Research On Modulation Strategy Optimization For Parallel Inverter Of Electric Propulsion System

Electric propulsion systems power electric aircraft.With the development of electric aircraft in the direction of manned and long range large,which puts forward more stringent requirements for the reliability and efficiency of the electric drive system.At present,three-phase inverter is an important part of electric propulsion system.Compared with other loading equipment,it has higher failure rate and energy consumption.Compared with a single three-phase inverter,the redundancy of multiple three-phase inverters in parallel is not only beneficial to the improvement of reliability,but also provides the possibility for the suppression of common mode voltage and ripple current by optimizing the modulation strategy.Therefore,a variable structure modulation strategy is proposed for three-phase inverter to suppress high and low frequency common mode voltage and switching losses.Based on this strategy,taking the parallel structure of two three-phase inverter as the research object,a staggered complementary variable structure modulation strategy is proposed to suppress high and low frequency common mode voltage and ripple current.Firstly,this paper introduces the basic vector synthesis principle and analyzes the relationship between modulation strategy and vector synthesis.The principles of common mode voltage,switching losses and ripple current are studied in this paper.This paper explores the variation law of common mode voltage and other characteristics under basic vector synthesis,and obtains mathematical models of basic vector synthesis and above characteristics,which provides theoretical basis for subsequent modulation strategy optimization.Secondly,the electric propulsion system still uses three-phase inverter as the driving topology,which is simple in structure and easy to get the relationship between the switching function and the output voltage.But the traditional modulation strategy does not take into account the suppression of common mode voltage and switching losses.Therefore,this paper makes a comparative study of four-vector synthesis and three-vector synthesis,and improves the traditional four-vector synthesis.According to the complementary characteristics of the two vector synthesis,a variable structure modulation strategy was proposed,which integrates the three-vector synthesis and the improved four-vector synthesis.With the rotation of the voltage vector,the two vector synthesis methods in the whole voltage vector circle can be automatically switched.The characteristics of this strategy are compared with the traditional strategy and verified by simulation.The results show that the strategy can suppress both high and low frequency common mode voltage and switching losses in the range of full modulation system.Due to the high performance requirements of electric propulsion system,this paper adopts the topology structure of two three-phase inverters in parallel.This structure has the possibility of eliminating common mode voltage,but the conventional modulation strategy has a high common mode voltage amplitude.Therefore,based on the variable structure modulation strategy,this paper proposes a staggered complementary variable structure modulation strategy for parallel inverter by changing the switching sequence synthesized of the improved four-vectors synthesis.This strategy extends the region where the traditional modulation is 1and redefines the modulation and vector sectors.The characteristics of this strategy are compared with traditional modulation strategy on parallel inverter,and built simulation model to verify it.The results show that the staggered complementary variable structure modulation strategy can eliminate the high and low frequency common mode voltage in the low modulation,suppress the high and low frequency common mode voltage in the high modulation,and suppress the ripple current in the full modulation.Finally,the paper designed and built the experimental platform.The experimental platform consists of RT-Lab,drive board,etc.,and the software and hardware debugging are finally completed on this platform.The experimental results verify the feasibility of the optimized modulation strategy under the above two topologies.