Experimental Studies on Fuzzy Logic Stabilization Control
for Energy Capacitor System
T. Hiyama
K. Tomsovic
E. Anami
hiyama@eecs.kumamoto-u.ac.jp
Dept. of Electrical & Computer Engineering
Kumamoto University
Kumamoto 860-8555, Japan
S. Yamashiro
M. Yamagishi
Dept. of Electrical & Electronic Eng.
Kitami Institute of Technology
Kitami 050-8507, Japan
M. Shimizu
Power Systems Co.
Yokohama 236, Japan
Abstract: A fuzzy logic based switching control system is
proposed for an electrical double-layer energy capacitor
system (ECS) to enhance the overall stability of electric power
systems. The real power flow signal at the location of the ECS
is utilized to generate switching control signals for the
regulation of charging or discharging power to and from the
ECS. The resulting switching can greatly improve the
damping of oscillations. To demonstrate the efficiency of the
proposed switching control, experimental studies have been
performed on a 5kVA one-machine infinite-bus laboratory
system by using a 70Wh (250kJ) ECS.
Results show that through the proposed control, damping
of oscillations can be greatly improved relative to the
performance by braking resistors. To demonstrate the
efficiency of the proposed switching control scheme,
experimental studies have been performed on a 5kVA onemachine infinite-bus system using a 70Wh (250kJ) ECS.
Keywords: Electrical double-layer capacitor, energy
capacitor system, fuzzy logic control, switching control,
stability enhancement.
The basic configuration of the proposed fuzzy logic
switching controller is shown in Fig. 1. The real power flow
signal Pe is utilized at the location of the ECS to generate the
switching control signal U. The proposed controller is
implemented by using a personal computer containing a
DSP board with A/D and D/A conversion interfaces.
II. FUZZY LOGIC SWITCHING CONTROL SCHEME
I. INTRODUCTION
Zs(k)
Due to the increasing size and complexity of electric power
systems, as well as the growth of electric power demand
without corresponding infrastructure upgrades, electric
power systems are often operated with lower stability
margins. Stability improvement is one of the major research
subjects for improved power system operation. This paper
investigates the stability enhancements using a new device,
the electrical double-layer energy capacitor system (ECS)
[1-3].
Switching control of braking resistors is one of the most
effective means to enhance the overall stability of electric
power system. In the case of the braking resistors, only the
absorption of real power is available whenever the braking
resistors are switched in; however, the injection of real
power is impossible. On the other hand, the absorption and
the injection of real power is possible by charging and
discharging real power to and from the ECS. In this paper, a
fuzzy logic based switching control system [4] is proposed
to enhance the overall stability of electric power systems.
The real power flow signal at the location of the ECS is
utilized to generate switching control signals for the
regulation of charging or discharging for the ECS.
As
-Pe(k)
Zp(k)
R1
I
R2
I
R3
Fuzzy Logic
Control
Rules
U(k)
R : Reset Filter I : Integrator
U : Switching Control Signal
Fig. 1. Proposed Fuzzy Logic Switching Controller
The proportional and integral signals of the generator speed,
derived from the real power flow signal Pe at the location of
the ECS through the filtering shown in Fig. 1, is utilized for
the switching control. Here, Zs and Zp are local measures of
the speed deviation and the phase deviation, respectively, at
the location of the ECS. This two dimensional information,
i.e., Zs and Zp, is utilized to generate the switching control
signal U to damp the oscillations.
In the fuzzy logic control scheme, the system state is
defined by the phase/speed/acceleration state p(k) as shown
in Fig. 2. The point p(k) is given by
p(k) = [ Zp(k) AsZs(k) ]
(1)
Then, the radius D(k) is found as
Paper ISAP2001 No. 50 accepted for presentation at the
IEEE ISAP2001 Conference, Budapest, Hungary, June 1821, 2001.
D ( k ) = Z p ( k ) 2 + ( As × Z s ( k )) 2
and the angle q(k) as
(2)
q ( k ) = tan -1 ( As × Z s ( k ) / Z p ( k ))
(3)
Note that Zs and Zp become zero at the final steady state
once the study generator is stabilized. Further note that As is
simply a scaling factor for the speed deviation signal Zs.
In Fig. 2, Sector A gives the region where deceleration
control is required, that is, charging operation is required for
the ECS in order to withdraw real power from the power
system. Sector B is the region where acceleration control is
required, that is, discharging operation is required in order to
inject real power flow into the power system.
