J. Electrical Systems 8-1 (2012): Regular paper

Transcription

1 K.R.M. Vijaya Chandrakala S. Balamurugan K. Sankaranarayanan J. Electrical Sytem 8- (22): Regular paper Damping of Tie-Line Power Ocillation in Interconnected Power Sytem uing Variable Structure Sytem and Unified Power Flow Controller Thi paper focue on ytematic approach toward damping the tie-line power ocillation in an interconnected thermal power plant uing Unified Power Flow Controller (UPFC) with Variable Structure Sytem (VSS) Controller. Conventionally, Automatic Generation Control (AGC) i carried out by primary governor control and econdary Proportional- Integral (PI) controller. PI controller help to damp out tie-line power and frequency ocillation when ubjected to unit tep load diturbance. Ziegler Nichol (ZN) method i ued in tuning the gain of conventional PI controller and VSS controller. Optimal witching of VSS controller i carried out by the performance indice. UPFC baed damping controller help to tabilize the tie-line power ocillation of power ytem. The imulation indicate reduced frequency and tieline power tranient with much fater ettling time i obtained by uing UPFC along with econdary VSS controller. Keyword: Automatic Generation Control, Proportional-Integral Controller, Variable Structure Sytem Controller, Unified Power Flow Controller.. INTRODUCTION Sytem load change in an interconnected power ytem lead to ocillation in frequency and mimatche in cheduled power interchange between the area. AGC help to damp out the frequency ocillation and match the power flow between the area within predetermined limit [- 3]. AGC cheme ha two main control loop namely, primary control and econdary control []. During load variation, the primary peed governor control generation, to match with the demand. With the help of peed controller, variou generator in the control area track the load variation and hare the load in proportion to their capacitie. The econdary controller will fine tune the frequency and tie line power. A per Cohn control trategy, the power flow control among the area become a joint undertaking for the control area [4]. The control area may contain thermal, hydro, nuclear or ga turbine plant. The inter connection between thee area of any combination i made by tie line. In thi paper, two area thermal power ytem i conidered. Depending upon the turbine type, the primary control loop repond within few econd. Though, primary AGC match generation with demand, there will be a dip in frequency. The peed changer etting control by econdary controller i ued to reet the error to zero in few minute. Thi fine adjutment carried out through integral action by controlling the peed changer etting i coniderably lower and goe into action only after the primary control loop ha completed it job [- 3]. Motly a econdary controller, PI controller i employed [4-9]. The PI controller i tuned uing variou technique [5-8]. Zeigler Nichol method i imple and effective in Correponding author : _balamurugan@cb.amrita.edu Department of EEE., Amrita School of Engineering, Amrita Vihwa Vidyapeetham, Coimbatore, India Copyright JES 22 on-line : journal/ergroup.org/je

2 Vijaya Chandrakala et al: Damping of Tie Line Power Ocillation Uing VSS and UPFC tuning. The high proportional gain creating teady tate error and high integral gain reulting tranient overhoot i highly undeirable [9- ]. Thi conflict i reolved by employing the principle of VSS controller [- 3]. The VSS controller applied to the twoarea power ytem improve the ytem performance [4]. Further improvement in the tie line power flow i achieved uing Flexible AC Tranmiion Sytem (FACTS) device. The Superconducting Magnetic Energy Storage (SMES) [5] appreciably improve the frequency ocillation but the tie line power variation are not much improved [6]. The tie line power ocillation are much damped out uing the Static Synchronou Serie Compenator (SSSC) [4][7]. The Unified Power Flow Controller (UPFC) [8-2] i ued in thi paper for damping the tie line power ocillation. Thi paper decribe about two area thermal power in ection 2. Variable Structure Sytem Controller and it tuning are explained in ection 3. The modeling of UPFC and it connection with two area thermal power ytem i furnihed in ection 4. The performance of UPFC along with VSS controller i decribed in ection MODEL OF TWO AREA THERMAL PLANT PRIMARY CONTROL 2.. Speed Governing Sytem Speed-governor in thermal power plant control the turbine peed with the help of fly ball and peed changer mechanim. The team input valve to the turbine i controlled by peed governor. The peed control in turn control the active power output of the ytem. During load variation, by controlling the team the real power i made to match with the demand. The governor indirectly meaure the mimatch between generation and demand by ening the change in frequency. Though, primary AGC match generation with demand, there will be error in frequency. Fine tuning of frequency i carried out by econdary controller which in turn control the peed changer etting. Mathematically, the peed governor function i expreed by the following equation; Δ Pg() = ΔPref() Δ f() () R 2.2. Hydraulic Valve Actuator Very large mechanical force are needed to poition the valve againt the high team preure inlet to turbine. Thee force are obtained via hydraulic amplifier. Mathematically, the hydraulic amplifier function i provided a; Δ PH() = ΔPg() (2) TH 2.3. Turbine-Generator Repone The prime mover driving a generator unit i a team turbine of thermal power plant. The turbine power i directly proportional to the flow of the team. Here, a non-reheat type turbine i ued. It relate the poition of the valve that control the emiion of team into turbine to the power output of the turbine. The mathematical equation of the turbine function i provided a; Δ PT() = ΔPH() TT (3) 86

