28

Interconnection of Oman and UAE Electric Power Systems

Omar H. Abdalla, Rashid Al-Badwawi, Hilal Al-Hadi,Hisham Al-Riyami, Ahmed Al-Nadabi, Karim Karoui,

Ariadne Szekut - Oman

GCC CIGRE - P.O. Box 697 Doha, Qatar Tel.: +974 44620497 Fax: +974 44620474

e-mail: gsgccc@cigre-gcc.org Website: www.cigre-gcc.orgPapers published in this proceedings represent the opinion & efforts of their authors without any responsibility towards the organizers

األوراق املنشورة في هذا اجمللد تعبر عن رأي وجهد مقدميها بدون أي مسؤولية للجهة املنظمة

22 - 24 November 2011 - Kuwait

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* P. O. Box 1224, P. C. 131, Al-Hamriya, Muscat, Sultanate of Oman, E-Mail: ohabdalla@ieee.org

Interconnection of Oman and UAE Electric Power Systems

Omar H. Abdalla*, Rashid Al-Badwawi*, Hilal Al-Hadi*, Hisham Al-Riyami*, Ahmed Al-Nadabi*, Karim Karoui** and Ariadne Szekut**

* Oman Electricity Transmission Company

(Sultanate of Oman) ** Tractebel Engineering

(Belgium)

Summary: The objective of this paper is to assess the impact of interconnecting Oman and United Arab Emirates (UAE) through the existing 220kV, 46.7km double circuit transmission line rated 762MVA for each circuit. A model is developed to represent both systems of Oman and UAE based on the EUROSTAG© software. Load flow and contingency studies are presented to evaluate the compliance of the interconnected system for security criteria N and N-1. Calculations are applied to two operating conditions (peak and minimum load) and for three energy exchange cases: no exchange and maximum exchanges in both directions. Short circuit studies are presented to show that breaking capacity of existing circuit breakers is not exceeded following the interconnection of the two systems, especially at the interconnector and neighboring substations. Transient stability studies are presented to demonstrate the intrinsic response of the interconnected system following the occurrence of large disturbances such as three-phase faults followed by the loss of faulted elements and the simultaneous loss of the largest generating units of both systems. The maximum power transfer limits between the two countries are determined through P/V curve calculations for import and export during peak and off-peak load conditions for N and N-1 contingency situations. These theoretical limits are significantly larger than the effective exchange capability. They give an indication of the margin with respect to the voltage collapse and loss of synchronism risk due to an excessive power exchange across the interconnector. The results indicate that both Oman and UAE systems could be operated satisfactorily with the existing 220kV interconnector.

Keywords: GCC Oman-UAE Interconnection, Load flow, Short circuit, Contingency, Voltage profile, Transient stability, Voltage stability.

1. INTRODUCTION There has been a considerable interest in the development of the GCC interconnection project during the recent three decades. Details of the progress of the GCC interconnection project can be found in the GCC Interconnection Authority (GCCIA) website [1]. Briefly, the project consists of three phases: Phase I: interconnecting the GCC North Grid which includes the State of Kuwait, Kingdom of Bahrain and State of Qatar and Kingdom of Saudi Arabia through a HVDC back-to-back interconnector. Phase 1 is operational. Phase II: interconnecting the GCC South Grid which consists of the independent systems in the UAE and the Sultanate of Oman. Phase III: interconnecting the GCC North and South Grids, thus completing the interconnection of the six Gulf States. Successful completion of phases III has been officially launched on the 20th of April 2011. The objectives of the GCC interconnection are to provide economical, operational and technological benefits to the GCC countries [1]. 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 previous study of the interconnection between Oman and Abu Dhabi transmission systems was performed in 2008 by Transco Abu Dhabi and eDF [2]. The results

30

showed that power exchanges between the two systems were limited due to weak transmission facilities at Al Wasit area on Oman side. Since then major transmission reinforcements have been implemented in Oman to cope with increasing demand and allowing significantly higher level of power exchange between Oman and UAE. This paper concentrates on the evaluation of the steady-state and dynamic performances of Oman transmission system when interconnected to the Abu Dhabi system [3] through the existing 220kV, 2x762 MVA, 46.7km transmission line between Oman and Abu Dhabi electricity systems. A detailed model of the transmission systems of Oman and UAE has been developed [3] to simulate the steady-state and dynamic system performances using the product grade software EUROSTAG©. Section 2 describes the methodology and the investigated phenomena. Section 3 describes the transmission system. Section 4 addresses modeling aspects of the Oman and UAE systems. Section 5 presents load flow and contingency analyses. Sections 6 presents short-circuit level calculations. Section 7 describes transient stability studies. Section 8 shows the maximum power transfer limits between the two countries. Section 9 describes briefly further studies including synchronization, system protection and defense plan. Section 10 summarizes the main conclusions.

