Simple Flywheel Energy Storage
using Squirrel-Cage Induction Machine
for DC Bus Microgrid Systems
Jae-Do Park
Dept. of Electrical Engineering
University of Colorado Denver
Denver, CO
Email: jaedo.park@ucdenver.edu
Abstract—A simple flywheel energy storage using a squirrelcage induction machine is proposed in this paper. The suggested
motor/generator system operates with an open-loop Volt/Hertz
control scheme and utilizes only the nameplate data as machine
parameters. Therefore complex controller tuning or machine
parameter measurement is not required. Also, any communication between storage units or with other controllers is not
necessary because the system uses bus voltage information for
charge/discharge operations. The proposed system has an advantage on parallel operation because adding/replacing of units are
straightforward. Hence it can easily operate with different types
of storage or distributed energy sources in DC bus microgrid
systems. Moreover, the proposed control scheme improves the
overall stability of the DC bus system. The proposed system has
been validated with Matlab simulation and an experimental setup
is under construction for verification.
I. I NTRODUCTION
Distributed generation systems have recently been intensively researched and developed, especially in conjunction
with renewable energy sources such as wind turbines and
photovoltaic systems. The advantages of distributed generation
systems include the capacity relief of transmission and distribution, better operational and economical efficiency including
effective use of waste heat, and improvement of reliability,
eco-friendliness and power quality [1]–[3]. Considering the
current energy situation and the energy policy of governments
worldwide, the penetration of distributed generation using the
renewable energy sources is expected to increase rapidly.
As a way to realize the distributed generation system, a
”microgrid” system that combines distributed energy sources
and loads as a small-scale power system can be used. The
microgrid approach reduces or eliminates central dispatch and
enhances the power quality to sensitive loads [4], [5]. A
microgrid is controlled and operated quite differently than a
conventional power systems: the distributed energy sources
are connected through power electronics converters and the
islanded operation is required as well as grid-connected operation [6].
Various kinds of dispachable and non-dispachable prime
movers can be included in a microgrid system, such as diesel
generators, microturbines, fuel cells, wind turbines and photovoltaics, to support different types of loads. Distributed energy
Fig. 1.
Conceptual diagram of a DC-bus microgrid system.
storage systems including batteries, flywheels and supercapacitors are also included. The energy storage capability is
important for microgrid operations because the stored energy
can be utilized for different purposes such as load support,
frequency control, power compensation, and voltage leveling.
This is especially true for systems using a high portion
of sustainable energy sources; for example, for a microgrid
system that is operating in a remote area isolated from a
power grid, storage devices play a critical role because of the
intermittent and uncontrollable nature of the renewable energy
sources.
A flywheel energy storage system using a squirrel-cage
induction machine is proposed in this paper. The proposed
system utilizes the squirrel-cage induction machine, which is
widely available and inexpensive, and the simple Volt/Hertz
control technique with just nameplate data as machine parameters. Therefore no complex parameter measurement is necessary and the system has an advantage on parallel operation
because adding/replacing units are straightforward. Hence it
can easily operate with different types of storage or distributed
energy sources in DC bus microgrid systems. Moreover, the
proposed control scheme improves the overall stability of the
DC bus system. The proposed system has been validated
with Matlab simulation and an experimental setup is under
construction for verification.
II. S YSTEM C ONFIGURATION
Fig. 2.
Induction machine per-phase equivalent circuit.
A. DC Bus Microgrid System
Among the microgrids that have been researched recently,
low-voltage DC (LVDC) bus-based systems have received
attention because of advantages such as fast control without
need for communication, a variety of DC energy sources and
loads, and efficiencies on system size and cost [7], [8]. A block
diagram of a typical DC bus microgrid system is shown in Fig.
1.
The LVDC systems utilize the voltage droop technique,
which uses the DC bus voltage as a command signal. DC bus
systems do not have the functional issues of AC systems such
as synchronization and reactive power compensation. Unlike
the large scale distribution systems, the LVDC microgrid
system does not have a resistive loss issue because the length
of the bus is much shorter. Also, sharing a DC bus has a
structural advantage because all the energy sources and loads
are connected via DC/DC or DC/AC voltage-source converters
that utilize DC voltage as their medium. Another layer of
DC/AC converters is necessary for some subsystems to make
an AC bus system.
Using the fast-acting power electronics converters, constant
voltage can be supplied to the loads regardless of some
fluctuations on the bus side and the renewable energy sources
can readily generate maximum power with maximum power
point tracking (MPPT) techniques.
