| |
High-voltage direct current[Categories: Electrical
engineering]
HVDC or high-voltage, direct
current electric
power transmission systems contrast with the more common alternating-current
systems as a means for the bulk transmission of electrial power.
The modern form of HVDC transmission used technology developed
extensively in the ansmission.
However, low voltage is convenient for
utilisation equipment such as lamps
and motors.
The principal advantage of AC is the use of transformers
to change the voltage at which power is used. No equivalent of the
transformer exists for direct current, so the manipulation of DC
voltages is considerably more complex. With the development of
efficient AC machines, such as the induction
motor, AC transmission and utilization became the norm (see
War
of Currents).
History of HVDC transmission An early method of
high-voltage DC transmission was developed by the Swiss engineer
Rene Thury [5]. This system used series-connected motor-generator
sets to increase voltage. Each set was insulated from ground and
driven by insulated shafts from a prime mover. An early example of
this system was installed in 1889 in Italy
by the Society Acquedotto de Ferrari-Gallieri. This system
transmitted 630 kW at 14 kV DC over a distance of 120 km.[6] Other
Thury systems operating at up to 100 kV dc operated up until the
1930s, but the rotating machinery required high maintenance and had
high energy loss. Various other electromechanical devices were
tested during the first half of the 20th century with little
commercial success[7].
The grid controlled mercury
arc valve became available for power transmission during the
period 1920 to 1940[9]. In 1941 a 60 MW, +/- 200 kV link was
designed for the city of Berlin
using mercury arc valves (Elbe-Project),
but owing to the collapse of the German government in 1945 the
project was never completed [8]. This system was to provide power
over a 115 km buried cable, which during wartime would be less
conspicuous as a bombing target. The equipment was removed to the
HVDC systems use only solid-state devices.
Advantages of HVDC over AC Transmission In a number of
applications HVDC is often the preferred option.
Undersea
cables. (e.g. 250 km Baltic
Cable between Sweden
and Germany
[3]).
Endpoint-to-endpoint long-haul bulk power transmission without
intermediate 'taps', for example, in remote areas.
Increasing the capacity of an existing power grid in situations
where additional wires are difficult or expensive to
install.
Allowing power transmission between unsynchronised AC distribution
systems.
Reducing the profile of wiring and pylons for a given power
transmission capacity.
Connection of remote generating plant to the distribution grid, for
example Nelson
River Bipole.
Stabilising a predominantly AC power-grid,without increasing maximum
prospective short circuit current
Long undersea cables
have a high capacitance.
While this has minimal effect for dc transmission, the current
required to charge and discharge the capacitance of the cable causes
additional power losses when the cable is carrying ac. In addition,
ac power is lost to dielectric
losses.
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. This voltage
determines the insulation thickness and conductor spacing. This
allows existing transmission line corridors to be used to carry more
power into an area of high power consumption, which can lower
costs.
Possible health advantages of HVDC over AC transmission A
high-voltage DC transmission line would not produce the same sort of
extremely
low frequency (ELF) electromagnetic field as would an equivalent
AC line. It is speculated by those who believe that ELF radiation is
harmful that such a reduction in EM fields would be beneficial in
terms of health. The benefits would extend only to those near the
transmission lines, as the electric and magnetic fields associated
with high current AC transmission lines do not travel far beyond the
actual lines themselves. These fields are, however, also associated
with electrical equipment and household appliances. It should be
noted that the current scientific consensus
does not consider ELF sources and their associated fields to
be particularly harmful.
DisadvantagesThe required static invertors are expensive
and cannot be overloaded very much. At smaller transmission
distances the losses in the static inverters may be bigger than in
an AC powerline, and the cost of the inverters may not be offset by
reductions in line construction cost.
In contrast to AC
systems, realizing multiterminal systems is complex, as is expanding
existing schemes to multiterminal systems. Controlling power flow in
a multiterminal DC system requires good communication between all
the terminals.
AC networks interconnections Ac transmission lines can only
interconnect synchronized AC networks, which oscillate at the same
frequency and in phase. Many areas which wish to share power have
unsynchronized networks. The power grids of the UK,
Northern Europe and continental Europe all operating at 50 Hz are
not synchronized. Japan
has 50 Hz and 60 Hz networks. Continental North America, while
operating at 60Hz throughout, is divided into regions which are
unsynchronised, East, West, Texas
and Quebec.
Brazil
and Paraguay
which share the massive Itaipu
hydroelectric plant, operate on 60Hz and 50Hz respectively. However,
HVDC systems makes it possible to interconnect unsynchronized AC
networks, and also adds the possibility of controlling AC voltage
and reactive power flow.
