Research On Neutral-point Potential Balancing And Zero-sequence Circulating Current Suppression In Three-phase Three-level Inverter

With the gradual depletion of non-renewable energy and the energy demand continues to grow,smart grid and new energy power generation technology is becoming a hot spot of present research.In order to alleviate the impact of a large number of new energy power generation access grid on the power grid,while improving the efficiency of power grids,a large number of energy storage systems have been applied.This dissertation focuses on the neutral-point(NP)potential balancing and the zero-sequence circulating currents(ZSCC)for the three-level inverter parallel system in the energy storage system.Firstly,the development of multi-level topology and existing modulation methods were elaborated,the existing three-level NP control and ZSCC suppression strategy are compared and analyzed.Secondly,the working principle of T-type three-level topology was elaborated,and a three-level space vector implementation method based on 60° coordinate system was introduced.A model for three phase grid – tied inverter was built,and related controller was designed.The DC side support capacitor is designed according to the NP current component,and the filter inductance parameter is designed according to the requirement of inductance ripple and grid current harmonics.Thirdly,the causes of the NP potential fluctuation of the three-level inverter are analyzed,and the NP potential is analyzed by its mathematical modeling.The traditional method for NP potential balancing is usually only to eliminate the DC component of the midpoint potential,while its ability to suppress the low frequency ripple is limited.A PR controller is proposed to increase the open-loop gain at the triple fundamental frequency,so as to realize the suppression of low-frequency ripple.The controller parameters are designed based on the Bode diagram.Finally,this dissertation built the mathematical model of ZSCC,the "coupling" relationship between the NP fluctuation and the ZSCC is analyzed.In order to reduce the impact of NP on ZSCC,the NP was connected to each other.The traditional ZSCC control strategies are usually by reducing the parameters difference between inverters,while the ZSCC will increase greatly when the inverter conditions changed.A quasi-PIR controller is used to suppress the low-frequency component of the ZSCC,and the infinite gain is achieved at the low frequency,so as to realize the astatic control.The performance of proposed strategy and traditional control was compared by simulation and experiment.The correctness of the theoretical analysis on the NP potential and the ZSCC were verified,and the effectiveness of the proposed strategies were also verified by simulation and experiment.