Title: high_voltage_direct_current
1High Voltage Direct Current
(HVDC)Transmission Systems An Overview.
2- 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
transmissions2. Back-to-back stations3.
Multi-terminal systems
3- 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.
4- 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 - Hewitts 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
5- 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.
6Aerial 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.
7Why HVDC? Its 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 cost2. Long distance water
crossing3. Lower losses4. Asynchronous
interconnections5. Controllability6. Limit
short circuit currents7. Environment
8- In general terms the different reasons/ADVANTAGES
for using HVDC can be divided in two main groups,
namely - HVDC is necessary or desirable from the technical
point of view (i.e. controllability). - HVDC results in a lower total investment
(including lower losses) and/or is
environmentally superior.
The Baltic Cable HVDC Link, Overview of Herrenwyk
station
9- 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
"break-even 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 over-stressed, since several other
factors, such as controllability, are important
in the selection between AC or HVDC.
10Typical investment costs for an overhead line
transmission with AC and HVDC.
11Relative 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.
12Typical tower structures and rights-of-way for
alternative transmission systems of 2,000 MW
capacity.
Source Arrillaga (1998)
13AC 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.
14AC 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.
15AC 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.
16Increased 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
172) 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.
18- 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
19Comparison of the losses for overhead line
transmissions of 1200 MW with AC and HVDC.
204)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
21- 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.
22- Normally a positive sequence program for example
ABBs SIMPOW (now transferred to STRI AB) or
PTIs 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.
23Control room with VDU displays and
mimic board at Talcher,India.
FennoSkan HVDC Station, Control room, Rauma,
Finland.
24- 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.
25- 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.
26- 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
27HVDC 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.
28- 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.
29- 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).
30Aspects on HVDC Classic performance
1.Reliability and availability 2.Losses
3.Disturbances 4.Fault performance
- 1.HVDC Classic reliability and availability
- 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.
31- 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 ABBs 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
32- 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.
33- 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.
34-
- 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 .
354.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.
36- 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 (lt 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.
37Special 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.
38Special 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.
39Renewable 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.
40HVDC 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.
41Transcontinental 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 250500 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.
42 HVDC Installations in the world today
43 New Technologies Needed?
- For transfers of 5,000 MW over 4,000 km, the
optimum voltage rises to 1,0001,100 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.
44HVDC links in India
45AC harmonic filter area at Talcher
Converter transformers of one pole at Talcher,
in front of the valve hall.
46(No Transcript)
47Rihand HVDC station, India
Rihand HVDC station, Valve Hall interior, India
48Vindhyachal HVDC 2250MW Back to back station,
India
Rihand HVDC station, Control room, India