Improved neutral point clamped three-level VSI topology construction in agricultural device

Abstract: A NPC three-level inverter is suitable to be used in the field of high power, high voltage. A DC side neutral-point N and three-phase bridge arm neutral-point O of a traditional NPC three-level inverter were linked together. The voltage of the three-phase bridge arm neutral-point would fluctuate, because of the DC side neutral-point voltage fluctuation. It would influence the efficiency of the NPC three-level inverter's work. The neutral-point voltage excessive fluctuation of the traditional neutral point clamped (NPC) three-level inverter would cause the total harmonic distortion (THD) of output voltage increase and switch devices damage. This problem limits its engineering applications in the field of agriculture. A NPC three-level inverter topology with a neutral-point voltage self-balancing function was proposed. It consisted of two parts. Part 1 was the traditional NPC three-level inverter. Part 2 was the active compensation voltage device, and was composed of a single-phase full-bridge inverter circuit, which was used to compensate for the voltage fluctuation of a three-phase bridge arm neutral-point in a traditional NPC three-level inverter. The active compensation voltage device is similar to a controllable voltage source. It was used in a series between point O and point N in a traditional NPC three-level inverter. The three-phase bridge arm neutral-point real-time voltage value was detected and compared with a given value. Then, a real-time compensation voltage was generated by the active compensation voltage device. The voltage of the three-phase bridge arm neutral-point was uo, the voltage of the DC side neutral-point was un, and the voltage of the DC side was us. The real-time compensation voltage was ub, . When the neutral-point voltage of part 1 fluctuates excessively, the active compensation voltage device would generate a real-time compensation voltage to keep neutral-point voltage stability, . Because coordinate transformation is not required, the control scheme is simple. Further, a theoretical analysis of the system stability was achieved. In order to verify the proposed control method, the system was simulated by using the "Power system Blockset" in the Matlab/Simulink environment. The parameters used for simulation are defined as follows: us=3000 V，udc=500 V, dc link capacitor, C=1000 uF, L=0.4 mH, the switching frequency of the NPC three-level inverter was 3kHz, and the switching frequency of the active compensation voltage device was 10 kHz. After the compensation, a voltage fluctuation value of the three-phase bridge arm neutral-point in the traditional NPC three-level inverter was limited under 3%. In addition, simulation results showed that the proposed topology has good dynamic performance. In conclusion, a NPC three-level inverter with a neutral-point voltage self-balancing function was proposed due to the problem of neutral-point voltage excessive fluctuation of a traditional neutral point clamped (NPC) three-level inverter. This structure consisted of an active compensation voltage device to eliminate the neutral-point voltage excessive fluctuation. Simulation results have shown that no matter how the load varies, the presented circuit structure can eliminate the three-phase bridge arm neutral-point voltage fluctuation effectively with quick response and good dynamic performance.