Unified Power Quality Conditioner (UPQC) During Voltage Sag and Swell

Transcription

1 udharshan ao Gandmen and Vjay Kumar K. 1 Unfed Power Qualty Condtoner (UPQC) Durng Voltage ag and well udharshan ao Gandmen and Vjay Kumar K. ABTACT: Ths paper deals wth conceptual study of unfed power qualty condtoner (UPQC) durng voltage sag and swell on the power dstrbuton network and researches profoundly on the coordnated control of UPQC (Unfed Power Qualty Condtoner). By analyzng the radcal reasons of couplng effect between UPQC seres unt and shunt unt, a smple and practcal coordnated control strategy for UPQC seres unt and shunt unt s proposed. Therefore, the complex degree of the whole UPQC control system s smplfed greatly. The coordnated control between UPQC seres unt and shunt unt s mplemented by the proposed strategy. Fnally, the unfed power qualty mult-functon control of UPQC s acheved. Based on njected voltage phase angle wth respect to the utlty or PCC voltage phase angle, thus the UPQC can work n zero actve power consumpton mode, actve power absorpton mode and actve power delverng mode. The seres actve power flter (APF) part of UPQC works n actve power delverng mode and absorpton mode durng voltage sag and swell condton, respectvely. The shunt APF part of UPQC durng these condtons helps seres APF by mantanng dc lnk voltage at constant level. Ths paper ntroduces a new concept of optmal utlzaton of a UPQC. The seres nverter of UPQC s controlled to perform smultaneous 1) voltage sag/swell compensaton and ) load reactve power sharng wth the shunt nverter. The actve power control approach s used to compensate voltage sag/swell and s ntegrated wth theory of power angle control (PAC) of UPQC to coordnate the load reactve power between the two nverters. The MATAB / MUNK results are provded n order to verfy the analyss. The author presents results wth balanced, unbalanced and nonlnear loads at load bus. n [1-9], several control methods have been appled n UPQC. Fg.1 shows a typcal man crcut topologcal structure of UPQC, the nner department of magned lne s UPQC, whch s composed by seres unt and shunt unt as well as DC storage unt. The seres unt has the functons of DV (Dynamc Voltage estorer) and DUP (Dynamc Unnterruptble Power upply), whle the shunt unt has the functons of VG (tatc Var Generator) and APF (Actve Power Flter), and the energy storage unt has the functons of BE (Battery Energy torage ystem) or super capactor energy storage system. One of the serous problems n electrcal systems s the ncreasng number of electronc components of devces that are used by ndustry as well as resdences. These devces, whch need hgh-qualty energy to work properly, at the same tme, are the most responsble ones for njectons of harmoncs n the dstrbuton system. Therefore, devces that soften ths drawback have been developed. One of them s the UPQC, t conssts of a shunt actve flter together wth a seres-actve flter. Ths combnaton allows a smultaneous compensaton of the load currents and the supply voltages, so that compensated current drawn from the network and the compensated supply voltage delvered to the load are snusodal, balanced and mnmzed. The seres- and shuntactve flters are connected n a back-to-back confguraton, n whch the shunt converter s responsble for regulatng the common DC-lnk voltage. Keywords : UPQC, Power Qualty, Dstrbuton ystem, ag, well and APF. 1. NTODUCTON Wth the ncrease n the complexon of the power dstrbuton system and the loads, t s very possble that several knds of power qualty dsturbances are n a dstrbuton system or a power load smultaneously, and t s therefore mportant to ntroduce UPQC (Unfed Power Qualty Condtoner). UPQC s the emergng devce of Custom Power, whch combnes the functons of seres voltage compensator, shunts current compensator and energy storage devce. Multple power qualty regulaton functons are mplemented n UPQC smultaneously, wth a hgher performance rato. udharshan ao Gandmen and Vjay Kumar K. are wth Department of Electrcal & Electroncs Engneerng, Dad nsttute of Engneerng & Technology, Vshakhapatnam, Andhra Pradesh, NDA., sudharshan14u@gmal.com, sudharshan14u@gmal.com The UPQC s one of the major custom power solutons, whch s capable of mtgatng the effect of supply voltage sag at the load end or at the pont of common couplng (PCC) n a dstrbuted network. t also prevents the propagaton of the load current harmoncs to the utlty and mproves the nput power factor of the load. The control of seres compensator (EC) of the UPQC s such that t njects voltage n quadrature advance to the supply current. Thus, the EC consumes no actve power at steady state. The other advantage of the proposed control scheme s that the EC can share the laggng VA demand of the load

2 udharshan ao Gandmen and Vjay Kumar K. 13 wth the shunt compensator (HUC) and can ease ts loadng. The UPQC employng ths type of quadrature voltage njecton n seres s termed as UPQC-Q. The VA requrement ssues of EC and HUCs of a UPQC-Q are dscussed. A PC-based new hybrd control has been proposed and the performance of the UPQC-Q. The UPQC s a versatle devce whch could functon as seres actve flter and shunt actve flter. UPQC can smultaneously fulfll dfferent objectves lke, mantanng a balanced snusodal (harmonc free) nomnal voltage at the load bus, elmnatng harmoncs n the source currents, load balancng and power factor correcton. Keepng the cost effectveness of UPQC, t s desrable to have a mnmum VA loadng of the UPQC, for a gven system wthout compromsng compensaton capablty. For UPQC, ts seres compensator and parallel compensator can be