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A number of conventional time-variable step size adaptive algorithms have been tried on the harmonic detection of the active power filter (APF), and experiments indicate that their performance is highly sensitive to the noise disturbance. The paper presents a novel adaptive algorithm using the coherence average estimation of signal K (n) to control the updating of the step size, where K (n) is the proportion of the error signal e (n) in the total signal S(n). It can provide a fast convergence at early stage of adaptation and ensure a small final maladjustment even under low signal-to-noise ratio (SNR) conditions. In this way more flexible control can be made between maladjustment and convergence time without the need to compromise one for the other. Theoretical analysis is also verified by simulations and experimental results. The convergence speed and steady state performance of the algorithm are analyzed, and one method to choose the parameter of the algorithm is provided.