Robust Multi-Objective Control of Power System Stabilizer Using Mixed H 2 /H ∞ and µ Analysis

International Journal of Electrical and Computer Engineering (IJECE)
Vol. 8, No. 6, December 2018, pp. 4800~4809
ISSN: 2088-8708, DOI: 10.11591/ijece.v8i6.pp4800-4809  4800
Journal homepage: http://iaescore.com/journals/index.php/IJECE
Robust Multi-Objective Control of Power System Stabilizer Using
Mixed H2/H∞ and µ Analysis
Javad Mashayekhi Fard
Department of Electrical Engineering, Sabzevar Branch, Islamic Azad University, Iran
Article Info ABSTRACT
Article history:
Received Jan 20, 2018
Revised Jul 22, 2018
Accepted Aug 10, 2018
In order to study the dynamic stability of the system, having a precise
dynamic model including the energy generation units such as generators,
excitation system and turbine is necessary. The aim of this paper is to design
a power stabilizer for Mashhad power plant and assess its effects on the
electromechanical fluctuations. Due to lack of certainty in the system,
designing an optimized robust controller is crucial. In this paper, the
establishment of balance between the nominal and robust performance is
done by the weight functions. In the frequencies where the uncertainty is
high, in order to achieve to the robust performance of the controller, µ
analysis is more profound, otherwise, in order to achieve to nominal
performance, robust stability, noise reduction and decrease of controlling
signal, the impact of the controller H2/H∞ is more profound. The results of
the simulation studies represent the advantages and effectiveness of the
suggested method.
Keyword:
H2/H∞/µ Controller
LMI
Multi-Objective Control
Power System Stabilizer
Copyright © 2018 Institute of Advanced Engineering and Science.
All rights reserved.
Corresponding Author:
Javad Mashayekhi Fard,
Department of Electrical Engineering,
Islamic Azad University,
Siadati Blv, Sabzevar, Iran.
Email: mashayekhi@iaus.ac.ir
1. INTRODUCTION
The operation of the power system is usually limited to the boundaries of the dynamic stability,
which is far from the limits of thermal stability. Nowadays, to improve the attenuation of the oscillations with
low frequencies, in most cases, power system stabilizers (PSS) are widely used in power systems. Designing
a proper stabilizer could decrease the limitations due to dynamic issues and help the system to get closer to
its nominal capacity. These stabilizers are usually designed according to the single machine, infinite bus of
the system in a definite working point. Therefore, it is possible that the stability of the system to be threaten
by changes in the parameters or equilibrium points of the system. In this paper, the system stability, in the
other words the stability of power system’s ability against the change of parameters are checked. As a result,
three main controlling goals will be obtained: strengthening the closed loop system, lower cost designing
strategies and improving the transient response. H2 control is used in the transient stage for presenting a fast
dynamic response to minimize the response energy of the impulse. While H∞ control is used in the stable
stage for reduction in the disturbance and protecting against tracking; which in turn would guarantee the
robust stability. In order to achieve to optimal performance, taking into account the effect of uncertainty
during system design period is required. But on the other hand it can lead to severe restrictions on the
controller that sometimes makes it an infeasible problem. So far, various studies have been conducted on the
stability and controlling of power systems.
The first formulation of the H∞ control problem was performed in 1981 by Zames. To date, large
numbers of researches have been performed for study of the robust control, the H2 control and H∞ control.
Doyle has analyzed the state space by using H∞ and H2 standard form and it’s solving. The conditions of

