Helwan University

From the SelectedWorks of Omar H. Abdalla

October 27, 2019

Technical Evaluation of 400 kV InterconnectorBetween North and South Grid of OmanHisham A. Al-RiyamiAdil G. Al-BusaidiMusabah N. Al-SayabiM. H. Al-HasniA. Szekut, et al.

Available at: https://works.bepress.com/omar/74/

Technical Evaluation of 400 kV Interconnector Between North

and South Grid of Oman

H. A. S. Al Riyami, G. Kh. Al Busaidi, M. N. Al Sayabi, M. H. Al Hasni

Oman Electricity Transmission Company (Muscat, Sultanate of Oman)

A. Szekut, J. Dubois, R. Bicskei, R. Fahmi,

Tractebel Engineering

(Brussel, Belgium)

O. H. Abdalla,

Faculty of Engineering, Helwan University

(Cairo, Egypt)

Summary:

The objective of this paper is to assess the technical

feasibility studies of a new 400 kV interconnection

between the following power systems in Oman: (i) the

Main Interconnected System (MIS) of the Northern

Region of Oman, (ii) the Petroleum Development

Oman (PDO) System, (iii) Dhofar system, and (iv) the

isolated system in Duqm. Three different options for

connecting the MIS, PDO, Dhofar and Duqm systems

are presented. The evaluation studies include steady

state analyses (load flow, short-circuit, voltage profile,

contingency, and power transfer capability) in addition

to transient analyses (rotor angle stability, voltage

stability and oscillation damping studies). Technical

criteria of the Transmission Security Standard and Grid

Code are considered. The best option of the 400kV

interconnector is identified based on the steady-state

and transient studies.

Keywords: 400 kV Interconnector of Oman,

Maximum Power Transfer, Technical Criteria,

Transmission Security Standard

1. INTRODUCTION Oman Electricity Transmission Company (OETC) owns

and operates the Main Interconnected System (MIS) and

the Dhofar system, located in the north and the south

regions of Oman, respectively [1]. At the moment, these

two systems are interconnected via the 132 kV network

of PDO (Petroleum Development Oman). Besides the

synchronous system, Oman also has isolated networks

which are locally fed by diesel generators in Duqm and

Mahout Areas.

A Memorandum of Understanding (MOU) has been

signed between OETC, Oman Power and Water

Procurement Company (OPWP) [2] and PDO for the

future interconnected 400 kV system of Oman between

MIS-PDO-Dhofar and the isolated region of Duqm. The

aim of the MOU is to provide economical and technical

potential of the interconnection.

Technical analyses play a key role in the interconnection

of different networks to make sure that no violation of

network constraints, considering the technical criteria of

Transmission Security Standard [3] and Grid Code [4].

Technical analyses include power flow study in normal

and N-1 conditions; short-circuit fault studies, total

transfer capability determination and voltage stability as

well as dynamic stability studies. These analyses

evaluate the impact of interconnection on power systems

to assist system planners in their decision making

process. The simulation studies are performed by using

the DIgSILENT software [5].

2. SYSTEM DESCRIPTION

A. Main Interconnected System & Dhofar

System

The MIS has three HV operating voltages, i.e. 400 kV, 220 kV and 132 kV. The MIS extends across the whole of northern Oman and interconnects bulk consumers and generators of electricity located in the Governorates of Muscat, Batinah South, Batinah North, Dhahirah, Buraimi, Dakhliyah, Sharquiya South and Sharqiya North. The MIS is currently interconnected with the system of the United Arab Emirates (UAE) via 220 kV link and with PDO via 132 kV link. While in Dhofar 132 kV system is employed [1].

Figure 1: Electrical networks of Oman.

A. Petroleum Development Oman System

The PDO transmission system is composed of 132 kV

lines and has an extension of near 800 km from north to

south [6]. The network is interconnected to the MIS

through a 132 kV single circuit line between Nahada

and Nizwa. The PDO system is also interconnected with

Dhofar system through 132 kV single circuit line

between Harwell and Thumrait [7].

B. Duqum system

Duqm system has one power station and ten 33/11 kV

step-down substations, The Duqm Power Station is

composed by nine diesel generators with a total

installed capacity of 66.326 MW.