As a small-scale power system, a microgrid can have
relatively higher load fluctuations, especially when it is not
operating in grid-connected mode. This is because the inertia
of the generators is not as large as that found in the largescale synchronous generators. Generation of power from the
renewable energy sources relies heavily on natural conditions
and they are intermittent. Hence energy storage devices are
required for stable operation of a microgrid system in either
grid-connected or islanded operations. A flywheel-based energy storage system is investigated in this paper.
wheel motor/generator [10]–[15]. PM machines have advantages such as lower rotor losses, high power factor, efficiency,
and power density. However, high-power magnets are costly
and they have an inherent disadvantage of spinning losses.
Synchronous homopolar machines, although they have been
researched for various applications, are not widely used in
practice. Synchronous reluctance machines can be a viable
choice for a flywheel motor/generator, but the machines are
not easily available.
Induction machine-based flywheel systems have been investigated and it has been suggested that the rugged and inexpensive induction machines are good candidates for high-power
flywheel motor/generators [14]–[16]. Field-oriented vector
controllers are generally used for faster dynamic response,
which require complex machine parameter measurements and
complicated controllers.
However, the majority of the fast disturbances are shorter
than several seconds [9] and storage devices needed to store
the intermittent generation of renewable energy sources do
not necessarily have to be fast if they are not focusing on
transient performance improvement. Considering the overall
cost, it would be more efficient for a microgrid system to
use a combination of faster storage devices for short transient
and slower but inexpensive storage units for massive energy
charge and discharge with renewable energy sources such as
wind turbines and photovoltaic systems.
To develop a cost-effective system, a squirrel-cage induction
machine is selected for the motor/generator of the flywheel
energy storage in this paper. Easy parallel operation is an
important factor in energy storage for higher capacity.
B. Flywheel Energy Storage
Advances in power electronics, magnetic bearings and flywheel materials have made flywheel systems a viable energy
storage option. Although it has higher initial cost than batteries, flywheel energy storage has advantages such as longer
lifetime, lower operation and maintenance costs, and higher
power density (typically by a factor of 5 to 10) [9]. Flywheel
systems have been utilized in many applications instead of or
in conjunction with batteries.
Machines such as permanent magnet (PM) machines, synchronous reluctance machines, synchronous homopolar machines and induction machines have been explored for fly-
C. Squirrel-Cage Induction Machine
1) Machine Modeling: A per-phase equivalent circuit of an
induction machine is shown in Fig. 2. Machine torque can be
derived as can be seen (1)-(4). The expressions can be applied
to the integral horsepower induction machines where the speed
is high enough for the resistive drop to negligible.
i2 =
em
R2
jX2 +
s
=
em
sωe
ωe jsX2 + R2
(1)
Fig. 3.
Block diagram of flywheel drive system.
III. C ONTROL T ECHNIQUE
Pm
P R2
2
=3
|i2 |
Tq =
ωm
2 ωsl
P
ωsl R2
= 3 λ2m
2
2
(ωsl L2 ) + R22
P
ωsl
≈ 3 λ2m 2
2
R2
A. Microgrid System Operation
(2)
(3)
(4)
where, Pm is machine power, P is machine poles, s is slip,
λm is air-gap flux, ωe is primary angular speed and ωsl is slip
angular speed.
As shown in (4), induction machine torque can be
controlled with the slip frequency if air-gap flux λm is kept
constant, which can be accomplished by constant voltage and
em
v1
frequency ratio
≈
. Although there are conditions such
ωe
ωe
as machine’s power rating and speed to make the expressions
valid, operating conditions of a flywheel energy storage
system well satisfy the conditions.
2) Volt/Hertz Control: Volt/Hertz control has been widely
used for induction machine speed control because of its
simplicity for the applications that tight torque response
for transient dynamics is not required. Field-oriented vector
control technique has been utilized for the applications that
need fast response, but it requires parameter measurements,
current feedback, machine model and controller tuning. On the
contrary, all of the information necessary to run an induction
machine in rated condition, such as voltage, speed and slip,
can be found on the nameplate of the machine for Volt/Hertz
control.
Many researches on flywheel energy storage systems have
utilized field-oriented controllers because the applications need
quite fast energy flow, for example, UPS application compensating the voltage dip. However, if an energy storage system
absorbs or releases the energy in slow dynamics, Volt/Hertz
control can be a valid candidate due to its simplicity and
inherent stability.
The proposed microgrid system shown in Fig. 1 utilizes the
DC bus voltage as a control signal. Hence, a prime mover,
such as a grid-connected converter or microturbine unit, does
not control the voltage tightly at a fixed reference voltage to
reflect the energy flow on the bus voltage if it is in the nominal
operating region above the minimum threshold.
