Analysis And Control Of Quasi-Z-Source Photovoltaic Inverter

Nowadays,photovoltaic industry has an increasing development due to the advantages of the solar energy,such as non-pollution,wide distribution,and etc.However,photovoltaic system has some drawbacks which are low efficiency,high cost and large space occupation.Therefore,it is essential to optimize the system.As the indispensable link,inverter plays a significant role in the system optimization.Nevertheless,the traditional single-state inverters lack boost link which results in a small operation range.Moreover,the two-state inverters increase cost and reduce efficiency.As a result,the quasi-Z-source inverter is proposed,which can not only solve the above disadvantages of traditional inverters,but also improve the system reliability.This paper is focused on studying the modulation methods and improved topologies of quasi-Z-source inverter.Firstly,a technical scheme which can improve voltage range and operation efficiency is proposed.Using a bidirectional power switch to connect negative terminals of cascade units,the technology can change topology structure in real time.When the input voltages of the proposed inverter are lower,the proposed inverter works under quasi-Z-source cascaded multilevel inverter mode to obtain wider operation range.When the input voltages of the proposed inverter are higher,the proposed inverter is transformed into equivalent quasi-Z-source H-bridge inverter mode to improve operation efficiency.The topology structure and operating principle are introduced in this paper.Meanwhile,the thesis proposes a SVPWM modulation strategy which is applicable to the proposed topology structure and analyzes the mechanism of efficiency improvement.Finally,the technical scheme can be verified with simulation.Secondly,a novel quasi-Z-source cascade multilevel inverter is proposed.On the basis of traditional quasi-Z-source cascade multilevel inverter analysis,a topology is presented,which uses bidirectional power switch to connect positive terminals of two quasi-Z-source inverters.Then,a plan,which uses positive and negative bidirectional power switches to connect the positive and negative terminals of quasi-Z-source inverters,is applied to obtain the novel quasi-Z-source cascade multilevel inverter.Furthermore,according to the modes of the proposed topology,a PS-SPWM strategy is presented.In order to verify the improvement of efficiency,the conduction loss of proposed topology is computed.Eventually,the performance of proposed topology is verified by simulation on the basis of output voltage,current and current harmonic distortion.Finally,a wavelet modulation strategy applied to a quasi-Z-source inverter was proposed.The regulating range of the shoot-through duty ratio could be increased by changing the locations of the sampling points set using traditional wavelet-modulation.The proposed modulation strategy divided the shoot-through zero vector into two portions,which were then individually inserted into both ends of the sampling time.Under the proposed strategy,the quasi-Z-source inverter could obtain a theoretical maximum constant shoot-through duty ratio without any extra switching frequencies.In addition,the proposed strategy was compared with traditional modulation strategies used in quasi-Z-source inverters.The boosting capacity,efficiency,harmonic and modulation ratio of the different modulation strategies for quasi-Z-source inverter were analyzed thoroughly.Finally,the performance of the proposed method was verified by simulation.Both the simulation results and theory verify that the strategy can reduce harmonic,increase voltage utilization rate,and enhance the boosting capacity.