High-resolution phase shift and digital implementation of a fuel cell powered UPS system

This paper presents the digital implementation of a single-phase fuel cell powered uninterruptible power supply (UPS) and distributed generation (DG) system. The power conditioning unit of the system consists of a voltage source inverter and a three-port bidirectional DC-DC converter interfacing the fuel cell and inverter with a supercapacitor. It is shown that the control of the whole system including both the DC-DC and DC-AC stages can be carried out by a single digital signal processor (DSP). In particular, in order to eliminate limit cycle oscillations in digitally controlled DC-DC converters, the generation of high-resolution phase shifts is explained in detail. The system is suitable for residential applications and can improve the power quality for the local user. Practical design issues such as the state-of-charge (SOC) management of the supercapacitor and prototype design details are discussed. Experimental results from a 3.5 kW prototype are presented to verify the effectiveness of the digitally implemented control scheme.