A medium-power current source converter (CSC)-based shunt active power filter (APF) system is designed and implemented to suppress the amplification of low-order harmonics at the medium-voltage (MV) interface bus between the distribution and transmission systems, owing to the presence of large shunt capacitor banks installed only for reactive power compensation. Four CSC-based APF modules designed at 1.0 kV are operated in parallel and connected to the 31.5-kV MV bus via a specially designed coupling transformer. In each APF module, a specially designed LC-type input filter eliminates the switching ripples, and active damping method embedded into the control software suppresses harmonic frequencies around the corner frequency of the input filter. The resulting system can operate at relatively high frequencies in the range from 2.0 to 3.0 kHz, depending upon which selected harmonics among 5th, 7th, 11th, and 13th are to be eliminated. Furthermore, in order to reduce the installed capacity of CSCs, selective harmonic amplification method is applied to the APF system described in the paper. MV APF system has been built as a mobile system for temporary connection to a problematic MV interface bus until a permanent solution is found for that location in the distribution system.