500 kv vsc transmission system for lines and cables
DESCRIPTION
HVDCTRANSCRIPT
B4-6
500 kV VSC Transmission System for lines and cables
B. JACOBSON, B. WESTMAN, M. P. BAHRMAN*
ABB
Sweden, USA*
In many cases an elevated transmission voltage is needed to reach an optimum balance between
power capacity, investment cost and losses. High voltage also implies large power blocks. The
maximum allowable block size is determined by total system response to loss of links or converters.
In weaker networks classic current source converters will require special measures to operate well.
Here the by now traditional advantages of voltage source converters (VSC) can be used to facilitate
addition of transmission capacity even to inject power into points with very low available short
circuit power. Other aspects of the VSC technology like relatively small foot print and the possibility
to connect isolated wind farms can be other factors leading to the selection of this particular
converter type.
By dividing the power of the VSC converters in two separate poles, a high utilization of converter
rating and optimal lines can be obtained without surpassing the reliability criteria for the network.
Two blocks of opposite polarity is preferred because they can balance out any ground current in
normal operation without the need for a third conductor or cable. A key component in this scheme
is the asymmetrical voltage source converter.
The paper will cover the status of the HVDC converter technology, particularily with respect to two
basic functional topologies namely asymmetric monopoles and the bipole configuration. Examples of
two operational asymmetric links and one asymmetric link under design and construction will be
covered detailing the specific requirements and operational duties that led to the selection of
asymmetric links. Aspects of cable connection as well as overhead lines will be covered. Finally some
future possible applications of this technology will be discussed.
2012 San Francisco Colloquium
Advances in voltage source converter (VSC) technologies
http : //www.cigre.org
1 StatusofthetechnologyVSC transmission has changed from a niche technology to being a large portion of the total HVDC
market in the past five year period. Some of the aspects bringing about this shift are increased
demand for advanced transmission, the build‐up of a large and varied pool of user experience, the
increase of power and voltage ratings, the reduction of losses and last but not least the availability of
more than a single supplier with now three vendors having delivered or been awarded large
international VSC transmission system contracts.
Examples of advanced transmission are interconnections between weak networks, power from
shore, multi‐infeed and wind power infeed, especially from offshore generation. The pool of user
experience needs not be listed here but can be found on vendor websites, e.g. [1].
Voltage ratings have gradually progressed. The highest pole voltage in operation for VSC is 350 kV
(Caprivi Link, Namibia) and the highest voltage in an awarded contract is 500 kV (Skagerrak 4,
Denmark‐Norway). The voltage level is not in principle limited by the converters, so it would be
feasible to build VSC converters to the 800 kV level, this being the highest direct voltage in
operation, albeit on a LCC (Line Commutated Converter or “Classic”) HVDC system. The experience
from the HVDC Classic insulation design could be adapted to VSC converters.
Cable voltages available is cited at 500‐550 kV from different vendors. Typical cable currents could
be 1400‐2000 A depending on thermal conditions and choice of conductor material, where copper
would give higher current capacity but also higher material cost and total weight. Increased
transmission capacity could be created by parallelling cables, especially for short bypass sections of
an otherwise overheadline transmission, for instance when passing a river or an urban area.
Figure 1 Evolution of losses for HVDC Light (VSC Transmission). HVDC Classic typical losses shown in green
for comparison.
In successive generations of VSC technology, the converter station losses have been reduced step by
step. The now more or less prevailing multi‐level technology has led to conversion losses of about
1% per station, which is a huge step from the early days of VSC when 3% was a typical value. To
come down to 1% or even less, advances in topologies (multilevel converters) and switching
strategies have to be paired with careful semiconductor selection and even optimization of the
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semiconductors and their driving for the specific application in HVDC. The semiconductor of choice
for VSC is the Insulated Gate Bipolar Transistor by virtue of low driving power, predictable behaviour
during transient events, large experience bank, comparatively low losses and good availability in
suitable ratings on the market.
In the different types of possible multi‐level technologies, there is the choice to use series
connection of semiconductors. The main reason for using series connection is reliability. When series
connecting several semiconductors in a converter cell, single device failures are acceptable and
operation of the converter can continue without need to bypass the cell with a faulty IGBT. Without
series connection, the affected cell needs to be bypassed with some type of device, often a
mechanical switch, otherwise operation of the converter has to be stopped because of the
unpredictable status of a converter cell with no means of control left. In order to take full advantage
of series connection specially designed IGBTs that fail to a stable short circuit are used.
2 ProjectswithasymmetricconvertersTo date three asymmetric VSC transmission systems are in operation/construction but the reasons
to select this design and the operation modes used vary:
Valhall (78 MW, 150 kV) was designed as an asymmetrical monopole to make optimum use
of the coaxial HVDC cable design.