regarded as two dc voltage nverter. Therefore, mantanng a constant value for dc voltage s necessary for UPQC to performance normally. The constant dc voltage s related to the power balance between UPQC and sources, namely when the nput actve power of UPQC s equvalent to ts consumpton theoretcally. o the controllng of dc voltage nvolves n the actve current of sources. f the nput sgnal of the controller s the error of dc voltage, ts output sgnal should be the actve current of sources. Under ths crcumstance, to get a mathematcal model of ts closed loop controller, fndng the dc voltages expressed as a functon of the actve current of sources s crtcal. Ths paper proposed a method of desgn the dc voltage controller by usng the small sgnal model of UPQC, wth whch mathematcal relatonshp between dc voltages and the actve current of sources s deduced. Based on that the mathematcal model of closed loop of dc voltage s promoted. Parameters of the controller s calculated wth t. A example s proposed n the last. The experment results show that the dc control system has good stablty margn and close-loop bandwdth, whch verfed the effectvely. UPQC s a seres-parallel element n the Flexble AC Transmsson ystem (FACT) famly. n ths paper, a UPQC wth cascaded multlevel nverter s proposed. Voltage sag, unbalance and load power factor n dstrbuton system s mtgated usng proposed multlevel UPQC. There s no need of usng transformer and flter when multlevel UPQC s appled and t s one of ts advantages. PWM (nusodal Natural Pulse Wdth Modulaton) scheme s used for pulse generaton to control multlevel nverters. The results showed the effectveness of the proposed method. For UPQC, ts seres compensator and parallel compensator can be regarded as two dc voltage nverter. Therefore, mantanng a constant value for dc voltage s necessary for UPQC to performance normally. The constant dc voltage s related to the power balance between UPQC and sources, namely when the nput actve power of UPQC s equvalent to ts consumpton theoretcally. o the controllng of dc voltage nvolves n the actve current of sources. f the nput sgnal of the controller s the error of dc voltage, ts output sgnal should be the actve current of sources. Under ths crcumstance, to get a mathematcal model of ts closed loop controller, fndng the dc voltages expressed as a functon of the actve current of sources s crtcal. Ths paper proposed a method of desgn the dc voltage controller by usng the small sgnal model of UPQC, wth whch mathematcal relatonshp between dc voltages and the actve current of sources s deduced. Based on that the mathematcal model of closed loop of dc voltage s promoted. Parameters of the controller s calculated wth t. A example s proposed n the last. The smulaton results show that the dc control system has good stablty margn and close-loop bandwdth, whch verfed the effectvty. n ths paper, a new methodology s proposed to mtgate the unbalanced voltage sag wth phase jumps by UPQC. UPQC s used to mtgate both voltage and current power qualty (PQ) problems. Durng the process of mtgaton, UPQC s supposed to nject real and reactve power nto the system to mtgate current (shunt) and voltage (seres) power qualty (PQ) problems. As per the senstve load concerns, deep and long duraton sags are more vulnerable than shallow and short duraton sags. To resolve these ssues a new methodology s proposed wth optmal Volt-Ampere (VA) loadng on UPQC to mtgate deep and long duraton unbalanced sag wth phase jumps. The proposed method has been valdated through detaled smulaton studes. Ths paper presents a new synchronous-reference frame (F)-based control method to compensate PQ problems through a three-phase four-wre UPQC under unbalanced and dstorted load condtons. The proposed UPQC system can mprove the power qualty at the pont of common couplng on power dstrbuton systems under unbalanced and dstorted load condtons. The smulaton results based on Matlab/mulnk are dscussed n detal to support the F-based control method presented n ths paper. The proposed approach s also valdated through expermental study wth the UPQC hardware prototype. The unfed power qualty condtoner s a power condtonng devce, whch s a ntegraton of back to back connected shunt actve power flter (APF) and seres APF to a common DC lnk voltage. For mprovement of power qualty (PQ) problems n a three -phase four-wre dstrbuton system, two topologes are proposed n ths paper. A comparatve analyss of these topologes along wth the most common four-leg voltage source nverter (V) based topology of four-wre UPQC s dscussed n ths work. The performance of each topology of UPQC s evaluated for dfferent PQ problems lke power-factor correcton, load balancng, current harmonc mtgaton, voltage harmonc mtgaton and source neutral current mtgaton. The synchronous reference frame (F) theory s used as a control strategy of seres and shunt APFs. The UPQC s used to mtgate the current and voltage-related power-qualty (PQ) problems smultaneously n power dstrbuton systems. Among all of the PQ problems, voltage sag s a crucal problem n dstrbuton systems. n ths paper, a new methodology s proposed to mtgate the unbalanced voltage sag wth phase jumps by UPQC wth mnmum real power njecton. To obtan the mnmum real power njecton by UPQC, an objectve functon s derved along wth practcal constrants, such as the njected voltage

