Undamped Control Strategy For The Inner Current Loop Of LCL-type PWM Rectifiers

LCL filter is widely used in novel PWM rectifier because of the better harmonicattenuation compared with the traditional single-inductor filter. However the LCL filterhas a resonant peak which may raise oscillations and even instability, and that increasesthe difficulty of control system design. This thesis focuses on the undamped controlstrategy for the inner current loop of three-phase LCL-Type PWM rectifiers that using thebridge side currents as feedback signals.The inner current loop mathematical model and inherent delay of digital control havebeen implemented and analyzed. The inherent delay of digital control was pointed out thathas an important role in the stability problem of the LCL-Type PWM rectifiers. Theimpacts of LCL resonant frequency and inherent delay on the system stability have beenanalyzed, and then deduced the boundary condition that damping isn’t required underideal conditions. The simulation results demonstrate the correctness of the conclusion.According to the actual inductance of LCL filter usually contain iron losses, establish theactual inductance mathematical model, investigate the joint influence of the inductors ironlosses, resonance frequency and digital control inherent delay on the stability margins ofsystem. Pointed out that with an actual LCL filter, the stability margins of system is larger.In order to suppress current harmonics that caused by voltage background harmonics, thispaper proposes a harmonic control strategy with PR controller based on the grid sidecurrent estimates, to overcome the introduction of noise problems that caused by fullfeed-forward of grid voltage control strategy when using the digital systems to achievedifferential links.Experimental was implemented on a22kW three-phase PWM rectifier with LCL filter.And the experimental results demonstrate the conclusion of the stability problem and thefeasibility of the harmonic control strategy with PR controller.