Abstract—This paper describes the development of a digital computer model representing the main electric transmission system in Oman. A commercially available DIgSILENT power system computation package is employed. A brief description of system is given, which includes generating units, transformers, transmission lines and loads. The system is connected with a number of internal industrial plants to exchange electric power in addition to the interconnection between Oman and UAE.

The model is used to perform computer studies such as load flows, short circuit analysis, stability, transient responses, etc. The simulation results are used for evaluating system performance and strategic planning purposes. Sample results are presented to show the capabilities of developed model.

I. INTRODUCTION LECTRICITY utilities over the world perform studies by using digital models to simulate the

electric power systems. These models are based on computer packages with versatile facilities to represent large scale power systems, including generators, turbines, exciters, transformers, transmission lines, loads and various control devices [1] – [8]. Historically, DC and AC network analyzers were employed by electricity utilities for studies such as load flow and short circuit analyses. Small-scale experimental power systems containing micro-generators where used by utilities and universities for dynamic and transient studies [9], [10]. A micro-generator is a small (3-5 kVA) synchronous machine specially designed to represent large generators. The parameter values of this small machine in per unit are similar to those of large generators. The main advantage of these small-scale real systems is the possibility of testing the performance of new devices in real time.

The above analog devices have been replaced by digital computer models representing large and more complex modern power systems. Since the beginning of digital

Manuscript received June 1, 2009. Prof. O. H. Abdalla is on leave from the University of Helwan,

Egypt. Since January 2007, he is with Oman Electricity Transmission Company, P. O. Box 1224, P. C. 131, Al-Hamriya, Muscat, Sultanate of Oman, (phone: 00968-92649686; fax: 00968-24493416; e-mails: ohabdalla@ieee.org)

H. Al-Hadi and H. Al-Riyami are with the Strategic Planning and Studies Section, Oman Electricity Transmission Company, P.O. Box 1224, P.C. 131, Al-Hamriya, Muscat, Sultanate of Oman, (e-mails: alhadi@omantel.com.om, hisham40142@ieee.org).

computers age, software programs have been developed and extensively used in the field of power system studies. In the early days, utilities and universities used to develop simulation programs by using common programming languages such as FORTRAN. Recently, commercially available professional software packages [1] – [8] are used for power system studies by electricity utilities over the world. These programs have all necessary functions required for various studies, planning, design and operation of modern electric power systems.

This paper presents the development of a digital computer model which represents the main transmission system in Oman, using a commercially available power system computation package. The model is used for the purposes of system planning and studies. The main transmission system extends over an area of circa 130 000 km2 in the Northern part of Oman [11]. The software package is briefly described in section II. The main transmission system is described in section III. Section IV describes system modeling and section V presents system studies and some samples of results.

II. SOFTWARE PACKAGE Oman Electricity Transmission Company (OETC) uses

a commercially available professional software package called “PowerFactory”. It is a DIgital SImulation and Electrical Network calculation program abbreviated as “DIgSILENT’. Details can be found in [4], [12]. Briefly, it is an interactive computation program used for power system planning and studies including steady-state and dynamic analyses. The main functions include load flow, short circuit, reliability, protection, optimization, stability among others. The network can be graphically represented by a single-line diagram entered through the mouse and keyboard. It can be interactively edited and the results are shown on the screen. Various data handling, documentation techniques, and interface for computer-based mapping systems are available.

A data manager is provided to simplify managing and browsing data in the object oriented database. The program stores the data as sets, e.g. Type and Element data set. When data is entered in the basic data set, it can be directly used for any calculation functions available in the program, e.g. load flow, short circuit, stability, etc. The program has a built-in library containing standard models and typical parameter data for basic power system elements. A user library can be established for the study system containing selected and new defined models.

Development of a Digital Model for Oman Electrical Transmission Main Grid

Omar H. Abdalla, Senior Member, IEEE, Hilal Al-Hadi, and Hisham Al-Riyami Member, IEEE

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ACTEA 2009 July 15-17, 2009 Zouk Mosbeh, Lebanon

978-1-4244-3834-1/09/$25.00 © 2009 IEEE 451

Muscat

Al Batinah South

Al Batinah North

Ad Dhahirah

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Al-Dakhiliah

Al Wusta

UAE

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Alkamil

JBB Ali

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Khaburah

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Sumail

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Filaj

Barka

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Seeb Main

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Fig. 1. Main Electricity Transmission System of Oman.

III. SYSTEM DESCRIPTION The existing 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 Batinah, Dhahirah, Dakhliyah and Sharquiya [13]. Fig. 1 shows a geo-schematic diagram of the system. It has two operating high voltages, i.e. 220 kV and 132 kV.

