Research On Control Scheme Of Single-Phase Shunt Active Power Fliters With LCL Filters

As the development of modern industry, more and more non-linear loads are used in power system, resulting in increasingly severe harmonic pollution. Shunt active power filter (SAPF), as the most effective method to eliminate harmonic, has been paid more and more attention recently. Traditional SAPFs usually adopt simple L filters to filter out the switch ripples. It is simple, reliable and easy to be controlled, but lack of high frequency attenuation capability. While the third order LCL filter could obtain better high frequency attenuation capability with small inductance, it gradually becomes the mainstream of output filter of SAPFs. This paper chose the single-phase SAPF with LCL filter as the research object. Works have been focused on the parameters design of LCL filter, active damping strategy and rapid output current control.In the LCL filter parameter design process, tradeoff between various performance indicators needed to be made. The step by step design method usually been adopted in this process. Although step by step design method is simple and direct, the influence of performance indicators is not clear in the design process, which usually causes some performance indicators unoptimum. This design method is helpless for parameter optimization. To overcome the defect of step by step design method, this paper proposed a graphic design method. Although the graphic design method still need to try-in in design process, however, with the help of the graphic, the tradeoff between various performance indicators can be easy to maded.Designing the current control for SAPF is a hard work for the ability to traking for harmonic current. Due to the perfect control precision and the flexible of resonant control, it is widely used in SAPFs. To apply the resonant control for high order harmonics, phase-lag compensation must be included in the current controller. The performance of different resonant control and their phase-lag compensation methods was discussed in this paper. The PR control cannot be used for high order harmonic compensation, and closed-loop system would amplify the current command which frequency is slightly bigger than the resonance frequency. With delay compensation, the PR control can be used for high order harmonic compensation, and the amplify phenomenon can be also mitigated. However, the frequency characteristics of closed-loop system are not as smooth as that of the VPI or P+VPI control, which sensitivity to grid frequency variation.In the resonant control, proportional controller is usually used to improve the responding speed. However, to achieve an adequate phase magin, the value of proportional parameter is usually low. Proportional control is insufficient for tracking high order harmonics. Excellent responding speed can only be achieved by carefully design the resanont controller parameters. In this paper, a novel parameter design method to achieve rapid current tracking is proposed. The design method is besed on an iterative algorithm to arrange the real part of dominant poles of closed-loop system. For the VPI control, system phase margin only slightly minish when dominant poles reach the possible furthest distance away from imaginery axis. So that position should be used to tune controller parameters, to ensure fastest responding speed. For the P+VPI control, the dominant poles cannot far away from the imaginary axis, otherwise system phase margin would reduce rapidly. To design the controller parameters, phase margin must be ensured. Then, the proportion and generalized integral coefficients can be obtained by tradoff the tracking speed of low and high frequency current command.To using the LCL filter, filter resonance must be damped. Current researchs of active damping method are focused on the capacitor current, since in the ideal condition only the proportional feedback of capacitor current can damping LCL resonance. However, in the practical system, the performance of this method would be changed. In this paper, the analysis of active damping effects of the capacitor current, capacitor voltage and grid-side current feedback method was discussed, by considering the one-step delay and the affect of zero-order holder. According to the ratio beteen resonant frequency of LCL filter and control frequency, the suitable range for the three active damping methods is discused. Capacitor current feedback method is suitable for 0.05<γ<0.1 and 0.35<γ< 0.4. Capacitor voltage feedback method is suitable for 0.1<γ< 0.225. Grid-side current feedback method is suitable for 0.225<γ<0.35. Since the grid-side current feedback method is especially suitable to SAPF, there is no need of extra sensors for additional states measurements and grid voltage disturbance rejection could be enhanced while offer a good resonance damping, the grid-side current feedback method is applied in a sigle-phase SAPF. The experimental results demonstrate the effect this damping method.Since the influence of grid harmonic on grid-side current is one of the main problems for grid-connected inverter, extending the grid-side current feedback method into the application of grid-connected inverter would be rewarding. This paper combined the lag control and grid-side current feedback to make it possible to be used in converters which yis, greater than 0.14. Experimental results on a single-phase grid-connected inverter prove the effectiveness of the proposed methods. For the y smaller than 0.14, additional one-step delay cannot damp LCL resonace effectively, and increase the delay time would further reduce the current control bandwidth. Therefore, this method is not suitable for the condition whereγis too small.