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Network Interconnection: Murraylink: the underground connection

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In early 2001, site preparation work began on what is believed to be the world's longest underground transmission system - the 180 km Murraylink interconnector in Australia. By 2002, this 200 MW link will enable power to flow from the state of Victoria to South Australia, where economic growth has placed a strain on power resources.

Murraylink is a privately funded transmission line interconnect project and will be fully underground for its 180 km route. The project is being developed by the Murraylink Transmission Company Pty Ltd (MTC) as an innovative solution to the electricity generation shortfall in South Australia.


Figure 1. HVDC Light technology has been applied around the world
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MTC awarded the contract to develop the interconnector to Hydro-Québec, and in March 2001, SNC-Lavalin, the Canadian energy infrastructure engineering and management company, became a shareholder in the project.

SNC-Lavalin will partner with TransÉnergie, the transmission and distribution subsidiary of Hydro-Québec, to develop Murraylink. The two companies are both 50 per cent shareholders in MTC, and the partnership will combine TransÉnergie's transmission expertise with the engineering and infrastructure management skills of SNC-Lavalin. Hi-tech engineering company ABB is providing the equipment for the link.

Murraylink will be a high voltage direct current facility with two above-ground converters. It will connect into the existing transmission network in Victoria at the existing Red Cliffs Transmission substation owned by SPI PowerNet, and will connect into the transmission network of South Australia at a new substation to be built as part of the project outside Berri for ElectraNet SA.

Murraylink is now under construction with site works having commenced at Berri, South Australia, in March 2001. The construction uses existing corridors and requires no private easements or use of private land. The 180 km route also has full environmental and planning approval in both states, and in January 2001 received formal 'committed project' status from Nemmco, the Australian electricity market controller.

"Nemmco does not award committed status to a project lightly," said Murraylink director Mike Farr. "TransÉnergie considers this a significant milestone for the Murraylink project."

TransÉnergie has been planning and developing Murraylink for two years and now has all planning and environmental permitting and licenses in hand to allow the construction and operation of the link to go ahead. MTC was the first private company to receive a SA Transmission Licence from the South Australia Independent Industry Regulator.

Tight supplies

Australia is enjoying a prolonged period of economic growth. With a well-balanced economy, it was relatively unaffected by the Asian economic crisis in spite of a drop in demand for its exports to Japan. Economic growth has fuelled demand for electricity, and some states, in particular Queensland and South Australia, are experiencing supply shortages.


Figure 2. Murraylink route: the construction uses existing corridors and requires no private easements or use of private land
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The states' transmission grids are not well interconnected, and isolated systems remain in Tasmania, Western Australia and the Northern Territory. Nevertheless, the country is working on improving interconnections and the creation of a national electricity market. Murraylink is an important part of this drive.

The tight electricity supply/demand balance in South Australia is not expected to significantly change for some years. Installed generation in 1999 was 2579 MW with a 1999-2000 peak system load of 2646 MW. Up to 500 MW of power can also be transferred from Victoria over the existing Heywood interconnector in order to meet peak demand.

The Murraylink interconnector will therefore provide an essential link into the state in the fast developing Riverland area. In addition, its near tri-state border location in Victoria will enable it to source power generated in Victoria, New South Wales and the Snowy Mountains region.

Power will also be able to flow from South Australia into Victoria as the need arises.

A complete system

MTC is jointly owned by Hydro-Québec and SNC Lavalin and will operate the link when it is commissioned in late 2001. In February 2001, MTC awarded ABB Transmission and Distribution a turnkey engineering, procurement and construction (EPC) contract to build Murraylink.


Figure 3. HVDC Light is well-suited to the demands of competitive power markets
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"We believe TransÉnergie has placed the world's biggest ever order for an underground power link," said Farr. "Murraylink is another example of TransÉnergie's philosophy that there is no longer a need for new overhead lines."

Under its contract, ABB will design, procure equipment, construct and commission Murraylink. It will provide a complete HVDC Light transmission system made up of extruded cables buried underground with a converter station at each end of the facility, substations and transformers. More than 360 km (400 drums) of direct current (dc) cable will be used in the project.

The converter stations convert the alternating current (ac) used in the local power grid to dc power for efficient power transmission enabling the operators to control the power flow accurately and rapidly in both directions. The converter stations are joined by two single core HVDC cables over which the power is transmitted.

The converter stations are connected to the existing transmission networks by short lengths of ac underground cables.

In Victoria, connection to the grid will be made at 220 kV to an existing transmission substation at Red Cliffs. In South Australia, Murraylink will connect into the 132 kV network at a new substation to be built for ElectraNet outside Berri, called the Monash substation. A short length of double circuit 132 kV overhead transmission line will be required to allow local utility ElectraNet to fully connect the new Monash substation with the grid.

The Monash substation and the overhead double circuit transmission lines will be constructed by ABB as part of the Murraylink project for ElectraNet.