3 udharshan ao Gandmen and Vjay Kumar K. 14 lmt on the seres actve flter, phase jump mtgaton, and angle of voltage njecton. Partcle swarm optmzaton (PO) has been used to fnd the soluton of the objectve functon derved for mnmzng real power njecton of UPQC along wth the constrants. Adaptve neuro-fuzzy nference systems have been used to make the proposed methodology onlne for mnmum real power njecton wth UPQC by usng the PO-based data for dfferent voltage sag condtons. The proposed method has been valdated through detaled smulaton and expermental studes. Ths paper presents a comprehensve revew on the UPQC to enhance the electrc power qualty at dstrbuton levels. Ths s ntended to present a broad overvew on the dfferent possble UPQC system confguratons for sngle-phase (twowre) and three-phase (three -wre and four-wre) networks, dfferent compensaton approaches and recent developments n the feld. t s notced that several researchers have used dfferent names for the UPQC based on the unque functon, task, applcaton or topology under consderaton. Therefore, an acronymc lst s developed and presented to hghlght the dstngushng feature offered by a partcular UPQC. n all 1 acronyms are lsted, namely, UPQCD, UPQC-DG, UPQC-, UPQC-, UPQC-MC, UPQC-MD, UPQC-M, UPQC-P, UPQC-Q, UPQC-, UPQC- and UPQCVAmn. More than 150 papers on the topc are rgorously studed and metculously classfed to form these acronyms and are dscussed n the paper. UPQC seres unt and shunt unt cannot only operate ndependently to realze ther own functons, but also be unfed to realze ther synthetc functons. To control the UPQC seres unt and shunt unt as a whole, t s necessary to solve ts coordnated control to make full use of ts great synthetc functons. n vew of ths, a coordnated control strategy of UPQC seres unt and shunt unt s proposed, and ts valdty s testfed. n addton, the controller desgn of UPQC seres unt and shunt unt s based on H model matchng technology about power qualty waveform trackng compensaton, whch has been stated n detal n [10]. Besdes, other control methods such as deadbeat control also can be appled nto the proposed coordnated control strategy to desgn the synthetc controller of UPQC. The confguraton of UPQC seres unt and shunt unt n dstrbuton system s shown n Fg.1, the seres unt operated as the controlled voltage source udv and the shunt unt as the controlled current source APF. UPQC seres unt and shunt unt are unfed, and then, there are nteractons due to the two knds of couplng between seres unt and shunt unt n the man crcut: The nteracton between the output voltage compensaton of the seres unt and the output current compensaton of the shunt unt due to ther electrc connecton wth the outer dstrbuted lne. The nteracton between ther nverters due to ther sharng wth the nner DC capactor of energy storage unt. The couplng nteracton between the seres unt and the shunt unt ncreases the complexon of UPQC unfed coordnated control. How to solve ths problem? An effectve method s ntroduced to control the couplng effect between seres unt and shunt unt, whch can make the UPQC functon unfed and dversty, moreover make the control manner smple and ndependent wth each of the unts. The control method that can handle the couplng effect between UPQC seres unt and shunt unt s addressed next. The UPQC, an ntegraton of shunt and seres APF s one of the most sutable as well as effectve devce n ths concern [8-1]. A UPQC tackles both current as well as voltage related power qualty problems smultaneously. ecently more attenton s beng pad on mtgaton of voltage sags and swells usng UPQC [8-1]. The common cause of voltage sag and swell s sudden change of lne current flowng through the source mpedance. Ths paper s based on the steady state analyss of UPQC durng dfferent operatng condtons. The purpose s to mantan snusodal source current wth unty power factor operaton along wth load bus voltage regulaton. The major concern s the flow of actve and reactve power durng these condtons, whch decde to amount of current flowng through the actve flters and through the supply. Ths analyss can be useful for selecton of devce ratngs. The use of nonlnear and mpact loads brng about harmoncs and reactve power loadng varance n power system, whch has a strong mpact on the other loads n the same system. Employment of UPQC (unfed power qualty condtoner) could decrease mpact on transmsson and dstrbuton harmoncs and neutral-lne current caused by unbalance and nonlnear load, enhance custom power qualty meanwhle supply balance and snusodal voltage to load and enhance power dstrbuton relablty [1]-[3] Fg.1 shows the crcut confguraton of the proposed UPQC, whch s a three-phase four-wre UPQC, beng formed of seres compensator and shunt compensator. Usually there are two control scheme of UPQC, one s most used,known as ndrect control strategy, n whch seres compensator work by way of voltage source compensatng manly voltage dstorton and fundamental wave devaton supplyng rated balance snusodal voltage for load and shunt compensator as current source compensatng the harmoncs, reactve current n load. The other s drect control strategy n whch seres compensator work as snusodal current source shunt compensator as snusodal voltage source. The power factor of power lne can be unty because of seres compensaton current havng the same phase wth system voltage and the load can get balance, rated snusodal voltage. Employng ths strategy, seres compensator solate the voltage dsturbance between power lne and load as well as shunt compensator prevent the reactve power, harmonc and neutral current on the load sde nto power lne.addtonally, another beneft from the drect control strategy s that t s not necessary to change the work mode when power lne dumpng or restorng, for shunt compensator all along s controlled as snusodal voltage source.[4]- [8].

