The City of Cairns in the far north of the state of Queensland on the east coast of Australia is a popular international gateway for tourists visiting the Great Barrier Reef and the nearby World Heritage-listed rainforests. With an increasing population and an electricity demand growing annually by 7.3% (the statewide average is 4.5%), Powerlink Queensland needed to upgrade its transmission system to supply a new substation to be constructed on the southern outskirts of the city. To obtain the right balance of necessary development in a sensitive environment, Powerlink established new technical and environmental care standards.
Negotiating Rights-of-Way A government-owned corporation, Powerlink is responsible for operation of one of the world's longest high-voltage networks. The network stretches 1700 km (1060 miles) from north of Cairns to the Gold Coast on the Queensland/New South Wales border (Fig. 1). The transmission system has 21 connected customers: nine generators, four industrial customers and eight distribution companies. According to 1997/1998 statistics, Powerlink has a system capacity of 5848 MW to supply up to 34,012 GWh of power. The company maintains 73 substations connected via 4394 km (2600 miles) of 110/132-kV lines and 4884 km (3035 miles) of 275-kV lines.
The existing supply to Cairns was via two double-circuit 132-kV lines: one double-circuit line routed along the coastal strip; and a single-circuit line from Chalumbin routed through the rainforest. The long-term development plan for this area identified the need for a new double-circuit 275-kV transmission line. The challenge for Powerlink was to negotiate a right-of-way for the new line through two diverse and fragile ecosystems-the World Heritage-listed Wet Tropics Rainforest and the Trinity Inlet estuarine area. Beginning in 1994, the three-year consultation process with local community groups, special interest groups and government agencies identified various environmental, social and other concerns that had to be addressed. Issues associated with the tropical climatic conditions of the area, such as high rainfall, cyclone wind conditions and high lightning activity, were also considered in designing the new line.
Where possible, Powerlink selected routes for the new transmission line that used the easement of the existing line. This approach minimized the need for excessive vegetation clearing and eliminated construction of additional access tracks. The company also adopted construction techniques that minimized the need for heavy vehicle and crane access to structure sites. Towers were designed so that subsequent routine maintenance could be carried out by helicopter. The specialty lattice steel structures are significantly taller than normal towers enabling lines to be strung above the 40-m (130-ft) mature forest canopy. On completion of the final stage of the new line in 2003, the existing transmission line will be redundant and removed. The existing easement will be rehabilitated and rainforest will be encouraged to regenerate across the cleared areas and access tracks, allowing up to 40 hectares (100 acres) of previously cleared easement land to regenerate into rainforest.
The final route of Stage One of the 275-kV line, from Springmount to Woree, emerged after a three-year consultation process. Beginning at Springmount, the line travels 40 km (25 miles) across the Tablelands then 16 km (10 miles) through the Wet Tropics Rainforest. Where the line comes out of the rainforest and enters the urban areas of the city, 2.6 km (1.6 miles) of the line were placed underground. The line is then routed for 9 km (5.6 miles) through the Trinity Inlet estuarine area through the future Woree substation site (to be established in 2003) before connecting to the existing substation as shown in Fig. 2.
During stage two, the remainder of the line from Springmount to Chalumbin will be constructed. Stage three of this project will involve the construction of the Woree substation, operation of the complete Chalumbin-Woree circuit at 275 kV and removal of the existing single-circuit 132-kV line through the Wet Tropics.
Wet Tropics Section The values associated with the Wet Tropics Rainforest are many and varied, and its listing on the World Heritage register emphasizes its environmental importance. Traditional transmission line construction methods, which would have required easement clearing and a network of access tracks for both construction and future maintenance, were unsuitable for an area with such significance. The best alternative for the new 16-km (10-mile) route traversing the Wet Tropics was to parallel an existing single-circuit 132-kV line. This offered relative ease of access for the new work, however, close proximity of the existing energized line required many construction outages. With the existing line already using the best tower positions, the 32 new towers, most at 75 m (250 ft) tall, were sited alongside the existing towers. The work to level the tower sites and prune the vegetation was restricted. All clearing was done by hand with no material allowed to fall outside the 25-m-by-25-m (80-ft-by-80-ft) construction site at each tower position. The total area cleared through the Wet Tropics amounted to 0.75 hectares (1.9 acres). The level work at the more inaccessible tower sites required significant changes to plants used for drilling footings and abnormally large leg extensions, up to 12 m (39 ft) in some cases.
The 75 m (250 ft) towers, which are more than double the height of the existing towers, were designed to enable the conductors to be strung above the 40 m (130 ft) rainforest canopy (Fig. 3). A helicopter-landing platform was incorporated into the tower design to permit access from the air for maintenance, avoiding the need to maintain ground access tracks (Fig. 4). Composite insulators were selected based on their superior pollution performance and minimum maintenance requirements in humid tropical areas. The insulators are lighter than ceramic or glass insulators and are light enough to be carried by maintenance crews in a single helicopter flight.
Foundations ranged from extended depth footings through to rock anchors and in most sites, standard tower boring equipment could not be used exclusively. The tower designs incorporated unusually large leg extensions to conform to natural slopes of the land (Fig. 5). Because additional earth-works and site leveling at tower positions were not permitted, Powerlink commissioned a new track-mounted borer capable of use in all planes. The machine's flexibility andmaneuverability marked a significant development in earthworks.
