Study On DC-Link Harmonics And Optimization Of Losses Distribution In Three-level Neutral Point Clamped Inverter

Three-level NPC inverter has the advantages that output voltage and current THD is low, semiconductor voltage stresses and switching losses halves compared with two-level inverter, so that reducing the volume and mass of passive component. Recent years, three-level NPC inverter is used in medium-voltage and low volatage 380V~690V applications, eg. pv grid-connected, UPS source and so on. DC-link current harmonics analytical expression and power losses formula that necessary to capacitor sizing and module selecting in traditional two-level inverter is developed. Three-level NPC inverter is not developed like that. This thesis research around the two aspects, sizing the DC-link capacitor in full power factor range of 100 k VA(380V) grid-connected inverter, and verifying power losses distribution and junction temperature in safe range.Firstly, several kinds of multi-level inverter topology are analyzed. Common modulations of three-level NPC inverter are analyzed and simulated with MATLAB/Simulink, as basis on DC-link current harmonics and power losses.Secondly, a “geometric wall” of three-level NPC inverter DC-link current is builded based on two-level inverter and DC-link current harmonics is obtained. A comparison between MATLAB/Simulink and analytical expression has been done. Sizing capacitor based on this current harmonics analytical expression.Thirdly, three-level NPC inverter power losses formulas with SPWM and third harmonic injected PWM, thermal module and junction temperature are derived. A losses and junction temperature simulation tool is builded with Math CAD, and it is verified by Semisel on-line simulation tool. The unequal losses distribution of three-level NPC inverter is analyzed, and ANPC topology is used to resolve the problem.Finally, DC-link current harmonics spectra and power losses distribution in different cases can be obtained by the DC-link capacitor spectra and power losses simulation tool above. Sizing capacitor and selecing power module by the methods, and a 100 k VA grid-connected three-level NPC inverter based on SK150MLI1066 T module is designed.