Also known as an uninterruptible power source or battery/flywheel, a UPS provides emergency power to load when the main power source fails. A UPS is different from an auxiliary or emergency power system or standby generator because it will provide near-instantaneous protection from power interruptions, by supplying energy stored in the batteries.
A UPS unit is typically used to protect hardware like computers, data centers, telecommunication equipment and any other equipment that could cause injuries, fatalities, serious business disruption or data loss if the there was an unexpected disruption to the power supply.
UPS units range in size from units designed to protect a single computer without a video monitor (around 200 volt-ampere rating) to large units powering entire data centers or buildings. The world’s largest UPS is a 46-megawatt Battery Electric Storage System (BESS), in Fairbanks, Alaska which powers the entire city and nearby rural communities during outages.
The main role of any UPS is to provide short-term power when the main power source fails. However, most UPS units are also able to correct common utility power problems, in varying degrees, including:
- Voltage spike or sustained overvoltage
- Momentary or sustained reduction in input voltage
- Noise, usually from equipment that is nearby
- Instability of the main frequency
- Harmonic distortion
UPS units are divided into categories based on which of the problems listed they address. The three general categories of modern UPS systems are “standby”, “line-interactive” or “on-line”.
Standby
The standby UPS (SPS) offers only the most basic features, providing surge protection and battery backup. The protected equipment is normally connected directly to incoming utility power. When the incoming voltage falls below or rises above a predetermined level the SPS turns on it internal DC-AC inverter circuitry, which is powered from an internal storage battery.
Line-interactive
The line-interactive UPS is similar in operation to a standby UPS, but with the addition of a multi-tap variable-voltage autotransformer. This is a special type of transformer that can add or subtract powered coils of wire, so it can increase or decrease the magnetic field and the output voltage of the transformer. This is also known as a Buck-boost transformer.
Online
In an online UPS, the batteries are always connected to the inverter, so that no power transfer switches are necessary. When the power goes out, the rectifier simply drops out of the circuit and the batteries keep the power steady and unchanged. When the power is restored, the rectifier resumes carrying most of the load and begins charging the batteries, though the charging current may be limited to prevent the high-power rectifier from overheating the batteries and boiling off the electrolyte.
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