A real-time fault diagnosis system for UPS based on FFT frequency analysis

UPS provides emergency power when utility power is not available, so the reliability of UPS is more important than inverter drive systems. In this paper, a fault diagnosis system for UPS is proposed using FFT frequency analysis of output current of inverter side of UPS under linear and nonlinear load conditions. Software PLL for precise synchronization of one period sampling and double buffer memory for real time processing are proposed. Experimental results show the increase of even harmonics including dc offset in case of fault conditions such as increase of resistance and delay or misfiring of IGBT turn-on, and prove the possibility of UPS fault diagnosis system if the criteria for fault decision are well defined.