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M. G. Rabbani, M. R. I. Sheikh, M.F. Hossain

Proceedings of the ICEECE
December 22-24, 2003, Dhaka, Bangladesh
**Email : tamanna86131@yahoo.com 102
A Novel Control Strategy for Superconducting Magnetic
Energy Storage Unit to Improve Power System Stability
M.G. Rabbani*, M.R.I. Sheikh**, and M.F. Hossain***
*Associate Professor **Assistant Professor ***Lecturer
Department of Electrical and Electronic Engineering
Rajshahi University of Engineering and Technology, Rajshahi – 6204, Bangladesh
ABSTRACT
In this paper a simple but novel control strategy is developed for Superconducting Magnetic Energy Storage (SMES) unit.
The SMES unit is used to improve the dynamic performance of power system. The gain of the controller is determined
depending on the power system and the capacity of the SMES unit. A simple delayed function is used to determine the
desired power from the SMES unit. Eigenvalue analysis and computer simulation results show that the proposed controller
is very robust in dealing different types of disturbances.
1. INTRODUCTION
During the past decade SMES Unit has attracted much
attention from the researchers. Especially high temperature
superconductor makes the SMES unit very attractive to the
utility engineers. SMES systems have the capability of
storing energy in their low resistance coils. This energy can
be supplied to the power system by the SMES or received
by it can be controlled by controlling the voltage across the
inductor which eventually controls by the firing angles of
the 12-pulse converter.
SMES has been used for different applications in power
systems [1-12]. Some researcheres have used the SMES
unit for the improvement in power system dynamic
performance[1-5]. It has also been used for the
improvement in damping of subsynchronous oscillations[6-
9]. Effect of SMES on Automatic Generation  Control has
also been investigated[10-12]. However, most of the SMES
controllers fall under the following two categories:
 Proportional  Integral (PI) / Proportional Integral
Derivative (PID) controller
 Intelligent such as Fuzzy and Neural Network based
controllers
The parameters of the PI or PID controllers are obtained by
pole placement method. In case of intelligent controllers,
most of the parameters are obtained on-line.
This paper introduces a novel idea of generating the
compensating power from SMES unit by using a simple
delay network. Pole placement technique is used to
dertermine the gain of the controller. The controller is
tested on a power system model and the results are
compared to that of a PI controller.
2. SYSTEM MODEL
The power system shown in Fig. 1 comprises a synchronous
generator connected to the infinite bus system through
transmission line and a SMES unit at this generator
terminal. The placement of the SMES unit is selected to
achieve the best performance from it while improving the
power system stabilization[2]. The power system is
equipped with a governor and a static excitation system[2].
Without the SMES unit, there are 10 system state variables.
The system is described by two-axis model[13]. Some
important equations are given below:
/
dE = [  Ed – ( Xq Xd ) Iq ] / Tq0 (1)
/
qE = [ EFD  Eq + ( Xq Xd ) Id ] / Td0 (2)
where EFD is the field excitation voltage, and Td0 and Tq0
represent d-aixs and q-axis transient time constant,
respectively. The swing and rotor angle equations can be
written as
 =  [Pm  Dg  Pe Psm ] / Mg (3)
 =   1                                                   (4)
where Pm, Dg and Mg are the output power of the reheat
steam turbine, damping coefficient and moment constant. Pe
is the electromagnetic power transferred in the air gap. Psm
is stored power into the SMES.  The terminal voltage which
describe the relation between transmission line and
generator are
Syn. Generator
G
SMES
UNIT
Vt
Fig. 1: The power system with the SMES unit
Id, Iq
Isd, Isq
Itd, Itq
Infinite bus
Re+jXe
ISBN: 984-31-1528-7
Proceedings of the ICEECE, December 22-24, 2003, Dhaka, Bangladesh
103
Y-Y/
Transformer
Fig. 2: Schematic diagram of the SMES unit
Ism
Vsm
Inductor12-pulse bridge
converter
Vd = Ed – Ra Id  Xd Iq
= – V sin () + Re Id + Le Iq (5)
Vq = Eq – Ra Iq + Xd Id
= V cos () + Re Iq - Le Id (6)
Vt2 = ( Vd2 + Vq2 )1/2 (7)
where Vt and V represent terminal voltage of the
synchronous generator and infinite bus voltage respectively.
3. THE SMES UNIT AND ITS CONTROL STRATEGY
The SMES inductor-converter unit consists of a dc
superconducting inductor, a 12-pulse cascaded bridge type
ac/dc converter and a Y-Y/ step down transformer as
shown in Fig. 2. Control of the converter firing angle (),
provides the means for continuous variation of the bridge
voltage Vsm over a wide range of positive and negative
values. If losses are assumed negligible, then, in per unit
[10],
csm0smsm RIcosVV  (8)
where  is the delay angle
Vsm is the per unit bridge voltage
Vsm0 is the no load per unit bridge voltage
Ism is the per unit bridge current, and
Rc is the per unit commutating resistance
For this lossless case the current Ism can be expressed as
sm
sm
sm Vdt
dIL  (9)
For “charging” at the maximum rate, Vsm should be held at
its maximum value, which corresponds to rectifier operation
with   0. The current then builds up nearly as linear
function of time until base current Ism = 1.0 p.u.  is reached.
Then  must be adjusted so that this normal or rated
current is held constant. From (8), this corresponds to  =
900 for the normal operating conditions. At any time during
the charging period, the stored energy is
2
smsmL IL2
1W  p.u.                             (10)
With constant Vsm, the inductor current Ism can be expressed
as
tL
VI
sm
sm
sm  p.u.                                (11)
which shows that the time variation of load current is linear
for a superconductive load
While in power system stabilization, the compensation Psm
provided by the SMES is continuously controlled
depending on the sensed frequency deviation signal f. The
incremental change in the SMES power is given by
 dc
sm/
sm ST1
KP (12)
where  Ksm = Gain of the control loop
and      Tdc = Convereter time delay
The proposed control strategy is shown in Fig. 3. The gain
of the controller Ksm can be determined by simulating a very
large disturbance in the system. The maximum deviation in
the system frequency can be obtained by two ways:
(i) if the system is unstable, then observe the maximum
deviation before the system becomes unstable.
(ii) if the system is stable, then observe the maximum
deviation in the simulation process.
Once the maximum value of the frequency deviation fmax
is selected, the gain of the SMES controller can be
calculated as
Ksm =
max
max,smmax,sm
f
IV
 (13)
In the present study, the value of Ksm is found to be 172.5.
When the system load increases the bus frequency falls.
Consequently, a negative power Psm, shown in equation
(12) is consumed by the SMES unit.
This means that a negative voltage is impressed across the
inductor. The inductor current being unidirectional,
Fig. 3: Superconducting magnetic energy storage unit
control system
Ism