A generator
connected to a long ac transmission line may become unstable and
fall out of synchronization with a distant ac power system. An HVDC
transmission link may make it economically feasible to use remote
generation sites. Wind
farms located off-shore may use HVDC systems to collect power
from multiple unsynchronized generators for transmission to the
shore by an underwater cable.
In general, however, an HVDC
power line will interconnect two AC regions of the
power-distribution grid. Machinery to convert between AC and DC
power adds a considerable cost in power transmission. The conversion
from AC to DC is known as rectification,
and from DC to AC as inversion.
Above a certain break-even distance (about 50 km for submarine
cables, and perhaps 600-800 km for overhead cables [3]), the lower
cost of the HVDC electrical conductors outweighs the cost of the
electronics.
The conversion electronics also present an
opportunity to effectively manage the power grid by means of
controlling the magnitude and direction of power flow. An additional
advantage of the existence of HVDC links, therefore, is potential
increased stability in the transmission grid.
Rectifying and inverting
Rectifying and inverting components Early static systems
used mercury
arc rectifiers, which were unreliable. Nevertheless some HVDC
systems using mercury
arc rectifiers are still in service in 2005. The thyristor
valve was first used in HVDC systems in the 1960s.
The thyristor is a solid-state semiconductor
device similar to the diode,
but with an extra control terminal that is used to switch the device
on at a particular instant during the AC cycle. The insulated-gate
bipolar transistor (IGBT) is now also used.
Because the
voltages in HVDC systems, around 500 kV in some cases, exceed the
breakdown voltages of the semiconductor devices, HVDC converters are
built using large numbers of semiconductors in series.
The
low-voltage control circuits used to switch the thyristors on and
off need to be isolated from the high voltages present on the
transmission lines. This is usually done optically. In a hybrid
control system, the low-voltage control electronics sends light
pulses along optical fibres to the high-side control
electronics. Another system, called direct light triggering,
dispenses with the high-side electronics, instead using light pulses
from the control electronics to switch light-triggered thyristors
(LTTs).
A complete switching element is commonly referred to
as a 'valve', irrespective of its construction.
Rectifying and inverting systems Rectification and
inversion use essentially the same machinery. Many substations are
set up in such a way that they can act as both rectifiers and
inverters. At the AC end a set of transformers, often three
physically separate single-phase transformers, isolate the station
from the AC supply, to provide a local earth, and to ensure the
correct eventual DC voltage. The output of these transformers is
then connected to a bridge rectifier formed by a number of valves.
The basic configuration uses six valves, connecting each of the
three phases to each of the DC rails. However, with a phase change
only every sixty degrees, considerable harmonics remain on the DC
rails.
An enhancement of this configuration uses 12 valves
(often known as a twelve-pulse system). The AC is split into
two separate three phase supplies before transformation. One of the
sets of supplies is then configured to have a star (wye) secondary,
the other a delta secondary, establishing a thirty degree phase
difference between each of the sets of three phases. With twelve
valves connecting each of the two sets of three phases to the two DC
rails, there is a phase change every 30 degrees, and harmonics are
considerably reduced.
In addition to the conversion
transformers and valve-sets, various passive resistive and reactive
components help filter harmonics out of the DC rails.
Configurations
Monopole and earth return In a common configuration, called
monopole, one of the terminals of the rectifier is connected to
earth ground. The other terminal, at a potential high above, or
below, ground, is connected to a transmission line. The earthed
terminal may or may not be connected to the corresponding connection
at the inverting station by means of a second conductor.
If
no metallic conductor is installed, current flows in the earth
between the earth electrodes at the two stations. The issues
surrounding earth-return current include
Electrochemical corrosion of long buried metal objects such as pipelines
Underwater earth-return electrodes in seawater may produce chlorine
or otherwise affect water chemistry.
An unbalanced current path may result in a net magnetic field, which
can affect magnetic navigational
compasses
for ships passing over an underwater cable.
These effects can
be eliminated with installation of a metallic return conductor
between the two ends of the monopolar transmission line. Since one
terminal of the converters is connected to earth, the return
conductor need not be insulated for the full transmission voltage
which makes it less costly than the high-voltage conductor. Use of a
metallic return conductor is decided based on economic, technical
and environmental factors[4].
Modern monopolar systems for
pure overhead lines carry typically 1500 MW. If underground or
seacables are used the typical value is 600 MW.
BipolarIn bipolar transmission a pair of conductors is
used, each at a high potential with respect to ground, in opposite
polarity. Since these conductors must be insulated for the full
voltage, transmission line cost is higher than a monopole with a
return conductor. However, there are a number of advantages to
bipolar transmission which can make it the attractive
option.