After the electrical sector was unbundled; Oman Electricity Transmission Company (OETC) has taken over the operation of transmission system on the 1st of May 2005. The present OETC transmission system consists of:

686 circuit-km of 220 kV overhead transmission lines

2837 circuit-km of 132 kV overhead transmission lines

12 circuit-km of 220 kV underground cables 14 circuit-km of 132 kV underground cables 5630 MVA of 220/132 kV transformer capacity 6970 MVA of 132/33 kV transformer capacity Two 220 kV interconnection grid stations Two 220/132 kV grid stations Four 220/132/33 kV grid stations Thirty one 132/33 kV grid supply point substations

The transmission system is interconnected at 220 kV

from Al-Wasit in Mahadah with the transmission system of the United Arab Emirate and through this interconnection it will form part of the GCC Grid that links the electricity supply systems of Oman, Kuwait, Saudi Arabia, Bahrain, Qatar and the UAE. This should provide increased security of supply and benefits to the member countries in the form of cost savings from the sharing of reserve capacity and energy resources. The interconnector will be brought into service when the Inter-Governmental agreement is signed.

The main transmission system is supplied with electricity generated from eight gas-based power stations located at Ghubrah (482MW), Rusail (684MW), Wadi Al-Jizzi (290MW), Manah (279MW), Al-Kamil (282MW), Barka AES (434MW), Barka SMN (683MW) and Sohar (590MW) [14]. In addition, the transmission system may be supplied from direct customers such as Sohar Aluminium Company and Petroleum Development of Oman (PDO) [15].

The bulk of the power transmitted through the main grid, is fed, through 220/132/33 kV and 132/33 kV grid stations, to the three distribution licence holders, i.e. Muscat Electricity Distribution Company, Mazoon Electricity Company and Majan Electricity Company. In addition to the distribution companies 11 large private customers are directly connected to the transmission system.

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In 2008 the system gross peak demand of 3139 MW occurred at 15:00 hours on 31 May, which was an increase of 13.2 % from 2007 peak demand. During 2008 the regulated units transmitted over the main grid was 13837.6 GWh; compared with 11380.5 GWh in 2007, resulting in a remarkable growth of about 21.6%.

IV. SYSTEM MODELING

A. Synchronous Generators The OETC system comprises 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 an 8th order dynamical model [16], [17] based on the two-reaction theory and Park’s transformation. The stator has three windings ABC, and the round rotor has the field winding in the d-axis. Also, it is assumed that the rotor has one damper winding in the d-axis and two damper windings in the q-axis. The program allows representation of the magnetic saturation effects in the simulation.

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). The program library contains various types of turbine standard models including IEEE representations [18] – [21]. The user can select the appropriate model and insert parameter values. Alternatively if a particular model is available, it can be used. Fig. 2 shows the selected models of the turbines and speed governors. The GAST model is used to represent all gas turbine and speed-governor systems, in the main grid, either open cycle or combined cycle types. The IEEE SGO model is used to represent steam turbine prime mover systems. These include conventional separate steam turbines or that part in a combined cycle configuration. 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. The objective is to achieve maximum efficiency.

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. The program provides IEEE representation models for individual types of excitation systems including automatic voltage regulators [22]. Fig. 3 shows the block diagram of the selected excitation system models. 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).

(a) GAST gas turbine and speed governor model

(b) IEEE SGO steam turbine and speed governor model

Fig. 2. Prime movers and speed governor models

(a) IEEE Type AC1 model.

(b) IEEE Type ST1 model.

Fig. 3. Block diagram of excitation systems.

The third type comprises a thyristor converter to drive

the synchronous generator field directly. The converter is

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fed from the generator terminals through a transformer. The IEEE Type ST1 model is used to represent the static excitation system and the brushless ET type, with different parameter values.

D. Transformers The generating units in the 8 power stations are

equipped with step-up transformers connecting the generators to the corresponding 132 kV or the 220 kV transmission network. 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 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. The program allows modeling of three-phase power transformers in positive and negative sequence forms with equivalent circuit parameters in ohms or in pu. The models include representation of the magnetization reactance and iron loss admittance in addition to the leakage reactances and winding resistances. O-load tap changers with their automatic control facilities; and off-load tap changers can be simulated in the transformer model. The representations include various connection types and vector groups of transformers. Earthing transformers with associated earthing resistors are also simulated in the model.

E. Transmission Lines Transmission lines can be simulated by either lumped

parameters or distributed parameters models. The selection depends mainly on the line length and the nature of required studies. For steady-state calculations, such as load flow or short-circuit analyses, lumped parameters are normally used. The OETC system comprises 53 double-circuit transmission lines; most of them are overhead lines and only few are cables. The majority of these lines are within the short length range; only few are in the medium length range. Therefore, lumped-parameters π-equivalent circuit models are used to simulate the lines. The program accepts transmission line parameter data (series resistance, series reactance, shunt capacitance, shunt admittance) per km. alternatively; tower configuration and conductor type can be used.