C. Mahout system

Mahout system has one power station that feeds three

main subsystems. Eight diesel generators compose the

main Power Station with a total installed capacity of 9.6

MW.

3. TECHNICAL CRITERIA

A. Voltage Criteria

The voltage range in the pre-contingency state at the

220 kV and 132 kV transmission levels is ±5% and in

the post contingency state is ±10%. At the 400 kV, the

voltage range in the pre-contingency state is 2.5% and

in the post-contingency state is ±5% as shown in [3].

Table I: Voltage Criteria

Nominal Voltage Pre Contingency

Voltage Range (kV)

Post Contingency

Voltage Range (kV)

400 kV 390 to 410 (±2.5%) 380 to 420 (±5%)

220 kV 209 to 231 (±5%) 198 to 242 (±10%)

132 kV 125.4 to 138.6 (±5%) 118.8 to 145.2 (±10%)

B. Thermal Criteria

The Transmission Security Standard (Condition 26 of

the OETC Transmission and Dispatch Licence) [3]

states that under prevailing system conditions and also

following a fault outage all system equipment shall be

within the relevant technical limits. The criteria deals

with the loading of the equipment in the healthy (N)

and in the post contingency single event (N-1, N-1-1)

states, assuming that the normal loading limits of the

equipment shall not be 100% or higher.

However, the applicable emergency overloading based

on the Transmission Master Plan for transformers and

overhead line reaches 115% and 110% respectively [8].

Mahout

MIS

PDO

Dhofar

Duqm

Interconnection

Option 1

Petroleum Development Main Interconnected System

Dhofar Duqm

Mahoot

Interconnection

Option 2

Interconnection

Option 3400kV Izki 400kV Izki 400kV Izki400kV Ibri IPP 400kV Ibri IPP 400kV Ibri IPP

400kV Nahadah 400kV Nahadah 400kV Nahadah

40

0k

V

Barik

40

0k

V

Ma

ho

ot

40

0k

V

Su

wa

iha

t

40

0k

V

Su

wa

iha

t

40

0k

V

Su

wa

iha

t

40

0k

V

Du

qm

40

0k

V

Du

qm

40

0k

V

Du

qm

40

0k

V

Harw

ee

l

40

0k

V

Harw

ee

l

40

0k

V

Harw

ee

l

400kV Ittin400kV Ittin 400kV Ittin

C. Fault Clearing Times Criteria

The main protection schemes and settings applied shall

ensure that fault clearance times (from fault inception to

circuit breaker arc extinction) of faulted lines and

equipment on the 400 kV, 220 kV and 132 kV systems.

The fault clearance time shall be no greater than 100 ms

at 400 kV and 120 ms at 220 kV and 132 kV [4]. The

fault clearance time for different voltage levels are

shown in Table II.

Table II: Fault Clearing Times

Voltage (kV) Main Protection Clearing Time (ms)

400 kV 100

220 kV 120

132 kV 120

D. Voltage Stability Criteria

Acceptable transient voltage recovery following fault

clearance is critical to prevent motor stalling and other

harmful effects on loads. The voltage stability criteria

require that the voltage after a fault should exceed:

• 0.7 p.u within 0.5 seconds after fault clearance;

• 0.8 p.u - 0.9 p.u by the end of a 10 second period

As required in the OETC TSS, three phase faults are to

be simulated to evaluate transient instability and the

associated critical clearance time on the transmission

system [3].

E. System Damping

The criterion defined in OETC TSS states that a system

is correctly damped if the resultant peak deviations in

machine rotor angle and/or speed after a 20 second

period of the outset event remain under 15% of the peak

deviations at the outset [3]. Figure 2 illustrates the

requirements for system damping.

Figure 2: Damping criteria.

F. Short Circuit Levels

The maximum three-phase and single-phase short-

circuit fault levels should be determined in accordance

with IEC 60909 and shall be maintained within

equipment fault rating [9].

4. FEASIBILITY STUDY

The Oman North to South 400 kV Interconnector

Transmission feasibility study has identified three

possible options for the 400 kV interconnector in order

to connect MIS (via Ibri and New Izki) to PDO (via

Suwaihat, Barik, Harweel and Nahadah) as well as

Dhofar System (via Ittin), Duqm area and Mahout area.

