Journal of Power and Energy Engineering, 2013, 1, 1-7
http://dx.doi.org/10.4236/jpee.2013.14004 Published Online September 2013 (http://www.scirp.org/journal/jpee) 1
Technical and Economical Merits of Power Systems
Interconnection
Abdullah M. Al-Shaalan
Electrical Engineering Department, College of Engineering, King Saud University, Riyadh, KSA.
Received August 13th, 2013; revised September 12th, 2013; accepted September 20th, 2013
Copyright © 2013 Abdullah M. Al-Shaalan. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.
ABSTRACT
This work aims at exploring the effects of the interconnection between isolated electric power systems upon some im-
portant aspects such as enhancing reliability levels along with reducing installation and operation costs. To discern the
advantages associated with this study, the developed methodology has been applied to three existing power systems in
the northern region of the Kingdom of Saudi Arabia presently within the concession domain of the Saudi Electric
Company (SEC). These systems have been established to meet the present loads and to withstand future electrical de-
mands for a period of time before additional g eneration and transmission reinforcements are needed. In this work, reli-
ability measures have been utilized to determine the period that these systems can fulfill the present and future loads
without affecting the reliability levels and the threshold that an additional capacity should be added to maintain those
required reliability levels. In application to the reliability criteria, technical, operational and economic advantages can
be realized, i.e., higher reliability levels and lower installation and operation costs after the proposed interconnection
between these selected isolated power systems take place.
Keywords: Reliability Index; Power System; Cost; Interconnection
1. Introduction
Interconnection of electrical power systems is an effec-
tive means of not only enhancing the overall system re-
liability but also reducing its operating reserve. The di-
versity existing between different systems in regard to
their load requirements and capacity outages will allow
the systems to assist each other in times of emergencies
and generation deficiencies. Also, it will permit systems
to operate in less reserve than what would be required for
being isolated at a given risk level. The benefits that may
accrue from systems interconnection depend mainly
upon the operating reserves in the individual systems,
their outage rates, the tie-line capacity and the type of
agreement among the systems regarding the size of the
emergency assistance.
From the preceding section, it is evident that reliab ility
is one of the most important criteria which must be taken
into consideration during all phases of power system
planning, design and operation. Reliability criterion is
required to establish target reliability levels and to, con-
sistently, analyze and compare the future reliability lev-
els with feasible alternative expansion plans. This need
has resulted in the development and applications of reli-
ability evaluation and modeling methodologies.
In power system reliability evaluation, the most widely
used reliability ind ex by electric co mpanies is th e Loss of
Load Expectation (LOLE). This index can be used for
specifying the appropriate timing for future capacity ad-
ditions as well as for comparing between various alterna-
tive expansion plans. It indicates the average power out-
ages time accumulated in a specified period (for example,
number of days in one year) due to capacity deficit. This
research will, also, demonstrate other complementary
reliability indices that are—to the author’s knowledge—
given less consideration in present realistic and applied
cases. These indices are the “Expected Demand Not Serv-
ed (EDNS)”, the “Expected Energy Not Served (EENS)”
and the “Energy Index of Reliability (EIR)”.
2. Survey to Some Existing Works on Power
Systems Interconnection
In this section, an overview of some existing works is
attempted to get acquaintance with other countries’ ex-
periences and practices in systems interconnections. The

