Transactions Template JOURNAL OF ENGINEERING RESEARCH AND TECHNOLOGY, VOLUME 3, ISSUE 3, SEPTEMBER 2016 44 Role of Flywheel Energy Storage System in Microgrid Salima Nemsi 1 , Seifeddine Abdelkader Belfedhal 2 , Linda Barazane 3 1 Laboratory of Electrical and Industrial Systems, University of Sciences and Technology Houari Boumediene, Algeria nemsisalima@yahoo.fr 2 Laboratory of Electrical Engineering and Plasma, University of Ibn Khaldoun, Algeria seifedinebelfedhal@yahoo.fr 3 Laboratory of Electrical and Industrial Systems, University of Sciences and Technology Houari Boumediene, Algeria lbarazane@yahoo.fr Abstract—Recently, the idea of electricity generation from one side has changed by introducing the concept of microgrid. The latter enables not only producer to be consumer or vice versa. But to aggregate different renewable energy sources like solar and wind in order to mitigate CO2 emission and produce clean energy, avoid big power losses, which are principally due both to the large electrical power produced in one place and long transmission lines .Nevertheless, this operation depends strongly on storage systems with power electronic converters for being reliable and controllable. In this context, a power electronic converter supplying a squirrel- cage induction machine coupled to a flywheel is proposed for study in this paper, This system is known as Flywheel Energy Storage System (FESS) and aims to improve the quality of electric power of grid or consumer by storing an amount of energy under kinetic form during high production of wind for example and to generate that quantity in case of deficit of primary source. The simulation results have been achieved using the software Matlab/Simulink. Index Terms—Microgrid, Flywheel Energy Storage System (FESS), Matlab/Simulink. I INTRODUCTION The worldwide is facing serious problems with electrical energy. From a side, the pollution caused by CO2 emission from fossil fuels and from other side, the depletion of tradi- tional sources like gas. Besides, the big losses, which are principally due both to the large electrical power produced in one place and long transmission lines. To overcome these drawbacks, a new form of electricity generation has been proposed known as Microgrid. Usually, the latter is composed by an aggregation of distributed genera- tion units, which depend essentially on renewable energy resources like wind and solar, loads and storage devices [1][2]. The whole system can be connected to either the main grid and known as grid-connected mode or works as autono- mous known as standalone mode [3][4]. An important feature of renewable energy resources is the fluctuation of the output power over time. Hence, the im- portance of storage systems within Microgrid appears espe- cially for boosting the power supplied by the Microgrid in grid-connected mode if the distributed generation sources are not supplying the expected level of energy due to their natural power variation [5]. Different types of storage exist, some are already used and others are under development and can be classified into two categories [6]: a/- Long term storage: Where the period of storage is above 10 minutes and the well-known types of long term storage are batteries, storage under potential form of water. b/- Short term storage: Where the period of storage is less than 10 minutes and well-known types of short term storage are Flywheel, super capacitor. In this context, the objective of this article is to study the Flywheel Energy Storage System (FESS) alone: the latter has many advantages like: simple maintenance, detailed knowledge of stored energy level, clean storage unlike bat- teries, independent lifetime duration of storage/retrieval cycles. This article is arranged as follows: Section II briefly de- scribes the main component parts of the FESS and its work- ing principle. In section III, the importance of such type of storage is presented. Section V is devoted to mathematical model of the whole system. The fifth Section shows the simulation results using Matlab/Simulink and discussion. Finally, conclusion is presented in Section VI. II CONSTITUTION AND WORKING PRINCIPLE OF FLYWHEEL STORAGE SYSTEM Figure 1 shows the main component parts of the storage Salima Nemsi, Seifedine Abdelkader Belfedhal and Linda Barazane / Role of Flywheel Energy Storage in Microgrid (2016 ) 45 system based on flywheel, which comprises the following elements: - A flywheel - A motor-generator - A power electronic converter As in the majority of the energy storage systems, there are a reversible transformation of energy. During storage, the electrical energy is converted into mechanical energy through the electric motor. The mechanical energy is stored in the flywheel