4 udharshan ao Gandmen and Vjay Kumar K. 15 Ths paper presents a method of detectng compensaton sgnals and a control scheme based on t. Because the p-q-r transformaton s sophstcated, ths paper presents a mproved p-q-r algorthm, whch smplfy the calculatons. Based on the mproved p-q-r theory, the calculatng method of compensatng current and voltage are proposed. Wth ntroducng ts prncple and control schematc dagram n detal, a composte control strategy combnng of the ordnary drect and ndrect control strategy s presented, too. mulaton results usng MATAB/ MUNK show that the harmonc current and reactve power of load as well as neutral current are compensated well.o the proposed strategy s feasble and effectve.. TEADY - TATE POWE FOW ANAY: The powers due to harmoncs quanttes are neglgble as compared to the power at fundamental component, therefore, the harmonc power s neglected and the steady state operatng analyss s done on the bass of fundamental frequency component only. The UPQC s controlled n such a way that the voltage at load bus s always snusodal and at desred magntude. Therefore the voltage njected by seres APF must be equal to the dfference between the supply voltage and the deal load voltage. Thus the seres APF acts as controlled voltage source. The functon of shunt APF s to mantan the dc lnk voltage at constant level. n addton to ths the shunt APF provdes the var requred by the load, such that the nput power factor wll be unty and only fundamental actve power wll be suppled by the source. The voltage njected by seres APF can vary from 0 0 to Dependng on the voltage njected by seres APF, there can be a phase angle dfference between the load voltage and the source voltage. However, n changng the voltage phase angle of seres APF, the ampltude of voltage njected can ncrease, thus ncreasng the requred kva ratng of seres APF [7]. Fg. : Equvalent Crcut Dagram n the followng analyss the load voltage s assumed to be n phase wth termnal voltage even durng voltage sag and swell condton. Ths s done by njectng the seres voltage n phase or out of phase wth respectve to the source voltage durng voltage sag and swell condton respectvely. Ths suggests the real power flow through the seres APF. The voltage njected by seres APF could be postve or negatve, dependng on the source voltage magntude, absorbng or supplyng the real power. n ths partcular condton, the seres APF could not handle reactve power and the load reactve power s suppled by shunt APF alone. The equvalent crcut of a phase for UPQC s shown n the Fg.. The source voltage, termnal voltage at PCC and load voltage are denoted by V s, V t and V respectvely. The source and load currents are denoted by s and respectvely. The voltage njected by seres APF s denoted by v r, where as the current njected by shunt APF s denoted by h. utable model for the analyss and control of the UPQC was qute dffcult to obtan, whch prohbted not only the analyss and comparson between exstng control strateges, but also the ndustral applcatons, as no generalzed method to desgn the control loop for dfferent dsturbances. n ths paper, a unfed DC voltage compensator desgn s proposed for UPQC based on the system nstantaneous energy equlbrum model. The man crcut model of UPQC s derved frstly, ncludng both the steady state model and the small sgnal model. ubsequently, four exstng control strateges for the shunt converter control are found and modeled n detal, whch are combned wth UPQC man crcut model, and the whole control system are obtaned accordngly. The UPQC whole system model are compared and evaluated n dfferent dsturbances. And then the unfed compensator desgn method for the DC lnk voltage control s proposed, the worst control strategy s then chosen as an example for the detaled compensator desgn, based on the newly proposed model. Fnally, the computer smulaton and prototype experment are done to verfy the valdty all the analyss and control. Fg. 3 shows varaton of angle durng dfferent modes of operatons of UPQC, represented by zones. Fgure conssts of seven zones of operatons. The x axs represents the reference load voltage whereas the shunt APF compensatng current can vary from 0 0 to Zone, and represents the case of pure resstve, nductve and capactve load respectvely. f the load s pure resstve, shunt APF does not nject any compensatng current snce there s no reactve power demand from the load, ths condton s represented by zone. Consderng the case of nductve load, the load var requrement s suppled by shunt APF by njectng 90 0 leadng current. The magntude of the compensatng current would depend on the vars to be compensated. Ths condton s represented by zone. Now, f the load s capactve one, theoretcally, the load would draw leadng current from the source,.e. load generates vars. Ths load generated vars are compensated by shunt APF by njectng 90 0 laggng current. The magntude of compensatng current depends on the vars to be cancelled out, represented by zone. Durng the operaton of UPQC n zone and larger the var compensaton more would be the compensatng current magntude.

