MINIMIZATION OF INTERHARMONIC CURRENTS FROM A CURRENT SOURCE AC DRIVEBY MEANS OF A SELECTIVE DC SIDE ACTIVE-FILTER

Current source a.c. drives draw a current waveform from the supply whi ch is both distorted and unsteady. The frequency spectrum of the suppl y current contains harmonic components and interharmonic components wh ich change frequency with the speed of the output motor. The increasin g number of large current source variable frequency a.c. motor drives in use on power systems is causing concern over the interharmonic curr ent distortion produced by the drives. If the interharmonic currents a re excessive and compensation is required, it is difficult to achieve attenuation of these components with conventional passive filters. Thi s paper describes how a single phase p.w.m. current source inverter co nnected in the d.c. link of a current source drive may be used to filt er out a.c. side current interharmonic components. A prototype filter has been built and tested on a low power model of a variable speed syn chronous motor drive system. Results show clearly that a significant r eduction in the interharmonic current magnitude can be achieved using the d.c. side active filter. The results of computer simulations show that the rating of the active filter should be approximately 1% of the drive rating. Hence, the active filter may be realised using commonly available power semiconductor devices.