2. METHODOLOGY 2.1 Study systems The study focuses on the year 2011 as an expected provisional first year of operation. This is justified as the impact of the interconnection on both systems especially the OETC one will be larger during the first year of operation. Later on, Oman and UAE power system will further develop. The impact of the interconnection on both systems will decrease or will be mainly influenced by the future interconnections between the two systems but also between the various GCCIA interconnected power systems.

2.2 Investigated phenomena and definitions The classical state of the art in power system analysis classifies the stability of power systems in different categories according to the behavior of the system. So the concepts of transient stability, small signal stability or frequency stability are defined. Each concept refers to a typical behavior of the power system and to typical disturbances, typical time frame. They are studied separately, but, in reality, the different kinds of dynamics are intermingled. The transient and frequency stability analysis are part of the so called dynamic stability. Transient stability is verified when the system is capable of bearing the consequences of a serious disturbance without incurring a loss of synchronism, and capable of returning to a steady state. The frequency stability is verified if,

following an active power unbalance, the frequency remains within the acceptable range. The power transfer limit concept is related to the interconnection and determines the maximum power that can be exchanged between the two interconnected systems (OETC and ENG) through the interconnector. It is assessed by the calculations of P/V curves and the maximum transmissible power is determined by the voltage collapse point (ΔV/ ΔP = or ΔP/ΔV = 0). This maximum is a theoretical point. Practically, the maximum power that can be effectively exchanged is lower to remain within the acceptable voltage range among other operational constraints. The short circuit analysis aims at the determination of the short circuit levels and checks that the fault currents remain compatible with the substation fault and breakers ratings when the two systems are operated interconnected. The impact of the interconnection is generally limited to the substations electrically very close to the interconnector. Also in the present study, the short circuit analysis is restricted to the 3-phased faults levels. In case of excessive single phase fault levels, the insertion of grounding impedance could solve the problem.

3. SYSTEM DESCRIPTION

3.1 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 [4]. Figure 1 shows a geo-schematic diagram of the main transmission system of Oman in 2011. The OETC system model is composed of three voltage levels: 220kV, 132kV and 33kV. In general, the lines are fitted with double circuit except for the interconnection with Petroleum Development of Oman (PDO) [5]. The substations of 220/132kV and 132/33kV present an arrangement of two transformers in parallel. The 33kV network is operated by the distribution companies. Only the 33kV primary substations pertaining to the 132/33kV transformation are represented in the model. Together with the downstream load, they are represented by an equivalent load model.

3.1.1 Power plants At the total, the OETC system supplied from eight power stations [6]: Rusail IPP (687MW) and Ghoubrah Power &

Desalination Plant (469MW) are situated in Muscat. Barka AES IWPP (434MW) and Barka SMN IWPP

(681MW) are situated in Al Batinah South region. Sohar IWPP (605MW) is situated in Al Batinah

North. Wadi Jizzi IPP (290MW) is situated in Al Buraimi. Manah IPP (279MW) is situated in Al Dakhliyah. Al Kamil IPP (297MW) is situated in Al Sharquiya.

31

Figure 1 : Oman main transmission system in 2011.

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. Generation dispatches for peak and off peak load situations have been determined to represent the Load Dispatch Center real practice.

3.1.2 Transmission system The existing transmission system consists of: 835 circuit-km of 220kV OH transmission lines 2970 circuit-km of 132kV OH transmission lines 12 circuit-km of 220kV underground cables 64 circuit-km of 132kV underground cables 6630 MVA of 220/132kV transformer capacity 9239 MVA of 132/33kV transformer capacity 150 MVA of 132/11kV transformer capacity Two 220kV interconnection grid stations Two 220/132kV grid stations Five 220/132/33kV grid stations Thirty eight 132/33kV grid stations One 132/11kV grid station

3.1.3 Distribution system and directly connected customers

The bulk of the power transmitted through the main grid, is fed, through 220/132/33kV, 132/33kV and 132/11kV 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 2010 the system peak demand of 3614MW occurred at 15:00 hours on the 1st of June. The minimum demand was 766MW occurred in winter at 03:00 on the 16th of January. A number of large private customers are connected directly to the transmission system either at 220kV or 132kV, 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

This line is not in operation in the present study

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Some customers have their own generation capability on site. For the peak situation 2011, it is expected that some of these customers inject electric power to the main transmission grid. 3.1.4 Shunt compensation In 2011, there are 630MVAr of capacitive shunt compensation installed in some grid stations at 33kV.

3.1.5 Oman-UAE interconnection line The interconnection is an existing 220kV, 46.7km transmission line between Al Foah substation in Abu Dhabi and Al Wasit (Mahadah) substation in Oman. The line consists of double circuits with twin Araucaria 821mm2 AAAC per phase. The thermal capacity of each circuit is 762MVA. The line is ready for operation upon finalizing the agreement between the two countries.

