Future approach to mitigate Inter-Area Oscillations in GCC Combined System

GCC Interconnection Authority,
P.O. Box 3894, Dammam 31481, KSA
Future approach to mitigate Inter-Area Oscillations in
GCC Combined System
Ossama Ahmed
Operation Planning Engineer
GCC Interconnection Authority
oahmed@gccia.com.sa
Hatim Elsayed
Head, Operational Planning
helsayed@gccia.com.sa
Ikram Rahim
Senior Operation Planning Engineer
irahim@gccia.com.sa
Nasser Al-Shahrani
Director, Operations and Control
nshahrani@gccia.com.sa
Abstract
Inter-Area power oscillations is one of the major concerns
for GCC Combined System Operators, as it is diminishing
tie-lines Available Transfer Capacity and threatening
System Security. This paper presents outcomes of recent
modal analyses study carried out on a Combined System
model. It also shows real-time oscillations recorded and
analyzed by Wide Area Monitoring System (WAMS)
owned by Gulf Cooperation Council Interconnection
Authority (GCCIA).
The damping characteristic of the inter-area mode is
dictated by the tie-line strength, the nature of the loads, and
the power flow through the interconnection and the
interaction of loads with the dynamics of generators and
their associated controls. Therefore, it is a very challenging
task to tune power system stabilizers (PSS) to damp
sufficiently well both local and inter-area modes of
oscillations based on local feedback signals only. The
secure operation of the Combined System requires
application of robust control strategy that is effective to
damp inter-area oscillations over wide operating range.
Referring to the recent development of WAMS, it is worth
to consider new methods to mitigate Inter-Area oscillations
in the Combined System. Tuning of PSS for damping Inter-
Area oscillations based on WAMS, and enhancement of
damping performance of generators through wide-area
controller, i.e. remote measured signals obtained using
synchronized Phasor Measurements Units (PMUs)
transmitted in real-time via communication network, are
two proposed options for study.
Keywords: GCCIA network, GCC Combined System, PSS
tuning, WAMS, LFO, OSM, PMU, FACTS, SVC
1. Introduction
Low-frequency oscillations (LFO) appear among parallel-
operated AC generators when it is subject to small
perturbations during normal operation i.e. forming a mass-
spring system. A fundamental factor in this problem is the
manner in which the power outputs of synchronous
machines vary as their rotors electromechanical oscillate
[1].
LFO problems may be either local or global in nature.
Local problems are usually associated with rotor angle
oscillations of a single power plant against the rest of the
power system. Such oscillations are in the range of 0.7-2 Hz
and called local plant mode oscillations. Stability
(damping) of these oscillations depends on the strength of
the transmission system as seen by the power plant, the
excitation control systems and plant output. Global
problems are caused by interactions among large groups of
generators and have widespread effects. Such oscillations
are in the range of 0.1-0.7 Hz and called interarea mode
oscillations. There are two distinct forms of inter-area
oscillation; one is associated with very low frequency
modes, which involve all generators in the system. The
system is essentially splits into two parts, with generators in
one part swinging against machines in the other part, the
frequency of these modes of oscillation is on order of 0.1-
0.4 Hz. The other form is associated with higher frequency
modes, that involve different generator subgroups swinging
against each others. The frequency of these modes of
oscillation is on order of 0.4-0.7 Hz. The characteristics of
inter-area mode oscillations are very complex and
significantly differ from those of local plant mode
oscillations [1]. Load characteristics, in particular, have a
major effect on the stability of interarea modes

