Transcript High Voltage Direct Current (HVDC)Transmission Systems: An Overview. Prepared By :

High Voltage Direct Current
(HVDC)Transmission Systems: An Overview.
Prepared By :
What is HVDC?
HVDC stands for High Voltage Direct Current and is today a well-proven
technology employed for power transmission all over the world. In total about 70,000
MW HVDC transmission capacity is installed in more than 90 projects.
The HVDC technology is used to transmit electricity over long distances by overhead
transmission lines or submarine cables.
It is also used to interconnect separate power systems, where traditional alternating
current (AC) connections can not be used.
There are three different categories of HVDC transmissions:
1. Point to point transmissions
2. Back-to-back stations
3. Multi-terminal systems
The development of the HVDC technology started in the late 1920s, and only
after some 25 years of extensive development and pioneering work the first
commercially operating scheme was commissioned in 1954.
This was a link between the Swedish mainland and the island of
Gotland in the Baltic sea.
The power rating was 20 MW and the transmission voltage 100 kV.
At that time mercury arc valves were used for the conversion between AC
and DC, and the control equipment was using vacuum tubes.
A significant improvement of the HVDC Technology came around 1970 when
thyristor valves were introduced in place of the mercury arc valves. This
reduced the size and complexity of HVDC converter stations substantially.
 The use of microcomputers in the control equipment in today's
transmissions has also contributed to making HVDC the powerful alternative in
power transmission that it is today.
Historical Perspective on HVDC Transmission
The first commercial electricity generated (by Thomas Alva Edison) was direct current (DC)
electrical power.
The first electricity transmission systems were also DC systems.
However, DC power at low voltage could not be transmitted over long distances, thus giving
rise to high voltage alternating current (AC) electrical systems.
Nevertheless, with the development of high voltage valves, it was possible to once again
transmit DC power at high voltages and over long distances, giving rise to HVDC transmission
systems.
Important Milestones in the Development of HVDC
technology
Hewitt´s mercury-vapour rectifier, which appeared in 1901.
Experiments with thyratrons in America and mercury arc valves in Europe before 1940.
First commercial HVDC transmission, Gotland 1 in Sweden in 1954.
First solid state semiconductor valves in 1970.
First microcomputer based control equipment for HVDC in 1979.
Highest DC transmission voltage (+/- 600 kV) in Itaipú, Brazil, 1984.
First active DC filters for outstanding filtering performance in 1994.
First Capacitor Commutated Converter (CCC) in Argentina-Brazil interconnection, 1998
First Voltage Source Converter for transmission in Gotland, Sweden ,1999
The Classic HVDC Transmission
Using HVDC to interconnect two points in a power grid, in many cases is the
best economic alternative, and furthermore it has excellent
environmental benefits.
The HVDC technology (High Voltage Direct Current) is used to
transmit electricity over long distances by overhead transmission
lines or submarine cables.
It is also used to interconnect separate power systems, where
traditional alternating current (AC) connections can not be used.
In a high voltage direct current (HVDC) system, electric power is taken from
one point in a three-phase AC network, converted to DC in a converter
station, transmitted to the receiving point by an overhead line or cable and
then converted back to AC in another converter station and injected into the
receiving AC network.
Typically, an HVDC transmission has a rated power of more than 100 MW
and many are in the 1,000 - 3,000 MW range.
Aerial overview of the
3000MW HVDC converter
station at Longquan, China(
Three Gorges-Changzhou
HVDC transmission)
HVDC transmissions are used for transmission of power over long or very
long
distances, because it then becomes economically attractive over conventional
AC lines.
With an HVDC system, the power
flow can be controlled rapidly
and accurately as to both the power level and the direction. This possibility is
often used in order to improve the performance and efficiency of the connected AC
networks.
Why HVDC?: It’s Advantages
Power stations generate alternating current, AC, and the power delivered to the
consumers is in the form of AC. Why then is it sometimes more suitable to use
direct current, HVDC, for transmitting electric power?
