Research On Capacitor Voltage Self-balancing Control For Multi-level Grid-tied Inverters

With the development of renewable power industry recently,the capacity of photovoltaic(PV)generation systems is increasing dramatically.Inverters have to reach the demands of high input voltage.Since the withstand voltage of power switches is limited,multilevel inverting technology is widely used in medium or high voltage applications.However,neutral point clamped(NPC)multilevel structure leads to capacitor voltage unbalancing problem.So it's necessary to explore a new economical and effective method to control the capacitor voltage.A capacitor voltage controlling method without sampling the two capacitor voltages respectively is studied in the paper.The method only needs to sample the input voltage,introduce the coefficient k into the modulation waveform,change k according to modulation index,and the capacitor voltage balancing can be achieved.Firstly,the structure of the five-level dual-buck full bridge inverter and the five-level full bridge inverter are illustrated and the operation modes of them are introduced.The relationship between capacitor voltage balancing and modulation index is analysed in detail.It is proved that the capacitor voltage can balance with the conventional carrier disposition pulse width modulation(PWM)when the modulation index is relatively high,with no relationship to power factor.Then,the operation modes with improved modulation strategy and provides the range of k in different modulation index are analyzed.The conclusion is reached that the capacitor voltage can balance with improved modulation strategy when the modulation index is relatively low,dispite of the power factor.According to the characters of improved modulation strategy,by building the mathematical model and calculating the loss,the software and hardware parameters in inverters are analyzed and designed.Finally,based on the result of parameters designation,a prototype of five-level dual-buck full bridge inverter and a prototype of five-level full bridge inverter are built.The steady and dynamic experimental results show that the improved modulation strategy introduced in this paper can control the capacitor voltage effectively in different work modes and different topologies.