Research On Parallel Dual-frequency Single-phase Grid-connected Inverter With A Hybrid Pulse Width Modulation Mode

Renewable energy and distributed power generation systems have developed rapidly,and grid-connected inverters are widely used as the interface between new energy power generation systems and the power grid.The current will be polluted by the switching harmonics of the grid-connected inverter.Usually,a low-pass passive filter is added to the output side of the inverter to ensure the quality of the grid-connected current.In order to reduce the size of the filter,the inverter usually uses a higher switching frequency.However,a high switching frequency will increase the loss of the converter and reduce the efficiency of the system.Low switching frequency is not conducive to the design of passive filters,which will cause the size of passive components to be very large.There is a contradiction between the efficiency of the inverter and the size of the filter.In view of the above problems,this thesis proposes a hybrid modulation parallel dual-frequency single-phase grid-connected inverter.The dual-frequency grid-connected inverter includes two units with different switching frequencies.The main unit adopts a low switching frequency and is responsible for transmitting the main energy of the inverter to the power grid.The current ripple of the low-frequency unit is large,so the auxiliary high-frequency unit generates the opposite current to compensate the harmonics of the low-frequency unit through a feedforward method.Although the current of the low-frequency unit is large,the switching loss is low at a low switching frequency.Although the high-frequency unit increases the cost of the inverter,it can actively and effectively suppress the harmonics of the grid-connected current,and the additional losses brought by it are very small.The high frequency unit uses a feedforward method to eliminate grid switching current ripple without detecting current harmonics.In the topology,because the low switching frequency is not conducive to the design and use of the LCL filter,the low-frequency unit uses the L filter.The high frequency unit uses an LCL filter to further reduce the amount of inductance and switching harmonics from the high frequency unit.The control adopts mixed modulation mode,that is,the low frequency unit adopts unipolar modulation,and the high frequency unit adopts bipolar modulation.This is beneficial to the reduction of the DC side voltage of the high-frequency unit and the generation of the harmonic elimination compensation current.This thesis first analyzes the working principle of the L + LCL type dual-frequency single-phase grid-connected inverter,introduces the filter parameter design of the inverter,and discusses the advantages of the hybrid modulation mode.Simulation and experimental results show that the current harmonics are effectively suppressed,and the efficiency analysis shows the advantage of the dual-frequency mode on efficiency.