Vsm
Ksm
dcsT1
1
 Vsm sL
1+ -
KIS

Ism0
Ism
Ism=Ism0+Ism
Psm
+ +
+
+
Psm
Psm0
+
+
Psm

Proceedings of the ICEECE, December 22-24, 2003, Dhaka, Bangladesh
104
application of negative voltage across it results in
withdrawal of energy.
Since the amount of stored energy is finite, the inductor
current falls. In actual practice, the inductor current should
not be allowed to reach zero to prevent the possibility of
discontinuous conduction in the present of large
disturbance[2,10]. To avoid such problems, the lower limit
to the inductor current is set to 30% of Ism0. It is desirable to
set the rated inductor current Ism0 such that the maximum
allowable energy absorption equals the maximum allowable
energy discharge. This makes the SMES equally effective in
damping swings caused by sudden increase as well as
decrease in load.  Thus, if the lower current limit is chosen
at 0.3 Ism0, the upper inductor current limit, based on the
equal energy absorption/discharge criterion becomes 1.38
Ism0 [10] when L = 0.5 H and Wsm = 4 MJ . When the
inductor current reaches either of these limits, the dc
voltage has to be brought to zero.
4. DETERMINATION OF KP AND KI
Y=CX
Where X=[   Efd VST Ed Eq Pm Pc PH
PRUsm Vsm]
B=














 46.38
.
.
.
0
0
0
U=Vsm
Y=
C=[1  0  .  . .  .  .  ]
Y(s)=CX(s)
X=AX+BU
SX(s)=AX(S)+BU(S)
X(S)=(SI-A)-1BU(S)= )s(YS
KKST1
ST I
p
W
w 

 
 X(s)=(SI-A)-1B H(s) CX(S)
[I-C(SI-A)-1 BH(s)]X(s) =0
The characteristic equation is
1-C(SI-A)-1 BH(s)=0
Substitution of i
H(i)= 




 i
I
p
wi
Wi
i
KKT1
T
B)AI(C
1
1=C( iI-A)-1 B. 





i
I
p
wi
wi KKT1
T
1=C( iI-A)-1 B. I
wi
w1
ip
wi
wi KT1
T)AIC(KT1
T

 
1=[Real (XX) + Imag(XX)] Kp +[Real(YY) + Imag(YY)]KI
1=Real(XX)KP+Real(YY)KI
0=Imag(XX)KP + Imag(YY)KI