5 udharshan ao Gandmen and Vjay Kumar K. 16 A UPQC that combnes the operatons of a Dstrbuton tatc Compensator (DTATCOM ) and Dynamc Voltage egulator (DV) together. We have analyzed the operaton of a UPQC that combnes the operatons of a Dstrbuton tatc Compensator (DTATCOM) and Dynamc Voltage estorer (DV) together. The seres component of the UPQC nserts voltage so as to mantan the voltage at the Pont of Common Couplng (PCC) balanced and free of dstorton. multaneously, the shunt component of the UPQC njects current n the a.c. system such that the currents enterng the bus to whch the UPQC s connected are balanced snusods. Both these objectves must be met rrespectve of unbalance or dstorton n ether source or load sdes. Fg. 3 : Zones n Operaton of UPQU Zone V and zone V represents the operatng regon of UPQC durng the voltage sag on the system for nductve and capactve type of the loads respectvely. Durng the voltage sag as dscussed prevously, shunt APF draws the requred actve power from the source by takng extra current from the source. n order to have real power exchange between source, UPQC and load, the angle Oh should not be 900. For nductve type of the load, ths angle could be anythng between 00 to 900 leadng and for capactve type of the load, between 0 0 to 90 0 laggng. Ths angle varaton manly depends on the 00 of sag need to be compensated and load var requrement. Zone V and zone V represents the operatng regon of UPQC durng the voltage swell on the system for nductve and capactve type of the loads respectvely. Durng the voltage swell as dscussed prevously, shunt APF feeds back the extra actve power from the source by takng reduced current from the source. n order to acheve ths angle, h would be between 90 0 to leadng and between 90 0 to laggng for nductve and capactve type of load respectvely. The proposed control strategy s amed to generate reference sgnals for both shunt and seres APFs of UPQC. n the followng secton, an approach based on F theory s used to get reference sgnals for the seres and shunt APFs. A. eference voltage sgnal generaton for seres APF The control strategy for seres AF s shown n Fg.4. nce, the supply voltage s dstorted, a phase locked loop (P) s used to acheve synchronzaton wth the supply voltage [8].Three-phase dstorted supply voltages are sensed and gven to P whch generates two quadrature unt vectors (snwt, coswt).the sensed supply voltage s multpled wth a sutable value of gan before beng gven as an nput to P. A dstorton free, balanced and a constant magntude three phase voltage has d component only, whle q and 0 component wll be zero. Hence, wth the help of unt vectors (snwt, coswt) obtaned from P, an nverse Parks transformaton s done for the desred peak value of the PCC voltage usng bellow eqn. 3. THE MPOVED P-Q- THEOY Voltage at three-phase a-b-c coordnates can be transformed to dq0 as The computed reference voltages from above eqn are then gven to the hysteress controller along wth the sensed three phase actual load voltages(vla, vlb and vlc).the output of the hysteress controller s swtchng sgnals to the sx swtches of the V of seres AF. The hysteress controller generates the swtchng sgnals such that the voltage at PCC becomes the desred snusodal reference voltage. Therefore, the njected voltage across the seres transformer through the rpple flter cancels out the harmoncs present n the supply voltage. The control scheme to ger the reference source ( sa, sb and sc ) usng F theory s depcted n Fg.5.

6 udharshan ao Gandmen and Vjay Kumar K. 17 Wth the help of unt vectors (snwt, coswt) the load currents are transformed n to d-q-0 components usng Park s transformaton as per the eqn. () Where s the compensatng current (a) (b) Fg. 4: Control cheme of eres APF usng F Theory (c) Fg.6. (a) The Equvalent Crcut Of oad And upply ystem; (b) Phasor of Uncompensated ne; (c) Phasor of the Compensated ne From above equaton V = E -V = Z = V, _= V Fg. 5 : Control cheme of hunt APF usng F Theory P V jq 4. MODENG OF DTATCOM 4.1. Voltage egulaton Wthout Compensator o that V j X P V jq Voltage E and V mean source voltage and PCC voltage respectvely. Wthout a voltage compensator, the PCC voltage drop caused by the load current, s as shown n Fg.6 (b) as V P V X Q J X P V Q V V X

7 udharshan ao Gandmen and Vjay Kumar K. 18 The voltage change has a component V n phase wth V and a component Vx, n quadrature wth V, whch are llustrated n Fg.6(b). t s clear that both magntude and phase of V, relatve to the supply voltage E, are the functons magntude and phase of load current, namely voltage drop depends on the both the real and reactve power of the load. The component V can be wrtten as : V 4.. Voltage egulaton Usng The DTATCOM Fg. 6(c) shows the vector dagram wth voltage compensaton. By addng a compensator n parallel wth the load, t s possble to make current of the compensator. Where s compensator current j X E V by controllng the Basc Operatng Prncple : Basc operatng prncple of a DATCOM s smlar to that of synchronous machne. The synchronous machne wll provde laggng current when under excted and leadng current when over excted. DTATCOM can generate and absorb reactve power smlar to that of synchronous machne and t can also exchange real power f provded wth an external devce DC source. Exchange Of eactve Power : f the output voltage of the voltage source converter s greater than the system voltage then the DATCOM wll act as capactor and generate reactve power(.e.. provde laggng current to the system) Exchange Of eal Power : as the swtchng devces are not loss less there s a need for the DC capactor to provde the requred real power to the swtches. Hence there s a need for real power exchange wth an AC system to make the capactor voltage constant n case of drect voltage control. There s also a real power exchange wth the AC system f DTATCOM d provded wth an external DC source to regulate the voltage ncase of very low voltage n the dstrbuton system or n case of faults. And f the VC output voltage leads the system voltage then the real power from the capactor or the DC source wll be suppled to the AC system to regulate the system voltage to the =1p.u or to make the capactor voltage constant. Hence the exchange of real power and reactve