a moing-aerage filter of 100 Hz [10] 104
The last method is adopted in this project. Note that sin(ωt +π/) in Fig. 5(a) is in phase with V u.. When (V ) dc > (V u ) dc the product of V u and i Z ( i Z ) forms positie actie power because i Z contains the same B. irculating-urrent ontrol Fig. 5(b) shows the MATLAB/SIMULINK Model-block diagram of the circulating-current control. The current minor loop forces i Z to follow its command i, producing the oltage command V A that is common to the three clusters.. Indiidual-Balancing ontrol Fig. 5(c) shows the MATLAB/SIMULINK Model-block diagram of the indiidual balancing control. It forms an actie power between the ac oltage of each bridge cell and the corresponding cluster current. The oltage commands B, ju B, and j B are gien by jw B ju B j B jw K ( 4 K ( 4 u K ( 4 w ju j jw ) i ) i u w ) i wu Z (4) The following equation is obtained from aboe equation: j1 B ju j1 B j (5) j1 B jw 0 component as V u. As a result, an amount of actie power flows into the u-phase cluster, thus leading to increasing (V u ) dc. On the other hand, the product of V u and i Z forms negatie actie power when (V ) dc < (V u ) dc, thus leading to decreasing (V u ) dc Hence, the sum of the oltage commands is equal to zero. This means that no interference occurs between the indiidual balancing control and the circulating-current control. D. Actie-power, Reactie-Power, and Oerall Voltage ontrols Fig. shows the MATLAB/SIMULINK Model-block diagram of the actie-power, reactie power, and oerall oltage controls, in which pand q represent the power commands of p and q at the P. The dc component of qis adjusted to control positiesequence reactie power keeping the relation of p= 0. On the other hand, a couple of second-order components (100 Hz) with the same amplitude but a phase difference of 90 are superimposed on p and q, respectiely, to control negatie-sequence reactie power. A lowfrequency component is superimposed on p to control actie power, keeping the relation of q= 0. The line toline oltage commands V u, V w, and V wu are determined by decoupled current control of the compensating currents. A oltage major loop intended for compensating the conerter loss is formed as shown in Fig. which forces (V ) dc to follow its command V. Fig. MATLAB/SIMULINK Model-Block Diagram of instantaneous actie and reactie power controls, and oerall oltage control 105
Fig. 7. MATLAB/SIMULINK Model-A Voltage command of each bridge cell. (a) u-phase. (b) -phase. (c) w-phase. Fig. 7 shows the MATLAB/SIMULINK Model-A oltage command of each bridge cell. The oltage command is normalized by each dc-capacitor oltage. Then, it is compared with a triangular waeform haing a maximal alue of 1 and a minimal alue of 1 with a carrier frequency of f V. SIMULATION RESULTS AND DISUSSIONS In the present work, 100-V 5-kVA downscaled model considered for the Modeling and Simulation of PWM STATOM for Reactie Power ontrol. The following section explains the results obtained in the simulation. Fig. 8 to 11 show the Simulation waeforms obtained from the 100-V 5-kVA downscaled model. All the Simulation waeforms were taken in a personal computer (P) through the MATLAB/Simulink R2010a with different sampling frequencies. Figs. 8, 9 and 11 had a sampling frequency of 100 khz, and Fig. 10 had a sampling frequency of 20 khz. A. Negatie-Sequence Reactie-Power ontrol The instantaneous actie and reactie power commands in the three-phase circuit p and q are gien by 5 p 5cos(2 t )[ kw] 5 5sin(2 t )[ kvar ] () q where the initial phase was set as φ = 5π/ so that the amplitude of i u has its maximal alue. Equation () and block diagram of instantaneous actie and reactie power controls, and oerall oltage control gie i d and i q as follows: 5 id 5cos(2 t 5 i q 5sin(2t )[ A] )[ A] (7) where the dc component of i d coming from the oerall oltage control is excluded from aboe equation. Applying the inerse d q transformation produces i u, i, and i w as i i i u w 41sin( t 41sin( t 41sin( t 2 )[ A] )[ A] 5 )[ A] It is obious from aboe equation that i u leads (8) iw by 120 o and i by 240 o, thus resulting in drawing the negatie-sequence reactie power from the ac mains. Fig. 8 shows the Simulation waeforms when the rated negatie-sequence reactie power of 5 kvar was controlled. The cluster oltage Vu is a seen-leel PWM waeform with a oltage step of 0 V (=V ), 10