3 J. Electrical Sytem 8- (22): The turbine power output i given a the input to the generator which in turn provide electrical power to the power ytem. The generator along with the power ytem with the proviion for load diturbance i repreented below; KP ΔPT() Δ PD() = Δf() (4) TP 2.4. Tie-line Practically, all power ytem are tied together with neighbouring area. The problem of load-frequency control become a joint undertaking in controlling the power flow on the inter-tie. In thi paper, a two-area thermal power plant i connected by a tie-line a hown in Fig.. The interconnected thermal power plant i ubjected to a unit tep load diturbance, the ytem frequency and tie-line power deviation are controlled. Area Steam Unit P tie 2 Area 2 Steam Unit Figure : Block diagram of two area interconnected power plant The mathematical equation of tie-line power i hown below; 2ΠT Δ Ptie2() = ( Δf () Δ f 2()) (5) Primary AGC mathematical model of a two-area thermal power plant i ued for imulation a hown in Fig. 2. R T H T T KP T P 2ΠT R2 TH T 2 T 2 KP2 T P2 Figure 2: Mathematical model of a two area thermal power plant without econdary controller 87

4 Vijaya Chandrakala et al: Damping of Tie Line Power Ocillation Uing VSS and UPFC 3. DEVELOPMENT OF SECONDARY VSS CONTROLLER 3.. PI controller Conventionally, PI controller act a econdary controller which et the turbine reference power of each area. When a two-area thermal power plant i ubjected to a unit tep load diturbance, the variation in one area affect the other area via tie-line in term of frequency and tie-line power. Thee two variation have to be combined linearly a one ignal which erve a the input to the econdary PI controller. The input of each controller i called a Area Control Error (ACE). Mathematically, the function of econdary PI controller i repreented a below; KI Δ Pref () = ( KP ) ACE() (6) The ACE adopted in two area ytem i developed by Cohn [4]. The proportional control i ued for increaing the loop gain to make the ytem le enitive to load diturbance. The integral control i ued to eliminate teady tate error [5]. The econdary PI controller reet both the frequency and tie-line power variation back to nominal value. According to Cohn control trategy [4], all the area in the network hould control both tie-line and frequency deviation when ubjected to tep load diturbance. The control trategy adopted in the repective area i hown below; ACE Ptie2 B f () = Δ Δ () (7) ACE 2 Ptie2 B2 f 2 () = Δ Δ () (8) 3.2. Variable Structure Sytem Controller In the PI controller, a high proportional gain K P reult in a large change in the output for a given change in the error which may lead the ytem to untable condition. Similarly, a high integral gain K I reult overhoot, increaing harply a a function of the gain which i highly undeirable. Thi conflict between the tatic and dynamic accuracy in uing econdary PI controller can be reolved by employing the principle of Variable Structure Sytem (VSS) controller [- 3] i.e., witching from P controller to PI controller when the ytem i from tranient to teady tate period. The block diagram repreentation of a VSS controller i hown in Fig. 3. Figure 3: Block diagram of VSS controller At the firt tage of the tranient period, when the error i ufficiently large, the control law applied i; Δ Pref () = KP. ACE() for ACE >ε (9) where, ε i a contant, and when error i mall the control law i; KI Δ Pref () = ( KP ). ACE() for ACE ε () 88

