Uninterruptable Power Supplies

There are three basic types of UPS: Standby (or Offline), Line Interactive, and Double Conversion (or Online).

Standby (Offline)

The Standby UPS consists of a basic battery/power conversion circuit and a switch that senses irregularities in the electric utility. The equipment to be protected is usually directly connected to the primary power source, and the power protection is available only when line voltage dips to the point of creating an outage. Some off-line UPS include surge protection circuits to increase the level of protection they offer.

In the case of power surges, a standby UPS passes the voltage surge to the protected system until it hits a predetermined level, usually around 115% of the input voltage. At the surge limit value, the unit then goes to battery. Although they do provide reasonably good protection against spikes and switching transients. However, they do not protect against sags, line noise, frequency variation or brownouts unless the battery is delivering power to the protected system.

If the UPS is forced to go to battery frequently, it can drain the battery, making it unavailable during blackouts. Since standby UPS provide only partial protection against many common power problems, they are most often used to shield a single user or less critical or sensitive equipment.

Line Interactive

Line Interactive UPS are hybrid devices that offer a higher level of performance by adding better voltage regulation and filtering features to the standby UPS design.

Like standby models, line interactive UPS protect against power surges by passingthe surge voltage to the equipment 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, 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 at set intervals to maintain the output voltage as the input voltage falls. It will eventually go to battery full-time once the input voltage reaches a pre-selected level. This sytem offers adequate protection as long as the power sags aren’t continuously changing, which may reduce battery time. If it is frequently going to battery, you run the risk of not having the batteries fully charged for use during a power outage.

For electrical line noise and frequency variation, line interactive UPS 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 operation.

Ferroresonant UPS, another hybrid technology, keeps the inverter in standby mode similar to line interactive and standby UPS. 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 small equipment or PCs briefly when a total outage occurs. This keeps the equipment supplied with power within most input requirements until the inverter is switched on.

However, ferroresonant UPS are not very effective against unstable frequency variations or sudden current changes. In general, ferroresonant UPS work best with most non-computer or on-critical technology, or with linear loads such as motors, heaters and lights.

Double Conversion (Online)

Double conversion UPS, often called “Online” provide the highest level of power protection and are an ideal choice for shielding hyour organization’s most important computing and equipment 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. Online UPS offer complete protection and isolation from all types of power problems – power surges, high-voltage spikes, switching transients, power sags, electrical line noise, frequency variations, brownouts and blackouts. In addition, they provide digital-quality power not possible with offline systems. For these reasons, they typically are used for mission critical applications that demand high productivity and system availability.

Double conversion UPS can be the most cost-effective way to ensure comprehensive power protection. Double conversion systems provide the same benefits of a standby UPS in conjunction with a line conditioner, at a price that is lower than buying the two components seperately. However, double conversion UPS also can be the most expensive solution initially, and they require regular battery maintenance and monitoring. UPS batteries can e very expensive, and are also temperature sensitive. They cannot be placed in harsh environments.

Because of expense and sensitivity of these units, they are normally used only if there is no other acceptable alternative to 100% continuous power.

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