This paper presents a unified design and implementation of active power filters (APF) for medium-voltage high-power applications. A fast and accurate harmonic extraction technique based on the time transformation algorithm (TTA) is developed. A novel approach is used to derive the mathematical model describing the relation between the output current and the control signal of the NPC inverter for active power filter. Accordingly, the current controller and voltage controller are designed using the derived model. The neutral-point voltage is maintained near zero using a feed-forward controller. The designed three-level APF is feasible to implement, comparatively cheaper and reduces the filter size. From both the simulation and experimental results, it is found that the developed system provides excellent performance in regulating voltage and reducing harmonics significantly, from 26.3% to 4.0%.