3.1.6 Security and operation criteria The N-1 security principle is applied in the OETC system for the 220kV and 132kV voltage level as planning and real time operational criteria. In some locations at peak load, the N-1 criterion cannot be fulfilled currently by OETC. The N-1 criterion cover the loss of a single line, a single transformer or a single generating unit (taking into account the reduced power of the steam turbine in case of loss of a gas turbine of a combined cycle) in the OETC system. The size of the secured generation loss incident for OETC corresponds to the loss of 220MW.

In order to cope with this partially satisfied criterion, it is proposed to replace the N-1 preventive security criterion by N-1 corrective security criterion. This implies that in case of contingency in area of the system where only the N criterion is satisfied, corrective measures such as generation rescheduling and/or load curtailment is required as it would have been the case without interconnection. 3.1.7 Frequency deviations Table II presents the frequency operating range requirements of the Oman grid code.

Table II: Frequency operating range – OETC

Conditions Frequency (Hz) Minimum Maximum

Normal operation 49.95 50.05 Exceptional condition 49.90 50.10 Disturbance (transiently) 48.00 51.50

3.1.8 Voltage deviations The grid code requires the voltage to remain inside a range of ±10% around the nominal voltage for the 220 and 132 kV voltage levels and ±6% around the nominal voltage for the 66kV, 33kV and 11kV voltage levels.

3.1.9 Specific operation constraints At peak load conditions all the generating units are connected and shared the total load according to their relative rated power. At low load conditions, at least one unit has to be online at each power plant location to secure adequate voltage controls and to respect water generation constraints, except in Wadi Jizzi. The most important plants regarding the voltage constraints are Al Kamil and Manah. The must-run constraints linked to water production are Al Ghoubrah, Barka AES and Sohar IWPPs.

3.2 Abu Dhabi system The Abu Dhabi power system is described in more details in [3] and [7]. Briefly, it is composed of the 400kV and 220kV voltage levels. Abu Dhabi Island has a meshed sub-transmission network operated with 132kV. 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 are 16 power plants connected to the Transco system totaling an installed capacity of approximately 12.3GW: Taweelah A1, A10, A2, B and Bext power stations

located in the center of the Abu Dhabi Emirate with a total installed capacity of 4.7GW.

Shuweihat, Madinat Zayed and Mirfa power stations located in the Western region with a total installed capacity of 1.9GW.

Sas Al Nakhel and Umm Al Nar power stations located in the Abu Dhabi Islands’ region with a total installed capacity of 2.5GW.

Al Ain power station located in the Al Ain region with a total installed capacity of 256MW.

Fujairah F1, F1 ext and F2 power stations are located in the Northern Emirates with a total installed capacity of 3GW.

There are six SVCs in operation in the Transco system: 2 x [-100;100] MVA at Mussafah 220kV substation; 2 x [-100;100] MVA at Al Ain South West 220kV

substation; 2 x [-45;90] MVA at Al Ain Power House 220kV

power station.

Their range is allocated partly (50%) to voltage control during steady state and partly (50%) to dynamic stability to enhance the voltage recovery after fault clearing.

33

4. SYSTEM MODELING 4.1 Synchronous Generators The OETC power system comprises 56 synchronous generators of a round-rotor type in the 8 power stations. The rating of these turbo-generators ranges from 13.4MVA for the smallest old unit to 280MVA 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.

4.2 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) [8]. 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.

4.3 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 [8]. 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).

4.4 Transformers The generating units in Oman power stations are connected to the 132kV or the 220kV transmission network through step up transformers. Auto transformers of 500MVA and 315MVA are used at the interconnection substations between the 220kV and 132kV transmission systems. At connection points with the distribution companies, 132/33kV two-winding transformers are used in the substations. Most of these transformers are 125MVA rating. In some smaller substations 63MVA, 40MVA or 15MVA ratings are used. Their 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.

4.5 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.

4.6 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 as shown in Figure 2. To match the load flow power factor, fixed shunt compensation is connected at the secondary of the service transformer.

Figure 2: Load model

The choices and related references for the adopted numerical values of the model are presented hereafter:

Transformer 33/11kV: The resistance and reactance are 0.01+j0.07 pu as per practical experience. Their loading is 50% and the secondary voltage is regulated at 1.03 pu. The transformers being rated at 11kV 6%, this assumes that the transformers are slightly compensating the voltage drop along the 11kV and 0.4kV OHL and cables.

