2. CONTENTS
Introduction
HVDC System overview
Motivations of HVDC Transmission
Components of HVDC Transmission system
Advantages and Disadvantages of HVDC Transmission system
Economical And Environmental Considerations
HVDC Applications
HVDC Light technology
Short circuit contribution of HVDC light
Conclusion
References
3. INTRODUCTION
The main HVDC project in inda as,
Assam to Agra in Uttar Pradesh, connecting 1,728 km, was started in
September 2013.
The HVDC Rihand–Delhi is a HVDC connection between Rihand and Dadri
(near Delhi) in India, put into service in 1990.
The HVDC interconnector will facilitate bidirectional control flow of power,
which means it can transfer power from the north-eastern region when it
is in surplus during the high hydro period of the year.
4. HVDC SYSTEM OVERVIEW
The system consists of three blocks: the two converter stations and the
DC line. Within each station block there are several components
involved in the conversion of AC to DC and vice versa.
The HVDC technology is used in transmission systems to transmit electric
bulk power over long distances by cable or overhead lines.
It is also used to interconnect asynchronous AC systems having the same
or different frequency.
5. The traditional HVDC system is built with line commutated current source
converters based on thyristor valves.
The operation of this converter requires a voltage source like synchronous
generators or synchronous condensers in the AC network at both ends.
The current commutated converters can not supply power to an AC system
which has no local generation.
The control of this system requires fast communication channels between
the two stations.
6. MOTIVATIONS OF HVDC TRANSMISSION
DC and AC conductors, either as overhead transmission lines or submarine
cables can have lower losses but at higher expense since the larger cross-
sectional area will generally result in lower losses but cost more.
When converters are used for d.c. transmission in preference to a.c.
transmission, it is generally by economic choice driven by one of the
following reasons;-
1. There are other environmental advantages to a d.c. transmission line
through the electric and magnetic fields being d.c. instead of ac.
2. cable transmission systems are in service whose length is in the
hundreds of kilometers and even distances of 600 km or greater have
been considered feasible.
3. if a DC converter station is located in each system with an
interconnecting DC link between them, it is possible to transfer the
required power flow even though the a.c. systems so connected remain
asynchronous.
7. TRANSMISSION LINE DELIVERY CAPABILITY
AC line distance effects:
Intermediate switching stations, e.g. every ~250 mi maximum
Lower stability limits (voltage, angle)
Increase stability limits & mitigate parallel flow with FACTS: SVC & SC
Higher reactive demand with load
Higher charging at light load
Parallel flow issues more prevalent
Thermal limit remains the same
8. DC line distance effects:
No distance effect on stability (voltage, angle)
No need for intermediate stations
No parallel flow issues due to control
Minor change in short circuit levels
No increase in reactive power demand
9. HIGHLIGHTS FROM THE HVDC HISTORY
The Transmission and Distribution of Electrical Energy started with
direct current. In 1882,a 50 km long 2 KV DC line was built between
Miesbach and Munich in Germany.
In an AC system voltage conversion is simple. An AC transformer allows
high power levels and high insulation levels within one unit and has low
losses. It is a relatively simple device which requires little maintenance
Further.
AC technology was introduced at a very early stage in the development
of electrical power systems . It was soon accepted as the only feasible
technology for generation , transmission and distribution of electrical
energy.
10. High-voltage AC transmission links have disadvantages, which may
compel a change to DC technology:
Inductive and Capacitive elements of overhead lines and cables put limits to
the transmission capacity and the transmission distance of AC transmission
links.
This limitation is of particular significance for cables Depending on the
required transmission capacity .the system frequency and loss evaluation
achievable transmission distance for an AC cable will be in the range of 40
to 100 km .It will mainly be limited by the charging current.
Direct connection between two AC systems with different frequencies is not
possible.
Direct connection between two AC systems with the same frequency or a
new connection within a meshed grid may be impossible because of system
instability,too high short-circuit levels or undesirable power flow scenarios.
