Power Quality Improvement in an Autonomous Microgrid under Nonlinear Load using Control Strategies

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1 Power Qualty Improvement n an Autonomous Mcrogrd under Nonlnear Load usng Control Strateges N.Chtra #1, A.Senthl Kumar *2, K.Prabaakaran #3 #1 Research Scholar, Anna Unversty & Asso. Prof. #3 Asst. Prof. # Electrcal & Electronc Engneerng, SKP Engneerng College Tamlnadu, Inda 1 psk_sva@hotmal.com 3 prabaakaran031@gmal.com *2 PostdoctoralResearch * Faculty of Engneerng & the Bult Envronment Tshwane Unversty of Technology, Pretora 0001, South Afrca. Abstract Mcrogrd has come out as one of the key spots n research on dstrbuted energy system. Snce the defnton of Mcrogrd s a paradgm for the frst tme, nvestgaton n ths area s growng contnuously and there are numerous research projects ths moment all over the world. Ths paper manly focuses the concentraton on power qualty enhancement n an autonomous mcrogrd along wth the nonlnear load. In ths paper, a novel control strategy s proposed partcularly when the mcrogrd s slanded. In partcular the attenton has been focused on the ssues of power qualty enhancement n mcrogrds wth DG nverters. f regulaton and power sharng are the two man performance parameters whch are consdered n ths work. The control strategy s composed of an nner current control loop and an outer power control loop based on a synchronous reference frame and the conventonal PI regulators. To verfy the advantages and effectveness of the control strategy, several smulaton have been carred out and typcal results have been presented wth bref dscusson. Keyword- Mcrogrd, Autonomous mode, Power qualty, Dstrbuted Generaton (DG), Nonlnear load. I. INTRODUCTION The Mcrogrd can generally be vewed as a cluster of DG that made nterconnected to the man utlty grd. Through some of the power electronc devces lke voltage source nverter (SI), the DGs are nterconnected to the grd. The development of mcrogrd represents a decentralzed nfrastructure envronment to the exstng utlty grd due to the rapd ncrement of demand. Two modes of operaton exsts n mcrogrd namely, grd connected and autonomous mode. Dstrbuted generaton system s based on renewable energy sources such as solar energy, wnd energy, hydro electrc power, fuel cell etc. It emerges as an alternatve quantty whch develops green energy scenaro for the extended supports of utlty grd [1]. These sources are made nterconnected through SI along wth pulse wdth modulaton (PWM) technque creates a non lnear voltagecurrent characterstc of power electronc components, whch affects the power qualty of the supply [2]. When the loads are made to transfer from grd connected to slanded, there arses power qualty problems. Ths ablty of slandng the generaton and load has the potental of hgh relablty of the man grd [3]. An example of mcrogrd s shown n fg.1. Concernng the nterfacng of a mcrogrd to the utlty system s an mportant area of study. It also helps to nvestgate the mpact of power qualty problems [4]. If unbalance n voltage s serous, the sold state crcut breaker (CB) connected between the mcrogrd and utlty grd wll open to solate the mcrogrd. When voltage unbalance s not so solemn, CB remans closed, resultng n sustaned mbalance voltage at the pont of common couplng (PCC) [5]. A robust control strategy s adopted to acheve hgh performance operaton and meet power qualty requrement. Consequently, the current control strategy of PWM SI system s one of the major aspects of modern power electronc converters. Current control strategy for SI s more responsble to mtgate the power qualty problems. SI s made nterconnected by wdely used PWM, that have nonlnear voltage - current characterstcs and hgh swtchng frequency whch affect the qualty of power supply [6]. Two man categores of current controllers are lnear controller based open loop PWM and non lnear controller based closed loop PWM. It s appled