Feeder: The full load voltage drop is 0.01 pu. The feeder impedance X/R ratio is 0.26. The adopted value of 0.26 is average value as the feeder includes two parts: 11kV and 0.4kV. It reflects mainly the 0.4kV impedance for which the X/R ratio are considerably lower than for MV and HV cables which is often higher than 1.

Motor: The proportion of motors is 75% and reflects the air conditioning share in the load mix. The motor loading is 100% during peak load periods. During winter periods, the loading decreases to 80% together with the number of AC units in service that is reduced by approximately 50%. The motor inertia is a best guess 0.5MWs/MVA. As per the literature, the inertia is mainly concentrated in the motor due to the relatively small inertia of the compressor. This trend is reinforced

M

Distribution feeder

InductionMotor Resistive

load

ance

Step-down transformer

34

by the separation between the compressor and the ventilation that take place in the split systems. The mechanical torque is the sum of a constant torque (75%) and a quadratic torque (25%). This is an approximation as a wide variety of load curves are observed.

5. LOAD FLOW AND CONTINGENCY STUDY

5.1 Objectives and methodology The objective is to evaluate the compliance of the interconnected system for to the security criteria N and N-1. Calculations are applied to two operating conditions (peak and light load) and for three energy exchanges levels: no exchanges (interconnection closed/opened) and maximum exchanges in both directions. The exchanges levels are determined by the generation increase in one side and a corresponding decrease in the other side. In case not enough generation capacity is available (it is the case of the OETC system in peak situation) the load is artificially decreased in order to export power while maintaining the voltage control at the main generation centers. This methodology is adapted to the determination of the power exchange capacity of the analyzed interconnector irrespective of the ability of the two systems to effectively be able to export the determined amounts of power exchange. The system adequacy is evaluated in N-1 for a set of critical contingencies: Loss of one circuit of the interconnection line; Loss of one heavy loaded circuit close located to the

interconnection; Loss of the largest generating unit of both systems

(220MW in Oman and 330MW in UAE)

5.2 Load flow and contingency results Isolated operation: In off peak situation both systems are adequate in N and N-1 conditions. At peak load, the results indicate that OETC system is adequate in N conditions. In N-1 conditions, some circuits become overloaded (125%) after the loss of the circuit in parallel. Dedicated reinforcements are planned in the near future. Till then, the OETC system would be operated with N-1 criterion partially respected and the interconnection maximum exchanges adjusted to satisfy a corrective N-1 criterion. The TRANSCO system satisfies the N and N-1 criteria except for one contingency (110 % of Imax). Interconnected operation: The maximum exchanges levels have been determined in both directions. The results are summarised in Table III. At the peak load, the maximum power flow from Abu Dhabi to Oman is constrained by the 132kV double circuit line between Al Wasit and Wadi Sa’a that reaches its full loading (100% of Imax) in N conditions. The maximum power flow from Oman to

Abu Dhabi is constrained by the loading of one circuit of the 220kV line between Al Wasit and Al Foah after the loss of the other circuit. At off peak-load, the maximum exchanges levels are determined by the loss of one interconnector. A sensitivity analysis has been performed to assess the impact of the planned 220kV line between Al Wasit and Ibri on the obtained exchanged levels. It indicates that the presence of this 220kV line would increase the OETC importing capacity from 515MW to 744MW, being limited only by the N-1 constraint over the interconnection lines.

5.3 Discussions of load flow and contingency analyses

The results of the load flow and contingency analysis suggest considering two criteria when defining the max exchange between the two systems: preventive N-1 in the parts of the system able to fulfill the N-1 criterion and corrective N-1 security in the parts of the system not able to fulfill the N-1 preventive criterion in islanded mode. This requires adequate alarm to be installed and corrective actions be implemented by system operator. They imply to reduce the import and increase the generation on the Western side of Oman or curtail the load supplied to Ibri, Dank and Al Hayl. Now, it is interesting to further investigate the real opportunities of power exchanges between Oman and UAE. To identify the times during which power can be exported from Oman to UAE, we refer to year 2010 actual generation and loading conditions. Figure 3 shows the recorded monthly generation and corresponding maximum and minimum loads during 2010. During all months (January to December) significant amount of power can be transferred from Oman to UAE at minimum loading conditions. At maximum loading conditions, available power could also be transferred, particularly during January, February, March, November and December. Figure 4 shows the daily available generation, maximum load, and minimum load during the week of the maximum demand in 2010 in Oman (Saturday 29-May to Friday 4-June). The load is high during working days (Saturday to Wednesday) and relatively lower during weekend days (Thursday and Friday) Thursdays are weekend days in Oman and working days in UAE, whilst Saturdays are working days in Oman and weekend days in UAE. Therefore, Oman could export power to UAE on Thursdays and import on Saturdays, e.g., on Thursday 3rd June 2010 Oman had more than 600MW that could have been exported to UAE.

Table III: Oman import and export capacity.

