The 3 Types of UPS Systems: Standby vs. Line Interactive vs. Online UPSs

The 3 Types of UPS Systems: Standby vs. Line Interactive vs. Online UPSs

Detectable power outages account for only a small percentage of power disturbances. Utility power is also plagued with surges, sags, electrical noise, harmonics, load fluctuations, and other interference. Even under normal utility operations — with no storm or lightning in sight — enterprise IT systems can be bombarded daily by conditions that damage critical components.

That’s why it’s important to select a nobreak eaton that offers more than just standby power. Here’s a look at the three types of UPS systems and how well they protect against power disturbances.

What Is a Standby (Offline) UPS?

When IT equipment is plugged into an offline UPS, it receives power directly from the input source rather than the UPS’s inverter output. The inverter, which converts the battery’s DC power to AC, is “offline.” The battery charger is connected to the input source so that it can keep the battery fully charged and ready to go when needed. When the input power fails, the UPS automatically connects the IT equipment load to the inverter’s output. This can take up to 10 milliseconds.

Furthermore, the offline UPS only has basic surge suppression and electromagnetic line noise filtering capabilities. IT equipment connected to an offline UPS is subjected to many of the power disturbances of the electric grid.

What Is a Line-Interactive UPS?

A line-interactive UPS has a smarter design in which the DC-to-AC power inverter is “inline” with the UPS’s output. When the input power source is functioning normally, the inverter is in AC-to-DC mode to charge the battery. If the input power source fails, the UPS’s transfer switch opens, and the inverter reverses so that the battery can power the IT equipment. The load transfer time for a line-interactive UPS is less than 4 milliseconds.

Because the inverter remains online, line-interactive UPSs minimize switching transients. They also provide under- and over-voltage protection as well as surge suppression and electromagnetic line noise filtering.

What Is a Double-Conversion (Online) UPS?

As the name implies, a double-conversion UPS converts power twice: A rectifier converts AC power from the primary source to DC power, which is stabilized and fed through an inverter that converts it back to AC power. If the main power source fails, the UPS automatically eliminates the rectifier to switch to battery power. Because the inverter is already engaged, transfer to the battery is virtually instantaneous.

The double-conversion process also minimizes the common power disturbances of the main supply, including harmonics and waveform distortions. As a result, online UPSs provide maximum protection for highly sensitive IT equipment.

Line-Interactive vs. Online UPSs

Generally, standby UPSs are inadequate for mission-critical data center applications due to potential disruptions during battery switchover and limited protection against power disturbances. Line-interactive UPSs and online UPSs are better choices, offering a comparable set of safeguards:

Surge/Noise Protection. Both line-interactive and online UPSs provide surge suppression and electromagnetic line noise filtering.

Voltage Regulation. Line-interactive UPSs detect when voltage deviates from preset thresholds and uses automatic voltage regulation to correct it. The double-conversion process of an online UPS regulates voltage somewhat more precisely.

Pure Sine Wave Output. The power supplies of some IT equipment require pure sine wave output, which is the optimal waveform of electricity. All online and many line-interactive UPSs offer this capability.

When choosing between line-interactive and online uninterruptible power supply, it often comes down to price. Line-interactive UPSs cost up to 40 percent less than online UPS systems. However, if you can’t afford even one millisecond of downtime, an online UPS may be right for you. Check out our nobreak 20kva guide for more information on how to select the perfect UPS for your environment.