Coherent noise rejection in a three-phase power inverter

Abstract

In this thesis, we discuss the design of a controller to reject the effects of high order harmonics in a three-phase power inverter. Specifically, coherent noise in the fifth harmonic is considered, as it seems to be dominant in most applications. The controller used in this power inverter operates in a reference frame synchronous with the 60 Hz line voltage. This transformation effectively changes the desired 60 Hz sine wave output into a DC value that has the same amplitude as the sine wave. The power inverter uses an optimal form of pulse-width modulation (PWM), called space vector modulation, which causes the harmonic noise. In order to reject the distortions introduced by the space-vector modulation process, a Linear Quadratic Gaussian (LQG) controller is designed with the sinusoidal disturbances modeled as uncontrollable modes of the system, which are observable from the input and output signals. The extra states in the state space model associated with the disturbance are estimated by the Kalman Filter and subtracted from the control input to compensate for the disturbance