2011 International Conference on Computational Intelligence and Communication Systems

The Effective Role of PSS in Damping Inter Area Mode of Oscillation Using
MATLAB/ Simulink

Mahiraj Singh Rawat R. N. Sharma

Department of Electrical Engineering Professor, Department of Electrical Engineering
National Institute of Technology, Hamirpur National Institute of Technology, Hamirpur
Himachal Pradesh-177005, India Himachal Pradesh-177005 India
Email: rawat.ms85@gmail.com Email: rnaresh@nitham.ac.in

Abstract— It is well established that fast acting exciter with phase with rotor speed deviation [7]-[9]. In a practical
high gain AVR can contribute to oscillatory instability in system, the various modes of oscillation can be
power system. This type of instability is characterized by low
frequency (0.2 Hz to 2.0 Hz) oscillations. This type of grouped into 3 broad categories.(I) Intra plant modes in
instability can endanger system security and limit power which only the generators in a power plant participate. The
transfer. In this paper Power System Stabilizers (PSS) are used oscillation frequencies are generally high in the range of 1.5
to damp inter area mode of oscillation in two area power to 3.0 Hz. (II) Local modes in which several generators in
system. Simulations are carried out in MATLAB/Simulink an area participate. The frequencies of oscillations are in the
environment for the two area system model with and without range of 0.8 to 1.8 Hz. (III) Inter area mode in which
PSS to analyze the effects of PSS on Inter area oscillations of generators over an extensive area participate. The
the system. oscillations frequencies are low and in the range of 0.2 to
Keywords-PSS; MATLAB/Simulink; Inter area oscillations;
0.5Hz.
Two area power system This paper investigates the performance of PSS for
damping the inter area oscillations of inter area power
I. INTRODUCTION system. The simulation results are tested on kundur’s two
Modern power system is a complex network comprising area four machines power system. The paper proceeds as
of numerous generators, transmission lines, variety of loads fallows. In section II & III, principal of operation, block
and transformers. As a consequence of increasing power diagram of PSS and tuning of PSS are presented. In Section
demand, some transmission lines are more loaded than was IV, simulation results that demonstrate the effectiveness of
planned when they are built. Interconnections of the power the PSS on two area power system are presented.
system are of great benefits but it also brings lot of new
problems such as low frequency oscillations which will II. PSS- BASIC PRINCIPAL
make the transmission capacity decreased [1]-[2]. Low
frequency oscillations consist of local modes and inter area
modes, which are associated with local generators and The basic function of a PSS is to add damping to
generators in different areas. Automatic Voltage Regulator generator rotor oscillations by controlling its excitation using
auxiliary stabilizing signal. To provide damping, the
(AVR) with PSS can make the system more stable [3]-[5].
stabilizer must produce a component of electrical torque in
The AVR provides regulation or maintenance of the
phase with rotor speed deviation. However for the practical
terminal voltage of the machine to which it is attached. In implementation, other signals such as bus frequency,
addition, a high-gain fast-response AVR improves large- electrical power, accelerating power are also used. The latter
signal transient stability in the sense that it increases the signal is actually synthesized by a combination of electrical
ability of the power system to maintain synchronism when and mechanical power signals. The mechanical power signal
subjected to severe transient disturbances, for instance can be obtained from the gate position in a hydraulic turbine
network faults. High-gain fast-response AVR action can, or steam pressers in steam turbine. The choice of control
however, lead to reduced damping of system signal for PSS can be based on the following criteria.
electromechanical modes of oscillation. Standard ways of
eliminating this loss of system damping are either to employ [1].The signal must be obtained from local measurements
transient gain reduction on the AVR or more commonly and easily synthesized.
attach a PSS to appropriate machines [6]. [2].The noise content of the signal must be minimal.
The basic function of a PSS is to add damping to Otherwise complicated filters are required which can
generator rotor oscillations by controlling its excitation introduce their own problems.
using auxiliary stabilizing signal. To provide damping, the [3].The PSS design based on a particular signal must be
stabilizer must produce a component of electrical torque in robust and reject noise. This implies that lead

978-0-7695-4587-5/11 $26.00 © 2011 IEEE 740

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DOI 10.1109/CICN.2011.160
 compensation must be kept to a minimum to avoid Where the time constant T1 to T4 are chosen to provide a
amplifying the noise. phase lead for the signal in the range of frequencies that rare
of interest (0.1 to 3.0 Hz). There are two PSS design criteria.

All the control signals considered rotor speed, frequency, [1]. The time constants, T1 to T4 are to be chosen from
electrical power are locally available. The speed signal can the requirement of the phase compensation to
be obtained from a transducer using a tooth wheel mounted
achieve damping torque.
on the shaft. Alternately it can be obtained from the angle of
the internal voltage which can be synthesized. The bus
frequency signal can be obtained by measuring the period [2]. The gain of the PSS is to be chosen to provide
using zero crossing detection. The power signal can be adequate damping of all critical modes under
derived from a Hall Effect transducer. PSS representation various operating conditions. It is to be noted that
(shown in fig.1) consists of five blocks: gain, a signal PSS is tuned at a particular operating condition
washout block, phase compensation block, torsional filter (full load conditions with strong or weak AC
and limiter.
system) which is most critical.

The tuning procedure for the phase compensator, the

following steps are carried out.

[1]. Identify the plant GEP(S).

