Research Of Control Strategy And Grid Filter In Shunt APF

Nowadays, widespread application of nonlinear loads leads to serious harmonic distortion in power grid and decreases the power quality, thus, more and more research is focused on harmonic attenuation. Small active power filter with superior compensation characteristics emerged to overcome the defects of traditional passive filter which is resonance-prone and bulky. However, three-phase converter in APF introduces switch frequency harmonics which will certainly increase total harmonic distortion of grid current. Thus, grid filter is utilized to reduce the harmonic injected to power grid. However, most research on active power filter focused on control strategy, few researches relates to grid filter, optimization of grid filter will certainly improve the compensation characteristic of active power filter.This paper analyzes the selection and operation principle of the active power filter, summarizes the detection strategies and current control algorithms frequently utilized in active power filter. Then, this paper assesses the single inductor filter and LCL filter from different respects, such as system stability, current suppression characteristic, damping loss, influence of reactance and harmonic content in the power source, presents the design methodology. Based on all the research on passive damping LCL filter, three active damping strategies for LCL filter is obtained through the analysis of closed-loop transfer function model. Furthermore, a novel LCL filter topology is proposed to get over the defects of traditional passive damping LCL filter. There is a damping resistor paralleled with grid side inductor which can reduce the damping loss and a damping inductor connected in series with filter capacitance which can accelerate attenuation at high frequency. MATLAB is utilized to analyze influence of inductor ratio, damping resistor and grid reactance and harmonic content on suppression characteristic and damping loss. Simulation is done based on the theoretical analysis, the result indicates that proposed filter could improve the harmonic attenuation with lower losses. The total harmonic distortion is reduced from 28% to 1.2%.Design and experiment of active power filter is fulfilled after all the theoretical analysis and simulation verification. The system is completed by dual-DSP28335-control. AD7606 controlled by CPLD and dual-port RAM are utilized for A/D conversion and communication respectively. Experiments of different grid filters including L filter, LCL filter and novel LCL filter are conducted under different grid voltages. The results confirm the theoretical expectations.