Research On High-Reliable Modulation And Control Technology Of Parallel Three-Level Rectifier For DC Charging System

The world today is facing serious challenges of energy crisis and environmental pollution.Striving to develop new energy vehicles is vital to solve this crisis and develop from a large automotive country to a strong automotive country.As an indispensable key infrastructure for electric vehicles,DC charging system has a profound impact on the rapid development of new energy vehicle industry.For the AC-DC converter of DC charging system,compared with conventional two-level rectifier and Vienna three-level rectifier,T-type three-level rectifier is more promising due to its advantages of lower harmonics,no current zero-crossing distortion and bidirectional energy flow.Therefore,T-type three-level rectifier has become a research hotspot in academia and industry at home and abroad.However,a number of relevant theories and technologies for T-type three-level rectifier and its parallel system have not been well addressed in practical application.For the single T-type three-level rectifier,neutral-point(NP)imbalance and inherent NP potential oscillation problems will lead to uneven device stress,poor power quality and low reliability.For parallel T-type three-level rectifiers(PT23LRs)system,zero-sequence circulating current(ZSCC)among three-level rectifiers will lead to AC current distortion and operation stability decrease.Furthermore,with significant increase in the number of power switches in PT23LRs,combining with complex and changeable operation conditions,the open-circuit fault(OCF)problem of power switches is highlighted.Specifically,the switch failure will further increase ZSCC of PT23LRs,which will deteriorate ac-side power quality sharply and pose serious threat to the safe and reliable operation of PT23LRs.To this end,the thesis takes T-type three-level rectifier and its parallel system as research object,and have conducted researches on NP potential balance control and oscillation suppression,ZSCC suppression and fault-tolerant control(FTC)of power switches.The main research contents and innovations are as follows:1.Aimed at the problem of NP potential imbalance and oscillation of T-type three-level rectifier,a hybrid control strategy for NP potential based on finite-time control is proposed.Firstly,the effects of all types of space vectors on NP potential are analyzed.Secondly,a finite-time controller with fast NP balance ability is designed,which regulates the dwell time of P-type and N-type redundant small vectors in real time to rapidly balance NP potential.Furthermore,supported by the mathematical models of NP potential,the adjustment of dwell time of three phase state O is precisely calculated,and supplemented by sinusoidal pulse width modulation strategy,NP potential oscillation suppression is realized.As a result,compared with conventional NP balance control methods,the proposed method has faster NP potential balance speed and better NP potential oscillation suppression.Finally,feasibility and correctness of the proposed method was verified by simulation and experimental results.2.Aimed at the problem of ZSCC and NP imbalance of PT23LRs,a coordinated control method for ZSCC suppression and fast NP balance is proposed.Firstly,the ZSCC model for PT23LRs is established,and a ZSCC controller based on "finite-time+feedforward" is designed to complete ZSCC suppression by adjusting the difference between the zero-sequence duty ratios of two rectifiers in real time.Secondly,a NP balance controller based on finite-time control is constructed to achieve fast NP balance.Notably,the output quantities of ZSCC controller and NP balance controller are decomposed skillfully and applied to two rectifiers respectively.As a result,the mutual non-influence of ZSCC suppression and fast NP balance is guaranteed,and low harmonic grid-connection and highly reliable operation of PT23LRs are achieved.Finally,the experimental results verified feasibility and correctness of the proposed method.3.Aimed at the problem of shutdown due to OCF of power switches during the operation of PT23LR system,the FTC method based on switching sequence reconstruction is proposed.Firstly,the characteristics of T-type three-level rectifier system when OCF occurs are analyzed in detail.Then,without affecting ZSCC suppression,based on the criterion of "area equivalence principle",the switching sequence is reconstructed by changing the state switching mode of fault phase in real time,so that FTC under OCF of power switches is realized.As a result,the sinusoidalized output of AC current of PT23LRs is restored,and the highly reliable continuous operation of PT23LRs is guaranteed effectively in the event of OCF during operation.Finally,feasibility and correctness of the proposed method was verified by experimental results.4.Aimed at the problem of ZSCC suppression of PT23LRs under fault condition,a novel carrier-based ZSCC suppression method is proposed.First of all,the double carriers of the fault phase of PT23LRs system are replaced by an amplitude-multiplied single carrier to effectively avoid faulty state.Secondly,ZSCC model of PT23LRs under fault condition is established,and a novel ZSCC controller based on "PI+feedforward" is constructed to achieve ZSCC suppression by adjusting three phase state transition time of the fault rectifier in real time.Finally,the dwell time of redundant small vectors of the non-fault rectifier is regulated to ensure the dc-side NP potential balance.As a result,the proposed method effectively restrains the ZSCC and balances the NP potential of PT23LRs under fault condition,which ensure the normal startup of system and improve the ac-side power quality of PT23LRs significantly during system operation.Besides,it also does not require any additional hardware.Finally,the experimental results verified feasibility and correctness of the proposed method.