The vast majority of electric power transmissions use three-phase alternating current.
The reasons behind a choice of HVDC instead of AC to transmit power in a specific
case are often numerous and complex. Each individual transmission project will
display its own set of reasons justifying the choice of HVDC, but the most common
arguments favoring HVDC are:
1. Lower investment cost
2. Long distance water crossing
3. Lower losses
4. Asynchronous interconnections
5. Controllability
6. Limit short circuit currents
7. Environment
In general terms the different reasons/ADVANTAGES for using HVDC can be divided in
two main groups, namely:
1. HVDC is necessary or desirable from the technical point of view (i.e. controllability).
2. HVDC results in a lower total investment (including lower losses) and/or is
environmentally superior.
The Baltic Cable HVDC Link, Overview of
Herrenwyk station
1) HVDC transmission for lower
investment cost.
A HVDC transmission line costs less than an AC line for the same
transmission capacity. However, the terminal stations are more expensive in
the HVDC case due to the fact that they must perform the conversion from
AC to DC and vice versa. But above a certain distance, the so called "breakeven distance", the HVDC alternative will always give the lowest cost.
The break-even-distance is much smaller for submarine cables (typically
about 50 km) than for an overhead line transmission. The distance depends
on several factors (both for lines and cables) and an analysis must be made
for each individual case.
The importance of the break-even-distance concept should not be overstressed, since several other factors, such as controllability, are important in the
selection between AC or HVDC.
Typical investment costs for an overhead line
transmission with AC and HVDC.
Relative Cost of AC versus DC
• For equivalent transmission capacity, a DC line
has lower construction costs than an AC line:
– A double HVAC three-phase circuit with 6
conductors is needed to get the reliability of a
two-pole DC link.
– DC requires less insulation ceteris paribus.
– For the same conductor, DC losses are less, so
other costs, and generally final losses too, can be
reduced.
– An optimized DC link has smaller towers than an
optimized AC link of equal capacity.
Typical tower structures and
rights-of-way for alternative
transmission systems of 2,000
MW capacity.
Source: Arrillaga (1998)
AC versus DC (continued)
• Right-of-way for an AC Line designed to carry 2,000
MW is more than 70% wider than the right-of-way
for a DC line of equivalent capacity.
– This is particularly important where land is expensive or
permitting is a problem.
• HVDC “light” is now also transmitted via
underground cable – the recently commissioned
Murray-Link in Australia is 200 MW over 177 km.
– Can reduce land and environmental costs, but is more
expensive per km than overhead line.
AC versus DC (continued)
• Above costs are on a per km basis. The
remaining costs also differ:
– The need to convert to and from AC implies the
terminal stations for a DC line cost more.
– There are extra losses in DC/AC conversion relative
to AC voltage transformation.
– Operation and maintenance costs are lower for an
optimized HVDC than for an equal capacity
optimized AC system.
AC versus DC (continued)
• The cost advantage of HVDC increases with the
length, but decreases with the capacity, of a link.
• For both AC and DC, design characteristics trade-off
fixed and variable costs, but losses are lower on the
optimized DC link.
• The time profile of use of the link affects the cost of
losses, since the MC of electricity fluctuates.
• Interest rates also affect the trade-off between
capital and operating costs.
Increased Benefits of Long-Distance
Transmission
• Long distance transmission increases
competition in new wholesale electricity
markets.
• Long distance electricity trade, including
across nations, allows arbitrage of price
differences.
• Contractual provision of transmission services
demands more stable networks.
• Bi-directional power transfers, often needed
in new electricity markets, can be
accommodated at lower cost using HVDC
2) HVDC cable transmissions for long distance
water crossing.
There are no technical limits for the length of a HVDC cable.
In a long AC cable transmission, the reactive power flow due to the large cable
capacitance will limit the maximum possible transmission distance. With HVDC
there is no such limitation, why, for long cable links, HVDC is the only viable technical
alternative.
