infrastructure//data center//power chain//UPS

A UPS (uninterruptible power supply, SAI in Spanish) is a power converter with an energy store, usually batteries, that keeps its load supplied without a break when the incoming power fails or misbehaves, and in a data center it is the stage of the power chain that covers the seconds between a utility failure and a running generator. It does not replace the generator: its batteries typically hold the full load for five to ten minutes, enough to bridge a generator start with a wide margin or to shut systems down cleanly if the generator never comes.

A UPS (uninterruptible power supply, SAI in Spanish) is a power converter with an energy store, usually batteries, that keeps its load supplied without a break when the incoming power fails or misbehaves, and in a data center it is the stage of the power chain that covers the seconds between a utility failure and a running generator. It does not replace the generator: its batteries typically hold the full load for five to ten minutes, enough to bridge a generator start with a wide margin or to shut systems down cleanly if the generator never comes.

The common large design is the double-conversion UPS. Incoming AC is rectified to DC, which charges the batteries, and an inverter rebuilds clean AC for the load all the time. When the utility fails the rectifier stops and the batteries feed the inverter directly, so the load sees no transfer at all; the price is the loss of two conversions on every watt, a few percent, which on a 50 MW site is megawatts of heat. Cheaper line-interactive and standby designs pass utility power straight through and switch to the inverter on a fault, which takes a few milliseconds, short enough for most servers and too long for some equipment.

battery at this load7.1 min generator on load10 s loadheld At 70 % load the battery lasts 7.1 min. The generator holds the load after 10 s, using 2 % of the battery. The load never sees the outage.

Ride-through is a race between the battery and the generator.

The battery's autonomy at the actual IT load must exceed the time the generator needs to start, reach speed and accept load (about ten seconds for a standby generator built to the strictest emergency class, longer for others), with margin for a failed first start. A battery sized for ten minutes at the design load gives far less at a higher load and less again as it ages.

The second job is power quality. A double-conversion UPS isolates the servers from sags, spikes and frequency excursions of the grid, which happen far more often than full outages.

Batteries are the weak element. Lead-acid strings degrade with temperature and age and fail cell by cell, so they are monitored and load-tested; lithium-ion packs are smaller and last longer but bring their own fire-safety requirements. A UPS never tested on battery is an assumption.

Data centers duplicate it. Two UPS systems feeding two independent paths to each server is the pattern of a 2N redundancy level; a shared N+1 UPS system with one spare module is cheaper and protects against a module failure but not against a fault in the common output.