A PLL-free robust control scheme with application to grid-connected fuel cell DGs under balanced and unbalanced conditions

Abstract

A robust control scheme for the power electronic converter interface of grid-connected fuel cell distributed generators is presented. The scheme enables them to operate under various limiting factors that can severely affect performance such as voltage sags, parametric uncertainties, unknown disturbances and noise. The control strategy adds low voltage ride-through ability to the grid-connected system without requiring a specialized phase locked loop that is often an integral part of the control systems used under unbalanced grid conditions, and therefore, simplifying its design and lowering its computational complexity. The dc-dc and dc-ac converters are controlled using two different controllers: the uncertainty and disturbance estimation controller for the dc-side voltage, and the repetitive controller for the grid-side currents. The control system is designed with the objectives of robustness, fast transient response, disturbance rejection and low voltage ride-through ability. A number of simulation case studies are presented using the SimPowerSystems™ toolbox of MATLAB/Simulink computing environment to highlight the performance of the proposed control scheme under both symmetrical and asymmetrical voltage sags,parametric uncertainties, and abrupt operating condition changes. The control system is also tested through digital implementation on a low-cost microcontroller to validate its performance.