Active power filter

a device for improving power quality

is used widely in various occasions

in which the harmonics detection plays an important role in the active filters. Aiming to such severe condition as voltage wave distortion and amplitude drop at the network side and the case of not locking voltage phase accurately by traditional phase locked loop（PLL）

a kind of Root-Mean-Square（RMS）harmonic detection method based on Decoupled Double Synchronous Reference Frame Software Phase Locked Loop（DDSRF-SPLL）is proposed. And it is compared with RMS harmonic detection method based on PLL in case of simulating voltage drop and phase shift. It is shown by the simulation result that this method is feasible and can detect the phase information of positive sequence fundamental voltage of grid voltage accurately at severe condition and

at the same time

it is used in the active power filter and desired compensation result of harmonics at the grid side is achieved.