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Transformerless Multilevel VoltageSource Inverter Topology
Definition
At present, renewable energies represent 25% of the global power generation capacity. The increase in clean energy facilities is mainly due to the high levels of pollution generated by the burning of fossil fuels to satisfy the growing electricity demand. The global capacity of generating electricity from solar energy has experienced a significant increase, reaching 505 GW in 2018. Today, multilevel inverters are used in PV systems to convert direct current into alternating current. However, the use of multilevel inverters in renewable energies applications presents different challenges; for example, gridconnected systems use a transformer to avoid the presence of leakage currents. The gridconnected systems must meet at least two international standards analyzed in this work: VDE 012611 and VDEARN 4105, which establish a maximum leakage current of 300 mA and harmonic distortion maximum of 5%. Previously, DC/AC converters have been studied in different industrial applications.
1. Introduction
Parameters  Description 

Highswitching frequency  Require fast switching and stray inductance should be minimized with the proper circuit. 
High dv/dt  The energy injected into the load must be a sinusoidal signal. When intermediate energy levels are not used, the load must support high dv/dt stress. 
Power loss  The fast switching causes a temperature increase in the semiconductor devices, which requires an adequate heat dissipation system. 
Electromagnetic Interference (EMI)  Electromagnetic interference problems increase with the switching frequency of semiconductors. 

Presents an overview of the current integration of RES with energy injection systems to the grid.

Provides an evaluation and comparison between three voltagesource multilevel inverter topologies.

Discusses about the modulation strategy in NPC inverters.

Presents future trends and research opportunities to contribute to the field.

Presents the challenges and issues concerning the interconnection between the inverters and the grid.

Summarizes more than 20 inverter application works in PV systems.
2. Multilevel VoltageSource Inverters
Parameters  Description 

Lowswitching frequency  The switching frequency is lower, since generally more switches are used to generate the scaled output levels. 
Low dv/dt of output voltages  The voltage stress is lower in each switch since the output levels are distributed among a greater number of semiconductors, thus obtaining a lower dv/dt of output voltage. 
Structure  Modular structure that allows increasing the number of input sources and output power. 
Power  Highoutput power without increasing the rating of the topology switches. 
Total Harmonic Distortion (THD)  Low THD due to a more sinusoidal signal. 
Reduced losses  Switching and conduction losses are low. 
Fault tolerant operation  Using an adequate control strategy and state redundancy. 
where: Vdc is the input power supply and Vo,max is the output voltage in the load.
The entry is from 10.3390/en13123261
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