small-disturbance stability of oman/uae interconnected system

Small-Disturbance Stability of Oman/UAE

Interconnected System

O. H. Abdalla*, R. Al-Badwawi, H. Al-

Hadi, H. Al-Riyami, and A. Al-Nadabi

Oman Electricity Transmission Co.,

Sultanate of Oman

* [email protected]

K. Karoui**, and A. Szekut

Tractebel Engineering,

Belgium

**[email protected]

A. Al-Hinai

Masdar Institute of Science &

Technology

United Arab Emirates

[email protected]

Abstract—This paper presents small-disturbance stability studies

of the interconnected power systems of the Sultanate of Oman

and the State of the United Arab Emirates. The two systems are

interconnected through the existing 220 kV double circuit

transmission line between Oman and Abu Dhabi leading to a

rather longitudinal structure power system. The objective of the

paper is to assess the inter-area oscillation damping of this

interconnected system during small amplitude disturbances by

using eigenvalue analysis. A detailed model of the interconnected

system is obtained including representation of generators, prime

movers, speed governors, exciters, automatic voltage regulators,

transmission lines, transformers, compensators and loads. The

system eigenvalues are calculated at various operating scenarios,

including peak and minimum loading conditions with zero and

maximum power exchanges in both directions and also during N

and N-1 conditions. The results have shown that both systems

have adequate damping before being interconnected. After

interconnection, the eigenvalues profile indicates that the system

will remain stable and satisfactorily damped for the considered

power exchanges.

Keywords-Oman-UAE Interconnection; Small-Disturbance

Stability; Eigenvalue Analysis, Inter-Area Ocillations.

I. INTRODUCTION

Interconnected power systems have been widely developed

in many parts of the world. The objectives of the

interconnecting power systems are to provide economical,

operational and technological benefits to the interconnected

countries. This will improve security of supply and system

reliability. The interchange of energy among the

interconnected grids will reduce the total operating costs. The

interconnection facilitates sharing of power generation

reserves and installed capacity which can lead to optimization

of investments in power generation and grid infrastructures. In

addition, the interconnection can provide alternative energy

sources to support individual systems during emergencies.

A regional example is the development of the Gulf

Cooperation Council (GCC) electricity interconnection project

which has been under considerable interest during the recent

decades. Details of the progress of the GCC interconnection

project can be found in the GCC Interconnection Authority

(GCCIA) website [1].

The interconnection between the power systems of the

Sultanate of Oman and United Arab Emirates (UAE) has been

successfully in continuous operation since the 14th of

November, 2011 through the existing 220 kV double-circuit

transmission line between Oman and Abu Dhabi electricity

systems.

A recent comprehensive study [2] was performed by the

Oman Electricity Transmission Company (OETC) and

Tractebel Engineering to assess the impact of OETC-Transco

Abu Dhabi interconnection on the two systems. Static and

dynamic analyses have been performed to assess the

behaviour of the interconnected system during both large and

small disturbances, determine maximum power exchanges,

establish synchronization and control requirements, and

identify the need for adapting existing defense plan and

emergency measures. Results are available in [2] and [3].

The configuration of the interconnected Oman/UAE system

will be a rather longitudinal structure power system. It is well

known that longitudinal systems are prone to inter-area

oscillations, which could affect the system operation.

Interconnection of power systems can change substantially the

existing oscillation modes and new inter-area oscillation

modes can be experienced.

The analysis of the small-disturbance stability deals with the

system stability against small amplitude disturbances such as

random variations in the demand or random switching

operations in the power system. Normally, the

electromechanical oscillations triggered by the small

amplitude disturbances shall show a minimum damping factor

ξ (e.g.: ξ > 5%) in all the credible operating conditions.

This paper concerns with small-disturbance stability of the

Oman/UAE interconnected systems. The objective is to assess

the inter-area oscillation damping during small amplitude

disturbances by using eigenvalue analysis [4]. A detailed

model of the transmission systems of Oman and UAE has

been developed [2] to simulate the steady-state and dynamic

system performances using the product grade software

EUROSTAG© developed by Tractebel Engineering. The

system eigenvalues are calculated at various operating

scenarios, including peak and minimum loading conditions

with zero and maximum power exchanges in both directions

and also during N and N-1 conditions.

Section II describes the transmission systems of Oman and

Abu Dhabi. Section III addresses modeling aspects of the two

systems. Section IV explores the small-disturbance

phenomena. Section V presents the results of the small-

disturbance stability studies. Section VI summarizes the main

conclusions.