Caprivi (250 MW, 300 kV) was designed as an asymmetrical monopole but is prepared for a
future additional converter and bipolar operation.
Skagerrak 4 (715 MW, 450 kV) is designed as a monopole and will be operated in bipolar
mode with Skagerrak 3 (which uses LCC technology).
2.1 ValhallThe Valhall platform complex has been in operation since 1982 but the facilities needed to be
upgraded and it was decided that the platforms should be supplied with power from the Norwegian
AC grid and to remove the existing gas turbines installed on of the platforms.
The link is 78 MW HVDC link and use asymmetrical 150 kV VSC converters and a 292 km long 150 kV
HVDC cable. The power coming from shore is converted in the offshore HVDC station to AC and then
distributed on 11 kV level to the hotel and production platforms. The majority of the load on the
platform complex consists of compressors and pumps.
Figure 2 The Valhall platform complex is located Figure 3 Lista onshore HVDC converter station.
roughly halfway between UK and Norway.
The VSC converter concept offers important advantages in offshore installations as space and weight
are key issues and cost drivers for the platform. The filters used in VSC design are all small and can
be made compact and light‐weight compared to other solutions.
The transmission link went into operation 2011.
2.2 CapriviLinkThe Caprivi Link Interconnector is rated 300 MW at 350 kV and the transmission distance is 950 km.
The link is interconnecting the AC networks of Namibia and Zambia and is important in the Southern
African Pool. This is the first project that uses VSC technology together with overhead lines.
The link has been design to allow a future bipolar extension and the DC overhead lines are built with
two poles from the start. The scheme can in the first phase be operated in three alternative
operational modes:
1. Metallic return using the second pole as return conductor
2. Earth return using one DC line conductor
3. Earth return using parallelled DC line pole conductors.
The losses are particularly low in operation mode 3.
The long overhead line is of course subject to lightning strikes but also to disturbances emanating
from fires in the brush vegetation in the arid parts of the transmission path. The need to come back
quickly to stable transmission even during adverse electrical network conditions has been solved,
partly by employing conventional high voltage dc breakers and partly by developing controls and
sequences that enable an adapted response to a wide range of contingencies.
The transmission link went into operation 2011.
Figure 4 Zambezi converter station located in the Caprivi region. The design allows integration of a future
second HVDC pole.
2.3 Skagerrak4In February 2011, an order for a 715 MW 500 kV VSC asymmetric system was awarded for an
additional parallel fourth interconnection between Norway and Denmark, the Skagerrak HVDC
interconnections. The three existing HVDC links in the system all use LCC technology and have a
capacity of 1050 MW. The two first links SK1 and SK2 have been in operation since mid 1970’s. The
new link SK4 will operate in bipolar operation with SK3 (in operation since the early 1990’s).
The new link’s HVDC voltage of 500 kV is to date the highest pole voltage for a VSC system. Particular
challenges in this project, apart from the record voltage, include parallel operation to three other
LCC HVDC systems and a requirement that the VSC system should be able to balance the resulting
current when one of the other poles go out of operation or when other conditions in operation so
demand. This has called for the first VSC system design that is required to reverse polarity. Also the
project requires the full use of the reactive power capability inherent in the VSC to enable
transmission even during degraded or abnormal network conditions.
3 PotentialNorthAmericanApplicationsfor±500kVVSCHVDC North America has abundant, high quality wind resources across the Great Plains on the centre of
the continent. Many of the major load centres, however, are located over 1000 km to the east or
west of this region. Transmission in this prime wind region is relatively sparse reflecting its
comparatively low population density. The networks across this region are separated by
asynchronous boundaries between the eastern and western interconnects and with the Electric
Reliability Council of Texas (ERCOT). Substantial transmission additions are required to integrate
these wind resources.
With its increased converter capacity and improved efficiency, the unique characteristics of VSC
based HVDC transmission make it an even more attractive for integration and delivery of renewable
energy from non‐traditional generation. Circuit capacity is now roughly equivalent to that of a 500
kV ac line but with lower line cost and lower line losses. In a traditional bipolar configuration,
transmission capacity is similar to that of a double circuit 500 kV ac line but at about half the line
cost. Higher transmission capacities are possible with parallel converters or alternative circuit
configurations, e.g. multiple symmetrical monopoles. Unlike with an ac line, however, there is no
variable reactive power demand or stability limitation due to distance effects. If the transmission
path crosses sensitive or congested areas, where permits and right‐of‐way acquisition are difficult to
obtain on a timely basis, underground transmission with solid dielectric, extruded dc cables can be
used for all or a portion of the route without distance limitation and with due consideration of
shared rights‐of‐way. Figure 5 shows the configuration for a bipolar VSC HVDC transmission link
with hybrid overhead – underground transmission. The link is equipped with provision for metallic
return switching.
