"Sharpening Skills.....
Serving Nation"
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459 (Online), Volume 4, Special Issue 1, February 2014)
International Conference on Advanced Developments in Engineering and Technology (ICADET-14), INDIA.
How HVDC is Beneficial To EHVAC
Shubham Tyagi1, Snigdha Sharma2, Ankur Kumar3, Danish Chaudhary4
Shubhamtyagi89@gmail.com1,snigdha.2015@gmail.com2,akvak9@gmail.com3,
Danishchaudhary89@gmail.com4
Abstract -- High voltage transmission is employed to
transfer the bulk power from sending end to receiving end
with lesser losses. Generally power is transferred in ac
because of the feasibility in stepping up the voltage and
stepping down. However with increase in voltage levels
beyond 400kV ac EHV transmission has certain
disadvantages such as corona loss, insulation requirement for
conductor, radio interference and heavy supporting
structures. Therefore for transmission with EHV voltages
HVDC transmission is economical. This paper discuss that
how HVDC plays a vital role in power system development.
This paper includes introduction of HVDC, layout,
applications and advantages of HVDC. The aim of this paper
is to review the merits of HVDC over EHVAC.
II. LAYOUT O F HVDC SUBSTATION
Keywords-- Corona Loss, EHVAC, HVDC, Thyristor
Valve.
I. INTRODUCTION
A HVDC uses direct current for the transmission of
electrical power. The first long distance transmission of
electrical power was demonstrated using direct current in
1882 but only 2.5 KW is transmitted. The advent of
thyristor valve and related technological improvements
over the last 18 years has been responsible for the
acceleration of the growth of HVDC technology is still
undergoing many changes due to continuing innovations
directed at improving reliability and reducing cost of
converter stations.
II.1 Converter unit
The HVDC power converter may be non-controllable if
constructed from one or more power diodes in series or
controllable if constructed from one or more thyristors in
series. The standard bridge or converter connection is
defined as a double-way connection comprising six valves
or valve arms (six pulse) that are connected as illustrated
below. When electric power flows into the dc valve group
from the ac system then it is considered a rectifier. If power
flows from the dc valvegroup into the ac system, it is an
inverter. Each valve consists of many series connected
thyristors in thyristor modules.
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"Sharpening Skills.....
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International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459 (Online), Volume 4, Special Issue 1, February 2014)
International Conference on Advanced Developments in Engineering and Technology (ICADET-14), INDIA.
II.3 DC reactor
The direct current equipment often includes a coil that
adds inductance in series with the DC line to help smooth
the direct current. The inductance typically amounts to
between 0.1 H and 1 H. The smoothing reactor can have
either an air-core or an iron-core. Iron-core coils look like
oil-filled high voltage transformers.
II.2 12 pulse converter
Nearly all HVDC power converters with thyristor valves
are assembled in a converter bridge of twelve pulse
configuration. The most common twelve pulse
configuration is the use of two three phase converter
transformers with one dc side winding as an ungrounded
star connection and the other a delta configuration.
Consequently the ac voltages applied to each six pulse
valve group which make up the twelve pulse valve group
have a phase difference of 30 degrees which is utilized to
cancel the ac side 5th and 7th harmonic currents and dc
side 6th harmonic voltage, thus resulting in a significant
saving in harmonic filters. Since the voltage rating of
thyristors is several kV, a 500 kV system may have
hundreds of individual thyristors connected in series.
II.4 DC switchyard
The DC switchyard contains all the external equipment
needed for an HVDC transmission: voltage dividers,
current measuring devices, smoothing reactors and dc
filters. Transmission with cables does not require any dc
filters. Transmissions with OH typically require dc filters to
prevent the harmonics generated by the converter to be
transported along the dc line. A harmonic filter for 12 th and
high pass is usually enough.
II.5 AC switchyard
The three-phase alternating current switch gear of a
converter station is similar to that of an AC substation. It
will contain circuit breakers for over current protection of
the converter transformers, isolating switches, grounding
switches, and instrument transformers for control,
measurement and protection. The station will also have
lightning arresters for protection of the AC equipment from
lightning surges on the AC system.
