Research On The Control Of Three-level Converter Under Unblanced Grid

With the development of renewable energy,photovoltaic systems and wind power systems are increasingly connected to the grid.As an important device of photovoltaic and wind power systems,the three-level converter is gradually replacing the two-level converter as the preferred choice.Three-level converter is more widely used in renewable energy grid-connected and high-power transmission systems.In practice,unbalanced power grid often occurs due to unbalanced impedance of transmission lines or unbalanced load of electricity.The instability of grid voltage makes the new energy power generation system face great challenges in grid-connected operation.In this paper,the three level grid-connected converter is taken as the research object.The working principle of the three-level converter is described firstly.Then,the realization of the control objectives of the converter under the unbalance of grid voltage is studied and the neutral point voltage fluctuation is analyzed.Finally,an improved modulation strategy is proposed to realize the neutral point voltage balance.Firstly,the topological structure and working principle of the three-level converter are described,and the inherent neutral point voltage fluctuation problem of the three-level converter is analyzed.Then the formula of neutral point current under the condition of grid balance is derived to analyze the neutral point voltage harmonics.Meanwhile,the traditional neutral point voltage balance modulation algorithm is introduced,and the key factors affecting its effectiveness are analyzed,which provides a basis for the subsequent analysis.Then,the realization of several control objectives of the converter under unbalanced grid voltage is studied.The mathematical model under unbalanced grid voltage is established and the instantaneous power analysis is carried out.The control structure of the converter is determined by the results of instantaneous power analysis.Then,according to different control requirements,tthe three phase grid-connected current balance,constant active power output,constant reactive power output and constant bus voltage output on the dc side are achieved respectively.The final result is verified by simulation.As for the neutral point voltage fluctuation under the unbalanced grid voltage,the formula of the neutral point current under the balanced situation of the power grid is derived to the unbalanced situation.And the harmonic analysis of the neutral point voltage is carried out to study the influence of the asymmetry degree of the power grid and reactive power on the mid-point voltage.Meanwhile,the limitations of the traditional neutral point balancing algorithm in the case of grid imbalance are studied and improved.A neural point voltage balancing strategy which is modulated only by zero vector,small vector and large vector is proposed.Finally,the traditional neutral point balance algorithm and the improved neutral point balance modulation algorithm are simulated on MATLAB/Simulink,and the effectiveness of the improved modulation algorithm is verified under the two conditions of unit power factor and certain reactive power.