| The Z-source inverter proposed by Professor Peng Fangzheng effectively overcomes the shortcomings of traditional current-type and traditional voltage-type inverters that can only only boost and lower the voltage,by introducing a Z-source network in front of the inverter bridge.For Z-source inverters,the bridge arms of the same phase of the inverter bridge are allowed to conduct at the same time,so no dead time needs to be introduced.At the same time,the system reliability is increased,and the output waveform quality is also improved.For wind power generation,photovoltaic power generation and other new energy power generation forms with large output voltage fluctuations,Z-source inverters have become particularly applicable.In addition to the advantages of the traditional Z-source inverter,the improved bidirectional Z-source inverter can also effectively reduce the voltage on the Z-source network capacitor.The improved bidirectional Z-source inverter effectively prevents the occurrence of abnormal working conditions by anti-paralleling a switch tube at the Z-source network diode,and enhances the reliability of the system.Based on the working principle of the improved bidirectional Z-source inverter,this articleanalyzes in detail the voltage and current of some branches while the traditional improved Z-source inverter working in abnormal conditions and the improved bi-directional Z-source inverter working in normal conditions.Then,the sine pulse width modulation and space vector pulse width modulationapplicable to the Z-source inverter are introduced.The inductor current ripple and capacitor voltage ripple of the Z-source network affect the selection of the inductor and capacitor of the Z-source network respectively,so this article has conducted in-depth research on the ripple characteristic of the improved Z-source inverter.The inductor current ripple and capacitor voltage ripple of Z-source network are related to the modulation strategies.This article analyzes inductor current ripple of the SPWM modulation method and the three typical SVPWM modulation methods,and proposes new modulation strategies to reduce the inductor current ripple.The inductor current ripples corresponding to the new modulation method and the typical modulation method are fully analyzed and compared.The capacitor voltage ripple of the Z-source network is related to the value of the Z-source network inductance.When the inductance is in a different range,the improved bidirectional Z-source inverter will have different working modes.When the improved bidirectional Z-source inverter is connected to the grid,in order to stabilize the output voltage of the inverter bridge,the DC link voltage of the inverter bridge needs to be stabilized first.The closed-loop control method for stabilizing the DC-chain voltage is introduced.DC-link voltage control method with double closed-loop of capacitor voltage and inductor current is proposed.The Z-source network and grid-side mathematical models are established.The feedforward decoupling strategy is introduced to realize the decoupling of the active and reactive components of the grid-connected current.Based on the instantaneous reactive power theory,the detection of load reactive current is realized.By controlling the active and reactive components of the grid-connected current,the function of reactive power compensation can be achieved while achieving grid-connection. |