11. ARGUMENTS FAVOURING HVDC:
The most common arguments favouring HVDC are:
Investment cost
Long distance water crossing
Lower losses
Asynchronous connection
Controllability
Limit short circuit currents
Environment
12. Engineers were therefore engaged over generations in the development of
a technology for DC transmissions as a supplement to the AC transmissions.
13. COMPONENTS OF HVDC TRANSMISSION SYSTEM
The Thyristor or IGBT(Insulated Gate Bipolar Transistor) valves make
the conversion from AC to DC and thus are the main component of any
HVDC converter. Each single valve consists of a certain amount of
series connected thyristors (or IGBTs) with their auxiliary circuits.
The Converter Transformers transform the voltage level of the AC
busbar to the required entry voltage level of the converter. The main
component of a converter station are:
Thyristor valves
VSC valves
14. The Smoothing reactor, which main functions are:
Prevention of the intermittent current
Limitation of the DC fault currents
Prevention of resonance in the DC circuits
The Harmonic Filters, on the AC side of a HVDC converter station, which have
two main duties:
To absorb harmonic currents generated by the HVDC converter
To supply reactive power
DC filters
Surge arrester
DC Transmission circuit
Control and Protection
15. SUBSTATION CONFIGURATION
The central equipment of a DC substation are the thyristor converters
which are usually housed inside a valve hall.Figure shows an example of
the electrical equipment required for a d.c. substation.
16. ADVANTAGES AND DISADVANTAGES OF HVDC
TRANSMISSION
ADVANTAGES
HVDC is the ability to transmit large amounts of power over long distances
with lower capital costs and with lower losses than AC. Depending on
voltage level and construction details, losses are quoted as about 3% per
1,000 km.
In a number of applications HVDC is more effective than AC transmission.
Examples include:
o Undersea cables
o Endpoint-to-endpoint long-haul bulk power transmission without
intermediate 'taps‘.
o Increasing the capacity of an existing power grid in situations where
additional wires are difficult or expensive to install.
o Power transmission and stabilization between unsynchronised AC
distribution systems
17. o Connecting a remote generating plant to the distribution grid.
o Stabilizing a predominantly AC power-grid.
o Reducing line cost.
o Facilitate power transmission between different countries that use AC at
differing voltages and/or frequencies.
o Synchronize AC produced by renewable energy sources.
Long undersea high voltage cables have a high electrical capacitance, since
the conductors are surrounded by a relatively thin layer of insulation and a
metal sheath.
DC cables have no such limitation. Although some DC leakage current
continues to flow through the dielectric, this is very small compared to the
cable rating.
HVDC can carry more power per conductor because, for a given power
rating, the constant voltage in a DC line is lower than the peak voltage in an
AC line.
18. The peak voltage of AC determines the actual insulation thickness and
conductor spacing. Because DC operates at a constant maximum voltage,
this allows existing transmission line corridors with equally sized
conductors and insulation to carry 100% more power into an area of high
power consumption than AC, which can lower costs.
The magnitude and direction of power flow through a DC link can be
directly commanded, and changed as needed to support the AC networks
at either end of the DC link
19. DISADVANTAGES
The disadvantages of HVDC are in conversion, switching, control,
availability and maintenance.
HVDC is less reliable and has lower availability than AC systems. mainly due
to the extra conversion equipment.
At smaller transmission distances the losses in the static inverters may be
bigger than in an AC transmission line.
In contrast to AC systems, realizing multiterminal systems is complex, as is
expanding existing schemes to multiterminal systems.
High voltage DC circuit breakers are difficult to build because some
mechanism must be included in the circuit breaker to force current to zero,
otherwise arcing and contact wear would be too great to allow reliable
switching.
Operating a HVDC scheme requires many spare parts to be kept, often
exclusively for one system as HVDC systems are less standardized than AC
systems and technology changes faster.
