Directlink (Terranora)

The Directlink HVDC project, also known as the Terranora interconnector, is an important electricity transmission system connecting the power networks of New South Wales and Queensland in Australia. This High Voltage Direct Current (HVDC) link allows for the exchange of energy between the two states, improving power quality, reducing losses, and increasing reliability.

Technical Specifications

The Directlink interconnector consists of three independent 60 MW 2-level Voltage Source Converter (VSC) links operating in parallel. Each link, labeled as System 1, System 2, and System 3, comprises a positive 80 kV and a negative 80 kV DC cable. In total, there are six 59-kilometer DC cables installed between Mullumbimby (NSW) and Bungalora (QLD).

The system utilizes 2-level VSC technology, developed by Hitachi Energy in the late 1990s. Directlink was the first station to use this technology in Australia and the second in the world, following Gotland, Sweden.

Converter Stations

The converter stations are located at Bungalora in northern NSW and Mullumbimby in Queensland. Each station contains three 65 MVA Voltage Source Converters. The converters use Insulated Gate Bipolar Transistor (IGBT) valves to convert electricity from AC to DC and vice versa.

The IGBT modules used in the converter stations are water-cooled and rated at 2.5 kV and 500 A. Multiple units are connected in series to achieve the required voltage rating.

Transmission Capacity

The total transmission capacity of the Directlink interconnector is 180 MW. Each of the three independent links can transmit 60 MW, operating at ±80 kV DC voltage.

Cable Technology

Directlink utilizes HVDC polymeric insulated cable technology for its DC cables. This underground cable system allows for minimal environmental impact and easier permit processes.

Grid Connection

On the New South Wales side, Directlink connects to a 132 kV AC grid, while on the Queensland side, it connects to a 110 kV AC grid. This interconnection helps solve capacity shortage problems in southern Queensland and surplus capacity issues in New South Wales.

Advantages of VSC Technology

The 2-level VSC technology used in Directlink offers several advantages over LCC: Precise control of power flow to match power needs and control network voltage. Support for weak power networks connected to the link. Modularity and standardized design, reducing construction and commissioning periods. Ability to be installed quickly in response to competitive market signals. Compact converter stations and low electrical losses.

Recent Developments

Hitachi Energy is currently working on upgrading one of the three HVDC transmission systems that form part of the interconnector. This upgrade involves replacing IGBT valves in the Bungalora converter station. The project is expected to be completed within three years and will be one of the world's first of its kind, mixing older and newer technologies to extend the life of the infrastructure.

Historical Context

Directlink commenced commercial operation in December 2000. It was initially built by TransÉnergie Australia, a subsidiary of the Canadian utility Hydro Québec and Country Energy. The Directlink Joint Venture (DJV) was later acquired by Energy Infrastructure Investments (EII) in 2006. Currently, the APA Group operates and maintains the facility on EII's behalf.

The Directlink HVDC project enables efficient energy exchange between New South Wales and Queensland while minimizing environmental impact.