What is a Line-interactive UPS, how does it work and what are its advantages and disadvantages?

What is a Line-interactive UPS, how does it work and what are its advantages and disadvantages?

The next step up the UPS ladder is the line-interactive UPS and it is the most common topology used for small business, web and departmental servers.

In the line-interactive topology, the inverter is always connected to the output of the UPS. Operating the inverter in reverse during times when the AC power is normal provides battery charging. When the input power fails, the transfer switch opens and power flows from the battery to the UPS output. With the inverter always on and connected to the output, this design provides additional filtering for incoming power compared to the standby topology.

In addition, the line-interactive topology also incorporates a multi-tap transformer to buck (reduce) or boost (increase) the voltage, thereby providing some degree of voltage regulation (also known as “Automatic Voltage Regulation”) as the input voltage varies. Voltage regulation is an important feature when low voltage conditions exist, otherwise the UPS would transfer to battery power and frequent battery usage can cause premature battery failure. The buck or boost range is typically limited to 10% and while some models will provide both buck and boost other less expensive models will just provide boost capability.

The topology of the line-interactive UPS is shown below:

Line Interactive UPS topology

The inverter in this topology can also be designed such that its failure will still permit power flow from the AC input to the output, thereby eliminating the potential of single point failure by providing two independent power paths.

High efficiency (typically 90%- 96%), small size, economic price point coupled with the ability to correct low or high line voltage conditions make this the dominant type of UPS in the 0.5-5 kVA power range.

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