Fig. 1. Oman main electricity transmission system in 2011.

II. SYSTEM DESCRIPTION

A. Oman network

The transmission system extends across the whole of

northern Oman and interconnects bulk consumers and

generators of electricity located in the Governorate of Muscat

and in the regions of Bureimi, Batinah, Dhahirah, Dakhiliyah

and Sharquiya [5].

Fig. 1 shows a geo-schematic diagram of the main

electricity transmission system of Oman in 2011. The system

was composed of three voltage levels: 220 kV, 132 kV and 33

kV. In general, the lines are fitted with double circuit except

for the single-circuit interconnection with Petroleum

Development of Oman (PDO) [6]. The substations of 220/132

kV and 132/33 kV present an arrangement of two

transformers in parallel. The 33 kV network is operated by the distribution

companies. Only the 33kV primary substations pertaining to the 132/33 kV transformation are represented in the model. Together with the downstream load, they are represented by an equivalent load model.

Power plants

In 2011, the OETC system was supplied from 8 power

stations [7]:

Rusail IPP (687 MW)

Ghoubrah Power & Desalination Plant (469 MW)

Barka-1 IWPP (434 MW)

Barka-2 IWPP (681 MW)

Sohar IWPP (605 MW)

Wadi Jizzi IPP (290 MW)

Manah IPP (279 MW) Al Kamil IPP (297 MW)

A number of temporary diesel-engine driven generators are

connected directly to the 33kV voltage level at some grid

stations to support central generation at summer peak demand

[8].

Two new combined cycle power plants have been

commissioned during the period 2012-2013; namely Barka-3

(745 MW) and Sohar-2 (745 MW). One large combined cycle

generating plant is under contraction and will be in operation

during the period 2013-2014; this is Sur IPP (2000 MW).

Transmission system

The 2011 transmission system consisted of:

835 circuit-km of 220 kV OH transmission lines

2970 circuit-km of 132 kV OH transmission lines

12 circuit-km of 220 kV underground cables

64 circuit-km of 132 kV underground cables

6630 MVA of 220/132 kV transformer capacity

This line was not in operation in the present study



Two 220 kV interconnection grid stations

Two 220/132 kV grid stations

Five 220/132/33 kV grid stations

Thirty eight 132/33 kV grid stations

One 132/11kV grid station

Distribution system and directly connected customers

The bulk of the power transmitted through the main grid, is

fed, through 220/132/33 kV, 132/33 kV and 132/11 kV grid

stations, to the three distribution licence holders, namely,

Muscat Electricity Distribution Company, Mazoon Electricity

Company and Majan Electricity Company, in addition to

directly-connected large private customers. In summer 2011

the system peak demand of 4000 MW occurred on the 18th of

June 2011. The minimum demand was 909 MW occurred in

winter on the 8th of January 2011. The summer peak demand

in 2012 was 4448 MW occurred at 15:00 on the 25th of July

2012, while the minimum demand was 1274 MW occurred on

the 29th of January 2012. A number of large private customers

are connected directly to the transmission system either at 220

kV or 132 kV, these are as shown in Table I.

TABLE I. DIRECTLY CONNECTED CUSTOMERS.

220 kV Connections 132 kV Connections

Sohar Aluminium Smelter

Shadeed Steel

Sohar Industrial Estate Sharq Steel

Petrulem Development of Oman

Oman Mining Company Aromatics

Sohar Refinery Company

OMIFCO VALE

Some customers have their own generation capability on

site. For the peak situation 2011, some of these customers

injected electric power to the main transmission grid.

Shunt compensation

In 2011, there are 630 MVAr of capacitive shunt

compensation installed in some grid stations at 33 kV.

Oman-UAE interconnection line The interconnection is through the existing 220 kV, 51.3 km

transmission line between Al Foah substation in Abu Dhabi and Al Wasit (Mahadah) substation in Oman. The line consists of two parts: Oman part is a double-circuit, 46.7 km line with twin Araucaria 821 mm2 AAAC conductors per phase, and the UAE part is a double-circuit, 4.6 km line with quad Dove 328 mm2 ACSR conductors per phase.