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As power systems are continuously expanding and
upgrading to cater the ever-growing power demand,
mitigating LFO becomes ever more vital, and there are
several reasons for that; for instance units’ dampers are no
longer effective to damp interarea oscillations because of
high external impedance. In addition, the spread of
automatic controls increases the probability of adverse
interactions. Moreover, the high loading of tie-line
increases both tendency to oscillate and the importance of
the oscillation [2].
Although small oscillations in each generator that may be
insignificant, they may add up to tie-line oscillation, whose
damping degradation lower the operational efficiency and
occasionally led to unstable system operation with major
consequence including grid breakup and wide spread
blackout.
Normally, these electromechanical oscillations shall show a
damping factor more than a pre-specified value according
to utilities practice in all the credible operating conditions.
Whenever the minimum oscillation damping isn’t fulfilled,
actions have to be taken in order to enhance the damping.
Usually, installation of supplementary excitation control,
PSS is simple and economical method for improving
oscillatory stability.
Fig.1. Power System Stability Classifications as in [3]
Most of modern power plants are installed with PSS.
Adequate tuning of PSS allows reaching the pre-established
threshold for the oscillation damping. When unacceptable
damping is observed, a PSS dedicated tuning is required in
order to damp both local and inter-area oscillations. Given
the configuration and complexity of the system, these
solutions might not always sufficient in some cases.
LFO study requires dynamic modeling of most of the power
system components. Once the mathematical model is
available different methodologies can be applied to study
the system oscillatory behavior in low frequency range.
Eigenvalues and time domain simulations are typically used
among the utilities to get a complete understanding of
system oscillatory phenomena [4].
Eigenvalue analysis
The small signal stability or LFO study of the system can
be determined by system eigenvalues at an operating point.
The relative participation of state variables and their
contribution in certain oscillation mode are given by the
corresponding elements in the right and left eigenvectors.
Hence, combination of left and right eigenvectors yields
participation factor matrix. The participation factor matrix
can be used to identify the dominant state variable in a
particular mode.
In order for the system to be stable or oscillation free, all
the eigenvalues should be located in the open left half
plane. This means that real part of the eigenvalues should
be negative and damping ratio should be positive, typically
more than 5%.
If at least one of the eigenvalues has positive real part, the
system is said to be unstable. More specifically, in
oscillatory unstable cases, a pair of complex eigenvalue (𝜆
= 𝜎 ± 𝑗𝜔) will appear with positive real part.
Frequency of the mode: 𝑓= ω/2𝜋 (1)
Damping ratio 𝜉= − 𝜎/√(𝜎2+𝜔2) (2)
Wide Area Measurement System-WAMS
The mathematical models used for modal analyses and real
time simulations cannot be assured to always be correct.
Furthermore, the number of cases studied cannot guarantee
to be enough. This brings up the need for online monitoring
system that provides early warning/detection of the
potentially dangerous situations that will not show up in
simulations because of errors in the mathematical models or
by omitting some operating conditions in the simulation
studies.
WAMS serve as a guarantee that, whatever models are
used, the true behavior of the system can always be
observed [5]. In many systems, the PMUs are now in
operation supplying data to the control centers in real-time
for this purpose.
Fig. 2. Power measurements vs simulation (reconstructing of
events in models in use) as in [6]
2. Topology of GCC combined system
The GCC combined system is characterized by several
blocks connected together by relatively long AC lines as
shown in below figure.

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Fig. 3. GCCIA network, the 400kV backbone of GCC combined
system
Fig. 4. System frequency and Interchange between MSs
It is well known that longitudinal systems are prone to
inter-area oscillations, and there might be conditions that
result in highly amplitude and poorly damped oscillations
which could affect system operation.
3. Inter-area oscillations in GCC Combine System
3.1 Modal analysis:
According to the Phase-I & III Operational Study results,
the following inter-area modes were present in the
interconnected system with Member States where mode is
observable.
0.25Hz 0.32Hz 0.42Hz 0.45Hz 0.5Hz
Kuwait     
Bahrain     
Qatar     
UAE    
Oman    
Recently, classical modal analyses have been done using
GCC Combined System mathematical model of 2015.
Below are the results of two setups used in the study.
Natural oscillatory frequencies:
The first model setup used to investigate the natural
oscillatory frequencies of the system as well their damping,
Only the dynamic model of the synchronous machine has
been considered, i.e. disabling Governor and AVR
controls of the generators. The Study identified six natural
inter-area modes of oscillations; these modes are shown
below table.
Mode
No.
Eigenvalue (σ + jω) (𝜉) (𝑓)
σ ω % Hz
1 -0.072 1.581 4.5 0.252
2 -0.017 2.045 0.8 0.325
3 -0.074 2.525 2.9 0.402
4 -0.042 2.802 1.5 0.446
5 -0.1 3.975 2.5 0.634
6 -0.081 3.987 2 0.634
Oscillatory frequencies with controllers enabled:
The second model setup used to investigate the oscillatory
behavior of the GCC Combined System with all controllers
enabled. The results showed that poorly damped modes
were present. It worth to mention that some modes were
well damped in the base case i.e. no power exchange,
became poorly damped when extreme power flows
exchanged between MSs. Summarized result is shown in
the below tables.