Int J Elec & Comp Eng ISSN: 2088-8708 
Robust Multi-Objective Control of Power System Stabilizer Using Mixed ... (Javad Mashayekhi Fard)
4801
solving problem and its solution using Hamiltonian matrix introduction are the highlights of this paper [1].
Also Doyle as well as a tutorial overview in the fractional linear transformations (LFTs) and the value of the
unique structure, μ, and linear matrix inequalities (LMIs) in the solution of LFT problems has offered [2].
H2/H∞, were combined by Rotea in this way, two important approaches were suggested, 1) optimal control
limit of H2 and H∞ (actually constrained optimization), and 2) at the same time optimal control of H2/H∞ [3].
Lanzon in his PHD thesis chooses the weight functions in μ and H∞ design [4]. Many of the power stabilizers
proposed for systems of the single machine are not able to resolve the interaction problems; while some of
the multi variable stabilizers are also lacking suitable robust stability. Studies on the stability are mostly
conducted on two transient and steady states. At operation condition, a power system is in its permanent state
[5]. When performance is in the permanent state, if a sudden change happens, the system will go toward
the disturbance.
Investigation of the classic stability [5], the optimization method with the help of pareto multi-
objective [6], the method of adaptive control [7], the nonlinear controller [8], using the parameters estimation
[9], robust controller H2/H∞ [10], the pole placement and application of the linear matrix inequality [11],
fuzzy and Neural network control [12], and Evolutionary algorithm [13] are among the works which had
been done. The problem of closed-loop identification of the Heffron-Phillips model parameters is of practical
importance since the data used for identification can be gathered when the machine is normally connected to
the power system [14]. In this paper, at first the power system was modeled. Then, the problem was
introduced. In this paper, at first, H2/H∞ controller was investigated with a new insight along with the new
controller, µ; and then these two different controllers were combined via the use of the weight matrices.
Solving this problem would be possible by application of the linear matrix inequality. The results indicate
that, the goals of H2/H∞/µ combination, including elimination of the perturbation effect, reducing the
controlling signal and accounting for the uncertainty during the system’s functionality investigation, were
properly realized.
2. POWER SYSTEM MODELLING
The stabilizers of the power systems are designed with the aim of improving the attenuation of the
low frequency oscillations of the system, based on the single machine, infinite bus model. The power system
stabilizer is a traditional and economic controller whose aim is to increase the dynamic stability of the power
system. By creating the damping electrical torque, the stabilizer of the power system will improve the
deviations of the rotors rotations. The mentioned equipment also optimizes and tunes the exciting voltage, by
creating the suitable voltage. The power plant of Mashhad city is located at the eastern part of the city at the
beginning of Sarakhs Boulevard. This is the oldest power plant of Khorasan province and has 8 electricity
generating units, 4 of them are working with steam and the other 4 ones are gaseous. The steam units consist
of two ELIN and two SKODA units, and the gaseous units include two BBC units and two ALSTOM units.
This power plant was established in 1964 and started its work in 1968. The exciting system of ALSTOM
gaseous units of the power plant of Mashhad are classified as the static type. Feeding of such exciting system
is done via power voltage transformer and three current transformers [15] with the capability of being
saturated. The controller part of the stimulation system of ALSTOM gaseous units includes 3 main control
modules. By elimination of the three controlling modules, in order to attenuate the oscillations, the power
system stabilizers could be applied. In studying the dynamic stability of the power networks, and also in the
cases where the changes and disturbances of the network are mainly partial and slow, the linear generator
model could be employed. In order to consider a synchronous generator, we use 3 rd order synchronous
generator model called Heffron-Philips model [11]. This model contains 3 state variables: ∆ωr, ∆δ, ∆Eq.
Considering the exciter model will lead to the introduction of the fourth state variable ∆Eb. In this
model, governing differential equations are linear around operating point. Figure 1 shows block-diagram of
linear mode of Heffron-Phillips model along with exciter and AVR Regarding the generator parameters
Hefron-Philips coefficients could be obtained by (1) [16]:
tde
qe
b
tde
d
teqde
ed
b
de
dd
q
de
eq
b
de
dq
eq
VXX
EX
KinE
VXX
X
VXXXX
XX
K
inE
XX
XX
Ki
XX
XX
KinE
XX
XX
XX
)(
,s
)()(
cosXE
-=K,
s,,s
cosEE
=K
6
qb
53
42
qb
1






















(1)
State space of equation of Figure 1 shows in (2).

 ISSN: 2088-8708
Int J Elec & Comp Eng, Vol. 8, No. 6, December 2018 : 4800 - 4809
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









0
T
K
00
00
H2
1
0
B,
T
1
T
KK
T
KK
0
T
1
KT
1
T
K
0
000
0
H2
K
H2
K
H2
K
A,
T
V
u,
E
E
x,
Cxy
BuAxx
A
A
AA
6A
A
5A
d3dd
4
b
21D
m
ref
b
q
r
ooo

(2)
Figure 1. Heffron-Phillips model
3. PROBLEM STATEMENT
3.1. H2/H∞ Controller
Existence of uncertainty created due to an uncertain and erratic input (noise and disturbance) and
Un-modeled dynamic cannot be described completely and precisely as a true system by a mathematical
modeling. On the other hand, a true system should contain the following important objects: robust stability,
robust and nominal performance, settling time, maximum over shoot and etc which try to gain these
objectives of the controlling problem [4]. The type of uncertainty is another important factor in the system
analysis. Consider additive uncertainty shown in Figure 3.
Figure 2. ΔM  Model Figure 3. Additive uncertainty
Objective 1: if 0 then 1