Switching
Line
・ Zs(k)
As
・
N ( q ( k ))- P( q ( k ))
× G( D( k )) × U max
N ( q ( k ))+ P( q ( k ))
= [ 1-2 P( q ( k ))] × G( D( k )) × U max
U ( k )=
(4)
where Umax is the maximum size of the switching control
signal. The proposed switching control scheme has three
basic adjustable control parameters As, Dr, and the overlap
angle a. The parameter a is fixed to 90 degrees in this study.
The switching controller has been implemented by using a
personal computer with a DSP board including A/D and D/A
conversion interfaces. The control program has been
developed in the Matlab/Simulink environment. The Real
Time Workshop component of Matlab allows for real time
operation of the DSP board. In the experimental setup,
discrete switching of the ECS is used as described in the next
section.
Sector A
a/2
a/2
III. EXPERIMENTAL STUDIES
p(k)
D(k)
・
q(k)
Zp(k)
O
45
Sector B
To demonstrate the efficiency of the proposed fuzzy logic
switching control scheme, experimental studies have been
performed on a 5kVA one-machine infinite-bus laboratory
system shown in Fig. 4. The ECS is connected at the
generator terminal as shown in the figure. The line
switching of one of the parallel transmission lines is used as
the disturbance for the experiments.
S
Fig. 2. Phase Plane for Fuzzy Logic Switching Control
220V
5kVA
Unit
grade
N(q )
1
a = 90
o
P(q )
AC/DC Conversion Unit
External
System
220V
Power
Source
ECS Energy Capacitor System
0
0
90
135 180
270
q [ degrees ]
315
360
grade
G(D(k))
1
0
Dr
Fig. 4. One-machine Infinite-bus Laboratory System
Currently, a discrete type control for the charging and for
the discharging power is available on the three phase AC/DC
conversion unit. The following discrete logic is used based
on the control signal U.
Distance D(k)
TABLE I DISCRETE REGULATION OF CHARGING
OR DISCHARGING POWER
Fig. 3. Angle and Radius Membership Functions
The two sectors are defined by using the angle membership
functions N(q(k)) and P(q(k)) while an additional
membership function G(D(k)) is needed for the control
signal. The function N(q(k)) gives the grade of charging,
and P(q(k)) gives the grade of discharging. The function
G(D(k)) determines the level of the control signal. The
respective membership functions are shown in Fig. 3.
A continuous switching control signal is determined by
using the following defuzzification function
If
If
If
If
If
If
U > 0.975 then 1.00 kW charging.
0.975 > U > 0.625 then 0.75 kW charging.
0.625 > U > 0. 25 then 0. 50 kW charging.
0.25 > U > - 0.15 then no charging or discharging.
- 0.15 > U > - 0.45 then 0.30 kW discharging.
- 0.45 > U then 0.60 kW discharging.
To implement this on the laboratory setup, the actual
switching control signals U1 to U4 for the conversion unit
are determined as shown in Table II.
TABLE II FUNCTION OF CONTROL SIGNALS U1 to U4
U1(V) U2(V) U3(V) U4(V)
0
5
5
0
5
5
5
0
5
0
5
0
0
0
5
5
0
0
5
5
5
5
5
5
0
5
Function
Conversion Off
Conversion On
1kW Charging
0.75kW Charging
0.5kW Charging
No Charging
or Discharging
0.3kW Discharging
0.6kW Discharging
Fig. 5 to Fig. 7 illustrate the stabilizing effects achieved by
the charging and discharging control for ECS, where the
setting of the generator real power output was varied from
3kW, 5.1kW, and 5.6kW, respectively. The fuzzy control
parameters are specified as: As = 0.5, Dr = 0.05.
The disturbance was applied by opening one of the
transmission lines and subsequently, reclosing the line.
From the top to bottom, the generator real power output and
the generator terminal voltage are shown, including the
control signals U1 to U4 when the stabilization control
operates. With the proposed fuzzy logic stabilization
control, the generator damping is highly improved for both
disturbances.
One of the specific features of the discrete type switching
control is the possibility of chattering. In the proposed fuzzy
logic control scheme, the speed deviation signal Zs and the
phase deviation signal Zp are utilized to generate the control
signal for ECS. Due to filtering and the control logic, the
size of these two signals Zs and Zp undergo no immediate
change after changing the operation mode of ECS from
charging to discharging or from discharging to charging.
Thus, the chattering problem is not observed throughout in
these experiments.
