Research On Load Adaptive Modulation Strategy Of Resonant DC Link Inverter

In today’s world,power conversion is becoming more frequent and the application of inverters is expanding day by day.Inverter plays an important role in photovoltaic connected grid,uninterruptible power supply,induction heating and other fields.With the development and progress of technology,higher and stricter requirements are put forward for the performance indexes of inverters by both industrial field and professional people.The new generation of inverters with safety,green,energy saving and reliability is expected by more and more people,and the appearance of soft switching inverters happens to meet these demands.Among the many types of soft switching inverter topologies,the resonant DC link inverter is favored in medium and small power applications due to its simple structure,low cost and convenient control.Based on the analysis and research of various typical resonant DC link inverter topologies and modulation strategies,aiming at the problems of the high current stress of auxiliary switch and the large circulation loss of auxiliary commutation circuit under the original modulation strategy and the unsatisfactory effect under improved modulation strategy,a perfect and improved modulation strategy is proposed in this thesis.By forbidding the operation of auxiliary switches during the circulation switching state,the current stresses of auxiliary switches and the circulation loss of the commutation are reduced.But the perfect and improved modulation strategy still exists:1)when the load current Io is reversed,the parameter design is complicated;2)the auxiliary commutation circuit has high operation frequency;3)the fixed time modulation strategy consumes energy in the circulation mode.So the load adaptive modulation strategy based on sawtooth carrier with positive and negative slopes alternately is proposed in this thesis.The proposed modulation strategy replaces the traditional triangular carrier wave by using a sawtooth carrier wave with positive and negative slopes alternately,and unifies the commutation mode of the auxiliary commutation circuit into the reverse current to positive current,thereby realizing energy bidirectional flow and facilitating parameter design.At the same time,the auxiliary commutation circuit operates only once at the vertical edge of the sawtooth carrier,and the operation frequency of the auxiliary commutation circuit is reduced to 1/6 of that under the traditional triangular carrier modulation.On these bases,the load adaptive modulation strategy is applied to dynamically adjust the switching points of auxiliary switches to realize the soft switching action while reducing the duration of the circulation mode,thereby improving the efficiency of soft switching inverter in the whole output power range.The thesis firstly analyzes the modulation strategy of novel resonant DC link inverter and points out its existing problems,and then the perfect and improved modulation strategy is proposed.Based on the detailed analysis of the perfect and improved modulation strategy,aiming at its shortcomings the load adaptive modulation strategy based on a sawtooth carrier wave with positive and negative slopes alternately is proposed.Then,the load current Io variation law,steady state characteristics,auxiliary commutation circuit operation cycle and DC link voltage utilization rate,loss distribution of the proposed soft switching inverter under load adaptive modulation strategy are deeply analyzed.Finally,the correctness and effectiveness of the theoretical analysis are verified by the simulation circuit built in MATLAB/Simulink environment and an experimental prototype of 10kW.