UPS manufacturers use these general technologies:
An offline/standby UPS offers basic features — surge protection and battery backup. The protected equipment normally connects directly to incoming utility power. When the incoming voltage falls below a predetermined level, the UPS turns on its internal DC-AC inverter circuitry powered from an internal storage battery. The UPS then mechanically switches the connected equipment to its DC-AC inverter output. The switchover time can be as long as 25 milliseconds, depending on the amount of time it takes the standby UPS to detect the loss of utility voltage. An offline system tends to have the lowest initial cost of all UPS systems.
An line-interactive UPS is similar to a standby UPS, but with the addition of a multi-tap, variable-voltage autotransformer. This transformer can add or subtract powered coils of wire, increasing or decreasing the magnetic field and the transformer's output voltage.
This type of UPS can tolerate continuous undervoltage brownouts and overvoltage surges without using its limited reserve battery power. It compensates by automatically selecting different power taps on the autotransformer. Depending on the design, changing the autotransformer tap can cause a brief disruption of output power.
An online UPS is ideal for environments where electrical isolation is necessary or for equipment that is sensitive to power fluctuations. Once previously reserved for large installations of 10 kilowatts (kW) or more, advances in technology now permit its use as a common consumer device, supplying 500 watts or less. The initial cost of an online UPS might be slightly higher than other options, but its total cost of ownership generally is lower, due to its longer battery life.
An online UPS might be necessary when:
The basic technology of an online UPS is the same as a standby or line-interactive UPS. But it typically costs much more because it a greater current AC-to-DC battery-charger/rectifier, with the rectifier and inverter designed to run continuously with improved cooling systems. It is called a double-conversion UPS because of the rectifier directly driving the inverter, even when powered from normal AC current.
In an online UPS, batteries always connect to the inverter, so no power-transfer switches are necessary. When a power loss occurs, the rectifier simply drops out of the circuit, and the batteries keep the power steady and unchanged. When power returns, the rectifier resumes carrying most of the load and begins charging the batteries, though the charging current might be limited to prevent the high-power rectifier from overheating the batteries and boiling off the electrolyte.
The main advantage of an online UPS is its ability to provide an electrical firewall between the incoming utility power and sensitive equipment. While the standby and line-interactive UPS merely filter input utility power, the double-conversion UPS provides a layer of insulation from power-quality problems. It allows control of output voltage and frequency, regardless of input voltage and frequency.
If a prolonged interruption of the electrical supply to the load that a UPS system supports is not acceptable, the system should be configured so technicians can service the UPS or even replace it without interrupting the downstream load it is supporting.
For smaller point-of-use systems located within equipment racks, this means the UPS supports a power distribution unit (PDU) that has two independent sources. For centralized systems, this means a make-before-break maintenance bypass switch located in an enclosure separate from the UPS.
