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A Control Strategy for Flywheel Energy Storage System for Frequency Stability Improvement in Islanded Microgrid

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The Micro-Grid (MG) stability is a significant issue that must be maintained in all operational modes. Usually, two control strategies can be applied to MG; V/f control and PQ control strategies. MGs with V/f control strategy should have some
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   Iranian Journal of Electrical & Electronic Engineering, Vol. 13, No. 1, March 2017  10 1. Introduction The Microgrid (MG) is a low voltage network that usu-ally consists of loads, one or more Distributed Generators(DGs) and energy storage systems. Photovoltaic Cell(PV), Fuel Cell (FC), Microturbine (MT), Wind Turbine(WT) and small synchronous generator are some of theseDGs which can be used in MGs. Besides these, BatteryEnergy Storage System (BESS), Flywheel Energy StorageSystem (FESS) and super capacitor are some examples of storage systems that may be used in a MG [1-5]. Fig. 1shows a typical MG. A MG can operate in two operational modes; grid con-nected and isolated modes. When the MG operates in gridconnected mode, the main grid supports MG stability anddictates the frequency and voltage in all over the MGwhich operates in PQ control mode. While when it is dis-connected from the main grid, it operates in V/f controlmode [6-8].FC, MT and other kinds of DGs with slow dynamicsare usually controlled by the PQ control strategy in bothMG operation modes. However, DGs with the capabilityof the V/f control strategy, should have a fast response toload changes. The FESS, BESS and diesel generators al-most have this characteristic and can swiftly change their generation or absorption [9-12].The BESS has been used in [9], to enhance the MG sta- bility. In this case, a strategy that acts based on frequencydeviations has been defined. Actually, the BESS has a basic role alongside other DGs with the PQ control strat-egy. Although batteries have some advantages like relia- A Control Strategy for Flywheel Energy Storage System forFrequency Stability Improvement in Islanded Microgrid A. A. Khodadoost Arani * , B. Zaker  * and G. B. Gharehpetian *(C.A.) Abstract: The Micro-Grid (MG) stability is a significant issue that must be maintained in all operationalmodes. Usually, two control strategies can be applied to MG; V/f control and PQ control strategies. MGswith V/f control strategy should have some Distributed Generators (DGs) which have fast responses versusload changes. The Flywheel Energy Storage System (FESS) has this characteristic. The FESS, which con-verts the mechanical energy to electrical form, can generate electrical power or absorb the additional power in power systems or MGs. In this paper, the FESS structure modeled in detail and two control strategies(V/f and PQ control) are applied for this application. In addition, in order to improve the MG frequency andvoltage stability, two complementary controllers are proposed for the V/f control strategy; conventional PIand Fuzzy Controllers. A typical low voltage network, including FESS is simulated for four distinct scenariosin the MATLAB/ Simulink environment. It is shown that fuzzy controller has better performance than con-ventional PI controller in islanded microgrid. Keywords: Flywheel Energy Storage System (FESS), Fuzzy Controller, Microgrid (MG), Voltage and Fre-quency Stability, Stored Energy. Iranian Journal of Electrical & Electronic Engineering, 2017.Paper received 12 October 2016 and accepted 8 March 2017.* The authors are with the Department of Electrical Engineering, Amirk-abir University of Technology, Tehran, Iran.E-mails: a.a.khodadoost@aut.ac.ir, zaker.behrooz@aut.ac.ir and grpt-ian@aut.ac.ir Corresponding Author: G. B. Gharehpetian. DCACPVDCACACDCMTACDCFESSLVMVDCACDCAC Transfer switch CHP   Fig. 1. Architecture of typical MG    D  o  w  n   l  o  a   d  e   d   f  r  o  m    i   j  e  e  e .   i  u  s   t .  a  c .   i  r  a   t   1   4  :   0   8   I   R   S   T  o  n   T   h  u  r  s   d  a  y   O  c   t  o   b  e  r   1   2   t   h   2   0   1   7   [   D   O   I  :   1   0 .   2   2   0   6   8   /   I   J   E   E   E .   1   3 .   1 .   