Power Control Method using Time Delay
Compensation Scheme Based on Smith predictor forFlywheel Power Leveling System
Kenta Tanaka

Jun-ichi Itoh

Soya Matsuo

Noboru Yamada

Nagaoka University ofTechnology
Niigata, Japank_tanaka@stn.nagaokaut.
ac.jp

Nagaoka University ofTechnology
Niigata, Japan
itoh@vos.nagaokaut.ac.jp

Nagaoka University ofTechnology
Niigata, Japan
soya_matsuo@stn.nagaokaut.
ac.jp

Nagaoka University ofTechnology
Niigata, Japan
noboru@nagaokaut.ac.jp

Abstract— This paper introduces the performance of a power
leveling system with a 3.0-MJ, 9500-r/min flywheel energy
storage. In term of the cost reduction, this system uses low cost
flywheel and the general purpose products. Therefore, the timedelay of the measurement device limits the control performance.
In order to overcome this problem, the time delay compensation
scheme based on Smith predictor is applied in the control inorder to improve the control performance of the power
regeneration. As a result, the overshoot of the regeneration
power is eliminated by applying the compensation. In addition,
the effectiveness of the power leveling control in a prototype isevaluated by experiment. As a result, it is confirmed that thepower fluctuation is reduced by 84.6 %.

I.

INTRODUCTION

Renewable energy systems, especially a wind turbine and aphotovoltaic cell, are generated power fluctuation due to themeteorological conditions. Therefore, these systems requireenergy buffers such as electric double layer capacitors(EDLC), batteries, or flywheels to suppress the powerfluctuations. For example, the battery can be realized a highenergy density at low cost. However, one of the problems inthe battery energy storage has a short life time. In particular,
the lifetime depends on ambient and the number of charge anddischarge time. In addition, the battery cannot cope with rapid
charge and discharge by a large internal resistance. The
replacement cost for the batteries is very expensive. On theother hand, the EDLC has a high charge and discharge
efficiency. Moreover, the rapid charge and discharge ispossible because the internal resistance is very small.
However, similar to the battery, the lifetime is decreased dueto the influence of the ambient temperature. On the otherhand, the flywheels has some advantages: (i) environmentalfriendly, (ii) low maintenance cost, (iii) long lifetime due to no
chemical structure, and (iv) the charge and discharge

characteristic of high cycle are excellent. Additionally, thekinetic energy which stored in the flywheel is proportional to
the square of the rotational speed. For this reason, ultra highspeed rotation using magnetic bearings have been studied inorder to achieve high energy density [1-6]. The magnetic
bearing to hold the rotating shaft without mechanical contact
has been studied. However the cost of the flywheel energy
system becomes expensive.
This paper presents how to achieve low cost flywheel
system with a control method of the regeneration power forthe power leveling system. The flywheel system is configured
from commercial products; which consist of a transducer, ageneral purpose inverters and an inexpensive microcomputer
without the specific device. Moreover, instead of the magnetic
bearing, the normal ball bearing is used to degrade the cost.
However, the inexpensive power detection device has a long
delay time. As a result, the control becomes unstable due tolong delay time. In this paper, in order to improve the response
performance of the regenerative power, the time delay
compensation scheme based on the Smith predictor [7][8] isapplied to the proposed flywheel system. The constitution ofthis paper is follows: at first, the proposed flywheel system isintroduced. Next, the principle of the power control method
which includes the time delay compensation scheme based on
the Smith predictor is discussed. Finally, the proposed control
method is demonstrated by the prototype flywheel energy
storage system.
II.

