Research On Neutral Point Self-balancing Characteristic Of Three-Level Grid-connected Converter

The neutral point clamped three-level converter which has been widely used in high voltage level and high-power rate system exposes the shortcomings of the unbalance of the neutral point potential.The unbalance of the neutral point potential will increase the device voltage stress and threaten the stability of the system,even seriously deteriorate the quality of the grid-connected current.Therefore,it is of high practical value to study the neutral point balance characteristics and principles of three-level converters.The three-level grid-connected converter is taken as the object in this paper to deduce the following research on the neutral point balance problem:Firstly,takes the three-level NPC converter as an example,the working principle and harmonic distribution of three-level converter under PD and APOD modulation has been elaborated.Then analyzes the output voltage harmonic distortion of these two modulations under the neutral point potential unbalance.The amplitude and distribution of harmonic distortion voltage and the relation between harmonic amplitude and modulation ratio caused by unbalanced of neutral point potential are calculated.The influence of neutral point potential unbalanced on grid-connected current is deduced.Finally,the simulation results on Simulink can verify the correctness of the conclusions.This chapter lays a theoretical foundation for the calculation of self-balancing time constant.Secondly,this section analyzes the traditional zero-sequence voltage injection neutral point balance control.The principle of zero-sequence injection control under sinusoidal pulse width modulation(SPWM)and centered space vector pulse modulation(CSVPWM)is elaborated.The DC zero-sequence voltage injection can effectively control the DC voltage unbalance of the neutral point potential.The optimal injection voltage and influencing factors of the control performance under the continuous modulation are analyzed.Then,the advantages of the combined method of neutral point balance control and discontinuous modulation were discussed.Based on the above research results the time of neutral point balance control is quantitatively calculated.The research shows that the power factor has a greater influence on the neutral point balance control and discovers three-level grid-connected converter neutral point potential self-balancing ability simultaneously.The simulation is used to verify the correctness of the zero-sequence injection neutral point balance control method.Thirdly,aiming at the problems exposed in the traditional neutral point balance control method,this chapter analyzes the self-balancing performance of three-level grid-connected converter topology on the other hand.Firstly,the neutral-point self-balance method of the traditional topology is analyzed and the contribution of grid-connected current and filter capacitor current to the neutral-point self-balance process is discussed.Then,based on the PD and APOD modulation harmonic distribution characteristics,the neutral point self-balancing time constant of traditional topology and limiting factors during self-balancing process is deduced.The correctness of the conclusions is verified by simulation and experimental results.Forthly,the improved neutral point self-balancing topology is proposed to solve the problem of long self-balancing time and the disadvantage that zero sequence current cannot play an effective role in self-balancing process.First,the principle of improved topology has been analyzed.It is explained in detail that the improvement of the filter capacitor zero-sequence current component is the key factor to promote the self-balancing performance.Then,the parameter design principles of the improved topology are explained and the neutral point self-balancing time constant of the improved topology is calculated combining with the characteristics of the harmonic voltage distribution of PD and APOD modulation.In addition,the active damping control under current oversampling is proposed to reduce the impedance parameters in the self-balance time constant which is aimed to weaken the influence of the system stray impedance and inaccurate parameters on the self-balancing performance.Finally,this section studied the reason of neutral point potential drift and the pulsation of the neutral point potential when the grid voltage appears symmetrical and asymmetric sag.The improved topology can solve the neutral point unbalance caused by the grid voltage sag and can adopte in saddle modulation.Simulation and experimental results verify the performance of the improved topology and above conclusions.Above all,the neutral point self-balancing time of the improved topology is close to the time of zero sequence injection method.But the self-balanced topology does not require neutral point voltage measurement and it is not affected by the power factor.The proposed topology has highly applicability and economic value.Figures: 57,Tables: 14,References: 66...