High Voltage Direct Current Transmission – A Review, Part I

High Voltage Direct Current Transmission –
A Review, Part I
Mohamed H. Okba 1 , Mohamed H. Saied 2 , M. Z. Mostafa 3 , and T. M. Abdel- Moneim 3
1 M.Sc. Candidate, Electrical Engineering Dept., Testing, Measurement, and Protection Sect., Egyptian Electricity Transmission
Co., Al Behira, EGYPT, Eng.Okba86@Gmail.com
2 Ph.D., Member, IEEE, GM, Electrical Engineering Dept., Abu-Qir Fertilizers & Chemical Industries Co., Alexandria, EGYPT,
Mohammed.Saied@Gmail.com
3 Full-Prof., Member, IEEE, Electrical Engineering Dept., Faculty of Engineering, Alexandria University, Alexandria, EGYPT
Abstract—Major milestones in the development of high
voltage direct current (HVDC) technologies and concepts were
achieved in 1950s. Thanks to the high power thyristor switches
(1960-70s), the HVDC technologies reached a significant degree of
maturity in 1980s. The classical HVDC uses thyristor-based
current-sourced line-commutated converter (LCC) technology.
The advent of power semiconductor switches in 1980-90s, with
turn on-off capabilities especially the IGBTs and IGCTs, and the
on-going progress in this field, have introduced the conventional
(two-level) voltage-source converter (VSC) technology and its
variety of configurations, multi-level and multi-module VSCs,
also as viable converter technologies for power system
applications.
The DC system is experiencing significant degree of reemergence
due to its potential to either directly address, or to
facilitate resolving a large number of existing and anticipated
interconnected AC power system steady-state and dynamic issues.
HVDC technology made possible to transfer bulk power over long
distances. In part I of this two-parts paper, comparative
evaluations, studies, and review of HVDC versus HVAC
transmission systems, are presented. Applications, different
schemes of HVDC systems are also outlined.
Index Terms— HVDC converters, HVDC converter
technologies, Hierarchal Level, HVDC system components,
HVDC schemes, HVDC transmission.
T
I. INTRODUCTION
he first electric generator was the direct current (DC)
generator, and hence, the first electric power transmission
line was constructed with DC. Despite the initial supremacy of
the DC, the alternating current (AC) supplanted the DC for
greater uses. This is because of the availability of the
transformers, poly-phase circuits, and the induction motors in
the 1880s and 1890s [1]-[2].The ever increasing penetration of
the power electronics technologies into power systems is
mainly due to the continuous progress of the high-voltage
high-power fully-controlled semiconductors [3]-[14].
Transformers are very simple machines and easy to be used
to change the voltage levels for transmission, distribution, and
stepping down of electric power. Induction motors are the
workhorse of the industry and work only with AC. That is why
AC has become very useful for the commercial and domestic
loads. For long transmission, DC is more favorable than AC
because of its economical, technical, and environmental
advantages. In general, high voltage direct current (HVDC)
transmission systems can be classified in several ways; on the
basis of cost, flexibility, and operational requirements.
The simplest HVDC scheme is the back-to-back
interconnection, where it has two converters on the same site
and has no transmission lines. These types of connections are
used as inter-ties between two different AC transmission
systems.
The mono-polar link connects two converter stations by a
single conductor line and the earth or the sea is used as the
returned path. The most common HVDC links are bipolar,
where two converter stations are connected with bipolar
conductors (±), and each conductor has its own ground return.
The multi-terminal HVDC transmission systems have more
than two converter stations, which could be connected is series
or parallel [15].
II. RELIABILITY AND CONTROLLABILITY EVALUATIONS OF
TRANSMISSION SYSTEMS
Modern power systems are very complex technical
structures. They consist of large number of interconnected
subsystems and components each of which interact with, and
influence, the overall systems reliability. One definition of
reliability is the ability of a component or a system to perform
required functions under stated conditions for a stated period
of time [16]. Reliability assessments of electrical systems are
performed in order to determine where and when new
investments, maintenance planning, and operation are going to
be made.
