Standalone DC Micro Grids for a Three-variable Optimal Energy Management Strategy

Transcription

1 Standalone D Micro Grids for a Three-variable Optial nery Manaeent Stratey Abstract: R.Nitish Kuar M.Tech(PS), Arjun ollee of Technoloy and Sciences, Batasinara. Due to substantial eneration and deand fluctuations in standalone reen icrorids, enery anaeent strateies are becoin essential for the power sharin and voltae reulation purposes. The classical enery anaeent strateies eploy the axiu power point trackin (MPPT) aloriths and rely on batteries in case of possible excess or deficit of enery. In order to realize constant current-constant voltae (IU) charin reie and increase the life span of batteries, enery anaeent strateies require bein ore flexible with the power curtailent feature. In this paper, a coordinated and ultivariable enery anaeent stratey is proposed that eploys a wind turbine and a photovoltaic array of a standalone D icrorid as controllable enerators by adjustin the pitch anle and the switchin duty cycles. XITING ONPT: The stability of a dc icrorid is easured in ters of the stability of its dc bus voltae level which is one of the ain control objectives. The rid voltae source converters (G-VSs) are the priary slack terinals to reulate the voltae level of rid-connected icrorids. Battery banks, on the other hand, are effective slack terinals for standalone icrorids their enery absorbin capacities are liited reardin a nuber of operational constraints. Rosaiah Mudiondla M.Tech(PD), Assistant Professor, Arjun ollee of Technoloy and Sciences, Batasinara. Proposed concept: The proposed stratey is developed as an online nonlinear odel predictive control (NMP) alorith. Applyin to a saple standalone dc icrorid, the developed controller realizes the IU reie for charin the battery bank. The variable load deands are also shared accurately between enerators in proportion to their ratins. The D bus voltae is reulated within a predefined rane, as a desin paraeter. INTRODUTION: The icrorid ay operate as an extension of the ain rid, (rid-connected) or as a standalone rid with no connection to the rid. Standalone dc icrorids have soe distinct applications autootive or arine industries, rural areas. Since ac systes suffer fro the need of synchronization of several enerators, dc icrorids are ore efficient due to the fact that dc enerators and storaes do not need ac-dc converters for bein connected to dc icrorids. The few issues reardin voltae reulation, power sharin, and battery anaeent, is severe in standalone reen icro rids that consist of only interittent solar and wind enery sources, and lead to the necessity of ore controllable strateies. The rid voltae source converters are the priary slack terinals to reulate the voltae level of rid-connected icrorids. Battery banks, on the other hand, are effective slack terinals for standalone icrorids. Pae 451

2 Their enery absorbin capacities are liited reardin a nuber of operational constraints, as explained later in this section. In order to reulate the voltae level of standalone dc icro rids. Present load sheddin strateies for the cases in which there is insufficient power eneration or enery storae. Present strateies that curtail the renewable power enerations of standalone dc icro rids if the battery bank cannot absorb the excess eneration. These curtailent strateies restrict the batteries charin rate by the axiu absorbin power.standalone dc icrorids are usually located in sall-scale areas where the power sharin between DGs can be anaed by centralized aloriths which are less affected by two issues: a) Batteries in charin ode are nonlinear loads causin distortions to the rid voltae b) The absolute voltae level of a standalone icrorid is shifted as the result of the load deand variation. A nuber of phenoena affect the batteries operation durin the charin ode [19]: 1) Applyin hih charin currents, the batteries voltaes quickly reach to the assin threshold. 2) The internal resistor and hence power losses and theral effects increase at hih SO levels. 3) Batteries cannot be fully chared with a constant hih charin current. Operational constraint, the axiu absorbed power by the batteries in order to protect the fro bein overchared. Therefore batteries act as nonlinear loads durin the charin ode. Dependin on the proportion of the power eneration to the load deand ratio within standalone D icrorids, three cases are possible. 1) Power eneration and load deand are balanced; 2) Load deand exceeds power eneration causes dc bus voltae to drop in absence of any load sheddin; and 3) Power eneration is hiher than load deand leads batteries to be overchared and bus voltae to clib. nery anaeent stratey (MS) is proposed, as its control objectives, three aforeentioned issues correspondin standalone dc icro rids; i.e., dc bus voltae reulation, proportional power sharin, and battery anaeent. In contrast to the strateies available in literature in which renewable enery systes (RSs) always operate in their MPPT ode, the proposed ultivariable stratey uses a wind turbine and a PV array as controllable enerators and curtails their enerations if it is necessary. The proposed MS is developed as an online novel NMP stratey that continuously solves an optial control proble (OP) and finds the optiu values of the pitch anle and three switchin duty cycles. It siultaneously controls four variables of icro rids: 1) Power coefficient of the wind turbine. 2) Anular velocity of the wind enerator. 3) Operatin voltae of the PV array and 4) harin current of the battery bank. It is shown that, eployin new available nonlinear optiization techniques and tools, the coputational tie to solve the resultin NMP stratey is in perissible rane. The proposed stratey ipleents the IU charin reie that helps to increase the batteries life span. Block diara: Fi. 1.Topoloy of a sall-scale and standalone dc icrorid. Pae 452

