Download Network Extensions to Remote Areas : Part 1 Planning

Network Extensions to Remote Areas
Part 1 – Planning Considerations
Prepared by :
Ranil de Silva and Andrew Robbie
Power Systems Consultants Australia Pty Ltd
For :
Australian Energy Market Operator Ltd
Reference :
JA00501-1
Revision :
Final Rev 0
26 November 2009
Network Extensions to Remote Areas
Part 1 – Planning Considerations
Revision Table
Revision
Final Rev 0
Issue Date
26 November 2009
Description
Final report
Reviewers
Name
Chris Collie-Holmes
Position
Electrical Engineering Manager - PSC
Date
26 Nov 2009
Approval
Name
Adam Peard
Position
Planning Specialist - AEMO
Date
26 Nov 2009
Power Systems Consultants Australia Pty Ltd.
PO Box 273, Flinders Lane PO, Melbourne,
Victoria 8009,
Australia (ABN 35-089-074-019)
Phone +61-1300-933 632
Fax +61-3-9654 6833
Web www.pscconsulting.com
Email [email protected]
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Part 1 – Planning Considerations
List of Contents
Executive Summary
1. Introduction
2. Background to Transmission Technologies
2.1. AC Transmission
2.1.1. Overview of AC Transmission
2.1.2. Limitations to Long Distance AC Transmission
2.1.3. Examples of Long Distance AC Transmission Schemes
2.2. Classical HVDC Transmission
2.2.1. Overview of HVDC Transmission
2.2.2. Examples of Long Distance HVDC Transmission Schemes
2.3. HVDC Transmission using Voltage Source Converters
2.3.1. Overview of VSC Schemes
2.3.2. Examples of VSC DC Schemes
3. Route Selection
3.1. Effects of Transmission Lines on Environment
3.1.1. Urban and Ecologically or Culturally Sensitive Areas
3.1.2. Visual Impact
3.1.3. Land Use
3.1.4. Access Roads
3.1.5. Corona Effects - Audible Noise and Radio Interference
3.1.6. Electric and Magnetic Fields
3.1.7. Earth Return Currents on HVDC Lines
3.1.8. Time Required to Gain Resource Consent
3.2. Effects of Environment on Line Design
3.2.1. Wind and Ice Loading
3.2.2. Atmospheric Pollution
3.2.3. Altitude
3.2.4. Earth Resistivity
3.2.5. Complex Terrain
3.2.6. Route Diversity
3.3. Choice of Single Circuit or Double Circuit Lines
3.4. Facilitating the Connection of Other Generation
4. Choice of AC or DC Transmission
4.1. Fundamental Limitations of AC or DC Transmission
4.2. Connecting Other Generation or Demand
4.3. Stability
5. Losses and Operating Voltage
5.1. I2R Losses
5.2. Corona Losses
5.3. Terminal Losses
6. Cost of Materials
7. Security
8. Power Quality
8.1. Voltage Waveform Distortion (Harmonics)
8.2. Voltage Unbalance
8.3. Voltage Flicker
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9. Staging Transmission Capacity
10. Connection Point
10.1.Regional Planning Criteria
10.2.Avoiding Congestion in the Shared Network
10.3.Substations
10.4.Fault Levels
11. Summary of Major Cost Considerations
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Executive Summary
This report forms Part 1 of a study into ‘Network Extensions to Remote Areas’.
The report discusses the considerations that should be taken into account
when planning electrical transmission schemes to connect remote generation
to the existing shared grid in Australia. Accompanying this report is Part 2 of
the study which provides a hypothetical example of planning to connect
remote geothermal generation at Innamincka to the shared grid.
This report focuses on technical planning considerations and also how these
considerations relate to cost. In particular, the following planning
considerations are likely to have the greatest impact on the cost of the
transmission connection :
a) Transmission route
 Minimization of route length
 Avoidance of sensitive areas
 Wind and ice loading
 Minimization of visual impact
 Cost of easements
 Complexity of terrain
 Route diversity
 Facilitating the connection of other generation
 Allowing for time to gain resource consents
b) Selection of HVDC transmission if there is no benefit in facilitating the
connection of other generation on the route and the distance is greater
than 600km – 800km
c) Problems with earth return currents for HVDC transmission
d) There is an economic trade-off between capital cost and the cost of
losses. Selecting a high transmission voltage and a heavy, low
resistance conductor bundle will minimize the cost of losses but will
significantly increase the capital cost.
e) The cost of materials, such as steel, which in turn is strongly influenced
by the general state of the international economy.
f) Limiting the effect of credible contingency events to levels that can be
adequately managed by the Australian Energy Market Operator
(AEMO).
g) Staging transmission capacity to match growth in generation
h) Selecting a connection point that does not have a ‘knock-on’ effect on
congestion in the shared network, which could require expensive deep
network augmentation, or result in stranded generation.
