LCL Filter

Abstract—A higher order passive power filter (LLCL-filter) for the grid-tied inverter is becoming attractive for industrial applications due to the possibility to reduce the cost of the copper and the magnetic material. However, similar to the conventional LCL-filter, the grid-tied inverter is facing control challenges. An active or a passive damping measure can be adopted to suppress the possible resonances between the grid and the inverter. For an application with a stiff grid, a passive damping method is often preferred for its simpleness and low cost. This paper introduces a new passive damping scheme with low power loss for the LLCL-filter. Also a simple engineering design criterion is proposed to find the optimized damping resistor value, which is both effective for the LCL-filter and the LLCL-filter. The control analysis and the power loss comparison for different filter cases are given. All these are verified through the experiments on a 2 kW single–phase grid-tied inverter prototype using proportional resonant controllers. It is concluded that compared with the LCL-filter, the proposed passive damped LLCL-filter can not only save the total filter inductance and reduce the volume of filter, but also reduce the damping power losses for a stiff grid application.