3 DSRIPTIONS: The standalone dc icrorid in above fiure is a sallscale icrorid for reote applications. The wind turbine operates at variable speeds and is connected to the electrical enerator directly, that is, the direct-drive couplin. The variable speed operation is ore flexible for the power anaeent and MPPT applications. Furtherore, direct-drive couplin is ore efficient and reliable and is ore popular for sall-scale wind turbines. In spite of hih cost, peranent anet synchronous enerators (PMSGs) are the ost doinant type of direct-drive enerators in the arket, chiefly due to hiher efficiency. MODLING: The first two constraints f1 and f2 are due to the fact that in standalone dc icrorids the su of the enerated, stored, and consued powers is always zero: Modelin of the Three Syste: A. Wind Branch. B. Battery Branch.. Solar Branch. A. Wind Branch Fi2. Modified version of the syste odel The authors in [20] presented a atheatical odel of standalone reen dc icrorids as hybrid differential alebraic equations (hybrid DAs). Fi. 2 suarizes a odified version of the proposed odel in [20]. Since this paper focuses on the case in which there is an excess power reater than or equal to the axiu possible absorbin rate of the battery bank, the hybrid nature of the battery bank operation is inored for the sake of siplicity. The differential and alebraic states, i.e., and, and the anipulated and nonanipulated control variables, naely, and, are detailed later throuhout the next sub-sections. In what follows, the followin notations are used to odel the standalone dc icrorid in Fi. 1 as DAs: Perforance of the wind turbines is easured as the power coefficient curve with respect to the tip speed ratio and pitch anle. quation (3) shows the power coefficient curve of three-blade wind turbines: Where aa and bita, respectively, are the tip speed ratio and pitch anle. Rad is the radius of the blades and p,ax is the axiu achievable power coefficient at the optiu tip speed ratio of aa out. quation (4) presents the connected PMSG enerator: Pae 453

4 nery anaeent strateies of icrorids ust estiate the dc bus voltae level deviation fro its set point in about every 5 10 s. It eans that except the anular velocity of the enerator (4a) all other fast voltae and current dynaics can be inored. For enery anaeent strateies, the averae odel of the buck converter is replaced with the steady-state equations for the continuous conduction ode (M) wheredw is the switchin duty cycle of the converter and all reainin paraeters are as depicted in Fi. 1. The averae dc output voltae of the rectifier, Vwt, in presence of the non-instantaneous current coutation is calculated as follows. bein identified for each type of battery while the axiu aount of the battery capacity, ax, internal resistor of battery, Rbat, and the battery constant voltae, V0, are iven by anufacturers. By inorin the discharin ode of the battery bank operation, the bi-directional converter acts as a boosttype converter [(8d) (8e)].. Solar Branch: The equivalent electrical circuit of the PV odule [27], [28] is used to atheatically odel the solar branch, consistin of a PV array and a boost converter [29]. q. (9) shows the characteristic equations of a PV array, consistin Npvp*Npvs of PV odules: B. Battery Branch: harin operation of a lead acid battery bank, consistin of (Nbatp*Nbats) batteries, is odeled as: Where Iph denotes the photocurrent and I0 is the diode reverse saturation current. Rs and Rsh, respectively, are the series and parallel equivalent resistors of each PV odule and all other paraeters are as follows: the voltae, current, and state of chare of the battery bank. If is the filtered value of the battery current with the tie constant of Ts and Qact is the actual battery capacity. The experiental paraeter P1 requires Pae 454

5 Siilar tothe wind branch,the averae odel ofthe boost converter is replaced with the steady-state equations for M oparin with the conventional ethods, NMPs are inherently nonlinear and ultivariable strateies that handle constraints and delays. There are three different techniques to discretize and solve OPs: ONTROLLR DSIGN: Optial ontrol Probles (OPs) OPs, as (11), ake explicit use of the syste odel, iven by (11b), in order to find an optial control law u*(.), which eets nuber of equality and inequality constraints. The ter optial here is defined with respect to a certain criterion that iplies the control objectives. This criterion is specified with a cost functional J, consistin of the Laranian ter labda and the terinal cost ter M. While the Laranian ter indicates the cost function durin the period of tie, the terinal cost penalizes final values. quations (11d) and (11e), respectively, forulate the final and initial constraints which ust be aintained by the optial solution. Moreover, (11) represents boxin constraints on the states and control variables: 1) Dynaic prorain ethod based on the Bellan's optiality principle. 2) Indirect ethod based on the Pontryain iniu principle. 3) Direct ethods that convert OPs into nonlinear optiization probles (NLPs) which are then solved by NLP solvers. ontrol Syste: Since it focuses on the charin ode of the battery operation, The proposed MS successively ets the estiated syste states,, as inputs and calculates the optial solution,, as outputs. The external state estiator and the predictor of the non-anipulated variables are out of the scope of this paper. step ahead predictions of the solar irradiance, wind speeds, and load deands are extracted either fro a eteoroloical center or an external predictor usin autoreressive-ovin-averae (ARMA) technique [37]. The bus voltae level of the icrorid,, is set externally and hence the developed controller can act as the secondary and priary levels of the hierarchical architecture [13]. The developed NMP controller consists of three entities: Nonlinear Model Predictive ontrol (NMP): OPs are open-loop strateies and are wrapped by a feedback loop to construct NMP strateies [30]. NMP strateies, which are also called as the recedin horizon control, continuously solve an OP over a finite-horizon T usin the easureents obtained at t as the initial values. Then the first optial value is applied as the next control sinal. 1) The dynaic optiizer that successively solves OP at each saplin tie h, 2) The atheatical odel of the syste to predict its behavior 3) The cost function and constraints of the relevant OP. The optial pitch anle is applied as a set point to an inner closed-loop controller. Moreover, the optial values of the switchin duty cycles are applied to the Pae 455