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3.2 GCCIA Wide Area Monitoring System:
It is worth mentioning that LFO modes, especially the inter-
area can be detected from measured data. WAMS
application is able to identify accurately the amplitude and
frequency of the main oscillatory modes; however,
identification of damping is difficult, particularly when the
magnitude of the oscillations is in the range compared to
random load variations [5]. The PMUs installed in GCCIA
Network (seven substations) provide data for the WAMS,
in which an Oscillatory Stability Management (OSM) tool
monitors the system oscillatory patterns by continuously
deriving a series of valuable stability indicators directly
from small perturbations with three distinct advantages over
model-based techniques:
 Continuous real-time dynamic status reporting
 Early warning of near-instability
 Continuous records of actual system damping
Frequency, Active Power (P) and Angle Difference are
used for the analysis. The analysis determines system
stability using continuous random perturbations that appear
in measured signals in the system to characterize observable
modes of oscillation, and raises Real Time Alerts and
Alarms for damping below 5% & 3% respectively.
Fig. 5. GCCIA wide area measurement system-WAMS
The OMS detects, the MSs contains the source of the Inter-
area modes, but it does not identify the plants involved in
the oscillations, as the input data of WAMS is limited to
GCCIA Network. The following are the result of OSM tool
at two different GCCIA network topologies. The objective
is to identify the MS with the smallest damping
contribution, as being the source of the oscillation. Since
there was no synchronize data available for the 220kV
UAE-Oman tie line, participation of both systems was
combined and analyzed as appeared at Silla 400k, and is
mentioned UAE in following graphs.
GCCIA Network in N condition
While the normal operation of the system in N condition
shows acceptable damping for all LFO modes, there have
been occasional cases of degraded damping. Of these cases,
most are stable but unusually poorly damped. The
following table and histogram present LFO modes that had
been detected by GCCIA WAMS at Silla 400kV substation,
while separate plots of each LFO mode are shown after.
Table 2. Modes band at Silla s/s on 03/05/2016, 00:00-23:59
Band (Hz)
No of
Samples
Max Amp.
(mHz)
Min
Damping
(%)
Max
Damping
(%)
0.15 - 0.3 1,575 0.57 7.2 23.7
0.3 - 0.45 1,752 0.43 6.5 18.5
Fig. 6. Inter-area modes appearance at Silla s/s on 03/05/2016

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a) Oscillatory Frequency Band (0.21 - 0.27) Hz
In this band, Kuwait, Bahrain and Qatar were oscillating
against UAE/Oman.
Fig. 7. Frequency Band (0.21 - 0.27) Hz on 03/05/2016 00:00-
23:59 hrs
b) Oscillatory Frequency Band (0.36 - 0.4) Hz
Qatar, UAE/Oman and Kuwait were oscillating against
each other with no contribution from Bahrain in this
frequency band.
Fig. 8. Frequency Band (0.36 - 0.4) Hz on 03/05/2016, 00:00-
23:59 hrs
GCCIA Network in N-1 conditions
It is worth pointing out that, critical outages on GCCIA
Network (400kV OHL) can expose new modes and drift
frequencies of N condition modes. These outages may also
enhance/reduce modes’ damping depending on weather the
pre-outage contribution was a negative or a positive
damping. The effects of single circuit outages are shown
below.
A. Outage of Ghunan_Salwa 400kV OHL
During single circuit outage of the central 400kV corridor
of Ghunan- Salwa (235 km) on 05/05/2016 00:00-23:59
hrs, dominant modes appeared differently:
Table 3. Modes band at Silla s/s on 05/05/2016 00:00-23:59
Band (Hz)
No of
Samples
Max
Amp.
(mHz)
Min
Damping
(%)
Max
Damping
(%)
0.15 - 0.3 248 0.54 8.0 22.4
0.3 - 0.45 1,444 0.46 5.3 15.0
a) Oscillatory Frequency Band (0.20 - 0.26) Hz
Fig. 10. Frequency Band (0.20 - 0.26) Hz during the outage of
Ghunan_Salwa on 05/05/2016, 00:00-23:59 hrs

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b) Oscillatory Frequency Band (0.33 - 0.37) Hz
Fig.11. Frequency Band (0.33- 0.37) Hz during the outage of
Ghunan_Salwa on 05/05/2016, 00:00-23:59 hrs
B. Outage of Al-Zour_Al-Fadhili 400kV OHL
During single circuit outage of the longest 400kV corridor
of Al-Zour_Al-Fadhili (292 km) on 12/04/2016 00:00-
23:59 hrs, dominant modes appeared differently:
Table 4. Modes band at Silla s/s on 12/04/2016 00:00-23:59
Band (Hz)
No of
Samples
Max
Amp.
(mHz)
Min
Damping
(%)
Max
Damping
(%)
0.15 - 0.3 555 0.75 10.1 27.2
0.3 - 0.45 2,358 0.42 4.3 17.1
a) The Oscillatory Frequency Band (0.22-0.28) Hz
Fig. 13. Frequency Band (0.22-0.28) Hz during the outage of Al-
Zour_Al-Fadhili on 12/04/2016, 00:00-23:59 hrs
Fig. 14. System frequency over span of 30 s, during the outage of
Al-Zour_Al-Fadhili
Compared to oscillatory band in N condition, this line
outage caused high amplitude and relatively low damping
in Kuwait.

