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RESEARCH PAPERS
INCREASING THE POWER TRANSFER CAPABILITY OF EXISTING
EHVAC LINES USING SIMULTANEOUS AC-DC TRANSMISSION
By
* Department of Electrical and Electronics Engineering, Eritrea Institute of Technology, Eritrea.
** Dairy Engineering, Sri Venkateswara Veterinary University, India.
Date Received: 16/06/2020 Date Revised: 08/07/2020 Date Accepted: 07/09/2020
SUNIL KUMAR JILLEDI * SHALINI J **
ABSTRACT
It is difficult to load long extra-high voltage (EHV) ac lines to their thermal limits as a sufficient margin is kept against
transient instability. The model proposed in the paper, it will be feasible to load these lines near to their thermal
permissible limits. This paper gives us the feasibility of converting a double circuit ac line into composite ac–dc power
transmission line, without constructing a separate dc line, to get the advantages of parallel ac–dc transmission for
improving stability and loadability of a transmission line. There is no need for the alteration of conductors, insulator strings,
and towers. An analytical model is established for the loadability and transient stability analysis of the simultaneous AC-
DC transmission system. The validation of these models is carried out by comparing the results obtained from the
application of the models with already published results. Simulation has been carried out in MATLAB software package.
Both loadability and stability models are also applied to a realistic system. The benefits of the simultaneous AC-DC system
are evaluated and the results are critically discussed.
Key words: Simultaneous AC-DC power, Lodability, Thermal limits, DC mix, Matlab.
transfer without any significant structural changes. This has
given rise to the introduction of HVDC systems. Conversion
into DC has considerably decreased per unit losses,
improved power quality, and emphasized the reliability of
the line. Despite the greater cost of conversion equipment,
HVDC systems have proven to be safe, cost-efficient, and
environmentally friendly. As compared to HVAC systems,
they are encountered with decreased stability problems.
However, HVDC System fizz to make any flattering
contribution to the system synchronizing torque and
ultimately raise the margin of instability. This provides the
alley for researchers and engineers to think beyond the
obvious and establish a scheme that would allow HVDC
power transfer across an already fully functioning HVAC line,
hence giving rise to the idea of combined HVAC and HVDC
transmission. Such a scheme would significantly help in
removing problems of transient instability. Control strategies
could be established and operated on the HVDC system to
magnify the synchronizing and damping torques. This
INTRODUCTION
If it wasn't for Electricity today's world economy would have
been in great danger. Ever since the introduction of
electricity, mankind has shown a great leap in agricultural
and industrial sectors. Electric power transmission is nothing
but transfer of electrical energy, from generating power
plants, usually sited in remote locations, to electrical
substations that are located near the demand center. It
has been observed that the transmission of bulk power
through the installed high capacity AC voltage lines
experiences a certain upper limit beyond which the system
runs into transient instability. Consequently, the lines are
never loaded up to their maximum thermal limit rather
much less than that. This is a major barrier for finding ways
and means to raise the capacity of the existing EHVAC
transmission line prototype. Moreover, environmental
constraints have greatly limited the realization of new
power corridors with increased load capacity. Hence the
solutions are more or less limited to enhancing power
39
l
i-manager’s Journal o , Vol. No. 4 l
n 7 Power Systems Engineering November 2019 - January 2020
RESEARCH PAPERS
INTRODUCTION
If it wasn't for Electricity today's world economy would have
been in great danger. Ever since the introduction of
electricity, mankind has shown a great leap in agricultural
and industrial sectors. Electric power transmission is nothing
but transfer of electrical energy, from generating power
plants, usually sited in remote locations, to electrical
substations that are located near the demand center. It
has been observed that the transmission of bulk power
through the installed high capacity AC voltage lines
experiences a certain upper limit beyond which the system
runs into transient instability. Consequently, the lines are
never loaded up to their maximum thermal limit rather
much less than that. This is a major barrier for finding ways
and means to raise the capacity of the existing EHVAC
transmission line prototype. Moreover, environmental
constraints have greatly limited the realization of new
power corridors with increased load capacity. Hence the
solutions are more or less limited to enhancing power
transfer without any significant structural changes. This has
given rise to the introduction of HVDC systems. Conversion
into DC has considerably decreased per unit losses,
improved power quality, and emphasized the reliability of
the line. Despite the greater cost of conversion equipment,
HVDC systems have proven to be safe, cost-efficient, and
environmentally friendly. As compared to HVAC systems,
they are encountered with decreased stability problems.
