PROJECT TITLE :
Active Damping of LLCL-Filter Resonance Based on LC-Trap Voltage or Current Feedback
LLCL-filter is an rising fourth-order filter proposed after the third-order LCL -filter. It uses smaller passive elements than the LCL-filter, but is still burdened by resonance complications when used with a grid converter. An LLCL-filtered converter should, hence, be passively or actively damped. Active damping is presently more efficient, and will 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 easy proportional damper can't be used with the capacitor current. Instead, a high-pass damper must be used, that as commonly known, might cause undesired noise complications relying on operating conditions. During this paper, the same capacitor current damper has been investigated for the LLCL -filter with its limitations clarified. Both cases of with and while not delays have been thought-about. The investigation has additionally been extended to contemplate the LC-lure voltage feedback, which based mostly on the formulated transfer functions and experimental results, permits a easy 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 will be simply drawn. These equivalences are, therefore, useful tools to the practicing engineers.
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