6 Discrete, Ts = 1e-06 s. powerui 1.2 Generator speed (pu) 3 Pitch anle 12 Generator speed (pu) Pitch anle (de) Wind speed (/s) Vabc_WF Fro6 Iabc_WF Fro7 PMSG Wind Turbine T A B Vabc Iabc Scope1 T (pu) A B Ma_V_I P_Q Discrete 3-phase Positive-Sequence Active & Reactive Power a b c A B BWF A B Three-Phase Parallel RL Load Scope4 DBR - Pulse Generator 1 z [S1] Fro NOT Loical Operator S1 [S2] Fro1 [S1] Goto [S2] Goto1 S2 L1 1 B R Rload v V1 [S4] Vdc Fro Pulse Generator1 S4 1 z S6 S3 1 z S5 Vpv pulse width odulators (PWMs) of the dc-dc converters. 1) ontrol Objectives: Three aforeentioned control objectives, i.e., dc bus voltae reulation, proportional power sharin, and ipleentin the IU reie to chare batteries, are forulated by two slack variables in (12) and (13) and the cost function in (14) TABL II:WIND TURBIN, PMSG, BATTRY STAK, AND PV PARAMTRS IN THIS STUDY XPRIMNTAL RSULTS: Siulation circuit desin in Matlab: - L3 [S3] Fro2 [S6] Fro5 NOT Loical Operator1 Pulse Generator2 [S5] Fro4 L2 Pbat [S3] Goto2 [S4] Goto3 Product1 NOT Loical Operator2 2 Vpv Ipv - Product PVA Bat [S6] Goto4 [S5] Goto5 Ppv _ Battery <SO (%)> <urrent (A)> <Voltae (V)> 2) Box onstraints: quation (15) adds the pitch anle control feature to the developed MS in order to liit the produced aerodynaic power by the wind turbine: The other box constraints on the anipulated variables and the syste states are forulated as follows: TABL I: DSIGN PARAMTRS AND TH OMPUTATIONAL TIM OF TH DVLOPD NMP ONTROLLR Pae 456

7 In order to address these objectives, the developed MS siultaneously controls the pitch anle of the wind turbine and the switchin duty cycles of three dcdc converters. It has been shown that the developed controller tracks the MPPs of the wind and solar branches within the noral conditions and curtails their enerations durin the underload conditions. RFRNS: [1] J. M. Guerrero, M. handorkar, T. Lee, and P.. Loh, Advanced ontrol Architectures for Intellient Microrids-Part I: Decentralized and Hierarchical ontrol, I Trans. Ind. lectron., vol. 60, no. 4, pp , [2] R. S. Balo, W. W. Weaver, and P. T. Krein, The load as an enery asset in a distributed D sartrid architecture, I Trans. Sart Grid, vol. 3, no. 1, pp , [3] J. M. Guerrero, P.. Loh, T. L. Lee, and M. handorkar, Advanced ontrol Architectures for Intellient Microrids-Part II: Power quality, enery storae, and A/D icrorids, I Trans. Ind. lectron., vol. 60, no. 4, pp , [4] N. htedarpour and. Farjah, ontrol stratey for distributed interation of photovoltaic and enery storae systes in D icro-rids, Renew. nery, vol. 45, no. 0, pp , Fi shown: Output wave fors of the siulation ONLUSION AND FUTUR WORKS: In this paper, we developed a novel optial MS that anaes the enery flows across a standalone reen dc icrorid, consistin of the wind, solar, and battery branches. A coordinated and ultivariable online NMP stratey has been developed to address, as the optial MS, three ain control objectives of standalone dc icrorids. These objectives are the voltae level reulation, proportional power sharin, and battery anaeent. [5] D. hen and L. Xu, Autonoous D voltae control of a D icrorid with ultiple slack terinals, I Trans. Power Syst., vol. 27, no. 4, pp , Nov [6] L. Xu and D. hen, ontrol and operation of a D icrorid with variable eneration and enery storae, I Trans. Power Del., vol. 26, no. 4, pp , Oct [7] S. Anand, B. G. Fernandes, and M. Guerrero, Distributed control to ensure proportional load sharin and iprove voltae reulation in low-voltae D icrorids, I Trans. Power lectro., vol. 28, no. 4, pp , Pae 457

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