(a) Line Opening Disturbance without ECS Control
(a) Line Opening Disturbance without ECS Control
(b) Line Reclosing Disturbance without ECS Control
(b) Line Reclosing Disturbance without ECS Control
(c) Disturbances with ECS Control
(c) Disturbances with ECS Control
Fig. 5. Stabilizing Effect
(Generator Output: 3.0kW)
Fig. 6. Stabilizing Effect
(Generator Output: 5.1kW)
particularly important for lower voltage applications, such
as, distribution systems with small dispersed generators. In
this case, the amount of damping needed may not be as great
while the cost restrictions are tighter than for the
transmission system.
(a) Line Opening Disturbance without ECS Control
(a) Line Reclosing Disturbance with PSS Control
(b) Line Reclosing Disturbance without ECS Control
(b) Line Reclosing Disturbance with PSS and ECS Control
Fig. 8. Stabilizing Effect with Single Phase Switching
(Generator Output Setting: 3.85 kW)
IV. CONCLUSION
(c) Disturbances with ECS Control
Fig. 7. Stabilizing Effect
(Generator Output Setting: 5.6kW)
Additional experiments have also been performed on the
5kVA laboratory system allowing different types of
disturbances, such as step changes of mechanical input to the
generator with different locations for the ECS. Highly
improved damping is also achieved for these disturbances
through appropriate charging and the discharging levels.
Finally, initial laboratory setup included only a singlephase inverter. This put forward the idea to the authors of
the possibility of using single phase switching to damp
oscillations. Response to a disturbance based on such single
phase control is shown in Fig. 8. Significant damping
improvement can again be seen. In this case, it must be
noted that the generator was equipped with a Power System
Stabilizer (PSS). The authors suggest two important
implications of this single-phase control. First, the threephase stabilizer can still perform adequately with up to two
of three phases out of service for maintenance or due to
failure. Second, a lower cost unit can be used that
implements only a single-phase injection. This may be
The performance of a fuzzy logic stabilization control has
been demonstrated through the experimental studies on the
5kVA laboratory generator by using the 70Wh (250kJ) ECS.
The damping of the generator oscillations is greatly
improved by the proposed schemes. In addition, the
robustness of the method was demonstrated using the same
controller with only a single phase injection. Further studies
are ongoing considering continuously variable power levels
of charging or discharging. Other possible considerations
for the controller are non-zero reactive power output of the
inverter for enhanced voltage control.
REFERENCES
[1] M. Okamoto, “A basic Study on Power Storage Capacitor
Systems”, Transactions of IEE of Japan, Vol. 115-B, No.5,
1995.
[2] M. Ohshima, M. Shimizu, M. Shimizu, M. Yamagishi, and M.
Okamura, “Novel Utility-Interactive Electrical Energy
Storage System by Electrical Double Layer Capacitors and
an Error Tracking Mode PWM Converter”, Transactions of
IEE of Japan, Vol. 118-D, No. 12, 1998.
[3] T. Hiyama, D. Ueno, S. Yamashiro, M. Yamagishi, and M.
Shimizu, “Fuzzy Logic Switching Control for Electrical
Double-Layer Energy Capacitor System for Stability
Enhancement”, Proceedings of the IEEE PES 2000 Summer
Meeting, Vol. 4, pp.2002-2007, 2000.
[4] T. Hiyama, M. Kugimiya, and H. Satoh, “Advanced PID type
fuzzy logic power system stabilizer”, IEEE Transactions on
Energy Conversion, Vol. EC-9, No. 3, Sept. 1994, pp.514520.
[5] T. Hiyama, S. Oniki, and H. Nagashima, “Evaluation of
advanced fuzzy logic PSS on analog network simulator and
actual installation on hydro generators”, IEEE Transactions
on Energy Conversion, Vol. 11, No. 1, March 1996,
pp.125-131.
APPENDIX: EXPERIMENTAL SYSTEM
The overview of 5kVA laboratory power system is shown
in Fig. A1. The 70Wh(250kJ) Energy Capacitor System is
also shown in Fig. A2 including the AC/DC conversion unit
and the switching controller.
DC100V 7kW
DC Motor
AC220 5kVA
Generator
Transmission Line
Fig. A1. Overview of 5kVA Laboratory System
Switching
Controller
ECS
AC/DC
Conversion
Unit
E
Fig. 4. Overview of Energy Capacitor System
and AC/DC Conversion Unit
The detailed specification of the Energy Capacitor System
is given in Table A.
TABLE A. SPECIFICATION OF ECS
Rated DC Voltage : 250 V
Rated DC Current : 10 A
Maximum DC Current : 30 A
Inner Resistance : 0.5 W at Rated Voltage
Capacity : 69.4 Wh
Acceptable Air Temperature : –10 to 40 degrees in Celsius
Size : 368.2mm(W) x 364.2mm(D) x 166.2mm(H)