F. Loads An electric power system normally includes residential,

industrial and commercial main load types. These may be represented as constant P & Q loads, voltage dependant loads or dynamical loads [23]. Induction motors represent a major portion of the total load. Selection of load representation method depends on the objective of the study and availability of accurate data. The software program allows selection of load representation type. In the studies presented here, the load at each substation is represented as constant P and Q model.

G. Shunt Capacitors A number of 132/33 kV substations are equipped with

capacitor banks at the 33 kV load side to provide reactive power and voltage support. The capacitor banks are arranged in a number of groups called steps (1 to 4); each has a capacity of 5 MVAr. They are connected in shunt at the 33 kV busbar and set to power factor control mode.

V. SYSTEM STUDIES

A. Demand Forecast According to Transmission and Dispatch License,

OETC shall ensure that the forecasts of electricity flows and loading on each part of the transmission system are consistent with the prevailing electricity demand forecasts prepared and used by the Oman Power and Water Procure Company (OPWP). The OPWP maximum demand forecast [14] is based on data supplied by the three distribution companies (i.e. Muscat EDC, Mazoon EC and Majan EC) and includes demand forecasts for the major customers that are, or will be, directly connected to the OETC transmission system at either 220 kV or 132 kV in their respective supply areas. The DISCOs demand forecasts are provided in terms of:

• The growth in major industrial loads. • The growth in major tourism loads. • The annual domestic growth. • The growth in agriculture loads. • The growth in other major loads.

The OPWP maximum demand forecasts relate to the

totals for each distribution company and do not indicate how this load is allocated to the individual grid supply points. The OETC forecast, however, is required to identify the future demand at individual grid stations at the system peak for each year over the study period, since this information is required for the power flow studies. Also, the minimum demand has been estimated for the study period and was found to be about 18% of the peak demand.

B. Power Flow and Voltage Profiles The extreme cases of maximum and minimum system

demand are used to define the performance of the OETC transmission system when it is most heavily stressed, and when it is operating at minimum load. From the response of the system to these extreme conditions it is possible to identify where the system is strong and has capacity to spare and, conversely, where it is weak and will need strengthening before new connections could be considered. The peak demand case identifies where there is a risk of overloading, of operation above firm capacity, or of system voltages falling below the voltage limits as defined in the Grid Code [24]. Table 1 summaries sample of the load flow results concerning percentage loading on transmission lines at system peak demand.

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TABLE I Sample of Percentage Line Loadings at Peak Demand

NA Not Applicable (1) Circuit rating is 82 MVA (2) Circuit rating is 89 MVA (3) Overhead line (4) Underground cable line (5) Circuit rating is 101 MVA The studies show that all 220 kV lines satisfy the (N-1)

transmission security standard except the SPS-SIS line. The percentage load on each circuit of this line will exceeds the firm capacity of 50% at the peaks of 2011-2013. Therefore, a plan has been set to construct an additional double circuit transmission line before the peak of 2011 to reduce the loading on each circuit and bring the lines in firm. In addition the new line will contribute in evacuating the power generated from the new planned power station at Sohar.

Also, the results listed in Table I show that some of the existing 132 kV lines are out of firm loaded. These lines are: Al Wasit-Burimi, Dank-Al Hail, Ibri-Dank and Mawalih-Seeb Main. Therefore, a plan has been set to upgrade the capacity of conductors of these lines to curry 261 MVA, thus maintaining (N-1) security standard.

The voltages at all busbars are calculated at both peak demand and minimum demand. The calculated voltage profiles at all busbars are within the allowable range.

C. Short Circuit Studies Three-phase and single-phase short circuit studies have

been performed to determine the fault level at each busbar in the transmission network. The results have shown that prospective fault level at three buses can exceed the rated values, especially single-phase short-circuit currents. Alternative solutions to this problem have been

investigated by using the software network model. One idea is to split the HV busbars at concerned power stations. The generators at Rusail and Ghubrah power stations are divided into two groups in order to reduce the number of parallel generators which can feed the short circuit at the particular busbar. Table II lists the results of the three-phase levels in kA before and after splitting the busbars. Table III lists the results of the single-phase short-circuit studies. The results indicate that the splitting arrangements can significantly reduce the fault level at concerned busbars to be within the design rating of the switchgear.