Oman Import/Export

Capacity (MW) At peak load At off peak load

Maximum import 515 680 Maximum export 750 658

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Figure 3: Actual generation, maximum load and minimum load during the year 2010 in OETC

Figure 4: Actual generation, maximum load and minimum load during the week of maximum demand in 2010 in OETC

6. SHORT-CIRCUIT ANALYSIS

6.1 Objectives and methodology of short-circuit analysis

The objective of the short circuit calculations is to check that breaking capacity of the existing circuit breakers is not exceeded following the interconnection of the two systems especially at the interconnector and neighboring substations. The short circuits calculations are performed, according to the IEC 60909 standard, in 2011 peak load conditions. The summer conditions are the most constraining due to the higher number of units in

operation leading to an increase of the short-circuit current.

6.2 Short-circuit results Obtained short circuit calculations indicate that: In 400kV: There is almost no increase in the fault current with the interconnection closed. The interconnection does not affect the 400kV short-circuit level in the UAE. In 220kV: An increase of the fault current is observed for the substations near the interconnection. The maximum short-circuit current in the 220kV of Oman remains around 15.7kA. The major increase is observed at Al Wasit (5.2kA). Two other substations that face an increase in short-circuit current in Oman are Al Wasit

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BB2 (2.93kA) and SIS (2.44kA). Again, the short-circuit currents remains well within the fault ratings of the switchgear in Oman. In the UAE, the maximum short-circuit current remains around 39kA. The highest increase amounts 0.95 kA at Al Foah substation. In 132kV voltage level: In OETC a limited increase of the short-circuit current is observed with a maximum increase of 2.76kA in Al-Wasit. All the short-circuit currents remain under the admissible short-circuit limit for Oman. The interconnection has no impact on the UAE short-circuit currents as no relevant increase is observed.

7. TRANSIENT STABILITY ANALYSIS 7.1 Objective and methodology of stability

analysis The objective of the transient stability study is to analyze the intrinsic response of the system following the occurrence of large disturbances. Especially the risk of cross border loss of stability is assessed. The calculations are performed through time domain dynamic simulations. The stability of the system is assessed for three phase faults cleared in base time (120ms) followed by the loss of the faulted element; and also, for the simultaneous loss of the two largest generating units of both systems. The following cases are studied. Fault studies: Three-phase faults incidents are simulated on the interconnection line, substations busbars and heavy loaded circuits of both systems. Generator outages: The two largest generating units connected to the TRANSCO and the OETC systems are simultaneously tripped. Critical Clearing Time (CCT): The CCT for the main busses of the interconnected system is also determined for faults located close to the power plants. On the Oman side, these busses are in 132kV - Al Kamil, Ghoubrah, Manah, Rusail and Wadi Jizzi and in 220kV – Barka and Sohar. On the Abu Dhabi side, the busses are in 220kV – Al Ain Power House, Mirfa and Umm Al Nar and, in 400kV, SAS Al Nakhel, Shuweihat and Taweelah. In order to assess the ability of the TRANSCO and OETC systems to remain synchronized, the transient stability assessment is performed considering a 100% impedance load type to filter the potential load related voltage stability problems.

7.2 Transient stability results The system is stable for all the simulated cases at peak load. During off peak situation, the system is also stable for all simulated cases. Some post contingency conditions present in some substations a slight under-voltage (just below 0.9 p.u.). Figure 5 shows the system response to a three-phase fault of 120ms at the OETC side of the 220kV interconnection line followed by faulted circuit outage.

Figure 5: Voltage, active and reactive power flow in the interconnection lines after 3ph-fault at Al Wasit. Except for the unit ST-98 of Taweelah Bext, that displays a low inertia, all machines present a CCT higher than 260ms as shown in Table IV.

7.3 Discussion of stability analysis The transient stability analysis demonstrates that the interconnected system is able to sustain three-phase faults cleared in base time without any loss of angular stability between the two systems. It is observed that the presence of the large portion of air conditioning (induction motors) in the load mix will most probably trigger local voltage instabilities that requires to be mitigated with dedicated under voltage load shedding (UVLS) or the installation of fast reactive power reserve.

8. MAXIMUM TRANSFER LIMITS 8.1 Objective and methodology The maximum transmissible power between OETC and TRANSCO systems across the interconnection has been performed.

0 4 8 12 16 20

-100

-0

100

s

MW

[C1] ACTIVE POWER : LINE 51ALW220-2607OHA -1 Unit : MW[C1] ACTIVE POWER : LINE 51ALW220-2607OHA -2 Unit : MW

0 4 8 12 16 20-0.0

0.5

1.0

s

p.u.

[C1] VOLTAGE AT NODE : 51ALW220 Unit : p.u.[C1] VOLTAGE AT NODE : 2607OHA Unit : p.u.

