Dynamic Parameter -PI Control Method of STATCOM for Voltage Stability: Self-Adjustable Approach

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1 ISSN (ONLINE): , ISSN (PRINT): Volume-6, Issue-1, January-February-2016 Inernaional Journal of Engineering and Managemen Research Page Number: Dynamic Parameer -PI Conrol Mehod of STATCOM for Volage Sabiliy: Self-Adjusable Approach Shivam Singh 1, Prof. A.P.Chaudhary 2 1,2 Deparmen of Elecrical Engineering, Sri San Gadge Baba College of Engineering &Technology, Bhusawal, Maharashra, INDIA ABSTRACT Power sysem volage sabiliy is an imporan facor for mainaining high sandards of power qualiy supplied o he consumer. Among many FACTS devices which can provide reacive power compensaion o mainain he volage sabiliy he STATCOM is quie fas and efficien. In he previous works regarding he subjec many conrol mehods of STATCOM have been described among which many are Proporional- Inegral Conrollers. Bu he problem wih hese previous works is ha hey obain he PI conrol parameers/gains by rial& error or by deailed sudies bu wih compromise in balance of performance and applicabiliy. Thus he conrol parameers for an opimal resul a a given operaing poin may no be efficien a a differen poin of operaion. This paper proposes a relaively new model of conrolling he STATCOM via dynamic PI conrol, which has he capabiliy o dynamically adjus and adap he conrol parameers during a disurbance such ha he resul/performance always maches he desired response, regardless of differen or changing poin of operaing condiions such a ransmission nework change, varying load levels, differen inial conrol parameers, consecuive and severe disurbances. While he radiional STATCOM conrol wih fixed and uned PI parameers may give a fine performance in he original sysem, bu may no perform as effecive as he proposed conrol mehod when he sysem condiions are changed. Keywords volage sabiliy, conrol mehod, STATCOM, PI Conrol. I. INTRODUCTION For beer securiy and reliabiliy in power sysems, volage sabiliy is an imporan and criical consideraion. In he las en years, STATCOM devices based on GTO have gained much ineres o improve volage sabiliy via reacive power conrol. Many works have been done in he pas, which have focused on he conrol logic design of he sacom device, however very few have been done which focus on seing of he proporional inegral gains/parameers. In some previous works PI conrol parameers are designed via rial and error approach. Bu even if he conrol gains are uned o sui a paricular scenario, he performance of he STATCOM will no be effecive if a considerable change in he sysem condiions happen such as line upgradaion or reiremen, or more worse, if ransmission opology is changed.. Differen from hese previous works, he aim of his paper is o propose a conrol mehod ha ensures fas and consisen desired response when he sysem operaion condiions are changed. In oher words, he change of he exernal condiion will no deeriorae he performance, such as slower response, overshoo, or even insabiliy o he performance. Base on his fundamenal goal, a dynamic parameer PI conrol of STATCOM for volage regulaion is presened in his paper. Wih his conrol mehod, he PI conrol parameers can be selfadjused auomaically and dynamically under differen disurbances in a power sysem. When a disurbance occurs in he sysem, he PI conrol parameers for STATCOM can be compued auomaically in every sampling ime period which can be adjused o rack he reference volage. This mehod is free off he limiaions imposed by human experience and judgmen in seing he iniial parameers. II. STATCOM MODEL: TRADITIONAL APPROACH a) Sysem Configuraion: 342 Copyrigh Vandana Publicaions. All Righs Reserved.