The 580 kilometer-long NorNed link will be the longest underwater high-voltage
cable in the world in 2007 and thereby surpassing the present longest, the Baltic
Cable transmission between Sweden and Germany with its 250 km.
3)HVDC transmission has lower losses.
HVDC transmission losses come out lower than the AC losses in practically all cases.
An optimized HVDC transmission line has lower losses than AC lines for the same power
capacity.
The losses in the converter stations have of course to be added, but since they are only
about 0.6 % of the transmitted power in each station, the total HVDC transmission
losses come out lower than the AC losses in practically all cases.
HVDC cables also have lower losses than AC cables.
An optimized DC line has lower losses than an AC line
Comparison of the losses for overhead line
transmissions of 1200 MW with AC and HVDC.
4)HVDC link for asynchronous interconnections
Many HVDC links interconnect incompatible AC systems
Several HVDC links interconnect AC systems that are not running in synchronism with
each other.
For example the Nordel power system in Scandinavia is not synchronous with the UCTE
grid in western continental Europe even though the nominal frequencies are the same.
And the power system of eastern USA is not synchronous with that of western USA.
The reason for this is that it is sometimes difficult or impossible to connect two AC
networks due to stability reasons. In such cases HVDC is the only way to make an
exchange of power between the two networks possible. There are also HVDC links
between networks with different nominal frequencies (50 and 60 Hz) in Japan and South
America.
For smaller asynchronous interconnections HVDC Light is the proper choice.
A HVDC link can be a firewall against cascading disturbances
5) HVDC transmission for controllability of power
flow.
Controllability: One of the fundamental advantages with HVDC is that it is very easy to
control the active power in the link.
In the majority of HVDC projects, the main control is based on a constant power
transfer. This property of HVDC has become more important in recent years as the
margins in the networks have become smaller and as a result of deregulation in many
countries.
An HVDC link can never become overloaded!
In many cases the HVDC link can also be used to improve the AC system performance by
means of additional control facilities.
Normally these controls are activated automatically when certain criteria are fulfilled.
Such automatic control functions could be constant frequency control, redistribution of
the power flow in the AC network, damping of power swings in the AC networks etc.
In many cases such additional control functions can make it possible to increase the safe
power transmission capability of AC transmission lines where stability is a limitation.
Today's advanced semi-conductor technology, utilized in both power thyristors and
microprocessors for the control system, has created almost unlimited possibilities for the
control of the HVDC transmission system.
Different software programs are used for different kind of studies.
Normally a positive sequence program for example ABB’s SIMPOW (now transferred to
STRI AB) or PTI’s PSS/E program is used for load-flow and stability studies.
For more detailed investigations of the performance of the inner control loops of the
converter and its interaction with nearby network is simulated in a full three-phase
representation program such as PSCAD/EMTDC.
PSCAD/EMTDC is used for detailed investigations of the performance of a HVDC link.
Control room with VDU displays and
mimic board at Talcher,India.
FennoSkan HVDC Station, Control room,
Rauma, Finland.
6)An HVDC transmission limits short circuit
currents.
An HVDC transmission does not contribute to the short circuit current of the
interconnected AC system.
When a high power AC transmission is constructed from a power plant to a major load
center, the short circuit current level will increase in the receiving system.
High short circuit currents is becoming an increasingly difficult problem of many large
cities.
They may result in a need to replace existing circuit breakers and other equipment if their
rating is too low.
If, however, new generating plants are connected to the load center via a DC link , the
situation will be quite different.
The reason is that an HVDC transmission does not contribute to the short circuit
current of the interconnected AC system.
7) Environmental benefits
Positive effects on the power systems: Many HVDC transmissions have been built
to interconnect different power systems by overhead lines or cables. By means of these
links the existing generating plants in the networks more effectively so that the building
of new power stations can be deferred. This makes economic sense, but it is also
good for the environment.
