Study On Low-Frequency Stability Analysis And Control Scheme Of Single-Phase AC-DC System

The single-phase Boost PFC and PWM rectifier systems are widely used in AC-DC power conversion,which can effectively solve the harmonic pollution problem of power electronics to the power grid when they operate at high power factor.However,low-frequency instability in a single-phase AC-DC system can cause a sharp decrease in the power factor correction effect.In order to avoid low-frequency instability in single-phase Boost PFC systems,the existing improved control strategies are mostly based on voltage feedback,but such methods will cause the system’s dynamic response performance to decrease.In addition,the low-frequency stability analysis and control for the two-stage PWM rectifier system with dq mode control still lacks relevant research reports,and the stability improvement control schemes available for reference have a complicated design process.Aiming at the above problems,this paper respectively analyzes the low-frequency stability based on the harmonic average model of the Boost PFC system and the impedance stability criterion of the two-stage PWM rectifier system,and proposes composite control schemes that consider stability,dynamic and steady-state performances.The main contents and research results are as follows:(1)A control scheme of proportional outer-voltage loop combined with third current-harmonic feedforward compensation is proposed.Aiming at the nonlinear low-frequency instability in the resistive load single-phase single-stage Boost PFC system,the large-signal harmonic averaged model is used to determine the stability boundary of the original controlled system and the controllable range of the proposed control.In the proposed control,the proportional outer-voltage loop makes the system always stable,and can guarantee fast dynamic response characteristics;the third current-harmonic feedforward compensation is responsible for improving the steady-state THD result.Based on UC3854 BN,this paper compares the conventional control with the proposed control,and the experimental results verify the excellent performance of the proposed control.(2)By analogy with the proposed control scheme in(1),the low-frequency instability behavior of the single-phase Boost PFC cascaded two-switch forward DC-DC converter system is controlled.Aiming at the nonlinear low-frequency instability of the two-stage Boost PFC system,the basis for the large-signal equivalent of a constant-power-load in the second-stage is analyzed first,and then the stability boundary of the originally controlled two-stage system and the single-stage system are compared using a harmonic average model.The similarities and differences verify the uniformity of the controllable range of the proposed control.Through experiments,this paper compares and analyzes the stability improvement effect and dynamic and steady-state performances of the proposed control scheme in two-stage system and single-stage system.(3)A control scheme of high-bandwidth outer-voltage loop combined with d-axis second current-harmonic feedforward compensation is proposed.Aiming at the two-stage system composed of single-phase dq mode control PWM rectifier and Buck converter,the stability margin of impedance between the first-stage and second-stage is analyzed using the Middlebrook small-signal stability criterion,and the improvement effect of the proposed control on low-frequency stability is determined.In the proposed control,the high-bandwidth outer-voltage loop is responsible for increasing the stability margin,and ensuring fast dynamic response characteristics;the d-axis second current-harmonic feedforward is responsible for improving the steady-state THD result.Based on Matlab Simulink simulation,this paper verifies the existence of low-frequency instability in the single-phase dq mode control two-stage PWM rectifier system,and the improvement of the proposed control scheme on stability,dynamic and steady-state performances.Low-frequency instability in a single-phase Boost PFC system occurs under light-load conditions,exhibiting a specific instability frequency,which is essentially nonlinear large-signal instability;low-frequency instability in a single-phase two-stage PWM rectifier system occurs under heavy-load conditions,exhibiting uncertain instability frequency,which is essentially linear small-signal instability.Nevertheless,by widening the outer-voltage loop and introducing current-harmonic feedforward compensation,both AC-DC systems achieve excellent low-frequency stability,and dynamic and steady-state performances.