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1. Introduction
This study into ‘Network Extensions to Remote Areas’ discusses
considerations that should be taken into account when planning electrical
transmission schemes to connect remote generation to the existing shared
grid in Australia.
Some of the remote generation sites being investigated by potential
generators are of the order of 1000km from major load centres in the shared
grid. Connection to the shared grid will require transmission lines that are
comparable to the longest transmission schemes presently in service
anywhere in the world.
It should also be noted that most of the considerations discussed in this report
are equally applicable to connecting local generation to the grid, or for general
transmission line applications.
This report forms Part 1 of the study and discusses the impact of various
planning considerations on the cost of the transmission connection.
Accompanying this report is Part 2 of the study which provides a hypothetical
example of planning to connect remote geothermal generation at Innamincka
to the shared grid.
2. Background to Transmission Technologies
There are 3 distinct types of transmission technology that are currently in use :
a) AC transmission
b) ‘Classical’ HVDC transmission using current source converters
c) HVDC transmission using voltage source converters
2.1. AC Transmission
2.1.1. Overview of AC Transmission
The electrical power system is based on AC generation, transmission, and
distribution because it has historically represented the most economic way of
delivering power to the demand. The economic benefits are largely due to the
use of transformers to match the efficiency of high voltage transmission with
the convenience and safety of low voltage generation and demand.
The large AC networks that exist today are made up from the merging of
smaller separated networks. In general AC networks can be easily extended
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by adding on more circuits, or tied together with circuits of sufficiently large
capacity. In principle there is no limit to the geographical extent of an AC
network provided that generation and demand are evenly distributed
throughout the network.
One of the primary reasons for the attractiveness of AC transmission
technology for connecting remote generation is that there is no major
departure from the familiar technology already prevalent in the network.
However long distance connections to remote generation have limitations on
power transfer which can be costly to overcome.
2.1.2. Limitations to Long Distance AC Transmission
The primary limitation to long distance AC transmission from remote
generation is that long AC lines require a large amount of expensive reactive
compensation to provide a high transfer capacity for active power. A lack of
reactive compensation will lead to stability problems during normal operation
and likely loss of synchronism in the event of a fault.
2.1.3. Examples of Long Distance AC Transmission Schemes
There are several AC transmission schemes presently in service which
transmit power over distances comparable to the transmission distances
being contemplated for connecting remote generation in Australia. Some
examples are :
a) A 735kV AC transmission system connects hydro generation at James
Bay and Churchill Falls / Manic-Outardes to load centres in Montreal
and Quebec City in Quebec. 10 x 735kV AC single circuit lines are
each capable of transmitting about 2,000MW over about 1000km using
series capacitor compensation and shunt static var compensators.
b) A 765kV AC transmission system connects hydro generation at Itaipu
to the load centre in Sao Paulo in Brazil. 3 x 765kV AC circuits transmit
about 6,300MW over about 900km using series capacitor
compensation.
c) A 500kV AC transmission system connects hydro generation at G.M.
Shrum and Peace Canyon to the load centre in Vancouver in British
Columbia. 3 x 500kV AC circuits transmit about 3,300MW over about
900km using series capacitor compensation.
Note that there are several meshed AC transmission networks that span
distances much greater than 1000km , for example in Europe, North America,
and the NEM in Australia. However these meshed networks have widely
dispersed generation and load centres , and do not present the same
technical issues as long distance point-to-point transmission schemes.
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2.2. Classical HVDC Transmission
2.2.1. Overview of HVDC Transmission
Most HVDC transmission schemes that are currently operating are based on
the ‘classical’ current source converters. The expense of these AC/DC
converters prohibits the widespread use of HVDC technology in transmission
and distribution systems, except for some specific purposes including :
a) Long distance point to point overhead transmission where HVDC
schemes can operate stably over long distances without the need for
expensive reactive compensation along the transmission line (as with
AC transmission). In addition HVDC transmission lines require fewer
conductors and have fewer losses than AC transmission lines of similar
voltage and power ratings.
b) Long underground or submarine cable connections where HVDC
schemes do not have to handle the AC reactive cable charging current
which limits AC active power transfer
c) Joining systems of different frequencies (50Hz and 60Hz)
d) Augmenting the AC network without increasing fault current levels
Apart from the expense of the AC/DC converters, the primary disadvantages
of classical HVDC schemes are :
a) There are technical difficulties with tapping the line to connect other
generation or feed demand along the route. (A few ‘multi-terminal’
HVDC schemes have been built, but with very limited success).
b) The converters must be connected to an AC system with a postcontingency system strength (fault level) about twice the power rating
of the HVDC converter.
c) The converters require reactive power support of about half the rated
active power. Much of this is typically supplied from harmonic filters.
