Active Damping of LLCL-Filter Resonance Based on LC-Trap Voltage or Current Feedback

LLCL-filter is an emerging fourth-order filter proposed after the third-order LCL-filter. It uses smaller passive components than the LCL-filter, but is still burdened by resonance complications when used with a grid converter. An LLCL-filtered converter must hence be passively or actively damped. Active damping is presently more efficient, and can easily be realized by feeding back a state variable. For the LCL-filter, the variable is usually its middle capacitor current. However, with computational delays considered, a simple proportional damper cannot be used with the capacitor current. Instead, a high-pass damper must be used, which as commonly known, may cause undesired noise complications depending on operating conditions. In this paper, the same capacitor current damper has been investigated for LLCL-filter with its limitations clarified. Both cases of with and without delays have been considered. The investigation has also been extended to consider LC-trap voltage feedback, which based on the formulated transfer functions and experimental results, permits a simple proportional damper to be used. Lesser noise complications and faster dynamic can then be achieved. Circuit equivalences for the presented dampers have also been derived, from which quick damping insights can be easily drawn. These equivalences are therefore helpful tools to the practicing engineers.