Modulation And Control Strategy Of Two-phase Three-level Converter

The power quality problems,such as harmonic,reactive power and unbalance in three-phase networks of in-phase traction power supply system,are increasingly serious.The no power intervals of neutral section restrict the sustained high speed running of train,even leading to serious trouble like stopping of heavy vehicles on the slope.The advanced co-phase traction power supply system based on two-phase to single phase converter studied by this paper could achieve the same single-phase working of the whole power supply system utilizing the existing traction transformer,thoroughly resolving the power quality and the neutral section problems.Firstly,the mathematic model of YNvd traction transformer is given and the topology,working principle and mathematical model of diode-clamped three-level two-phase to single-phase converter which is connected with the transformer are analyzed.In term of the rectifier,the two-phase DQ decoupling control strategy is proposed by this paper.By two-phase decoupling and reverse decoupling transform,this strategy could control the active and reactive power respectively,dispel static error and ensure the two phase equilibrium to guarantee the three-phase current of primary side for YNvd transformer balance and unity power factor running of the whole system.Secondly,a two-phase SVPWM voltage-balance algorithm of two-phase cascade converter is proposed.The decomposition and synthetization of reference vectors are achieved in two-phase coordinate system and vector space.The unbalance of dc capacitor voltage for each phase is resolved through the equalizing the dc capacitor voltage using the vector which is the highest voltage-balance vector.Meanwhile,choosing optimized switch path,only one bridge’s switch works in one moment,ensuring no jump for port voltage.In addition,this paper analyses the harmonic characteristics of the two-phase converter verify the correctness of harmonic characteristics,two-phase DQ decoupling control and two-phase SVPWM modulation based on Matlab/Simulink.The simulation results reveal that the DC ripple of the two-phase converter when the difference of input voltage phase angle is 90 ° will be canceled out and the dynamic and static performance of the two-phase converter adopting the two-phase SVPWM modulation will be fine.At the same time,the advanced co-phase traction power supply system based on two-phase to single phase converter possess the four-quadrant running ability and is able to convert active power and compensate reactive power bidirectionally.Finally,the experiment was conducted to contrast the dynamic and static performance of in-phase supply system and advanced traction power supply system based on before and after improvement two-phase to single-phase converter.The current of in-phase supply system in three-phase grid is deeply unbalance and has severe distortion;the problem of no-load current and severe reactive current in three-phase grid is existing in the before improvement two-phase to single-phase converter;the three-phase grid current in the after improvement two-phase to single-phase converter is balancing and has good harmonic characteristics and quick dynamic response.