containing much less harmonic oltages as well as much less common-mode oltages than traditional two-leel oltage-source PWM conerters. Since the carrier frequency of each chopper cell is 2 khz, the equialent switching frequency of the cluster is 12 khz (2 khz x ). The compensating currents i u, i, and i w agree well with their current commands. The waeform of i u can be considered as a sinusoidal waeform with a fundamental component of 50Hz. The total harmonic distortion alue of i u is low.the u-phase cluster acts as an inductor because i u lags V u by 90 o On the other hand, the - and w-phase clusters act as a capacitor because i w leads V w by 90 and i wu leads V wu by 90 o. The dc-capacitor oltages V 1u, V 1, and V 1w contain both dc and ac components, in which the oltage control regulates the dc component at 0 V. The ac component consists of a second-order (100 Hz) frequency component. B. Actie-Power and Reactie-Power ontrols Flicker compensation of arc furnaces requires to control positie-sequence reactie power, negatie-sequence reactie power, and low-frequency actie power at the same time. Fig. 9 Waeforms when a positie-sequence capacitie reactie power of 1.7 kvar, a negatiesequence reactie power of 1.7 kvar, and a10-hz actie power of 1.7 kw were simultaneously controlled with a condition of = 1.4 Mf As an example, p and q are gien by t p 1.7 sin( ) 1.7 cos(2t 5 )[ kw] 5 5 q 1.7 sin(2t ) 1.7[ kvar] (9) Fig. 9 shows the Simulation waeforms when a positie sequence capacitie-reactie power of 1.7 kvar, a negatie sequence reactie power of 1.7 kvar, and a 10- Hz actie power of 1.7 kw were simultaneously controlled with a condition of = 1.4 mf (H = 5 ms). Since three different operating modes are intermixed, the amplitude and phase of the currents are changing dynamically as shown in Fig. 9. The amplitude of i Z in Fig. 9 is one-third of that in Fig. 8 because the negatie sequence reactie power in Fig. 8 is reduced to one-third of that in Fig. 8 Fig 8 Waeforms when the rated negatie-sequence reactie power of 5 kvar was controlled with a condition of = 1.4 mf 107
Fig. 9 indicates that the amplitude of the 10-Hz component is much larger than those of the 100-Hz components because it is inersely proportional to ω or ω 0. Hence, larger capacitors are required to control lowfrequency actie power.. Low-Frequency Actie-Power ontrol To control a 1-Hz instantaneous actie power of 5 kw, pand q are gien by t p 5sin( )[ kw] 50 q 0[ kvar] (10) Fig. 10 shows the Simulation waeforms in which each of the dc capacitors was replaced with = 0.9 F (H = 2.9 s). Both compensating and cluster currents form 1- Hz enelopes as shown in Fig. 10, in which the amplitude of the cluster currents was 1/ of that of the compensating currents. arefully looking into Fig. 10 reeals that the amplitudes of both cluster and compensating currents when p>0 are larger than that of the currents when p<0 due to the conerter loss. The relation of i Z = 0 always exists because no negatiesequence reactie power was controlled. The dccapacitor oltages V 1u, V 1, and V 1w contain both 1- and 100-Hz components in which the former is much larger than the latter. Fig 10 Waeforms when a 1-Hz actie power of 5 kw was controlled with a condition of = 0.9 F D. Transient-State Performance It is clear from Fig. 11 that the STATOM can achiee fast negatie-sequence reactie-power control without delay time. Howeer, the waeforms of V 1u, V 1, and V 1w show that a maximal oltage difference of 12V (20%) occurs during the transient state. To increase the capacitance of each dc capacitor is required to decrease the oltage difference during the transient period. Fig. 11 shows the Simulation waeforms when the negatie-sequence reactie power was increased from 2.5 to5 kvar in 20 ms, kept constant for 20 ms, and decreased from 5 to 2.5 kvar in 20 ms with a condition of = 1.4 mf VI. ONLUSION This paper has discussed Modeling and Simulation of PWM based STATOM using an MM-SDB for Reactie Power ontrol, with focus on operating principle and performance, the simulation results obtained from the 100V 5kVA downscaled model has led to the following conclusions. 1) The SDB has a capability to control negatiesequence reactie power with the help of the circulating current among the delta-connected clusters. 108
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