usng feedback current control loop [7]. Generally non lnear controller wth hysteress current control (HCC) s used for three phase grd connected SI system. The error sgnal s fed to the HCC and compensaton of current error s made along wth the generaton of PWM sgnal wth acceptable error. Current error sgnal s controlled by self regulatng the delay current, results n generaton of large rpple current leadng to the development of huge total harmonc dstorton (THD) [8]. Even though lnear current controller based on space vector pulse wdth modulaton (SPWM) are adopted and current error sgnals are compensated wth conventonal PI regulator or predctve control algorthm. Ths ISSN : ol 5 No 4 Aug-Sep

2 type of controller provdes an excellent control of current sgnal along wth proper compensaton leadng to the development of low rpple [9]. Recently, the power controller based on nner current control loop has been nvestgated for better mcrogrd confguraton. The controller s descrbed wth the am of ensurng the dynamc stablty of the system and provdng all the nformaton needed for analyss and desgn [10-11]. It s mportant to analyse the power qualty ndces when loads are shfted or changed, whch results n unbalance n voltage and frequency [12]. Power qualty ssues are made hgh concern on dfferent types of loads wth DGs, by controllng two ndependent parameters - voltage magntudes and system frequency that leads to the power sharng among the DGs [13-14]. Analyss of mcrogrd n grd connected and autonomous mode has been nvestgated wth an algorthm n [15].Wth proper control methods n mcrogrd result n mprovement of power qualty of the network [16]. Fg.1. An Example of Mcrogrd. The proposed controller s made externally nterfaced to the current controller, whch s based on synchronous reference frame. Hence the controller s adopted wth conventonal PI regulator along wth feed forward compensaton n order to acheve hgh dynamc response. When the mcrogrd swtches to slandng mode or load changng condton, the f control mode s adopted to regulate the mcrogrd voltage and frequency regulaton along wth PQ control mode to produce sustaned output power. The man objectve of ths work s to mprove the qualty of power supply by attanng adequate mcrogrd voltage and frequency along wth tolerable power sharng among the DG unts. The organzaton of paper s as follows. Secton II descrbes the modellng of mcrogrd n an autonomous mode wth SI. Secton III explans the desgn of Control strateges. In secton I smulaton results are analysed to verfy the am of these controllers. Fnally concluson s outlned at secton. A. Three phase grd connected SI mode II. MODELLING OF AUTONOMOUS MICROGRID Fg.2. SI model wth grd connected A classcal model of three phase grd connected SI along wth the control technque and flter are shown n Fg.2.where R & L represent equvalent resstance and nductance of the flters, C s the flter capactance. The state space representaton of system equvalent crcut n abc reference frame s gven by [17]. ISSN : ol 5 No 4 Aug-Sep

3 aasaa d R 1 b b sb b dt = L + L c c sc c Usng Park s transformaton, Eq. (1) can be framed as follows, R ω d d L d 1sdd = dt + q R q L sq q ω L Where ω s the angular frequency. The Park s Transformaton s defned as, Where dq (1) 0 = Tabc (3) da dq0 = q, abc = b T = 0 c cosθ cos( θ ) cos( θ+ ) snθ sn( θ ) sn( θ+ ) (2) (4) (5) III. DESIGN OF CONTROLLER A. Proposed Power Control Scheme The proposed power controller s shown n fg.3. The power controller comprses of two PI regulator whch helps to generate the reference sgnals and along wth dq reference frame. Ths controller has the capablty of respondng to sudden changes lke load changng or swtchng to slandng operaton mode. Here the DG has been adopted wth f power control strategy, when the mcrogrd s slanded. The conventonal PI regulator helps to regulate the voltage & frequency of the mcrogrd n order to produce the reference current vector