A SVC Based Control Algorithm for Load Balancing

Transcription

1 Proeedings of the 7th WSEAS nterntionl onferene on Power Systems, eijing, hin, September 5-7, 7 A S sed ontrol Algorithm for od lning AHAD KAZEM kzemi@iust..ir A. MORAD KOOH Arshmordi@ee.iust..ir Ele. Eng. Deprtment, rn University of Siene nd Tehnology Abstrt: - An lgorithm for pplying fixed pitor-thyristor-ontrolled retor (F-TR type of stti vr ompenstor (S for dynmilly blning system is introdued. With developed individul phse ontrol sheme, n S n redue negtive-sequene urrent used by the lod to improve system blning. A ompt ontrol lgorithm for retive power ompenstion nd lod blning with the stti vr ompenstor (S in three-phse systems is used in this pper. The lultion of ompenstion suseptnes is bsed on the riterion of unity power ftor nd zero sequene urrents fter ompenstion. This Method is simulted on the EEE node system blning nd totl hrmoni distortion (THD is studied in unblned lod node nd Substtion bus nd results is presented. Key-words: - Distribution system, Negtive-sequene urrent, Stti vr ompenstor, od blning NTRODUTON The lod is lwys hnging nd is normlly unblned in distribution system. mblne loding ondition uses negtive sequene urrent. Negtive sequene urrent will inrese the resistive losses in field winding nd dmper winding of genertor []. esides it my use overheting nd instility problems. n distribution system, unblned loding inreses power loss in the line nd prevent using the whole pity of line. Furthermore it uses voltge symmetry t utility ends. oltge symmetry of n order of only.5% my results in 5% inrese in motor temperture rise [], Power eletronis devies pper to be sensitive to voltge symmetry too. Nmely supply symmetry my use generting urrent hrmonis of non-hrteristi orders suh s third order hrmoni []-[4]. There is lso mehnism in systems with indution motors tht mgnify the voltge symmetry. onsidering these problems in three-phse system, it is neessry to blne the lod so tht tive powers in three phses re equl. n priniple, the urrent of n unblned lod n be divided into zero, positive, nd negtive-sequene omponents. The zero-sequene urrent only influenes the viinity of the lod nd n be bloked by deltwye grounded trnsformer. f devie ould be found to orret the negtive-sequene urrent in onjuntion with the zero-sequene urrent bloking in the distribution system, one ould obtin blned system, s viewed from the trnsmission level. n this pper, the k ground of lod blning with S nd different onfigurtion of S is Reviewed, n prt. n prt, theory of S ompenstion is summrized. n prt, ompenstion suseptne is ome nd the formultion is given. n prt, se study is presented nd simultion re done. n prt, the results of simultion re presented. AK GROUND Thyristor-ontrolled suseptne (TS iruits, or ommonly lled stti vr ompenstors (Ss, hve been undergoing development sine the 97's [5]-[6]. The very first bsi struture developed ws just like the one presented in the ove pper, nmely, TS nd pitor in prllel [7]. However, the pplitions of suh TS's hve been limited to minly blned systems. t hs been well known tht thyristor swithed indutor is gret soure of urrent hrmonis, in prtiulr, the third hrmoni. A single-phse TS with bsi struture my hve urrent hrmoni distortion (HD of the order of 5% t some firing ngles [7]. When three of suh TS's re onneted in delt onfigurtion, with symmetril ontrolling of firing ngles, this ompenstor is very effetive for ompensting the retive power of the lod without the injetion of third hrmoni urrent to the system. However, it my not be dequte for use in n unblned system, sine it requires, most likely, n symmetri ontrolling of firing ngles. When this ours, the third hrmoni urrents generted by three TS brnhes re in phse, but different in their mgnitudes, nd therefore, tremendous mount of HD is introdued (minly, the third hrmoni into the system. There is nother potentil problem worth mentioning when using TS with suh struture, nmely, resonne between the shunt pitor nd the system indutne my our. All of these ove-mentioned drwks redue the ppliility of suh TS.

