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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] JA00501-1 Final Rev 0 26 November 2009 Page 2 of 26 Network Extensions to Remote Areas 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 JA00501-1 Final Rev 0 26 November 2009 Page 3 of 26 Network Extensions to Remote Areas Part 1 – Planning Considerations 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 JA00501-1 Final Rev 0 26 November 2009 Page 4 of 26 Network Extensions to Remote Areas Part 1 – Planning Considerations 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. JA00501-1 Final Rev 0 26 November 2009 Page 5 of 26 Network Extensions to Remote Areas Part 1 – Planning Considerations 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 JA00501-1 Final Rev 0 26 November 2009 Page 6 of 26 Network Extensions to Remote Areas Part 1 – Planning Considerations 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. JA00501-1 Final Rev 0 26 November 2009 Page 7 of 26 Network Extensions to Remote Areas Part 1 – Planning Considerations 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. JA00501-1 Final Rev 0 26 November 2009 Page 8 of 26 Network Extensions to Remote Areas Part 1 – Planning Considerations 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 : JA00501-1 Final Rev 0 26 November 2009 Page 9 of 26 Network Extensions to Remote Areas Part 1 – Planning Considerations 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. JA00501-1 Final Rev 0 26 November 2009 Page 10 of 26 Network Extensions to Remote Areas Part 1 – Planning Considerations 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 JA00501-1 Final Rev 0 26 November 2009 Page 11 of 26 Network Extensions to Remote Areas Part 1 – Planning Considerations 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. JA00501-1 Final Rev 0 26 November 2009 Page 12 of 26 Network Extensions to Remote Areas Part 1 – Planning Considerations 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. JA00501-1 Final Rev 0 26 November 2009 Page 13 of 26 Network Extensions to Remote Areas Part 1 – Planning Considerations 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. JA00501-1 Final Rev 0 26 November 2009 Page 14 of 26 Network Extensions to Remote Areas Part 1 – Planning Considerations 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 JA00501-1 Final Rev 0 26 November 2009 Page 15 of 26 Network Extensions to Remote Areas Part 1 – Planning Considerations 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 JA00501-1 Final Rev 0 26 November 2009 Page 16 of 26 Network Extensions to Remote Areas Part 1 – Planning Considerations 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. JA00501-1 Final Rev 0 26 November 2009 Page 17 of 26 Network Extensions to Remote Areas Part 1 – Planning Considerations 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. JA00501-1 Final Rev 0 26 November 2009 Page 18 of 26 Network Extensions to Remote Areas Part 1 – Planning Considerations 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 JA00501-1 Final Rev 0 26 November 2009 Page 19 of 26 Network Extensions to Remote Areas Part 1 – Planning Considerations 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. JA00501-1 Final Rev 0 26 November 2009 Page 20 of 26 Network Extensions to Remote Areas Part 1 – Planning Considerations 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. JA00501-1 Final Rev 0 26 November 2009 Page 21 of 26 Network Extensions to Remote Areas Part 1 – Planning Considerations 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. JA00501-1 Final Rev 0 26 November 2009 Page 22 of 26 Network Extensions to Remote Areas Part 1 – Planning Considerations 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 JA00501-1 Final Rev 0 26 November 2009 Page 23 of 26 Network Extensions to Remote Areas Part 1 – Planning Considerations 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. JA00501-1 Final Rev 0 26 November 2009 Page 24 of 26 Network Extensions to Remote Areas Part 1 – Planning Considerations 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). JA00501-1 Final Rev 0 26 November 2009 Page 25 of 26 Network Extensions to Remote Areas Part 1 – Planning Considerations 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. JA00501-1 Final Rev 0 26 November 2009 Page 26 of 26