Helwan University From the SelectedWorks of Omar H. Abdalla October 27, 2019 Technical Evaluation of 400 kV Interconnector Between North and South Grid of Oman Hisham A. Al-Riyami Adil G. Al-Busaidi Musabah N. Al-Sayabi M. H. Al-Hasni A. Szekut, et al. Available at: hps://works.bepress.com/omar/74/
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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.
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].
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.