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Power Electronics and Controls for Large Wind Turbines and Wind Farms 189
obtained if a part of the generator phases is out of operation [52]. On the other hand, doubled cable
length is needed; extra cost, weight, loss, and inductance can be the major drawbacks. Moreover,
paralleling the converter cells will be difficult in order to further extend the power capability.
Some benchmarking studies of potential converter topologies for wind power applications under
either normal or abnormal operating conditions have been conducted in [48, 53], which are not
included in this chapter.
8.4.4 Future Converter Topologies
8.4.4.1 Cascaded H-Bridge Converter with Medium-Frequency Transformers
A configuration that shares the similar idea with next-generation traction converters [54, 55] and
is also proposed in the European UNIFLEX-PM Project [56] could be an interesting solution for
the future WTS. It is based on a structure of the BTB cascaded H-bridge converter, with galvanic
isolated DC/DC converters as interface. The transformer size can be significantly reduced in both
weight and volume due to high-frequency operation. Moreover, it can be directly connected to
the distribution power grid (10–20 kV) with high output voltage quality using a filterless design
and having redundant ability. This solution would become attractive for future large WTs if it can
be placed in the nacelle, where the bulky line frequency transformer can be replaced by the more
compact and flexibly configured power semiconductor devices—leading to a promising increase
of the power density.
8.4.4.2 Modular Multilevel Converter
Another potential configuration for the future wind turbines shares the similar idea with some of the
new and emerging converters used for high-voltage direct current (HVDC) transmission [57, 58].
One advantage of this configuration is easily scalable voltage/power capability; therefore, it can
achieve very high power conversion at dozens of kV with good modularity and redundant perfor-
mance. The output filter can also be eliminated because of the increased voltage levels.
It can be seen that the topologies with multiconverter cells have modular and fault-tolerant
abilities, which may contribute to achieving higher reliability and power capability. But, on the other
hand, these configurations have significantly increased component count, which could compromise
the system reliability and significantly increase the cost. The overall merits and defects of these mul-
ticell converters used in the wind power application still need to be further evaluated—also because
the technologies for power semiconductor devices are developing rapidly.
8.5 POWER ELECTRONIC SOLUTIONS FOR WIND FARM
As the WT capacity is getting larger and larger, on one hand, the high cost of energy will require
the transmission of wind power as efficient as possible, and on the other hand, the more significant
impacts to the power grid would require the WTs to play more active role in the power grid accord-
ing to grid codes. As a result, the design and configuration of wind farms, which normally involve
very larger-scale wind power integration, are becoming critical to achieve both highly efficient wind
power delivery and grid code compatibility.
8.5.1 Solutions for Wind Power Transmission
Wind farms may have significant impacts on the grids, and therefore, they play an important role in
the power quality and the control of the grid systems. The power electronics technology is again an
important part of the system configurations and control of wind farms in order to fulfill the growing
demands. Some existing and potential configurations of wind farms are shown in Figure 8.14.