Power system reliability is often divided by the two
functional aspects of system adequacy and security. Adequacy
is the ability of the power system to supply the aggregate
electric power and energy requirements of the customer at all
times, taking into account scheduled and unscheduled outages
of system components. Security is the ability of the power
system to withstand sudden disturbances such as electric short
circuits or non-anticipated loss of system components [16].
A reliability model that includes the whole complexity of
the entire electrical power system would be impossible to
implement. The analysis would be far too complex and the
results would be very difficult to interpret. Instead it is
preferable to separate the system into three hierarchal levels
(HL): generation (HL1), generation and transmission (HL2),
and distribution (HL3). Each level can then be modeled and
evaluated individually [16]. A study of HL2 is also referred to
as a composite system reliability assessment and this can
include both adequacy and security analysis. Reliability
assessments of HVDC systems can be modeled and evaluated

separately and then included into HL2 to evaluate the effect of
the overall system reliability. In reliability assessments of such
HVDC systems, it is of great importance to know the
technicalities of the system, in order to model it. The next
section describes the HVDC systems details.
The IEEE Standard is a guide for the evaluation of the
HVDC converter stations reliability [17]. It promotes the basic
concepts of reliability, availability, and maintainability (RAM)
in all phases of the HVDC station’s life cycle. The intention of
introducing these concepts of RAM in HVDC projects is to
provide help in: i) Improving RAM for stations in service, ii)
Calculating and comparing RAM for different HVDC designs,
iii) Reducing costs, iv) Reducing spare parts, and v) Improving
HVDC converter specifications [17]-[18]. In [19]-[28], several
researches have been published covering the area of assessing
the reliability of the HVDC system as a single system.
On the other hand, the controllability of HVDC links offers
firm transmission capacity without limitation due to network
congestion or loop flow on parallel paths. Controllability
allows the HVDC to ‘leap-frog’ multiple ‘choke-points’ or
bypass sequential path limits in the AC network. Therefore, the
utilization of HVDC links is usually higher than that for extra
high voltage (EHV) AC transmission lowering the
transmission cost per MWh. By eliminating loop flow,
controllability frees up parallel transmission capacity for its
intended purpose of serving intermediate load and providing
an outlet for local generation [29].
III. AC VERSUS DC TRANSMISSION
As the rapid development of renewable energy generation,
like wind and solar power generation, and high electrical
power generated at long-distances, it is urgent to feed these
distributed energy back to power grid through an economic
and environmental way. Actually, AC is very familiar for
industrial and domestic loads, but it has some limitations for
long transmission lines. Moreover, as the city power load is
increasing, the capacity of grid need to be expanded, despite
that the overhead AC lines have already occupied much
transmission space. In a word, a new transmission approach is
needed to solve these and other problems, the DC
transmission, which is being used in several projects [30],
[31]-[32], and [33]-[34].
Switching surges, for example, are the serious transient
over voltages for the high voltage transmission lines. In case of
AC transmission the peak values are 2 to 3 times normal crest
voltage, where for DC transmission it is 1.7 times normal
voltage. In addition to, the HVDC transmission has less corona
and radio interferences than that of HVAC transmission line
[35]-[37]. In the following section, comparisons of the HVDC
with the conventional AC transmission systems are carried out.
A. Transmission Costs Comparison
The cost of any AC or DC transmission lines usually
includes the cost of main components, such as; right-of-way
(ROW), which is the amount of landscape that might be
occupied during installations of towers, conductors, insulators,
terminal equipment, in addition to the operational costs such as
losses of transmission lines. For given operational constraints
of both AC and DC lines, DC lines has the ability to carry as
much power with two conductors as AC lines with three
conductors of the same size. Moreover, DC lines require fewer
infrastructures than AC lines, which will consequently reduce
the cost of DC lines' installation.