TABLE II SAMPLE OF 3-PHASE SHORT CIRCUIT LEVELS IK” (KA)

Grid Station Before Splitting

After Splitting

220 kV Busbar, 40kA Fault Rated MSQ 15.2 13.5 132 kV Busbar, 31.5kA Fault Rated Rusail 40.7 NA Rusail Busbar - A NA 23.9 Rusail Busbar - B NA 17.6 Ghoubrah 33.0 NA Ghoubrah Busbar - A NA 14.6 Ghoubrah Busbar - B NA 19.8 MSQ 31.8 20.8 Bousher 28.1 14.7 Wadi Adai 25.8 18.5 Wadi Al Kabir 17.0 13.5 Mawallah 21.6 12.8 Mobellah 30.3 22.3 Seeb 14.9 10.1 Jahloot 09.5 08.2 Falaj 22.5 16.8

TABLE III SAMPLE OF 1-PHASE SHORT CIRCUIT LEVELS IK” (KA)

Grid Station Before Splitting

After Splitting

220 kV Busbar, 40kA Fault Rated MSQ 14.7 13.2 132 kV Busbar, 31.5kA Fault Rated Rusail 48.3 NA Rusail Busbar - A NA 27.9 Rusail Busbar - B NA 21.1 Ghoubrah 40.3 NA Ghoubrah Busbar - A NA 18.2 Ghoubrah Busbar - B NA 23.4 MSQ 36.1 24.8 Bousher 29.5 17.4 Wadi Adai 28.6 22.0 Wadi Al Kabir 17.4 14.8 Mawallah 22.0 14.5 Mobellah 29.4 23.4 Seeb 14.2 10.7 Jahloot 10.2 09.2 Falaj 23.7 19.0

D. Transient Analysis Various types of transient analyses can be performed

by using the developed model. These include transient stability, transient response to network faults such as short circuits, line outages, generating unit tripping, etc.

Line Circuit Loading at Max Demand (%) 2009 2010 2011 2012 2013

220 kV Lines, 762 MVA Circuit Rating Al Wasit-SIS NA NA 11.08 12.92 18.10 Airport Heights-Filaj NA 43.20 49.07 41.68 28.72 Airport Heights-MSQ NA 38.70 42.44 34.47 21.36 Filaj-MIS 14.58 06.59 NA NA NA Filaj-Blue City NA NA 08.76 09.37 15.99 Blue City-MIS NA NA 10.62 10.92 13.53 MIS-SIS 05.34 18.84 16.07 20.95 21.48 SPS-SIS 26.32 46.77 54.47 68.59 70.44 SPS-SIA (A) 11.94 09.80 16.40 16.66 16.20 SPS-SIA (B) NA NA 05.07 08.72 09.99 Jahloot-Sur NA NA NA 13.70 14.62 132 kV Lines, 261 MVA Circuit Rating Al Kamil-JBB Ali 22.36 24.09 27.84 25.33 29.26 Al Kamil-Mudharib 18.47 24.15 29.95 28.62 28.80 Al Kamil-Sur 22.01 20.05 38.56 01.04 05.47 Al Wasit-Buraimi(1) 66.66 76.22 NA NA NA Al Wasit-Buraimi NA NA 27.59 30.20 32.74 Dank-Al Hail(2) 41.02 19.43 NA NA NA Dank-Al Hail NA NA 08.01 08.27 08.34 Filaj-Muladha 13.47 11.75 12.49 14.47 26.55 Ghubrah- MSQ(3) 29.22 NA NA NA NA Ghubrah- MSQ(4) NA 16.04 23.84 16.74 43.19 Ibri-Dank(2) 26.45 52.33 NA NA NA Ibri -Dank NA NA 19.19 15.29 14.85 Izki- Mudabi 19.18 25.86 49.07 10.94 15.36 Izki-Nizwa 18.78 15.98 25.18 NA NA Mawellih-Seeb Main(5) 72.61 NA NA NA NA Mawellah-Seeb Main NA 28.18 29.76 31.71 32.24 MIS-Khabourah 13.39 04.41 07.60 03.87 13.03

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Fig. 4 shows the rotor angle responses due to tripping of a 300 MW generating unit at the planned Barka III power plant which is expected to be commissioned in 2012. The studies help in selection of unit size and predicting future system performance. G1 and G2 are the steam-turbine and gas-turbine generators at Sohar Power Station. Other transient system studies have been made, but due to space limitation are not included in this paper. 24 12 0.0 2.5 5.0 Time (s)

Fig. 4 Generator rotor angle responses to 300 MW unit tripping.

VI. CONCLUSION The paper has presented the development of a digital

model for the main transmission system of Oman based on a commercially available power system simulation package. The model has been successfully used in present and future system performance studies, thus facilitating planning of the power grid. Simulation results have been presented to show the present model capabilities. These include both steady state (load-flow & short-circuit) and transient analyses. Some problems have been identified such as over-firm line loadings and prospective higher short-circuit levels. Practical solutions to these problems have been investigated. Proposed solutions include upgrading the conductors of loaded transmission lines and splitting the busbars at some power stations. Further improvements in the model capability are in progress to facilitate other system studies such as contingency and optimization techniques.

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G1 Rotor Angle (deg)

G2 Rotor Angle (deg)

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