20. ECONOMICAL AND ENVIRONMENTAL
CONSIDERATIONS
ECONOMIC CONSIDERATIONS
• A study for Oak Ridge National Laboratory reported on a survey to 3
suppliers of HVDC equipment for quotations of turnkey costs to supply two
bipolar substations for four representative systems. Each substation
requires one d.c. electrode and interfaces to an a.c. system with a short
circuit capacity four times the rating of the HVDC system.
• Transmission line costs cannot be so readily defined. Variations depend on
the cost of use of the land, the width of the right-of-way required, labor
rates for construction, and the difficulty of the terrain to be crossed.
• The cost advantage of d.c. transmission for traversing long distances is that
it may be rated at twice the power flow capacity of an a.c. line of the same
voltage.
• When electricity must be transmitted by underground or undersea cables,
a.c. cables become impractical due to their capacitive charging current
21. ENVIRONMENTAL CONDITIONS
The electrical environmental effects from HVDC. transmission lines can be
characterized by field and ion effects as well as corona effects
o FIELD AND CORONA EFFECTS
The field and corona effects of transmission lines largely favor d.c.
transmission over AC transmission. The significant considerations are as
follows:
1. For a given power transfer requiring extra high voltage transmission
2. The steady and direct magnetic field of a d.c. transmission line near or at
the edge of the transmission right-of-way will be about the same value
in magnitude as the earth’s naturally occurring magnetic field.
3. The static and steady electric field from d.c. transmission at the levels
experienced beneath lines or at the edge of the right-of-way have no
known adverse biological effects.
4. The ion and corona effects of d.c. transmission lines lead to a small
contribution of ozone production to higher naturally occurring
background concentrations
22. HVDC APPLICATIONS
Long Distance Bulk Power Transmission
Cable Transmission
Asynchronous Ties
Offshore Transmission
Power Delivery to Large Urban Areas
23. HVDC LIGHT TECHNOLOGY
HVDC Light represents electric power transmission by HVDC based on voltage
source converters.
This newly developed technology has various interesting characteristics that
make it a very promising tool for transmission of electric power to distant
loads, where no other transmission is possible or economic.
This refers specifically to the generation by internal control of the phase
voltages in the inverter, that could serve the loads in the connected AC
network.
New DC power cables based on a modified triple extrusion technology and a
specially designed DC material have been developed. DC power cables with
ratings 30 MW at 100 kV can be accomplished weighting only 1 kg/m.
For the future both powers and voltages will increase and extension to pure
DC networks will be possible
24. HVDC LIGHT TRANSMISSION SYSTEM
The HVDC Light transmission system mainly consists of two cables and two
converter stations. Each converter station is composed of a voltage source
converter (VSC) built up with IGBTs, phase reactors, ac filters and
transformer, as shown in Fig.
25. • By using pulse width modulation (PWM), the amplitude and phase
angle (even the frequency) of the converter AC output voltage can
be adjusted simultaneously.Since the AC side voltage holds two
degrees of control freedom, independent active and reactive power
control can be realized.
• Under the normal operation condition, the VSC can be seen as a
voltage source.
• Under abnormal operation conditions, for instance, during an ac
short-circuit fault, the VSC may be seen as a current source, as the
current capacity of the VSC is limited and controllable.
26. ADVANTAGES
Reduced environmental impact
Faster and easier issue of permits using DC underground cables.
The system reliability is enhanced with reduced risk of damage from
natural causes such as storms, wind, earthquakes and fire. You simply bury
it and forget it.
Operation and maintenance costs of the transmission easement are
virtually eliminated as there is no need for long term contracts to
maintain.
The width of the corridor to install the underground cable can be as narrow
as 4 meters, which will give greater flexibility with the selection of a
transmission route.
27. SHORT CIRCUIT CONTRIBUTION OF HVDC LIGHT
STUDIED AC SYSTEMS
o The studied AC system has a mixture structure in radial and mesh
connection It includes high, medium and low voltage buses.
o The AC transmission lines are modeled with π-link. The loads are constant
current loads.Three types of fault, namely, the close-in fault; the near-by
fault and the distant fault.