power of the voltage source converter wth AC system s the major requred phenomenon for the regulaton n the transmsson as well as n the dstrbuton system. For reactve power compensaton, DTATCOM provdes reactve power as needed by the load and therefore the source current remans at unty power factor (UPF). nce only real power s beng suppled by the source, load balancng s acheved by makng the source reference current balanced. The reference source current used to decde the swtchng of the DTATCOM has real fundamental frequency component of the load current whch s beng extracted by these technques. A TATCOM at the transmsson level handles only fundamental reactve power and provdes voltage support whle as a DTATCOM s employed at the dstrbuton level or at the load end for power factor mprovement and voltage regulaton. DTATCOM can be one of the vable alternatves to VC n a dstrbuton network. Addtonally, a DTATCOM can also behave as a shunt actve flter, to elmnate unbalance or dstortons n the source current or the supply voltage as per the EEE-519 standard lmts. nce a DTATCOM s such a multfunctonal devce, the man objectve of any control algorthm should be to make t flexble and easy to mplement n addton to explotng ts mult functonalty to the maxmum. The man objectve of any compensaton scheme s that t should have a fast response, flexble and easy to mplement. The control algorthms of a DTATCOM are manly mplemented n the followng steps: Measurements of system voltages and current and sgnal condtonng Calculaton of compensatng sgnals Generaton of frng angles of swtchng devces Generaton of proper PWM frng s the most mportant part of DTATCOM control and has a great mpact on the compensaton objectves, transent as well as steady state performance. nce a DTATCOM shares many concepts to that of a TATCOM at transmsson level, a few control algorthms have been drectly mplemented to a DTATCOM, ncorporatng Pulse Wdth Modulaton (PWM) swtchng, rather than Fundamental Frequency swtchng (FF) methods. Ths project makes attempt to compare the followng schemes of a DTATCOM for reactve power compensaton and power factor correcton based on: 1. Phase hft Control. Decoupled Current Control (p-q theory) 3. egulaton of ac bus and dc lnk voltage 4. ynchronous eference Frame (F) Method 5. Adalne Based Control Algorthm (n ths paper we are not dscussng about ths controller) The performance of DTATCOM wth dfferent control schemes have been tested through dgtal smulatons wth the dfferent system parameters. The swtch on tme of the DTATCOM and the load change tme are also mentoned. Phase hft Control : n ths control algorthm the voltage

8 udharshan ao Gandmen and Vjay Kumar K. 19 regulaton s acheved n a DTATCOM by the measurement of the rms voltage at the load pont and no reactve power measurements are requred. Fg.7 shows the block dagram of the mplemented scheme. Fg. 7 : Block Dagram Of Phase hft Control nusodal PWM technque s used whch s smple and gves a good response. The error sgnal obtaned by comparng the measured system rms voltage and the reference voltage, s fed to a P controller whch generates the angle whch decdes the necessary phase shft between the output voltage of the VC and the AC termnal voltage. Ths angle s summed wth the phase angle of the balanced supply voltages, assumed to be equally spaced at 10 degrees, to produce the desred synchronzng sgnal requred to operate the PWM generator. n ths algorthm the D.C. voltage s mantaned constant usng a separate dc source. Decoupled Current Control p-q Theory : Ths algorthm requres the measurement of nstantaneous values of three phase voltage and current. Fg.5. shows the block dagram representaton of the control scheme. The compensaton s acheved by the control of d and q. Usng the defnton of the nstantaneous reactve power theory for a balanced three phase three wre system, the quadrature component of the voltage s always zero, the real (p) and the reactve power (q) njected nto the system by the DTATCOM can be expressed under the dq reference frame as: _ ~ p p p and _ ~ q q q Where _ p and q are the average part and ~ p and ~ q are oscllatory part of real and reactve nstantaneous powers. The compensatng currents are calculated to compensate the nstantaneous reactve power and the oscllatory component of the nstantaneous actve power. n ths case the source transmts only the non-oscllatng component of actve power. Therefore the reference source currents s coordnate are expressed as: s s 1 v v v v and s _ p 0 n α-β These currents can be transformed n a-b-c quanttes to fnd the reference currents n a-b-c coordnate. sa sb sc Where o s the zero sequence components whch s zero n 3-phase 3-wre system and the correspondng block dagram s shown n Fg o nce vq=0, d and q completely descrbe the nstantaneous value of real and reactve powers produced by the DTATCOM when the system voltage remans constant. Therefore the nstantaneous three phase current measured s transformed by abc to dqo transformaton. The decoupled d- axs component d and q axs component q are regulated by two separate P regulators. The nstantaneous d reference and the nstantaneous q reference are obtaned by the control of the dc voltage and the ac termnal voltage measured. Thus, nstantaneous current trackng control s acheved usng four P regulators. A Phase ocked oop (P) s used to synchronze the control loop to the ac supply so as to operate n the abc to dqo reference frame. The nstantaneous actve and reactve powers p and q can be decomposed nto an average and an oscllatory component. Fg. 8 : Block Dagram Of Decoupled Theory Based Control Of DTATCOM ynchronous otatng Frame Theory : The synchronous reference frame theory s based on the transformaton of the currents n synchronously rotatng d-q frame. Fg.9 explans the basc buldng blocks of the theory. f θ s the

