Study On Control Scheme Of High-power Four-quadrant Converter

Four-Quadrant Converter (4QC) is an important part of AC traction drive system. Theconventional diode rectifier or thyristor controlled rectifier causes serious harmonicpollution to the catenary. PWM rectifier, with its lower harmonic content, controllablepower factor and bi-directional energy flow, has been widely used in mega-watt classhigh-power AC traction drives.On account of different switch modes, the operation principle of a two-level4QC isanalyzed. By introducing switching function, the state-space averaging model of4QC isestablished, while the open-loop stability is also proven. Small perturbation is then appliedwith the elimination of time-variances. Linear amplitude-phase steady-state model andsmall-signal model is finally obtained, and the design procedure of passive parts ispresented.Departure from the principle of indirect current control, an active-reactive decouplingmodel is introduced. Inspired by the time-domain current response characteristics, atransient-free indirect current control is proposed. Current can be imediately transit fromone steady state to another, and it greatly improves the dynamic response of conventionalindirect current control. The current response have little relationship with inductanceparameter, which overcomes the fundamental defects of indirect current control. Thiscontrol scheme can be used as an alternation for4QC, and can be extend to three-phasesystem. A wide-range linearization of4QC is achieved by dc link voltage squared control.During practical operation of electric locomotives, AC side line parameters vary withthe grid topology. In order optimizing the operation status of4QC, the circulation currentand harmonic spectrum of a multi-winding carrier phase-shift multiple system is analyzed,while an active-reactive decoupling model-based direct current control and circulationsuppression scheme is introduced. By introducing the closed-loop of active current and reactive current amplitude, grid side active and reactive power can be independently andaccurately controlled.A repetitive dc link voltage predictor is proposed in solving the dc link voltagepulsation problem of4QC. The predictor capitalizes on the repetitiveness of the dc linkripple and can predict the average value of the dc link voltage in the next switching periodwith precision. An area equalization-based algorithm is then used to calculate the requiredpulse width, a variable switching frequency application is given and the prediction error isanalyzed. By mentioned method, the clumsy resonant inductor in dc busbar could beuninstalled, which helps to improve the power density and reliability.