Single-Stage Grid-Connected Multilevel Converters: Topologies and Control Strategies
- Publication Type:
- Thesis
- Issue Date:
- 2023
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The global need for sustainable energy supplies is one of the major drivers in the power electronics industry. In this work, the main challenges and objectives for multilevel power converters in grid-connected applications have been elaborated. To address the identified requirements and challenges, a family of efficient multilevel converters with the single-stage dynamic voltage-boosting feature, reduced number of circuit components, modular structure, and bidirectional operation is presented. The aforementioned advantages make this converter a suitable candidate for renewable energy applications. Moreover, to further improve the performance of the proposed converter, an active power decoupling (APD) strategy for single-phase grid-connected inverters is presented to cancel out the double-line frequency ripple in the input current profile. This enables the converter to be employed in a broad range of grid-connected applications with improved efficiency without adding extra components to the circuit. The experimental results show more than 1% improvement in conversion efficiency over the whole power range, only by applying the proposed control strategy. Additionally, a flexible APD control strategy is applied to continuously adjust the tradeoff between the DC input current ripple and voltage stress on the circuit components, while retaining the power quality requirements of a standard grid-connected inverter. The voltage stress on the circuit components can be reduced by up to 17% with the proposed method compared to the conventional APD approaches. Furthermore, to address the scalability requirement of the single-stage DC-AC converters, a new transformerless grid-connected inverter with a common-grounded circuit architecture and single-stage dynamic voltage boosting gain is proposed. The key features of the presented inverter are the reduced current stress profile, modularity, uniform peak voltage stress on the switches, higher power handling capability, and bidirectional power flow operation. Through a modular design with a phase-shifted modulation, the injected grid current can be shared among the modules, while the size of the grid-interface filters can be reduced. The working principle and the generalized form of the converter are discussed, and some simulation and experimental results are presented to validate its feasibility, achieving more than 95% efficiency in grid-connected mode at 120 V DC input voltage over a broad power range. Finally, the conclusions and recommended future works are provided.
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