I
P
K
K
Imag(YY)(XX)Imag
(YY)Real)XX(alRe
0
1






 
0
1
Imag(YY)(XX)Imag
(YY)Real)XX(alRe
K
K 1
I
P
 KP=  45.99 and,  KI=376.4
5. EIGENVALUE ANALYSIS
Table 1 shows the system eigenvalues for two initial
operating conditions. It can be seen that for P0 = 1.0 p.u.,
the real part of the oscillatory mode is very small. The
system has positive real parts with P0 = 1.2 unit. However,
the overall performance is improved with SMES unit. For
both initial operating conditions, the real part of the
electromechanical mode has been shifted significantly to a
safe negative value when SMES unit is added. In order to
examine the control ability of the proposed controller at any
other load conditions, the real part of the eigenvalues of the
electromechanical mode is also studied. It is seen that the
system is stable upto P0 =1.128 p.u., without the SMES unit.
With the SMES unit, the system is highly stable even at P0
=1.7 p.u. The proposed controller extends the stability
margin considerably.
6. COMPUTER SIMULATION AND
PERFORMANCE ANALYSIS
In order to demonstrate the beneficial damping effect of the
proposed controller, computer simulations based on system
non-linear differential equations are carried out for different
load conditions. Both small and large disturbances are
considered. The differential equations are solved under
MATLAB environment. All the non-linearities such as
exciter ceiling voltage, SMES voltage limits, inductor
current limits have been included.
The use of SMES unit for small disturbance has been
studied. The disturbance considered is a three phase
symmetrical fault near the infinite bus cleared after 0.1 sec
with the post fault transfer impedance increased by 50%. It
reflects a reduction in power transfer capability due to
isolation of part of transmission circuitry. Figure 4 shows
the system performance without SMES unit. We see that the
system is oscillatory in nature without SMES unit. For
comparison purposes, the performance of the traditional PI
controller and the proposed controller is shown in fig.5.
Figure 5 shows the system performances with the SMES
unit following the large disturbance (P0 = 1.0 p.u) with the
proposed mode of control and that with the conventional PI
controller. The damping of the system frequency is not
satisfactory without the SMES unit. With the addition of
SMES, the damping is improved significantly. At the initial
period, the frequency deviation with PI and proposed
controller are almost same. It is due to the delay time Tdc
which is accounted for the SMES power transfer to the
system.
Table 1: System Eigenvalues
Proceedings of the ICEECE, December 22-24, 2003, Dhaka, Bangladesh
105
Without SMES
P0=1.0 p.u
With SMES Without SMES
P0=1.2 p.u.
With SMES
217.80
41.55
19.45
.09j10.2
11.63
4.32
2.25
1.51
.13
217.89
41.95
20.22
17.92j33.13
38.46
8.14
6.98
 3.17
1.530.47
.13
217.89
41.70
19.47
.05j10.4
11.59
4.30
2.35
1.50
.13
217.89
41.96
20.22
17.83j33.07
38.46
8.05
7.06
2.95
1.680.41
.13
Electromechanical mode denoted in bold
However, as the time increases, the proposed controller
shows a clear edge over PI controller. Though both the
controllers make use of the same maximum SMES power,
but due to the efficient harnessing of the SMES power Psm
by proposed one, a better performance is obtained. It is
evident from the Fig. 5 that the settling time of the speed
reduced substantially when the proposed controller is used.
0.98
1
1.02
1.04
Speed (p.u)
Without SMES
0 0.5 1 1.5 2 2.5 30.5
1
1.5
2
2.5
Rotor Angle Deviations(rad)
Time in sec
Fig. 4: System performance without SMES unit
7. CONCLUSION
In this paper, the SMES unit is applied for the improvement
of power system damping. A simple control strategy for the
SMES unit is applied. The damping of the synchronous
generator is greatly improved by the SMES unit with
proposed mode of control system. The simulation results
show that the SMES performs better when equipped with
the proposed controller. The power compensation of the
SMES unit is directly obtained depending on the speed
deviation, which makes it more sensitive to the disturbance.