1) Economic Considerations:
For a given transmission task, feasibility studies are carried
out before the final decision of implementing of a HVAC or
HVDC system. Whenever long distance transmission is
discussed, the concept of “break-even distance” arises. This is
where the savings in HVDC line costs offsets the higher
converter station costs. Fig.1.a shows typical cost comparison
curves between AC and DC transmissions, considering:
Terminal station costs,
Line costs, and
Capitalized value of losses.
The DC curve is not as steep as the AC curve because of
considerably lower line costs per kilometer. For long AC lines,
the cost of intermediate reactive power compensation has to be
taken into account. The break-even distance is in the range of
500 to 800 km depending on a number of other factors, like
country-specific cost elements, interest rates for project
financing, loss evaluation, cost of right-of-way, etc. [38]-[42].
Fig. 1.b shows the power capacity versus distances for both
AC and DC systems.
1) Environmental Issues:
A HVDC transmission system is basically environmentfriendly,
because the improved energy transmission
possibilities contribute to a more efficient utilization of
existing power plants. The land coverage and the associated
right-of-way cost for a HVDC overhead transmission line is
not as high as that of an AC line [40]-[41]. This reduces the
visual impact and saves land compensation for new projects. It
is also possible to increase the power transmission capacity for
existing rights of way.
(a)
(b)
Fig. 1. Comparison between AC and DC systems, (a) Cost comparison curves,
(b) Power capacity versus distances.

Tower structures of DC and AC overhead transmission
lines are shown in Fig. 2.There are some environmental issues
must be considered for the converter stations. These issues are
focused in [43]-[45]. The use of ground or sea return paths in
monopolar operation, electromagnetic compatibility, visual
impact, and audible noise are explained in [46]-[48]. In [49],
an overview of the engineering methods, tools, and design
solutions is introduced. Verification methods used in HVDC
converter stations design considering acoustic requirements are
also explained.
IV. VOLTAGE STABILITY OF HVDC VERSUS HVAC
INTERCONNECTIONS
Long transmission lines are required to deliver the power to
the major load centers or the nearest connection point of the
existing transmission network. For long transmission of bulk
power several technical and economic issues have to be
considered before an optimal decision can be made. Voltage
stability in general is one of the main technical issues to be
considered [50]-[51]. Several methods, used to obtain the
stability margin of a HVDC system, are well presented in the
literature [51]-[59]. The most common voltage stability indices
used for HVDC systems are maximum available power
(MAP), critical effective short circuit ratio (CESCR) and
voltage stability factor (VSF).
The maximum power method, which determines MAP and
the voltage sensitivity method to determine VSF are best
described in [52]. These two methods coincide, i.e. the MAP
point is reached when VSF nears infinite, if the converters are
operated in constant extinction angle and constant power
control mode. The basic P-V stability equations are also
derived taking into account load characteristics and system
parameters. These methods are applied in [53] to determine the
most unfavorable load characteristics with respect to degrading
power/voltage stability margins. This is done by analyzing the
impact of load characteristic on maximum power instability
(dP/dI) and MAP of the HVDC system. The Short Circuit
Ratio (SCR) or CESCR are also considered as stability factors
for an HVDC system, but only appropriate to evaluate the
impacts of AC system on the stability margin of HVDC [60].
Authors in [59] introduced a new index (dQt/eig_min) for
voltage stability analysis of AC/DC systems. This index is
used to classify the system into soft and non-soft modal
systems. The latter is defined as the system with constant
dQt/eig_min for all the SCRs and vice versa for the former.
This index also serves as a basis to decide the type of reactive
power compensation and HVDC control strategy.
Fig.2. Typical transmission line structures for approximately 1000 MW.
While the above mentioned indices can be used to compare
voltage stability margins between HVDC systems, they are not
applicable for HVAC and HVDC comparison. In [56], authors
extend the conventional point of collapse (PoC) method
developed for AC systems to determination of saddle-node
bifurcation in systems including HVDC links. In [57], a
comparison of the performance of the PoC and continuation
methods for large AC/DC systems is presented. The proposed
continuation method is applied in the two free softwarepackages
for stability studies; (UWPflow) and (PSAT) [61]-
[63].