2 ISSN (ONLINE): , ISSN (PRINT): The figure below shows an equivalen circui of he STATCOM. The resisance represened by R s in series wih he VSI represens he sum of inverer conducion losses wih ransformer winding resisance losses. The inducance represens he inducance caused by leakage flux of he ransformer. The parallel combinaion of R c and he capacior C is he sum of swiching and power losses respecively d d = i ds i qs V dc R s ω 3K 2C cos α ω R s K cos α K sin α 3K 2C sin α 1 R c C 1 V dl V ql 0 i ds i qs V dc (5) Figure 1: STATCOM model In figure 1 he hree phase STATCOM oupu volages are shown by V as, V bs, and V cs. The 3-phase bus volages are shown by V al, V bl, and V cl. The 3-phase STATCOM oupu currens are shown by i as, i bs, and i cs. b) STATCOM dynamic model The mahemaical expression for hree phases of STATCOM can be represened by he following form di as d = R si as + V as V al (1) Where i ds and i qs are d and q currens corresponding o i as,i bs and i cs. K is a facor ha relaes he dc volage o he peak phase-o-neural volage on he ac side; V dc is he dcside volage; α is he phase angle a which he STATCOM oupu volage leads he bus volage; ω is he synchronously roaing angle speed of he volage vecor; V dl andv ql represen he d and q axis volage corresponding o V al, V bl and V cl and axis volage corresponding o, and. Since V ql =0, based on he insananeous acive and reacive power definiion, (6) and (7) can be obained as follows: p l = 3 2 V dl i ds q l = 3 2 V dl i qs (6) (7) di bs d = R si bs + V bs V bl (2) The radiional mehod of conrol is derived based on above wo equaions and he radiional STATCOM conrol block diagram is shown as below: di cs d = R si cs + V cs V cl (3) d 1 d 2 CV dc 2 = V as i as + V bs i bs + V cs i cs V dc 2 R c (4) The equaions from (1) o (4) can be rewrien based on abc/dq ransformaion as: Figure 2: Tradiional STATCOM Conrol Block Diagram 343 Copyrigh Vandana Publicaions. All Righs Reserved.

3 ISSN (ONLINE): , ISSN (PRINT): As shown in he figure above, he measured bus volage is firsly compared wih he reference bus volage and hen he resuling signal is passed down o PI volage regulaor which convers he differenial signal ino reference recive curren i q wih he help of Regulaion slope Kd. The reference reacive curren is compared wih acual STATCOM reacive curren and is passed on o PI curren regulaor which hen convers he differenial signal ino reference angle by he same process an in previous sage. The PLynchronizes he bus volage wih he STATCOM volage. The combined angle of α+θ is fed ino Converer gae paern logic o obain he oupu volage of STATCOM. III. DYNAMIC PI CONTROL: SELF ADJUSTABLE APPROACH A) Concep of proposed approach If he operaing condiions of he proposed approach changes,i.e if for insance a change in load or ransmission nework akes place, he STATCOM wih fixed PI conrol parameers may no perform in desired way. A dynamic PI conrol mehod is proposed in his secion in order o mee he desired response and o avoid he rial and error approach of finding suiable parameers for PI conrollers when a new STATCOM is insalled in a power sysem. Wih his mehod, he dynamicselfadjusmen of Pi conrol parameers can be realized. A dynamic PI conrol block for STATCOM is shown in Fig. 3. as below. Figure 3: Dynamic Parameer PI conrol block diagram V m, V ref, I qref and I q are all in per uni values. The proporional and inegralpars of he volage regulaor gains are denoed by K pv and K iv respecively. Similarly, he gainsk pi and K ii represen he proporional and inegral pars, respecively, of he curren regulaor. Here we have se he allowable volage error K d o zero. All oher parameers such as K pv K iv K pi and K ii can be assigned any arbirary value such as simply 1.0. The desired curve which is chosen for seing as he reference volage in he ouer loop is depiced as shown below. I is an exponenial curve in erms of he volage growh. The blue line shows he reference volage or he ideal response we would wan from he sysem under a disurbance. The doed line is he measured volage. We can clearly see ha he measured volage is coninuously being correced o follow he blue line via he dynamic PI Figure 4 : Reference volage curve conrol mehod unil i reaches he red line of 1 p.u value of sysem volage, i.e he voage before he disurbance in he sysem or seady sae volage. We can choose any oher curve, he only requiremen being ha he measured volage reurns back o seady sae value. The process of volage conrol hrough Dynamic PI conrol mehod is as follows: 1) In he firs sep, he bus volage V m is measured. 2) If he measured volage V m V ss, he measured value is compared wih he reference value and he difference is used as an inpu o he ouer loop of he conrol sysem in he PI volage regulaor block where he volage difference is convered ino q-axis reference curren i qref. 3) In he nex loop he q-axis curren hus obained is compared wih he q-axis curren i q. Using he similar conrol mehod like he one for he previous loop, he parameers can be adjused based on he error. Then, a suiable angle can be found and evenually he dc volage in STATCOM can be modified such ha STATCOM provides he exac amoun of reacive power injeced ino he sysem o keep he bus volage a he desired value. 344 Copyrigh Vandana Publicaions. All Righs Reserved.