2.2.2. Examples of Long Distance HVDC Transmission Schemes
There are several classical HVDC transmission schemes which transmit
power over distances comparable to the transmission distances being
contemplated for connecting remote generation in Australia. Some examples
are :
a) The Pacific DC Intertie is a +/-500kV 3100MW HVDC bipole connecting
generation in Oregon over about 1360km to the load centre in
California. This runs in parallel with a 500kV AC scheme.
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b) Two +/-600kV 3150MW HVDC bipoles connect hydro generation at
Itaipu over about 800km to the load centres in Sao Roque and Sao
Paulo in Brazil. (There is also a 765kV AC scheme connecting Itaipu to
Sao Paulo.)
c) A +/-500kV 3000MW HVDC bipole connects hydro generation at Three
Gorges over 940km to the load centre in Guangdong in China.
d) A very similar +/-500kV 3000MW HVDC bipole connects hydro
generation at Three Gorges over 890km to the load centre in
Changzhou in China.
e) A +/-800kV 5000MW HVDC bipole is presently being constructed to
connect hydro generation in Yunnan over 1400km to the load centre in
Guangdong in China. The scheme is expected to be commissioned in
2010.
f) A +/-800kV 6400MW HVDC bipole is presently being constructed to
connect hydro generation in Xianjiaba over 2070km to the load centre
in Shanghai in China. The scheme is expected to be commissioned in
2011.
2.3. HVDC Transmission using Voltage Source Converters
2.3.1. Overview of VSC Schemes
HVDC transmission using voltage source converters is a relatively recent
technology made possible by the development of high power electronic
switches such as GTO’s and IGBT’s.
VSC converters offer benefits over classical HVDC converters :
a) VSC converters can operate with AC systems that have a very low or
zero system strength (systems with no synchronous generators)
b) VSC converters can provide dynamic reactive support to the AC
system, even with zero active power transfer
c) In principle, VSC converters could be easily tapped onto the DC circuit
to make up a multi-terminal scheme. However no multi-terminal VSC
scheme is yet in operation.
On the other hand VSC converters have some disadvantages compared with
classical HVDC converters :
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a) VSC converters are more expensive than classical current source
HVDC converters
b) VSC converters have higher conversion losses than classical current
source HVDC converters
c) With present VSC technology, a DC fault must be cleared by tripping
the VSC converter off the AC system. This requires a significantly
longer restart delay compared with classical HVDC schemes which
typically resume normal power transfer in about 100-200ms.
2.3.2. Examples of VSC DC Schemes
Being a relatively new technology, there are few VSC DC schemes currently
operating. There is one VSC scheme (Caprivi Link) presently under
construction which will transmit power over a distance comparable to the
transmission distances being contemplated for connecting remote generation
in Australia. As yet there are no multi-terminal schemes in operation. Some
examples of VSC DC schemes are :
a) Directlink is a +/-80kV 180MW VSC link connecting Queensland to
New South Wales with 59km long cables. It is made up of 3 x +/-80kV
60MW links operating in parallel.
b) Murraylink is a +/-150kV 220MW VSC link connecting South Australia
to Victoria with 180km long cables.
c) Cross Sound is a +/-150kV 330MW VSC link connecting Connecticut to
Long Island with 84km long cables.
d) Caprivi Link is a +/-350kV 600MW VSC link presently being
constructed to connect Zambia and Namibia with a 970km overhead
line. The first pole with 300MW capacity is expected to be
commissioned by the end of 2009, the second pole is yet to be
committed.
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3. Route Selection
The transmission line route is likely to have a major impact on the cost of
connecting remote generation to the shared grid. In extreme cases, if a cost
effective route is not available then the remote generation scheme may not be
commercially viable.
3.1. Effects of Transmission Lines on Environment
3.1.1. Urban and Ecologically or Culturally Sensitive Areas
There are generally extremely strong objections to routing transmission lines
through or near existing urban areas or ecologically sensitive or culturally
sensitive areas. Sensitive areas would include :
a)
b)
c)
d)
Protected rainforests
Scenic areas associated with tourism
Heritage sites
Lakes and shoreline
Avoiding these areas could significantly increase the length of the
transmission line which would increase costs due to :
a)
b)
c)
d)
Additional towers and conductor
Additional easement length
Additional losses
Additional reactive compensation for AC transmission
3.1.2. Visual Impact
Overhead transmission lines are visually obtrusive and the visual impact
generally extends several km beyond the easement itself. The consenting
process for the transmission line will typically include objections from
landowners whose property are not directly affected by the easement but are
within sight of the line. The addition of a new transmission line will invariably
reduce the value of properties within sight of the line. Depending on local
consenting processes, there may be an additional cost associated with
compensating these landowners.