5 J. Electrical Sytem 8- (22): The parameter of econdary VSS controller are tuned uing ZN method [7]. In ZN method, the proce i kept under cloed loop Proportional (P) control, the gain of the P controller at which the loop ocillate with contant amplitude being referred a the ultimate gain ( K cu ). Time between peak at thi etting in the utained ocillation i ultimate time period ( T u ). The proportional gain K P and integral gain K I are tuned uing Kcu andt u. ZN tuned value of econdary VSS controller i hown in Tab. I. Table. I: Tuned value of KP and KI for econdary VSS controller VSS controller K P K I ZN Tuning Method The threhold or witch value of VSS controller ε i optimized uing the performance indice for witching the ZN tuned value in the VSS controller. The performance indice [8] are a follow: a. Integral Square Error (ISE) b. Integral Time Abolute Error (ITAE) c. Integral Time Square Error (ITSE) The performance indice for different value of ε are tabulated in Tab. II. Table. II: Optimal witch value of VSS controller ε ISE ITAE ITSE From the table, the optimal witching threhold value i found to be.2 uing all the performance indice. The mathematical model of a two-area thermal plant with econdary controller i hown in Fig. 4. B R T H T T KP T P 2ΠT TH 2 TT 2 R2 KP2 T P2 B2 89

6 Vijaya Chandrakala et al: Damping of Tie Line Power Ocillation Uing VSS and UPFC Figure 4: Mathematical model of a two-area thermal power plant with econdary controller 4. UNIFIED POWER FLOW CONTROLLER Unified Power Flow Controller (UPFC), the FACTS device i a fat acting compenator. It control the power ytem parameter in term of voltage, phae angle and impedance. It can be ued not only for power flow control but alo for tabilizing the power ytem [8]. UPFC improve both teady tate and dynamic performance of the ytem. UPFC with damping controller further improve the dynamic performance of the ytem [9]. UPFC with damping controller i connected in erie with the tranmiion line or in a tie-line. It provide damping of tie-line power ocillation with fat control of voltage in maintaining the tability of the ytem. Schematic block of UPFC baed damping controller i hown in Fig. 5. Gain Wah-out Phae lead Compenator Figure 5: Block diagram of UPFC baed damping controller UPFC comprie of high pa filter known a ignal wah-out which prevent teady change in frequency by modifying input parameter. The wah out time contant Tw i not enitive and could lie in the range of to 2. T w of i taken a wah-out time contant. The phae compenator i a lead compenator whoe time contant are choen o that the ytem i fully compenated. The damping controller parameter are determined by mean of phae compenation technique [9]. The UPFC baed damping controller relate frequency deviation Δ f in term of damping controller m B. The tranfer function of the ytem relating equivalent electrical output power Δ Pe produced by the damping controller m B i hown in Fig. 6. Figure 6: Mathematical model of electrical power Δ Pe produced by 9

7 J. Electrical Sytem 8- (22): damping controller m B The mathematical model of a two-area thermal plant with econdary controller and UPFC baed damping controller i hown in Fig. 7. Figure 7: Mathematical model of a two-area thermal power plant with econdary controller and UPFC baed damping controller 5. SIMULATION RESULTS The two area thermal power plant hown in Fig. 2 i developed uing MATLAB / Simulink [2], and i ubjected to a unit tep load diturbance in area alone. The ytem repone i hown in Fig. 8. The repone how teady tate error pertaining to ocillation in frequency and tie-line power. Later, econdary PI controller, i dicued in ection 3.2 i included to the two-area thermal power plant a hown in Fig. 4. The interconnected thermal plant i ubjected to a unit tep load diturbance in area alone. The repone with econdary PI controller provide better reult with reduced peak overhoot attaining zero teady tate error, when compared with the open loop repone a hown in Fig. 8. del f (p.u.) Without Secondary Controller Secondary PI Controller del Ptie2 (p.u.) del f2 (p.u.) - Time (ec) x -3 Without Secondary Controller Secondary PI Controller - Time (ec) Without Secondary Controller Secondary PI Controller - Time (ec)

