Types of UPS or uninterruptible power supplies

Introduction to UPS or Uninterruptible Power Supply

An uninterruptible power supply (UPS) is a system that protects electronic equipment from power fluctuations. A UPS is a device that is interfaced between the electric network (connected to utility power eg. ESKOM in South Africa) and the equipment that need protecting.

Types of UPS or uninterruptible power supplies

UPS or uninterruptible power supply – Types

There are various types of UPS or uninterruptible power supplies and design approaches used to implement UPS systems, each with its own unmistakable performance characteristics. Listed below are the most common design approaches:

  • Standby
  • Line interactive
  • Standby-ferro
  • Double conversion on-line

The standby UPS

The Standby UPS is the most common type used for Personal Computers. At minimum even with a very small unit a standby ups will provide you with enough power where your computer could be gracefully shut down or sent into hibernation mode and brought back into operation once the power outage or other power situation was resolved. With proper filter and surge circuitry, a standby ups can also provide adequate noise filtration and surge suppression.

Line interactive UPS

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.

Standby-ferro UPS

At one time, the Standby-Ferro UPS was the dominant form of UPS in the 3-15kVA range. This design depends on a special saturating transformer that has three power connections. The main power supply comes from AC input, through a transfer switch, then through the transformer and to the output. In the case of a power failure, the transfer switch is opened, and the inverter picks up the output load.

The inverter, in the Standby-Ferro design, is in the standby mode, and is energized when the input power fails, which opens the transfer switch. The transformer has a special “Ferro-resonant” capability, which provides limited voltage regulation and output waveform “shaping” The isolation from AC power transients provided by the Ferro transformer is as good as, or better than any filter available.

Double conversion on-line UPS

This is the most common type of UPS above 10kVA (Volt-Ampere). Double Conversion On-Line UPS is the same as the Standby UPS except that the primary power path is the inverter instead of the AC mains.

In the design of Double Conversion On-Line operation, failure of the input AC does not cause activation of the transfer switch, because the input AC is not the primary source, but is rather the backup source. Therefore, during an input AC power failure, on line operation results in no transfer time. The on line mode of operation exhibits a transfer time when the power from the primary battery charger/battery/inverter power path fails.


What is a Standby-Ferro UPS
Double Conversion On-Line UPS