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.  

 

 

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for a Microgrid with Multiple Distributed Generation 

Units―, IEEE TRANSACTIONS ON POWER SYSTEMS, 

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[2] J. M. Guerrero, N. Berbel, J. Matas. L. García de Vicuña 

and J. Miret, ―Decentralized Control for Parallel Operation 

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[3]  J. M. Guerrero, J. Matas, L. García de Vicuña, M. Cas-

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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)

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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
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 Salima Nemsi, Seifedine Abdelkader Belfedhal and Linda Barazane / Role of Flywheel Energy Storage in Microgrid (2016 ) 
 

  

50  

VOL. 53, NO. 5, Oct 2006.    

 

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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.