8 Vijaya Chandrakala et al: Damping of Tie Line Power Ocillation Uing VSS and UPFC Figure 8: Comparion repone of two area thermal power plant with open loop and econdary PI controller Further, the econdary PI controller i replaced by VSS controller a hown in Fig. 4 i imulated. The repone of the plant with econdary PI controller and VSS controller i compared and furnihed in Fig. 9. del f (p.u.) Secondary PI Controller Time (ec) del Ptie2 (p.u.) del f2 (p.u.) 4 x -3 Secondary PI Controller Time (ec) Seconday PI Controller Time (ec) Figure 9: Comparion repone of two area thermal power plant with econdary PI controller and VSS controller VSS controller relatively reduce the tranient in frequency and tie-line power with zero teady tate error at fater rate. From the Fig. 9, it how the ocillation in tie-line power till perit for a while. Therefore, VSS controller with UPFC damping controller i included in two-area thermal power plant ubjected to tep load diturbance in area alone a hown in Fig. 7 i imulated and it comparion repone i hown with VSS controller in Fig.. del f (p.u.) del Ptie 2 (p.u.) del f2 (p.u.). -. with UPFC Time (ec) x -3 with UPFC Time (ec) with UPFC Time (ec) Figure : Comparion repone of two area thermal power plant with VSS controller and VSS controller with UPFC. Uing UPFC, the frequency ocillation are controlled in ame phae with VSS controller. The improvement in tie-line power ocillation provide much better ytem repone with VSS and UPFC in an interconnected thermal power plant than any other controller. 92

9 J. Electrical Sytem 8- (22): CONCLUSION The ytematic procedure for improving the ytem dynamic performance of an interconnected thermal power plant i preented in thi paper. Two-area thermal power plant when ubjected to unit tep load diturbance lead to frequency and tie-line power ocillation with teady tate error and tranient overhoot. The tranient ocillation in frequency and tie-line power i reduced uing ZN tuned econdary PI controller. In econdary PI controller, a high proportional gain reult in a large change in the output during the teady tate and high integral gain reult overhoot during the tranient period. Thu, it i overcome by uing VSS controller. A VSS controller witche in between P and PI controller, it help in achieving improved ytem frequency and tie-line power repone. But, the tranient tie-line power ocillation perit for a while which i not deirable. Therefore, UPFC connected in interconnected tie-line help to tabilize the ytem tie-line power ocillation at a fater rate. The two-area thermal power plant with VSS controller and UPFC yield much better fater repone in term of reduced peak overhoot, control the tranient frequency ocillation and reduce the tie-line power ocillation with zero teady tate error. LIST OF SYMBOLS - - H - T - P - P - PD R, R 2 : Speed regulation of thermal ytem; = 2Hz/pu MW T : Hydraulic amplifier time contant; =.8 ec T : Non-reheat turbine time contant; =.3 ec K : Power ytem gain contant; = T : Power ytem time contant; = 2 ec Δ, Δ PD2 : Change in load demand power in area and area2 repectively; =. p.u. A : Synchronizing power coefficient; = - δ : Operating voltage angle of the tie-line; = 45 - T : Synchronizing coefficient; =% of area capacity =. Co δ 2 =.77 - B, B 2 : Frequency bia contant; =.425 p.u. MW/Hz - Δ Pref, Δ Pref 2 : Change in reference power in p.u.; - Δ Pg : Change in governor power of the thermal ytem in p.u.; - Δ PH : Change in hydraulic valve power of the thermal ytem in p.u.; - Δ PT : Change in turbine power of the thermal ytem in p.u.; - Δ f, Δ f 2 : Change in frequency of area and 2 repectively in Hz; - Δ Ptie2 : Change in tie-line power in p.u.; - : Laplace operator; - K : Contant of the UPFC baed damping controller ; - K pd =.323; K pb =.6667; K 2 =.4567; K 6 =.834; K 3 =.625; K a = ; T a =. ec; K qb =.68; K qd =.524; K vb = -.97; vd K 8 = 26; 9.; REFERENCES T do ' = 5.44 ; K = -.7; K = -.7; K cb =.763; T =.3383 ec; T 2 =.76 ec; K dc = [] O. I. Elgerd, Electric Energy Sytem Theory An Introduction, Second Edition, Tata Mc Graw-Hill, 2. [2] A. J. Wood and B. F. Wollenberg, Power Generation Operation & Control, John Wiley & Son,

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