However, HVDC System fizz to make any flattering
contribution to the system synchronizing torque and
ultimately raise the margin of instability. This provides the
alley for researchers and engineers to think beyond the
obvious and establish a scheme that would allow HVDC
power transfer across an already fully functioning HVAC line,
hence giving rise to the idea of combined HVAC and HVDC
transmission. Such a scheme would significantly help in
removing problems of transient instability. Control strategies
could be established and operated on the HVDC system to
magnify the synchronizing and damping torques. This
would further stabilize the AC system and eliminate any
requirement for extra reactive power in converter
controllers. Problem statement in the existing transmission
system, long extra-high voltage (EHV) ac lines cannot be
loaded to their thermal limits to keep sufficient margin
against transient instability. With the scheme proposed in
this paper, it is feasible to load these lines near to their
thermal limits. The conductors are allowed to carry usual ac
along with dc superimposed on it. By doing so, the
capacity of the transmission lines can be increased by
nearly 70% of that if only AC is transmitted.
1. Literature Review
Constantly increasing demand along with limitations of
constructing new transmission infrastructures has increased
the need to make use of the power transmission systems at
their maximum level. Increasing the transmission capacity
of the existing transmission line has never been more
important because of the rising cost of building new
transmission lines and the difficulties to obtain new
transmission way. That's why power system engineers are in
continuous search for effective ways to obtain the full
capacity of the existing transmission lines. Ever y
transmission line has an upper limit for loadability mainly
governed by three influential factors namely; thermal limit,
voltage drop limit, and steady-state stability limit (Gutman
et al., 1979). The length of the transmission lines also
supervises the loadability limiting factors, and hence
thermal limit, voltage drop limit, and steady-state stability
limit factors are applicable for up to 80, 320, and beyond
320 KM length of the transmission line, respectively (Kundur,
1993). Constructing a new transmission line and operating
it in parallel with the present AC transmission system can
increase the power transfer. One method is using a parallel-
small power DC link and is presented (Lucas & Peiris, 2001).
The parallel DC link can improve the loadability and
dynamic stability of the AC transmission system. A second
line, working in parallel, can raise the power transmission
capacity and guarantee the service continuity during
maintenance and it can meet the future demand (Bakshi,
2009). Construction of a new transmission line will further
strengthen the existing AC transmission system, increase
the operational reliability of the system, and overcome the
overall transmission system restrictions (Ingemansson et al.,
2012). Froma stability point of view, DC link parallel
operation with AC transmission line (i.e. AC-DC parallel
transmission system) is more advantageous than AC-AC
parallel transmission lines. In the case of the AC-DC parallel
l
i-manager’s Journal o , Vol. No. 4 l
n 7 Power Systems Engineering November 2019 - January 2020
40
RESEARCH PAPERS
INTRODUCTION
If it wasn't for Electricity today's world economy would have
been in great danger. Ever since the introduction of
electricity, mankind has shown a great leap in agricultural
and industrial sectors. Electric power transmission is nothing
but transfer of electrical energy, from generating power
plants, usually sited in remote locations, to electrical
substations that are located near the demand center. It
has been observed that the transmission of bulk power
through the installed high capacity AC voltage lines
experiences a certain upper limit beyond which the system
runs into transient instability. Consequently, the lines are
never loaded up to their maximum thermal limit rather
much less than that. This is a major barrier for finding ways
and means to raise the capacity of the existing EHVAC
transmission line prototype. Moreover, environmental
constraints have greatly limited the realization of new
power corridors with increased load capacity. Hence the
solutions are more or less limited to enhancing power
transfer without any significant structural changes. This has
given rise to the introduction of HVDC systems. Conversion
into DC has considerably decreased per unit losses,
improved power quality, and emphasized the reliability of
the line. Despite the greater cost of conversion equipment,
HVDC systems have proven to be safe, cost-efficient, and
environmentally friendly. As compared to HVAC systems,
they are encountered with decreased stability problems.