4 ISSN (ONLINE): , ISSN (PRINT): B) Derivaion of parameer calculaing equaions The required parameers need o be consanly calculaed o ge a dynamic value. The dynamic values of K p and K i for boh inner and ouer loops need o be ime dependen equaions in such a way ha hey help in minimizing he error beween reference volage and measured volage. Since he inner loop conrol is similar o he ouer loop conrol, he mahemaical mehod o auomaically adjus PI conroller parameers in he ouer loop is discussed in his secion for illusraive purposes. A similar analysis can be applied o he inner loop. Firs of all, he d and q componens of line volage can be calculaed as follows:- HereT s = sample ime, se o s. In his sysem discree ime inegraor block is used o simulae a purely discree sysem. By Forward- Euler mehod, he resuling equaion for oupu of discree-ime inegraor block a is y = y T s + K iv T s T s V T s (12) Where y = K iv Puing +T s V y T s = K iv T s d and T s y T s = I qref V T s d So, V dl V ql 0 = V al V bl V cl V m = V dl 2 + V ql 2 The reference volage as described previously can be se as: V ref = V ss V ss V m e τ (8) (9) (10) In he above equaion, V ss is he desired seady sae volage, se o 1.0 p.u in his discussion.v m is measured volage ;τ = 0.01 seconds. V ref is as shown in figure. When he sysem is in normal operaing condiion hen V m will be 1 p.uand hence, V ref = 1 p.u. This implies ha K pv and K iv will no change and he STATCOM will no undergo any reacive power exchange wih he sysem bus o mainain he volage a he same. Bu if here is a disurbance in he sysem, he K pv and K iv will become self-adjusable due o equaion (10) and he STATCOM will provide he necessary reacive power o improve he volage profile. The error beween he reference volage and measured volage, is denoed by ΔV. Based on he dynamic PI conrol model, he following equaion can be obained a any ime insan. +T s ΔV K pv + K iv ΔV d = I qref + T (11) s (11) can be re-wrien as: +T s V K pv + K iv V K iv T s V T s d T s = I qref + T s I qref For a very shor period of ime, we can consider K iv = K iv T s Hence, (13) can also be re wrien as V K pv + K iv 345 Copyrigh Vandana Publicaions. All Righs Reserved. +T s A d d = I qref + T s I qref (13) (14) Where A = V V T s The desired values of K pv and K iv can b found ou if we can deermine wo ideal raios I qref + T s I qref and V K iv K pv Assuming a he ideal response, we have I qref + T s I qref = R V (15) Since he sysem is generally sable, we assume ha he bus volage will come back o 1 pu in 5τ. Also, since he iniial value I qref 0 =0, he equaion (11) can be wrien as

5 ISSN (ONLINE): , ISSN (PRINT): ΔV o K pv o + K iv o = R V o o +5τ ΔV d o Here, o = ime when disurbance occurs. Puing K iv o = 0, we ge K pv o = R (17) (16) Puing K pv ( 0 )= 0, we hen have K iv o = ΔV o R o +5τ (18) ΔV d o By above wo equaions he ideal raio m V = K iv o can be obained. K pv o Equaion (15) can be wrien as- I qref + 5τ I qref = k v V o (19) K pi = k I I q I q +m +Ts I Bd K ii = m I K pi (26) (25) C) Flowchar of dynamic PI conrol Flowchar of he proposed mehod is as shown in he figure 5. The dynamic PI conrol mehod sars by measuring he bus volage V m and hen comparing i wih he seady sae value V ss. If he measured value is equal o he seady sae value, i means here is no disurbance and volage is sable so all he parameer values K pv, K iv K pi and K ii are kep he same as before. Here, k v is he ideal raio, k V = I qref + T s I qref V According o he sysem bus capaciy and STATCOM raingδv max can be obained So, V o = k ΔV V ΔV o K pv