Conventional repetitive controllers are not suitable for applications were reference or disturbances are periodic where its fundamental frequency is subject to variation. However, this is a situation commonly encountered in applications of pulse-width modulated inverters feeding nonlinear loads or connected to the grid. Several approaches to mitigate this problem have been proposed in the literature, but there is no studies with comparative analysis among them. This paper provides a comparative analysis of three modified repetitive controllers developed to solve this problem. The following methods are covered in the analysis: i) continuous adjustment of the sampling rate; ii) truncation of the number of samples per reference cycle to the nearest integer and iii) adjustment of a fictitious sampling rate in order to maintain constant the number of samples of the controller per reference cycle. These existing approaches are depicted in details and its performances are discussed based on the experimental results obtained in two experimental prototypes, which had been carefully designed to operate at low and high switching frequencies.
Repetitive control; digital control; PWM inverter; AC power source
