ALTERNATIVE WAYS TO ENHANCE PERFORMANCE OF BTB HVDC SYSTEMS DURING POWER DISTURBANCES. Pretty Mary Tom 1, Anu Punnen 2.

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1 ALTERNATIVE WAYS TO ENHANCE PERFORMANCE OF BTB HVDC SYSTEMS DURING POWER DISTURBANCES Pretty Mry Tom, Anu Punnen Dept.of Electricl n Electronics Engg. Sint Gits College of Engineering,Pthmuttm,Kerl,Ini prettytom8@gmil.com ABSTRACT- This pper proposes two controllers to mitigte the DC link voltge oscilltions in HVDC systems uring power isturbnces. The comprehensive nlyticl esign proceure is represente for both the controllers. The effectiveness of the controllers re verifie n evlute through simultion. Key wors BTB HVDC system, BckStepping Controller n Integrtor Fctor Controller.. INTRODUCTION As the globl popultion continues to grow, resources re becoming even more stretche.growing popultion nee more ln s well s equte electricl n communiction services n these must be provie in wy tht complies with the enviornmentl regultions. A power system epens on stble n relible control of ctive n rective power to keep its integrity.losing this control my le to system collpse. High Voltge Direct Current trnsmission is n economicl wy for long istnce trnsmission or interconnection of synchronous systems with ifferent fequencies. It is esirble to hve the high power electronics bse systems vilble uring ifferent power system fults. If the protection mesures trip the converter system it tkes severl frction of n hour epening on the size of the converter to ischrge the DC link n check the helthiness of the whole system. Hence severl prcticl methos hve been propose n implemente to keep the system live by suppressing minly the huge currents tht the semiconuctor switches supply uring fult. These fult currents re generte minly ue to inherent ely effect of switching on how the controller observes the isturbnces. In ition the overvoltges tht cn occur uring fult my excee the switching forwr/reverse blocking cpbility. Some of the vluble control structures for VSC bse Bck to Bck HVDC systems hve been propose bse on set of stnr Proportionl Integrl controllers, but DC link voltge control hs been foun s the min ifficulty. The oscilltory nture of the DC link voltge cnnot be vnishe with ny of the controllers. The other rwbck is tht the DC link ynmics will be observble to the lo sie. So these re the mjor rwbcks in HVDC systems uring power isturbnces.two controllers re propose to mp the DC link voltge oscilltions n lso to blnce the lo sie voltges.. VSC MODELING The Voltge Source Converter (VSC) moelling is one to obtin the stte spce eqution with which the controller cn be esigne.the implementtion of VSC in HVDC trnsmission for conversion of three phse AC to DC,permit continous n inepennt control of rel n rective power n lso provie ynmic voltge regultion. The figure below shows the schemtic representtion of voltge source converter moel. Fig. Voltge source converter moel In figure, E bc is the three phse source voltge, L s is the source inuctnce, R s is the source resistnce, V bc is the input voltge to the converter, C DC is the DC link cpcitor, R p is the loss resistor n V DC the c voltge obtine fter rectifiction. By pplying kirchoffs voltge lw to the moel the expression for inuctor current is obtine s shown in eqution n the expression for cpcitor voltge is obtine s eqution. lbc R s E bc Vbc I bc t L L L s s s

