System Losses And Thermal Analysis On NPC Three-level Converter

As three-level converter has widely used in kinds of industrial and other areas, to achieve system optimization, we need to analyze kinds of the property of NPC three-level converter in detail. It is sensible to study on converter. Although the analyses on losses of two-level converter have become very mature, they can not be completely used in three-level converter.This paper introduces kinds of PWM control algorithms in three-level converter, and compares some of them by simulation, we presented the IGBT and power diode working waveform, and brought in the thermal equivalent model of power devices. The different losses status among power devices at several levels has been analyzed. The results indicate that unequal losses distribution in different power devices of NPC three-level converter with different system index. At last, it shows the relationship between the different work index and the losses of power device.This paper presents a new method to solve the neutral-point voltage unbalanced problem which can be used in each control algorithm. The easy realization and good effect are benefit from the avoidance of complex computing in control IC and new computing when the load properties changing. In addition, the dead-time compensation was realized by driver pulses transformation in FPGA.Based on above-mentioned analysis, this paper presents the design principles of every hardware model and software algorithm in detail. Additionally it introduces NPC three-level converter which put DSP&FPGA as control center. Its capacity is50kVA in which DSP performs the data collection and analysis and output control of driver pulses is achieved by FPGA in the way of data transformation between DSP and FPGA.At last, this thesis introduces different experiments with different control strategy, load properties and modulation index. In experiment, driver pulses output sequence has good consistency. Output voltage and current waveforms look great, and the balance of neutral-point voltage has been realized. The result proves the correctness in design of control algorithm. Through the measurement and analysis of the experimental data, it has been proved that the correctness of theoretical analysis on the relationship between system parameters and losses distribution in this text.