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Received Jan 22, 2018; Revised Apr 6, 2018; Accepted Apr 17, 2018
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1. Introduction
Nowadays, the contradictions between the consumption pattern of the traditional petrochemical energy and the economic development and environmental protection are becoming more and more prominent. People gradually realize the importance of taking sustainable development road, vigorous developing, and utilizing of the renewable energies. Among the renewable energies, the solar energy represents the largest and the most commonly distributed resource. The photovoltaic power generation technology using the solar cells effectively absorbs the solar energy and changes it into electricity. The grid-connected inverter is the key component and important equipment in a photovoltaic grid-connected system.
In the design of general inverters, synthetically considering the cost-effective factors, insulated-gate bipolar transistor (IGBT) represents the most employed device. However, due to the nonlinearity of the IGBT’s conduction voltage drop, it does not significantly increase with the increase of current, thus ensuring that the inverter still presents a relatively low loss and high efficiency at the maximum load condition. However, since the European efficiency is mainly related to the efficiency of the inverter at different light-load, the aforementioned characteristics of the IGBT represent the disadvantage of the photovoltaic grid-connected inverter. For light loads, the turn-on voltage drop of the IGBT does not significantly reduce, which in turn reduces the European efficiency of the inverter. In contrast, due to linear conduction voltage drop of the MOSFET, it provides lower turn-on voltage drop for light loads. Considering the excellent dynamic characteristics and high frequency work ability, MOSFET becomes the first choice for photovoltaic inverting [1].
Three-level inverter has been widely used in the middle and high voltage large capacity AC speed regulating fields, since its output has higher power quality, lower harmonic contents, better electromagnetic compatibility, lower switching losses, and other advantages. However, it still suffers from some key problems, including the simplification of the three-level algorithm, neutral point voltage control in the overmodulation region, and the stability of the system at high voltage. In view of the above problems, this paper studies the structure and principle of the...