The controllability of HVDC transmission mitigates inadvertent flow on parallel paths and bypasses
congestion thereby reserving established transmission capacity for its intended purpose.
Controllability tends to firm up HVDC transmission capacity since congestion on parallel paths is less
likely to cause curtailment during periods of high wind production. Power flow on the dc
transmission link can automatically track variable aggregated wind plant generation keeping it off
parallel paths.
Figure 5 ± 500 kV VSC HVDC link with hybrid OVHD – U/G transmission and metallic return switching
provision.
Conventional HVDC transmission requires reactive power compensation. Large concentrations of
shunt compensation lower voltage stability, increase load rejection over‐voltages and lower the
system resonance frequency for low order harmonics. Consequently conventional HVDC must have
relatively strong points for network interconnection. Wind or PV solar generation do not
appreciably strengthen the ac network. These issues are mitigated with VSC HVDC since there is no
need for reactive power compensation. Therefore much weaker network conditions can exist at the
points of interconnection without being supplemented with conventional generation or other
synchronous machines. Even radial outlet transmission from wind plants is possible with VSC HVDC
as evidenced by various offshore wind projects. Furthermore, the dynamic voltage support
capability of the converters and use of choppers can improve recovery of the wind farm from ac
system disturbances to better fulfill grid code requirements. With conventional HVDC, temporary dc
line faults are cleared quickly by converter phase control with no need for circuit breaker action.
Rapid clearing of dc line faults with VSC requires dc circuit breakers.
Figure 6 illustrates some of the large HVDC projects for accessing remote wind resources on the US
Great Plains which are currently under active development. More lines would be required if EHV ac
transmission were used. The ac system characteristics at the sending end for many of these projects
require additional investment to strengthen the system interconnection. This is especially true since
local conventional generation may not be dispatched when the wind is blowing strongly. Use of VSC
HVDC would substantially reduce the amount of additional investment. Furthermore, the variable
O&M expenses for following the wind generation would be less since there would be no need for
commensurate reactive power compensation.
Underground
Figure 6 Proposed ± 500 kV HVDC links for wind plants. Annual average wind speed at 80 m per AWS
Truewind, NREL.
4 AlternativesforCongestedRights‐of‐Way It is often challenging to terminate high capacity transmission lines at key substations within a major
load centre due to land use constraints, congested rights‐of‐way and reliability concerns. With
HVDC transmission, possibilities exist to work around some of these constraints. One option is to
transition to underground cable transmission along a shared utility corridor, beside roadways or
along transmission lines. A second option is to transfer loads from lower voltage circuits to higher
voltage circuits and use the freed‐up right‐of‐way for an HVDC line. A third option is to convert
existing EHV transmission lines to dc to increase their power transfer capacity.
Conversion of an ac line to a dc line to increase transfer capacity has been analyzed extensively over
the years but has never been implemented. [2]
Some lines have been designed with provision for eventual conversion to HVDC. Analysis must take
into account corona and field effects, insulation properties, clearances and conductor current rating
in order to determine the possible increase in capacity. In the United States, the National Electric
Safety Code provides clearance to ground criteria consisting of a defined reference height plus an
electrical component of clearance. The reference height depends on the land use of the underlying
terrain traversed. The electrical component of clearance depends on the maximum ac or dc
operating voltage and an overvoltage factor. For a long bipolar HVDC line the overvoltage factor is
about 1.75 times the operating voltage. This factor is lower than that for an EHV ac line. Composite
long rod insulators have superior pollution performance and can have longer creepage distance for a
given connecting length. Insulator replacement can allow raising the transmission voltage and
thereby the power capacity. Replacing the outer crossarms with horizontal ‘vee’ composite
insulators raises the conductor anchor point and eliminates conductor swing at the tower to obtain a
higher operating voltage for a given conductor sag. By making these insulation changes to convert a
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single circuit 345 kV ac line with two conductor bundles to a bipolar HVDC line for example, one
could reasonably expect to increase the transfer capability by about 80% above the ac line thermal
rating.
5 ConclusionsVSC is an innovative but mature technology that enables new uses in transmission.
Ratings are already today sufficient both in terms of current and voltage to match the near‐future
needs of large scale transmission, in North America as well as in the rest of the world. For example,
a 500 kV, 700 MW monopolar cable system has already been ordered in North Europe.
The asymmetric converter in a bipolar arrangement can give an attractive combination of high
power, reasonable block size and total system reliability.
Both overhead lines and cables are available giving the planner larger freedom to select an optimum
transmission path.
Possible uses in North America could be new wind power infeed and relief of congestion in the
transmission system
[1] www.abb.com/hvdc Internet website
[2] K. Halson, F. Loudon, I. Gutman, J. Lundquist; “Feasibility of upgrading 300 kV AC lines to DC for increased
power transfer capability”, B2‐110 CIGRE 2008