B. Abu Dhabi system

The Abu Dhabi power system is described in more details in

[3] and [9]. Briefly, it is composed of the 400 kV and 220 kV

voltage levels. Abu Dhabi Island has a meshed sub-

transmission network operated with 132 kV. The main load

centers are Abu Dhabi Island and surrounding areas and Al

Ain city. The transmission system is operated by Transco Abu

Dhabi. The system is connected to the rest of Emirate systems

to form the Emirate National Grid (ENG), which consists of

the following systems:

Abu Dhabi Water and Electricity Authority (ADWEA)

Dubai Electricity and Water Authority (DEWA)

Federal Electricity and Water Authority (FEWA)

Sharjah Electricity and Water Authority (SEWA)

There were 16 power plants connected to the Transco

system totaling an installed capacity of approximately 12.3

GW [3] and [9] in 2011.

III. SYSTEM MODELLING

A. Synchronous Generators

The OETC power system of 2011 comprised 56

synchronous generators of a round-rotor type in the 8 power

stations. The rating of these turbo-generators ranges from 13.4

MVA for the smallest old unit to 280 MVA for the largest unit

in the system. Each generator is represented by a dynamic

model based on Park’s equations. It is assumed that the rotor

has one damper winding in the d-axis and two damper

windings in the q-axis. All the generating units are equipped

with automatic voltage regulator and over and under

excitation limiters.

B. Prime mover and governor systems Most generating units in the OETC system are driven by gas

turbines in an open cycle basis. Some are driven by steam

turbines and few use combined cycle (gas plus steam) [10].

These include conventional separate steam turbines or that

part in a combined cycle configuration. To achieve maximum

efficiency, in the combined cycle power plant, the governor

valve of the steam part is made insensitive to frequency

variations, since the frequency response is usually achieved

through the speed governor of the gas turbine part. Fig. 2 and

Fig. 3 show the gas and steam turbine block diagrams.

C. Excitation systems Various types of excitation systems are employed to provide

the DC field magnetization for the synchronous generators.

These include rotating and static types [10]. The IEEE Type

AC1 model is used to represent a brushless Permanent Magnet

Generator (PGM) excitation system. It comprises a rotating

diode system feeding the field of the synchronous generator

from an AC exciter whose field is driven by a thyristor

converter fed from a PMG. The second type of excitation

systems (brushless ET) is similar to the first one mentioned

above, but the converter is supplied from the generator

terminals via an Excitation Transformer (ET). Fig. 4 and Fig.

5 show the block diagrams of the excitation systems.

D. Transformers The generating units in Oman power stations are connected

to the 132 kV or the 220 kV transmission network through

step up transformers. Auto transformers of 500 MVA and 315

MVA are used at the interconnection substations between the

220 kV and 132 kV transmission systems. At connection

points with the distribution companies, 132/33 kV two-

winding transformers are used in the substations. Most of

these transformers are 125 MVA rating. In some smaller

substations 63 MVA, 40 MVA or 15 MVA ratings are used.

Fig. 2. GAST gas turbine and speed governor model.

Fig.3. IEEE SGO steam turbine and speed governor model.

Fig. 4. IEEE Type AC1 model.

Fig. 5. IEEE Type ST1 model.

The transformer models include the magnetization reactance

and iron loss admittance in addition to the leakage reactances

and winding resistances. On-load tap-changers with their

automatic control facilities; and off-load tap changers are

simulated in the transformer model. The representations

include various connection types and vector groups of

transformers. Transformers neutral with equipped earthing

resistors are also simulated in the model.

E. Transmission lines In Oman, the main transmission system comprises double-

circuit transmission lines; most of them are overhead lines and

only a few are cables. The majority of these lines are within

the short length range; only a few are in the medium length

range. Lumped-parameters π-equivalent circuit models are

used to simulate the lines.

F. Loads The system dynamic behavior is highly dependent on the

assumptions adopted for the load. The load model structure is

composed of a step down transformer connected to an

equivalent LV feeder supplying in parallel a rotating load and

an impedance load. To match the load flow power factor,

fixed shunt compensation is connected at the secondary of the

service transformer [3].

IV. PHENOMENA BACKGROUND

A. Linear phenomena

Small signal stability concerns the study of the small

fluctuations around an operating point. This analysis of small

fluctuations allows the linearization of the equations that

describe the phenomena. The small fluctuations are caused by

the slight disturbances that occur at any time in a network (as

a result of switching of loads, control actions etc.). These

disturbances differ essentially from those considered in

transient, voltage or frequency stability studies, since in the

latter cases the disturbances have large amplitudes.

