Adaptive Low-Pass Filter Based DC Offset Removal Technique for Three-Phase PLLs

The presence of dc offset in the inputs of a phase-locked loop (PLL) introduces fundamental frequency oscillations in the estimated quantities. Due to their low frequency in a synchronous reference frame, removal of these oscillations is a challenging task. Recent design studies of pre/in-loop filtering based advanced PLLs show that incorporation of dc offset removal in a filtering stage reduces the bandwidth of the PLL. This degrades the dynamic performance of the PLL and results in slower response time. To tackle this issue, a simple yet effective dc offset removal technique based on adaptive low-pass filters is introduced in this letter. The proposed technique can be applied as an add-on to any PLL structures without altering their design. Therefore, its application has a minimal effect on the dynamic performance of the PLL under study. The effectiveness of the proposed technique is evaluated experimentally by applying it to different PLL structures.