A Resonant Controller with Robust Features for Digital Implementations at Low Sampling Frequency

A resonant controller with robust features for digital implementations at low sampling frequencies is proposed. The proposed controller is based on a phase-locked loop (PLL) approach and is derived by transforming the equations of the conventional resonant controller from Cartesian to polar coordinates. The realization of the proposed controller in discrete-time domain involves nonlinear equations with trigonometric functions. This implies slightly higher computational load when compared to the conventional controller. The achieved robustness, however, is significantly high and outweighs the computational issue. In some typical applications, for example, the conventional resonant controller starts losing its accurate performance as soon as the ratio of sampling frequency to the resonance frequency decreases below fifty. This number becomes as low as ten for the proposed controller. The proposed controller is also readily upgraded to become frequency-adaptive; a feature that is desirable in the control systems that operate at uncertain or slowly varying frequency such as the power system.