Figure 1. Victoria’s and South Australia’s reserve margins are predicted to fall over the next few years.
The UK’s National Grid Company is currently developing a 600 MW subsea transmission link in Australia which will allow power to be traded between Victoria and Tasmania for the first time. By the end of 2003 the Basslink interconnector will enhance security of supply on both sides of the Bass Strait, protecting Tasmania against the risk of drought and protecting Victoria and the southern states of Australia against the forecasted shortage of peak load power.
Wholesale electricity prices fell in Australia the mid-1990s because of reforms and privatization in the government-owned industry. However, prices have risen recently because of the shortage of generation capacity in Victoria and South Australia. Victoria has recently experienced periods where it has hit its capped price of A$5000 per MWh ($2540/MWh), which is expected to rise to A$10 000 per MWh next year.
A demand forecast from Nemmco, the government owned organization that operates Australia’s National Electricity Market (NEM) and the power system, has shown that demand in Victoria is increasing by approximately 300 MW per year, and the reserve margin will be reached by the end of this summer and deteriorate thereafter. By 2005 there will be insufficient supply to meet demand.
According to a Nemmco report, Basslink appears to be the only project sufficiently advanced to address this shortfall for the 2004/5 summer. Basslink will help maintain downward pressure on electricity prices by introducing capacity for the two-way exchange of competitively priced power.
The interconnector will allow Tasmania to import cheap off-peak power and conserve its valuable water resources. Hydro Tasmania, Tasmania’s largest energy supplier, will be able to export hydroelectric power during periods of peak demand, and the link will generate competition in Tasmania’s electricity supply system.
The Basslink route
Figure 2. On the Tasmanian side, Basslink terminates at the George Town sub-station.
Basslink will run from Loy Yang in Gippsland, Victoria across the Bass Strait to Bell Bay in Tasmania. When implemented, the 280 km undersea cable component will be among the longest of its type in the world.
The link will allow a maximum of 600 MW to flow north from Tasmania to Victoria for up to ten hours, and allow southbound flow at a maximum of 300 MW.
The interconnector will be constructed by the Tas-Vic consortium consisting of Siemens, Pirelli and Sumitomo, who are all specialists in High Voltage Direct Current (HVDC) transmission and cabling technology.
The proposed route on the Victoria side begins at the Loy Yang power station to the adjacent DC/AC converter station. From here an HVDC overhead line carries the load to a 50 m easement through the Mullungdung forest. Here, the forest vegetation will provide a screen for the overhead lines. The HVDC lines will then continue past the Gippsland highway to the transition station.
The transition station is located 7 km from the coast. This is the point where the HVDC overhead lines are transferred to underground cables. Subsea cables will then be used to continue the route through the Bass Strait to Tasmania.
On the Tasmanian side, underground cables will extend from the coast at Four Mile Bluff for 1.7 km to the transition station where the underground cables will be transferred to HVDC overhead lines.
An HVDC overhead line will be used to connect the transition station to the DC/AC converter station which is located approximately 5.5 km southeast of George Town. The route then continues via High Voltage Alternating Current (HVAC) overhead lines to the George Town substation.
The converter stations have been sited near the existing AC transmission infrastructure at Bell Bay and Loy Yang. The existing high voltage transmission lines are used from this point.
Figure 3. In Victoria, overhead lines will be screened by natural vegetation
Siemens is responsible for the on-land portion of the EPC contract, and will supply equipment for substation upgrades and alterations, the converter stations, the transition stations, the overhead transmission lines and the sub-sea electrodes.
Substations: The existing 500 kV substation at Loy Yang in Victoria will need an additional two-circuit-breaker bay within the existing configuration to handle the additional pressure expected from the Basslink interconnector.
The George Town substation will require the construction of one breaker configuration for the Basslink feeder. In addition to this, Transend – the current owner and user of the George Town substation – has chosen to reconfigure the substation from the existing double busbar arrangement to a ‘one and a half switch’ arrangement.
These substations will be used for switching the power lines and is where the transformation of voltage occurs.
Overhead lines: The HVAC and HVDC overhead lines required between Victoria and Tasmania will use conductors (HV and electrode) to carry the electricity. The conductors will be made of aluminium alloy as it is lightweight, flexible and has a low electrical resistance. The interconnector will use three types of overhead transmission lines: 400 kV DC, 550 kV AC and 200 kV AC. The overhead cables will be supported by galvanised steel lattice towers, which will use reinforced concrete foundations to secure the towers.
Porcelain or glass disc insulators or porcelain longrod insulators will be used to support and insulate the conductors. Groundwire will be used to protect the infrastructure from lightning strikes and provide a medium for supporting the fibre optic cable which will provide communication and real-time monitoring of the interconnector.
Transition stations: Transition stations are required where there is a transition from overhead lines to underground cables. They will contain a strain tower where the overhead line will be terminated. A conductor will connect the transmission line to an insulator and surge arresters and then to a bushing. The underground cable is terminated at the bushing. Monitoring and communication equipment will be housed in a separate building.
Underground transmission cables: There will be around 8.7 km of underground HVDC transmission cable used in this project. This cable will consist of a 2000 mm2 copper conductor surrounded by an insulating layer, with additional layers for mechanical protection and a waterproof outer sheath.
Subsea cable: An HVDC sea cable will be used along the Bass Strait. This 300 kV subsea cable will be approximately 120 mm in diameter and about 280 km in length. It will consist of a copper conductor, which will be 42 mm in diameter, and conductor screen. The copper conductor and copper screen will be well protected from the underwater elements by layers of paper insulating tape, a lead alloy sheath, a polyethylene sheath and galvanised steel wire.
Electrodes: An electrode system will be required on each side of the Bass Strait to provide a return path for current.
A cathode will be installed at the Victoria end of the Bass Strait which will consist of a bare copper conductor with a cross section of about 300 mm2 laid in a circle about 1 km in diameter with several insulated connections from the electrode cable to the copper ring.
An anode will be installed off the shore of Tasmania comprising an array of up to 60 anode elements, each consisting of graphite and coke in concrete housing.
Converter stations: As DC current is a more effective way of transporting power over long distances, the converter stations at Loy Yang and near George Town will convert AC to DC and visa versa. These stations will consist of connections to AC transmission lines, circuit breakers, filter banks, capacitor banks, a converter transformer, a smoothing reactor, a building to house the thyristor valve modules, and connections to DC transmission lines. The conversion from AC to DC or visa versa is carried out by the thyristor valves, which can function as either a rectifier or an inverter, depending on the direction of the power flow.
The final proposed route seeks a balance between environmental issues and visual amenity. The developers have also taken landowner constraints into consideration. The original route directly affected 112 landowners, and this has been reduced to 28 by planning the existing route. All affected landowners will be compensated for the disruption.
Basslink has also tried to screen the disruption caused by the building of the transmission lines by disguising pylons from public view with natural vegetation and by designing a revegetation scheme to rehabitat vegetation disrupted by the construction.
Although there have been objections to the link, there is little doubt on the need for extra power in southern Australia.The high demand for electricity in Victoria and South Australia and the forecast that the state will fall below reserve power supply levels has prompted the construction of many new generation plants.
Even with this new capacity, Basslink is still expected to be one of the most beneficial sources of power to the area. Research for Basslink by Intelligent Energy Systems has forecast that Basslink will cause a 6.7 per cent reduction in Victoria’s wholesale prices on commissioning which will later be reduced to a 1.4 per cent cut over the next seven years as more generation capacity is added to the area.