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b) The Oscillatory Frequency Band (0.36-0.40) Hz
Zour_Al-Fadhili on 12/04/2016, 00:00-23:59 hrs
c) The Oscillatory Frequency Band (0.5-0.7) Hz
Compared to complete network oscillatory modes in N
condition, line outage had spread out a low damped local
oscillatory mode in Kuwait.
The below locus plot shows the trend of this local mode
while crossing amplitude/damping alert and alarm
boundaries.
Fig. 17. Band (0.5-0.7) Hz Locus Plot with damping/amplitude
alert and alarm boundaries
Zour_ Al-Fadhili on 12/04/2016, 00:00-23:59 hrs
4. Future approach to mitigate Inter-Area Oscillations
in GCC Combined System:
The remedial measures for LFO can be classified in two
broad categories, at operational level, the measures include
re-tuning units’ excitation control system and PSS, Re-
dispatching of generators, and adjusting of load changers,
while, load shedding is used as the last defense action to
damp critical LFO. At planning stage, the measures include
adding new devices such as PSS, FACTS controllers,
Superconducting Magnetic Energy Storage (SMES) or
flywheel [4].
4.1 WAMS for inter-area oscillation monitoring
In many cases, simulation results can be different from real
system behavior. Measurement-based oscillation
monitoring using WAMS has been used to complement
dynamic modelling, and it has proved to be a valuable
application of WAMS.
There are several inter-area modes of oscillation over GCC
Combined System crossing the boundaries of responsibility.
All system operators have an interest in knowing whether

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there is a significant contribution within their own area of
responsibility.
In case of well-known LFO modes of oscillation, system
operator can design rules to mitigate the risk of degraded
damping progressing to instability. However, impermanent
poorly damped LFO modes only occur under particular
network conditions. The trigger of those oscillations could
be due to many factors related to the time of the incident,
like status of PSSs and network topology change.
Currently, GCCIA-WAMS gets data from GCCIA
Network; therefore, the OSM cannot identify the
generator(s) where investigation and control tuning are
most appropriate. The source location approach could be
applied to GCC Combined System by sharing a high-level
set of voltage phasor measurements from different blocks
[2]. This sharing is feasible as the data shared is not
commercially/politically sensitive i.e. it does not contain
power information. The system operators can combine a
high-level observation on interconnection wide oscillations
with a detailed identification of sources within their own
blocks.
It is important that future changes in the system do not
result in instability. WAMS covering GCC Combined
System will identify any significant changes oscillatory
behavior, and will provide valuable insight into the
dynamic performance of the system.
4.2 Tuning of PSS for damping Inter-Area oscillations
based on WAMS
The effectiveness of the PSS depends on the choice of its
parameters. There is no unique way of tuning a power
system stabilizer; however, in the industry there have been
created appropriate tools for tuning the PSS but when
tuning, it is important to ensure that the controller:
 Has the intended effect on all the modes that it is
designed for
 Has no significant destabilizing effect
 Works effectively over a wide range of system
conditions.
The PSS tuning procedure customarily used by engineers
normally aims at fulfilling the following three goals:
1. The PSS should be tuned in such a way that the local
mode oscillations, i.e., those with the frequencies above
0.7Hz, are well damped.
2. The tuning of the PSS should also provide adequate
damping for inter-area mode oscillations. That is, the
oscillations in the frequency range from 0.1 to 0.7Hz have
to be properly damped.
3. The PSS should not over-modulate the terminal voltage
of the generator when the generator is in steady-state
operation.
In general, it is possible to equip the existing excitation
systems easily with a global synchronizing clock and the
functionality of a PMU. Providing that multiple generator
excitation systems can exchange this information (possibly
with many other PMUs) among each other, it gives new
opportunities for tuning power system stabilizers to damp
also inter-area modes. The idea behind the wide-area
approach to power system stabilizers is in principle very
simple: Extend the number of signals, which can be
considered as a feedback for control (i.e. incl. the remote
ones available through PMUs) and select between them
carefully those with the highest observability of all critical
oscillatory modes to be damped. Hence, the selection of the
proper feedback signal is herewith the first and very
important step in the stabilizing controller design. This idea
has been evaluated on a realistic power system model as in
[7]. However, it is important to realize that this system has
also a higher technical complexity and cost.
4.3 Improvement of System Damping
Voltage Feedback for Damping Improvement
PSS is supplementary excitation control device equipped
with generating units; it provides an additional input signal
to the automatic regulator of the excitation system. The
conventional PSS is essentially decentralized, using some
local measurement, such as accelerating power, rotor speed
deviation, or frequency deviation, as the feedback signal
due to communication restriction. It can be expected that a
PSS will yield more damping when both local and remote
(wide area) signals are utilized as feedback signals [8].
Fig.18. Dual–input Power System Stabilizer-PSS
The main idea is to enhance generator unit damping to both
local and inter-area modes by utilization properly selected
remote feedback WAMS signals with a view to advance the
controlling capability of PSS by including both local and
remote voltage information as additional input signals.
However, it is important to realize that this system has also
a higher technical complexity and cost.
FACTS for Damping Improvement
The secure operation of the electric power systems requires
application of robust controllers to damp inter-area
oscillations. The traditional approach to address LFO
problem is to equip PSSs in generating units. However, the
continuing extensions of electric networks and increases in

