Design of Resonant Controllers for Compensation of Third Harmonic Ripple in the DC Capacitors Voltages of NPC Converters

This paper proposes a method to design an adaptive proportional-resonant (PR) controller for equalize and suppress the third harmonic in the capacitors voltages of NPC converters applied to a wind energy conversion system. This problem with NPC converters capacitors voltages occurs in all applications, but when the frequency of the AC system varies, as occurs in wind systems, a fixed frequency resonant controller could not be sufficient. Then, using a PR controller with adaptive resonant frequency is possible to improve the controller performance. A mathematical model for the capacitors voltages dynamic behavior is developed, as well as the control system used to equalize these voltages and suppress the third harmonic. A quasi-PR controller is used, which allows to increase the bandwidth, make it less sensitive to frequency variations. At least, is a method to design the controllers parameters. In order to verify the system operation and the controllers performance, results from simulations in PSCAD/EMTDC are used.