2 Proeedings of the 7th WSEAS nterntionl onferene on Power Systems, eijing, hin, September 5-7, 7 To overome the resonnt problem between the shunt pitor nd the system indutne, series indutor n be inserted. To eliminte the third hrmoni urrent generted by TS, hrmoni filter tuned to the third hrmoni frequeny n be employed. A number of TS's with vrious strutures, s shown in Fig., hve been developed, studied, nd presented [6],[8]. With the onsidertions of minimizing hrmoni injetion to the system nd voiding the resonne between the ompenstor's pitor nd system indutor with minimum inrese of iruit omplexity, it ws onluded tht TS_S_PF, hving firly simple struture, ould possibly redue the HD to the order of %. A ompenstor with three TS_S_PF's in delt onfigurtion, ould be used to blne n unblned system dynmilly nd, therefore, ws termed n "dptive blning ompenstor" (A [6], [9]. obtined nd the negtive-sequene urrent n be ompensted by n S. This ide is illustrted in + A A [ ] + + o ( + j where is the symmetril omponent opertor nd the dmittne of the S is j, e ( ( j, nd ( j. Eqution ( n be divided into rel nd imginry prts nd will hve three vriles nd two equtions. An dditionl onstrint hs to be dded to obtin unique solution. From the prtil point of view, the following onstrint is seleted: + + K ( Fig. rious iruit onfigurtion of TS,s. THEOR OF S OMPENSATON To void the omplition of other zero-sequenerelted problems, delt-onneted S is used in the development., every brnh hs fixed pitor nd thyristor-ontrolled retor. Eqution ( is the equivlent suseptne or dmittne of eh brnh by ontrolling the firing dely ngles (α of thyristors. A bidiretionl symmetril ontrol is normlly dopted to void even-order hrmonis π α sin α ( α ( πω Where is the suseptne of the fixed pitors nd is the originl indutne of the thyristor ontrolled retor. As shown in Fig., the zero-sequene urrent n be bloked by the delt-onneted trnsformer, but the negtive-sequene urrent n still pss through the trnsformer nd go into the trnsmission system to deteriorte the performne of genertors or other mhines. Therefore, the purpose of this pper is to derive n lgorithm to blok the negtive-sequene omponent by djusting the suseptne of the S. Where is suseptne of ompenstor between phses nd b, is suseptne of ompenstor between phses b nd, is suseptne of ompenstor between phses nd. f the power ftor of the lod needs to be improved t the sme time, the right side of eqution ( n be hnged to the required vlue. For simpliity, K is set to zero in the following derivtion. ombining equtions ( nd (, one n get ll suseptnes of the S from mesured phse voltges nd urrents t the primry of the trnsformer[]. 4 OMPENSATON SUSEPTANE The digrm of the studied system is shown in Fig., where substtion feeds eletri power to n unblned lod through three-phse system. t is ssumed tht both the substtion bus voltges nd trnsmission line impednes re blned. The supersripts S, T,, nd, used in this pper, represent the substtion, trnsmission line, ompenstor, nd lod, respetively. For the purpose of the S is to redue the negtive sequene urrents nd imginry prt of positive sequene urrents, hene, the usul trnsformtion mtrix [] is defined t first to relte the phse nd sequene quntities s shown in Eq. (4. [ T] (4 Theoretilly, omplete ompenstion n be

3 Proeedings of the 7th WSEAS nterntionl onferene on Power Systems, eijing, hin, September 5-7, 7 Fig. A three phse system with n S t the lod bus. ( ( ( ( (7 Fig. nstlltion of S to blne the distribution lod. Where is voltge between phses nd b, Where is voltge between phses b nd, Where is voltge between phses nd, vlue of voltge, is zero sequene is positive sequene vlue of voltge, is positive sequene vlue of voltge. n Fig., it is ssumed tht the lod is liner, nd then the lod urrent n be expressed s b (5 Applying the symmetril omponent trnsform in Eq. (4 to both side of Eq. (6, we hve + ( ( + ( ( (6 The negtive sequene voltge is introdued by unequl voltge drops on the trnsmission lines with unblned urrents flowing. n generl, the negtive sequene voltge is smller thn its positive sequene ounterprt. To simplify the nlysis proedure, the negtive sequene voltge is negleted, tht is n order to redue the negtive sequene urrents nd imginry prt of the positive sequene urrents, it is required tht m( m( m ( (8 ( α euse n idel S ontins no power-onsuming element, it n be obtined from Eq. (6 tht. Hene, the ompenstion suseptnes ( of the S n be desribed by m( m( m( m( m( m( nd s: (9 Substituting Eq. (6 into Eq. (9, the ompenstion suseptnes n be expressed in terms of symmetril omponents of the lod urrents. v m v [( + ( ] [( + ( ] ( m Four vriles, mgnitudes nd ngles of nd, re required to obtin the urrent informtion in Eq. (. euse the sum of three-phse urrents is zero, the distribution ondition of three-phse urrents n be obtined by mesuring only two line urrents.

