The Mechanism And Suppression Of Grid-Side Low-Order Harmonics Current Of Single-Phase PWM Rectifier

Single-phase PWM rectifier is widely used in traction main drive system and auxiliary power supply system in the field of AC drive of electric locomotive.In order to reduce the harmonic pollution to the traction power grid,it is of great practical significance to study the mechanism and suppression of grid-side low-order harmonics current of single-phase PWM rectifier.The control strategy of the single-phase PWM rectifier and the generation mechanism of the DC voltage ripple are analyzed.Under the double closed-loop control strategy of the voltage outer loop and the current inner loop,the mathematical model of the grid-side third harmonic current is deduced,and the magnitude of the grid-side third harmonic current generated by the control system is quantitatively calculated by the DC voltage ripple.The suppression algorithm using the filter in the voltage outer loop is studied,and the experimental results verify that it can effectively reduce the grid-side third harmonic current caused by the DC voltage ripple.The suppression algorithm using the parallel resonance controller in the current inner loop is studied,and the experimental results verify that it can effectively suppress the grid-side low-order harmonics current.The bridge arm voltage harmonics of the single-phase PWM rectifier directly affect the harmonics of grid-side current.Therefore,the principle of the grid-side current harmonics caused by pulse width modulation is studied.The theoretical spectrum of the bridge arm voltage of the single-phase PWM rectifier is analyzed by double Fourier series,and the influence of different sampling methods,DC voltage ripple and dead time effect on the harmonic components and amplitude of the bridge arm voltage are studied.The modulation compensation strategy and the dead zone compensation strategy are studied respectively.Experiments show that the above two compensation strategies can suppress the grid-side current harmonics induced by DC voltage ripple and dead time in PWM.In applications with low switching frequency,the increase of control delay leads to the decrease of the ability of parallel resonant controller to suppress the grid-side current harmonics.The effectiveness of different discretization schemes for digital resonant controllers at low switching frequency is investigated.A scheme of layered discretization directly using predistortion bilinear transformation method for the bottom integrator in the resonant controller is studied,and an accurate discretization model is established.This scheme ensures the control performance of the resonant controller at low switching frequency after discretization,and is also beneficial to deal with the frequency fluctuation of the grid voltage.Experiments are carried out at switching frequencies of 450 Hz and250Hz,and the discrete parallel resonance controller can effectively suppress the loworder harmonic components of the grid-side current,which verifies its control performance at low switching frequency.A single-phase PWM rectifier experimental platform is built.Simulations and experiments are carried out on the relevant strategies for suppressing low-order current harmonics in the grid-side current of single-phase PWM rectifiers at low switching frequencies,which verifies the correctness of the theoretical analysis and the effectiveness of the suppression strategies.