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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.
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