| In the case of energy shortage and serious environmental pollution,new energy photovoltaic power generation has a good application prospect.As the core part of photovoltaic power generation,inverter is particularly important in the energy conversion process.In addition to the same advantages as other multilevel inverters,cascaded multilevel inverters also have the advantages of easy modularization and expansion in structure,no need capacitors for voltage sharing,fewer devices and so on,which is very suitable for photovoltaic power generation field.The traditional cascaded H-bridge multilevel inverter can only operate in step-down condition and has the problem of leakage current exceeding the standard,so its application in photovoltaic field is limited.In order to solve the problems above,a single module Cuk inverter is proposed in this thesis which can realize voltage step up and down and suppress leakage current by combining symmetrical Cuk circuit design with full-bridge.the output side of the topology is connected in series,therefore a cascaded multilevel Cuk inverter with leakage current suppression ability is proposed which can realize output voltage step up and down.The passive component parameters of the proposed inverters are small,there are no electrolytic capacitor,meanwhile the modulation strategy and control method are also simple.First,the operating principles of single-module Cuk inverter and cascaded multilevel Cuk inverter are analysized in detail.There are two operating modes: DC side inductance current continuous mode and DC side inductance current discontinuous mode.According to current waveforms,the proposed inverters can operate in DC inductor current continuous mode and discontinuous mode.The modulation strategy and voltage step up and down capability are analyzed respectively.Then the common mode circuit is modeled and analyzed,which theoretically explains the inverter has the ability to obtain the expected voltage gain and suppressed leakage current.Moreover,the mathematical model of the topology is established by using the state space average method,and the closed-loop regulator is also designed and simulated.On the basis of theoretical analysis and simulation verification,finally the experimental prototype of single-module inverter and cascaded multilevel inverter is built,and the open-loop,closed-loop and grid-connected experiments are carried out.The experimental results show that the inverter has good voltage step up and down ability,and can effectively suppress the system leakage current,which verifies the correctness of the theoretical analysis. |
