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Understanding the different types of UPSes

March 2, 2023
Read Time 5 mins

By Sascha Schmidt-Ries, Product  Manager

In today’s highly connected age, businesses expect that power will not only always be available, but it will also be provided at a constant, acceptable voltage, current, and frequency.

Unfortunately, in South Africa this isn’t the case. Load shedding, which was introduced in 2007, is here to stay, with a recent announcement saying we can expect a constant stage two or three for the next few years.

When the power is off, business processes cease, revenue is lost in sales, productivity drops and ultimately customers become dissatisfied. Similarly, surges in power cause damage to expensive electronic equipment and circuit boards.

This is why many businesses are turning to an uninterruptible power supply (UPS) which provides guaranteed power protection for all connected electronics. Should a power interruption occur, or if it fluctuates outside of safe levels, a UPS will immediately provide clean battery backup power and surge protection.

Covered in this article

Types of UPS
Standby UPS
Line interactive UPS
Standby-ferro UPS
Double conversion on-line UPS
Delta conversion on-line UPS

Types of UPS

However, not all UPSes are the same, and the various types of these devices can cause confusion for businesses.

There is a wide range of design approaches that are used to implement UPS systems, each with unique performance characteristics.

The most common design approaches are:

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


Standby UPS

Standby UPS is the most common type of UPS used for desktop PCs, and less demanding home and professional environments. This type of UPS delivers short-term, battery-sourced power during outages. With standby UPS, hardware receives utility power, under usual conditions, through a direct AC connection.

In a nutshell, the standby unit and its inverter are on hold until backup power is required. Some standby UPSes also have the ability to protect data and sensitive equipment from surges, spikes, and dips.

Switchover times happen in a fraction of a second after failure, with response times varying between offerings. In the event of a lengthy outage, the battery backup power enables safe shutdowns, to ensure both equipment and data are protected.

Line interactive UPS

A line interactive UPS regulates voltage automatically. Line interactive technology responds to high and low voltage conditions intuitively. In addition, these units support systems during outages without draining the battery. With a line interactive UPS, the electrical source is the first line of power, although an inverter or converter technology enables the unit’s battery to be charged during normal operations.

Should the input power fail, the transfer switch opens and the power flows from the battery to the UPS output. With the inverter always on and connected to the output, this type of UPS offers additional filtering and promises reduced switching transients when compared with the standby UPS topology.

Line interactive UPS systems are able to protect sensitive equipment should a brownout or blackout occur. While these units are more costly than standby models, they are more affordable than certain other types of UPS. These UPSes can maintain performance in the event of a low-voltage situation as well as during short-term power failures. If a prolonged outage occurs, the battery power facilitates the safe shutting down of devices.

This type of UPS is the most popular model used for small businesses, web, and departmental servers. With this UPS, the battery-to-AC power converter (inverter) is constantly connected to the output of the UPS. Operating the inverter in reverse when the input AC power is on as normal enables battery charging too.

In addition, the line interactive design normally features a tap-changing transformer, which adds voltage regulation by adjusting transformer taps as the input voltage varies. Voltage regulation is a crucial feature when low voltage conditions happen, without them, the UPS would transfer to the battery and then eventually “down” the load.

This can result in premature battery failure, although, the inverter can also be designed to ensure that its failure will still allow power flow for the AC input to the output, which removes the potential of a single point failure and, in essence, provides for two independent power paths.

The high efficiency, small size, low cost, and high reliability, together with its ability to correct low or high line voltage conditions, make this the most popular type of UPS in the 0,5 to 5 kVA power range.

Standby-ferro UPS

At one time, the standby-ferro UPS was the dominant form of UPS in the 3 to 15 kVA sector.  The standby-ferro design relies on a special saturating transformer that has three windings, or power connections.

The primary power path is from AC input, via a transfer switch, through the transformer, and then to the output. In the event of a power failure, the transfer switch is opened and the inverter picks up the output load.

With this type of UPS, the inverter is in standby mode, and is energised should the input power fail and the transfer switch be opened. The transformer has a unique "ferro-resonant" capability which gives limited voltage regulation and output waveform "shaping".

In addition, the isolation from AC power transients provided by the ferro transformer is as good as or possibly even better than any other filter available. However, the ferro transformer itself creates severe output voltage distortion and transients, which can be more detrimental than a poor AC connection.

Although it is a standby UPS by design, this type of UPS generates a vast amount of heat due to the ferro-resonant transformer being foundational inefficient. Another downside is that these transformers are also bigger in size when compared to regular isolation transformers, and are often fairly bulky and heavy.

On the plus side, stands-ferro UPSes boast high reliability and excellent line filtering. This design has very low efficiency combined with instability when used with certain generators and newer power-factor corrected computers, which has seen the popularity of this design drop dramatically.

Double conversion on-line UPS

 This type of UPS is the most common type of UPS above 10 kVA.  A double conversion on-line UPS converts power twice. Initially, the AC input, with all its voltage spikes, distortion, and other anomalies, is converted into DC. The UPS then employs a capacitor to stabilise this DC voltage and store energy drawn from the AC input.

Next, DC is converted back into AC which is tightly regulated by the system. This AC output can also have a different frequency from the AC input. Every bit of the power provided to the load equipment goes through this double conversion process when AC input is present.

Should the AC input go out of a certain range, the UPS will draw power from its battery to ensure that the UPS output is unaffected. In a wide range of double conversion on-line designs, this transition inside the UPS between AC input and battery takes only several milliseconds.

This type of UPS provides almost ideal electrical output performance, however, the continual wear and tear on the power components lowers reliability in comparison to other designs.

Delta conversion on-line UPS

Finally, the delta conversion on-line UPS is a more recent technology that was designed to eliminate the disadvantages of the double conversion on-line design and comes in sizes ranging from 5 kVA to 1,6 MW.

Much like the double conversion on-line design, the delta conversion on-line UPS always has the inverter supplying the load voltage. In addition, the additional delta converter also contributes power to the inverter output. Should AC failure or disturbance occur, this UPS exhibits behaviour that is identical to the double conversion on-line UPS.

Delta conversion UPSes are also able to save energy by carrying the package for only the difference between the start and endpoints. While the double conversion on-line UPS converts the power to the battery and back again, the delta converter is able to move components of the power from input to the output.

The delta converter acts to control the input power characteristics. This active front end draws power in a sinusoidal manner, lessening harmonics reflected onto the utility, and ensuring that utility and generator system compatibility is optimal.

Another function of this UPS is to control the input current to regulate the charging of the battery system.  It enables the same output characteristics as the double conversion on-line design, but the input characteristics are usually different. Delta conversion on-line designs provide dynamic power factor corrected input but eliminates the inefficient use of the filter banks that one has with traditional solutions.

The most compelling benefit is a dramatic reduction in energy loss. The input power control also makes the UPS compatible with all generator sets and lowers the need for wiring and generator oversizing. This type of delta conversion on-line technology is the only core UPS technology that is patented, and therefore it is unlikely to be available from many UPS suppliers.

To find a UPS system that meets your unique requirements, contact us today.

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