as kinetic energy of a rotating mass. During the discharge of FESS, the mechanical energy is converted into electrical energy through the electric generator. The operating speed is imposed by the power electronic convert- er, which imposes the direction of transfer of energy through the electrical machine [6]. U stands for dc voltage link. III IMPORTANCE OF FLYWHEEL STORAGE SYSTEM In order to illustrate the behavior of FESS in a Mi- crogrid, we propose the schematic depicted in Figure 2 where the Microgrid in our case depend only on one type of renewable energy which is wind turbine connected to the grid in presence of a FESS. We suppose that the wind profile enables to generate an active power PWIND. The latter has variable values due to the random character of the wind. On the other hand, the grid must receive a smoothed power [6]. And knowing the power that must be delivered to the grid Preg, the FESS reference power can be determined as follows: ( ) If the reference power is positive, there is an excess of energy must be stored under kinetic form and the asynchro- nous machine works as motoring operation. Else, the asyn- chronous machine works as a generator where we have en- ergy to deliver. IV FLYWHEEL ENERGY STORAGE SYSTEM MODEL In this part, the modeling of all parties constituting the FESS will be presented. A Flywheel In this paragraph, the value of the inertia of the flywheel according to the power storing and which can be restored in a timely manner will be determined. The relationship that related the power to energy is the following [8]: ( ) With: PF: Maximum power deliverable by the storage system (equal to the nominal power of the asynchronous machine coupled to the flywheel) [W]. EF: Energy stored [J]. Then, the relationship between energy, inertia and angular velocity is: DC AC Control Motor/Generator Flywheel U P Safety and vaccum envelope Magnetic bearings Figure 1: Flywheel Energy Storage System constitution [7] Figure 2 Example of Flywheel Energy Storage System associated to wind energy [7] Electrical Grid Wind Turbine F E S S PWIND Preg Pref t t 0 0 PWIND Preg Pref Energy storage Energy delivery http://www.google.dz/imgres?sa=X&biw=1311&bih=620&tbm=isch&tbnid=w70DQUGHdKP02M:&imgrefurl=http://www.leblogenergie.com/2008/03/11/le-stockage-dne/&docid=vMrOrItN-FoAcM&imgurl=http://www.leblogenergie.com/files/2012/07/beaconsmartenergy25_2.gif&w=150&h=306&ei=LUaFUoqbKuTCigLuooCABg&zoom=1&ved=1t:3588,r:78,s:0,i:320&iact=rc&page=5&tbnh=189&tbnw=93&start=75&ndsp=23&tx=42&ty=65 Salima Nemsi, Seifedine Abdelkader Belfedhal and Linda Barazane / Role of Flywheel Energy Storage in Microgrid (2016 ) 46 ( ) Where: ΩF: Flywheel angular velocity in [rad/s]. JF: Flywheel moment of inertia expressed in [kg.m 2 ]. The moment of inertia of the flywhell is a key parameter because it characterizes the ability of storage (or restitution). By grouping equations (2) and (3), we get the following equation: ( ) Passing to small changes, we have: ( ) t: Time variation during charge or discharge for maxi- mum power [s]. ΩF: Small variation in angular velocity about an operat- ing point, in [rad/s]. ( ) ( ) ( ) Where: ΩFmax: Maximum flywheel angular velocity in [rad/s]. ΩFmin: Minimum flywheel angular velocity in [rad/s]. B Asynchronous machine The asynchronous machine is chosen according to these advantages in terms of simplicity and robustness of the rotat- ing parts. B.1 Electrical equations in the dq reference We use the model of the MAS in the Park reference. Flux and currents are given by the following system [6][8][9]: [ ] [ ( ) 0 ( ) 0 ] [ ] [ 0 0 0 0 0 0 ] [ ]( ) ( 0) ( ) Where: Rs, Rr: Stator and rotor phase resistances. Ls, Lr: Stator and rotor phase inductances. M: Mutual inductance. vds,vqs: Direct and quadrature components of stator volt- age. ids,iqs: Direct and quadrature components of stator current. ds, qs: Direct and quadrature components of the rotor flux. p: Number of pole pairs. s: Pulsation of the field in the stator reference frame. B.2 Control To determine the control (reference voltages to be ap- plied to the converter) of the asynchronous machine, we choose to work with rotor flux oriented control because equations are simpler compared to control stator flux or air gap flux oriented [6]. The positin of the reference is obtained to cancel the quadratic component of the flux rotor. There- fore, qlign the rotor flux vector with the axis of the Park reference. Suppose: ( ) 0 ( ) We obtain the following equations: [ ] [ 0 ] [ ] [ 0 0 0 0 0 0 ] [ ] ( ) The reference flux is imposed by the field weakening law of the asynchronous machine as follows [6]: { | | | | | | ( ) Konowing that: Salima Nemsi, Seifedine Abdelkader Belfedhal and Linda Barazane / Role of Flywheel