Chris Bannigan, Converteam UK Limited, UK
Designers love to design and engineers love to engineer, so those of us who have spent our lives in the field of rotating electrical machines – motors and generators – have constantly sought out improvements and enhancements for these most fundamental of electrical components.
|Inside the generator room at E.ON’s Hirschaid hydropower prior to the upgrade|
However, many now consider that conventional, copper-based machines have reached the limits of their theoretical development potential and that the future lies in the implementation of new technologies that offer the promise of step changes in power density and rating per unit of cost.
One such step looks set to be in the development of a new breed of generators, which make use of high temperature superconducting (HTS) materials in lieu of copper. The development of these machines holds the promise of a step change in efficiency together with size and weight reductions up to of 70 per cent when compared to a conventional solution.
These remarkable size and weight savings are due in large part to the fact that superconductors have markedly smaller cross-sectional areas when compared to copper conductors, thereby permitting many more turns for a given coil envelope, producing a more powerful magnet.
In early 2010, Converteam UK Limited will undertake full scale testing of what is thought to be the world’s first HTS hydro generator for a commercial installation. The generator is being manufactured by Converteam as part of a European Union (EU) part-funded project and will be installed at the Hirschaid run-of-river hydroelectric power plant in Bavaria, Germany, which is owned and operated by E.ON Wassercraft GmbH.
E.ON’s Hirschaid Power Plant
The Hirschaid power plant, located on the river Regnitz approximately ten km south of Bamberg and built in the 1920s, satisfies the electrical power requirement for around 2000 households. The three existing generators are driven by twin Francis turbine units – an inward flow reaction turbine which combines radial water inflow and axial water outflow – and are each rated at 1.25 MW.
E.ON, as owners and operators of the plant have elected to replace one of the three existing generators with a machine based on HTS technology. The new unit will have an upgraded power rating of 1.7 MW, compared to the existing 1.25 MW and, crucially, the HTS machine will be accommodated within the footprint of the original generator and will require no significant infrastructure modifications. The new generator will be designated as the plant’s baseload generator and, as such, it is anticipated that baseload generation will increase by in excess of 30 per cent when compared to the original generator configuration.
With more than 100 years of experience in the design, manufacture and support of rotating electrical machines, Converteam UK was selected to lead the process of developing this ground-breaking generator, which has become known as Hydrogenie.
After extensive evaluation of the proposed Hydrogenie design partial EU funding was obtained under the 6th Framework Programme (FP6) which aims to encourage transnational programmes of research and technological development. Under the terms of the funding a consortium was formed under the leadership of Converteam – see the table below for more details.
The art of superconductors
The properties, and potential benefits of, superconducting materials have been known to science and engineering for a century or more. Superconductivity is a phenomenon that occurs in certain materials at very low temperatures. Superconductors at low temperature, unlike conventional copper conductors, have zero electrical resistance and are therefore able to transmit electrical power without losses due to resistance. As a consequence, such materials exhibit power densities many times that of conventional conductors.
In a superconducting material resistance drops suddenly to zero when the conductor is cooled at or below its critical temperature, whereas conventional materials (such as copper) although they exhibit a gradual decrease in resistance that corresponds with decreased temperature, including absolute zero, they always retain some of their resistive qualities.
The critical temperature at which superconducting qualities are exhibited is a key factor in the evolution of commercially viable HTS equipment. The earliest observations of superconductors (i.e.mercury) required cooling to 4 K (-269 ºC). In contrast, today’s commercially available superconductors can, depending on the application, operate in relatively balmy temperatures in excess of 50 K, hence giving rise to the term ‘high temperature superconductor’.
|Figure 2: The fully-assembled rotor for the Hydrogenie generator|
The superconductors used in Hydrogenie are known as first generation (1G) conductors. These have been available commercially for many years and are based on multiple filaments of a ceramic material, known as BSCCO, which are embedded within a silver matrix. This material is manufactured via a multiple rolling process followed by a controlled heat treatment process.
Second generation (2G) HTS wires, based on a ceramic known as YBCO, are gradually becoming commercially available and look set to further tip the balance toward the widespread implementation of HTS products because of their more favourable pricing. The price, in terms of current carrying capacity, of 2G HTS wire is expected to become competitive with conventional copper within the next five to ten years.
