Uninterrupted Power supply, What is an uninterrupted power suppy

Uninterrupted Power Supply (UPS) speaks to what is an uninterrupted power supply.

There is often a UPS located between the generator and the emergency power system. The UPS or battery backup is equipment that provides instant emergency power when utility power fails.

The on-battery runtime of most uninterruptible power sources is relatively short 5 to15 minutes being typical for smaller units allowing time to bring an auxiliary power source on line, or to properly shut down the protected equipment.

The uninterrupted power supply is mostly used to protect computers, data center and communication equipment for obvious reasons. The UPS equipment come in all different sizes depending on the equipment to be protected.

UPS Functions

The Uninterrupted power supply can also correct common utility power problems such as:

Power failure: defined as a total loss of input voltage.

Surge: defined as a momentary or sustained increase in the mains voltage.

Sag: defined as a momentary or sustained reduction in input voltage.

Spikes, defined as a brief high voltage excursion.

Noise, defined as a high frequency transientor an oscillation, usually injected into the line by nearby equipment.

Frequency instability: defined as temporary changes in the mains frequency.

Harmonic distortion: defined as a departure from the ideal sinusoidal waveform expected on the line.

The uninterrupted power supply is categorized depending on which of the above problems are addressed.

The categories are on-line, line-interactive or standby. Generator concerns that a UPS system can resolve:

Frequency variations

The voltage and frequency of the power produced by a generator depends on the engine speed. The speed is controlled by a system called a governor. Some governors are mechanical, some are electronic.

The job of the governor is to keep the voltage and frequency constant, while the load on the generator changes. This may pose a problem where, for example, the startup surge of an elevator can cause short "blips" in the frequency of the generator or the output voltage, thus effecting all other devices powered by the generator.

Power factor

A problem in the combination of a "double conversion" uninterrupted power supply and a generator is the voltage distortion created by the UPS. The input of a double conversion UPS is essentially a big rectifier. The current drawn by the UPS is non-sinusoidal.

This causes the voltage from the generator also to become non-sinusoidal. The voltage distortion then can cause problems in all electrical equipment connected to the generator, including the UPS itself.

This level of "noise" is measured as a percentage of "Total Harmonic Distortion of the current" (THD(i)). Classic UPS rectifiers have a THD(i) level of around 25-30%. To prevent voltage distortion, this requires generators more than twice as big as the UPS.

There are several solutions to reduce the THD(i) in a double conversion UPS:

Passive power factor correction: (Passive PFC) Classic solutions such as passive filters reduce THD(i) to 5-10% at full load. They are reliable, but big and only work at full load, and present their own problems when used in tandem with generators. An alternative solution is an active filter.

Through the use of such a device, THD(i) can drop to 5% over the full power range. The newest technology in double conversion UPS units is a rectifier that doesn't use classic rectifier components (Thyristors and Diodes) but high frequency components (IGBTs).

A double conversion UPS with an IGBT rectifier can have a THD(i) as small as 2%. This completely eliminates the need to oversize the generator (and transformers), without additional filters, investment cost, losses, or space.

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