Technical and E c o n o mical Merits of Power Systems Interconnection
2
author in [1] proposed a flexible AC power transmission
link technology for linking two asynchronous independ-
ent power systems. The proposed flexible asynchronous
ac link (FASAL) system essentially consists of a rotating
transformer which is put in the ac tie-line between two
separate power systems or grids. The direction and the
magnitude of power flow are controlled by controlling
voltage and/or frequency. Simulink model of proposed
FASAL system has been developed for the analytical
study and the result verifies the power transfer capab ility
of the proposed system.
In Reference [2], the author suggested that for evalua-
tion of the synchronous interconnection of the African
regional power pools interconnection, security issues
should be taken care of before any economic analysis is
done. Power system security studies have been used in
industries only at the time of planning. This paper also
addresses a future real time security system in operation
of liberated power pools.
In [3], the author stated the advantages, requirements
and problems arising fro m the interconnection of electric
power systems in the Arab world. He also presented the
planning principles for grid systems, procedures for
power system development planning, technical aspects of
interconnection and possibilities of increasing intercon-
nection capacity by limited means. The results of studies
investigating the feasibility of interconnecting the power
networks of the six countries, which form the Gulf Co-
operation Council (GCC), were presented. The author
also showed existing and future interconnections of po-
wer systems in the Arab world.
In [4], the author emphasized the need, especially in
developing countries, for consolidating the dispersed
electric utilities in the isolated regions as a prerequisite
for future interconnecting these reg ions via local national
grids and with other neighboring countries.
In [5], the author presented methodologies and tech-
niques that can be adopted and used in the quantitative
assessment of power system reliability and its application
to the cost/benefit evaluation in system generation ex-
pansion planning particularly for interconnected power
systems.
In [6], the authors proposed a PMU (phasor measure-
ment unit) based on monitoring and estimation scheme of
power system small-signal stability in Singapore-Malay-
sia interconnection power system through a 50 Hz and
500 kV transmission line. Two PMUs are installed in the
power system interconnection network of Singapore-
Malaysia. One PMU is located in Singapore and the
other one is in Malaysia (Penang). Both PMUs measure
the single-phase voltage phasor. The data filtering tech-
nique based on Fast Fourier Transform (FFT) was em-
ployed to extract oscillation data for single mode. Finally,
some analysis results of monitoring and estimation of
Singapore-Malaysia interconnected power system based
on application practice of the Campus-WAMS were pre-
sented and analyzed.
3. The Developed Methodology for the
Proposed Study
For the purpose of this study, a computer program has
been developed (Figure 1) which can model and simu-
late the methodologies and techniques adopted and
demonstrated in this work. The following steps describe
the principal operations of this developed computer pro-
gram.
3.1. When Systems Are Isolated
1) Specifying the data relevant to each system under
study.
2) Building the Capacity Outage Probability Table
(COPT) for each system, using the Forced Outage Rates
(FOR’s) pertinent to each generating unit in each system.
3) Convolving the COPT with the Load Duration
Curve (LDC) for each individual system.
4) Evaluating the risk index level “Loss-of-Load-Ex-
pectation (LOLEe)” for each individual system and then
comparing it with the risk index level prescribed by the
Management (LOLEp).
5) If the LOLEe exceeds the LOLEp, extra unit(s) must
be added to the existing system capacity until the risk
index level, prescribed by the management, is satisfied,
otherwise proceed to the next year and repeat the same
process with the new forecasted load.
6) Evaluating other complementary and essential indi-
ces such as the Expected Demand Not Served (EDNS),
Expected Energy Not Served (EENS) and the Energy
Index of Reliability (EIR). These indices are used as evi-
dent indicators to enable the system planner to discern
between the planning outcomes and hence select the
most reliable and least cost among them. These indices
are utilized to vindicate the economic and technical mer-
its of interconnecting power systems rather being dis-
persed and isolated.
7) Assessing and estimating the overall system cost
based on the added generating unit(s) to the system at
every stage of the planning horizon.
3.2. When Systems Are Interconnected
1) Specifying the data relevant to each system under
study.
2) Building the Capacity Outage Probability Table
(COPT) for the combined systems (i.e. combine capacity
states with their associated probabilities).
3) Repeating the above mentio ned steps (c-g). The ca-
pacity assistance CT is a capacity which can be trans-
erred through the tie-line and added to the existing f

Technical and E c o n o mical Merits of Power Systems Interconnection
3
(a) (b)
Figure 1. Flowchart processes of the adopted computational algorithm. (a) Isolated systems; (b) Interconnected systems.
capacity model of the assisted system. For the evaluation
of the risk index level, the assisted system proceeds as in
the case of the isolated system risk index evaluation.
4. Application of the Developed Methodology
to Real-Case Systems
The previous modeling techniques have been substanti-
ated and then implemented on real and practical electric
power systems serving three large cities in the northern
region of the Kingdom of Saudi Arabia, namely, Hail,
Jouf and Tabouk and designated in this study as (A), (B)
and (C) respectively. These cities are characterized by
high population density, multiple of governmental, in-
dustrial and agricultural projects, so, high future demand
is anticipated. Recently, the existing power systems in
these cities have been transferred to the domain of the
Saudi Electric Company (SEC) that has been established
in 1999.
The generation expansion planning considered for this
study spans over the nex t eight years (2012 -2020) to spe-
cify the timings of capacity additions and to determine

Technical and E c o n o mical Merits of Power Systems Interconnection
4
the reliability levels for each system before and after the
proposed interconnection. The target is to explore and
investigate the technical and economic merits that might
ensue as a result of systems interconnection.
4.1. Technical Advantages of Systems
Interconnection
The aim of this study is to specify the reliability levels
for each system individually as a result of future load
growth over the next eight years (with the assumption of
no units addition to the system) and the expected dete-
rioration of reliability levels as a result of diminishing
reserve and capacity deficit. After specifying the year
that reliability level has exceeded the prescribed reliabil-
ity level, capacity addition (new generating units) can be
decided upon or interconnection with another system can
be an optional solution.
Figure 2 shows the results of the study that has been
conducted on the three systems: (A), (B) and (C) to in-
vestigate their reliability levels as being isolated over the
next eight years using the LOLE criterion. It is clear from
the figure that if the required level of the LOLE is set at
0.1 days/year, all systems will exceed that prescribed risk
level. Therefore, new generating units must be added to
each system to improve their risk levels and avoid power
outages and service interruptions.
After adding concept of systems interconnection, the
study has been conducted and the results are shown in
Figure 2 where it can be realized that reliability levels
have been improved after system interconnection. For
instance, system (A) will need no capacity addition until
the year 2014, and with respect to systems (B) and(C)
they will exceed their reliability limits at years 2015 and
2016 respectively.
To demonstrate and ensure the benefits of intercom-
nection between systems in enhancing and improving
their reliability levels, the effect of interconnection upon
other complementary reliability indices stated previously,
i.e. EDNS, EENS and EIR has been investigated. System
(A), being the largest among the three systems, has been
selected for this demonstration. Figure 3 shows the re-
sults of the investigation that declares to what extent the
size of the expected demand not served (EDNS) has been
reduced after systems interconnection. Figure 4 also
shows the reduction in the expected energy not served
(EENS) due to the effect of interconnection. The EIR
index was also investigated and the results are shown in
Figure 5 where it is obvious from the figure that the en-
ergy loss has decreased and reliability indicato r has risen
to the better level after interconnection between the sys-
tems.
4.2. Economic Advantages of Systems
Interconnection
The purpose of this part of this study is to assess the
economic advantages that may yield as a result of inter-
connection between the above mentioned three systems.
A study, which spans for eight years (2012-2020), has
been conducted. The LOLE index has been specified for
the three systems and set at 0.1 d/y and to be constant
over that selected planning period (this number is con-
sidered to be reasonable as being ad opted in industrial as
well as fast developing countries). The study was con-
cerned about that threshold year that reliability will dete-
riorate (i.e. LOLE exceeds the prescribed specified limit)
as a result of future load growth. To correct the risk level
and maintain it within the prescribed limit set by the
management, additional generating units must be added
(in case of system being isolated) or should be intercon-
nected with other system(s) after the latter is reinforced
Figure 2. Systems LOLE levels be for e and afte r interc onne c t ion.