o +K iv o max R o +5τ ΔV d o Based on he equaions (16),(19),(20) and (21), k V can be calculaed as k v = R V 0 K pv 0 V 0 + K iv o +5τ 0 ΔV d o (20) (21 ) V So, o calculae he PI conroller parameers based on (14), we can ge +T s V K pv + m V K pv A d = k V V (22) Therefore K pv nd K iv can be calculaed as k V V K pv = +T V + m s V A d K iv = m V K pv Using he similar derivaion, he values of K pi and K ii can be derived as (23) (24) Figure 5 : Flowchar of dynamic conrol mehod However if V m V ss, he K pv and, K iv are adjused based on equaions (23) and (24). Then from he volage regulaor block, he required I qref is obained and is compared wih I q and wih he similar process in he earlier block, K pi and K ii are calculaed based on equaion (25) and (26). Then, he phase angle α is 346 Copyrigh Vandana Publicaions. All Righs Reserved.

6 ISSN (ONLINE): , ISSN (PRINT): deermined and is hen passed hrough a limier for oupu, which hen finally decides he reacive power oupu from he STATCOM. In he nex sep if V is no wihin a olerance hreshold, which is a very small value such as p.u., he volage regulaor block and curren regulaor blocks are again processed in he loop unil he change is less han he given hreshold. Thus, he values fork pv, K iv, K pi and K ii are mainained. If here is he need o coninuously perform he process of volage conrol, which is generally he case, hen he process reurns o he measured bus volage. Oherwise, he volage-conrol process is deacivaed. IV. SIMULATION RESULTS wo successive disurbances are simulaed a 0.2 s and 0.25s. CASE STUDY 1- Response of original/iniial sysem In he original model, we have kep K pv = 12. K iv = 3000, K pi = 5 and K ii = 40. All he parameers are kep unchanged.. Iniially he volage surce a 1.0 pu, a volage base 500 kv. In his case if we se R=1, hen we have iniial m v = Since, in his case,δv 0 = V max and k V = , based on (23)-(26), we have V K pv = +T V s A d (27) The sysem which is o be sudied is as shown in he figure below. K iv = K pv I q K pi = +T I q s B d (28) (29) K ii = K pi (30) Fig 6 Sysem o be sudied In he above figure, a ±100-MVAR STATCOM is implemened wih a 48-pulse volage source converer and is conneced o a 500-kV bus. This sandard sysem is in he Malab/Simulink library. All he machines which are used in he simulaion are dynamical models. We will simulae he above sysem in STATCOM bus volage regulaion mode and he radiional fixed PI parameer approach will be compared by he proposed dynamic PI conrol mehod. Firs of all we will ry o compare hese wo mehods in firs case where original model is as shown in figure 6. Then we will ry o compare hese wo mehods by considering some changes in he operaing condiions of he sysem, - such as by changing he parameers o a differen value han in he original model. Also, we will es he efficiency of boh he mehods by considering a change in load, a change in ransmission nework, and response under a sudden large disurbance. Assuming he seady sae volage,v ss = 1.0 pu. In case sudy 1,2 and 3 a disurbance is simulaed a 0.2 s a he source. In case sudy 4, a volage rise is caused by assuming a volage disurbance a 0.2 seconds under a change in ransmission nework. In case sudy 5 Based on equaions (27) (30), he dynamic PI conrol sysem is designed. The resuls are shown in Figure 7 and 8. The observaions are abulaed in Table 1. As eviden from he resuls, he dynamic PI conrol sysem can achieve faser response han he original sysem. Figure 7 : Response of original sysem by radiional conrol 347 Copyrigh Vandana Publicaions. All Righs Reserved.