0 4 8 12 16 20

-100

-0

100

s

Mvar

[C1] REACT. POWER : LINE 51ALW220-2607OHA -1 Unit : Mvar[C1] REACT. POWER : LINE 51ALW220-2607OHA -2 Unit : Mvar

37

Table IV: CCT results for OETC and TRANSCO.

Peak Situation Three-phase short circuit at busses

OETC→UAE 658MW

No exchange

UAE→OETC 680 MW

132kV: AlKamil, Barka, Ghoubrah, Manah, Rusail, or Wadi Jizzi ˃300 ms ˃300 ms ˃300 ms

220kV: Sohar 270-275 ms (SPS ST1)

260-268 ms (SPS ST1)

275-280 ms (SPS ST1)

220kV: Al Ain Power House, Mirfa, or Umm Al Nar 400kV: Shuweihat ˃300 ms ˃300 ms ˃300 ms

SAS Al Nakhel 400kV 275-281 ms (Taweelah ST-98)

281-287 ms (Taweelah ST-98)

275-281 ms (Taweelah ST-98)

Taweelah 400kV 167-175 ms (Taweelah ST-98)

168-175 ms (Taweelah ST-98)

167-175 ms (Taweelah ST-98)

Table V: Maximum power transfer limits.

Cases Incident Peak load (MW) Off peak load (MW)

OETC→UAE UAE→OETC OETC→UAE UAE→OETC

N 1505 1445 1108 1110

N-1 Ckt-1-220kV line Al Wasit-Al Foah 1087 1067 863 876

These maximum values are theoretical limits. They give an indication of the margin with respect to the voltage collapse and loss of synchronism risk due to an excessive power exchange across the interconnector. Two values are determined corresponding to both directions in N and in the most impacting N-1 contingency situations. The transfer limit is reached by a global and slow change modification of the active generator power set points both sides of the interconnection. The resulting P/V curves and the voltage collapse point (ΔV/ΔP = or ΔP/ΔV = 0) are derived and corresponds to the maximum transmissible power.

8.2 Results of maximum transfer The P/V curves are presented in Figure 6 in peak load situation and in Figure 7 in the off peak load situation. The achieved values are presented in Table V and indicate that the exchange limits (see also values listed in Table III) are driven by thermal constraints and not stability constraints. The main maximum transfer limit limiting factor is the loss of one circuit of the interconnector. The off peak limits are lower than the peak limits because there is a lower number of units in service leading to a lower short circuit power and therefore a reduced ability to supply reactive power to the interconnector.

9. ADDITIONAL STUDIES In addition to the studies described here, other important studies have been performed to provide a complete picture on the feasibility of successful interconnection of the two systems [3]. These studies are briefly described below.

9.1 Small-disturbance stability The models of the Oman and Abu Dhabi systems indicate an adequate damping before being interconnected. After interconnection, the calculated eigenvalues profile indicates that the system will remain well damped for the considered power exchanges. Also, the damping remains adequate in N-1 contingency conditions.

9.2 Primary frequency control The interconnection of the two systems requires the definition of common rules for primary reserves and the determination of the expected contribution of both systems to the primary frequency control. The results have shown that the interconnection between the two systems permits to share the reserve especially the spinning reserve and its cost.

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(6-a): OETC exports towards ENG

(6-b): OETC imports from ENG

Figure 6: PV curves for peak load conditions

(7-a): OETC exports towards ENG

(7-b): OETC imports from ENG

Figure 7: PV curves for off peak load conditions.

0 500 1000 1500

0.4

0.6

0.8

1.0

p.u.

[OETC_UAE] ACTIVE POWER IN THE INTERCONNECTION Unit : MW [OETC_UAE] VOLTAGE AT NODE : 51ALW220 Unit : p.u.[OETC_UAE] VOLTAGE AT NODE : 2607OHA Unit : p.u.

0 200 400 600 800 1000

0.6

0.7

0.8

0.9

1.0

MW

p.u.

[OETC_UAE_N-1_1] ACTIVE POWER : LINE 51ALW220-2607OHA -2 Unit : MW[OETC_UAE_N-1_1] VOLTAGE AT NODE : 51ALW220 Unit : p.u.[OETC_UAE_N-1_1] VOLTAGE AT NODE : 2607OHA Unit : p.u.

0 200 400 600 800 1000 1200 1400

0.8

0.9

1.0

p.u.

[UAE_OETC] ACTIVE POWER IN THE INTERCONNECTION Unit : MW [UAE_OETC] VOLTAGE AT NODE : 51ALW220 Unit : p.u.[UAE_OETC] VOLTAGE AT NODE : 2607OHA Unit : p.u.

0 200 400 600 800 1000

0.4

0.6

0.8

1.0

MW

p.u.