and. Fg.3.Power Controller Scheme B. Current Control Scheme The block dagram of current control loop s shown n Fg.4. The current control loop s desgned based on synchronous reference frame. Ths controller s desgned to ensure perfect follow up and short transents of the nverter output current. In order to mplement park s transformaton n the current controller the phase locked loop (PLL) s essental to measure the voltage phase angle. The elmnaton of current error s done by the conventonal PI regulator. Therefore, the controller output s represented n dq reference frame, followed by nverse park s transformaton that generate the needed sx pulse for SPWM to fre IGBT. ISSN : ol 5 No 4 Aug-Sep

4 Fg.4. Current Controller Scheme In dq reference frame based on eq. (2), reference voltage sgnal can be expressed as follows, * 0 * d Kp ωls d Kp d K 0 Xd sd = + + * ωls Kp + q 0 Kp * 0 K Xq q q sq (6) I. SIMULATION RESULTS Several smulatons are carred out n MATLAB/ Smulnk envronment for the complete layout of an nverter based DG n an autonomous mode, to study the effectveness of the controllers under non lnear load. The model parameters are defned as follows: nverter based DG s desgned for an about 4 kw and are connected to the load. Each DG unt s represented by DC voltage source along wth SI. The samplng frequency and the swtchng frequency are 100 KHz & 10 KHz respectvely. Fg.5 to Fg.11 shows the results of the smulaton models. All the results are n p.u. system and the followng objectve are nvestgated. A. Actve & Reactve Power Delvered by DGs. In ths work, the mcrogrd operaton has been nvestgated durng autonomous mode. The power dspatch among the DGs s shown n fg.5 and fg.6. In ths fg, t s assumed that DG 1 and DG 2 were generatng 0.92 p.u. and 0.4 p.u. respectvely whch s feedng nto the RL load. To study the effect on the unbalance characterstc of the network, the RL load has been replaced wth the nonlnear load (nducton motor). The nonlnear load s a combnaton of unbalanced and harmonc load. It s assumed that at 2.0 sec, load change s appled n the dstrbuton network. In order to establsh voltage and frequency close to ther reference values, the power controllers of both the DG unts are swtched to f control mode. However at the load change, the frst DG unt operates n f control mode to mantan voltage anf frequency regulaton, whereas the second DG unt swtch over to PQ control mode n order to mantan constant output power. Fg.5. Actve Power Delvered by DG 1 n slanded mode. Fg.6. Actve Power Delvered by DG 2 n slanded mode. As shown n Fg.7 & 8. The reactve power are set to 0.3 p.u. and 0.26 p.u. respectvely by DG 1 and DG 2.Durng ths stage both DG unts operate n f control mode. At 2.0 sec load varaton s made and the reactve power suppled to the load are shown. ISSN : ol 5 No 4 Aug-Sep

5 Fg.7. Reactve power delvered to load by DG 1 Fg.8. Reactve power delvered to load by DG 2. B. oltage and Frequency Regulaton In ths mode, the ref and Fref of both DG unts are set to 1 p.u. To evaluate the control strategy, smulaton s started n sland mode. The mcrogrd voltage and frequency are regulated at 1 sec. At 2 sec, the mcrogrd undergoes the load change. In order to mtgate the voltage drop and to avod a severe devaton of the frequency caused by the load change, both DG unts adopt f power control mode. It can be notced that the power controller restores the mcrogrd voltage and frequency close to ther reference values at 3.0sec and 2.5 sec respectvely. As a result, f control mode offers an excellent behavour and mantans the mcrogrd voltage equal to 0.9p.u. and frequency around 0.95 p.u. as shown n fg.9 and fg.10 respectvely. Fg.9. Mcrogrd voltage regulated by f control Fg.10. Mcrogrd frequency regulated by f Control mode C. Speed Regulaton Fg.11. shows the speed of the nducton motor. Due to the load varaton at 2.0 sec, the DG 1 operates under f control mode n order to regulate the speed at 