Medium-voltage (MV) multilevel converters are considered a promising solution for large scale photovoltaic (PV) systems to meet the rapid energy demand. This article focuses on reviewing the different structures and the technical challenges of modular multilevel topologies and their submodule circuit design for PV applications. The unique structure of the converter’s submodule provides modularity, independent control of maximum power point tracking (MPPT), galvanic isolation, etc. Different submodule circuits and MPPT methods to efficiently extract the PV power are reviewed. The integration of the multilevel converters to PV systems suffers unbalanced power generation during partial PV shading conditions. Several balancing strategies to solve this problem are presented and compared to give a better understanding of the balancing ranges and capabilities of each strategy. In addition, the paper discusses recent research advancements, and possible future directions of MV converters-based large-scale PV systems for grid integration.

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Medium Voltage Large-Scale Grid-Connected Photovoltaic Systems Using Cascaded H-Bridge and Modular Multilevel Converters: A Review

The rise in renewable energy has increased the use of DC/AC converters, which transform the direct current to alternating current. These devices, generally called inverters, are mainly used as an interface between clean energy and the grid. It is estimated that 21% of the global electricity generation capacity from renewable sources is supplied by photovoltaic systems. In these systems, a transformer to ensure grid isolation is used. Nevertheless, the transformer makes the system expensive, heavy, bulky and reduces its efficiency. Therefore, transformerless schemes are used to eliminate the mentioned disadvantages. One of the main drawbacks of transformerless topologies is the presence of a leakage current between the physical earth of the grid and the parasitic capacitances of the photovoltaic module terminals. The leakage current depends on the value of the parasitic capacitances of the panel and the common-mode voltage. At the same time, the common-mode voltage depends on the modulation strategy used. Therefore, by the manipulation of the modulation technique, is accomplished a decrease in the leakage current. However, the connection standards for photovoltaic inverters establish a maximum total harmonic distortion of 5%. In this paper an analysis of the common-mode voltage and its influence on the value of the leakage current is described. The main topologies and strategies used to reduce the leakage current in transformerless schemes are summarized, highlighting advantages and disadvantages and establishing points of comparison with similar topologies. A comparative table with the most important aspects of each converter is shown based on number of components, modes of operation, type of modulation strategy used, and the leakage current value obtained. It is important to mention that analyzed topologies present a variation of the leakage current between 0 to 180 mA. Finally, the trends, problems, and researches on transformerless grid-connected PV systems are discussed.

Leakage Current Reduction in Single-Phase Grid-Connected Inverters—A Review

In many emerging applications, a dc–dc converter with a wide range of voltage conversion is necessary to equalize the output voltage level from various low voltage sources. To meet this purpose, as well as to reduce the number of power conversion stages in high step-up operation, a wide range of voltage gain dc–dc converter is proposed in this article. This converter is structured as an optimized integration of active switched-inductor configuration, and ladder-type voltage multiplier is merged with the regenerative-boost arrangement. Thereby, due to such an organization of structure, the proposed converter is developed the extremely high voltage-gain with comparatively fewer components. It has the advantages of clamped the input peak current, and higher voltage gain can be achieved at lower duty-ratios. This article describes the comprehensive steady-state analysis and performance characteristics in detail. Moreover, a detailed comparative performance analysis is carried out with existing recent literature. Finally, to validate the theoretical analysis and test feasibility and suitability of the proposed dc–dc converter, experimental results were observed using a 300 W fabricated prototype model.

Active Switched-Inductor Network Step-Up DC–DC Converter With Wide Range of Voltage-Gain at the Lower Range of Duty Cycles

The soft switching flyback inverters still cannot provide high efficiency and low output current total harmonic distortion (THD) in all load ranges. Therefore, a new flyback inverter with soft switching for photovoltaic AC module applications is presented in this study. The introduced inverter is simple and a small auxiliary circuit is added to the conventional topology of the flyback inverter. Since the primary current of the transformer is not affected by the auxiliary branch, the output current THD does not increase. In addition, all of the high-frequency switches of the proposed topology are switched under soft switching condition, which allows high-switching frequency. Hence, high efficiency, as well as compact design, can be achieved in the proposed inverter. Moreover, the voltage overshoot of the main switch during the turn-off process is limited. This effect decreases the conduction losses because the lower voltage rating switch can be used. Furthermore, a new control method is presented, which provides high efficiency in all load ranges. The operations of the introduced flyback inverter and component selection have been discussed in detail. The performance of the proposed inverter with the auxiliary circuit and corresponding controllers are validated with the help of experimental results.

