Uninterruptible power supply units are system formed by power electronics converters to supply sinusoidal voltages to feed critical loads. In this paper, a fixed switching frequency model predictive control strategy is presented for the control of the output voltage in an LC filter connected to a three-level NPC converter. The control objectives of the system are the tracking of the voltage reference and balance of the voltages of the dc-link capacitors. The mathematical model of the converter and the LC filter is developed and the control strategy is explained. Simulation results obtained in the Matlab/Simulink enviroment are presented to validate the control strategy