All of the renewable energy source units are operating in the
power mode, which is generating its maximum power when
the energy is available. Hence the bus voltage will rise above
the nominal operating range if the generation is larger than
the load power consumption. The storage devices detect the
excessive energy in the bus and absorb it. If the generated
energy is large enough to exceed the maximum threshold, the
grid-connected converter can push it back to the grid.
When the DC bus voltage gets lower than the discharge
threshold due to the increased load or decreased generation,
the energy stored in the storage units are discharged to the bus
to maintain the bus voltage at the minimum level. If all the
energy storages and generators are not able to hold the bus
voltage at the minimum level, load shedding can be initiated
by disconnecting some of the power electronics converters
supporting lower priority loads. If there are different kinds of
storage or load control units connected to the bus, the priority
can be easily controlled by setting the thresholds differently.
The units in the microgrid are autonomously operating using
the bus voltage without communicating between units or to a
central controller; hence addition, replacement or removal of
the units can be done easily without any major change in the
control configuration unlike the centrally controlled system.
B. Control of Flywheel Energy Storage System
The control block diagram of the proposed flywheel drive
system is shown in Fig. 3. The controller is consisted of three
parts, i.e. mode control, slip control and voltage control, to
generate the proper voltage and frequency for the flywheel
induction motor/generator. The low-pass filter filters out the
high frequency voltage fluctuations.
TABLE I
I NDUCTION M ACHINE PARAMETERS
Rated Power
Rated Voltage (line-to-line)
Rated Speed
Stator Resistance R1
Stator Leakage Reactance X1
Rotor Resistance R2
Rotor Leakage Reactance X2
Mutual Reactance Xm
Fig. 4.
Controller State Machine.
TABLE II
O PERATION MODE THRESHOLDS
CHARGE READY VchgR
CHARGE Vchg
IDLE
DISCHARGE READY VdisR
DISCHARGE Vdis
Fig. 5.
Bus voltage thresholds for control.
The mode control determines the operating mode based on
the bus voltage. The operating modes of the proposed flywheel
energy storage system are as follows and the state machine
of the mode control can be seen in Fig. 4. Each mode has
associated voltage levels pre-defined in the mode control, as
shown in Fig. 5.
• IDLE
• CHARGE READY
• CHARGE
• DISCHARGE READY
• DISCHARGE
The system is in IDLE mode when the bus voltage is in nominal range between mode thresholds for
CHARGE READY and DISCHARGE READY. In IDLE
mode, PWM is disabled and grid-tied converter or microtubine
generator forms the grid depending on the operation mode, i.e.
grid-connected or islanded mode.
When the bus voltage is increased above VchgR , the threshold of CHARGE READY mode, the voltage control starts
to increase the voltage in order to build rated flux while
maintaining zero slip. The rated flux will be built in the
induction machine when the bus voltage reaches Vchg .
∗
vm
∗
,
vm
Vrated vdc − VchgR
=
·
· fm
Frated Vchg − VchgR
50 Hp
460 V
1705 rpm
0.087 Ω
0.302 Ω
0.087 Ω
0.302 Ω
13.08 Ω
rated frequency, respectively. If the operating speed is above
the rated speed, Vrated needs to be reduced to operate in field
weakening mode.
The machine voltage reaches the proper level for the rated
flux according to the rotating speed when the bus voltage gets
to the charge threshold and the machine is ready to absorb
the power from the bus. In CHARGE mode, the slip control
increases the slip based on the excessive voltage above Vchg .
KIS
· (vdc − Vchg )
(6)
fslip = KP S +
S
∗
fm
= fm + fslip
(7)
Z
∗
θe = 2πfm
dt
(8)
∗
, KP S , KIS denote the frequency command, prowhere fm
portional and integral slip control gain, respectively. Basically
the slip control resets the slip to zero if the voltage decreases
below the CHARGE threshold Vchg . However, a hysteresis
needs to be implemented between modes for smooth mode
transition.
The voltage control receives the calculated slip frequency
fslip and machine rotating frequency fm and generates appropriate voltage based on the rated Volt/Hertz ratio to maintain
the rated flux in the machine so that the torque can be
controlled just by slip. The direct and quadrature voltages in
stationary reference frame and three-phase voltage commands
∗
can be generated as follows using vm
and angle information.