However, HVDC System fizz to make any flattering
contribution to the system synchronizing torque and
ultimately raise the margin of instability. This provides the
alley for researchers and engineers to think beyond the
obvious and establish a scheme that would allow HVDC
power transfer across an already fully functioning HVAC line,
hence giving rise to the idea of combined HVAC and HVDC
transmission. Such a scheme would significantly help in
removing problems of transient instability. Control strategies
could be established and operated on the HVDC system to
magnify the synchronizing and damping torques. This
would further stabilize the AC system and eliminate any
requirement for extra reactive power in converter
controllers. Problem statement in the existing transmission
system, long extra-high voltage (EHV) ac lines cannot be
loaded to their thermal limits to keep sufficient margin
against transient instability. With the scheme proposed in
this paper, it is feasible to load these lines near to their
thermal limits. The conductors are allowed to carry usual ac
along with dc superimposed on it. By doing so, the
capacity of the transmission lines can be increased by
nearly 70% of that if only AC is transmitted.
1. Literature Review
Constantly increasing demand along with limitations of
constructing new transmission infrastructures has increased
Figure 1. Basic Scheme for Simultaneous Ac-Dc Transmission
41
l
i-manager’s Journal o , Vol. No. 4 l
n 7 Power Systems Engineering November 2019 - January 2020
RESEARCH PAPERS
INTRODUCTION
If it wasn't for Electricity today's world economy would have
been in great danger. Ever since the introduction of
electricity, mankind has shown a great leap in agricultural
and industrial sectors. Electric power transmission is nothing
but transfer of electrical energy, from generating power
plants, usually sited in remote locations, to electrical
substations that are located near the demand center. It
has been observed that the transmission of bulk power
through the installed high capacity AC voltage lines
experiences a certain upper limit beyond which the system
runs into transient instability. Consequently, the lines are
never loaded up to their maximum thermal limit rather
much less than that. This is a major barrier for finding ways
and means to raise the capacity of the existing EHVAC
transmission line prototype. Moreover, environmental
constraints have greatly limited the realization of new
power corridors with increased load capacity. Hence the
solutions are more or less limited to enhancing power
transfer without any significant structural changes. This has
given rise to the introduction of HVDC systems. Conversion
into DC has considerably decreased per unit losses,
improved power quality, and emphasized the reliability of
the line. Despite the greater cost of conversion equipment,
HVDC systems have proven to be safe, cost-efficient, and
environmentally friendly. As compared to HVAC systems,
they are encountered with decreased stability problems.
However, HVDC System fizz to make any flattering
contribution to the system synchronizing torque and
ultimately raise the margin of instability. This provides the
alley for researchers and engineers to think beyond the
obvious and establish a scheme that would allow HVDC
power transfer across an already fully functioning HVAC line,
hence giving rise to the idea of combined HVAC and HVDC
transmission. Such a scheme would significantly help in
removing problems of transient instability. Control strategies
could be established and operated on the HVDC system to
magnify the synchronizing and damping torques. This
would further stabilize the AC system and eliminate any
requirement for extra reactive power in converter
controllers. Problem statement in the existing transmission
system, long extra-high voltage (EHV) ac lines cannot be
loaded to their thermal limits to keep sufficient margin
against transient instability. With the scheme proposed in
this paper, it is feasible to load these lines near to their
thermal limits. The conductors are allowed to carry usual ac
along with dc superimposed on it. By doing so, the
capacity of the transmission lines can be increased by
nearly 70% of that if only AC is transmitted.
1. Literature Review
Constantly increasing demand along with limitations of
constructing new transmission infrastructures has increased
the need to make use of the power transmission systems at
their maximum level. Increasing the transmission capacity
of the existing transmission line has never been more
important because of the rising cost of building new
transmission lines and the difficulties to obtain new
transmission way. That's why power system engineers are in
continuous search for effective ways to obtain the full
capacity of the existing transmission lines. Ever y
transmission line has an upper limit for loadability mainly
governed by three influential factors namely; thermal limit,
voltage drop limit, and steady-state stability limit (Gutman
et al., 1979). The length of the transmission lines also
supervises the loadability limiting factors, and hence
thermal limit, voltage drop limit, and steady-state stability
Figure 2. Equivalent Circuit of the AC-DC System.
