Research On Transformerless Three-Phase Three-Level Grid-Connected Inverters

With the development of society and technology, energy demand is increasing, but the fossil resources growing shortage, the development of new energy sources has become the vital subject of today’s society. Because of its unique advantages, solar energy plays an increasingly important role, and photovoltaic energy as one of the main forms of power generation, has become a hot research. As a core unit of energy conversion, the cost and efficiency of grid-connected inverter is the key to the whole photovoltaic power generation system. Therefore, the development of low-cost and high-efficiency photovoltaic power generation systems is the pursuing aim for researchers.Firstly, the operation principle of the NPC-I(Neutral Point Clamped, NPC) type three-phase three-level (TL) inverter and NPC-T type three-phase three-level inverter are analyzed in detail in this paper. Taking NPC-T three phase TL inverter as an example, the control principle has been discussed thoroughly, and a improved SVPWM (Space Vector Pulse Width Modulation) algorithm is proposed to suppress common leakage current compared to the conventional SVPWM algorithm. However, this improved modulation strategy might cause the NP voltage distortion of the bridge leg in the process of soft-start in which the modulation ratio is small, which is caused by no use of the redundant small vectors for suppressing common leakage current. In order to solve this problem, a twelve-sector SVPWM algorithm has been proposed in this paper. This proposed novel SVPWM algorithms do not employ small vectors, so aforementioned NP voltage distortion of the bridge leg does not exist. And the twelve-sector SVPWM modulation strategy can also effectively suppress common mode leakage current.The dynamic processes of current commutation have been discussed in detail based on the NPC-I type three-phase TL and NPC-T type three-phase TL topology, and on this basis the corresponding power losses of power devices have been summarized according to various current commutation modes. And then the power loss analysis has been done in NPC-I type three-phase TL and NPC-T type three-phase TL topology by adopting the same power devices. It can be concluded that the efficiency of NPC-T type three-phase TL topology is superior to the NPC-I type three-phase TL when designed at the power rating and the switching frequency in this paper.A17KW Transformerless three-phase three-level grid connected inverter experimental prototype composed of NPC-T type circuit is developed based on simulation study, the design method of NPC-T type circuit has been given, including inductor design, the section of power devices and the DC-link capacitors. Finally, the simulation and experimental results verify the proposed modulation strategy and design method.