Selection and Sizing of Uninterruptible Power System

It is always confusing when you need to choose an Uninterruptable Power Supply (UPS) as each different UPS system has its own distinct performance characteristics. The common UPS designs are listed as follows

  1. Standby
  2. Line Interactive
  3. Standby-ferro
  4. Double conversion on-line
  5. Delta conversion on-line

Standby UPS

The Standby UPS is the most common used for stand-alone devices and desktop computer. This UPS is primary connected to the AC source and the transfer switch will switch the load to the battery/ inverter should the primary source fail. The UPS is known as Standby because the inverter only starts when the power failure. High efficiency, small size and low cost are the benefits for this type of UPS.

Standby UPS Diagram

Line Interactive UPS

The Line Interactive UPS design is commonly used for small business, web and departmental servers. The UPS inverter is always connected to the output of the UPS whereby providing AC power charging to the battery during normal times. When the AC power fails, the transfer switch opens, and power will flow from the battery to the load. As the inverter is always connected to the output, this UPS design will provide additional filtering and reduced switching transients as compared to the Standby UPS.

High efficiency, small size, low cost and high reliability coupled with the ability to correct voltage fluctuation makes it a preferred choice for 0.5-5KVA range.

Line Interactive UPS Diagram

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 output of the UPS is connected to the main power supply via the transfer switch and the transformer.In the event of a power failure, the transfer switch is opened, and the inverter picks up the output load.

The isolation from AC power transients provided by the Ferro transformer is as good as, or better than any filter available.However, the Ferro transformer itself creates severe output voltage distortion and transients, which can be worse than a poor AC connection. Even though it is a standby UPS by design, the Standby-Ferro generates a great deal of heat because the Ferro-resonant transformer is inherently inefficient. These transformers are also large relative to regular isolation transformers; the unit is generally large and heavy. Therefore Standby-Ferro UPS systems are no longer commonly used as they can be fundamentally unstable when operating a modern computer power supply load

Standby-Ferro UPS Diagram

Double Conversion On-Line UPS

This UPS design is commonly used in UPS above 10KVA. This design is quite similar to a standby UPS except that the primary path is an inverter instead of an AC main. In the event of a power failure, there is no transfer time as the input AC is charging the battery source which provide power for the output inverter. This type of UPS provides a n ideal electrical output performance but the constant wear on the power components reduce its reliability as compared to other designs. The input power drawn by the large battery charge also may be non-linear therefore can interfere with building power wiring or cause problem with the standby generator.

Double Conversion On-Line UPS Diagram

Delta conversion On-line UPS

This is the newer UPS technology is for the 5KVA- 1.6MW UPS to eliminate the drawbacks of the Double Conversion On-line UPS. This design exhibits the same behavior in the event of a power failure as the Double Conversion On-line UPS except that there is an additional delta converter to the inverter output incorporated in the circuitry.

The Delta Converter has two function. It controls the input power characteristics by drawing the power in a sinusoidal manner so to minimize the harmonics onto the utility. This will ensure optimal utility and generator system compatibility to reduce heating and system wear in the power distribution system. The second function is to control the input current and regulate the charging of the battery system. This type of UPS is also highly efficient due to the design is without the use of filter banks therefore is able to deliver power to the load with much greater efficiency as compared to the Double Conversion UPS.

Delta Conversion On-Line UPS Diagram


Each design of the UPS has its advantages and disadvantages depending on its different uses as no single type of UPS is ideal for all applications. To ease your selection for the right UPS, the following table is a summary of the different types of UPS for your different application and budget.

Summary of the Different Types of UPS

Some Tips From Us

  1. Tabulate a list of all the equipment to be supported by the Uninterruptible Power Supply company in Singapore.
  2. Confirm the power rating of each device to be backup. The rating can be watts or VA. Add them together.
  3. Multiply the total power consumption by a safety factor of 1.3 to cater for in-rush current surge and future expansion.
  4. Check for the requirement of the back-up time for the equipment. More space (additional external battery pack) is required if longer backup time is needed.
  5. Depending on the requirement, select either an offline, online or line-interactive Uninterruptible Power Supplies accordingly.
  6. Depending of site situation and configuration, centralise  Singapore Uninterruptible Power Supply maybe a more cost effective solution than multiple Uninterruptible Power Supply approach.
  7. Centralise Uninterruptible Power Supply will require “hard-wirings”, eg isolator, power cables, distribution board, etc
  8. De-centralise Uninterruptible Power Supply installation will be straight-forward and easier to implement.
  9. Check the incoming power availability (Single-phase or Three-phase breaker) in order to determine if electrical wirings is required.
  10. Always ensure there is 24 hours air-conditioning in the room.