OK, so we’ve looked at what a UPS is, what it does, and some of the different forms a UPS system can take.
Before we move on to other topics, I wanted to cover the major components of a modern, online UPS – Otherwise known as a ‘double conversion’ UPS.
Aside from being the only true form of UPS – i.e. completely free from load breaks – online systems comprise the entirety of our UPS product range, so it only makes sense to explain how they actually work!
This is going to be the short, easy-to-understand version of the story – If you’re interested in the complete and (extremely) thorough version, as well as a lot more besides, I strongly recommend picking up a copy of the UPS Handbook.
The Anatomy of an Online UPS
An online UPS comprises four main parts: A rectifier/charger block, a battery, an inverter and a static switch. To see how these components fit together, have a look at the following simple block diagram.
Rectifier/Charger block– Although I’ve listed them together, the rectifier and charger can either be separate modules or combined as a single power block. They perform the same function either way, so for the sake of simplicity they are displayed in our block diagram as a single component.
The function of this combined power block is to take AC (Alternating Current) power from the mains, and convert it to DC (Direct Current) power, which is required to charge the battery. From there the charger directs power to the battery in order to maintain charge, and the rectifier provides a stable DC current directly to the inverter, all via the DC Bus.
You’ll notice that the DC Bus didn’t get its own explanatory section in this article, and that’s because it’s simply a fancy name given to the connection between power block, battery and inverter.
During times of power outage or fluctuation (typically outside +10% to -20% of normal) the charger shuts down, and the battery provides DC power to the inverter instead.
Battery– As previously discussed in the article on battery maintenance, most UPS systems (ours included) utilise lead-acid batteries. Since we’ve already covered the inner workings of UPS batteries I won’t bore you with it again – If you haven’t already, I’d suggest following the link and having a quick read.
Inverter– The inverter provides the second half of our double conversion process, by converting DC power from the rectifier/battery back to AC power. We needed DC power to charge the battery, but it’s AC power that’s required by your critical loads, so this secondary conversion process is vital.
One of the main benefits of an online UPS is that it provides the greatest level of protection to your critical load from power ‘events’ such as spikes, sags, surges, electrical noise, and so on. This is because, unlike the other types of UPS described in our last article, all of the power supplied to your critical load has been routed through the UPS, and thus undergone the double conversion process from AC to DC and then back to AC.
Static Switch– Of course, there is one scenario we haven’t discussed yet, and it’s a particularly important one to consider when implementing an online UPS.
We know what happens when there’s a power failure, but what happens if there’s a UPS failure? With an online system, all power to your critical load comes via the UPS… so if that fails it’s going to be just like having a power failure without a UPS, right?
Well thankfully not, and that’s down to the static switch.
The switch has two modes; by default the switch will accept AC power from the UPS, but it also has the option to accept AC power directly from the mains, or ‘bypass’.
Switching over to mains power is far from ideal, so in the event your critical loads are transferred to raw mains power it will be accompanied by an alarm or warning condition to ensure the UPS issue is resolved as quickly as possible.
Keeping Bypass to a Minimum
When I say that switching over to raw mains power is far from ideal, I’m not being flippant.
If you think back to my earlier article explaining the function of a UPS, you’ll recall that protecting against power disturbances is one of the most important functions of a UPS.
During periods of UPS failure, this vital task is foregone.
Our fancy UPS is temporarily relegated to little more than ornamental status.
This is why uptime is so important. At Kohler Uninterruptible Power we’re proud to say that our standalone solutions offer 99.999% (five nines) uptime, with an amazing 99.9999% (six nines) in the case of our modular systems.
To put that in terms that are easier to understand, our modular systems boast an average of just 31 seconds downtime per year, or more than six minutes over an average 10-year service contract. Standalone systems follow-up with an impressive average of just over five minutes downtime per year, or under an hour during the course of the same 10-year contract.
Of course these are simply averages – I’m not suggesting that a UPS system failure can be resolved in mere minutes. Technology has come on a lot in recent years, but we’re not quite there yet.
The time taken to resolve a modular UPS failure is substantially lower than with a standalone UPS, and it is often possible to repair or replace individual modules without ever needing to switch over to bypass power.
It is for these reasons that Kohler Uninterruptible Power have increasingly recommended modular systems to our clients in recent years.