Research On The Control Method Of Neutral-Point Voltage For NPC Three-level Grid-connected Inverter Under Non-ideal Grid Conditions

The neutral point clamped(NPC)three-level grid-connected inverter has been widely accepted and investigated for medium or high voltage and high power conversions,however,there is the neutral point(NP)voltage unbalance problem in NPC three-level grid-connected inverter.Because of the transient grid faults,the access of high power single phase load,the non-ideal grid appears,which makes the NP voltage unbalance problem more serious and complex.In this paper,the NP voltage unbalance problem under unbalanced grid conditions is researched and the control method for this problem is proposed.In this paper,the mathematical model of NPC three-level grid-connected inverter under unbalanced grid conditions is established,and the grid current control system is designed based on this mathematical model.The expression of NP voltage under unbalanced grid conditions is derived in this paper,also,the control principle for NP voltage are analyzed.In order to solve the NP voltage problem under unbalanced grid conditions,a new method is proposed,this method dynamically calculates the zero-sequence component by region selections based on the Sinusoidal Pluse Width Modulation(DCR-SPWM),and the zero-sequence component is superposed on the modulation waves to achieve the NP voltage balance.In order to simplify the NP voltage control system,optimize the performance under the severe unbalanced grid conditions,the proportional-integral control based on the hybrid parameter(HP-PI)is designed in this paper,according to different grid conditions,this controller changes the control parameters to control the NP voltage effectively.The experimental prototype of NPC three-level grid-connected inverter based on DSP-CPLD architecture and the control frameware are designed to test the proposed method in this paper.The results of experiment show that two methods proposed in this paper can balance the NP voltage effectively under different grid conditions,and the quality of grid current is improved,also,the proposed method has good performs a good dynamic characteristics.