Such analysis is mandatory following network

modifications like interconnection or putting into service new

generating units which could significantly affect the existing

rate of damping. It must be observed that small signal stability

is a necessary condition for a satisfactory operation of a power

system.

B. Stability criterion

The study of linear systems supplies small-disturbance

stability criterion: the eigenvalues are identified and it is

verified that all the real parts are negative. Indeed, in response

to a disturbance, there is an exponential that relates to each

eigenvalue. In general, the eigenvalue has the following form:

λ = σ ± j ωd (1)

The real part σ = ζ ωn should have a negative sign for the

system to be stable. If the real part is positive, the exponential

will be increasing and the system will be unstable. ωd is the

damped natural frequency and ωn is the undamped natural

frequency. ζ is the damping ratio.

C. Evaluation of damping

When the system is stable, the analysis of the eigenvalues

(their damping ratio) indicates the degree of damping of the

system. The requirement thereby is that the phenomena get

damped rapidly enough. Given the uncertainties regarding

some parameters, the cases where an eigenvalue would be

close to zero, either negative, or the positive, would both be

considered unacceptable, although purely mathematically the

first would be stable, the second not. Generally, the system

will be considered stable when there is a sufficient stability

margin (the real part is sufficiently negative).

D. Transfer functions and dominant modes

There are many eigenvalues in a large system. To each

eigenvalue relates a level of damping (potentially negative

when stable) and a natural frequency. The whole forms a

"specific oscillatory mode".

As a first approximation, the specific oscillation modes

obtained with the extended electromechanical model

represented by the differential-algebraic equations can be

classified into four groups:

- Inter-area modes : their frequency is generally comprised

between 0.1 and 1 Hz, they relates to the natural

oscillation between set of units forming together coherent

electrical areas;

- Electromechanical modes : their frequency is around 1

Hz, and they relate to the natural oscillations of the

generating units;

- Modes relating to the damper windings : these are real

and highly damped;

- Modes relating to control systems (speed or voltage) :

these can be found within the entire frequency range,

depending on the characteristics of the systems;

- Other modes: they cannot be related directly to any

precise cause. In line with the network and stator algebraic equations that

result from the phasor assumption, the local high frequency modes (more than 10 Hz) especially the electromagnetic transients induced by the L and C’s branches related modes are not computed because not needed in such an interconnection study.

V. RESULTS

The calculations of the eigenvalues are performed when the

system is in steady state or when its fluctuations are small in

amplitude (in this case the linearity assumption remains

valid). The eigenvalues are calculated for the following six

system situations:

1) Peak load – 0 MW Exchange

- OETC system isolated

- UAE system isolated

- OETC and UAE system interconnected

2) Peak load – OETC imports 515 MW from ENG

- System in normal state (N)

- System in N-1: loss of circuit 1 of the interconnection

lines

- System in N-1: loss of the 400 kV line between Dhama

and Taweelah

3) Peak load – OETC exports 760 MW towards ENG



lines

- System in N-1: loss of the 220 kV line between Al Wasit

and SIS

4) Minimum load – 0 MW Exchange

- OETC system isolated

- UAE system isolated

- OETC and UAE system interconnected

5) Minimum load – OETC imports 680 MW from UAE



lines

- System in N-1: loss of the 400 kV line between Dhama

and Taweelah

6) Minimum load – OETC exports 658 MW towards ENG



line - System in N-1: loss of the 220 kV line between Al Wasit

and SIS.

The eigenvalues spectrums have been calculated for the

various considered cases. All eigenvalues are found stable and

damped. This is visualized in the following graphs where the

eigenvalues are displayed in the second quadrant of the

complex plan (Real part in 1/s and Imaginary part in rad/s).

The shaded area of the second quadrant complex plan that

corresponds to values of damping ξ equal or inferior to 5%

has been superposed to each graph. Eigenvalues in this area

have a too low damping factor and requires supplementary

damping measures such as PSS retuning etc. Some of the

simulated cases are presented hereafter in Fig. 6 to Fig. 11 for

peak and minimum load situations. In all cases the system is

stable.

It should be emphasized that the results presented here are

based on the model of Oman and UAE electricity systems

only. The systems of the other GCC countries are not

included. Recent records have shown that there are some

damped inter-area oscillations when Oman and UAE systems

are interconnected with the rest of GCC systems, e.g. Kuwait,

Bahrain, Qatar and KSA. These inter-area oscillations in

Oman and UAE systems are currently under investigation

jointly by the two parties.

