Research And Optimization Design On Losses Of Three-Level Converters

Multi-level converters have shown superior performance in terms of higher output voltage,larger capacity,less stress on devices,lower switching losses,and lower device power rating,compared to the traditional two-level converter with low output voltage and small capacity.However,the number of power devices used in the multi-level converter is larger.Some problems will result in shorter lifetime,limited output power and switching frequency,which include power losses,junction temperature fluctuation,and the unbalance distribution of power losses.Therefore,the analysis for power losses is critical,and the optimization for the power losses and junction temperature of devices have probability to improve the lifetime and reliability of the power devices in converters.Three-level converters are used widely among multi-level converters.A calculation and simulation model is built for the characteristics of power losses of IGBTs in converter system.Based on that,the optimization design for the three-level converter is carried out,and the researches are below:(1)Two solutions are designed for three-level Neutral Point Clamped(3L-NPC)converters.One of them is a losses improvement solution with unfixed switching frequency,where the switching frequency is unfixed in a long period,considering the fluctuation of junction temperature of the 3L-NPC converter in the wind turbine system with fluctuated wind power.The solution aims at decreasing the fluctuation of losses and junction temperature.The other is a losses improvement strategy,where unfixed switching frequency is used within a fundamental cycle.The strategy aims at redusing switching losses of power devices.The effectiveness of the two solutions are verified through the built simulating model.(2)A strategy of dispersing loss distribution is developed based on a traditional PWM strategy for three-level Active Neutral Point Clamped(3L-ANPC)converter.Through analisising the losses of power devices in 3L-ANPC,the strategy is designed with no added devices.It aims at dispersing the switching losses between devices,reducing the power losses and junction temperature of the most stressed device.A 3LANPC converter model is built and the effectiviness of the strategy is verified through simulation.(3)The operating characterics are analyzed when open-error occurs in the 3L-ANPC converter.The losses are researched in fault-tolerant control strategy.In the meanwhile,the strategy of unfixed switching frequency within a fundamental cycle is used in the fault-tolerant control,which aims at reducing the switching losses and junction temperature of the operating devices.