Research On Modulation And Control Technology Of Three Phase Current Source Converter

With the intensification of carbon emissions and the significant deterioration of the global climate,it is imperative to vigorously develop low-carbon applications,such as electric vehicles,multi electric aircraft and uninterruptible power supply(UPS).As the core device of low-carbon application,electric energy converter can be divided into voltage source converter(VSC)and current source converter(CSC).Compared with the VSC,the CSC has the advantages of wide range output,inherent short-circuit protection,adjustable power factor,fast dynamic response and high reliability,so it is widely used in some important industrial occasions.In these industrial applications,the operating conditions of CSC can be divided into rated load condition,light load condition and discontinuous current condition.With the advancement of industrialization of CSC,a series of key technical problems in CSC have been exposed one after another under different load conditions.How to solve these key technical problems is the core to promote the development of CSC.By means of modulation and control technology of three-phase CSC,aiming at optimizing the output waveform quality under different working conditions for three-phase CSC,the key technical problems in the three-phase CSC will be studied in this thesis.The main contents are as follows.(1)Firstly,the working principle of three-phase CSC is analyzed,the general mathematical model and small signal mathematical model of three-phase CSC are established,and the AC side equivalent circuit and DC side equivalent circuit models of three-phase CSC are constructed.Based on the small signal mathematical model,the key factors affecting the output waveform quality of three-phase CSC are revealed,which lays a theoretical foundation and solution idea for solving the key technical problems existing in three-phase CSC.(2)Under the rated load condition,the DC link inductor current ripple of three-phase CSC is small,the influence of grid voltage on three-phase CSC is mainly considered.Unbalanced grid voltage will cause double frequency pulsation of output power for threephase CSC,resulting in double frequency pulsation of DC link output voltage and DC link inductor current.Meanwhile,the third harmonic content of grid current will be increased and the reliability of the system will be reduced.Therefore,taking the AC side active power of three-phase CSC as the control object,combined with the instantaneous power theory,a constant active power control scheme is proposed to keep the input side active power of three-phase CSC constant under the condition of unbalanced grid voltage,so as to suppress the double frequency pulsation of DC link output voltage and inductor current.Also,the third harmonic of grid current will be reduced.On this basis,in order to simplify the algorithm and improve the suppression effect of output voltage double frequency pulsation,an output voltage pulsation suppression scheme without positive and negative sequence separation calculation is proposed in this thesis.By introducing DC link inductor current feedback and reducing DC link inductor current pulsation,the DC link output voltage pulsation and grid current harmonic are reduced.The output waveform quality is improved.The system reliability is also improved.(3)When the grid voltage is disturbed,the DC link output voltage of three-phase CSC will fluctuate with a long dynamic adjustment process.Excessive DC link output voltage drop will cause shutdown or damage of electrical equipment,which will seriously affect the production efficiency.Therefore,combined with the instantaneous power theory,taking the grid voltage feedback and DC link load current feedback as the core technology,a DC link output voltage zero dynamic control scheme for three-phase CSC is proposed by this thesis,which can realize the DC link output voltage zero dynamic control under the instantaneous disturbance of grid voltage.(4)Under light load condition,the DC link inductor current ripple of three-phase current source converter is large.Excessive DC link inductor current ripple will affect the quality of grid current waveform,increase system loss and reduce efficiency.Therefore,a ripple suppression technology of DC link inductor current for three-phase CSC is proposed.Combined with the 12 sector space vector modulation strategy,three adjacent active vectors are used to synthesize the reference vector.By optimizing the vector arrangement sequence,the DC link inductor voltage change rate is essentially reduced,so as to reduce inductor current ripple.Therefore,the quality of grid current waveform is optimized and the overall performance of the system is improved.(5)Under discontinuous current condition,the existing calculation methods of dwell time of space vector modulation strategy for three-phase CSC will fail,which will lead to grid current distortion and system out of control.Therefore,this thesis deeply analyzes the time-domain expression of DC link inductor current under discontinuous DC link inductor current condition.Based on this,guided by the reference vector synthesis mechanism,a new dwell time calculation method of space vector modulation strategy is proposed,which can be used in different working conditions.So,the three-phase CSC can work well with the continuous inductor current condition and discontinuous inductor current condition,the output waveform quality of the three-phase CSC under discontinuous inductor current condition is optimized,and the multi-condition operation ability of the three-phase CSC is achieved.(6)In order to verify the effectiveness of the solutions to the above four key technical problems,based on TMS320F28335 DSP and XC6SLX9 FPGA,an experimental prototype of three-phase CSC is bulit,and a control circuit is designed for the three-phase CSC.The experimental platform has the functions of overvoltage protection,overcurrent protection,short circuit protection and so on.Based on the experimental platform,considering the dynamic and steady-state characteristics of different time scales,combined with the theoretical analysis and experimental verification,the four key technical schemes proposed in this thesis are explored in a multi-dimensional and in-depth manner,and the results prove the effectiveness of each scheme.