Design and implementation of a discrete-time H-infinity controller for uninterruptible power supply systems

This study provides a digital ℋ_∞ controller design suitable for uninterruptible power supply inverters, yielding results that comply with the IEC 62040-3 Standard. An augmented state space model for the system is given, taking into account the delay from the digital implementation of the control signal, and resonant controllers that ensure asymptotic tracking of the reference and good rejection of load disturbances. It is shown a case where an ℋ_∞ optimal state feedback controller cannot provide good results because of the high control gains. To solve this problem, an ℋ_∞ suboptimal controller is proposed, taking into account a prescribed bound for the ℋ_∞ norm of the closed-loop system and a parameter for limitation of the norm of the control gain vector. This suboptimal controller provides gains that lead to suitable results, complying with the constraints from the IEC 62040-3 Standard. These results are compared with those from a widely used state feedback controller, providing superior performance, even with a much simpler design procedure. Simulation and experimental results have a good correspondence. Finally, the study illustrates that the ℋ_∞ norm of the closed-loop system can be seen as the output impedance over the frequency. From this information, using the small gain theorem, one can get the admittance value of the linear loads that preserve stability when connected to the uninterruptible power supply.