2 V I V I lbc X C R. C C DC DC DC lo DC p DC DC To mke benefit of ll ecoupling n constnt properties of two phse system compre to the three phse, the inuctor current n cpcitor voltge is converte to q reference frme, n the former n lter cn be obtine s equtions 3 n 4. I R s E V I W I q 3 t L L L s g s l R E V I W I t L L L q S q q q q s s s By pplying power blnce to the moel, n eqution for the DC link voltge cn be obtine s in eqution5. V E I V P D C D C 3 lo 5 t C R C C D C p D C D C The generl form of stte spce eqution is shown in eqution6, where X(t) is the stte vrible vector, U(t) is the input vector n e(t) is the isturbnce vector, X t AX t BU t Ke t 6 I E V X t I U t e t E q q V q V P D C l o Here the losses in the interfce tnsformer is neglecte, E q is lso consiere s zero since E is ligne with xis in the synchronous frme. Hence the stte spce equtions of the system tking n q components of current n DC link voltge s the stte vribles cn be represente by equtions 7,8 n 9. x (t) = x + x + 3 e + 4 u (t) (7) x (t) = x + x + 3 e + 4 u (t) (8) x 3(t) = 5 e x + 6 x e 3 (9) In the bove equtions the vlues to 7 cn be obtine by performing clcultions with the system prmeters. 3. CONTROLLER DESIGN During power system isturbnces the DC link voltge rops, but it shoul be mintine t constnt vlue inorer tht the esire power trnsfer tkes plce through the trnsmission line. The pper iscusses two controllers to mintin DC link voltge t require level. 4 A. Integrtor Fctor Controller(IFC) In IFC we select the input to the moultor in orer to get the esire response, which is to mintin the DC link voltge.firstly the controller is esigne for I q (x ) ynmics. The objective for I q controller is to hve ecouple n isturbnce rejection chrcteristics. This criteri is obtine if its input U hs the form of eqution. The first term in this eqution is responsible for the response time of the stte n the rest is for ecoupling, isturbnce rejection n commn following. 3 U t f X X - e +t X ref 4 4 Seconly the controller is esigne for the first stte vrible X. Here lso the objective is to hve ecouple n isturbnce rejection chrcteristics, however, this stte is controlle to mintin the require ynmics for X 3 lso s X 3 oes not ccess to irect input to control the stte irectly. Hence, the input for X shoul hve the form s in eqution. 3e U t W t f X X To crete n input s well s cncel out the non linerity of x 3 it is ssume X t e t t The vlue of x (t) cn be clculte from the bove equtions s eqution3. 4 f x t x t e t f f 4 4 e e 4w T 3 t The require vlue of new input which cn control the DC link voltge is then clculte s eqution4. W t t t u u f e t e t e t e t With the help of new control input W (t) stte X 3 cn be controlle irectly, n the expression for X 3 cn be written s eqution 5 n the new control input is given in eqution 6. x t x e e 7 3 t f x t x ref 5 4

3 Figure shows the block igrm of integrtor fctor control which is use to generte switching signls to the converter. V c V cr P lo 6 4 Moultor The vlues of f n f 3 re selecte so s to obtin the require input to the moultor.the constrint is tht the moe ssocite with Z shoul be fster thn tht of Z 3 where Z n Z 3 hve the nture of the current n DC link voltge respectively, n hence the controller follows the well known rule in power electronics to esign the current controller fster thn the voltge one. Iq Iqref I V cref V c 9 Fig Integrtor fctor control moel P lo E The stte now hs virtul input which cn control the stte irectly. Among mny possibilities of input choice typicl form of input signl is selecte to control the response time, isturbnce rejection n commn following. I Iq Moultor B. Bck Stepping Controller(BSC) Iqref In BSC control signls re esigne for nonliner systems by recursive technique in which we cn esign feebck controls n fins Lypunov functions for set of n incresingly complex systems, the lst system being of interest.the funmentl of the ie cn be interprete s the locl control of the sttes tht o not ccess to irect input. Here, some sttes re use s pseuo control to stbilize other sttes by introucing some virtul stte vribles representing the ifference between the ctul n virtul control. In this cse the secon stte vrible cn be controlle irectly n so the system reuces to two stte system. The chnge of the coorintes to Z in eqution7 inictes tht X shoul tke whtever vlue to mke the error Z in eqution8 null corresponing to chieve the reference Z 3. Z = X (7) z = x α(z, e ) (8 ) The virtul control is chosen s in eqution9 to meet the requirement viz choosing the correct Lypunov function n mking their erivtives zero, thereby controlling the thir stte vrible which is the DC link voltge. Hence the input signl is chosen sw (t) s in eqution. (Z, e ) = f Z e (9) W (t) = z z α(, ) + α(, ) () Fig 3 Bck Stepping Control moel 4. PERFORMANCE OF PROPOSED CONTROLLERS The system performnce uner ifferent fult conitions re stuie with prmeters s shown in tble. Line to line voltge Lekge inuctnce DC link voltge Loss resistor Line frequency. V.36mH V 54Ω 6Hz Tble. Simultion prmeters An HVDC system is moele with the bove prmeters n the DC link voltge of the system is obtine s shown in figure4. Then the system is subjecte to single phse to groun fult n three phse fult then the DC link voltge is seen to rop. Then the controllers re implemente to provie switching signls to the converters n performnce is stuie through simultion. The performnce of the controllers is ienticl n the oscilltions o not

