Research On High Performance Modulation Strategy For Non-isolated Two Parallel Converters

Vigorously promoting scientific and technological innovation in the field of energy,promoting the innovative development of energy conservation and carbon reduction industries,and accelerating the green transformation and upgrading of traditional industries are the key support for China to achieve the strategic objectives of "emission peak" and "carbon neutrality".In this context,high-power energy conversion equipment based on power electronic technology will be at the core of scientific and technological innovation in the field of energy in the future.It can be predicted that high-power converter equipment with fully controlled devices will lead the technological revolution in the field of power conversion.At the same time,the industrial application field puts forward more stringent requirements for high-power converter equipment.While taking into account the traditional control indicators such as the dynamic and static performance,it emphasizes the "fine" management of the performance and energy consumption of high-power converter equipment.Under this background,this paper takes non-isolated two parallel converters suitable for low/medium voltage high current converter equipment as the research object,discusses the important topic of high performance and low energy consumption based on modulation technology optimization,and makes a detailed analysis of the key performance indexes(such as voltage error,current ripple,zero-sequence circulating current and switching loss).Multiple performance optimization PWM strategies are proposed based on its three-level characteristic,which enriches the selection of modulation strategies for non-isolated two parallel converters.Firstly,in order to optimize the current ripple and switching loss of non-isolated two parallel converters,a modulation strategy based on common-mode injection(CMIPWM)is proposed.The internal relationship between voltage vector sequence and the two indicators is investigated.The voltage vector sequences for optimizing switching loss are proposed,and the current ripple caused by each vector sequence is further evaluated.Based on this,each 60°sector is divided into 8 sub-regions,and each sub-region employs the respectively optimal vector sequence with the minimized current ripple,so as to realize the joint optimization of switching loss and current ripple.At the same time,this paper proposes a carrier implementation method based on common-mode injection,so that the proposed modulation method can be implemented simply and conveniently with simple logical comparison.Theoretical analysis and experimental results verify the performance of the proposed modulation strategy.Secondly,a unified modulation theory of improved discontinuous pulse width modulation(DPWM)methods with suppressed zero-sequence circulating current is proposed in this paper.In two-level converter,six DPWM methods are proposed to minimize the switching loss under different power factor angles.However,when applied to non-isolated two parallel converters,the combination of six traditional DPWM methods of two-level converter and the interleaved carriers will cause large zero-sequence circulating current.This paper deeply discusses the coupling mechanism between traditional DPWM methods,interleaved carriers and zero-sequence circulating current peaks.It reveals that the six traditional DPWM methods share two vector sequence databases under the framework of interleaved carriers.Therefore,this paper proposes a unified correction matrix which modifies the modulation waves and switching action rules of the traditional DPWM methods to replace the vector combinations that cause large zero-sequence circulating current,and obtains six improved DPWM methods.In addition,a unified modulation strategy of the improved DPWM methods is proposed,which can conveniently switch among the six improved methods conveniently by flexibly selecting the common-mode injection coefficient.Finally,theoretical analysis and experimental results show that the improved modulation methods significantly suppress the zero-sequence circulating current but do not affect the current quality and switching loss.Thirdly,a modulation strategy which deeply optimizes the AC output voltage and current quality is proposed.Based on the coupling mechanism between voltage vector sequence and various performance indexes,the comprehensive design principle of optimized vector sequence is proposed,and the optimized vector sequences are obtained,which reduces voltage error,current ripple and zero-sequence circulating current simultaneously,and does not increase switching loss.The comprehensive optimization of key performance indexes is realized.In addition,the unified implementation method based on interleaved carrier is proposed,which simplifies the implementation process.The effectiveness and performance of the proposed modulation strategy are verified by theoretical numerical analysis and experimental results.