A nonlinear programming approach for estimating the
voltage stability in AC/DC systems based on the above
mentioned algorithms is presented in [59] where PoCs are
found by solving an optimization problem for several test
systems. However, more in-depth analytical explanation is
required, and control issues of HVDC systems need to be
considered. Inappropriate control schemes of firing, extinction,
and overlap angles results in commutation failure or
singularity in the Jacobian matrix. Therefore, PoC based on
this method is not reliable to be used in the comparison of
voltage stability of HVDC and HVAC systems. The dVac/dq
factor at a particular bus is a commonly used voltage stability
index in both AC and DC systems [51], [54]-[55]. However, it
has never been used for comparison purposes between HVAC
and HVDC systems.
V. ADVANTAGES AND DISADVANTAGES OF HVDC
Although the rationale for selection of HVDC is often
economic, there may be other reasons for its selection. In many
cases more AC lines are needed to deliver the same power over
the same distance due to system stability limitations.
Furthermore, the long distance AC lines usually require
intermediate switching stations and reactive power
compensation. This can increase the substation costs for AC
transmission to the point where it is comparable to that for
HVDC transmission [29].
HVDC may be the only feasible way to interconnect two
asynchronous networks, reduce fault currents, utilize long
cable circuits, bypass network congestion, share utility rightsof-way
without degradation of reliability, and mitigate
environmental concerns. In all of these applications, HVDC
nicely complements the AC transmission system. The
following points highlight different advantages and
disadvantages of the HVDC systems [29].
A. Advantages
1) Greater power per conductor.
2) Simpler line construction and smaller transmission
towers.
3) A bipolar HVDC line uses only two insulated sets of
conductors, rather than three.
4) Narrower right-of-way.
5) Require only one-third the insulated sets of conductors as
a double circuit AC line.
6) Approximate savings of 30% in line construction.
7) Ground return can be used.
8) Each conductor can be operated as an independent
circuit.
9) No charging current at steady state.

10) No Skin effect.
11) Lower line losses.
12) Line power factor is always unity.
13) Line does not require reactive compensation.
14) Synchronous operation is not required.
15) Distances are not limited by stability.
16) May interconnect AC systems of different frequencies.
17) Low short-circuit current on D.C line.
18) Does not contribute to short-circuit current of an AC
system.
19) Controllability allows the HVDC to ‘leap-frog’ multiple
‘choke-points’.
20) No physical restriction limiting the distance or power
level for HVDC underground or submarine cables
21) Can be used on shared ROW with other utilities
22) Considerable savings in installed cable and losses costs
for underground or submarine cable systems [29].
B. Disadvantages
1) Converters are expensive.
2) Converters require much reactive power.
3) Multi-terminal or network operation is not easy.
4) Converters generate harmonics and hence, require filters.
5) Break-even distance is influenced by the costs of right-ofway
and line construction with a typical value of 500 km
[38]-[40].
VI. APPLICATIONS OF HVDC TRANSMISSION SYSTEMS
HVDC has gradually become a mature technology for AC
system interconnection since the commissioning of the first
commercial project between Mainland Sweden to Gotland
island in 1954 [30]. The applications of HVDC technology are
justified by some special conditions where HVDC is the most
feasible or may be the only solution. Such applications include
bulk power transmission over long distances, sub-marine cable
transmission, and asynchronous systems inter-connection
[64].HVDC transmission applications can be broken down to
the following different basic categories [29], [37] AND [64].
A. Long Distance Bulk Power Transmission
As shown above, HVDC transmission systems often
provide a more economical alternative to AC transmission, for
exploiting the high electrical power generated at long-distances
and bulk-power delivery from clean remote resources, such as;
hydroelectric developments, mine-mouth power plants, solar,
large-scale wind farms, or major hot-rock geothermal energy.
