Research On The Control Strategy Of NPC/H Bridge Five-level Converter

Benefiting from the advantages like low voltages on switch devices, low harmonics in output voltage, etc., multi-level converters have been gaining priority for high-power power electronics devices. Based on the NPC/H bridge five-level converter, this project aims to research on the problems concerned with its modulation. In particular, the capacitor power balancing, output voltage and current harmonics, switch frequency, and the common-mode voltage are emphasized. The application of current predictive control in multi-level converter is also investigated to improve the dynamic performance of the variable frequency drive.Firstly, the main circuit topology of the NPC/H bridge is analyzed. The impact of connection or disconnection between the output side neutral point of the series 12-pulse diode rectifier and the load neutral pointon the AC side current harmonicsis focused. The mechanism of how the inverter operating mode affects the capacitor voltage is also studied, which prepares for the following modulation methodology analysis.Secondly, taking the 60 degree coordinate systembased seven-segment modulation strategyas an example, the constrain of the common mode voltage on multi-level converter, relationship between the modulation frequency ratio and switching transition,the capacitor voltage balancing problem of the NPC/H bridge five-level converter are explored. Simulation studies have been carried out to validate the theory and lay the foundation for the other multi-level modulation algorithm.Thirdly, conventional PI control has a time delay of one control period, which affects the dynamic performance of the variable frequency drives. The current predictive control is capable of multi-step prediction and compensates the drawbacks of the PI control dynamic performance. However, conventional current control is generally used for two-level or three-level converter, and the rolling optimization is the key factor which limits the development of predictive control in multi-level applications. Based on the 60 degree coordinate system based seven-segment modulation strategy, this project reduces to three optimizing voltage vectors and simplify the calculation of predictive control. Hence, the predictive control can be extended to multi-level applications. Furthermore, considering that common DC bus does not present in the NPC/H bridge five-level converter, an independentvoltage predictive control algorithm is proposed. The proposed algorithm is featured by its independent phase current control(Each phase is optimized three times iteratively) and no override transition. The effectiveness of the two methods are verified though simulations.Lastly, using asynchronous motor as a control object, the mathematical model of the predictive control based motor has been established and the inner current loop has been rebuilt. In order to reduce the hardware cost, the current model flux observer is used. Simulations have been carried to study the rotating speed and torque of the motor, current tracking performance, and the voltage switching frequency. Compared with the conventional PI control, the voltage switching frequency is lower with the proposed strategy. The software flowchart for the DSP & FPGA control system has been designed. On the NPC/H bridge five-level converter experimental platform, the 60 degree coordinate system based seven-segment modulation strategy and current predictive control algorithm have been validated and matched with the simulation results.