Research On Three Phase T-type Three Level Photovoltaic Inverter

The DC/AC inverters are critical components in the rapidly growing market of grid-connected Photovoltaic(PV) applications,development and research on it has been concerned by scholars around the world.Three-level inverters output power are widely used in the field of new energy generation, because of its high output power quality, large output capacity, etc., In recent years T-type three-level topology has been rapidly developed in so many three-level circuit topologies, which is an improved Neutral Point Clamped(NPC) three level topology that has the advantages of fewer switching devices and loss distribution balance.Therefore,the research of this paper is based on the three phase T-type three-level inverter.Firstly, basic on analyzing switching and conduction mode of three phase T-type three level inverter, this paper calculates the conduction loss and switching loss of the inverter by using loss characteristic parameter, and the principle and implement method of Space Vector Pluse Width Modulation(SVPWM) are studied for three level inverter.Secondly, for the problems that there is common mode current in transformerless three phase T-type three level photovoltaic inverters, the paper builds the common mode equivalent model and analyzes the influence factors of common mode current. In order to solve this problem, two improved methods based on the traditional three level SVPWM are gived,which can reduce the common mode voltage through the switch sequence and basic vectors reasonable selection and sorting, thereby, the two methods realize the common current suppression, and their effectiveness is verified through simulations.Thirdly, for the problem of neutral point potential imbalance, this paper analyzes the influence of basic vectors to neutral point potential and summarizes the other causes leading to the imbanlance. By the basis of the common mode current suppression methods, and using different effects of the neutral point potential made by active and negative small vectors, a neutral point potential balancing method based on SVPWM strategy is given, and implementation steps and simulation results is given in detail.Finally, a platform of three phase T-type three level inverter has been developed, using TMS320F28335 DSP as the control core, the design of software and hardware are introduced. All modified modulation strategies are studied on the platform, and experimental results verify the correctness and feasibility of the theoretical analysis.