9 udharshan ao Gandmen and Vjay Kumar K. 0 transformaton angle, then the currents transformaton from α-β to d-q frame s defned as: d q cos sn sn cos Fg. 9 : Block Dagram For ynchronous Frame Theory F solator extracts the dc component by low pass flters (PF) for each d and q realzed by movng averager at 100Hz. The extracted DC components ddc and qdc are transformed back nto α-β frame as shown below: dc dc cos sn sn cos From here the transformaton can be made to obtan three phase reference currents n a-b-c coordnates usng. The reactve power compensaton can also be provded by keepng q component zero for calculatng reference currents. 5. MODENG OF DV Power qualty has a sgnfcant nfluence on hgh-technology equpments related to communcaton, advanced control, automaton, precse manufacturng technque and on-lne servce. For example, voltage sag can have a bad nfluence on the products of semconductor fabrcaton wth consderable fnancal losses. Power qualty problems nclude transents, sags, nterruptons and other dstortons to the snusodal waveform. One of the most mportant power qualty ssues s voltage sag that s a sudden short duraton reducton n voltage magntude between 10 and 90% compared to nomnal voltage. Voltage sag s deemed as a momentary decrease n the rms voltage, wth duraton rangng from half a cycle up to one mnute. Deep voltage sags, even of relatvely short duraton, can have sgnfcant ddc qdc costs because of the prolferaton of voltage-senstve computer-based and varable speed drve loads. The fracton of load that s senstve to low voltage s expected to grow rapdly n the comng decades. tudes have shown that transmsson faults, whle relatvely rare, can cause wdespread sags that may consttute a major source of process nterruptons for very long dstances from the faulted pont. Dstrbuton faults are consderably more common but the resultng sags are more lmted n geographc extent. The majorty of voltage sags are wthn 40%of the nomnal voltage. Therefore, by desgnng drves and other crtcal loads capable of rdng through sags wth magntude of up to 40%, nterrupton of processes can be reduced sgnfcantly. The DV can correct sags resultng from faults n ether the transmsson or the dstrbuton system. The voltage generated by power statons has a snusodal waveform wth a constant frequency. Any dsturbances to voltage waveform can result n problems related wth the operaton of electrcal and electronc devces. Users need constant sne wave shape, constant frequency and symmetrcal voltage wth a constant rms value to contnue the producton. Ths ncreasng nterest to mprove overall effcency and elmnate varatons n the ndustry have resulted more complex nstruments that are senstve to voltage dsturbances. The typcal power qualty dsturbances are voltage sags, voltage swells, nterruptons, phase shfts, harmoncs and transents. Among the dsturbances, voltage sag s consdered the most severe snce the senstve loads are very susceptble to temporary changes n the voltage. Voltage sag (dp) s a short duraton reducton n voltage magntude between 10% to 90% compared to nomnal voltage from half a cycle to a few seconds. The characterzaton of voltage sags s related wth the magntude of remanng voltage durng sag and duraton of sag [, 5]. The magntude has more nfluence than the duraton on the system. Voltage sags are generally wthn 40% of the nomnal voltage n ndustry. They can cause damaged product, lost producton, restartng expenses and danger of breakdown. Motor startng, transformer energzng, earth faults and short crcut faults wll cause short duraton ncrease n current and ths wll cause voltage sags on the lne. The wde area soluton s requred to mtgate voltage sags and mprove power qualty. One new approach s usng a DV [1, 8]. The basc operaton prncple s detectng the voltage sag and njectng the mssng voltage n seres to the bus as shown n Fg.1. DV has become a cost effectve soluton for the protecton of senstve loads from voltage sags. Unlke UP, the DV s specfcally desgned for large loads rangng from a few MVA up to 50MVA or hgher [5]. The DV s fast, flexble and effcent soluton to voltage sag problems, [4, 8]. 6. EUT The performance of the desgned DV s evaluated by usng

10 udharshan ao Gandmen and Vjay Kumar K. 1 the Matlab / mulnk program as a The proposed UPQC and ts control schemes have been tested through extensve case study smulatons usng Matlab. n ths secton, smulaton results are presented, and the performance of the proposed UPQC system s shown n Fg. 10 and the control model n smulnk as shown n Fg. 11. load on Feeder s fully protected aganst dstorton, sag/swell, and nterrupton. Furthermore, the regulated voltage across the senstve load on Feeder1 can supply several customers who are also protected aganst dstorton, sag/swell, and momentary nterrupton. Therefore, the cost of the MC-UPQC must be balanced aganst the cost of nterrupton, based on relablty ndces, such as the customer average nterrupton duraton ndex (CAD) and customer average nterrupton frequency ndex (CAF). t s expected that the MC-UPQC cost can be recovered n a few years by chargng hgher tarffs for the protected lnes. The performance of the MC-UPQC under a fault condton on Feeder s tested by applyng a three-phase fault to ground on Feeder between 0.3s<t<0.4 s. mulaton results are shown n Fg. 1. Fg. 10 : Matlab/mulnk Fle of UPQC Fg. 11 : Controller of UPQC The dstorted nonlnear load current s compensated very well, and the total harmonc dstorton (THD) of the feeder current s reduced from 8.5% to less than 5%. Also, the dc voltage regulaton loop has functoned properly under all dsturbances, such as sag/swell n both feeders. Fg1: mulaton esults For An Upstream Fault On Feeder: BU Voltage, Compensatng Voltage, And oads 1 and Voltages 6.1. Upstream Fault on Feeder When a fault occurs n Feeder (n any form of -G, --G, and ---G faults), the voltage across the senstve/crtcal load s nvolved n sag/swell or nterrupton. Ths voltage mperfecton can be compensated for by VC. n ths case, the power requred by load s suppled through VC and VC3. Ths mples that the power semconductor swtches of VC and VC3 must be rated such that total power transfer s possble. Ths may ncrease the cost of the devce, but the beneft that may be obtaned can offset the expense. n the proposed confguraton, the senstve/crtcal 6.. oad Change To evaluate the system behavor durng a load change, the nonlnear load 1 s doubled by reducng ts resstance to half at t=0.5 s. The other load, however, s kept unchanged. The system response s shown n Fg. 13. t can be seen that as load 1 changes, the load voltages reman undsturbed, the dc bus voltage s regulated, and the nonlnear load current s compensated.