PROTOTYPE FLYWHEEL SYSTEM

Fig. 1 shows the photograph of the proposed flywheel
system of which has the specification as shown in Table 1.
The general purpose ball bearings is used to reduce the costs
and to improve the performance because the rotational speed

is suppressed within 9500 r/min. Moreover, the outward formof the flywheel is 569×511×600 mm. Therefore, the energy
density is 17.2 kJ/dm3. On the other hand, the energy densityof the EDLC is 36.0 kJ/dm3 [9]. Therefore, it is approximately
twice of the prototype system.
Fig. 2 shows the block diagram of the prototype flywheel
system including the accessories parts. Inexpensive power
meters which has a long dead time is applied in this system.
Additionally, the rotational speed command of the flywheel iscontrolled by an inexpensive PIC microcontroller. In addition,
the vacuum pump and the oil cooler are implemented toreduce the windage loss and heat generation. As mechanicalfeatures, the vacuum is kept by a small vacuum pumpbecause the flywheel has been stored in a sealed container. In
this structure, it is possible to reduce the windage loss which
occurs at high speed rotation region. Furthermore, overtemperature of the bearing and the motor can be prevented bythe oil cooler.
Note that this flywheel system assumes that several
numbers of flywheels are used in one system in order toincrease energy capacity. Therefore, an oil pump and avacuum pump can be used for some flywheels.
III.

POWER CONTROL METHOD WITH SMITH PREDICTORIn the flywheel system, when induction machine isoperated as a generator during deceleration, kinetic energy isconverted into the electrical energy. On the other hand, duringacceleration, the electrical energy is stored as kinetic energy,
which is working as a motor. In general, inexpensive general
purpose inverter can only provide speed command.
Therefore, the charging and discharging power is controlled
by the speed of the flywheel.
The kinetic energy of a body of revolution is expressed by(1).

Flywheel

Induction
MotorVacuum
caseFig. 1. Photograph of the prototype Flywheel System.
This system adopts general purpose ball bearings inorder to reduce costs and improve performance because
the rotational speed is suppressed within 9500 r/min.
Table 1. Specification of the Flywheel unit.
Rated voltage

200V

Rated current

126A

Rated rotation speed

9500rpm

Accumulated energy

3.0MJ

Outward form

569×511×600mm

Energy density

17.2kJ/dm3

Weight of the FW

241kg

Diameter of the FW

45cm

InverterFRENIC5000G 11Controller

PIC16F887

1
E = Jω 2 ............................................................. (1)2where, J is the moment of inertia[kgm2] and ω is the angularvelocity of the body revolution[rad/s]. In addition, the speedcommand ω* which shown in Fig. 2 can be obtained bytransforming (1).
On the other hand, the output power of the flywheel can beobtained by differentiating the both sides of (1) from therelation between the rotational kinetic energy and power.
Thus, the output power is expressed by (2).
dω
P = Jω
................................................................... (2)dt

Accordingly, the regenerative power of the flywheel is
proportional to the product of the rotational angular velocity
and angular acceleration. The speed command value for theregenerative power of the flywheel is given by (3).

Sampling Time

20ms

Delay Time TL

400ms

Fig. 2. Configuration of the Flywheel system. In thissystem, the vacuum pump and the oil cooler are
implemented to reduce the windage loss and heatgeneration.

ω* =

1 P*
dt ........................................................... (3)∫J ωowhere, ωo is the rotational angular velocity of the previous
sampling time and P* is the output of the PI controller.

ωo1
Js

Furthermore, the discrete system of equation (3) is givenby (4).
ω n = ω n－1 +1 P*
Δt ........................................... (4)J ω n－1

where,ωn-1 is the speed command value of the previous
sampling time and Δt is the sampling time of the controller.

(a) Without smith predictor.

In this system, the regenerative power is obtained from aninexpensive power meter that the sampling time is 400 msec.
In such a system, the control may become unstable by setting
a high proportional gain to improve the response. In thispaper, the control performance is improved by the Smith
delay time compensation scheme.

ωoFig. 3(a) shows the block diagram of the typical feedback
control system. Note that the change of the rotational angularvelocity is sufficiently slower than the processing time of thecontrol system. Therefore, the rotational angular velocity ofthe previous sampling time is treated as a constant value ωo.
The characteristic equation of this control system is given by(5).