IMPACT OF STRENGTH OF AC NETWORKS
o The possible maximum relative short circuit current increment (∆Imax) is
determined by the short circuit ratio(SCR).
o Supposing that the ∆Imax is defined as it is found that the ∆Imax is
inversely in proportional to the SCR as the solid curve shown in Fig
28. The maximum possible short circuit current increment is in the boundary
defined by the two dashed curves. AC networks with SCR equal to
1.85,3.14 and 12 have been simulated and the results are also shown in
figure with black dots.
29. THE IMPACT OF CONTROL MODES
o The current is mainly limited by the impedances of transmission lines and
transformers when a short circuit occurs.
o It is important to notice that the change of short circuit current and the
variation of bus voltages usually go hand in hand. The increase of short
circuit current, namely, the increase of short circuit capacity, will improve
the voltage stability and minimize the reduction of bus voltage due to
faults.
o With Uacctrl control mode, the reactive current generation will be
automatically increased when the AC voltage decreases.
THE IMPACT OF OPERATION POINTS
o As it has been discussed, the maximum possible short circuit increment
(∆Imax) due to HVDC Light is determined by the SCR.
o It will occur if the VSC is operating at zero active power, namely, it is
operating as an SVC or STATCOM. Fig.9.2 shows the characteristic of short
circuit current contribution versus the load level
30. o The two dashed curves are the result by taking into account the
transformer winding ratio variation due to the tap-changer. AC networks
with SCR equal to 3.14 has been simulated. For different load levels the
observed short circuit currents,during a 3-ph close fault, are marked with
black dots.
31. CONCLUSION
A high-voltage, direct current (HVDC) electric power transmission system uses
direct current for the bulk transmission of electrical power, in contrast with the
more common alternating current systems.
HVDC systems are less expensive and suffer lower electrical losses. For shorter
distances, the higher cost of DC conversion equipment compared to an AC
system may be warranted where other benefits of direct current links are
useful.HVDC systems remain the best economical and environmentally friendly
option for the above conventional applications.
HVDC Light is a new technology that has been specifically developed to match
the requirements of the new competitive electricity markets. It provides the
ability to connect renewable generation to the AC grid.
32. The technical merits are that by virtue of their standardised prefabricated
modular constructions which lead to short delivery times, it is relocatable
and can be expanded to meet growing demand.
A pair of lightweight DC cables can be laid direct in the ground in a cost-
effective way which is comparable to or less than a corresponding total life
cycle cost of AC overhead line.
For these reasons HVDC Light provides an important role as a business
concept and opens up new opportunities for both investors and
environmentalist.
33. REFERENCES
G. Asplund, “Application of HVDC Light to Power System
Enhancement”, presented at IEEE/PES Winter Meeting,
Singapore,January 2000.
M. P. Bahrman, B. K. Johnson, “The ABCs of HVDC transmission
technologies,” IEEE Power & Energy, vol. 5, pp.32-44, Apr. 2007.
J. Zhu, H. Chao, R. Mukerji, D. Wang and L. Brown, “Economic
assessment for transmission upgrades in a deregulated market,”
2006 Session, CIGRE C1-115.
34. M. P. Bahrman, B. K. Johnson, “HVDC Transmission overview”,IEEE
SIEMENS, “High voltage direct current transmission - proven technology for
power exchange,” Mars 2007, brochure from SIEMENS, Source
G. Asplund, “Application of HVDC Light to Power System Enhancement”,
presented at IEEE/PES Winter Meeting, Singapore,January 2000.
U. Axelsson, A. Holm, C. Liljegren, M. Åberg, K. Eriksson and O.Tollerz, “The
Gotland HVDC LIGHT Project – Experiences from Trial Commercial Operation”
Presented at CIRED Conference, Amsterdam,The Netherlands, June 18-21,
2001.