Figure 1 Block Diagram of PSS [2]. Choose the time constant from the phase
compensation technique, describe earlier and from
The stabilizer gain KSTAB determine the amount of
damping introduced by the PSS. The signal washout block the knowledge of GEP(s).
serves as a high pass filter with the time constant TW high
enough to allow signals associated with oscillation in r to [3]. Select the PSS gain such that it is a fraction of the
pass unchanged. Without it steady changes in speed would gain corresponding to instability. This can be
modify the terminal voltage. The value of TW is not critical determined from root loci to maximize the
may be in the range of 1 to 20 sec. The phase compensation damping of the critical (least damped) mode.
block provide the appropriate phase lead characteristics to
compensate for the phase lag between the exciter input and The transfer function of the filter can be expressed as 
the generator electrical torque. 
The torsional filter in the PSS is essentially a band reject ߱௡ଶ
or a low pass filter to attenuate the first torsional mode ‫ܶܮܫܨ‬ሺܵሻ ൌ ଶ
‫ ݏ‬൅ ʹߦ߱௡ ‫ ݏ‬൅ ߱௡ଶ
frequency. The output of the PSS must be limited to prevent 
the PSS acting to counter the action of AVR. For example,
The criteria for designing of the torsional filter are
when load rejection takes place, the AVR acts to reduce to
reduce the terminal voltage when PSS action call for higher
value of the terminal voltage (due to the increase in speed or [1]. The maximum possible change in damping of any
frequency). torsional mode is less than some fractional of the
inherent torsional damping.

III. TUNNING OF PSS [2]. The phase lag of the filter in the frequency range of
1 to 3 Hz is minimized.
In practice two or more first order blocks may be used to
achieve the desired phase compensation. The phase IV. SIMULATION RESULTS
compensator used in industry is made up to two lead-lag
stages and has the following transfer function
In this paper we have taken the two area system for
study. The system comprises two similar areas connected by
ሺͳ ൅ ‫ܶݏ‬ଵ ሻሺͳ ൅ ‫ܶݏ‬ଷ ሻ a weak tie (as shown in fig2). Each area consist of two
ܶ௦ ൌ generators, each having a rating of 900 MVA and 20 kV.
ሺͳ ൅ ‫ܶݏ‬ଶ ሻሺͳ ൅ ‫ܶݏ‬ସ ሻ
The system contains eleven buses and two areas, connected
by weak tie between bus 7 and 9. Two static loads are
connected at bus7 and 9. The system has fundamental
frequency of 60 Hz.

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 oscillatory instability is to provide damping for generator
power oscillations. This is conveniently done by providing
power system stabilizers (PSSs) which are supplementary
controller in the excitation system.
The basic function is to add damping to the generator
rotor oscillations by controlling its excitation using
auxiliary stabilizing signals. However for practical
implementation other signal such as bus frequency,
electrical power, accelerating power are also used. In
Simulink model to damp Inter area oscillaitons under heavy
Figure 2 Kundur’s Two area four machines system
load conditions we are using PSS in each generating plant
(as shown in figure 4).
Matlab/ Simulink model (shown in figure 3) of Kundur’s Simulation results (figure 7-9 ) shows that when we are
two area system consists of Steam turbine & governor using PSS in each generating plant the inter area oscillations
System, Synchronous Generator and excitation system. In have damped and system becomes stable after 8 seconds. To
Simulink model transmission lines are represented by
equivalent  model.

Figure 3 Simulink Model diagram of kundur’s two area system

Loading of generator or tie line is one of the factors check the performance of PSS under disturbance, a three
which cause low frequency oscillations in the system. These phase symmetrical short circuit fault of 20 cycle duration
types of low frequency oscillations can endanger system occur nearer to bus 4. Fault is cleared in 4 cycle duration
security and limit power transfer. Fast acting exciter with after initiation of the fault. Simulation results are shown in
high gain AVR can contribute to oscillatory instability in figure 10-12.
power system.
This type of instability is characterised by low frequency
(0.2 to 2 Hz) oscillations. The simulation results from figure V. CONCLUSION
5 and 6 shows the low frequency oscillations in Kundur’s In this paper, PSS is used in order to damp low frequency
two areas system of increasing amplitude. oscillations. The dynamics of the system is compared with
The simulation results of various parameter shows that and without the presence of PSS in the system. It is clear
when the tie line is heavily loaded the two area system goes from the simulation result that PSS is effective device to
to low frequency oscillations with increasing amplitude. A damp inter area oscillations even in the disturbance
cost effective and satisfactory solution to this problem of condition.

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 0.015

0.01

PSS output voltage Vs (pu)

0.005

-0.005

-0.01

-0.015 Vs(G1)
Vs(G2)
-0.02 Vs(G3)
Vs(G4)
-0.025
0 2 4 6 8 10
Time(s)

Figure 7 PSS output voltage at each generator

1.035

Figure 4 Subsystem of Generator 1 in Kundur’s two areas system with PSS

Terminal voltage of Generator (pu)

1.03

1.025
45.5
G1
45 Vt(G1)
G2 1.02
Vt(G2)
44.5 G3
Vt(G3)
G4 1.015
Load Angle delta (deg)

Vt(G4)
44

43.5 1.01

43
1.005
42.5
1
42 0 2 4 6 8 10
Time(s)
41.5

41 Figure 8 Terminal voltage of generators ( two area system with PSS)

40.5
0 5 10 15 20
Time in seconds 42.8

Figure 5 Load angles of generators when tie line is heavily loaded 42.78

1.045
Load Angle delta (deg)

42.76
1.04
Terminal voltage of Generator (pu)

42.74
1.035

1.03 42.72

1.025 Vt(G1) 42.7

Vt(G2) G1
1.02
Vt(G3) 42.68
1.015 Vt(G4)

1.01 42.66

1.005 0 2 4 6 8 10
Time in seconds
1

0.995 Figure 9 load angle of generator 1 (two area system with PSS)
0 5 10 15 20
Time(s)
Figure 6 Terminal voltages of generators when tie line is heavily loaded

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G1
42
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