Double Conversion On-Line UPS

This is the most common type of UPS above 10kVA (Volt-Ampere). Double Conversion On-Line UPS is the same as the Standby UPS except that the primary power path is the inverter instead of the AC mains. In the design of Double Conversion On-Line operation, failure of the input AC does not cause activation of the transfer switch, because the input AC is not the primary source, but is rather the backup source. Therefore, during an input AC power failure, on line operation results in no transfer time. The on line mode of operation exhibits a transfer time when the power from the primary battery charger/battery/inverter power path fails. This can occur when any of the blocks in this power path fail. The inverter power can also drop out briefly, causing a transfer, if the inverter is subjected to sudden changes in the load, or if the inverter experiences an internal control “glitch”. Contrary to popular belief, Double Conversion On-Line UPS systems do exhibit a transfer time, and in actual installations may transfer as frequently as standby type UPS systems; however on line UPS transfers are not related to AC input power failures as they are in a standby UPS. Both the battery charger and the inverter convert the entire load power flow in this design, which causes undesirable heat and results in reduced efficiency. Due to practical design constraints, UPS below 10kVA that are represented as Double Conversion On-Line UPS are almost always actually of the Standby On-Line Hybrid type UPS. The Double Conversion On-Line UPS provides nearly ideal electrical output performance. However, the constant wear on the power components can reduce reliability and the energy consumed by the electrical power inefficiency is a significant part of the life-cycle cost of the UPS. Also, the input power drawn by the large battery charger is often non-linear and can interfere with building power wiring.