VI. CONCLUSINS

The eigenvalues of the OETC and ENG system models

indicate an adequate damping before being interconnected.

After interconnection, the eigenvalues profile indicates that

the system remains stable and satisfactorily damped for the

considered power exchanges.

No clear inter-area oscillation mode between OETC and

ENG system appears. Rather, some damped oscillation modes

take place between OETC and ENG systems. In the

considered cases, the low frequency modes (characteristic of

long electrical distance inter-area oscillation between large

groups of generating units) appear damped (i.e. low amplitude

eigenvectors).

The damping remains adequate in N-1 contingency

conditions especially the most severe contingencies. These

results are rather logical due to the short electrical distances

between the OETC main generation centers and the

TRANSCO connected large generation centers.

Further small-signal stability analyses of Oman and UAE

systems are currently in progress including the effects of

interconnection with the rest of the GCC countries.

ACKNOWLEDGMENT

The authors are grateful to OETC and Transco Abu Dhabi for kindly providing data and use of results of the studies done by OETC to facilitate performing the interconnection study.

Fig. 6. Peak load – 0 MW Exchange, OETC and UAE system interconnected.

Fig. 7. Peak load – OETC imports 515 MW from ENG, System in N-1:

loss of circuit 1 of the interconnection lines.

Fig. 8. Peak load–OETC exports 760 MW towards ENG, System in N-1: loss

of circuit 1 of the interconnection lines.

REFERENCES

[1] Gulf Cooperation Council Interconnection Authority (GCCIA) website, http://www.gccia,com.sa

[2] OETC & Tractebel Engineering, “Oman – UAE interconnection itudies: Final report”, OETC, pp. 1-209, September 2010.

[3] Omar H. Abdalla, Rashid Al-Badwawi, Hilal Al-Hadi, Hisham Al-Riyami, Ahmed Al-Nadabi, Karim Karoui, and Ariadne Szekut, “Interconnection of Oman and UAE electric power systems”, Proceedings of the GCC Power 2011 Conference & Exhibition, Kuwait 20– 23, Paper No. A103, November 2011.

[4] O. H. Abdalla, S. A. Hassan, and N. T. Tweig, “Coordinated stabilization of a multimachine power system”, IEEE Trans. On Power Apparatus and System, Vol. PAS-103, No. 3, pp. 483-494, March, 1984.

[5] OETC: The annual five-year transmission system capability statement (2010-2014), pp.1-136, http://www.omangrid.com

[6] Al-Busaidi, and I. French, “Modeling of petroleum development Oman (PDO) and Oman electricity transmission company (OETC) power systems for automatic generation control studies”, Proceedings of the Intternational Conference on Communication, Computer, and Power, ICCCP’09, SQU, Muscat, Oman, 15-18 Feb., 2009. (Available online) IEEE Explore

Fig. 9. Minimum load – 0 MW Exchange, OETC and UAE system

interconnected.

Fig. 10. Minimum load – OETC imports 680 MW from ENG, System in N-1: loss of circuit 1 of the interconnection lines.

Fig. 11. Minimum load – OETC exports 658 MW towards ENG, System in N-1: loss of circuit 1 of one interconnector circuit.

[7] Oman Power & Water Procurement Company, “OPWP’s 7-year

statement 2011-2017”, pp.1-55, http://www.omanpwp.com.om

[8] O. H. Abdalla, Rashid Al-Badwawi, Hilal Al-Hadi, and Hisham Al-Riyami: “Steady-State and Dynamic Performance of Oman Transmission System with Diesel-Engine Driven Distributed Generation” Proceedings of the “46th International Universities Power Engineering Conference (UPEC 2011)”, South Westphalia University of Applied Sciences, Soest, Germany from 5th to 8th September 2011. (Available online) IEEE Explore

[9] Transco Abu Dhabi, “Five year electricity planning statement (2010-2014)”, Vol. 1, pp. 1-80, Dec 2009. http://www.transco.ae

[10] O. H. Abdalla, Hilal Al-Hadi, and Hisham Al-Riyami, “Development of a digital model for Oman electrical transmission main grid”, Proceedings of the 2009 International Conference on Advanced Computations and Tools in Engineering Applications), ACTEA, NDU, Lebanon, 15-18 July, 2009, pp. 451-456. (Available online) IEEE Explore

small-disturbance stability of oman/uae interconnected system

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