Control And Modulation Algorithms Of AC-Drive Locomotive PWM Rectifiers For Harmonic Characteristics Improvement

With the rapid development of electric railways in China,electric locomotive traction drives,which traditionally used dc drives,are being upgraded to modern ac drives.AC locomotives still inject a certain amount of low-order current harmonics to the supply network polluting the supply networks.Excessive low-order harmonics may sometimes lead to distortion of the supply voltage that can cause starting faults of trains.Furthermore,when the electrical matching between ac locomotives and supply networks loses stability,very small content of high-order current/voltage harmonics generated by ac locomotives may be amplified drastically,causing resonance in traction power supply systems(TPSSs).This has occurred in more than 10 railway lines in China having severe effects to the safe and stable operation of railway systems.The ac locomotive is the harmonic source in locomotive-network coupling system and its harmonic characteristics depend on the control and modulation of the PWM rectifier on the network-side of the traction drive system.Considering such harmonic issues in real applications,this work engages to control and modulation algorithms of locomotive PWM rectifiers for the purposes of harmonic improvement and resonance suppression.Mathematic models of single-phase PWM rectifiers for two typical topologies,two-level H-bridge(HB)and three-level neutral point clamped(NPC)converters,are derived.Three kinds of conventional control strategies and carrier-based PWM(CBPWM)schemes commonly used in ac locomotives are analyzed.By using Double Fourier Series(DFS)to analyze the CBPWM process,the high-order current harmonic characteristics is obtained and the generation mechanism of low-order current harmonics is derived from the view-point of control.Based on the measured data of the CRH380AL type electric multiple units(EMUs),the analytical solutions,simulation results and measured data of ac locomotive current harmonics are synthetically analyzed.Internal Model Theory proves that the sinusoidal model in the current controller is necessary for the sinusoidal current zero steady-state error tracking and sinusoidal voltage disturbance rejection.Therefore,a multiple quasi-proportional resonant(M-Q-PR)controller is designed in the current loop to regulate the fundamental and harmonic components of ac current and a multiple notch filter(M-NF)is introduced in the dc voltage feedback loop to remove the effects of dc voltage ripple to ac the current reference.Consequently,a M-Q-PR+M-NF control strategy for single-phase PWM rectifier is proposed to suppress the low-order harmonics of the line current of ac locomotives.Simulation and experimental results validate the effectiveness and feasibility of the proposed method.Coupling mechanism of harmonics in the locomotive-network system and resonance characteristics are analyzed through simplified equivalent circuit models.Based on selective harmonic elimination PWM(SHE-PWM)technique and the resonance characteristics from real TPSSs,a windowed SHE-PWM(WSHE-PWM)is proposed.WSHE-PWM eliminates all the low-order harmonics under 1000 Hz and provides 500 Hz bandwidth of harmonic eliminating capability in the frequency range from 1000 Hz to 3500 Hz,covering different resonant frequencies of different supply sections.By reducing the number of eliminated harmonics,effectively relaxing the restrictions of SHE-PWM,relative wide range continuous solutions of WSHE-PWM are obtained.These solutions are applied directly in a conventional closed-loop control system.Comparative simulation studies prove that WSHE-PWM is an effective alternative that overcomes the resonance suppression limitations of the conventional PWM.The effectiveness of WSHE-PWM is further validated on an experimental equivalent resonant circuit.Consisting of finite control set model predictive control(FCS-MPC)and WSHE-PWM,a WSHE-MPC control strategy is proposed.Compared to conventional double closed-loop control strategies,WSHE-MPC controls multiple variables under a single closed-loop control structure by designing a dynamic reference current,and does not use PI controllers providing faster dynamic response.Compared to the standard FCS-MPC method,WSHE-MPC provides constant switching frequency and PWM spectrum by including WSHE-PWM patterns into the cost function as a constraint to control input.Simulation results validate the effectiveness and feasibility of the proposed control strategy.