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line loading have shown that PSS alone is often not enough
and even detrimental [5]. This has opened the door for
FACTS devices installed in the system for dynamic voltage
support and enhancement of transfer capacities to add
damping on LFO modes when equipped with a properly
designed damping controller [9].
5. Conclusions
Modal analysis alone is not sufficient for full LFO analyses,
as it represents the standard and most common situations in
which the real system is operating. Always there will be a
need for online monitoring and detection of the potentially
dangerous situations that will not show up in simulations
because of errors in the models or by omitting some
operating conditions in the simulation studies.
An accurate WAMS is able to identify all LFO modes; and
it provides relevant power system information useful for
mathematical model validation as well. Due to the lack of
data from PMU’s within MSs, the OSM tool points only to
the beyond of interface-substations as the source of
oscillations, whereas if that synchronized data was
available, the OSM can pinpoint close to the main
generators participating in the oscillation.
The LFO analyses is ongoing task as all modes must
continue to be managed to ensure that subsequent changes
in the grid do not result in making them destabilized. Those
changes can lead to different interactions even between the
old plants and the system.
In the future GCCIA might install Flexible AC
Transmission Systems (FACTS) and Static Var
Compensator (SVC) devices in GCCIA network for
dynamic voltage support; fortunately, these devices can
serve as powerful actuators to control inter-area
oscillations, provided they are equipped with appropriate
controller extensions.
6. References
[1] P. Kundur, Power System Stability and Control, New
York: McGraw-Hill, 1994.
[2] N. Shwal D. Wilson, M. Parashar, Identifying Sources
of Oscillations Using Wide Area Measurements,
CIGRE US National Committee, 2014.
[3] J. Machowski, J. W. Bialek, J. R. Bumby, POWER
SYSTEM DYNAMICS Stability and Control, John
Wiley & Sons, Ltd. 2008.
[4] K. Prasertwong, Understanding Low-Frequency
Oscillation in Power Systems, International Journal of
Electrical Engineering Education · July 2010.
[5] K. Uhlen, Warland, J. O. Gjerde, M. Uusitalo, A. B.
Leirbukt, P. Korba “Monitoring Amplitude,
Frequency and Damping of Power System
Oscillations with PMU Measurements” Presented at
IEEE 2008.
[6] Samuelsson, O. Wide Area Measurements of Power
System Dynamics; The North American WAMS
Project and its Applicability to the Nordic Countries,
1999.
[7] P. Korba and V. Knazkins and M. Larsson, Power
System Stabilizer with Synchronized Phasor
Measurements, 17th Power Systems Computation
Conference, 2011.
[8] Peihwa HUANG, TaHsiu TSENG, Voltage Feedback
Controller for Power System Damping Improvement,
Journal of Computational Information Systems,2010
[9] R. Majumder, B. C. Pal, C. Dufour, P. Korba, Design
and Real-Time Implementation of Robust FACTS
Controller for Damping Inter-Area Oscillation, IEEE
transactions on power systems, VOL. 21, NO. 2,
MAY 2006

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