FS (nominal performance).
1
)(

 GKIS (S is sensitivity function).
Objective 2: if 0 then system is robust stability. KKGIM
1
)(

 ,
1)()())(( 

MSjjif  (3)
Objective 3: n is white noise with one PSD (power spectral density). H2 norm, caused due to decrease in the
controlling signal. 1
2
1

H
nU
RT (To minimize U1 variance with noise input). F(s), R(s) and γ(s) are weighting
function) from Parseval equation and objective 3. Then we have three tasks for controller design

4803
( 1FS 

, 1M)S( 

, 1T
1
nU 

), such that,
1
)G,K(1RT
)G,K(M
)G,K(FS
nU














(4)
Problem (4) shown in Figure 4. Rotea and Doyle offer two other methods for solve this problem.
[1]-[3]. A large class of system with uncertainty can be treated as LFT (Linear fractional Transformation).
LFT model is shown in Figure 3. W: the disturbance signals to the system which won’t be a function of states
of system, Z: the variable that will be controlled, P: the nominal open loop system, Y: the system measurable
output. To transform the changed diagram of Figure 4 to the LFT model, we will write the problem in the
standard form, and then solve it by using of Riccati equation [17]. The (4) LFT model is practicable in Figure
5 and can be used to design a controller. State space of Figure 5 is written in (5).
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



1nU3
2
1
22212
12111
21
RT
M
FS
Z
Z
Z
Z
uDWDxCy
uDWDxCz
uBWBAxx
1
2
3
0 0 0 0
0 0
0 00 0 0
00 0 0
1 2
0 0
0 0 0
0 0 0
0 0 0
T
f f ff ff
W
R R R RR
CL
f f
R
A B Bx x
B C A B B D B Dx x f r n u
x x BA
x x B BA
A
B B
D C Cz
z C
z C
y C
C
  

 
  
  
 



      
       
       
         
      
  
  
  
     
1
2
0 0
0
1
Tf f f
f
R R W
R
CL
x D D D D D
x D r n u
x D D
x D D
D
C


 
 
 
 

 
   
    
    
     
   
(5)
Determining three weight functions, specified in Figure.4, contain special importance. Using robust
optimal state feedback method for (4) equations.
Figure 4. LFT Model Figure 5. Graphical model of problem (4)
3.2. µ Controller
Here we try to assess robust performance of this closed-loop system by using µ-analysis. Robust
performance condition is equivalent to the following structured singular value µ test [2].
1
( , ) ( )wz PT M M W   


        (6)
The complex structured singular value ( )M is defined as
 
1
( ) min ( ) det( ) 0M I M


    
Lower and
Upper bond of µ can be shown to be
1
( ) ( ) min ( )P UM M DMD 

  .
3.2.1. D-K iteration
Unfortunately, it is not known how to obtain a controller’s achieving path directly to the structured
singular value test. But we can obtain the lower and upper bounds of µ. This method taken here is the so-

 ISSN: 2088-8708
4804
called D-K iteration procedure. The D-K iteration involves a sequence of minimizations over either K or D
while holding the other fixed, until a satisfactory controller is constructed. First, for D = I fixed, the
controller K is synthesized using the well-known state-space H∞, optimization method. LFT form of Figure 3
is written in equations (7) [17], [18].
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

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


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



U
W
P
DII
DII
I00
x
C
C
0
y
z
q
,
U
W
P
]B00[Axx (7)
3.3. New approach: H2/H∞, μ combination
Now, we tend to synthesize two collectors according to Figure 6. As mentioned before, the
availability of robust performance causes extreme limitation on the controller, which sometimes prevents it
from reaching a possible condition. Also, availability of nominal performance means considering operation
without uncertainty, and it is usual that the essence of uncertainty has decisive effect on the operation. So, we
tend to balance between robust and nominal performance. W1 and W2 are weight functions. Having this data,
we can determine which frequencies have more uncertainty effect, with regard to the controller effect of μ.
Of course, it is of importance to mention that robust performance contains nominal performance, so,
controller coefficient of μ should be smaller than H2/H∞ controller coefficient.
Problem 1: Determine W1 and W2, in a way that an additive uncertainty system contains robust stability.
1
1 1 2 2 1 1 2 2( ) ( )
1
M W K G W K G I W K W K
M


   