Figure 3. Pure AC Power Transmission System
l
i-manager’s Journal o , Vol. No. 4 l
n 7 Power Systems Engineering November 2019 - January 2020
42
RESEARCH PAPERS
INTRODUCTION
If it wasn't for Electricity today's world economy would have
been in great danger. Ever since the introduction of
electricity, mankind has shown a great leap in agricultural
and industrial sectors. Electric power transmission is nothing
but transfer of electrical energy, from generating power
plants, usually sited in remote locations, to electrical
substations that are located near the demand center. It
has been observed that the transmission of bulk power
through the installed high capacity AC voltage lines
experiences a certain upper limit beyond which the system
runs into transient instability. Consequently, the lines are
never loaded up to their maximum thermal limit rather
much less than that. This is a major barrier for finding ways
and means to raise the capacity of the existing EHVAC
transmission line prototype. Moreover, environmental
constraints have greatly limited the realization of new
power corridors with increased load capacity. Hence the
solutions are more or less limited to enhancing power
transfer without any significant structural changes. This has
given rise to the introduction of HVDC systems. Conversion
into DC has considerably decreased per unit losses,
improved power quality, and emphasized the reliability of
the line. Despite the greater cost of conversion equipment,
HVDC systems have proven to be safe, cost-efficient, and
environmentally friendly. As compared to HVAC systems,
they are encountered with decreased stability problems.
However, HVDC System fizz to make any flattering
contribution to the system synchronizing torque and
Figure 3. Simulink Model of Simultaneous AC-DC Transmission
Table 1. Effect of the Values of k, α, x on Loadability
K αxCombined Power Flow
(MW)
Pure AC System Power Flow
(MW)
0.01
0.495
0.55
0.62
0.99
0.65
0.74
0.99
0.99
0.99
0.99
0.42
0.4241
0.99
0.5
0.5065
0.99
0.99
0.99
0.99
755.79
758.4
770.8
897
900.5
909.05
1325
1375
1418
751.77
758.4
1628
889
900.5
1645.7
1871
1866
1859
758.4
758.4
758.4
900.5
900.5
900.5
1300
1375
1420
758.4
758.4
758.4
900.5
900.5
900.5
1850
1866
1890
0.36
0.43
0.54
0.575
0.5968
0.36
0.43
0.8298
084
0.8556
43
l
i-manager’s Journal o , Vol. No. 4 l
n 7 Power Systems Engineering November 2019 - January 2020
RESEARCH PAPERS
INTRODUCTION
If it wasn't for Electricity today's world economy would have
been in great danger. Ever since the introduction of
electricity, mankind has shown a great leap in agricultural
and industrial sectors. Electric power transmission is nothing
but transfer of electrical energy, from generating power
plants, usually sited in remote locations, to electrical
substations that are located near the demand center. It
has been observed that the transmission of bulk power
through the installed high capacity AC voltage lines
experiences a certain upper limit beyond which the system
runs into transient instability. Consequently, the lines are
never loaded up to their maximum thermal limit rather
much less than that. This is a major barrier for finding ways
and means to raise the capacity of the existing EHVAC
transmission line prototype. Moreover, environmental
constraints have greatly limited the realization of new
power corridors with increased load capacity. Hence the
solutions are more or less limited to enhancing power
transfer without any significant structural changes. This has
given rise to the introduction of HVDC systems. Conversion
into DC has considerably decreased per unit losses,
improved power quality, and emphasized the reliability of
the line. Despite the greater cost of conversion equipment,
HVDC systems have proven to be safe, cost-efficient, and
environmentally friendly. As compared to HVAC systems,
they are encountered with decreased stability problems.