The visual impact of the transmission line can be reduced by integrating the
line into the landscape, taking advantage of natural landscape features, or by
camouflaging the line. :
a) In general reducing the height of the towers will reduce the visual
impact of the line. However this will tend to increase the cost of the line
as more towers are required, sited closer together to maintain ground
clearances. A double circuit line will have taller towers than two single
circuit lines but require less easement area. Note that an HVDC bipole
line typically has shorter towers than a double circuit AC line and is
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consequently less obtrusive whilst providing the same level of N-1
security.
b) The visual impact of the line can be reduced by avoiding breaking the
skyline. This may require shorter towers or a longer less direct route,
both of which will increase the cost of the line.
c) It may be possible to camouflage the line by using trees or by coating
the tower with a paint colour that blends with the background. These
can be relatively inexpensive means of reducing the visual impact.
d) Poles may have less visual impact than lattice towers when the line
appears in the foreground. However poles are generally more
expensive than lattice towers because poles are shorter, requiring
shorter span lengths.
e) In extreme cases underground cable can be used for some sections of
line. Cable is often used in the last few km of line entering a substation
because of the urban surroundings, or to avoid overhead lines
encroaching on each other. However the cost of underground cabling
can be of the order of 10 times the cost of overhead line. There are
also additional costs associated with protection systems for the cable
and the increased repair times for a cable compared with an overhead
line.
3.1.3. Land Use
The cost of easements is very dependent on the use of the land :
a) The lowest cost easements are through barren land that has little use.
b) Easements through agricultural land can incur additional costs due to
loss of usable land under towers, increased ground clearances for farm
machinery, and possible crop damage during line maintenance.
c) Easements through commercial forestry land can incur additional costs
due to the inability to grow trees under the line.
d) It may be possible to reduce cost by making use of existing
infrastructure corridors that already contain other transmission lines,
motorways, or railways.
3.1.4. Access Roads
It may be necessary to build new access roads to construct and maintain the
transmission line. New access roads will have an impact on the environment
and add to the cost of the line. In some cases landowners will view the access
roads as being beneficial for the land owner’s private use.
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In some cases it may be more economic to use helicopter access for
construction and maintenance rather than build an access road.
3.1.5. Corona Effects - Audible Noise and Radio Interference
The corona associated with high voltage AC and DC transmission lines will
produce audible noise and radio interference.
In the case of AC transmission lines the corona effects get significantly worse
with rain or frost. These effects can be reduced by the following measures, all
of which will increase the cost of the AC line :
a)
b)
c)
d)
Increase conductor size
Increase clearance to ground
Increase number of subconductors in a bundle
Increase distance between phase conductors
In general, the amount of corona mitigation will vary for different sections of
the line. Sections close to populated areas will typically require more corona
mitigation, and consequently cost more.
In contrast corona effects are less severe for DC transmission lines. Audible
noise and radio interference are only associated with the positive pole and
tend to decrease with rain.
3.1.6. Electric and Magnetic Fields
There is growing debate regarding the health effects of electric and, more so,
magnetic fields associated with AC transmission lines. As a consequence
there is a trend to require reduced magnetic field levels at the boundary of the
easement. This results in increased cost due to purchasing wider easements
or changes to line design.
In contrast there is less concern regarding the health effects of electric and
magnetic fields associated with DC transmission lines. The electric field is
associated with ion concentration which is similar to naturally occurring ion
concentrations. The magnetic field strength is similar to the earth’s magnetic
field strength. Consequently the costs associated with mitigating electric and
magnetic fields associated with DC transmission lines are significantly less
than mitigations associated with AC transmission lines.
3.1.7. Earth Return Currents on HVDC Lines
Balanced AC transmission lines typically have insignificant earth currents. On
the other hand, HVDC transmission lines in monopole operation or
unbalanced bipole operation require a return path for the pole current which
can be via the earth or via a metallic return.
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a) HVDC earth return is the least expensive option. Earth electrodes or
sea electrodes are used to connect the neutral side of the converters to
earth which has a very low return resistance , and consequently low
losses. The main environmental issues with earth return currents are
the possibility of corrosion on gas or water pipelines near the
electrodes, and dc current flow through the earthed neutrals of
transformers causing magnetic saturation. Both of these issues can be
mitigated by locating the electrode in a low resistivity area far from
pipelines or transformers, possibly requiring a long electrode line to be
built.
b) If an earth return is prohibited then a more expensive metallic return is
required. The metallic return requires a conductor of the same current
rating as the high voltage pole conductor, but with less insulation. For
monopole operation, the overall losses using metallic return are about
double the losses using earth return.