Energy Storage in Microgrid (2016 ) 47 PI Calculation iqs_ref Calculation Γ - Decoupling System PI Flux Estimator PI - abc dq abc dq Flywheel Converter Asynchronous machine ( ) rn: Nominal rotoric flux [Web]. sn: Nominal statoric flux [Web]. Where: √ ( ) With: vs: Rms value of simple statoric voltage [V]. s: Grid pulsation equal to 314.16 rad/s. The reference direct statoric current is given by: ( ) ( ) PI: Proportienal integral regulator. We estimate the value of rotoric flux through the follow- ing equation: ( ) s: Laplace operator. We want to control the power of the asynchronous ma- chine coupled to the flywheel. From a reference power, one can deduce the electromagnetic torque reference of the ma- chine leading the flywheel by measuring the rotational speed. The expression of the electromagnetic torque can be calculated by [10]: ( 0) C Converter We define voltages modulated by the converter in the Park reference and applied to the stator of the asynchronous machine by the following system [9][11]: [ ] [ ] ( ) Figure (3): Flywheel Energy Storage System control [8] With: vd-reg and vq-reg represent converter adjusting tension in the Park reference. As well, the current modulated by the converter is given by: [ ][ ] ( ) The control of the converter associated with the asyn- chronous machine is derived by reversing the system in Equation (21): ( ) ( ) Knowing that: ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( 0) And ( ) The globale FESS control scheme is depicted in Figure (3). Due to the large size of that scheme, it is placed at the end of this article. Figure (3): Flywheel Energy Storage System control [8] Salima Nemsi, Seifedine Abdelkader Belfedhal and Linda Barazane / Role of Flywheel Energy Storage in Microgrid (2016 ) 48 V SIMULATION RESULTS Figures (4) to (10) illustrate the operation of FESS in a period of 60 seconds. The initial velocity of the Flywheel is fixed at 1500 rpm and the reference power is equal to the nominal power of the asynchronous machine 450 kW. 0 10 20 30 40 50 60 140 160 180 200 220 240 260 280 300 320 Time (s) F ly w h e e l s p e e d ( ra d /s ) ref Figure (4): Flywheel rotation speed 0 10 20 30 40 50 60 -6 -4 -2 0 2 4 6 x 10 5 Time (s) P o w e r (W ) Figure (5): Power storage system 0 10 20 30 40 50 60 0 50 100 150 200 250 300 Time (s) D ir e c t c u rr e n t (A ) i ds-ref i ds Figure (6): Direct statoric current 0 10 20 30 40 50 60 -300 -200 -100 0 100 200 300 Time (s) Q u a d ra tu re c u rr e n t (A ) i qs-ref i qs Figure (7): Quadrature statoric current 0 10 20 30 40 50 60 0 0.5 1 1.5 2 2.5 3 3.5 4 Time (s) F lu x ( W b ) ref Figure (8): Rotoric flux 0 10 20 30 40 50 60 -3000 -2000 -1000 0 1000 2000 3000 Time (s) V o lt a g e ( V ) a n d C u rr e n t (A ) v s (V) i s (A) Figure (9): Statoric current and voltage Salima Nemsi, Seifedine Abdelkader Belfedhal and Linda Barazane / Role of Flywheel Energy Storage in Microgrid (2016 ) 49 The flywheel rotation speed is shown in Figure (4). Note that the speed goes from 1500 to 3000 rpm in 30 seconds. This corresponds well to storage. Then this speed goes from 3000 to 1500 rpm in 30 seconds to restore 450 kW. The power storage system is shown in Figure (5). It is requested in this simulation to store 450 kilowatts during the first 30 seconds and return 450 kW in 30 seconds remaining. Looking at this figure, we see that the reference power is followed. We also note that the reference power is reversed when the speed of the flywheel reaches a high or low limit (see Figures 4 and 5).Therefore, we ask the asynchronous ma- chine to provide or consume nominal power of 450 kilo- watts. A positive power corresponds to a power consumed by the machine and a negative power represents a power supplied by the machine. Figures (6), (7) and (8) respectively show the evolution of the direct, quadrature current and flux of the asynchro- nous machine, there is a good follow instruction. A second observation that can be drawn from these fig- ures depends on direct current and its relationship with the flux, it is found from the change in the direct current com- ponent, which is the image of the flux. During storage, the current is in phase delay with the voltage where the machine acts as a motor (see Figure 10) and in return, the current is ahead of phase with the voltage where the machine works as a generator (see Figure 11) allows it to justify the two modes of operation of the asyn- chronous machine. V CONCLUSION In this article, we have presented the FESS as a solution to store electrical energy as a kinetic form in periods of excess of production of renewable energies sources and to restore it in the case of deficit following the random charac- teristic of such alternatives system. Initially a general