Director’s Blog: UPS Systems: What’s the Difference?

Following the last article, where we discussed exactly what a UPS is (and isn’t), I thought it would be useful to consider some of the different forms these uninterruptible power systems can take. This is going to be a bit technical at times, but there is a lot of confusion over the different types of UPS available, and hopefully I can clear some of that up.

So without further ado…

UPS System Size

The first thing to consider when weighing up the benefits of different UPS systems is size; systems range from units designed to support a single desktop PC to those built to support data centres.

Let me put that in perspective.

A basic desktop PC might run on a 400w power supply, which conservatively would require a 640vA (Volt-Amperes) UPS to allow time for safe shutdown in the event of power failure.

At the other end of the scale, KOHLER Uninterruptible Power’s PW 9500DPA can be scaled up to 500kvA (Kilo-volt-amperes) per frame, with the potential for six frames to be combined in parallel. That’s a total of 3MvA (mega-volt-amperes) – Over 4,687 times the output capacity of the system from our first example.


In terms of size, UPS systems can be roughly categorised as follows:

· Micro (Up to 1000vA) – Typically designed as an uninterruptible power supply for PC to accommodate a single desktop.
· Mini (500 – 2000vA) – Suitable to supply a file server or complete workstation, e.g. a PC and peripherals such as a basic printer.
· Medium (3 – 20kvA) – Sufficient to power an office network, small server farm, or similar.
· High-power (30 – 500kvA per frame) – Systems of this size can accommodate major data centres.

Further to the size of output required, it is also vital to consider the voltage required by critical loads. Electricity is invariably provided at either 230 or 240 volts (single-phase), or at 400 and 415 volts (3-phase) – single-phase providing for most domestic and small business purposes, and 3-phase being required for industrial or larger commercial uses.
Your critical loads will require one or the other, so it’s important to know which!

Three System Topologies

I want to include a small disclaimer before I get into this: There are more than three types of UPS system.

A wide variety of hybrid systems – mostly falling within the line interactive category mentioned below – have been developed over the years, but have now largely been superseded by modern, online systems.

With this in mind, here are the three primary types of UPS system:

Offline Systems – Usually a small unit which maintains battery charge whilst inactive, switching over to a backup battery power supply in the event that mains voltage fails or falls outside a pre-set tolerance.

Inevitably there is a brief load break during this switchover period (around 2-10ms), and consequently some purists argue that offline systems are really a form of standby power supply rather than a true UPS. This load break also occurs when the system switches back to mains power, so overall power protection is minimal compared to other systems.

Offline systems are typically inexpensive, as they are cheap to produce, and are also more efficient than other systems, as much of the UPS is inactive whilst mains power is within tolerance. These however are not suitable for critical loads.

Line Interactive Systems – Similar to offline systems, this term describes a variety of hybrid systems in which a transformer provides voltage regulation during normal mains usage. The most common types of line interactive system include either a buck/boost or ferro-resonant transformer.

Transformers enable a UPS to maintain a steady output voltage to critical systems – without switching to and from battery power – whilst the mains voltage is within the range of approximately +20% to -30%, resulting in far fewer load breaks and reduced battery wear.

Online Systems – In an online UPS, the battery charger is replaced by a rectifier/charger block, and the system also includes an inverter which constantly provides processed power to its load. During normal operation, mains power is converted from AC to DC by the rectifier in order to charge the battery, and is then converted back to AC by an inverter before it reaches the load.

This process is often referred to as ‘double conversion’.

If the mains supply falls outside pre-set parameters (typically +10% to -20%) or fails entirely, the battery begins to discharge, and the load is unaffected.
The important thing to note here is that power always flows through the UPS – the only difference in the event of a mains failure is that the battery will discharge – and consequently there will never be a load break.

The first online UPS systems, developed in the 1970s, included an output transformer which stepped-up voltage provided by the inverter to a level compatible with the supported load. These systems were widely produced – particularly at the very top of the output power range.

Advances in power semiconductor and transistor technology, however, have enabled the development of transformerless UPS systems. Providing all the benefits of an online UPS, these systems also offer improved efficiency, reduced operating costs, physical size and weight reduction, less noise, enhanced battery life and lower capital costs.
For these reasons, in 1995 KOHLER Uninterruptible Power launched our first 3-phase transformerless UPS in the UK and Ireland, and these days our product range is exclusively transformerless.

Removing Single Points of Failure

When selecting a UPS system, there is one further thing to consider: Standalone, or modular UPS systems?

Standalone systems function exactly as described above – They provide the benefits of modern technology, and several units can be operated in tandem (known as horizontal scaling) to provide for a greater load.

Modular systems perform the same function, but nearly the entire UPS is divided into modules (hardware and software) to minimise single points of failure – If a single module is faulty, that module can be replaced in isolation without even needing to power down the system. This is known as ‘hot swapping’.

As a result of this, modular UPS systems boast greater availability than their standalone counterparts, as although the mean time between failures is slightly lower, the time taken to fix those failures is vastly reduced. This adds up to a 99.9999% (six nines) availability for modular systems, compared to 99.999% (five nines) for standalones.

In addition, modular systems can be scaled both horizontally and vertically, allowing them to be tailored much more accurately to a client’s needs.

Modular UPS Systems

The above image shows a PW 9500DPA system with three 100kvA modules. As you can see, there is space for 2 further modules within this rack (vertical scaling), and it is also possible to combine six UPS units in parallel (horizontal scaling). In practice, this makes it possible for the product to accommodate power requirements anywhere between 100kvA and 3MvA (3000kvA) – Something that was practically science fiction in the not too distant past!

With all this said, the choice of modular vs. standalone will usually come down to the situation at hand, it’s very much a case of picking the right tool for the right job. KOHLER Uninterruptible Power offers both modular and standalone systems in various sizes, and we’ve implemented both for clients in recent years with great success.

At KOHLER Uninterruptible Power we aim to provide our customers with an unrivalled single source for all their power protection needs. We offer a wide range of UPS services, from initial site surveys and UPS system design to implementation, maintenance and repairs. Want to find out more, or book a free UPS site survey? Get in touch.

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