3.1.8. Time Required to Gain Resource Consent
Overhead transmission lines can typically require 5 -10 years to gain resource
consent whilst subsequently requiring only 2-5 years to construct. The
potential delays in consenting need to be taken into consideration such that,
ideally, the transmission connection is operating a little before the generation
is commissioned.
Given the long consenting delays, consideration should be given to gaining
consent for future line upgrades as part of the initial consent.
3.2. Effects of Environment on Line Design
3.2.1. Wind and Ice Loading
Transmission lines are subjected to mechanical loading due to the conductors
as well as wind and ice. Sections of line that are in areas prone to cyclones or
icing will need a combination of shorter spans and tower strengthening which
will increase the cost of the line.
3.2.2. Atmospheric Pollution
Natural atmospheric pollution such as salt contamination near the coast , or
smoke pollution from bush fires, as well as industrial atmospheric pollution
can contaminate the surface of insulators on the transmission lines. Line
sections in areas that are prone to atmospheric pollution will require greater
insulator creepage distances to prevent flashovers, which will increase the
cost of the line.
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Corrosive environments such as windy locations near the coast, or areas
prone to sand abrasion, can also result in loss of galvanizing. Towers in
areas with little corrosion may typically have lives of 50 – 100 years, whilst
towers in areas with significant corrosion may typically have lives of 10 – 15
years. Increased galvanizing coatings will increase the cost of the line.
3.2.3. Altitude
Transmission line sections at high altitudes will require greater electrical
clearances to limit corona and to prevent over-voltage flashovers. This will
increase the cost of the line.
3.2.4. Earth Resistivity
Effective lightning performance of transmission lines requires low footing
resistances at the base of towers. If the earth resistivity is high then a
lightning stroke to the earthwire may result in a backflashover to the phase
conductors. Reducing the footing resistance in areas of high earth resistivity
will add to the cost of the line.
3.2.5. Complex Terrain
Transmission line sections that run through complex terrain are likely to
require a greater proportion of strain towers, relative to lines that run through
relatively flat land. Each extra strain tower will add to the cost of the line,
being of the order of twice the expense of a suspension tower.
3.2.6. Route Diversity
Some transmission line sections may run through areas that are associated
with low probability risks that could have a high impact on the reliability of the
transmission scheme.
For example if the line runs through an area that is prone to bush fires then a
double circuit line would be less reliable than two single circuit lines on diverse
routes. However route diversity will increase the cost of the transmission
scheme.
3.3. Choice of Single Circuit or Double Circuit Lines
Typical transmission schemes are made up of either single circuit AC lines,
double circuit AC lines, or bipole HVDC lines.
Ignoring easements, one double circuit AC line has a comparable cost to two
single circuit AC lines with the same rating. The most significant differences
are :
a) One double circuit AC line has about half the easement cost of two
single circuit AC lines
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b) The double circuit AC line is typically taller than the single circuit AC
lines and may have more visual impact
c) The double circuit AC line is less reliable than two single circuit AC
lines and is more prone to faults due to lightning or failure due to wind
loading, ice loading, or fire.
3.4. Facilitating the Connection of Other Generation
There may be significant overall economic benefits in selecting a transmission
route that runs through areas that have potential for future generation
development (or demand development). The time and cost associated with
consenting and building a long transmission line tend to inhibit remote
generation development. Consequently the additional cost of a longer route
that enables other generation may help to make the transmission scheme
commercially more viable and more justifiable in the consenting process.
4. Choice of AC or DC Transmission
Both AC and DC transmission schemes have been used to connect
generation to the grid. Section 2 briefly compared the two technologies and
this section discusses the decisive factors that are likely to influence the
choice between AC and DC transmission for connecting generation in the
Australian context.
4.1. Fundamental Limitations of AC or DC Transmission
In general, power transfer on AC transmission connections is limited by the
thermal capacity of the conductors (for short lines), or by stability (for long
lines).
In the case of short AC lines the power transfer capacity can be increased by
raising the voltage, adding conductors, or adding parallel circuits.
In the case of long AC lines the power transfer capacity can be increased by
raising the voltage, using series compensation to electrically ‘shorten’ the line,
or adding new parallel circuits. In practice it is generally most economic to
firstly utilize the highest practical voltage, then to apply series compensation
(limited to about 75%), and then to add parallel circuits. (In principle, dynamic
shunt compensation could be used to increase stable power transfer to any
level, however the cost of such shunt compensation rapidly becomes
prohibitive).