There is an obvious environmental benefit by not having to build a new power station,
but there are even greater environmental gains in the operation of the interconnected
power system by using the available generating plants more efficiently.
The greatest environmental benefit is obtained by linking a system, which has
much hydro generation to a system with predominantly thermal generation. This has
the benefit of saving thermal generation ( predominately at peak demand ) by hydro
generation.
Also the thermal generation can be run more efficiently at constant output and
does not have to follow the load variations. This can be done easily with the hydro
generation.
A DC line can carry more power than an AC line of the same size.
The figure above compares two 3,000 MW HVDC lines (for the Three Gorges Shanghai transmission, China) to five 500 kV AC lines that would have been
used if AC transmission had been selected
HVDC technology.
The conceptual design of the classic HVDC converter stations of today dates back from the mid
1970's, when thyristor valves were taking over in place of the mercury arc valves. But there
has been a dramatic development in the performance of HVDC equipment and systems.
A HVDC converter station uses thyristor valves to perform the conversion from AC to DC and
vice versa.
The valves are normally arranged as a 12-pulse converter.
The valves are connected to the AC system by means of converter transformers.
The valves are normally placed in a building and the converter transformers are located just
outside.
The 12-pulse HVDC converter produces current harmonics (11th, 13th, 23rd, 25th, 35th,
37th etc.) on the AC side.
These harmonics are prevented from entering into the connected AC network by AC filters, i.e.
resonant circuits comprising capacitors, inductances (reactors) and resistors.
The filters also produce a part of the reactive power consumed by the converter.
The HVDC converter also gives rise to voltage harmonics on the DC side (12th, 24th, 36th
etc.).
A large inductance (smoothing reactor) is always installed on the DC side to reduce the ripple
in the direct current.
In addition, a DC filter is also normally needed to reduce the level of harmonic currents in
the DC overhead line.
The harmonics may otherwise cause interference to telephone circuits in the vicinity of the DC
line.
The power transmitted over the HVDC transmission is controlled by means of a control
system.
It adjusts the triggering instants of the thyristor valves to obtain the desired combination of
voltage and current in the DC system.
Several other apparatus are needed in a converter station, such as circuit breakers, current
and voltage transducers, surge arresters, etc.
The conceptual design of the classic HVDC converter stations remained unchanged until 1995,
when ABB introduced HVDC with Capacitor Commutated Converters (CCC).
Aspects on HVDC Classic performance
1.Reliability and availability
2.Losses
3.Disturbances
4.Fault performance
1.HVDC Classic reliability and availability
A. Transmission configuration
The Reliability and availability requirements on a particular HVDC transmission are particularly
high for links supplying major parts of a load (e.g. a city or an island) or evacuating a major
power plant. Where it is essential to have at least 50% power if an outage occurs and for large
size transmissions, a bipolar HVDC transmission is the natural choice. For network
interconnections of moderate size often a monopolar configuration is chosen.
Bipolar HVDC converter stations are designed such that there shall be no risk of having a
forced outage of both poles at the same time.
The most probable type of line fault: a ground fault due to lightning, affects only one pole.
Bipolar HVDC line faults only happens in case of a fallen line tower.
Since bipolar faults are very rare, one can regard a HVDC bipole as being equivalent to a
double circuit AC line from the reliability point of view.
B. Maintenance & Spares.
Modern HVDC converter stations require little maintenance. Most HVDC stations schedule an
annual maintenance period at a time when the utilization of the transmission is low.
For a bipolar link, one pole can be serviced while the other pole is live.
Maintenance can also be performed on redundant equipment, such as ABB´s MACH 2TM ,
when the link is in full operation.
The majority of equipment in a converter station is normal high-voltage and low-voltage
equipment ( breakers, disconnectors, transformers, capacitors, reactors, low-voltage power
distribution and motor control systems, etc) that require normal service.
To further reduce the scheduled, and forced, outage time a facility for Remote Fault Tracing
and Maintenance is included where the station can be monitored from virtually any remote
location.