https://ietresearch.onlinelibrary.wiley.com/doi/epdf/10.1049/iet-rpg.2019.0365

Soft switching flyback inverter for photovoltaic AC module applications

In this study, a new multi-input high step-up inverter, based on isolated soft-switching DC-DC converter blocks is proposed. Each of these blocks can provide zero-voltage and zero-current switching for its semiconductors, which improve power efficiency. The interesting feature of this DC-DC converter is using bidirectional switches to generate both positive and negative output voltage levels in each DC-DC block with an appropriate control scheme. Each DC-DC converter operates by simple pulse width modulation control through fixed frequency and has two degrees of freedom, which provide the capability of output voltage regulation or maximum power point tracking. The proposed inverter consists of the cascaded connection of these DC-DC converters at their output terminal. This inverter can operate with high voltage gain, where the output voltage of each DC-DC converter is regulated. Furthermore, it can generate more output voltage levels with less number of DC-DC blocks. All these advantages make the proposed inverter suitable for photovoltaic power conditioning systems. In this study, the theoretical analysis with steady-state waveforms, design constraints, resonant tank analysis and comparison study are given. Then, experimental results of both the proposed inverter and its DC-DC blocks are presented.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-pel.2018.5801

Multi-input high step-up inverter with soft-switching capability, applicable in photovoltaic systems

For photovoltaic applications, the flyback microinverter with pseudo-dc-link is popular as a simple topology but brings large transformer turns ratio and thus large leakage inductance, which would deteriorate the converter efficiency. To solve this issue, based on the nonisolated pseudo-dc-link structure, this paper proposed a hybrid boost–flyback/flyback (BF/F) microinverter. This new topology is operated at the BF mode for the most segment of a half grid cycle and the F mode for the rest. During the BF mode, high voltage gain with low voltage stress is easily available in minimized transformer turns ratio. Besides, the leakage energy is recycled and the turn- off voltage spike of the main switch is clamped, as a result of a passive snubber inherently contained in this mode. Given that the BF mode is lack of step-down function, the F mode is developed to regulate the output voltage even for values lower than the input voltage. The operation and characteristic of the hybrid BF/F microinverter in boundary conduction mode are analyzed in detail, and the mathematical expression of reference current is derived theoretically to guarantee high power quality. Finally, a 240 W prototype was implemented to validate the theoretical analysis and the benefits of the proposed topology.

A Hybrid Boost–Flyback/Flyback Microinverter for Photovoltaic Applications

Based on the combination of boost-flyback and flyback converter, a dual-mode mirco-inverter with pseudo-dc-link was proposed in this paper. This new topology operates at boost-flyback (BF) or flyback (F) mode alternatively dependent on a pair of double-line frequency complementary switches. During the BF mode, high voltage gain with low voltage stress on the power devices is obtained owing to the serial output of the boost and flyback sub-converter. Furthermore, this mode also provides a passive snubber to recycle the energy stored in the leakage inductor of transformer and clamp the turn-off voltage spike of main switch. The F mode enables the dual-mode converter to regulate the output voltage lower than the input voltage, making it possible to generate a rectified sinusoidal waveform at the dc link. The operation of the dual-mode mirco-inverter with peak current control in boundary conduction mode (BCM) is described in detail, and the mathematical expression of reference current aiming to lower total harmonic distortion (THD) is derived theoretically. Finally, a 240W prototype was implemented to validate the theoretical analysis and the benefits of the proposed topology.

Yanshen Hu

Papers

A Hybrid Boost–Flyback/Flyback Microinverter for Photovoltaic Applications

A Novel Dual-mode Micro-inverter for Photovoltaic AC Module Applications