HVDC Light

The HVDC Light technology to be used for the project has been developed and successfully applied around the world by ABB. HVDC Light represents advanced generation dc transmission technology. It provides a link that is fully controllable in both magnitude and direction of power flow and is generally compliant with environmental constraints around the world.

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The same technology was applied to the DirectLink project linking the electricity grids of Queensland and New South Wales, Australia, which was developed by TransÉnergie US and TransÉnergie Australia. HVDC Light technology will be applied to the Cross Sound project also being developed by TransÉnergie in the USA.

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HVDC Light is well suited to meeting the demands of competitive power markets. It has a standardized design, power ratings of up to 300 MW and short delivery times. In addition to the converters being easy to control, HVDC Light allows independent control of active and reactive power, feeding of power into passive networks and power quality control.


Figure 4: HVDC Light gives the operator control of both active and reactive power
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HVDC Light makes use of advanced semiconductor technology: insulated gate bipolar transistors (IGBT). The converters are voltage source converter (VSC) type and they operate at a very high frequency using pulse width modulation (PWM). The result is converters that are extremely fast in control of active and reactive power with little need for auxiliary equipment such as filters and switches. The converter stations are therefore small and compact compared to conventional HVDC technology.

Control of active and reactive power: The control makes it possible to create any phase angle or amplitude, which can be done almost instantly, allowing both active and reactive power to be controlled independently. No reactive power compensation equipment is therefore needed at the station, only an ac-filter is installed. While the transmitted active power is kept constant, the reactive power controller can automatically control the voltage in the ac network.

Reactive power generation and consumption can be used for compensating the needs of the connected network within the rating of a converter. As the rating of the converters is based on maximum currents and voltages the reactive power capabilities of a converter can be traded against the active power capability.

Power quality control: The HVDC Light converter has a switching frequency in the kHz range, 40 times faster than a phase commutated converter operated at 50 Hz. This offers new levels of performance in terms of power quality control such as flicker and mitigation of voltage dips and sags, and harmonics caused by disturbances in the power system. In the presence of a fault which would normally lead to an ac voltage decrease, the converter can be rapidly deblocked and assist with voltage support to avoid severe disturbances in local industries that are sensitive to voltage dips.

Modular design: HVDC Light technology has a modular design and is factory tested before shipment. This allows faster installation and commissioning times than for traditional HVDC technology. The design also allows facilities to be relocated.

Robust against grid alterations: The fact that an HVDC Light converter can feed power into a passive network makes it very robust and easily able to accommodate alterations in the ac grid to where it is connected. This is a very valuable property in a deregulated electricity market where ac network conditions will change more frequently than in a regulated market.

Cables: The HVDC Light extruded cable is the outcome of a comprehensive development programme, and has a high mechanical strength, high flexibility and low weight. Extruded HVDC Light cable systems in bipolar configuration have both technical and environmental advantages.

The cables are small yet robust and can be installed by plowing, making installation fast and economic.

Consultation

For the full 180 km of the HVDC cable route, Murraylink has required no easements over private land. The cables generally follow existing road corridors and travel under the Sturt and Calder highways, rail lines and the Murray River.

The route was developed after extensive community consultation and discussions with representatives of various agencies including Transport, Native Vegetation, Native Heritage and Aboriginal communities. The route borders National parks and areas subject to international conservation treaties without impacting on them.

Through the route selection process and the judicial choice of cable installation the overall impact on vegetation has been kept to a minimum. A contract has been established with Land Care Groups and the local native vegetation agencies that ensures that all revegetation along the route results in a net gain in native vegetation.

The cables will generally be direct buried with excavated soils used as a back fill. Where the cables cross the Murray river, the highways and the rail, directional bore techniques will be used.

Directlink: an innovative solution

Directlink is a 180 MW HVDC Light project that linked the regional electricity markets of New South Wales and Queensland. Directlink was developed by a joint venture between NorthPower, a New South Wales utility, and Hydro-Québec, and was commissioned in December 1999. It is a non-regulated project and operates as a generator by delivering energy to the highest valued regional market. By directly participating in the spot market, Directlink earns a market-based return, which can include substantial revenues during periods of scarcity in either Queensland or New South Wales, when the market clearing prices rise.

Directlink was designed to ease the pressure on Queensland's overtaxed electricity supply system and enables NorthPower to provide a more reliable and secure electricity supply. The link also boosts supply services to the Tweed Heads region of New South Wales. Its construction involved burying an underground electricity transmission cable to connect the New South Wales and Queensland electricity grids, allowing power to be traded between the two states for the first time.

Directlink has added about 180 MW of energy supply to the Queensland system - enough to meet the electricity needs of 100 000 households. While this contribution is relatively small in the context of Queensland's total power demand, it will help overcome supply shortfalls.

Directlink employs the HVDC Light technology. The system is composed of two HVDC Light converter stations, at Terranora and Mullumbimby, and a total of 354 km of HVDC Light cables. The Voltage Source Converter terminals can act independently, allowing each to provide ancillary services (such as VAR support) in the networks to which Directlink is connected.

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