- A phenomenon that occurs when using the DQ Transformation to obtain the compensation references in active filter applications is presented in this paper, in which new harmonics that had not existed originally in the system, are injected to it. Although these new injected harmonics have very low magnitude, they do exist and correct processing must be assured in order not to increase their magnitudes. The phenomenon is generated by the filtering action at the dq reference frame; the filter does not have an ideal behavior, it inserts a gain or attenuation and a phase-shift, different for each frequency component. The phenomenon takes place if only the d signal is filtered out to separate the dc component. But, if both d and q components are filtered, the phenomenon disappears. However, in fact the THD of the compensated variables are lower using the scheme that injects new harmonics than using the one that does not. This phenomenon is formally analyzed under general three-phase unbalanced conditions. © 2015 IEEE.