A Study Of Over-modulation Impact On The MSI Series Micro-Grid

When the Micro Source Inverter Series Micro Grids(MSI-SMGs)system is performing the power balance control,it is easy for MSI going into the over-modulation state,which cause the amplitude of fundamental output voltage of the system is no longer linearly related to the modulation degree.Besides,the harmonic harmonic content increases,stability decreases or even instability.This thesis studies over-modulation impact on the MSI-SMGs system.Firstly,the structure and modulation strategy of the micro-source inverter series micro-grid system is introduced,the characteristics of the system output voltage and power are analyzed.The mechanism of over-modulation in power balance control is expounded,which lays a foundation for studying the influence of over-modulation on the system.Secondly,based on the phase shift and amplitude variation Carrier PWM modulation method,the Double Fourier method is applied to obtain the mathematical expression of the output voltage of the MSI-SMGs under over-modulation condition.The impact on the system output voltage fundamental component and harmonic component is analysed in detail.The results show that with the increase of overshoot,the growth ratio of the fundamental amplitude gradually slows down,but the fluctuation is smaller than 28%;the harmonic amplitude of fundamental multiples increases first and then remains unchanged;the carrier harmonic amplitude tends to zero;the harmonic content of the system output voltage increases.Then,both in the normal modulation and the over modulation states,the slightly biased linearized model of the output voltage models and the transfer functions are given.In order to make the output voltage of the system follow the given value quickly and accurately,the voltage-current double closed-loop control strategy is adopted,and the PI regulator is designed for correction.Based on the designed control strategy,the system characteristic roots,step response curves and the frequency characteristics of the both states are analyzed in detail.and the conclusion that over-modulation will affect the system stability and anti-interference capacity is drawn.Finally,the simulation software is used to build the simulation model of five-micro-source inverter series microgrid system.The simulation results proved that the theoretical analysis of the influence of overmodulation on the amplitude and harmonic of system output voltage in this thesis was correct,verifies the effectiveness of the control strategy.