III. MERITS O F HVDC OVER EHVAC
III.1 Corona losses:
DC corona losses are of less concern to the design of the
conductor bundles, since the increase during rain is much
smaller than with AC, only about 2-3 times. Also Corona
Loss is directly proportional to frequency. Therefore in DC
line corona loss will be lesser compared to AC line. When
comparing HVDC and EHVAC line with regard to power
losses, the main difference is that corona losses of HVDC
lines are much less sensitive to variations in weather
conditions.
III.2 Stability Limits:
The power transfer in AC lines is dependent on the angle
difference between the voltage phasors at the two ends. For
a given power level, these angle increases with distance.
The maximum power transfer is limited by the
considerations of steady state and transient stability.
However this is not the case with DC.
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"Sharpening Skills.....
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International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459 (Online), Volume 4, Special Issue 1, February 2014)
International Conference on Advanced Developments in Engineering and Technology (ICADET-14), INDIA.
III.3 Skin effect:
The skin effect is the tendency of an alternating electric
current to distribute itself within a conductor so that the
current density near the surface of the conductor is greater
than that at its core. It causes the effective resistance of the
conductor to increase with the frequency of the current.
Skin effect is due to eddy currents set up by the AC current.
This effect can be minimized by using ACSR (Aluminum
conductor steel reinforce) conductors which has the
property to minimize the skin effect. But it increase the cost
compare to normal conductors. Skin effect does not exist in
HVDC.
III.4 Reliability:
The reliability of dc transmission systems is quite good
and comparable to that of Ac systems. It must be
remembered that the performance of the thyristor valves is
much more reliable than mercury arc valves and further
development in devices control and protection is likely to
improve the reliability level. For example, the development
of direct light triggered (LTT) is expected to improve
reliability because of the elimination of the high voltage
pulse transformers and auxiliary supplies for turning on the
device. Both energy availability and transient reliability of
existing dc systems with thyristor valves is 95% or more.
III.5 Reactive power compensation:
The maintenance of constant voltages in ac at the two
ends requires reactive power control from inductive to
capacitive as the line loading is increased. The reactive
power requirements increase with the increase in the line
lengths. Although dc converter stations require reactive
power related to the line loadings, the line itself does not
require reactive power. The steady state charging currents
in ac lines pose serious problems in cables this puts the
break even distance for the cable transmission around 40
km. Unlike AC line DC line does not require any reactive
power compensation devices. This is because of the
absence of charging currents and power factor operation.
III.6 Economic considerations:
The cost of transmission line includes the investment
and operational costs. The investment includes costs of
Right of Way (ROW), transmission towers, conductors,
insulators and terminal equipment.
The operational costs include mainly the cost of losses.
The power losses in case of power cables is also very less
for dc transmission. The trend of power electronic
components, for use in the main circuit of an HVDC
transmission, being developed means that the relative cost
of HVDC transmissions is reduced as the components
become cheaper as a result of continuing innovative
technological developments. Thus a large converter station
is today cheaper in current dollars compared with the
situation 20 years ago. The dc line is less costly compared
with an 800 kV ac line. On the other hand, the converter
station cost offsets the gain in reduced cost of the
transmission line. Thus a short line is cheaper with ac
transmission, while a longer line is cheaper with dc.
IV. APPLICATIONS O F HVDC
IV.1 Asynchronous connection
Because HVDC allows power transmission between
unsynchronized AC distribution systems, it can help
increase system stability, by preventing cascading failures
from propagating from one part of a wider power
transmission grid to another. Changes in load that would
cause portions of an AC network to become
unsynchronized and to separate, would not similarly affect
a DC link, and the power flow through the DC link would
tend to stabilize the AC network. The magnitude and
direction of power flow through a DC link can be directly
controlled, and changed as needed to support the AC
networks at either end of the DC link. This has caused
many power system operators to contemplate wider use of
HVDC technology for its stability benefits alone.
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"Sharpening Skills.....
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International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459 (Online), Volume 4, Special Issue 1, February 2014)
International Conference on Advanced Developments in Engineering and Technology (ICADET-14), INDIA.
