According to one aspect, embodiments of the invention provide a UPS comprising a delta transformer having a primary winding and a secondary winding, the primary winding coupled between an input and an output and the secondary winding having a first end and a second end, a delta inverter coupled between a DC bus and the secondary winding, a short circuit control circuit selectively coupled between the first end and the second end of the secondary winding, a main inverter coupled between the DC bus and the output, and a controller configured to control, in a bypass mode of operation, the short circuit control circuit to couple the first end of the secondary winding to the second end such that the secondary winding is short circuited and unconditioned output AC power, derived from the input AC power via the primary winding, is provided to the output.