Getting from the 1986 discovery of ceramics that exhibit high temperature superconductivity to their practical application in useable cables has not been easy. But now, over 20 years on, a transmission-voltage HTS link is in operation. The next challenge is to get the costs down.
On 25 June the world’s first transmission-voltage (138 kV) high temperature superconductor (HTS) cable was inaugurated, at Holbrook in the centre of Long Island. Hosted by Long Island Power Authority (LIPA) the installation aims to provide real-life experience with HTS cable within an operating transmission system.
At 600 m it is also the longest HTS link to date, and, while we are on the superlatives, its power rating – 574 MW – is greater than that of all previous HTS installations combined.
Attention is now shifting to the next step at Holbrook: a second phase of the project, LIPA II, in which second generation (2G) HTS conductors are to be deployed – the first application of 2G conductors at transmission voltage.
These 2G materials promise dramatic cost reductions, which are going to be needed if HTS is ever to become part of the transmission system designer’s portfolio of technologies, in particular fulfilling its potential as a means of upgrading overstretched and congested urban power networks.
The second phase of the project also aims to develop and demonstrate a jointing technique for the HTS cables, allowing much longer HTS links, of the order of several kilometres, to be contemplated – made up of 600 m lengths. In addition LIPA Phase 2 will demonstrate the fault limiting characteristics of the second generation wire. It will also have a repairable cryostat. The cryostat is an insulating envelope that surrounds the cable core, consisting of two concentric stainless steel corrugated tubes with a vacuum in the space between them – somewhat like a very elongated thermos flask.
The existing cable at Holbrook uses tapes of the first generation, ie bismuth based, material, employing hair thin BSCCO (Bi2Sr2CaCu2O8) filaments embedded within a silver matrix, laminated with brass – about 0.2 Troy ounces of silver per m – the purpose of the silver matrix and brass being to give the cable the required flexibility.
The new cable will employ tapes of the second generation, yttrium based, material, YBCO (YBa2Cu3O7). The AMSC brand name is 344 superconductors, deriving from the fact the AMSC 2G wires are 4.4 mm wide and three-ply in structure, consisting of three layers: a central layer of nickel/tungsten coated with YBCO, sandwiched between two layers of stainless steel or brass. Silver is still employed, to ensure good jointing between the layers and to allow the passage of oxygen between the layers, but only a microscopic deposition is required, resulting in much lower material requirements per m, and thus reduced costs.
The partners in the second phase will be the same as the first: Long Island Power Authority (LIPA), local utility host, transmission system owner and operator; American Superconductor Corporation (AMSC), project prime contractor and owner and developer of the HTS wire technology as well as manufacturer of the HTS wire; Nexans, designer, manufacturer and installer of the cable system that incorporates the HTS wires, including cryostat, plus outdoor terminations; Air Liquide, refrigeration system provider; and the US Department of Energy (DoE), sponsor.
The DoE, which has provided $27.5 million of the $58.5 million total project cost for Phase 1, says it sees HTS cables as a core component of the modern electricity superhighway, in particular one that is free of bottlenecks and can readily transmit power to customers from remote generating sites, such as wind farms.
An attraction of HTS cables is of course their ability to deliver large amounts of power through small corridors, allowing, for example, better use of existing rights of way – a scarce and increasingly valuable commodity. The conductor at the heart of an HTS cable can carry about 150 times the current of a copper conductor of equivalent dimensions. This translates into an HTS cable that can carry 3-5 times more power than a conventional copper cable of equivalent size operating at the same voltage. In other words HTS opens up the possibility of boosting transmission capacity by increasing the current rather than upgrading to a higher voltage system. The power transmission capacity of a 345 kV conventional line, for example, can be achieved with a 138 kV HTS cable.
Phase 1 of the LIPA HTS project, which connects the Authority’s Holbrook substation to its 138 kV network comprises three HTS cables (one for each phase) running in parallel in an existing right of way (in underground HDPE conduits). It was energised on 22 April 2008.
The operating current is 2400 A and the design fault current is 51000 A @ 12 line cycles (200 ms).
The cable configuration is of the very low impedance, coaxial cold dielectric, type, one phase per cable. The table on p 20 compares the electrical properties of such an HTS cable with those of conventional cables.
Three separate Nexans locations contributed to the manufacturing of the cable for the LIPA I project: Halden, Norway (cable core); Hanover, Germany (cryogenic envelope); and Calais, France (terminations).
LIPA Phase II launched
Phase II of the LIPA project calls for the replacement of one (or maybe all) of the existing HTS cable system’s three 600 m phases with new cable incorporating second generation HTS wire. The cable system will also incorporate American Superconductor’s Secure Super Grids concept. Introduced by AMSC in May 2007, Secure Super Grid is essentially a package of systems including second generation conductors and ancillary controls which aims to help utilities deliver more power through the grid and suppress power surges that can disrupt service, making use of the inherent fault current limiting capabilities of the second generation cable.
Another application of the AMSC Secure Super Grid system, being developed in parallel with but independently of LIPA II, is Project HYDRA, which aims to help keep the lights on in Wall Street during extreme events (see panel above). But unlike LIPA II, the HYDRA project is at distribution voltage, namely 13.8 kV.
For LIPA Phase II, the US Department of Energy, through its National Energy Technology Laboratory, is expected to provide AMSC with $4 million in federal funding to cover the period up completion of its first project budget period, expected to end in September 2008. Upon successful completion of key project milestones and sustained execution of a viable business strategy, as much as $5 million in additional Department of Energy funding may be made available for continued implementation of the two-and-a-half-year long LIPA II project, up to March 2010, subject to availability of funds appropriated by the US Congress.
Another feature of LIPA Phase II will be development of a lower cost, more reliable and more efficient refrigeration system, including technology of the helium Turbo-Brayton type, which Air Liquide says is “extremely well suited to this application.”
Currently at Holbrook a helium Brayton cycle refrigeration system is used to cool the liquid nitrogen that circulates in the cable. This refrigeration system was originally employed at the short-lived DTE Frisbie HTS project (abandoned due to cryostat leaks). The cooling capacity of the Frisbie system was upgraded (from 4.3 kW to 5.4 kW) and installed at the LIPA facility. In the existing Brayton system, helium is compressed by two compressors working in parallel. It enters a vacuum insulated cold box where it is pre-cooled and then expanded in two turbo-expanders. Liquid nitrogen returning from the cable is re-cooled in the cold box by counter flow heat exchange with cold helium.
The refrigeration system at Holbrook is redundant. In case of a failure in the Brayton cycle system a back-up system using an open loop of liquid nitrogen with a large storage tank kicks in. The open loop system can also be used in conjunction with the Brayton cycle refrigerator should the cooling demand from the cable require it.
Looking beyond the Holbrook 2G cable installation there is also a proposal for a 6.5 mile HTS link between two LIPA substations – with a possible completion target date of 2015.