| Along with the gradual depletion of fossil fuels and the increasing environmental pollution,all countries around the world are facing increasingly serious energy and environmental pressure.The utilization of the renewable energies such as solar energy and wind energy is an important way to solve environmental pollution and energy crisis.Hence,developing green and clean energy has become the key breakthrough to solve the contradiction between development and energy environment problems.As the interface facility of energy conversion,inverter plays a very important role in the renewable energy generation system and three-level inverters have been widely utilized due to the merits of good waveform quality,low switching stress.The power rating of one single inverter is limited and parallel operation of them can effectively increase system power scale.Parallel operation also brings several merits to the inverter system such as high reliablity and flexibility.Thus paralled inverters have received many concerns and studies.However,zero-sequence circulating current(ZSCC)will be generated when multiple inverters are connected in parallel.The ZSCC will lead to the distortion of output current,reduction of the system efficiency,and even break down problems.Therefore,it has to be well suppressed.In this paper,the T-type three-level inverter is used as the research object.The problem of ZSCC suppression has been researched.The mathematical model of the ZSCC is established and its mechanism is analyzed.Corresponding control strategies have also been proposed.This paper is organized as follows:1)The working principle of T-type three-level inverter is analyzed and its mathematical models in different coordinate systems are established.The space vector pulse width modulation(SVPWM)is researched as well as its carrier based implementation.The normalization process,the current feed-forward decoupling and the grid voltage feed forward are utilized when designing the current controller.2)The mathematical models of the two-parallel and multi-parallel inverter system are raised considering the difference of the filter parameters and the unbalance condition of neutral-point potential.The influence of unbalanced neutral-point potential to ZSCC is analyzed.A more reasonable three-level parallel topology is proposed.For the multi-parallel inverter system,the coupling relationship among different inverters is also analyzed.3)An improved model predictive control method is proposed for the ZSCC suppression in two-level parallel system and the effectiveness of this method is verified by experiments.For the three-level parallel system,two ZSCC suppression strategies based on optimal control and PI + feed-forward control are proposed respectively.The optimal control variable is obtained by the ZSCC model and is added to the modulation wave.The PI + feed-forward control method employes feed-forward scheme to suppress the 3rd order harmonic component and to eliminate the ZSCC jumps.4)Two ZSCC suppression strategies for multi-parallel inverter system are proposed since multi-parallel system is more practical and more useful.The optimal control method of the two-parallel system is extended to multi-parallel system.The ZSCC variables are reorganized to solve the coupling problem of the ZSCC of each inverter and the PI + feed-forward strategy could be used after the reorganization process.For the start stop condition of an inverter while the parallel system is running,this paper also proposes corresponding control scheme to ensure no ZSCC spikes occur during the transient procedure.5)The experimental platform of parallel inverter system based on dSPACE is built and the related experiments are conducted.The software architecture and hardware circuit of the system are introduced and analyzed,and the effectiveness of the proposed algorithm is verified by experiments. |
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