1550 RPM. Fg.11. Speed Regulaton. CONCLUSION The power qualty mprovement n an autonomous mcrogrd under nonlnear load has been dscussed n ths paper. A novel power control strategy has been employed for the operaton of the mcrogrd. The control strategy make use of two control loops: nner current control loop and the outer power control loop. In order to mantan the mcrogrd voltage and frequency wthn the regulatng lmt when the mcrogrd operates n slandng mode, f power control strategy s mplemented. Smlarly to mantan the sustaned output power, the PQ power control strategy s adopted by the second DG unt durng the load change. The smulaton results prove that the proposed control strateges offer an excellent response n voltage & frequency regulaton under slandng mode and also n attanng adequate power sharng. ISSN : ol 5 No 4 Aug-Sep

6 REFERENCES [1] R.H. Lasseter, Mcrogrds, Power Engneerng Socety, Wnter Meetng. 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Adapa, Control of parallel connected nverters n standalone AC supply systems, IEEE Transactons on Industry Applcatons 29 (1), pp , [15] N.L. Soultans, S.A. Papathanasou, N. Hatzargyrou, A stablty algorthm for the dynamc analyss of nverter domnated unbalanced L mcrogrds, IEEE Transactons on Power Systems 22 (February (1)), pp, , [16] R. Lasseter, P. Pag, Provdng premum power through dstrbuted resources, n: Proc. of IEEE 33rd Hawa Int. Conf. on System Scences, [17] Il-Yop C, Wenxn L, Cartes DA, Schoder K, Control parameter optmzaton for a mcrogrd system usng partcle swarm optmzaton. In: ICSET IEEE nternatonal conference on sustanable energy technologes, pp , BIBLIOGRAPHY Dr. A.Senthl Kumar, obtaned s Bachelor s Degree (1996) n Electrcal and Electroncs Engneerng n frst class from Unversty of Madras, Chenna, Taml Nadu. He obtaned s Master s degree (2000) n Power Electroncs and Drves n frst class from Bharathdasan Unversty, Trchy, Taml nadu and also he obtaned s Master s degree (2006) n Human Resource Management n frst class from TNOU, Chenna. He completed hs Doctoral degree (2010) n the area of Electrcal Engneerng from Indan Insttute of Technology Roorkee, Roorkee, Uttarakhad, Inda. He obtaned many awards and certfcates durng M.E and Ph.D studes. He has 16 years of teachng and research experence. He has publshed 5 papers n nternatonal journals and presented 20 papers n nternatonal and natonal conferences. He has attended many nternatonal semnar and workshops. He s a lfe member of many professonal body membershps lke ISTE, IEI, CSI, IAENG, IACSIT etc., He vsted foregn country such as Hong Kong and Chengudu, chna, fnancal support funded by DST and CSIR. He has delvered state of the art lectures n many educatonal nsttutons and professonal socetes. He has also currently dong ongong project funded by AICTE worth of 22 lakhs. Hs research nterests nclude Multphase Machnes, Power Electroncs, Renewable Energy Generaton Source, Mcrocontroller & LSI applcaton n Power Electroncs & Electrc Drves, Actve Flters Stablty and System Analyss. Currently he s dong Postdoctoral Fellow n Centre for Energy and Electrcal Power, Electrcal Engneerng Department, Faculty of Engneerng and the Bult Envronment, Tshwane Unversty of Technology, Pretora, South Afrca. He also works as Professor/EEE at elammal Engneerng College, Chenna, Taml Nadu. N.Chtra obtaned her B.E., Electrcal and Electroncs Engneerng degree from Unversty of Madras n 1999 and completed her M.E n Power System from Annamala Unversty n 2001.She has 12 years of Teachng experence. She has publshed 3 papers n Internatonal Journal and 11 papers n Internatonal & Natonal conferences. She s a Ph.D research scholar of Anna Unversty, Chenna. At present she s workng as Assstant Professor n EEE department of SKP Engneerng College, Truvannamala, Inda. She s a member of IEEE, IETE. Her areas of nterest are Soft computng, Smart Grd, Power Systems, Control Systems. ISSN : ol 5 No 4 Aug-Sep