view of the constituent parts of this system and its operating principle has been shown. Then, each part of FESS have been modeled separately including: Flywheel, asynchronous machine and its control and power converter. Finally, the results using Matlab/Simulink soft- ware justify the advantages of the Flywheel Energy Storage System either in storage period where the system works as a motor and stocks 450 kW as a kinetic form or in restitution period where the system works as a generator. REFERENCES [1] F. Katiraei and M. R. Iravani, ―Management Strategies for a Microgrid with Multiple Distributed Generation Units―, IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 21, NO. 4, Nov 2006. [2] J. M. Guerrero, N. Berbel, J. Matas. L. García de Vicuña and J. Miret, ―Decentralized Control for Parallel Operation of Distributed Generation Inverters in Microgrids Using Resistive Output Impedance―, IECON 2006 - 32 nd Annual Conference on IEEE INDUSTRIAL ELECTRONICS, 2006. [3] J. M. Guerrero, J. Matas, L. García de Vicuña, M. Cas- tilla, and J. Miret, ―Wireless-Control Strategy for Parallel Operation of Distributed-Generation Inverters―, IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, 20 20.01 20.02 20.03 20.04 20.05 20.06 20.07 20.08 -3000 -2000 -1000 0 1000 2000 3000 Time (s) V o lt a g e ( V ) a n d C u rr e n t (A ) v s (V) i s (A) Figure (10): Zoom of statoric current and voltage during storage 40 40.01 40.02 40.03 40.04 40.05 40.06 40.07 40.08 -3000 -2000 -1000 0 1000 2000 3000 Time (s) V o lt a g e ( V ) a n d C u rr e n t (A ) v s (V) i s (A) Figure (11): Zoom of statoric current and voltage during restitution http://ieeexplore.ieee.org/xpl/mostRecentIssue.jsp?punumber=4152824 http://ieeexplore.ieee.org/xpl/mostRecentIssue.jsp?punumber=4152824 Salima Nemsi, Seifedine Abdelkader Belfedhal and Linda Barazane / Role of Flywheel Energy Storage in Microgrid (2016 ) 50 VOL. 53, NO. 5, Oct 2006. [4] J. Kim, J. M. Guerrero, P. Rodriguez, R. Teodorescu and K. Nam, ―Mode Adaptive Droop Control With Virtual Out- put Impedances for an Inverter-Based Flexible AC Mi- crogrid―, IEEE TRANSACTIONS ON POWER ELEC- TRONICS, VOL. 26, NO. 3, Mar 2011. [5] M. A. Abusara, S. M. Sharkh, ―Control of Line Interac- tive UPS Systems in a Microgrid―, IEEE International Sym- posium on Industrial Electronics, 2011. [6] G. O. Cimuca, ―Système inertiel de stockage d’énergie associé à des générateurs éoliens―, PhD thesis in Electrical Engineering, Lille University, 2004. [7] H. BEN AHMED, B. MULTON, N. BERNARD, C. KERZREHO, ―Le stockage inertiel électromécanique―, Revue 3EI N°48, pp. 18-29, Mar 2007. [8] A. Davigny, ―Participation aux services systèmes de fermes d’éoliennes à vitesse variable intégrant du stockage inertiel d’énergie―, PhD thesis in Electrical Engineering―, Lille University, 2004. [9] S. Belfedhal, M. Berkouk, ―Modeling and Control of Wind Power Conversion System with a Flywheel Energy Storage System―, International Journal of Renewable Energy Research, IJR, Vol.1, No3, pp.43-52, 2011 [10] D. Leclercq, ―Apport du stockage inertiel associé à des éoliennes dans un réseau électrique en vue d’assurer des services systèmes―, PhD thesis in Electrical Engineering, Lille University, 2004. [11] L. Leclercq, B. Robyns J.M. Grave, ―Control based on fuzzy logic of a flywheel energy storage system associated with wind and diesel generators―, Mathematics and Com- puters in Simulation, Vol. 63, pp. 271–280, 2003. Salima Nemsi received her B.Sc., M.Sc. degrees in electrical engineering, in 2008 and 2011 respectively, from university of Science and Technology Houari Boumediene, Algiers, Algeria, where she is currently working toward the Ph.D. degree. Since 2013, she has been working full-time as a researcher at Renewable Energy Development Center, Algiers, Algeria. Her research inter- est include Photovoltaic and Wind Energy, storage systems, auton- omous and grid integration, DC-DC and DC-AC converters. Seifedine Abdelkader Belfedhal received his B.Sc., M. Sc., degrees in electrical engineering, in 2007 and 2010 respec- tively, from Ibn Khaldoun University, Tiaret, Algeria. He is a PhD student at the same university. His research interest in- clude Wind Energy, power conversion, energy management and power converters. Linda Barazane Professor at university of Science and Tech- nology Houari Boumediene, Algiers, Algeria, received her Engineer degree and M. Sc., in Electrical Engineeering from the National Polytechnic School of Algiers (ENP), Algeria, in 1989 and 1993, respectively. She received the doctorate de- gree in Electrical Engineering Departement of university of Science and Technology Houari Boumediene, in 2006. Her research interest are in Fuzzy logic systems, electrical and renewable energy.