Research On Control Strategy Of Z-source Three-level Photovoltaic Inverter

With the rapid development of economic for increasing demand of energy and the large-scale utilization of fossil energy,China is facing increasingly severe.pressure of energy shortage and environmental pollution.It is necessary to vigorously develop renewable energy to improve the energy supply structure.In recent years,the consumption of China's renewable energy has been increased,of which the increasing of solar energy is the fastest.Therefore,it is very important to improve the performance of photovoltaic power generation system.As the core component of photovoltaic power generation system,inverter is an important bridge to realize energy conversion.Three-level inverters are widely used in photovoltaic power generation systems because of their high waveform quality and low voltage stress.However,because of the voltage fluctuation of photovoltaic,it is necessary to add an extra boost link before the inverters are used.However,the two-stage structure increases the cost and reduces the efficiency of system.Z-source three-level inverters have the capability of voltage boosting,which can solve the problem of two-stage structure mentioned above.This topic takes Z-source three-level inverters as research object,and the main research content are as follows:Firstly,the working principle of Z-source NPC three-level inverters is introduced,the quivalent circuits in shoot-through and non-shoot-through state are analyzed,the mathematical relationship between output voltage and shoot-through duty cycle of Z-source network is deduced,and the modulation method of Z-source three-level inveter based on the phase disposition carrier-based approach is introduced in detail,which lays a foundation for the analysis and improvement of control strategy.Secondly,the causes of unbalanced neutral-point potential and the influence of shoot-through states on neutral point potential are analyzed,and the mathematical model of average neutral-point current is deduced.In order to solve the problem of low-frequency neutral-point voltage oscillations,a hybrid control strategy for eliminating low-frequency oscillations is proposed,which combines the advantages of zero-sequence component injection and double signals modulation method.By simplifying the classification,calculating the optimal value of zero-sequence voltage compensation signal which is needed to eliminate oscillations under different conditions,by dynamic setting of the amplitude limitation of through-through duty cycle,the coordinated control of compensation signal injection and Z-source network boosting is realized.According to the switching conditions,the switching between compensation signal injection and double signals modulation method is carried out to eliminate low-frequency oscillations.Thirdly,in order to reduce the switching loss of the inverters,this paper proposes a control strategy for reducing the switching loss of Z-source three-level inverters considering the characteristics of Z-source three-level inverters.By calculating the clamping zero-sequence component combining with the duty cycle of shoot-through state,clamping the switches of different phase legs in different intervals,the coordination control between the introduction of shoot-through states and the reduction of switching loss is realized.Furthermore,the problem of neutral-point potential is analyzed and solved when using the proposed strategy.A new clamping zero-sequence component is defined,and its influence on neutral-point potential is analyzed.According to the shift direction of neutral-point voltage and the averaged neutral-point current value,the appropriate zero-sequence component is selected to control neutral-point potential,so as to realize optimization of switching loss reduction and neutral-point potential balance.Finally,the design of experimental platform for Z-source NPC three-level inverters is introduced,including hardware design and software design.The experimental platform of Z-source NPC three-level inverters is set up,and the programs of control strategy are written based on DSP.The validity of the proposed control strategy is verified by experimental results.