Abstract
Photovoltaic (PV) conversion systems are in continuous development due to their increasingly competitive prices. The traditional configuration of large-scale PV plants is based on high-power central inverters, which have reduced their cost by increasing their power rating. However, this cost reduction is expected to saturate in the near future, mainly due to an increase in the cost of the dc wiring. Cascaded conversion systems have appeared as potential solutions to continue reducing the PV plant cost. They consist of several conversion units whose ac outputs are connected in series. This enables the power-rating reduction of each individual conversion unit, while maintaining the power rating of the conversion structure. Thus, the conversion units are placed closer to the PV panels, reducing the dc wiring cost. In this paper, a novel three-phase topology for medium-voltage cascaded conversion systems is presented. The proposed topology is formed of several conversion units, each one with a reduced number of conversion stages, namely, dc/ac, medium-frequency isolation and ac/ac. Moreover, thanks to its sequential operation and modulation technique, zero-voltage switching and zero-current switching are achieved in all conversion stages. In this way, with respect to the configuration with central inverters, the proposed topology has the advantages of cascaded conversion systems. In comparison to previously investigated cascaded topologies, the proposed topology also presents promising characteristics, representing a potential cost reduction and efficiency increase. An experimental validation of the topology is carried out in a laboratory prototype consisting of three conversion units.
Highlights
Solar photovoltaic (PV) renewable energy systems are expanding in the power sector due to its competitive prices [1]
A reduction in the number of PV modules connected to each conversion unit is achieved, making it possible to place the conversion unit closer to the PV modules. This drastically reduces the dc-wiring cost as its length is highly reduced. As with these topologies the conversion system generates in medium voltage, the new ac wiring used to connect the ac output of the conversion units has a much lower cross section than the dc wiring that is used to connect the PV modules to the central inverter in the traditional configuration of large-scale PV plants
They include several conversion units whose ac outputs are connected in series
Summary
Solar photovoltaic (PV) renewable energy systems are expanding in the power sector due to its competitive prices [1]. This drastically reduces the dc-wiring cost as its length is highly reduced As with these topologies the conversion system generates in medium voltage, the new ac wiring used to connect the ac output of the conversion units has a much lower cross section than the dc wiring that is used to connect the PV modules to the central inverter in the traditional configuration of large-scale PV plants. The main disadvantage of medium-voltage cascaded conversion systems in comparison to the traditional configuration with a central inverter is the increased number of conversion stages, which increases the cost and reduces the efficiency of the conversion unit. An alternative used to mitigate the power imbalance between phases is presented in [27], where a zero-sequence injection method is proposed This technique has the same limitations as single-phase CHB conversion units, so it cannot completely extinguish the circulating currents. To the best of the authors’ knowledge, there are no publications concerning MV-MMC conversion systems formed of three-phase conversion units
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