At some tower positions, assembly work using cranes within the tower frame was possible up to approximately 30 m (100 ft); however, the upper section of the towers had to be pre-assembled off-site and flown into position by helicopters. Although this technique is not new, it was used extensively throughout all sections of the project. The total project incorporated more than 300 lifts. Helicopters were also used to string the conductor, both to minimize ground movements and to minimize damage to trees and conductors. Stringing operations were limited to one central winch position near a public viewing area.
Underground Cable Section Where the line emerges from the rainforest and enters the urban areas of Cairns, a 2.6-km (1.6-mile) double-circuit underground cable was installed to lessen the visual impact on the local community. In addition to increasing the project cost by A$14 million (US$8.7 million), this is the first double-circuit 275-kV XLPE cable installed in Australia.
The cable installation comprises two circuits of three 1200-sq mm (1.9-sq inch) single-core cables laid in trefoil, each with a rating of approximately 1000 A. In general, the circuits were installed at 970-mm (38-inch) centers with a cover of approximately 900 mm (35 inch) of specially chosen material (i.e. sand, road base). Depending on the location, mechanical protection was provided by polymeric cable covers and concrete. Special directional boring techniques were used to install ducts under the six-lane arterial highway and the main railway to Cairns. Extra care was required to avoid other services, in particular the fiber-optic communications cable connecting North Queensland to the rest of Australia. Olex Cables manufactured the cables in custom-built lengths to suit three-joint bays per circuit. Pirelli provided the joints and the outdoor terminations for connection to the overhead transmission line.
As part of the route approval agreement, approximately 10 km (6.2 miles) of the existing double-circuit 132-kV line was relocated from residential areas to the edge of Trinity Inlet. The redundant line was then removed.
Mangrove Wetlands Section Mangroves are tough tidal trees growing to a height of 25 m (80 ft) with a tangle of exposed roots providing a cradle for diverse sea life. To implant towers in a sensitive way across 2.5 km (1.5 miles) of dense mangrove forests in Trinity Inlet, which is bounded by water on one side and property on the other, posed additional construction challenges.
To minimize ground impact, Powerlink constructed temporary bridging on removable piles and false jetties to access four tower positions. Where necessary, mangroves were pruned rather than removed. The bridges were built at 1.5 m (5 ft) above the mudflats with the mangroves pruned to 1.2 m (4 ft). After construction of the tower bases was complete, the bridges were removed. Figure 6 shows the temporary access bridge used for personnel and machinery access to these tower positions. The remaining 19 towers in this section were sited on outcrops of degraded land. In many towers, two legs were on dry land and two were in the mud. In these cases temporary bridging was also used to access the legs located in the mud.
Compact towers with horizontal-V synthetic insulators and non-reflective conductors were used throughout the Trinity Inlet to reduce the visual impact in an area. The line was designed so that the conductors would be strung above the existing 25-m (80-ft) tall mangrove forest canopy.
Environmental Protection Powerlink employed a full-time independent environmental consultant for the project to ensure all aspects of the Environmental Management Plan were honored. Close liaison was maintained with all agencies whose approvals were essential to the project's success, such as the Wet Tropics Management Authority and the state departments of Primary Industries, Forestry and Fisheries. In the Wet Tropics section, Powerlink employed a team from the Department of Environment to manage the vegetation rehabilitation required at each tower position in the rainforest. Returning each site to the pre-construction state involved transplanting plants collected from the area surrounding the tower position. Soil genetics and micro-topology were important factors in the re-vegetation process. Powerlink also made similar arrangements with the Department of Primary Fisheries (Northern Fisheries) to replant and undertake mangrove regeneration research in the mangrove wetlands sections.
World Class Project Powerlink faced a demanding challenge in negotiating a transmission line route that balanced the needs of the community in the region with substantial environmental issues. To meet these goals, Powerlink developed innovative methods to minimize the environmental impact of its work in close co-operation with all relevant authorities. The cost of developing and adopting these innovative techniques has been considerable. The Wet Tropics and Trinity Inlet sections of the transmission line cost up to three times more than a typical expenditure for a comparable open terrain line. However, Powerlink sees this as an essential cost of business. This world-class project has set new standards for Powerlink and other utilities in installing infrastructure in environmentally sensitive areas.
Construction of Stage One from Springmount to Woree (project cost A$60 million [US$ 37.2 million]) began in January 1997 and was commissioned in June 1998. Stage two, the construction of the Springmount-to-Chalumbin section of the line, is scheduled for completion in late 2001. Stage three, the establishment of a Woree substation, uprating of the line to 275 kV and the removal of an existing 132-kV line, is planned for 2003.
Simon Bartlett is the manager of business development and asset manager for Powerlink Queensland. His 25 years with the electricity industry in Australia and overseas, span generation and transmission over the full spectrum from planning through to operations. His business background includes managing the sale of the Queensland government's 1700-MW Gladstone Power Station in Central Queensland and business development activities for Australia's fastest-growing transmission business.