Hydrogenie in a nutshell
The Hydrogenie machine was initially intended as a technology demonstrator for HTS technologies, and for the many associated design features that need to be developed in order to enable a working generator based on superconductors to be constructed and put into service. The 1.7 MW, line voltage 5.25 kV, 28 pole, 214 rpm Hydrogenie generator features HTS rotor coils and a conventional stator.
|Figure 3: Schematic of the Hydrogenie superconducting generator|
The design and manufacture of Hydrogenie has been a complex process and many technical challenges have been encountered and overcome, particularly in terms of the cooling and insulation of the rotor’s superconducting coils, and the development of a rotating cryostat within which the generator’s rotor itself is housed.
The rotor’s field coils are cooled to the requisite temperature of about 30 K using high-pressure helium gas transferred from static cryo-coolers, situated outside the machine, to the rotor via a custom-designed and manufactured rotating coupling supplied by our project partner Stirling Cryogenics and Refrigeration BV.
The HTS coils use a Bi-2223 tape 1G conductor and are insulated using Multi Layer Insulating (MLI) blankets (sometimes known as superinsulators) in order to reduce radiation heat loads onto the cryogenically cooled components. The coils are positioned over laminated iron poles to form a rotor assembly, which is enclosed within a sealed vacuum chamber that rotates with the shaft.
A part of Converteam’s overall investment in the Hydrogenie generator was the construction of a new cryogenic test laboratory at its facility in Rugby, UK. This test facility enables extensive testing of the HTS coils under the most extreme conditions that might be anticipated once the generator goes into service at the power plant.
Field excitation for the HTS coils is provided via electronic circuitry, which is mounted on the external surface of the vacuum chamber. The temperature and voltage of each individual HTS coil is continuously monitored and is fed into the control and protection circuitry, which is also located on the outside of the vacuum chamber. Communication between the HTS machine and the automatic voltage regulator (AVR) is provided by means of a wireless link. A rotating brushless interface, rather than slip rings, was selected to provide a robust and reliable means of providing electrical power to this circuitry.
What Next? More Small Steps?
The Hydrogenie successfully completed factory testing towards the end of 2009, and will be installed at the Hirschaid plant during the summer of 2010, where it will undergo extensive field testing in a commercial environment. Converteam, together with its consortium collaborators, is enormously proud of the achievements to date and these have recently been recognized when, in November 2009, the Converteam Hydrogenie project was named as a winner of the 2009 Innovation Award in the Power and Energy category by the UK’s Institution of Engineering and Technology.
|Figure 4: Converteam’s cryogenic test facility at Rugby formed part of the company’s investment in the Hydrogenie generator|
So one might be tempted to conclude that the project is finished and that there are no further developments to ‘develop’ or engineering challenges to ‘engineer’. Wrong!
Installation and testing of the Hydrogenie generator is likely to result in further learning and further challenges – that is simply in the nature of doing something for the first time. Confidence is high though and there is justifiable optimism that such challenges and obstacles will be handled professionally and that superconducting machines will be proven as a commercially viable proposition.
As 2G HTS conductor technology continues to mature it seems likely that HTS generators and motors will have an impact on several markets where traditional machines are currently the only choice. Renewable energy markets may be one of those most likely to benefit in the short-to-medium term. The business of recovering electrical energy from wind seems like a likely candidate that could reap massive benefits from improvement in generator efficiency and reductions in size and weight.
At present it may be said that there is something of a glass ceiling in wind turbine design which limits, with a few notable exceptions, turbine output to around 6 MW. Using current estimates, a 10 MW Converteam direct drive superconducting generator would be almost equal in size and mass to a 5 MW geared generator employing ‘traditional’ copper technology.
Studies have shown that, once the commercial price of 2G HTS becomes competitive, the lifetime cost of operating such wind turbines may be significantly less that conventional machines. The reduction in nacelle weight means that towers will not need to be substantially bigger than the current machines.
Converteam believes that HTS machines and their integration within electrical systems have the potential to massively impact the industries in which they are introduced. The impressive power density, high efficiency and reduced cost of these machines will make them highly attractive to a wide variety of users who would benefit from direct drive rather than intermediate systems, such as those processes which employ hydraulics or gear boxes.
The technology of high temperature superconductors is now very much out of the laboratory and is fast becoming a commercial reality. Hydrogenie may be one small step in this technology’s evolution but perhaps it should be realistically viewed as a critical part of the giant leap towards commercialization of one of the most exciting developments in the field of electrical engineering.