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Power electronic control in electrical systems 219

Fig. 6.43 Five-level flying capacitor converter phase leg.

. All semiconductors have equal switching frequency which is equal to the carrier
frequency.
. The topology is modular.
However, a number of issues must be resolved in order for such a topology to become
a commercially viable alternative to conventional VSC topologies for high power
applications. These issues can be summarized as follows:
. The number of high voltage capacitors is considered quite high. Furthermore,
taking into account that they need to conduct the full load current at least part of
the switching cycle makes them very expensive with high values.
. The topology is subject to faults and does not have tolerance to them, therefore
appropriate control and monitoring is required.
. The flying capacitors initially have zero charge and starting the converter is not a
trivial issue. This needs to be seriously addressed.