[UAE_OETC_N-1_1] ACTIVE POWER : LINE 2607OHA -51ALW220-2 Unit : MW[UAE_OETC_N-1_1] VOLTAGE AT NODE : 51ALW220 Unit : p.u.[UAE_OETC_N-1_1] VOLTAGE AT NODE : 2607OHA Unit : p.u.

0 200 400 600 800 1000

0.2

0.4

0.6

0.8

1.0

p.u.

[OETC_UAE] ACTIVE POWER IN THE INTERCONNECTION Unit : MW [OETC_UAE] VOLTAGE AT NODE : 51ALW220 Unit : p.u.[OETC_UAE] VOLTAGE AT NODE : 2607OHA Unit : p.u.

0 200 400 600 800

0.4

0.6

0.8

1.0

MW

p.u.

[OETC_UAE_N-1_1] ACTIVE POWER : LINE 51ALW220-2607OHA -2 Unit : MW[OETC_UAE_N-1_1] VOLTAGE AT NODE : 51ALW220 Unit : p.u.[OETC_UAE_N-1_1] VOLTAGE AT NODE : 2607OHA Unit : p.u.

0 200 400 600 800 1000

0.2

0.4

0.6

0.8

1.0

p.u.

[UAE_OETC] ACTIVE POWER IN THE INTERCONNECTION Unit : MW [UAE_OETC] VOLTAGE AT NODE : 51ALW220 Unit : p.u.[UAE_OETC] VOLTAGE AT NODE : 2607OHA Unit : p.u.

0 200 400 600 800

0.2

0.4

0.6

0.8

1.0

MW

p.u.

[UAE_OETC_N-1_1] ACTIVE POWER : LINE 2607OHA -51ALW220-2 Unit : MW[UAE_OETC_N-1_1] VOLTAGE AT NODE : 51ALW220 Unit : p.u.[UAE_OETC_N-1_1] VOLTAGE AT NODE : 2607OHA Unit : p.u.

N-1: ckt 1 220 kV line Al Wasit – Al Foah

N state

N state

N-1: ckt 1 220 kV line Al Wasit – Al Foah

N state

N-1: ckt 1 220 kV line Al Wasit – Al Foah

N state

N-1: ckt 1 220 kV line Al Wasit – Al Foah

39

Table VI: Power flow variation in the interconnection lines in peak load following secured event occurrence.

Peak Situation Incident OETC→UAE 750MW

No Exchange

UAE→OETC 515MW

Loss of Barka GT1+1/2 ST1 = (230 MW) ΔP = 182MW ΔP = 193MW ΔP = 200MW Loss of Taweelah-B GT95+1/3 ST98 = (340 MW) ΔP = 38MW ΔP = 41.25MW ΔP = 40MW Table VI shows the power flow variation in the 220kV interconnection line at peak load following occurrence of generation outages. The corollary is that the proportion of the power import following a generation loss in an area grows with the ratio between the total interconnected generation capacity and the area generation capacity. This imposes to keep a provision for the primary frequency control.

9.3 Synchronization process The simulations results have shown that both systems are able to withstand the synchronization process with large difference of voltage amplitude (10% of the nominal voltage in 220kV), voltage angle (30°) and frequency (0.6Hz). The following synchro-check device settings permit to perform a satisfactory synchronization with an adequate margin and a limited stress imposed to the system: Max Voltage difference ΔV = 5 % = 11kV, where

VOETC>VUAE; Max Voltage angle difference Δφ = 10°; Max Frequency difference Δf = 200mHz.

9.4 System protection and defense plan A large number of severe contingencies have been simulated to locate the coherent group of generators following a loss of synchronism. These have been achieved by the simulation of three-phase faults cleared in backup time close to 220kV substations leading to the loss of two circuits or one 220kV circuit and one transformer. A large number of cases lead to the loss of synchronism of few generators that should be detected and tripped by their out of step protections. In some situations the system splits into two coherent groups of generators: The two groups correspond to the two OETC and

TRANSCO systems. The mitigating measure is the installation of a loss of synchronism protection on the interconnector

The two groups do not always correspond to the Oman and Abu Dhabi systems. In some cases, the Sohar and Wadi Jizzi power stations remain coherent with the TRANSCO system. The mitigation measure would imply to split the Oman system through a dedicated system protection scheme. It implies also to be able to resynchronize the two systems at the splitting locations.

