Tech Talk: UPS Bypass Synchronisation

This ‘Tech Talk’ video explores the reasons why a UPS might switch to static bypass and explains the importance of bypass synchronisation to ensure no break power transfer.

A static bypass is a crucial element of all dual conversion online uninterruptible power supply (UPS) system, both single-phase and three-phase types. Its vital role is to automatically and seamlessly transfer the load to the mains supply if there’s a fault or internal failure.

When does a UPS transfer to bypass supply? Well, there are several scenarios. The inverter or rectifier may fail. It may be a fault like an overload or a short-circuit on the downstream side of the UPS. In some cases, it might not even be related to a mechanical fault. Service engineers can switch to bypass mode during UPS maintenance.

In this video, Jason Yates our Technical Services Manager explains the main components of a UPS bypass, the battery and the static switch. He also highlights the role of another core cog, namely the thyristors that conduct the AC voltage waveform.

Jason stresses the importance of bypass synchronisation, namely the output voltage waveform from the inverter must be in sync with the supply voltage waveform fed in from the bypass switch. If the bypass isn’t synchronised in such a way – also known as in-phase – the UPS system won’t be able to deliver smooth, instantaneous no break power transfer.

Video Transcript

Hi and welcome to this week’s Tech Talk.

Today we’re going to be looking at the static bypass, which forms part of all online dual conversion UPS systems, whether they be three-phase UPS or single-phase UPS.

Now on the board here I’ve got a basic line diagram of an online dual conversion UPS system. The online dual conversion UPS system operates simply by taking the power from the mains supply on this side, converting that power from AC to DC via the rectifier, then we reinvert that power from DC back to AC again by the inverter in order to supply power to our critical load.

The static bypass, as the name infers, effectively bypasses the entire UPS system here from the input side here to the output side over here.

Depending on the configuration of the UPS, whether it be a single input UPS or a dual input UPS system, will determine whether or not the bypass supply here will either take the power from the same source as the rectifier, in the case of a single input UPS, or if it’s a dual input UPS or a dual-fed UPS, then effectively there are two supplies coming into the UPS system here and therefore the bypass supply here is on its own dedicated source, and the rectifier is also on its own dedicated source, hence the term dual input. In terms of resilience, the dual input option is the preferred method.

When Will A UPS Transfer To Bypass Mode?

The purpose of the actual static bypass itself is to transfer the load seamlessly, that is with no break, from the output of the inverter here on to the bypass line here whether it be automatically or manually, should the needs arise.

Now the reasons that can cause the UPS system to automatically transfer the load from the inverter onto the bypass line and vice versa, such things as if the inverter or the rectifier here were to fail, or there was a problem with the controlling architecture within the UPS system which meant that the output voltage waveform from our UPS here was having a problem or was beginning to collapse, then the UPS system can seamlessly transfer the load on to the static bypass line and hold it there until either the matter is resolved, or if the matter can’t be resolved, it will maintain it until further action can be taken by the intervening service engineer.

Another reason why it would transfer to bypass should be if we have a fault or an issue on the downstream side of the UPS, such as an overload or a short-circuit. Again the UPS system can transfer the load cleanly on to the static bypass line here in order for the fault to be cleared, and then it can return back online again once that fault is cleared.

Sometimes it could be activated manually by a service engineer. Now if a service engineer is maintaining the UPS system, he will transfer the load onto the static bypass switch here in order so that you can transfer it onto the main wraparound bypass and therefore provide a completely seamless transfer.

Then he will return this back via the static switch when he’s putting the UPS back into circuit once he’s completed his maintenance, and therefore the static switch allows the user or a service engineer to seamlessly transfer the load onto the bypass and vice versa should they need to.

What Are The Main Components Of A UPS Bypass?

Now the bypass itself is made up of two primary components. You have the battery protection and you have the main static switch. The battery protection there is basically there to prevent the UPSs from back-feeding power through the bypass line here when there’s no main supply here. If there’s no main supply here and the UPS is supplying the connected load of either from the batteries, then effectively we don’t want any power trying to flow back through the static bypass switch there, therefore the battery protection device prevents us.

The battery protection device is typically made up of either a contactor, which will physically open and therefore we can’t pass any current back, or it will be via a current device so that if we measure any current flowing in the bypass line when we are on battery operation, then the UPS can take the appropriate action.

The static bypass switch itself is so-called because it effectively has no moving parts. It’s static and it’s not like a mechanical switch. It basically comprises two components referred to as thyristors. Now thyristors over many, many years have proven themselves to be a highly-reliable component and a perfect application, or perfect component for this kind of application.

In this case, we have two thyristors in parallel with one another but facing in opposite directions. The purpose of that is because the upper thyristor here effectively conducts the positive half of the AC voltage waveform, whereas the lower thyristor here conducts the lower half of the AC voltage waveform.

When they’re combined and both switched on they effectively act like a switch or in this case the static switch bypassing the voltage from the inputs here to the output side over here.

Why Is UPS Bypass Synchronisation Important?

One of the main things that must happen within any UPS system to allow a zero-break transfer is that the output voltage waveform from our inverter here must always be in synchronisation with the supply voltage waveform that’s coming in via our bypass switch.

If that’s not in sync, unfortunately, the UPS can never make a seamless transfer. Some UPSs will simply not transfer to bypass or put in a design-dedicated break within the supply in order to make sure that no problems occur and no damage to the load occurs.

In this case, the UPS system itself will always try to maintain that the inverter outputs, again on this side, remains synchronised and basically by being synchronised that will allow the UPS system to instantaneously transfer the load from the inverter onto the bypass line and vice versa.

If you don’t know what I mean by synchronisation or in-phase, then what we can do now we can take a look at an oscilloscope and I can physically show you what the waveforms look like when they’re in phase or out of phase.

Okay, so if we take a look at the oscilloscope, the waveform that you’ve got across the top here is the waveform that is supplying the bypass line. The waveform along the bottom is the waveform that is being produced by the inverter. Now if I move those two waveforms over one another, you can see clearly that they’re practically identical and hence in phase with one another.

If I instruct the UPS system to disable the synchronisation like so, you will see that the two waveforms now stray apart. Under these conditions, the input and output are no longer synchronised and hence the bypass line would be unable to transfer instantaneously.

However, if I now force the UPS system to re-synchronise again like so, you can see that the UPS system re-synchronises the inverter output and hence an instantaneous transfer to static bypass can take place.

Well, I hope you enjoyed that Tech Talk. If you have a question that you would like us to answer here at Riello UPS, then please drop us an email to Thank you.