Minimisation of interharmonic currents from a current source AC drive by means of a selective DC side active filter

Current source AC drives draw a current waveform from the supply which is both distorted and unsteady. The frequency spectrum of the supply current contains harmonic components and interharmonic components which change frequency with the speed of the output motor. The increasing number of large current source variable frequency AC motor drives in use on power systems is causing concern over the interharmonic current distortion produced by the drives. If the interharmonic currents are excessive and compensation is required, it is difficult to achieve attenuation of these components with conventional passive filters. This paper describes how a single phase PWM current source inverter connected in the DC link of a current source drive may be used to filter out AC side current interharmonic components. A prototype filter has been built and tested on a low power model of a variable speed synchronous motor drive system. Results show clearly that a significant reduction in the interharmonic current magnitude can be achieved using the DC 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