The operating conditions of UPS inverters and the
fact that the load is unknown lead to great difficulties in fault
diagnosis. This paper proposes the use of a fault diagnosis
algorithm based on estimated pole voltage error for UPS inverters
based on the 3-level Neutral-Point-Clamped topology. This algorithm
is independent from the load and is not affected by
unbalanced three-phase operation. This algorithm provides very
fast and reliable results and requires no additional sensors or
hardware. Simulation results are presented to demonstrate the
algorithm’s speed and robustness.