Multilevel converters are widely used in medium and high voltage applications. Their high performance, power quality, efficiency and smaller filters make them interesting for renewable energy distribution systems. In utility- scale photovoltaic plants, these topologies could provide multiple benefits since they are able to connect string of photovoltaic panels to independent modules. However, high floating voltages caused by high number of modules limit multilevel converters in medium and high voltage applications, since all of them are not suitable to provide isolation to each module. To offer a solution, this paper presents a novel multi-modular converter that provides multiple isolated modules connected in series through low frequency transformers to operate at medium voltage levels. This topology is able to achieve the power balancing between the connected modules and independently adjust the dc voltage of each module by means of controlling a circulating current which flows through the arms. Furthermore, the topology implemented in photovoltaic renewable energy systems and the control strategy required to regulate the circulating and the output current are presented. The main principle behind this concept and the performance of the converter are evaluated and validated through simulation and experimental results.

The increasing participation of renewables in power systems has forced system operators to strengthen grid codes requiring renewable power plants (RPPs) to provide fast frequency support such as inertia response and power oscillation damping. To cope with such new requirements, this article proposes the implementation of a virtual synchronous power plant controller (VSPPC) for RPPs. The VSPPC takes the most from emulating the behavior of a synchronous generator, especially in the case of grid events, as it permits emulating inertia and provides power oscillation damping capabilities. If compared with conventional control schemes, the main advantage of the VSPPC lays in the fact that it does not require to implement any modification in the existing controllers of the plant's converters, which are often controlled as grid-following generation units. This feature makes VSPPC attractive for incorporating advanced grid supporting functionalities in utility-scale RPPs. In this work, the main principles behind the VSPPC and comparative analysis based on simulation and experimental results are provided to validate its performance.

The integration of renewable energies based on photovoltaic power plants is increasing exponentially fast. In consequence, stability problems associated to the lack of inertia is becoming a serious concern. Transmission and distribution system operators are demanding more requirements in photovoltaic plants to provide grid functionalities in terms of frequency and voltage support. Therefore, power converters based on virtual synchronous controllers have been proposed to emulate the dynamic behavior of synchronous generators. Control strategies such as the Synchronous Power Controller has been proposed for photovoltaic applications to fulfill this requirement. However, most of these control strategies are integrated locally as an add-on firmware embedded in each converter, providing the demanded support in their connection point. For photovoltaic power plants, the inherent synchronous characteristic is expected at the connection point with the electrical grid, which is not always the connection point of each power converter, thus, unexpected power support levels can be obtained. The aggregation of several power converters operating in a coordinated synchronous action would solve this issue, due to the fact that they can provide voltage and frequency support in a specific point. In this paper a centralized control strategy based on a synchronous central architecture is proposed. The control strategy emulates an aggregated synchronous control to regulate a photovoltaic power plant as a coordinated system. Simulation and experimental results validate the effectiveness of the control proposed.

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.

Single-phase photovoltaic (PV) energy conversion systems are the main solution for small-scale rooftop PV applications. Some multilevel topologies have been commercialized for PV systems and an they are attractive alternative to implement small-scale rooftop PV applications. Efficiency, reliability, power quality and power losses are important concepts to consider in PV converters. For this reason this paper presents a comparison of four multilevel converter based in the T-type topology proposed by Conergy. The presented control scheme is based in single-phase voltage oriented control and simulation results are presented to provide a preliminary validation of each topology. Finally a summary table with the different features of the converters is provided.

The series-connection of modules in multilevel converters are prone to energy imbalances in the dc capacitor due to the differences between the power absorbed and consumed. In renewable energy applications where the primary source is directly connected to each module, energy imbalances can be even worse if the primary sources are affected by unpredictable weather conditions. Therefore, control strategies are required to compensate such energy imbalances, while maintaining the correct converter operation. Focusing our attention on a cascaded transformer multilevel inverter called Isolated Multi- Modular Converter, this paper introduces the combination of two control strategies aimed at providing a wide range of operation under imbalanced energy states. A general analytical model, including the regulation capability and differences with an existing strategy are presented to demonstrate the performance of the control proposed. The effectiveness of the proposal is validated through experimental results based on a three-phase multilevel prototype.