In contrast, the power transmission capacity of HVDC lines of any length is
ultimately limited by the voltage drop due to conductor resistance and the
thermal capacity of the conductors. The HVDC power transfer capacity can be
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increased by raising the voltage, adding conductors, or adding parallel
circuits.
The cost of converters makes DC transmission more expensive over shorter
distances, but the requirement for more line easements makes AC
transmission more expensive over long distances. For long distance point to
point transmission over land, classical HVDC transmission tends to be more
economic than AC transmission for distances above 600km – 800km, for
comparable levels of voltage, capacity, security, and losses.
4.2. Connecting Other Generation or Demand
The decisive factor that would favour AC transmission over classical HVDC
transmission over long distances in excess of 600km – 800km is the relative
ease of connecting other generation or demand part way along the line. The
potential to connect other generation may help to make the AC transmission
scheme commercially more viable than DC and more justifiable in the
consenting process.
It is possible that future multi-terminal VSC DC transmission schemes will
allow generation or demand to be connected part way along the line, however
no such scheme is yet in commercial operation.
4.3. Stability
The controllability of HVDC transmission can provide added benefits by
modulating the DC power transfer to stabilize oscillations between generators
on the shared AC network. There is a potential cost benefit if this relieves
congestion on the AC network due to stability constraints.
On the other hand AC transmission requires mitigations to preserve stable
power transfer over long distances. Series capacitors are typically added to
increase the stable power transfer capacity. The addition of series capacitor
compensation to a long line can typically increase the transfer capacity by
about 50% at a cost of the order of 10% of the line cost.
In conjunction with the series capacitors, intermediate switching stations are
typically added to bus circuits together such that an AC fault does not trip the
entire length of the AC circuit. Three evenly spaced intermediate switching
stations can typically increase the post-contingency capacity of the overall AC
transmission scheme by about 40%.
If series capacitor compensation or large HVDC converters are installed then
consideration should be given to the possibility of sub-synchronous resonance
(SSR) between the series compensated line or HVDC converters and nearby
thermal generators with long shafts. SSR issues can typically be mitigated by
modifications to exciter controls or HVDC controls at a relatively low cost.
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5. Losses and Operating Voltage
In addition to the capital cost of the transmission scheme, the cost of
transmission losses is a decisive factor in the commercial viability of the
remote generation.
5.1. I2R Losses
Transmission line losses are usually dominated by conductor I2R losses.
Adding more conductors reduces the resistance and losses by an inverse
factor whilst increasing voltage reduces current and losses by an inverse
squared factor. Consequently as the transmission distance increases, it
becomes more economic to select a higher voltage to minimize losses.
Typical I2R losses for an AC or HVDC transmission scheme of 1000km are
about 4% - 8% at rated load.
Increasing the operating voltage increases the cost of the towers due to the
increased air clearance required to withstand over-voltages. In the case of
EHV AC lines the decisive factor is usually switching overvoltages whilst for
comparable voltage DC lines the switching over-voltages are lower and the
decisive factor is usually lightning over-voltages.
Stringing a heavy, low resistance conductor bundle will reduce the cost of
losses but will significantly increase the capital cost of the line due to the cost
of the conductor and the extra tower strengthening or span reduction required
to handle the heavy conductor bundle.
5.2. Corona Losses
For low voltage AC lines, corona losses are negligible. However for typical
EHV AC lines, corona losses are about 1kW/km in fine weather, 70kW/km in
rainy weather, and rise to 100kW/km in frosty weather. In the case of DC lines
corona losses do not vary so much with weather, typically ranging from about
1kW/km in fine weather, about 2kW/km in rainy weather, and rise to about
5kW/km in frosty weather.
Both AC and DC corona loss increases with altitude. If the line design is kept
the same then the corona loss at 2000m will typically be 4 times the corona
loss at sea level. For EHV AC lines the worst case corona loss is comparable
to the rated I2R loss.
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5.3. Terminal Losses
The terminal substation losses associated with AC transmission are typically
about 0.2% - 0.3% of the transmission capacity per substation at rated load,
mostly due to transformer losses.
Classical HVDC converter station losses are typically 0.6% - 0.7% per
converter station at rated load, whilst VSC DC converter station losses are as
high as 2% per converter station at rated load.
6. Cost of Materials
The cost of the transmission connection will be strongly influenced by the cost
of materials, such as steel, which in turn is strongly influenced by the general
state of the international economy.
Recent years have seen a rapidly growing economy associated with high
material prices and a shortage of manufacturing capacity, followed by a
collapsing economy associated with low material prices and excess
manufacturing capacity.
7. Security
Requirements for security and reliability will strongly influence the cost of the
transmission scheme.