The configuration of three options summarized in the

following:

The first option considers the minimum investment required to interconnect new 400 kV grid station at Duqm to new 400 kV grid station at Suwaihat and to reinforce the existing 132 kV transmission infrastructure between PDO and Dhofar systems via Harweel and Ittin grid stations to 400 kV system. In addition, a new 400 kV at Nahadah will be connected via loop in-out connection from existing Ibri-New Izki 400 kV line.

The second option uses the same infrastructure proposed in the first and in addition, it considers a complete 400 kV backbone running next to PDO 132 kV transmission lines. The interconnection will be supported by three intermediate substations at Barik, Suwaihat and Harweel.

The third option also uses the same infrastructure proposed in first option and in addition, it considers complete 400 kV backbone running near the coast of Oman with three intermediate substations at Mahout, Duqm and Harweel as shown in Figure 3.

Figure 3: Three potential options of 400kV

interconnector.

100%

<15%

In order to carry out technical analyses, different

assumptions have been taken into consideration: peak

demand for all systems shall be considered for 2021 year,

and generation plan for the same year. Figure 4 illustrates

peak demand and available generation plan until 2030,

where there is surplus in generation planned capacity

from 2016 to 2024; however it is expected to have deficit

in generation planned capacity from 2025 onwards. The

400 kV double circuits overhead lines are Quad Yew 400

kV type with 1773 MVA thermal rating at each circuit

and due to the long distances involved, these options

required shunt compensation designed to compensate the

generated reactive power of the lightly loaded elements

as shown below. The shunt compensation will be placed

at both sides of the 400 kV double circuits to absorb the

reactive power compensation generated at worst

condition when no power is transferred. In addition,

some options might require series compensation to

reduce voltage drops along the line and to increase the

transfer capability of the interconnection.

Figure 4: Available Capacity versus Demand.

Table III: New 400kV OHL and shunt Compensation

400 kV

OHL

Length

(km)

Option

1

Option

2

Option

3

Compensation

(MVAr)

Nahada-Barik

181 x 4 x 65

Barik-

Suwaihat 127 x 4 x 45

Suwaihat-

Harweel 281 x 4 x 100

Harweel-

Ittin 144 x x x 4 x 50

Suwaihat-Duqm

165 x x x 4 x 60

Izki-

Mahout 250 x 4 x 90

Mahout-

Duqm 156 x 4 x 55

Duqm-

Harweel 379 x 4 x 135

5. RESULTS

The system model [10] has been updated to include

all new expations.

A. Short circuit levels

Short circuit analyses have been carried out for all

busbars in terms of single phase and three phase faults.

The results indicated that the fault levels are maintained

within rating of busbars and some busbars faults can be

reduced by applying busbar splitting method. Table IV

shows the results on critical busbars.

Table IV: 3Ph and 1Ph fault levels

Busbar Rating

(kA)

Option 1 Option 2 Option 3

3PH 1PH 3PH 1PH 3PH 1PH

Ik" (kA)

Ik" (kA)

Ik" (kA)

Ik" (kA)

Ik" (kA)

Ik" (kA)

220kV

Misfah 50 39.6 44.6 40.3 45.1 40.5 45.3

220kV SIS

40 36.9 34.7 36.9 34.7 36.9 34.7

220kV

Sur PS 50 39.0 46.5 39.5 47.0 39.6 47.1

132kV Harweel

25 14.6 16.0 20.2 21.0 19.1 19.9

B. Voltage profile

The voltage profile has been carried out and results

indicated that the voltage is maintained well within the

voltage criteria deviations mentioned previously for all

three options. Samples of voltage profile are depicted in

Figure 5.

Figure 5: Samples of voltage profile.