Technical and E c o n o mical Merits of Power Systems Interconnection 5
Figure 3. EDNS before and after interconnection.
Figure 4. EENS before and after interconnection.
Figure 5. EIR before and after interconnection.

Technical and E c o n o mical Merits of Power Systems Interconnection
6
by new generating units. For estimating the cost of the
added units before and after interconnection, the proc-
esses shown and described in Section 4 have been util-
ized. Figure 6 portrays the findings of this study where
they postulate that each system will benefit from being
interconnected and there will be substantial savings [in
Million USD (MUSD)] in both capital (fixed) and oper-
ating (variable) costs. This can be estimated as 56%, 50%,
and 60% for systems (A), (B) and (C) [i.e. Hail, Jouf and
Tabouk] respectively. Furthermore, there will be an im-
provement in the three systems reliability levels as ana-
lyzed and discussed in the preceding section.
4.3. Optimal Reliability Level
Evaluation of optimal reliability levels is a major step in
power system planning process to ensure continuous and
quality service with reasonable cost. The syste m planners
perform sensitivity analysis based on economic varia-
tions, installation and transmission costs. Therefore,
LOLE reliability index has been applied for system (A)
electric system using the economic concepts and the re-
liability criterion shown in Type equation here [4,5]. In
the analysis, new generating un its of 6 8 MW (identical to
the present installed units in system A) have been added
to the system when reliability levels deteriorate below
the prescribed level. To arrive at the most appropriate
range of reliability levels, system cost (SC) has been
weighted with the outages cost (OC). System cost in-
cludes unit installation cost as well as the fuel and main-
tenance cost. Outages cost represents the cost of losses
suffered by the society (all classes of customers) due to
insufficient capacity and consequently, energy curtail-
ment. The total system cost (TSC) depicts the overall
cost endured by the customers in return of power supply
and its availability.
In an attempt to arrive at the most optimal reliability
level that ensure the least system cost, the above men-
tioned costs have been investigated employing system
(A). The results of the investigations are illustrated by
Figure 7 where it manifests that system cost (SC) in-
creases as reliability level increases but the outage cost
(OC) decreases as a result of reliability improvement due
to more system investment and adequate generating ca-
pacity additions. The most optimal range of reliability
levels, as depicted by the figure, varies between 0.338
and 0.675 days/year. However, in some cases adding new
capacity may not signify the ideal solution to meet in-
creasing future loads and maintain better reliability lev els.
Therefore, it is better to enhance operating unit’s per-
formance through regular preventive maintenance. Also,
it is an imperative to establish a good co-operation be-
tween the supply side (electric company) and the demand
side (the consumers) through well-coordinated load ma-
nagement strategies, improving system load factor and
correction of power factor. Hence, system will be capa-
ble of meeting loads efficiently and reliably particularly
in power system interconnection.
5. Conclusion
In this study, reliability and costs criteria have been es-
tablished and applied for power systems planning and
interconnection. To substantiate the developed method-
ology, three electric power systems in the northern region
of the Kingdom of Saudi Arabia have been selected as a
Figure 6. Cost (MUS$) for isolated and interconnected systems.

Technical and E c o n o mical Merits of Power Systems Interconnection 7
Figure 7. Variation of reliability levels with total system
cost.
model for this study. These systems, at the present time,
are isolated, dispersed and subjected to random failures.
The results display the advantages and benefits that may
accrue from systems interconnection in upgrading reli-
ability levels, reducing systems cost, limiting service
cease and mitigating energy interruptions.
6. Acknowledgements
The Author thanks Al-Zamil Chair for Electricity Con-
servation, College of Engineering, King Saud University
for supporting this work.
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