This transmission is established using fewer lines with HVDC
than with AC transmission.
B. Cable Transmission
Unlike the case for AC cables, there is no physical
restriction limiting the distance or power level for HVDC
underground or submarine cables. Underground cables can be
used on shared ROW with other utilities, without impacting
reliability concerns over use of common corridors. Saving
advantages of underground and submarine cable systems ‘have
been shown previously, knowing that depending on the power
level to be transmitted; these savings can offset the higher
converter station costs at distances of 40 km or more.
On the other hand, for AC transmission over a distance,
there is a drop-off in cable capacity due to its reactive
component of charging current, since cables have higher
capacitances and lower inductances than AC overhead lines.
Although this can be compensated by intermediate shunt
compensation for underground cables at increased expense, it
is not practical to do so for submarine cables [65]-[66].
C. Asynchronous Ties
With HVDC transmission systems, interconnections can be
made between asynchronous networks for more economic or
reliable system operation. The asynchronous interconnection
allows interconnections of mutual benefit while providing a
buffer between the two systems. Often these interconnections
use back-to-back converters with no transmission line [67].
Asynchronous HVDC links effectively act against propagation
of cascading outages in one network from passing to another
network.
Higher power transfers can be achieved, with improved
voltage stability in weak system applications, using capacitor
commutated converters. The dynamic voltage support and
improved voltage stability offered by voltage source converter
(VSC) based converters permits even higher power transfers
without as much need for AC system reinforcement. VSC
converters do not suffer commutation failures, allowing fast
recoveries from nearby AC faults. Economic power schedules
which reverse power direction can be made without any
restrictions since there is no minimum power or current
restrictions [68].
D. Offshore Transmission
Self-commutation, dynamic voltage control, and black-start
capability allow compact VSC HVDC transmission to serve
isolated and orphaned loads on islands, or offshore drilling and
production platforms over long distance submarine cables.
This capability can eliminate the need for running uneconomic
or expensive local generation or provide an outlet for offshore
generation such as that from wind.
The VSC converters can operate at variable frequency to
more efficiently drive large compressor or pumping loads
using high voltage motors. Large remote wind generation
arrays require a collector system, reactive power support, and
outlet transmission. Transmission for wind generation must
often traverse scenic or environmentally sensitive areas or
bodies of water. Many of the better wind sites with higher
capacity factors are located offshore. VSC based HVDC
transmission not only allows efficient use of long distance land
or submarine cables but also provides reactive support to the
wind generation complex and interconnection point [29].
E. Power Delivery to Large Urban Areas
Power supply for large cities depends on local generation
and power import capability. Local generation is often older
and less efficient than newer units located remotely. Air
quality regulations may limit the availability of these older
units. New transmission into large cities is difficult to site due
to right-of-way limitations and land use constraints. Compact
VSC-based underground transmission circuits can be placed on
existing dual-use rights-of-way to bring in power, as well as to
provide voltage support allowing a more economical power
supply without compromising reliability. The receiving

terminal acts like a virtual generator delivering power and
supplying voltage regulation and dynamic reactive power
reserve. Stations are compact and housed mainly indoors
making siting in urban areas somewhat easier. Furthermore,
the dynamic voltage support offered by the VSC can often
increase the capability of the adjacent AC transmission [29].
These applications can be summarized as follows:
1) Power transmission of bulk energy through long distance
overhead lines.
2) Power transmission of bulk energy through sea cables.
3) Fast and precise control of energy flow over back-to-back
HVDC links, creating a positive damping of electromechanical
oscillations, and enhancing the network
stability, by modulating the transmitted power.
4) Linking two AC systems with different frequencies using
asynchronous back-to-back HVDC links, which have no
constraints with respect to systems' frequencies or phase
angles.
5) Multi-terminal HVDC links are used to offer necessary
strategically and political connections in the traversed
areas of the potential partners, when power is to be
transmitted from remote generation locations, across
different countries, or different areas within one country.