4 Proeedings of the 7th WSEAS nterntionl onferene on Power Systems, eijing, hin, September 5-7, 7 4 Hene, only two single phse P-Q meters, with voltge-oils onneted to phse nd nd urrentoils onneted to phse nd s tht shown in Fig., the so lled two-wttmeter onnetion method, re used to quire the four urrent vriles in Eq. (. n prtil implementtion, the voltge signl in Eq. ( is the verge of three line to line voltges mesured by three-phse voltge trnsduer. Sine the lod bus voltge is ssumed blned, the omplex form of the signls mesured by the P-Q trnsduers n be desribed s: ( ( ( + ( ( + ( + ( From Eq. (, the positive nd negtive sequene urrents n be expressed in terms of tive nd retive powers. ( ( ( ( ( ( ( - Define four outputs of P-Q trnsduers s: ( ( ( ( P jq P jq ( where ( P + P nd ( Q + Qd re the tive nd retive powers onsumed by lods, respetively. Substituting Eq. ( nd Eq. ( into Eq. (, we get the on-line ompenstion formuls for the S. The ompenstion formul shown in Eq. (4 is very ompt nd need only two single phse P- Q trnsduers, so tht it is very suitle for on-line ontrol. ( ( ( ( Q P ( p + Q ( p + P (4 5 ASE STUD The implemented S is instlled in distribution system simultor s shown in Fig.4. EEE node distribution system represents the distribution system simultor. An unblned lod is dded to node 675. The S tht onsists of - onneted thyristor ontrolled retors nd fixed pitors (TR-F is instlled t the lod node (node no 675. The trnsduer modules whih onsist of line-to-line tive power (P, retive power (Q, nd voltge ( trnsduers re instlled ner the unblned lod. A triggering signl is sent to eh thyristor to ontrol the ondution ngle of the TR in order to give the required ompenstion suseptnes. The tul vlues of retive powers nd voltges n be obtined similrly. These tul vlues re used to lulte ompenstion suseptnes of the S using Eq. (4. The needed suseptne of eh phse of the TR is given by the following. ( σ ( P, Q, (5 TR S The reltionship between ondution ngles nd orresponding suseptnes of the TR is shown in Eq. (. F Fig. 4 EEE node network. 6 SMUATON RESUTS Equtions (, (5 nd ( re used to blne lod urrents of node 675, while Ss re onneted to this node. THD, urrent wveforms, nd RMS urrent of nodes 675 nd 65 (Substtion bus re studied before nd fter Ss onnetion presented in fig. 5 nd 6. THDs before nd fter S onnetion is shown in tle. t n lso be observed tht the S genertes hrmoni urrents. The indued hrmoni urrents from n S would flow to the power distribution system nd hrm the power qulity. Hrmonis mitigtion tehniques suh s hrmoni filters should be employed in the design of S.

5 Proeedings of the 7th WSEAS nterntionl onferene on Power Systems, eijing, hin, September 5-7, ONUSON An lgorithm for pplying fixed pitor-thyristorontrolled retor (F-TR type of stti vr ompenstor (S for dynmilly blning system is proposed in this pper. Sine the phse suseptne of the S n be obtined from very simple funtion of voltge nd power signls, the lgorithm is very suitle for on-line ontrol. The required mesurement equipments re two single-phse P-Q trnsduers nd three-phse voltge trnsduer whih re ommerilly ville. Simultion results show tht, with the S, THD is gretly redued. The ontrol pproh n be esily dopted to other types of S. Referenes: [] Dniel Kuy-hyn hen, Design of n On-line Miroproessor sed ndividul Phse ontrol of Stti r ompenstor", EEE nt. onf. On dvnes in power system ontrol, opertion, nd mngement, Nov. 99 Hong. [] EPR report No. EM-7, "Tehniques for Energy onservtion in A Motor-driven Systems". [] vmini, R., nd Smith, W.J, "Third Hrmoni urrent from Unblned A/D onverters", Pro. EE,98,,(,pp [4] ude,., nd Shrf, A.M., "Unhrteristi Hrmonis used by A System mblne nd the Effet of Smoothing Retors", Pro. of the rd nt. onf. On Hrmonis in Power Systems, Nshville,USA, 988. Fig. 5 Unblned lod urrent without S [5]. Gyugyi, "Priniples nd pplitions of stti, thyristor-ontrolled shunt ompenstors," EEE Trns. Power App. Syst., vol. PAS-97, Sept./Ot. 978, pp [6]. S. zrneki, G. hen, Z. Stroszzyk, nd S. M. Hsu, "ompenstors with thyristor swithed indutors operted under degrded power qulity: Performne nd its improvement," in Pro. PQA 4th nt. onf. Power Qulity, Applitions nd Perspetives, My 9-, 995. [7] Shih-Min Hsu, "Disussion of Using A Stti r ompenstor to lne A Distribution System", EEE Trns. ol. 5, No. 5, 999, pp Fig. 6 od urrent with S Tle.RMS,THD,st, nd & rd Hrmoni of urrent in phse A [8]. S. zrneki nd S. M. Hsu, "Thyristor ontrol suseptnes for blning ompenstors operted under nonsinusoidl onditions," Pro. nst. Elet. Eng. Elet. Power Applit., vol. 4, no. 4,July 994, pp [9]. S. zrneki, S. M. Hsu, nd G. hen, "Adptive blning ompenstor," EEE Trns. Power Delivery, vol., July 995, pp [] J.H.hen,W.J.ee,M.S.hen,"Using A Stti s ompenstor To lning A Distribution System", EEE Trns. ol5,no.,999, pp []. J. Wu,. M. iw, nd S.. ee, "Miroproessor-sed Stti Retive Power ompenstors for Unblned lods," Eletri Power Systems Reserh, ol., No., 994