Uninterruptible power supply is an electrical apparatus that provides emergency power to a load when the input power source, typically mains power, fails. In that it will provide near-instantaneous protection from input power interruptions, by supplying energy stored in batteries or a flywheel. A UPS is typically used to protect computers, data centers, telecommunication equipment or other electrical equipment where an unexpected power disruption could cause injuries, fatalities, serious business disruption or data loss.
Common power problems:
The primary role of any UPS is to provide short-term power when the input power source fails. However, most UPS units are also capable in varying degrees of correcting common utility power problems:
- Voltage spike or sustained Over voltage
- Momentary or sustained reduction in input voltage.
- Noise, defined as a high frequency transient or oscillation, usually injected into the line by nearby equipment.
- Instability of the mains frequency.
- Harmonic distortion: defined as a departure from the ideal sinusoidal waveform expected on the line.
Types of UPS systems
There are mainly 3 types:
- Online/double conversion
- Offline/ standby
- Line interactive
An on-line UPS uses a “double conversion” method of accepting AC input, rectifying to DC for passing through the rechargeable battery (or battery strings), then inverting back to 120 V/230 V AC for powering the protected equipment.
Offline / Standby:
The offline / 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 a predetermined level the SPS turns on its internal DC-AC inverter circuitry, which is powered from an internal storage battery. The UPS will be designed to power certain equipment, such as a personal computer, without any objectionable dip or brownout to that device.
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, thereby increasing or decreasing the magnetic field and the output voltage of the transformer. This is also known as a Buck–boost transformer.