2   ]   Iranian Journal of Electrical & Electronic Engineering, Vol. 13, No. 1, March 2017  11  bility, high rating (series and parallel implementation),cost-effective and high energy density, they have somedrawbacks such as low power density, higher maintenancerequirements and environmental issues and concerns. Al-though the Battery can discharge rapidly like FESS, itcannot charge rapidly [13]. On the other hand, in comparison to BESS, the FESSis environmentally friendly, requires no remarkable main-tenance and has high power density. Due to these advan-tages, the FESS can be used for different applications in power systems and MGs such as power quality improve-ment [15, 16] and stability enhancement [17]. In [18], the FESS modeling has been introduced andused for a MG including a diesel generator and criticalloads. In [19, 20], the authors have tried to mitigate fre-quency oscillations by using FESS in a power system in-cluding wind power plants. That scheme required a smartcommunication network among sources. Indeed, theFESS had a complementary role in this case. The appli-cation of the FESS in [21-22], causes more penetration of renewable energies, and indeed the FESS has increasedreliability in MG by improving its stability. The Authorsshow that application of the FESS in distribution systemshas improved voltage and frequency control [23]. In some works such as [36-37], a fuzzy controller for the FESS has been used to regulate the power in presenceof wind power generation units. The wind generation unitshave many fluctuations. The application of fuzzy con-troller has improved the power generation curve of theFESS with these units. In [23], a survey of FESS applications and differentcontrol strategies has been presented. A FESS with V/f control and PQ control strategies has been used in [24]and the results on the MG stability have been presentedfor these control strategies. However, the model appliedfor FESS, is an ideal DC voltage source connected to aninverter. Therefore, the dynamics of FESS has been neg-lected. Also, some limitations of state-of-charge and speedhave been disregarded. Actually, the authors have neg-lected the primary dynamics of the FESS and conse-quently the dynamic behavior of the MG has been ignoredin the studied cases. Since the FESS is not a long-term en-ergy storage device and has a low capacity of energy, theresults of the detailed model are different from this model.This is important for maintenance studies, as well. Due tothe limited energy capacity of the FESS, a charge and dis-charge rule should be determined for it. In this paper, a comprehensive and detailed model of the FESS is initially introduced. Then, different controlstrategies (PQ and V/f) are applied to this model. In addi-tion, a complementary scheme is proposed and added toFESS with the V/f control strategy. To compare the effectof applied strategies on the MG frequency and voltagestability, the FESS with three control strategies, is studiedin the low voltage network of CIGRE [25]. The simula-tion results are presented in the fourth section of the paper.In each simulation scenarios, the voltage and frequencyhave been analyzed. In addition, the power and energy of the FESS, as an important factor for this energy storagesystem, have been discussed. An appropriate long termcontrol scheme on electrical power of the FESS and other DGs should be applied to maintain the stability of MG.Finally, a conclusion of the paper is presented. 2. FESS Structure Briefly, a massive disk, which is connected to an elec-trical machine shaft and its velocity and power flow (toor from MG) can be controlled, is called FESS. The elec-trical model of a FESS consists of an electrical machine,a back to back converter and its control system and a mas-sive disk (flywheel). Fig .2 shows a general scheme of FESS. The stored energy in the FESS can be computed asfollows:(1)where, Jf (kg.m 2 ) and ω (rad/s) are the flywheel inertiamoment and speed, respectively. In low speed FESS, thevelocity approximately reaches up to 10000 rpm and thusfor storing more energy, based on (1), the inertia of thedisk should be increased [13, 22, 26].In this case the in-ertia has been increased, therefore, a low speed FESS has been used.Based on (1), the State-of Charge (SoC) of the FESScan be defined as follows:(2)where E0 is the initial stored energy in the FESS and ωcan be achieved in each time which is sensed by a speedsensor, consequently the SoC of the FESS can be com- puted comfortably.In this study, an Induction Machine (IM) is used in theFESS scheme. For modeling of the inertia of the disk, anadditional inertia has been set for IM. Also, the FESS hasa back to back converter, including two converters and aDC link. One of these converters (machine-side conver-tor) controls IM, and adjusts its speed while the other con-verter connects the FESS to the MG. According to thedifferent conditions such as MG operation mode, state-of-storage charge etc., the direction of the power flow inthe FESS is determined and it is implemented by control-ling two converters. 