Superscript ’s’ denotes stationary reference frame.
s
∗
vds
= vm
cos θe
(9)
∗
vm
sin θe
s
vds
(10)
s
vqs
=
vas =
(5)
fm , Vrated and Frated denote the machine stator
where
voltage command, rotating frequency, rated stator voltage and
560 V
540 V
520 − 540 V
520 V
500 V
√
1 s
3 s
vbs = − vds
+
vqs
2
2
√
1 s
3 s
vcs = − vds
−
v
2
2 qs
(11)
(12)
(13)
Fig. 6. Top: DC bus voltage (a)Vchg (b)VchgR (c)VdisR (d)Vdis
Bottom: DC bus currents (a)iload (b)idc .
Fig. 8.
Fig. 9.
Fig. 7.
Three-phase stator voltage and currents.
Command voltage peak and slip.
Similarly, in DISCHARGE READY mode, slip is kept at
zero, and the voltage command will be given as
∗
vm
=
Machine torque and flywheel speed.
Vrated vdc − VdisR
·
· fm .
Frated Vdis − VdisR
(14)
And slip frequency is calculated as follows in DISCHARGE
mode.
KIS
fslip = KP S +
· (vdc − Vdis )
(15)
S
Although a speed sensor is assumed in the proposed system,
a speed estimation technique is readily applicable, especially
if the speed is changing slowly due to the large inertia.
All the control variables such as, Vrated , Frated , rated slip
and slip limiter can be obtained or calculated from induction
machine nameplate. The service factor of the machine can be
used to determine the slip limiter so that the machine can be
operating safely in overload range.
Energy storage system’s input and output impedance can be
controlled by the gains and limiter in slip control because the
slip determines the power the system takes or releases.
C. Stability Consideration
It is well known that DC power distribution systems can
have stability issues due to the negative impedance of the
connected converters, even if the individual subsystems are
stable [17], [18]. The input impedance of the converter can
be expressed as follows, where ∆ denotes the deviation from
the steady state operating point values. The input impedance
of the converters becomes negative when they are operating
in constant power mode.
2
Zi =
(vdc )
∆vdc
=−
∆idc
Po
(16)
The power electronics converters can tightly control their
output power as almost constant, and the negative impedance
affects the DC bus stability adversely. It has also been suggested that the output impedances of the sources Zo should be
smaller than input impedance of the loads Zi for the overall
stability of the DC bus [19].
|Zo | << |Zi |
(17)
Although this is not a direct issue for the proposed energy storage system because it does not operate in constant
power mode, its effect on overall system stability needs to
be considered. The proposed system controls the power with
the slip and constant Volt/Hertz ratio, which keeps the torque
proportional to the slip. When the flywheel energy storage
system charges the energy from the bus, slip is proportional
to the bus voltage. Hence, the power that storage system
takes from the bus is proportional to the bus voltage and
the impedance of the system is always positive. On the other
hand, the slip is inversely proportional to the change of the bus
voltage which lowers the overall source impedance because the
DC current the storage system discharges increases as the bus
voltage decreases. Therefore, the energy storage system can
improve the overall stability of the system in either operating
mode.
IV. S IMULATION R ESULT
The proposed flywheel energy storage system has been
simulated with Matlab. A 50Hp induction machine as the
motor/generator and a flywheel with 23.5 kgm2 moment of
inertia has been utilized. The simulated system can store 2220
kJ at 4150 rpm and supply rated power for 1 minute. The
parameters for the system simulation are shown in Table I and
II.
The 20 kW and 30 kW of load increase and renewable
generation is simulated at 0.1 sec, 2 sec, 5 sec and 7 sec,
respectively. As voltage decreases below VdisR , the machine
voltage increases to build up the flux then slip decreases to
generate power. Likewise, when the bus voltage increases over
VchgR and Vchg , machine voltage and the slip increase.
As can be seen in Fig. 6, the bus voltage is maintained
at Vdis and the load current is compensated in DISCHARGE
mode and the flywheel storage takes the energy generated by
renewable energy sources in CHARGE mode. The output of
the voltage control and slip control is shown in Fig. 7. The
machine torque and the variation of the flywheel speed show
the energy exchange in Fig. 8.
V. C ONCLUSION
A Volt/Hertz control-based flywheel energy storage using
a squirrel-cage induction machine has been suggested in this
paper. The proposed energy storage system is simple and costeffective. It has utilized only the nameplate data as machine
parameters and does not require complex measurements and
tuning. The charge/discharge operation that is based only on
bus voltage information without any communication between
storage units or with other controllers has an advantage of
parallel operation and makes adding/replacing units straightforward. The proposed system has been validated with Matlab
simulation and an experimental setup is under construction for
verification.
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