Research On Three-level Adjustable Two-stage Inverter

The abundant reserves and non-pollution make solar energy an ideal green energy source.At present,solar power generation is widely used in solar energy.Under the influence of large variation of solar cell output voltage,single-stage three-phase inverter structure is usually used in high-voltage photovoltaic grid-connected occasions.When the output voltage is low(such as 270 V),both of the current and the power loss are too high.Boost two-stage inverter is usually used in low-voltage grid-connection,the efficiency of this structure is not so high due to two transformations of boost and inverter.To solve the problems above,this paper fully considers the working relationship between the front and back stages,presenting an efficient new two-stage inverter structure.The main research contents are as follows:(1)Presenting a new three-level adjustable two-stage inverter.The front circuit adopts the booster circuit with double power overlapping supply,which makes the current and switching amplitude of the booster greatly reduce compared with the ordinary boost converter.Two buck converters are also used to supply power to the midpoint of the back stage,which can effectively adjust the midpoint voltage.(2)A ZCS is added in the double power charging and discharging path,it can reduce the front stage power loss of two stage inverter structure.(3)In order to reduce the power loss of the back stage,a joint control method of the front and back stages is presented.When the back stage switching tube does not operate,it is equivalent to one less transformation,which can reduce the switching loss and magnetic loss of back-stage inverter to a very low level without maintaining the midpoint voltage balance.The circuit and control method are simulated by MATLAB,their feasibility is proved.(4)Establishing a power loss mathematical model of the two-stage inverter structure.The maximum conversion efficiency is 98.79%,which is higher than the traditional two-stage boost inverter.According to the calculation formula of China gridconnected photovoltaic weighted efficiency,the conversion efficiency of each power point is calculated,geting the weighted efficiency is 98.47%.(5)Designing the hardware of this inverter with low power prototype.It includes main control circuit,measuring circuit and inverter circuit.The new inverter presented in this paper can reduce the power loss of front and back circuits at the same time,and is expected to replace the traditional boost two-stage inverter,single-stage inverter(output up to 540V)and distributed inverter.