| Inverter is an important energy transformation instrument,it is widely used in daily life of human beings.The derivation of the traditional modulation strategy for unipolar and bipolar single-phase inverter is based on the assumption that the DC-link voltage is ideally constant,the modulation index is usually a constant in the interval [0,1].Under actual operating conditions,because of the power coupling,and make DC-link voltage of the inverter contains a large amount of the double line frequency voltage ripple.With traditional modulation strategy which with constantm modulation index,the voltage ripple in DC-link causes the increase of the third harmonic in the AC side,declines the power quality in the output voltage,and the total harmonics distortion(THD)will increase,the output power quality of inverter will be reduced.In order to solve the problem of power quality degradation caused by power coupling,this thesis conducts the following research.By means of the double Fourier series method,the harmonic characteristics of output voltage is analyzed.To prove the limitation of the traditional inverter modulation strategy with constant modulation index under the condition of high DC-link voltage ripple rate.After analyzing the influence of the double line frequency voltage ripple on output voltage,an improved inverter modulation strategy which based on the pulse modulation index is proposed.In order to prove the correctness of the improved modulation strategy,the harmonic characteristics and THD of output voltage before and after the improvement are compared,and make simulation with MATLAB/Simulink software.The third-order generalized integrator is applied to the traditional PLL phase to extract DC-link voltage ripple parameters,which used to construct the pulse modulation index.Finally,an experimental prototype is designed to verify the correctness and feasibility of the proposed strategy. |
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