Summary of discussions on Electrical Conversion System
Chairman:
Speakers:
Report:
Sjoerd de Haan (TU Delft)
Henk Polinder (TU Delft)
Jan Pierik (ECN)
Johan Morren (Essent)
Bart Ummels (TU Delft)
Ralph Hendriks (TU Delft)
It was recognized that Permanent Magnet Synchronous Generators (PMSG) are a promising
technology for wind power applications, both in direct-drive systems and in systems using a singlestage gearbox. The main advantage is the high power density of the machine. Using a transverseflux design could offer additional benefits. The prices of rare-earth magnets have decreased by more
then an order of magnitude in the last ten years. The main disadvantage of PMSG is the risk of
demagnetization caused by too high temperatures or high currents. These problems, however, can
be overcome and do not necessarily have to be show-stoppers. Olesen (Vestas) remarked that from
an industry point of view a transition to PMSG, direct-drive technology is unlikely because of the
‘lock-in’ in doubly-fed induction generator (DFIG) technology. Many companies went through a
learning process and now have a well developed product. In addition all design tools, logistics and
maintenance processes are developed for this technology. The main obstacle for considering a
transition to this technology is the uncertainty about the total lifecycle costs, and possibly
complicated maintenance (replacement of bearings was indicated as possibly problematic). Further
the realization of the full-size power electronic converters in the multi-MW power range was
considered a problem. For offshore applications the PMSG is more promising then on shore
(reliability, transportation).
Many dynamic modeling studies of wind turbines and wind farms have been performed over the last
years. A problem identified by the researchers was the lack of data available for model verification.
Both wind turbine manufacturers (Vestas, GE) and utilities agree that good models are crucial for
having a clear understanding of the interaction between their wind turbines and the grid, but for
confidentiality reasons data is seldom shared. For researchers verification of models would indicate
whether they are on ‘the right track’ and could e.g. develop simpler models that are more suited for
simulations of large systems while still having an acceptable time response.
The interaction between wind farms and power systems is a research topic that needs more
attention. To get a better understanding of how the control systems of the individual turbines and
the wind farm controller influence each other, modeling and simulation is essential. To investigate
the interaction between controllers of wind farms and the controllers of the grid is considered a
challenge. Cooperation with industry is again necessary to provide for model verification. With more
advanced control algorithms, wind turbines and complete wind farms can provide ancillary services
to the grid, e.g. by providing reactive power or participate in power/frequency control. To study the
impacts of these advanced control strategies on a system level, more modeling efforts are required.
Another topic that is of interest to research institutes and industry is the impact of large amounts of
wind power on power system balancing. For this type of problems, other models are required, that
adequately incorporate the ‘slow dynamics’ of the power system (mechanical time constants),
whereas for the electrical part a quasi steady-state approach is applicable. With this type of
modeling, for instance the effects of wind forecasting on power system operation can be studied.