10. CONCLUSIONS

Load flow and N-1 contingency analysis have been performed on peak and light load conditions of year 2011 on a power system model of OETC (Oman) and the ENG system. The latter is composed of a detailed representation of TRANSCO (Abu Dhabi) and an equivalent representation of the other systems composing the ENG interconnection. Obtained results indicate that the N-1 criterion cannot be fulfilled during the whole year especially the 132kV corridors between Al-Wasit and Ibri that is operated in N conditions only. Imposing the N-1 criterion would restrict significantly the import capabilities of the OETC system. The maximum exchange capabilities have therefore been determined in N-1 in the TRANSCO system and with a partially fulfilled N-1 criterion in the OETC system. Under these conditions, the maximum gross exchange capabilities of the interconnector (i.e. the transfer reliability margins not included) are: At peak load 515MW (OETC import) – 750MW

(OETC export); At off-peak load 680MW (OETC import) – 658MW

(OETC export).

The 750, 680 and 658MW limits are imposed by the thermal capability of the interconnector in N-1. The 515MW limit is imposed by the N criterion that is not anymore respected on the 132kV double circuit corridor Al-Wasit – Wadi-Sa’a. The presence of Wadi-Jizzi and Sohar power plants on the OETC side and the 400/220kV transformers at Dahma permit to control the voltage and should permit to control and limit the exchange of reactive power at the various loading conditions. The scheduled power exchange must permit the normal operation of the primary frequency control. This implies that a transfer margin should be provisioned to permit the adequate power transfer following the loss of the largest unit in both systems leading to the net exchange capabilities. Those margins amount: At peak load ΔP = 200MW on the OETC importing

side leading to 315MW net import capability; At off peak load ΔP = 0MW on the OETC exporting

side when the two interconnector circuits are in operation and ΔP = 42MW when only one interconnector circuit is in operation leading to 750MW respectively 708MW net export capability.

The completion of the 220kV Al-Wasit – Ibri corridor will remove the OETC import bottleneck, thus allowing

40

operating the OETC-ENG interconnection up to its thermal limits. The three-phase faults short circuit levels have been calculated. Obtained results indicates that the impact of the interconnection on the fault level will remain limited in amplitude, localized around the interconnection substations and will remain compatible with the breaker equipment’s capabilities. Maximum transfer capacities have been calculated across the interconnector in N and N-1 conditions. Obtained results indicate that the active power stability limits are much larger than the obtained thermal limits (i.e. 1500MW (N) and 1070MW (N-1) in comparison to 750MW at maximum). This confirms that the interconnector will be mainly driven by thermal constraints rather than stability and synchronizing torque constraints. Finally it is worth noting that, a new 400kV, 400MW GCC interconnector is currently being considered between Sewihan 400kV grid station in UAE and Mahadah (Al Wasit) 220kV grid station in Oman. The project consists mainly of a double circuit transmission line between Sewihan and Mahadah and a 400/220kV grid station at Mahadah. Initial studies of this interconnection are described in [9]. The operation of this project is expected to be in 2014, thus completing the GCC interconnection project. The results presented in this paper and those presented in [9] have shown that the interconnection of Oman and UAE systems either through the existing 220kV or through the new 400kV GCC interconnector will provide economical, operational and technological benefits to Oman and UAE, and ultimately all GCC countries. This will improve security of supply and system reliability.

11. ACKNOWLEDGEMENT The authors are grateful to Transco Abu Dhabi for kindly providing data of the UAE system to perform the interconnection study.

12. REFERENCES [1] Gulf Cooperation Council Interconnection Authority (GCCIA)

website: http://www.gccia,com.sa. [2] Transco Abu Dhabi and eDF: Study Operational Network

Studies Final Report - OMAN Interconnection, Transco Abu Dhabi, June 2008, pp. 1-263.

[3] OETC & Tractebel Engineering, Oman – UAE Interconnection Studies: Final Report, OETC, September 2010, pp. 1-209.

[4] OETC: The Annual Five-Year Transmission System Capability Statement (2010-2014), pp.1-136, http://www.omangrid.com.

[5] Al-Busaidi, and I. French: Modeling of petroleum development Oman (PDO) and Oman electricity transmission company (OETC) power systems for automatic generation control studies, Proc. Int. Conf. on Communication, Computer, and Power, ICCCP’09, SQU, Muscat, Oman, 15-18 Feb., 2009. (Available online) IEEE Explore

[6] Oman Power & Water Procurement Company, OPWP’s 7-Year Statement 2011-2017, http://www.omanpwp.com.om

[7] Transco Abu Dhabi, Five Year Electricity Planning Statement (2010-2014), Vol. 1, pp1-80, Dec 2009. http://www.transco.ae.

[8] 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.

[9] O. H. Abdalla, Rashid Al-Badwawi, Hilal Al-Hadi, and Hisham Al-Riyami: Performance of Oman Transmission System with the 400kV Gulf Cooperation Council Electricity Interconnection, (Proceedings of the 2011 IEEE GCC Conference), Dubai, United Arab Emirates, 19-22 February 2011. (Available online) IEEE Explore.