Direct drive superconducting wind generators - Wind Power Generation and Wind Turbine Design - page 310

One common characteristic of these materials is they were all complex copper
oxides and, more signifi cantly, all ceramic materials. While it was relatively easy
to manufacture such materials in bulk form, the technology to produce fl exible
wires that would be of use in electrical machine windings proved to be a consider-
able challenge. After considerable effort, the HTS material Bi 2 Sr 2 Ca 2 Cu 3 O x
(more commonly referred to as BSCCO-2223) was successfully manufactured
into practical wires during the 1990s.
3.3 HTS rotating machines
Rotating machines utilizing HTS materials have been under development for
nearly 20 years, following the discovery of HTS materials in the late 1980s. HTS
machines can use either HTS wires [19], or bulk HTS material [20], or even a com-
bination of both [21]. Various types of HTS rotating machine topology have been
proposed, including synchronous, homopolar [22] and induction [23]. Most large
HTS machines projects to date have used a topology similar to conventional large
synchronous machines, with a DC fi eld winding on the rotor wound with HTS
wire, and a copper AC stator winding at conventional temperature, as in [19].
The largest HTS machine that has been built and tested so far is a 36.5 MW, 120 rpm
ship propulsion motor designed by American Superconductor (AMSC), and manu-
factured by AMSC, Northrop Grumman Corporation and Electric Machinery (now
part of the Converteam group) for the US Navy [24]. This motor completed full
load testing in January 2009. It has a rated torque of 2.9 million Nm, comparable
to that of a 4 MW wind turbine. The 36.5 MW machine was the follow-on from a
scaled prototype 5 MW, 230 rpm machine design and manufactured by AMSC and
ALSTOM Power Conversion Ltd. (now Converteam Ltd.). The 5 MW machine
was tested at full torque at ALSTOM, Rugby, UK [ 25 ], and at full load under
simulated ship at sea conditions over a period of 1 year in the C.A.P.S. facility at
Florida State University [ 26 ].
4 HTS technology in wind turbines
4.1 Benefi ts of HTS generator technology
HTS technology allows rotating machines to be constructed with signifi cant
increases in power density compared to conventional or permanent magnet
machines. This advantage becomes greater as the size of the machine increases
[27]. High power density is the result of the high current density that can be
obtained in HTS coil, reducing the space required for the rotor fi eld coils. The
copper coils in a conventional machine typically operate with a current density
between 3 and 5 A/mm 2 , while the current density in the wire in a HTS coil can
operate at 200 A/mm 2 or more. In HTS wire this is known as the 'engineering
current density' which is the current density in the full cross section of the wire,
but as the HTS material only forms a small proportion of the cross sectional area
of the wire, the current density in the HTS material itself is much higher, up to
20,000 A/mm 2 . Additionally, the ability to place many Amp-turns of fi eld winding