AEMO manages security by dispatching generation and reserves such that in
the event of any single credible contingency :
a) There are sufficient FCAS (Frequency Control Ancillary Services)
reserves to maintain the frequency within acceptable limits
b) Transmission line and transformer flows remain within acceptable limits
c) Bus voltages remain within acceptable limits
d) The system is stable in terms of transient stability (generators remain in
synchronism), oscillatory stability (electromechanical oscillations are
well damped, and voltage stability (no voltage collapse or overvoltage
issues)
The design of the transmission connection to remote generation will need to
take into account the way in which AEMO manages security. Some examples
of how security requirements will affect the cost of the transmission scheme
are :
a) If the transmission connection to remote generation consists of a single
circuit then tripping the circuit will result in a loss of power injection to
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the system. AEMO will need to dispatch sufficient FCAS reserves to
cover this loss and maintain the frequency. If the loss of power is
higher than the existing single credible contingency then FCAS costs
may rise. This can be mitigated by installing additional circuits which
will limit the loss of power for a single circuit trip.
b) Some parts of the network are already heavily constrained as a result
of AEMO's requirement to manage system security. The generation
connection should consider appropriate connection points on the
network that will facilitate the intended generation injection without
significant additional constraint (and ideally reduce system constraints).
Selection of ideal connection points may require transmission
connections over much longer distances than the nearest connection to
the existing system.
c) If a transmission connection consists of multiple circuits then each
circuit should be rated to handle the increased power that may flow in
the event of tripping any other single circuit (at least for a limited
duration – typically 15 minutes). The change in power flow may also
require additional reactive support to maintain acceptable bus voltages.
d) Power transfer on long distance AC transmission connections are
typically limited by stability considerations. There may be a significant
expense associated with plant required to maintain stability such as
series capacitors or intermediate switching stations.
8. Power Quality
The design of the transmission connection should not degrade the power
quality on the network. The following aspects of power quality should be
addressed, however they do not generally impose a major cost on a long
distance transmission scheme.
8.1. Voltage Waveform Distortion (Harmonics)
An excessive level of harmonic voltages can lead to overheating equipment
and maloperation of control systems. Voltage waveform distortion associated
with long distance transmission connections are generally associated with
harmonic currents generated by HVDC converters, or by Static Var
Compensators. The harmonic voltages are limited by installing harmonic
filters.
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8.2. Voltage Unbalance
Excessive voltage unbalance on the network can lead to overheating on
generator and motor windings, and maloperation of control systems. The
phase conductors on AC transmission lines are not positioned symmetrically,
consequently the associated impedances are asymmetric which can lead to
unbalanced voltages on the network, especially for long lines. This unbalance
is generally corrected by installing transposition towers at intervals along the
line which periodically swap the positions of the conductors and minimize the
unbalance.
8.3. Voltage Flicker
An excessive level of voltage fluctuations is seen as an irritating lighting
‘flicker’ by humans. Voltage fluctuations associated with long distance
transmission connections are generally associated with switching circuits and
switching reactive compensation (such as shunt reactors or capacitors). In the
case of HVDC transmission there will be voltage fluctuations associated with
switching harmonic filters and starting and stopping. The level of voltage
fluctuations can be limited by reducing the size of individual reactive
compensation or harmonic filter banks (which increases cost because more
banks are needed to supply the total amount of required compensation).
Voltage fluctuations can also be limited by installing dynamic reactive support
such as SVC’s.
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9. Staging Transmission Capacity
If the remote generation is being implemented in stages then it may be
economic to also stage the transmission capacity to match the generation.
This may have cost benefits if there are several years between the
implementation of generation stages , or if there is uncertainty regarding the
ultimate development of the remote generation.
AC transmission schemes can be staged by :
a) Constructing the line for operation at high voltage (say 765kV) but
initially operating at a lower voltage (say 500kV) to match the voltage of
the connection point in the shared network. This allows the cost of the
765/500kV transformers to be deferred. Ultimate operation at 765kV
will provide lower losses and higher transmission capacity.
b) Initially stringing a double circuit line with only one circuit can lower the
initial capital cost by about 25%. The overall line capacity can be
doubled by later stringing the second circuit at a deferred cost.
HVDC transmission schemes can also be staged by :
a) Constructing an HVDC bipole line but initially operating as a monopole
allows the cost of the second stage converters to be deferred, and
initial losses can be halved by paralleling the conductors. Alternatively
the cost of the second pole conductor can be deferred.
b) Converters can be added in either parallel or series with the original
converters allowing either current or voltage upgrades. The design of
the transmission line will need to allow for these upgrades.
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10. Connection Point
The sending end of the transmission scheme is fixed at the site of the remote
generation, but there can be many options for connecting the receiving end to
the shared AC network. This section discusses considerations related to the
point of connection.