IV.2 Cable system
Long undersea / underground high voltage cables have a
high electrical capacitance compared with overhead
transmission lines, since the live conductors within the
cable are surrounded by a relatively thin layer of insulation
and a metal sheath. The geometry is that of a long co-axial
capacitor. The total capacitance increases with the length of
the cable. This capacitance is in a parallel circuit with the
load. Where alternating current is used for cable
transmission, additional current must flow in the cable to
charge this cable capacitance. This extra current flow
causes added energy loss via dissipation of heat in the
conductors of the cable, raising its temperature. Additional
energy losses also occur as a result of dielectric losses in
the cable insulation.
However, if direct current is used, the cable capacitance
is charged only when the cable is first energized or if the
voltage level changes; there is no additional current
required. For a long AC powered undersea cable, the entire
current-carrying ability of the conductor would be needed
to supply the charging current alone. This cable capacitance
issue limits the length and power carrying ability of AC
powered cables. DC powered cables are only limited by
their temperature rise and Ohm's Law. Although some
leakage current flows through the dielectric insulator, this
is small compared to the cable's rated current.
V.2 Murraylink project
The Murraylink 220 MW ±150 kV HVDC Light bipolar
interconnector is believed to be the world’s longest
underground power transmission system, connecting the
Riverland region in South Australia and Sunraysia region
in Victoria through converter stations at Red Cliffs in
Victoria and Berri in South Australia. The controllable
interconnection allows power to be traded in either
direction between the two States, and provides enough
electricity to meet the needs of around 200,000 households.
V. W ORLD LARGEST HVDC P ROJECT
V.1 Itaipu project
The Itaipu HVDC Transmission Project in Brazil, owned
by Furnas Centrais Elétricas S.A. in Rio de Janeiro (an
Elétrobras company), is by far the most impressive HVDC
transmission in the world. It has a total rated power of 6300
MW and a world record voltage of ±600 kV DC. The Itaipu
HVDC transmission consists of two bipolar DC
transmission lines bringing power generated at 50 Hz in the
12600 MW Itaipu hydropower plant to the 60 Hz network
in São Paulo, in the industrial centre of Brazil.
V.3 Rio Madeira project
The Rio Madeira HVDC system is a 7,100 MW ± 600
kV high-voltage direct current transmission system in
Brazil being built to export electricity from new hydro
power plants on the Madeira River in the Amazon Basin to
major load centers in southeastern Brazil – a distance of
2,375 km, making it the longest transmission link in the
world when commissioned in 2013-2014.
Lord Krishna College of Engineering (An ISO 9001:2008 Certified Institute) Ghaziabad, Uttar Pradesh, INDIA.
Page 424
"Sharpening Skills.....
Serving Nation"
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459 (Online), Volume 4, Special Issue 1, February 2014)
International Conference on Advanced Developments in Engineering and Technology (ICADET-14), INDIA.
Several projects has been established all around the
world using HVDC to increase bulk transmission of
electricity,
underground
transmission and using
asynchronous connections.
REFERENCES
[1]
[2]
[3]
VI.
CONCLUSION
HVDC has many advantages over EHVAC and therefore
it is widely used now a days. However, it can not be a
substitute for EHVAC but with HVDC we can improve the
syste’s reliability as well as its efficiency.
[4]
[5]
Narain G. Hingorani in IEEE Spectrum magazine, 1996
The HVDC Transmission Québec–New England". ABB Asea
Brown Boveri. Retrieved 2008-12-12
Guide to the specification and design evaluation of AC filters for
HVDC systems, CIGRÉ Technical Brochure No. 139, 1999
DC side harmonics and filtering in HVDC transmission systems,
CIGRÉ Technical Brochure No. 092, 1995
Arrillaga, Jos; High Voltage Direct Current Transmission, second
edition, Institution of Electrical Engineers, ISBN 0 85296 941 4,
1998.
Lord Krishna College of Engineering (An ISO 9001:2008 Certified Institute) Ghaziabad, Uttar Pradesh, INDIA.
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