The scheme proposed in the present paper makes effective
use of active power modulation of the SMES unit. Hence,
its economic advantage is expected to be stronger than that
of earlier schemes. The control strategy is simple and does
not require heavy computation, therefore, implementation is
feasible.
0.9
5
1
1.0
5
Speed (p.u)
With SMES
0
1
2
Rotor Angle Deviations(rad)
-
0.5
0
0.
5
Psm(pu)
0 0.
5
1 1.
5
2 2.
5
3-0.5
0
0.
5
Vsm(pu)
Time in sec
Fig. 5: System performance of PI controller (.....) and of
proposed controller (       ) with the SMES unit
REFERENCES:
[1] Y. Mitani ,  K. Tsuji, and  Y.  Murakami, “Application
of  Superconducting  Magnetic Energy Storage to
Improve  Power System Dynamic Performance”, IEEE
Trans. on  Power System, 3(1988),  1418-1425.
[2] C. J. Wu and Y. S. Lee, “Application of
Superconducting Magnetic Energy Storage to Improve
the Damping of Synchronous Generator”, IEEE Trans.
on Energy Conversion, Vol. 6, No. 4, December 1991,
pp. 573-578.
Proceedings of the ICEECE, December 22-24, 2003, Dhaka, Bangladesh
106
[3] M. G. Rabbani, J. B. X Devotta and S. Elangovan, “A
Fuzzy Set Theory Based Control of Superconducting
Magnetic Energy Storage (SMES) Unit to Improve
Power System Dynamic Performance”, International
Journal of Electric Power System Research, Vol. 40,
February 1997, pp. 107-114
[4] M. G. Rabbani, J. B. X Devotta and S. Elangovan, “An
Adaptive Fuzzy Controlled Superconducting Magnetic
Energy Storage Unit for Power Systems” published in
the International Journal of Energy Conversion and
Management.
[5] M. G. Rabbani, J. B. X Devotta and S. Elangovan, “An
Efficient Fuzzy Controlled System for Superconducting
Magnetic Energy Storage Unit” published in the
International Journal of Electrical Power and Energy
Systems.
[6] Y. S. Lee and C. J. Wu, “Application of
Superconducting Magnetic Energy Storage Unit on
Damping of Turbogenerator Subsynchronous
Oscillation”, IEE Proceedings - C, Vol. 138, No. 5,
September 1991, pp. 39-44.
[7] L. Wang, S. M. Lee and C. L. Huang, “Damping
Subsynchronous Resonance Using Superconducting
Magnetic Energy Storage Unit”, IEEE Trans. on
Energy Conversion, Vol. 9, No. 4, December 1994, pp.
770-777.
[8] C. J. Wu and Y. S. Lee, “Application of Simultaneous
Active and Reactive Power Modulation of
Superconducting Magnetic Energy Storage to damp
Turbine-Generator Subsynchronous Oscillations”,
IEEE Trans. on Energy Conversion, Vol. 8, No. 1,
March 1993, pp. 63-70.
[9] A Rahim, A.M. Mohammad and M. R. Khan, “Control
Subsynchronous Resonant Modes in Series
Compensated System through Superconducting
Magnetic Energy Storage Units”, IEEE Trans. on
Energy Conversion, Vol. 11, No. 1, March 1996, pp.
175-180.
[10]S. Banerjee, J. K.   Chatterjee, and  S. C. Tripathy,
“Application  of  Magnetic  Energy Storage  Unit  as
Load  Frequency  Stabilizer”,  IEEE  Trans. on  Energy
Conversion, Vol. 5, No. 1, March 1990, pp. 46-51
[11]S. C. Tripathy, R. Balaubramanian and P. S.
Chandramohanan Nair, “Adaptive Automatic
Generation Control with Superconducting Magnetic
Energy Storage”, IEEE Trans. on Energy Conversion,
Vol. 7, No. 3, September 1992, pp. 434-441.
[12]S. C. Tripathy, K. Kalantar and R. Balaubramanian,
“Dynamics and Stability of Wind and Diesel Turbine
Generators with Superconducting Magnetic Energy
Storage Unit on an Isolated Power System”, IEEE
Trans. on Energy Conversion, Vol. 6, No. 4, December
1991, pp. 579-585.
[13]P.M. Anderson and A. A. Fouad, Power System
Control and Stability, Iowa State University press,
Ames, Iowa,1977.
APPENDIX
System data and initial conditions[2,13]
All parameters are expressed in p.u. unless stated otherwise.
Generator and transmission line
Base 160 MVA, 15 kV.      Generator 160 MVA, 15 kV,
0.85 p.f.        Exciter 375 V, 926 A
Xd  0.245      Xd  1.70      Xq  1.64          Ra 
0.001096       Mg  4.74         Dg  0 Re  0.02
Xe  0.4 Tq0  0.075 s Td0  5.9 s
SMES Unit [6]
Ism0  0.6495  p.u Vsm0  0 p.u.        Lsm  0.5 H
Wsm0  6.0 MJ        Tdc = 0.026 secs


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