Under normal load, negligible earth-current flows, as in the case of
monopolar transmission with a metallic earth-return; minimising eart
return loss and environmental effects.
When a fault develops in a line, with earth return electrodes have
been installed at each end of the line, current can continue flow
using the earth as a return path, operating in monopolar
mode.
Since for a given power rating bipolar lines carry only half the
current of monopolar lines, the cost of the second conductor is
reduced compared to a monopolar line of the same rating.
In very adverse terrain, the second conductor may be carried on an
independant set of transmission towers, so that some power may
continue to be transmitted even if one line is damaged.
A
bipolar system may also be installed with a metallic earth return
conductor.
Bipolar systems may carry as much as 3000 MW at
voltages of +/-533 kV. Submarine cable installations initially
commissioned as a monopole may be upgraded with additional cables
and operated as a bipole.
Back to backA back-to-back station is a plant in
which both static inverters are in the same area, usually even in
the same building and the length of the direct current line is only
a few meters. HVDC back-to-back stations are used for
coupling of electricity mains of different frequency (as in
Japan)
coupling two networks of the same nominal frequency but no fixed
phase relationship
different frequency and phase number (for example, as a replacement
for traction
current converter plants)
different modes of operation (as until 1995/96 in Etzenricht,
Dürnrohr
and Vienna).
In
contrast to HVDC long-distance lines, the DC voltage in the
intermediate circuit can be selected freely at HVDC back-to-back
stations because of the short conductor length. The DC voltage is as
low as possible, in order to build a small valve hall and to avoid
parallel switching of valves. For this reason at HVDC back-to-back
stations the strongest available static inverter valves are
used.
Corona dischargeCorona
discharge is the creation of ions
in a fluid
(such as air)
by the presence of a strong electromagnetic
field. Electrons
are torn from unionised
air, and either the positive ions or else the electrons are
attracted to the conductor, whilst the charged particles drift. This
effect can cause considerable power loss, create audible and
radio-frequency interference, generate toxic compounds such as
oxides of nitrogen and ozone, and lead to arcing.
Both AC and
DC transmission lines can generate coronas, in the former case in
the form of oscillating particles, in the latter a constant wind.
Due to the space
charge formed around the conductors, an HVDC system may have
about half the loss per unit length of a high voltage AC system
carrying the same amount of power. With monopolar transmission the
choice of polarity of the energised conductor leads to a degree of
control over the corona discharge. In particular, the polarity of
the ions emitted can be controlled, which may have an environmental
impact on particulate condensation (particles of different
polarities have a different mean-free path). Negative
coronas generate considerably more ozone than positive
coronas, and generate it further downwind of the power
line, creating the potential for health effects. The use of a
positive
voltage will reduce the ozone impacts of monopole HVDC power
lines.
Applications
Overview The controllability of current-flow through HVDC
rectifiers and inverters, their application in connecting
unsynchronized networks, and their applications in efficient
submarine cables mean that HVDC cables are often used at national
boundaries for the exchange of power. Offshore windfarms also
require undersea cables, and their turbines
are unsynchronized. In very long-distance connections between just
two points, for example around the remote communities of Siberia,
Canada,
and the Scandinavian
North, the decreased line-costs of HVDC also makes it the usual
choice. Other applications have been noted throughout this
article.
The development of insulated
gate bipolar transistors and gate
turn-off thyristors has made smaller HVDC systems economical.
These may be installed in existing AC grids for their role in
stabilizing power flow without the additional short-circuit current
that would be produced by an additional AC transmission line. One
manufacturer calls this concept "HVDC Light", and has extended the
use of HVDC down to blocks as small at a few tens of megawatts and
lines as short as a few score kilometres of overhead line.
System configurations A HVDC link in which the two AC-to-DC
converters are housed in the same building, the HVDC transmission
existing only within the building itself, is called a
back-to-back HVDC link. This is the common configuration for
interconnecting two unsynchronised grids or for changing frequency
or for stabilizing an AC network.
HVDC back-to-back stations
can also be designed to deliver single phase AC. This is required
for Traction
current converter plants.
The most common configuration
of an HVDC link is a station-to-station
link,
where two inverter/rectifier
stations are connected by means of a dedicated HVDC link. This is
also a configuration commonly used in connecting unsynchronised
grids, in long-haul power transmission, and in undersea
cables.
Multi-terminal HVDC links, connecting more than two
points, are rare. The configuration of multiple terminals
can be series,
parallel,
or hybrid (a mixture of series and parallel). Parallel configuration
tends to be used for large capacity stations, and series for lower
capacity stations. An example is the 2000 MW Quebec
- New England Transmission system opened in 1992, which is
currently the largest multi-terminal HVDC system in the world.
[3]
|
| |