0

100

200

300

400

500

600

700

800

900

1000

Po

we

r fl

ow

at

Vo

ltag

e C

om

plia

nce

lim

it (

MW

)

Option 1 Option 2 Option 3

C. Contingency analysis

The load flow and security analyses assessed the whole

power system of Oman including MIS, PDO, Dhofar,

Duqm and Mahout subsystems and the 400 kV

interconnection. The security assessments are performed

based on the detailed criteria and corrective actions after

the contingency events. The assessment results show that

some violations occurred in PDO system specifically in

N-1 condition of 132 kV lines under option-1 only. As

option-1 does not offer 400 kV backbone network

through the middle of Oman, Therefore, the power flows

through the 132 kV network, inherently yields to

overloading of a few lines e.g. 132 kV Amal Power

Station -Amin-1 and NimrW-Amin-1 as shown in Table

V. These lines require reinforcement in terms of an

additional 132 kV parallel circuits.

In case of the MIS, for interconnection Option 1 and

interconnection Option 2 the 220 kV Misfah- Airport

Heights circuit and Airport Height power transformer

has minor overloading. However, the overloading is

below the applicable emergency overloading limits.

Therefore, no reinforcement is required in the MIS.

Further, it is worth to mention that Dhofar, Duqm and

Mahout Subsystems have no violations.

Table V: Critical contingencies

System Contingency Overloaded

Element

Option 1

Option 2

Option 3

Loading post

incident (%)

Loading post

incident (%)

Loading post

incident (%)

PDO

132kV AMIN-AMIN2

132kV NimrW-AMIN-1

119.6 N/A N/A

Amal Power Station -AMIN-2

Amal Power

Station -AMIN-1

133.4 N/A N/A

MIS

220/132kV 2x500MVA

Airport Height (1)

220/132kV 2x500MVA

Airport Height TX

101.8 102 102.5

220kV OHL Airport Height-

Misfah(1)

220kV OHL

Airport Height-Misfah

100.9 104.6 N/A

D. Maximum Transfer limits

The power transfer between two subsystems can be

increased to such a value that there is a binding security

limit such as voltage compliance limits, thermal limits

and voltage collapse. The limits were assessed by adding

fictitious loads at LV side of 400/132 kV transformers. In

the importing region (sink), a positive load with 0.95

lagging power factor and in the exporting regions

(source), a negative load with unit power factor are

added. During Simulation, variation of the fictitious

loads forces the power flow from exporting region to

importing region until there is a violation in above

mentioned limits and thus the maximum transfer limit is

determined [11]. Figure 6 shows power flow at voltage

compliance limit for the three options.

Figure 6: Power flow at voltage compliance limit for the

three options.

E. Transient stability

Transient stability is ensured when the system can

withstand the consequences of a severe disturbance, such

as a fault without loss of synchronism, and to return to

steady state. The calculations concerning the stability of

the system with respect to short circuit disturbances are

performed through time domain dynamic simulations.

Based upon the interconnection arrangements, critical

contingencies are simulated as following:

Three-phase faults are simulated for 100 ms at

400 kV interconnection busbars.

Three-phase faults are simulated for 100 ms at

50% length of the 400 kV interconnection

overhead lines followed by the tripping of the

same circuit.

The amount of graphics from results generated is

massive; however one simulated case would be presented

as following:

Fault at 400 kV circuit between MIS-PDO

The example case considers the interconnection option

2. Three-phase fault is simulated for 100 ms at the 50%

length of the Nahada-Ibri IPP 400 kV interconnection

overhead line followed by the tripping of the same

circuit. The three-phase fault was applied at 1 second,

immediately after the fault clearance the same single

circuit was tripped. The analysis ran for 30 seconds, all

the generator rotor angles and the 400 kV voltages are

monitored.

0

100

200

300

400

500

600

700

800

900

1000

Po

we

r fl

ow

at

Vo

ltag

e C

om

plia

nce

lim

it (

MW

)

Option 1 Option 2 Option 3

Figure 1: Power Flow at Voltage Compliance Limit for Three Options

0

100

200

300

400

500

600

700

800

900

1000

Po

we

r fl

ow

at

Vo

ltag

e C

om

plia

nce

lim

it (

MW

)

Option 1 Option 2 Option 3

Figure 2: Power Flow at Voltage Compliance Limit for Three Options

Figure 7: Voltage of the interconnection 400 kV

busbars.

Figure 8: MIS rotor angles option 2.

