Research Of LCL Output Filter Applied To Single-phase Active Power Filter

LCL filter on the one hand can provide better attenuation effect on harmonics generated by VSC, on the other hand, the filter’s output current is the control current. LCL filter affects both the content of high-frequency harmonics in output current and the accuracy of the converter. However, the LCL filter’s resonance will cause the system to shocks or instability without being suitably controlled. In this dissertation an active damping control strategy is used to suppress resonance by designing a discrete time-domain current controller, which has optimized state feedback pole placement and deadbeat current control, with good damping, enough bandwidth and rapid transient over-current protection.The paper first establishes active power filter (APF) simulation model for electrified railway and discrete state space model for the LCL filter, taking account of the impact of the system delay. Then the filtering effect brought by the parameters of the filter is analyzed, after which the parameter values of the LCL filter are determined according to requirements on the decay rate of high harmonic and harmonic compensation speed.The eigenstructure assignment method is taken to control the system’s dynamic performance in order to stabilize the system and the feature vector can constrain the transient response curve. After the current deadbeat control being satisfied, the damping control is introduced to suppress the resonant peaks, followed by the servo control with integrator to deal with the damping control’s steady-state error. The requirement of each state variable is different, so using the linear quadratic optimal control to search the optimal control vector, in accordance with the corresponding weights matrix. It makes the complex computing process simplified, more intuitive.The grid disturbance always bring trouble on the track of instruction current, asymptotic robust tracking controll is applied to suppress the external interference to the object current. In addition, one of the design objectives is to achieve robust stability particularly to parameter errors in the inductor on the grid side, because of the values often change when the grid is in failure or affected by the environment.Finally the entire system model of APF used in the electrified railway is established in MATLAB/simulink environment. The dynamic and static performance is analyzed to verify the correctness of the design and theoretical analysis after simulation.