However, HVDC System fizz to make any flattering
contribution to the system synchronizing torque and
ultimately raise the margin of instability. This provides the
alley for researchers and engineers to think beyond the
obvious and establish a scheme that would allow HVDC
power transfer across an already fully functioning HVAC line,
hence giving rise to the idea of combined HVAC and HVDC
transmission. Such a scheme would significantly help in
removing problems of transient instability. Control strategies
could be established and operated on the HVDC system to
magnify the synchronizing and damping torques. This
would further stabilize the AC system and eliminate any
requirement for extra reactive power in converter
controllers. Problem statement in the existing transmission
system, long extra-high voltage (EHV) ac lines cannot be
loaded to their thermal limits to keep sufficient margin
against transient instability. With the scheme proposed in
this paper, it is feasible to load these lines near to their
thermal limits. The conductors are allowed to carry usual ac
along with dc superimposed on it. By doing so, the
capacity of the transmission lines can be increased by
nearly 70% of that if only AC is transmitted.
1. Literature Review
Constantly increasing demand along with limitations of
constructing new transmission infrastructures has increased
the need to make use of the power transmission systems at
their maximum level. Increasing the transmission capacity
of the existing transmission line has never been more
important because of the rising cost of building new
transmission lines and the difficulties to obtain new
transmission way. That's why power system engineers are in
Table 2. Comparative Data Analysis
30
40
44.47
50
60
3729
3800
3828
3860
3910
3481
3481
3481
3481
3481
248
319
347
379
429
-99
-28
0
32
82
-99
-28
0
32
82
Angle δ
(degree)
Power Flow in (MW)
Total Power
(MW)
DC Power
(MW)
AC Power
(MW)
P - P
comb comb2 P - P
ac ac2
Table 3. Line Loading for Different Transmission Angle
30
45
60
75
1961
2030
2074
2099
1505.4
1505.4
1505.4
1505.4
30.26%
34.25%
37.77%
39.43%
Transmission Angle
(degree)
Combined Power
(MW)
Pure AC Power
Flow (MW)
Increased
Loadability
(%)
Table 4. Line Loading for Different DC Voltage Mix
10
20
30
40
45
49.5
1505.4
1505.4
1505.4
1505.4
1505.4
1505.4
DC Voltage Mix
(%)
1791.43
2498.96
2905.42
3296.83
3492.53
3658.12
Combined Power
(MW)
Pure AC Power
Flow (MW)
l
i-manager’s Journal o , Vol. No. 4 l
n 7 Power Systems Engineering November 2019 - January 2020
44
RESEARCH PAPERS
INTRODUCTION
If it wasn't for Electricity today's world economy would have
been in great danger. Ever since the introduction of
electricity, mankind has shown a great leap in agricultural
and industrial sectors. Electric power transmission is nothing
but transfer of electrical energy, from generating power
plants, usually sited in remote locations, to electrical
substations that are located near the demand center. It
has been observed that the transmission of bulk power
through the installed high capacity AC voltage lines
experiences a certain upper limit beyond which the system
runs into transient instability. Consequently, the lines are
never loaded up to their maximum thermal limit rather
much less than that. This is a major barrier for finding ways
and means to raise the capacity of the existing EHVAC
transmission line prototype. Moreover, environmental
constraints have greatly limited the realization of new
power corridors with increased load capacity. Hence the
solutions are more or less limited to enhancing power
transfer without any significant structural changes. This has
given rise to the introduction of HVDC systems. Conversion
into DC has considerably decreased per unit losses,
improved power quality, and emphasized the reliability of
the line. Despite the greater cost of conversion equipment,
HVDC systems have proven to be safe, cost-efficient, and
environmentally friendly. As compared to HVAC systems,
they are encountered with decreased stability problems.
However, HVDC System fizz to make any flattering
contribution to the system synchronizing torque and
ultimately raise the margin of instability. This provides the
alley for researchers and engineers to think beyond the
obvious and establish a scheme that would allow HVDC
power transfer across an already fully functioning HVAC line,
hence giving rise to the idea of combined HVAC and HVDC
transmission. Such a scheme would significantly help in
removing problems of transient instability. Control strategies
could be established and operated on the HVDC system to
magnify the synchronizing and damping torques. This
would further stabilize the AC system and eliminate any
requirement for extra reactive power in converter
controllers. Problem statement in the existing transmission
system, long extra-high voltage (EHV) ac lines cannot be
loaded to their thermal limits to keep sufficient margin
against transient instability. With the scheme proposed in
this paper, it is feasible to load these lines near to their
thermal limits. The conductors are allowed to carry usual ac
along with dc superimposed on it. By doing so, the
capacity of the transmission lines can be increased by
nearly 70% of that if only AC is transmitted.
