Due to rapid deployments of power electronics in renewable energy sources and new loads, high levels of harmonics causes problems in current distribution systems. This paper presents an advanced harmonic extraction technique for detecting and extracting selected harmonics in a medium-voltage high-power system. A time-transformation technique is developed which can accurately extract the desired harmonics only. It decomposes the current into multiple components and is therefore, accurate and selected compensation characteristics depending upon the application can be realized. From simulation and experimental results, it is found that the developed technique can correctly detect and significantly reduce selected harmonics. This technique is feasible and can be implemented in any three-phase three-level active power filters.