1 + C(s)G(s)e-TLS = 0 .................................................. (5)In a typical feedback control system, the system becomes
unstable due to the phase lag. This is caused by the delay timeelement.

1Js(b) With smith predictor.
Fig. 3. Block diagram of time delay compensation scheme
based on Smith predictor. This compensation eliminates theeffect of delay time by estimating the output of thecontrolled object G(s) after the lapse of delay time.

Fig. 3(b) shows the block diagram with the Smith predictor
to power control. This compensation eliminates the effect ofdelay time by estimating the output of the controlled object
G(s) after the lapse of delay time. The characteristic equation
is given by (6).

predictor. The simulation conditions are as follows; the steadyrotation speed is 6000 r/min and the load is changed from 0.0
kW to 2.0 kW. From the simulation results, a large overshootoccurs when the Smith predictor is not applied. By contrast,
the overshoot does not occur when the Smith predictor isapplied. Therefore, it is confirmed that the response
performance is improved by the Smith predictor.

1 + C ( s )G ( s ) = 0 ....................................................... (6)Next, the effectiveness of the Smith predictor is verified by
the experiment.

Therefore, the control performance can be improved fromthis equation. In other words, the delay time element has been
removed from the characteristic equation.

Fig. 5 shows the step response of the output power. Notethat the experimental conditions are similar to the simulation.

In this control system, the speed command ω*^ is obtainedby (2) from the estimation of the output power Ps. Note thatPinv is detected value of the power meter and Pd is the output^power after the lapse of dead time. In addition, P s is
calculated by the inertia J and the rotation speed command ω*from (1).
IV.

SIMULATION AND EXPERIMANTAL VERIFICATION

Fig. 4 illustrates the simulation results of the regenerative
power from the flywheel system with and without the SmithIn addition, the gain of PI controller is determined by thelimit sensitivity method [10]. The inverter used in theexperiment is FRENIC5000G11 (Fuji Electric), themicrocontroller is PIC16F887 (Microchip Technology), andthe sampling time is 20msec.
According to Fig. 5(a), an overshoots is approximately 500
W because the power measurement system has delay time.
On the other hand, it is confirmed that the overshoots does
not occur by using Smith predictor in Fig. 5(b). From theseresults, the effectiveness of the compensator is confirmed bysimulations and experiments.

Fig. 4. Step response of the regeneration power on thesimulation results. The simulation conditions are asfollows; the steady rotation speed is 6000 r/min and theload is changed from 0.0 kW to 2.0 kW.

Regeneration Power[kW]32Pload

1Pinv

0-10

5

1015Time[s]2025(a) Without Smith compensation scheme.
Regeneration Power[kW]32Pload1Fig. 7 shows the frequency characteristics of the prototype
system. From the results, frequency response after applying
the Smith predictor is 0.18Hz. Power generation system using
renewable energy power generator varies greatly depending
on weather conditions. In the change of the amount of solar
radiation and wind direction are short period fluctuations andlong-term fluctuations are combined. Therefore, the prototype
system can be applied to compensate for power fluctuations
which is several tens of seconds [11][12].
V.

Pinv

0-10Fig. 6. Analytical results of the steady state losses. In thisexperiment, the rotation speed is 9500 r/min, the vacuum
case is kept at 800 Pa.
Fig. 6 shows the analytical result of the steady state losses.
In this experiment, the rotation speed is 9500 r/min, thevacuum case is kept at 800 Pa. As a result, the flywheel
losses in the steady state are as follows; the bearing loss is19 %, the iron loss is 76 % for the induction motor. From theanalysis, it is confirmed that the motor loss is dominant in thestandby mode. Therefore, it is necessary to reduce the motor
loss to improve the efficiency of the flywheel system.

51015Time[s]2025(b) With Smith compensation scheme.
Fig. 5. Step response of the regeneration power on theexperimental results. The experimental conditions are as
follows; the steady rotation speed is 6000 r/min and theload is changed from 0.0 kW to 2.0 kW.
Next, the prototype flywheel system is evaluated by loss
analysis.