7 ISSN (ONLINE): , ISSN (PRINT): K ii = K pi (34) Based on equaions (31) (34), he dynamic PI conrol sysem is designed. The resuls are shown in Figure 7 and 8. The observaions are abulaed in Table I. As eviden from he resuls, he original model is unable o make he bu volage ge back o 1 pu, and hence he STATCOM performs poorly. Alhough, in he case of dynamic PI conrol, he STATCOM responded smoohly as desired and volage is revived back o 1 pu quickly wihin 0.1 seconds. Figure 8 : : Response of original sysem by dynamic conrol TABLE I : COMPARING ORIGINAL AND DYNAMIC CONTROL Original Conrol Dynamic Conrol Lowes volage afer disurbance p.u p.u Figure 9 :Response of sysem wih original conrol and changed PI parameer values Time (sec) when V= seconds seconds Δ o reach V= seconds seconds CASE STUDY 2- Changed PI Parameers In his case, all he oher sysem parameers remain unchanged while he PI conroller parameers for he original sysem are changed o K pv = 1 K iv = 1, K pi = 1 and K ii = 1 The dynamic conrol parameers which depend on he pos-faul condiion are given as V K pv = +T V s A d (31) Figure 10 : Response of sysem wih dynamic conrol and changed PI parameer values K iv = K pv (32) I q K pi = +T I q s B d (33) CASE STUDY 3- Changed load condiion In his scenario, he original PI conroller parameers are kep o heir original values, i.ek pv = 12. K iv = 3000, K pi = 5 and K ii = 40. The load is changed from 300 o 400 MW. In his case, we have given he dynamic conrol gains by 348 Copyrigh Vandana Publicaions. All Righs Reserved.

8 ISSN (ONLINE): , ISSN (PRINT): V K pv = V T s A d (35) K iv = K pv (36) I q K pi = +T I q s B d (37) K ii = K pi (38) Based on equaions (35) (38), he dynamic PI conrol sysem is designed. The resuls are shown in Figure 7 and 8. The observaions are abulaed in Table II. I is eviden from he resuls ha he dynamic PI conrol provides a beer performance han radiional conrol under changed load condiion. TABLE II : COMPARING ORIGINAL AND DYNAMIC CONTROL IN THE CASE OF CHANGE OF LOAD Original Dynamic Conrol Conrol Lowes volage afer disurbance Time (sec) when V= pu pu seconds seconds Δ o reach V= seconds seconds CASE STUDY 4 Change in ransmission nework In his case, he PI conroller parameers remain unchanged, as in he original sysem, bu here line 1 is swiched off a 0.2 seconds o represen a differen nework which approximaely represens a scheduled ransmission mainenance. Here, we ge, V K pv = V T s A d (39) K iv = K pv (40) Figure 11 : Response of sysem wih original conrol and changed load condiion I q K pi = +T I q s B d (41) K ii = K pi (42) Based on equaions (31) (34), he dynamic PI conrol sysem is designed. The resuls are shown in Figure 7 and 8. The observaions are abulaed in Table III. Figure 12 : Response of sysem wih dynamic conrol and changed load condiion 349 Copyrigh Vandana Publicaions. All Righs Reserved.