Spares are normally provided based on experience.
For some items which are essential for the operation and which may cause extensive
downtime if failure happens, a complete unit is normally provided for each station. This is
normally the case for: converter transformers, smoothing reactors, wall bushings, instrument
transformers, filter reactors and resistors, etc
2.HVDC Classic transmission losses
DC Line
An optimized HVDC transmission line has lower losses than AC lines for same power capacity.
DC cables
HVDC cables also have lower losses than AC cables.
One reason for this is that there is no dielectric losses in an DC cable as there are in AC cables.
Also the full current capacity can be used for the power transmission as there is no 50 or 60
Hz charging current that causes conductor losses without any contribution to the active power.
Converter station
The losses, in the HVDC Classic converter stations amount to about 0.6 - 0.7 % of the rated
HVDC transmission capacity (per station) at rated load.
The no-load (standby) losses are about 0.1 %. The main contributors to these losses are the
converter transformers (» 50 %) and the thyristor valves (» 30 %). The rest comes from the
AC filters, the smoothing reactor, the station service power and the DC filter.
Loss minimization in AC network
If the HVDC link is operated in parallel with AC lines, there is a possibility to adjust the power
on the HVDC link to minimize the total grid losses.
3.Disturbances in HVDC Classic transmissions
The AC/DC conversion process gives rise to electromagnetic harmonics of various
frequencies. These harmonics must be dealt with in order not to cause disturbances with
communication equipment. A converter station also has equipment that generates acoustic
noise that can be disturbing to people in the neighborhood.
Telephone interference
Frequencies between 100 Hz and up to say 3 kHz, i.e. harmonics within the audible range, can
cause telephone interference to people close to the DC and AC lines coming from the converter
station.
The disturbance is then magnetically induced in the telephone cable (or wires) running at
some distance from the high voltage line.
In order to prevent this AC filters and DC filters that suppresses these frequencies are
included in the station.
Telephone interference from HVDC stations are relatively rare.
PLC interference
If power line carrier communications (in the range from 20 - 40 kHz up to about 200 kHz) are
used in the AC grid (or on the DC line) high frequency noise from the HVDC converter might
cause interference. To prevent this, a PLC filter can be installed.
Radio interference (RI)
High frequency noise from the HVDC converter might also cause radio interference in the
AM bands (150 kHz - 30 MHz) in the vicinity of the converter station.
FM radio, TV and mobile phones occupy higher frequencies and are not disturbed.
The way to avoid radio interference is proper screening of the valve buildings (or outdoor
valves).
In addition small RI-filters are normally provided that take care of the RI noise that
escape from the building via the AC and DC bushings.
Audible noise
The audible noise that a HVDC converter station emits to the surroundings comes mainly
from the converter transformers, the valve cooling fans, the smoothing reactors and the
AC and DC filters.
There are a number of methods to mitigate the noise: 1) orient disturbing equipment
away from the most sensitive sound direction, 2) use of low noise level equipment, 3)
screening or enclosing equipment .
4.HVDC Classic fault performance
DC overhead line faults
When a fault (flash-over) occurs on a AC line, there are circuit breakers that disconnects the
line. It is then normally automatically re-connected again.
There are no DC breakers in the HVDC converter stations, so when a fault occurs on a DC line
another method must be applied.
The fault is detected by the DC line fault protection. This protection orders the rectifier into
inverter mode and this discharges the line effectively. After some 80 - 100 ms the line is charged
again by the rectifier. If the fault was intermittent, due to e.g. a lightning strike, then normally
the line can support the voltage and the power transmission continues. Full power is then
resorted in about 200 ms after the fault.
But if the fault was due to contaminated line insulators there is a risk that re-charging of the
line results in a second fault.
Many HVDC transmissions are designed such that after a number of failed restart attempts the
following attempts are made with reduced voltage (80 %).
It should be pointed out that the DC line fault clearing does not involve any mechanical action
and therefore is faster than for an AC line.