VERIFICATION OF POWER LEVELING CONTROL

Fig. 8 shows the power leveling control system. Note thatthe rotation speed is 5500 r/min at steady state. In theexperiment, the power fluctuation is reproduced by a stepdown chopper. In addition, the flywheel power command
value is extracted from a high pass filter (HPF) that cutoff
frequency is set to 0.0025 Hz. The prototype system isintended to compensate for a short cycle of the powerfluctuation. Therefore, the cutoff frequency of the HPF is setto low frequency.
Fig. 9(a) shows the power fluctuation pattern which uses apart of generation power by PV panels. From this figure, thegeneration power of the solar cell has a rapid powerfluctuation of approximately 2 kW.

(a)Fluctuation pattern(a)Gain characteristic.

(b) Output power of the Flywheel(b)Phase characteristic.
Fig. 7. Bode diagrams of the proposed control system. It is
confirmed that the response frequency of the prototype
flywheel system is 0.18Hz.

(c) Grid power

Fig. 8. Configuration of the experimental system for thepower fluctuation compensation. By using the step down
chopper, this system simulates the power fluctuations
which caused PV.
Fig . 9(b) shows the output power of the flywheel and Fig.
9(c) shows the grid power. From these results, the fluctuationof generated power is compensated by the output power ofthe flywheel. Therefore, the sudden fluctuations due to powerfluctuation does not occur on the grid power.
Fig. 9(d) shows the rotation speed of the flywheel. As aresult, it is confirmed that the sudden speed variation does notoccurs. According to the Fig. 9(c), the high frequency ripple
occurs in the grid power. This ripple is caused by theinfluence of the power detection delay time. This problem can(d)Rotation speed
Fig. 9. Experimental results of the power fluctuationcompensation control when applying Smith predictor.
(Rotation speed is 5500r/min)be improved by the optimization of the control gain.
Fig. 10 shows the harmonic analysis of the fluctuation
pattern and the grid power. The experimental conditions are
similar to Fig. 9. As the result, it is confirmed that thefluctuation pattern has large frequency component between
0.01 Hz and 0.004 Hz. By contrast, the grid powerfluctuations over 0.004 Hz is reduced 84.6 % by the power
leveling control.
VI.

CONCLUSION

This paper discussed a low cost power leveling system with
a 3.0 MJ, 9500 r/min flywheel energy storage by a
combination of the general purpose products.
As a result, in the regenerative power control, theovershoot of regeneration power was improved by using
Smith predictor. In addition, the effectiveness of the power
leveling control in the prototype was evaluated byexperiment. From the harmonic analysis, it is confirmed thatthe power fluctuation is reduced by 84.6 %.

Fig.10. Harmonic analysis during the power fluctuationcompensation. The experimental conditions are similar toFig. 9.

In future work, the design method of the low loss flywheel
system is established.

[5] Frank N. Werfel, Uta Floegel-Delor, Thomas Riedel, Rolf Rothfeld,
Dieter Wippich, Bernd Goebel, Gerhard Reiner, and Niels
Wehlau:’’Towards High-Capacity HTS Flywheel Systems’’, IEEETransactions on Applied Superconductivity, Vol.20, No.4, pp.2272-2275(2010)ACKNOWLEDGMENT

Results of a Compact Disk-Type Motor/GeneratorUnit with
Superconducting Bearings for Flywheel Energy Storage Systems with
Ultra-Low Idling Losses’’, IEEE Transactions on Applied
Superconductivity,, Vol.21, No.3, pp.1497-1501(2011)

A part of this study was supported by Industrial Technology
Grant Program in 2009 from New Energy and Industrial
Technology Development Organization (NEDO) of Japan.

[6] C. Zhang, K.J. Tseng, T.D. Nguyen, S. Zhang:”Design and loss analysis
of a high speed flywheel energy storage system based on axial-flux
flywheel-rotor electric machines”, IPEC-Sapporo, pp.886-891(2010)

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