11 udharshan ao Gandmen and Vjay Kumar K Unbalance Voltage The control strateges for shunt and seres VCs, whch are ntroduced n ecton, are based on the d q method. They are capable of compensatng for the unbalanced source voltage and unbalanced load current. To evaluate the control system capablty for unbalanced voltage compensaton, a new smulaton s performed. n ths new smulaton, the BU voltage and the harmonc components of BU1 voltage are smlar to those gven n ecton V. However, the fundamental component of the BU1 voltage an unbalance factor voltage s gven by s an unbalanced three-phase voltage wth of 40%. Ths unbalance The smulaton results show that the harmonc components and unbalance of BU1 voltage are compensated for by njectng the proper seres voltage. n ths fgure, the load voltage s a three-phase snusodal balance voltage wth regulated ampltude. The smulaton results for the three-phase BU1 voltage seres compensaton voltage, and load voltage n feeder 1 are shown n Fg. 14. Fg. 14 : BU1 Voltage, eres Compensatng Voltage, and oad Voltage n Feeder1 Under Unbalanced ource Voltage 7. CONCUON n ths paper, a new confguraton for smultaneous compensaton of voltage and current n adjacent feeders has been proposed. The new confguraton s named multconverter unfed power-qualty condtoner (MC -UPQC). Compared to a conventonal UPQC, the proposed topology s capable of fully protectng crtcal and senstve loads aganst dstortons, sags/swell, and nterrupton n twofeeder systems. The dea can be theoretcally extended to multbus/multfeeder systems by addng more seres VCs. The performance of the MC-UPQC s evaluated under varous dsturbance condtons and t s shown that the proposed MC-UPQC offers the followng advantages : 1) Power transfer between two adjacent feeders for sag/swell and nterrupton compensaton; ) Compensaton for nterruptons wthout the need for a battery storage system and, consequently, wthout storage capacty lmtaton; 3) harng power compensaton capabltes between two adjacent feeders whch are not connected. Fg. 13 : mulaton esults For oad Change: Nonlnear oad Current, Feeder1 Current, oad 1 Voltage, oad voltage, and dc-lnk Capactor Voltage 8. EFEENCE [1] D. D. abn and A. undaram, Qualty enhances relablty, EEE pectr., vol. 33, pp , Feb

12 udharshan ao Gandmen and Vjay Kumar K. 3 [] M. astog,. Nak, and N. Mohan, A comparatve evaluaton of harmonc reducton technques n threephase utlty nterface of power electronc loads, EEE Trans. nd. Appl., vol. 30, no. 5, ep./oct [3] F. Z. Peng, Applcaton ssues of actve power flters, EEE nd. Appl. Mag., vol. 4, no. 5, pp. 1 30, ep../oct [4] H. Akag, New trends n actve flters for power condtonng, EEE Trans. nd. Appl., vol. 3, no. 6, pp , Nov./Dec [5]. Gyugy, C. D. chauder,.. Wllams, T.. etman, D.. Torjerson, and A. Edrs, The unfed power flow controller: A new approach to power transmsson control, EEE Trans. Power Del., vol. 10, no., pp , Apr [6] H. Fujta and H. Akag, The unfed power qualty condtoner: The ntegraton of seres and shunt actve flters, EEE Trans. Power Electron., vol. 13, no., pp , Mar [7] A. Ghosh and G. edwch, A unfed power qualty condtoner (UPQC) for smultaneous voltage and current compensaton, Elect. Power yst. es., pp , 001. [8] M. Aredes, K. Heumann, and E. H. Watanabe, An unversal actve power lne condtoner, EEE Trans. Power Del., vol. 13, no., pp , Apr [9]. Gyugy, K. K. en, and C. D. chauder, nterlne power flow controller concept: A new approach to power flow management n transmsson systems, EEE Trans. Power Del., vol. 14, no. 3, pp , Jul [10] B. Fardanesh, B. hperlng, E. Uzunovc, and. Zelngher, Mult-converter FACT Devces: The generalzed unfed power flow controller (GUPFC), n Proc. EEE Power Eng. oc. ummer Meetng, 000, vol. 4, pp [11] A. K. Jndal, A. Ghosh, and A. Josh, nterlne unfed power qualty condtoner, EEE Trans. Power Del., vol., no. 1, pp , Jan [1] M. Hu and H. Chen, Modelng and controllng of unfed power qualty compensator, n Proc. 5th nt. Conf. Advances n Power ystem Control, Operaton and Management, Hong Kong, Chna, 000. udharshan ao Gandmen born on 7 th eptember 1987, receved B.Tech Degree from Chatanya Engneerng College, Vsakhapatnam, AP, nda n 009 & he s pursng M.Tech from Dad nsttute of Engneerng & Technology, Anakapall, Vsakhapatnam,AP,nda n the department of Electrcal & Electroncs Engnnerng. Hs area of nterests on electrcal Drves. K. Vjay Kumar born on 1 st June 1968, receved B.Tech Degree from Nagpur Unversty n 1990 & M.Tech n the year of 001 from Andhra Unversty. Presently workng as Assocate Professor & Head of the Department n Dad nsttute of Engneerng & Technology, Anakapall, Vsakhapatnam, AP, nda. Her area of nterests on electrcal machnes.