Uninterruptible Power Supplies: A Critical Power Play
Part 1: UPS: Standby Systems Ideal for Small Applications
Part 2: Line-Interactive UPS Offer Higher Reliability
Part 3: True Online UPS Offer Higher Power Quality, Compact Size
Part 4: Standby Online Hybrid UPS Improve Energy Efficiency
Part 5: Flywheel UPS Require Less Maintenance
Part 6: Maintenance Requirements for Uninterruptible Power Supplies
Part 7: PRODUCT FOCUS: Uninterruptible Power Supplies
True Online UPS Offer Higher Power Quality, Compact Size
By Greg Livengood - February 2010
Under normal operation, a true online UPS runs continually on the battery via the inverter, while the line power runs the battery charger. In some cases, this type of UPS is called a double conversion, or double conversion online.
For a true online UPS, no transfer time occurs upon loss of utility power. The system provides power-factor correction and frequency regulation, in addition to surge suppression and power filtering.
In rare occurrences, the inverter fails. In that case, the UPS will switch to power provided directly from utility power via the UPS power filter/surge suppressor.
Converting all the power from AC to direct current (DC) and back to AC lowers efficiency and raises heat output. One method to reduce this inefficiency is to replace the battery charger with a delta-conversion online UPS. Instead of providing all the output from the battery under normal circumstances, some of it goes directly from the delta-converter from the input line power. In the event of a power failure, the unit operates like a regular true online UPS.
Typical applications include large servers, data centers and large sensitive equipment, and the typical size ranges from 5-5,000 kVA. The advantages of true online UPS include:
- Greater reliability because of redundant system components and pathways
- Higher power quality because power output is isolated from power input. The unit also provides power-factor correction and frequency regulation.
- Compact physical size, compared to line-interactive systems.
Among its disadvantages are: low energy efficiency, generally 85-92 percent; higher heat output; higher initial and operating cost; frequent battery replacement, typically every five years; and greater maintenance requirements because components have shorter service lives than line-interactive units.