10.1. Regional Planning Criteria
The Jurisdictional Planning Bodies develop Planning Criteria, which are
intended to discharge their obligations under the Rules and relevant regional
transmission planning standards. The Jurisdictional Planning Bodies must
consider their Planning Criteria when assessing the need to increase network
capability and developing network extensions.
The transmission connection from remote generators would need to consider
some of the Planning Criteria, where necessary. This is likely to depend on
the owner of the connection.
10.2. Avoiding Congestion in the Shared Network
Ideally, the transmission scheme should be connected to the shared network
at a point that does not aggravate congestion issues in the shared network.
The congestion generally stems from the requirement to maintain N-1 security
which limits the power transfer on circuits to a level that can handle a single
credible contingency without loss of supply to demand. A good connection
point is likely to be close to a load centre as opposed to a generation centre
so as to avoid additional loading on the transmission network.
If congestion is an issue then deep network augmentations may be very
expensive. Depending on relative fuel costs, the new generation may be
stranded or may displace existing generation.
10.3. Substations
If the remote generation capacity is smaller than a few hundred MW then it
may be possible to avoid the cost of a receiving terminal substation by tee-ing
into an existing AC circuit. However if the remote generation capacity is larger
than a few hundred MW then N-1 security requirements generally result in the
need for a substation or switching station.
If an AC transmission scheme is selected that matches the voltage of the
shared network at the connection point, then a switching station is sufficient
and the cost of transformers can be avoided. A substation will be required to
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connect an AC transmission scheme of different voltage, or an HVDC
transmission scheme.
The substation may be based on Air Insulated Switchgear (AIS) or Gas
Insulated Switchgear (GIS) and consideration should be given to :
a) AIS substations require considerably more land area than GIS
substations
b) AIS components are much more interchangeable than GIS
components
c) AIS components are more prone to atmospheric pollution than GIS
components
d) In the case of EHV substations, the overall cost of AIS (including land
requirements) tends to be slightly more expensive than GIS
If the substation is close to an urban area then GIS becomes much more
favourable.
The design of the substations should also take into account the reliability and
maintainability of equipment. For example spare transformers should be
considered due to the long lead time required for purchasing replacement
transformers. Also standardized bus configurations such as breaker-and-ahalf or double-breaker should be considered to provide improved reliability
and security.
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10.4. Fault Levels
Short circuit currents at the site of the remote generation may be very high if
the generation includes a large amount of synchronous machines (this would
include typical geothermal generators but not typical wind generators). Circuit
breakers and switching equipment at the site of the remote generation will
need to be rated to handle these short circuit currents.
In the case of an AC transmission line the transmission connection will also
transfer some of this short circuit current to the shared network. The short
circuit current injected from the remote generator will add to current injections
from existing generators and the combined short circuit current must not
exceed the rating of circuit breakers and other equipment. There may be a
significant cost associated with ‘re-breakering’ (Upgrading existing circuit
breakers to handle the higher levels of short circuit current).
In the case of an HVDC transmission line the short circuit current injected into
the shared network will be almost zero after a half-cycle, therefore there will
be no issue with exceeding the short circuit current rating of existing circuit
breakers. On the other hand a classical HVDC converter requires a postcontingent short circuit level of at least twice the DC power transfer in order to
help stable recovery after a fault. If the short circuit level at the point of
connection is too low then there will be additional costs associated with
increasing the system strength (for example by adding synchronous
condensers).
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11. Summary of Major Cost Considerations
This report has discussed a variety of planning considerations for connecting
remote generation. The discussion suggests that the following considerations
are likely to have the greatest impact on the cost of the transmission
connection :
a) Transmission route
 Minimization of route length
 Avoidance of sensitive areas
 Wind and ice loading
 Minimization of visual impact
 Cost of easements
 Complexity of terrain
 Route diversity
 Enablement of other generation
 Allowing for time to gain resource consents
b) Selection of HVDC transmission if there is no benefit in enabling other
generation on the route and the distance is greater than 600km –
800km
c) Problems with earth return currents for HVDC transmission
d) There is an economic trade-off between capital cost and the cost of
losses. Selecting a high transmission voltage and a heavy, low
resistance conductor bundle will minimize the cost of losses but will
significantly increase the capital cost.
e) The cost the cost of materials, such as steel, which in turn is strongly
influenced by the general state of the international economy.
f) Limiting the effect of credible contingency events to levels that can be
adequately managed by the Australian Energy Market Operator
(AEMO).
g) Staging transmission capacity to match growth in generation
h) Selecting a connection point that does not have a ‘knock-on’ effect on
congestion in the shared network, which could require expensive deep
network augmentation, or result in stranded generation.
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