2 1 2  FESS   f    EJ        20 0 ( ) 11 ( % ) ( )2  FESS   f    Et SoCJt  EE          D  o  w  n   l  o  a   d  e   d   f  r  o  m    i   j  e  e  e .   i  u  s   t .  a  c .   i  r  a   t   1   4  :   0   8   I   R   S   T  o  n   T   h  u  r  s   d  a  y   O  c   t  o   b  e  r   1   2   t   h   2   0   1   7   [   D   O   I  :   1   0 .   2   2   0   6   8   /   I   J   E   E   E .   1   3 .   1 .   2   ]   Iranian Journal of Electrical & Electronic Engineering, Vol. 13, No. 1, March 2017  12 3. Modeling of FESS3. 1. Machine-side converter control To control the machine-side converter, the direct field-oriented control (DFOC) is used. The detailed procedureof this control method has been stated in and shown in Fig3. For an IM with p pairs of pole, the operation equationin the stator reference frame, as space vectors, can be writ-ten as follows [27-29]:(3) (4) (5)(6)(7)where, R  r  and R  s are the rotor and stator resistance, re-spectively, and Lr and Ls are the rotor and stator induc-tance, respectively. M is the no-load inductance of the IMand φ, i, and v indicate the flux, current and voltage andσ is defined as follows:(8)In the rotor reference frame, considering the d-q frameand developing the above equations, the torque and fluxcan be stated as follows:(9)(10)Equations (9) and (10) that are the basic equations inthe FOC method show that in the rotor flux referenceframe, the rotor flux and torque can be obtained. An im- portant outcome is achieved showing that the decoupledcontrol of the rotor flux and torque by using d and q com- ponents of the stator current is practical. However, thismethod requires knowledge of rotor flux position. The block diagram of the Direct Field Oriented Control(DFOC) is shown in Fig. 3.In this method, the reference of the rotor flux and ma-chine torque is estimated by measuring the rotor speedand stator current. Then, the reference of the flux andtorque, obtained at a higher control level, is comparedwith the estimated values. Thereafter, to set the flux andtorque of the machine in a predefined value, a referencevoltage is generated and finally this voltage is deliveredin machine terminals by using a Space Vector Pulse WidthModulation (SVPWM) technique [15]. 3. 2. Grid-side convertor control The grid-side converter is controlled by a vector-ori-ented control (VOC) method. A Phase Locked Loop(PLL) is used in this control method for grid synchroniza-tion. This method has been stated in detail in [29] and itsstructure is shown in Fig. 4.An important issue is the determination of referencevalues of currents in this method, which is shown in Fig.    s s s s d  vRi dt        0  r r r r m d   Rij dt               s s s r   LiMi        r  r r s  LiMi        ( . )   r s s r   M T   p  j LL        2   1  s r   M  LL        1   rd rd sd r r  d   M i dt           32  rd s q r   M T   p i L      Fig. 2. General scheme of FESS    D  o  w  n   l  o  a   d  e   d   f  r  o  m    i   j  e  e  e .   i  u  s   t .  a  c .   i  r  a   t   1   4  :   0   8   I   R   S   T  o  n   T   h  u  r  s   d  a  y   O  c   t  o   b  e  r   1   2   t   h   2   0   1   7   [   D   O   I  :   1   0 .   2   2   0   6   8   /   I   J   E   E   E .   1   3 .   1 .   2   ]   Iranian Journal of Electrical & Electronic Engineering, Vol. 13, No. 1, March 2017  13 5.These references, can be determined for maintaining DCvoltage of the DC link in nominal value or supplying adistinct value of active and reactive power or both. The power calculation unit operates as follows:(11)(12) 3. 3. Control of FESS In this section, the proposed control structure is de-scribed. Two control strategies can be applied on FESS:V/f and PQ control strategies. In the V/f control strategy,the FESS unit has been used as a reference voltage andfrequency unit in MG. Therefore, the grid-side converter must control the frequency and voltage. The reference of the voltage and frequency are achieved based on droopequations as follows [30-35]:(13)(14)where, f* and V* are the no-load frequency and voltagerespectively, and m and n are droop coefficients. Also,complementary control includes two PI controllers theinput of which is the feedback of the frequency and volt-age, which are added to the no-load frequency and volt-age. In addition a fuzzy controller can be used for this purpose, as has been shown in Fig. 6. The fuzzy controller which has been designed in this *  ff   m  P       * VVn Q       sd q sq d  Q = v i -v i    sd d s qq  P  =vi+vi   Fig. 3. Block diagram of FOC [38].       + Fig. 5. Reference currents determination in grid-side con-verter controller.   Frequency  NB    NM     NS     ZE     PS     PM     PB   PB  P   M    P  S    Z   E    Z   E    N  S    N   M     N df/dt  PB  P   M    P  S    Z   E    N  S    N   M    NB  Z    P   M    P  S    Z   E    Z   E    N  S     M   B  NB  P    Table 1 . Fuzzy control rules for computation   LPF + n  + m  _  _   P  LPF +90 ° V 0 I 0  Power Calculation Unit Q    E*  E* sin (  t  )+  f*  is ps k k  s   f*   _ + Controller k  3 k  2 k  1  Switch   Fig. 6. V/f control strategy with complementary control.   Fig. 4. Control of grid-side convertor [38].    D  o  w  n   l  o  a   d  e   d   f  r  o  m    i   j  e  e  e .   i  u  s   t .  a  c .   i  r  a   t   1   4  :   0   8   I   R   S   T  o  n   T   h  u  r  s   d  a  y   O  c   t  o   b  e  r   1   2   t   h   2   0   1   7   [   D   O   I  :   1   0 .   2   2   0   6   8   /   I   J   E   E   E .   1   3 .   1 .   2   ]   Iranian Journal of Electrical & Electronic Engineering, Vol. 13, No. 1, March 2017  14  paper has two input and one output. All inputs and outputhave been normalized in range of [-1, 1]. Input 1 repre-sents the normalized value of frequency changes in MGwhich has seven Membership Functions (MFs). The MFshave been selected as triangular shape and their overlapfirstly has been selected 50% and then has changed by tryand error by expert person.For input 2, three MFs have been selected and the out- put has seven MFs. Fig. 7 show the input and output MFs.Table 1 lists the rules of fuzzy inference system.Application of the droop and complementary controlsretrieve the frequency to its nominal value. The droopmethod leads to stable operation of the V/f controlled in-verter. Due to a sudden change of active power in the MG,the V/f controlled DG (that is, the FESS) should respondto it. Based on the droop method by increasing the gener-ation of the FESS, the frequency is dropped. Indeed therelation between the changes of frequency with respectto active power changes are always negative, as follows: (15)This ensures the stability of the system. On other hand,the complementary control helps to set the frequency inan acceptable range by changing no-load frequency andleads to a stable operation mode. As can be seen, an in-crement in P leads to a frequency drop. As a result, theinput of the PI controller is increased which causes no-load frequency change.To ensure that the voltage of the DC link has no fluc-tuation, the voltage control of the DC link is essential. Themachine-side converter controls the power flow betweenDC link and machine and tries to keep it constant bychanging the machine speed. This strategy is shown inFig. 8. If FESS is adjusted by the PQ control, the grid-side converter works as DC link voltage stabilizer and themachine-side converter sets the disk speed. The referencespeed is determined using Pref. To calculate the speedreference (ωref), firstly, the energy reference of the FESSis computed by using Pref and then, ωref can be written,as follows:(16)(17) 4. Simulation Results In the simulated MG, a PV and FC are associated bythe PQ control strategy based on dynamic modeling ap- plied in [9]. The nominal ratings of PV and FC are 1.5kVA and 2 kVA respectively. Three scenarios are studiedin the simulations. The first scenario has a FESS with PQcontrol. The second one includes the FESS with V/f con-trol strategy and the third scenario consists of a FESS withV/f control strategy and complementary control scheme.In all of the scenarios, the MG is connected to the main 1 0 1   t  t t   E E   P   dt r  e  f r  e  f       0  f   P     2  E r  e  f     =r  e  f   J  f   w   ( c )    N Z   P 1-1  0 (a)   (  b )   Fig. 7. Membership function of ; a: Input1 (frequency of MG), b: Input2 (changes of frequency) and c: Output.   Fig. 8. The overall control structure of FESS.    D  o  w  n   l  o  a   d  e   d   f  r  o  m    i   j  e  e  e .   i  u  s   t .  a  c .   i  r  a   t   1   4  :   0   8   I   R   S   T  o  n   T   h  u  r  s   d  a  y   O  c   t  o   b  e  r   1   2   t   h   2   0   1   7   [   D   O   I  :   1   0 .   2   2   0   6   8   /   I   J   E   E   E .   1   3 .   1 .   2   ]
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