Robust control of single phase UPS

This paper deals with the design of a robust control scheme for single phase uninterruptible power supplies (UPS). UPS systems must provide sinusoidal output voltage irrespective of operating conditions, e.g. in presence of nonlinear and variable loads. In this paper, variations of the operating conditions are modelled as perturbations of the plant transfer function, so that control design can be approached by robust control techniques. Modelling plant and perturbations according to the linear fractional transformation (LFT) standard structure, allows the use of μ-synthesis to design a robust controller, able to guarantee the specified performances in any operating condition, including nonlinear or step-changing loads. In addition, the paper shows how the main drawback of the robust controller, that is its high order, can be compensated by order reduction techniques, thus allowing a simple implementation. The superiority of the reduced order robust controller is demonstrated by a comparison with a conventional controller