Research On Transformerless Three-phase Three-level Thotovoltaic Grid System

With the development of the society, the renewable energy has become the focusin our life and has been paid much attention. And the solar energy, an inexhaustibleenergy, has received more and more attention. The photovoltaic grid connectedsystem develops rapidly. And as the core part of PV system, converter has become aresearch focus at home and abroad. Compared with the traditional two-level converterthe multilevel converter has been widely used because of its higher quality of theoutput waveform.Based on the analysis of several topology of multilevel converter and itsadvantages and disadvantages, in the paper, the common diode-clamped three-levelinverter is chosen as the main circuit of the inverter unit. At the same time, the paperintroduces its working principle, mathematical model, and analysis the relationshipbetween SPWM and SVPWM—two kinds of modulation strategy.Aiming at the neutral point potential characteristic of diode-clamped three-levelinverter, this paper analyzes its causes, and recommends three methods to solve theproblem. They control the neutral point potential in qualitative and quantitativerespectively. This paper focus on the analysis of the zero sequence voltage injectionmethod which control the neutral point potential quantitatively and can be applied toboth SPWM and SVPWM. At last the simulation results verify the correctness of thetheory analysis.The mathematical model of the three-level converter in two-phase rotatingcoordinates is derived based on the switch function. The voltage and current doubleclosed-loop control system is designed according to feedforward decouple controlstrategy. This paper analysis the causes of the leakage current, which exists in thetransformerless three-phase three-level photovoltaic grid system inherently. Theessential reason of leakage current is the existence of common-mode voltage. In thispaper, the common-mode voltage is suppressed with software and hardwarerespectively. The software solution contains the carrier same phase modulation,carrier opposite phase modulation strategy and the single carrier modulation strategy.Furthermore, the hardware solution is an improved topology. Finally this paper bringsup a method of the combination of the modulation strategy and the improved topology.Simulation results validate the correctness of theory analysis.Modulation strategy of the research and the control method are verified with theexperimental platform and the results verify the correctness of this thesis.