Voltage depression occurs during the three-phase fault

and swiftly after the transient event it recovers and

stabilizes within the voltage planning criteria. The 400

kV interconnection busbar voltages are depicted in

Figure 7.

The rotor angles of the MIS’ generators during the

simulation are depicted in Figure 8. The fault occurs

close to Ibri IPP; therefore the swings with the highest

amplitude are experienced at that location. The steam

turbines of Sur power plant are representing the reference

machine, thus their rotor angle remain constant.

The generators in PDO and Dhofar networks also

respond to the disturbance. All generators remained

stable throughout the dynamic simulation as shown in

Figure 9 and Figure 10 respectively.

In summary; the voltage stability and generators’ rotor

angles stability for the three 400 kV interconnection

options have been simulated and the interconnected

system is found to be stable. In all cases, the oscillations

are well damped respecting the criteria defined in the

TSS.

Figure 9: PDO network rotor angles option 2.

Figure 10: Dhofar network rotor angles option 2.

6. CONCLUSION AND

RECOMMENDATIONS

The paper presents an overview of the technical analysis

for the three options of the 400 kV MIS-PDO-Dhofar-

Duqm interconnector in steady state and dynamic

conditions. Based on the 2021 year data, the results

carried out for all potential options in light with

contingencies using DIgSILENT Power Factory

software. In terms of transfer capacity limits, option 2

provided the highest transfer among the three options.

Further, the static analysis showed that in healthy state

the system voltages are all within the limits stipulated by

the planning standards of the different subsystems. In

addition, the N-1 security analysis concluded that

reinforcements are required in PDO network under

option 1, however these reinforcements are not required

if option 2 or 3 is selected as the majority of the power

is transferred to the 400 kV circuits.

Moreover, short circuit assessment indicted that single

phase and three phase faults are maintained within the

rating short circuit for all busbars. In the transient

stability analysis, the dynamic behavior of the integrated

Omani system was assessed by the simulation of faults

at the busbar and line for the three interconnection

options. Voltage stability and generators’ rotor angle

stability were evaluated and found to be stable for the

simulation cases in both 2020 off-peak and 2021 peak

regimes. The system is sufficiently damped.

Considering the above studies and simulation results, the

paper recommends option 2 as the preferred option from

a technical point of view.

7. REFRENCES

[1] Oman Electricity Transmission Company, Five-

Year Annual, Transmission Capability Statement

(2018-2022), 2017. (Available online):

http://www.omangrid.com.

[2] “OPWP 7-Year Statement 2018-2024,” Oman

Power and Water Procurement Company,

(Available online): http://www.omanpwp.co.om.

[3] Transmission Security Standard, Oman

Electricity Transmission Company, July 2016.

(Available online): http://www.omangrid.com.

[4] Connection Conditions the Grid Code for the

Sultanate of Oman, Oman Electricity

Transmission Company, May 2016. (Available

online): http://www.omangrid.com.

[5] “PowerFactory DIgSILENT User Manual,”

http://www.digsilent.de.

[6] A. 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, Sultan Qaboos University,

Muscat, Oman, 15-18 Feb., 2009.

[7] Tractebel Engineering S.A., “Evaluation and

Utilization of MIS-Duqm-PDO-Dhofar 400 kV

Interconnector," Belgium, 2017.

[8] Oman Electrical Master Plan Study (2014-2030),

Oman Electricity Transmission Company, 2016.

[9] International Standards IEC 60909, International

Electrotechnical Commission.

[10] O. H. Abdalla, Hilal Al-Hadi, and Hisham Al-

Riyami: “Development of a Digital Model for

Oman Electrical Transmission Main Grid”, Proc.

of the 2009 International Conference on

Advanced Computations and Tools in

Engineering Applications, ACTEA, pp. 451-456,

Notre Dame University, Louaize, Lebanon, 15-18

July, 2009. (Available online) IEEE Explore.

[11] H. A. S. Al Riyami, A. G. Kh. Al Busaidi, M. N.

Al Sayabi, M. H. Al Hasni, A. Szekut, J. Dubois,

R. Bicskei & R. Fahmi," Assessment of

Maximum Power Transfer limits of Oman North

to South 400 kV Interconnector Transmission

Project," in GCC Cigre 2018-Kuwait.