Tech Talk: How Does Online UPS Provide No Break Power Transfer?

Learn how an online UPS’s dual conversion technology helps deliver no break power transfer if there’s a fault or failure with the mains supply.

An uninterruptible power supply (UPS) carries out the crucial role of maintaining a continuous power supply to the critical load at all times, even if there’s disruption to mains electricity. Even a seemingly infinitesimal break in continuity can prove catastrophic.

Because it uses dual conversion technology, the output from an online UPS in unaffected even if mains power fails. In this latest ‘Tech Talk‘, Jason Yates, Technical Services Manager for Riello UPS, uses an oscilloscope to describe the process of no break power transfer in an informative yet easily-understood way.

He also explores the importance of UPS batteries and capacitors and how they both – in very different ways – help to ensure a disruption-free flow of electricity.

Video Transcript

Hi and welcome to this week’s Tech Talk.

Today we’re going to be looking at one of the ways in which an online dual conversion UPS system provides protection to the critical load which is connected to its output.

Now the primary purpose of the UPS is to ensure that, should the incoming mains supply to the UPS system fail, then the UPS will ensure that the supply to the critical load remains present at all times.

This is referred to as no-break because when the main supply fails the output to the critical load remains completely unaffected. Exactly what do I mean by no break? Well if we take a look at the oscilloscope I can show you in a little bit more detail.

OK, so if we take a look at the oscilloscope here the waveform that we have across the top here is effectively the mains supply that’s coming into the UPS. The waveform along the bottom is the waveform that is being supplied to the critical load and hence the protected side of the UPS.

Now if I slow the trace down like so and then fail the main supply to the UPS, I can stop the trace and effectively capture the moment when the main supply fails.

If I zoom into that moment like so, you can see that even though the main supply has failed the output from the UPS system remains completely unaffected. If I return that process, in that we will return the mains supply back on to the UPS like so, if I slow down my trace once again and reapply that mains supply and again stop the trace.

If I zoom in to that trace, again even though the main supply has returned, the output voltage waveform from our UPS system remains completely unaffected.

If we take a look at the diagram of the board here this effectively is a basic block diagram of an online dual conversion UPS system. The online dual conversion UPS system operates simply by taking the voltage from the incoming mains supply converting that voltage from AC to DC by the rectifier, then we re-invert that voltage back from DC back to AC again by the inverter in order to supply that voltage to the critical load.

In between the rectifier output and the inverter input are connected the batteries. Depending on the type of UPS will determine how those batteries are connected. Some systems will use a direct connection where the batteries are connected directly onto the DC bus, some systems will use an IGBT-based (insulated-gate bipolar transistor) switching technology via a bus-boost system, and some systems will just use a thyristor-based switching mechanism.

Regardless of which, when the mains supply on the incoming side of the UPS system fails, the power being provided from the rectifier will also fail. Now the purpose of the rectifier to ensure that the inverter which is providing our AC voltage is provided with a continuous source of electrical energy. If the mains supply fails that is no longer available and therefore the UPS system must absorb that power from the connected batteries.

However, once the mains supply fails, there is a delay in which the UPS system can transfer the power from the output of the rectifier on to the batteries. If the delay is too big, a break in the supply to the critical load will occur.

Inside each UPS system, there are devices referred to as capacitors, and its symbol basically is similar to a battery but with two vertical lines at the same length. A capacitor is very similar to a battery, however a battery is a chemical reaction which is providing power for a prolonged period of time, whereas a capacity will only give us back what we put into it minus losses etc.

The purpose of a capacitor is to ensure that whilst we’re transferring between the rectifier output and on to the batteries the capacitor will hold up that DC rail for a long enough time so that we can make that transfer and the inverter does not see any disruption in the source of electrical power.

Therefore the inverter can continue operating and continue providing the protected load with a clean source of electrical power. When the mains supply returns again the process is simply returned, and so therefore inside the UPS we have banks of capacitors which can bridge that gap when the mains supply fails and that we transfer onto the battery and vice versa.

Well I hope you found this Tech Talk useful and if you have a question that you would like us to answer here at Riello UPS, then please email us Thank you.