1. Literature Review
Constantly increasing demand along with limitations of
constructing new transmission infrastructures has increased
the need to make use of the power transmission systems at
their maximum level. Increasing the transmission capacity
of the existing transmission line has never been more
important because of the rising cost of building new
transmission lines and the difficulties to obtain new
transmission way. That's why power system engineers are in
continuous search for effective ways to obtain the full
capacity of the existing transmission lines. Ever y
transmission line has an upper limit for loadability mainly
governed by three influential factors namely; thermal limit,
voltage drop limit, and steady-state stability limit (Gutman
et al., 1979). The length of the transmission lines also
supervises the loadability limiting factors, and hence
thermal limit, voltage drop limit, and steady-state stability
limit factors are applicable for up to 80, 320, and beyond
320 KM length of the transmission line, respectively (Kundur,
1993). Constructing a new transmission line and operating
it in parallel with the present AC transmission system can
increase the power transfer. One method is using a parallel-
small power DC link and is presented (Lucas & Peiris, 2001).
The parallel DC link can improve the loadability and
dynamic stability of the AC transmission system. A second
line, working in parallel, can raise the power transmission
capacity and guarantee the service continuity during
maintenance and it can meet the future demand (Bakshi,
2009). Construction of a new transmission line will further
strengthen the existing AC transmission system, increase
the operational reliability of the system, and overcome the
overall transmission system restrictions (Ingemansson et al.,
2012). Froma stability point of view, DC link parallel
operation with AC transmission line (i.e. AC-DC parallel
transmission system) is more advantageous than AC-AC
parallel transmission lines. In the case of the AC-DC parallel
45
l
i-manager’s Journal o , Vol. No. 4 l
n 7 Power Systems Engineering November 2019 - January 2020
RESEARCH PAPERS
INTRODUCTION
If it wasn't for Electricity today's world economy would have
been in great danger. Ever since the introduction of
electricity, mankind has shown a great leap in agricultural
and industrial sectors. Electric power transmission is nothing
but transfer of electrical energy, from generating power
plants, usually sited in remote locations, to electrical
substations that are located near the demand center. It
has been observed that the transmission of bulk power
through the installed high capacity AC voltage lines
experiences a certain upper limit beyond which the system
runs into transient instability. Consequently, the lines are
never loaded up to their maximum thermal limit rather
much less than that. This is a major barrier for finding ways
and means to raise the capacity of the existing EHVAC
transmission line prototype. Moreover, environmental
constraints have greatly limited the realization of new
power corridors with increased load capacity. Hence the
ABOUT THE AUTHORS
Mr. Sunil Kumar Jillediwas born in Tirupathi, India. He received his B.Tech in the Department of Electrical and Electronics from
Anna University, Chennai, in 2006 and M.Tech from Sri Venkateswara University, Tirupathi, in 2011. Pursuing Ph.D. in OPJS University,
India. Currently working as Senior Lecturer in Eritrea Institute of Technology, Asmara, Eritrea. Previously worked as Lecturer in
Adama science and technology university, Adama, Ethiopia. His research interests include Power Systems, Renewable Energy,
Fuzzy Logic, Neural Networks, Flexible AC Transmission System (FACTS). Up to now, 30 International journals are in credit, 6
International conferences. He is working as a reviewer for many journals like the International Journal of Electrical Power &
EnergySystems (Elsevier), International Journal of Scientific and Engineering Research, (IJ-ETAETS), Global Journal of Researches in
Engineering, United States, International Journals of Engineering & Sciences.
l
i-manager’s Journal o , Vol. No. 4 l
n 7 Power Systems Engineering November 2019 - January 2020
46
