Delay lines; Harmonic compensation; Periodic disturbances; Positive real (PR) functions; Power system harmonics; Repetitive control Active filters; Adaptive control systems; Control system analysis; Feedback control; Harmonic analysis; Optical bistability; Passivation; Reusability; System stability; A-stability; Control designs; Control schemes; Current harmonics; Delay lines; Digital implementation; Feedback-control systems; Feedforward paths; Fundamental frequencies; Harmonic compensation; Harmonic components; Open loops; Passivity properties; Periodic disturbances; Positive real (PR) functions; Power system harmonics; Repetitive control; Shunt active filters; Six-pulse converters; Control system stability