4 pper in the prescence of propose controllers.the phse compenstion is mjor contribition of the propose controllers to suppress the DC link voltge oscilltions. DC link voltge with IFC 5. RESULTS AND DISCUSSION The HVDC system uner ifferent fult conitions re observe to check the fesibility of the propose controllers. Vc(V) DC link voltge with BSC DC link voltge of BTB HVDC system Vc(V) Vc(V) Fig 4 DC link voltge of the system.single phse to groun fult DC link voltge of BTB HVDC system subjecte to single phse to groun fult Fig6 Effect of controllers on DC link voltge uner single phse to groun fult. Three phse fult When the HVDC system is subjecte to three phse fult,there is much severe fll in DC link voltge s shown in figure7. DC link voltge when subjecte to three phse fult Vc(V) Vc(V) Fig5 DC link voltge uner single phse to groun fult When the HVDC system is subjecte to single phse to groun fult the fll in DC link voltge is unvoible which is shown in figure5. When the controllers re implemente the DC link voltge is mintine t the require vlue, which is shown figure 6. Also it cn be seen tht the sttes ynmic vritions for the chnge in commn re ienticl for both controllers. The performnce improvement is obvious with the controllers Fig7 DC link voltge uner three phse fult In the cse of most severe cse of three phse fult lso the effectiveness of the controllers cn be seen in mitigting the rop in DC link voltge s in figure 8.

5 Vc(V) Vc(V) Fig 8 Effect of controllers on DC link voltge subjecte to three phse 3. Effect on lo ynmics Vbc(V) - - DC link voltge with IFC DC link voltge with BSC Lo sie ynmics when subjecte to three phse fult It cn be seen tht there re istortions in lo sie voltge n current uring both single phse to groun fult s well s three phse fult s shown in figure9. With the implementtion of the controllers IFC n BSC in the system, the istortions in the lo sie is lso effectively reuce. The simultion results proves the effectiveness of the controllers in mintining the DC link voltge n thereby reucing istortions in the lo sie voltge. The effect of both the controllers is ienticl in mintining DC link voltge n lo sie ynmics. 6. CONCLUSION The pper proposes lterntive wys of enhncing the performnce of n HVDC system. The HVDC system is moele n is subjece to fult conitions, the resulting oscilltions in DC link voltge n the effect on lo sie ynmics re mpe with the controllers which is prove by simultion in MATLAB. Thus mintining the DC link voltge, effective trnsmission of rel power is me possible through the HVDC system.it hs been prove nlyticlly tht the implementtion of the propose controllers hs enhnce the performnce of the VSC bse HVDC system when subjecte to power system isturbnces in the inverter sie. 7. REFERENCES [] B.Prkhieh n S.Bhttchry Resilient Opertion of Voltge source BTB HVDC Systems uner power system isturbnces, IEEE conf. On AC DC trnsmission,9. Ibc(A) Ibc(A) Lo ynmics with BSC [] M. Hgiwr n H. Akgi, An pproch to regulting the DC-link voltge of voltge source BTB system uring power line fults, IEEETrns. on Inustry Applictions, Vol. 4, No. 5, 5. [3] M. Hgiwr, H. Fujit, n H. Akgi, Performnce of selfcommutte BTB HVDC link system uner single-line-to-groun fult conition, IEEE Trns. on Power Electronics, Vol. 8, No., 3. Vbc(V) Fig 9 Comprison of lo sie ynmics without n with the controllers uring single phse togroun fult. [4] A. Yzni n R. Irvni, Dynmic moel n control of the NPC-bse bck-to-bck HVDC system, IEEE Trns. on Power Delivery, Vol., No., 6. [5] B. Prkhieh n S. Bhttchry, Performnce n control of bck-tobck voltge source converter

6 systems uner norml n fult conitions, th FACTS Users Group Meeting, New Orlens, 7 [6] L. Yng, T. Zho, J. Wng n A. Hung, Design n nlysis of 7 kw DC/DC converter for soli stte trnsformer using kv SiC power evices, IEEE Power Electronics Specilist Conference, PESC 7, Orlno, 7. APPENDIX The prmeters for IFC re: The prmeters for BSC re: = The vlues to 7 cn be obtine from yhe equtions given below = = = = = = 3 = By substituting the vlues from system prmeters the vlues to 7 cn be clculte n re me use of in the controller esign.