Off-line consist of a battery, and a switch that senses irregularities in the electric utility. The computer is usually connected directly to the utility that serves as the primary power source, and power protection is available only when voltage dips to the point of creating an outage. Some off-line UPSs include surge suppression circuits, and some possess optional built-in power line conditioners to increase the level of protection they offer.
In the case of power surges, an off-line UPS passes the surge voltage to the protected system until it hits a predetermined level, around 115% of the input voltage. At the surge limit value, the unit then goes to battery. With high-voltage spikes and switching transients, they give reasonably good coverage. For power sags, electrical line noise and brownouts, off-line UPSs protect only when the battery is delivering power to the protected system.
A similar limitation exists in the case of frequency variation. An off-line UPS protects only if the inverter is operating and on battery. If the input frequency varies outside the device’s range, the unit is forced to go to battery to regulate the output to the computer. In very unstable conditions, this may drain the battery, making it unavailable during a blackout.
Since off-line UPSs provide only partial protection against many common power problems, they are most often used to shield single-user PCs and other less-critical applications.
This type of UPS are hybrid devices that attempt to offer a higher level of performance by adding voltage regulation features to conventional off-line designs.
Like off-line models, line-interactive UPS protect against power surges by passing the surge voltage to the computer until it hits a predetermined voltage, at which point the unit goes to battery. They provide moderate protection against high-voltage spikes and switching transients, although, again, not with complete isolation.
With power sags, a line-interactive UPS may use a tapped transformer to provide the voltage levels needed to maintain output voltage. Essentially, the unit switches to battery to adjust the tap location down at certain intervals to maintain the output voltage as the input voltage falls, eventually going to battery full-time once the input voltage reaches a pre-selected level. This system offers adequate protection as long as the power sage aren’t continuously changing, which may reduce the battery time. In this case it is being used frequently and may not recharge itself in time for use in a power outage.
For electrical line noise and frequency variation, these devices work only when the inverter is operating and the battery is the power source, which may drain the battery during prolonged unstable conditions that typically occur during generator operations. In the case of brown-outs, some line-interactive UPSs have tapped transformers that protect against this problem. Again, this may reduce battery hold-up time if a power outage occurs before the recharge is complete.
Ferroresonant UPSs, another hybrid technology, keep the inverter in standby mode similar to line-interactive and standby UPSs. The protected system however, is powered from the utility through the ferroresonant transformer. The transformer provides voltage regulation and power conditioning for disturbances such as electrical line noise. The ferroresonant transformer also maintains a reserve of energy that is usually sufficient to power most computers, i.e. PCs, briefly when a total outage occurs. This keeps the computer supplied with power within most input requirements until the inverter is switched on.
In cases where the input frequency is unstable, the ferroresonant topology will subject the load to some voltage excursions in addition to frequency fluctuations. When the frequency variations exceed a pre-set limit, the unit will supply stable voltage and frequency to the load through the back-up battery and inverter.
Ferroresonant technology is also sensitive to sudden current surges on the load input currents. this may cause a significant change in the output voltage of the UPS. In general, ferroresonant UPSs work best with most non-computer technology or linear loads such as motor, beaters and lights.
On-line UPSs provide the highest level or power protection and are the ideal choice for shielding your organization’s most important computing installations. This technology uses the combination of a double-conversion (AC to DC/DC to AC) power circuit and an inverter, which continuously powers the load, to provide both conditioned electrical power and outage protection. On-line UPSs offer complete protection and isolation from all types of power problems — power surges, high-voltage spikes, switching transients, power sags, electrical line noise, frequency variation, brownouts and blackouts. In addition, they provide digital-quality power not possible with off-line systems. For these reasons, they typically are used for mission-critical applications that demand high productivity and systems availability.
On-line UPSs are also the most cost-effective way to ensure comprehensive power protection. On-line systems provide the same benefits of a standby UPS in conjunction with a line conditioner, at a price lower than the cost of both components.