Design and analysis of output filter for 3-phase UPS inverter

This paper proposes a design method of the L-C output filter for three-phase UPS inverter. The harmonic components of the output voltages are dependent on the L-C value and the system time constant defined by the switching frequency, the speed of control processor the controller gain, etc., but their characteristics are complicated and entangled with each other. In this paper the relation between the L-C value and the system time constant are described with the closed form and the filter values can be calculated to meet the limit of voltage THD considering the system time constant. The schematic design procedure is given by a single flow chart and the proposed filter design algorithm is verified through the simulation and experiments.