9 ISSN (ONLINE): , ISSN (PRINT): CASE STUDY 5- Response under wo successive disurbances In his paricular condiion, wo disurbances are simulaed successively, firsly by creaing a volage disurbance a 0.2 seconds and hen swiching off one line a 0.25 seconds. The resuls are displayed in he figures 15 and 16 from which i is obvious ha he dynamic conrol has a faser response han he original one and is hence more effecive in hese siuaions. Figure 13: Response of sysem wih original conrol and changed nework condiion Figure 14: Response of sysem wih dynamic conrol and changed nework condiion In his case he STATCOM absorbs he reacive power from he sysem, herefore he disurbance gives a volage rise a subsaion A from 1 o 1.01 pu. The radiional conrol is unable o handle his scenario and over absorpion of reacive power by STATCOM occurs. While in he Dynamic PI conrol mehod gives a much effecive and faser response. TABLE III : COMPARING ORIGINAL AND DYNAMIC CONTROL IN CASE OF CHANGED NETWORK Original Dynamic Conrol Conrol Lowes volage afer disurbance Time (sec) when V= pu pu seconds seconds Δ o reach V= seconds seconds V. CONCLUSION Many works in he lieraure regarding volage sabiliy using STATCOM have already discussed various aspecs of STATCOM conrol. Bu very few works have been done in he pas which alk abou changing he parameers of PI conrol loops dynamically in he STATCOM conrol sysem. In his paper we compared he old radiional mehod of STATCOM conrol wih he proposed mehod of dynamic conrol which will self adjus 350 Copyrigh Vandana Publicaions. All Righs Reserved.

10 ISSN (ONLINE): , ISSN (PRINT): he parameers o obain a quick and smooh response. Toal 5 case sudies were sudied in his paper esing he sandard sysem under various differen operaing condiions and in all he case sudies, we observed ha dynamic PI conrol mehod is beer han he old mehod of saic PI conrol. REFERENCES [1] Yao Xu; Fangxing Li, "Adapive PI Conrol of STATCOM for Volage Regulaion," in Power Delivery, IEEE Transacions on, vol.29, no.3, pp , June 2014 [2] A. Jain, K. Joshi, A. Behal, and N. Mohan, Volage regulaion wih STATCOMs: Modeling, conrol and resuls, IEEE Trans. Power Del., vol. 21, no. 2, pp , Apr [3] D. Soo and R. Pena, Nonlinear conrol sraegies for cascaded mulilevel STATCOMs, IEEE Trans. Power Del., vol. 19, no. 4, pp , Oc [4] F.Liu, S.Mei, Q.Lu, Y.Ni,F.F.Wu, and A.Yokoyama, Thenonlinear inernal conrol of STATCOM: Theory and applicaion, In. J. Elec. Power Energy Sys., vol. 25, no. 6, pp , [5] C. Hochgraf and R. H. Lasseer, STATCOM conrols for operaion wih unbalanced volage, IEEE Trans. Power Del.,vol.13,no.2,pp , Apr [6] G. E. Valdarannma, P. Maavalli, and A. M. Sankonic, Reacive power and unbalance compensaion using STATCOM wih dissipaiviy based conrol, IEEE Trans. Conrol Sys. Technol., vol. 19, no. 5, pp , Sep [7] H. F. Wang, Phillips-Heffron model of power sysems insalled wih STATCOM and applicaions, Proc. Ins. Elec. Eng., Gen. Transm. Disrib., vol. 146, no. 5, pp , Sep [8] H. F. Wang, Applicaions of damping orque analysis o sacom conrol, In. J. Elec. Power Energy Sys., vol. 22, pp , [9] Y.Han,Y.O.Lee,andC.C.Chung, Modified non-linear damping of inernal dynamics via feedback linearisaion for saic synchronous compensaor, IET Gen. Transm. Disrib., vol. 5, no. 9, pp , [10] H. Norouzi and A. M. Sharaf, Two conrol schemes o enhance he dynamic performance of he STATCOM and SSSC, IEEE Trans. Power Del., vol. 20, no. 1, pp , Jan [11] Narain G Hingorani, Laszlo Gyugyi, Undersanding FACTS: Conceps and Technologies of Flexible AC Transmission Sysems, IEEE Press, 1s ediion,2000. [12] K.R Padiyar, FACTS Conrollers in Power Transmission and Disribuion, New Age Inernaional Press, 1s ediion, Copyrigh Vandana Publicaions. All Righs Reserved.