(8)
Figure 6. Controller H2/H∞/µ
3.3.1. Robust optimal state feedback with H2/H∞, μ combination
We now attempt to follow the analysis of the conditioning of the pole placement problem.
Researchers shown a number of robust performance indices have been considered in optimization approaches
for control system design [18]. In robust control using H∞ optimization, the objectives are expressed in terms
of the H∞-norm of transfer functions. One of the objectives is the following:

















KS
S
supmin
K
, where 1 1
[ ( ) ]S I j I A BK
 
   . In this paper we assume that the state of the generalized
plant G is available for feedback. To be more precise let a state-space description of P (figure 3) is given by
(LFT Model):
1 1 2 2( )
W
W
x AX BU B W
Y X
U KX
x A BW K BW K X B W
  


   
(9)
The signal W denotes disturbance. The signals U and Y denote the control input and the measured
output, respectively. Next to gaining K1 by H2/H∞ and K2 by μ analysis, we tend to determine weight
functions, using linear matrix inequality.

4805
Lemma1: (bounded-real lemma) given a constant 0 , for system, M(s) = (A, B, C) the following two
statements are equivalent, 1) this system is stable 

)(sM , 2) there exists a symmetric positive
definite matrix Q, such that: [19]
1
1
0
0
T T
q
T T
p q
q q
A Q QA QB Cp
B Q I D
C D I
Q





 


 
 
 
 
 
(10)
Lemma2: Consider the feedback system of Figure.3, where G is given by (9). Then, a given controller K is
admissible and close loop system is robust stability and desired performance if and only if there exists
1 2W and W solving the following LMI problem:
1 2 1 2
1 2
0
0 , 0 , 0
T T T T
W
T
W
A BY BY Y B Y B A B C
B I D
C D I
Y Y


        
 
  
 
  
 
   
Where,
1 1 1 1
1 1 1 2 2 2 .,W Y K W Y K
   
    1 2K and K Design with equations 5 and 7 and controller achieves
1 1 2 2K W K W K . 
Lemma 2, it helps to solve of problem 1. Mashayekhifard et al. presented Robust multi-objective static output
feedback with H2/H∞, μ combination [20].
4. METHODOLOGY
a. To design the H2/H∞ for the process with uncertainty. (It helps to select the weighting function properly).
For H2 /H∞ design can use Rotea and Doyle method. ([3], [8]) or use 1
),(
),(
),(












GKRT
GKM
GKFS
 and obtained 1K .
For ,F and R we use inverse sensitivity function. Or use Automatic Weight Selection Algorithm [4],
[21].
b. To design the µ controller for the process with uncertainty (if the process is unstable, at first must be
stabilize). D-K iteration method can be used to improve the performance of the controller design for the
system. Peak value of the µ (D-K iteration) bound should be less than one, and obtained 2K .
c. Order reduction method can be used to reduce the order of the 1 2K ,K .
d. 1 2,W W are given with LMI (12) then the robust stability of the system has to be established.
e. H infinity norm of 2W must be smaller than 1W .
f. 1 1 2 2K W K W K  .
This controller (K) has robust stability and desired performance.
5. RESULTS OF SIMULTION
First H2/H∞ controller and then µ is designed. After that the order of I+GK was reduced by the help
of the residual method. Regarding the practical considerations and by application of the inverse of the
sensitivity functions, the weight functions were selected with the form of I
s
s
R
200
2


 , I
4s5.0
)1s1.0(4
F


 ,
I
s
s
10
100


 . K1 and K2 are determined according to the equations 5 and 7, while w1 and w2 were defined
regarding equation 12. According to Figure 6 and section 3.A and 3.B, k was designed. The simulations were
done by MATLAB software and toolboxes of LMI [22], Robust multiobjective control toolbox [23] and µ

 ISSN: 2088-8708
4806
[24] were employed. In the designing process, we used Heffron-Phillips model which is a reduced order
model. In order to estimate our designs through simulations, we use complete model of power system
containing synchronous generator, exciter system, governor, turbine, 3-phase transformer, transmission line,
load and infinite bus. For comparison purposes, we compare the variations of before and after 3-phase fault
occurring in the middle of transmission line. Three-phase fault occurs at 0.5 sec. and is gone within 0.55 sec.
In addition, the comparison of the singular values for controlling signals related to three types of design is
depicted in Figure 7. The results show that the largest amount of the control signal is related to µ controller
and the lowest amount was associated with H2/H∞. Step response of the closed loop system for the three
controllers shown in Figure 8.
Figure 8 indicates that the best function of the controller is for µ while H2/H∞ shows the weakest
performance. It could also be noted that since the system has multiple inputs and outputs, the sensitivity and
weight functions have the matrix form. The results verify the success of combining the robust and nominal
performance with each other. Reaching to the mentioned objectives with the minimum controlling signal is
one of the advantages of H2/H∞/µ controller. Most of the robust controllers have high orders and controlling
signals. But this new approach did well in this regard. H2/H∞ controller has the order of 7, and µ controller’s
order in 10, due to use of order reduction method, the order of the H2/H∞/µ controller is 5. For further
investigation of three controller, the form of the waves related to rotor angle and speed are shown according
to 2% p.u increase in the input voltage of the system in Figure 9 and 10, respectively. The mentioned figures
indicate for H2/H∞/µ controller the attenuation rate of 3 s and low oscillation. Variations of rotor speed before
and after 3-phase fault shown in Figure 11.
Figure 7. Singular value for controlling signal (H2/H∞, µ, H2/H∞/µ)
Figure 8. Step response of close loop system (H2/H∞, µ, H2/H∞/µ)

