Abstract
Multilevel converters are widely considered to be the most suitable configurations for renewable energy sources. Their high-power quality, efficiency and performance make them interesting for PV applications. In low-power applications such as rooftop grid-connected PV systems, power converters with high efficiency and reliability are required. For this reason, multilevel converters based on parallel and cascaded configurations have been proposed and commercialized in the industry. Motivated by the features of multilevel converters based on cascaded configurations, this work presents the modulation and control of a rooftop single-phase grid-connected photovoltaic multilevel system. The configuration has a symmetrical cascade connection of two three-level T-type neutral point clamped power legs, which creates a five-level converter with two independent string connections. The proposed topology merges the benefits of multi-string PV and symmetrical cascade multilevel inverters. The switching operation principle, modulation technique and control scheme under an unbalanced power operation among the cell are addressed. Simulation and experimental validation results in a reduced-scale power single-phase converter prototype under variable conditions at different set points for both PV strings are presented. Finally, a comparative numerical analysis between other T-type configurations to highlight the advantages of the studied configuration is included.
Highlights
Rooftop photovoltaic (PV) energy conversion systems, have become a well-established technology in the industry
Considering the advantages and features previously presented regarding the 3L-TNPC and symmetrical cascaded configurations, this paper described and validated the 5L-CTNPC topology for rooftop PV applications by using a cascaded connection of two 3L-TNPC legs which was firstly introduced in [20] as a cascade 3L-TNPC converter
Each 3L-TNPC leg operates as a string inverter connected to a single potential-induced degradation converter which is fed by one PV string
Summary
Rooftop photovoltaic (PV) energy conversion systems (less than 20 kW), have become a well-established technology in the industry. T-type NPC (5L-HTNPC), was presented in the recent literature [16] This topological variation is built with a 3L-TNPC leg and a two-level leg inverter, forming a five-stepped voltage waveform in the AC terminals. Focusing on the second alternative, cascade MLI can be developed by using symmetrical or asymmetrical voltage levels and by using different type of topologies such as: Full H-Bridge, 3L-TNPC converters or by performing a hybrid configuration [17]. Considering the advantages and features previously presented regarding the 3L-TNPC and symmetrical cascaded configurations, this paper described and validated the 5L-CTNPC topology for rooftop PV applications by using a cascaded connection of two 3L-TNPC legs which was firstly introduced in [20] as a cascade 3L-TNPC converter.
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