6) Link renewable energy sources, such as hydroelectric,
mine-mouth, solar, wind farms, or hot-rock geothermal
power, when are located far away from the consumers.
7) Pulse-Width Modulation (PWM) can be used for the VSC
based HVDC technology as opposed to the thyristor
based conventional HVDC. This technology is well
suited for wind power connection to the grid.
8) Connecting two AC systems without increasing the short
circuit power, that the reactive power does not get
transmitted over a DC links. This technique is useful in
generator connections, various applications of an HVDC
system shown in Fig. 3.
Fig. 3. Various applications of HVDC systems.
VII. DIFFERENT HVDC SCHEMES [69]-[73]
A. Back-To-Back Converters
The "Back-to-back" indicates that the rectifier and inverter
are located in the same station. Back-to-back converters are
mainly used for power transmission between adjacent AC
grids which cannot be synchronized. They can also be used
within a meshed grid in order to achieve a defined power flow.
B. Monopolar Long-Distance Transmissions
For very long distances and in particular for very long sea
cable transmissions, a return path with ground/sea electrodes
will be the most feasible solution. In many cases, existing
infrastructure or environmental constraints prevent the use of
electrodes. In such cases, a metallic return path is used in spite
of increased cost and losses.
C. Bipolar Long-Distance Transmissions
A bipolar is a combination of two independent poles in
such a way that a common low voltage return path, if
available, will only carry a small unbalance current during
normal operation. This configuration is used if the required
transmission capacity exceeds that of a single pole. It is also
used if requirement to higher energy availability or lower load
rejection power makes it necessary to split the capacity on two
poles. During maintenance or outages of one pole, it is still
possible to transmit part of the power. More than 50% of the
transmission capacity can be utilized, limited by the actual
overload capacity of the remaining pole, while require only
one-third the insulated sets of conductors compared to a
double-circuit AC line.
Other advantages of a bipolar solution over a solution with
two monopoles are reduced cost, due to one common or no
return path, and lower losses. In [74]–[76] the bipolar HVDC
system configuration has been modeled. The reliability models
in these three papers are similar to each other but the
objectives in the papers differ.
1) Bipolar With Ground Return Path:
This is a commonly used configuration for a bipolar
transmission system. The solution provides a high degree of
flexibility with respect to operation with reduced capacity
during contingencies or maintenance, upon a single-pole fault,
the current of the sound pole will be taken over by the ground
return path and the faulty pole will be isolated. Following a
pole outage caused by the converter, the current can be
commutated from the ground return path into a metallic return
path provided by the HVDC conductor of the faulty pole.
2) Bipolar With Dedicated Metallic Return Path For
Monopolar Operation:
If there are restrictions even to temporary use of electrodes,
or if the transmission distance is relatively short, a dedicated
LVDC metallic return conductor can be considered as an
alternative to a ground return path with electrodes.
3) Bipolar Without Dedicated Return Path For Monopolar
Operation:
A scheme without electrodes or a dedicated metallic return
path for monopolar operation will give the lowest initial cost;
Monopolar operation is possible by means of bypass switches
during a converter pole outage, but not during an HVDC
conductor outage.

Fig. 4. Different HVDC Schemes.
A short bipolar outage will follow a converter pole outage
before the bypass operation can be established. Fig. 4 shows
different HVDC Schemes [29].
D. Multi-terminal HVDC System
In this configuration, there are more than two sets of
converters. A multi-terminal CSC-HVDC system with 12pulse
converters per pole is shown in Fig. 5. In this case,
converters 1 and 3 can operate as rectifiers, while converter 2
operates as an inverter. Working in the other order, converter 2
can operate as a rectifier and converters 1 and 3 as inverters.
By mechanically switching the connections of a given
converter, other combinations can be achieved [77].
Fig. 5. Multi-terminal CSC-HVDC system- parallel connection.
VIII. CONCLUSION
Comparative evaluations, studies, applications, different
schemes, and review of HVDC versus HVAC transmission
systems, are presented in this part of the two-parts paper.
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