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Figure 9. Rotor angle with 2% (p.u) change Figure 10. Rotor speed with 2% (p.u) change
Figure 11. Variations of rotor speed before and after 3-phase fault
6. CONCLUSION
Providing the spare parts and resolving the errors in the excitation system are among the most
important problems of the old power plants. For this reason, replacement of the control section of the
excitation system seems necessary. For attenuating the oscillations by controlling the excitation process, the
stabilizers of the power systems are used. The aim of this paper is to design a robust stabilizer of the power
system for the power plant of Mashhad city. First the parameters of Hefron Philips model was derived and
obtained, since there is no certain model of the system in hand, the robust performance is considered. By
robust performance, it means by consideration of the uncertainty the errors of the system be minimized. In
order to investigate the robust performance, µ analysis was used. Generally, existence of the robust
performance results in the severe limitations on the controller which is sometimes making it an unfeasible
issue, and if it could be feasible the order of the controller would become higher and the resulted control
signal would be increased which would lead to saturation of the actuator. In order to decrease the controlling
signals, it is needed to use to controllers of µ and H2/H∞ for the performance of robust and its stability.
Designing the filters or in the other words weight functions have also crucial role in determination of the
closed loop response. In this content, first, three weight functions were designed for H2/H∞ controller and
then two weight functions by LMI were designed for balancing between H2/H∞ and µ. Due to multi variable
system of the weight functions, they were plotted in the form of matrix and the singular values. The results
show that the closed loop was stabilized despite of the existence of uncertainty and has the desirable
performance. Moreover, the response of the closed loop and controlling signal of the combined controller
(H2/H∞/µ), is between the two other controllers. The angle and speed of rotor verifies the effectiveness and
advantages of the suggested method.

 ISSN: 2088-8708
4808
7. APPENDIX
A. Nomenclature
Xd direct axis reactance of synchronous machine (p.u)
X/
d direct axis transient reactance of synchronous
machine (p.u)
Xq quadrature axis reactance of synchronous machine
X/
q quadrature axis transient reactance of synchronous
machine
Xe transmission line reactance
T/
do direct-axis transient open circuit time constant
K1 to K6 Heffron-Phillips model coefficient
KA DC gain of the AVR
TA time constant of the AVR
KD PSS gain
H inertia constant
Eb exciter Output Voltage
Eq voltage proportional to direct axis Flux
linkages
δ (t) rotor angle
ωr (t) speed of the rotor
Tm mechanical/electrical torque
Iq generator stator current
Vt Teminal voltage of synchronous machine(p.u)
∆ Denotes small perturbation in the variable from
steady state value
fb Synchronous Generator
B. Machine data
Xd X/
d Xq X/
q Xe T/
do K1 K2
2.013 0.3 1.76 0.65 0.68 0.53 0.55 1.2
K3 K4 K5 K6 KA TA KD H
0.66 0.7 0.095 0.815 50 0.5 7.1 3.5
ACKNOWLEDGMENT
This research was financed from the budget of Islamic Azad University, Sabzevar branch-Iran.
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BIOGRAPHIES OF AUTHORS
Javad Mashayekhi Fard received the B.S. degree in power engineering from the Islamic Azad
University, Bojnourd, Iran, in 2003, The MS degree in control engineering from Islamic Azad
University, South Tehran branch, Iran in 2006, and his Ph.D in Control Engineering from Islamic
Azad University, Science and Research Branch, Tehran, Iran in 2013. He is currently an
Assistant Professor in the Electrical Engineering Department at Islamic Azad University,
Sabzevar, Iran. His research interests include optimal and robust control, process control, and
multivariable control systems.

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