What is an On-line UPS, how does it work and what are its advantages and disadvantages?

What is an On-line UPS, how does it work and what are its advantages and disadvantages?

This is the most common UPS topology above 10kVA and is designed to provide continuous power protection against all power problems to mission critical equipment in data centers and server rooms. This topology ensures a consistent quality of power supply regardless of disturbances in the incoming mains. There are actually 2 sub-categories of On-line UPS known as – Double Conversion On-line and Delta Conversion On-line, with the former being the more popular technology.

In the Double Conversion On-line topology, the primary power path to the loads is the inverter instead of the AC mains, thus the inverter is ON 100% of the time and hence the term on-line. The name double-conversion arises from the operation of the device. It first converts line voltage (AC) into battery-compatible low-voltage DC using a rectifier. The rectifier supplies power to the inverter in addition to charging the batteries. The DC from the batteries is then converted by an inverter back to the highly controlled and regulated AC. This process thus effectively removes any electrical disturbance on the utility (input) side of the UPS.

The topology of the online UPS is shown below:

Online UPS topology

The Double Conversion UPS is the only truly uninterruptible system because the inverter is always connected to the load and is always drawing power from the battery source irrespective of whether the mains power is present or not. Therefore, during an input AC power failure, on-line operation results in no transfer time. This topology also provides protection against all forms of power irregularity including surges, spikes, over-voltages, sags, brownouts and blackouts. This topology also maintains a significantly closer output voltage tolerance compared to the line-interactive topology as it uses solid-state voltage regulators instead of transformer taps to cope with under and over voltages.

While the Double Conversion On-line topology provides nearly ideal electrical output, the constant wear on the power components reduces its reliability and the reduced efficiency (typically 80 – 90%) results in higher life cycle costs for the UPS.

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