The DC fault current is also lower than the AC fault current and therefore the dead time
before the restart is shorter than for an AC line!
The reduced voltage restart is also unique for HVDC.
DC cable faults
Cable faults are very rare. They are as a rule caused by mechanical damage. Therefore
submarine DC cables are often buried (except in deep waters) to prevent damage from anchors
and trawls. The same protection action occurs as for a DC line but without the restart attempt.
AC network faults
When a temporary fault occurs in the AC system connected to the rectifier, the HVDC
transmission may suffer a power loss. Even in the case of close single-phase faults, the link may
transmit up to 30 % of the pre-fault power. As soon as the fault is cleared, power is restored to
the pre-fault value.
When a fault occurs in the AC system connected to the inverter, a commutation failure can
occur interrupting power flow.
If the AC-fault is temporary the power is restored as soon as the fault is cleared.
A distant fault with little effect on the converter station voltage (< 10 percent) does not
normally lead to a commutation failure.
 A CCC (Capacitor Commutated Converter) HVDC converter can tolerate about twice this
voltage drop before there is a risk of commutation failure.
Converter station faults
HVDC converter stations are provided with an elaborate protection system that is designed to
detect fault conditions or other abnormal conditions that might expose equipment to hazard
and/or cause unacceptable disturbances. The faulty equipment is taken out of service by the
protection system.
Special Applications of HVDC
• HVDC is particularly suited to undersea
transmission, where the losses from AC are
large.
– First commercial HVDC link (Gotland 1 Sweden, in
1954) was an undersea one.
• Back-to-back converters are used to connect
two AC systems with different frequencies – as
in Japan – or two regions where AC is not
synchronized – as in the US.
Special Applications (continued)
• HVDC links can stabilize AC system frequencies
and voltages, and help with unplanned
outages.
– A DC link is asynchronous, and the conversion
stations include frequency control functions.
– Changing DC power flow rapidly and
independently of AC flows can help control
reactive power.
– HVDC links designed to carry a maximum load
cannot be overloaded by outage of parallel AC
lines.
Renewable Energy & HVDC
• HVDC seems particularly suited to many
renewable energy sources:
– Sources of supply (hydro, geothermal, wind, tidal)
are often distant from demand centers.
– Wind turbines operating at variable speed
generate power at different frequencies, requiring
conversions to and from DC.
– Large hydro projects, for example, also often
supply multiple transmission systems.
HVDC & Solar Power
• HVDC would appear to be particularly relevant
for developing large scale solar electrical
power.
• Major sources are low latitude, and high
altitude deserts, and these tend to be remote
from major demand centers.
• Photovoltaic cells also produce electricity as
DC, eliminating the need to convert at source.
Transcontinental Energy Bridges
• Siberia has large coal and gas reserves and could
produce 450-600 billion kWh of hydroelectricity
annually, 45% of Japanese output in 1995.
– A 1,800 km 11,000MW HVDC link would enable
electricity to be exported from Siberia to Japan.
– Siberia could also be linked to Alaska via HVDC.
• Zaire could produce 250–500 billion kWh of
hydroelectricity annually to send to Europe (5-6,000
km) on a 30-60,000 MW link.
• Hydroelectric projects on a similar scale have been
proposed for Canada, China and Brazil.
HVDC Installations in the world today
New Technologies Needed?
For transfers of 5,000 MW over 4,000 km, the
optimum voltage rises to 1,000–1,100 kV.
Technological developments in converter
stations would be required to handle these
voltages.
Lower line losses would reduce the optimum
voltage.
However, environmentalist opposition and
unstable international relations may be the
biggest obstacle to such grandiose schemes.
HVDC links in India
AC harmonic filter area at Talcher
Converter transformers of one pole at
Talcher, in front of the valve hall.
Rihand HVDC station, India
Rihand HVDC station, Valve Hall interior,
India
Rihand HVDC station, Control room, India
Vindhyachal HVDC 2*250MW Back to
back station, India